GRAPHENE NANOSTUCTURING WITH THE HELIUM ION MICROSCOPE

Transcription

1 GRAPHENE NANOSTUCTURING WITH THE HELIUM ION MICROSCOPE WANG YUE (B.Eng.(Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2015

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3 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. WANG YUE 13 th May 2016

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5 Acknowledgements It is my great pleasure to express my sincere gratitude to all those who supported me during my pursuit of this Ph.D; it would not have been possible to write this dissertation without the help of these kind people. Foremost, I would like to express my deepest gratitude to my supervisor Dr. Daniel S. Pickard for his continuous support of my Ph.D study, his guidance, inspiration, enthusiasm, and unsurpassed knowledge, and his financial support. His mentorship encouraged me to grow not only as an experimentalist, but also as an instructor and independent thinker. He provided the opportunity to develop individuality and self-sufficiency by allowing me to work with great independence and be immersed in a huge amount of resources. I also express my appreciation to the other two committee members, Prof. John Thong and Prof. Albert Liang, for their encouragement and insightful comments. I would also like to thank all my fellow labmates in Plasmonics and Advanced Imaging Technology Laboratory for the stimulating discussions, for the sleepless nights we spent working together, and for all the fun we had over the past few years. I am most grateful to Dr. Ai Zhongkai for working with me enthusiastically in various aspects. I thank Dr. Vignesh Viswanathan for the pioneer work on this project, and the various types of help provided along the way. I am indebted to Dr. Hao Hanfang, Gu Jing, Shamir Yusuf, Nabeel Shami and everyone who works or has worked in this lab, who were always willing to help and give their best suggestions. It would have been a lonely lab without them. I would also like to thank Dr. Andersson for his help and suggestions. Finally, I would like to thank my parents. Their patience, encouragement, and love have continuously motivated me, and without their love and understanding I would not have completed this work.

6 Table of Contents Abstract...ix List of Figures...vi List of Symbols...ix List of Abbreviations... x Chapter 1. Introduction Lattice Structure and Electronic Structure Nanopatterned Graphene and Applications Fabrication Techniques of Graphene Nanostructures Characterization Techniques of Graphene Organization of Thesis Chapter 2. Sub-10-nm Direct Patterning of Graphene Nanostructures using Focused Helium Ion Beam Introduction Helium Ion Microscope Graphene Imaging Using HIM (Secondary Electron Mode) Graphene Patterning Using HIM (Sputtering) Direct Patterning of Graphene Nanostructure Using Focused Helium Ion Beam HIM Graphene Patterning Dose Control HIM Graphene Patterning Strategy Patterning Cut Width Selected Results of Direct Patterning Method Contamination from Hydrocarbons Chapter 3. Analysis of Helium Ion Irradiation Induced Graphene Damage Introduction Helium Ion Beam Induced Graphene Damage Raman Analysis of Helium Ion Irradiation Induced Graphene Damage Raman Spectroscopy for Graphene Characterization Probing Controlled Damage of Graphene Introduced by Helium Ion Irradiation 48 i

7 3.3.3 Evolution of the Raman Spectra for Graphene under Controlled Helium Ion Irradiation Conclusion Chapter 4. Atomic-Hydrogen-Assisted Nano-Scale Graphene Patterning Using Helium Ion Microscope Introduction Instrumentation Setup for Atomic Hydrogen Etching System Experimental Results and Discussions Local Defect Introduction Defect Density Etching Resolution Layer-Number-Dependent Etching Edge Selectivity Other Factors Conclusion Chapter 5. Graphene Device Fabrication Techniques Introduction Transfer-Free Suspended Graphene Sample Preparation Technique Novel TEM Sample-Preparation Technique Preliminary TEM Results of Graphene Deposited on the Novel Substrate Other Approaches Resist-Free Graphene Device Fabrication Technique for Electronic Applications Fabrication of GNR-FET Electrode Fabrication Conclusion Chapter 6. Summary and Future Work Summary Future Work Graphene NEMS Nano-Patterning of Other 2D-Materials References ii

8 Appendix A: TRIM Simulation Setup Appendix B: Graphene Sample Preparation Appendix C: NPGS Setup Appendix D: Raman Peak Fitting iii

9 Abstract Graphene has attracted tremendous interest in the past decade due to its superior physical properties and broad range of potential applications. Many of these applications require the engineering of graphene at the nanometer scale. However, the nanostructures accessible by conventional fabrication methods are either limited by the lithographic resolution or limited to geometries such as dots, hexagons, or ribbons. In this thesis, we demonstrate a resist-free, top-down direct patterning method for arbitrary sub-10-nm graphene nanostructures using a helium ion microscope. The small probe size of the He + beam (<0.5 nm) enables the fabrication of complex structures and devices with high resolution on both supported and suspended graphene samples. However, the low sputtering yield requires a high fluence of incident helium ions (~10 18 He + /cm 2 ) to completely sputter the carbon atoms in the cut-region of graphene. Under such a high fluence, the backscattered helium ions cause significant damage to the graphene adjacent to the helium ion exposed area, degrading the device s properties. Another key contribution of this work is the demonstration of a means to substantially avoid this ion induced damage by combining directed damage with a selective atomic hydrogen post-etch. This combined damage/etch technique eliminates the need for direct sputtering in the cut-regions, thereby reducing the ion fluence required to clear a device by up to two orders-of-magnitude. The process consists of deliberately damaging the graphene in the desired cut-regions with a relatively low dose (~10 16 He + /cm 2 ), iv

10 followed by exposure to atomic-hydrogen, which selectively etches the carbon atoms exhibiting broken bonds in the damaged regions. Since the etch is highly selective relative to the graphene, the fabrication of arbitrary nanoscale features is possible with high resolution, well-defined edges, and limited residual damages. Raman characterization indicates a fluence of He + or less results in negligible damage caused by the backscattered helium ions. Therefore, the graphene nanostructures fabricated through this method are well-suited for electronic applications. In addition to fabrication methods using helium ion irradiation, we also explore techniques for resist free device fabrication. These include: 1) A novel suspended graphene sample preparation method which avoids wet chemical graphene transfer, thereby eliminating the introduction of contaminants; and 2) A stencil mask based contamination-free electrode deposition method to enable fabrication of defect-free electronic devices. v

11 List of Figures Figure 1-1 Schematic drawing of a hexagonal lattice of graphene Figure 1-2 Graphene energy bands near Fermi level Figure 1-3 Schematic of graphene nanoribbon (GNR) with different edge configurations Figure 1-4 Edge-configuration-dependent and ribbon-width-dependent bandgap for graphene nanoribbons Figure 1-5 Graphene nanostructure fabrication techniques Figure 1-6 Characterization techniques for graphene Figure 2-1 A heterostructure graphene device fabricated using the helium ion microscope Figure 2-2 The Helium ion microscope Figure 2-3 Trajectories of 30 kev He + (left) and 5 kev e - (right) incident on 300 nm SiO 2 on Si substrate, calculated using TRIM and CASINO software package respectively Figure 2-4 HIM image of mechanical exfoliated graphene flake deposited on SiO 2 /Si substrate patterned with nanodots Figure 2-5 Average fraction of lost atoms (vacancy-type defects) of suspended graphene exposed with helium ion beams of 30 kev and 35 kev Figure 2-6 Ribbon array used to determine the required patterning dose on suspended graphene Figure 2-7 Illustration of graphene nanoribbon patterning procedure Figure 2-8 HIM image of patterned graphene ribbon on SiO 2 /Si substrate with heavy incident helium ion dose and large exposure area Figure 2-9 HIM patterning isolation test Figure 2-10 Graphene ribbons defined by 2-nm cut width Figure 2-11 Examples of graphene nanostructures fabricated by HIM direct sputtering method Figure 2-12 Suspended graphene nanoribbon fabricated and imaged using HIM.. 34 Figure 2-13 HIM image of dot array with 10 nm dot diameter and 50 nm spacing Figure 2-14 Star-pattern fabricated and imaged using HIM Figure 2-15 Raman spectrum of graphene sample before and after Evactron clean Figure 2-16 Customized heater stage mounted on the x-y-rotational-stage in HIM Figure 2-17 Raman spectrum of graphene sample before and after in situ heating Figure 3-1 Ribbons with width varies from 5 nm to 30 nm on graphene sample deposited on bulk silicon capped with 285 nm SiO 2 with cut width of 2 nm.. 42 Figure 3-2 TRIM simulation of He + bombardment to SiO 2 /Si substrate Figure 3-3 Typical Raman spectra of single-layer graphene with and without vi

12 He + -irradiation induced defects, obtained using 532 nm excitation laser Figure 3-4 Raman characterization of graphene under different helium ion dose exposure Figure 3-5 Graphene Raman G-peak and D'-peak under 35 kev helium ion exposure with increasing dose Figure 3-6 I D /I G plot under helium ion beam exposure with different area dose Figure 3-7 I D /I G ratio and fraction of remaining graphene under increasing helium ion irradiation at stage II Figure 3-8 FWHM of Raman G-peak and 2D-peak of few-layer graphene exposed with 35 kev helium ions Figure 4-1 Schematic of the two-step atomic-hydrogen-assisted HIM graphene patterning process Figure 4-2 Raman spectrum of graphene flake before and after 1 hour atomic hydrogen treatment Figure 4-3 Schematic of the home-built atomic hydrogen system Figure 4-4 Overview of the home-built atomic hydrogen system Figure 4-5 Atom Source Figure 4-6 Residual gas analyzer (RGA) chamber-component monitoring during the atomic hydrogen plasma process Figure 4-7 Temperature calibration of the button heater Figure 4-8 Array of nm box patterned with different helium ion area doses on a multi-layer graphene Figure 4-9 Atom loss and amorphization of graphene by helium ion irradiation calculated for 30 kev and 35 kev incident helium ions using ion beam damage simulation for graphene Figure 4-10 I D /I G plot under helium ion beam exposure with different area dose before and after 2hr atomic hydrogen exposure Figure 4-11 Arrays of ribbons of 20 nm widths and 500 nm lengths defined with different cut widths ranging from 5 nm to 40 nm Figure 4-12 HIM image of 15 nm ribbon array with cut-width ranging from 1 nm to 20 nm Figure 4-13 HIM images of box arrays patterned with different sizes on supported and suspended few layer graphene flakes Figure 4-14 HIM image of 15 nm ribbon array defined by two separate cuts with increasing cut width on suspended graphene Figure 4-15 HIM images of arrays of ribbons of 20 nm width and 500 nm length defined by different cut widths and helium ion doses, for both suspended mono-layer graphene and multi-layer graphene Figure 4-16 Angle dependent sputtering fraction, amorphization area fraction, and backscattered ion distribution under 35 kev helium ion irradiation with incident helium ion dose of ions/cm Figure 4-17 Backscattered helium ion density on substrate with different thicknesses calculated by TRIM simulations Figure 5-1 Bright-field TEM image of suspended graphene after forming gas vii

13 annealing Figure 5-2 Images of MCP substrate for suspended graphene compatible with TEM studies Figure 5-3 TEM image of suspended graphene with laser-cut micro-channel plate as supporting grid Figure 5-4 Schematic of the customized suspended graphene supporting grid for TEM study Figure 5-5 Schematic of simple back-gate GNR-FET Figure 5-6 Schematic illustration of GNR-FET fabrication process Figure 5-7 Illustration of nanoribbon fabrication process Figure 5-8 Schematic of electrode deposition process using nanostencil mask Figure 5-9 Exploded view of the components for the illustration of stencil mask alignment procedure Figure 5-10 Stencil mask alignment procedure Figure 5-11 Stencil mask for electrode deposition Figure 5-12 Images of GNR-FET fabricated using atomic-hydrogen-assisted etching and nanostencil electrode deposition at different field of view Figure 6-1 Drum structure fabricated on suspended graphene for NEMS application Figure B-1 Graphene sample preparation and identification Figure B-2 Raman Characterization of graphene with different thicknesses Figure C-1 Definition of center-to-center and line-spacing in NPGS viii

14 List of Symbols Symbol Description Unit E L laser excitation energy ev I D relative intensity of Raman D-peak arb. units I G relative intensity of Raman G-peak arb. units K conical point L graphene nanoribbon length nm n D defect density cm -2 W AC graphene nanoribbon width with armchair edge nm W ZZ ribbon width with zigzag edge nm α primitive translation vectors indicates the 2D-real space unit vector Γ Brillouin zone center Γ 2D the full width at half maximum of Raman 2D-peak cm -1 Γ G the full width at half maximum of Raman G-peak cm -1 ix

15 List of Abbreviations AFM Ar Au a:c Bi 2 Te 3 C CH 4 CMOS Cr CVD FET FOV FIB FWHM Ga + GFIS GNR GNR-FET H HIM He + HR-TEM H + H 2 ITO MCP atomic force microscope argon gold amorphous carbon bismuth telluride carbon methane Complementary metal-oxide-semiconductor chromium chemical vapor deposition field effect transistor field of view focused ion beam full width at half-maximum gallium ion gas field ion source graphene nanoribbon graphene nanoribbon field effect transistor hydrogen helium ion microscope helium ion high-resolution transmission electron microscope hydrogen ion molecular formula of hydrogen tin-doped indium oxide micro-channel plate x

16 MoS 2 MoSe 2 MOSFET NH 3 NP NEMS NM O PDMS PMMA RGA RIE SAMA SEM Si SFIM SF 4 SiO 2 Si 3 N 4 SNR STM TEM UHV WS 2 molybdenum disulfide molybdenum diselenide metal-oxide-semiconductor field-effect transistor ammonia nanoperforation nanoelectromechanical systems nanomesh oxygen polydimethylsilosane Poly(methyl methacrylate) residual gas analyzer reactive ion etching surface-assisted molecular assembly scanning electron microscope silicon scanning field ion microscope sulfur tetrafluoride silicon dioxide silicon nitride signal to noise ratio scanning tunneling microscope transmission electron microscope ultra-high vacuum tungsten disulfide xi

17 Chapter 1. Introduction Graphene has become one of the most investigated materials since its first isolation in 2004 [1]. The superior physical properties, such as exceptionally high carrier mobility at room temperature ( cm 2 V -1 s -1 ) [2], superior thermal conductivity ( Wm -1 K -1 ) [3], fractional quantum Hall effects [4, 5], chiral tunneling [6, 7], and good optical transparency over a broad wavelength range (97.7%) [8] have resulted in enormous potential applications in many emerging fields, ranging from high frequency electronics to chemical sensors, light emitting devices, energy, and touch panels [9]. 1.1 Lattice Structure and Electronic Structure Figure 1-1 Schematic drawing of a hexagonal lattice of graphene. Its unit cell (yellow) contains two carbon atoms. The primitive translation vectors and indicate the 2D-real space unit vectors. The distance between the nearest carbon atoms is 1.42 Å [10]. 1

18 Graphene, a single atomic layer of graphite, consists of a two-dimensional hexagonal lattice of sp 2 -hybridized carbon atoms with a carbon carbon distance of 1.42 Å and lattice constant of 2.46 Å [10, 11], as shown in Figure 1-1 [12, 13]. Although graphene strictly refers to a single-layer material, bior multi-layer graphene samples are being studied with equal interest [14]. Figure 1-2 Graphene energy bands near Fermi level. Black hexagons represent represent the electronic Brillouin zones of graphene, red rhombus represents the first-phonon Brillouin zone, and the conical shape represents the electronic dispersion. The phonon wave vectors connecting electronic states in different valleys are labeled in red. Graphene has two atoms per unit cell, and thus six normal modes at the Brillouin zone center Γ. This conical shape is the same at Κ and Κ' points. When the Fermi level is above the Dirac point, the carriers are electron-like, while when the Fermi level is below the Dirac point, the carriers are hole-like. Figure reproduced from [15]. The fascinating properties of graphene are attributed to its unique electronic structures: the linear dispersion relation near the Dirac points. As shown in Figure 1-2, in each Brillouin zone, the conduction band and the valence band cross at two conical points, K and Κ'. A linear dispersion relation is evident about the K and Κ' points for electrons and holes, causing the electrons in a pristine graphene sheet to behave like massless Dirac fermions [16]. This conical dispersion is minimal at Κ and Κ' points, which coincides with the 2

19 Fermi level, and separates conduction and valance bands, revealing a zero band gap in the graphene. Owing to this dispersion relation, the density of states of graphene is linear with energy, and vanishes at the Dirac points. 1.2 Nanopatterned Graphene and Applications Graphene exhibits lots of unique properties and quantum mechanical phenomenon when engineered into the nanometer scale. In this section, we summarize some of properties and potential applications of the mesoscopic graphene. The early investigations explored graphene nanoribbons (NRs) with nanometer scale width resulting in an energy bandgap [17, 18]. These studies have been further studied for nanoelectronic applications [1, 19-21]. Sub-5-nm GNRs are necessary for a sizeable bandgap and ON/OFF ratio [22-25], with up to 10 6 ON/OFF ratio reported for ultra-small devices [26-29]. Density functional theory studies [30, 31] and experimental studies [18] have shown that the bandgap of a GNRs scale inversely proportional to the ribbon width [28] and is significantly influenced by the edge configurations [32]. 3

20 Figure 1-3 Schematic of graphene nanoribbon (GNR) with different edge configurations. (a) Armchair-GNR of length L and width W AC. (b) Zigzag-GNR of length L and width W ZZ. The edge configurations in graphene can be classified into two types: armchair and zigzag, Figure 1-3, defined by the orientation of the hexagons relative to the ribbon length, with crystallographic orientations differing by 30 [33]. Figure 1-4 shows the bandgap being inversely proportional to the width of the nanoribbon for zigzag and armchair nanoribbons, with different origins of bandgap opening [34, 17]. Armchair-GNRs are semiconducting due to the quantum confinement of charge carriers, while zigzag-gnrs possess a magnetic order when narrower than 7 nm [35, 36], which may induce a bandgap for the otherwise metallic structure caused by the flat band edge state. This magnetic moment formed at the graphene zigzag edges in addition with 4

21 its gate-tunable carrier concentration and high electronic mobility makes graphene promising for spintronics devices [37]. Figure 1-4 Edge-configuration-dependent and ribbon-width-dependent bandgap for graphene nanoribbons. The armchair-gnr presents a quantum confinement bandgap inversely proportional to the ribbon width (where n = 1, 2, 3, ). The zigzag-gnr has a bandgap governed by the emerging edge magnetism, and a sharp semiconductor (antiferromagnetic) to metal (ferromagnetic) transition is revealed. Data points were calculated using the mean field Hubbard model. This figure is replotted from [17]. Other nanostructures, such as nanomeshes (GNMs) [38-40], nanoperforations (NP) [41], and antidot lattice were also explored for bandgap opening, with critical dimension of graphene in the sub-10 nm regime. Graphene nanostructures are also promising candidates for single-electron transistors [42] and quantum dot devices [43, 44], the performances of which are also largely affected by the edge configurations [45-47]. In addition to its fascinating electrical properties, nanostructured graphene possesses a tunable optical absorption [48-50], which enables a broadband 5

22 optical response from microwave to ultraviolet, including the range which is commonly used for fiber-optic communications [51]. This, combined with ultra-high carrier mobility, makes graphene a potential candidate for optical applications, including ultrafast photodetectors [52], modulators [53], terahertz wave detectors [54], and tunable fiber mode-locked lasers [19, 49]. Pure monolayer graphene photodetectors have been created by introducing electron-trapping centers and creating a bandgap in graphene through bandgap engineering, demonstrating the potential of graphene as a promising material for efficient optoelectronic devices [55]. Applications of graphene in nanophotonics and optical applications such as that of graphene plasmonics [56], have also attracted much interest, due to its unique electrical tunability, which is not possible with conventional metals [57], long plasmon lifetime [58-63] which is inversely proportional to the confinement, and high degree of electromagnetic confinement [61, 64]. The electromagnetic response of a patterned graphene structure moves from the terahertz frequency regime to the mid-infrared regime as the device dimension shrinks from micron to nanometer scale [60, 62, 63, 65]. In this spectral regime, graphene possess a light confinement volume 10 6 times smaller than the diffraction limit, which makes graphene an attractive platform for nanophotonics. In devices such as GNRs, the q factor is inversely proportional to the ribbon width for localized plasmons [66]. This scalable response could be utilized for applications such as plasmonic waveguides, modulators, 6

23 detectors from sub-terahertz to mid-infrared regimes, and mid-infrared photonic devices. Graphene-based nanoelectromechanical systems (NEMS) have been studied [67] with the aim of making mechanical resonators from the large surface area material. Coupled with its superior electronic properties, graphene-based NEMS can exhibit sensitivity much higher than competing technologies [68]. Having the largest specific surface area (2630 m 2 g -1 [69]) of all materials makes graphene a good candidate for sensors. As the interaction with chemical species depends strongly on the presence of defects, nanostructures of graphene have also been introduced to enhance the gas sensitivity [70-73]. Other phenomena such as induced magnetism or a strong anisotropy of quantum transport have been predicted for nanopatterned graphene [74, 75]. 1.3 Fabrication Techniques of Graphene Nanostructures All these applications discussed above require the graphene to be engineered into nanometer scale. In this regime, the critical dimension and edge quality directly affect the property of graphene. The graphene nanostructures are non-trivial to synthesize experimentally, due to the difficulties in controlling their width, edge geometry, and chemical functionalization etc., which cause large variation between devices and seriously degrade the ON/OFF ratios of the logic device [76-82]. As a result, it is highly desirable to minimize any 7

24 edge roughness and have uniformly organized GNRs for electronic applications. In this section, we summarize some of the established fabrication techniques to realize graphene nanostructures with critical dimensions below 50 nm. Several top-down lithographical method such as electron-beam lithography [21], nanosphere lithography [83, 84], nanoimprint lithography [85], and copolymer lithography [39, 41], have been employed for restructuring graphene. Direct patterning by transmission electron microscope (TEM) [86] and scanning tunneling microscope (STM) [87] have also been demonstrated. Other methods such as sonochemical breaking of chemically derived graphene [28], metal nanoparticle catalysed cutting [88], unzipping of carbon nanotubes along the axial direction [89], and cutting graphene sheets with a focused charged particle beam [90] have been studied as well. Bottom-up methods are also developed such as direct chemical synthesis [91], direct chemical vapor deposition (CVD) growth [92], and surface-assisted molecular assembly (SAMA) [93, 94]. Figure 1-5 summarizes and illustrates some of these nanofabrication techniques. 8

25 Figure 1-5 Graphene nanostructure fabrication techniques. (a) Electron beam lithography followed by reactive ion etching. (b) Direct milling using ion beams. (c) Direct surface structure modification using STM. (d) Metal nanoparticle catalyzed cutting. (e) Sonochemical breaking of chemically derived graphene. (f) Unzipping from carbon nanotubes [21, 91, 28, 88, 89, 90, 87, 95]. The most conventional method for graphene nanostructure fabrication is top-down plasma etching, which make use of the resist [21, 96], metals [97], 9

26 nanowires [29] or copolymers [39, 41] as masks for the following reactive ion etching (RIE) process to etch away the graphene in the exposed region. The biggest drawback of this top-down plasma etching method is the poor edge quality, which can severely degrade the electronic properties, especially for sub-20-nm nanostructures [98]. Limited by the lithography resolution, the conventional e-beam lithography followed by RIE method is only reliable for patterns ~20 nm, while sub-5-nm GNRs are desirable for high ON/OFF ratio FETs at room temperature. Moreover, the commonly used resist for e-beam lithography such as poly(methyl methacrylate) (PMMA) can lead to the presence of unwanted residues which are difficult to fully remove, and ultimately reduce the carrier mobility [99]. The use of nanowires instead of resist as the etching mask pushes the patterning resolution to ~8 nm, also avoids the contamination problem, but with the difficulty of controlling the position of the nanowires. Additional step of gas-phase anisotropic etching can further push the resolution of the graphene nanostructure down to ~4 nm with well-defined zigzag edges [97, 100], although the range of possible patterns that can be generated are still limited by the lithography resolution to a certain extent. The metal nanoparticle catalysed cutting method can result in well-defined edges, but the difficulties in controlling the mobile metal particles resulting in the irregular shape which is poorly suited to integrated device applications. Chemical sonication of expandable graphite can result in GNRs as small as ~2 10

27 nm in width [28], although the size and position of the resultant graphene nanoribbon cannot be controlled precisely. TEM and STM can directly pattern graphene with atomic precision, but these techniques cannot be scale up for practical applications [101, 102]. Therefore, there is still a need to develop a clean patterning technique that can fabricate sub-10 nm graphene nanostructures with high precision. In this thesis, we address this deficiency with a top-down contamination free method of graphene nanopatterning using a helium ion microscope (HIM). With this method, fabrication graphene features down to ~4 nm with well-defined edge structures is demonstrated. 1.4 Characterization Techniques of Graphene The characterization of graphene is an important aspect of developing our proposed nanofabrication technique, in order to study the quality and properties of the fabricated graphene nanostructures. Properties such as the number of graphene layers, the structure and morphology of the graphene sheet, the purity of graphene sample in terms of defects, and other property changes in terms of doping, functionalization, strain etc. must be studied to thoroughly evaluate the fabrication capabilities and its limitations. 11

28 Figure 1-6 Characterization techniques for graphene. (a) Optical image of graphene with different numbers of layers deposited on SiO 2 /Si substrate. (b) SEM image of graphene ribbon between electrodes using InLens detector. (c) STM atomic level images with lattice structure [103]. (d) TEM electron diffraction pattern of graphene. (e) AFM images of graphene morphology [28]. (f) Raman spectroscope image of defect free graphene. Characterization methods of graphene usually involve measurements based on various microscopic techniques such as using an optical microscope, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission 12

29 electron microscopy (TEM), scanning tunneling microscopy (STM), and Raman spectroscopy, all of which are briefly illustrated in Figure 1-6. Optical microscope is the simplest tool for identifying graphene flakes and detecting the number of graphene layers. This method is based on the contrast arising of the reflected light from each interface [104]. Silicon with 90-nm or 285-nm SiO 2 capping layer is commonly used for high contrast optical imaging based on Fresnel s law [ ]. The major drawback of this method is that it does not provide sufficient resolution to study the nanoscale properties of graphene. SEM can provide more detailed surface information as well as the number of graphene layers due to the linear relationship between the secondary electron intensity excited from the sample surface and the number of graphene layers [108]. However, the resolution is not high enough to provide atomic level imaging to study the atomic arrangement of graphene, point defects and edge configurations, which can be achieved using a TEM or STM. Identification of defects, doping [109] and direct counting of graphene layers [110] is possible with these instrumentations. AFM can provide information about graphene s structural, mechanical, and electrical properties using various modes and configurations of operation [111]. It does not directly probe the point defects in graphene, but may provide quantitative information about the disorders [112]. Raman spectroscopy is one 13

30 of the most widely used techniques for graphene-defect identification. It is a non-destructive, easy-access tool for detecting the structural and electronic properties of graphene [15]. Other than microscopic techniques, electrical transport measurements for individual nanosystems can provide indirect information about the graphene s qualities, such as the level of irradiation-induced defects. This is very important for electronic applications, but requires a more complex device-fabrication process. In this work, the optical microscope and Raman spectroscopy were employed for graphene sample characterization. A novel transfer-free TEM sample preparation method has been developed for TEM study of the graphene after ion-irradiation. Further, a novel defect-free, contamination-free graphene device fabrication method was also developed for electron transport measurement for electronic applications. 1.5 Organization of Thesis The main objectives of this project were to develop a novel technique to fabricate defect-free sub-10-nm features and devices on graphene with precisely controlled arbitrary shapes. This thesis is organized into six chapters, with this chapter (Chapter 1) being the introduction. It introduces graphene, its unique properties and potential applications when engineered to nanostructures. Fabrication techniques to realize these nanostructures are 14

31 discussed. Finally, various graphene characterization methods are summarized. Chapter 2 reports a direct patterning technique to fabricate sub-10-nm graphene nanostructure using the helium ion microscope. The patterning and imaging paradigms of the helium ion microscope for graphene are described. The patterning capability and some direct applications of this technique are demonstrated. The patterning strategy with ion beam parameters optimized for both supported and suspended graphene samples has been described. Finally, the challenges of this direct patterning method are discussed. Chapter 3 discusses the graphene damage under helium ion irradiation. The extent of damage arising from the backscattered induced defects has been evaluated. Graphene damage as a function of incident helium ion fluence has been quantitatively studied by analyzing the data obtained from Raman spectroscopy. The Raman data is used as a guideline for controlled modification of graphene with minimized backscattered ion induced damage. Chapter 4 presents an atomic-hydrogen-assisted method of graphene patterning in sub-10-nm scale using the helium ion microscope. The combination of defects introduced by the helium ion irradiation and the subsequent gas-phase chemical etching provides a solution to the problem of backscattered-ion-induced damage, enabling fabrication of graphene nanostructure with well-defined edges. A customized atomic hydrogen etching 15

32 system has been built for this experiment, and working parameters and experimental details have been presented. Chapter 5 introduces resist-free methods for graphene device fabrication in order to further study the graphene properties for direct applications. A novel transfer-free suspended graphene sample preparation technique is developed, which is compatible with TEM measurements. A contamination-free device fabrication method is presented for electron transport studies, with the entire fabrication process free of organic resist exposure. Chapter 6 concludes the thesis by summarizing the work presented, highlighting the key achievements and proposing future research directions that have spawned from this work. 16

33 Chapter 2. Sub-10-nm Direct Patterning of Graphene Nanostructures using Focused Helium Ion Beam 2.1 Introduction Figure 2-1 A heterostructure graphene device fabricated using the helium ion microscope. Graphene heterostructure with sub-10-nm ribbon width, quantum dots, and 18 electrodes with independent control of the E-field at each device element for highly localized control. (a) Helium ion microscope image demonstrating voltage contrast and isolation of the (odd) electrodes with positive potential. (b) Schematic of the device and ribbon inset (prior to electrodes). A top-down resist-free direct graphene patterning method using the helium ion microscope is presented in this chapter. The high surface sensitivity and small probe size of He + (< 0.75 nm [113]) enabled imaging an atomic layer of carbon and fabrication of complex structures and devices on both supported and suspended graphene flakes. To gain a sense of appreciation of the potential 17

34 of this fabrication technique, a heterostructure device with independent side-gate controls has been fabricated, as demonstrated in Figure 2-1, to show the degree of complexity of the device that could be fabricated, which is almost inaccessible with other techniques. Various nanostructures such as graphene nanoribbons and nanopore arrays have been fabricated with high resolution and precision, to demonstrate the capability of the helium ion microscope for a wide range of applications. 2.2 Helium Ion Microscope In this work, a helium ion microscope (Zeiss ORION Plus) was used both as an imaging tool similar to a scanning electron microscope to obtain high-resolution graphene images, and as a lithography tool similar to a gallium focused-ion beam system to controllably modify the graphene surface with high precision. 18

35 Figure 2-2 The Helium ion microscope. Schematic of the Helium ion microscope column, with the image of the equipment shown on the upper left, and image of the trimer structure captured under scanning field ion microscope (SFIM) shown on the upper right. The Helium Ion Microscope (HIM) is based on a Gas Field Ion Source (GFIS), that utilizes a cryogenically cooled sharp tungsten tip in an Ultra-High Vacuum (UHV) system with a sharpened radius in the order of 100 nm [114]. By biasing the tip positively with a high voltage with respect to grounded electrode, a strong electrical field is produced at the sharpest points of the tip, which is sufficient to ionize the neutral gas atoms around the tip by the process of electron tunneling. The resulting positive ion is immediately accelerated 19

36 away from the tip and focused onto the sample. In the HIM, the tip of the tungsten needle is sharpened by stripping individual atoms away until an atomic pyramid with three atoms at the very end of the tip is created; this is referred to as a trimer, as shown in Figure 2-2. The advantage of this trimer structure is that the top most ionization discs begin emitting ions at a relatively low voltage while all the other atoms in the rest of the tip are not yet capable of emitting. This ensures that the field ionization of helium ions takes place predominantly at these three topmost atoms, and therefore the helium ion current emission is predominantly drawn from the trimer atoms. Under normal operating conditions, the brightest atom of the trimer is selected and aligned with the optical column, which acts as a virtual single atomic tip (< 1 Å [115]), providing high brightness and high resolution in the HIM. Typical operating voltages are around kv. Due to the high mass of helium ions compared with electrons (~7292 ), HIM exhibits negligible diffraction effects. Therefore it is possible to focus the helium ion beam to a much smaller probe size, with an ultimate resolution of 0.5 nm or better Graphene Imaging Using HIM (Secondary Electron Mode) As an imaging tool, the HIM is well suited to graphene imaging compared to the conventional scanning electron microscope. Compared to low energy electrons, i.e. 5 kev, as shown in Figure 2-3, which provides reduced interaction volume and high surface sensitivity than that of the high energy 20

37 electrons, the helium ions have a much narrower interaction volume where secondary electrons are limited to few nanometers from the surface. Combined with a sub-nanometer scale probe size, this provides extreme surface sensitivity and high lateral resolution while imaging, superior to that of a conventional SEM. The secondary electron yield of helium ions ranges from three to nine secondary electrons per incoming helium ion for carbon materials depending on the substrate [114, 116]. This abundance of secondary electrons in combination with small probe size enables high contrast and high resolution imaging with lower incident ion dose. 21

38 Figure 2-3 Trajectories of 30 kev He + (left) and 5 kev e - (right) incident on 300 nm SiO 2 on Si substrate, calculated using TRIM and CASINO software package respectively. The detailed simulation setup is given in Appendix A. The schematic of the secondary electron beam profile with rough beam size are shown on the top. The lateral straggle of the electron near the sample surface is quite large compared with the helium ion. The cascade of electron production (SE2) over a large volume limits the resolution of SEM, while the helium ions penetrate deeper into the substrate with a sharper beam profile and smaller interaction volume, which ensures that the secondary electrons emitted at or near the surface come from a very small area compared with SEM. Figure 2-4 shows one example of a graphene HIM image obtained under secondary electron imaging mode providing detailed surface information and superior contrast. In this typical secondary electron imaging mode, the operating helium ion energy is around 30 to 45 kev with the imaging current ranging from 0.2 to 1 pa. The HIM produces images with a higher Signal to Noise Ratio (SNR) as the incident helium ion fluence is increased. However, the higher dose will lead to local modification of the sample [117] due to material sputtering, and therefore a relatively low current is used for imaging 22

39 while a higher current and longer dwell time could be utilized for graphene patterning, as discussed in the following sections. Figure 2-4 HIM image of mechanical exfoliated graphene flake deposited on SiO 2 /Si substrate patterned with nanodots. There is superior contrast between the graphene and substrate, with great surface information; folds, cracks and patterned nanostructures in graphene can be clearly observed Graphene Patterning Using HIM (Sputtering) As a lithography tool, the HIM has superior resolution compared to the commonly used Gallium Focused Ion Beam (Ga + -FIB) system. Due to the small probe size and surface interaction area, the HIM provides higher fabrication precision and allows a factor of 4 times reduction in feature size compared to the best resolution reported by Ga + -FIB patterning [118, 119]. The low mass compared to gallium ions leads to a lower energy transfer, and hence lower sputtering rate. But this also implies a better control over the milling process, as the sputtering event are much more likely occur close to the beam axis. 23

40 Moreover, with the same incident energy, helium ions produce much less lattice damage than gallium ions. Compared with 30keV gallium beam incident in silicon substrate, the energy loss rate for helium ions is 106eV/nm, with 5% of this energy dissipated through ion s interaction with nuclei, and 95% goes to electron, while for gallium ions, the energy loss rate is 1210 ev/nm, with 90% of this energy dissipated through the ion s interaction with nuclei [115]. Therefore, at the same energy, helium ions introduce less lattice damage compared to gallium ions. In addition, helium ion beam patterning of graphene devices do not suffer from the ionic contaminations which are introduced by the focused gallium ion beam [120]. Besides, the graphene patterning and imaging can be carried out in situ, which makes HIM is an attractive and important tool for imaging and fabricating graphene structures in nanometer scale. The sputtering mechanism, patterning strategy and dose control involved in the helium ion microscope graphene patterning process will be discussed in depth in the following sections. 24

41 2.3 Direct Patterning of Graphene Nanostructure Using Focused Helium Ion Beam For the graphene nanofabrication techniques summarized in Chapter 1, the resultant nanostructures either suffer from poor edge quality, such as in the top-down plasma etching method, or are limited by the lithography resolution. Some chemical methods can result in graphene nanostructures with high resolution and well-defined edge configurations, but the geometries of the nanostructures that can be generated are limited. To address this deficiency, my supervisor Dr. Pickard has introduced a direct patterning method for graphene nanostructure fabrication using the helium ion microscope back in 2009 [121], with the capability of fabricating sub-10 nm graphene features with arbitrary shape and incredibly high precision. Independent works have been reported by other groups [43, 119, ]. Extremely fine cuts on graphene with ribbon widths down to 4 nm and 8 nm on suspended and supported graphene samples have been successfully demonstrated by our group [121]. This method is based on utilizing a 30 to 45 kev focused helium ion beam to directly sputter carbon atoms away from predetermined areas of graphene sheets, either suspended or supported on a substrate. With the incident ion fluence and beam writing strategy well controlled, limitless nanostructures on graphene can be created that are almost impossible to create with other techniques. 25

42 For this helium ion beam direct patterning method, the pattern is transferred onto the graphene surface by direct material modification based on sputtering. Two main factors have to be considered for this patterning process, the incident helium ion fluence and the helium ion beam milling strategy, both of which will significantly affect the quality of the resultant nanostructures HIM Graphene Patterning Dose Control HIM graphene nanopatterning involves direct carbon atom sputtering, defect formation and recombination upon energetic helium ion irradiation. Due to graphene s excellent heat and charge conductivity, the ion irradiation induced damage in graphene is governed by knock-on atom displacements [126]. As sp 2 -bonded nanostructured carbon, the graphene lattice has the unique ability to reorganize its structure by forming nonhexagonal rings, which restructures the lattice by creating a modified coherent network that can retain many of the lattice s original properties. Molecular dynamics simulations have provided great insight into irradiation-induced modifications of graphene [127, 128]. As shown in Figure 2-5, the simulation shows the sputtering fraction of suspended graphene under helium ion irradiation is energy dependent, and increases nonlinearly with exposed helium ion dose. Within the HIM operating energy range, graphene exposed with primary helium ion beam will be fully removed only when the incident helium fluence is as high as ~ ions/cm 2. 26

43 Figure 2-5 Average fraction of lost atoms (vacancy-type defects) of suspended graphene exposed with helium ion beams of 30 kev and 35 kev. Simulation is performed with ion beam damage simulator for graphene [127, 128], with a normal incidence and assuming a uniform exposure; the result is averaged over 20 times. Experimental results are in agreement with these theoretical simulations. As shown in Figure 2-6, an array of 50-nm-long ribbon sets were patterned on suspended graphene flakes with increasing incident helium ion dose. From top left to bottom right, as the total number of incident helium ions increased, more and more carbon atoms in the exposed region were sputtered away. At an area dose of ion/cm 2, a sharp and clear ribbon pattern could be obtained for this particular suspended graphene sample. 27

44 Figure 2-6 Ribbon array used to determine the required patterning dose on suspended graphene. Ribbon arrays with area dose range from ions/cm 2 to ions/cm 2 have been patterned. At a lower dose, the carbon atoms in the exposed area have not been totally removed. As the area dose increases, a clearer cut is obtained. All units in ions/cm 2. Small variations in the ion fluence are observed from sample to sample, due to the different number of layers and the varying surface cleanness of the graphene sample. In all these experiments, the patterning current was kept between 0.5 pa and 1 pa, which provides the required ion fluence within a time frame where the beam drift and other issues are not significant. 28

45 2.3.2 HIM Graphene Patterning Strategy With the proper patterning dose, the patterning order (i.e. beam-moving direction) was found to be very crucial to the fabrication of a nanostructure with high fidelity. For example, in the case of GNRs, it was found during our experiment that if the milling on one side of the ribbon was performed only after the other side was completed, the ribbon defined by these two cuts would be easily broken due to the unbalanced stress on the nanoribbon. Also, if the patterning time is relatively long, the beam will encounter a slight drift over time, and therefore patterning one side of the ribbon after the other may make it difficult to precisely control the ribbon width. As a result, it is very important to control the beam moving direction and order during the ribbon formation process to minimize any lateral stress on the ribbon and eliminate any detrimental effects due to beam drift. During this experimental programme, a unique strategy for graphene nanostructure patterning was developed, as illustrated in Figure 2-7. The cuts defining both sides of a ribbon were etched sequentially with beam moving towards the ribbon center ( 1 followed by 2 as indicated in the figure). Once both sides of the cuts were cleared, the beam was then moved down to repeat the same process to delineate the ribbon. The helium ion beam was externally controlled by a patterning generator (NPGS), which provided precise control of the beam blanking, dwell time, and the beam position. The graphene sheet 29

46 within the area defined by NPGS will be exposed to helium ion beams with the designed order and direction. More detailed information about patterning beam control is provided in Appendix C. Figure 2-7 Illustration of graphene nanoribbon patterning procedure. Bold arrows denote the beam-writing direction; numbers denote the writing sequences. Graphene in the helium-ion-exposed area will be sputtered away, leaving a nanoribbon defined by two cut lines. Courtesy of Dr. Pickard Patterning Cut Width Generally, wider cut width is preferred for graphene patterning to ensure the ribbon isolation and to prevent reconstructions of etched gaps. However, more helium ions are required for large area graphene removal due to the larger cut width. For graphene exfoliated on the commonly used SiO 2 /Si substrate, the increased area dose will introduce micro-bubble formation on the substrate caused by the high fluence helium ion implantation into the substrate [129]. The micro-bubbles formed will deform the graphene by applying tensile stress 30

47 to the nanostructure above the bubble, as shown in Figure 2-8. Figure 2-8 HIM image of patterned graphene ribbon on SiO 2 /Si substrate with heavy incident helium ion dose and large exposure area. Graphene ribbon deformed due to tensile stress from the micro-bubble formed underneath. To avoid this substrate bulging effect and to minimize the total ion fluence, the patterns that can be fabricated are limited to thin lines for graphene deposited on the SiO 2 /Si substrate using this direct sputtering method. Instead of sputtering all the carbon atoms and leaving the desired pattern, the pattern is isolated from the graphene sheet by thin line cuts. Enclosed patterns are used to check the minimum fluences required to isolate the pattern from the rest of the graphene sheet, as shown in Figure 2-9. If the enclosed part shows darker contrast compared to the rest of the graphene sheet after HIM image acquisition, we can assume that the center part has been totally isolated from the bulk graphene due to the charging effect arisen from the insulating SiO 2 substrate during HIM imaging process. 31

48 Figure 2-9 HIM patterning isolation test. Enclosed patterns such as circles and squares are often used to test the cut quality. After patterning the circle, if the enclosed region gets charged up after an image acquisition, we can confirm that the graphene enclosed by the circle pattern is isolated from the bulk graphene sheet and, therefore, that the cut is thoroughly through. From several iterations, 2-nm cut widths were found to be an optimal choice which is wide enough to isolate the features and prevent reconstructions of the etched gap, while minimizing the patterning dose and hence the total ion implanted in the substrate and patterning time, as demonstrated in Figure Figure 2-10 Graphene ribbons defined by 2-nm cut width. 10 nm, 20 nm and 30 nm graphene ribbons are fabricated by HIM direct patterning method with 2 nm cut-width. 32

49 2.3.4 Selected Results of Direct Patterning Method With the patterning parameters precisely controlled, extremely sharp features down to sub-10 nm scale could be fabricated by this HIM direct sputtering method on both suspended and supported graphene, with the complexity and resolution almost impossible to access through other techniques. Figure 2-11 Examples of graphene nanostructures fabricated by HIM direct sputtering method. (a) Graphene nanoribbon heterostructures with quantum dots patterned on suspended graphene sheet. (b) Same structure as (a) patterned on graphene deposited on SiO 2 /Si substrate. Both structures are patterned with a helium ion area dose of ions/cm 2, with cut width fixed at 2 nm. All the samples were prepared by mechanical exfoliation of natural graphite, with detailed sample preparation method discussed in Appendix B. 33

50 Figure 2-12 Suspended graphene nanoribbon fabricated and imaged using HIM. (a) Single graphene nanoribbon with different width sections, which could be fabricated with precise dose control and the proper writing strategy. (b) Ultra-large aspect ratio graphene nanoribbon, which could be fabricated by optimizing the patterning conditions. Courtesy of Dr. Pickard. As shown in Figure 2-12, a narrow ribbon on suspended graphene with different width on each section could be obtained by precise dose control and carefully designed writing strategy. The finest ribbon obtained could be as small as 5 nm. An ultra-large aspect ratio ribbon on suspended graphene could also be fabricated without break-down. This opens up many possibilities for graphene-based electronic device fabrication. Nanometer-sized dot arrays could also be fabricated, as demonstrated in Figure 2-13, enabling potential applications in graphene plasmonics and high current GNR-FETs. 34

51 Figure 2-13 HIM image of dot array with 10 nm dot diameter and 50 nm spacing. The dot array could be fabricated for potential application in graphene plasmonics, and could also be patterned on the GNR to enhance the maximum current it can carry. 2.4 Contamination from Hydrocarbons All these results presented above were obtained from clean graphene samples that required special cleaning processes. The sample cleanness significantly affects the patterning quality, as shown in Figure When patterning the graphene samples without any cleaning process, hydrocarbon deposition due to ion beam induced polymerization is observed instead of material removal. 35

52 Figure 2-14 Star-pattern fabricated and imaged using HIM. (a) Designed pattern with proper HIM cutting. (b) Instead of milling, a deposition effect is apparent after patterning, due to ion-beam-induced hydrocarbon deposition. Under the helium ion exposure, amorphous carbon is deposited on the ion beam scanned area, which originates from residual hydrocarbons from the air transfer and sample loading process. Due to the high mobility of these hydrocarbons, they continue to diffuse into the interaction region, with increasing deposition thickness under continued ion irradiation. The resultant deposited feature is much larger than the helium ion exposed region, since the amorphous carbon deposition process originates from both the interaction of the primary beam and secondary electrons with the hydrocarbon species adsorbed on the sample surface [130], which has a larger extended area compared to the primary helium ion beam. 36

53 Figure 2-15 Raman spectrum of graphene sample before and after Evactron clean. The black curve was obtained from the sample prepared by standard mechanical exfoliation process followed 3 hours annealing in 95% argon, 5% hydrogen forming gas at 450 C. The red curve was obtained after the sample was treated in the HIM chamber with Evactron cleaning. After 4 times, 10 minutes cycles, distinctive D-peak was observed in the Raman spectrum. Studies have shown that hydrocarbon contamination formed on the surface of graphene upon exposure to ambient air [131]. Therefore, to prevent this hydrocarbon deposition effect, a clean patterning environment is required. Evaluation of the Evatron plasma clean [132] in situ in the HIM did not yield favorable results, since the oxygen and hydroxyl radicals formed by the plasma damages the graphene sample [133] while removing residual hydrocarbon contaminations, as observed in the emergence of Raman D-peak in Figure

54 Figure 2-16 Customized heater stage mounted on the x-y-rotational-stage in HIM. An alternative approach for hydrocarbon removal is by in situ heating of the sample. A custom designed heater stage was installed for this purpose, as shown in Figure After the sample is transferred into the HIM chamber, an in situ heat treatment is applied to remove the hydrocarbons and moisture attached to the graphene surface before patterning. After heating the sample at ~250 C for 4 mins, the chamber pressure will rise rapidly due to the release of the hydrocarbons and water vapors from the sample surface followed by drop in the pressure indicating removal of volatile contaminants. When the chamber pressure drops back to 10-7 torr level after the sample cools down, a second heat treatment is applied to ensure a clean surface. After two heat cycles, most of the hydrocarbons attached are removed. No additional Raman D-peak is observed after the in situ heating, indicating no damage was introduced by this heating process, as shown in Figure

55 Figure 2-17 Raman spectrum of graphene sample before and after in situ heating. A heat treatment is carried out in vacuum (~ torr) at ~250 C for 5 minutes. And no significant change in the Raman spectrum was observed. Although this helium ion beam direct pattering method allows the fabrication of graphene features at sub-10-nm scale, the device fabricated do not show favorable transport properties [43, 119, ]. This may result from the defects caused by primary helium ions and backscattered helium ions which significantly affect the patterning quality, change the electronic properties of graphene, and hence degrade the performance of the fabricated devices. Therefore, it is essential to identify these problems and minimize effects caused. The following chapters will discuss the damages caused by both the primary beam and backscattered ions, as well as the proposed method to minimize these damages. 39

56 Chapter 3. Analysis of Helium Ion Irradiation Induced Graphene Damage 3.1 Introduction As discussed in Chapter 2, due to the low sputtering yield of helium ions for graphene (~ to 0.02 atoms/ion [134]), a high fluence of helium ions (~ ions/cm 2 ) is required for this direct carbon sputtering method. Under such high fluence, the backscattered helium ions as well as the sputtered substrate atoms introduce significant damage and degrade the properties of the nanometer-scale patterned graphene by simply damaging the C C bonds, not necessarily sputtering the carbon atoms from the graphene surface. In this chapter, the damage caused by the backscattered helium ion will be discussed with the assist of Raman analyses, and the possible solution is proposed to minimize the damages. 3.2 Helium Ion Beam Induced Graphene Damage HR-TEM studies have revealed an amorphous region of ~3 nm on the patterned edges induced by ion bombardment [135]. With our fabricated GNR width in the sub-10-nm regime, this primary beam induced amorphization almost covered the entire region of the ribbon and significantly deteriorates the edges, which affects the carrier transport and degrade the performance of the fabricated devices. 40

57 Other than the primary beam induced amorphization, for the supported graphene sample, which is widely used for device fabrication, the main obstacle is the unwanted damages introduced by the backscattered helium ions and the sputtered substrate atoms. Compared to the primary beam, the backscattered ions and the sputtered substrate atoms extend over a large region beyond the exposed pattern area, and hence cause larger extent of damage. The sputtered substrate atoms cause damage to the supported graphene through various collisions, which increases the extent of damage made by primary beam bombardment compared to the suspended samples. The backscattered helium ions are the main source of graphene damage, since they have a relatively high yield compared with the sputtered substrate atoms (~0.014 for backscattered He +, ~ for Si, and ~ for O). As a result, under the same optimized patterning condition, narrow ribbon with a clear cut is very difficult to obtain on graphene deposited on SiO 2 /Si substrate due to the high density of defects caused by the backscattered helium ions in the ribbon region, as shown in Figure 3-1 (a). 41

58 Figure 3-1 Ribbons with width varies from 5 nm to 30 nm on graphene sample deposited on bulk silicon capped with 285 nm SiO 2 with cut width of 2 nm. (a) HIM image of the ribbons. (b) Sputtered carbon atom density of the same ribbon array with the incident helium ion dose of ions/cm 2 with 35 kev beam energy calculated by TRIM [136] simulations. The details of the simulation are presented in the Appendix A. Figure 3-1 (b) shows the sputtered carbon atoms distribution of a ribbon array defined with 2 nm cut slot. Although only the 2 nm wide slot is exposed with the primary helium ion beam, the adjacent region of the exposed area also shows a high density of carbon atoms sputtering, which is caused by the backscattered helium ions and sputtered substrate atoms, not the primary beam. Based on TRIM simulation, for the incident helium ion with energy of 30 kev, 98.7% of the carbon removal outside the primary beam exposed area is due to backscattered helium ions, while 1.3% carbon removal is due to sputtered substrate atoms. 42

59 Figure 3-2 TRIM simulation of He + bombardment to SiO 2 /Si substrate. (a) Ray-tracing plot of incident He + distribution in Si substrate with 300 nm SiO 2 capping layer through a 0.34 nm thick carbon layer. (b) HIM image of the fabricated 20nm ribbon defined by 2nm cut slot with incident helium ion fluence of ions/cm 2 and beam energy of 30 kev. (c) Backscattered helium ion density calculated with TRIM simulation under the same beam condition as in (a). (d) The backscattered helium ion density calculated with lowered incident helium ion dose, ions/cm 2. In order to minimize this backscattered ion induced damage, several attempts have been made. Figure 3-2 (a) illustrates the incident helium ion distribution inside the target SiO 2 /Si substrate. It is obvious that with higher incident beam energy, the incident ions penetrate deeper into the substrate. Hence the number of backscattered ions decreases accordingly, which implies less damage to the graphene surface, as the backscattered ions are the main source of the damage to the supported graphene, other than the primary beam. 43

60 To analyze the effect of the backscattered ions, the statistical distribution of backscattered ions for 20-nm-wide GNR with 2nm cut width has been calculated, as illustrate in Figure 3-2 (b)-(d). For direct sputtering (area dose in the range of ~ ions/cm 2 ) with 30 kev beam energy and 2 nm cut width, which was usually employed in our direct patterning experiments, the backscattered ion density in the primary-beam-exposed region was around ions/cm 2 and about ions/cm 2 in the adjacent 100 nm region around the initial incident positions. This simulation result suggests that, even with such a narrow cut width, the graphene located near the cut edge will still be exposed to helium ions, not through the primary beam, but through the backscattered ions. If we lower the incident helium ion fluence from ions/cm 2 to ions/cm 2, the number of backscattered ions will also decrease by two orders of magnitude, to ~10 13 ions/cm 2, which will significantly reduce the graphene damage from the backscattered ions. Therefore, in order to minimize the backscattered-ion-induced damage, a lower incident ion fluence and higher incident beam energy are preferred. However, the operation of the HIM is limited to a beam energy within the range of 30 kev to 45 kev. Alternatively, lowering the incident helium ion fluence and optimization of the design of patterns are two ways to reduce the damage caused by the backscattered ions. 44

61 3.3 Raman Analysis of Helium Ion Irradiation Induced Graphene Damage In order to find the proper incident helium ion dose to avoid the backscattered helium ion induced damage outside the primary beam exposed region, Raman characterization has been carried out on graphene exposed with different helium ion fluence, to work out the range where the backscattered helium ions will not introduce significant damage to graphene. This servers as a guideline to avoid the backscattered helium ion induced damage problem, and hence enabling a controlled modification of graphene. In this section, the correlation between the incident helium ion dose and the graphene disorder caused by the helium ion irradiation has been analyzed. The defect formation mechanism has been discussed in agreement with the two stage classification of graphene amorphization [137]. The Raman analysis of graphene under helium ion irradiation provides a means to understand the helium ion beam graphene patterning process Raman Spectroscopy for Graphene Characterization Raman spectroscopy is employed to evaluate the graphene quality and quantitatively study the damage caused under different helium ion fluences. The ability to non-destructively probe graphene defects makes Raman spectroscopy particularly important to our experimental programme. 45

62 This technique is a well-established technique for investigating the properties of graphene because the intrinsic dispersion of the π electron in graphene makes all wavelengths of incident radiation resonant [138], and offers information about both the atomic structure and electronic properties of the graphene sample under investigation. It is widely used to determine the number and orientation of graphene layers [139, 140] by inelastic Raman scattering. Moreover, the ability to probe the quality and type of graphene edges [141], structural damage [142], effects of perturbations [15] such as electric and magnetic fields [143], strain [144], doping [145], and functional groups [146] introduced during the processing of the graphene sample, makes it an important tool to characterize the material after each processing step. Figure 3-3 Typical Raman spectra of single-layer graphene with and without He + -irradiation induced defects, obtained using 532 nm excitation laser. Graphene can be identified by the position and shape of its G-peak (~1580 cm -1 ) and 2D-peak (~2680 cm -1 ). The D-peak (~1350 cm -1 ) only presents in the defected graphene, and is not visible in pristine graphene. 46

63 Figure 3-3 shows a typical graphene Raman spectrum obtained with a 532 nm excitation laser. The main feature of the Raman spectrum of graphene consists of a G-peak located at ~1580 cm -1, which is caused by the in-plane optical vibration, and a 2D-peak located at ~2680 cm -1, which is caused by the second-order zone-boundary phonons [147]. The G*-peak, located at ~ 2450 cm -1, originates from a combination of the zone-boundary in-plane longitudinal acoustic phonon and the in-plane transverse optical phonon modes [148]. The position of the G*-peak red-shifts as the number of graphene layers increases [149]. The D-peak located at ~1350 cm -1 originates from the first-order zone-boundary phonons. It is absent in defect-free graphene, but exists in the presence of defects or at the graphene edge, since the defects and unsaturated edge atoms provide the missing momentum in order to satisfy momentum conservation in the Raman scattering process [150]. The D-peak intensity, or D-peak to G-peak intensity ratio (I D I G ), provides a convenient measure for estimating the amount of disorders in graphene [151]. Under helium ion irradiation, the D-peak to G-peak intensity ratio (I D I G ) indicates the degree of amorphization of the irradiated graphene as a function of the incident helium ion dose, which is the key parameter for evaluating the graphene disorder. By analyzing the existence, relative intensity and position of the Raman peaks, a considerable amount of information about the graphene quality before and after helium ion irradiation could be obtained. 47

64 3.3.2 Probing Controlled Damage of Graphene Introduced by Helium Ion Irradiation In our experiment, a WITec Alpha 300 confocal Raman spectroscopy equipped with 532 nm (2.33 ev) excitation laser was employed. The measurements were performed at room temperature under ambient conditions. A 100 objective lens is used to obtain a spot size of ~400 nm on the graphene surface under investigation. The laser power is kept well below 1 mw to avoid sample heating. Due to the diffraction limited laser spot size, nanometer-scale fine features cannot be examined directly by analyzing the Raman spectrum obtained due to the existence of the dangling bonds at the edge. Therefore, we applied helium ion exposure to a large area (large enough to cover the laser spot) to emulate the effect of damage introduced to graphene under different fluences of helium ion irradiation. The helium beam current was kept at 1 pa, the distance between each beam exposure spot which defined by center-to-center and line-spacing were distributed uniformly to ensure a uniform exposure inside the defined region. As shown in Figure 3-4, an array of 4 4 µm boxes on graphene were exposed to different incident helium ion doses, following which the Raman spectrum of each exposed area was obtained and analyzed. 48

65 Figure 3-4 Raman characterization of bi-layer graphene under different helium ion dose exposure. The graphene flake was exposed with helium ion doses ranging from ions/cm 2 to ions/cm 2 with 35 kev helium ion beams. As shown in Figure 3-4, when we expose a pristine graphene flake to energetic helium ions, the D band scattering is activated, leading to a detectable D-peak above a threshold ion fluence. The relative intensity of the D-peak increases significantly as we increase the incident helium ion dose, with a simultaneous decrease in the intensity of the 2D-peak. Above ions/cm 2, the Raman peaks start to broaden significantly, as the defects caused by the helium ion irradiation have a direct impact on the width of the G-peak and 2D-peak [152, 142]. 49

66 Figure 3-5 Graphene Raman G-peak and D'-peak under 35 kev helium ion exposure with increasing dose. The unit for helium ion dose is in ions/cm 2. As shown in Figure 3-5, the D'-peak centered at ~1620 cm -1, is also presented, and increases with an increasing incident helium ion dose [152, 153, 140].. It originates from single-phonon intra-valley process, where defects provide the missing momentum [142, 15, 154]. Further increase in the helium ion dose causes merging of the G-peak and D'-peak. At the dose of ions/cm 2, these two peaks overlap. A redshift of the G-peak is also observed with increasing helium ion dose exposure. For our HIM direct patterning method discussed in Chapter 2, with the patterning dose applied (~ ions/cm 2 ), the backscattered helium ion density is larger than ions/cm 2 (Figure 3-2), which shows significant D-peak. This indicates a high degree of disorder in the graphene adjacent to 50

67 the cutting slot, which degrades the performance of the as-made graphene devices. In order to minimize the damage caused by the backscattered ions, we need to understand the relation between defect formation and the incident energetic ion dose Evolution of the Raman Spectra for Graphene under Controlled Helium Ion Irradiation Luccheses et al. have proposed a model to aid the interpretation of Raman I D ratio of defective graphene [153], providing a method to quantitatively I G obtain the defect density introduced by ion irradiation. They proposed that activation of D-peak takes place within the region defined by the structurally disordered area from the ion impact and the vicinity crystalline area which is locally-activated by the point defect. When the incident ion fluence is low, we can assume no interactions between each defective point, I D I G increases proportional to the number of defects. As the incident ion fluence increases, more defects are created and the neighboring D-peak activated regions begin to overlap, causing a decrease in I D with increasing incident ion fluence. I G 51

68 Figure 3-6 I D I G plot under helium ion beam exposure with different area dose. Pristine graphene shows no D-peak. As the incident helium ion dose increases, the D-peak to G-peak intensity ratio increases correspondingly for ion fluence lower than ~ ion/cm 2. Above this point, the ratio is saturated or even drops with a higher irradiation dose (> ~ ions/cm 2 ), which could be explained by the complete amorphization of the graphene structure, with the loss of long-range periodic order [155]. As shown in Figure 3-6, our sample followed the expected I D I G upon kev helium ion irradiation, where single and double vacancies are produced in graphene lattice according to the theoretical calculations [128, 127]. Careful peak fitting is performed to calculate I D I G at different ion fluences (Appendix D). For graphene exposed with low helium ion fluence, < ions/cm 2, the ion irradiation induced defect density is low. In this regime, an increasing I D with increasing incident ion dose is expected. Beyond ions/cm 2, I D reaches a maximum and begins to drop with further increase in the incident I G I G 52

69 helium ion dose. In this regime, the ordered and disordered regions created by ion impact overlap. At even higher dose, the lattice structure of graphene can no longer be retained, leading to complete amorphization. This non-monotonic behavior is in conceptual agreement with the phenomenological three-stage model of amorphization trajectory of carbon materials relating the Raman parameters to the fraction of sp 2 of disordered carbon established by Ferrari et al. [137], with stage I: from graphene to nanocrystalline graphene, stage II: from nanocrystalline graphene to low sp 3 amorphous carbon, and stage III: from amorphous carbon to tetrahedral amorphous carbon, with the first two stages relevant to graphene. In stage I, under low helium ion dose exposure, ion fluence < ions/cm 2, D-peak and D'-peak appears and the D-peak to G-peak intensity ratio (I I ) increases proportionate to the incident helium ion dose, and also proportionate to the number of defects. In this region, the total area contributing to scattering is proportional to the number of defects; that is I is proportional to the total number of defects probed by the laser spot, while I is proportional to the total area probed [153]. As the incident helium ion dose is further increased, the relative intensity ratio saturates or even drops when the graphene structure becomes completely amorphous (loss of the long-range periodic order [155]). The D-peak activated regions start to overlap and eventually saturate. When two defects are closer 53

70 than the average distance an electron-hole pair travels before scattering with a phonon [152, 141], their contribution will not sum independently anymore, leading to a maximum D-peak intensity. We consider ions/cm 2, the dose of the maximum in I I, as the transition between stage I and stage II disorder for our sample, where the bonding begins to change from sp 2 to sp 3 configurations. At the end of stage I, G and D' peaks also start to overlap (Figure 3-5). In stage II, ion fluence > ions/cm 2, upon further increase in the incident helium ion dose, the defect-activated region created by ion bombardment overlaps, and the graphene starts to be dominated by the structural disorders. In this regime, no more well-defined second-order peaks are observed, but a broad feature modulated by the 2D, D+D' and 2D' bands. Above the onset of stage II, increasing helium ion dose created more defects, but leading to a decrease in I D, due to the full amorphization and partial sputtering of the graphene under such high ion dose exposure [153], as shown in Figure 3-7. In this regime, with increasing incident helium ion fluence, sp 2 domains become smaller, and the graphene lattice structure becomes more distorted until it opens up. Since the G-peak is related to the motions of sp 2 carbons, we assume I G is roughly constant as a function of disorder. With the loss of sp 2 rings, I D will decrease with respect to I G and I D I G is proportionate to the number of ordered rings [152]. Our findings correlates well with Luccheses s phenomenological model based 54

71 on 90eV Ar + bombarded on graphene with different fluences [153]. For our work, to cause the similar I D I G response, higher fluence of helium ion dose is required compared to the work reported. About two orders of magnitude lower Ar+ dose is required compared to He +, to initiate the same level of graphene damage. TRIM simulation suggests that the sputtering yield of carbon atoms for 30keV He + and 90eV Ar + are atoms/ion and atoms/ion respectively, but more lattice damage is caused by Ar + due to heavier mass. Calculations using ion beam damage simulator for graphene shows that under the same 30keV He + exposed area is about 0.18% with barely no vacancy-type defect, while the total amorphized area under 90eV Ar + exposed area is about 23.7% with 3.75% vacancy-type defect. Figure 3-7 I D I G ratio and fraction of remaining graphene under increasing helium ion irradiation at stage II. I D I G decreases with increasing incident helium ion dose, as the portion of remaining graphene decreases with increasing helium ion dose. 55

72 The Full Width at Half Maximum (FWHM) Γ of the Raman G-peak (Γ G ) and 2D-peak (Γ 2D ) is another parameter that could be used to measure the structural disorder [156]. As shown in Figure 3-8, Γ G and Γ 2D always increase with increasing number of disorders. Combining I I and Γ allows distinguishing between stage I and stage II, since the graphene with different extent of damage could possess the same I I, but not the same Γ, with the Γ value always larger in stage II compared to stage I. Figure 3-8 FWHM of Raman G-peak and 2D-peak of few-layer graphene exposed with 35 kev helium ions. Cançado et al. have derived a simple equation to quantify the defect density (n D ) introduced by ion irradiation from I D I G [152], for graphene damage in stage I, assuming no interactions between each D-peak activated regions: n D (cm 2 ) = (7.3 ± 2.2) 10 9 E L 4 I D I G 56

73 where E L is the laser excitation energy. Therefore, the density of the backscattered ion induced damage for our helium ion direct patterning method (Figure 3-2, Figure 3-6) calculated from the I D is ~10 11 cm -2, which will significantly affect the electron transport I G properties with about 4 mobility reduction [157, 158] of the fabricated graphene structures. All these analyses above assumed a uniform helium ion exposure. However, in our nano-feature fabrication, we fixed the beam center-to-center and line-spacing at 0.5 nm, to ensure a full coverage of the helium ions over the defined area. In this case, more local defects will be created compared to the uniform exposure case discussed, hence an even higher defect density is expected. 3.4 Conclusion In this chapter, the damages caused by primary helium ion beam and backscattered helium ions were identified. By analysing the I D I G ratio under different fluence of helium ion irradiation, the correlation between the incident helium ions and the beam induced disorder in graphene has been obtained. For the area dose applied for the helium ion beam direct patterning method (~ ions/cm 2 ), while the graphene in the helium ion exposed area is fully removed, the backscattered ions introduce disorders in the adjacent area of the 57

74 ion beam exposure sites, with backscattered ion density of ions/cm 2. Graphene sample under such ion fluence exposure falls within the stage I of the amorphization trajectory, according to Ferrari s terminology, and shows a significant Raman D-peak. As a result, this helium ion direct sputtering method cannot be used to straightforwardly fabricate devices for electronics applications without introducing damage. To solve this backscattered-ion-induced damage problem, we have explored an atomic-hydrogen-assisted etching method to further reduce the patterning helium ion dose required to the regime where the backscattered helium ion causes negligible D-peak, and hence make the helium ion microscope graphene patterning suitable for electronic device fabrication. This will be illustrated in detail in the following chapter. 58

75 Chapter 4. Atomic-Hydrogen-Assisted Nano-Scale Graphene Patterning Using Helium Ion Microscope 4.1 Introduction In previous chapters, we demonstrated the top-down resist-free fabrication method of graphene nanostructures using high-fluence focused helium ions irradiation, and highlighted the most significant drawback of this fabrication method: the backscattered-ion-induced damage of the device, and the edge degradation leading to poor electrical properties. To avoid the damage caused by the backscattered ions, a lower dose is preferable in the nanofabrication of graphene using the helium ion microscope. But the lowered helium ion dose is insufficient for graphene patterning, only leads to defect introduction. Atomic hydrogen is reported to selectively attack the defected graphene. Earliest report by McCarroll et al. on the chemistry of atomic hydrogen etching of natural graphite crystals [159] demonstrated the reaction occurs at preferred sites (i.e. dislocation cores and their immediate neighborhood), producing a pit with perpendicular orientation in the graphite basal plane. The carbon atoms in the edge of the etched pit were arranged in a zigzag form with labile atoms in alternative positions. Therefore, we proposed a method to pattern graphene, utilizing the selective atomic hydrogen etching of the defected graphene, to realize graphene 59

76 patterning using lower helium ion dose, hence reduce the backscattered helium ion induced damage. The working principle of this method is illustrated in Figure 4-1, with the combination of defects induced by the helium ion irradiation and the subsequent gas-phase atomic hydrogen etching. Figure 4-1 Schematic of the two-step atomic-hydrogen-assisted HIM graphene patterning process. (a) Graphene flake is exposed to helium ions with designed pattern, in this case, a nanoribbon; defects have been created in the helium ion exposed area with minimum fluence employed. (b) After helium ion exposure, atomic-hydrogen etching is applied to remove the carbon atoms in the helium ion exposed area; the etching initiates from the defect site and progresses to finally form graphene nanostructures bonded with zigzag edges. Patterns are not to scale. This atomic-hydrogen-assisted etching of graphene can be described as hydrogenation (C H covalent bond formation) and volatilization (C C bond breakage) of reactive carbon sites by the hydrogen radicals [160]. The presence of bond disorders and functional groups [161, 162] at the edges and defect sites of graphene further increases the chemical reactivity of the carbon atoms than the perfectly bonded sp 2 carbon atoms in the unexposed basal plane. Although the reconstructed defects after helium ion exposure have no 60

77 dangling bonds, the non-hexagonal rings of these sp 2 -hybrized carbon atoms locally increase the reactivity of the structure and allow absorption of other atoms on graphene [163, 164]. The experimental evidence indicates methane [165, 159] to be the main reaction product, which is in accord with thermodynamic expectations of equilibrium situations: 4 The gaseous methane is volatile, dissociates from the graphene surface, leaving no residual after the etching, which ensures a clean process. Raman data also confirm that no additional D-peak is introduced in the basal plane after the atomic hydrogen etching process (Figure 4-2), indicates no graphane formed after this process [166]. 61

78 Figure 4-2 Raman spectrum of graphene flake before and after 1 hour atomic hydrogen treatment. The atomic hydrogen treatment was performed at 450 C. After etching, the Raman spectrum shows the appearance of a negligible D-peak, indicates no graphane formed after etching at this high temperature [166]. A red-shift of G-peak is also observed, which could be due to strain in the graphene sheets, sometimes observed after the high-temperature reaction process [97]. Anisotropic [165, 167] and isotropic [168] etching of graphene by hydrogen plasma forming hexagonal or circular patterns have been reported, depending on the ion energy [166]. Unlike the conventional approach of e-beam lithography followed by reactive ion etching, which results in poorly defined edges, the hydrogen plasma etching method allows the fabrication of defect-free edges. However, the positioning resolution and pattern density are limited by the lithography step. Combined with the small probe size of helium ions (<0.5nm), our proposed method enables us to construct complex graphene structures and devices with precise control, while minimizing the backscattered ion induced damage effect by using lower helium ion dose. 62

79 The previous chapters have already presented the patterning capabilities and introduction of controlled defect through ion irradiations. In this chapter, we focus on the atomic-hydrogen etching process, highlighting instrumentation setup, and the parameters that affect the etching. 4.2 Instrumentation Setup for Atomic Hydrogen Etching System A systematic study of atomic-hydrogen-assisted graphene etching process after helium ion exposure was performed in a home-built ultra-high vacuum system, as shown in Figure 4-3 and Figure 4-4. Our home-built system comprises a vertically placed sample holder (equipped with a HeatWave Labs C UHV button heater) in an UHV chamber with the atomic hydrogen source output port directly facing the sample surface. A Tectra plasma source equipped with ion trap is used to generate atomic hydrogen (Figure 4-5). The plasma source is operated in the atomic beam mode, in which the beam consists of atoms and molecules in various excited and neutral states. An ion trap is used to deflect the ions out of the beam by applying an electrical field, to ensure that the flux directed to the sample surface contains only atomic hydrogen. A helium leak check was performed before the experiment to make sure there was no detectable air leaks in the system, ensuring a pure hydrogen environment during the 63

80 experiment (Figure 4-6). The base pressure of our plasma chamber is at ~10-9 torr level with an ion pump. Figure 4-3 Schematic of the home-built atomic hydrogen system. From left to right: a loadlock is attached for sample loading and unloading, and gate valves are used to ensure the chamber is always under vacuum. A heater stage is placed in the main chamber, attached to an x-x-z-r-t 5-axis position manipulator. The sample is loaded on the heater stage by the transfer arm and then rotated by the position manipulator to face the atomic hydrogen generator side. The atomic hydrogen generator with hydrogen supply and water cooling is attached at the right side. An ion trap is installed at the plasma gun head to make sure all the radicals entering the chamber are in the neutral state. The relative position between the plasma gun head and sample surface can be adjusted using an x-y-z position manipulator. Overview of the setup is shown in Figure 4-4. In our experiment, the distance between the sample surface and the plasma gun head was kept at ~15 cm (the position can be changed by the stage to effectively increase or decrease the atomic hydrogen flux density on the sample surface). The chamber pressure was varied from torr to torr with hydrogen flow by manually adjusting the leak valve on the plasma source (Figure 4-5). Hydrogen gas was generated using a hydrogen 64

81 generator (LNI-CARR-250), with a purity of % at pressure of ~21 psi. The plasma source was operated at 40 ma magnetron anode current with 2.45 GHz microwave. The ion trap was turned on to the maximum voltage of 2 kv to ensure that the graphene sample was only exposed to neutral hydrogen radicals. Figure 4-4 Overview of the home-built atomic hydrogen system. An LNI hydrogen generator is used to generate % high purity hydrogen as the hydrogen source. A Tectra Gen2 plasma source equipped with ion trap is used in atom-source mode to generate atomic hydrogen. A Residual Gas Analyzer (RGA) and ion gauge are installed to monitor the chamber s gaseous components and pressure. An ion pump is used to maintain the chamber in UHV state, while a turbomolecular pump is used during the experimental process. With controlled experimental setup, the atomic hydrogen flux at the sample surface was in the order of atoms/cm 2 /s. This hydrogen flux was chosen in terms of removing graphene in the patterned region within a reasonable 65

82 time frame, while still being able to keep etching of the non-patterned regions to a minimum. Figure 4-5 Atom Source. Plasma is created in a coaxial waveguide by evanescent wave coupling of microwave energy at 2.45 GHz, which is excited in the gun head. In our experiments, the plasma source was operated in the atom-source mode. In this mode, a specially designed aperture plate inhibits ions from escaping from the plasma source, while allowing reactive neutrals and atomic hydrogen, to escape and form the dominant beam fraction. The emitted particles are largely thermalised through multiple collisions on passing through the aperture. The attached ion trap at the end of the plasma gun can completely remove the residual ion content from the beam, to eliminate the reaction with H + [169]. Figure 4-6 Residual gas analyzer (RGA) chamber-component monitoring during the atomic hydrogen plasma process. Hydrogen radicals were the main component in the chamber, while no pronounced nitrogen and oxygen component was observed. The plot was measured under torr chamber pressure with continuous hydrogen supply. 66

83 The reaction rate of atomic hydrogen graphene etching is temperature dependent. In accord with thermodynamic expectations of equilibrium situations of methane [165, 159], the etching rate reaches its maximum when the sample is heated up to around 450 C; this is caused by the thermodynamic instability of methane at this temperature. Also, at 450 C and above, the hydrogenated graphene will dehydrogenate [166]. Therefore, in order to maximize the etching rate and prevent the formation of graphane, 450 C were applied in our atomic-hydrogen-etching process. The graphene sample was loaded on a Molybdenum heater stage and exposed to atomic hydrogen for various times at an elevated temperature to ensure that the dangling bonds generated in situ were passivated with H [170]. The actual temperature on the sample surface was carefully calibrated with in situ thermocouple measurements, as shown in Figure

84 Figure 4-7 Temperature calibration of the button heater. The thermocouple was attached to the sample holder in contact with the molybdenum plates; thermocouple readings were recorded with each supply current of the button heater. (a) Thermocouple reading vs. time plot under different current supply. In order to keep the sample heated at ~450 C, the current supply to the heater should be kept at ~3 A. (b) Image of the sample holder attached with a thermocouple during the heating process. 4.3 Experimental Results and Discussions Here, we demonstrate that exposing the graphene sample to atomic hydrogen flux is an effective method of achieving the selective etching of pre-damaged graphene by helium ions. Many variables can be manipulated in the experiment, such as the incident helium ion dose for defect introduction, 68

85 hydrogen density (chamber pressure), and etching time. In this section, each of these parameters, and selected results of the atomic-hydrogen-assisted etching will be discussed. All experiments were carried out with a helium ion beam current of 1 pa and beam energy ranging from 30 to 45 kev, to ensure a small probe size Local Defect Introduction The optimization of incident helium ion dose is critical for this atomic-hydrogen-assisted etching method. The incident helium ion fluence should be high enough to introduce local defects to graphene surface in the helium ion exposed region, to serve as the reaction cores for carbon removal in the subsequent atomic hydrogen etching process, while also should be low enough to eliminate the backscattered-ion-induced damage to the graphene adjacent to the helium ion incident site. We adopted a nm box array which exposed to helium ion beam with doses ranging from ions/cm 2 to ions/cm 2 to work out the proper dose range for this defect introduction step, as shown in Figure 4-8. With an area dose of around ions/cm 2, a square with well-defined edges could be obtained after atomic hydrogen treatment, which suggests that the optimized area dose should be around this value. 69

86 Figure 4-8 Array of nm box patterned with different helium ion area doses on a multi-layer graphene. (a) HIM image of the box array after treatment with atomic hydrogen at 400 C for 90 minutes using full-power microwave. (b) Backscattered ion distribution of the box array calculated with TRIM simulation. The helium ion dose units are in ions/cm 2. 70

87 Ion beam damage simulations [127, 128] were performed to illustrate the sputtering and amorphization under different helium ion doses (Figure 4-9), which helps us to understand the etching behavior observed. Figure 4-9 Atom loss and amorphization of graphene by helium ion irradiation calculated for 30 kev and 35 kev incident helium ions using ion beam damage simulation for graphene. The incident helium ion beam is normal to the graphene surface with uniform exposure, and the current density is kept at 1 µa/cm 2. The results are averages over 20 simulations [127, 128]. According to Figure 4-9, assuming a 35 kev helium ion beam exposed uniformly onto graphene, for the incident helium ion fluence < ions/cm 2, the average fraction of amorphized region under the helium ion exposure area is only 0.4% and the average fraction of lost carbon atoms is even smaller. Raman spectra before and after atomic hydrogen treatment under these helium ion dose exposure have been studied. As shown in Figure 71

88 4-10, for the incident helium ion dose smaller than 1x10 15 ions/cm 2, the atomic hydrogen treatment at elevated temperature leads to a decrease in D-peak to G-peak intensity ratio, due to the self-healing and thermal annealing effect [ ], which overcomes the etching effect under this defect density and time frame. As a result, after atomic hydrogen etching, no box features is created, as shown in Figure 4-8 (a) first row. Figure 4-10 I D I G plot under helium ion beam exposure with different area dose before and after 2hr atomic hydrogen exposure. Further increase in the helium ion dose leads to the increase of both the amorphization rate and sputtering rate, which creates more reaction sites and leads to a higher reaction rate, and box features become visible. For the incident helium ion fluence between ions/cm 2 to ions/cm 2, the average fraction of lost atom is within the range of 2-20% and average amorphization area is within the range of 15-85%. Under such helium ion 72

89 fluences, the amorphization rate is sufficiently high to enable the etching effect from the defects sites. While the backscattered ion density induced from the primary beam outside the defined box remains low (<10 13 ions/cm 2 ), this will not lead to unwanted carbon removal, as shown in Figure 4-8 (a) second row. However, as the incident helium ion dose increases, the backscattered ion density also increases, which causes unwanted carbon atoms removal in the adjacent of the exposed area, and loses the edge selectivity of the atomic hydrogen etching. As shown in Figure 4-8 (b), with incident ion fluence > ions/cm 2, the backscattered ion density is > ions/cm 2. Since the backscattered helium ion has lower energy compared with the primary beam, the backscattered ions generated with the density of ions/cm 2 would also cause a finite rate of amorphization, which leads to etching effect in the subsequent atomic hydrogen treatment. As shown in Figure 4-8 (a), for the box exposed with helium ion dose > ions/cm 2, unwanted carbon removal appears around the defined box feature, which has the same distribution trend as the simulated backscattered ion as shown in Figure 4-8 (b). As a result, the optimized helium ion dose for this atomic-hydrogen-assisted patterning method should fall within ions/cm 2 range. Around this dose, the sputtering rate of the carbon atoms remains low, while the averaged amorphization fraction within the helium ion exposed region is between 20% 73

90 and 80% (Figure 4-9). The backscattered ion density is in the range of ions/cm 2, and gives negligible sputtering and amorphization to graphene. Raman analysis also proves that no significant D-peak is introduced after exposed with such ion density (Figure 3-4). Therefore, for this defect-introducing process, ions/cm 2 is chosen, considered to be a good compromise between high incident area dose density and low backscattered ion density for supported graphene (i.e. deposited on SiO 2 /Si substrate) patterning. The subsequent atomic-hydrogen etching will initiate at the helium-ion-induced defect pit, and continuously remove the carbon atoms from the helium ion exposed area, resulting a well-defined nanostructure. The optimization of etching time is needed for better control of the etching process Defect Density With the incident helium ion dose within the optimized range, further studies have been performed to find the relationship between defect density and etching rate. As a comparison of the upper and lower rows in Figure 4-11 shows, the area exposed to a higher helium ion dose requires a shorter etching time. This could be explained by the higher fraction of amorphization created under ion beam exposure; as calculated in Figure 4-9, about 43.5% of the graphene is amorphized under an area dose of ions/cm 2, while about 67.3% of it is amorphized under an area dose of ions/cm 2. 74

91 Higher amorphization means higher defect density, and therefore more reaction sites and a higher etching rate, leading to an earlier complete removal. Figure 4-11 Arrays of ribbons of 20 nm widths and 500 nm lengths defined with different cut widths ranging from 5 nm to 40 nm. (a) HIM image taken right after the helium ion ribbon patterning, with ions/cm 2 (top row) and ions/cm 2 (bottom row). (b) Image taken after 2 hours of atomic hydrogen etching. (c) Image taken after 4 hours of atomic hydrogen etching. All the images were taken at different locations on the same sample flake with all the experimental parameters kept the same to avoid double exposure of the helium ions to the area of interest. 75

92 As a comparison of Figure 4-11 (b) and Figure 4-11 (c) shows, even with a lower area dose, the patterns will eventually be etched under longer atomic hydrogen exposure, due to the finite number of defects created under such helium ion dose exposure. Thinner ribbons will also be obtained by further etching initiated from the ribbon edges. The hydrogen radical will selectively remove the carbon atoms from both the defect sites and the graphene edges due to the high reactivity at these unsaturated carbon atoms [159, 165]. Therefore, the etching time should be precisely controlled to preserve the desired feature size. Similar phenomena may also be observed for supported graphene samples. Compared with the suspended sample, a supported graphene sample exhibits a lower etching rate, as shown in Figure This could be explained by the suspended graphene s larger contact area with the atomic hydrogen radicals, compared with that of the graphene samples deposited on the substrate. We can also observe that for the ribbons patterned with incident helium ion fluence of 7.5x10 16 ions/cm 2 and 1x10 17 ions/cm 2, after atomic hydrogen treatment, the ribbons are stitching together. This could be explained by the higher etching rate caused by the larger fraction of amorphized area when exposed with higher helium ion fluence. Like the direct patterning method, when one side of the ribbon is released before the other, it tends to bend towards the unrevealed edge due to the unbalanced stress on the ribbon, causing the stitching. 76

93 Figure 4-12 HIM image of 15 nm ribbon array with cut-width ranging from 1 nm to 20 nm. HIM images obtained after helium ion exposure with increasing irradiation dose on supported (a) and suspended (c) graphene. (b) and (d): HIM image obtained after 40-min atomic hydrogen treatment of the graphene flakes in (a) and (c). All units in ions/cm 2. 77

94 4.3.3 Etching Resolution Figure 4-13 HIM images of box arrays patterned with different sizes on supported and suspended few layer graphene flakes. (a) Boxes with sizes ranging from nm to nm were exposed to helium ions with an area dose of ions/cm 2 on supported graphene film. (b) The same patterns as in (a), made in suspended graphene film. (c) and (d) The resultant square shape patterns after 80 minutes of atomic hydrogen etching of the same sample as shown in (a) and (b) respectively. The white dots shown in (b) and (d) are the SiO 2 residue from the photolithography process used to fabricate the hole for graphene suspension (Appendix B), and do not affect any properties of the graphene under study. In this section, we will explore the finest feature that can be obtained by this nanofabrication method. Both supported and suspended graphene samples have been studied. As shown in Figure 4-13, after atomic hydrogen etching, a 78

95 box as small as nm can be obtained with high fidelity for both supported and suspended graphene. In order to minimize the damage caused by the sputtered and backscattered ions while keeping the helium ion exposure time to a reasonable time frame, a small incident helium ion dose and narrow cut width are preferred. However, under the same incident helium ion fluence, there is a cut-width threshold, below which the thermal annealing effect will overcome the etching effect during the atomic hydrogen treatment process with elevated temperature, while above which the atomic hydrogen etching will lead to carbon removal in the helium ion exposed area and create a graphene nanostructure in a given time frame. Figure 4-14 HIM image of 15 nm ribbon array defined by two separate cuts with increasing cut width on suspended graphene. From left to right: the cut width defining the ribbon varies from 5 nm to 40 nm accordingly. The helium dose of all these patterns was kept at ions/cm 2. The image was obtained after 2 hours of atomic hydrogen etching. Ribbon arrays with increasing cut widths were fabricated to find the suitable cut width for this atomic-hydrogen-assisted graphene patterning method. As shown in Figure 4-14, on a suspended graphene sample exposed with

96 10 16 ions/cm 2 helium ions, 15-nm-wide ribbons can be obtained for cut widths of 10 nm and above, after 2 hours of atomic hydrogen treatment. For the left-most ribbon with 5 nm cut width, no distinct ribbon can be observed after the atomic hydrogen etching under the same time scale. This could be explained by the graphene defect migration [171, 172] and self-healing [173] at elevated temperature. At low defect density, the vacancies in graphene can migrate, reconstruct to sp 2 -hybridized carbon atoms to form coherent network, or capture the unbounded carbon atoms diffused on the surface with self-healing, to prevent reaction with hydrogen radicals [ ]. With narrow cut (< 5 nm) under incident helium ion area dose of ions/cm 2, the self-healing and thermal annealing effect overcomes the etching effect after 2 hours treatment, and therefore no etched ribbon can be observed. Upon further increasing the defected area under such helium ion dose, healing cannot prevent amorphization anymore. As a result, a sufficient large cut width is required. However, a large cut width also means a longer helium ion exposure time under the same conditions, which will increase the chance of beam drifting during the patterning process. For the supported graphene sample, a large cut width will also suffer more from the substrate effects. Therefore, we usually design our patterns using fine lines to outline the desired feature, to avoid large area helium ion exposure. Thus, we choose 10 nm as the cut width for this atomic-hydrogen-assisted etching method. 80

97 4.3.4 Layer-Number-Dependent Etching The number of graphene layers is another factor which can affect the etching process, and so the relationship between the atomic hydrogen etching rate and thickness of graphene has been studied experimentally, as shown in Figure Figure 4-15 HIM images of arrays of ribbons of 20 nm width and 500 nm length defined by different cut widths and helium ion doses, for both suspended mono-layer graphene and multi-layer graphene. Suspended multi-layer graphene (a) and mono-layer graphene (c) exposed with ions/cm 2 (top row), and ions/cm 2 (bottom row). From left to right: the cut width defining the ribbon varies from 5 nm to 40 nm accordingly. (b) and (d) The multi-layer graphene shown in (a) and (c) after 2 hours of atomic hydrogen etching respectively. For the ribbons fabricated under the same patterning condition and etching condition, much higher etching rate for single-layer graphene than that of multi-layer graphene is observed, which is consistent with Yang et al. s 81

98 experimental data on etching graphene using hydrogen plasma [165]. The possible reason for this higher etching rate of single-layer graphene might be the larger cross-sectional area for the chemical reaction, compared with that of multi-layer graphene, when exposed to the atomic hydrogen flux. For the supported case, the substrate-induced roughness may also result in a higher etching rate for the single-layer graphene than for the multi-layer graphene. Furthermore, Raman studies show a higher D-peak to G-peak intensity ratio of supported single-layer graphene than that of multi-layer graphene under the same patterning and etching conditions, which indicates a higher defect density and the presence of the substrate effect on the defect production [112] Edge Selectivity Since this atomic hydrogen etching is selective to defected graphene, after etching, the fabricated nanostructures should have negligible number of defects. Zigzag-terminated edges will be dominated after this atomic hydrogen etching process. The formation of zigzag-terminated edges results from the high chemical reactivity compared with the armchair edges [ ], since each carbon atom of the zigzag edge has an unpaired electron that is active to combine with another reactant, while the triple covalent bond between the two open carbon atoms of each edge hexagon ring of the armchair edges makes the armchair-terminated edges more stable in chemical reactivity [177]. The calculation of bond dissociation energy for the C-H bonds formed suggested 82

99 that, in terms of the reactivity towards a hydrogen radical, zigzag edges have the highest reactivity (2.86eV), compared with armchair edges (1.55eV). The activation energy of atoms to form zigzag or armchair edges is much lower than other edge configurations. The formation of other types of edges are rare, due to the low activation energy required to form zigzag or armchair edges compared with other edge configurations [174]. Therefore, the resultant edge structures should be zigzag dominated. Furthermore, this atomic hydrogen etching leads to hydrogen-terminated carbon atoms at the edge, with the C H bond in σ/σ* states, which does not mix with π/π* states, and hence makes no contribution to the electronic states near Fermi level [178, 179], making this method suitable for electronic applications Other Factors When fabricating a graphene nanostructure with highly orientated edges and a minimum number of defects with this atomic-hydrogen-assisted etching method, there are many more parameters that can be manipulated to optimize the patterning process, such as the graphene crystal orientation, incident helium ion beam angle, substrate material and thickness etc. The crystallographic orientation of the graphene might be decisively important for the electrical properties of the nanoscale patterned graphene devices [180, 83

100 181]. The study of direct graphene modification using TEM suggested that patterning along certain crystallographic direction leads to different edge configurations. Hence patterning along the graphene crystalline axis is preferred. The crystalline orientation of the graphene sample could simply be estimated from its long-straight edges after exfoliation, as they tend to be either armchair or zigzag [182], or could be identified roughly by adding a pre-etching step with circular hole pattern, as it tends to form a hexagon after reaction with active hydrogen radicals [165, 167]. However, for our helium ion beam patterning, the primary beam will cause a region of ~2-3 nm amorphization near the exposure edge, therefore the resultant edge will not be perfectly zigzag or armchair orientated. After the atomic hydrogen etching, the zigzag-terminated edges should be dominant, as discussed in previous session. The incident angle of the helium ion beam is another parameter we can vary, as helium ions incident in a grazing angle interact with a longer row of atoms in a graphene sheet compared with the normal incident beam, and hence increase the sputtering yield of carbon atoms [127]. Therefore, patterning graphene at a grazing angle will lower the helium ion dose required. However, for the supported graphene samples, i.e. those deposited on the SiO 2 /Si substrate, backscattered-ion-induced damage should be taken into consideration. Figure 4-16 shows the backscattered helium ion density distribution for a nm box area exposed with helium ions with different incident angles, 84

101 calculated by TRIM simulation and ion beam simulator. With the same helium ion fluence, as the incident beam angle increases, the sputtering fraction and amorphized area fraction also increases, which increase the etching rate for the subsequent atomic hydrogen treatment. However, with a larger incident angle, the backscattered ion density also increases, since the ions that have penetrated into the substrate are closer to the surface in the tilted case. The distribution of the backscattered ions also differs a lot compared with the normal incident case, which may leads to an uneven etching for the atomic hydrogen treatment. Therefore, if we want to take advantage of the high sputtering rate and amorphization rate for the tilted incidence, the incident helium ion dose and incident angle should be carefully chosen to eliminate the effect of backscattered ion induced damage. 85

102 Figure 4-16 Angle dependent sputtering fraction, amorphization area fraction, and backscattered ion distribution under 35 kev helium ion irradiation with incident helium ion dose of ions/cm 2. (a) Backscattered helium ion density when exposed the silicon substrate capped with 285nm SiO 2 layer calculated by TRIM simulations. A nm box area has been exposed to primary helium ion beam. From top left to bottom right, the incident helium ion beam is 0, 27.5, 57.5, and 87.5 normal to the graphene basal plane. (b) Atom loss and amorphization of graphene by helium ion irradiation calculated with helium ion beams incident at different angles normal to the graphene basal plane using ion beam damage simulator for graphene. The helium ion beam incident with uniform exposure and current density is kept at 1 µa/cm 2. The results are averaged over 20 simulations [127, 128]. In addition, different substrate materials and thicknesses could be applied to improve the etching rate or decrease the backscattered ion density. The etching 86

103 anisotropy for single-layer graphene could be improved with a smoother substrate such as mica [167]. The backscattered helium ion density could also be further reduced by using a thinner substrate as expected. As indicated in Figure 4-17, for the same patterning condition, a thinner substrate gives a lower backscattered ion density, although the change is not significant. Therefore, fully suspended graphene is preferred to avoid any substrate effect, which increases the difficulties in the graphene sample preparation. A novel suspended graphene sample preparation technique will be introduced in the following chapter. Figure 4-17 Backscattered helium ion density on substrate with different thicknesses calculated by TRIM simulations. (Top row) Backscattered He + density distribution on bulk silicon substrate capped with 90 nm SiO 2 layer with incident beam energy of 30 kev and 45 kev. (Bottom row) Backscattered He + density distribution on 110 nm Si membrane capped with 90 nm SiO 2 layer with incident beam energy of 30 kev and 45 kev. The cut width of the pattern was kept at 10 nm and the ribbon width was 20 nm, with an incident helium ion dose of ions/cm 2 for all patterns. 87

104 4.4 Conclusion In conclusion, with this atomic-hydrogen-assisted etching approach, we can lower the incident helium ion fluence by up to two orders-of-magnitude for graphene patterning compared with the direct sputtering method, to a regime where the backscattered ion damage is insignificant. From controlled experiments, we have observed that for this atomic-hydrogen-assisted etching method, a 10 nm cut width exposed with ~ ions/cm 2 helium ions and treated in atomic hydrogen at 450 C is the optimum parameter to obtain the graphene nanostructures with high fidelity while minimizing the damage caused by backscattered ions, and the etching rate is dependent on the number of graphene layers. The main advantage of this patterning method is its ability to pattern both supported and suspended graphene down to sub-10-nm scale without residual damage. The atomic hydrogen treatment will remove any amorphization at the edge, revealing zigzag-dominated hydrogen-terminated edges. This discovery represented a major advance in our experimental process, and our technique could be expanded to multiple graphene device concepts. The hydrogen passivated zigzag edges, which are spin polarized and semiconducting [183, 184], make it possible for magnetism and electric-field effects to be explored with regard to graphene-based nanodevices. 88

105 Chapter 5. Graphene Device Fabrication Techniques 5.1 Introduction In the previous chapters, we have demonstrated a fabrication method with sub-10-nm resolution using the helium ion microscope assisted with atomic hydrogen etching. Characterization technique employed so far does not have the resolution to thoroughly study the hydrogen etched graphene edge structures. Therefore, TEM, with the atomic scale resolution, is a very important tool for understanding the graphene defects in the development of our graphene patterning method. A fully suspended graphene sample is required for graphene TEM studies, because TEM only works for ultra-thin, electron transparent samples, as it detects the transmitted electron beam from the sample [185]. The commonly used TEM sample preparation method for graphene involves graphene transfer from substrate to TEM supporting grid using organic films [ ], which introduces stubborn polymer residues to the sample surface, and strongly hinders the atomic structural characterizations of graphene. To solve this problem, a novel transfer-free graphene TEM sample preparation method is proposed, enabling direct study of the graphene structure after helium ion patterning without extra transfer steps. This method could also be used to fabricate fully suspended graphene sample to avoid any substrate effect and for studies such as NEMS. 89

106 For the electrical characterization and actuations of NEMS structures, electrodes should be deposited in contact with the graphene devices. However, the conventional e-beam lithography followed by lift-off process introduces polymer residues, which severely degrade the electrical properties. To eliminate this problem, the device fabrication process requires development of a custom stencil mask deposition of metal contact, to realize a cleaner method to fabricate devices for electron transport studies without the influence of unwanted contaminations. 5.2 Transfer-Free Suspended Graphene Sample Preparation Technique Owing to its high spatial resolution, TEM enables direct visualization of the structural defects, the edge atom arrangement, and identify the number of layers of graphene [186]. The presence of any supporting film or membrane will introduce a background signal which will be sufficiently high to distort the signal from the graphene sample under investigation [187]. Therefore, a fully suspended graphene sample is required which deviates from our current fabrication process. 90

107 Figure 5-1 Bright-field TEM image of suspended graphene after forming gas annealing. Suspended graphene was prepared through mechanical exfoliation, and transferred using poly(bisphenol A carbonate) [188]. The image was obtained by FEI titan at 80kV. Polymer residual was observed on the graphene surface after forming gas annealing. The commonly used approach for TEM sample preparation involves a temporary supporting organic film such as polymethyl methacrylate (PMMA) or polydimethylsilosane (PDMS) [ ] which adheres to the graphene surface; the graphene flake is transferred from the substrate to the TEM grid, and the film is removed after the transfer. Even though the organic film is removed afterwards, the process produces stubborn residues which are difficult to remove, and the wet chemical steps to remove the resist may also introduce contaminations to graphene, as shown in Figure 5-1. Although many examples of atomic-level imaging of graphene films have been published, such images were taken within small clean regions which were surrounded by polymer residues [99]. However, with our graphene patterning process, we cannot predict the locations of these clean regions beforehand to position the 91

108 patterns within these regions, and the clean regions also may not be large enough to place the entire pattern within. Therefore, a much cleaner graphene transfer method is required to solve this problem. In this section, a novel TEM sample-preparation method is introduced with some preliminary TEM results obtained from the suspended graphene samples fabricated using this proposed method Novel TEM Sample-Preparation Technique Besides transferring the graphene flake onto the TEM substrate after exfoliation, an alternative approach for TEM sample fabrication is to directly exfoliate the graphene flake onto the TEM-compatible supporting grid, to eliminate graphene exposure to organic contamination during transfer. However, commercially available TEM grids are typically less than 1 µm thick and therefore cannot survive the force during the mechanical scratch process. A thicker supporting structure, such as a silicon disk with a thickness of a few hundred microns, would be robust enough to survive the process, but generating a dense array of through holes with diameters smaller than 10 µm is a challenging fabrication task, even with Reactive Ion Etching (RIE) [192]. Our approach employs commercially available Micro-Channel Plate (MCP) [193](Longlife TM MCP with Nichrome electrodes), which has a thickness of around 300 µm and arrays of densely packed 5 µm through holes at 6 µm spacing, as shown in Figure 5-2. The MCP is laser cut into the 92

109 TEM-compatible sized disk (of 3 mm diameter) for graphene TEM study. A vacuum chuck is employed to hold the laser-cut MCP disk for the graphene mechanical exfoliation process. Optical contrast between MCP and graphene is then utilized to identify graphene through the optical microscope (Figure 5-2 (b)). Forming gas annealing could be applied to remove the tape residues from the exfoliation process, since the substrate material can withstand high temperature and is not reactive with gas hydrogen. Due to this hydrogen-inactive property, the MCP could also be applied as the supporting grid for the suspended graphene samples used in studies of atomic-hydrogen-assisted etching nanofabrication technique. 93

110 Figure 5-2 Images of MCP substrate for suspended graphene compatible with TEM studies. (a) Optical images of a commercially available Micro-Channel Plate (MCP) laser cut into a 3 mm diameter round disk to fit standard TEM holders. The MCP substrate featured an array of 5 micron capillaries with a 6 micron center-to-center spacing, with a thickness of around 300 µm. (b) Optical image of graphene flake deposited on MCP Preliminary TEM Results of Graphene Deposited on the Novel Substrate TEM imaging was carried out for the suspended graphene samples fabricated using this laser-cut micro-channel plate as the supporting grid. Figure 5-3 is a preliminary TEM micrograph for the as-made suspended graphene flake. The number of graphene layers can be obtained by counting the number of dark lines on the edge in the TEM bright-field image, Figure 5-3 (a), and the graphene crystal organization can be obtained by the electron diffraction 94

111 pattern, Figure 5-3 (c), where the highly crystallized graphene shows six-fold symmetry. The bright-field images and diffraction patterns obtained using the proposed structure, shows the ability of this structure for TEM measurement. Figure 5-3 TEM image of suspended graphene with laser-cut micro-channel plate as supporting grid. (a) Bright-field image of graphene on MCP obtained with FEI-Titan at 80kV. (b) High magnification image of the sample. (c) Electron diffraction pattern of suspended graphene sheet recorded with a JEOL-2010F at 200kV. It shows the typical six-fold symmetry expected for crystalline graphene and graphite. 5.3 Other Approaches Although much cleaner surface is observed for TEM sample prepared with our proposed direct exfoliation method compared to the conventional transfer method, atomic resolution image has not been obtained yet. This may due to the problems associated with our current TEM supporting grid prototype: one is the large aspect ratio of the through holes, causing charging effect when electrons hit the side wall, Figure 5-3(a); the other is the thickness of the supporting grid is too large, we have to thin down the supporting grid by mechanical polishing, which introduces lots of contaminations on the surface, and hindered the atomic structures. 95

112 Further modifications could be made to improve this TEM substrate design. Taking the half angle of TEM into consideration, with a 10 mrad half angle for TEM transmission mode, the aspect ratio of the capillary hole should be larger than 1:100 (hole radius : thickness), i.e. at least 5 µm diameter holes are required for 250 µm thick disk, smaller aspect ratio will cause charging effect on the side walls and distort the images. To address this problem, a customized capillary holes structure could be fabricated with a similar structure to that of MCP, with full control over the hole aspect ratio and disk thickness, as proposed in Figure 5-4. The disks are fabricated by fiber pulling, which enables a much more economical mass production. With the addition of a solid frame around the capillary holes, the sample is robust compare to the laser-cut MCP, and easier to handle without breakage. Moreover, the hole size and spacing could be varied to ensure a sufficient aspect ratio for imaging, and to increase the adhesion area of the graphene to the substrate which increases the mechanical exfoliation yield. 96

113 Figure 5-4 Schematic of the customized suspended graphene supporting grid for TEM study. 97

114 5.4 Resist-Free Graphene Device Fabrication Technique for Electronic Applications To study the performance of the device fabricated using our helium ion beam patterning method, electronic transport study is required for better understanding, improvement, and application of these structures. In this section, we introduce a contamination-free fabrication method to make graphene nanostructure-based electronic devices, which do not expose to organic resist during the entire fabrication process Fabrication of GNR-FET Figure 5-5 Schematic of simple back-gate GNR-FET. The back-gate field effect transistor has a graphene nanoribbon as a conducting channel, SiO 2 as the insulator layer, and Si substrate serving as a back-gate. Metal contacts are deposited which serves as the source and drain. The optical image of the fabricated device is shown on the right. We fabricate a simple back-gated GNR-FET to demonstrate the device fabrication ability of our method, as shown in Figure 5-5. The GNR-FET 98

115 consists of a GNR as the channel which is fabricated by the atomic-hydrogen-assisted etching process, Au with an adhesive Cr layer as the source and drain metal contacts, which is fabricated through thermal evaporation with a nanostencil mask, a p-type bulk silicon substrate serving as back-gate, and 90nm or 285 nm SiO 2 on top of the bulk silicon as gate dielectric. Figure 5-6 and Figure 5-7 illustrate the steps for the GNR-FET fabrication process. There are two main steps in this device-fabrication process: (i) graphene nanoribbon fabrication and (ii) the subsequent electrode deposition. 99

116 Figure 5-6 Schematic illustration of GNR-FET fabrication process. (a) Mechanical exfoliated graphene flake deposited on Si/SiO 2 substrate, used as a starting material. Forming gas annealing is performed to remove any tape residues. After transferred into HIM chamber, in situ heat treatment is applied to remove any hydrocarbons and moisture adhering to the graphene surface. (b) The graphene flake is exposed to helium ion beams for large-scale trench patterning. The trench structure, usually ribbon in micrometer scale, is used as a registration marker for subsequent nanoribbon patterning. (c) The atomic hydrogen treatment is performed for the patterned graphene flake to remove the carbon atoms in the pre-defined area, creating micro-sized trenches on graphene, as shown in (d). (e-f) Repeat step (b-c) for small nanoribbon patterning, with the pre-etched trenches as a registration marker to avoid any unnecessary helium ion exposures in the nanoribbon area. (g) The resultant nanoribbon. (h) Metal contacts are then thermally deposited with good alignment with the nanoribbons fabricated, to realize a back-gated GNR-FET device. 100

117 Figure 5-7 Illustration of nanoribbon fabrication process. (a) Optical image of a mechanically exfoliated graphene flake deposited on Si/SiO 2 substrate. (b) The shaded area (blue) is exposed to the helium ion beam and followed by atomic hydrogen etching for large-scale trench fabrication. (c) HIM image of the corner of a trench fabricated through step (b). (d) Position the nanoribbon pattern in between the pre-patterned trenches, expose the pattern with helium ion beam, and followed by atomic hydrogen etching. For steps (b) and (d), a 1 pa current with ions/cm 2 helium ion area dose is used for patterning. (e) HIM image of 20 nm GNR resultant from step (d). (f) Optical image of the resultant graphene nanoribbons with electrodes. Au/Cr contacts are deposited on the patterned graphene flake, making contact with the micron-sized ribbon with nanoribbons placed in between the electrodes, for further electrical property characterization. Starting from a graphene flake deposited on the SiO 2 /Si substrate, the device fabrication involves two separate steps: device isolation and the critical nanoribbon fabrication. Large-scale trench patterns are fabricated with low helium ion dose exposure ( ions/cm 2 ), followed by atomic hydrogen etching (2 hours). This pre-etched micro-sized trench is used both to divide the large graphene flake into different sections for multi-device fabrication, and also serves as the registration marker for the subsequent nanoribbon device 101

118 patterning, to avoid any unnecessary helium ion exposure to the region of interest. The trench pattern design varies from sample to sample, depending on the shape of each exfoliated graphene flake, and the number of devices intended to fabricate. After the large-scale trench patterning, the nanometer-scale ribbon patterns are fabricated in selected sections defined by the pre-defined trenches. The nanoribbon position can be precisely controlled with the pre-etched trenches as a registration marker, to avoid any exposure to the device region. With the subsequent atomic hydrogen etching, the nanometer-scale ribbon with well-defined edges will be obtained. Subsequently the electrodes are deposited for transport measurements Electrode Fabrication Conventional pattern transfer for electrode deposition using e-beam lithography has multiple processing steps which introduce contamination on the graphene surface. TEM studies have shown that resists such as PMMA introduce contamination on the graphene surface which is difficult to remove with subsequent processing [99]. This residual resist between the graphene and metal contacts, as well as between the substrate and metal contacts, will degrade the performance of the device. Our approach employs a nanostencil mask for resist free metal contact 102

119 deposition. In addition to mitigating of resist contaminants, the nanostencil mask can potentially be reused, significantly reducing the effort required for mask fabrication. Figure 5-8 demonstrates the electrode deposition process using this nanostencil method. Figure 5-8 Schematic of electrode deposition process using nanostencil mask. (a) Stencil mask with pre-cut patterns is aligned to the desired location. (b) Thermal evaporation of contact metals. (c) The stencil mask is removed after the deposition, leaving a metal pattern with the desired shape on the substrate. In general, it is easier to deposit electrodes before graphene patterning since the metal electrodes could be used as a registration marker for the subsequent helium ion exposure. However, graphene experiences up to a few orders of magnitude lower contact resistance when graphene edges are contacted with metal other than the inert graphene surfaces, which requires introduction of graphene edges in graphene-electrode contact region [194, 195]. Although this contact resistance issue is not discussed in this thesis, in our experiment, we still choose to deposit electrode after all features are fabricated to make this electrode deposition procedure compatible with further studies. On the other hand, electrode deposition after graphene nanoribbon patterning process makes alignment of the electrode a challenge. Therefore, special alignment setup is designed to address this problem. 103

120 Figure 5-9 Exploded view of the components for the illustration of stencil mask alignment procedure. (a) Customized stencil mask flexure holder. (b) Stencil mask. (c) Supporting structure which connected to the x-y-z stage. Stencil mask holder is clamped to (c) with (d). (e) Sample holder for 5 mm silicon chip. (f) Clamps to hold stencil mask (a) to (e) after alignment. (g) 5 mm silicon chip. (h) Push clip to hold sample (g) in place without using tapes. (i) Supporting base mounted on the microscope stage. 104

121 Figure 5-10 Stencil mask alignment procedure. (a) Assembly for stencil mask alignment. Nanostencil mask is held by a specially designed flexure mask holder (b), and attached to the x-y-z micro-motion stage, with free motions above the graphene sample. (b) Top and bottom view of aluminum flexure stencil mask holder. Flexure A flexes in the radial direction, which holds the stencil mask in place without any tape. Flexure B flexes up and down in the normal direction, which prevents the membrane from being broken by the large force exerted in the normal direction during the alignment process. The graphene sample is mounted on a 1 by 3 sample holder (c) with a push chip to prevent any contaminations introduced by tape. (d) The whole assembly is mounted under an optical microscope; both the electrode patterns on the stencil mask and the graphene patterns underneath can be observed by adjusting the focusing. The position of the nanostencil mask is manipulated until the electrode pattern is aligned with the graphene nanostructure. The mask is lowered until it touches the graphene sample surface, and then clamped together with (c). After that, the sample holder (c) is released from the base, and the whole assembly can then be placed into the thermal evaporator for electrode deposition. 105

122 Figure 5-9 and Figure 5-10 illustrate the stencil-mask alignment setup and procedure for the contact deposition. The graphene sample is mounted on a cover-slip-sized base plate with push clips, which can be loaded to the thermal evaporator. A flexure stage is designed to hold the nanostencil mask, which is attached to the x-y-z manipulator with clamps, and can move freely above the graphene sample. The use of the flexure stage ensures that the stencil mask can be held without the introduction of any possible contaminant, such as tape adhesive (Figure 5-10 (b) flexure A); it also prevents the breakage of the nanostencil membrane when it makes contact with the sample surface (Figure 5-10 (b) flexure B). After alignment, the flexure holder with stencil mask is clipped down onto the graphene sample holder, and then the whole assembly is transferred to the thermal evaporator for contact deposition. This assembly is compact and compatible with existing deposition system. Owing to low contrast and small size of the graphene nanostructure, the mask chosen for the nanostencil should be optically transparent for the graphene structure to be positioned underneath, and also large enough to have a sufficient Field of View (FOV) for the mask-alignment process. For the ease of stencil-mask fabrication and alignment, the electrodes are deposited through two steps: center small electrode deposition using the silicon nitride membrane making contrast with the graphene, and large electrode deposition, which overlaps with the small electrode to extend the center small electrode to a scale of a hundred microns for further processing. 106

123 A commercially available thin silicon nitride membrane is utilized for center small electrode deposition, Figure 5-11 (a), with a 200 nm-thick optically transparent window with 250 µm FOV. The membrane window is milled with a two-terminal electrode structure of ~2 µm in width at the center with a focused Ga + beam, as shown in Figure 5-11 (b). The membrane with electrode pattern is then aligned to the graphene nanostructures for thermal evaporation, creating 2 to 3 µm wide and 30 µm long metal electrodes in direct contact with the patterned graphene sample. Figure 5-11 Stencil mask for electrode deposition. (a) Standard silicon nitride support film for TEM (PELCO ), with a µm window of 200 nm thick membrane. (b) SEM image of stencil mask made by focused gallium beam (FEI Quanta dual beam) on the Si 3 N 4 membrane in (a) for center electrode deposition; the cut width in the center is about 1 µm with 2 µm spacing. Prior to FIB patterning, a thin layer of carbon (~ 15 nm) is deposited by thermal evaporation, to minimize sample charging during stencil mask fabrication. (c) Optical image of nickel stencil mask made by laser-cutting for electrode extension; the narrowest cut width is ~10 µm. The center shaded area is large enough to cover the patterned graphene flake underneath, while the four slots overlap with the electrode deposited with (b). The large square shape is used for making metal pads for wire-bonding or direct probing. Large metal pads (a few hundred microns) are usually required for measurement through direct probing or packaging via wire-bonding. The large 107

124 electrode deposition is realized by a nickel stencil mask, Figure 5-11 (c), which is made from a 100-µm-thick nickel disk and laser-cut with an electrode pattern with critical dimension of ~10 µm. The sample alignment procedure is the same as the center small electrode deposition. Figure 5-12 Images of GNR-FET fabricated using atomic-hydrogen-assisted etching and nanostencil electrode deposition at different field of view. (a) Optical image of the device wire-bonded on the chip carrier ready for measurement. (b) Optical image of GNR-FET device. (c) Optical image of the center small electrodes region. (d) High magnification image of the center region. (e) SEM image of the ribbon region. After graphene ribbon patterning using atomic-hydrogen-assisted etching and electrode deposition using the nanostencil method, simple back-gate GNR-FET can be fabricated and made ready for further electrical characterization, as shown in Figure It can be directly measured using a probe station, or it can be wire-bonded on a chip carrier for test-fixture 108

125 measurement. As shown in Figure 5-13, the sheet resistance R s of this 20 nm-wide, 500 nm-long GNR calculated by R s = R W L, is ~ Ω/. Since the contact resistance is a significant portion of the measured resistance, the actual sheet resistance should be much lower than the calculated value. The mobility, determined by μ=(n s er s ) -1, is around cm 2 V -1 s -1, assuming a charge density of cm -2, it is more than one order of magnitude lower than graphene sheet. Figure 5-13 Transport measurement of the fabricated sample. A slight non-linearity at low bias is observed. The scaling nonlinearity may attribute to conductance suppression in the first graphene layer, owing to interactions with the substrate for multi-layer graphene. No gate voltage dependence of the conductance is observed for this sample, while gate tenability is an essential characteristic of high-quality graphene. This independence of the conductance with back-gate voltage may attribute to the relatively small gate voltage window applied in this measurement. The 109

126 gate-bias window to observe expected ambipolar behavior in graphene was reported to be as high as ± 50V for 300nm thick gate-oxide [196]. The charge neutrality point is shifted beyond the gate voltage range used in our measurement. Therefore, further electrical transport studies are necessary for understanding the charge transport mechanism and the quality of the fabricated ribbon. 5.5 Conclusion In conclusion, this chapter introduces several novel techniques for graphene device fabrication. These include a transfer-free graphene TEM sample preparation method, which enables the study of helium ion induced damage without introducing contaminations during the sample preparation; as well as a stencil mask based contamination-free GNR-FET fabrication techniques, which enables characterization of the electron transport properties of the nanostructures fabricated with the developed atomic-hydrogen-assisted graphene patterning method. With these approaches, the intrinsic properties as well as the potential applications of the helium ion patterned graphene nanostructures can be further studied. 110

127 Chapter 6. Summary and Future Work 6.1 Summary This dissertation has demonstrated a direct patterning technique to fabricate sub-10-nm graphene nanostructures using the helium ion microscope. The small probe size and interaction volume of helium beam enables patterning of graphene with high precision and resolution. Complex structures with arbitrary shape can be fabricated which is almost not accessible through other methods. We have identified and addressed the problems associated with ion induced damage in this direct patterning method, which limits the applications which are sensitive to graphene defects. Raman analyses have been carried out to find the range of helium ion dose where the backscattered ions will not cause significant damage to graphene. Subsequently a gas-phase atomic hydrogen assisted graphene etching method was developed to improve the fabrication process by lowering the incident helium ion fluence by an order or more in magnitude, which minimize the backscattered-ion-induced damage to the supported graphene, and also results in well-defined edges. An alternative sample preparation method compatible with TEM investigations after each process step and for suspended devices has also been demonstrated. In addition, a resist-free electronic device fabrication method has been proposed to eliminate possible contamination and defects introduced during the fabrication process which may degrade the performance of graphene devices. 111

128 The main contributions of this thesis are as follows: 1. We demonstrated a top-down method to directly pattern graphene nanostructures scaling down to sub-10 nm dimensions using helium ion microscopy, for both suspended and supported graphene samples. High vacuum tube furnace has been built for graphene sample cleaning. 2. Limitations arising due to primary beam and backscattered-ion-induced graphene damage problem during the helium ion direct sputtering process for graphene samples were identified and studied. The combination of low dose helium ion irradiation induced local defect and gas-phase atomic hydrogen etching was explored to selectively remove defect induced regions and fabricate functional defect-free graphene nanostructures. Custom designed ultra-high vacuum system has been built for atomic hydrogen treatment. Three-wavelength micro-raman system has been designed and built for Raman analysis. 3. A resist-free technique employing stencil mask based metal electrode deposition was developed to fabricate contacts and electrodes, enabling the study of the electrical properties of the fabricated devices without deleterious effects from contaminations. 112

129 Stencil mask alignment assembly has been built and developed for metal contact deposition. 4. We proposed a novel transfer-free method employing micro-channel plate as substrate to make suspended graphene samples suitable for TEM study. With this approach, we can eliminate the organic residues introduced in the graphene-transfer step. Also, with the free-standing graphene, various studies can be carried out without the effect of backscattered-ion-induced damage. 6.2 Future Work With this thesis presents the ability of nanostructuring graphene using helium ion microscope, it establishes the enormous potentials for various 2D material nanostructures and devices studies using the focused helium ion beam. Some of the applications that could be carried out in the near future will be discussed in this section Graphene NEMS Graphene is an ideal material for applications in nanoelectromechanical systems due to its single atomic thickness, robustness and stiffness. Graphene has a high elastic modulus (E Y 1TPa) and high strain limit (~25%) [197] compared with the conventional materials that used for NEMS resonators. Coupled with its excellent electronic properties, graphene is an attractive 113

130 material for development of the tunable and stretchable two-dimensional NEMS that could be used in applications such as sensing, which can exhibit sensitivity much higher than competing technologies [68]. The development of oscillators based on NEMS resonators made from graphene has been developed as new frequency references for stable oscillators. Self-sustaining feedback oscillators can be created using graphene membrane resonators that can vibrate at radiofrequencies up to 110 MHz and can be electrostatically tuned by around 10% [198]. The small on-chip footprints makes an important advance in developing such oscillators, but numerous challenges in fabrication process still need to be addressed. With our patterning ability, graphene NEMS could offer potential to extend these capabilities. 114

131 Figure 6-1 Drum structure fabricated on suspended graphene for NEMS application. (a) Circular diaphragm flexure. (b) Symmetrical multi-folded flexure. (c) Nested planar diaphragm structures with sub-10-nm features in the inner structure (d), which demonstrate the range of dimensions achievable with this technique [199]. Figure 6-1 shows flexural structures fabricated to study the mechincal properties of graphene for applications in NEMS. The drum structure [200] fabricated exhibits uniaxial tensile stress and hence the membrane stretches nonlinearly in the longitudinal direction upon actuation with the designed diaphragm flexure pattern [199]. Diaphragm structures with sub-10-nm critical dimension can be achieved Nano-Patterning of Other 2D-Materials Although graphene has been widely studied because of its outstanding 115

132 properties, it has to be engineered to open a bandgap for logic electronic applications. However, bandgap engineering deteriorates the unique transport properties of graphene [201, 202]. Searching for other 2D materials with suitable bandgaps is therefore an important aspect of future studies. In 2005, other semiconducting 2D materials of different transition metal dichalcogenide, such as Molybdenum disulfide (MoS 2 ), Bismuth Telluride (Bi 2 Te 3 ) [203, 204], Molybdenum diselenide (MoSe 2 ), and tungsten disulfide (WS 2 ) were proposed; these have also drawn considerable attention more recently [ ], along with phosphorene and silicene. MoS 2 MOSFETs have recently been experimentally demonstrated, showing reasonable mobilites and high ON/OFF ratios [ ]. Strain engineering of these 2D materials could also be a relevant topic, as it influences the electronic and magnetic properties of the fabricated nanostructures. It has been reported that the electronic and magnetic properties of MoS 2 nanoribbons can be manipulated by strain engineering, showing potential applications in spintronics and photovoltaic cells [212]. Our developed method for graphene nano-patterning should also apply to other 2D materials; the electrical and mechanical properties of these materials could be examined for both supported and suspended cases. 116

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150 [220] J. Nabity, " JC Nabity Lithography Systems, [Online]. Available: 134

151 Appendix A: TRIM Simulation Setup TRIM (Transport of Ions in Matter) [136] simulations were implemented to illustrate the ion trajectories inside the substrate and the level of sputtered and backscattered ions introduced by the primary helium ion beam. TRIM calculates the interactions of energetic ions with amorphous targets using a quantum mechanical treatment of ion-collisions. Although it is designed for simulating a bulk amorphous target under ion bombardment, it still provides a reasonable scale of the sputtering yield of the thin layer of carbon deposited on the substrate via helium ion bombardment, and of the ion trajectories inside the substrate. Hence it can be used both as a guideline for patterning ion dose selection and to analyze the level of damage caused by backscattered ions and sputtered atoms from the substrate. For our TRIM simulation setup, we used a thin layer of carbon atoms with a thickness of 3.4 Å [213] (theoretical density of 2.265g/cm 3 ) to simulate the graphene layer, which was placed on a 90 nm or 300 nm thick SiO 2 (density of 2.2 g/cm 3 for dry thermal oxide [214]) on a bulk silicon substrate (density of 2.33 g/cm 3 [215]). The 90 nm and 300 nm oxide layers were chosen as these are commonly used in experiments, because they provide a good optical contrast with graphene. 135

152 Appendix B: Graphene Sample Preparation In our experiment, standard mechanical exfoliation of natural graphite was performed to fabricate both supported and suspended graphene samples. For both cases, a 250-µm- or 500-µm-thick p-type boron doped <100> silicon wafer with 90-nm or 285-nm dry chlorinated thermal oxide was used. To improve the surface hydrophilicity for better graphene adhesion and surface cleanness, piranha clean (a mixture of sulfuric acid and 30% hydrogen peroxide with volume ratio of 3:1) was applied to the substrate [216]. Exfoliated graphite flakes were then deposited on top of the clean wafer, and the different types of graphene, of single- and multi-layer thicknesses, were identified using the optical microscope [217], the process is illustrated in Figure B

153 Figure B-1 Graphene sample preparation and identification. (a) Schematic of graphene mechanical exfoliation process: bulk natural graphite is transferred to blue tape (Nitto Denko), thinned by being transferred between the tapes and finally transferred to the substrate. (b) Optical images of mono-layer graphene, bi-layer graphene, and multi-layer graphene on Si substrate capped with 285 nm SiO 2 layer. The lightest pink-colored triangle shape enclosed by the yellow dotted line represents single-layer graphene. As the layer number increases, the color becomes darker. The white circles are the etched holes on the substrate, with 3 µm diameter and 10 µm pitch; cross-section of the etched hole is shown in (c). The graphene film above the etched hole region is hence free-standing. The same mechanical exfoliation method was also used to prepare the suspended graphene sample. Graphene was peeled on the substrate with a photolithographically fabricated 3 µm hole array with 10 µm period, as shown in Figure B-1 (b)-(c). The oxide layer in the circle region was removed with SF 4 dry etching, and so the exfoliated graphene deposited on top of the etched 137

154 hole was suspended. After the graphene exfoliation process, optical images were used to locate graphene flakes. Figure B-2 Raman Characterization of graphene with different thicknesses.(a) Optical image of graphene flake under test; Raman spectrums are obtained for areas 1 to 4 after forming gas annealing. (b) Raman spectra obtained for each area shown in (a). (c) The 2D bands from (b) magnified to show the different FWHMs of each area, with different number of layers. (d) The four fitted components of the 2D band for the bi-layer graphene. Thermal anneal in forming gas (mixture of 95% argon and 5% hydrogen) was then applied to remove most of the tape-adhesive residues and hydrocarbons on the sample surface. Raman analyses were performed after the forming gas anneal to identify the number of graphene layers and to probe the graphene quality. As shown in Figure B-2 (b), no D-peak was presented, which 138

155 indicates no extra defects were introduced to the graphene flake with the anneal process. The number of graphene layers can also be obtained by analyzing the Raman G-peak to 2D-peak intensity ratio and 2D band full width at half-maximum (FWHM) [140, 218, 219] as shown in Figure B-2 (c) and (d). 139

156 Appendix C: NPGS Setup In our experiment, a Nanometer Pattern Generation System (NPGS) [220] patterning generator was used to precisely control the beam for helium ion lithography it controlled the beam blanking, beam dwell time and beam moving direction. The graphene within the area defined in the NPGS was exposed to the helium ions. The beam current used for patterning in all experiments was in the range of 0.5 pa to 1 pa, which is high enough to provide a sufficient signal-to-noise ratio (SNR) while maintaining a good beam size. The area dose was calculated based on the center-to-center and the line spacing as indicated in Figure C-1, with the following formula: Area Dose = (Beam Current) (Exposure Time) (Center to Center) (Line Spacing) In our patterning process, both center-to-center and line spacing were usually set at 0.5 nm. The helium ion fluence is controlled by varying the dwell time. Figure C-1 Definition of center-to-center and line-spacing in NPGS. 140