Nanomaterials Synthesis and Characterization by Liquid Phase Exfoliation and Vapor Liquid Solid Methods

Transcription

1 Nanomaterials Synthesis and Characterization by Liquid Phase Exfoliation and Vapor Liquid Solid Methods BY TARA FOROOZAN B.S., International University of Imam Khomeini, Iran, 2012 THESIS Submitted as partial fulfillment of the requirements for the degree of Master of Science in Materials Engineering to the Graduate College of the University of Illinois at Chicago, 2016 Chicago, Illinois Defense Committee: J. Ernesto Indacochea, Chair and Advisor Amin Salehi-Khojin, MIE, Advisor Michael J. McNallan

2 To my dear family, And my loving fiancé, Soroosh ii

3 ACKNOWLEDGMENTS This thesis paper could not be written without the help and support of my advisors Dr. Amin Salehi-Khojin and Dr. Ernesto Indacochea who served as my supervisors and gave me support and assistance in every single steps of my graduate studies. My special gratitude also goes to my lab mates, Amir, Poya, Reza, Mohammad, Bijandra and Aditya for their help and support. iii

4 AUTHORS CONTRIBUTIONS The High Quality Black Phosphorous Atomic Layers by Liquid Phase Exfoliation CHAPTER 2 is from the papers I have contributed to and the written permission from journals (see appendix) are also attached. The contributions of the co-authors are listed below: Authors contributions in chapter 2: A. Salehi-Khojin, P. Yasaei, B. Kumar, and T. Foroozan conceived the idea. A. Salehi-Khojin led the material synthesis, fabrication, characterizations (except TEM) and experiments. T. Foroozan performed the liquid exfoliation. T. Foroozan and B. Kumar performed the absorption spectroscopy experiments. D. Tuschel, P. Yasaei and T. Foroozan performed Raman spectroscopy experiments. P. Yasaei performed SEM, AFM and sensing experiments. M. Asadi performed DLS. R. F. Klie C. Wang and A. Nie performed TEM imaging and analysis. Authors contributions in chapter 3: A. Salehi-Khojin and T. Foroozan conceived the idea. T. Foroozan performed the nanowires growth, optimization, SEM and AFM imaging. P. Phillips and S. Sharifi performed the EDS and TEM imaging. iv

5 TABLE OF CONTENTS PAGE CHAPTER INTRODUCTION The Rise of nanomaterials Future of integrated circuits... 6 CHAPTER HIGH QUALITY BLACK PHOSPHOROUS ATOMIC LAYERS BY LIQUID PHASE EXFOLIATION Introduction Literature Review Towards the Mass Production of 2D Materials Liquid Phase Exfoliation (LPE) of Black Phosphorous (BP) Characterization of exfoliated BP flakes Optimization of the Exfoliation Process Applications of liquid phase exfoliated BP Stable and Selective Humidity Sensing Using Stacked Black Phosphorus Flakes Selective ionic transport pathways in atomically thin BP Polymer nanocomposites containing BP nanoflakes Summary and Conclusion CHAPTER v

6 SULFUR ASSISTED VAPOR LIQUID SOLID (VLS) GROWTH OF SILICA NANOWIRES BY CHEMICAL VAPOR DEPOSITION (CVD) Abstract Introduction Literature Review Growth of One Dimensional Materials Experimental Procedure Characterization of the nanowires Growth mechanism of the nanowires Optimization of the process Effect of seeding concentration in the growth of SiOx nanowires Effect of the nanowires growth on the substrate surface Reducing effect of Sulfur on the growth of SiOx nanowires Growth time effect on the growth of SiOx nanowires Summary and Conclusion CHAPTER CONCLUSION AND FUTURE STUDIES CITED LITERATURE VITA TABLE OF TABLES vi

7 Table 1. Hansen solubility parameter and surface tension of examined solvents Table 2. Tensile modulus of the PMMA nanocomposites TABLE OF FIGURES Figure 1. Schematic image of 0D, 1D, 2D and 3D nanostructures assemblies Figure 2. Moore's Law (Transistors per Microprocessor), ][28]... 7 Figure 3. Intel R&D microstructures road map... 8 Figure 4. Schematic of some ultrathin 2D materials[35] Figure 5. Optical image of solutions at different experimental steps. (A) Optical image of samples after one day Figure 6. SEM images of the product obtained in DCB solution. a) Low and b) highmagnification. Scale bars are 1 um and 200 nm Figure 7. Polymerization of NVP in presence of black phosphorus. A) Shows the polymerization of NVP and its conversion to PVP. b) Optical images of the viscous solution obtained from sonication of BP in NVP solution Figure 8. Steps involved in production of nanoflakes and films. a) Schematic and real image of exfoliation process b) film production Figure 9. a) Perspective illustration of the puckered structure along the y-direction.[106] b) Top view illustrating the honeycomb structure. The blue box represents an individual unit cell of BP. C) Signature Raman peaks obtained from BP nanosheets Figure 10. Morphological and Raman characterization of the exfoliated BP sheets. a) Low magnification SEM image of the on chip separated flakes showing coffee-rings on S/SiO2 substrate. (Scale bar is 200 μm). b) High magnification SEM image from center of the ring (scale bar is 200 nm). c) Raman spectra of synthesized BP sheets at different orientations showing the anisotropic nature of BP flakes. d) SEM image of individual BP flakes on the produced rings (scale bar is 200 nm). e) AFM image and height profile of the area shown in D. f) Histogram of the flakes thickness distribution measured by AFM height measurement of 70 flakes from both DMF and DMSO solutions Figure 11. SEM images of BP Nano flakes. (A), (B), and (C) are obtained from DMSO solution. Scale bars are 10 um, 10 um, and 200 nm respectively. (D), (E), and (F) were taken from sample prepared in similar way using DMF solution. Scale bars are 1 um, 0.5 um, and 200nm, respectively vii

8 Figure 12. Comparison between DMF and DMSO solutions. (a) Photograph of the solutions after sonication (left) and after centrifugation and supernatant collection (right). (b) Optical absorption spectra indicating optical bandgap of mono to 5 layer BP flakes in solution. c) Normalized absorbance intensity vs. characteristic length of the cell (A / l) in different concentrations for = 1176 nm (E = 1.05 ev) and extracted extinction coefficient (α) from their slope. (d) DLS histogram showing the particle size distribution Figure 13. Transmission electron microscopy (TEM) images of BP nanoflakes. The scale bar is 100 nm Figure 14. HRTEM image and FFT of a monolayer BP. The scale bar is 2nm. FFT in (B) shows the presence of BP lattice and hydrocarbon on the surface of the BP flakes and also shows that (I110/I200) is greater than one, confirming the presence of monolayer. The ratio was measured to be 2.7 ± 0.2 after background removing which is close to which is the computational value BP monolayer[56] Figure 15. FFT and HRTEM of BP nanoflakes. A) TEM image of a BP flake scale bar is 200 nm. B) TEM image and FFT of the same flake showing the existence of monolayer BP on the region of 80 nm 80 nm. The scale bar is 20 nm. C) HRTEM image of the monolayer BP together with TEM simulation (upper right inset) and a filtered image (lower left inset). Scale bar is 1 nm Figure 16. Schematic of atomic structures together with TEM image simulations of mono and multilayer BP viewing from (001) direction. (A, B) mono and multilayer crystal structure of BP drawn by VESTA. (C, D) TEM image simulations of monolayer and multilayer BP showing different structures of hexagonal and orthogonal for monolayer and multilayer, respectively Figure 17. A) EELS map and high-angle annular dark-field (HAADF) image of BP (scale bar is 200 nm). Each pixel size of the EELS map is 22 nm. E) An individual EELS spectrum of the EELS map IS shown in B Figure 18. Energy-dispersive X-ray spectroscopy (EDX) analysis performed on BP sheets. phosphorus (P) peak is the largest one and the peaks associated to Carbon and Copper come from the TEM grid, originate from the used transmission electron microscopy (TEM) carbon grid and weak Chromium (Cr) and Titanium (Ti) peaks are because of the contamination coming from the sonication tip Figure 19. Optical image of the solutions with single chunk and with ground material (A) before solvent addition (B) after solvent addition. After (C) 1 hour (D) 3 hours (E) 6 hours (F) 12 hours of sonication viii

9 Figure 20. Schematic showing the differences between solutions, suspension, precipitate and supernatant during the centrifugation process Figure 21. AFM height mapping and profile from samples of (A-B) 2000 rpm centrifugation for 30 minutes and (C-D) 500 rpm for 30 minutes. As it can be seen 500 rpm is not very effective for separation of thin flakes from bulky material, while 2000 rpm was effective for this purpose Figure 22. (a) BP film on a PTFE membrane filter. (b) Film cut into in size and attached on a tape as support. (c) Using Ga-In as electrical connections Figure 23. a) Image of the BP device used for sensing experiments. b) BP film response to injected analytes showing 5 fold drain current increase upon injection of water Figure 24. Sensing stability of the BP film after 3 months of exposure to ambient condition Figure 25. Sodium transport pathway in BP atomic layers with respect to the sodium source contact location. (a) Sodium contact interface normal to the [100] direction. Inset shows corresponding electron diffraction pattern of the few-layer phosphorene in Panel a. (b) Sodium contact interface parallel to the [100] direction. Inset shows corresponding electron diffraction pattern of the few-layer phosphorene in Panel b Figure 26. PMMA nanocomposites. From left to right: pure PMMA and nanocomposite containing 0.05 and 0.1% phosphorene nanoflakes, respectively Figure 27. Tensile strength measurement of prepared nanocomposites Figure 28. SEM imaging of the embedded BP flakes into the polymer matrix showing the incoherency in dispersion of flakes into the structure and existence of preferred directions for crack growth. Scale bar is 1um Figure 29. Schematic illustration of SiO2 nanowires growth. In this reaction the growth is catalyzed by gold-silicon oxide droplet on the surface of the wafer Figure 30. Schematic and picture of the CVD furnace used for the silica growth Figure 31. Si and SiO2 substrates after growth of nanowires Figure 32. Morphology and density of the structure. SEM image of the grown silica nanowires in low (left) and high (right) magnifications showing the high density of the grown material. The scale bars are 1um Figure 33. Transmission electron microscopy image of a typical nanowire on which the electron dispersive mapping was performed. Inset shows the IPA solution containing grown nanowires. Scale bar is 500nm Figure 34. EDS mapping of the grown structure, showing the existence of both Si and O elements in the structure forming SiOx structure ix

10 Figure 35. Crystal structure of the grown material. Diffraction pattern obtained from grown SiOx (lower left) and Pt/SiO2 (upper right) showing the amorphous structure for nanowires and crystalline structure for the catalyst droplet Figure 36.Electron Dispersive Spectroscopy (EDS) spectrum obtained from nanowires on copper TEM grid showing Si, O, Pt elements existing in the grown structure and Cu/ C existing on the grid Figure 37. AFM imaging showing the morphology of the grown nanowires. A) In enough feed stock. B) In high feed stock. Insets show the height profile of the nanowires. Scan size is 1.3um Figure 38. Phase diagram of Si-Pt Figure 39. Effect of Pt seeding concentration on the growth of the nanowires. a-d shows low to high concentration of sputtered Pt. Scale bars are 1um Figure 40. Substrate morphology after growth. Morphology of the Si/SiO2 substrate after superficial scratching of the grown material from the surface. Scale bar is 1um Figure 41. Surface morphology of the annealed substrate without having Sulfur involved in the process. Scale bar in 1 um Figure 42. SEM image of the substrate after 30 minutes of growth. Scale bar in 1 um x

11 LIST OF ABBREVIATIONS 1D 2D AFM BP CHP CVD DCB DLS DMF DMSO EDX EELS FFT HAADF HRTEM LPE IPA LPCVD MoS2 one-dimensional two-dimensional atomic force microscopy black phosphorus N-Cyclohexyl-2-pyrrolidone chemical vapor deposition 1,2-Dichlorobenzene dynamic light scattering dimethylformamide dimethyl sulfoxide energy dispersive X-ray spectroscopy electron energy loss spectroscopy fast Fourier transform High angle annular dark field high resolution transmission electron microscopy Liquid Phase Exfoliation isopropyl alcohol low-pressure chemical vapor deposition Molybdenum disulfide xi

12 NIR NF NMP NVP PL PMMA PTFE SAED SEM SLS Tungsten disulfide STEM TEM TMDC VLS VSLS near infrared Nano-flake N-Methyl-2-pyrrolidone N-Vinylpyrrolidone photoluminescence Poly(methyl methacrylate) Polytetrafluoroethylene selected area electron diffraction scanning electron microscopy solid liquid solid WS2 scanning transmission electron microscopy transmission electron microscopy transition metal dichalcogenides Vapor Solid Liquid Vapor Solid Liquid Solid xii

13 SUMMARY Recently many researches have been focused on nano materials because of the crucial role of dimensionality in the fundamental properties of materials. Graphene is the first 2D material explored experimentally. Its unique properties and also developed methods in synthesis of its ultrathin layers led to the exploration of other 2D materials such as Molybdenum disulfide (MoS2) and phosphorene. Another type of nanomaterials are the ones with one dimensionality, like nanowires and nanotubes with unique electron transportation and structural anisotropy needed for different applications in electronics, optoelectronics and energy storage. While nanomaterials are unique and exciting, one big challenge is production of these materials in large quantities to be used in real word applications. Different methods have been used for synthesis of the low dimensional materials among which we are focusing on Liquid Phase Exfoliation (LPE) and Vapor Liquid Solid (VLS) - Chemical Vapor Deposition (CVD) techniques, which are among the most promising approaches used for production of nanomaterials. Success in the mass production of desired nanomaterials will make them lead the industry in the 21st century. In this work, Liquid phase exfoliation (LPE) of Black Phosphorous (BP) atomic layers down to monolayer and also Sulfur assisted Vapor Liquid Solid (VLS) - chemical vapor deposition (CVD) of Silica nanowires are proposed and investigated. xiii

14 In chapter 2, using compatible solvent and suitable ultrasonic energy and time, BP nanosheets have been synthesized. The process considering solvent, exfoliation and centrifugation time has been optimized. Optical microscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), Raman spectroscopy, photoluminescence measurement (PL), UV-Vis- near IR spectroscopy and Dynamic Light Scattering (DLS) have been used to study the properties and characteristics of the produced sheets. The nanosheets have been used as films for humidity detection sensors and also as reinforcing material in nanocomposite polymers. Sodium transfer pathway in the synthesized nanoflakes has also been investigated using in-situ TEM. Specifically in chapter 3, Sulfur assisted Vapor Liquid Solid (VLS) technique has been employed for the synthesis of SiOx nanowires in the presence of Sulfur (S) promoter gas. The process considering growth time and temperature, seeding concentration, and sulfur have been studied. In this study, different characterization methods such as SEM, TEM, AFM and EDS have been performed for analyzing the grown structures and optimizing the growth process. xiv

15 CHAPTER 1 INTRODUCTION 1.1 The Rise of nanomaterials In the last decade, evolution of nanoscience and nanotechnology have gained high attention of scientists. Nanostructures are the ones with at least one dimension less than 100nm. In this dimension the number of atoms are countable and because of the quantum effects the properties of such materials are different from their bulk. The nanostructured materials are divided into three different types based on their dimensions: zero dimensional (0D) like nanoclusters, one-dimensional (1D) like nanowires and fibers, two-dimensional (2D) like Graphene, molybdenum disulfide (MoS2) and three-dimensional (3D) like nanopowders and nano-fibrous. 3D nanomaterials are composed of nanomaterials with 0D, 1D or 2D dimensions which are in close contact with each other leading to 3D structures with Nano sized grains inside. A schematic of these structures are shown in Figure 1. 1

16 2D 3D 0D 1D Figure 1. Schematic image of 0D, 1D, 2D and 3D nanostructures assemblies. In December 1959, Richard P. Feynman [1]introduced the importance of nanomaterials in his work There s Plenty of Room at the Bottom. He stated What could we do with layered structures with just the right layers? What would the properties of materials be if we could really arrange the atoms the way we want them However, the highest motivation for studying the 1D materials started by discovery of Carbon Nanotubes (CNTs) having exciting properties due to quantum confinement effect. This phenomena affects mechanical, physical, electrical and optical properties like melting point, tensile strength, thermal conductivity, and light polarization along the longitudinal axes of 1D materials. Later on different one dimensional materials were synthesized and investigated in the past decades. These structures are appealing for studying the effect of dimensionality (quantum confinement). They can also be used as interconnects in electronics, optoelectronics and micromechanical systems[2]. 2

17 In 1964, Wagner and Ellis[3] proposed the growth of silicon whiskers, which have been widely used to guide the growth of various kinds of one-dimensional nanostructures. The reason for the natural 1D growth is the highly anisotropic bonding within the crystal structure. 1D nanowires, nanoribbons (NRs)[4] and nanomeshes (NMs)[5] with a confinement in two directions are examples of 1D structures. Nanowires are classified in different groups considering their electrical conductivity. Au, Ag, Ni are among metallic nanowires and Si, ZnO, and GaN are among semiconductors, while SiO2 is being considered as insulating nanowires. Because of these exiting properties and their applications in nano science, researchers are putting high efforts on studying these materials and opening the new routes for using these materials in industrial scale. 1D materials can play an important role in interconnects and also functional components in nano-devices used in electronics, optoelectronics, electrochemistry and sensing. In order to widely employ these interesting materials in the real world industry, the synthesis and mass production of these materials is of high importance. 2dimensional (2D) materials are one layer thick atoms having nanoscale dimensions in two directions. The layered materials are van der Waals solids consisting of weak, noncovalent bonding between layers and strong in-plane covalent bonding. These materials have the ability to be sheared parallel to the in-plane direction. So the bulk of these materials can be exfoliated into single or few-layers. The properties of layered materials have been known since 400 C.E. when the Mayans started using layered clays for making dyes. Later it was developed and some scientific researches were performed on 3

18 understanding the properties of these layered materials. Experiments were designed to exfoliate them into individual sheets with interesting properties. Robert Frindt applied his prediction by insulating thin MoS2 films using adhesive tape and synthesize monolayers of MoS2 by Li intercalation[6]. However, it did not gain much attention until the extraordinary transport properties of individual graphene sheets were investigated in 2004 by Novoselov et al[7]. The discovery of graphene caused huge explosion of interest in exploring other 2D materials and understanding their properties to be used in future circuits. Novoselov et al simply exfoliated graphite into sheets of down to single layer using scotch tape. This mechanical exfoliation technique opened up the research path into 2D materials with unique properties and won the noble prize in D materials are composed of two different groups. First, 2D allotropes, which are single elemental 2D materials such as graphene, germanene, silicene, borophene, stanene, phosphorene which are single layers of carbon, germanium, silicon, boron, tin and phosphorous[8]. Second, 2D compounds, which are composed of more than one element. Some examples of this later group are Hexagonal boron nitride (hbn) and Transition metal Di-chalcogenides (TMDCs) like Molybdenum disulfide (MoS2) and Tungsten disulfide (WS2). Nowadays a lot of articles are getting published on the synthesis and properties of these materials because of their unique characteristics which are different from their bulk. For example, the carrier mobility of 5000 m 2 V -1 S -1, young modulus of 1 TPa and 3000 W/mK thermal conductivity is observed for a monolayer sheet of graphene [9]. Also monolayer TMDs such as MoS2 and Phosphorene, an elemental P-type semiconducting material, own tunable 4

19 direct bandgap[10][11]. This change in band gap opens up their application in optoelectronics where a direct band gap is needed. NbS2 monolayer is a transition metal dichalcogenides (TMDCs) with nonmagnetic and superconducting behavior at low temperatures of about 6 K[12], and hbn is an insulating material [13]. The assembly of multilayers of TMDCs, graphene, boron nitride (BN) and other 2D systems, vertically or laterally bound together by weak van der Waals forces, can modulate the optical, electrical, thermal and physical properties which can all be explained by quantum mechanics. They can play an important role in applications such as photovoltaic devices[14], lithium ion batteries[15], hydrogen evolution catalysis[16], transistors[17], photodetectors[11], DNA detection[18], and memory devices[19]. One of the most important application of nanomaterials are in microelectronics where smaller means better performance, more circuits in each chip and lower cost[20]. Also in information storage the miniaturization will play an important role[21]. These unique properties have attracted various researchers in the fields of physics, chemistry, materials science, and engineering and evolved the new research field named Nano science. The mass production and controllable preparation of the nanostructures are really important for advancements of nanotechnology and also nanoscience. One of the important effects of exfoliation is increasing the exposed surface area of the material. This increase in surface area will be a benefit in different applications. In catalytic materials, it will lead to enhancement in their chemical and physical reactivity[22]. Also in composites where higher stiffness and strength is required, the decrease in the thickness of the fillers will 5

20 help the enhancement of these properties[23]. Another importance of these 2D materials are in electronics where electrons are constrained and adopt a 2D wave function different from their bulk materials, which has the 3D wave function. This leads to a change in the band structure of material and different electronic properties[24]. These appealing properties in 2D materials have made them so attractive for both research and industry, that a lot of funding goes into investigating their research. Therefore relevant studies on the production and characterization of these materials are among the major priorities in the field of nanotechnology. 1.2 Future of integrated circuits According to Moore s law, the number of transistors in chips doubles every 18 months[25]. Shrinkage in size of the chips and increase in number of transistors per chip is pushing the silicon industry towards its size limitations. Increase in the number of transistors per chip can be seen in Figure 2 which follows the Moor s law. 6

21 Figure 2. Moore's Law (Transistors per Microprocessor), ][27] Si transistors cannot scale down indefinitely, and further shrinkage of their size will compromise their performance. So, microelectronics industry is looking into materials beyond silicon in order to secure the future higher technology computers. The most recent prediction of size limits for integrated circuits can be seen in Figure 3[28]. 7

22 In production Figure 3. Intel R&D microstructures road map Because of this limitation, nanomaterials beyond silicon are being widely studied as a way to overcome the technological limitations of silicon-based technology at the atomic scale. 8

23 CHAPTER 2 HIGH QUALITY BLACK PHOSPHOROUS ATOMIC LAYERS BY LIQUID PHASE EXFOLIATION (Previously published as High quality black phosphorous atomic layers by liquid phase exfoliation. P. Yasaei,, B. Kumar, T. Foroozan, C. Wang, M. Asadi, D. Tuschel, J. E. Indacochea, R. Klie, A. Salehi- Khojin. Advanced Materials, 2014, 27 (11), ) 2.1 Introduction Discovery of 2D nanomaterials like graphene and TMDCs have gained much attention due to their unique properties which can be employed in different fields of electronics, optoelectronics, sensing, electrochemistry, flexible devices and etc. Graphene has the highest carrier mobility but the absence of a bandgap limits its performance in semiconductor industry[24]. On the other hand TMDCs possess a tunable bandgap changing with number of layers; however, the mobility of TMDCs are much lower than graphene that prevent their performance in high frequency applications where higher mobility is needed[29]. The recent discovery of phosphorene (one layer thick black phosphorous) has filled the space between graphene and TMDCs owing to its relatively high mobility and tunable direct bandgap making it a good candidate for the semiconducting industry[30]. BP is also permissible to be used in electrocatalysis[31], energy generation[32], chemical/bio-sensing[33] and storage systems[34]. In these applications there is no need for atomically thin flakes, but it is important to have them in the form of embedded structures, composites or films. 9

24 In this report we have used liquid exfoliation technique for mass production of highly crystalline atomically thin BP flakes and showed its promising performance in different applications such as composites, sensing and batteries. 2.2 Literature Review After the exfoliation of graphene from graphite in 2004, the research progress on 2D materials increased extraordinary. Exploring other 2D materials properties became of major interest for Scientifics. As some examples of these 2D materials are: hexagonal boron nitride (h-bn), transition metal dichalcogenides (e.g., MoS2, WS2, MoSe2, WSe2, etc.), graphitic carbon nitride (g-c3n4), layered metal oxides, layered double hydroxides (LDHs) metal organic frameworks (MOFs), covalentorganic frameworks (COFs), black phosphorus (BP), silicone, and MXenes[35]. 10

25 Figure 4. Schematic of some ultrathin 2D materials[35]. The development of a facile method for synthesis of these materials is a major challenge for using them in real applications. One of the important discoveries regarding the production of 2D materials was understanding that the layered crystals get exfoliated in some liquids. Regarding this discovery different methods involving ion intercalation, surface passivation and oxidation[36] have been used. The exfoliated sheets are known as nanosheets and nano is used to refer to the thickness of the produced sheets. There are different types of materials that can be exfoliated into nanosheets such as graphene, boron nitride, transition metal dichalcogenides (TMDCs), metal halides, clays, layered double hydroxides, ternary transition metal carbides/nitrides and metal oxides[37]. 11

26 There are different methods for liquid exfoliation. One of them is oxidation of the material at first and then dispersion of the oxide in the proper solvents[38]. In this method the material will be treated with oxidizers which results in the attachment of the hydroxyl groups to material. This increases the hydrophilicity of the surface and lets the water molecules intercalate between the layers of the materials and produces oxide material nanosheets. Then the material can be reduced using reducing agents but will cause defects on the surface structure of the material changing the properties from the pristine material. Another method used for this purpose is called intercalation[36]. In this method guest molecules named inclusion complexes are absorbed between the layers of materials. These ionic species increase the space between the layers and weaken the boding between them resulting in the reduction of the energy barrier needed for exfoliation. After this intercalation, introducing some energy between layers like thermal shock or ultrasonication will separate the sheets from each other producing nanosheets. The issue for this method is sensitivity of the ionic compounds to ambient condition. Liquid phase exfoliation using ultrasonic waves is another method used recently for exfoliation of materials with less defects than other exfoliation techniques in liquid[39]. This is a non-chemical, non- covalent solution based process which is insensitive to ambient condition. In this method the ultrasonic waves generate cavitation bubbles which produce high energy jets while collapsing. This energy can break the wan der Waals forces between the layers and separates them from each other by means of ultrasonic energy. The theoretical calculations have revealed that if the surface energy of the material is similar 12

27 to that of the solvent, the state of the energy difference between the exfoliated material and re-aggregated one will be so small that the sheets will not have the tendency for reaggregation. The other benefit of this method is the possibility for mass production of sheets with much less defects than other exfoliating methods involving chemical reactions and covalent bindings between the solvent and exfoliating material. The energy needed for exfoliating layered materials can be identified by surface energy. Surface energy is the needed energy for removing one layer of the material from its bulk divided by twice the surface area of the monolayer. Using proper solvents with desirable surface energy together with enough sonication time and energy, one can disperse layers of material in solvents. The exfoliated sheets can be then vacuum filtered, drop casted, spray/ spin coated in different structures such as films, hybrids and composites for different applications from sensors[40] to conductive composites[23], energy generation[41] and transparent electrodes[42] Towards the Mass Production of 2D Materials In order to employ 2D materials for industrial applications, the most important barrier is the mass production of them. Different approaches have been used to fabricate the few layer thick samples. The first method used to insulate a few layers of these materials was mechanical exfoliation[43]. In the mechanical exfoliation technique, the weak bonds between the layers of bulk material are peeled off using scotch tape. By putting the tape containing exfoliated material on the surface of a receiving substrate and removing 13

28 it very slowly, the material will be transferred to the surface of desired substrate. After that it is time to locate the flakes using optical microscopy which is a time consuming approach. Also the typical size of the mechanical exfoliated flakes is about 1-3 µm which is not useful for lots of industrial applications. The advantage of this method is the pristine quality of as exfoliated material which is appealing for research stage but as mentioned the yield is low and industrialization of these materials need large area and mass production techniques which mechanical exfoliation is incapable of. Other efforts for synthesis of these materials have been made by means of physical or chemical reactions. Solvent-assisted exfoliation[37], chemical exfoliation via lithium intercalation[44] wet chemical synthesis[45] and chemical vapor deposition[46] are examples of the routes developed for synthesis of these materials for different applications. Synthetic methods of 2D materials can be classified into two groups of top-down (e.g., mechanical, chemical exfoliation,) and bottom-up (e.g., direct wet chemical synthesis and chemical vapor deposition) routes. These methods all have pros and cons and considering the application one should choose the desired method for production of these materials. As mentioned previously, although mechanical exfoliation gives pristine monolayers useful for understanding the actual properties of material, the inability of this method for mass production and large area synthesis is its main disadvantage. The intercalation of TMDCs by ionic species exfoliates the material in liquid and was first demonstrated in the 1970s[47]. Submerging of the TMDC powder in a solution 14

29 of a lithium-containing compound such as n-butyllithium for days allows lithium ions to intercalate into the layers of bulk material. Exposing the intercalated material to water separates the sheets by formation of H2 gas as a result of the reaction between water and lithium[6]. The disadvantage of ionic intercalation is that first, Li compounds are flammable under ambient conditions and as a result the work needs to be done under inert gas; Second, Li is expensive and use of this material for exfoliation is not economical[29] and also the sheets will be defective by this method. The disadvantage of chemical exfoliation is that in this approach the Li compounds functionalize the materials chemically with compounds such as hydroxyls and epoxides to stabilize the layers[38]. This covalent bonding between the reducers and material disrupts the structure of material which affects the desired electronic properties. Nowadays, ultrasonic assisted liquid phase exfoliation (LPE)[48], is one of the most promising methods used for mass production of 2D materials. In this method the shear force and cavitation bobbles produced from the propagation of sonication waves break the van der Waals force between the layers but cannot break the covalent intraleyer bindings. Using this method layered materials get exfoliated with minimum defects on the nanosheets produced because of the non-covalent and non-chemical interaction between the bulk material and liquid. The liquid-phase exfoliation is related to solution thermodynamics. The free mixing energy ( Gmix) is given by: 15

30 Where Hmix is the enthalpy of mixing and Smix is the entropy of mixing. For negative Gmix which is needed for good mixing, large Smix is needed. However, nanosheets are large and the Smix is small, so Hmix needs to be small. Hmix is related to solvent and nanosheets surface tension as mentioned in this equation. TNS is the nanosheet thickness, S and NS are the solvent and nanosheet surface energy, and is the dispersed nanosheet volume fraction. Having S and NS close to each other minimizes the enthalpy of exfoliation. Even when the the surface energy of the solvents matches the nanosheets, sometimes the concentration of nanosheets is very low. For this reason, Hansen parameter ( T), which is related to other solvents interaction parameters of polar-bonding ( P), hydrogen-bonding ( H) and dispersions ( D) is proposed[49]. As a result, to minimize the enthalpy energy of exfoliation, three solubility parameters of the solvent should be close to those of the solute for best solvent exfoliation. Considering the recent discovery of black phosphorous as a promising material for use in different applications such as storage systems, embedded structures and sensors, the 16

31 exfoliation of this material is of major interest. In this chapter I have focused on the exfoliation of black phosphorous into mono and few layer phosphorene using liquid phase exfoliation and some applications of the produced sheets in humidity sensing, polymer nanocomposites and Na- ion batteries have been investigated as well. 2.3 Liquid Phase Exfoliation (LPE) of Black Phosphorous (BP) In this chapter, we used the liquid phase exfoliation technique for the mass production of highly crystalline atomically thin BP sheets in organic solvent. Ultrasonic energy is used for breaking the van der Waals force between the layers of bulk BP and separating them in the solution. Typically, ultrasonication is a top down method which mechanically exfoliates the layered crystals to produce flakes with the size of few hundred nanometers which can be used in inkjet printing, spray coating and etc. In this method molecules of the solvent are adsorbed on the exfoliated sheets by non-covalent interaction which is less damaging than chemical exfoliation, in which covalent bonding between liquid and solid material is involved. We investigated the exfoliating ability of several solvents of different chemical families with different chemical and physical properties (e.g., Hansen solubility parameters). The groups we considered are as follows: These were mainly: (1) Alcohol (e.g., methanol), (2) Chloro-organic solvent (e.g., 1,2- Dichlorobenzene), (3) Ketone (e.g., acetone), (4) aliphatic pyrrolidone (e.g., NCyclohexyl- 2-pyrrolidone), (5) N-alkyl-substituted amides (e.g., Dimethylformamide) and (6) Organosulfur compound (e.g., Dimethyl sulfoxide) having different surface tensions (

32 to dynes/cm) and polar interaction parameters (2.98 to 9.3) which are important for the investigation of the exfoliating abilities of different solvents. The Hansen parameters of these solvents can be seen in Table 1. These solvents have been frequently employed for exfoliating layered materials in previous works[50]. 18

33 Table 1. Hansen solubility parameter and surface tension of examined solvents Solvents Surface Dispersive Polar hydrogen bond interaction tension force interaction (δh) (MPa1/2) (dynes/cm) (δd) (MPa1/2) (δp) (MPa1/2) Methanol (MeOH) Ethanol (EtOH) Acetone prapanol (IPA) Dimethyl sulfoxide (DMSO) Dimethylformamide (DMF) 1,2-Dichlorobenzene (DCB) N-Vinylpyrrolidone (NVP) Cyclohexyl-2-pyrrolidone (CHP) For this purpose 0.5 mg of bulk black phosphorous (purchased from Smart Elements) was grinded by a mortar and pestle and immersed in 10 ml of different solvents and were then covered by parafilm in order to decrease the chance of contaminating the 19

34 solution and also exposure to air. Then we introduced a small hole on the parafilm for entering the sonication tip into the solution and performing the sonication in a Sonics Vibra-Cell sonicator (130 W) for about 15 hours. The optical image of solutions we obtained from sonication of BP in different solvents can be seen in Figure 5. Figure 5. Optical image of solutions at different experimental steps. (A) Optical image of samples after one day Some solvents such as N-Cyclohexyl-2-pyrrolidone (CHP) and Isopropanol (IPA) could not exfoliate the BP to a noticeable amount after many hours, so we considered them as non-exfoliating solvents for BP bulk material. In some other solvents such as 1,2- Dichlorobenzene (DCB), or N-Vinylpyrrolidone (NVP), we noticed chemical reaction instead of physical separation of the layers. For understanding the reaction that was involved, we refer to the first principle study calculations[51] suggesting that several chemical species, such as H, F, and Cl can strongly bind to the phosphorous atom acting as scissors to separate upper and lower parts of the puckered honeycomb structure of BP. Thus, we assume that DCB breaks down the BP layers into atomically small pieces not breaking the van der Waals force between them to separate the layers. The SEM image 20

35 of drop casted solution obtained from BP particles in DCB can be seen in Figure 6. We can note that instead of crystalline separated flakes, agglomerated particles of amorphous structure are seen. Figure 6. SEM images of the product obtained in DCB solution. a) Low and b) high-magnification. Scale bars are 1 um and 200 nm. In case of NVP, we observed a drastic increase in viscosity of the solution. In this case we assume that this chemical reaction is because of the high polymerizing ability of NVP that can form high molecular weight polyvinylpyrrolidone (PVP) as seen in Figure 7. 21

36 Figure 7. Polymerization of NVP in presence of black phosphorus. A) Shows the polymerization of NVP and its conversion to PVP. b) Optical images of the viscous solution obtained from sonication of BP in NVP solution. In case of DMSO and DMF solutions crystalline and transparent nanoflakes were obtained, which shows the ability of these solvents to exfoliate BP into atomically thin sheets. In order to understand the mechanism of exfoliating the bulk BP a schematic of the process in which a chunk of bulk material converts to exfoliated nanosheets in the solvent is shown in Figure 8a. At the beginning a chunk of BP is immersed in DMF or DMSO solution and by using ultrasonication energy, using the Sonics Vibra-Cell sonicator (130 W), as a source for breaking down the van der Waals forces between the layers we could separate the nanosheets of BP with different thicknesses down to monolayer. In order to 22

37 separate the thick sheets from the thinner ones, centrifugation with Eppendorf 5424 Centrifugation machine (250W) in 2000rpm for 30 minutes is performed on the solutions. This way the supernatant consisting of the thinner flakes will be collected for further experiments. Considering the fact in some applications in which films of exfoliated flakes are needed, we produced the films BP NFs as can be seen in Figure 8b. For this purpose we used hydrophilic polytetrafluoroethylene (PTFE) filter papers of 0.1 μm pore size and filtered the solution through them using a vacuum filtration setup washed thoroughly with pints of ethanol and IPA. We used PTFE membrane as this is among few filter membranes which is compatible with DMF and DMSO and does not degrade and dissolve during the filtration process. Ethanol and water are the best solvents for DMF and DMSO with low evaporation points that can remove the residue of these solvents on filter. However, we just used ethanol and not water for the washing step since it s proven that BP degrades upon exposure to water molecules, which we don t want. The purpose for using IPA in the last step of washing is removing the ethanol residue. Filtering 3ml of a typical DMF solution containing 0.2 mg BP results in a typical film shown in Figure 8b. 23

38 a b Figure 8. Steps involved in production of nanoflakes and films. a) Schematic and real image of exfoliation process b) film production In order to compare the ability of these two solvents, careful study has been performed using SEM, AFM, optical absorption spectroscopy and DSL. c 2.4 Characterization of exfoliated BP flakes Black Phosphorous is composed of a puckered honey comb structure as seen in Figure 9. The puckered structure along the y-direction is shown in Figure 9a. Figure 9b is the top view showing a honeycomb-like structure and the box in blue represents an individual unit cell of single-layer BP. The atoms in this structure can have different in 24

39 plane and put of plane vibration modes. In order to confirm the crystallinity of the flakes after exfoliation and also film production, Raman spectroscopy was collected on individual flakes and films obtained from the BP NFs. Both emperiments represent the signature Raman peaks of BP at wavenumbers of 360, 437, and 466 cm 1, showing Ag (outof-plane mode), B2g and Ag2 (inplane modes) shown in Figure 9c, respectively. Figure 9. a) Perspective illustration of the puckered structure along the y-direction.[106] b) Top view illustrating the honeycomb structure. The blue box represents an individual unit cell of BP. C) Signature Raman peaks obtained from BP nanosheets. For characterizing the individual flakes we used on chip separation method. For this purpose we used the dispersion of BP NFs in IPA and using a polyethylene pipette drop casted a small amount of solution on a Si/SiO2 substrate of 270 nm thermally grown oxide, and dried. We used different methods for drying the substrates. In order to have bigger flakes we used light/hot plate (90 C) as the dryer and the flakes remained on the 25

40 surface in circular pattern because of coffee-ring effect [52]. This method is used to have size- sorted BP flakes. The droplet size, concentration of solution and drying temperature all have effect on the formation of the rings. Flakes with larger size stay on the rings while remaining residue and smaller flakes move to the center of the ring, shown in Figure 10a. The other method used for having more individual flakes on substrate was drop casting the IPA/NFs solution on substrate and letting it dry naturally. This was done under hood with a cap covering the surface to minimize deposition of residual particles in the air. The samples were then rinsed gently with methanol, ethanol, deionized (DI) water, and IPA. N2 was blown on the sample in order to remove the remaining solvent residues during evaporation stage. Scanning electron microscopy (SEM) images of the sample dried using coffee ring method shows densely packed BP nanosheets in the center (Figure 10b) and individual flakes in the outer rings (Figure 10d). Atomic force microscopy (AFM) height measurement were also performed on same flakes showing thickness ranging from 5.8 to 11.8 nm (Figure 10e). Thickness distribution of the flakes in DMF and DMSO solutions were compared by performing low magnification AFM mapping of the samples. The thickness histogram obtained from 70 individual flakes (Figure 10f) show that in DMF, more than 20% of the considered flakes have thickness of less than 5 nm, while DMSO solution contains the flakes with thickness of about nm. Considering these results we chose DMF for the rest of our experiments. 26

41 BP is known for its anisotropic structure[53], which leads to different optical and electrical performance in different directions. For this purpose performing the linearly polarized Raman spectroscopy on the flakes with different sample orientations was interesting to show the anisotropic nature of the BP exfoliated sheets. As shown in Figure 10c, the intensities of the three vibrational modes change by changing the orientation of the sample. We can see the Ag1 and Ag2 modes trend for 180 degree rotation of the sample in right side of Figure 10c. The Ag modes should be maximized when the laser polarization is parallel to X-axis of the BP crystal[54]. Considering this fact one can determine the flakes orientation using polarized Raman spectroscopy. Figure 10. Morphological and Raman characterization of the exfoliated BP sheets. a) Low magnification SEM image of the on chip separated flakes showing coffee-rings on S/SiO2 substrate. (Scale bar is 200 μm). b) High magnification SEM image from center of the ring (scale bar is 200 nm). c) Raman spectra of synthesized BP sheets at different orientations showing the anisotropic nature of BP flakes. d) SEM image of individual BP flakes on the produced rings (scale bar is 200 nm). e) AFM image and height profile of the area shown in D. f) Histogram 27

42 of the flakes thickness distribution measured by AFM height measurement of 70 flakes from both DMF and DMSO solutions In order to confirm the result from thickness of 70 individual flakes, we performed SEM imaging and we noticed that smaller, but thinner flakes exist in case of DMF. For this purpose we used the substrates with alignment marks and deposited the flakes on them to have a better contrast for comparison. By looking at the transparency of the flakes in exposure to electron beam in same condition (20 kv acceleration voltage, and 30 um aperture size) we found that the ability of DMSO in exfoliating the BP is lower than DMF, and thicker, but bigger flakes can be obtained from this solvent (Figure 11). Figure 11. SEM images of BP Nano flakes. (A), (B), and (C) are obtained from DMSO solution. Scale bars are 10 um, 10 um, and 200 nm respectively. (D), (E), and (F) were taken from sample prepared in similar way using DMF solution. Scale bars are 1 um, 0.5 um, and 200nm, respectively. In order to measure the thickness of a typical film we dispersed the wet washed film into IPA using sonication and re-filtered the flakes onto a mixed cellulose membrane 28

43 filter. Then we cooled them in liquid nitrogen to make them more fragile and broke the filter to have access to the cross section of the membrane. Using SEM we measured the thickness of the film to be ~26 μm. Characterizing the samples in the solution level is also of major importance. We used the solutions of DMSO and DMF, shown in Figure 12a, which are BP nanoflake dispersions after 15h sonication (left image) and after the centrifugation (right image) for our characterizations in this level. For measuring the concentration of our solution, we used 0.2 mg of chunk BP and immersed it in 10 ml of solutions. We sonicated the samples using a Sonics Vibra-Cell sonicator (130 W) for 15 hours and then centrifuged the solution in an Eppendorf 5424 Centrifugation machine (250 W) for 30 min in 2000 rpm. Collecting the top 50% of the solution helped us separate the thinner flakes from the thicker ones. The Supernatant was then filtered using a Sigma Aldrich vacuum filtration system on a hydrophilic polytetrafluoroethylene (PTFE) membrane filter of 0.1 μm pore size. We washed the produced film thoroughly using ethanol, water, and isopropyl alcohol (IPA) to make sure the solvent residue is removed from the surface, and dispersed the stacked flakes in IPA. We used another filter membrane of mixed cellulose which is compatible with IPA and measured the weight of the membrane using a weighing machine with resolution of 0.01mg. The IPA solution containing the BP NFs were again filtered on the mixed cellulose membrane with known weight. Afterwards we let the membrane dry and measured it again with the same weighing machine. The difference between the weight of the membrane before and after filtration show approximately the amount of BP we had in 29

44 the solution. Dividing this amount to the volume of the solvent we used for the sonication, we calculated the concentrations of the solutions to be about 10 μg ml 1. For solution level characterization of our samples, we performed optical absorption spectroscopy for the range of 0.5 to 1.4 ev which covers the bandgap spectra of BP from bulk to monolayer. BP atomic layers have a direct thickness dependent bandgap ranging from 0.3 ev for bulk to about 1 ev in monolayer[55]. Optical absorption spectroscopy have been used to characterize the BP flakes dispersed in DMF and DMSO with considering the bandgap dependency of BP. We focused on the wavelength of nm (NIR) to determine the peaks showing the band gap of BP nanoflakes. Optical absorption spectroscopy from both DMF and DMSO solutions show several peaks at 1.38, 1.23, 1.05, 0.85, and 0.72 ev (labeled as numbers 1 5 in Figure 12b) in the NIR range, which are associated with the enhanced light absorption by mono, to five-layers thick BP nanoflakes, respectively. The location of these peaks are so close to the position of the photoluminescence peaks related to mono- to five-layers mechanically exfoliated BP flakes[54]. As it can be seen in Figure 12b the peaks labeling 1 and 2 are having much smaller intensities compering to the rest of the peaks. These peaks at 1.38 and 1.23 ev which are associated with the bandgap of one and two layer thick BP implies the low yield of mono and bilayers flakes in our samples. The smaller peaks compared to other peaks implies that the yields of mono- and bilayers are lower than other atomic layers. Considering the Lambert Beer law (A/l =αc, where α is the extinction coefficient), concentration of the dispersion in a solution is related to the absorbance of the material in 30

45 the solution, and also the absorbance is directly related to the length of the line path. Considering this law the normalized absorption intensity over the length of the cell (A/l) at λ= 1176 nm were measured for both DMF and DMSO solutions at different concentrations (C), and a linear trend was observed for A / l versus concentration (Figure 12c). This trend shows the well-dispersion of nanoflakes in both solutions. The extinction coefficients for DMF and DMSO solutions were also calculated from the slope of the Figure 12c to be 4819 and 5374 ml mg 1 m 1, respectively. Considering this coefficient we can calculate the concentration of the dispersions in both DMF and DMSO solutions. This method is more precise than vacuum filtration and weighing method, which is time consuming. Also, the loss of material during the process is so probable. As it can be seen in Figure 12d, the size distribution of BP sheets was also measured by dynamic light scattering (DLS) spectroscopy, showing the average flake sizes of about 190 for DMF and 532 nm for DMSO solution, suggesting the existence of smaller flakes in DMF solution compared to DMSO. 31

46 Figure 12. Comparison between DMF and DMSO solutions. (a) Photograph of the solutions after sonication (left) and after centrifugation and supernatant collection (right). (b) Optical absorption spectra indicating optical bandgap of mono to 5 layer BP flakes in solution. c) Normalized absorbance intensity vs. characteristic length of the cell (A / l) in different concentrations for = 1176 nm (E = 1.05 ev) and extracted extinction coefficient (α) from their slope. (d) DLS histogram showing the particle size distribution. In order to study the shape, quality, crystallinity and number of layers in our exfoliated flakes, transmission electron microscopy (TEM) was performed on individual flakes. Figure 13 shows TEM image of some random BP nanoflakes on a lacy carbon support. 32

47 Figure 13. Transmission electron microscopy (TEM) images of BP nanoflakes. The scale bar is 100 nm. In order to identify the BP flakes, fast Fourier transform (FFT) was used on flakes with thicknesses down to a single layer. As suggested theoretically by Castellanos-Gomez et al.[56], the intensity ratio of (110) to (200) diffraction peaks is more than one for monolayer and less than one for multilayer BP. This ratio has been experimentally verified for multilayers in reference 56. In our images taken from monolayer BP, the I110 / I200 is about 2.7 ± 0.2, in good agreement with the proposed computational value of The high resolution TEM image together with the DFT patterns from monolayer BP has been shown in Figure 14. As it can be seen, the intensity ratio of the (110) and (200) peaks is clearly greater than one. 33

48 Figure 14. HRTEM image and FFT of a monolayer BP. The scale bar is 2nm. FFT in (B) shows the presence of BP lattice and hydrocarbon on the surface of the BP flakes and also shows that (I110/I200) is greater than one, confirming the presence of monolayer. The ratio was measured to be 2.7 ± 0.2 after background removing which is close to which is the computational value BP monolayer[56]. In order to show the crystallinity of the flakes, FFT patterns was collected from different parts of a flake, as shown is Figure 15b. All of them show identical features showing the single crystalline nature of the monolayer flakes. Also, high-resolution TEM imaging was performed on the flakes in order to resolve the atomic structure of the flakes. As it can be seen in Figure 15c(upper right), the TEM simulations from the (001) direction using the BP lattice parameters[57], completely matches the atomic structure of monolayer BP. Also you can see the filtered image in the lower left inset of Figure 15c. 34

49 Figure 15. FFT and HRTEM of BP nanoflakes. A) TEM image of a BP flake scale bar is 200 nm. B) TEM image and FFT of the same flake showing the existence of monolayer BP on the region of 80 nm 80 nm. The scale bar is 20 nm. C) HRTEM image of the monolayer BP together with TEM simulation (upper right inset) and a filtered image (lower left inset). Scale bar is 1 nm. We should note that for monolayer BP, the simulated image shows hexagonal structure (like graphene), and orthogonal structure for multilayers as a result of ABA stacking order for multilayers (Figure 16). 35

50 Figure 16. Schematic of atomic structures together with TEM image simulations of mono and multilayer BP viewing from (001) direction. (A, B) mono and multilayer crystal structure of BP drawn by VESTA. (C, D) TEM image simulations of monolayer and multilayer BP showing different structures of hexagonal and orthogonal for monolayer and multilayer, respectively. In order to understand the quality of the BP flakes energy-dispersive X-ray spectroscopy (EDX) analysis, together with electron energy loss spectroscopy (EELS) have been performed on the existing BP sheets on lacy carbon TEM grid. As it can be seen in Figure 17a, EELS elemental mapping from the BP flake shows only existence of phosphorous without any impurities. Comparison between HAADF and spectrum images obtained from the same flake shows a pure phosphorous distribution on the considered area. An individual EELS spectrum of the EELS map also confirms the existence of pristine phosphorous atoms and absence of the Px Oy peak loss in Figure 17b. Interestingly, repeating the EELS analysis on samples of more than 1 month old, revealed that the flakes remain intact in solution which is a main benefit of this production method for BP. 36

51 Figure 17. A) EELS map and high-angle annular dark-field (HAADF) image of BP (scale bar is 200 nm). Each pixel size of the EELS map is 22 nm. E) An individual EELS spectrum of the EELS map IS shown in B. Having energy-dispersive X-ray spectroscopy (EDX) analysis as a confirmation of the EELS result is shown in Figure 18. Phosphorus (P) peak is the largest one which shows the main element existing on the flakes and the peaks associated to Carbon and Copper come from the TEM grid. They originate from the transmission electron microscopy (TEM) carbon grid, and weak Chromium (Cr) and Titanium (Ti) peaks are because of the contamination coming from the sonication tip. 37

52 Figure 18. Energy-dispersive X-ray spectroscopy (EDX) analysis performed on BP sheets. phosphorus (P) peak is the largest one and the peaks associated to Carbon and Copper come from the TEM grid, originate from the used transmission electron microscopy (TEM) carbon grid and weak Chromium (Cr) and Titanium (Ti) peaks are because of the contamination coming from the sonication tip. 2.5 Optimization of the Exfoliation Process In order to have the best exfoliating condition, we need to optimize our experimental condition. Considering the better yield of DMF compared to DMSO we chose DMF for our further characterizations and also optimization of the exfoliation process. For studying the effect of the sonication power, we chose15, 30, 65 and 130 W for both sonication bath (1510 Branson) and sonication probe (Vibra Cell Sonics 130W). For this purpose we compared the solutions from sonication bath and sonication probe after 20 hours of sonication and 24 hours of settlements. For lower energies of 13, 30 and 65 W, the color of the solution was changed from brown to yellow after 24 hours of settlement, with visible sedimentation at the bottom of the container. However, for 130 W 38

53 the color almost stayed unchanged with small amount of sedimentation. Considering these results, we can say that higher sonication power is favorable, and therefore, we fixed the power at 130 W for the rest of our characterizations and optimization process. In order to optimize the time of sonication we used a 0.2 mg single chunk of BP crystal in 10 ml of solvent and studied the solutions in 27 hours, with intervals of 3 hours. After about 10 hrs, the bulk BP was completely disappeared, and further sonication lead to production of thinner flakes with smaller sizes. After 15 hours, the thickness didn t change much, but the average size decreased from 190nm in 15 hrs., to 100 nm in 27hrs. Considering this result, we fixed the time of bulk sonication to 15hrs. However, we should note that sonication time depends on different parameters, such as size and surface area of the bulk material in the solution. Also in order to decrease the sonication time needed for optimized sonication, we grinded the bulk BP using a mortar and pestle and sonicated the powered BP. We noted that time of sonication decreased significantly as the surface area of the material has increased a lot. As it can be seen in Figure 19, it took about 3-6 hours, depending on amount of grinding, for the grinded BP to give similar results as of single chunk after 15 hours sonication. The consumed ultrasonic energy by the sonication tip was and ~1 MJ after 15 hours and ~0.4 MJ after 6 hours, which shows the lower energy consumption using the ground material as the source for exfoliation. 39

54 Figure 19. Optical image of the solutions with single chunk and with ground material (A) before solvent addition (B) after solvent addition. After (C) 1 hour (D) 3 hours (E) 6 hours (F) 12 hours of sonication. Another important parameter for optimization of the produced solution is the centrifugation time and rate. Centrifugation is a process in which centrifugal force is used to separate the denser/heavier compounds than lighter ones. Heavier material which is composed of higher number of layers move to the lower part of the centrifugation tube, while lighter material consisting of less number of layers move towards the upper part of the tube. The more bulky material stays at the bottom of the tube, and the more exfoliated material stays on the upper part. The precipitate remaining at the bottom is referred to as a pellet and the liquid remaining above the solid is called the 'supernatant. The schematic of the samples before and after centrifugation is shown in Figure

55 Figure 20. Schematic showing the differences between solutions, suspension, precipitate and supernatant during the centrifugation process For optimizing the rate of centrifugation, We centrifuged the solutions at 500, 2000 and 4000 rpm for 30 minutes and the supernatant liquid was then withdrawn with a Pasteur pipette and characterized using AFM. As it can be seen in Figure, the height measurement from the sample centrifuged at 500 rpm shows a large number of thick flakes (50-100nm), while the AFM height scan images from sample with 2000 rpm rotation speed show the flakes thickness of about 5 nm. The 4000 rpm was also ineffective, and very low concentration of the flakes were found on the substrate after drop casting. So, we chose 2000 rpm for further characterizations. 41

56 Figure. AFM height mapping and profile from samples of (A-B) 2000 rpm centrifugation for 30 minutes and (C- D) 500 rpm for 30 minutes. As it can be seen 500 rpm is not very effective for separation of thin flakes from bulky material, while 2000 rpm was effective for this purpose. In summary our optimization experiments resulted in fixing the experimental parameters for the case of 0.2 mg ground bulk BP on 6 hours of sonication at 130 W and for the single chunk 15 hours of sonication and centrifugation with 2000 rpm in 30 minutes. 2.6 Applications of liquid phase exfoliated BP Liquid phase exfoliation of BP opens up different potentials for this material to be used in nanoelectronics[10], optoelectronics [58], energy storage systems[59], sensing [60], etc. 42

57 In most of these applications there is no need for this material to be atomically thin and the preferred structures are in the forms of composite and thin films. For this regard, the application of BP exfoliated flakes, as a selective and stable humidity sensor in the form of film, reinforcement element for polymer composites and its sodium ion transport pathway for use in rechargeable ion batteries has been studied. Regarding this work, I have had collaborations in three different works regarding the applications of the exfoliated BP in humidity sensors, rechargeable- ion batteries, and also polymer nanocomposites, which are discussed briefly Stable and Selective Humidity Sensing Using Stacked Black Phosphorus Flakes (Previously published as Stable and Selective Humidity Sensing Using Stack of Black Phosphorus Flake. P. Yasaei, A. Behranginia, T. Foroozan, M. Asadi, K. Kim, F. Khalili-Araghi, A. Salehi-Khojin. ACS Nano, 2015, 9 (10), pp ) In this section the humidity sensing properties of film of BP NFs have been studied[61]. For this purpose, the BP film sensors were fabricated easily from the vacuum filtration of dispersed BP flakes in IPA on PTFE membrane filters (Figure 21a), as discussed previously. Then, they were cut into desired geometry and attached on a tape for support (Figure 21b). Using Ga-In eutectic, the electrical connections were established as the last step of fabrication (Figure 21c). Next, the devices are cut into desired sizes and two probe measurement, together with the pulse injection method was used for testing the sensing performance of the BP films 43

58 Figure 21. (a) BP film on a PTFE membrane filter. (b) Film cut into in size and attached on a tape as support. (c) Using Ga-In as electrical connections. (Figure 22a). 0.2 to 5 μl of the analytes like water vapor, alcohols (ethanol, isopropanol), ketones, toluene, acetone), and benzenes (dichlorobenzene) were injected using the injection unit of a gas chromatography system (HP 6890). In all the experiments a constant (DC) bias of 0.5 V was applied. Interestingly, 5 fold drain current increase was observed by injection of water vapor, at least two orders of magnitude larger than other analytes. Full recovery of 1-5 seconds happened after the injection (Figure 22b). Figure 22. a) Image of the BP device used for sensing experiments. b) BP film response to injected analytes showing 5 fold drain current increase upon injection of water. 44

59 The sensing stability of our BP film sensors were also studied by comparing the responses immediately after fabrication of the sensor, with the ones after 3 months exposure to ambient conditions (25 C and 25±12% RH). Interestingly, the behavior of the sensor remain almost intact after 3 months exposure to ambient condition (Figure 23). Figure 23. Sensing stability of the BP film after 3 months of exposure to ambient condition. The selective response of the BP film against water vapor together with its response stability for several months are highly important for real applications of humidity detection sensors Selective ionic transport pathways in atomically thin BP (Previously published as Selective Ionic Transport Pathways in Phosphorene. A. Nie, Y. Cheng, S. Ning, T. Foroozan, P. Yasaei, W. Li, B. Song, A. Salehi-Khojin, F. Mashayek, R. Shahbazian-Yassar. Nano Letters, 2016, 16 (4), pp ) 45

60 By means of in-situ transmission electron microscopy (TEM), the transport pathway of sodium ions was studied in mono and few-layer black phosphorous (BP). Results show that the sodium ions prefer to migrate along the [100] direction, which is a zigzag direction and they are not willing to move along the armchair direction, as shown in Figure 24. Figure 24. Sodium transport pathway in BP atomic layers with respect to the sodium source contact location. (a) Sodium contact interface normal to the [100] direction. Inset shows corresponding electron diffraction pattern of the few-layer phosphorene in Panel a. (b) Sodium contact interface parallel to the [100] direction. Inset shows corresponding electron diffraction pattern of the few-layer phosphorene in Panel b. 46

61 2.6.3 Polymer nanocomposites containing BP nanoflakes Black phosphorous nanoflakes also have the ability to be used in embedded structures such as polymer nanocomposites as reinforcement. For this purpose the BP nanocomposite were fabricated using PMMA polymer as matrix. For this purpose 10 gr PMMA powder was dispersed in 150 ml of acetone, and dissolved by placing the container beaker on a 35C hot plate, having magnets spinning inside the container for better dissolve. Then, a known concentration of BP flakes dispersed in acetone (the method for concentration calculation has been mentioned previously) was added to the solution and dispersed by means of magnet spinning. The prepared solution was then poured in petri dishes with smooth flat surface in order to increase the uniformity of the produced films. The petri dish was covered with parafilm, introducing small holes on to let the sample get exposed to air, having low air turbulence on the surface in order to decrease the chance of having shrinkage on the surface. Then the prepared samples were placed in hood for 2 days to dry. Figure 25 shows the prepared polymer nanocomposites having different concentration of BP. 47

62 Figure 25. PMMA nanocomposites. From left to right: pure PMMA and nanocomposite containing 0.05 and 0.1% phosphorene nanoflakes, respectively. The prepared composites were then cut into standard sized specimens of e 0.1 mm thickness, 12 mm width and 50 mm gage length using D882 standard, which is designed specifically for thin sheets and films less than 1 mm thick for tensile test characterization. As it can be seen in Figure 26, elongation of the nanocomposites containing BP flakes have decreased a lot compared to pure PMMA. Also, the tensile strength has not changed much. This suggests that the BP nanosheets is not improving the tensile properties of the PMMA polymer. Figure 26. Tensile strength measurement of prepared nanocomposites The young modulus of the nanocomposites have increased by increasing the amount of BP nanosheets, as it can be seen in Table 2. 48

63 Table 2. Tensile modulus of the PMMA nanocomposites In order to understand the reason for the decreased elongation behavior of the nanocomposites, SEM imaging was performed on a typical prepared nanocomposite. As it can be seen in Figure 27, the BP flakes are not embedded well into the PMMA matrix, and we can clearly see the crystalline edge of the flakes with bigger sizes. The smaller flakes have also agglomerated and introduced lines of flakes touching each other, which is not desired in composites. This way there will be a preferred direction for the crack growth, which leads to failure of the structure earlier than expected. For improving the reinforcement properties of this composite, other fabrication techniques such as polymerization is needed which is out of scope of our lab expertise and ability. 49

64 Figure 27. SEM imaging of the embedded BP flakes into the polymer matrix showing the incoherency in dispersion of flakes into the structure and existence of preferred directions for crack growth. Scale bar is 1um. 2.7 Summary and Conclusion As conclusion, we have performed large scale production of BP nanoflakes with high purity and crystallinity using liquid-phase exfoliation technique, being among the first works done on liquid-phase exfoliation of BP. Interestingly, the dispersed flakes in the solution remained intact after 2 months of being left over in ambient condition. This result shows that liquid exfoliation is a promising method for production of BP Nano sheets, and can be protected from degradation in solution form, considering the fact that BP is an air/ water sensitive material. Also, our exfoliated BP flakes have been shown to be promising in different applications such as humidity sensing, Sodium-ion batteries and polymer nanocomposites. 50

65 BP sensor shows ultrasensitive and selective behavior upon exposure to water with a really small drift over time. Also it has been shown that sodium ions prefer to move in the zigzag direction in phosphorene rather than the armchair, showing anisotropic migration of sodium ions in the BP structure. The incorporation of BP in polymer nanocomposites shows the compatibility of this material to PMMA polymer; however, the mechanical properties of the polymer was not improved because of the aggregation of the flakes in lines, causing preferred directions for the crack growth in the composite. 51

66 CHAPTER 3 SULFUR ASSISTED VAPOR LIQUID SOLID (VLS) GROWTH OF SILICA NANOWIRES BY CHEMICAL VAPOR DEPOSITION (CVD) 3.1 Abstract Nanofibers of amorphous SiO2 were synthesized by thermal processing of a Si/SiO2 substrate at 1000 C in the presence of Sulfur (S) as a reducer and nitrogen as carrier gas. Sputtered Pt nanoparticles were used as the catalyst for the growth of nanofibers. The substrate was the only Silicon source for the growth of nanowires. In this process, Sulfur was used to reduce the energy needed for the materials growth and lower the reaction temperature. The nanostructures grown were characterized by Transmission Electron Microscopy (TEM), Electron dispersive Spectroscopy (EDS), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The structural characterizations suggests that the grown material is amorphous Silica (SiOx). The nanowires growth is consistent with the Vapor- liquid- solid (VLS) growth mechanism. The Pt/Si alloy droplets are formed from the reaction between silicon sputtered Pt particles acting as nucleation points which remain connected to the wires till the end of the process. The effect of Sulfur, Pt concentration, time and temperature of growth have been investigated and the growth mechanism is also proposed. The results show that growth time of 5 minutes, growth temperature of 1000C and 1.5 mins Pt sputtering are the optimum parameters for the highest density of the fibers in our growth condition. Also, presence of Sulfur plays an 52

67 important role in our growth and no fibers will be grown if we don t add Sulfur in the chamber. 3.2 Introduction In the last decade, evolution of nanoscience and nanotechnology have gained high attention of scientists. Nanostructures are the ones with at least one dimension less than 100nm. In this dimension the number of atoms are countable and because of the quantum effects the properties of such materials are different from their bulk. Nanowires are structures having diameter scales of 10 9 meters (nanometers). They call them nanowires when the ratio of length to width is greater than 1000, and they count as 1D materials. Nanowires have gained intense attention during the past decades. Nanowires can solve scientific questions regarding one dimensional systems, in which electrons are quantum confined, and they occupy energy levels different from the bulk materials. They also have lots of applications in integrated Nano systems as interconnects or main components. Different types of nanowires such as superconducting (YBCO), metallic (Pt), semiconducting (Si), and insulating (SiO2) exist. Silicon and Oxygen are among the most abundant elements on the earth. Silica as their combination has significant use in microelectromechanical systems (MEMS) and complementary metal oxide semiconductors (CMOS)[62]. It can be used as insulating medium in circuits, masking material for etching applications[63], and also energy storage[64]. In addition to the unique physical, chemical, and mechanical properties of SiOx nanowires, the size of these materials is comparable to visible light in the range of 400 to 650 nm wavelength, which 53

68 shows the ability of these nanowires to handle light and be used in optoelectronics. Although Si is being widely used in different applications, its luminescence efficiency is low. However, SiO2 has much better luminescence owing to its low refractive index[65] which can be used as coating in LEDs for improving the illumination uniformity[66]. Silica nanowire-based optoelectronic devices including blue, green, red and ultraviolet lightemitting diodes (LEDs)[66], and photodiodes[67] have been demonstrated already. The reason for these emissions can be oxygen deficient centers or the natural points with oxygen vacancies[68]. Also, the surface of silicon oxide can react with compounds like carboxyl and amine, and can also be modified chemically and physically to become functional for use in bio-chemical sensing[69]. In the following sections, vapor phase fabrication methods for the growth of nanowires are discussed. 3.3 Literature Review Nanowires are the product of anisotropic and 1D growth in scale of nanometers. The growth of these nanowires in a controlled manner is the main key for using them in industrial scale Growth of One Dimensional Materials Nanowires are synthesized by two different basic approaches: top-down and bottom-up. In the top-down method, a bulk material is used to synthesize the nanowires in which different techniques such as complicated etching and lithography or electrophoresis are used[70]. Although this method has been used a lot in the last half century, the more 54

69 viable methods needs to be used for mass production of these materials. It has been proven that the bottom up method can overcome the limitations that top-down fabrication methods can have. The bottom-up method uses constituent adatoms for the growth of nanowires. Nanowires using this method can be solution or vapor based. The main solution base method is the solvothermal method. In this method precursors and reactants are mixed together for templating the growth. Further reactions for the growth of nanowires happen at elevated temperatures. The drawback for this method is the low yield and purity of the grown structures. Also the solvents involved in the reactions are not environment friendly. Vapor based methods are the most promising methods for the growth of nanowires. Different techniques such chemical vapor deposition (CVD)[71], pulsed laser deposition (PLD)[72], molecular beam epitaxy (MBE)[73] have been used. In this method vapor is the main key for the production of nanostructures. This vapor phase can be generated in different ways. Thermal evaporation, laser ablation and chemical reaction are among the ways vapor can be introduced to the system. These bottom-up methods involve two major steps of nucleation and growth. When the concentration of atoms/ions/ molecules become sufficient enough, they aggregate together making the initial block for further growth of the structure. These nuclei serve as seeds for the growth of desired 1D structures. The produced gas will be carried to the desired growth location by means of a carrier gas in a tube furnace. 55

70 For our nanowires the approach used for the growth of nanowires is Chemical vapor deposition (CVD). CVD is the most promising method for the mass production of thin films and nanomaterials for industrial applications. It is a chemical process for production of high quality solid monocrystalline, polycrystalline, amorphous, and epitaxial materials mostly used in microfabrication processes[74]. This method has recently shown promise to synthesize nanowires[75]. In a typical CVD method, appropriate substrates (e.g. copper, sapphire, glass, Si/SiO2) are exposed to defined precursors, in vapor state, in the reaction chamber. The precursors react and/or decompose in the reaction chamber or on the surface of the substrate, and deposit as the desired compound in solid state, which is nucleation step for the growth. The thermodynamically/kinetically stable structure then starts to grow in the form of thin films, flakes, rods, wires, particles etc.[76]. Volatile compounds will also be produced as a result of reaction between precursors, which will be removed by the flow of the carrier gas in the tube furnace. CVD method is so sensitive to process conditions, and small changes in experimental conditions can lead to changes in structure, density, size and shape of the grown structures. Parameters affecting the growth are evaporating temperature (Te), deposition temperature (Td), amount of precursors, substrate, the composition of the precursors and carrier gases (Ar or Ar/H2), flow rate, source substrate distance, and catalyst. Regarding this, it is important to study the parameters controlling the growth of nanowires. Modification of 56

71 these parameters are of major importance for the growth of desired material with high crystallinity and purity. There are different CVD growth methods such as low pressure chemical vapor deposition (LPCVD)[77], atmospheric pressure chemical vapor deposition (APCVD)[78], metal organic chemical vapor deposition (MOCVD)[79] and laser assisted chemical vapor deposition (LCVD)[80] depending on the growth condition. Different mechanisms can be involved in the CVD growth of silica nanowires such as screw dislocation-driven growth [81], oxide assisted growth (OAG)[82] and self-catalyzed growth on the substrate. The latter mechanism is named catalytic CVD, in which vapor liquid solid (VLS), vapor solid (VS) and solid liquid solid (SLS) are among mechanisms responsible for the growth[83]. Below, VLS and SLS mechanisms which are responsible in our case are discussed. In the direct Vapor Solid (VS) growth[84], the vapor will be produced by evaporation and will be condensed on the surface of the substrate. The nucleation starts on the defect sites or by intrinsic anisotropic growth. In this method the use of other precursors than the source material is needed to provide the needed vapor for the growth. Oxide assisted [85] and carbothermal[86] are among the VS methods used for production of nanowires. Solid Liquid Solid (SLS)[87] mechanism is another method used for production of nanowires which does not involve the vapor phase. The SLS is another mechanism 57

72 proposed for the growth of whiskers in which there is no vapor phase involved. The only phases involved in the growth are solid and liquid, which are the substrate and droplet alloy (metal/precursor). In this mechanism, the substrate itself is the only source of material needed for growth, and no other precursor is involved. Yan et al[88],yu et al[87], and Paulose et al[89] are among researchers who proposed the SLS growth mechanism for nanowires. In this mechanism, the introduced metal catalyst forms a liquid alloy droplet of relatively low freezing temperature, which is the same as the VLS mechanism. Particles composed of the metal particle saturated with mother material act as catalyst for the growth of nanowires. The Si from the substrate material diffuses into the molten eutectic alloy making it supersaturated. Then, the excess materials precipitate out of the molten particle, leading to the growth of nanowires. These particles remain on the surface of the grown material during the whole process, and can be seen as of the nucleation points for the growth of nanofibers. The schematic for the growth of nanowires by SLS technique can be seen in Figure 28. In this case, we can still say that the mechanism involved in the growth is VLS. The vapor phase, in this case, will be the vapor reactant from substrate that exists in the surrounding of the droplet in the growth temperature being involved in the reaction. As an example of this work, some researchers have used silicon wafer as the only source for the growth of silica nanowires. However, they have used N2/ 3% H2 flow as the reducing gas at C growth temperature[89]. The growth temperature on silica nanowires is expected not to be less than 847 C [90], which is the eutectic temperature in the Pt-Si system. 58

73 The solid-liquid-solid (SLS) growth approach does not need any gas precursor involved in the growth such as SiH4 or SiCl4. Because of this, SLS is being considered a much more straightforward technique for synthesis of nanowires. Pt Figure 28. Schematic illustration of SiO2 nanowires growth. In this reaction the growth is catalyzed by goldsilicon oxide droplet on the surface of the wafer. Another method for having the anisotropic growth is introduction of a liquid solid interface in the system. Vapor liquid solid (VLS)[91] growth is a mechanism used for the growth of nanowires proposed by Wagner et al[3] in early 1960s. This method is much easier than the rest of the methods used for nanowires growth and high purity and density is another benefit of this method. VLS is being widely used in laboratories because of its simplicity and versatility. In the VLS growth mechanism, the nanowires are formed from precipitation of the reactant material out of supersaturated metal seed/reactant alloy droplets. In VLS method all three phases of vapor, liquid and solid are involved and the process can be described in 59

74 three simple steps. The first step is preparation of a droplet, which consists of the alloy of reactant material and metal particle (Au/Pt/Ag/Al/Cu), acting as catalyst and nucleation point to initiate the growth, and also to facilitate the decomposition and activation of the reactant. The catalyst should be an inert material not able to react with the desired grown material. Also, it needs to be able to form a solution with the material we want to be grown at the growth temperature and solve the precursor in a wide range of temperatures. The equilibrium vapor pressure of the catalyst around the droplet alloy should be low that the droplet does not evaporate or shrink till the end of the growth[92]. The formed droplets have lower melting point due to the formation of eutectic composition. The second step is having a vapor substance to be absorbed on the surface of the droplet and diffused into it. The driving force for the growth of nanowires is the supersaturation of the metal droplet seeds. It happens by absorption of the gaseous reactants existing in the surrounding atmosphere, when the partial pressure of the reactant is enough in the particular temperature. The amount of supersaturation affects the morphology of the grown structure. Low supersaturation leads to the growth of 1D structures. Increase to medium level supersaturation depending on the feed stock to the nuclei leads to formation of bulk material, and further supersaturation results in formation of powders[2]. These particles define the direction of the nanowires growth and also their diameter size. Finally supersaturation of the droplet with the introduced vapor is initial to start the nucleation at the liquid/solid interface leading to the axial growth of the material in one direction. Owing to the introduced liquid-solid and liquid-vapor interfaces, the reaction energy is much 60

75 lower than the case of just having two phases of vapor and solid involved in the reaction. The growth starts only from the areas already activated and is a heterogeneous growth. In most of the works done regarding the VLS growth of nanowires, the source for the Si vapor phase is supplied from a powder target or from silane gas[93]. Also, oven-laser ablation method was used having Si powder or Silane gas as Si source for the growth[94]. Using Ar/methane[95] gas and carbon seed[96] are among other approaches used for the synthesis of silica nanowires by VLS technique. Givargizov studied the growth model in more depth and discussed the kinetics involved in this method[91]. By means of in-situ TEM, Yang et al[97] has also investigated the growth mechanism. In this chapter, we study the sulfur (S) assisted VLS growth of silica nanowires. The nanowires are characterized using SEM, AFM, TEM and EDS; and different parameters such as growth temperature, time, seeding concentration and sulfur effect are evaluated. 3.4 Experimental Procedure Si wafer with 270nm thermally grown SiO2 on top was used as the starting substrate. First, the wafer was cut into pieces of about 15*15 cm 2. Then, the substrates were immersed in acetone for 10 mins, washed with isopropanol (IPA), immersed in IPA for 10 mins, and blow dried with nitrogen gas. Pt was deposited on the surface as nucleation points using a sputter coater system (Anatech Hummer Model 6.2). The sputter coater was first evacuated down to 30mtorr and then purged with Ar. The deposition was performed in 100mtorr Ar pressure having 40 ma current for 0.5, 1, 1.5, and 2 mins, in order to have 61

76 different concentrations of catalyst, with a deposition rate of 3nm/min. The substrate was then placed upside up on the Alumina crucible and pushed to the middle of the three zone MTI model OTF-1200X CVD furnace, shown in Figure 29. 1gr of Sulfur flakes >99.99% (purchased from Sigma Aldrich) were placed in another Alumina crucible and pushed into the outer part of the tube furnace, 10cm away from the beginning of 1 st zone. The schematic of the tube furnace containing Si/SiO2 substrate and Sulfur flakes is shown below. Figure 29. Schematic and picture of the CVD furnace used for the silica growth. 62

77 The chamber was fist evacuated down to 100 mtorr having 50 sccm N2 gas purging into the chamber. The chamber was purged for 10 minutes to get rid of any potential contaminations. Then, the 2 nd and 3 rd zone of the furnace were heated up to 1000 C in an hour with a ramping rate of 16.5 C /min. Then, the sulfur was engaged in the process by increasing the temperature of the 1 st zone of the furnace to 600 C and covering the outer part of the tube furnace, where sulfur flakes are, with Aluminum foil. This way, escape of the heat in that area will be decreased and the temperature of the sulfur boat will increase by having the reflection of heat onto the flakes. The temperature was maintained for 15 mins at 1000 C followed by rapid cooling of the furnace. The growth was performed on both Si (111) and also Si/SiO2 substrates. Figure 30. shows the substrates after the growth, and as it can be seen, a thin white-grey colored layer is observed on both substrates after the growth. Figure 30. Si and SiO2 substrates after growth of nanowires 63

78 The samples were used for studying the morphology, structure, and composition of the grown materials. SEM, TEM, EDS and AFM techniques were used for characterizing the grown structure. In this study, different parameters such as the effect of temperature, time, Sulfur precursor, and catalyst concentration was investigated on the growth. 3.5 Characterization of the nanowires SEM images from the grown structure on the Si/SiO2 substrate shows the high density of this woven structures having Pt as nucleation points and catalyst for the growth. Figure 31 shows a typical image of this woven structure in high and low magnifications. Low magnification imaging shows the length of the nanowires to be between a few micrometers to millimeters long. From the high magnification imaging the approximate width of the nanowires is estimated to be from a few nanometers to a few hundred nanometers. Figure 31. Morphology and density of the structure. SEM image of the grown silica nanowires in low (left) and high (right) magnifications showing the high density of the grown material. The scale bars are 1um. 64

79 In order to identify the chemical composition of the grown material, Transmission Electron Microscopy (TEM) and Electron Dispersive Spectroscopy (EDS) were performed on the nanofibers. For this purpose, the wafer having grown material on top was immersed in IPA and bath sonicated for half an hour to have nanowires released into the IPA, a solvent with low evaporation temperature (shown in Figure 32 inset). Then, using a polyethylene pipette, a small amount of solution was drop casted on a holy carbon TEM grid and air dried completely prior to be placed in the TEM machine for further analysis. Figure 32 shows the TEM image from a typical nanowires from which the Electron Dispersive Spectroscopy (EDS) mapping was performed to identify the composition of the nanowire. Figure 32. Transmission electron microscopy image of a typical nanowire on which the electron dispersive mapping was performed. Inset shows the IPA solution containing grown nanowires. Scale bar is 500nm. 65

80 EDS mapping shown in Figure 33 suggests that the nanowires are composed of a mixture of Si and O, slightly more O-rich than SiO2 (about 0.15% more oxygen that SiO2). Figure 33. EDS mapping of the grown structure, showing the existence of both Si and O elements in the structure forming SiOx structure. The additional oxygen existing in the structure can come from the oxygen existing in the system. This can be because of the system leakage and the residual oxygen in the carrier gases, which leads to further oxidation of the material at high annealing temperatures. The composition is consistent all over the nanowire and no core shell structure is observed. 66

81 Also, the amorphous structure of nanowires was confirmed by diffraction patterns obtained from both nanowires and also Pt/SiO2 particles, used as the catalyst and nucleation point for the growth of the nanowires. As it can be seen in Figure 34, the diffraction pattern from the nanowires does not show any crystallinity in the structure. Inversely, we can see the crystalline structure in the Si/Pt alloy particles. Figure 34. Crystal structure of the grown material. Diffraction pattern obtained from grown SiOx (lower left) and Pt/SiO2 (upper right) showing the amorphous structure for nanowires and crystalline structure for the catalyst droplet. 67

82 The EDX analysis in Figure 35 shows the existence of Si/ O/ Pt peaks coming from the grown structure, and C/ Cu peaks initiating from the nature of the TEM holy carbon grids. Figure 35.Electron Dispersive Spectroscopy (EDS) spectrum obtained from nanowires on copper TEM grid showing Si, O, Pt elements existing in the grown structure and Cu/ C existing on the grid. Figure 36 illustrates the two different morphologies of the grown SiOx nanowires studied by Atomic Force Microscopy (AFM). It has been proven that different chemical environments can affect the morphology of the grown nanowires[98]. With a sufficient amount of reactant, supersaturation of the droplets leads to fast 1D growth of the material on the truncating facets, and a much slower 2D growth on the main facet. The morphology of a typical grown nanowire having sufficient feedstock can be seen in Figure 36a, having a 30nm diameter size. On the other hand, the high amount of sulfur precursor (in our case 2 gr) added to the system as a reducing agent, can increase the amount of volatile SiO2 68

83 available in the system during the growth, leading to mass transfer promotion and a higher growth rate of the material in both directions. Having feedstock higher than needed will cause the nanowires to grow under kinetic conditions rather than thermodynamic. This condition leads to the formation of stepped nanowires having a considerable growth on the main facet. The morphology of the grown nanowires in this condition can be seen in Figure 36b. The height profile of the nanowires in the inset shows 30 and 60 nm height and 50 nm width for the grown nanowire. Considering the fact that future labs will be "labs-on-a-chip", i.e., laboratories being the size of a computer chip, direct access to channel surfaces are essential. These structures will give scientists the ability to work with much smaller volumes of fluids, which leads to shorter reaction times and also reduced costs of bio/chemical analysis. Nowadays, this platform is becoming viable by advanced lithographic methods, which is expensive and also time consuming. Here, we are showing that having careful control on the parameters involving in the growth, one can grow structures containing nanochannels with minimal effort and work. This work can provide a route towards open nanofluidics and microfluidics systems[99]. 69

84 a) b) Figure 36. AFM imaging showing the morphology of the grown nanowires. A) In enough feed stock. B) In high feed stock. Insets show the height profile of the nanowires. Scan size is 1.3um. 3.6 Growth mechanism of the nanowires The proposed mechanism for our growth is Sulfur assisted VLS. First, the thin Pt catalyst film deposited using the sputter coater system breaks into nano-sized droplets at high temperatures. Si and SiO2 may react with each other on the surface of the substrate at high temperatures as follows: Si +SiO2 2SiO (1) Actually, the SiO above the wafer is metastable with respect to the Si/SiO2 and will decompose to Si and SiO2. SiO Si + SiO2 (2) If there is no gold droplets available, Si and SiO2 will deposit on the wafer. However, existence of the gold droplets as a catalyst will make Si/O and Pt form the Pt/Si alloy above the eutectic point. At about 300C Pt and Si will form Silicide (Pt2Si). Moving to higher 70

85 temperatures, the thin film of silicide transforms to clusters of molten PtSi (having formation of Pt6Si5 formation as an intermediate phase) in temperatures around 1000C[100], which is our growth temperature. The phase diagram of the Pt-Si system can be seen in Figure 37 for a better understanding of the mechanism. Figure 37. Phase diagram of Si-Pt However, the amount of SiO coming from the reaction (1) is not enough for the growth of silica nanowires[101]. We should consider that Silicon may react with oxygen in two different ways depending on the annealing temperature and also partial pressure of the Oxygen in the furnace[102], passive and active oxidation as mentioned below. 71

86 Si +O2 SiO2 Passive oxidation (3) 2Si + O2 2SiO Active Oxidation (4) Passive oxidation happens when a high partial pressure of oxygen is available in the system. On the other hand, active oxidation happens when the partial pressure of oxygen is low (below 10 parts per million), which can be found in typical vacuum conditions. We should consider that our vacuum system is not complete. Before, having N2 gas in the system, the lowest pressure we could observe was about 90 mtorr, which proves the existence of oxygen in our system. The Oxygen partial pressure can also increase in higher temperatures as well. We should also note that commercially available Nitrogen and Argon gases usually contain a small amount of Oxygen, which is enough for oxidizing many materials, including Si, at high temperatures. In our study, the active oxidation of Silicon is responsible for the growth of silica nanowires. Reactions (1) and (4) provide enough SiO in the system needed for the growth of Silica nanowires. SiO will be dissolved in the molten droplet, will eventually precipitate out of the supersaturated droplet, and will act as nucleation point for the growth of nanowires. The other parameter involved in our growth is the existence of Sulfur as a reducing agent. My hypothesis is that the Sulfur reacts with the silicon on the surface of the substrate at high temperatures and produces solid silicon disulfide (SiS2) (Reaction 5). The boiling point of SiS2 is about 1100C, and will sublime above 940C. Considering our growth 72

87 temperature of 1000C, the SiS2 will be converted into vapor SiS2, and will react with the oxygen in the furnace, producing SiO2 vapor (reaction 6) needed for the growth of silica nanowires by the VLS mechanism. The excess sulfur gas will be removed from the system by N2 carrier gas. Si + 2S SiS2 ΔG = -187 Kj/mol (5) SiS2 + O2 SiO2 + 2S ΔG = -543 Kj/mol (6) This reaction reduces the energy needed for the growth of nanowires and will increase the kinetic growth of the material compared to similar works, using the SLS technique without the existence of Sulfur[87]. The existence of the liquid solid contact line, together with the precipitation at the vapor-liquid interface and providing Si/O-rich interface (coming from reactions 2, 4, 6) lowers the barrier for nucleation and the growth start. Under these conditions, we can say that the only Si/O source for the growth of silica nanowires comes from the substrate, and the existence of S helps the evaporation of the silicon substrate, having more SiO vapor involved in the reaction. Considering these observations, we can say that the most appropriate term for this mechanism can be VSLS, because of the important role that the Si/O vapor phase plays in the growth of nanowires. The growth stops when the two conditions are met. It can happen when the temperature goes lower than the melting point of the formed alloy droplet (while the furnace starts to cool down) or when the nanowires reach out of the hot zone, and not enough Si vapor is present for further growth. 73

88 3.7 Optimization of the process The CVD method is sensitive to process conditions. Small changes in experimental conditions can alter the shape, density, and size of the grown structures. Therefore, it is important to study the parameters controlling the growth and also to modify the growth for desired density, purity, and size. The parameters that need to be considered are evaporation temperature (Te), deposition temperature (Td), amount of precursors, reactants, composition of the precursors, substrate, carrier gas, flow rate, and the source substrate distance. Below, we investigate different parameters such as seeding concentration, growth time, temperature, effect of sulfur on the growth, and also substrate Effect of seeding concentration in the growth of SiOx nanowires In order to see the effect of Pt density on the growth of nanowires, different sputtering times of 0.5, 1, 1.5 and 2 minutes were performed using Anatech Hummer Model 6.2. Sputter coater, having 30 ma current and 100 mtorr working pressure. The deposition rate was calculated to be about 3 nm/min in defined condition. As it can be seen in Figure 38, by increasing the Pt sputtering time from 0.5 to 1.5 minutes, the concentration of the grown nanowires are increased; however, having 2 mins of sputtering, the length of the grown material has been decreased, with the same growth time of 15 mins. 74

89 Figure 38. The effect of Pt seeding concentration on the growth of the nanowires. a-d shows low to high concentration of sputtered Pt. Scale bars are 1um. 2 minutes sputtering (6nm deposition) results with the covering of most of the Si substrate surface with decomposed pt nanoparticles, not having much Si wafer exposed to the sulfur. As a result the amount of SiS2 vapor decreases, and less SiO will be available for the growth. This results in the lower growth rate and smaller final average length of the nanowires. Having about 10 nm deposition of Pt film, can lead to the re-solidification of Pt2Si layer, preventing the growth of nanowires[100]. So we can conclude that there is an 75

90 optimum metal deposition at 1.5 minutes (4nm), under the defined condition, for the highest density of nanowires Effect of the nanowires growth on the substrate surface In order to see the effect of the growth on the surface of the substrate, the grown sample was superficially scratched and the morphology of the surface was characterized using SEM. As it can be seen in Figure 39 the substrate s surface has been roughened due to consumption of Si, as the source for the growth, from the substrate surface. Figure 39. The substrate morphology after growth. Morphology of the Si/SiO2 substrate after superficial scratching of the grown material from the surface. Scale bar is 1um Reducing effect of Sulfur on the growth of SiOx nanowires In order to evaluate the reducing effect of Sulfur on the growth of nanowires, the same growth condition (1000C temperature for 15 mins, having 4nm Pt deposition) was maintained, except that we did not add any Sulfur in the chamber during the procedure. As 76

91 it can be seen in Figure 40 there was no nanowires grown after 15mins annealing of the sample at 1000 C. It shows the significant reducing effect of sulfur on our procedure. During this process only Pt particles enlargement happens by absorption of SiO2 from the surface and accumulation of the particles at high temperatures, in order to reduce the surface energy. However, the energy needed for the growth of nanowires will not be provided without sulfur being involved in this process. Figure 40. Surface morphology of the annealed substrate without having Sulfur involved in the process. Scale bar in 1 um Growth time effect on the growth of SiOx nanowires Another parameter we found important in our process, is the growth time. For this purpose we increased the growth time from 15 to 30, to 45 minutes. Interestingly, having 30 minutes of growth results in a highly roughened surface (Figure 41b) having nanowires evaporated already. Moving to higher process time (45 mins) results in the complete 77

92 evaporation of the substrate. As a result we can conclude that there is an optimum growth time of 15 minutes for the best result. Figure 41. SEM image of the substrate after 30 minutes of growth. Scale bar in 1 um. Increasing the time makes the nanowires thermodynamically unstable and leads to evaporation of nanowires and then the substrate itself. Accordingly higher amount of SiS2 vapor will lead to higher etching rate of the substrate and finally vanishing the substrate. Having sulfur as the reducing agent also decreases the needed time and temperature for the growth of nanowires, compared to other works having lower growth density at higher growth temperature (1100C) and time (30minutes) [103], [104]. 78

93 3.8 Summary and Conclusion In summary, silica nanowires have been grown by sulfur assisted VLS mechanism, having only substrate as the source for the growth of nanowires. Morphology of the grown nanowires and the effect of temperature, time, seeding concentration and sulfur have been studied. Our study shows that sulfur has significant reducing effect on the growth of silica nanowires. The growth time and temperature is lower than what was previously published on the growth of silica nanowires[87][102][103]. Also the proper control on the growth parameters can lead to inherent nano-channel formation on the grown nanowires, useful for applications where open accessible nano-channels are needed[99]. 79

94 CHAPTER 4 CONCLUSION AND FUTURE STUDIES In this study, we have synthesized two different types of nanomaterials (2D phosphorene nanosheets and1d silica nanowires) by means of two convenient methods used for synthesis of nanomaterials. In chapter 2, Black phosphorous bulk material was exfoliated to BP nanosheets down to monolayer by ultrasonic assisted liquid exfoliation (LPE) technique. The process was optimized for the best exfoliating condition, and the produced nanoflakes were studied by means of different characterization techniques to understand the crystallinity, morphology, size distribution and structure of them. The results show the highly pure and crystalline structure of the nanoflakes which remain intact in liquid for months. Also different applications of the produced BP nanosheets were shown in humidity sensors (as sensing component), lithium ion batteries (as anode material) and embedded structure (as reinforcing material in polymer nanocomposites). A future improvement to this work can be the study of the exfoliating ability of other exfoliation techniques, such as microwave assisted[105] synthesize of phosphorene sheets in ionic liquids. This method might improve the yield and size of the exfoliated material for applications in which monolayers with high surface area are needed specifically. 80

95 In chapter 3, SiOx nanowires were synthesized by means of sulfur assisted VLS- CVD technique, having Pt nanoparticles as catalysts. Using sulfur as a reducing agent for the growth of nanowires has improved the growth rate and also decreased the time and temperature of the growth compared to previous works[100,101]. Structural and morphological analysis were performed on the grown material using different characterization instruments. The growth mechanism was proposed and the process was optimized considering time, temperature and seeding concentration. Considering the possible growth of Si nanowires in an oxygen free atmosphere, one of the improvements to this work can be production of Si nanowires with a similar approach. Improving the sealing of the heating furnace can help to prevent the presence of Oxygen in the furnace. Using etched silicon substrate (with HF wet etching, or dry plasma etch using SF6 gas), without inherent SiO2 topping layer and having the growth process in a sealed ampoule at higher growth temperatures (1200 C), can prevent the oxygen contamination and improve the crystallinity of grown material. 81

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106 Appendix Here, I have attached the written permission from journals of the papers I have contributed to and have been used in this dissertation. 92

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