Grpahene Synthesis by CVD. QingkaiYu Ingram School of Engineering Texas State University at San Marcos

Transcription

1 Grpahene Synthesis by CVD QingkaiYu Ingram School of Engineering Texas State University at San Applied Nanotech Inc, July 25 th, 2011

2 Acknowledgement Texas State University Dr. Zhihong Liu Austin Williamson University of Houston Wei Wu Zhihua Su Dr. Peng Peng Dr. Jiming Bao Dr. Shin-shem Pei Carl Zeiss SMT, Inc. Dr. Dongguang Wei Purdue University Dr. Yong P. Chen Luis A. Jauregui Robert Colby Jifa Tian Helin Cao Deepak Pandey Jack Chung Brookhaven National Laboratories Dr. Eric Stach Argonne National Laboratories Dr. Nathan Guisinger Dr. Jongweon Cho Rensselaer Polytechnic Institute Dr. Jie Lian Gongkai Wang SEMATECH Dr. Chanro Park Dr. Pat Lysaght Dr. Casey Smith

3 Outline Background of graphene synthesis on metals by CVD. Synthesis of continuous graphene films. Synthesis of graphene islands for study on grain boundary of graphene. Characterization of grain boundary by TEM, STM, Raman, Electrical transport. Synthesis of single crystal graphene arrays. Conclusions. 3

4 Background of Graphene

5 Dimensionality of Carbon Diamond, at least 3000 years ago in India. Graphite, A.D in England. Graphene, 2004 at Manchester Univ. by Geim and Novoselov Nobel Prize. Fullerene, 1985 at Rice Univ. Kroto, Curl and Smalley shared Nobel Prize in Carbon Nanotube, 1952 in Soviet by Radushkevich and Lukyanovich, but this discovery was largely unnoticed in NEC by Iijima.

6 Graphene Structure Structure relation between graphene and other carbon materials. Zigzag edge is more stable. Geim et. al., Nat Mater 6 (2007) 183. Girit et. al., Science 323 (2009) 1705

7 Extraordinary Properties Strongest material ever measured: σ=42 N/m, E=1.0 TPa. Thinnest material, largest area: one atomic layer, 2630 m 2 /g. Very flexible: flexible devices, wearable devices. Optical transparent: 97% transmission. Record thermal conductivity: 5000 W/mK. Negative thermal expansion coefficiency. Extremely high electrical carrier mobility: two order higher than Si. Sustain high current density: six order of magnitude higher than Cu. Zero effective mass: a cute toy for solid-state physicist. Ballistic transport of electron: submicron.

8 Applications Transparent conductor Energy Storage Composite materials Transistor MEMS Chemical/bio-sensor

9 Graphene Synthesis Method Quality Size Transfer Scalable Exfoliation of graphite Best Microns Yes No Reduction of exfoliated graphene oxide Desorption of Si from SiC Worst Microns Yes Yes Good SiC Single Crystal No Yes Growth on metal Good > foot Yes Yes Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, Geim, A. K. Science 2009, 324, (19), Park, S.; Ruoff, R. S. Nat. Nanotech. 2009, 4, Li, X. S., et. al., Science (2009), 324,

10 Graphene Synthesis on Metals by CVD Heating Metal Substrate Exhausted gases Ar+H 2 +CH 4 Typical recipe: Heat samples 1.5 hour at 1000 C with argon and hydrogen. Feed methane into chamber for 20 minutes for carbon dissolution. Ambient pressure. 10

11 Our Research Roadmap Where have we been and where are we going? CVD graphene has raised great hope for graphene synthesis owing to its large area, well-controlled thickness, high quality, and low cost. Here six key issues on CVD graphene are listed, which represent the past, present and future research goals of Dr. Yu s team. 1. Large area. (solved ) 2. Uniform thickness. (largely solved) 3. Controllable GB and single crystal. (current effort) 4. Bandgap engineering. (just beginning) 5. Doping and defect engineering. (just beginning) 6. Large volume manufacture. (not yet started) 11

12 Large Area and Uniform Thickness

13 Graphene Synthesis on Nickel Graphene Synthesis on Ni by CVD G D 2D C-Ni phase diagram Raman spectra Cooling rate can determine the thickness of graphene layers. In Raman spectra, I 2D /I G ~2 for monolayer and I 2D /I G <1/2 for graphite. 13

14 Graphene Synthesis on Ni by CVD Locally good few layer graphene. Large scale: highly non-uniform. 0.80nm

15 Graphene Synthesis on Copper Graphene Synthesis on Cu by CVD Extremely low carbon dissolvability and no stable copper carbides. Good catalyst for decomposing hydrocarbn gas. Cheap large single crystal. arxiv v1 published April 10th,

16 Graphene Synthesis on Cu by CVD 4 inch Cu foil PMMA/graphene float on ferric nitrate solution PMMA/graphene transferred to a 6 wafer PMMA/ Graphene/ Cu foil/ Graphene

17 NEXAFS Study on CVD Synthesized Graphene π* σ* Angle-resolved C K-edge near edge x-ray absorption fine structure (NEXAFS) spectra Dichroic ration (DR)=(I -I )/(I +I ) Monolayer graphene on Cu c=o I and I are the integrated intensity of the π* resonances with x-ray incident angles at 90 and 0, respectively. DR=0 for completely random alignment of π-orbitals and DR=-1 for perfect alignment. Monolayer graphene on SiO 2

18 Grain Boundary and Single Crystal

19 Graphene Synthesis C adatom concentration on metal 1 2 C nucl 3 4 C supersatur 5 C eq CH 4 gas concentration in chamber Gas on Time Gas off C nucl : carbon adatom concentration for graphene nucleation. C supersatru : concentration of supsaturated carbon adatom under certain hydrocarbon gas pressure. C eq : equilibrium concentration between carbon adatom and graphene

20 TEM The left panel shows simulation of SAED from hexagon graphene islands with zigzag and armchair edges, respectively. (a) A montage of bright field TEM images spliced together to show an example island of single-crystalline graphene. A schematic outline has been included based on the adjoining SAED pattern (inset) to demonstrate that the edges are much nearer to zig-zag than armchair. (b) Bright field TEM image of merging islands of graphene and SAED patterns (c,d) from the individual islands demonstrate that each is primarily comprised of a single crystal of graphene, and that the two islands are rotated from each other by approximately

21 STM (a) Graphene (b) (c) Z A Z Z A Cu Z Scanning tunneling spectroscopy of graphene Island on Cu foil: (a) STM topography (sample tip bias V b =-2 V, I=50 pa) of a 5 x 5μm 2 patch of graphene resting on a Cu foil. (b) Atomic resolution STM topography image of the graphene honeycomb lattice close to the bottom edge of graphene island in the black square shown in (a). (c) Atomic resolution STM topography image of the graphene honeycomb lattice at the right edge of graphene island in the white square shown in (a). 21

22 Raman Mapping (a) I D (x, y) (b) I G (x, y) I 2D (x, y) (c) 2 μm Spatially dependent Raman Spectroscopy of merged graphene islands. (a) Raman mapping of the amplitude of the D peak, defined as the maximum within (1300 to 1400 cm -1 ) baseline. (b) Raman mapping of the amplitude of the G peak defined as the maximum within ( cm -1 ) baseline. (c) Raman mapping of the 2D defined as the maximum within ( cm -1 ) - baseline. Spatial and Spectra resolution are 0.4µm and 2.48 cm -1 respectively in all Raman mappings. The power of the 532nm laser is ~1mW to reduce any heating effect on graphene. 22

23 Electrical Transport (a) (b) T = 4.3K μm T = 300K (c) Electrical measurements. (a) Optical image of a merged islands device fabricated on a 300nm SiO 2 wafer with e-beam defined electrical contacts Cr/Au (5/35nm). (b) 4 probes Current-Voltage curves taken in each single island and cross boundary. (c) Magnetoresistance (Rxx ) measured at 4.3K taken from each single islands and cross boundary using lock-in technique by applying 1uA as excitation current. 23

24 Single Crystal Graphene Advantages: No grain boundary. Significantly reduce defects. Identical lattice orientation. Is it possible to grow single-crystalline graphene on polycrystalline copper substrate? If graphene epitaxially grows on copper, within a copper grain, all graphene flakes can merge to each other without orientation mismatch; different copper grains have different crystalline orientations. The grain boundary of copper substrate will be replicated on graphene. If graphene non-epitaxially grows on copper, grain boundary of graphene can be induced when graphene flakes merge to each other. one graphene flake can grow across grain boundary of copper without changing its orientation. 24

25 Single Crystal Graphene SEM image of as-grown hexagonal graphene islands whose edge orientations are approximately aligned with each other. SEM image was taken in a range of single Cu grain. Scale bar is 10 µm. SEM image of as-grown graphene islands whose edge orientations are NOT aligned with each other (except for the two islands labeled as #1 and #2). SEM images was taken in a range of single Cu grain. Scale bar is 10 µm. #1 #2 25

26 Single Crystal Graphene SEM image showing hexagonally-shaped GSC s can grow across Cu crystal grain boundaries (indicated by red arrows). 26

27 Single Crystal Graphene 20 µm Ambient pressure Low pressure Gas flow and pressure can lead to different shape of graphene islands 27

28 Single Crystal Graphene Graphene seeds exfoliated from HOPG can be transferred to Cu substrates. C adatom concentration on metal 1 2 C nucl 3 4 C supersatur 5 C eq CH 4 gas concentration in chamber HOPG seeds Time Gas on Gas off Single seed! Single crystal! 28

29 Graphene Arrays by CVD Graphene Seeds We now offer a strategy to avoid GB s in graphene devices by the growth of graphene single crystal (GSC) array. The great advantage of GSC array is that all the GSCs are addressable for the fabrication of graphene-based devices. Therefore it is easy to large-scale fabricate all the devices within the GSCs to avoid the negative influence of GBs. (a) (b) (c) with seeds without seeds 29

30 Graphene-based Chemical and Bio-sensors Field effect transistor (FET) structure with Pd nanoparticles decoration on graphene for detecting H 2. The change of carrier density in graphene induced by palladium hydride (PdH x ). Sensor response to 0.05% hydrogen, the resistance of the sensor increased 1% in less than 30 second

31 Gas flow and pressure can affect the shape of graphene islands. Ambient pressure is necessary for graphene islands with hexagonal shapes. Each graphene island with hexagonal shape is a single crystal. The orientations of the edges of graphene islands are along zig-zag direction. Graphene GBs are the locations with the high concentration of defects. Graphene GBs have negative influence on conductivity of graphene. Seeded growth is a promising approach for the synthesis of array of single crystal graphene. 31

32 Thank you for your attention! Questions? 32