Nanoimprinting of 2-D Graphene Nanowires Jeremy R. Roth, Yen Peng Kong, Albert F. Yee Challenges Objective Establish a process for patterning graphene using Nano-Imprint Lithography for use in nano-scale electronic circuitry. Introduction Researchers have found that graphene, single layer graphite, has unique electrical and quantum properties.1-3 Graphene’s carbon nanotubelike scale and electrical behavior make it attractive for 2-D electronic circuits.4 However a method to commercially pattern graphene must be developed if carbonbased electronics are to go beyond mere laboratory curiosities. This research is an effort to evaluate the viability of Nano-Imprint Lithography (NIL) as a technique for patterning graphene using certain polymers as precursors (see Patterning Process). Challenges were to find a material and graphitization process that are compatible with NIL and would result in single layer graphene. This requires: 1. A precursor that may: a. be patterned via an existing NIL technique, or b. be incorporated into a new NIL technique 2. A NIL-compatible material process with reasonable processing parameters (i.e. temperature, pressure) 3. A reliable approach to characterizing structures and properties Patterning Process NIL Polymers Literature Graphitizing Polymers Results Conclusion Conventional Graphitization Non-catalytic graphitization of PAN, PS and PVA was attempted. Raman spectra for PAN at heat treatment temperatures from 500 to 800oC most closely match Figure 6. Raman spectra of PAN heat treated at that of glassy carbon15 rather 500 to 800 C for 10 minutes. than graphite or graphene. A decrease in intensity with respect to the Si peak can be seen likely due to thinning from oxidation in the mostly inert atmosphere of the TGA. Spectra for other materials was not possible due a failed laser on the raman spectrometer used. 1. Graphitization of polymers alone requires prohibitively high temperatures and high vacuum, making it difficult to combine with a commercial process such as NIL. o Catalyzed Graphitization Graphitic C layer (formed upon heat treatment) Conventional Graphitization* Material Selection Spin Coat onto Si Mold** Catalytic Graphitization 7 (a) NIL Transfer to Insulating Substrate Catalytic Graphitization Heat Treatment (500-900C) Figure 1. Georgia Tech. lithographypatterned graphene and graphene transistor from silicon carbide.5 Graphitization Heat Treatment (1400-3000C + UHV) Candidate Materials NIL Transfer to Insulating Substrate Raman / SEM / AFM / STM Characterization Common polymers like polyacrylonitrile (PAN), polystyrene (PS) and polyvinyl alcohol (PVA) were first considered for precursor materials to produce graphene. Further materials were considered based on their ability to graphitize after Franklin.6 Figure 3. Process for graphitizing NIL patterned polymer precursors for both catalyzed and conventional graphitization approaches. * Not pursued due to lack of High Temperature / UHV facilities ** Spin coating was done on flat wafers for simplicity. *** PVD films of Ni were created initiallyto study diffusion 12, 13. Steps in italics indicate future work. Nano-Imprint Lithography14 Step 3. Mold is released, nano-scale features remain PVC6 PVC7 PVC8 Anthracene9 Diamond8 Poly-11 paraphenylene Pitch10 Napthalene7 Black 7 Pitch Coke Step 1. Polymer is spin-coated onto treated mold and positioned into NIL press over patterning substrate Figure 7. (a) Schematic of PAN/C:Ni systems after Sinclair12. (b) Raman spectrum of Ni-C PVD system.* SEM image of Ni-C PVD system (c) heated to 700oC (d). In figures 7(c)(d) and 7(b)(c) carbon appears to have diffused through the nickel layer and nucleated on the surface. Raman spectra for each are also similar to that for glassy carbon15. This is not necessarily a negative result for graphite as carbon below that did not diffuse through the nickel could be the major source for these spectra, masking any graphitic carbon formed on the surface. *Thanks to Renishaw LLC for the raman data in figures 7(b) and 8(a) 8 (b) Step 2. Polymer-coated mold is pressed against patterning substrate w/ specific temperature and pressure Raman Spectroscopy 8(c) Petroleum Coke7 Welsh Coking Coal6 8 (a) 1000oC 1500oC 2000oC 2500oC 3000oC 3500oC Graphitization Temperature Figure 2. Known graphitizing materials and their graphitization temperature S ummer U ndergraduate 2 R esearch 0 F ellowship in 0 I nformation 6 T echnology ranges.6-11 Figure 4. Raman scattering of monochromatic laser light from aromatic carbon sp2 hybrid bond15. Figure 5. Raman spectra of graphite and graphene16. Future Work • Study further carbon-catalyst systems and configurations. • Graphitize nano-imprint patterned polymeric carbon precursors and analyze integrity of pattern after heat treatment. Acknowledgments Polymer Carbonization Heat Treatment (500-900C) 7 (b) Pitch Coke6 Petroleum Coke7 7 (d) Si Wafer 3. SEM and raman spectroscopy have proven reliable tools for characterizing these material processes. • Use E-beam epitaxy to create single crystal systems similar to the PVD systems shown in figure 6(a) in an effort to create a continuous graphene sheet. 7 (c) 6nm Ni PVD Layer 4nm C PVD / carbonized PAN layer Spin Coat onto (111) Epitaxial Catalyst Mold*** 2. Catalytic graphitization presents a promising avenue for lowering processing temperature, making graphene more compatible with NIL Figure 8. (a) Raman spectrum of Ni-PAN PVD system.* (b) SEM image of Ni heated to 800oC for 10 minutes. (c) Ni-PAN PVD system, PAN carbonized at 700oC for 10 minutes, PVD Nickel applied and heated to 800oC for 10 minutes . [email protected] · www.research.calit2.net/students/surf-it2006 · www.calit2.net My deepest gratitude to Said Shokair and UROP, Dr. Tai Chen, Professor Eric Potma and Hyun Min Kim in the department of Chemistry, the Mecartney / Mumm group and the Calit2 staff for all their help and support. References 1. Berger, C. et al. Electronic confinement and coherence in patterned epitaxial graphene. SCIENCE 312, 1191-1196 (2006). 2. Zhang, Y., Tan, Y., Stormer, H. & Kim, P. Experimental observation of the quantum Hall effect and Berry's phase in graphene. NATURE 438, 201-204 (2005). 3. Novoselov, K. et al. Two-dimensional gas of massless Dirac fermions in graphene. NATURE 438, 197-200 (2005). 4. J. Hass, C. A. J., R. Feng, T. Li, X. Li, Z. Song, C.Berger, W.A. de Heer, P.N. First, and E.H. Conrad. Highlyordered Graphene For Two Dimensional Electronics. 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