Large-scale pattern growth of graphene films for stretchable transparent electrodes

1. Large-scale pattern growth of graphene films for stretchable transparent electrodes Daniel S. Wood Ph.D. Student ECE 3-24-10

2. Outline • Creating Graphene • CVD – The Basics • CVD – The Process • Transfer Process • Characterizing Graphene • Electronic Properties • Conclusions

3. Creating Graphene : CVD - The Basics • CVD – Chemical Vapor Deposition • Process of chemically reacting a volatile compound of a material to be deposited in conjunction with other gases, to produce a nonvolatile solid that deposits atomistically on substrate. • Industry uses • Solid-state devices • Ball bearings and cutting tools • Rocket engines • Nuclear reactor components • Advantages • No vacuum • Small power requirement • Affordable equipment and operating costs

4. Creating Graphene : CVD - The Basics • Convective and diffusive transport of reactants from the gas inlets to the reaction zone • Chemical reactions in the gas phase to produce new reactive species and by-products • Transport of the initial reactants and their products to the substrate surface • Adsorption (chemical and physical) and diffusion of these species on the substrate surface • Heterogeneous reactions catalyzed by the surface leading to film formation • Desorption of the volatile by-products of surface reactions • Convective and diffusive transport of the reaction by-products away from the reaction zone

5. Creating Graphene : CVD - The Process • Why CVD? • Soluble growth: cheap synthesis, low electrical conductivity • Epitaxial growth : high quality graphene, strong substrate interaction • Past Problems: carbon forms crystals not sheets, HNO3 etch causes H2 bubbles CVD Deposition of CH4/H2/Ar on 300 nm Ni for ~7 min. Crystalproblem  Heat to 1000C, reducing 10C/s with Ar. Graphene thickness controlled by Ni thickness and deposition time Transfer / Etch problem Used FeCl3 or HF/BOE. Dry Transfer : PDMS stamp

6. Results : Characterizing Graphene • Optical /Raman • Bright areas are monolayers. Confirmed by Raman spectroscopy • AFM • Ripples show difference in thermal expansion of Ni and graphene • SEM Clear contrast between graphene layer thickness • TEM Confirmed <10 layers of graphene (mostly bilayers)

7. Results : Electrical Properties of Graphene • Transmittance • Transferred to a quartz plate • Transmittance was 80% • Graphene is 2.3%  6-10 Layers • Can be increased by deposition and Ni thickness, UV etching. • UV and Resistance • Mobility • Transferred to Si waferwith oxide layer • Half integer quantum hall effect was observed monolayer graphene • Graphene is comparable electronically to mechanically cleaved

8. Results : Electrical Properties of Graphene • Strain Induced Resistance • Bend test • 0 to 2.3mm curvature  no change • 2.3 to 0.8mm curvature  50 times resistance • Original properties achieved upon unloading • Stretching • < 6% strain  recovers original properties, but degrades of iterations • Pre-strained substrates  increase durability of graphene to 25 % strain

9. Conclusions • New method to develop high quality graphene • CVD on Ni substrate • PDMS stamp • Thickness control by substrate thickness, deposition time UV exposure • Transmittance consistent with other data • Mobility shows high quality monolayer graphene • Strain induced resistance • Flexible electronics • Change in band structure

10. Questions?