||Helin Cao, Qingkai Yu, Luis Jauregui, Deepak Pandey, Robert Colby, Jifa Tian, Nathan Guisinger, Eric Stach, Steven Pei, Yong Chen
We have synthesized large scale (up to several inches) graphitic and graphene thin films by chemical vapor deposition (CVD) on polycrystalline Ni (1,2) and Cu (3) foils and systematically characterized their structural and electronic properties after transferred onto insulating SiO2 (on doped Si) substrates. For films grown on Ni (1,2), structural characterizations by atomic force microscopy (AFM), scanning tunneling microscopy (STM), cross-sectional transmission electron microscopy (XTEM)(4) and spectroscopic Raman mapping confirm that such large scale graphitic thin films contain both thick graphite regions and thin regions of few layer graphene. The films also contain many wrinkles, with sharply-bent tip and dislocations revealed by XTEM, yielding insights on the growth and buckling processes of the film. Measurements on mm-scale back-gated transistor devices fabricated from the transferred film show ambipolar field effect with resistance modulation ~50% and carrier mobilities reaching ~2000 cm^2/Vs. We also demonstrate quantum transport of carriers with phase coherence length over 0.2 μm from the observation of 2D weak localization in low temperature magneto-transport measurements. Our results show that despite the non-uniformity and surface roughness, such large-scale, flexible thin films can have electronic properties promising for device applications. For films grown on Cu (3), we show they consist dominantly of monolayer graphene as indicated by Raman mapping. STM imaging shows monolayer graphene lattice. Low temperature transport measurements are performed on micro devices fabricated from such CVD graphene, displaying ambipolar field effect (with on/off ratio ~5 and carrier mobilities up to ~3000 cm^2/Vs) and “half-integer” quantum Hall effect, a hall-mark of intrinsic electronic properties of monolayer graphene. We also observe weak localization and extract information about phase coherence and scattering of carriers by disorder in the graphene. Finally, we have measured the thermal conductivity of suspended CVD graphene to be ~3000 W/m-K, comparable with that of exfoliated graphene, by combining electronic transport and Raman thermometry (5). 1. Q. Yu et al., Applied Physics Letters, 93, 113103 (2008); 2. H. Cao et al., Journal of Applied Physics, 107, in press (2010) DOI: 10.1063/1.3309018; 3. H. Cao et al., Applied Physics Letters, accepted (2010) (arXiv: 0910.4329); 4. R. Colby et al., Diamond and Related Materials 19, 143 (2010); 5. L. Jauregui et al., ECS Transactions (Proc. ECS-217), in press (2010).