Graphene

The research in our group on graphene aims to understand its unique electronic properties using a combination of electrical transport measurements and scanning probe microscopy. Currently, we are focused on looking at exfoliated graphene on various substrates. We have examined both mono and bilayer graphene using scanning tunneling spectroscopy. We also have ongoing projects looking at new ways to control and image the electronic properties of graphene using scanning probe microscopy.

Monolayer Graphene on Boron Nitride

Recently we have obtained atomic resolution topography and spectroscopy images of graphene on hexagonal boron nitride. We have found that the graphene is extremely flat, about 10 times flatter than on silicon dioxide. The 1.8% mismatch between the graphene and boron nitride lattices causes a Moire pattern to form based on the relative orientation of the two lattices. On the right is an example of a topographic image showing the hexagonal lattice of graphene and a longer wavelength Moire pattern.

Beyond the topography measurements, we have also mapped the local density of states in the graphene. We have found that the boron nitride reduces the density flucuations (electron and hole puddles) by two orders of magnitude compared to graphene on silicon dioxide. The image below shows a comparision of the potential fluctuations in graphene on silicon oxide (left) and boron nitride (right). We observe that there are less puddles on boron nitride and they have much smaller potential variation. This work was published in Nature Materials.

 

 

 

 

 

 

 

Monolayer Graphene on Silicon Oxide

We have achieved atomic resolution imaging of monolayer graphene using scanning tunneling microscopy. The images show the full honeycomb lattice associated with graphene unlike in thicker samples which only show a triangular lattice. On the right is an example of a topographic image of the graphene lattice which also shows large height variations due to the underlying silicon dioxide substrate. We were the first group to produce spatially resolved scanning tunneling spectroscopy results for monolayer graphene on silicon dioxide. We found that the density of states is not uniform but rather breaks up into a series of electron and hole doped regions. The origin of these electron puddles can come from the curvature of the film or charged impurities. Based on recent work correlating the topography with the local density of states, we have found that most of the puddles seem to arise from charged impurities rather than the curvature of the graphene itself. This work was published in Physical Review B and featured in the May 2009 issue of Physics. The results of the research were also highlighted by a press release from the American Institute of Physics.

Bilayer Graphene

In bilayer graphene unlike monolayer graphene it is possible to open a controllable band gap by the application of an electric field perpendicular to the graphene. Using scanning tunneling spectroscopy, we have shown that the electric field from a back gate can open a gap in bilayer graphene. The figure on the right shows that the minumum in the density of states (red) which corresponds to the Dirac point shifts and changes with as a function of the gate voltage. The shift of the Dirac point is linear with gate voltage (top right figure) as expected for bilayer graphene as compared to monolayer graphene which has a square root dependence. The width of the gap (bottom right figure) is also a function of gate voltage with a minimum occuring when the Dirac point as at the Fermi energy. This research appeared in Applied Physics Letters.

References

J. Xue, J. Sanchez-Yamagishi, D. Bulmash, P. Jacquod, A. Deshpande, K. Watanabe, T. Taniguchi, P. Jarillo-Herrero, B.J. LeRoy, "Scanning tunnelling microscopy and spectroscopy of ultra-flat graphene on hexagonal boron nitride," Nature Materials advance online publication 13/02/2011.

A. Deshpande, W. Bao, Z. Zhao, C.N. Lau, and B.J. LeRoy, "Spatial mapping of the Dirac point in monolayer and bilayer graphene," IEEE Transactions on Nanotechnology 10, 88-91 (2011).

W. Bao, G. Liu, Z. Zhao, H. Zhang, D. Yan, A. Deshpande, B.J. LeRoy, and C.N. Lau, "Lithography-free Fabrication of High Quality Substrate-supported and Suspended Graphene devices," Nano Research 3, 98 (2010).

A. Deshpande, W. Bao, Z. Zhao, C.N. Lau, and B.J. LeRoy, "Mapping the Dirac point in gated bilayer graphene," Appl. Phys. Lett. 95, 243502 (2009).

A. Deshpande, W. Bao, F. Miao, C.N. Lau, and B.J. LeRoy, "Spatially resolved spectroscopy of monolayer graphene on SiO2," Phys. Rev. B 79, 205411 (2009).