Evidence for Strain Induced Conductance Modulations in Single-Layered Graphene on SO2

Citation

Lai, Andrew P. (2009) Evidence for Strain Induced Conductance Modulations in Single-Layered Graphene on SO2. Senior thesis (Major), California Institute of Technology. doi:10.7907/W14N-1749. https://resolver.caltech.edu/CaltechTHESIS:02132018-124452832

Abstract

The unique physical properties of graphene and its potential uses in nanoscale devices make it a compelling subject of study^1. Graphene, a two dimensional crystal which consists of a single layer of carbon atoms bonded together in a hexagonal lattice, has a conical band structure at low energies. Therefore, the charge carriers of graphene obey the Dirac equation for relativistic particles and can be thought of as massless Dirac fermions ^1,2,3 However, recent scanning tunneling microscopy (STM) studies of graphene on silicon dioxide substrates have found corrugations in the graphene samples, and more importantly have observed deviations in the tunneling spectra from the expected Dirac-like behavior ^(4,5,6). Several possible explanations exists for this observed deviation from Diraclike behavior, including phonon-mediated inelastic tunneling ^4,7, charge impurities ⁶, and changes to the band structure to due the underlying substrate ⁸.

We present scanning tunneling spectroscopy and microscopy data of graphene on a silicon dioxide substrate at a temperature of 77 K. The topographical studies reveal surface corrugations in graphene due to rippling and partial conformation to the underlying silicon dioxide substrate. In the tunneling spectra, a deviation from the expected Dirac-like behavior is observed. A lack of correlation between the Dirac voltage and conductance maps at a low bias voltage indicates charge impurities are not the primary cause for deviations from Dirac-like behavior. A strain map is computed from the topography and green's functions are fitted to estimate the contribution of phonon coupling to the tunneling conductance. We find that regions of higher strain correspond to higher phonon frequencies, indicating that phonon-mediated inelastic tunneling is a major contributor to the deviation from Dirac-like behavior found in tunneling spectra.