Synopsis: The ripple effect?

Scanning tunneling spectroscopy establishes the dominant mechanism of electron scattering when graphene is placed on a substrate.
Synopsis figure
Illustration: A. Deshpande et al., Phys. Rev. Lett. (2009)

Graphene is considered a two-dimensional form of carbon but it is never atomically flat. Nanometer-scale deformations or ripples exist in graphene regardless of whether it is suspended or placed on a substrate such as SiO2. These ripples are responsible for the unusual metallic behavior of graphene, even for very low concentrations of carriers where electron localization may be expected. In addition to intrinsic ripples, graphene is also susceptible to adsorbed impurities and defects.

In an article appearing in Physical Review B, Aparna Deshpande and collaborators from the University of Arizona, Tucson, and the University of California, Riverside, have spatially mapped the local density of electronic states in graphene on an SiO2 substrate to better understand the different scattering mechanisms that govern electronic transport. With scanning tunneling spectroscopy, they achieve an unprecedented spatial resolution of the local density of states, which they obtain by measuring the differential conductance as a function of position. The presence of charge inhomogeneity results in a shift in the Dirac point—where the conduction band meets the valence band—to finite bias voltage.

The graphene ripples have a height variation of about 5Å over an area of 30×30nm2 and their lateral extent follows the corrugation of the SiO2 substrate. If intrinsic ripples were the only factor determining electron scattering, then highly curved regions would appear to be electron-doped and flat regions hole-doped. However, electron and hole puddles do not correlate with the curvature measured in topographic images. Instead, the Dirac point shift is consistent with long-range scattering from random charged impurities at the graphene-substrate interface. These impurities also contribute to graphene’s finite minimum conductivity that exists even at zero voltage bias. – Sarma Kancharla


Announcements

More Announcements »

Subject Areas

Graphene

Previous Synopsis

Next Synopsis

Materials Science

Making room for holes

Read More »

Related Articles

Viewpoint: Chasing the Exciton Condensate
Semiconductor Physics

Viewpoint: Chasing the Exciton Condensate

Unusual interactions between charges have been observed in two closely separated graphene bilayers, a promising system in which to create a condensate of electron-hole pairs. Read More »

Focus: New Form of Carbon Stores Lots of Gas
Graphene

Focus: New Form of Carbon Stores Lots of Gas

Carbon honeycomb, a new carbon structure, could store large amounts of hydrogen gas, which may benefit fuel cell technology. Read More »

Synopsis: Graphene Majoranas
Graphene

Synopsis: Graphene Majoranas

Graphene could host Majorana quasiparticles if brought into contact with a conventional superconductor. Read More »

More Articles