Origin of Spatial Charge Inhomogeneity in Graphene

TL;DR

This paper introduces a new technique to map the Dirac point in graphene, revealing that charge inhomogeneities originate from impurities below the surface rather than topographical features, affecting electron behavior and mobility.

Contribution

A novel Dirac point mapping method is used to identify the microscopic origin of charge inhomogeneities in graphene as impurity-induced rather than topography-induced.

Findings

Charge fluctuations are caused by below-surface impurities.

Impurities induce standing wave patterns in graphene.

Wave patterns can be modulated by electric gating.

Abstract

In an ideal graphene sheet charge carriers behave as two-dimensional (2D) Dirac fermions governed by the quantum mechanics of massless relativistic particles. This has been confirmed by the discovery of a half-integer quantum Hall effect in graphene flakes placed on a SiO2 substrate. The Dirac fermions in graphene, however, are subject to microscopic perturbations that include topographic corrugations and electron density inhomogeneities (i.e. charge puddles). Such perturbations profoundly alter Dirac fermion behavior, with implications for their fundamental physics as well as for future graphene device applications. Here we report a new technique of Dirac point mapping that we have used to determine the origin of charge inhomogeneities in graphene. We find that fluctuations in graphene charge density are not caused by topographical corrugations, but rather by charge-donating impurities below the graphene. These impurities induce unexpected standing wave patterns due to supposedly forbidden back-scattering of Dirac fermions. Such wave patterns can be continuously modulated by electric gating. Our observations provide new insight into impurity scattering of Dirac fermions and the microscopic mechanisms limiting electronic mobility in graphene.