Dual-probe spectroscopic fingerprints of defects in graphene

TL;DR

This paper develops a theoretical dual-probe spectroscopic method to analyze nanoscale defects in graphene, revealing quantum interference effects and transport anisotropies, and demonstrating its usefulness for characterizing extended defects and nanostructures.

Contribution

The paper introduces a real space Green's function approach for dual-probe conductance calculations applicable to extended samples like graphene sheets.

Findings

Quantum interference fingerprints around single-site defects identified.

Transport anisotropies in pristine graphene characterized.

Fano resonances in graphene antidots depend on edge geometry.

Abstract

Recent advances in experimental techniques emphasize the usefulness of multiple scanning probe techniques when analyzing nanoscale samples. Here, we analyze theoretically dual-probe setups with probe separations in the nanometer range, i.e., in a regime where quantum coherence effects can be observed at low temperatures. In a dual-probe setup the electrons are injected at one probe and collected at the other. The measured conductance reflects the local transport properties on the nanoscale, thereby yielding information complementary to that obtained with a standard one-probe setup (the local density-of-states). In this work we develop a real space Green's function method to compute the conductance. This requires an extension of the standard calculation schemes, which typically address a finite sample between the probes. In contrast, the developed method makes no assumption on the sample size (e.g., an extended graphene sheet). Applying this method, we study the transport anisotropies in pristine graphene sheets, and analyze the spectroscopic fingerprints arising from quantum interference around single-site defects, such as vacancies and adatoms. Furthermore, we demonstrate that the dual-probe setup is a useful tool for characterizing the electronic transport properties of extended defects or designed nanostructures. In particular, we show that nanoscale perforations, or antidots, in a graphene sheet display Fano-type resonances with a strong dependence on the edge geometry of the perforation.