Band structure engineering of epitaxial graphene on SiC by molecular doping

TL;DR

This study demonstrates that molecular doping with F4-TCNQ effectively neutralizes intrinsic n-type doping in epitaxial graphene on SiC, enabling control over its electronic properties through non-covalent functionalization.

Contribution

It introduces a novel method of molecular doping using F4-TCNQ to tune the electronic structure of epitaxial graphene on SiC, including achieving charge neutrality and band gap modulation.

Findings

Charge neutrality achieved in monolayer graphene via F4-TCNQ doping.

Band gap increase in bilayer graphene with F4-TCNQ deposition.

Doping effect remains stable in air and up to 200°C.

Abstract

Epitaxial graphene on SiC(0001) suffers from strong intrinsic n-type doping. We demonstrate that the excess negative charge can be fully compensated by non-covalently functionalizing graphene with the strong electron acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ). Charge neutrality can be reached in monolayer graphene as shown in electron dispersion spectra from angular resolved photoemission spectroscopy (ARPES). In bilayer graphene the band gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band gap. The reduction of the charge carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy. The structural and electronic characteristics of the graphene/F4-TCNQ charge transfer complex are investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The doping effect on graphene is preserved in air and is temperature resistant up to 200\degree C. Furthermore, graphene non-covalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultra-high vacuum but also by wet chemistry.