Revealing the nature of defects in quasi free standing mono-layer graphene on SiC(0001) by means of Density Functional Theory

TL;DR

This study uses Density Functional Theory to analyze the structural and electronic properties of defects in hydrogen coverage in quasi free standing monolayer graphene on SiC, linking these defects to observed STM features and potential electronic applications.

Contribution

It provides a systematic DFT analysis of H vacancy defects in QFMLG, correlating defect configurations with STM images and electronic properties, advancing understanding of defect structures.

Findings

Large H vacancies correlate with specific STM features.

Vacancy size and shape tune localized electronic states.

Hydrogen vacancies tend to aggregate, affecting defect formation.

Abstract

Quasi free standing monolayer graphene (QFMLG) grown on SiC by selective Si evaporation from the Si-rich SiC(0001) face and H intercalation displays irregularities in STM and AFM analysis, appearing as localized features, which we previously identified as vacancies in the H layer coverage [Y Murata, et al. Nano Res, in press, DOI: 10.1007/s12274-017-1697-x]. The size, shape, brightness, location, and concentration of these features, however, are variable, depending on the hydrogenation conditions. In order to shed light on the nature of these features, in this work we perform a systematic Density Functional Theory study on the structural and electronic properties of QFMLG with defects in the H coverage arranged in different configurations including up to 13 vacant H atoms, and show that these generate localized electronic states with specific electronic structure. Based on the comparison of simulated and measured STM images we are able to associate different vacancies of large size (7-13 missing H) to the different observed features. The presence of large vacancies is in agreement with the tendency of single H vacancies to aggregate, as demonstrated here by DFT results. This gives some hints into the hydrogenation process. Our work unravels the structural diversity of defects of H coverage in QFMLG and provides operative ways to interpret the variety in the STM images. The energy of the localized states generated by these vacancies is tunable by means of their size and shape, suggesting applications in nano- and opto-electronics.