Modeling vacancies and hydrogen impurities in graphene: A molecular point of view

TL;DR

This study uses a molecular cluster approach to investigate how vacancies and hydrogen impurities affect the electronic and magnetic properties of graphene, revealing impurity-induced states and magnetic moments.

Contribution

It introduces a molecular cluster modeling method to analyze impurity effects in graphene, providing insights into electronic structure changes due to specific impurities.

Findings

Impurities increase the density of states near HOMO level.

A zero-energy mode appears only with substitutional protons.

Magnetic moments are induced in certain impurity configurations.

Abstract

We have followed a "molecular" approach to study impurity effects in graphene. This is thought as the limiting case of an infinitely large cluster of benzene rings. Therefore, we study several carbon clusters, with increasing size, from phenalene, including three benzene rings, up to coronene 61, with 61 benzene rings. The impurities considered were a chemisorbed H atom, a vacancy, and a substitutional proton. We performed HF and UHF calculations using the STO-3G basis set. With increasing cluster size in the absence of impurities, we find a decreasing energy gap, here defined as the HOMO-LUMO difference. In the case of H chemisorption or a vacancy, the gap does not decrease appreciably, whereas it is substantially reduced in the case of a substitutional proton. The presence of an impurity invariably induces an increase of the density of states near the HOMO level. We find a zero mode only in the case of a substitutional proton. In agreement with experiments, we find that both the chemisorbed H, the substitutional proton, and the C atom near a vacancy acquire a magnetic moment. The relevance of graphene clusters for the design of novel electronic devices is also discussed.