Underlying Mechanisms on Tunable Electronic Structures of Graphene Quantum Dots Doped with Nitrogen and Sulfur Heteroatoms

Abstract

Graphene quantum dots (GQDs) are a new class of quantum dots with unique chemical and physical properties, and significant developments have been made on their electronic characteristic structures for wide applications in electronics, energy conversion and storage devices. Doping heteroatoms into graphene nanostructures is an efficient way to tune electronic structures. However, different synthesis methods have resulted in different doping configurations to affect electronic properties and thereby hindered systematically understanding the mechanisms of GQDs doped with heteroatoms. Herein, the electronic mechanism of GQDs doped with N and S was studied by density functional theory (DFT). The formation energies, electronic structures, and electrostatic potentials of the doped GQDs were calculated to reveal effects of different doping types on electronic properties. For the N/S co-doped GQDs, graphitic N and pyridine N were considered to substitute carbon atoms in the plane while the -C-S-C- and -C-SO2-C- at the edge. The formation energy calculated by DFT indicates that heteroatom doping tends to be doped at the basal plane. And surface doping could increase charge density of carbon atoms connected to nitrogen atoms. The introduction of -C-S-C- into the graphitic N and pyridine N structures reduces the energy difference between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO). However, the addition of oxidized S into the N-doped structure increases the HOMO–LUMO energy gap. For the electrostatic potential diagram, the introduction of sulfur-containing groups can enhance the charge density around nitrogen, which suggests that heteroatom co-doped GQDs have improved electron transports. Therefore, this work provides valuable information on understanding electronic properties of N/S co-doped GQDs for the applications in nanoelectronic devices and give guidance for developing methods to controllably synthesize GQDs with well-defined and desirable properties towards specific purposes.