Effect of Nitrogen Position in Tailoring the Electronic Properties of Graphene Quantum Dot: A DFT Investigation

Abstract

Graphene quantum dots (GQDs) are nanoscale materials with tunable optical and electronic properties due
to quantum confinement and edge effects. Nitrogen doping has proven especially effective in enhancing
these properties, improving conductivity, reactivity, and fluorescence. Studying how nitrogen atoms, when
doped at different sites in GQDs, affect their electronic structure is essential for advancing their use in
applications like sensing, imaging, and energy storage. This study presented a comparative analysis of the
electronic properties of pristine, edge-doped (GQD-Nₑ), and center-doped (GQD-Nc) nitrogen graphene
quantum dots (GQDs), with a focus on their band structure and density of states (DOS) using density
functional theory (DFT). The band structure of the pristine GQD reveals a direct band gap of approximately
2.18eV, indicative of its semiconducting behavior. Upon nitrogen doping, significant modifications in the
electronic structure are observed. Edge doping introduces localized states near the Fermi level (EF),
resulting in a substantial narrowing of the band gap and enhanced electronic states in the mid-gap region.
This is attributed to the higher electronegativity and lone pair electrons of nitrogen atoms at the edges,
which distort the local electronic potential. In contrast, center doping yields a moderate increase in states
near the EF, maintaining a more ordered band structure and preserving much of the original semiconducting
character. The DOS analysis supports these findings, showing a high density of electronic states around
EF in GQD-Nₑ and a moderate increase in GQD-Nc compared to the pristine system. These results
demonstrate that nitrogen doping provides a tunable route to engineer the electronic properties of GQDs,
with edge doping favoring enhanced conductivity and center doping offering balanced semiconducting
behavior for potential nanoelectronic and optoelectronic applications.