Optical and electrical properties of pristine and transition Metals-doped graphene quantum dots as gas sensors toward H2O and NO2: A first principles study

• The adsorption of NO 2 and H 2 O molecules on pure structure is weak, while transition metal-doped (Pt and Pd atoms) structures make the adsorption strong. • The preference of NO 2 and H 2 O molecules is in the center of hexagon and in the middle of C-C bond, respectively. • While H 2 O is weakly adsorbed, NO 2 shows moderate and strong adsorptions. • The optical variations of Pd-doped device toward gas molecules are more prominent than Pt-doped device. In this work, we theoretically investigate the variations in the absorbance spectra of hexagonal graphene quantum dots (GQDs) under the influence of different gas molecules to evaluate their potential for optical gas sensing . The absorbance of gas molecules such as NO 2 and H 2 O has been analyzed using density functional theory (DFT) calculations. This paper explores the optical and electrical properties of GQDs with a particular focus on the impact of Pt and Pd dopants on the spectral characteristics of GQDs, emphasizing their role in enhancing adsorption energies and gas selectivity. Our findings indicate that the most favorable doping site within the GQD structure is the center of a carbon hexagon (hollow site, H). The adsorption energy of NO 2 on pristine GQDs at the top site (T) is determined to be −0.16 eV, while doping with Pt and Pd significantly increases this value to approximately −2.0 eV. Additionally, charge transfer calculations reveal that in Pt-GQDs, −0.562e is transferred from the GQDs to the transition metal, whereas in Pd-GQDs, the transferred charge is −0.203e. The most favorable adsorption configurations for NO 2 and H 2 O on Pd-, Pt-doped, and pristine GQDs correspond to the O and H orientations, respectively. A significant shift in the Fermi level toward the conduction band is observed upon NO 2 adsorption, whereas the effect is minimal for H 2 O. The results demonstrate that Pt and Pd-doped GQDs exhibit higher adsorption energies, indicating their impact in gas-sensing applications. Pd-doped GQDs show stronger sensitivity to NO 2 , which makes it a promising candidate for efficient gas detection. Optical analysis reveals a blue shift in the absorption peaks around 18 eV, providing insights into electronic transitions induced by gas interactions. Furthermore, in Pt- and Pd-doped GQDs, new absorption peaks emerge in the low-energy range (2–5 eV), indicating they are suitable for gas sensing applications within this region of the electromagnetic spectrum.