DFT Investigation of the Effect of CO Adsorption Sites on the Structural, Electronic, and Optical Properties of Ge-doped Si Quantum Dots

Density functional theory (DFT) calculations were used to investigate how CO adsorption sites affect the structural, electronic, charge transfer, vibrational, and optical properties of Ge-doped Si quantum dots. Three adsorption sites, namely hollow, top, and valley, were investigated. Among the three configurations, the top site exhibits the strongest adsorption, with an adsorption energy of -0.915 eV, whereas the valley site shows the weakest interaction, with E ads = -0.124 eV. CO adsorption at the top site produces the largest geometric deformation and surface buckling, indicating a stronger interaction between CO and the SiGe surface. CO adsorption substantially modifies the electronic structure of the quantum dot. The HOMO-LUMO gap decreases, while the density of states near the Fermi level increases, particularly for the top configuration. HOMO-LUMO distributions, charge density differences, and Bader charge analysis further reveal pronounced charge redistribution and orbital overlap at the top site. Adsorption was also found to increase low frequency soft vibrational modes and reduce the dynamical stability of the system. The optical properties of the system, including the dielectric function, absorption coefficient, reflectivity, and joint density of states (JDOS), show the appearance of new peaks or changes in peak intensity, most clearly at the top site. These findings demonstrate that the adsorption site strongly influences the gas-surface interaction and the resulting electronic and optical responses. The results suggest that the investigated SiGe quantum dot model may be relevant to CO sensing and related nano-optoelectronic applications.