This thesis theoretically investigates the photocatalytic activity an‎d nonlinear optical (NLO) properties of two-dimensional (2D) monolayer quantum dots using computational methods based on Density Functional Theory (DFT). The research is divided into two main sections. In the first section, novel quantum dots, including C₃P₄, C₃As₄, Ge₃N₄, Ge₃P₄, Ge₃As₄, Si₃N₄, Si₃P₄, an‎d Si₃As₄, inspired by the g-C₃N₄ structure, were introduced an‎d studied. The results demonstrated that these quantum dots possess suitable ban‎d gaps (within the visible light range), enhanced optical absorption, an‎d efficient electron-hole separation, making them suitable for photocatalytic applications such as water splitting an‎d CO₂ reduction. Notably, Ge₃P₄ an‎d Si₃N₄ quantum dots exhibited superior charge separation an‎d visible light absorption compared to g-C₃N₄. Additionally, calculations of polarizability (α₀) an‎d hyperpolarizability (β₀) indicated improved NLO properties in these structures compared to g-C₃N₄. In the second section, the effects of doping with silicon (Si) an‎d germanium (Ge), along with various functional groups (-OH, -COOH, -F, -NH₂, -SH, -SCH₃, -Cl), on the properties of g-C₃N₄-based quantum dots were investigated. The results showed that doping at the middle (M) position, compared to the triangular (T) position, in g-C₃N₄ led to a more significant reduction in the ban‎d gap to below 3 eV an‎d enhanced visible light absorption. This ban‎d gap reduction was particularly pronounced in Si-doped g-C₃N₄ with -SH, -SCH₃, an‎d -COOH functional groups. Doping with Ge an‎d Si, especially at the middle position, significantly improved electron-hole separation. Specifically, the charge transfer length index (D) increased to 5.59 Å with Si doping in the presence of the -SH functional group at the middle position. The t index rose to 3.99 Å with simultaneous Si doping an‎d the -SCH₃ functional group at the middle position, an‎d the electron-hole overlap index (Sr) reached a minimum of 0.11 a.u. with simultaneous Si doping an‎d the -COOH functional group at the middle position, indicating reduced electron-hole recombination an‎d enhanced photocatalytic efficiency. Analysis of absorption spectra further confirmed that these structures exhibit improved optical absorption compared to g-C₃N₄. This research highlights the high potential of the proposed an‎d modified quantum dots for photocatalytic applications.

حسين فرخ پور

عليرضا نجفي چرمهيني , محمدمحسن مومني هامانه