Etude des propriétés physiques des clusters d’Or dopés pour les applications optoélectroniques

Abstract

This thesis investigates the physical characteristics and structural stability of pure gold clusters and nanoclusters (Aun), with a particular focus on the effects of metal doping using titanium (Ti) and platinum (Pt). The research explores doping with Ti (TiAun, n=1-17) and Pt (PtTiAun, n=1-16), emphasizing their applications in detectors and biosensors. The study employs density functional theory (DFT) with generalized gradient approximation (GGA) as implemented in the SIESTA simulation package. Time-dependent density functional theory (TDDFT) utilizes long-range corrected hybrid functionals, including B3LYP and CAM-B3LYP, where "CAM-B3LYP" denotes a variable with a corrected approach modified by adjusting the range separation parameter, computational resources from the HPC facility. (HPC Ab initio simulations were performed using computational resources from the HPC facility (DGRSDT) at the University of Batna 2 (Algeria) /ROZA Deutsche Forschungsgemeinschaft (DFG, GERMANY RESEARCH FOUNDATION) in OLDENBURG) were utilized for HPC Ab-initio simulations using the GAUSSIAN software package, implementing linear response theory. This approach determines excited-state properties by analyzing the electron density's response to time-dependent perturbations. Calculations employed first-principles wave function-based electronic correlation methods using a standard Hamiltonian within the Born-Oppenheimer approximation. The study examined the equilibrium geometry, electronic structure, and magnetic properties of TiAun, PtTiAun, Aun+1(n = 1–17), and Aun+2 (n = 1–16) groups. The stability of the systems was evaluated through binding energies, bond lengths, second-order energy differences, and HOMO-LUMO electronic energy gaps. The findings reveal differences in the formation of planar, two-dimensional, and three-dimensional structures. As energies increase with larger group sizes, the groups gain energy as they expand. Theoretical gaps between HOMO and LUMO are prevalent with stable MAu. N groups (M = Ti, Pt) decrease with increasing group size. Vertical electron affinity (VEA), vertical ionization potential (VIP), and chemical hardness (η) indicate that certain MAun groups exhibit unique properties. Substituting Ti and Pt atoms modifies the magnetic moments of pure gold clusters. Excited-state calculations were performed for ground-state-optimized geometries of Aun, TiAun, and PtTiAun n=1-6 clusters using the configuration interaction (CI) methodology at different levels. While clusters exhibit a strong redshift compared to pure gold, dominant peaks in PtTiAu4-6 show a slight blue shift relative to more active TiAu4-6 systems. The Pt component, with its denser d-shell, stabilizes reactions by enhancing low-energy transitions. The absorption spectrum of PtTiAu4-6 extends into the near-infrared region up to 700 nm, indicating low-energy charge transfer (CT) states induced by Ti, though their oscillator strength is weaker compared to Au or Pt transformations. Vibrational analysis shows that molecular symmetry influences Aun spectra, with symmetric Au3 exhibiting limited activity (due to mutual exclusion), unlike the complex spectra of lower-symmetry Au4-6 clusters. Ti addition alters the bonding environment, causing a blue shift in the main IR absorption from (165 cm-1 in Au4 to 340-380 cm-1 in TiAu3) due to high-frequency MAun stretching modes. The Ti and Pt; Ti alloy clusters show denser Raman features and intense low-frequency modes (50-150 cm-1), confirming enhanced polarizability and plasmonic coupling, which are crucial for superior SERS applications.