A density functional theory study of structural, electronic, optical and magnetic properties of small Ag–Cu nanoalloys

The structures and properties of 13-atom silver and copper bimetallic clusters are systematically investigated by density functional theory (DFT) in the theoretical frame of the generalised gradient approximation (GGA) exchange-collection function. Optical absorption, Raman spectra, vibrational spectra, as well as electronic and magnetic properties are calculated by DFT/GGA and semi-core pseudopotentials. The following lowest-energy structures in the 13-atom Ag–Cu clusters are obtained: cuboctahedron for pure Ag₁₃, icosahedrons for pure Cu₁₃, Ag₁Cu₁₂, Ag₆Cu₇ and Ag₁₂Cu₁; and amorphous motifs for Ag _m Cu_13−m when m = 2–5 and 7–11. Ag₂Cu₁₁, Ag₇Cu₆ and Ag₁₁Cu₂ are magic clusters. The Ag₂Cu₁₁ cluster exhibits high energetic stability, strong electronic stability, multipole surface plasmon resonance (SPR) mode and small dipole moment. The Ag₆Cu₇ cluster is a Janus-separated cluster that possesses the strongest electronic stability with a band gap of 0.424 eV and a vertical ionisation potential of 5.8417 eV. The amorphous Ag₇Cu₆ cluster shows an Ag–Cu alloyed motif. The blue shift of the maximum SPR peak becomes increasingly evident as silver atoms are added. All Raman and vibrational spectra exhibit many significant vibration modes within the wavenumber ranges of 0–270 and 0–306.55 cm⁻¹, respectively. Ferroelectric and ferromagnetic behaviours are observed in the 13-atom Ag–Cu nanoalloys, indicating their new potential applications in nonlinear optical devices.