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Ligand-protected Au nanoclusters have attracted tremendous interest due to their atomically precise structural determination and controllable optical properties. However, the origin of their optical features is not well understood. Herein, we address the effects of charge state and type of ligands on the structural, electronic, and optical properties of the Au 25 (SR) 18 nanocluster, using first principles calculations. In particular, we analyze in detail the optical absorption spectra of phenylethanethiol-capped [Au 25 (SCH 2 CH 2 Ph) 18 ] q nanocluster (q = -1, 0, and 1) and demonstrate that the nanocluster undergoes geometric changes when increasing its charge state, yielding the development of additional photoabsorption features. Moreover, by comparing the properties of the anionic [Au 25 (SR) 18 ] 1- nanoclusters (with SR = -SCH 2 CH 2 Ph, -SCH 3 and -SH) we demonstrate that accounting for full (-SCH 2 CH 2 Ph) compared to simplified (-SCH 3 and -SH) ligands leads to significant modifications in the shape of the main absorption features, such as the emergence of step-like absorption shoulders. In addition, we show that the energy of the main absorption peaks of the [Au 25 (SR) 18 ] 1- significantly blue-shifts when using -SH as model ligands. Overall, our computational work aids the experimental characterization of optical properties of ligand-protected nanoclusters and highlights the importance of accounting for full ligands in optical spectra calculations.

In this work, we used density functional theory (DFT) and linear response time-dependent DFT (LR-TDDFT) to investigate the stability, electronic structure, and optical properties of Au-doped [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3- nanoclusters (BDT: 1,3-benzenedithiol; TPP triphenylphosphine) with x = 0-5. The aim of this work is to shed light on the most favorable doped structures by comparing our results with previously published experimental data. The calculated relative energies, ranging between 0.8 and 10 meV per atom, indicate that several doped Ag 29-x Au x nanoclusters are likely to co-exist at room temperature. However, only the Au-doped [Ag 29-x Au x (BDT) 12 (TPP) 4 ] 3- nanoclusters that have direct bonding between Au dopants and phosphines display an enhancement in the electronic transitions at ∼450 nm. This agrees with the main spectral absorption features that have been experimentally reported for the mixture of Au-doped silver Ag 29-x Au x nanoclusters. In addition, the formation of the Au-TPP bond could prevent cluster degradation starting from the detachment of the phosphine molecules, since the Au-TPP bond is stronger by ∼0.4 eV than the analogous Ag-TPP one. Thus, the results presented here show the important role of Au-TPP bonding in determining the stability and optical properties of thiolate/phosphine-protected Ag 29-x Au x nanoclusters.

Quantum spin Hall (QSH) insulators exhibit a bulk insulting gap and metallic edge states characterized by nontrivial topology. Here, we used first-principles calculations to investigate the electronic and topological properties of halogenated silicon germanide (X2-SiGe, with X = F, Cl, and Br) monolayers, which we found to be trivial semiconductors with energy band gaps ranging from 500 meV to 900 meV. Interestingly, we found that under 8% strain, X2-SiGe monolayers behave as QSH insulators with global band gaps between 53 meV and 123 meV. The underlying mechanism of the topological phase transition is the strain-induced s-p band inversion. The nontrivial topological features for the strained X2-SiGe monolayers were further confirmed by the presence of topologically protected edge states that form a single Dirac cone in the middle of the bulk band gaps. Therefore, our results reveal that this new family of QSH insulators is promising for room temperature applications in spintronics and quantum computation devices.