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Understanding the critical roles of ligands (e.g. thiolates, SR) in the formation of metal nanoclusters of specific sizes has long been an intriguing task since the report of ligand exchange-induced transformation of Au38(SR)24 into Au36(SR')24. Herein, we conduct a systematic study of ligand exchange on Au38(SC2H4Ph)24 with 21 incoming thiols and reveal that the size/structure preference is dependent on the substituent site. Specifically, ortho-substituted benzenethiols preserve the structure of Au38(SR)24, while para- or non-substituted benzenethiols cause its transformation into Au36(SR)24. Strong electron-donating or -withdrawing groups do not make a difference, but they will inhibit full ligand exchange. Moreover, the crystal structure of Au38(SR)24 (SR = 2,4-dimethylbenzenethiolate) exhibits distinctive ππ stacking and "anagostic" interactions (indicated by substantially short AuH distances). Theoretical calculations reveal the increased energies of frontier orbitals for aromatic ligand-protected Au38, indicating decreased electronic stability. However, this adverse effect could be compensated for by the AuH-C interactions, which improve the geometric stability when ortho-substituted benzenethiols are used. Overall, this work reveals the substituent site effects based on the Au38 model, and highlights the long-neglected "anagostic" interactions on the surface of Au-SR NCs which improve the structural stability.

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.

Engineering the surface ligands of metal nanoparticles is critical in designing unique arrangements of metal atoms. Here, we report the synthesis and total structure determination of a large box-shaped Ag 67 nanocluster (NC) protected by a mixed shell of thiolate (2,4-dimethylbenzenethiolate, SPhMe 2 ) and phosphine (triphenylphosphine, PPh 3 ) ligands. Single crystal X-ray diffraction (SCXRD) and electrospray ionization mass spectrometry (ESI-MS) revealed the cluster formula to be [Ag 67 (SPhMe 2 ) 32 (PPh 3 ) 8 ] 3+ . The crystal structure shows an Ag 23 metal core covered by a layer of Ag 44 S 32 P 8 arranged in the shape of a box. The Ag 23 core was formed through an unprecedented centered cuboctahedron, i.e., Ag 13 , unlike the common centered Ag 13 icosahedron geometry. Two types of ligand motifs, eight AgS 3 P and eight bridging thiols, were found to stabilize the whole cluster. The optical spectrum of this NC displayed highly structured multiple absorption peaks. The electronic structure and optical spectrum of Ag 67 were computed using time-dependent density functional theory (TDDFT) for both the full cluster [Ag 67 (SPhMe 2 ) 32 (PPh 3 ) 8 ] 3+ and a reduced model [Ag 67 (SH) 32 (PH 3 ) 8 ] 3+ . The lowest metal-to-metal transitions in the range 500-800 nm could be explained by considering the reduced model that shows almost identical electronic states to 32 free electrons in a jellium box. The successful synthesis of the large box-shaped Ag 67 NC facilitated by the combined use of phosphine and thiol paves the way for synthesizing other metal clusters with unprecedented shapes by judicious choice of thiols and phosphines.

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.

The spillover mechanism of molecular hydrogen on carbon nanotubes in the presence of catalytically active platinum clusters was critically and systematically investigated by using density-functional theory. Our simulation model includes a Pt4 cluster for the catalyst nanoparticle and curved and planar circumcoronene for two exemplary single-walled carbon nanotubes (CNT), the (10,10) CNT and one of large diameter, respectively. Our results show that the H2 molecule dissociates spontaneously on the Pt4 cluster. However, the dissociated H atoms have to overcome a barrier of more than 2 eV to migrate from the catalyst to the CNT, even if the Pt4 cluster is at full saturation with six adsorbed and dissociated hydrogen molecules. Previous investigations have shown that the mobility of hydrogen atoms on the CNT surface is hindered by a barrier. We find that instead the Pt4 catalyst may move along the outer surface of the CNT with activation energy of only 0.16 eV, and that this effect offers the possibility of full hydrogenation of the CNT. Thus, although we have not found a low-energy pathway to spillover onto the CNT, we suggest, based on our calculations and calculated data reported in the literature, that in the hydrogen-spillover process the observed saturation of the CNT at hydrogen background pressure occurs through mobile Pt nanoclusters, which move on the substrate more easily than the substrate-chemisorbed hydrogens, and deposit or reattach hydrogens in the process. Initial hydrogenation of the carbon substrate, however, is thermodynamically unfavoured, suggesting that defects should play a significant role.

Molecular dynamics simulations using quantum mechanics for the electronic system, i.e., within the Born-Oppenheimer or related Car-Parrinello approximation, became feasible and popular in recent years for very large systems. The most common setup for these simulations is the supercell method in conjunction with the Γ-point approximation. Here we provide a tool which is useful to choose the supercell of the considered system such that it makes it appear to have either an as large as possible band gap (optimized for Car-Parrinello setup) or the metallic character reflected at the Γ point (e.g., fold the Dirac point to the Γ point for graphene and carbon nanotubes) in order to monitor the metallic character in a trajectory. We address carbon nanotubes, graphene, and inorganic TS2 analogues with T = Re, Nb. We further provide a simple Hückel code, which allows checking the electronic states close to the Fermi level within the Γ-point approximation, and we test its predictions against the density-functional-based tight-binding approach.