Neutral and Anionic Gold Decamers: Planar Structure with Unusual Spatial Charge-Spin Separation

The lowest-energy structure of the gold cluster Au10: planar vs. nonplanar?

The onset of the transition from 2D to 3D structures in pure gold clusters remains a matter of continuing debate. In this theoretical study we revisit several planar and non-planar structural motifs of the size Au₁₀ with the aim to clarify this issue. Computations using a long-range corrected exchange-correlation functional LC-BLYP, coupled-cluster theories CCSD(T) and PNO-LCCSD(T)-F12 reveal that, at variance with previous reports on the preference of a planar elongated hexagon, both planar and nonplanar isomers of the neutral Au₁₀ are energetically degenerated up to 300 K. Its 3D equilibrium geometry is a core-shell structure which can be built up from a trigonal prism by capping four extra Au atoms outside. A comparison to the available experimental vibrational spectra allows us to argue that both lowest-lying 2D and 3D isomers of Au₁₀ likely coexist in the molecular beam during measurement of its FIR spectra. This result also suggests that the 2D-3D transition of neutral gold clusters occurs at the size Au₁₀.

Density-functional tight-binding approach for metal clusters, nanoparticles, surfaces and bulk: application to silver and gold

Density-functional based tight-binding (DFTB) is an efficient quantum mechanical method that can describe a variety of systems, going from organic and inorganic compounds to metallic and hybrid materials. The present topical review addresses the ability and performance of DFTB to investigate energetic, structural, spectroscopic and dynamical properties of gold and silver materials. After a brief overview of the theoretical basis of DFTB, its parametrization and its transferability, we report its past and recent applications to gold and silver systems, including small clusters, nanoparticles, bulk and surfaces, bare and interacting with various organic and inorganic compounds. The range of applications covered by those studies goes from plasmonics and molecular electronics, to energy conversion and surface chemistry. Finally, perspectives of DFTB in the field of gold and silver surfaces and NPs are outlined.

Rashba Interaction and Local Magnetic Moments in a Graphene-BN Heterostructure Intercalated with Au

We intercalate a van der Waals heterostructure of graphene and hexagonal boron nitride with Au, by encapsulation, and show that the Au at the interface is two dimensional. Charge transfer upon current annealing indicates the redistribution of the Au and induces splitting of the graphene band structure. The effect of an in-plane magnetic field confirms that the splitting is due to spin splitting and that the spin polarization is in the plane, characteristic of a Rashba interaction with a magnitude of approximately 25 meV. Consistent with the presence of an intrinsic interfacial electric field we show that the splitting can be enhanced by an applied displacement field in dual gated samples. A giant negative magnetoresistance, up to 75%, and a field induced anomalous Hall effect at magnetic fields <1  T are observed. These demonstrate that the hybridized Au has a magnetic moment and suggests the proximity to the formation of a collective magnetic phase. These effects persist close to room temperature.

DFT insights into the cycloisomerization of ω-alkynylfuran catalyzed by planar gold clusters: mechanism and selectivity, as compared to Au(i)-catalysis

A detailed reaction mechanism of Au_3–10-catalyzed cycloisomerization of ω-alkynylfuran was systemically investigated at the TPSSh/def2-TZVP levels.

Pool-BCGA: a parallelised generation-free genetic algorithm for the ab initio global optimisation of nanoalloy clusters

The Birmingham cluster genetic algorithm is a package that performs global optimisations for homo- and bimetallic clusters based on either first principles methods or empirical potentials. Here, we present a new parallel implementation of the code which employs a pool strategy in order to eliminate sequential steps and significantly improve performance. The new approach meets all requirements of an evolutionary algorithm and contains the main features of the previous implementation. The performance of the pool genetic algorithm is tested using the Gupta potential for the global optimisation of the Au10Pd10 cluster, which demonstrates the high efficiency of the method. The new implementation is also used for the global optimisation of the Au10 and Au20 clusters directly at the density functional theory level.

Novel hexagonal polytypes of silver: growth, characterization and first-principles calculations

We report a study of the relative effects of dimensional and kinetic constraints on the stabilization of metastable, polytypic forms of metallic silver. We show that the hexagonal 4H polytype (hitherto observed only in size-constrained systems) can be produced in the form of bulk thin films by suitably slowing down the growth kinetics. Further, using extremely slow growth conditions, we have been successful in depositing a novel, two-dimensional, metastable polytype (2H) of silver, which is highly reactive (easily oxidized) and has a density 23% lower than normal silver. First-principles calculations based on density functional theory confirm that the 4H structure is relatively stable. However, local stability analysis via a determination of the phonon dispersion of the 2H structure reveals that it is only marginally stable with an energy surface that is rather flat or weakly varying with respect to many of the modes. This makes a large contribution to the configurational entropy and is probably the reason for the metastability of the observed 2H polytype with an unusually large lattice constant along the c-direction.

Hollow polyhedral structures in small gold-sulfide clusters

Using ab initio methods, we investigate the structural evolution of a family of gold-sulfide cluster anions (Au(m)S(n)(-)). We show that this family of clusters exhibits simple size-evolution rules and novel hollow polyhedron structures. The highly stable Au(m)S(n)(-) species such as Au(6)S(4)(-), Au(9)S(5)(-), Au(9)S(6)(-), Au(10)S(6)(-), Au(11)S(6)(-), Au(12)S(8)(-), and Au(13)S(8)(-) detected in the recent ion mobility mass spectrometry experiment of Au(25)(SCH(2)CH(2)Ph)(18) (Angel et al. ACS Nano2010, 4, 4691) are found to possess either quasi-tetrahedron, pyramidal, quasi-triangular prism, or quasi-cuboctahedron structures. The formation of these polyhedron structures are attributed to the high stability of the S-Au-S structural unit. A unique "edge-to-face" growth mechanism is proposed to understand the structural evolution of the small Au(m)S(n)(-) cluster. A 3:2 ratio rule of Au/S is suggested for the formation of a hollow polyhedron structure among small-sized Au(m)S(n) clusters.

Structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3) clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 non local hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Ru(n)Au(m)](0/+) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and compared to those of pure gold and ruthenium clusters. Assignments of all principal electronic transitions are given and interpreted in terms of contribution from specific molecular orbital excitations.

Potential energy surface and reactive collisions for the Au+H(2) system

A global potential energy surface is obtained for the ground state of the endoergic Au((2)S)+H(2)(X (1)Sigma(g) (+))-->AuH((1)Sigma(+))+H((2)S) reaction. The global potential is obtained by fitting highly correlated ab initio calculations on the system, using relativistic pseudopotential for the gold atom. Several electronic states are calculated correlating with Au((2)S)+H(2), Au((2)D)+H(2), and H(2), Au((2)P)+H(2) asymptotes. These states show several conical intersections and curve crossings along the minimum energy reaction path which are analyzed in detail. One of them gives rise to an insertion well in which there are important contributions from the Au((2)D) and Au((2)P) states of gold, which is interesting because it is analog to the deep chemisorption well appearing in larger gold clusters. Quantum wave packet and quasiclassical trajectory dynamical calculations performed for the reaction at zero total angular momentum are in good agreement, provided that a Gaussian binning method is used to account for the zero-point energy of products. Finally, integral and differential cross sections are calculated for the reaction with quasiclassical trajectories. Two different reaction mechanisms are found, one direct and the second indirect, in which the Au atom inserts in between the two hydrogen atoms because of the existence of the insertion well discussed above.