Tube-like Gold Clusters M2@Au17 q (M = W, Mo; q = 0, ±1): Structure, Electronic Property, and Optical Nonlinearity.

Density functional theory (DFT) calculations are carried out to determine the geometries and electronic and nonlinear optical (NLO) properties of the doubly doped gold clusters in three charge states M2 @ Au17 q with M = W, Mo and q = 0, ±1. At their lowest-lying equilibrium structures, the impurities that are vertically encapsulated inside a cylindrical gold framework, significantly enhance the stability and modify properties of the host. The presence of M2 units results in the formation of a tube-like ground state, which is identified for the first time for gold clusters. Having 30 itinerant electrons, the electron shell of M2@Au17 - can be described as 1S21P61D102S2{1F xz 2 21F yz 2 2}1F z 3 2{1F xyz 21F z(x 2-y 2) 2}{1F y(3x 2 -y 2),1F x(x 2 -3y 2)}. The species is thus stabilized upon doping, but it is not a magic cluster. The optical transitions are shifted to the lower-energy region upon doping Mo and W atoms into Au17 q . The static and dynamic NLO properties of M2@Au17 q are also computed and compared to those of the pure Au19 q (having the same number of atoms) and an external reference molecule, i.e., para-nitroaniline (p-NA). For hyperpolarizabilities, the doped clusters possess smaller values than those of their pure counterparts but much larger values than the p-NA. Of the doubly doped systems, the neutral M2@Au17 exhibits particularly high first and second hyperpolarizability tensors. The doped cluster units can also be used as building blocks for the design of gold-based nanowires with outstanding electronic and optical characteristics.

The smallest triple-ring tubular gold clusters M2@Au15 with M = Mo, W: stability, electronic properties and nonlinear optical response.

The smallest triple ring tube-like gold clusters M2@Au15q with M = Mo, W and q = 1, 0, -1 are reported for the first time. Incorporation of an M2 dimer results in a remarkable modification of both atomic and electronic structures of the gold host. While the bare Au15 cluster exhibits a 3D cage shape, the doubly doped clusters M2@Au15 in all charge states are found to prefer a tubular form composed of three five-membered Au rings in an anti-prism arrangement and stabilized by an M2 unit placed inside the tube-like Au15 gold framework. The equilibrium geometry of both M2@Au15 and M2@Au15- is not much modified upon electron detachment from or attachment to their pure gold counterpart. The anion M2@Au15- with 28 itinerant electrons establishes an electron shell configuration of 1S21P61D102S21F8, in which the 1F shell splits into four different sub-levels. These stable clusters are thus not magic. Computed results on the first and second hyper-polarizability parameters of the doped clusters show a strong dependence on the charge. Overall, the neutral M2@Au15 is found to exhibit a particularly strong nonlinear optical (NLO) response. These clusters can also be extended to 1D nanowires, providing helpful guidance for the design of novel gold-based nanowires with rich optoelectronic properties.