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

The smallest triple ring tube-like gold clusters M@Au with M = Mo, W and = 1, 0, -1 are reported for the first time. Incorporation of an M dimer results in a remarkable modification of both atomic and electronic structures of the gold host. While the bare Au cluster exhibits a 3D cage shape, the doubly doped clusters M@Au 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 M unit placed inside the tube-like Au gold framework. The equilibrium geometry of both M@Au and M@Au is not much modified upon electron detachment from or attachment to their pure gold counterpart. The anion M@Au with 28 itinerant electrons establishes an electron shell configuration of 1S1P1D2S1F, 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 M@Au 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.