Experimental and computational simulations revealed that boron clusters, which favor planar (2D) structures up to 18 atoms, prefer 3D structures beginning at 20 atoms. Using global optimization methods, we found that the B20 neutral cluster has a double-ring tubular structure with a diameter of 5.2 Å. For the B 20 ؊ anion, the tubular structure is shown to be isoenergetic to 2D structures, which were observed and confirmed by photoelectron spectroscopy. The 2D-to-3D structural transition observed at B20, reminiscent of the ring-to-fullerene transition at C20 in carbon clusters, suggests it may be considered as the embryo of the thinnest single-walled boron nanotubes.photoelectron spectroscopy ͉ density functional calculation ͉ global minimum search S mall atomic clusters often exhibit structures and properties remarkably different from those of their bulk counterparts. For example, the most stable form of carbon is graphite, consisting of layers of two-dimensional (2D) graphene sheets. Yet small carbon clusters form chains, rings, and fullerenes (1-5). Boron, carbon's lighter neighbor, is also a strongly covalent material consisting of B 12 icosahedral cages (6-8). But small boron clusters were predicted to be planar (9-11), in stark contrast to the bulk three-dimensional (3D) cages. Planar boron clusters have been recently produced in the gas phase and experimentally confirmed up to B 15 (12)(13)(14). However, it is still unclear at what critical size the 2D-to-3D structural transition occurs. We show from concerted photoelectron spectroscopy (PES) and global geometry optimization theoretical studies (15-17) that the transition occurs at the size of 20 atoms. The B 20 neutral cluster is found to overwhelmingly favor a double-ring tubular-type structure over any 2D isomers, whereas in the anion the tubular and several 2D structures are close in energy. The 2D-to-3D transition at B 20 is reminiscent of the ring-to-cage transition at C 20 , which forms the smallest fullerene (5). The tubular B 20 is the smallest stable 3D boron cluster and can be viewed as the embryo of the thinnest boron nanotube, with a diameter of 5.2 Å. Methods PES.The experiments were carried out by using a magnetic-bottle time-of-flight PES apparatus equipped with a laser vaporization supersonic cluster source (15, 17). B n Ϫ cluster anions were produced by laser vaporization of a disk target made of enriched 10 B isotope (99.75%) in the presence of a helium carrier gas and were analyzed with a time-of-flight mass spectrometer. The B 20 Ϫ clusters were mass-selected and decelerated before irradiation by a photodetachment laser beam. Photoelectrons were collected at nearly 100% efficiency by the magnetic bottle and analyzed in a 3.5-m-long electron flight tube. The photoelectron spectra were calibrated by the known spectrum of Rh Ϫ , and the energy resolution of the apparatus was ⌬E k ͞E k ϳ 2.5%, i.e., 25 meV for 1-eV electrons. Effort was devoted to control the cluster temperatures (Fig. 4, which is published as supporting information on th...

The fullerenes are the first ''free-standing'' elemental hollow cages identified by spectroscopy experiments and synthesized in the bulk. Here, we report experimental and theoretical evidence of hollow cages consisting of pure metal atoms, Au n ؊ (n ‫؍‬ 16 -18); to our knowledge, free-standing metal hollow cages have not been previously detected in the laboratory. These hollow golden cages (''bucky gold'') have an average diameter >5.5 Å, which can easily accommodate one guest atom inside.anion photoelectron spectroscopy ͉ density functional calculation ͉ hollow gold cages ͉ lowest-energy clusters

We performed a global-minimum search for low-lying neutral clusters (Au(n)) in the size range of n=15-19 by means of basin-hopping method coupled with density functional theory calculation. Leading candidates for the lowest-energy clusters are identified, including four for Au(15), two for Au(16), three for Au(17), five for Au(18), and one for Au(19). For Au(15) and Au(16) we find that the shell-like flat-cage structures dominate the population of low-lying clusters, while for Au(17) and Au(18) spherical-like hollow-cage structures dominate the low-lying population. The transition from flat-cage to hollow-cage structure is at Au(17) for neutral gold clusters, in contrast to the anion counterparts for which the structural transition is at Au(16) (-) [S. Bulusu et al., Proc. Natl. Acad. Sci. U.S.A. 103, 8362 (2006)]. Moreover, the structural transition from hollow-cage to pyramidal structure occurs at Au(19). The lowest-energy hollow-cage structure of Au(17) (with C(2v) point-group symmetry) shows distinct stability, either in neutral or in anionic form. The distinct stability of the hollow-cage Au(17) calls for the possibility of synthesizing highly stable core/shell bimetallic clusters M@Au(17) (M=group I metal elements).