Structural Identification of Caged Vanadium Doped Silicon Clusters

Abstract

The geometry of cationic silicon clusters doped with vanadium, Si n V þ (n ¼ 12-16), is investigated by using infrared multiple photon dissociation of the corresponding rare gas complexes in combination with ab initio calculations. It is shown that the clusters are endohedral cages, and evidence is provided that Si 16 V þ is a fluxional system with a symmetric Frank-Kasper geometry. DOI: 10.1103/PhysRevLett.107.173401 PACS numbers: 36.40.Mr, 33.20.Ea, 61.46.Bc The ongoing trend towards further miniaturization in microelectronics triggers a quest for nanostructured building blocks. Given the importance of silicon in the semiconductor industry, it is straightforward to consider small silicon particles for nanostructuring. The search for silicon building blocks was initiated in the 1990s and revealed strong size dependencies and cluster geometries that do not correspond to bulk fragments [1]. Unfortunately, elemental silicon clusters have dangling bonds, which renders them chemically reactive and therefore not suitable as nanoscale building blocks [2]. In contrast to carbon, silicon prefers the formation of bonds through sp 3 hybridization, resulting in three-dimensional structures [3]. Ion mobility studies demonstrated that silicon structures follow a prolate growth sequence and no fullerenelike caged particles are formed [4,5]. However, incorporating a metal atom or hydrogenation can saturate the dangling bonds and induce the formation of caged silicon clusters [3,6,7].Theoretical investigations of doped silicon clusters have considered dopants from almost every group of the periodic table [6]. Most interestingly, it is predicted that transition-metal atoms possibly stabilize the clusters and induce the formation of symmetric endohedral cages with the dopant atom at the center of the cage, which is of relevance for novel silicon based nanostructured devices [8][9][10][11][12]. For example, fullerenelike cages and Frank-Kasper (FK) polyhedrons, which are tetrahedrally close-packed structures containing interpenetrating polyhedra with coordination numbers 12, 14, 15, or 16 [13], are predicted for Ti and V doped silicon clusters with at least 12 Si atoms [10,14,15]. However, if there are insufficient Si atoms to fully enclose the dopant atom, basketlike structures are formed [11]. Although mass spectrometry and photodissociation experiments [8,[16][17][18][19] show an enhanced stability of specific transition-metal doped silicon clusters, no single experiment has yet provided detailed information on their structure. Up to now, mainly indirect evidence is found for the formation of symmetric species by photoelectron and x-ray spectroscopy studies [20][21][22] and chemical probe methods [20,23].In this Letter, the structure of size selected endohedrally doped silicon clusters is obtained by combining experimental infrared multiple photon dissociation (IR-MPD) spectroscopy with quantum chemical calculations. It will be shown that the V dopant atom in the cationic Si n V þ (n ¼ 12-16) clusters locates at the center ...

Structural determination of niobium-doped silicon clusters by far-infrared spectroscopy and theory

“…Starting from n = 10, one finds new structural forms with the Nb atom embedded in the surface in a 4-fold (sometimes square) coordination, such structures are different from the V-doped silicon clusters, e.g., for which the Si 12 V + cluster is a distorted V-doped hexagonal prism. 26 In general, the Nb and V doped silicon clusters show similar structures, except for n = 10 and 12. Probably, the larger atomic radius of Nb becomes a relevant factor for the structures of the larger clusters.…”

“…The structure of Si 12 Nb + (iso4) is very different from that of Si 12 V + , which has been predicted to contain an endohedral V atom based on the absence of Ar complex formation, 52 and was later shown to have a hexagonal prism structure. 26 …”

“…[6][7][8][9][10][11][12] Different from carbon clusters that usually show sp 2 hybridization, bonding in pure silicon clusters occurs through sp 3 hybridization and thus they are more chemically reactive due to the existence of unsaturated dangling bonds on the clusters' surface, making them less suitable as nanoscale building blocks. 13 However, it has been found that silicon clusters can be stabilized by doping with transition metal (TM) atoms, [14][15][16] and by now a great number of studies, both experimentally [17][18][19][20][21][22][23][24][25][26][27][28][29] and theoretically, [30][31][32][33][34][35][36] have explored the structures and electronic properties of TM-doped silicon clusters for their potential use in silicon-based nanomaterials. In fact, doping silicon clusters with appropriate transition metal atoms [37][38][39][40][41] can reduce the number of dangling bonds on the cluster surface or even fully saturate them via pd hybridization, and thereby change the geometrical structures and chemical reactivities compared to pure silicon clusters.…”

“…[23][24][25][26][27][28][29] However, from most experimental studies detailed geometrical information cannot be directly deduced and comparison with theoretically predicted properties is required for structural assignments. For example, from experimental infrared spectra obtained using either IR multiple photon dissociation or IR-UV two-color ionization together with theoretical studies the structures for neutral Si n TM (TM = V, Mn, Co) 23,24 and cationic Si n TM + (TM = V, Mn, Cu, Ag, Au) [25][26][27][28][29] have been determined. The comparison with the experimental findings also allows to test the suitability of different theoretical approaches for predicting the ground state structures.…”

The Geometric Structure of Silver‐Doped Silicon Clusters

“…[16] One possible solution to overcome this deficiency is to add transition metal dopant atoms to the silicon clusters, which is known to induce the formation of stable and unreactive cage-like structures. [17][18][19][20][21][22] It is however not clear if coinage metal (Cu, Ag, and Au) dopants can induce cage formation for these silicides.…”