Synthesis and Characterization of Metal-Encapsulating Si₁₆ Cage Superatoms

Abstract

Nanoclusters, aggregates of several to hundreds of atoms, have been one of the central issues of nanomaterials sciences owing to their unique structures and properties, which could be found neither in nanoparticles with several nanometer diameters nor in organometallic complexes. Along with the chemical nature of each element, properties of nanoclusters change dramatically with size parameters, making nanoclusters strong potential candidates for future tailor-made materials; these nanoclusters are expected to have attractive properties such as redox activity, catalysis, and magnetism. Alloying of nanoclusters additionally gives designer functionality by fine control of their electronic structures in addition to size parameters. Among binary nanoclusters, binary cage superatoms (BCSs) composed of transition metal (M) encapsulating silicon cages, M@Si, have unique cage structures of 16 silicon atoms, which have not been found in elemental silicon nanoclusters, organosilicon compounds, and silicon based clathrates. The unique composition of these BCSs originates from the simultaneous satisfaction of geometric and electronic shell-closings in terms of cage geometry and valence electron filling, where a total of 68 valence electrons occupy the superatomic orbitals of (1S)(1P)(1D)(1F)(2S)(1G)(2P)(2D) for M = group 4 elements in neutral ground state. The most important issue for M@Si BCSs is fine-tuning of their characters by replacement of the central metal atoms, M, based on one-by-one adjustment of valence electron counts in the same structure framework of Si cage; the replacement of M yields a series of M@Si BCSs, based on their superatomic characteristics. So far, despite these unique features probed in the gas-phase molecular beam and predicted by quantum chemical calculations, M@Si have not yet been isolated. In this Account, we have focused on recent advances in synthesis and characterizations of M@Si BCSs (M = Ti and Ta). A series of M@Si BCSs (M = groups 3 to 5) was found in gas-phase molecular beam experiments by photoelectron spectroscopy and mass spectrometry: formation of halogen-, rare-gas-, and alkali-like superatoms was identified through one-by-one tuning of number of total valence electrons. Toward future functional materials in the solid state, we have developed an intensive, size-selected nanocluster source based on high-power impulse magnetron sputtering coupled with a mass spectrometer and a soft-landing apparatus. With scanning probe microscopy and photoelectron spectroscopy, the structure of surface-immobilized BCSs has been elucidated; BCSs can be dispersed in an isolated form using C fullerene decoration of the substrate. The intensive nanocluster source also enables the synthesis of BCSs in the 100-mg scale by coupling with a direct liquid-embedded trapping method into organic dispersants, enabling their structure characterization as a highly symmetric "metal-encapsulating tetrahedral silicon-cage" (METS) structure with Frank-Kasper geometry.

Structural growth pattern of neutral and negatively charged yttrium-doped silicon clusters YSin0/− (n=6–20): from linked to encapsulated structures

“…Nanoclusters have been one of the central issues of nanomaterial sciences because of their unique structures and properties. [7][8][9][10] Consequently, it is very important to probe the evolution of yttrium silicide aggregates in the transition from the molecular to the condensed phase.…”

“…11,12 Ren and co-workers 13 probed the geometries and electron properties of YSi n with n up to 16 by means of a relativistic density functional investigation and concluded that (i) the spin-orbit correction is small and negligible; (ii) the most stable structures keep analogous frameworks to the Si n+1 cluster for n ¼ 1-14, but encapsulated geometries starting from n ¼ 15. For anions, silicon clusters doped with yttrium atoms, YSi n À (n ¼ [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] were generated by a double-laser vaporization technique and their electronic and geometric stabilities were characterized with adsorption reactivity and photoelectron spectroscopy (PES). 2,3 In addition, the structure of the YSi n À clusters was deduced by a chemical probe method, 2,3 and it was found that the YSi n À clusters evolved from incomplete cage-type structures to cage-type structures from n ¼ 16 to n ¼ 20.…”

Revisit of large‐gap Si16 clusters encapsulating group‐IV metal atoms (Ti, Zr, Hf)

“…They were first predicted theoretically by Kumar and Kawazoe in 2001, and subsequently prepared in laboratory with laser ablation and magnetron cosputtering techniques by Nakajima's and others’ groups . Recently, Nakajima's group have synthesized the Ti@Si 16 and Ta@Si 16 clusters in a large scale (100 mg) by scaling up the clean dry‐process with a high‐power impulse magnetron sputtering . Moreover, the Ta@Si 16 clusters were deposited on highly oriented pyrolytic graphite or C 60 terminated surface, which exhibited excellent thermal and chemical stabilities; the latter system formed a heterojunction with spontaneous polarization and can serve as charge separation layers in nanoscale devices …”

Enhanced oxygen reduction activity of platinum subnanocluster catalysts through charge redistribution

“…Pt NC catalysts were prepared by soft-landing of size-selected NCs using a high-flux single-size NC generation system, nanojima s -NAP01, 31,32 combined with a quadrupole mass filter (Fig. S2, ESI †), where the collision energy of the NCs was kept under 1 eV per atom to avoid fragmentation during the deposition.…”