The electronic structure of Si clusters strongly depends on their bond network. Various Si solids including amorphous Si and Si-cage compounds can be considered to be cluster solids not only from their geometries but also from the electronic structure since many important features of cluster electronic structure are conserved in their solid phases. Si clusters and their solids are scientifically interesting and technologically very important in terms of the systems.

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The structure of crystalline Si is the well-known diamond structure, in which each atom is tetrahedrally coordinated; however, small clusters of Si have unusual “compact structures” similar to close packing of shperes. This paper discusse how stability and structure of small Si clusters are affected by hydrogenation.It is shown by growth experiments of Si_nH_x+ ions in an ion trap and ab initio theoretical calculations that Si_nH_x+, has two different stable structures. One of them is the bulk fragment structure whose dangling bonds are terminated by H atoms, while the other is the compact structure with almost no hydrogen. These results verify that the bulk-like structure is stabilized when the dangling bonds are terminated.

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Electronic properties of small silicon clusters (n≤11), produced by a laser vaporization method, were investigated by photoelectron spectroscopy of their anions. The doping of F or Na atoms enables us to substract or add an electron from/into the silicon clusters without any serious geometric rearrangement, revealingelectronic structures of the silicon clusters. Both the doped F and Na atoms areattached onto the surface of the silicon clusters, which is in agreement with abinitio theoretical calculation on electronic and geometric structures of the Si-F and Si-Na binary clusters. The cluster size dependence of HOMO-LUMO gaps in the neutral silicon clusters were measured experimentally with the F atom doping, and the electronic structures above the LUMO of the neutral silicon clusters were revealed with the Na atom doping. The doping of C atoms, furthermore, induces some geometric change of the silicon clusters, and the substitution of Si atoms to C atoms results in a phase transition.

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Silicon clathrate compounds are Si analogs of carbon cluster C₆₀; those consist of polyhedral cages of Si₂₀ and Si₂₄, which are linked three dimensionally by sharing faces. The Barium containing silicon clathrate compound, Na_xBa_ySi₄₆, shows superconductivity below 4 K. This is the first superconductor with a sp³-Si network. Silicon clathrate compounds are prepared from Zintl phases containing [Si₄]^4- cluster anions. Another kind of Zintl phase, CaSi₂ contains corrugated Si layers with Ca atoms between them. A layer structured polysilane (SiH)_n was derived from CaSi₂ by removing Ca in a concentrated HCl solution at -30°C. The polysilane is a semiconductor with a band ga of about 2.5 eV.

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The discovery of superconducting transition in C₆₀ solid and Si clathrate solid has largely contributed to the development of the studies of cluster-solids. Both solids are packed with cluster of icosahedral symmetry, C₆₀ soccer-ball and Si₂₀ dodecahedron, respectively. Icosahedral boron solids are packed with B₁₂ icosahedral cluster and the only elemental semiconducting solids of group III element. They have topologically similar crystalline structure to those of C₆₀ solid and Si clathrate solid but form peculiar 3-center covalent bonds which may give peculiar electronic structures and properties to the boron solids. In this article we report the electronic-property investigation on metal-doped β-rhombohedral boron (β-rh. B) solid (most commonly known phase among the icosahedral boron solids) and present the feature of the electronic structure. We have investigated electrical conduction, magnetic susceptibility, nuclear magnetic resonance, electron energy loss spectroscopy, and ultra-violet photo-emission spectroscopy for Li-, Cu-, Ni-, and V-doped β-rh. B. Interestingly metal transition was absent even after several at.% of metal-doping, but we found the density of localized states at the Fermi energy to have a peak with respect to metal concentration. Based on the property, we discuss the possibility of filling localized intrinsic acceptor band originated from the uppermost bonding orbital of B₁₂ icosahedral cluster that is split by the Jahn-Teller distortion. In another form of icosahedral boron solids, however, α-rhombohedral boron was predicted to show metal transition after Li doping by band calculation.

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A high concentration of hole is generated without annealing, by high dose ion-implantation of boron into silicon substrate. A model has been proposed that B₁₂ clusters are generated in crystalline Si and act as double acceptors.Recently, we investigated the atomic and electronic structures for B₁₂ clusters by the ab initio calculation. It was found that the clusters capture the valence electrons, which results in the generation of unoccupied level in valence band of crystalline Si and the generation of holes. It was also found thatthe icosahedral B₁₂ in Si is more stable than the cubo-octahedral B₁₂.

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By means of a gel-growth method well habitted orthorhombic Ca-tartrate single crystals grow several millimeters in size. The dominant faces developed are {110} and {101}. It has been found that an extremely diluted HCI solution of HCI : H₂O=1: 500 is an excellent dislocation etchant for the Ca-tartrate crystals. With etch pits the crystallographic orientation and face indices are assigned easily, because their shapes and orientations are proper and characteristic to the individual faces. On the matched crystal faces cut by cleavage there occur surface structure revealing traces of modification due to dislocations. It has been found that the cleavage plane produces extended giant-step-line structures at passing a swarm of spiral dislocations. Some of them are over 300 nm high and can be observed easily by optical microscopy. This would provide an evi-dence that the lattice shift in the bulk due to spiral dislocation is released as a step structure on the free surface.

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For the purpose of confirming the growth mechanism of organic ultrathin films prepared by organic molecular beam deposition (OMBD), we employed a new combined system of total reflection X-ray diffraction method (TRXD) and OMBD. By using this system (TRXD-OMBD), the in-situ observation of cohesive structures and growth modes in organic ultrathin films were performed. In this study, the in-plane structure and orientation of fullerene (C60) ultrathin films epitaxially grown on oriented silver (Ag) surfaces were examined during the deposition. It was confirmed that the C60 ultrathin film formed the hexagonal closed packed (hcp) structure and the epitaxial structure of C60 on the Ag (111) surface occurred at the thickness of a few nanometer, namely the (2√3× 2√3) R30° structure as a stable state. Further, the growth modes of C60 ultrathin films were examined by the in-situ observation of total reflection fluorescence X-rays of Ag. In this paper, we noted the possibility of various analysis on organic ultrathin films and surfaces by our TRXD-ONBD system.

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We have constructed a scanning tunneling microscope operating in an ultra high vacuum at temperatures down to 2 K. We describe this from technical points of view.

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