Synthesis of metal encapsulated Si clusters and its application to formation of interfacial nanostructures

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The objective of this research project is to investigate the possibility of a cage cluster composed of 12 Si atoms encapsulating a transition metal atom M, MSi_<12> as a building block of nanostructures on surfaces or interfaces of Si. We obtained the following achievements.1. We have developed a multi-pole ion trap for synthesis of clusters with a specific atomic composition. This trap consists of 36 pole electrodes arranged in a square lattice surrounded by a cage electrode, and has the ability to confine ions with a wide range of mass values and extract efficiently ions having a mass larger than a specific value.2. Our ab-initio calculations demonstrated that the structural stability of the MSi_<12> cluster originates in the efficient covalent bonding of the central M atom with the surrounding Si atoms accompanied with charge transfer.3. It has been shown by scanning tunneling microscopy and X-ray photoelectron spectroscopy that the reaction of SiH_4 molecules with Mo atoms deposite … More d on Si (111)7x7 surfaces yields MoSi_n clusters with a semiconducting band gap.4. We predict by first principles calculations that an atomically-thin layered compound, (MoSi_<12>)_n is a semiconductor with the energy band gap larger than 0.5 eV. This material is composed of a layer of Mo atoms sandwiched by two layers of Si atoms, and is characterized by the fact that the electronic structure can be tuned by replacing the Mo atoms with other transition metals.5. The charge transfer doping has been demonstrated for carbon-nanotube transistors and Si surfaces. This doping method utilizes the principle that the charge transfer is induced and electrons or holes are generated when solid surfaces are deposited with clusters or molecules having a sufficiently low ionization potential or a large electron affinity. We actually demonstrated the hole carrier generation using such clusters as TaF_6 and C_<60>F_<36> and analyzed the relation between the induced charge density and the energy levels of clusters. Less