From first-principles calculations, we predict that specific transition metal (TM) atom-adsorbed silicon nanowires have a half-metallic ground state. They are insulators for one spin direction, but show metallic properties for the opposite spin direction. At high coverage of TM atoms, ferromagnetic silicon nanowires become metallic for both spin directions with high magnetic moment and may have also significant spin polarization at the Fermi level. The spin-dependent electronic properties can be engineered by changing the type of adsorbed TM atoms, as well as the diameter of the nanowire. Present results are not only of scientific interest, but also can initiate new research on spintronic applications of silicon nanowires.

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The adsorption of hydrogen molecules on the titanium metallocarbohedryne (met-car) cluster has been investigated by using the first-principles plane wave method. We have found that, while a single Ti atom at the corner can bind up to three hydrogen molecules, a single Ti atom on the surface of the cluster can bind only one hydrogen molecule. Accordingly, a Ti 8 C 12 met-car can bind up to 16 H 2 molecules and hence can be considered as a high-capacity hydrogen storage medium. Strong interaction between two met-car clusters leading to the dimer formation can affect H 2 storage capacity slightly. Increasing the storage capacity by directly inserting H 2 into the met-car or by functionalizing it with an Na atom have been explored. It is found that the insertion of neither an H 2 molecule nor an Na atom could further promote the H 2 storage capacity of a Ti 8 C 12 cluster. We have also tested the stability of the H 2 -adsorbed Ti 8 C 12 metcar with ab initio molecular dynamics calculations which have been carried out at room temperature.

This paper investigates atomic structure, mechanical, electronic, and magnetic properties of silicon nanowires ͑SiNW͒ using first-principles plane-wave calculations within density-functional theory. We considered bare, hydrogen-terminated, and 3d-transition metal ͑TM͒ adsorbed SiNWs oriented along ͓001͔ direction. We also studied Cr interstitial impurity. Nanowires of different sizes are initially cut from the bulk Si crystal in rodlike forms, and subsequently their atomic structures are relaxed before and also after the termination of surface dangling bonds by hydrogen atoms. We first presented an extensive analysis of the atomic structure, stability, elastic, and electronic properties of bare and hydrogen-terminated SiNWs. The energetics of adsorption and resulting electronic and magnetic properties are examined for different levels of 3d-TM atom coverage. Adsorption of TM atoms generally results in magnetic ground state. The net magnetic moment increases with increasing coverage. While specific SiNWs acquire half-metallic behavior at low coverage, at high coverage ferromagnetic nanowires become metallic for both spin directions, and some of them have very high spin polarization at the Fermi level. Our results suggest that the electronic and spintronic devices with conducting interconnects between them can be fabricated on a single SiNW at a desired order. We believe that our study will initiate new research on spintronic applications of SiNWs.