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Silicon exhibits a large variety of different bulk phases, allotropes, and composite structures, such as, e.g., clathrates or nanostructures, at both higher and lower densities compared with diamond-like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, which are non-toxic and earth-abundant, with properties tailored precisely towards specific applications. We illustrate how such novel Si based materials either in the bulk or as nanostructures may be used to significantly improve the efficiency of solar energy conversion devices.

The deformation behavior and mechanical properties of a tilted Si nanorod array structure on Cu/Ti bilayer film coated Si substrate were studied for the first time by coupled atomic force microscopy and nanoindentation techniques. The individual Si nanorods fabricated by an oblique angle deposition technique are composed of many fine Si nanofibers with the diameter ranging from 10 to 50 nm. They are not brittle, but ductile. The ductile metallic Cu/Ti bilayer film roots contribute remarkably to the mechanical robustness of the Si nanorods. The toughening mechanism of such Si-based nanoanodes has been elucidated by experimental mechanics studies.

During the oblique angle co-deposition of Si and Cu, the nanoculumn tilting angle β changes systematically with the relative composition of Si and Cu when the vapor incident angle θ is fixed. This result demonstrates that the β-θ relationship does depend closely on the material property and composition, which is very different from the predictions of the well-known “universal” models, such as tangent or cosine rules. A statistic model based on the size, spacing, and stacking of Si and Cu atoms has been proposed and the predicted result agrees well with the experimental data.

We present a theoretical study of the thermal conductivity (κ) of amorphous silicon (a-Si) based on molecular and lattice dynamics. We find that the majority of heat carriers are quasi-stationary modes; however the small proportion (≃3%) of propagating vibrations contributes to about half of the value of κ. We show that in bulk samples the mean free path of several long-wavelength modes is on the order of microns; this value may be substantially decreased either in thin films or in systems with etched holes, resulting in a smaller thermal conductivity. Our results provide a unified explanation of several experiments and show that kinetic theory cannot be applied to describe thermal transport in a-Si at room temperature.

By assuming the H diffusion coefficient and H adsorption rate to be exponentially and linearly dependent on H concentration, a physical model is developed to predict the hydrogenation process of Mg nanoblades. The predicted H uptake curves agree well with the experimental data from V-coated Mg nanoblades. The obtained H diffusion coefficients in MgHx between Mg and MgH2 have nearly three orders of magnitude variation. The characteristic time of H surface adsorption is longer than that of H diffusion in Mg but shorter than that in MgH2 for 100 nm thick nanoblades. Thus, as it proceeds, the hydrogenation process gradually changes from surface reaction-limited to diffusion-limited. In both one- and two-dimensional simulations, it is shown that a hydride shell is not formed during hydrogenation. In contrast, a hydride core is formed during dehydrogenation. The strong (exponential) concentration dependence of H diffusion coefficient throws profound influence on the stability and instability of a diffusion front, i.e., a H diffusion front in hydrogenation, and a H-vacancy diffusion front in dehydrogenation. In the latter case, the front tends to corrugate forming islands when the H2 release rate is high.

The effect of underlayer thin films on the sensitivity of Ag nanorod surface-enhanced Raman scattering (SERS) substrates was studied both theoretically and experimentally. With the same Ag nanorod film, different materials (Ag, Al, Si, and Ti) with different thicknesses (25, 100, and 400 nm) were used as underlayers to alter the reflectivity systematically. The SERS intensity was found to increase linearly with the underlayer reflectivity, which can be explained by a modified Greenler’s model due to the contribution of reflected electric field from the substrate. This finding can be used to design high enhancement SERS substrates.

The hydrogenation of Mg film and Mg nanoblade array fabricated on Ti coated Si substrates has been studied and compared. The nanoblades start to absorb hydrogen at a temperature between 250 and 300°C, which is much lower than 350°C for Mg film. However, the saturated total hydrogen uptake in nanoblades is less than half of that in the film, resulting from MgO formation by air exposure. The nanoblade morphology with large surface area and small hydrogen diffusion length, and the catalytic effect of Ti layer, are two main reasons for the nanoblade hydrogenation behavior.

Using an oblique angle codeposition technique, 0–8at.% Ti has been doped into Mg and aligned nanostructure arrays, composed of hexagonal Mg single crystals with preferred orientations, are formed. Ti doping produces nanorod structures instead of the nanoblade structures that are formed from pure Mg deposition. This is due to the pinning effect of Ti for Mg adatom diffusion. There is a linear shift of the Mg diffraction peaks toward the higher 2θ angles with the Ti dopant concentration. This is possibly caused by the partial substitution of large Mg atoms by small Ti atoms.