Structural and Optical Properties of Nanostructures: Can Chirality Affect the Optical Response of Silicon Nanowires?

PI: Traian Dumitrica

There is a great need for major advances in alternative energy technologies. State-of-theart solar cells are made of multiple crystalline materials (Si, cadmium telluride, etc.) of different band gaps, which allow for efficiently absorbing the incident solar light at multiple wavelengths. The high cost associated with crystalline materials prohibits the widespread use of solar fuel generation. To bring down the cost, an intensely researched possibility is replacing the crystalline materials currently in use with nano-crystalline ones, which are much cheaper to grow and can conduct energy more efficiently.

Si nanodots and nanowires (NWs) are appealing choices not only because their nontoxicity and ideal interface compatibility with Si-based technology but also because the emergent optic response, which is qualitatively different from the unremarkable bulk one. For example, it is well known that 2-8 nm in diameter Si nanodots exhibit quantum confinement and photoluminescence. This phenomena does not occur in crystalline bulk, where the absorbed optical energy would be normally squandered in ways other than the release of photons. Among the many challenges in integrating Si nanomaterials in solar cells, the optical gap engineering is of paramount importance. An intensely researched direction is doping in order to control the electronic states around the last occupied first empty levels. However, introducing impurities in nanostructures proves to be a difficult task. A less explored possibility, which is in the focus of the previous and proposed Seed, is instead to modify the electronic confinement via symmetry changes.

Context and Proposed Research: We have previously embarked in a systematic study of the structural-optical properties relationship in Si nanostructures via parameter-free modeling of the fundamental interplay between the classical ionic and quantum-mechanical electronic subsystems. The key insight was that the gap and optical response could be tuned effectively in Si nanodots via symmetry changes. Encouraged by this result we are proposing to continue this theme in Si NWs.

Fig. 1. (a) Pristine and (b) screw dislocated Si NWs (side views) of 1.8 nm in diameter. The screw axis coincides with the NW's central axis. Note that the small Eshelby twist caused by axial glide with <110> Burgers vector b, changes translational symmetry and makes inapplicable standard atomistic treatments.