Silicon Clathrate Quantum Dots and the Allotropic Dependence of Quantum Confinement

Clathrate quantum dots have yet to be synthesized but represent an entirely new direction harnessing the promise of quantum dot assemblies. Of particular interest is the idea that clathrate nanocrystals may be able to carry out multiple-exciton generation for a slice of the solar spectrum that makes them more technologically relevant than their diamond silicon counterparts. Nine types of silicon clathrate nanocrystals are computationally characterized and compared. The relationship between crystal structure and quantum confinement is quantified by analyzing sets of quantum dots associated with each clathrate crystal type. The computational results allow a relationship to be constructed between dot size and energy gap that gives a linear correlation between quantum confinement sensitivity and bulk crystal effective mass. All clathrates are found to have a confinement sensitivity less than that of diamond silicon. Bulk properties (gap and effective mass) can therefore be used to identify clathrate semiconductors with promising optoelectronic properties. For example, the combination of low bulk band gap and relatively low confinement sensitivity causes quantum dots constructed from type VII clathrate to have energy gaps smaller than those constructed from diamond silicon, making type VII worthy of consideration for efficient multiple exciton generation and other optoelectronic applications.