Theoretical study of the dependence of confinement energy on the size of Quantum dots (QDs) using quantum mechanical approach is presented. A simple model obtained for confinement energy is generally found to be in good agreement with the predicted inverse quadratic dependence on the dot size. Thus optical and electronic behavior of QDs can be engineered during manufacturing to meet specific applications. It is found that energy levels of the charge carriers within QDs are increased yielding to discrete energy states for electrons and holes. Therefore, QDs can emit and absorb light at specific wavelengths which are related to QD size. The plots for ground state confinement energy as a function of dots radius for CdSe, GaAs and ZnS QDs show monotonous decay curves. Thus as the dot radius increases, the ground state confinement energy decreases exponentially but never reaches zero. Thus, charge carriers in quantum dots possess non-zero minimum energy state in consistence with the infinite potential well. It can be observed that making the dot size large enough the effect of size on confinement energy is very small for different QDs. This is because as QDs grow larger their energy levels move closer and form a near continuum. In addition, among the QDs considered the degree of confinement on the CdSe and GaAs QDs strongly indicates that their optical wavelength can be extended to match the solar spectrum for multi-junction solar cells applications. This improves the solar to electricity conversion efficiency by harvesting multiple portions of solar spectrum.

Novel properties of nano-scale semiconductors based on the surface and quantum effects have been studied and applications identified. Spherical potential well model is used to study quantum effect whereas basic geometrical models are used for the surface effect. We have shown such effects to be the fundamental factors responsible for the novel nanosized semiconductor characteristics different from the bulk of same material. It is found that the surface area to volume ratio follows inverse power law. Thus at nanoscale, the surface to volume ratio increases significantly to enhance chemical reactivity. In addition, the increased surface area makes most nananocrystals highly soluble in liquid and dramatically lowers their melting temperature. The result strongly suggests also that the shape of the nanoparticles influences the surface area which has huge impact on their properties and performance. Our results of quantum size effect reveal that spatial confinement of charge carriers within semiconductor nanocrystals significantly modulates their properties such as size dependent absorption and emission spectra with non-zero discrete electronic transition energies as well as their blue shift band gaps. Thus by changing the size of the particle, we can literally fine-tune a material property of interest such as optical, electrical, and surface area. Specifically we found that InAs and InSb nano semiconductor optical absorption spectrum, in contrast to their bulk, can be tuned in broad range of UV to IR regions which are favorable operating wavelengths for nano photonic technology such as IR photo detectors and full spectrum solar cells applications.

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