Theoretical investigation of energy gap bowing in CdSxSe1-x alloy quantum dots.

To investigate energy gap bowing in homogeneously alloyed CdSxSe1-x quantum dots (QDs) and to understand whether it is different from bulk, we perform density functional theory based electronic structure calculations for spherical QDs of different compositions x (0 ≤ x ≤ 1) and of varying sizes (2.2 to 4.6 nm). We find the bowing constant to be slightly higher than in bulk for different sizes of quantum dots. The change in the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of QDs mainly arises due to the change in the LUMO energies. Upon comparison, the highest occupied molecular orbital (HOMO) energies remain almost the same. This observation is in contrast to the results for bulk CdSxSe1-x (J. Appl. Phys., 2000, 87, 1304). We identify the change in the lattice constant on alloying as the main factor affecting the hybridization of the anion-cation state, which in turn results in bowing of the HOMO-LUMO gap. To understand the shape dependence of the band gap, we perform electronic structure calculations for pyramid-shaped and cubic QDs of different compositions and of two different sizes. The study of l-decomposed partial charge density and Bader charge analysis is useful to understand the difference in the nature of bonding with changing size and composition. The results presented will assist in experiments and hence can lead to the possible applications of CdSxSe1-x QDs.

Probing Absolute Electronic Energy Levels in Hg-Doped CdTe Semiconductor Nanocrystals by Electrochemistry and Density Functional Theory.

The absolute electronic energy levels in Hg-doped CdTe semiconductor nanocrystals (CdHgTe NCs) with varying sizes/volumes and Hg contents are determined by using cyclic voltammetry (CV) measurements and density functional theory (DFT) -based calculations. The electrochemical measurements demonstrate several distinct characteristic features in the form of oxidation and reduction peaks in the voltammograms, where the peak positions are dependent on the volume of CdHgTe NCs as well as on their composition. The estimated absolute electronic energy levels for three different volumes, namely 22, 119 and 187 nm(3) with 2.7±0.3 % of Hg content, show strong volume dependence. The volume-dependent shift in the characteristic reduction and oxidation peak potential scan can be attributed to the alteration in the energetic band positions owing to the quantum confinement effect. Moreover, the composition (Cd/Hg=98.3/1.7 and 97.0/3.0) -dependent alteration in the electronic energy levels of CdHgTe NCs for two different samples with similar volumes (ca. 124±5 nm(3) ) are shown. Thus obtained electronic energy level values of CdHgTe NCs as a function of volume and composition demonstrate good congruence with the corresponding absorption and emission spectral data, as well as with DFT-based calculations. DFT calculations reveal that incorporation of Hg into CdTe NCs mostly affects the energy levels of conduction band edge, whereas the valence band edge remains almost unaltered.