The band gap bowing effect in oleic acid-stabilized CdS x Se 1−x alloy quantum dots (Q-dots) with varying composition has been studied experimentally by means of cyclic voltammetry and theoretically using density functional theory based calculations. Distinct cathodic and anodic peaks observed in the cyclic voltammograms of diffusing quantum dots alloy are attributed to the respective conduction and valence band edges. The quasi-particle gap values determined from voltammetric measurements are compared with interband transition peaks in UV−vis and PL spectra. Electronic structure for alloy Q-dots is determined computationally with projector augmented wave method for a particular size of dots. The band gap bowing is observed predominantly in the conduction band states. The bowing parameter determined experimentally (0.45 eV) has been found to be in good agreement with the one estimated from DFT (0.43 eV).

Computational detailsElectronic structure calculations based on density functional theory (DFT) [1] are carried out using accurate plane augmented wave (PAW) method [2] as implemented in Vienna Ab initio Simulations Package (VASP) [3]. The exchange-correlation energy functional as given by Perdew, Burke and Ernzerhof (PBE) [4] is used, since it is known to provide descent estimates for electronic properties. The valence electronic configurations used for Zn and S are 4s 2 3d 10

Variation in composition provides an additional degree of freedom in nanocrystals design.

To investigate energy gap bowing in homogeneously alloyed CdSSe 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 CdSSe (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 CdSSe QDs.

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.