Optical measurements on ZnS nanoclusters have been carried out to investigate surface effects along with quantum size effects. ZnS nanocrystals have been synthesized in the range of 1.5–2.5 nm, using different chemical methods as well as electronic passivating procedures. The size of nanoparticles has been estimated from empirical pseudopotential calculations. We have obtained a significantly narrower size distribution of ZnS nanocrystals than reported in earlier published results. We observed band gap luminescence in mercaptoethanol capped ZnS nanocrystals. Effects of various defect levels on the luminescent behavior of ZnS nanoparticles have been examined
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
We present results of a study of small stoichiometric CdnT en (1≤n≤6) clusters and few medium sized non-stoichiometric CdmT en [(m, n = 13, 16, 19); (m =n)] clusters using the Density Functional formalism and projector augmented wave method within the generalized gradient approximation. Structural properties viz. geometry, bond length, symmetry and electronic properties like HOMO-LUMO gap, binding energy, ionization potential and nature of bonding etc. have been analyzed. Medium sized non-stoichiometric clusters were considered as fragments of the bulk with T d symmetry. It was observed that upon relaxation, the symmetry changes for the Cd-rich clusters whereas the Te-rich clusters retain their symmetry. The Cd-rich clusters develop a HOMO-LUMO gap due to relaxation whereas there is no change in the HOMO-LUMO gap of the Te-rich clusters. Thus, the symmetry of a cluster seems to be an important factor in determining the HOMO-LUMO gap.
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