Sougata Pal

Size-dependent properties of ZnmSn clusters: A density-functional tight-binding study

We present the results of our theoretical calculations on structural and electronic properties of ligand-free Zn(n)S(n) [with n ranging from 4 to 104 (0.8-2.0-nm diameter)] clusters as a function of size of the clusters. We have optimized the structure whereby our initial structures are spherical parts of either zinc-blende or wurtzite structure. We have also considered some hollow bubblelike structures. The calculations are performed by using a parametrized linear combination of atomic orbitals-density-functional theory-local-density approximation-tight-binding method. We have focused on the variation of radial distribution function, Mulliken populations, electronic energy levels, band gap, and stability as a function of size for both zinc-blende and wurtzite-derived ZnS clusters. We have also reported the results of some nonstoichiometric Zn(m)S(n) (with m+n=47, 99, 177) clusters of zinc-blende modification.

Self-Consistent-Charge Density-Functional Tight-Binding Parameters for Cd-X (X = S, Se, Te) Compounds and Their Interaction with H, O, C, and N.

Parameters for CdX, SeX, and TeX (X = H, C, N, O, S, Se, Te, and Cd) have been generated within the self-consistent-charge density-functional tight-binding (SCC-DFTB) framework. The approach has been tested against ab initio density-functional theory calculations for the relevant bulk phases, surfaces, nanowires, and small molecular systems. The SCC-DFTB approach reproduces structural, electronic, and energetic properties very well, demonstrating that the developed parameters are fully transferable among different chemical environments.

Nonadiabatic Molecular Dynamics for Thousand Atom Systems: A Tight-Binding Approach toward PYXAID

Excited state dynamics at the nanoscale requires treatment of systems involving hundreds and thousands of atoms. In the majority of cases, depending on the process under investigation, the electronic structure component of the calculation constitutes the computation bottleneck. We developed an efficient approach for simulating nonadiabatic molecular dynamics (NA-MD) of large systems in the framework of the self-consistent charge density functional tight binding (SCC-DFTB) method. SCC-DFTB is combined with the fewest switches surface hopping (FSSH) and decoherence induced surface hopping (DISH) techniques for NA-MD. The approach is implemented within the Python extension for the ab initio dynamics (PYXAID) simulation package, which is an open source NA-MD program designed to handle nanoscale materials. The accuracy of the developed approach is tested with ab initio DFT and experimental data, by considering intraband electron and hole relaxation, and nonradiative electron-hole recombination in a CdSe quantum dot and the (10,5) semiconducting carbon nanotube. The technique is capable of treating accurately and efficiently excitation dynamics in large, realistic nanoscale materials, employing modest computational resources.

Theoretical study of structural, electronic, and optical properties of Zn m Se n clusters

We present density-functional theoretical results of structural, electronic, and optical properties of ligand-free

A complete set of self‐consistent charge density‐functional tight‐binding parametrization of zinc chalcogenides (ZnX; X=O, S, Se, and Te)

{\mathrm{Zn}}_{m}{\mathrm{Se}}_{n}

A search for lowest energy structures of ZnS quantum dots: Genetic algorithm tight-binding study

clusters as a function of size of the cluster. We have optimized the structure, whereby our initial structures are spherical parts of either zinc blende or wurtzite crystal structure and we have studied systems up to almost 200 atoms. The calculations were performed by using density-functional tight-binding (DFTB) method. The results include the radial distribution of atoms and of Mulliken populations, the electronic energy levels (in particular the highest occupied molecular orbital and lowest unoccupied molecular orbital), the band gap and the stability as a function of size and composition. In addition to structural and electronic properties we present electronic excitation spectra obtained from time-dependent density functional response theory.

Electronic structure and band gap engineering of CdTe semiconductor nanowires

We have developed a complete set of self‐consistent charge density‐functional tight‐binding parameters for ZnX (X = Zn, O, S, Se, Te, Cd, H, C, and N). The transferability of the derived parameters has been tested against Pseudo Potential‐Perdew, Burke and Ernzerhof (PP‐PBE) calculations and experimental values (whenever available) for corresponding bulk systems (e.g., hexagonal close packing, zinc‐blende, and wurtzite(wz)), various kinds of nanostructures (such as nanowires, surfaces, and nanoclusters), and also some small molecular systems. Our results show that the derived parameters reproduce the structural and energetic properties of the above‐mentioned systems very well. With the derived parameter set, one can study zinc‐chalcogenide nanostructures of relatively large size which was otherwise prohibited by other methods. The Zn‐Cd parametrization developed in this article will help in studying large semiconductor hetero‐nanostructures of Zn and Cd chalcogenides such as ZnX/CdX core/shell nanoparticles, nanotubes, nanowires, and nanoalloys.

Ligand mediated tuning of the electronic energy levels of ZnO nanoparticles

The lowest energy structures of ZnS quantum dots of different sizes have been determined by an unbiased search using genetic algorithm (GA) coupled with the density-functional tight-binding method. The GA search converges to a rather new ringlike configurations of ZnS quantum dots. We have studied the structural, electronic, and optical properties of these ringlike clusters and compared these properties with those of other reported structures of ZnS quantum dots, namely, hollow, zinc-blende, wurtzite, and rocksalt structures.

Sub-Picosecond Auger-Mediated Hole-Trapping Dynamics in Colloidal CdSe/CdS Core/Shell Nanoplatelets

The structural and electronic properties of (100) faceted CdTe nanowires with hexagonal or triangular cross sections were investigated using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. The formation energies and band gap of CdTe nanowires are studied as a function of both nanowire size and surface atom ratio. The atomic relaxations of the surface of the (100) CdTe nanowires are compared with the corresponding (100) CdTe surface. The surface strain was eliminated by passivating the dangling bonds with hydrogen atoms. The passivation of the dangling bonds has only little influence on the band gap resulting only in an increase of about 0.06 eV as compared to unpassivated nanowires. However, it had a significant influence on the highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO). We also investigated the effect of the adsorption of dicarboxylic acid derivatives on the (100) surface of the hexagonal unpassivated CdTe nanowire with a goal to engineer the band gap. From the band alignment we conclude that the hybrid systems NW-DCDC (di-cyano di-carboxylic acid) and NW-DNDC (di-nitro di-carboxylic acid) represent a type II surface characterized by the presence of molecular states in the gap which reduce the optical gap and may be suitable for use in nanowire-dye sensitized solar cells.

Electronic structure and bandgap engineering of CdTe nanotubes and designing the CdTe nanotube–fullerene hybrid nanostructures for photovoltaic applications

The surface capping of nanoparticles is one of the important ways through which one can alter electronic energy levels and hence enable the development of novel nanostructures with desired properties and specific applications. By using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method we envisage the role of the ligand in engineering the electronic structure of ZnO nanoparticles. Significant differences are observed in the electronic structure of ZnO nanoparticles because of the variation of the nanoparticle–ligand bonding interactions. We found that –OH passivated ZnO quantum dots (QDs) are the most stable, followed by –NH2 passivated QDs, and –SH passivated QDs are the least stable. The study of the HOMO–LUMO gap and excitation spectra show that there is a clear blue shift in the absorption spectra of the QDs as compared to bare ones and the extent of the blue shift sensitively depends on the nature of the passivating ligands. The maximum blue shift occurs in –OH passivated QDs.