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Saha S.,Visva Bharati University | Sarkar S.,Visva Bharati University | Pal S.,University of Gourbanga | Sarkar P.,Visva Bharati University
RSC Advances | Year: 2013

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. © 2013 The Royal Society of Chemistry. Source


Sarkar S.,Visva Bharati University | Saha S.,Visva Bharati University | Pal S.,University of Gourbanga | Sarkar P.,Visva Bharati University
RSC Advances | Year: 2014

The electronic structure of CdTe nanotube-fullerene nanocomposites has been explored. Within this context, the structural and electronic properties of isolated 1010 faceted CdTe nanotubes (NTs) with hexagonal cross-sections were first investigated using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. The possibility of band gap engineering of clean CdTe nanotubes is explored by varying either the size or wall thickness of the NTs. However, the efficient modification of the band gap can be attained by introducing the molecular states of fullerene into the band gap region of the CdTe NTs. The effects of the modulation of the band alignment through the variation in the wall thickness of the CdTe NTs on the electron injection rate from the NT to C60 in hybrid systems have been explored and we also found that the light harvesting efficiency of these nanohybrids can be maximized by increasing the concentration of the C60-thiol moieties. The position of the electronic energy levels offers information about the electronic structure of the hybrid systems such as whether it constructs type I or type II hetero-junctions, which bears key information for their application in photovoltaics. We also studied the electronic structure of CdTe-fullerene hybrid nanostructures with a series of fullerenes of different compositions. This journal is © the Partner Organisations 2014. Source


Saha S.,Visva Bharati University | Sarkar S.,Visva Bharati University | Pal S.,University of Gourbanga | Sarkar P.,Visva Bharati University
Journal of Physical Chemistry C | Year: 2013

By using the self-consistent charge density functional tight binding method we studied the electronic structure of ZnO/ZnS core/shell nanowire as a function of both core radius and shell thickness. By studying the band energy alignment, band structure, density of states, and band edge wave functions we envisage the efficacy of this particular nano heterostructure in dye sensitized solar cell. The strong localization of valence band maximum and conduction band minimum in ZnS shell and ZnO core, respectively, irrespective of core radius and shell thickness clearly indicates the spatial charge separation in this system. This spatial charge separation decreases the charge recombination rate thereby increasing the chance of better photovoltaic performance. We also investigated the electronic structure of anthraquinone (AQ) acid dye molecule-ZnO/ZnS nanowire composite system. We demonstrated that whether the composite system will form type I or type II band alignment that very much depends on the thickness of the ZnS shell and the nature of the functional group (electron withdrawing or electron donating) attached to AQ acid molecule. © 2013 American Chemical Society. Source


Sarkar S.,Visva Bharati University | Saha S.,Visva Bharati University | Pal S.,University of Gourbanga | Sarkar P.,Visva Bharati University
Journal of Physical Chemistry C | Year: 2012

Thiol-capped CdTe quantum dots (QDs) have recently become the subject of intense investigation because of their diverse applications ranging from optoelectronic devices to the fabrication of solar cells. To achieve the desired functionalities, one must have clear understanding of the electronic structure of thiol-capped CdTe nanocrystals which is still lacking in the literature. In this paper, we have explored the electronic structure of relatively large thiol-capped CdTe QDs by taking advantage of the efficacy of the SCC-DFTB method. Our emphasis will be on the stability, charge transfer, density of states, and HOMO-LUMO gap of the dot as a function of both size and morphologies. We also studied the electronic structure of CdTeQD-carbon nanotube (CNT) nanocomposites. The effects of modulation of the band alignment through the variation in size of CdTe QDs on the electron injection rate from CdTe QD to CNT in CdTeQD-CNT nanohybrids have been explored. © 2012 American Chemical Society. Source

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