Sunandan Sarkar

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.

Theoretical prediction of a new two-dimensional carbon allotrope and NDR behaviour of its one-dimensional derivatives

By using state of the art theoretical methods we have predicted a new two-dimensional (2-D) carbon allotrope. This new planar carbon framework is made of hexagons, octagons and pentagons and hence named as HOP graphene (HOPG). The possibility of existence of HOPG is evident from its dynamical stability as confirmed by phonon-mode analysis and also from an energetic point of view since it is energetically more favorable than recently synthesized graphdiyne. The band structure shows the metallic behaviour of this new form of carbon allotrope. We also explored the electronic structure and transport properties of a 1-D derivative (nanoribbon) of HOPG. Most of the nanoribbons exhibit multiple negative differential resistance (NDR) behaviour with high peak to valley ratio.

Band gap engineering of graphene–CdTe quantum dot hybrid nanostructures

We report the results of our theoretical studies on the electronic structure of graphene–CdTe quantum dot (QD) hybrid nanostructures. We put emphasis on the possibility of engineering the electronic energy levels of hybrid systems either through the variation of the size of the CdTeQD or by controlling the H coverage of graphene. We also extend our study to see the effect of Se doping on the electronic energy levels of graphene–CdTeQD hybrid nanostructures. It is well known that for solar cell applications the composite system should have a type-II band alignment that hinders the recombination of charge carriers, thereby improving the photovoltaic performance. By analyzing the electronic energy levels of the composite systems we have shown that one can engineer the band gap of the system by controlling either the size of the QD or percentage of the hydrogen atoms on graphene to achieve the type-II band alignment. Based on the relative positions of the frontier energy levels we offer qualitative understanding of the dynamics of electron transfer from QDs to the graphene and also the dynamics of recombination of the charge carriers.

Probing the spectral response of a new class of bioactive pyrazoline derivative in homogeneous solvents and cyclodextrin nanocavities: a spectroscopic exploration appended by quantum chemical calculations and molecular docking analysis

Time-dependent density functional theory (TD-DFT) calculations and molecular docking analysis provide valuable insights, in addition to experimental evidence, on the newly synthesized pyrazoline derivative 5-(1′-(4-bromophenyl)-3a′,4′,5′,6′,6a′-hexahydrocyclopentapyrazoline)-3-methyl-1-phenyl-1H-pyrazole-4-carbonitrile (PZ) as a fluorescence recognition probe and its photophysical signature in homogeneous and heterogeneous cyclodextrin (CD) environments, in addition to its insertion mechanism inside CD. The spectral trends of PZ do not appear to originate only from changes to the solvent polarity, but also indicate hydrogen bonding interactions with a homogeneous medium. The encapsulation of PZ within supramolecular α-, β- and γ-CD hosts was investigated using fluorescence spectroscopic techniques. The results show the formation of both 1 : 1 and 1 : 2 PZ–CD inclusion complexes with β-CD and only a 1 : 1 complex with γ-CD. The measured lifetimes and steady state anisotropy values also show the same trend and reveal the mode of interaction of the probe with the CD moiety. Furthermore, molecular docking studies performed via molecular mechanics methods (MMC) indicate that the pyrazoline moiety of PZ is most likely oriented towards the dip inside the cyclodextrin cavity. Solvent-dependent spectral data using TD-DFT calculations on the optimized ground and excited state structures of PZ were found to correlate well with the experimental findings.

Electronic structure and band gap engineering of CdTe semiconductor nanowires

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.

Ligand mediated tuning of the electronic energy levels of ZnO nanoparticles

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.

Energetics and electronic structure of encapsulated graphene nanoribbons in carbon nanotube.

We report results of our total energy electronic structure calculation of encapsulation of graphene nanoribbon (GNR) in the carbon nanotube (CNT). The encapsulation of both coronene and perylene based graphene nanoribbons in zigzag (n,0) carbon nanotubes (where n ranges from 14 to 18 for perylene based nanoribbon and from 16 to 20 for coronene based nanoribbons) is an exothermic process. Our study shows that in certain cases arm-chair GNR (aGNR) encapsulated CNT results in type II band alignment and may be useful in the application in solar cells. We have also studied the potential of this composites for hydrogen storage. We found that the encapsulated GNR composite systems have higher hydrogen adsorption energies than the individual components of either GNR and CNT. The hydrogen molecules oriented perpendicular to GNR are found to be more stable as compared to hydrogen molecules parallel to GNR.

Electronic structure and transport properties of sulfur-passivated graphene nanoribbons

Electronic structure of newly synthesized sulfur-terminated graphene nanoribbons (S-GNRs) has been presented from the calculations based on ab initio density functional theory and non-equilibrium Greens function (NEGF) method. The calculations reveal that zigzag-edged S-GNRs (Z-S-GNRs) are thermodynamically more stable than armchair edged S-GNRs (A-S-GNRs). It has been observed that the band gap of S-GNRs depends both on ribbon width and edge symmetry. The calculated band gap, in case of A-S-GNRs, is also supported by the presence of threshold bias in the I-V characteristics obtained from NEGF formalism. It is shown that all A-S-GNRs having width up to 1.50 nm are semiconducting but the Z-S-GNRs of similar widths are metallic. For A-S-GNRs, the width dependent band-gap hierarchy follows three different trends which seem to be different from that of H-passivated GNRs. The band-gaps for A-S-GNRs arise from both quantum confinement as well as crucial effect of edge, where the passivating S atoms play an imp...

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

The electronic structure of CdTe nanotube–fullerene nanocomposites has been explored. Within this context, the structural and electronic properties of isolated 100 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.

Understanding the electronic structure of CdSe quantum dot-fullerene (C60) hybrid nanostructure for photovoltaic applications

By using the density-functional tight binding method, we studied the electronic structure of CdSe quantum dot(QD)-buckminsterfullerene (C60) hybrid systems as a function of both the size of the QD and concentration of the fullerene molecule. Our calculation reveals that the lowest unoccupied molecular orbital energy level of the hybrid CdSeQD-C60 systems lies on the fullerene moiety, whereas the highest occupied molecular orbital (HOMO) energy level lies either on the QD or the fullerene depending on size of the CdSe QD. We explored the possibility of engineering the energy level alignment by varying the size of the CdSe QD. With increase in size of the QD, the HOMO level is shifted upward and crosses the HOMO level of the C60-thiol molecule resulting transition from the type-I to type-II band energy alignment. The density of states and charge density plot support these types of band gap engineering of the CdSe-C60 hybrid systems. This type II band alignment indicates the possibility of application of thi...