Research in our group involves exploring novel physics and chemistry at the nanoscale using theoretical tools. In particular we are interested in how properties (e.g. structural, electronic, vibrational, magnetic and chemical) change upon reducing size or lowering dimensionality (particularly in the nano-scale) and how the changes in these properties affect the phenomena associated with these low dimensional (e.g. nanowires, nanotubes, surfaces and clusters) materials. To address such issues we perform first principles calculations using quantum mechanical density functional theory (for ground state properties), density functional perturbation theory (for vibrational properties) and time dependent density functional theory (for excited state properties). Although, we are primarily a computational group, we collaborate with experimental groups both @ IISER Pune and outside.

Using the above methods, we try to achieve the following goals: (a) understand aspects of chemical bonding and microscopic couplings that are essential to the specific properties of materials, (b) obtain information about the atomistic structure and electronic states which are often hard and sometimes inaccessible to experiments and (c) design new materials and/or modify existing materials to yield materials with desired properties. We are primarily interested in materials with applications in heterogeneous catalysis, photovoltaic cells, and thermoelectrics.

Some examples of problems of recent interests are:

(1) Physics and chemistry of layered materials: Group IV layered materials like graphene, silicene and germanene have been of interest to researchers. They have unique properties like high carrier mobility, thermal conductivity, and mechanical strength that are not observed in the parent 3D structure. Although they have interesting properties which can be used for devices, these are zero band gap (semimetal) materials. However, for use in devices it is necessary to have a finite band gap to switch on and off the devices. Further in order to make devices they need to be put on a substrate which might alter their properties. In constrast hexagonal boron nitride, which is isoelectronic to graphene has a large band gap. In my group we are trying to understand the properties of these materials using first principle density functional theory based calculations. We aim to address the following questions:

(a) Can the band gap be opened while maintaining their properties? A common way to tune their properties is through functionalization by chemical means. We are looking into how the properties of these materials change when we functionalize them with H. Further we are also looking into defects in these materials. We are trying to find ways through which defects can be manufactured in these materials such that they give rise to certain desired properties. Additionally we are also investigating stacking of these materials into bilayers.

(b) How the properties of the pristine materials and their modified forms change when they they are grown on substrates? Presently we are investigating changes in the properties of these materials when they are put on ferromagnetic substrates. Further, we also see the tunability of their properties by modifying the substrate. A summary of our finding are given below. Please refer to J. Phys. Chem. Lett., 3, 2852 (2012), Phys. Rev. B, 86, 121411(R) (2012) and Phys. Rev. B, 87, 235440 (2013) for further details.

In addition to these monoatomic layerd materials, materials with 3-4 atomic layers thick are also interesting. Presently in collaboration with an experimental group in Italy we are looking into Mn-doped GaSe thin films.

(2) Defects in semiconductor nanostructures:

Semiconductors like PbS, CdS, CdSe, etc. are important candidates for applications in photovoltaics. When one moves from bulk to lower dimensions, for example nanowires and nanotubes, clusters, etc., the bulk properties change significantly and sometimes new properties are also observed. These change in their properties affect the functionalities of these materials. Further one can engineer or tune their band gap by forming core-shell nanoparticles of these material. Moreover, these materials are prone to defects that significantly affect their optical properties. The nature of the defects and the position of defect states in the nanostructures are also quite different than those in the bulk. In collaboration with Dr. Shouvik Datta's group we have investigated the role of defects and strain in the optical properties in CdS nanotubes and CdTe/CdS core-shell nanoparticles. For CdS nanotubes grown in S rich condition, we have shown that it is the Cd vacancies rather than the S interstitials (which are more abundant ones) that determine the optical properties. For details please refer to J. Phys. Chem. C, 118, 21604 (2014). A summary of our results are given below.

(3) Photocatalytic water splitting:

In collaboration with the experimental group of Dr. Nandini at the National Chemical Laboratory, we are presently looking into photocatalytic water splitting by ZnO nanostrucutres modified by conjugated organic molecules. Surface modification of ZnO nanoparticles is identified as a method of modulating surface sites advantageously. ZnO nanoparticles of two different sizes are surface modified with a conjugated organic moiety to enable electron conduction and transfer. Enhanced H2 evolution from water-methanol mixtures was observed in the composite systems compared to pristine ZnO under visible light irradiation without any cocatalyst. The system is also marginally active in water splitting in pure water without any sacrificial agents. Photophysical characterization indicates that even though reducing size into the nanoregime affects the band gap detrimentally, modifications by simple conjugated organic molecules assist in enhanced visible light activity. The experimental observations are corroborated with computational studies, which also point to a localization of valence band maximum of the interface on the organic moiety and conduction band minimum on ZnO. Details can be found in J. Phys. Chem. C, 119, 3060 (2015).

(4) Heterogeneous Catalysis:

Catalysts play an important role in several industrial processes. The catalytic process can be classified as homogeneous and heterogeneous. In the former case, both the reactants and the catalysts are in the same phase, while in the later, they are in different pahses. We are primarily interested in the following areas of heterogeneous catalysis: (I) Rational design of catalysis, (ii) tuning reactivity of catalysts by alloying; both surface and nano alloys, (iii) replacement of costly transition metal catalysts with cheaper ones and (iv) role of support in altering reactivity of catalysts. Presently we are looking into selective hydrogenation of acetylene to ethylene on intermetallic PdGa surfaces and sub-nano clusters.

(5) Computational Spectroscopy:

We use time dependent density functional theory and constrained density functional theory to calculate excited state properties of systems. For example presently we are studying the photophysics of ellipticine molecule, a probable anti-cancer drug, in presence of different solvents. Additionally we are looking into charge transfer excitations in polypeptides. Further we also compute core level shifts and near-edge X-ray absorption spectra to understand and/or interpret similar spectra obtained from experiments which are often difficult to interpret.

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