Study of atomic & Molecular Clusters and functional nano-materials using molecular dynamics (Softwares: LAMMPS, OVITO, VMD, Python)

In the Future research project, I will try to focus on search of more stable perovskites with better properties, i.e rapid rise in energy conversion efficiency for optoelectronic and photonic devices such solar cells, photo detectors, light emitting diode and field effect transistor. Our approach is to change various components in these compounds to search for a combination that is more stable and maintains high efficiency for energy conversion using first principles calculations based on density functional theory (DFT). In the current density functional theory, we will consider spin orbit effect and GW approximation for correct prediction of electronic band structure and band gap. The specific objectives are:

1. Obtain optimized stable structures for various configurations which contains different components.

2. To understand the valence band offset of the absorber by calculating electronic band structures and density of states.

3. To understand the optical properties.

4. To investigate frequency and temperature dependent dielectric properties.

5. To investigate the vibrational properties and phonon dispersion curves to find dynamical stability.

6. To calculate and model electronic transport properties.

7. To discuss and pass the first principles results to our experimental collaborators interested to build 2D hybrid organic –inorganic perovskites.

I have been mainly studying the structural and dynamical properties of low-dimensional quantum charged fluids, with particular emphasis on the role of the dynamical character of many-body correlation effects in the double quantum-well and double quantum-wire systems (at zero/finite temperature, with/without magnetic field). The study includes both the electron-electron and electron-hole bilayer systems. The theoretical procedure used is self-consistent generalized quantum/dynamic version of Singwi, Tosi, Land, and Sjolander (qSTLS) approach within the framework of linear response theory. We have calculated various static and dynamic properties of these systems and the results are found to be in good agreement with the computer simulation experiments, whenever available. We find that, in addition to the density of carriers, the lateral width of the wire provides an extra control on the strength of many-body correlations. This result could be of immense experimental use in fabricating the strongly correlated many-body system even at moderately high electron densities by adjusting the width of the confining potential. The possibility of a phase transition into a density modulated ground-state is also investigated for both the electron-electron and electron-hole double quantum-well and double quantum-wire systems, another unique and important feature of the qSTLS theory is that, in both the electron-electron and electron-hole double quantum well/wire systems, it exhibits an instability towards a coupled Wigner Crystal (WC) ground-state, below a critical density and in the close proximity of the double quantum-well/wire systems. Moreover, at densities higher than the critical density for the onset of the WC phase, it indicates a transition to a charge density wave (CDW) ground-state. Recently, we have also explored the feasibility of the existence of a spin- polarization transition in the symmetric electron-electron and electron-hole bilayers. Interestingly enough, a spin-polarization transition is found to take place in both the electron-electron and electron-hole bilayers from the unpolarized to polarized liquid well before the unpolarized liquid could actually make transition to the WC ground state. The polarized electron-electron and electron-hole bilayers systems too support the CDW and WC instabilities, but the crossover density is now lowered in comparison to their respective unpolarized counterparts.

Solar cells with high efficiency and long term stability at low cost have been a long standing dream. The realization of such practical goals depends on the proper architecture, process and key materials as solar cells are typically constructed from light harvesting multilayer heterostructures with electron and hole transporting layers as a major component. In this regard, a new solid state solar cell material “perovskite” has attracted great deal of attention. However, in a very short span of six years the efficiency of perovskite solar cell is reported 20.1%. This can be attributed to the effective integration of structure/ architecture deposition process and composition of perovskite materials. However, the water solubility which makes these materials easy to fabricate leads to their destruction. The lifetime of compounds in air range only up to a few weeks to months and to mere hours for the lead and tin based perovskite compound [6]. Therefore, apart from understanding its electronic, optical and dielectric properties, the current researches in this area mainly focus on the stability of these compounds and replacing lead (Pb) by other transition metals to avoid the environment and health concerns.

Electronic-structure calculation and quantum Monte Carlo(QMC) simulation:

• Most properties of solids and molecules are determined by the behavior of the electrons that bind their atoms together. The ability to make quantitative predictions about this behavior is therefore of great importance in a wide range of sciences, from solid-state physics to biochemistry. However, calculating the distribution and energies of electrons in materials—the electronic structure—is a nontrivial problem because of the need to simulate large numbers of strongly interacting particles.

• Quantum Monte Carlo (QMC) methods enable the calculation of the electronic structures of solids and molecules with unrivaled accuracy. The methods are stochastic, generating random sets of electron coordinates with the appropriate distribution. Useful quantities, such as energies, are recovered from these data using statistical methods. All my QMC calculations are carried out using the CASINO code, of which I am one of the collaborator of authors team.

Atomic Clusters and Functional Nano-materials:

• Quite a large number of atomic and molecular cluster series are investigated toward their fundamental understanding and novel applications. A number of exceptionally stable Aromatic and Jellium clusters, e.g. Be32-, Si4Mg3, Ga4Mg3, Be2S2, Mg2Se2, Cu4Sn4, Ag4Sn4 etc. are invented towards developing their assembled materials.

• Noncollinear magnetism for a series of iron and chromium clusters along with their anions and oxides are investigated and found to be useful for spintronics applications. Metal nitride and carbide (M3N and M2C2 ; M=Sc, Y) encapsulated fullerenes (C40, C80) and their boron analogues (B40, B80) are investigated, where boron analogues are found to be superior Lanthanide nitrides (L3N; L=Eu, Gd and Tb) encapsulated B80 and its Carbon analogue (C80) are investigated for their useful application as imaging contrast agents.

Nano-, 2D and Bulk Materials:

• Some of the assembled nano-, ultrathin 2D and bulk materials are developed from the identified cluster building units, e.g. CuSn (sc), AgSn (sc), 2D h-BeS and h-MgSe, bulk h-BeS and h-MgSe, etc. and lot more are currently being explored.

Single/Parallel Molecular Electronic Devices:

• Electronic transport through single or parallel DNA base pairs, e.g. Adenine-Thymine, Guanine-Cytosine etc. is explored and it is observed that transport enhances about hundred times in parallel architecture compared to their single base pair systems.

• Development of preferable inorganic and hybrid (organic/inorganic) single molecular electronic systems with respect to their prototypical organic analogues.

Quantum Toxicology:

• Density functional theory based quantum chemical descriptors are explored to explain biological activity and toxicity of various series of organic and inorganic compounds by developing Quantitative structure activity relationships (QSAR).

• The biological activity was studied for series of various human hormones and ACAT inhibitors, whereas toxicity was investigated for series of PCB, PHDF, PHDF, phenols, alcohols etc.

• The developed potential descriptors for biological activity and/or toxicity include Electrophilicity index (ω), Fukui function (fk), Charge transfer (∆N) etc.