Rectifiers formed between Organic and Inorganic Semiconductors: Characterization by Scanning Tunneling Spectroscopy

Rectifying junctions are fundamental building blocks for basic electronics. In traditional rectifiers based on inorganic semiconductors, directionality of current flow arises due to the depletion layer or potential barrier developed at the junction. In organic rectifiers, molecular orbitals, namely lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO), enforce directional flow of carriers. Hence assemblies of organic molecules as well as junctions between two nanostructures are of utmost importance for both molecular and nanoscale electronic research. In forming donor/acceptor assemblies that act as molecular rectifiers, we have introduced magnetic organic molecules as electron-donating and electron-accepting moieties. We have characterized the molecular assemblies formed on an electrode with a scanning tunneling microscope tip. Such donor/acceptor assemblies with a control over the orientation of moments of the components provided unique systems to study the effect of the nature of alignment on molecular rectifiers. We have observed that the rectification ratio increased in junctions with moments of the components being parallel to each other. We have formed pn- and np-junctions between monolayers of p- and n-type nanocrystals that exhibit current rectification in the nanodiodes when characterized with a scanning tunneling microscope (STM) tip. With the use of ferromagnetic nanocrystals, we study the effect of mutual alignment of magnetization vectors on current rectification in the junction between the two nanocrystals. We show that when the magnetization vectors of the p- and of the n-type nanocrystals are parallel to each other (and both face toward the apex of the STM tip), tunneling current in both bias modes increases with correspondingly a higher rectification ratio. We have also formed hybrid nanodiodes with magnetic organic molecules and diluted magnetic semiconductors. The rectification ratio was enhanced when the magnetic moment of the component materials align along a particular direction with the application of magnetic field. The current as well as rectification ratio became enhanced due to the flow of spin polarized electron flow along a particular direction. We have mapped band-edges across pn-junction that was formed as an heterostructure in a single nanorod or in a heterodimers system in an ultrahigh vacuum scanning tunneling microscope (UHVSTM) at 77 K. From scanning tunneling spectroscopy and correspondingly the density of states (DOS) spectra, we determined the conduction and valence band-edges at different points across the junction and also the individual materials. We could map the band-diagram of the heterostructure junctions to bring out the salient features of a diode, such as p- and n-sections, band-bending, depletion region in the nanoscale. The width (of the depletion region) and the energy-offset at the interface depended on the size of the semiconductors.Research was carried out under the supervision of Prof. A J Pal of Solid State Physics division under SPS [School of Physical Sciences]Research was conducted under CSIR fellowshi

STUDIES ON MAGNETIC AND DIELECTRIC PROPERTIES OF OXIDES AND NANOCOMPOSITES WITH MESOPOROUS STRUCTURE

The present thesis deals with the study of dielectric, ferroelectric and magnetic property of oxides and nanocomposites with mesoporous structure. The synthesis, characterization and the study of different physical properties of the mesoporous oxides and nanocomposites are the principal objectives of this thesis. To explore the field is not only of immense interest to the scientists, but also has potential for wide applicability. The materials have been synthesized using the soft chemical route. The characterization and detailed analysis of the observed physical properties have been discussed in the different chapters.The research was carried out under the supervision of Prof. Dipankar Chakraborty of MLS and Prof. A. Bhaumik of Materials Science division under SMS [School of Materials Science]The research was conducted under CSIR research fellowship and also grant from Indo-Australian project supported by Department of Science and Technology, New Delhi and Nano Science and Technology Initiative programme of the Department of Science and Technology, New Delhi for providing instrumental facilities

Study of Stochastic Processes in Confined Geometry

It has been choosen the overdamped dynamics of a Brownian particle in a two-dimensional bilobal confinement as the fundamental model to carry out desired investigations. This is an entropic analogue of a bistable energetic potential which is the general set-up to examine phenomena where barrier crossing is somehow involved, such as, transformation of reactant to product, phase transition, stochastic resonance, resonant activation, hysteresis, logical stochastic resonance, erasure of a bit of memory, and many others. The overdamped Brownian dynamics perfectly incorporates the fluctuations appearing in the nonequilibrium situations and portrays the features of the trajectories away from equilibrium which are responsible for the barrier crossing to occur.The research was carried out under the supervision of Prof. D S Ray of Physical Chemistry division under SCS [School of Chemical Sciences

Theoretical Study of Structure, Interaction and Dissociation of Van Der Waals Complexes

The present dissertation entitled “Theoretical study of structure, interaction and dissociation of van der Waals complexes” is submitted to fulfill the requirements for the degree of Doctor of Philosophy (Science) of Jadavpur University. The study has been done for the investigation of spectroscopy, molecular properties, interaction, stability, potential energy surface, and dissociation of weakly bound molecules and complexes having astrophysical, atmospherical and biological interest using high-level theoretical methods. All the studies have been done under the supervision of Professor Abhijit Kumar Das in the Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India. This thesis contains six chapters. Chapter I contains a brief review of the van der Waals complexes appeared in astrophysical, atmospherical, interstellar media and biological systems. The second chapter contains the role of computational science explaining the fundamental review of the basic theoretical methods. Chapter III, IV & V deal with the rare-gas and halogen gas containing weakly bound molecules and van der Waals complexes. The last Chapter, Chapter VI contains the detailed theoretical analysis of the light cation dihydrogen and carbonyl sulfide and acetylene containing van der Waals complexes using dispersion corrected functionals. Most of the results incorporated in this thesis have been published in reputed international journals, a list of which is given in the next section.The research was carried out under the supervision of Prof. A K Das of the Spectroscopy division under SPS [School of Physical Sciences]The research was carried out under IACS research grant and fellowshi

Functional Soft Nano-hybrids: Synthesis and Biological Applications

Nature implements the route of self-assembly in several fundamental processes. Researchers around the globe are always fascinated by the subtle and intricate mysteries of Nature. They try to mimic natural ways by building supramolecular self-assembly of molecules, which bear resemblance to those occurring in Nature. In this regard, amphiphilic molecules comprised of polar hydrophilic head and hydrophobic tail self assembles in water to form different supramolecular structures. These structures are widely diverse and are utilized to understand structurefunction relation of biological processes. On the other hand, the birth of nanotechnology has revolutionized almost every domain of research, especially from material science to biomedicinal arena. However, the potential of any nanomaterial is extremely constricted without the use of supramolecular chemistry. From the very synthesis and stabilization of any nanomaterial the invisible bonds play the central role. Hence the fundamental process of self assembly is the key towards utilizing nanomaterials in almost any direction of research. In this respect, carbon nanomaterials and metal nanoparticles have gained major attention owing to their amazing optical and electronic properties. Recently, it has also gained huge impetus in biomedicinal arena. The present thesis gives an overview on the development of novel self-assembled aggregates with a particular focus on gelation and some of their task specific applications. Also, it deals with amalgamation of supramolecular self-assembled systems with nanomaterials like carbon nanotube, graphene, silver nanoparticles (AgNPs) and thereby developing soft nanocomposites having superior physicochemical and biochemical properties.Research was conducted under the supervision of Prof. P K Das of Biological Chemistry division under SBS [School of Biological Sciences]Research was carried out under CSIR fellowshi

Diverse role of nonmuscle myosin IIs in cellular dedifferentiation and blebbing

Nonmucle myosin IIs (NM-IIs) are the most important actin based motor proteins that are widely distributed throughout the entire organism and play distinct roles in cell adhesion, migration, division etc. NM IIs have been shown to be localized into stress fibre, sarcomere and most likely maintain the cortical tension. In this thesis, we have studied the localization profile of major NM-IIs (NM-IIA and NM-IIB) isoforms and effect of phosphorylation status of myosin regulatory light chain (RLC20), which is known to induce NM-IIs activity, in 3- methylcholanthrene (3MC) treated C2C12 myotubes. We have found that 24h post 50nM 3MC treated myotubes exhibit discrete localization of NM-II isoforms in the sarcomere and/or stress fiber in cytosol- NM-IIA accumulates at the centre of midbody, whereas NMIIB is at the cortex of midbody. Immunoblot analysis of phosphorylation status of RLC shows that there is 5.6 ± 0.5 fold reduction in 3MC treated myotubes in comparison to vehicle treated myotubes during the fragmentation step of myotube dedifferentiation. In contrast, expression level of myogenic factors like MyoD, Myogenin and cell cycle regulatory proteins like Cyclin D, Cyclin E remain unchanged as assessed by real-time PCR array analysis during the fragmentation step of myotubes. Interestingly, addition of myosin light chain kinase inhibitor, ML-7, enhances the fragmentation about 45 ± 3.5 %, whereas phosphatase inhibitor perturbs the 3MC induced fragmentation of myotubes about 20 ± 3.4 In this thesis, we have also studied the functional role of NM-II isoforms in blebbing. We found that the three isoforms of NM-II have differential role in bleb formation. We have shown that ectopically expressed GFP-tagged NM-II isoforms exhibit different types of membrane protrusions such as multiple blebs, lamellipodia, combination of both or without having any protrusions in a human breast tumor cell line MCF-7 as revealed by time lapse video microscopy. Quantification suggests that 49% of GFP-NM-IIA, 26% of GFP-NM-IIB and 19% of GFP-NM-IIC1 expressing MCF-7 cells show multiple bleb formation. Interestingly, expression of phospho-dead mutant of regulatory light chain (RLC), but not the phospho-mimic or wild-type, could reduce the bleb formation to 8% in MCF-7 cells. When we induce the bleb formation by disruption of cortex using 405 nm laser light, we find that all the three GFP-tagged NM-II isoforms can re-appear and form filaments at different degree into the growing bleb. GFP-NM-IIB can form filament into the blebs in 48 % of GFP-NMIIB expressing cells compared with GFP-NM-IIA and -II-C1 which form filament only in 10 % and 3 %, of GFP-NM-IIA and GFP-NM-IIC1 expressing cells, respectively. These studies suggest that myosin IIs have differential role in bleb dynamics.Research was conducted under the supervision of Prof. S S Jana of the Biological Chemistry division under SBS [School of Biological Sciences]Research was carried out under CSIR fellowship and gran

Multi-reference coupled-cluster studies on the effect of dynamical and non-dynamical correlation on molecular energies and properties

In this thesis it has been formulated and implemented a suite of related multi-reference coupled cluster theories to describe open-shell molecular systems taking care to maintain spin-adaptation of the wave function and incorporate the effects of electron correlation and orbital relaxation to the greatest extent possible within the limits of computational and theoretical viability.Research was carried out under the supervision of Prof. Debasish Mukherjee and Dr. Ankan Paul of RCAMOS under SCS [School of Chemical Sciences]Research was conducted under grant of CSIR, India for the Shyama Prasad Mukherjee Fellowship, and DST, India. Also there are sponsorship from abroad , i.e. CEFIPRA/IFCPAR for funding academic visits to Toulouse, France, CTCC, Oslo, Norway

Dynamics and Phases of Strongly Correlated Systems

Non-equilibrium dynamics of an isolated quantum system driven through a quantum critical point shows Kibble-Zurek scaling. This scaling form is controlled by the critical exponents of the universality class of the quantum phase transition. We develop a projection operator formalism for studying both the zero temperature equilibrium phase diagram and the non-equilibrium dynamics of the Bose-Hubbard model. Our work shows that the method provides an accurate description of the equilibrium zero temperature phase diagram of the Bose-Hubbard model for several lattices in two- and three-dimensions (2D and 3D).We show that the accuracy of this method increases with the coordination number z0 of the lattice and reaches to within 0:5% of quantum Monte Carlo data for lattices with z0 = 6. We compute the excitation spectra of the bosons using this method in the Mott and the superfluid phases and compare our results with mean-field theory. We also show that the same method may be used to analyze the non-equilibrium dynamics of the model both in the Mott phase and near the superfluid-insulator quantum critical point where the hopping amplitude J and the on-site interaction U satisfy z0J=U 1. In particular, we study the non-equilibrium dynamics of the model, both subsequent to a sudden quench of the hopping amplitude J and during a ramp from Ji to Jf characterized by a ramp time t and exponent a: J(t) = Ji +(Jf Ji)(t=t)a. We compute the wave function overlap F, the residual energy Q, the superfluid order parameter D(t), the equal-time order parameter correlation function C(t), and the defect formation probability P for the above-mentioned protocols and provide a comparison of our results to their mean-field counterparts. We find that Q, F, and P do not exhibit the expected universal scaling. We explain this absence of universality and show that our results for linear ramps compare well with the recent experimental observations. We have generalized our above mentioned work to develop a time-dependent hopping expansion technique for studying the non-equilibrium dynamics of strongly interacting bosons in an optical lattice in the presence of a harmonic trap characterized by a force constant K. We show that after a sudden quench of the hopping amplitude J across the superfluid (SF)-Mott insulator(MI) transition, the SF order parameter jDr(t)j and the local density fluctuation dnr(t) exhibit sudden decoherence beyond a trap-induced time scale T0 K 1=2. We also show that after a slow linear ramp down of J, jDrj and the boson defect density Pr display a novel non-monotonic spatial profile. Both these phenomena can be explained as consequences of trap-induced time and length scales affecting the dynamics and can be tested by concrete experiments. We also study the statistics of the work distribution P(w) in a d dimensional closed quantum system with linear dimension L subjected to a periodic drive with frequency w0. We show that the corresponding rate function I(w) = ln[P(w)=L0]=Ld after a drive period satisfies an universal lower bound I(0) nd and has a zero at w = QLd=N, where nd and Q are the excitation and the residual energy densities generated during the drive, L0 is a constant fixed by the normalization of P(w), and N is the total number of constituent particles/spins in the system. We supplement our results by calculating I(w) for a class of d-dimensional integrable models and show that I(w) has oscillatory dependence on w0 originating from Stuckelberg interference generated due to double passage through critical point/region during the drive. We suggest experiments to test our theory.Research was conducted under the supervision of Prof. Krishnendu Sengupta of the Theoretical Physics division under the SPS [School of Physical Sciences]Research was carried out under IACS fellowship and gran

Studies of Multi-functional Metal-Organic Frameworks and Complexes using Polycarboxylic Acids and pyrazole based Ligands

The design and synthesis of polyhedral coordination cages1 has become a major part of research endeavour in recent years. The supramolecular coordination complexes (SCCs) refer to discrete metallacycles and metallacages which can be obtained by self-assembly reactions. A coordination cage can be defined as a type of coordination compounds with cavities that can engage in host-guest chemistry.2 They usally consist of several metal centres connected with organic ligands. Early efforts in the development of coordination-driven self-assembly reactions and the formation of SCCs were largely structural in nature. The major aim to achieve specific metallacycles and cages through various, now well-defined, assembly strategies including edge-directed, face-directed, symmetry-adapted, and weaklink approaches, all unified under the theme of ―directional bonding‖. Many of these methods are rooted in two component assemblies, wherein one metal acceptor and one organic donor are used in specific ratios to form highly symmetric metallacycles or cages, with examples spanning many polygons and polyhedra. The metallacycles and metallacages are very useful in host-guest chemistry, apart from that they are also well documented in the literature in molecular sensors, in luminescence, molecular optics, magnetism, homo and heterogeneous catalysis etc.The studies of polynuclear lanthanide based cage complexes are at the early stage of research. Polynuclear lanthanide cages or clusters have attracted research interest because of their versatile architecture (geometry) and photophysical properties. Polynuclear lanthanide cages or clusters have widespread applications in sensors, labels for biomolecules, stains for cellular imaging, display devices, redox-active switches, magnetocaloric effects and single molecule magnets (SMMs). Due to the magnetic anisotropy arising from large unquenched orbital angular momentum and unpaired f-electrons lanthanide complexes (especially Dy) are excellent candidate for the high energy barrier for reversal of magnetization.3 On the other hand, molecular magnetic materials which show large magnetocaloric effect (MCE)4 are of 9 immense research interest for their cryogenic applications. Gd3+ ion one of the most commonly found element which are well documented in the literature having large magneto caloric effect (MCE). Such kind of fascinating properties arises due to large spin-only magnetic moment (S=7/2), quenched orbital momentum and weak super exchange interactions which is responsible for a large entropy change. Therefore it has been found that Gd(III) based complexes showed super cooling properties for magnetic based coolants with highly entropy change value.The research was conducted under the supervision of Dr. Raju Mondal of Inorganic Chemistry under SCS [School of Chemical Sciences]The research was conducted under CSIR grant and fellowshi

Study of Nonlinear Dynamics of Some Chemical Reactions and Reaction-Diffusion Systems

The account depicted in the preceding section makes it clear that the phenomena associated with nonlinear chemical dynamics are essentially manifestations of the nonlinearity of the chemical system under farfrom- equilibrium condition. Chemical systems are naturally diverse and under far-from-equilibrium condition the possible permutations of their disposition become manifold. Since, it is a formidable task to cover all forms of nonlinearity the present thesis focuses primarily on chemical systems where autocatalysis functions as the common thread of nonlinearity. The central theme of the first four chapters of this dissertation is the presence of at least one autocatalytic step in the concerned chemical reaction in view of the fact that it renders the system nonlinear. Three very distinct yet interesting phenomena associated with nonlinear chemical systems are covered in the first three chapters while the last in this group provides a recipe for simplification of nonlinear chemical reactions. Spatially extended chemical reactions form the other half of the thesis where reaction-diffusion models have been considered to study spatiotemporal organization. The following pages provide a succinct summary of the investigations that are covered in this thesis. 1.3.1 Oscillatory chemical reaction induced dynamical hysteresis in a polymer gel Hysteresis is a distinct phenomenon usually associated with ferromagnetic and ferroelectric substances[39, 40, 41, 42, 43, 44, 45]. This fact is attributed to the assertion that the output of the system depends not only on the present input but also on its past values. However, it should be kept in mind that such a phenomenon is not confined to magnetic substances and is observed in stochastic systems[46, 47, 48, 49], selforganizing avalanches[50], and other contexts[39]. In the same spirit mechanical systems such as polymer gels have been found to exhibit static hysteresis in volume-temperature curves[51, 52]. Here it is noteworthy to acknowledgeThe research was conducted under the supervision of Prof. D. S Ray, Physical Chemistry division, SCS [School of Chemical Sciences