Photocatalysis is a process to convert light energy into chemical energies. This advanced process has been extensively applied in different areas, such as water splitting to evolve hydrogen, organic/ inorganic pollutants decomposition, artificial photosynthesis (CO2 reduction), disinfection, heavy metal recovery, organic synthesis and nitrogen fixation (reduction). The difficulty for photocatalysis applied in practical is primarily due to the low quantum yield as for the high recombination of photogenerated charge carriers. Various strategies have been implemented to overcome these challenges. As recently developed advanced materials, two dimensional materials have attracted lots of attentions as for their superiorities such as large specific surface area and high conductivity. These advantages for two dimensional materials make them be promising cocatalysts in enhance catalytic activity. In this thesis, various two dimensional materials (such as MoS2, SnS, BN as well as C3N4) other than graphene were prepared and investigated in the promotion of photocatalytic activity. Specifically, the focus of present work is on two dimensional materials enhanced photocatalysis in environmental remediation, including organic pollutants detoxification as well as bacteria inactivation. It was found that two dimensional materials, including MoS2, SnS, BN, may be excellent candidates as cocatalysts to enhanced visible-light-driven photocatalytic activity. And g-C3N4 as an effective photocatalyst exhibited excellent photocatalytic oxidation activity, and its activity can be further enhanced with surface modification by hydroxyl functional groups (a modification method reported in the thesis). Suggestions for future work were also proposed in this thesis.

The 5d transition metal oxides have drawn substantial interest for predictions of being suitable candidates for hosting exotic electronic and magnetic states, including unconventional superconductors, magnetic skyrmions, topological insulators, and Weyl semimetals. In addition to the electron-electron correlation notable in high-temperature 3d transition metal superconductors, the 5d oxides contain a large spin-orbit interaction term in their ground state, which is largely responsible for the intricate phase diagram of these materials. Iridates, or compounds containing 5d iridium bonded with oxygen, are of particular interest for their spin-orbit split Jeff = 1/2 state, which is partially filled without the presence of any additional electron correlation. However, the comparable energetics between a small, finite electron correlation energy and the spin-orbit interaction make the band structure of iridates amenable to small perturbations of the crystalline lattice and ideal for exploring the interplay between these two interactions. While altering the spin-orbit interaction strength of iridium is tenably not feasible, the electron correlation energy can be tuned using a variety of experimental techniques. In this dissertation, the electronic and magnetic properties of iridates at various electron correlation energies are studied by altering the epitaxial lattice strain, dimensionality, and the radius size of the A-site cation. These parameters tune the effective electronic bandwidth of the system, which is inversely proportional to the effective electron correlation energy. The lattice strain and the cationic radius size achieve this by altering the Ir-O-Ir bond angle between nearest neighbor Ir ions. In the case of dimensionality tuning, the effective bandwidth is controlled via the coordination number of each Ir ion. In the first study, a metal-to-insulator transition is observed in thin films of the semi-metallic SrIrO3 as in-plane compressive lattice strain is increased. This observation is consistent with the expectation of compressive lattice strain increasing the effective correlation energy; however, optical spectroscopy spectra reveal the increase is not sufficient for opening an insulating Mott gap. In the second part, the effective correlation energy is adjusted using a dimensional confinement of the layered iridate Sr2IrO4. Here, the coordination number of each Ir ion is reduced using an a-axis oriented superlattice of one-dimensional IrO2 quantum stripes, where several emergent features are revealed in its insulating Jeff = 1/2 state. In the final study, the effective correlation is tuned in a series of mixed-phase pyrochlore iridate thin films, where the Ir atoms take a corner-shared tetrahedral configuration. Here, a transition between conducting to insulating magnetic domain walls is revealed as the correlation energy is increased via A-site chemical doping. Each of these studies sheds light on the pronounced role the effective correlation energy plays in determining the local subset of phases predicted for iridates and related systems featuring strong spin-orbit interactions.

The observation of graphene’s extraordinary electrical properties has stirred great interest in two dimensional (2D) materials. The rapid pace of discovery for low dimensional materials with exciting properties continue with graphene allotropes, multiple polymorphs of borophene, germanene, and many others. The future of 2D materials goes beyond synthesis and characterization of free standing materials and on to the construction of heterostructures or sophisticated multilayer devices. Knowledge about the resulting local structure and composition of such systems will be key to understanding and optimizing their performance characteristics. 2D materials do not have a repeating crystal structure which can be easily characterized using bulk methods and therefore a localized high resolution method is needed. Electron microscopy is well suited for characterizing 2D materials as a repeating coherent structure is not necessary to produce a measureable signal as may be the case for diffraction methods. A unique opportunity for fine local scale measurements in low dimensional systems exists with a specific class of materials known as ferecrystals, the rotationally disordered relative of misfit layer compounds. Ferecrystals provide an excellent test system to observe effects at heterostructure interfaces as the whole film is composed of interdigitated two dimensional layers. Therefore bulk methods can be used to corroborate local scale measurements. From the qualitative interpretation of high resolution scanning transmission electron microscope (STEM) images to the quantitative application of STEM energy dispersive X-ray spectroscopy (EDX), this thesis uses numerous methods electron microscopy. The culmination of this work is seen at the end of the thesis where atomically resolved STEM-EDX hyperspectral maps could be used to measure element specific atomic distances and the atomically resolved fractional occupancies of a low dimensional alloy. These local scale measurements are corroborated by additional experimental data. The input of multiple techniques leads to improved certainty in local scale measurements and the applicability of these methods to non-ferecrystal low dimensional systems.

In this study, the thermal properties of low dimensional materials such as graphene and boron nitride nanotube were investigated. As one of important heat transfer characteristics, interfacial thermal resistance (ITR) between graphene and Cu film was estimated by both experiment and simulation. In order to characterize ITR, the micropipette sensing technique was utilized to measure the temperature profile of suspended and supported graphene on Cu substrate that is subjected to continuous wave laser as a point source heating. By measuring the temperature of suspended graphene, the intrinsic thermal conductivity of suspended graphene was measured and it was used for estimating interfacial thermal resistance between graphene and Cu film. For simulation, a finite element method and a multiparameter fitting technique were employed to find the best fitting parameters. A temperature profile on a supported graphene on Cu was extracted by a finite element method using COMSOL Multiphysics. Then, a multiparameter fitting method using MATLAB software was used to find the best fitting parameters and ITR by comparing experimentally measured temperature profile with simulation one. In order to understand thermal transport between graphene and Cu substrate with different interface distances, the phonon density of states at the interface between graphene and Cu substrate was calculated by MD simulation.As another low dimensional material for thermal management applications, the thermal conductivity of BNNT was measured by nanoscale thermometry. For this work, a noble technique combining a focused ion beam (FIB) and nanomanipulator was employed to pick and to place a single BNNT on the desired location. The FIB technology was used to make nanoheater patterns (so called nanothermometer) on a prefabricated microelectrode device by conventional photolithography processes. With this noble technique and the nanoheater thermometry, the thermal conductivity of BNNT was successfully characterized by temperature gradient and heat flow measurements through BNNT.

Over the past two decades, controlling thermal transport properties at the nanoscale has become more and more relevant. This is mostly motivated by the need of developing novel energy-harvesting techniques based on thermoelectricity and the necessity to control the heat dissipation in semiconductor devices. In this field, two major research lines can be identified: On one side 'phononics', which aims at developing devices such as thermal diodes, thermal transistors, and thermal logic gates, among others, and on the other side, phonon engineering aiming at controlling heat transport by producing or structurally modifying heterostructures made of novel nanomaterials (e.g., two-dimensional (2D) materials, nanotubes, organic systems). In order to gain insight into the factors controlling nanoscale heat flow and to be able to design highly-efficient thermal devices, the development of new computational approaches is crucial. The primary goal of the present thesis is the implementation of new methodologies addressing classical and quantum thermal transport at the nanoscale. We will focus on three major issues: (i) We will study thermal rectification effect in nanodevices made of novel 2D materials by means of nonequilibrium molecular dynamics simulations. The influence of structural asymmetry and substrate deposition on the thermal rectification will be investigated. (ii) To address quantum ballistic thermal transport in nanoscale systems, we will implement a nonequilibrium Green's functions (NEGF) treatment of transport combined with a density-functional based approach. Here, we will explore the dependence of the thermal transport properties of 2D materials and nanotubes on different intrinsic (structural anisotropy and grain boundaries) and external (molecular functionalization, strain engineering, and doping) factors. Finally, (iii) a time-dependent NEGF formalism will be developed and implemented to probe the transient and steady thermal transport in molecular junctions. In short, our results show that the mechanisms governing the thermal rectification effect in the 2D thermal rectifiers proposed in this work are shape asymmetries, interface material (planar stacking order), and changes in the degree of spatial localization of high-frequency modes (under nonequilibrium heat transport conditions). The rectification effect can be also controlled by substrate engineering. Moreover, we found that quantum ballistic thermal transport in 2D puckered materials displays an anisotropic behavior. The presence of structural disorder in the form of grain boundaries in graphene reduces overall its thermal transport efficiency. Dynamical disorder induced by coupling to a thermostat has however a weaker effect, suggesting that structural defects are playing a major role. External factors have a noticeable influence on the heat transport in new 2D materials and BNC heteronanotubes. On the other hand, we have also been able to characterize, from a quantum point of view, the phonon dynamics in carbon-based molecular junctions. We expect that the results obtained within this thesis will yield new insights into the thermal management of low-dimensional materials, and thus open new routes to the design of thermoelectric and phononic devices. / In den letzten zwei Jahrzehnten hat die Kontrolle der thermischen Transporteigenschaften im Nanobereich immer mehr an Bedeutung gewonnen. Dies ist vor allem auf die Notwendigkeit zurückzuführen, neue Energiegewinnungstechniken zu entwickeln, die auf Thermoelektrizität basieren, sowie auf die Problematik, die Wärmeabfuhr in Halbleiterbauelementen kontrollieren zu müssen. In diesem Bereich lassen sich zwei große Forschungslinien identifizieren: Auf der einen Seite 'Phononik', die unter anderem auf die Entwicklung von Bauelementen wie thermischen Dioden, Transistoren und Logikgattern abzielt, und auf der anderen Seite die Phononentechnik, die den Wärmetransport durch Herstellung oder strukturelle Modifikation von Heterostrukturen aus neuartigen Nanomaterialien (z.B. zweidimensionalen (2D) Materialien, Nanoröhren, organischen Systemen) steuert. Um einen Einblick in die Faktoren zu erhalten, die den Wärmefluss im Nanobereich steuern, und um hocheffiziente thermische Bauteile entwickeln zu können, ist die Entwicklung neuer Berechnungsansätze entscheidend. Das Hauptziel der vorliegenden Arbeit ist die Implementierung neuer Methoden, die sich mit dem klassischen und dem quantenthermischen Transport auf der Nanoskala befassen. Wir werden uns auf drei Hauptthemen konzentrieren: (i) Wir werden den thermischen Rektifikationseffekt in Nanobauteilen aus neuartigen 2D-Materialien mit Hilfe von Nichtgleichgewichts-Molekulardynamiksimulationen studieren. Der Einfluss von Strukturasymmetrie und Substratablagerung auf die thermische Rektifikation wird untersucht. (ii) Um den quantenballistischen Wärmetransport in nanoskaligen Systemen anzugehen, werden wir eine NEGF-Behandlung (Nichtgleichgewichts-Greensche Funktionen) des Transports in Kombination mit einem dichtefunktionalen Ansatz implementieren. Hier wird die Abhängigkeit der thermischen Transporteigenschaften von 2D-Materialien und Nanoröhrchen von verschiedenen intrinsischen (strukturelle Anisotropie und Korngrenzen) und externen (molekulare Funktionalisierung, Stammtechnik und Dotierung) Faktoren untersucht. Schließlich wird (iii) ein zeitabhängiger NEGF-Formalismus entwickelt und implementiert, um den transienten und stetigen Wärmetransport in molekularen Verbindungen zu untersuchen. Unsere Ergebnisse zeigen, dass die wesentlichen Mechanismen für die thermische Gleichrichtung in 2D thermischen Gleichrichtern durch Asymmetrien der Bauteilform, das Interface-Material (planare Stapelung Reihenfolge), und änderungen im Grad der räumlichen Lokalisierung von Hochfrequenz-Modi (unter Nicht-Gleichgewicht Wärmetransport-Bedingungen) gegeben sind. Der Gleichrichteffekt kann auch durch die Wahl des Substrats gesteuert werden. Darüber hinaus haben wir festgestellt, dass der quantenballistische Wärmetransport in 2D-Puckered-Materialien ein anisotropes Verhalten zeigt. Das Vorhandensein von strukturellen Störungen in Form von Korngrenzen in Graphen reduziert insgesamt die Effizienz des Wärmetransports. Dynamische Störungen, die durch die Ankopplung an einen Thermostaten hervorgerufen werden, haben jedoch eine schwächere Wirkung, was darauf hindeutet, dass strukturelle Defekte eine große Rolle spielen. Externe Faktoren haben einen nachweislichen Einfluss auf den Wärmetransport in neuen 2D-Materialien und BNC-Heteronanotubes. Weiterhin konnten wir auch die Phononendynamik in kohlenstoffbasierten molekularen Verbindungen quantitativ charakterisieren. Wir erwarten, dass die Ergebnisse dieser Arbeit neue Erkenntnisse über das Wärmemanagement von niedrigdimensionalen Materialien liefern und damit neue Wege für das Design von thermoelektrischen und phononischen Bauelementen eröffnen.

Semilocal density functionals up to the generalized gradient approximation (GGA) level cannot accurately describe band gaps of bulk solids. Meta-GGA density functionals with a dependence on the kinetic energy density ingredient (τ) can potentially give wider band gaps compared with GGAs. The recently developed TASK meta-GGA functional yields excellent band gaps of bulk solids. The accuracy of the TASK functional for band gaps of bulk solids cannot be straightforwardly transferred to low-dimensional materials due to reduced screening in low-dimensional materials. We have developed mTASK from TASK by changing (a) the tight upper-bound for one or two-electron systems (h0X) from 1.174 to 1.29 and (b) the limit of the interpolation function fX(α → ∞) of the TASK functional that interpolates the exchange enhancement factor FX(s,α) from α = 0 to 1, so that mTASK has the screening appropriate for low-dimensional materials. These two conditions guarantee the increased nonlocality within the generalized Kohn-Sham scheme in the mTASK functional and yield a better description of band gaps of low-dimensional materials. We computed the band gaps of bulk solids from mTASK having a wide range of gaps such as Ge, CdO, ZnS, MgO, NiF, Ar. The improvement in the band gaps from mTASK is more consistent than TASK for the large-gaps crystals. We have studied the band structures in two forms of transition metal dichalcogenide (TMD) monolayers, i.e., monolayer hexagonal (1H) and monolayer trigonal (1T) and their nanoribbons. The mTASK functional systematically improves the band gaps and is in close agreement with the experiments or the hybrid level HSE06 density functional for 2D single-layer and nanoribbon systems. In the second part of this assessment, we explore the large tunability of band gaps and optical absorption of phosphorene nanoribbons under mechanical bending from first-principles. Bending can induce an unoccupied edge state in armchair phosphorene nanoribbons. The electronic and optical properties of nanoribbons drastically change because of this edge state. GW-Bethe–Salpeter equation calculations for armchair phosphorene nanoribbons at different bending curvatures show that the absorption peaks generally shift toward the high energy direction with increasing curvature. Our study suggests that bright excitons can also be formed from the transition from the valence bands to the edge state when the edge state completely separates out from the continuum conduction bands. We systematically study the role of the edge state to form bound excitons at large curvatures. Our analysis suggests that the optical absorption peaks of zigzag phosphorene nanoribbons shift toward the low-energy region, and the height of the absorption peaks increases while increasingthe bending curvature. In the third part of this assessment, we extend our study of phosphorene nanoribbons to MoS2 nanoribbons under bending from GW and Bethe-Salpeter equation approaches. We find three critical bending curvatures for armchair MoS2 nanoribbons, and the edge and non-edge band gaps show a non-monotonic trend with bending. The edge band gap shows an oscillating feature with ribbon width n, with a period of ∆n=3. The binding energy and the lowest exciton energy decrease with the curvature. The large tunability of optical properties of bent MoS2 nanoribbon is applicable in tunable optoelectronic nanodevices. / Physics

The properties of bulk 3D materials of metals or semiconductors are manifested with various length scales(e.g., Bohr excitonic radius, magnetic correlation length, mean free path etc.) and are important in controlling their properties. When the size of the material is smaller than these characteristics length scales, the confinement effects operate reflecting changes in their physical behavior. Materials with such confinement effects can be designated as low dimensional materials. There are exceedingly large numbers of low dimensional materials and the last half a century has probably seen the maximum evolution of such materials in terms of synthesis, characterization, understanding and modification of their properties and applications. The field of” nanoscience and nanotechnology”, have become a mature field within the last three decades where, for certain application, synthesis of materials of sizes in the nanometer range can be designed and controlled. Interface plays a very important role in controlling properties of heterogeneous material of every dimensionality. For example, the interface forms in 2D thin films or interface of heterogeneous nanoparticles(0D). In recent times, a large number of remarkable phenomena have triggered understanding and controlling properties arises due to nature of certain interface. In the field of nanoparticles, it is well known that the photoluminescence property depends very strongly on the nature of interface in heterostructured nanoparticles. In the recent time a large variety of heterostructured nanoparticles starting from core-shell to quantum dot-quantum well kind has been synthesized to increase the photoluminescence efficiency up to 80%. Along with improvement of certain properties due to heterostructure formation inside the nanoparticles, the techniques to understand the nature of those interfaces have improved side by side. It has been recently shown that variable energy X-ray Photoemission Spectroscopy (XPS) can be employed to understand the nature of interfaces (internal structure) of such heterostructure nanoparticles in great detail with high accuracy. While most of the previous studies of variable energy XPS, uses photonenergies sensitive to smaller sized particle, we have extended the idea of such nondestructive approach of understanding the nature of buried interfaces to bigger sized nanoparticles by using photon energy as high as 8000eV, easily available in various 3rd generation synchrotron centers. The nature of the interface also plays an important role in multilayer thin films. Major components of various electronic devices, like read head memory devices, field effect transistors etc., rely on interface properties of certain multilayer thin film materials. In recent time wide range of unusual phenomenon such as high mobility metallic behavior between two insulating oxide, superconductivity, interface ferroelectricity, unusual magnetism, multiferroicity etc. has been observed at oxide interface making it an interesting field of study. We have shown that variable energy photoemission spectroscopy with high photon energies, can be a useful tool to realize such interfaces and controlling the properties of multilayered devices, as well as to understand the origin of unusual phenomenon exists at several multilayer interfaces. Chapter1 provides a brief description of low dimensional materials, overall perspective of interesting properties in materials with reduced dimensionality. We have emphasized on the importance of determining the internal structure of buried interface of different dimensionalities. We have given a brief overview and importance of different interfaces that we have studied in the subsequent chapters dealing with specific interfaces. Chapter 2 describes experimental and theoretical methods used for the study of interface and self-assembly reported in this thesis. These methods are divided into two categories. The first section deals with different experimental techniques, like, UV-Visible absorption and photoluminescence spectroscopy, X-Photoelectron Spectroscopy(XPS), X-Ray diffraction, Transmission Electron Microscopy(TEM) etc. This section also includes brief overview on synchrotron radiation and methods used for detail analysis of interface structure using variable energy XPS. In the second part of this chapter, we have discussed theoretical methods used in the present study. \ In Chapter 3A we have combined low energy XPS, useful to extract information of the surface of the nanoparticles, with high energy XPS, important to extract bulk information and have characterized the internal structure of nanoparticle system of different heterogeneity. We have chosen two important heterostructure systems namely, inverted core-shell(CdScore-CdSeshell) type nanoparticles and homogeneous alloy(CdSeS)type nanoparticles. Such internal structure study revealed that the actual internal structure of certain nanomaterial can be widely different from the aim of the synthesis and knowledge of internal structure is a prerequisite in understanding their property. We were able to extend the idea of variable energy XPS to higher energy limit. Many speculations have been made about the probable role of interface in controlling properties, like blinking behavior of bigger sized core-shell nanoparticles, but no conclusive support has yet been given about the nature of such interface. After successfully extending the technique to determine the internal structure of heterostructured nanoparticles to very high photon energy region, we took the opportunity to determine the internal structure of nanoparticles of sizes as large as 12nm with high energy photoemission spectroscopy for the first time. In Chapter 3B we emphasize on the importance of interface structure in controlling the behavior of bigger sized nanoparticles systems, the unsettled issues regarding their internal structure, and described the usefulness of high energy XPS in elucidating the internal structure of such big particles with grate accuracy to solve such controversies. The existence of high density storage media relies on the existence of highly sensitive magnetic sensors with large magnetoresistance. Today almost all sensor technologies used in modern hard disk drives rely on tunnel magnetoresistance (TMR) CoFeB-MgO-CoFeB structures. Though device fabrication is refined to meet satisfactory quality assurance demands, fundamental understanding of the refinement in terms of its effect on the nature of the interfaces and the MgO tunnel barrier leading to improved TMR is still missing. Where, the annealing condition required to improve the TMR ratio is itself not confirmatory its effect on the interface structure is highly debatable. In particular, it has been anticipated that under the proposed exotic conditions highly mobile B will move into the MgO barrier and will form boron oxide. In Chapter 4 we are able to shed definite insights to heart of this problem. We have used high energy photoemission to investigate a series of TMR structures and able to provide a systematic understanding of the driving mechanisms of B diffusion in CoFeBTMR structures. We have solved the mix-up of annealing temperature required and have shown that boron diffusion is limited merely to a sub-nanometer thick layer at the interface and does not progress beyond this point under typical conditions required for device fabrication. We have given a brief overview on the evolution of magnetic storage device and have described various concepts relevant for the study of such systems. The interface between two nonmagnetic insulators LaAlO3 and SrTiO3 has shown a variety of interface phenomena in the recent times. In spite of a large number of high profile studies on the interface LaAlO3 and SrTiO3 there is still a raging debate on the nature, origin and the distribution of the two dimensional electron gas that is supposed to be responsible for its exotic physical properties, ranging from unusual transport properties to its diverse ground states, such as metallic, magnetic and superconducting ones, depending on the specific synthesis. The polar discontinuity present across the SrTiO3-LaAlO3 interface is expected to result in half an electron transfer from the top of the LaAlO 3 layer to each TiofSrTiO3 at the interface, but, the extent of localization that can make it behave like delocalized with very high mobility as well as localized with magnetic moments is not yet clear. In Chapter 5 we have given a description of this highly interesting system as well as presented the outcome of our depth resolved XPS investigation on several such samples synthesized under different oxygen pressure. We were able to describe successfully the distribution of charge carriers. While synthesizing and understanding properties of nanoparticles is one issue, using them for device fabrication is another. For example, to make a certain device often requires specific arrangements of nanoparticles in a suitable substrate. Self-assembly formation can be a potential tool in these regards. Just like atom or ions, both nano and colloidal particles also assemble by themselves in ordered or disordered structure under certain conditions, e.g., the drying of a drop of suspension containing the colloid particles over a TEM grid. This phenomenon is known as self-assembly. Though, the process of assembly formation can be a very easy and cost-effective technique to manipulate the properties in the nano region, than the existing ones like lithography but, the lack of systematic study and poor understanding of these phenomena at microscopic level has led to a situation that, there is no precise information available in literature to say about the nature of such assembly. In Chapter 6 we have described experiments that eliminate the dependence of the self-assembly process on many complicating factors like substrate-particle interaction, substrate-solvent interaction etc., making the process of ordering governed by minimum numbers of experimental parameter that can be easily controlled. Under simplified conditions, our experiments unveil an interesting competition between ordering and jamming in drying colloid systems similar to glass transition phenomenon Resulting in the typical phase behavior of the particles. We establish a re-entrant behavior in the order-disorder phase diagram as a function of particle density such that there is an optimal range of particle density to realize the long-range ordering. The results are explained with the help of simulations and phenomenological theory. In summary, we were able to extend the idea of variable energy XPS to higher energy limit advantageous for investigating internal structure of nonmaterial of various dimensionalities and sizes. We were able to comprehend nature of buried interface indicating properties of heterostructures quantum dots and thin films. Our study revealed that depth resolved XPS combined with accessibility of high and variable energies at synchrotron centers can be a very general and effective tool for understanding buried interface. Finally, we have given insight to the mechanism of spontaneous ordering of nanoparticles over a suitable substrate.

Discovery of grapheme has paved way for experimental realization of many physical phenomena such as massless Dirac fermions, quantum hall effect and zero-field conductivity. Search for other two dimensional (2D) materials led to the discovery of boron nitride, transition metal dichalcogenides(TMDs),transition metal oxides(MO2)and silicene. All of these materials exhibit different electronic and transport properties and are very promising for nanodevices such as nano-electromechanical-systems(NEMS), field effect transistors(FETs),sensors, hydrogen storage, nano photonics and many more. For practical utility of these materials in electronic and photonic applications, varying the band gap is very essential. Tuning of band gap has been achieved by doping, functionalization, lateral confinement, formation of hybrid structures and application of electric field. However, most of these techniques have limitations in practical applications. While, there is a lack of effective method of doping or functionalization in a controlled fashion, growth of specific sized nanostructures (e.g., nanoribbons and quantum dots),freestanding or embedded is yet to be achieved experimentally. The requirement of high electric field as well as the need for an extra electrode is another disadvantage in electric field induced tuning of band gap in low dimensional materials. Development of simpler yet effective methods is thus necessary to achieve this goal experimentally for potential application of these materials in various nano-devices. In this thesis, novel methods for tuning band gap of few 2D materials, based on strain and stacking, have been proposed theoretically using first principles based density functional theory(DFT) calculations. Electronic properties of few layered nanomaterials are studied subjected to mechanical and chemical strain of various kinds along with the effect of stacking pattern. These methods offer promising ways for controlled tuning of band gap in low dimensional materials. Detailed methodology of these proposed methods and their effect on electronic, structural or vibrational properties have also been studied. The thesis has been organized as follows: Chapter1 provides a general introduction to the low dimensional materials: their importance and potential application. An overview of the systems studied here is also given along with the traditional methods followed in the literature to tune their electronic properties. The motivation of the current research work has also been highlighted in this chapter. Chapter 2 describes the theoretical methodology adopted in this work. It gives brief understanding of first principles based Density Functional Theory(DFT) and various exchange and correlation energy functionals used here to obtain electronic, structural, vibrational and magnetic properties of the concerned materials. Chapter 3 deals with finding the origin of a novel experimental phenomenon, where electromechanical oscillations were observed on an array of buckled multiwalled carbon nanotubes (MWCNTs)subjected to axial compression. The effect of structural changes in CNTs in terms of buckling on electronic properties was studied. Contribution from intra-as well as inter-wall interactions was investigated separately by using single-and double-walled CNTs. Chapter 4 presents a method to manipulate electronic and transport properties of graphene bilayer by sliding one of the layers. Sliding caused breaking of symmetry in the graphene bilayer, which resulted in change in dispersion in the low energy bands. A transition from linear dispersion in AA stacking to parabolic dispersion in AB stacking is discussed in details. This shows a possibility to use these slid bilayers to tailor graphene based devices. Chapter 5 develops a method to tune band gap of bilayers of semiconducting transition metal dichalcogenides(TMDs) by the application of normal compressive strain. A reversible semiconductor to metal(S-M) transition was reported in this chapter for bilayers of TMDs. Chapter 6 shows the evolution of S-M transition from few layers to the bulk MoS2 under various in-plane and out of plane strains. S-M transition as a function of layer number has been studied for different strain types. A comparison between the in-plan and normal strain on modifying electronic properties is also presented. Chapter 7 discusses the electronic phase transition of bulk MoS2 under hydrostatic pressure. A hydrostatic pressure includes a combined effect of both in-plane and normal strain on the structure. The origin of metallic transition under pressure has been studied here in terms of electronic structure, density of states and charge analysis. Chapter 8 studies the chemical strain present in boron nitride nanoribbons and its effect on structural, electronic and magnetic properties of these ribbons. Properties of two achiral (armchair and zig-zag) edges have been analyzed in terms of edge energy and edge stress to predict stability of the edges. Chapter9 summarizes and concludes the work presented in this thesis.

<p>  </p> <p>Magnetism plays a vital role in the technologies of today, and materials used for magnetic applications largely consist of solid state phases. Intermetallic chalcogenides are one such material which have exhibited a full range of properties useful for a variety of applications requiring soft magnets, superconductors, magnetocalorics, and even rarer magnetic phenomenon such as 1D Heisenburg magnetic chains. Solid state chemists continue to develop new synthesis methods for chalcogenides as they produce both new phases and modifications of existing phases, usually with the express intent of improving their physical and chemical properties. Low dimensional chalcogenides often have predictable structure-property relationships which when understood aids in these efforts of optimizing existing materials.</p> <p>In this work, we have synthesized novel, low-dimensional Tl1-xAxFe3Te3 (A = K, Na)-based magnetocalorics for magnetic refrigeration technologies utilizing a variety of synthetic methods. Doping of alkali metals into the thallium site simultaneously reduces the toxicity and cost of the material, and also modifies their crystal structures leading to changes in their magnetic properties including ordering temperature, magnetic anisotropy, magnetic hysteresis, coercivity, and magnetic entropies. Most notably, the magnetic ordering temperature has been boosted from 220 K of the prior known TlFe3Te3 phase up to 233 K in the new Tl0.68Na0.32Fe2.76Te3.32 phase, further towards room temperature which is required for the commercialization of magnetic refrigerants for home appliances. There exist strong magnetostructural correlations for most of the alterations in the magnetic properties, and relationships have been modelled where trends exist to match the magnetism to the changes in the unit cell of the structure.</p> <p>New synthetic methods were also developed for the ternary TBi4S7 (T = transition metal) phase which exhibits a pseudo-1D structure of Heisenberg antiferromagnetic chains. These synthetic techniques resulted in more consistent high purity of phases than methods reported previously in literature. Attempts at synthesizing new phases were made, and crystallographic and composition analysis methods suggested the synthesis of a new Mn1-xCoxBi4S7 phase, though magnetic impurities prevented characterization of this new material’s magnetic properties. </p>

Low-dimensional (D) materials, such as graphene, transition metal dichalcogenides and chalcogenide nanowires, are attractive for spintronics and valleytronics due to their unique physical and chemical properties resulting from low dimensionality. Emerging concepts of spintronics devices will greatly benefit from using 1D and 2D materials, which opens up new ways to manipulate spin. A majority of 1D and 2D materials is non-magnetic, thus their applications in spintronics are limited. The exploration, design and synthesis of new 1D and 2D materials with intrinsic magnetism and high spin-polarization remains a challenge. In addition, the valley polarization and spin-valley coupling properties of 2D materials have attracted great attention for valleytronics, which not only manipulates the extra degree of freedom of electrons in the momentum space of crystals but also proposes a new way to store the information. The computational investigation of magnetic and electronic properties of low-dimensional materials is the subject of this thesis. We have systematically investigated geometric, electronic, magnetic and valleytronic properties of several 2D and 1D materials by using the density functional theory. These investigations not only theoretically show rich and adjustable magnetic properties of...