Theoretical and experimental work on superconductivity has won a number of Nobel prizes in Physics, beginning with the prize to Kamerlingh Onnes in 1913 that included the initial discovery of superconductivity in mercury. Superconductivity has since been at the forefront of research in condensed matter physics. Furthermore, since the first isolation of graphene by Geim and Novoselov in 2004, there has been growing interest in other monolayer and few-layer crystals. Like graphene, other materials can be exfoliated due to the weak van der Waals interactions between layers, primarily the transition metal dichalcogenides (TMDs). Atomically flat and chemically stable thin two dimensional (2D) layers of TMDs have opened up new opportunities for discovering exciting new physics and ultimately developing thin flexible devices. Defect-free exfoliated TMDs are regarded to be ideal materials for use as channels for field effect transistors (FET), which have been shown to possess remarkable electronic properties. Recent advances in field effect-based TMD devices have been achieved using ionic liquid gating and the formation of electrical double layers. Using the techniques previously developed for isolating graphene, few-layer crystals of 1T- and 2H-TaS2 have been obtained in this project to be used as channel materials for FET and ionic field effect transistor (iFET) devices that incorporate DEME-TFSI ionic liquids as a top gate to control the carrier density. In the first experimental chapter (chapter 5) iFETs using a 1 μm thin film of a highly boron-doped diamond (BDD) as the channel material are introduced and the influence of top gating on the transition temperature using a DEME-TFSI ionic liquid is studied. An enhancement in the Tc of the BDD sample under positive top gate potentials is shown as a result of electron doping at the grain boundaries leading to stronger coupling between the grains. The following chapter (Chapter 6) describes low temperature measurements of graphene FET (GFET) devices. These devices were fabricated to enable a reliable and effective calibration for the DEME-TFSI top gate specific capacitance against the known back gate capacitance. This represents a valuable reference for ionic liquid gating studies of TMD materials. The last experimental chapter describes the electrical properties of few-layer 1T-TaS2 (initial section) and 2H-TaS2 (final section) samples used as channels in FET devices. Charge density wave (CDW) transitions in 1T- and 2H-TaS2 are investigated and gating measurements using ionic liquids on these samples are described and summarised. Although no gate influence was seen on the CDW in 2H-TaS2 , a suppression of the CDW transition in cooling cycles of a 1T-TaS2-based FET sample was observed. This suppression demonstrates that accumulation of additional charge carriers in the sample drives it into a metallic state. In a ∼15 nm 2H-TaS2 FET device, strong enhancement of the superconducting critical temperature from 0.8 to 4.7 K is observed with DEME-TFSI top gating. The influence of an additional back gate potential on the device enhances the transition temperature still further up to 5 K. This indicates a co-operative effect between the top and back gates of the sample. It was also demonstrated that 2H-TaS2 crystals are susceptible to intercalation by DEME+ cations in the ionic liquid; a clear enhancement of Tc was observed after simply placing a drop of ionic liquid on a 2H-TaS2 flake without application of a top gate bias. This research project has studied superconductivity in 2D materials and illustrates the capability of ionic liquid gating as a versatile tool to modify the carrier concentration and enhance the critical temperature of a wide range of different materials.

Transition metal Dichalcogenide MoS2in the monolayer and few-layer form have generated intense interest in the fundamental and applied research community due to its surprisingly strong light-matter interactions, strong excitonic effect, and unique elec-tronic and chemical activity at the edges. In this thesis work, I have conducted a series of synergistic experimental and computational investigations focused on understanding the fundamental optical properties of few-layer MoS2(experiment with supporting computational calculations) and its potential application into the electrochemical reduction of CO2(computational)In the first part of the thesis, I show that sulfur vacancies affect the optical properties of few-layer thin films deposited using magnetron sputtering. In particular, I show that sulfur vacancies can obscure the well-defined A/B excitons in MoS2. Next, while contributing with the process of developing high-quality MoS2films, I designed an approach to accurately determine the optical constants by combining transmission spectroscopy with spectroscopic ellipsometry. The method, which we call Transmission-assisted spectroscopic ellipsometry (TASE), is demonstrated on high-quality MoS2films deposited on transparent and absorbing substrates. Next, Transmission spectroscopy combined with the Kramers-Kronig consistent optical model was employed to determine the complex dielectric function of few-layer MoS2in the broadband energy range of 0.7-6.5 eV. Optical transitions leading to peaks in the dielectric functions are assigned to the band structure. In particular, a new peak is observed and assigned at 4.5 eV in few-layer MoS2. Finally, I have examined the effectiveness of doped MoS2on the catalytic activity for CO reduction using density functional theory method. The structural calculation shows that doping Mo edge site of MoS2with transition metals that have higher work function than Mo atom results a lowering in the CO adsorption energy which suppresses the dissociation reaction and enhances the hydrogenation reaction. The Bader charge analysis shows that the dopant atom does not contribute to CO adsorption directly but it reduces the charge density at the edge atom that is indicated from the Density of states.

Graphene and layered two-dimensional (2D) materials have set a new paradigm in modern solid-state physics and technology. In particular their exceptional optical and electronic properties have shown great promise for novel applications in light detection. However, several challenges remain to fully exploit such properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors (PDs), the efficient extraction of photoexcited charges and ultimately the environmental stability of such atomically-thin materials. In order to overcome the aforementioned limits, novel approaches to tune the properties of graphene and semiconducting \ce{HfS2} are explored in this work, using chemical functionalisation and laser-irradiation. Intercalation of graphene with \ce{FeCl3} is shown to lead to a highly tunable material, with unprecedented stability in ambient conditions. This material is used to define photo-active junctions with an unprecedented LDR via laser-irradiation. Intercalation with \ce{FeCl3} is also used to demonstrate the first all-graphene position-sensitive photodetector (PSD) promising for novel sensing applications. Finally, laser-irradiation is employed, to perform controlled oxidation of ultra-thin \ce{HfS2}, which leads to induced strain in the material and a consequent spatially-varying bandgap. Such structure is used to demonstrate, for the first time, efficient extraction of photogenerated carriers trough the so-called ``charge-funnel'' effect, paving the way to the development of ultra-thin straintronic devices.

In this dissertation work, the study focuses on large areal growth of MoS2 monolayers and a study of the structural, optical and electrical properties of such monolayers before and after transfer using a polymer-lift off technique. This work will discuss the issue of contact resistance and the effect of defects (both intrinsic and extrinsic) on the overall quality of the monolayer films. The significance of this dissertation work is that a reproducible strategy for monolayer MoS2 film growth and quantification of areal coverage as well as the detrimental effects of processing on device performance is presented.

The ability to convert electrical energy into mechanical motion is of significant interest in many energy conversion technologies. For more than a decade Ionic polymer-metal composite (IPMC) as an electroactive smart polymer material has been extensively studied and has shown great potential as soft robotic actuators, artificial muscles and dynamic sensors in the micro-to-macro size range. IPMC consists of an ion exchange polymer membrane sandwiched between two noble metal electrodes on either side of the membrane. Under applied potential, the IPMC actuator results in bending deformation because of ion migration and redistribution across its surface due to the imposed voltage. Nafion are highly porous polymer materials which have been extensively studied as the ion exchange membrane in IPMC. Nafion has also been mixed with carbon nanotubes, graphene, and metallic nanoparticles to improve actuation and bending characteristics of electro-mechanical actuators. For the first time, liquid phase exfoliated Transition Metal Dichalcogenides (TMDs)-Nafion nanocomposite based electro-mechanical actuators has been studied and demonstrate the improvement in the electromechanical actuation performance. In this thesis, we create a 2D model of the TMD-Nafion based electromechanical actuator in COMSOL Multi-physics software. The behavior of the model is examined at different electric potentials, frequencies, and actuation lengths. The simulation results were compared with the experimental data for validation of the model. The data showed improvement in the actuation for TMD-Nafion actuator when compared with pure Nafion actuator. The improvement in the actuation was due to the increase in diffusivity of the TMD-Nafion actuator in comparison with pure Nafion actuator. This increase in the diffusivity as seen in the model is because of the new proton conducting pathways being established with the addition of TMDs. The model also shows an increase in the stress and strain values with the incorporation of TMDs. With the same length of the actuator we were able to obtain more stress and strain with the addition of TMDs. This helps in improving the performance of the actuator as it would be able to handle more stress cycles which also increases the life of the actuator.

Group III-nitride materials have attracted great attention for applications in high efficiency electronic and optoelectronics devices such as high electron mobility transistors, light emitting diodes, and laser diodes. On the other hand, group VI transition metal dichalcogenides (TMDs) in the form of MX2 has recently emerged as a novel atomic layered material system with excellent thermoelectric, electronic and optoelectronic properties. Also, the recent investigations reveal that the dissimilar heterojunctions formed by TMDs and III-nitrides provide the route for novel devices in the area of optoelectronic, electronics, and water splitting applications. In addition, integration of III-nitrides and TMDs will enable high density integrated optoelectronic circuits and the development of hybrid integration technologies. In this work, we have demonstrated kinetically controlled growth processes in plasma assisted molecular beam epitaxy (PAMBE) for the III-nitrides and their engineered heterostructures. Techniques such as Ga irradiation and nitrogen plasma exposure has been utilized to implement bulk GaN, InGaN and their heterostructures in PAMBE. For the growth of III-nitride based heterostructures, the in-situ surface stoichiometry monitoring (i-SSM) technique was developed and used for implementing stepped and compositionally graded InGaN-based multiple quantum wells (MQWs). Their optical and microstrain analysis in conjunction with theoretical studies confirmed improvement in the radiative recombination rate of the graded-MQWs as compared to that of stepped-MQWs, owing to the reduced strain in graded-MQWs. Our achievement also includes the realization of the p-type MoS2 by engineering pristine MoS2 layers in PAMBE. Mainly, Ga and nitrogen plasma irradiation on the pristine MoS2 in PAMBE has resulted in the realization of the p-type MoS2. Also, GaN epitaxial thin layers were deposited on MoS2/c-sapphire, WSe2/c-sapphire substrates by PAMBE to study the band discontinuity at GaN/TMDs heterointerface. The determination of band offset parameters for both GaN/MoS2 and GaN/WSe2 heterostructures revealed realization of type-II band alignment. Also, heterojunctions such as AlGaN/MoS2 is implemented to achieve type-I heterojunction. This work may open up a new avenue towards photonic quantum devices based on the integration of III-nitrides with 2D TMDs.

No description available.

La miniaturisation des MOSFET a permis une forte diminution des transistors et des puces, ainsi qu’une augmentation exponentielle des capacités de calcul. Cette miniaturisation ne peut néanmoins continuer ainsi: de nos jours, un microprocesseur peut contenir des dizaines de milliards de transistors et la chaleur dégagée par ces composants peut fortement détériorer ses performances. De plus, du fait de leur principe même de fonctionnement, la tension d’alimentation des MOSFET ne peut être réduite sans en impacter les performances. De nouvelles architectures telles que le TFET -basé sur l’effet tunnel bande-à-bande et pouvant fonctionner à des tensions d’alimentation très basses- ainsi que de nouveaux matériaux pourraient donc apporter une alternative au MOSFET silicium. Les monocouches de dichalcogènures de métaux de transitions (TMDs) -des semiconducteurs à bande interdite directe d’environ 1 à 2 eV- possèdent un fort potentiel pour l’électronique et la photonique. De plus, dans le cas de contraintes appropriées, ils peuvent conduire un alignement de bandes présentant un broken-gap; cette configuration permet de surpasser les limites habituelles du TFETs, à savoir de faibles courants dus à l’effet tunnel sur lequel ces dispositifs reposent. Dans ce travail de thèse, des hétérojonctions planaires de TMD sont modélisées via une approche atomistique de liaisons fortes, et une configuration broken-gap est observée dans deux d’entre elles (MoTe2/MoS2 et WTe2/MoS2). Leur potentiel dans le cadre de transistors à effet tunnel (TFETs) est évalué au moyen de simulations de transport quantique basées sur un modèle TB atomistique ainsi que la théorie des fonctions de Green hors-équilibre. Des TFETs type-p et type-n basés sur ces hétérojonctions sont simulés et présentent des courants ON élevés (ION > 103 µA/µm) ainsi que des pentes sous-seuil extrêmement raides (SS < 5 mV/dec) à des tensions d’alimentation très faibles (VDD = 0.3 V). Plusieurs architectures novatrices basées sur ces TFETs et découlant de la nature 2D des matériaux utilisés sont également présentées, et permettent d’atteindre des performances encore plus élevées. / Nowadays, microprocessors can contain tens of billions of transistors and as a result, heat dissipation and its impact on device performance has increasingly become a hindrance to further scaling. Due to their working mechanism, the power supply of MOSFETs cannot be reduced without deteriorating overall performance, and Si-MOSFETs scaling therefore seems to be reaching its end. New architectures such as the TFET, which can perform at low supply voltages thanks to its reliance on band-to-band tunneling, and new materials could solve this issue. Transition metal dichalcogenide monolayers (TMDs) are 2D semiconductors with direct band gaps ranging from 1 to 2 eV, and therefore hold potential in electronics and photonics. Moreover, when under appropriate strains, their band alignment can result in broken-gap configurations which can circumvent the traditionally low currents observed in TFETs due to the tunneling mechanism they rely upon. In this work, in-plane TMD heterojunctions are investigated using an atomistic tight-binding approach, two of which lead to a broken-gap configuration (MoTe2/MoS2 and WTe2/MoS2). The potential of these heterojunctions for use in tunnel field-effect transistors (TFETs) is evaluated via quantum transport computations based on an atomistic tight-binding model and the non-equilibrium Green’s function theory. Both p-type and n-type TFETs based on these in-plane TMD heterojunctions are shownto yield high ON currents (ION > 103 µA/µm) and extremely low subthreshold swings (SS < 5 mV/dec) at low supply voltages (VDD = 0.3 V). Innovative device architectures allowed by the 2D nature of these materials are also proposed, and shown to enhance performance even further.

No description available.

It has been known since the 1930’s that protons and neutrons, collectively called as nucleons, are not “point-like” elementary particles, but rather have a substructure. Today, we know from Quantum Chromodynamics (QCD) that nucleons are made from quarks and gluons, with gluons being the elementary force carriers for strong interactions. Quarks and gluons are collectively called as partons. The substructure of the nucleons can be described in terms of parton correlation functions such as Form Factors, (1D) Parton Distribution Functions (PDFs) and their 3D generalizations in terms of Transverse Momentum-dependent parton Distributions (TMDs) and Generalized Parton Distributions (GPDs). All these functions can be derived from the even more general Generalized Transverse Momentum-dependent Distributions (GTMDs). This dissertation promises to provide an insight into all these functions from the point of view of their accessibility in experiments, from model calculations, and from their direct calculation within lattice formulations of QCD. In the first part of this dissertation, we identify physical processes to access GTMDs. By considering the exclusive double Drell-Yan process, we demonstrate, for the very first time, that quark GTMDs can be measured. We also show that exclusive double-quarkonium production in nucleon-nucleon collisions is a direct probe of gluon GTMDs. In the second part of this dissertation, we shift our focus to the “parton quasi-distributions”. Over the last few decades, lattice QCD extraction of the full x-dependence of the parton distributions has always been prohibited by the explicit time-dependence of the correlation functions. In 2013, there was a path-breaking proposal by X. Ji to calculate instead parton quasi-distributions (quasi-PDFs). The procedure of “matching” is a crucial ingredient in the lattice QCD extraction of parton distributions from the quasi-PDF approach. We address the matching for the twist-3 PDFs gT (x), e(x), and hL(x) for the very first time. We pay special attention to the challenges involved in the calculations due to the presence of singular zero-mode contributions. We also present the first-ever lattice QCD results for gT (x) and hL(x) and we discuss the impact of these results on the phenomenology. Next, we explore the general features of quasi-GPDs and quasi-PDFs in diquark spectator models. Furthermore, we address the Burkhardt-Cottingham-type sum rules for the relevant light-cone PDFs and quasi-PDFs in a model-independent manner and also check them explicitly in perturbative model calculations. The last part of this dissertation focuses on the extraction g1T (x,~k2⊥) TMD for the very first time from experimental data using Monte Carlo techniques. This dissertation therefore unravels different aspects of the distribution functions from varied perspectives.