Two dimensional transition metal dichalcogenides provide a host of unique optoelectronic properties, attributed to their two dimensional nature and unique band structure, making them promising for future optoelectronics device applications. In the work presented in this thesis, we focus on the theoretical understanding and modelling of the optoelectronic properties of monolayer transition metal dichalcogenides, their heterostructures and multilayers. We studied the relaxation rates of photo-excited carriers leading to the formation of electron-hole pairs and their subsequent radiative recombination, resulting in emission of light. We find sub-ps relaxation times, attributed to the strong coupling of carriers with optical phonons, allowing the efficient formation of strongly bound multi-particle complexes such as excitons, trions and biexcitons, which can recombine radiatively if allowed by selection rules. We classify the various complexes according to their optical activity, and predict using diffusion quantum Monte Carlo calculations the resulting photoluminescence spectra in these materials. We proposed a novel, material specific, Auger process in WS2 and WSe2 involving dark excitons, which dominates over radiative processes for relatively low carrier densities, providing an explanation to the observed low quantum efficiencies in these materials. In the same pair of materials, we have shown how the ground state dark trions and biexcitons can become bright and recombine radiatively through an electron-electron intervalley scattering process, resulting in new observable lines in the photoluminescence spectra of these materials. The ability to form van der Waals heterostructures of two or more layers of these materials, allows for new degrees of freedom to be explored and utilised. The heterobilayer system made of MoSe2/WSe2 has a type-II band alignment, allowing for the formation of interlayer bound complexes with carriers localized on opposite layers. We studied the bound complexes formed in this bilayer system, localized on donor impurities. We used quantum Monte Carlo methods to obtain binding energies and wave functions, and calculated the radiative rates and doping dependent photoluminescence spectra of these complexes for closely aligned layers, and asymptotic behaviour for strongly misaligned layers. Finally, we studied few-layers of 2H-stacked transition metal dichalcogenides. The van der Waals quantum well structure results in the splitting of the conduction and valence bands into multiple subbands with energy spacings covering densely the infrared to far-infrared spectral range. We developed a hybrid k.p-tight binding model parameterised by DFT calculations of monolayer and bulk crystals of the studied materials. We used the model to describe the subband dispersions, transition energies, phonon induced broadening and resulting absorption lineshapes for both p-doped and n-doped few-layer films.

Chalcogen-rich transition-metal dichalcogenide (TMD) magnetron sputtering targets were custom manufactured via ball milling and sintering in the interest of depositing p-type chalcogen-rich films. Room temperature radio frequency (RF) magnetron sputtering produced ultra-thin amorphous precursor of WSx and MoSx (where x is between 2-3) on several different substrates. The influence of working pressure on the MoS3 content of the amorphous films was explored with X-ray photoelectron spectroscopy (XPS), while the physical and chemical effects of sputtering were investigated for the WSx target itself. The amorphous precursor films with higher chalcogenide content were chosen for laser annealing, and their subsequent laser annealing induced phase transformations were investigated for the synthesis of polycrystalline 2H-phase semiconducting thin films. The role of laser fluence and the number of laser pulses during annealing on phase transformation and film mobility was determined from Raman spectroscopy and Hall effect measurement, respectively. Hall effect measurements were used to identify carrier type and track mobility between amorphous precursors and crystalline films. The p-type 2H-TMD films demonstrates the ability to produce a scalable processing criterion for quality ultra-thin TMD films on various substrates and in a method which is also compatible for flexible, stretchable, transparent, and bendable substrates.

We discuss the effect of elastic deformations on the electronic properties of atomically thin materials, with a focus on bilayer graphene and MoS2 membranes. In these materials distortions of the lattice translate into fictitious gauge fields in the electronic Dirac Hamiltonian that are explicitly derived here for arbitrary elastic deformations, including in-plane as well as flexural (out-of-plane) distortions. We consider bilayer graphene, where a constant fictitious gauge field causes a dramatic reconstruction of the low energy trigonally warped electronic spectrum inducing topological transitions in the Fermi surface. We then present results of ballistic transport in trigonally warped bilayer graphene with and without strain, with particular focus on noise and the Fano factor. With the inclusion of trigonal warping the Fano factor at the Dirac point is still F = 1/3, but the range of energies which show pseudo diffusive transport increases by orders of magnitude compared to the results stemming out of a parabolic spectrum and the applied strain acts to increase this energy range further. We also consider arbitrary deformations of another two-dimensional membrane, MoS2. Distortions of this lattice also lead to a fictitious gauge field arising within the Dirac Hamiltonian, but with a distinct structure than seen in graphene. We present the full form of the fictitious gauge fields that arise in MoS2. Using the fictitious gauge fields we study the coupling between electronic and mechanical degrees of freedom, in particular the coupling between electrons and excited vibrational modes, or vibrons. To understand whether these effects may have a strong influence on electronic transport in MoS2 we calculate the dimensionless electron-vibron coupling constant for all vibron modes relevant for electronic transport. We find that electron-vibron coupling constant is highly sample specific and that the longitudinal stretching mode is the vibron with the dominant coupling. This however reaches maximum values which are lower than those observed in carbon nanostructures.

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.

Two dimensional (2D) materials are poised to revolutionize the future of optics and electronics. The past decade saw intense research centered around graphene. More recently, the tide has shifted to a bigger class of two-dimensional materials including graphene but more expansive in their capabilities. The so called ‘2D material zoo’ includes metals, semi-metals, semiconductors, superconductors and insulators. The possibility of mixing and matching 2D materials to fabricate heterostructures with desirable properties is very exciting. To make devices with superior electronic, optical and thermal properties, we need to understand how the electrons, phonons and other quasi particles interact with each other and exchange energy in the femtosecond and nanosecond timescales. To measure the timescales of energy distribution and dissipation, I used ultrafast pump-probe spectroscopy to perform time-domain measurements of optical absorption. This approach allows us to understand the impact of manybody interactions on the bandstructure and carrier dynamics of 2D materials. After a brief introduction to femtosecond laser spectroscopy, I will explore the transient absorption dynamics of three classes of 2D materials: intrinsic graphene, graphene-hBN heterostructures and Transition Metal Dichalcogenides (TMDs). We will see that using pumpprobe measurements around the high energy M-point of intrinsicgraphene, we can extract the value of the acoustic deformation potential which is vital in characterizing the electron-acoustic phonon interactions. In the next part of the thesis, I will delineate the role of the substrate in the cooling dynamics in graphene devices. We will see that excited carriers in graphene on hBN substrates cool much faster that on SiO2 substrates due to faster decay of the optical phonons in graphenehBN heterostructures. These results show that graphene-hBN heterostructures can solve the hot phonon bottleneck that plagues graphene devices at high power densities. In the last part, I will demonstrate the role of phonon induced bandgap renormalization in the carrier dynamics of TMD materials and measure the timescale of phonon decay through the generation of low-energy phonons and transfer to the substrate. This study will help us understand carrier recombination in TMD devices under high-bias conditions which show great potential in opto-electronic applications such as photovoltaics, LEDs etc.

Recently, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) materials have drawn significant attention due to their unique optical and electrical properties1, 2. This offers unique opportunities for the next-generation electronic and optoelectronic devices3. Specifically, recent innovations in the big-data-driven prediction of new 2D materials, integration of new device architectures, interfacial engineering of contacts between semiconductor/metals and semiconductor/dielectrics as well as encapsulation in hexagonal boron nitride4, 5 have further propelled the electrical mobility to be on a par with or even beyond the silicon (Si) counterpart. These strategies hold tantalizing prospects on extending the Moore's law. Yet, there is still room for improvement before 2D TMDCs become truly technologically relevant. The challenge lies in the full validation of the intrinsic charge transport that is associated with the specific nature and ordered arrangement of atoms in the atomically thin crystal lattice. This requires, the controlled stitch of both metals and chalcogenides in an atom-by-atom fashion. To this end, a variety of synthetic approaches have been developed, this includes but not limited to chemical vapor deposition (CVD) 6, 7, mechanical exfoliation8 and solution-based exfoliation9. Among which, CVD shows better controllability over thicknesses, geometric shapes, sizes, and qualities through manipulation of the growth factors, e.g., growth temperature, pressure, precursor ratio, and gas carrier. These complex growth environments will significantly confound the scalability, crystallinity, defect density, and reproducibility of the CVD approach. Therefore, an impetus exists to gain fundamental insights into the universal growth mechanism that is currently lacking and therefore curbs the realization o the controlled epitaxy of high-mobility three-atom-thick semiconducting TMDCs films with wafer-scale-homogeneity. In this thesis, a mechanistic study toward revealing the epitaxy growth mechanism is established to include 1) epitaxy growth of multilayer, 2) epitaxy growth of heterostructures, and 3) epitaxy growth of high quality (exceedingly low defect density) of 2D TMDCs materials through a controlled CVD strategy.

In this thesis, two-dimensional materials such as graphene are tested for their suitability for opto-electronic applications using terahertz time domain spectroscopy (THz-TDS). This ultrafast all-optical technique can probe the response of novel materials to photoexcitation, and yield information about the dynamics of the material systems. Graphene grown by chemical vapour deposition (CVD) is studied using optical-pump THz-probe time domain spectroscopy in a variety of gaseous environments in Chapter 4. The photoconductivity response of graphene grown by CVD is found to vary dramatically depending on which atmospheric gases are present. Adsorption of these gases can open a local bandgap in the material, allowing stimulated emission of THz radiation across the gap. Semiconducting equivalents to graphene, molybdenum disulphide (MoS₂) and tungsten diselenide (WSe₂), grown by CVD, are investigated in Chapter 5. These members of the transition metal dichalcogenide family show sub-picosecond responses to photoexcitation, suggesting promise for use in high-speed THz devices. In Chapter 6, an alternative production route to CVD is studied. Liquid-phase exfoliation offers fast, easy production of few-layer materials. THz spectroscopy reveals that the dynamics of these materials after photoexcitation are remarkably similar to those in CVD-grown materials, offering the potential of cheaper materials for future devices. Finally in Chapter 7, it is shown that carbon nanotubes can be used to make ultrafast THz devices. Unaligned, semiconducting single walled carbon nanotubes can be photoexcited to produce an ultrafast, dynamic THz polariser. The work in this thesis demonstrates the potential for these novel materials in future opto-electronic applications. THz spectroscopy is shown to be an important tool for the characterisation of new materials, providing information that can be used to understand the dynamics of materials, and improve production methods.

L’étude intitulée « optical properties of thin layers of transition metal dichalcogenides » vise les phénomènes physiques émergeant à la limite de la bidimensionnalité, lorsque l’épaisseur du composé ciblé est à l’échelle atomique. Les effets de la dimensionnalité réduite sur les propriétés physiques furent initialement explorés dans le graphène. Les études concernant ce composé se concentrent surtout sur ses propriétés de transport puisque le graphène lui-même n’a pas bande interdite. Les sc-TMD, en plus de présenter une structure atomique et électronique similaire au graphène (vallée aux points K dans la zone de Brillouin), peuvent également être produit sous forme de couche monoatomique. Ainsi, plusieurs études révélèrent que ces composés en couche mince combinent des propriétés découlant de leur caractère 2D en plus des caractéristiques typiques des semi-conducteurs. De plus, la dimensionnalité de ces composés joue un rôle important dans la structure électronique. Plus précisément, les sc-TMD présentent dans le régime tridimensionnel un gap indirect qui devient direct lorsque les composés sont sous la forme d’une monocouche. Cette thèse est une étude complète des propriétés optiques des composés sc-TMD. Le manuscrit en question est divisé en cinq parties : trois sections principales précédées par une introduction. L’ensemble est complété par une annexe présentant des études complémentaires sur un autre composé 2D : le nitrure de bore hexagonal (h-BN).Introduction :Les propriétés fondamentales des composés étudiés sont présentées en mettant l’accent sur celles qui jouent un rôle important dans la réponse optique de sc-TMD. Plus précisément, on y retrouve des informations sur la structure cristalline et la structure de bandes électroniques. Cette section détaille également le processus de préparation des échantillons ainsi que les divers montages expérimentaux utilisés.Chapitre 1 : caractérisation optique de base des excitons en résonance dans les couches et les multicouches de sc-TMD.La réponse optique des composés (surtout le MoSe2 et le WSe2) obtenue par spectroscopie de réflexion et par spectroscopie d’émission est interprétée. En particulier, l’impact du nombre de monocouches et de la température sur celle-ci est discuté. De plus, une étude complémentaire de ces propriétés optiques résolue temporellement y est insérée.Chapitre 2 : Spectroscopie Zeeman des excitons résonants sous champ magnétique.L’évolution des résonances optiques en fonction d’un champ magnétique appliqué perpendiculairement aux couches est l’objet de cette section. Un modèle phénoménologique décrivant la dépendance en champs magnétique de l’énergie des états électroniques est dérivé directement des résultats expérimentaux présentés dans cette section. L’effet de pompage optique est également étudié dans la monocouche de WSe2, effet qui est très sensible aux champs magnétiques.Chapitre 3 : Émetteurs de photons uniques dans les couches minces de sc-TMD.La découverte de raies d’émission fines et localisées sur de minces cristallites de sc-TMD est présentée, suivie d’une étude approfondie sur leur nature et leurs propriétés. Entre autres, leur évolution en fonction de la température, leur sensibilité aux champs magnétiques appliqués et leur polarisation sont discutées. Finalement, la spectroscopie de corrélation de photon est utilisée pour vérifier le caractère « source de photon unique » de ces émetteurs.Annexe A : Émetteurs de photons uniques dans le nitrure de bore hexagonal.Le h-BN partage de nombreuses caractéristiques avec les sc-TMD tout en se distinguant de ceux-ci par la présence d’un gap électronique significativement plus grand. Certaines régions cristallines se comportent comme des défauts ponctuels, dans les matériaux caractérisés par leur large gap, en présentant des raies d’émission fines. Ces régions partagent une similarité frappante avec les émetteurs de photons uniques observés dans le WSe2. / The research reported in the thesis entitled ‘Optical properties of thin layers of transition metal dichalcogenides’ focuses on physical phenomena which emerge in the limit of two-dimensional (2D) miniaturisation when the thickness of fabricated films reaches an atomic scale. The importance of such man-made structures has been revealed by the dynamic research on graphene: a single atomic plane of carbon atoms arranged in honeycomb lattice. Graphene is intrinsically gapless and therefore mainly explored with respect to its electric properties. The investigation of semiconducting materials which can also display the hexagonal crustal structure and which can be thinned down to individual layers, bridges the concepts characteristic of graphene-like systems (K-valley physics) with more conventional properties of semiconductors. This has been indeed demonstrated in a number of recent studies of ultra-thin films of semiconducting transition metal dichalcogenides (sc-TMD). Particularly appealing, from the point of view of optical studies, is a transformation of the bandgap alignment of sc-TMD films, from the indirect bandgap bulk crystals to the direct bandgap system in single layers. The presented thesis work provides a comprehensive optical characterisation of thin structures of sc-TMD crystals. The manuscript is divided into five parts: three main chapters with a preceding introduction and the appendix reporting the supplementary studies of another layered material: hexagonal boron nitride.Introduction. The fundamental properties of the investigated crystals are presented, especially those which are important from the point of view of optical studies. The discussion includes information on the crystal structure, Brillouin zone and electronic band structure. Also, the general description of the samples’ preparation process and experimental set-up is provided.Chapter 1. Basic optical characterisation of excitonic resonances in mono- and multi-layers of sc-TMDs. The optical response, as seen in the reflectance and luminescence spectra of thin sc-TMDs is analysed (mostly for MoSe2 and WSe2 materials). The impact of the number of layers and temperature on the optical resonances is studied and interpreted in details. The complementary time-resolved study is also presented.Chapter 2. Zeeman spectroscopy of excitonic resonances in magnetic fields. The evolution of the optical resonances in an external magnetic field, applied perpendicularly to the layers of sc-TMD materials is investigated. Based on these results, a phenomenological model is developed aiming to describe the linear with magnetic field contributions to the energy of individual electronic states in fundamental sub-bands of sc-TMD monolayers. Furthermore, the effects of optical pumping are investigated in WSe2 monolayers, which can be tuned by tiny magnetic fields.Chapter 3. Single photon sources in thin sc-TMD flakes. The discovery of localised narrow lines emitting centres has been in thin sc-TMD flakes is presented. An investigation of their fundamental properties is discussed. This includes the measurements of temperature and magnetic field evolution of the photoluminescence lines, and the analysis of the polarisation properties and the excitation spectra as well as photon correlation measurements.Appendix A. Single photon emitters in boron nitride crystals. Hexagonal boron nitride also belongs to the family of layered materials, but it exhibits much larger band gap than semiconducting transition metal dichalcogenides. A narrow lines emitting centres has been observed in boron nitride structures, which reveal multiple similarities to defect centres in wide gap materials. They are characterised in a similar manner as the emitting centres in WSe2.

abstract: Ultrasonication-mediated liquid-phase exfoliation has emerged as an efficient method for producing large quantities of two-dimensional materials such as graphene, boron nitride, and transition metal dichalcogenides. This thesis explores the use of this process to produce a new class of boron-rich, two-dimensional materials, namely metal diborides, and investigate their properties using bulk and nanoscale characterization methods. Metal diborides are a class of structurally related materials that contain hexagonal sheets of boron separated by metal atoms with applications in superconductivity, composites, ultra-high temperature ceramics and catalysis. To demonstrate the utility of these materials, chromium diboride was incorporated in polyvinyl alcohol as a structural reinforcing agent. These composites not only showed mechanical strength greater than the polymer itself, but also demonstrated superior reinforcing capability to previously well-known two-dimensional materials. Understanding their dispersion behavior and identifying a range of efficient dispersing solvents is an important step in identifying the most effective processing methods for the metal diborides. This was accomplished by subjecting metal diborides to ultrasonication in more than thirty different organic solvents and calculating their surface energy and Hansen solubility parameters. This thesis also explores the production and covalent modification of pristine, unlithiated molybdenum disulfide using ultrasonication-mediated exfoliation and subsequent diazonium functionalization. This approach allows a variety of functional groups to be tethered on the surface of molybdenum disulfide while preserving its semiconducting properties. The diazonium chemistry is further exploited to attach fluorescent proteins on its surface making it amenable to future biological applications. Furthermore, a general approach for delivery of anticancer drugs using pristine two-dimensional materials is also detailed here. This can be achieved by using two-dimensional materials dispersed in a non-ionic and biocompatible polymer, as nanocarriers for delivering the anticancer drug doxorubicin. The potency of this supramolecular assembly for certain types of cancer cell lines can be improved by using folic-acid-conjugated polymer as a dispersing agent due to strong binding between folic acid present on the nanocarriers and folate receptors expressed on the cells. These results show that ultrasonication-mediated liquid-phase exfoliation is an effective method for facilitating the production and diverse application of pristine two-dimensional metal diborides and transition metal dichalcogenides. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2018

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22988号 / 理博第4665号 / 新制||理||1669(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柳瀬 陽一, 教授 田中 耕一郎, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM