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Defects in materials significantly influence their properties and enhance functionality. Hybrid functionals like HSE06, though effective for describing defects, face challenges in geometry optimization for large supercells. The r2SCAN+rVV10+U+Ud method provides a computationally efficient alternative. By selecting appropriate U and Ud values for the d orbitals of host and defect atoms, this method accurately describes defects in materials. Our study on small polaron defects in layered transition-metal oxides demonstrates this. Using literature values for U and Ud, we investigated birnessite (KnMnO2, n = 0.03) and KnNiO2, n = 0.03. With one K atom intercalated in a supercell, both materials show a localized eg polaronic state on the transition metal ion reduced by the K atom, when the geometry is calculated using published U values. The expected Jahn-Teller distortion is not observed when U=Ud=0. In layered cobalt oxide with additional potassium ions (KnCoO2, n = 1.03), a single extra K atom in the supercell leads to four localized electrons in the band gap, using standard U values, and even for U=Ud=0. Monolayer MoS2 exhibits intriguing properties and potential technological applications when subjected to strain. A recent experimental study reported that the bandgap of monolayer MoS2 on a mildly curved graphite surface decreases by 400 meV/% strain under biaxial strain with a Poisson’s ratio of 0.44. We conducted density functional theory (DFT) calculations on a free-standing MoS2 monolayer using the generalized gradient approximation (GGA) PBE, the hybrid functional HSE06, and many-body perturbation theory with the GW approximation using PBE wavefunctions (G0W0@PBE). Our findings indicate that under biaxial strain with the experimental Poisson’s ratio, the bandgap decreases at rates of 63 meV/% strain (PBE), 73 meV/% strain (HSE06), and 43 meV/% strain (G0W0@PBE), which are significantly lower than the experimental rate. Additionally, PBE predicts a reduction rate of 90 meV/% strain for a Poisson’s ratio of 0.25. Spin-orbit correction (SOC) has minimal impact on the bandgap or its strain dependence. We also observed a semiconductor-to-metal transition at 10% tensile biaxial strain and a shift from a direct to an indirect bandgap, aligning with previous theoretical studies. / Physics

Since the discovery of graphene, substantial effort has been put toward the synthesis and production of 2D materials. Developing scalable methods for the production of high-quality exfoliated nanosheets has proved a significant challenge. To date, the most promising scalable method for achieving these materials is through the liquid-based exfoliation (LBE) of nanosheetsin solvents. Thin films of nanosheets in dispersion can be modified with additives to produce 2D inks for printed electronics using inkjet printing. This is the most promising method for the deposition of such materials onto any substrate on an industrial production level. Although well-developed metallic and organic printed electronic inks exist on the market, there is still a need to improve or develop new inks based on semiconductor materials such as transition metal dichalcogenides (TMDs) that are stable, have good jetting conditions and deliver good printing quality.The inertness and mechanical properties of layered materials such as molybdenum disulfide (MoS2) make them ideally suited for printed electronics and solution processing. In addition,the high electron mobility of the layered semiconductors, make them a candidate to become a high-performance semiconductor material in printed electronics. Together, these features make MoS2 a simple and robust material with good semiconducting properties that is also suitable for solution coating and printing. It is also environmentally safe.The method described in this thesis could be easily employed to exfoliate many types of 2D materials in liquids. It consists of two exfoliation steps, one based on mechanical exfoliation of the bulk powder utilizing sand paper, and the other inthe liquid dispersion, using probe sonication to liquid-exfoliate the nanosheets. The dispersions, which were prepared in surfactant solution, were decanted, and the supernatant was collected and used for printing tests performed with a Dimatix inkjetprinter. The printing test shows that it is possible to use the MoS2 dispersion as a printed electronics inkjet ink and that optimization for specific printer and substrate combinations should be performed. There should also be advances in ink development, which would improve the drop formation and break-off at the inkjet printing nozzles, the ink jetting and, consequently, the printing quality. / Sedan upptäckten av grafen har mycket arbete lagts på framställning och produktion av 2D-material. En viktig uppgift har varit att ta fram skalbara metoder för produktion av högkvalitativa  nanosheets via exfoliering. Den mest lovande skalbarametoden hittills har varit vätskebaserad exfoliering av nanosheets i lösningsmedel. Tunna filmer av nanosheets i dispersion kan anpassas med hjälp av tillsatser och användas för tillverkning av halvledare strukturer med inkjet-skrivare, vilket är den mest lovande metoden för på en industriell produktions nivå beläggaden typen av material på substrat. Även om det finns välutvecklade metalliska och organiskabläck för tryckt elektronik, så finns det fortfarande ett behov av att förbättra eller utveckla nya bläck baserade på halvledarmaterial som t.ex. TMD, som är stabila, har goda bestryknings  egenskaper och ger bra tryckkvalitet. Den inerta naturen tillsammans med de mekaniska egenskaperna som finns hosskiktade material, som t.ex. molybdendisulfid (MoS2), gör demlämpliga för flexibel elektronik och bearbetning i lösning. Dessutom gör den höga elektronmobiliteten i dessa 2D-halvledaredem till en stark kandidat som halvledarmaterial inom trycktelektronik. Det betyder att MoS2 är ett enkelt och robust material med goda halvledaregenskaper som är lämpligt för bestrykning från lösning och tryck, och är miljömässigt säker.Den metod som beskrivs här kan med fördel användas föratt exfoliera alla typer av 2D-material i lösning. Exfolieringensker i två steg; först mekanisk exfoliering av torr bulk med sandpapper, därefter används ultraljudsbehandling i lösning för att exfoliera nanosheets. De dispersioner som framställts i lösning med surfaktanter dekanterades och det övre skiktetanvändes i trycktester med en Dimatix inkjet-skrivare.Tryckprovet visar att det är möjligt att använda MoS2 -dispersion som ett inkjet-bläck och att optimering för särskildaskrivar- och substratkombinationer borde göras, såsom förbättringav bläcksammansättningen med avseende på droppbildning och break-off vid skrivarmunstycket, vilket i sin tur skulleförbättra tryckkvaliteten. / KM2 / Paper Solar Cells

The ability to convert photons into mechanical motion is of significant importance for many energy conversion and reconfigurable technologies. Establishing an optical-mechanical interface has been attempted since 1881; nevertheless, only few materials exist that can convert photons of different wavelengths into mechanical motion that is large enough for practical import. Recently, various nanomaterials including nanoparticles, nanowires, carbon nanotubes, and graphene have been used as photo-thermal agents in different polymer systems and triggered using near infrared (NIR) light for photo-thermal actuation. In general, most photomechanical actuators based on sp bonded carbon namely nanotube and graphene are triggered mainly using near infra-red light and they do not exhibit wavelength selectivity. Layered transition metal dichalcogenides (TMDs) provide intriguing opportunities to develop low cost, light and wavelength tunable stimuli responsive systems that are not possible with their conventional macroscopic counterparts. Compared to graphene, which is just a layer of carbon atoms and has no bandgap, TMDs are stacks of triple layers with transition metal layer between two chalcogen layers and they also possess an intrinsic bandgap. While the atoms within the layers are chemically bonded using covalent bonds, the triple layers can be mechanically/chemically exfoliated due to weak van der Waals bonding between the layers. Due to the large optical absorption in these materials, they are already being exploited for photocatalytic, photoluminescence, photo-transistors, and solar cell applications. The large breaking strength together with large band gap and strong light- matter interaction in these materials have resulted in plethora of investigation on electronic, optical and magnetic properties of such layered ultra-thin semiconductors. This dissertation will go in depth in the synthesis, characterization, development, and application of two- dimensional (2D) nanomaterials, with an emphasis on TMDs and molybdenum disulfide (MoS2), when used as photo-thermal agents in photoactuation technologies. It will present a new class of photo-thermal actuators based on TMDs and hyperelastic elastomers with large opto-mechanical energy conversion, and investigate the layer-dependent optoelectronics and light-matter interaction in these nanomaterials and nanocomposites. Different attributes of semiconductive nanoparticles will be studied through different applications, and the possibility of globally/locally engineering the bandgap of such nanomaterials, along with its consequent effect on optomechanical properties of photo thermal actuators will be investigated. Using liquid phase exfoliation in deionized water, inks based on 2D- materials will be developed, and inkjet printing of 2D materials will be utilized as an efficient method for fast fabrication of functional devices based on nanomaterials, such as paper-graphene-based photo actuators. The scalability, simplicity, biocompatibility, and fast fabrication characteristics of the inkjet printing of 2D materials along with its applicability to a variety of substrates such as plastics and papers can potentially be implemented to fabricate high-performance devices with countless applications in soft robotics, wearable technologies, flexible electronics and optoelectronics, bio- sensing, photovoltaics, artificial skins/muscles, transparent displays and photo-detectors.

Les travaux réalisés portent sur l'étude de l'émission de photons induite par un microscope à effet tunnel fonctionnant sous ultravide et à température ambiante. Sur des surfaces métalliques, le phénomène étudié est lié à la désexcitation radiative de modes plasmons localisés entre la pointe et la surface et excités par des électrons transitant inélastiquement dans la jonction. Dans un premier temps, ce phénomène a été étudié sur des surfaces d'or en fonction de différents paramètres pouvant varier dans la jonction (milieu, pointe, courant tunnel, tension de polarisation). Par la suite, des spectres localisés ont pu être obtenus en localisant la pointe sur des zones données de la surface. Des modifications de la répartition en énergie des photons émis ont été observées et attribuées soit à la surface étudiée (morphologie, cristallographie, contamination), soit à la pointe utilisée (matériau, géométrie de l'apex). Enfin, le dispositif a été appliqué à l'étude de nano-objets métalliques constitués d'îlots d'or de taille nanométrique déposés sur un substrat semi-conducteur (MoS2). Dans ce cas, la position du seuil haute énergie du spectre permet de déterminer l'énergie réellement injectée dans la jonction. Les résultats obtenus montrent qu'il est possible de déterminer les propriétés électriques de l'interface entre l'îlot et le subtrat grâce à la mesure de ce seuil. Des caractéristiques I(V) de type redresseur ont pu être observées lors de la réduction en taille de ces îlots, faisant de ce dispositif un outil de caractérisation électrique de nano-composants.

There has been growing interest in two-dimensional (2-D) crystals beyond graphene for next-generation nano-electronics. Transition metal dichalcogenides have been most widely studied, for their semiconducting characteristics and hence, potential applications. This interest has fueled many efforts to establish methods for synthesis of MoS2 layers, a most promising candidate, in controlled numbers over large areas. One of the most scalable methods is chemical vapor deposition (CVD). The current approaches to growth from the vapor phase are by and large very empirical. This thesis is hence concerned with the predictive synthesis of n-layered MoS2 using CVD uniformly over large areas and the correlation of growth parameters with the structural and electronic properties of the deposited films. A simple, relatively non-toxic and non-pyrophoric chemistry, consisting of Mo(CO)6 and H2S was first chosen for vapor phase synthesis. This chemistry allowed synthesis of MoS2 from precursors located outside of the growth reactor, a necessary condition for electronics device technology. Iterative thermodynamic modeling of the Mo-S-C-O-H system and growth was then done to identify the appropriate CVD process windows for the growth of pure MoS2, departures from stoichiometry, contamination and breakdown of equilibrium modelling. Remarkable agreement between theoretical modelling and actual growth has been observed leading to predictable deposition. Within these thermodynamic windows, the gas phase supersaturation were then reduced to obtain better kinetic control over crystal growth. It is shown that control of supersaturation at the very initial stages of growth is critical to reduce the nucleation density and hence obtain monolayers with small defect densities. In addition, it is shown that at higher temperatures the kinetics of nucleation and growth are determined by the supersaturation on the growth surface. Physico-chemical modelling reveals that this steady state supersaturation is determined by the kinetics of adsorption and desorption. All of this understanding has been used to realize a variety of structures from discrete crystalline islands- 30 nm to 150 microns- to deposits with controlled number of layers – n =1 to 6 or greater- uniformly over large areas on quartz and sapphire. Gas phase chemistry also affects the electrical characteristics of the as deposited layers. It is shown, for the first time, that by changing gas phase Mo to S ratios the stoichiometry of the deposited layers MoS2 can be made metal or chalcogen deficient. This yields MoS2 that can be either p-type or n-type. p-type and n-type MoS2 with mobilities up to 7.4 cm2/Vs and 40 cm2/Vs respectively are demonstrated. FETs fabricated on MoS(2-x) samples (increasing x) with varying stoichiometry showed a maximum on-current of 18 μA (4.5 μA/μm) in vacuum and 0.6 μA (0.15 μA/μm) in air for a drain bias Vds = 1 V. Sulphur deficiency also affect reliability. While samples with a higher concentration of sulphur vacancies have higher mobility in vacuum, the mobility degrades significantly in air and gets reversed on annealing in H2S. The details of such correlation between growth and electrical characteristics are discussed in this thesis.

Semiconductor nanostructures have attracted considerable research interest due to their unique physical and chemical properties at nanoscale which open new frontiers for applications in electronics and sensing. Zinc oxide nanostructures with a wide range of applications, especially in optoelectronic devices and bio sensing, have been the focus of research over the past few decades. However ZnO nanostructures have failed to penetrate the market as they were expected to, a few years ago. The two main reasons widely recognized as bottleneck for ZnO nanostructures are (1) Synthesis technique which is fast, economical, and environmentally benign which would allow the growth on arbitrary substrates and (2) Difficulty in producing stable p-type doping. The main objective of this research work is to address these two bottlenecks and find a solution that is inexpensive, environmentally benign and CMOS compatible. To achieve this, we developed a Sonochemical method to synthesize 1D ZnO Nanorods, core-shell nanorods, 2D nanowalls and nanoflakes on arbitrary substrates which is a rapid, inexpensive, CMOS compatible and environmentally benign method and allows us to grow ZnO nanostructures on any arbitrary substrate at ambient conditions while most other popular methods used are either very slow or involve extreme conditions such as high temperatures and low pressure. A stable, reproducible p-type doping in ZnO is one of the most sought out application in the field of optoelectronics. Here in this project, we doped ZnO nanostructures using sonochemical method to achieve a stable and reproducible doping in ZnO. We have fabricated a homogeneous ZnO radial p-n junction by growing a p-type shell around an n-type core in a controlled way using the sonochemical synthesis method to realize ZnO homogeneous core-shell radial p-n junction for UV detection. ZnO has a wide range of applications from sensing to energy harvesting. In this work, we demonstrate the successful fabrication of an electrochemical immunosensor using ZnO nanoflakes to detect Cortisol and compare their performance with that of ZnO nanorods. We have explored the use of ZnO nanorods in energy harvesting in the form of Dye Sensitized Solar Cells (DSSC) and Perovskite Solar Cells.

We have deposited bulk, monolayer and few-layers as well as large-scale 2D layered MoS2 thin films by pulsed DC magnetron sputtering from an MoS2 target. MoS2 has gained great attention lately, together with other layered Transition Metal Dichalcogenides (TMDCs), for its unique optical and electrical properties with thickness-dependent bandgap. MoS2 also transitions from an indirect to a direct bandgap when thinned down to monolayer. This is intriguing in the fabrication of novel solar cells and photodetectors. Sputter-deposition has the advantage of producing large-scale, high-quality films, which is paramount for layered MoS2 to be applicable on an industrial level. The quality in terms of crystallinity and c⊥-texture of sputtered bulk MoS2 was evaluated as a function of several deposition process parameters: process pressure, substrate temperature and H2S-to-Ar ratio. X-ray Diffraction (XRD) results revealed that the high substrate temperature of 700 °C together with reactive H2S process gas improved the quality regardless of pressure. However, the quality was slightly improved further with increasing pressure up to 50 mTorr. We also found that the quality improved with increasing temperature up to 700 °C using pure Ar as the process gas. Rutherford Backscattering Spectrometry (RBS) analysis showed that with the addition of H2S the stoichiometry of MoSx improved from MoS1.78 using pure Ar to fully stoichiometric MoS2.01 at 40% H2S in the H2S/Ar mixture. Cross-sectional Transmission Electron Microscopy (TEM) imaging revealed the high-quality 2D layered structure of the MoS2 films and a maximum thickness of 5 nm of c⊥-growth MoS2 before the onset of the undesirable c∥-growth. These results provide a solution with respect to the ongoing challenge of obtaining high quality and good stoichiometry of sputtered TMDC films at elevated temperatures. Formation of monolayer and few-layers MoS2 was confirmed by Raman and Photoluminescence (PL) spectroscopy. The peak separation of the E12g and A1g Raman-active modes for MoS2 monolayer was measured to 19.3 cm-1 on SiO2/Si, increases substantially in the transition to bilayer MoS2 and exhibits bulk values from four layers MoS2 and above. This result serves as a good indicator of monolayer as well as few-layers MoS2 formation. The monolayer film exhibits a strong photoluminescence peak at 1.88 eV owing to its direct optical bandgap, as compared to the indirect one of bilayer and thicker films. X-ray Photoelectron Spectroscopy (XPS) spectra of the monolayer MoSx film indicate successful sulfurization of the molybdenum atoms and absence of residual sulfur. XPS also showed ideal stoichiometric MoS2.03 ± 0.03 of the monolayer film. Furthermore, a uniform MoS2 monolayer was successfully grown on a 4" SiO2/Si wafer, demonstrating the large-scale uniformity that can be achieved by sputter-deposition, making it highly applicable on an industrial level.

As part of the EU SmartVista project to develop a multi-modal wearable sensor for health diagnostics, field-effect transistor (FET) based biosensors were explored, with glucose as the analyte, and carbon nanotubes (CNTs) or monolayer MoS2 as the semiconducting sensing layer.  Numerous arrays of CNT-FETs and MoS2-FETs were fabricated by photolithographic methods and packaged as integrated circuits.  Functionalization of the sensing layer using linkers and enzymes was performed, and the samples were characterized by atomic force microscopy, scanning electron microscopy, optical microscopy, and electrical measurements. ON/OFF ratios of 102 p-type and < 102 n-type were acheived, respectively, and the work helped survey the viability of realizing such sensors in a wearable device. / EU Horizon 2020 - SmartVista (825114)