Dissertations / Theses on the topic 'Tungsten Disulfide'

The objective of this research was to generate shock-resistant materials based on inorganic fullerene type tungsten disulfide (IF-WS2) and carbon nanocomposite structures for personal protection armor systems. The aim was to develop a new generation of composites that combine the known energy absorbing properties of carbon nanofibers, with the shock absorbing properties reported for IF-WS2 structures. Various methods were explored to generate the desired WS2carbon fiber composite. Experimentation revealed that in situ growth of carbon fibers from a nickel catalyst with tungsten disulfide particulates had to be performed from particular precursors and fabrication conditions to avoid undesirable byproducts that hinder fiber growth. As a result, tungsten oxide was used as tungsten source, nickel as carbon fiber growth catalyst, ethylene as hydrocarbon and fuel rich oxidative conditions for growth, all followed by a sulfurization process. Fabrication of fibers was performed at moderate temperatures (ca. 550 degrees C) with a sulfurization step at 900 degrees C in a tubular furnace. Microstructural characterization of the samples was primarily conducted using X-ray diffraction and electron microscopy. In order to determine more properties of the nanocomposites, the samples were dispersed into an epoxy matrix. Nano-indentation was utilized as a method of determining mechanical properties of the composites while a gas gun was used to determine shock propagation effects. The inclusion of WS2/C nanocomposites into epoxy matrixes showed a significant improvement in modulus and hardness values when compared to bare carbon fiber epoxy composites. WS2/C fiber epoxy nanocomposites preserved their integrity during gas gun tests while samples without WS2 fractured.

Les normes environnementales toujours plus sévères associées à une constante augmentation de la demande énergétique rendent nécessaires les actions à entreprendre en vue d’améliorer l’efficacité énergétique. Dans le domaine du transport, responsable à lui seul de plus de la moitié de la consommation des ressources pétrolières, les actions visant à optimiser la consommation énergétique se traduisent par la mise en place de nouvelles technologies hybrides, par un travail sur l’aérodynamisme des véhicules, la conception de pneumatiques plus performants ou bien encore la réduction de la taille des moteurs et des boites de vitesse afin de réduire le poids des véhicules. Cependant, selon le VTT (Centre de recherche technique finlandais), dans les voitures particulières, les pertes par frottement représentent un tiers de la consommation totale de carburant. Plusieurs millions de litres de carburant sont ainsi utilisés dans le monde chaque année pour vaincre les forces de frottement. Pour cette raison la recherche en tribologie dans le domaine de l’automobile s’est plus particulièrement focalisée ces dernières années sur le développement de matériaux à faible coefficient de frottement ainsi que lubrifiants plus performants. Ce travail de thèse a pour objectif d’étudier le potentiel des nanoparticules de dichalcogénures métalliques en tant qu’additifs de lubrification pour applications automobiles dans le but de développer de nouveaux lubrifiants hautement performants. Ces nanoparticules, synthétisées pour la première fois dans les années 90, ont déjà montré des propriétés tribologiques intéressantes lors de tests effectués en laboratoire, en régime de lubrification limite. Toutefois, leur utilisation dans des systèmes mécaniques réels nécessite une meilleure compréhension de leurs performances dans des conditions plus proches de la réalité, à savoir en présence de surfaces rugueuses ainsi qu’en présence d’additifs couramment utilisés dans les lubrifiants industriels. Au cours de ce travail, nous nous sommes focalisés sur des nanoparticules de bisulfures de tungstène produites industriellement. Le contexte industriel de ce travail de thèse ainsi que les bases de la tribologie seront exposés dans un premier chapitre consacré à l’état de l’art. Dans ce même chapitre, une revue bibliographique des propriétés lubrifiantes des nanoparticules de bisulfures métalliques (MoS2, WS2) observées lors d’essais tribologiques effectués en laboratoire sera également présentée. L’influence de certains paramètres tels que les conditions d’essais, l’effet de la température, de la concentration, de la cristallinité sera également présentée dans ce premier chapitre.Les résultats de mon travail de thèse seront présentés à travers deux grandes parties. Dans une première, les caractéristiques morphologiques et chimiques des nanoparticules étudiées seront présentées. Leurs propriétés tribologiques dans l’huile de base en présence de surfaces en acier de différentes rugosités seront discutées. Enfin, leurs performances en présence d’additifs couramment utilisés dans les applications industrielles ont également été étudiées. Tous ces essais ont été réalisés en régime de lubrification limite et à 100°C. Dans une seconde partie, le potentiel des nanoparticules pour une application boîte de vitesses a été exploré. Des essais tribologiques ont été réalisés à la fois dans une huile de base ainsi qu’en présence d’un cocktail d’additifs, tout d’abord à l’échelle du laboratoire puis lors de tests réels effectués avec des boites de vitesses utilisées dans l’automobile. Les résultats montrent que les nanoparticules peuvent être utilisées pour augmenter la durée de vie des engrenages
The growing environmental concerns, along with the continuous increase of energy demand, have encouraged research to improve energy efficiency in every technological field. In the transport industry, responsible of more than half of the world’s oil consumption, manufacturers have bet on hybrid fuel technologies, more aerodynamic car profiles, innovative tires and even downsizing of engines and gearboxes to reduce the weight of the vehicles to face the problem. However, according to VTT Technical Research Centre of Finland, in passenger cars one third of fuel consumption is due to friction loss. This means that several millions of liters of fuel are used every year to overcome friction around the world. As a consequence, reduction on the friction losses would have a direct impact in oil consumption. For this reason, research in the tribology field has specially focused in the development of low friction materials and more efficient lubricants. This work investigates the potential of metal dichalcogenide nanoparticles as lubricant additives for automobile applications with the aim of formulating more efficient lubricants. These nanoparticles which were synthetized for the first time in the 90’s have shown interesting tribological properties when added to base oil under specific laboratory test conditions. However, their future use in real-life mechanical systems needs a better comprehension of their behavior on rough surfaces and in the presence of additives commonly used in industrial lubricants.Industrially produced tungsten disulfide nanoparticles were used in this work. First of all, the industrial context of this work and the basis of tribology science in general and of tribology in the automotive industry in particular are exposed in the state of the art part. In this section, a literature review of the lubricating properties of laboratory scale produced metal dichalcogenides nanoparticles of tungsten and molybdenum disulfide is exposed. The effect of different conditions (temperature, concentration in oil, contact pressure, among others) is also presented in this first section. The research work done for this thesis is divided in two main parts. In the first one, the nanoparticles were first morphologically and chemically characterized and their tribological potential in base oil was investigated on smooth and rough surfaces under different test conditions. Then, their tribological behavior in the presence of additives that are commonly used in industrial applications, in the boundary lubrication regime and at 100°C was studied.In the second part, the use of nanoparticles for a gearbox application was explored. The potential of the nanoparticles in base oil and in the presence of a commercial package of additives for this application was studied, first at the laboratory scale, and then in scaled-up tests with gearboxes used in cars. The results suggest that nanoparticles can be used to increase life span of the mechanical parts of gears

Introduction Radioisotopes play an important role in nuclear medicine and represent powerful tools for imaging and therapy. With the extensive use of 99mTc-based imaging agents, therapeutic rhenium analogues are highly desirable. Rhenium-186 emits therapeutic − particles with an endpoint-energy of 1.07 MeV, allowing for a small, targeted tissue range of 3.6 mm. Additionally, its low abundance γ-ray emission of 137.2 keV (9.42 %) allows for in vivo tracking of a radiolabeled compounds and dosimetry calculations. With a longer half-life of 3.718 days, synthesis and shipment of Re-186 based radiopharmaceuticals is not limited. Rhenium-186 can be produced either in a reactor or in an accelerator. Currently, Re-186 is produced in a reactor via the 185Re(n,γ) reaction resulting in low specific activity which makes its therapeutic application limited.[1] Production in an accelerator, such as the PETtrace at the University of Missouri Research Reactor (MURR), can theoretically provide a specific activity of 34,600 Ci.mmol−1 Re[2], which represents a 62 fold increase over reactor produced 186Re. The studies reported herein focused on the evaluation of accelerator-based reaction pathways to produce high specific activity (HSA) 186Re. Those pathways include proton and deuteron bombardment of tungsten and osmium targets by the following reactions: 186W(p,n)186Re, 186W(d,2n) 186Re, 189Os(p,α)186Re, and 192Os(p,α3n)186Re. Additional information on target design related to the determination and optimization of production rates, radionuclidic purity, and yield are presented. Material and Methods Osmium and tungsten metals are very hard and thus very brittle. Attempts at pressing the pure metal into aluminum backings resulted in chalky targets, which easily crumbled during handling. Osmium disulfide (OsS2) and tungsten disulfide (WS2) were identified to provide a softer, less brittle chemical form for targets. OsS2 and WS2 targets were prepared using a unilateral press with a 13 mm diameter die to form pressed powder discs. A simple target holder design (FIG. 1) was implemented to provide a stabilizing platform for the pressed discs. The target material was sealed in place with epoxy using a thin aluminum foil pressed over the target face. Initial irradiations of OsS2 were performed using the 16 MeV GE PETtrace cyclotron at MURR. Irradiations were performed for 30–60 minutes with proton beam currents of 10–20 µA. Following irradiation, the OsS2 targets were dissolved in NaOCl and the pH adjusted using NaOH. The resultant aqueous solution was mixed with methyl ethyl ketone (MEK), with the lipophilic perrhenate being extracted into the MEK layer and the osmium and iridium remaining in the aqueous layer. The MEK extracts were then passed through an acidic alumina column to remove any remaining osmium and iridium. Determination of rhenium and iridium activities was done by gamma spectroscopy on an HPGe detector. Preliminary irradiations on WS2 targets were performed at MURR with the beam degraded to 14 MeV with a proton beam current of 10 µA for 60 minutes. After irradiation, WS2 was dissolved using 30% H2O2 with gentle heating and counted on an HPGe detector to determine the radio-nuclides produced. Results and Conclusion Thin natOsS2 targets were produced, irradiated at 16 MeV for 10 µAh, and analyzed for radiorhenium. Under these irradiation conditions, rhenium isotopes were produced in nanocurie quantities while iridium isotopes were produced in microcurie quantities. Future studies with higher proton energies are planned to increase the production of rhenium and decrease the production of iridium. After optimizing irradiation conditions, enriched 189Os will be used for irradiations to reduce the production of unwanted radionuclides. A liquid-liquid extraction method separated the bulk of the rhenium from the iridium. The majority of the rhenium produced was recovered in the first organic aliquot with little iridium observed while the majority of the iridium and osmium was retained in the first aqueous aliquot. Target production with WS2 was successful. A thin target of natWS2 was produced and irradiated at 14 MeV for 10 µAh. Under these irradiation conditions, several rhenium isotopes were produced in microcurie quantities. Target parameters to maximize 186Re production remain to be determined before enriched 186W targets are used for irradiations to reduce the production of unwanted radionuclides. In conclusion, the potential production routes for accelerator-produced high specific activity 186Re are being evaluated. Cyclotron-based irradiations of natOsS2 targets established the feasibility of producing rhenium via the natOs(p,αxn)Re reaction. Current results indicate higher proton energies are necessary to reduce the production of unwanted iridium isotopes while increasing the production of rhenium isotopes. Preliminary irradiations were performed using the 50.5 MeV Scanditronix MC50 clinical cyclotron at the University of Washington to determine irradiation parameters for future higher energy irradiations (20–30 MeV). A rapid liquid-liquid extraction method isolated rhenium from the bulk of the iridium and osmium following irradiation. Preliminary studies indicate WS2 may also provide a suitable target material to produce 186Re via the (p,n) reaction pathway.

This dissertation tested the hypothesis that pulsed laser deposition (PLD) could be used to create targeted dopant profiles in few layered WS2 films based on congruent evaporation of the target. At the growth temperatures used, 3D Volmer-Weber growth was observed. Increased energy transfer from the PLD plume to the growing films degraded stoichiometry (desorption of sulfur) and mobility. Sulfur vacancies act as donors and produce intrinsic n-type conductivity. Post deposition annealing significantly improved the crystallinity, which was accompanied by a mobility increase from 6.5 to 19.5 cm2/Vs. Preparation conditions that resulted in excess sulfur, possibly in the form of interstitials, resulted in p-type conductivity. Current-voltage studies indicated that Ohmic contacts were governed by surface properties and tunneling. Extrinsic p-type doping of few layered WS2 films with Nb via pulsed laser deposition using ablation targets fabricated from WS2, S and Nb powders is demonstrated. The undoped controls were n-type, and exhibited a Hall mobility of 0.4 cm2/Vs. Films doped at 0.5 and 1.1 atomic percentages niobium were p-type, and characterized by Fermi levels at 0.31 eV and 0.18 eV from the valence band edge. That is, the Fermi level moved closer to the valence band edge with increased doping. With increased Nb doping, the hole concentrations increased from 3.9 x1012 to 8.6 x1013 cm-2, while the mobility decreased from 7.2 to 2.6 cm2/Vs, presumably due to increased ionized impurity scattering. X-ray photoelectron spectroscopy indicates that Nb substitutes on W lattice sites, and the measured peak shifts toward lower binding energy observed corresponded well with the UPS data. Throughout, a clear correlation between degraded stoichiometry and decreased mobility was observed, which indicates that point defect and ionized impurity scattering is a dominant influence on carrier transport in PLD few-layered WS2 films. The approach demonstrates the potential of PLD for targeted doping of transition metal dichalcogenides.

Tungsten disulfide is a proven material as a low friction solid coating. The material is well characterized and has proven its capabilities the last century. Graphene is this centurys most promising material with electrical and mechanical properties. With it the 2D material revolution have started. In this thesis I present a feasible way to sputter tungsten disulfide on graphene as a substrate with little damage to the graphene from energetic particles and a straight forward method to quantize the damage before and after deposition. Further I investigate compositional changes in the sputtered films depending on processing pressure and how tungsten disulfide film thickness and the amount of graphene damage affects the materials low friction capabilities. It is shown that graphene is not a viable substrate for a low friction tungsten disulfide film and that tungsten disulfide is an excellent material for low friction coatings even down too a few nanometers and that the films behavior during load in the friction testing significantly depends on the processing pressure during sputtering.

Coatings are often used on critical components in machines and engines to reduce wear and to provide low friction in order to reduce energy losses and the environmental impact. A triboactive coating not only provides this desired performance, it also actively maintains the low friction by a structural or chemical change in a very thin top layer of these already micrometer thin coatings. This so-called tribofilm is often 5-50 nm thick and can be formed either from the coating itself or by a reaction with the counter surface or the surrounding atmosphere, i.e. gas, fuel, oil, etc. The tribofilm will maintain the wanted performance for as long as the system is not chemically disturbed. This thesis provides a detailed overview of the functionality of triboactive low-friction coatings, in many different systems. The majority of the tribofilms discussed, formed in very different environments, are built up by tungsten disulfide (WS2), which is a material similar to graphite, with a lamellar structure where strongly bonded atomic planes may slip over each other almost without resistance. The major difference is that WS2 is an intrinsically triboactive material, while graphite is not. However, graphite and other carbon-based materials can be made triboactive in certain atmospheres or by addition of other elements, such as hydrogen. The remarkable affinity and driving force to form such WS2 low-friction tribofilms, regardless of the initial states of the sulfur and tungsten, and even when the forming elements are present only at ppm levels, is a recurrent observation in the thesis. Addition of an alloying element to sputtered coatings of WS2 can improve its mechanical and frictional properties significantly. Several promising attempts have been made to find good candidates, out of which a few important ones are investigated in this thesis. Their achievable potential in friction reductions is demonstrated. By reducing friction, energy losses can be avoided, which also results in lower particle and exhaust emissions, which directly reduces the environmental impact. Triboactive coatings are shown to be a promising route to significantly improve tribological applications and allow more environmental friendly and energy efficient vehicles.

Le but principal de ces travaux de thèse fut la conception et la réalisation de biocapteurs par utilisation de méthodes de transduction sans marquage, comme la spectroscopie d’impédance électrochimique (EIS), pour la détection de cible d’intérêts. Pour cela, différentes architectures moléculaires, spécifiques à la molécule d’intérêt ciblée, ont été développées afin de permettre la transduction du signal issu de la reconnaissance entre le biorécepteur et son substrat, et conduire ainsi à la détection de la cible.Les systèmes mis au point reposent sur l’intégration de nanomatériaux, tels que les nanotubes de carbones ou le disulfure de tungstène, pour assurer l'immobilisation de l'entité biospécifique à la surface du capteur. L’intérêt de ces matériaux est multiple puisqu’ils permettent une très forte augmentation de la surface spécifique du système et sont également mis à contribution lors de la fonctionnalisation de la surface de l’électrode. Un des grands défis rencontré dans le développement des biocapteurs étant la stratégie d'immobilisation de l'entité biospécifique sur la surface du capteur.Ces travaux se sont donc dans un premier temps intéressés à la réalisation et à la caractérisation de films minces de ces nanomatériaux ainsi qu’à leur transfert à la surface d’une électrode. Dans ce contexte, le but est de concevoir des bioarchitectures poreuses à base de polymères fonctionnels électrogénérés autour des nanostructures de carbone permettant la pénétration de grandes biomolécules comme des anticorps pour développer des immunocapteurs de haute performance.La seconde partie de ce travail s’est donc orientée vers la conception de biocapteurs par utilisation de ces différents matériaux. La fiabilité du procédé de la construction de ces nanostructures poreuses a été validée par la conception de systèmes immunologiques pour la détection de l’anticorps de l’antitoxine du choléra et l’anticorps de la toxine de la dengue.Enfin, un dernier biocapteur enzymatique, s’appuyant sur l’utilisation de nano-bâtonnets de disulfure de tungstène, a été développé. Ce dernier permet la détection de deux molécules d’intérêts, à savoir le catéchol et la dopamine, par utilisation de la polyphénol oxydase
The main purpose of this work was the design and the development of biosensors by using non-marking transduction methods, such as electrochemical impedance spectroscopy (EIS), for the detection of targets of interests. To this end, various molecular architectures have been developed to allow the transduction of the signal resulting from the recognition between the bioreceptor and its substrate, and thus lead to the detection of the target.The systems developed are based on the integration of nanomaterials, such as carbon nanotubes or tungsten disulfide, to ensure the immobilization of the biospecific entity at the surface of the sensor. The advantages of these materials are multiples, since they allow a very large increase in the specific surface area and are also used in the functionalization of the surface of the electrode. Indeed, one of the major challenges encountered in the development of biosensors is the strategy involved in the immobilization of the biospecific entity on the surface of the sensor.This work was initially interested in the realization and characterization of thin films of these nanomaterials as well as their transfer to the surface of an electrode. In this context, the aim is to design porous bioarchitectures based on electrogenerated functional polymers around carbon nanostructures allowing the penetration of large biomolecules such as antibodies to develop high-performance immunosensors.The second part of the work was oriented towards the design of biosensors using these different materials. The reliability of the process has been validated by the design of immunological systems for the detection of the anti-cholera toxin antibody and dengue toxin antibody.Finally, a last enzymatic biosensor, based on the use of tungsten disulfide nano-sticks, has been developed. The latter allows the detection of two molecules of interest, catechol and dopamin, by the use of polyphenol oxidase

The graphene discovery led to advances in exfoliation and synthetic techniques, and the lack of a bandgap in graphene has stimulated the research for new 2D semiconducting materials. Transition metal dichalcogenides (TMDCs), semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), have recently been isolated. TMDCs exhibit a unique combination of atomic-scale thickness, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, valleytronics and optoelectronics. According to optical measurements, single-layer WS2 sheets exhibit a direct band gap of at least 2.0 eV. Because of its strong spin-orbit coupling induced valence band splitting, WS2 shows spin-valley coupling, even in few-layer sheets , which may allow easier observation of the valley Hall effect than in the other TMDCs. The thesis reviews the theoretical background of TMDCs and their optoelectronic properties. It also reports on the fabrication of field-effect transistors based on few-layer sheets of WS2 and the investigation of their electronic transport properties. Particularly the project focuses on improving the interface between the metal contact and WS2 sheet, where annealing improves the contact transparency. Together with van der Pauw geometry, annealing allows four-terminal measurements to be performed and the pristine properties of the material to be recovered at room temperature, where the devices show n-type behaviour and a linear I-V curve. The promising improvements and the electronic properties shown in this thesis make WS2 interesting for future applications in valleytronic devices.

The purpose of this thesis was to compare graphene nanoplatelets (GNP) and WS2 as solid lubricant additives to aluminum in order to reduce friction and wear. The central hypothesis of this work relied on lubricating properties of 2D materials, which consist layers that slip under a shear force. Two aluminum composites were made (Al-2 vol.% GNP and Al-2 vol.% WS2) by spark plasma sintering. Tribological properties were evaluated by ball-on-disk wear tests at room temperature (RT) and 200°C. WS2 not only presented the lowest COF (0.66) but also improved the wear resistance of aluminum by 54% at RT. Al-2 vol.% GNP composite displayed poor densification (91%) and low hardness resulting in poor wear resistance. The wear rate of Al-2 vol.% GNP composite increased by 233% at RT and 48% at 200°C as compared to pure aluminum. GNP addition also resulted in lower COF (0.79) as compared to pure aluminum (0.87).

This thesis describes the fabrication and characterization of two-dimensional transition dichalcogenides (2D TMDs) nanolayers for various applications in electronic and opto-electronic devices applications. In Chapter 1, crystal and optical structure of TMDs materials are introduced. Many TMDs materials reveal three structure polytypes (1T, 2H, and 3R). The important electronic properties are determined by the crystal structure of TMDs; thus, the information of crystal structure is explained. In addition, the detailed information of photon vibration and optical band gap structure from single-layer to bulk TMDs materials are introduced in this chapter. In Chapter 2, detailed information of physical properties and synthesis techniques for molybdenum disulfide (MoS2), tungsten disulfide (WS2), and molybdenum ditelluride (MoTe2) nanolayers are explained. The three representative crystal structures are trigonal prismatic (hexagonal, H), octahedral (tetragonal, T), and distorted structure (Tʹ). At room temperature, the stable structure of MoS2 and WS2 is semiconducting 2H phase, and MoTe2 can reveal both 2H (semiconducting phase) and 1Tʹ (semi-metallic phase) phases determined by the existence of strains. In addition, the pros and cons of the synthesis techniques for nanolayers are discussed. In Chapter 3, the topic of synthesized large-scale MoS2, WS2, and MoTe2 films is considered. For MoS2 and WS2 films, the layer thickness is modulated from single-layer to multi-layers. The few-layer MoTe2 film is synthesized with two different phases (2H or 1Tʹ). The all TMDs films are fabricated using two-step chemical vapor deposition (CVD) method. The analyses of atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and Raman spectroscopy confirm that the synthesis of high crystalline MoS2, WS2, and MoTe2 films are successful. The electronic properties of both MoS2 and WS2 exhibit a p-type conduction with relatively high field effect mobility and current on/off ratio. In Chapter 4, vertically-stacked few-layer MoS2/WS2 heterostructures on SiO2/Si and flexible polyethylene terephthalate (PET) substrates is presented. Detailed structural characterizations by Raman spectroscopy and high-resolution/scanning transmission electron microscopy (HRTEM/STEM) show the structural integrity of two distinct 2D TMD layers with atomically sharp van der Waals (vdW) heterointerfaces. Electrical transport measurements of the MoS2/WS2 heterostructure reveal diode-like behavior with current on/off ratio of ~ 104. In Chapter 5, optically uniform and scalable single-layer Mo1-xWxS2 alloys are synthesized by a two-step CVD method followed by a laser thinning. Post laser treatment is presented for etching of few-layer Mo1-xWxS2 alloys down to single-layer alloys. The optical band gap is controlled from 1.871 to 1.971 eV with the variation in the tungsten (W) content, x = 0 to 1. PL and Raman mapping analyses confirm that the laser-thinning of the Mo1-xWxS2 alloys is a self-limiting process caused via heat dissipation to SiO2/Si substrate, resulting in fabrication of spatially uniform single-layer Mo1-xWxS2 alloy films.

For sustainable development, it is highly important to limit the loss of energy and materials in machines used for transportation, manufacturing, and other purposes. Large improvements can be achieved by reducing friction and wear in machine elements, for example by the application of coatings. This work is focused on triboactive coatings, for which the outermost layer changes in tribological contacts to form so-called tribofilms. The coatings are deposited by magnetron sputtering (a physical vapor deposition method) and thoroughly chemically and structurally characterized, often theoretically modelled, and tribologically evaluated, to study the connection between the composition, structure and tribological performance of the coatings. Tungsten disulfide, WS2, is a layered material with the possibility of ultra-low friction. This work presents a number of nanocomposite or amorphous coatings based on WS2, which combine the low friction with improved mechanical properties. Addition of N can give amorphous coatings consisting of a network of W, S and N with N2 molecules in nanometer-sized pockets, or lead to the formation of a metastable cubic tungsten nitride. Co-deposition with C can also give amorphous coatings, or nanocomposites with WSx grains in an amorphous C-based matrix. Further increase in coating hardness is achieved by adding both C and Ti, forming titanium carbide. All the WS2-based materials can provide very low friction (down to µ<0.02) by the formation of WS2 tribofilms, but the performance is dependent on the atmosphere as O2 and H2O can be detrimental to the tribofilm functionality. Another possibility is to form low-friction tribofilms by tribochemical reactions between the two surfaces in contact. Addition of S to TiC/a-C nanocomposite coatings leads to the formation of a metastable S-doped carbide phase, TiCxSy, from which S can be released. This enables the formation of low-friction WS2 tribofilms when a Ti-C-S coating is run against a W counter-surface. Reduced friction, at a moderate level, also occurs for steel counter-surfaces, likely due to formation of beneficial iron sulfide tribofilms. The studied coatings, whether based on WS2 or TiC, are thus triboactive, with the ability to form low-friction tribofilms in a sliding contact.

Plazmonika, tedy vědní obor zabývající se interakcí světla s kovovými materiály, nabízí ve spojení s nanotechnologiemi nezvyklé možnosti, jak světlo ovládat a využívat. Výsledkem tohoto spojení může být například zaostřování světla pod difrakční limit, zesilování emise nebo absorbce kvantových zářičů, či extrémně citlivá detekce molekul. Tato práce se zabývá zejména možnostmi využití plazmoniky pro vývoj plošných optických prvků, tzv. metapovrchů, a pro fotokatalytické aplikace založené na plazmonicky generovaných elektronech s vysokou energií, tzv. horkých elektronech. Nejprve jsou vysvětleny teoretické základy plazmoniky a je poskytnut přehled jejích nejvýznamnějších aplikací. Poté jsou představeny tři studie zabývající se využitím plazmonických nanostruktur pro ovládání fáze a polarizace světla, pro vytváření dynamicky laditelných metapovrchů, a pro foto-elektrochemii s horkými elektrony. Společným prvkem těchto studií je pak používání pokročilých, resp. v rámci těchto oblastí netradičních, materiálů, jako např. oxidu vanadičitého nebo dichalkogenidů přechodných kovů.

The thesis deals with study of thin layers of transition metal dichalcogenides, especially of molybdenum disulfide. Nanostructures were fabricated on two-dimensional crystals of MoS2 and WSe2. Within followed analysis attention was paid to the photoluminescence properties. In the thesis transition metal dichalcogenides are reviewed and description of the modified process of preparation by micromechanical exfoliation is given.

Elektrochemické metody nacházejí využití v mnoha aplikacích (např. senzorice, skladování el. energie nebo katalýze). Jejich nespornou výhodou je nízká finanční náročnost na přístrojové vybavení. Abychom lépe porozuměli procesům probíhajícím na elektrodách, je dobré znát elektronickou pásovou strukturu materiálu elektrody. Úkolem této práce je vyhodnotit výstupní práci a pozici hrany valenčního pásu nových materiálů pro elektrody, konkrétně cínem dopovaného oxidu india pokrytého nanotrubicemi sulfidu wolframičitého. Ultrafialová fotoelektronová spektroskopie a Kelvinova silová mikroskopie jsou metody použité pro tuto analýzu. Zvláštní důraz je kladen na přípravu vzorků elektrod pro měření, aby nedošlo k nesprávné interpretaci výsledků vlivem vnějších efektů jako je např. kontaminace nebo modifikace povrchu.

Ce travail de thèse a permis d’obtenir une caractérisation à l’échelle atomique des feuillets (Co)MoS2 et (Ni)WS2 de catalyseurs d’hydrotraitement supportés sur alumine et de déterminer l’impact d’un additif organique, l’acide citrique (CA). Dans cette étude des techniques avancées ont été mises en œuvre telles la caractérisation par adsorption de CO à basse température suivie par spectroscopie IR (IR/CO), des calculs DFT et des observations par microscopie électronique haute résolution en mode transmission et par STEM HAADF.Sur les catalyseurs non promu à base de W et de Mo, l’adsorption de CO permet de discriminer les sites des bords M- et des bords S- des feuillets de TMS. La détermination des coefficients d’extinction molaire des bandes liées au CO adsorbé permet de déduire la morphologie des feuillets. L’HR STEM HAADF confirme que l’addition de CA modifie la morphologie des feuillets TMS d’un triangle tronqué à une forme hexagonal.Sur les catalyseurs promus NiW et CoMo, la microscopie electronique montre que l’addition de CA diminue la taille des feuillets de TMS et conduit aussi à la création de petits clusters (<1 nm). L’analyse de particules isolées par HR STEM HAADF permet d’identifier la nature des atomes du bord du feuillet de TMS. Ainsi la microscopie et l’IR/CO mettent en évidence que au sein d’un même feuillet sulfure peuvent coexister des bords totalement promus, et partiellement promus
This work was focused on obtaining an atomic scale characterization of the (Co)MoS2 and (Ni)WS2 slabs of hydrotreating catalysts supported on alumina and to determine the impact of citric acid (CA) addition. In this study, advanced techniques were implemented such as low temperature CO adsorption followed by IR spectroscopy (IR/CO), DFT calculations and high resolution electron microscopy observations in transmission mode and by STEM HAADF.On the non-promoted W and Mo catalysts, CO adsorption makes it possible to discriminate between the sites of the M- and S- edges of the TMS slabs. The determination of the molar extinction coefficients of the adsorbed CO bands allows the morphology of the slabs to be deduced. HR STEM HAADF confirms that the addition of CA modifies the morphology of the TMS slabs from a truncated triangle to a hexagonal shape.On the promoted NiW and CoMo catalysts, electron microscopy shows that the addition of CA decreases the size of the TMS slabs and also leads to the creation of very small clusters (<1 nm). The analysis of isolated particles by HR STEM HAADF allows identifying the nature of the atoms on the edge of the TMS slabs. Thus microscopy and IR/CO show that within the same sulfide slabs, can coexist edges that are fully promoted and partially promoted

碩士
國立臺灣科技大學
電子工程系
107
The dissertation demonstrates the fabrication and discusses the characteristics of heterojuction diode, homojunction diode, and bipolar junction transistor (BJT). To start with, WSe2 and MoS2 were grown by chemical vapor transport (CVT). Material composition structure and hexagonal structure of WSe2 and MoS2 were respectively verified through Raman spectroscopy and X-ray diffraction analysis. The semiconductor behaviors of p-type WSe2 and n-type MoS2 were demonstrated by field effect transistor (FET) measurement, respectively. Through mechanical exfoliation, we limited the thickness of WSe2 and MoS2 sheets from 1 to 3 μm, and vertically stacked the sheets to form a pn heterojunction diode, which presented a significant rectification and low cut-in voltage. Indeed, the half-wave rectification experiment indicated that the pn heterojunction diode featured significant rectifying behavior under 1-6 Vpp and 1-1000 Hz. On the other hand, we used the oxygen plasma treatment to transform the characteristics of MoS2 from n-type to p-type and demonstrated the charge neutrality point (CNP) shift from -80 V to +102 V by the FET measurement, successfully. In order to fabricate a pn homojunction diode, we defined the junction on the surface of MoS2 with a mask for doping precisely. The pn homojunction diode shows excellent rectifying behavior with 1-10 Vpp and 1-1000 Hz by the half-wave rectification experiment. Finally, starting from the concept of pn homojunction diode, it is further fabricated into a pnp BJT. The base width of the BJT was defined by photolithography and the behavior of the n-type semiconductor was maintained in the base. Emitter and collector of the BJT were heavily doped p-type and lightly doped p-type with oxygen plasma, respectively. It is observed that it has a stable active region, saturation region, and cut-off region. In this experiment, a variety of transistors were fabricated by using molybdenum disulfide and tungsten diselenide in a simple method through vertical stacking, oxygen plasma doping, and photolithography. The dissertation demonstrates that Transition metal dichalcogenides (TMDs), vertically stacked and doped with oxygen plasma, have great potential for pn diodes which feature significant pn characteristics and rectifying behavior.

碩士
中原大學
物理研究所
106
Layer dependent properties of WS2 have gained popularity through the years. Its Quantum dot (QD) derivative however is less explored brought by the long synthesis process that is currently employed and available. The growing popularity of this nanostructure requires for a facile and fast synthesis process. In this study, the main focus were the following: synthesis of WS2 QDs (undoped and doped), unraveling the origin of excitation-dependence (independence) photoluminescence for undoped and diethylriamine(DETA)-doped WS2 QDs, and Photo I-V measurements (negative photoconductivity). WS2 powder in Ethanol were used to synthesize the undoped WS2 QDs via pulsed laser ablation for 30 mins, the DETA-doped WS2 QDs were fabricated using the same manner but with the introduction of varying DETA concentration. After the introduction of the dopant, a huge increase in PL (76 folds) was observed alongside with excitation-wavelength independence contrary to the PL excitation-wavelength dependence of the undoped sample, which is suggested to arise from the difference in the recombination mechanism of carriers from undoped and DETA-doped WS2 QDs. On the other hand, the photo I-V curves reveal a decrease in current under illumination, NPC phenomenon. It was observed that NPC enhances with respect to illumination intensity, when exposed to atmospheric condition and DETA doping. The light induced detrapping of electrons induced by water and/or oxygen molecules is suggested to be responsible for the NPC phenomenon in WS2 QDs. Furthermore, the fabricated WS2 QDs (undoped and doped) possess potentials that are yet to be discovered for future applications.

碩士
國立臺灣海洋大學
光電科學研究所
106
The nonlinear optical properties of two different concentration of Tungsten Disulfide dispersion were investigated using the Z-scan technique with femtosecond laser pulses at 790 nm. Concentration 0.026mg/1mL nonlinear refractive index n2 and real part of the third-order nonlinear susceptibility χ(3) of Tungsten Disulfide dispersion were (2.334±0.101)×10^(-11)(esu) and (8.742±0.381)×10^(-12)(esu), respectively. Concentration 1mg/1mL nonlinear refractive index n2 and real part of the third-order nonlinear susceptibility χ(3) of Tungsten Disulfide dispersion were (9.5±0.18)×10^(-11)(esu) and (3.68±0.07)×10^(-11)(esu) , respectively. The result demostrate reverse saturable absorption of 0.026mg/1mL Tungsten Disulfide dispersion and 1mg/1mL Tungsten Disulfide dispersion with a two photon absorption coefficient, β were (3.362±0.063)×10^(-10) (cm⁄W) and (6.816±0.208)×10^(-8) (cm)⁄W) , respectively. We speculate that when nonlinear changes due to increased concentration of the sample, a solvent tungsten disulfide volume fraction increases, the number of molecules in dispersion, tungsten disulfide increases, and the distribution becomes more uniform. Resulting in a more dense array of molecules, leading to an increase in the nonlinear coefficient. .

碩士
國立臺灣科技大學
應用科技研究所
105
Stimuli-responsive or “smart” biomaterials are of great interest in the field of biotechnology and biomedicine. Drug delivery system based on external stimulus-responsive materials for controlled and long-term drug release under an action external action offer the promising of new treatment. Here we present, electrically triggered drug release form WS2 nanocomposite composed of two dimensional materials integrate with conducting polymer such as polypyrole. In this study, three thiol compounds was used to study the exfoliation of WS2. The exfoliation of WS2 few layers were characterized by Raman and Uv-vis spectrum. In addition, the polypyrole was then added in the exfoliated WS2. The current results showed that therapeutic molecule only released from nanocomposite, no passive drug release is expected. Currently, we still continue to utilize the ultrasound waves to reduce the size decrease the height of the WS2 nanosheet to enhanced drug loading in system to achieve enhanced drug release.

碩士
國立臺灣大學
電子工程學研究所
105
In this thesis, the mechanically exfoliated 2D material WS2 nanosheet was successfully used to fabricate thin film transistor. Using 3M scotch tape method and PDMS stamp can avoid the residues of 3M scotch tape being left on the surface of WS2. Using optical microscopy and atomic force microscopy, the WS2 flakes with appropriate thickness can be chosen. Ohmic contact of WS2 TFT can be achieved by low work function metal Chromium. The highest on/off current ratio of MoS2 TFT was up to 7 order of magnitude and the mobility of 38 cm2/V-sec was achieved. The electronic and physical properties of WS2 are greatly dependent on the layer thickness and owing to the scotch tape method , the thickness of the WS2 flakes is random. Therefore , in order to get the appropriate thickness of WS2 , CF4 plasma is used to control the thickness of WS2 flakes by reactive ion etching (RIE). From XPS spectra after etching, the Fermi level of the WS2 moves downward valence band which represents the p-doping effect. In I-V characteristics, the threshold voltage shifts to higher positive voltage about 27 V and 1.94×1012 cm-2 induced carrier charge density are achieved after 4 sec CF4 gas plasma etching. In addition, owing to the damage of the WS2 surface after etching, the on/off current ratio and mobility both decreased. In general, the mobility of WS2 TFT on the SiO2 substrate measured in experiments is much lower than the theoretical calculation and it can be attributed to the defects existing in the WS2/SiO2 interface. Hence, the hexagonal boron nitride (h-BN) is an ideal substrate because it provides an atomically flat surface without dangling bonds and charged impurity. In I-V characteristics, the on/off current ratio is up to 107 and the field-effect mobility is 106 cm2/V-sec. The mobility is enhanced by 179%. Besides, it is found that the oxygen and water molecules are easily adsorbed at the WS2 surface in air, which would lead to p-doping effect and hysteresis in devices. The hysteresis effect and the humidity issue can be reduced by annealing. Further, the hysteresis effect can be completely eliminated with h-BN substrate because it has few charge impurities.

碩士
國立臺灣海洋大學
光電科學研究所
107
Transition metal dichalcogenides (TMDCs), such as tungsten disulfide (WS_2). Similar to that of graphene, the atomic thickness of TMDCs significantly limits their optoelectronic applications. TMDCs is two-dimensional layers semiconductor, layers and layers are stacked on each other by Van der Waals force. When thinned from bulk to atomic-layer thickness, the material transforms from indirect to direct bandgap semiconductor. Recent advances in nanoscale materials characterization and few layer TMDCs’ unique optical properties make them a research hot-spot in nonlinear optics. In this work, the nonlinear refractive index of few-layers WS_2 on fused silica substrates has been characterized with two-beam coupling measurement under 790nm femtosecond pulsed laser excitation, where the laser repetition rate is 80MHz. To see if the light intensity affects the nonlinear refractive index of the sample, different laser intensities are used. The few-layer tungsten disulfide on fused silica substrate of the two photon absorption coefficient  was estimated to be 7.66±1.26)×〖10〗^(-7) (cm/W) and the nonlinear polarizability 〖Im[χ〗^((3))] was estimated to be（2.05±0.34）×〖10〗^(-8)（esu）, the nonlinear refractive index n_2 at 780 nm were estimated to be (4.0±1.22)×〖10〗^(-12) （〖cm〗^2/W） and (1.21±0.36)×〖10〗^(-17) (m^2/V^2 ).

abstract: A highly uniform and repeatable method for synthesizing the single-layer transition metal dichalcogenides (TMDs) molybdenum disulfide, MoS2, and tungsten disulfide, WS2, was developed. This method employed chemical vapor deposition (CVD) of precursors in a custom built cold-wall reaction chamber designed to allow independent control over the growth parameters. Iterations of this reaction chamber were employed to overcome limitations to the growth method. First, molybdenum trioxide, MoO3, and S were co-evaporated from alumina coated W baskets to grow MoS2 on SiO2/Si substrates. Using this method, films were found to have repeatable coverage, but unrepeatable morphology. Second, the reaction chamber was modified to include a pair of custom bubbler delivery systems to transport diethyl sulfide (DES) and molybdenum hexacarbonyl (MHC) to the substrate as a S and Mo precursors. Third, tungsten hexacarbonyl (WHC) replaced MHC as a transition metal precursor for the synthesis of WS2 on Al2O3, substrates. This method proved repeatable in both coverage and morphology allowing the investigation of the effect of varying the flow of Ar, varying the substrate temperature and varying the flux of DES to the sample. Increasing each of these parameters was found to decrease the nucleation density on the sample and, with the exception of the Ar flow, induce multi-layer feature growth. This combination of precursors was also used to investigate the reported improvement in feature morphology when NaCl is placed upstream of the substrate. This was found to have no effect on experiments in the configurations used. A final effort was made to adequately increase the feature size by switching from DES to hydrogen sulfide, H2S, as a source of S. Using H2S and WHC to grow WS2 films on Al2O3, it was found that increasing the substrate temperature and increasing the H2S flow both decrease nucleation density. Increasing the H2S flow induced bi-layer growth. Ripening of synthesized WS2 crystals was demonstrated to occur when the sample was annealed, post-growth, in an Ar, H2, and H2S flow. Finally, it was verified that the final H2S and WHC growth method yielded repeatability and uniformity matching, or improving upon, the other methods and precursors investigated.
Dissertation/Thesis
Doctoral Dissertation Physics 2019

博士
國立交通大學
材料科學與工程學系所
105
Two-dimensional layered transition metal dichalcogenides (TMDs) materials such as Molybdenum disufide (MoS2) have been recognized as one of the low-cost and highly efficient electrocatalysts for hydrogen evolution reaction (HER). On the other hand, TMDs have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. The crystal edges, rather than the basal planes, of MoS2 have been identified as the active sites for HER performance, but they only account for a small percentage of the surface area, of MoS2 monolayer. Here, we report a simple and efficient approach that involves using a remote hydrogen-plasma process to creating S-vacancies on the basal plane of a monolayer crystalline MoS2; this process not only can generate high density of S-vacancies but also can maintain the morphology and structure of MoS2 monolayer, as confirmed with Atomic force microscopy (AFM) characterizations. The density of S-vacancies (defects) on the basal plane of MoS2 monolayers resulting from the remote hydrogen-plasma process can be tuned and play a critical role in HER, as evidenced by the results of electrical measurements. A lowered overpotential, from 727mV to 183mV, and a decreased Tafel slope, from 164mV/dec to 77mV/dec, as compared to those of a pristine MoS2 monolayer are  observed. We found  several times enhancement in the capacitance of the hydrogen- plasma-treated  MoS2 monolayer from the electrical double layer capacitance (EDLC) measurement, Moreover, the stability test shows these materials have high durability in acid environment. The H2-plasma-treated MoS2 also provides an excellent platform for systematic and fundamental study of defect-property relationships in TMDs, which provides insights for future applications including electrical, optical and magnetic devices. Second, we report a two-step epitaxial growth of lateral heterojunction WSe2-MoS2 monolayer with an atomically sharp interface, instead of preferred TMD alloy, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface which can be evidenced by high resolution TEM.  From the electrical transport curves, we found the lateral heterostructure WSe2-MoS2 monolayer display apparent p-n junction and thus photovoltaic effect. Our spatially connected TMD lateral heterojunctions are potential candidates for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors.

博士
國立交通大學
材料科學與工程學系所
107
In organic photovoltaic (OPV) devices, more effective transfer of dissociated electrons and holes from the active layer to the respective electrodes will result in higher fill factors (FF) and short-circuit current densities (Jsc) and, thus, enhanced power conversion efficiencies (PCE). The PCE of OPVs is affected not only by active layer but also transport layer. In my study, I specialize in developing the electron transport layer (ETL) by doping two-dimensional (2D) transition-metal dichalcogenide (TMD) materials for OPV devices. In the first part study, we incorporated molybdenum disulfide (MoS2) nanosheets into sol–gel processing of zinc oxide (ZnO) to form ZnO:MoS2 composites for use as ETLs in inverted polymer solar cells featuring a binary bulk heterojunction active layer. We could effectively tune the energy band of the ZnO:MoS2 composite film from 4.45 to 4.22eV by varying the content of MoS2 up to 0.5 wt%, such that the composite was suitable for use in bulk heterojunction photovoltaic devices based on poly[bis(5-(2-ethylhexyl)thien-2-yl)benzodithiophene–alt–(4-(2-ethylhexyl)-3-fluorothienothiophene)-2-carboxylate-2,6-diyl)] (PTB7-TH):phenyl-C71-butryric acid methyl ester (PC71BM). In particular, the PCE of the PTB7-TH:PC71BM (1:1.5, w/w) device incorporating the ZnO:MoS2 (0.5 wt%) composite layer as the ETL was 10.1%, up from 8.8% for the corresponding device featuring ZnO alone as the ETL—a relative increase of 15%. Incorporating a small amount of MoS2 nanosheets into the ETL altered the morphology of the ETL and resulted in enhanced current densities, fill factors, and PCEs for the devices. We used ultraviolet photoelectron spectroscopy (UPS), synchrotron grazing-incidence wide-/small-angle X-ray scattering (GIWAXS/GISAXS), atomic force microscopy (AFM), and transmission electron microscopy (TEM) to characterize the energy band structures, internal structures, surface roughness, and morphologies, respectively, of the ZnO:MoS2 composite films. For the second part study, a new universal ETL that involves doping hydrogen-plasma treated tungsten disulfide (WS2) nanosheets into ZnO for polymer/fullerene or small molecule OPVs was prepared. A hydrogen-plasma treatment was used to alter the structures of WS2 nanosheets such that the W6+ content was converted into W4+; then ZnO:WS2 nanosheets composites were prepared to form ETLs. The energy band of the ZnO:WS2 films could be tuned from 5.15 to 4.60 eV by varying the concentration of the WS2 nanosheets up to 0.5 wt%. It was found that ZnO:WS2 ETLs exhibited superior charge transport properties than those of the pristine ZnO layer because of the structure changes, as determined from the X-ray scattering characterizations. OPVs incorporating active layers of PTB7-TH/PC71BM and PTB7-TH/IDIC blends exhibited their power conversion efficiencies of 10.3% and 6.7%, respectively, with the incorporation of 0.3 wt% of the WS2 nanosheets, up from 8.9% and 5.4% for the corresponding devices featuring pristine ZnO—relative increases of 16% and 24%, respectively. This study demonstrates the effectiveness of hydrogen-plasma treatment for altering the surface structures of 2D TMD nanosheets, and paves a way for the composite ETLs for use in OPVs.