Supercapacitors are an emerging energy storage device that have gained attention because of the large specific power, at a reasonable specific energy, that they exhibit. These energy storage devices could be used alongside of or in the place of traditional electrochemical battery technologies to power reliable electrical devices. The performance of supercapacitorsis largely determined by electrode properties including the surface area to volume ratio, the electrical conductivity, and the ion diffusivity. Nanomaterial synthesis has been proposed as a method of enhancing the performance of many macroscopic supercapacitor electrodes due to the high surface area to volume ratio and unique tunable properties that are often size or thickness dependent for many materials. Specifically, carbon materials (such as carbon nanotubes), metal organic frameworks, (such as ZIF-8), and transition metal dichalcogenides (such as molybdenum disulfide) have been of interest due to their conductivity, large surface area, and ion diffusivity that they exhibit when one or more of their characteristic lengths is on the order of several nanometers.<div><br></div><div>For the experiments, a carbon nanotube-/ZIF-8-/MoS2framework was synthesized into an electrode material. This process involved first dispersing the carbon nanotubes in DMF using ultrasonication and then modifying the structure with polydopamine to create a binding site for the ZIF-8 to attach to the carbon nanotubes. The ZIF-8 was synthesized by combining 1,2,4-Triazole-3-thiol and ZnCl under 120 degrees Celsius. Afterwards, the MoS2was associated with the carbon nanotube and ZIF-8 framework by a disulfide bond with the sulfur vacancy of the MoS2andthe sulfide group of the ZIF-8. Finally, the sample solution was filtered by vacuum filtration and then annealed at 110 degrees Celsius before being deposited on a nickel foam substrate and tested in a 3-electrode electrochemical cyclic voltammetry study.<br></div><div><br></div><div>The resulting materials were found to have a capacitance of 262.15 F/g with corresponding specific energy and specific power values of 52.4Whr/kg and 1572W/kg. Compared to other supercapacitor research materials, this electrode shows a much larger capacitance than other exclusively carbon materials, and comparable capacitance values to the ZIF-8 and MoS2materialswith the added benefits of an easier and faster manufacturing process. Overall, the electrodes developed in this study, could potentially reduce the cost per farad of the supercapacitor to be more competitive energy storage devices<br></div>

As the family of two−dimensional(2D) materials has grown, two−dimensional heterostructure devices have emerged as great alternatives to replace conventional electronic materials and enable new functionality such as flexible and bendable electronics. The fabrication and performance of these devices depend critically on the understanding and ability to manipulate the mechanical interplay between the stacked materials. In this dissertation, we investigate adhesive interactions and determine the shear modulus of heterostructure devices made from Molybdenum Disulfide (MoS2). MoS2 has been attracting attention recently due to its semiconductor nature (having a direct band gap of 1.9 eV) along with its exceptional mechanical strength and flexibility. As the first step of our research, we suspended MoS2 flakes grown through chemical vapor deposition (CVD) over substrates made of metal (gold, titanium, chromium), semiconductor (germanium, silicon), insulator (silicon oxide), and semi-metal (graphite). Then, by creating pressure differences across the membrane, we forced MoS2 to bulge upward until we observe separation from the surface of the substrates. We demonstrated that MoS2 on graphite has the highest work of separation within the tested surface materials. Furthermore, we measured considerable adhesion hysteresis between the work of separation and the work of adhesion. We proposed that surface roughness and chemical interactions play a role in surface adhesion and separation of 2D materials. These experiments are critical to guiding the future design of electrical and mechanical devices based on 2D materials. Next, we measured the effective shear modulus of MoS2/few−layer graphene (FLG) heterostructures by employing a blister test. Again, by introducing a pressure differential across the suspended MoS2 membrane over the FLG substrate, the MoS2/FLG heterostructure peeled off from the silicon oxide surface once the critical pressure is exceeded. Incorporating a modified free energy model and Hencky’s axisymmetric membrane solution, we determine the average effective shear modulus of the heterostructure. This is the first experimental measurement of the shear modulus of heterostructure devices using a blister test and this platform can be extended to determine the shear modulus of other 2D heterostructures as well. / 2024-05-24T00:00:00Z

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Abstract This thesis is dedicated to realizations and physical understanding of electronic and photocatalytic properties after decorating transition metals to the semiconducting molybdenum disulfide. Synthesized via facile wet chemical methods, the MoS₂-Au, MoS₂-Au-Ni and MoS₂-Ag-Ni composites were formed as binary or ternary compounds. The Au nanoparticles are stably joined to the MoS₂ matrix without deteriorating layered structures of the host. After introducing the Au nanoglue as a common buffer, a metallic contact is reached between Ni and MoS₂, and attributed to new electron migration channel via MoS₂ edge contact. Adapting the Ag as the buffer element can attach the Ni to the basal plane of the MoS₂ beside edge contact. The Ni-Ag-MoS₂ composite effectively splits water under visible light irradiation and produce hydrogen. The excellent photocatalytic activity is attributed to effective charge migration through dangling bonds at the MoS2-Ag-Ni alloy interface and the activation of MoS₂ basal planes. / Original papers The original publications are not included in the electronic version of the dissertation. W. Cao, V. Pankratov, M. Huttula, X. Shi, S. Saukko, Z. Huang, M. Zhang. Gold nanoparticles on MoS2 layered crystal flakes. Materials Chemistry and Physics, 158, 89−95 (2015). DOI: 10.1016/j.matchemphys.2015.03.041 X. Shi, S. Posysaev, M. Huttula, V. Pankratov, J. Hoszowska, J.-Cl. Dousse, F. Zeeshan, Y. Niu, A. Zakharov, T. Li, O. Miroshnichenko, M. Zhang, X. Wang, Z. Huang, S. Saukko, D. L. González, S. van Dijken, M. Alatalo, W. Cao. Metallic contact between MoS₂ and Ni via Au nanoglue. Small, 14, 1704526 (2018). DOI: 10.1002/smll.201704526 http://jultika.oulu.fi/Record/nbnfi-fe2018060525279 X. Shi, M. Huttula, V. Pankratov, J. Hoszowska, J.-Cl. Dousse, F. Zeeshan, Y. Niu, A. Zakharov, Z. Huang, G. Wang, S. Posysaev, O. Miroshnichenko, M. Alatalo, W. Cao. Quantification of bonded Ni atoms for Ni-MoS₂ metallic contact through X-ray photoemission electron microscopy. Microscopy and Microanalysis, 24, 458−459 (2018). DOI: 10.1017/S1431927618014526 http://jultika.oulu.fi/Record/nbnfi-fe2018082834233 X. Shi, M. Zhang, W. Cao, X. Wang, M. Huttula. Efficient photocatalytic hydrogen evolution via activated multilayer MoS₂. Manuscript. X. Shi, Z. Huang, M. Huttula, T. Li, S. Li, X. Wang, Y. Luo, M. Zhang, W. Cao. Introducing magnetism into 2D nonmagnetic inorganic layered crystals: a brief review from first-principles aspects. Crystals, 8, 24 (2018). DOI: 10.3390/cryst8010024 http://jultika.oulu.fi/Record/nbnfi-fe201802153441

Les enjeux environnementaux actuels, ainsi que la hausse continue du prix du pétrole, ont incité les constructeurs automobiles du monde entier à améliorer le rendement de leurs véhicules. Les propriétés tribologiques des lubrifiants des moteurs et boîtes de vitesses ont une influence considérable sur le rendement global des véhicules. Ils réduisent en effet le frottement généré par un grand nombre de contacts, et permettent parfois la réduction de la taille de différents composants en leur conférant une meilleure résistance à l’usure. Les avancées récentes en termes de synthèse de nanoparticules ont ouvert de nouvelles perspectives en termes d’additivation de lubrifiants avec, par exemple, la découverte des excellentes propriétés tribologiques des nanoparticules inorganiques de type fullerène comme le disulfure de molybdène ou de tungstène. L’objectif de ce manuscrit est d’évaluer le potentiel tribologique des nanoparticules IF-MoS2 dans l’optique d’une application automobile. L’influence de la taille et de la structure des nanoparticules a d’abord été étudiée. Les nanoparticules peu cristallines se sont révélées être plus aptes à maintenir un tribofilm performant sur des surfaces en acier dans des conditions de lubrification limite, indépendamment de leur taille. Toutes les nanoparticules testées ont cependant atteint des performances équivalentes lorsqu’une recirculation de l’huile était imposée, permettant de maintenir une alimentation continue du contact en nanoparticules. Une fois incorporées dans une formulation d’huile complète, les nanoparticules IF-MoS2 perdent leurs propriétés tribologiques. Les dispersants contenus dans l’huile, bien que permettant une bonne dispersion des IF-MoS2, semblent responsables de leur inefficacité en empêchant la formation de tribofilms sur les surfaces antagonistes. Une fois correctement dispersées, les nanoparticules pénètrent toujours le contact et se retrouvent bien exfoliées. Une adsorption excessive des dispersants sur les feuillets de MoS2 ainsi libérés et/ou sur les surfaces en acier semble nuire à l’adhésion du tribofilm. Un équilibre entre dispersion des nanoparticules et performance tribologique a ensuite été trouvé, en utilisant de très faibles concentrations de dispersants. Le comportement des huiles dopées en nanoparticules dans des conditions plus proches d’une application automobile a finalement été exploré. Les IF-MoS2 ont permis une réduction significative du frottement et de l’usure à température ambiante et en roulement/glissement, à la fois pour des surfaces lisses et rugueuses. Les risques associés à la présence de nanoparticules dans l’huile dans les régimes de lubrification en film complet ont été partiellement levés. Aucun impact significatif n’a en effet été constaté sur le coefficient de frottement pour l’ensemble des conditions d’essais retenues. Le potentiel des nanoparticules IF-MoS2 pour la protection des surfaces soumises à la fatigue de contact a enfin été démontré. / The growing environmental concerns, along with the continuous increase in the price of fossil fuels, have highly motivated car manufacturers worldwide to improve the efficiency of their vehicles. The tribological properties of engine and gearbox lubricants have a significant impact on the global efficiency of vehicles, as they contribute to reducing friction in many contacts and allow the downsizing of various components by providing their surfaces with anti-wear protection. The recent breakthroughs in nanoparticle synthesis have opened new prospects in terms of lubricant additivation, with the discovery of the excellent friction and wear reducing properties of nanoparticles such as Inorganic Fullerene-like (IF-) molybdenum or tungsten disulfides. The tribological potential of IF-MoS2 for automobile applications was investigated in this work. The respective influences of nanoparticle size and structure were first of all studied, revealing that poorly crystalline nanoparticles were more efficient in maintaining low-friction tribofilms on steel substrates in severe boundary lubrication regimes regardless of size (for the range studied). All the nanoparticles tested however showed similar performances when proper oil recirculation was ensured, providing a continuous feeding of the contact in nanoparticles. The IF-MoS2 nanoparticles lost their lubricating abilities when added to fully-formulated lubricants. This behavior was attributed to the presence of dispersants in the oil, which dispersed the nanoparticles effectively but prevented them from forming tribofilms on the rubbing surfaces. The well-dispersed IF-MoS2 were shown to enter the contact and exfoliate, but an excessive adsorption of the dispersants on the released MoS2 platelets and/or the steel surfaces is thought to prevent tribofilm adhesion. A balance between nanoparticle dispersion and tribological performance was then found, by using very low concentrations of dispersants. The behavior of nanoparticle-doped oils in various scenarios related to automobile applications was finally explored. The IF-MoS2 provided significant friction and wear reduction at ambient temperature and in milder rolling/sliding test conditions, for smooth and rough surfaces. The risks related to the presence of nanoparticles in the oil in full-film lubrication regimes were partially lifted, with no significant influence on friction witnessed for all the test conditions considered. The ability of IF-MoS2 nanoparticles to protect steel surfaces from surface-initiated Rolling Contact Fatigue was finally shown.

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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.

Cette thèse est consacrée à l’évaluation du potentiel d'un semi-conducteur 2D, le disulfure de molybdène (MoS2) monocouche, en tant que matériau de canal de type N pour l’électronique flexible. Ce semi-conducteur d'épaisseur nanométrique est stable chimiquement, robuste mécaniquement et possède une bande interdite directe de 1,9 eV. Le travail réalisé couvre en premier lieu la synthèse de monocouches de MoS2 par dépôt chimique en phase vapeur (CVD) et leur caractérisation. Les monocouches synthétisées ont été intégrées avec succès en tant que matériau de canal dans des transistors de type N stables à l'air. L'étude a mis en évidence l'impact sur les performances de l'environnement et des résistances aux interfaces métal/MoS2. Des mobilités électroniques de 20 cm²/(V.s) associées à des rapports ION/IOFF > 106 ont été obtenus. Ces performances ont permis l’intégration du MoS2 monocouche dans des transistors flexibles. Ce travail a été combiné à d’importants efforts sur l’intégration de films minces organiques électrogreffés en tant que diélectrique de grille, y compris sur substrat flexible. Dans un domaine encore jeune mais en rapide évolution, ces travaux montrent la viabilité de l’option MoS2 monocouche pour l’électronique flexible, notamment en combinaison avec les diélectriques minces organiques. / This PhD thesis is dedicated to the assessment of the potential of monolayers of molybdenum disulfide (MoS2) as a N-type channel material for flexible electronics. This 2D semiconductor of atomic-scale thickness is chemically stable, mechanically robust and has a direct bandgap of 1.9 eV. This work includes the synthesis of MoS2 monolayers by Chemical Vapor Deposition (CVD) and the characterization of this material. The MoS2 monolayers were integrated in air-stable N-type transistors. The study highlighted the impact on the device performances of both the environment and the resistances at the MoS2/metal interfaces. Electronic mobilities of 20 cm²/(V.s) in combination with ION/IOFF ratios > 106 were achieved. These performances allowed integrating MoS2 monolayers in flexible transistors. This work was combined with the study of electrografted organic ultrathin films used as gate dielectrics and their integration in MoS2 transistors. This thesis shows that MoS2 monolayers are a viable option for flexible electronics operating at low bias, in particular when they are associated with ultrathin organic dielectrics.

To achieve sustainable production of H2 fuel through water splitting, low-cost electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are required to replace Pt and IrO2 catalysts. Here, for the first time, we present the interface engineering of novel MoS2/Ni3S2 heterostructures, in which abundant interfaces are formed. For OER, such MoS2/Ni3S2 heterostructures show an extremely low overpotential of ~218 mV at 10 mA cm-2, which is superior to that of the state-of-the-art OER electrocatalysts. Using MoS2/Ni3S2 heterostructures as bifunctional electrocatalysts, an alkali electrolyser delivers a current density of 10 mA cm-2 at a very low cell voltage of ~1.56 V. In combination with density function theory (DFT) calculations, this study demonstrates that the constructed interfaces synergistically favor the chemisorption of hydrogen and oxygencontaining intermediates, thus accelerating the overall electrochemical water splitting.