R. I. Romanov

Ion-assisted deposition of MoSx films from laser-generated plume under pulsed electric field

The thickness profiles and compositional distributions of MoSx films deposited from a plume generated by pulsed laser irradiation of the MoS2 target were investigated at a varying fluence and constant laser pulse energy. It was shown that films with stoichiometric composition were formed at sufficiently low fluence (near the ionization threshold), and increasing fluence caused intricate nonmonotonic variations of the compositional distribution. A substantial deviation of the film composition from stoichiometric and a significant radial gradient of the sulfur concentration over the substrate surface (1<x<3, where x is the ratio of concentrations of S and Mo atoms, x=S/Mo) were found. These phenomena were caused by: (1) the incongruent target evaporation; (2) the mass dependence of the angular distribution of the ablated particles; and (3) the selective sulfur sputtering and desorption induced by energetic particles (ions, excited atoms) of the laser-generated plume. When the laser fluence was low, films of...

Effect of the pulsed laser deposition conditions on the tribological properties of thin-film nanostructured coatings based on molybdenum diselenide and carbon

The structural state and tribological properties of gradient and composite antifriction coatings produced by pulsed laser codeposition from MoSe2(Ni) and graphite targets are studied. The coatings are deposited onto steel substrates in vacuum and an inert gas, and an antidrop shield is used to prevent the deposition of micron-size particles from a laser jet onto the coating. The deposition of a laser jet from the graphite target and the application of a negative potential to the substrate ensure additional high-energy atom bombardment of growing coatings. Comparative tribological tests performed at a relative air humidity of ∼50% demonstrate that the “drop-free” deposition of a laser-induced atomic flux in the shield shadow significantly improves the antifriction properties of MoSex coatings, decreasing the friction coefficient from 0.07 to 0.04. The best tribological properties, which combine a low friction coefficient and high wear resistance, are detected in drop-free MoSex coatings additionally alloyed with carbon (up to ∼55 at %) and subjected to effective bombardment by high-energy atoms during growth. Under these conditions, a dense nanocomposite structure containing the self-lubricating MoSe2 phase and an amorphous carbon phase with a rather high concentration of diamond bonds forms.

Formation of the Chemical Composition of Transition Metal Dichalcogenide Thin Films at Pulsed Laser Deposition

The formation of the chemical composition of dichalcogenide films at pulsed laser deposition in vacuum and in rarefied gases (Ar, H2) is investigated with MoSex thin-film coatings. It is found that deposition in gases increases the selenium concentration and somewhat flattens the composition over the substrate surface. To elucidate the mechanisms underlying the MoSex film formation, a computer model is used that simulates the motion of a pulsed laser-initiated atomic flux through a rarefied gaseous medium. Using this model, the energy and angular parameters of atomic Mo and Se fluxes toward the substrate are calculated. It is shown that the expansion dynamics of laser plume components (Mo and Se) and the selective sputtering of selenium are the main factors governing the formation of the chemical composition and its distribution over the substrate. The influence of the sort of gas on the efficiency of atomic flux slowdown and scattering and on material losses during deposition is considered.

Study and simulation of the growth of solid lubricant MoSe x coatings during pulsed laser deposition

The chemical composition and tribological properties of the thin-film diselenide molybdenum coatings deposited by pulsed laser deposition in vacuum and a rarefied inert gas (argon) atmosphere are studied. Upon deposition in a gas at a pressure of ∼2 Pa, stoichiometric coatings with improved antifriction properties as compared vacuum-deposited coatings form. However, a too strong increase in the argon pressure (to ∼10 Pa) degrades the tribological properties of the coating. Structure formation in the MoSex coatings grown by pulsed laser deposition on an unheated substrate is investigated. Deposition in vacuum or argon at a pressure of 2 Pa leads to formation of rather smooth coatings with a dense amorphous structure containing molybdenum nanoinclusions. Deposition at a high argon pressure results in a developed surface relief and a loose coating structure. A mathematical model is developed using the kinetic Monte Carlo method in order to describe structure formation in the coatings that grow during physical deposition of an atomic flux. A comparative analysis demonstrates satisfactory agreement between the simulated and experimentally studied structures in the coatings created by pulsed laser deposition at various gas pressures.

The formation of a hybrid structure from tungsten selenide and oxide plates for a hydrogen-evolution electrocatalyst

It has been found that the pulsed laser deposition of a thin tungsten selenide film, followed by thermal treatment at 550°C in an Ar + O2 mixture of gases, results in the formation of a hybrid structure that is made up of ultrathin WSe2 and WO3–y platelets. The structural and size characteristics of the nanoplatelets deposited on microcrystalline graphite provide the effective hydrogen evolution reaction in a 0.5 M H2SO4 solution, with the cathode current made about seven times higher at a potential of–100 mV and the slope of the Tafel characteristic reduced from 340 to 90 mV/dec.

Pulsed laser deposition of thin-film coatings using an antidroplet shield

The formation of thin-film coatings of molybdenum diselenide is studied during the deposition of a laser-induced material flux using a shield that is placed on the path of the expansion of this flux in order to trap the droplet fraction. To increase the efficiency of atomic scattering into the shadow zone (behind the shield), deposition is carried out in an inert gas (argon). As the argon pressure increases to 2 Pa, low-density coatings with a developed surface relief form in the shadow zone. When a negative bias voltage is applied to a substrate, the quality of the coating increases substantially. Numerical experiments based on the combination of two computer models that describe physical processes on the atomic level using Monte Carlo methods are performed to reveal the factors that affect the thickness, chemical composition, and structure of the MoSex coatings deposited in the shadow zone. The results of calculating the dynamics of the laser-induced atomic flux in a chamber with a shield are used to simulate the coating growth. The deposition of a scattered atomic flux under conditions of surface bombardment by incident particles is shown to substantially increase the coating density and to smooth away the surface relief.

Formation of thin catalytic WSe x layer on graphite electrodes for activation of hydrogen evolution reaction in aqueous acid

The possibility of obtaining new relatively inexpensive electrode materials to provide enhanced efficiency of hydrogen evolution reaction (HER) in an aqueous acid solution was investigated. For this purpose, the surface properties of cathodes made of microcrystalline graphite were modified by pulsed laser deposition of thin films of WSex. The structure, morphology, and chemical composition of the thin film coatings were varied by changing the deposition conditions and subsequent heat treatment. The compact and dense structure of the film in an amorphous and crystalline state did not result in a marked positive impact on the character of the HER process, which was investigated in 0.5 M H2SO4 solution at room temperature. Formation of thin layers consisting of nanocrystalline “petals” WSe2 caused an increase in cathodic current by more than 6 times (at a voltage of–150 mV), and the Tafel slope of the voltage vs. current curve was reduced by about 80 mV/dec. The conditions were determined to produce on the surface of the graphite cathode a high density of new catalytically active sites that formed on edges of molecular planes forming a layered structure characteristic of WSe2 nanocrystals.

Nanostructured catalyst for hydrogen electrochemical reduction based on molybdenum diselenide thin films

Thin-filmed dichalcogenides of transition metals, in particular, MoSe2, are considered as potentially active materials in the hydrogen evolution reaction, which can compete with expensive platinum. It has been established that the laser deposition technique ensures the formation of nanocomposite films containing a high density of Mo nanoparticles in the MoSex shell. Deposition of Mo nanoparticles increase roughness and induce activation of a surface. This manifests itself in formation of “edge” sites in the MoSex shell and on edges of the basis planes in MoSex nanocrystals oriented normally to the film surface. In a 0.5 M H2SO4 solution, on carbon cathodes coated with MoSex films, the hydrogen overvoltage dropped to −0.17 V and the current density doubled.

Effect of hydrogen on the electrical characteristics of structural elements of the Pt/WOx/6H-SiC

The formation conditions of the Pt/WOx/SiC thin-film system on a silicon carbide (6H-SiC) single crystal are optimized. The prepared system possesses stable characteristics and makes it possible to effectively record hydrogen at low concentrations in air at a temperature of ∼350°C, as well as to hold hydrogen in the WOx lattice at room temperature for a long time. The voltage shift of reverse portions of the current–voltage characteristics at a hydrogen concentration of ∼0.2% reach 6.5 V at a current of 0.4 µA because of large series resistance, which is defined by space-charge regions in WOx and SiC. Structural-phase investigations of the oxide layer are performed under various effect modes of the hydrogen-containing medium on the Pt/WOx/SiC system. A correlation in the variations of its electrical properties (ability to accumulate charge and vary the resistivity) and structural state of the oxide layer is revealed. An explanation for the variation in the current transport through the Pt/WOx/SiC and its contact regions (barrier layers) under the effect of hydrogen is proposed.

Functional micro- and nanostructured layers based on tungsten oxide for high-temperature hydrogen detectors on the “Pt-metal oxide-SiC” platform

The features of the structural formation and chemical composition of thin films of tungsten oxide grown under various conditions of pulsed laser deposition on a substrate of single crystal silicon carbide followed by annealing have been investigated. To obtain doped films of tungsten oxide deposited by the laser plume expanding from a tungsten target, an additional deposition of Pt, Ti, and Ta atoms was carried out. In some cases a thin film of a catalytically active metal (platinum) was deposited on the oxide layer formation. The structural state of the films was investigated by X-ray diffraction, electron and atomic force scanning microscopy, and Raman spectroscopy. The gas-sensing properties of the Pt-oxide-SiC structures were investigated by measuring the current-voltage characteristics at 300°C in air and air mixtures with hydrogen (2 vol %). It has been established that the deposited metal oxide films are significantly different in morphology and structure on micro- and nanoscales. This had a significant impact on the magnitude of the response to hydrogen and the mechanisms determining the gas-sensing properties: current passage in oxide and the magnitude of the potential barriers at the interfaces of the structure.