V. Yu. Fominski

Pulsed laser deposition of MoSx films in a buffer gas atmosphere

Abstract The influence of a buffer (inert) gas atmosphere on structure, composition and tribological properties of MoSx films formed by pulsed laser deposition with various laser influence was investigated. The films were deposited at room temperature from an MoS1.8 target under vacuum conditions (4 × 10-4 Pa) and in Ar atmosphere at a gas pressure varying from 0.7 Pa to 4 Pa. Ablation under vacuum conditions using a relatively low laser fluence resulted in the deposition of amorphous films with conservation of stoichiometry. According to XPS analysis data, such films may contain many defects of chemical nature. Deposition at higher laser fluences resulted in substantial sulphur depletion and, under certain conditions, crystallization of the films. These microcrystalline films exhibited a lower friction coefficient than amorphous ones. Ablation at high laser fluences in Ar atmosphere allowed us to deposit MoSx films with various stoichiometries (x ⩽ 2.2) and tribological properties superior to those of the films deposited under vacuum conditions. These films have the amorphous structure and chemistry (according to XPS measurements) inherent in perfect MoS2.

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.

Peculiarities of Pulsed Ion Implantation from a Laser Plasma Containing Multiply Charged Ions

A mathematical model describing the dynamics of a pulsed laser plasma with multiply charged ions, as well as the formation of the accelerated ion flow in an external magnetic field, is developed. Experimental studies and mathematical simulation by the particle-in-cell method are used to determine the role of multiply charged ions in the process of ion implantation into a silicon substrate from the pulsed plasma containing singly and doubly charged titanium ions. The plasma spreads between parallel-plate electrodes (Ti target and Si substrate) along the normal to the surface of the target. Ions are accelerated by high-voltage negative pulses applied to the substrate. It is found that doubly charged ions effectively participate in the implantation process when an external electric field is applied very soon after the laser action on the target. The application of a high-voltage pulse with an amplitude of 50 kV 0.5 μs after a laser pulse leads to ion implantation with an energy close to 100 keV. With increasing delay in the application of the high-voltage pulse, the upper boundary of the energy spectrum of implanted ions is displaced towards lower energies. Comparison of the depth profiles of titanium distribution in silicon calculated from the results of simulation are compared with the experimental profiles shows that the model developed here correctly describes the formation of the high-energy component of the ion flow, which is responsible for defect formation and doping of deep layers of the substrate.

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.

The formation of the two-way shape memory effect in rapidly quenched TiNiCu alloy under laser radiation

The effect of pulsed laser radiation (λ = 248 nm, τ = 20 ns) on structural properties and shape memory behavior of the rapidly quenched Ti50Ni25Cu25 alloy ribbon was studied. The radiation energy density was varied from 2 to 20 mJ mm−2. The samples were characterized by means of scanning electron microscopy, x-ray diffraction, microhardness measurements and shape memory bending tests. It was ascertained that the action of the laser radiation leads to the formation of a structural composite material due to amorphization or martensite modification in the surface layer of the ribbon. Two methods are proposed which allow one to generate the pronounced two-way shape memory effect (TWSME) in a local area of the ribbon by using only a single pulse of the laser radiation. With increasing energy density of laser treatment, the magnitude of the reversible angular displacement with realization of the TWSME increases. The developed techniques can be used for the creation of various micromechanical devices.

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.

Structure and catalytic properties of MoSe x thin films containing Mo nanoparticles in electrochemical production of hydrogen in solution

The introduction of molybdenum nanoparticles in MoSex thin films formed by pulsed laser deposition led to changes in the film structure. The base planes of the layered atomic packing of the MoSeх matrix around Mo nanoparticles rotated; as a consequence, the edge sites that formed during the “breaking” of the Se–Mo–Se layered atomic packing came out to the film surface. At high nanoparticle concentrations, this effect led to high density of edge sites possessing increased catalytic activity (compared with that of the base planes) for initiating the electrochemical evolution of hydrogen in a 0.5 M H2SO4 solution. Voltammetric measurements at room temperature showed that when the carbon cathode was coated with MoSex thin films under optimum conditions, the hydrogen overvoltage considerably decreased, and the cathodic current increased. The results indicate that developments in the field of preparation of nanostructured electrodes based on layered transition metal dichalcogenides show promise as an alternative to expensive electrodes based on platinum group metals for electrocatalysts of hydrogen evolution.