Argon Background Gas Research Articles

Simulation of emission spectra from nonuniform reactive laser-induced plasmas.

We demonstrate that chemical reactions leading to the formation of AlO radicals in plasmas produced by ablation of aluminum or Ti-sapphire with ultraviolet nanosecond laser pulses can be predicted by the model of local thermodynamic equilibrium. Therefore, emission spectra recorded with an echelle spectrometer and a gated detector were compared to the spectral radiance computed for uniform and nonuniform equilibrium plasmas. The calculations are based on analytical solutions of the radiation transfer equation. The simulations show that the plasmas produced in argon background gas are almost uniform, whereas temperature and density gradients are evidenced in air. Furthermore, chemical reactions exclusively occur in the cold plume periphery for ablation in air. The formation of AlO is negligible in argon as the plasma temperature is too large in the time interval of interest up to several microseconds. Finally, the validity of local thermodynamic equilibrium is shown to depend on time, space, and on the elemental composition. The presented conclusions are of interest for material analysis via laser-induced breakdown spectroscopy and for laser materials processing.

Nanocrystalline diamond films prepared by pulsed electron beam ablation on different substrates

Abstract

Slowing of femtosecond laser-generated nanoparticles in a background gas

The slowing of Pt nanoparticles in argon background gas was characterized by Rayleigh scattering imaging using a plume of nanoparticles generated by femtosecond laser through thin film ablation of 20 nm-thick Pt films. The ablation was performed at threshold laser energy fluences for complete film removal to provide a well-defined plume consisting almost entirely of nanoparticles traveling with a narrow velocity distribution, providing a unique system to unambiguously characterize the slowing of nanoparticles during interaction with background gases. Nanoparticles of ∼200 nm diameter were found to decelerate in background Ar gas with pressures less than 50 Torr in good agreement with a linear drag model in the Epstein regime. Based on this model, the stopping distance of small nanoparticles in the plume was predicted and tested by particle collection in an off-axis geometry, and size distribution analysis by transmission electron microscopy. These results permit a basis to interpret nanoparticle propagation through background gases in laser ablation plumes that contain mixed components.

The Morphology and Electrochemical Properties of Pt and GDC Nano Composite Films Using Co-Sputtering Method

Solid oxide fuel cell (SOFC) has a potential of next generation energy conversion device for high energy conversion rate and clean emission. However it requires high temperature above 800oC due to low kinetics of electrolyte and electrode materials. Lowering operating temperature to 400~600oC allows high system stability, various material selection, fast start-up and convenient sealing. There were several studies to lower the operating temperature with structural and material approach. Among these, application of nano fabrication to electrode and electrolyte is one of the most promising candidates because thickness of electrolyte and TPB density of electrodes are strongly affects to the cell performance. Up to now, several structures and materials for low temperature operation have been presented however, PVD based nano-porous Pt is dominantly used for cathode because conventional mixed conducting oxides shows poor electronic conduction and large overpotential at a certain temperature between 400~600oC. Pt electrodes for LT-SOFCs are usually fabricated by physical vapor deposition (PVD) to increase TPB density because it can easily control grain and pore size into nanometer scale. However increased surface energy and mobility of nano sized Pt accelerates coarsening even at a low temperature operation. In a metallurgical research area, several methods to enhance heat resistance of metals and one of the most effective methods are known as dispersion hardening. By uniform dispersion (<1% by weight) of thermally stable oxide nano particles (<50nm) in to metals, it prevents thermal degradation even in a high temperature.In this research we used GDC as a dispersion oxide material to enhance physical and electrochemical property of Pt because it has high ionic conductivity and electronic conductivity among oxide electrolytes.Pt & GDC nano composite electrodes were fabricated under co-sputtering using Pt and GDC target. 50mtorr of argon background gas pressure was maintained during sputtering and various sputtering power between 50~200W on Pt and GDC were applied to differentiate the composition.Fabricated electrodes are treated at 500oC for an hour to characterize morphological change and analyzed by electron scanning microscopy. Finally, fabricated electrodes are applied into YSZ pellet and evaluated electrochemical properties.

Spatial characterization of laser induced Yb plasma in argon using optical emission spectroscopy: Pressure effect

• Laser induced Yb plasma at different pressure of Argon is spatially investigated. • O-TOF profiles and excitation temperature are used to characterize the plasma. • At 100 Pa of argon background gas, shock wave begins to be formed. • Drag force expansion describes the plasma propagation at pressures bigger than 1 Pa. • Two components of velocity distribution of the Yb atoms are estimated. Spatial and temporal behavior of laser induced Ytterbium plasma plume is studied using optical emission spectroscopy technique. A third harmonic Nd:YAG nanosecond laser was used to generate Yb plasma plume at different Argon background pressures (1, 10, 10 2 , 10 3 and 10 4 Pa). The plasma dynamics was investigated based on the spatial behavior of the excitation temperature coupled with optical time of flight (O-TOF) profiles of neutral Yb emission line (555.65 nm) along the propagation axe of the plasma plume. Drag force model was appropriate to describe the propagation dynamics at all pressures except of the lowest one (1 Pa) where free expansion model is dominant. The velocity distribution of Yb I atoms were extracted using two terms of Shifted Maxwell–Boltzmann (SMB) distribution. The correlation between the spatial comportment of both excitation temperature and O-TOF profiles is discussed.

On the preparation of iron pyrite from synthetic and natural targets by pulsed electron deposition

We report on the preparation of iron pyrite (<TEX>$FeS_2$</TEX>) using pulsed electron ablation of two targets, namely, a mixture of sulfur and iron compound target, and a natural iron pyrite target. Thin films of around 50 nm in thickness have been deposited on glass substrates under Argon background gas at 3 mTorr, and at a substrate temperature of up to <TEX>$450^{\circ}C$</TEX>. The thin films have been analyzed chemically and examined structurally using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and visible Raman spectroscopy. The morphology and thickness of the films have been assessed using scanning electron microscopy (SEM) and visible spectroscopic reflectance. The preliminary findings, using a synthetic target, show the presence of iron pyrite with increasing proportion as substrate temperature is increased from <TEX>$150^{\circ}C$</TEX> to <TEX>$250^{\circ}C$</TEX>. The data have not shown any evidence of pyrite in the deposited films from a natural target.

Spectroscopic characterization of laser-produced GaAs plasma in helium and argon background gases

Optical emission from laser produced plasma of GaAs was studied for various helium and argon background pressures. The pressure was changed in the range from 5×10−3 to 100mbar. The time evolutions of selected atomic and ionic spectral transitions were measured and compared. Several different maxima in the temporal evolution of As and Ga optical emission were observed and explained by the processes in plasma. These processes also explain some of the pressure dependencies of the integrated emission intensities. It was found that while argon background gas was more efficient in exciting plasma species, helium led to more uniform species’ ratios over larger pressure range.

Pulsed laser deposition of thin carbon films in a neutral gas background

We studied carbon film deposition using a laser-produced plasma, in argon and helium background gas, at pressures between 0.5 and 700 mTorr. A Nd : YAG, 370 mJ, 3.5 ns, at 1.06 µm, operating at 10 Hz, with a fluence of 6.7 J cm−2 was used. The laser plasma was characterized using space resolved OES and a fast response Faraday cup. The resulting carbon films were analysed using AFM, Raman spectroscopy, XPS and SIMS. The structural properties of the carbon films were found to be strongly correlated with the laser carbon plasma composition. Films with a relatively high content of sp3, characteristic of DLC, were obtained at pressures below 200 mTorr. For these conditions the characteristic carbon ion energies in the expanding laser plasma were of the order of 100 eV. At higher pressures sp2 bonds, associated with amorphous carbon, were dominant, which coincides with a high content of C2 molecules in the laser plasma, and a characteristic carbon ion energy around 20 eV.

Measurements of the Ion Species of Cathodic Arc Plasma in an Axial Magnetic Field

Metal and gas ion species and their charge state distributions were measured for pulsed copper cathodic arcs in argon background gas in the presence of an axial magnetic field. It was found that changing the cathode position relative to anode and ion extraction system, as well as increasing the gas pressure, did not affect much the arc burning voltage and the related power dissipation. However, the burning voltage and power dissipation greatly increased as the magnetic field strength was increased. The fraction of metal ions and the mean ion charge state were reduced as the discharge length was increased. The observations can be explained by the combination of charge exchange collisions and electron impact ionization. They confirm that previously published data on characteristic material-dependent charge state distributions (e.g., Anders and Yushkov in 2002) are not universal but valid for high vacuum conditions and the specifics of the applied magnetic fields.

Effect of Argon Ambient Gas Pressure on Plume Expansion Dynamics

In this paper we present a numerical modeling of a nanosecond laser pulse interaction with a titanium target. We investigate the vapor plume formation and the influence of the ambient gas pressure on plume expansion dynamics. The vapor plume formation depends on the results of the heat transfer in the solid target modeling. The solid-liquid phase change is modeled by a two dimensional approach using an enthalpy formulation. The resulting plume expansion in the argon background gas is studied using the species transport model. The algebraic equations are discretized by the finite volume method implemented by Fluent CFD software [1]. The calculation results of plume expansion velocity, density, temperature and degree of ionization in the plume are presented.

Microscopic description of the thermalization process during pulsed laser deposition of aluminium in the presence of argon background gas

The presence of background gases is typical in most pulsed laser deposition (PLD) applications and there is a need for methods which do not suffer from the oversimplified assumption of collisionless transfer of the target atoms onto the substrate in analytical descriptions. In this work, we give a microscopic description of a thermalization process by following the history of each ablated particle by Monte Carlo calculations. TRIM code (SRIM2010), which is capable of quantum mechanical treatment of ion–atom collisions, has been used in our simulations. Various kinetic parameters of ablated Al ions during target-to-substrate transfer have been calculated to demonstrate the efficient role of background gas atoms in thermalization of ions. Moreover, the growing parameters of interest in PLD have been calculated to achieve the optimal deposition conditions in the presence of a background gas. A base pressure of 1 × 10−1 Torr and 2–3 cm of target-to-substrate distance have been found to be the optimal conditions in PLD of Al in Ar gas environment. Our model can be used to obtain the first estimates of nonreactive PLD parameters, such as the background gas pressure and the target-to-substrate distance for the growth of even more complex materials in the presence of different background gases.

On the plume splitting of pulsed laser ablated Fe and Al plasmas

A time resolved imaging study of pulsed laser ablated Fe and Al plasma plumes with specific interest in the splitting of plumes into the slow and fast moving components as they expand through the background argon gas at different pressures is reported. The material ablation was achieved using a Q-switched Nd:YAG (yttrium aluminum garnet) laser operating at 532 nm with a pulse duration of ∼8 ns full width at half maximum and a fluence of 30 Jcm−2 at the target surface. Typical time resolved images with low magnification show that the splitting occurs at moderate background gas pressures (0.5 and 1.0 mbar for Fe, and 0.2 mbar for Al plasma plumes). The plume splitting did not occur for higher background gas pressures.

Particle simulation of femtosecond laser stimulation of electrical discharges in small gaps

A particle-in-cell/Monte Carlo collision simulation study of femtosecond laser stimulation of electrical discharges in submicron gaps between platinum scanning tunneling microscope cathode tips and gold film anodes in atmospheric pressure argon gas is described. The breakdown potential of gaps containing either preloaded electrons or preloaded neutral and ionized platinum atoms and electrons at various densities were compared to the breakdown potential of gaps with background argon gas alone. It was found that gaps preloaded with sufficient densities of either electrons or partially ionized electrode materials broke down at cathode potential of −80 V while gaps with background argon gas alone required applied cathode potentials between −200 to −250 V to cause a breakdown. Also, partially ionized material was much more effective at stimulating breakdown than electrons alone. The density of preloaded partially ionized platinum required to lower breakdown cathode potential to −80 V was approximately 12.5 times smaller than that required for the preloaded electrons alone in the same size gap. Analysis of the mechanism by which the preloaded partially ionized material promoted breakdown showed that laser ablated ions located near the cathode tip increased the local field at the cathode surface and the field-emitted electron flux from the cathode. This higher electron flux ionized background argon gas and preloaded platinum which resulted in gap breakdown. The threshold density of partially ionized platinum for gap breakdown varied approximately linearly with applied cathode potential.

Photoconductivity of CdTe nanocrystal films in a simple multilayer device structure

The photoconductivity of CdTe nanocrystal films was investigated by employing aZnO/CdTe/In multilayer device structure. CdTe was deposited on a ZnO electrode by apulsed electron-beam technique at argon background gas pressures of 9, 13 and 17 mTorr.Using two photo-excitation sources (visible and near-infrared), the device with the CdTedeposited at 17 mTorr demonstrated the highest photocurrent to dark current ratio,suggesting the highest quantum efficiency among the three different devices. It alsodemonstrated the highest short circuit photocurrent and the fastest photocurrent decay.These results are attributed to the formation of more nanocrystals at 17 mTorr withenhanced optoelectronic properties.

Nonthermal plasma synthesis of size-controlled, monodisperse, freestanding germanium nanocrystals

Germanium nanocrystals may be of interest for a variety of electronic and optoelectronic applications including photovoltaics, primarily due to the tunability of their band gap from the infrared into the visible range of the spectrum. This letter discusses the synthesis of monodisperse germanium nanocrystals via a nonthermal plasma approach which allows for precise control of the nanocrystal size. Germanium crystals are synthesized from germanium tetrachloride and hydrogen entrained in an argon background gas. The crystal size can be varied between 4 and 50nm by changing the residence times of crystals in the plasma between ∼30 and 440ms. Adjusting the plasma power enables one to synthesize fully amorphous or fully crystalline particles with otherwise similar properties.

Optical and magnetic properties of Co-doped TiO2 thin films grown by pulsed laser deposition

Co-doped TiO2 thin films were grown by pulsed laser deposition onto (0001)Al2O3 substrates, heated at 310°C, using a KrF excimer laser. The ablation of polycrystalline targets of TiO2 rutile containing a few mol% of Co3O4 was performed in a background gas mixture of argon and hydrogen at total pressure of 10Pa, for H2 flow rates of 0.0, 0.1 and 0.2sccm. X-ray diffraction analysis showed that the as-deposited films consist of TiO2 rutile-based nanocrystallites with preferred orientation along the [101] direction. The chemical composition of the samples was evaluated by Rutherford backscattering measurements, revealing that Ti distribution is homogeneous on the surface and along film depth, and that the Co:Ti ratio is 0.03:0.97 for all the films investigated. In this paper, we focus on the influence of hydrogen addition to argon background gas on the optical properties of the Co-doped TiO2 films. Results of magnetic characterisation will also be presented.

Expansion of a laser-produced silver plume in light background gases

The expansion of a silver ablation plume in a helium and an argon background gas has been studied over the pressure range 10-6 to 1 mbar. The angular distribution of silver atoms deposited on an array of quartz-crystal microbalances as well as time-of-flight signals of the plume ions in both gases were measured. There is a distinct sharpening of the angular distribution of the ablated silver atoms in the helium gas within a narrow range from 0.2 to 0.4 mbar. In contrast, the width of the angular distribution in the argon gas increases throughout the interval. The time-of-flight signals show a peak of fast ions which gradually merges into a more complex structure.

Pulsed laser deposition of InP thin films on sapphire(1 0 0 0) and GaAs(1 0 0)

Indium phosphide thin films were deposited on sapphire and GaAs(1 0 0) substrates by pulsed laser deposition using a XeCl excimer laser in argon background gas and in high vacuum. The grown film structure and morphology were observed by reflection high-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The films were grown on sapphire were found to be disordered with the argon gas background enhancing the particulate formation. Epitaxial InP(1 0 0) film with oriented polyhedral islands were grown on hydrogen-cleaned GaAs(1 0 0) at a substrate temperature of ∼573 K at 1 × 10 −8 Torr background pressure. By varying the deposition parameters, relatively smooth InP films with average surface roughness of about 4–12 nm were obtained.

Pulsed laser deposition of InP thin films on sapphire(1 0 0 0) and GaAs(1 0 0)

Indium phosphide thin films were deposited on sapphire and GaAs(1 0 0) substrates by pulsed laser deposition using a XeCl excimer laser in argon background gas and in high vacuum. The grown film structure and morphology were observed by reflection high-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The films were grown on sapphire produced a reflection electron diffraction pattern of a disordered film. Introduction of an argon gas background enhanced the particulate formation. Epitaxial InP(1 0 0) film with oriented polyhedral islands were grown on hydrogen-cleaned GaAs(1 0 0) at a substrate temperature of ∼573 K at 1×10 −8 Torr background pressure. By varying the deposition parameters, relatively smooth InP films with average surface roughness of about 4–12 nm were obtained.

Fabrication of crystallized boron films by laser ablation

Polycrystalline β-rhombohedral boron films mixed with amorphous boron phase have been successfully fabricated on quartz substrates using pulsed laser ablation in a quartz glass tube chamber placed in an electric furnace. The crystallinity of the films strongly depended on the temperature of the furnace and the pressure of background argon gas. High temperature and high pressure in the chamber were suitable for crystallized boron film preparation. The best crystalline films (without B 2O 3 phase formation) were obtained at 1000°C, 100 Pa. XPS measurements demonstrated that the major contaminants were carbon and oxygen, and the atomic ratio of oxygen to boron was 0.05 under the preparation conditions of well-crystallized films. The surface roughness of the films decreased by lowering laser energy to 150 mJ/pulse under the same pressure and temperature conditions.