Low Argon Research Articles

On the Effect of Randomly Oriented Grain Growth on the Structure of Aluminum Thin Films Deposited via Magnetron Sputtering

The microstructure of aluminum thin films, including the grain morphology and surface roughness, are key parameters for improving the thermal or electrical properties and optical reflectance of films. The first step in optimizing these parameters is a thorough understanding of the grain growth mechanisms and film structure. To investigate these issues, thin aluminum films with thicknesses ranging from 25 to 280 nm were coated on SiOx/Si substrates at ambient temperature under high-vacuum conditions and a low argon pressure of 3 × 10−3 mbar (0.3 Pa) using the radio frequency magnetron sputtering method. Quantitative analyses of the surface roughness and nanograin characteristics were conducted using atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray diffraction. Changes in specular reflectance were measured using ultraviolet–visible and near-infrared spectroscopy. The low roughness values obtained from the AFM images resulted in high film reflectivity, even for thicker films. TEM and AFM results indicate monomodal, randomly oriented grain growth without a distinct columnar or spherical morphology. Using TEM cross-sectional images and the dependence of the grain size on the film thickness, we propose a grain growth mechanism based on the diffusion mobility of aluminum atoms through grain boundaries.

INFLUENCE OF THE RATIO OF GASE CONSUMPTION N<sub>2</sub>/Ar AND MAGNETRON POWER ON THE DENSITY AND STOICHIOMETRIC COMPOSITION OF TIN<sub>X</sub> FILMS SYNTHESIZED BY MAGNETRON SPUTTERING METHOD

The films of titanium nitride were deposited by direct current magnetron sputtering on the surface of singlecrystalline silicon samples in an Ar-N2 atmosphere for use as a diffusion barrier. The thickness and density of films were measured by X-ray reflectometry. The design of the MAGNA TM-200-01 installation has been changed to increase the supply of nitrogen into the chamber. The influences of sputtering conditions, including the flow rate of nitrogen and argon gases and their N2 /Ar ratios in the range of 1–60 in the chamber, magnetron power of 690–1400 W on the formation of TiNx films, their density and stoichiometric composition, were studied. It is shown that the value of x is affected not only by the N2 /Ar gas flow rate ratio, but also by the magnetron power. At the sputtering parameters 1200 W, N2 /Ar = 30, 0.8 Pa, 320 s and 100°C, a maximum density of 5.247 g/cm3 of a film was achieved, which corresponds to the composition TiN0.786 = Ti56N44. The presence of nanocrystalline film of titanium nitride and the absence of a nanocrystalline titanium phase were confirmed by photographic X-ray diffraction. It was found that for the synthesis of titanium nitride as close as possible to the stoichiometric composition TiN0.770 - TiN0.786, it is necessary to use magnetron power in the range of 900–1200 W, nitrogen rate of 30 cm3 /min with low argon flows of 1–5 cm3 /min.

Novel age constraints on offshore islets around Taiwan with implications for the Northern Taiwan Volcanic Zone

Determining the exact age of volcanic activity in the Northern Taiwan Volcanic Zone (NTVZ) is essential for comprehending Taiwan's tectonic evolution. The timing and duration of the volcanic processes that formed the NTVZ remain uncertain despite the availability of geochronological, geochemical, and geophysical data. While extensive efforts have been directed towards investigating the age of the Tatun Volcanic Group, the primary feature of the NTVZ, many other aspects require further examination. This study concentrates on two offshore islets in the northern region of the NTVZ. We present new 40Ar/39Ar ages of 0.252 ± 0.018 Ma for a lava flow, representing an early stage of volcanic activity, and 0.118 ± 0.012 Ma for a dike, representing the latest stage of volcanic activity around Pengjia Islet. A sample collected from Mianhua Islet did not yield a reasonable age due to high uncertainty, likely attributable to young and low radiogenic argon. Field observations support the interpretation that volcanic activity on both islets is considerably younger than previously estimated. This study corroborates the hypothesis from geophysical studies that submarine NTVZ features to record a potentially active magmatic reservoir offshore of northern Taiwan. By refining our understanding of spatio-temporal patterns in volcanic activity, this study contributes to volcanic hazard assessment in Taiwan and surrounding areas.

The DarkSide-20k underground argon procurement chain

The DarkSide-20k experiment searches for dark matter by looking for interactions of WIMPs in a 50-tonnes target of liquid argon using double-phase time projection chamber (TPC) technology. The key component of the experiment is low radioactivity argon depleted in the isotope 39Ar. Unfortunately, 39Ar is naturally present in atmospheric argon and it is constantly produced due to the interaction with cosmic rays. Finding a source of depleted argon was the first step of the Collaboration when it started to size the DarkSide-20k detector.The procurement chain begins with the Urania plant in Colorado, which can produce argon with a purity of 99.99% from a crude CO2 stream extracted from a deep well, at a rate of about 250 kg per day. The plant has already been fabricated while the site is being prepared for installation. After the extraction and purification of 120 t of underground argon (UAr), it will be transported to Sardinia, Italy, where the ARIA plant, consisting of a 350 m cryogenic distillation column, will further remove impurities up to 2 orders of magnitude. The final purity is expected to be 99.9999%. After the ARIA purification stage, the ultra-pure UAr will be delivered to the Laboratori Nazionali del Gran Sasso, L'Aquila, Italy where it will fill the DarkSide-20k TPC.The ARIA plant has already been fully fabricated and is now in the installation phase in a mine shaft. A shorter version, about 26 m high, has been tested over the last three years with very positive results. We were able to measure the separation factor between two isotopes of nitrogen (29N2, 28N2) and three of argon (40Ar, 38Ar, 36Ar) thus validating the capability of performing isotopic distillation with ARIA.

Interface engineering and heterometal doping Cu3Se2/WSe2 for efficient charge transfer and nonlinear optical performance

Integrating interface engineering and precious heterometal doping with semiconductors is a feasible pathway to construct efficient charge transfer and improve the nonlinear optical (NLO) performance of materials. Here, we report a bimetallic selenide type-II heterostructure of Cu3Se2/WSe2 (CWS) with a simple one-step magnetron co-sputtering of Cu and WSe2 targets at low argon flow rates. Se first nucleates and precipitates to form nanospheres at low flow rates. Due to the electronegativity of Cu being greater than that of W and Cu prioritizes attachment to the outside of the Se nanospheres and promotes the formation of the nanorods-grown Cu3Se2. A combined analysis of X-ray photoelectron spectroscopy and X-ray diffraction data was used to investigate the minor shift in the W 4 f/Se 2p ratio revealing that electrons shifted from Cu3Se2 to WSe2. The diffraction peaks of CWS and the valence electronic structure of Cu1+/Cu2+ confirmed the implantation of Cu3Se2 nanorods on WSe2. The NLO properties of CWS composite films were modulated by varying the power of Cu. More importantly, the nonlinear absorption coefficient of the CWS composite films formed after Cu3Se2 promotion is much better than that of other unmodified pure WSe2 films. The photogenerated carrier dynamics of CWS composite films were investigated by femtosecond transient absorption spectroscopy, and density functional theory calculations show that CWS composite films inhibit efficient electron transfer photogenerated carrier complexation. This work proposes a few strategies for designing heterogeneous structures with interface engineering and precious metal doping to promote adjustable electron transfer dynamics and achieve high NLO conversion efficiency.

Conditions and thermophysical properties for transport of hydrocarbons and natural gas at high pressures: Dense phase and anomalous supercritical state

Hydrocarbons and natural gas, like other fluids exhibit anomalous behavior in the vicinity of their critical points (Pc, Tc), where flow oscillations, thermal instabilities, and decreased heat transfer can occur. This work, based on Peng-Robinson equations of state, demonstrates that in the case of mixtures, the cricondenbar, cricondentherm, and a part of the dense phase belong to this anomalous region. Consequently, the most desirable conditions for the transport of supercritical natural gas (SNG) are: above the critical pressure and critical temperature, above the cricondenbar and cricondentherm, and beyond the anomalous state. For methane, such conditions exist at pressure, P > 5 MPa and temperature, T > - 30 °C whereas for average composition of natural gases from US/Canada, West Asia, and North Sea, P ≥ 6 MPa and T > −30 °C may be appropriate/safe for SNG transport. Corresponding gray zones which will require special design considerations are also identified. Reduced pressure, (P/Pc) of 1.15 and reduced temperature, (T/Tc) of 1.25 and 1.4 may be appropriate to delimit the Unsafe Zone/Gray Zone and Gray Zone/Safe zone. In addition, the specific volume and kinematic viscosity show asymptotic behavior at high SC pressures that can have important implications in the transport of hydrocarbons and natural gas. Moreover, the critical temperature, cricondentherm, and anomalous region of natural gas can be moved to lower temperatures by adding one or more modifier gases with low Tc, e.g., methane, nitrogen, and argon; or, higher using gases with high Tc such as ethane, propane, and carbon dioxide. The pipelines carrying SNG, with Tc-modifications if needed, can therefore pass through polar, tropical, arid, and desert-like conditions.

On the importance of excited state species in low pressure capacitively coupled plasma argon discharges

In the past three decades, first principles-based fully kinetic particle-in-cell Monte Carlo collision (PIC/MCC) simulations have been proven to be an important tool for the understanding of the physics of low pressure capacitive discharges. However, there is a long-standing issue that the plasma density determined by PIC/MCC simulations shows quantitative deviations from experimental measurements, even in argon discharges, indicating that certain physics may be missing in previous modeling of the low pressure radio frequency (rf) driven capacitive discharges. In this work, we report that the energetic electron-induced secondary electron emission (SEE) and excited state atoms play an important role in low pressure rf capacitive argon plasma discharges. The ion-induced secondary electrons are accelerated by the high sheath field to strike the opposite electrode and produce a considerable number of secondary electrons that lead to additional ionizing impacts and further increase of the plasma density. Importantly, the presence of excited state species even further enhances the plasma density via excited state neutral and resonant state photon-induced SEE on the electrode surface. The PIC/MCC simulation results show good agreement with the recent experimental measurements in the low pressure range (1–10 Pa) that is commonly used for etching in the semiconductor industry. At the highest pressure (20 Pa) and driving voltage amplitudes 250 and 350 V explored here, the plasma densities from PIC/MCC simulations considering excited state neutrals and resonant photon-induced SEE are quantitatively higher than observed in the experiments, requiring further investigation on high pressure discharges.

Direct Detection of Dark Matter with DarkSide-20k

DarkSide run since mid 2015 a 50-kg-active-mass dual phase Liquid Argon Time Projection Chamber (TPC), filled with low radioactivity argon from an underground source and produced world class results for both the low mass (< 20 GeV/c2) and high mass ( > 100 GeV/c2) direct detection search for dark matter. The next stage of the DarkSide program will be a new generation experiment involving a global collaboration from all the current Argon based experiments. DarkSide-20k, is designed as a 20-tonne fiducial mass dual phase Liquid Argon TPC with SiPM based cryogenic photosensors, and is expected to be free of any instrumental background for an exposure of >100 tonne year. Like its predecessor, DarkSide-20k will be housed at the INFN Gran Sasso (LNGS) underground laboratory, and it is expected to attain a WIMP-nucleon cross section exclusion sensitivity of 7.4 × 10−48 cm2 for a WIMP mass of 1 TeV/c2 in a 200 t yr run. DarkSide-20k will be installed inside a membrane cryostat containing more than 700 t of liquid Argon and be surrounded by an active neutron veto based on a Gd-loaded acrylic shell. The talk will give the latest updates of the ongoing R&D and prototype tests validating the initial design. A subsequent objective, towards the end of the next decade, will be the construction of the ultimate detector, ARGO, with a 300 t fiducial mass to push the sensitivity to the neutrino floor region for high mass WIMPs.

Effect of argon sputtering pressure on the electrochemical performance of LiFePO4 cathode

The development of a novel small-scale electrical power source technology is essential in the design of microelectronic devices. In this sense, this study presents the optimization of the sputtering deposition parameters to grow a LiFePO4 cathode, a candidate material for the development of solid-state lithium batteries. Specifically, it was evaluated the effect of argon deposition pressure on the electrochemical properties of LiFePO4 cathode. By combining X-ray photoelectron spectroscopy with electrochemical characterization methods, it was revealed that argon deposition pressure modulated the degree of the re-sputtering effects. In addition, it was found that high argon deposition pressure leads to the growth of LiFePO4 films with oxygen and phosphor deficiency. The absence of these atoms caused the decrease in the LiFePO4 electrochemical activity. Conversely, deposition parameters based on low argon sputtering pressure significantly reduced the degree of the re-sputtering effects, which in turn favored the growth of stoichiometric LiFePO4 films with an enhanced electrochemical activity.

New physics searches in a low threshold scintillating argon bubble chamber measuring coherent elastic neutrino-nucleus scattering in reactors

The sensitivity to New Physics of a low threshold scintillating argon bubble chamber measuring coherent elastic neutrino-nucleus scattering in reactors is reported. Namely, light scalar mediators, sterile neutrino oscillations, unitarity violation, and non-standard interactions are studied. The results indicate that this detector could be able to set stronger constraints than current limits set by the recent COHERENT measurements. Considering the best scenario, a 100 kg detector located 30 m from a 2000 MW$_{th}$ reactor, a sterile neutrino search would cover most of the space parameter allowed from the reactor anti-neutrino anomaly fit. Unitarity violation studies could set constraints on $\alpha_{11}$ more stringent than the current oscillation experiments fit. A low threshold argon detector with very low backgrounds has the potential to explore New Physics in different scenarios and set competitive constraints.

Oblique angle co-deposition of nanocolumnar tungsten thin films with two W sources: Effect of pressure and target current

Two series of tungsten thin films are sputtered on silicon and glass substrates by oblique angle co-deposition technique with an original configuration. Two opposite distinct tungsten targets are simultaneously used, both tilted with an oblique angle of 80°. The growth is performed at low (0.33 Pa) and high (1.5 Pa) argon sputtering pressure and the current intensity applied to the targets varies between 50 and 250 mA. The effect of these deposition parameters on the films microstructure and electrical properties is investigated by scanning and transmission electron microscopy, X-ray diffraction and pole figures, and van der Pauw method. Due to self-shadowing effect, all tungsten sputtered thin films are porous and columnar. At low pressure, the columnar tilt angle β can be tuned with the target current intensity until obtaining vertical columns. X-ray diffraction and pole figure analyses point out a A15 crystal structure (β-W) and a uniaxial fiber texture with a <100> growth direction. In contrast, tungsten thin films deposited at high pressure present a cauliflower structure and are poorly crystallized. Both deposition parameters also affect the films electrical resistivity and anisotropy. These behaviors are discussed and linked to the microstructure and crystallography.

Efficient neutralization of core ionized species in an aqueous environment.

Core ionization dynamics of argon-water heteroclusters ArM[H2O]N are investigated using a site and process selective experimental scheme combining 3 keV electron irradiation with Auger electron-ion-ion multi-coincidence detection. The formation of Ar 2p-1 vacancies followed by non-radiative decay to intermediate one-site doubly ionized states Ar2+(3p-2)-ArM-1[H2O]N and subsequent redistribution of charge to the cluster environment are monitored. At low argon concentrations the emission of an [H2O]n'H+/[H2O]n''H+ ion pair is the dominant outcome, implying on high efficiency of charge transfer to the water network. Increasing the condensation fraction of argon in the mixed clusters and/or to pure argon clusters is reflected as a growing yield of Arm'+/Arm''+ ion pairs, providing a fingerprint of the precursor heterocluster beam composition. The coincident Auger electron spectra, resolved with better than 1 eV resolution, show only subtle differences and thereby reflect the local nature of the initial Auger decay step. The results lead to better understanding of inner shell ionization processes in heterogeneous clusters and in aqueous environments in general.

Plasma surface treatment of polystyrene in a low power low frequency argon glow discharge

A low power low frequency (50 Hz) argon plasma was used to treat the surface of polystyrene (PS) films. Plasma treatment for 3 min produced the best hydrophilic enhancement and it corresponded to doubling of the surface energy. At 45 Pa and flowrate of 44 sccm, a ‘saturation’ region in terms of lowest water contact angle (WCA) attained (WCA = 21°±3°), occurred at low discharge power of 0.15–0.25 W (applied power of 3–8 W). The induced hydrophilicity was attributed mainly to the oxygen-containing functional groups incorporated on the PS surface during the plasma treatment. Oxygen was speculated to be sourced from the dissociation of residual water vapour in the reactor chamber. Upon exposure to the laboratory environment, rapid hydrophobic recovery of the treated samples occurred but WCA stabilized after 72 h and remained at >30° below its value at pristine condition.

Effect of nitrogen doping on the structure of metastable β-W on SiO2

Metastable β-W phase exhibits a large spin Hall effect which has attracted a great interest for spin-orbit torque-based memory applications. Here we study the growth and structure of W films prepared by magnetron sputtering under different argon and nitrogen gas mixture conditions. X-ray diffraction and sheet resistivity show that low argon with low nitrogen gas flow stabilizes the β-W phase in thick films. High argon gas flow promotes the growth of porous β-W films which increases the oxygen content and the resistivity of the films. It is observed that the N2 doping of the W films reduce the perpendicular magnetic anisotropy of an adjacent CoFeB layer.

Modelling and measurements of gas tungsten arc welding in argon–helium mixtures with metal vapour

Argon–helium mixtures in gas tungsten arc welding of an iron workpiece are investigated using an axisymmetric computational model that includes the cathode, workpiece, and arc plasma in the computational domain. The three-gas combined diffusion coefficient method is used to treat diffusion of helium, argon, and iron vapour. Calculations for argon–helium mixtures without metal vapour are performed; good agreement with previous numerical results is found. A transition from a helium-like to an argon-like arc occurs when the argon mole fraction increases above about 0.3. Calculations for a wide range of argon–helium mixtures including iron vapour are then performed. Adding helium to argon alters the arc properties and affects the weld geometry. Iron vapour cools the arc for all argon–helium mixtures. Iron vapour is present above the workpiece, near the cathode and in the arc fringes for very low argon mole fractions. As the argon mole fraction increases, the iron vapour becomes increasingly confined to the region above the workpiece, with small amounts near the cathode tip. Emission spectroscopy measurements of arcs in argon–helium mixtures with water-cooled copper and uncooled iron workpieces were performed. The measured distributions of atomic helium and iron emission show good agreement with the predictions of the model.

Current–voltage characteristics of an impulse magnetron discharge in target material vapor

The magnetron discharge with hot (uncooled) target in an impulse mode has been experimentally investigated. The I–V characteristics have been measured depending on the magnetic field strength for three target materials: copper, chromium, and silicon. For melted copper and hot chromium targets, stable gasless (no argon) operation of the magnetron has been demonstrated with maximum impulse power densities about 2.5 kW/cm2 (averaged over the racetrack area). For silicon target, maximum impulse power density was 1.5 kW/cm2 at low argon pressure (0.1 Pa). The magnetic field dependences of discharge parameters have shown the associated changes in differential plasma impedance.

Bio-inspired synthesis of silver nanoparticles using Hibiscus Tiliaceus L. flower extracts for improved optical characteristics

In this work, we report a simple, facile, environment friendly and green approach to synthesize silver nanoparticles using aqueous flower extracts of Hibiscus Tiliaceus L. The morphology (shape/size) and crystal structure of as-synthesized silver nanoparticles (NPs) were determined by transmission electron microscopy and x-ray diffraction. The UV–Vis spectroscopy was used to record the surface plasmon resonance peak, while spectro-fluorophotometry was used to examine the fluorescence emission spectra of silver NPs. Moreover, the optical nonlinearities of silver NPs were investigated using z-scan technique with low power argon laser beam at wavelengths 488 nm and 514 nm in continuous-wave regime and at 532 nm in the pulsed laser regime. Our results indicate that the nonlinear absorption in silver NPs is reverse saturation absorption of the order of 10–3 cm/W and the negative nonlinear refractive index was determined as of the order of 10–8 cm2/W. Moreover, the optical nonlinear properties were used to demonstrate optical limiting abilities of the synthesized silver NPs. The results presented in this study indicate that bio-synthesized silver NPs are excellent candidates as a nonlinear material and can be widely used for waveguide applications.

Incompatibility of argon during magma ocean crystallization

We report results from multi-anvil (MA) and laser-heated diamond anvil cell (LH-DAC) experiments that synthesize high-pressure phases, including bridgmanite, ferropericlase, stishovite, and ultramafic liquid, in the presence of an argon-rich fluid. The goal of the experiments is to constrain the equilibrium distribution of argon in magma ocean environments. Argon concentrations in LH-DAC experiments were quantified by electron microprobe analysis, while argon concentrations in MA experiments were quantified by laser-ablation mass spectrometry and electron microprobe analysis. Our LH-DAC experiments demonstrate that argon solubility in ultramafic liquid is near or above 1.5 wt.% at conditions between 13–101 GPa and 2300–6300 K. Argon concentrations in bridgmanite and ferropericlase synthesized in LH-DAC experiments range from below detection to 0.58 wt.%. Argon concentrations in bridgmanite and ferropericlase synthesized in MA experiments range from below detection to 2.16 wt.% for electron microprobe measurements and laser-ablation measurements. We interpret this wide range of argon concentrations in minerals to reflect the variable presence of argon-rich fluid inclusions in analytical volumes. Our analyses therefore provide upper limit constraints for argon solubility in high-pressure minerals (<0.015 wt.%) across all mantle pressures and temperatures. The combination of relatively high argon solubility in ultramafic liquid (∼1.5 wt.%) and low argon solubility in minerals implies argon incompatibility (Dbridgmanite−meltAr < 0.01, Dferropericlase−meltAr < 0.01) during magma ocean crystallization and that the initial distribution of argon, and likely other neutral species, may be controlled by liquids trapped in a crystallizing magma ocean. We thus predict a basal magma ocean would be enriched in noble gases relative to other regions of the mantle. Moreover, we predict that the noble gas parent-daughter ratio of magma ocean cumulates pile will increase with crystallization, assuming refractory and incompatible behavior for parent elements.

Dark matter detection capabilities of a large multipurpose Liquid Argon Time Projection Chamber

Liquid Argon Time Projection Chambers are planned to comprise a central role in the future of the U.S. High Energy Physics neutrino program. In particular, this detector technology will form the basis for the 40 kton Deep Underground Neutrino Experiment (DUNE) . In this paper we take as a starting point the dual phase far detector design proposed by the DUNE experiment and ask what changes are necessary to allow one of the four 10 kt modules to be sensitive to heavy Weakly Interacting Massive Particle (WIMP) dark matter. We show that with control over backgrounds and the use of low radioactivity argon, which may be commercially available on that timescale, along with a significant increase in light detection, one DUNE-like module gives a competitive WIMP detection sensitivity, particularly above a dark matter mass of 100 GeV.

Effects of deposition and annealing conditions on the crystallisation of NiTi thin films by e‐beam evaporation

This Letter demonstrates the effects of deposition and annealing conditions on the crystallisation of nickel–titanium (NiTi) thin films deposited by the e-beam evaporation. NiTi thin films of 0.5 μm thick are fabricated on P-type (110) silicon substrates using the e-beam evaporation technique. With the characterisations using the X-ray diffraction method, the results indicate that low e-beam deposition rate and argon employed annealing procedures can be beneficial to the formation of NiTi crystalline structure. Additionally, the as-deposited NiTi thin films without annealing procedures are amorphous, when deposited on the substrates at low temperatures. The differential scanning calorimetry studies show NiTi thin films formed by the e-beam evaporation are Ni-rich. Furthermore, SEM images illustrate that high-precision patterned NiTi thin film microstructures can be fabricated by combining with the e-beam evaporation and lift-off resist methods, without any wet or dry etching procedures. The proposed fabrication process for NiTi thin film microstructures shows great potential for application in shape memory alloy medical devices, as well as the micro-robotics.