Jahn-Teller effect and features of divalent copper ion behavior in multicomponent borate crystals.

Crystals of YGa3(BO3)4, YAl3(BO3)4, EuGa3(BO3)4 and EuAl3(BO3)4 with copper alloy were studied by electron paramagnetic resonance and X-ray diffraction analysis. The lattice parameters and coordinates of copper-doped boron atoms were determined. The study of EPR spectra showed that copper is in the divalent state and replaces aluminum ions with C2 node symmetry. In YAl3(BO3)4:Cu crystals, a ligand structure exists due to the interaction of copper electrons with yttrium nuclei. The parameters of the spin Hamiltonian describing the behavior of the Cu2+ spectrum have been determined. The deviation of the Z-axis spectra from the C3 axis by 54(1)° is due to Jahn-Teller vibronic interaction and monoclinic distortion. In the EuGa3(BO3)4 crystal, a new spectrum 2 was found, which also belongs to divalent copper but is observed at an excited state 31 cm-1 away from the ground state. Above 70 K, an isotropic EPR line with a width of 450 Gs, g = 2.1, appears and exists up to room temperature.

Surface-Enhanced Raman Spectroscopy and Artificial Neural Networks for Detection of MXene Flakes' Surface Terminations.

The properties of MXene flakes, a new class of two-dimensional materials, are strictly determined by their surface termination. The most common termination groups are oxygen-containing (=O or -OH) and fluorine (-F), and their relative ratio is closely related to flake stability and catalytic activity. The surface termination can vary significantly among MXene flakes depending on the preparation route and is commonly determined after flake preparation by using X-ray photoelectron spectroscopy (XPS). In this paper, as an alternative approach, we propose the combination of surface-enhanced Raman spectroscopy (SERS) and artificial neural networks (ANN) for the precise and reliable determination of MXene flakes' (Ti3C2Tx) surface chemistry. Ti3C2Tx flakes were independently prepared by three scientific groups and subsequently measured using three different Raman spectrometers, employing resonant excitation wavelengths. Manual analysis of the SERS spectra did not enable accurate determination of the flake surface termination. However, the combined SERS-ANN approach allowed us to determine the surface termination with a high accuracy. The reliability of the method was verified by using a series of independently prepared samples. We also paid special attention to how the results of the SERS-ANN method are affected by the flake stability and differences in the conditions of flake preparation and Raman measurements. This way, we have developed a universal technique that is independent of the above-mentioned parameters, providing the results with accuracy similar to XPS, but enhanced in terms of analysis time and simplicity.

Microstructure and physical properties of black-aluminum antireflective films.

The microstructure and physical properties of reflective and black aluminum were compared for layers of different thicknesses deposited by magnetron sputtering on fused silica substrates. Reflective Al layers followed the Volmer-Weber growth mechanism classically observed for polycrystalline metal films. On the contrary, the extra nitrogen gas used to deposit the black aluminum layers modified the growth mechanism and changed the film morphologies. Nitrogen cumulated in the grain boundaries, favoring the pinning effect and stopping crystallite growth. High defect concentration, especially vacancies, led to strong columnar growth. Properties reported for black aluminum tend to be promising for sensors and emissivity applications.

Plasmon assisted Ti3C2Tx grafting and surface termination tuning for enhancement of flake stability and humidity sensing performance.

Humidity sensors play a critical role in monitoring human activities, environmental health, food processing and storage, and many other fields. Recently, some 2D materials, particularly MXenes, have been considered as promising candidates for creating humidity sensors because of their high surface area, surface-to-bulk ratio, and excellent conductivity, arising from the high concentration and mobility of free electrons. In this work, we propose the plasmon-assisted surface modification and termination tuning of common MXene (Ti3C2Tx) to enhance their response to humidity and increase their stability against oxidation. Hydrophobic (-C6H4-CF3) and hydrophilic (-C6H4-COOH) chemical moieties were covalently grafted to the Ti3C2Tx surface using plasmon-mediated diazonium chemistry. In situ Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) measurements, performed at different humidity levels indicate that surface modification significantly affects penetration of water molecules in Ti3C2Tx films. As a result, the sensitivity of the flakes to the presence of water molecules was significantly altered. Additionally, proposed surface grafting commonly proceeds on the less stable MXene surface sites, where flake oxidation commonly initiates. As a result of the modification, such "weak" and more chemically active sites were blocked and Ti3C2Tx stability was significantly enhanced.

Silver Nanoparticles for Fluorescent Nanocomposites by High-Pressure Magnetron Sputtering.

We report on the formation of silver nanoparticles by gas aggregation in a reaction chamber at room temperature. The size distribution of nanoparticles deposited on a silicon substrate for various lengths of an aggregation (high-pressure) chamber was investigated by atomic force microscopy. Nanoparticles were characterized by scanning and transmission electron microscopy and spectral ellipsometry. The physical shape of the nanoparticles and its distribution was correlated with their optical properties. Metal-dielectric nanocomposites were deposited employing simultaneous deposition of Ag NPs via high-pressure magnetron sputtering and the dielectric matrix was deposited via thermal evaporation. Pure and Eu-, Er-, and Yb-doped lithium fluoride was used as the dielectric host matrix. Optical transmittance of lithium fluoride containing silver nanoparticles was measured and their theoretical absorption cross-section calculated. The nanoparticles were also embedded in Eu3+-doped downshifting and Er3+- and Yb3+-doped up-conversion materials to study their influence on emission spectra. Spectra of identical layers with and without nanoparticles were compared. Their transmittance at various annealing temperatures is also presented.

Surface Enhancement Using Black Coatings for Sensor Applications.

The resolution of a quartz crystal microbalance (QCM) is particularly crucial for gas sensor applications where low concentrations are detected. This resolution can be improved by increasing the effective surface of QCM electrodes and, thereby, enhancing their sensitivity. For this purpose, various researchers have investigated the use of micro-structured materials with promising results. Herein, we propose the use of easy-to-manufacture metal blacks that are highly structured even on a nanoscale level and thus provide more bonding sites for gas analytes. Two different black metals with thicknesses of 280 nm, black aluminum (B-Al) and black gold (B-Au), were deposited onto the sensor surface to improve the sensitivity following the Sauerbrey equation. Both layers present a high surface roughness due to their cauliflower morphology structure. A high response (i.e., resonant frequency shift) of these QCM sensors coated with a black metal layer was obtained. Two gaseous analytes, H2O vapor and EtOH vapor, at different concentrations, are tested, and a distinct improvement of sensitivity is observed for the QCM sensors coated with a black metal layer compared to the blank ones, without strong side effects on resonance frequency stability or mechanical quality factor. An approximately 10 times higher sensitivity to EtOH gas is reported for the QCM coated with a black gold layer compared to the blank QCM sensor.

Langmuir Probe Perturbations during In Situ Monitoring of Pulsed Laser Deposition Plasmas.

The recent advancements in pulsed laser deposition (PLD) control via plasma diagnostics techniques have been positive and raised questions on the limitation of some techniques, such as the Langmuir probe (LP). The particularities of laser-produced plasma can lead to incorrect interpretation of collected electrical signal. In this paper, we explored the limitations of LP as a technique for in situ PLD control by performing investigations on several metallic plasmas, expanding in various Ar atmosphere conditions. Sub-microsecond modulation was seen in the reconstructed IV characteristics attributed to non-equilibrium dynamics of the ejected charges. A perturbative regime was recorded for Ar pressures higher than 2 Pa, where ionic bursts were observed in the electron saturation region. This perturbation was identified as a plasma fireball. A non-linear multifractal model was developed here to explore these new regimes of the LP. The strange attractors characterizing each fireball were reconstructed, and their evolution with the Ar pressure is discussed. Both short- and long-time non-linear behavior were correlated via probe bias, and the pressure effect on the strange attractor's defining the fireball-like behavior was investigated. A good correlation was noticed between the simulated data and experimental findings.

Effect of Twinning on Angle-Resolved Photoemission Spectroscopy Analysis of Ni49.7Mn29.1Ga21.2(100) Heusler Alloy.

To explain the observed features of k-space photoelectron images taken on off-stoichiometric Heusler Ni49.7Mn29.1Ga21.2 single-crystals in the cubic austenitic and pseudotetragonal martensitic phases, the images were simulated theoretically. Despite the moderate structural difference of both phases, there is large difference in photoemission spectra. Analysis of the final states' structure, matrix elements, and interface barrier scattering was performed to interpret discrepancies between the external photoemission of the austenite and martensite. The missing signal at the surface-normal emission of the martensitic phase is, ultimately, explained by repeated scatterings of escaping electrons on the interfaces between nanotwins.

A grease for domain walls motion in HfO2-based ferroelectrics.

A large coercive fieldECof HfO2based ferroelectric devices poses critical performance issues in their applications as ferroelectric memories and ferroelectric field effect transistors. A new design to reduceECby fabricating nanolaminate Hf0.5Zr0.5O2/ZrO2(HZZ) thin films is used, followed by an ensuing annealing process at a comparatively high temperature 700 °C. High-resolution electron microscopy imaging detects tetragonal-like domain walls between orthorhombic polar regions. These walls decrease the potential barrier of polarization reversal in HfO2based films compared to the conventional domain walls with a single non-polar spacer, causing about a 40% decrease inEC. Capacitance versus electric field measurements on HZZ thin film uncovered a substantial increase of dielectric permittivity near theECcompared to the conventional Hf0.5Zr0.5O2thin film, justifying the higher mobility of domain walls in the developed HZZ film. The tetragonal-like regions served as grease easing the movement of the domain wall and reducingEC.

In Situ Monitoring of Pulsed Laser Annealing of Eu-Doped Oxide Thin Films.

Eu3+-doped oxide thin films possess a great potential for several emerging applications in optics, optoelectronics, and sensors. The applications demand maximizing Eu3+ photoluminescence response. Eu-doped ZnO, TiO2, and Lu2O3 thin films were deposited by Pulsed Laser Deposition (PLD). Pulsed UV Laser Annealing (PLA) was utilized to modify the properties of the films. In situ monitoring of the evolution of optical properties (photoluminescence and transmittance) at PLA was realized to optimize efficiently PLA conditions. The changes in optical properties were related to structural, microstructural, and surface properties characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The substantial increase of Eu3+ emission was observed for all annealed materials. PLA induces crystallization of TiO2 and Lu2O3 amorphous matrix, while in the case of already nanocrystalline ZnO, rather surface smoothening0related grains' coalescence was observed.

On the Dynamics of Transient Plasmas Generated by Nanosecond Laser Ablation of Several Metals.

The dynamics of transient plasma generated by UV ns-laser ablation of selected metals (Co, Cu, Ag, Bi) were investigated by the Langmuir Probe method in angle- and time-resolved modes. Multiple ionic and electronic structures were seen for all plasmas with some corresponding to anions or nanoparticle-dominated structures. The addition of an Ar atmosphere energetically confined the plasma and increased the charge density by several orders of magnitude. For pressure ranges exceeding 0.5 Pa fast ions were generated in the plasma as a result of Ar ionization and acceleration in the double layer defining the front of the plasma plume. Several correlations between the target nature plasma properties were attempted. The individual plasma structure expansion velocity increases with the melting point and decreases with the atomic mass while the corresponding charged particle densities decrease with the melting point, evidencing the relationship between the volatility of the sample and the overall abated mass.

Role of the paramagnetic donor-like defects in the high n-type conductivity of the hydrogenated ZnO microparticles.

The magnetic and electronic properties of the hydrogenated highly conductive zinc oxide (ZnO) microparticles were investigated by electron paramagnetic resonance (EPR) and contactless microwave (MW) conductivity techniques in the wide temperature range. The EPR spectra simulation allowed us to resolve four overlapping EPR signals in ZnO microparticles. The Lorentzian EPR line with isotropic g-factor 1.9623(5) was related to the singly ionized oxygen vacancy. Another Lorentzian line with g|| = 1.9581(5), g⊥ = 1.9562(5) was attributed to the zinc interstitial shallow donor center, while EPR signal with g|| = 1.9567(5), g⊥ = 1.9556(5) and Gaussian lineshape was assigned to the hydrogen interstitial shallow effective-mass-like donor. The EPR signal with g|| = 1.9538(5), g⊥ = 1.9556(5) and Lorentzian lineshape was tentatively attributed to the shallow donor center. The charge transport properties in ZnO microparticles have been investigated by the contactless MW conductivity technique at T = 5-296 K. Two conduction mechanisms, including ionization of electrons from the shallow donors to the conduction band and hopping conduction process, have been distinguished. The hopping conduction process follows Mott's variable-range hopping T-1/4 law at T = 10-100 K. The evaluated values of the average hopping distance (15.86 Å), and hopping energy (1.822 meV at 40 K) enable us to estimate the donor concentration in the investigated ZnO microparticles as ~ 1018 cm-3.

Optical emission spectroscopy of nitrogen species and plasma plume induced by laser ablation combined with pulse modulated radio-frequency discharge.

Combined laser ablation and pulse modulated radio-frequency (RF) discharge for deposition of CNalpha films was studied by the use of optical emission spectroscopy. Chemically active nitrogen was produced by RF discharge, concentrated between two small electrodes. Influence of RF power, nitrogen pressure, modulation frequency and pulse rate on nitrogen species production was researched. For the same system plasma expansion rate, kinetic energy and concentration of carbon ions emitted by laser from graphite target were determined by Langmuir probes measurement.