Preparation and properties of La1 − xAgyMnO3 + δ thin epitaxial films

We report here the preparation and properties of La_1 − xAg_yMnO_3 + δ thin epitaxial films. The original two-step preparation procedure was developed. At first, La_1 − xMnO_3 + δ were grown epitaxially by metal-organic chemical vapor deposition on the single-crystal substrates (001) and (110) SrTiO₃, (001) LaAlO₃, (111) and (001) ZrO₂(Y₂O₃). Treatment by the vapor of the metallic silver in the oxygen atmosphere (at 1 bar and 20 bar) was the second step resulting in the selective absorption of silver by La_1 − xMnO_3 + δ phase. The value of y depended on the process conditions and revealed different kinetics of the silver absorption for (001) and (110) orientation of La_1 − xMnO_3 + δ films. The films prepared were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersion X-ray analysis, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, electrical resistivity and magnetoresistance measurements in a four-probe configuration. We have found that metal-insulator transition temperature (T_p) in the series La_1 − xAg_xMnO_3 + δ possessed a maximum of 380 K at x = 0.15. Thus, T_p of La_1 − xAg_xMnO_3 + δ films was significantly higher than ever reported in the literature for the La_1 − xAg_xMnO_3 + δ ceramics. La_1 − xAg_xMnO_3 + δ films demonstrated the important role of the ferromagnetic fluctuations above Curie temperature T_c resulting in the sign change of the resistivity curve temperature slope dR / dT and a significant shift of T_p well above T_c. The maximum of the magnetoresistance on the temperature scale was close to dR / dT maximum. The intrinsic magnetoresistance values as high as 22% at 310 K and 50% at 280 K were measured in the magnetic field of 1 T in the series of La_1 − xAg_yMnO_3 + δ epitaxial films.     

Study on the chemical stability of Y-doped BaCeO3 − δ and BaZrO3 − δ films deposited by aerosol deposition

Barium cerate (BaCeO₃) has high proton conductivity but rather poor chemical stability in CO₂-containing atmospheres. Barium zirconate (BaZrO₃), in contrast, is a rather stable material, but exhibits poor sinterability. In the present work, powders of Y-doped BaCeO₃ and BaZrO₃ were synthesized via the solid solution reaction method, and dense ceramic membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ were prepared by the aerosol deposition method at room temperature. Aerosol deposition method is a technique that enables the fabrication of ceramic films at room temperature with a high deposition rate as well as strong adhesion to the substrate. The powders and aerosol-deposited membranes were characterized by X-ray diffraction, particle size analysis, scanning electron microscopy, and X-ray elemental mapping. The chemical stability of powders and aerosol-deposited membranes with BaCe_0.9Y_0.1O₃ and BaZr_0.85Y_0.15O₃ against water and carbon dioxide has been investigated, and it was found that BaZr_0.85Y_0.15O₃ materials showed a better chemical compatibility.     

Polarized spectroscopic properties of Ho-doped LuLiF4 single crystal for 2 μm and 2.9 μm lasers

Polarized spectroscopic properties of a Ho³⁺-doped LuLiF₄ (Ho:LuLF) single crystal grown by the Czochralski method have been investigated as a promising material for 2 μm and 2.9 μm lasers. The Judd-Ofelt (J-O) model has been applied to the analysis of the polarized room temperature absorption spectra to establish the so-called J-O intensity parameters. Based on the calculated parameters, we determined the emission probabilities, branching ratio and radiative lifetime for the Ho³⁺ transitions from the excited state manifolds to the lower-lying manifolds. Ho:LuLF crystal shows long fluorescence lifetime of ⁵I₇ manifold (16 ms) and broad absorption and emission spectra, which exhibit strong polarization characteristics. Stimulated emission cross-sections spectra of the ⁵I₆ → ⁵I₇ and ⁵I₇ → ⁵I₈ transitions were derived and compared with those of the other well-known Ho³⁺-doped laser crystals.     

Optical constants and band gap determination of Pb0.95La0.05Zr0.54Ti0.46O3 thin films using spectroscopic ellipsometry and UV–visible spectroscopy

We report the structural evolution and optical properties of lanthanum doped lead zirconate titanate (PLZT) thin films prepared on Pt/TiO₂/SiO₂/Si substrates by chemical solution deposition. X-ray diffraction demonstrates the post-deposition annealing induced crystallization for PLZT films annealed in a temperature (T_a) range of 550–750 °C. PLZT films annealed at higher temperature exhibit polycrystalline structure along with larger grain size. Optical band gap (E_g) values determined from UV–visible spectroscopy and spectroscopic ellipsometry (SE) for PLZT films were found to be in the range of 3.5–3.8 eV. E_g decreases with increasing T_a. The optical constants and their dispersion profiles for PLZT films were also determined from SE analyses. PLZT films show an index of refraction in the range of 2.46–2.50 (λ = 632.8 nm) with increase in T_a. The increase in refractive index at higher T_a is attributed to the improved packing density and crystallinity with the temperature.     

AC impedance and dielectric spectroscopic studies of Mg<Superscript>2 + </Superscript> ion conducting PVA–PEG blended polymer electrolytes

Polyvinyl alcohol (PVA)–polyethylene glycol (PEG) based solid polymer blend electrolytes with magnesium nitrate have been prepared by the solution cast technique. Impedance spectroscopic technique has been used, to characterize these polymer electrolytes. Complex impedance analysis was used to calculate bulk resistance of the polymer electrolytes. The a.c.-impedance data reveal that the ionic conductivity of PVA–PEG–Mg(NO₃)₂ system is changed with the concentration of magnesium nitrate, maximum conductivity of 9·63 × 10^− 5 S/cm at room temperature was observed for the system of PVA–PEG–Mg(NO₃)₂ (35–35–30). However, ionic conductivity of the above system increased with the increase of temperature, and the highest conductivity of 1·71 × 10^− 3 S/cm was observed at 100°C. The effect of ionic conductivity of polymer blend electrolytes was measured by varying the temperature ranging from 303 to 373 K. The variation of imaginary and real parts of dielectric constant with frequency was studied.     

Direct observation of oxygen vacancy and its effect on the microstructure, electronic and transport properties of sputtered LaNiO3 − δ films on Si substrates

Highly (100)-oriented LaNiO₃ films with different oxygen content were deposited on Si substrates by radio frequency sputtering and post-annealed in oxygen and vacuum conditions. The formation of oxygen vacancies is directly observed by a decrease of lattice oxygen ratio in O 1s core-level photoelectron spectroscopy. X-ray diffraction measurement indicates that low oxygen pressure during the deposition or annealing process has a significant influence on the lattice constant of LaNiO₃ films. Further valence-band spectra and transport measurements demonstrate that the oxygen vacancies also have a significant influence on the electronic structure and transport behaviors of final LaNiO₃ films.     

Infrared to ultraviolet upconversion fluorescence of Gd in β-NaYF4 microcrystals induced by 1560 nm excitation

Under 1560 nm excitation, ultraviolet (UV) upconversion (UC) emissions of Gd³⁺ and Er³⁺ ions were observed in β-NaYF₄:Yb³⁺/Gd³⁺/Er³⁺ microcrystals, which were synthesized through a facile EDTA-assisted hydrothermal method. Experimental analysis exhibited that these UV emissions came from high-order UC processes. In the Yb³⁺-Gd³⁺-Er³⁺ codoped system, ground state absorption (GSA) and excited state absorption (ESA) of Er³⁺, ESA of Gd³⁺, energy transfers (ETs) from Yb³⁺ to Er³⁺ and from Er³⁺ to Gd³⁺ ions worked simultaneously in populating these high-energy states of Er³⁺ and Gd³⁺ ions.     

A Monolayer MoS2 FET with an EOT of 1.1 nm Achieved by the Direct Formation of a High-κ Er2O3 Insulator Through Thermal Evaporation

Achieving the direct growth of an ultrathin gate insulator with high uniformity and high quality on monolayer transition metal dichalcogenides (TMDCs) remains a challenge due to the chemically inert surface of TMDCs. Although the main solution for this challenge is utilizing buffer layers before oxide is deposited on the atomic layer, this method drastically degrades the total capacitance of the gate stack. In this work, we constructed a novel direct high-κ Er₂O₃ deposition system based on thermal evaporation in a differential-pressure-type chamber. A uniform Er₂O₃ layer with an equivalent oxide thickness of 1.1 nm was achieved as the gate insulator for top-gated MoS₂ field-effect transistors (FETs). The top gate Er₂O₃ insulator without the buffer layer on MoS₂ exhibited a high dielectric constant that reached 18.0, which is comparable to that of bulk Er₂O₃ and is the highest among thin insulators (< 10 nm) on TMDCs to date. Furthermore, the Er₂O₃/MoS₂ interface (D_it ≈ 6 × 10¹¹ cm⁻² eV⁻¹) is confirmed to be clean and is comparable with that of the h-BN/MoS₂ heterostructure. These results prove that high-quality dielectric properties with retained interface quality can be achieved by this novel deposition technique, facilitating the future development of 2D electronics.     

Polarization strategies to improve the emission of Si-based light sources emitting at 1.55 μm

We present a electroluminescence (EL) study of the Si-rich silicon oxide (SRSO) LEDs with and without Er³⁺ ions under different polarization schemes: direct current (DC) and pulsed voltage (PV). The power efficiency of the devices and their main optical limitations are presented. We show that under PV polarization scheme, the devices achieve one order of magnitude superior performance in comparison with DC. Time-resolved measurements have shown that this enhancement is met only for active layers in which annealing temperature is high enough (>1000 °C) for silicon nanocrystal (Si-nc) formation. Modeling of the system with rate equations has been done and excitation cross-sections for both Si-nc and Er³⁺ ions have been extracted.     

Crossover of magnetoresistance from positive to negative in La0.5Sr0.5CoO3 − σ/Si heterostructure

The magnetoresistance (MR) properties of a heterostructure fabricated by depositing a La_0.5Sr_0.5CoO_3 − σ film on an n-type Si substrate have been studied. The heterostructure exhibits a good rectifying behavior. A negative MR at T = 210 K and a positive MR at T = 300 K are observed for all bias currents whereas; for temperatures ranging from 240 to 280 K the MR changes from being positive to negative with the increase of the bias current. The observed behavior of the MR effect is discussed in terms of current-induced ferromagnetic spin order.     

Study of YBaCo4O7 + δ thin films grown by sputtering technique on (1012)-oriented sapphire substrates

We report the growth of thin films of the cobaltite YBaCo₄O_7 + δ by means of the dc magnetron sputtering technique at high oxygen pressure onto r (1012) sapphire substrates. The films were characterized according to their structural, morphological, electrical, magnetic, and optical properties. An analysis of the X-ray diffraction pattern indicates that the films grown on r-sapphire substrates are single phase polycrystalline. Despite the high growth temperature (850 °C), no indication of interface reaction (formation of BaAlO₄ or Y₂O₃) is detected. Measurements of resistivity as a function of temperature reveal a semiconductor-like character of the grown films. No indication of possible transitions is observed in the temperature range 50-300 K. The electronic transport mechanism seems to be dominated by Mott variable range hopping (VRH) conduction. Fitting the VRH model to the experimental data allows one to estimate the density of states of the material at the Fermi level N(E_F). The resistivity measured in magnetic fields as strong as 5 T increases notably, and positive magnetoresistance values as high as ~ 60% at 100 K are obtained. Magnetization measurements show well defined hysteresis loops at 300 K and 5 K. Nevertheless, calculated values of the magnetization have ended up being too small for the ferro- or ferrimagnetic states. Raman spectra, in turn, allow one to identify bands associated with vibrating modes of CoO₄ and YO₆ in tetrahedral and octahedral configurations, respectively. Additional bands which seem to stem from Co ions in octahedral configuration are also clearly identified. Measurements of transmittance and reflectance show two well defined energy gaps at 3.7 and 2.2 eV.     

Effect of some VA group modifiers on R2O3 (R = Sb,Bi)–ZnF2–GeO2 glasses doped with CuO by means of spectroscopic and dielectric investigations

Transparent glasses 40Sb₂O₃–20ZnF₂–(40 − x)GeO₂:xCuO, and 40Bi₂O₃–20ZnF₂–(40 − y)GeO₂:yCuO with x = 0, 0.6 and 0 ≤ y ≤ 1 wt% were prepared by melt quenching technique and were characterized by XRD and differential thermal analysis. Spectroscopic studies like optical absorption, FTIR, Raman, EPR and dielectric parameters (such as ?′, loss(tan δ), and σ_ac) were carried out to examine the modifier and dopant effect on zinc germanate glass network. Optical absorption and EPR data have revealed that the environment of Cu²⁺ ions is more ionic in bismuth series rather than antimony glasses. Reduced bismuth ions have been found in pure and at lower concentration of dopant in Bi₂O₃ mixed glasses, which are useful for IR amplifications. FTIR and Raman spectra have indicated the conversion of GeO₄ to GeO₆ structural units by forming cross linking bonds like Bi–O–Ge, Ge–O–Cu, etc., and open the glass network with integration of Bi₂O₃ and CuO doping. It is also confirmed by decreasing T_g and E_g values. The temperature dependence of dielectric parameters at different frequencies was interpreted in terms of structural changes in the glass network.     

FT-IR,Raman and UV–VIS spectroscopic studies of copper doped 3Bi<Subscript>2</Subscript>O<Subscript>3 </Subscript>·B<Subscript>2</Subscript>O<Subscript>3</Subscript> glass matix

FT-IR, Raman and UV–VIS experimental results were presented for xCuO·(100-x)[3Bi₂O₃·B₂O₃] glass system, where 0 ≤ x ≤ 50 mol%. The FT-IR measurements indicate the presence in xCuO·(100-x)[3Bi₂O₃·B₂O₃] glasses of BO₃, BO₄ units, BiO₃ pyramidal and distorted BiO₆ octahedral units and their dependence of CuO content. The Raman scattering data indicate that for 0 ≤ x ≤ 10 mol% the structure of studied glasses consist from BiO₃ pyramidal and distorted BiO₆ octahedral units, ring and chain type of metaborate groups, ortoborate and pyroborate groups. For higher concentration the Raman spectra suggest that the structure become more disordered. The FT-IR and Raman bands characteristic for CuO were not directly evidenced, but the absorption bands specific for the glass matrix are influenced by the presence of copper ions in the glass network structure. The optical absorption confirms the presence of Cu²⁺ in the CuO doped 3Bi₂O₃·B₂O₃ glass matrix.     

Morphology and structure of La1.85Sr0.15CuO4 + δ thin films deposited on (100) SrTiO3 substrates by dc magnetron sputtering

La_1.85Sr_0.15CuO_4 + δ thin films were deposited by dc magnetron sputtering. Relaxing time intervals were introduced in the deposition process for improving the quality of the thin films. Atomic force microscopy and X-ray diffraction were used to examine the morphology and structure of the thin films. The study reveals that the time intervals inserted in the deposition process improve the morphology of the thin films, and shows that the thicker the film, the better the crystalline quality and the superconducting property for both deposition processes.     

Evaluation of free carrier losses to 1.54 μm emission in Si/Si:Er nanolayers on SOI substrate for optical gain observation

In this study we use a combination of variable stripe and shifting excitation spot methods to evaluate linear low temperature (4.2 K) optical properties of Si/Si:Er multinanolayer grown on SOI substrate. In particular, Er-1 luminescence at 1.54 μm under continuous wave excitation was examined. Absorption coefficients of 22.4 ± 4.2 and 45.1 ± 4.2 cm⁻¹ at photon fluxes of 2.4 × 10¹⁹ and 2.4 × 10²⁰ cm² s⁻¹, respectively, have been established. These are approximately three times higher than those found earlier for a similar structure grown on Si substrate.     

Preparation,characterization and dielectric properties of Ba<Subscript>3−x</Subscript>Sr<Subscript>x</Subscript>LiM<Subscript>3</Subscript>Ti<Subscript>5</Subscript>O<Subscript>21</Subscript>[M=Nb and Ta,x = 0 to 3] ceramics

The ceramic compositions Ba_3−xSr_xLiM₃Ti₅O₂₁[M=Nb and Ta, x = 0 to 3] were prepared through conventional solid state ceramic route. A detailed study has been carried out to correlate the structure of Ba_3−xSr_xLiM₃Ti₅O₂₁[M=Nb and Ta, x = 0 to 3] with respect to their dielectric properties. The structure and microstructure of ceramic samples were studied using powder X-ray diffractometer and Scanning Electron Microscopic techniques. The dielectric properties of the sintered ceramic compacts have been studied. The Ba-rich compositions were identified as promising candidates for high frequency applications whereas the Sr-rich compositions were excellent ionic conductors and can be commercially exploited for applications in solid-state batteries.