Enhanced performance of sandwich structure Pb0.8La0.1Ca0.1Ti0.975O3 thin film for pyroelectric applications

Using the advantages of low-temperature crystallization and high orientation in Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ (PLCT) film, a dense PLCT/porous PLCT/dense PLCT sandwich structure was obtained in the present study. It is found that dense PLCT layer can both sustain the porous density in the core layer and also lead to better preferential orientation of the sandwich structure. In the sandwich structure, low dielectric constant (ε_r = 43) and leakage current density (J < 9 × 10^− 5A/cm²) are simultaneously achieved. Because of high orientation in sandwich structure, the pyrocoefficient (p > 185 µC/m² K) is still keeping a relatively large value. The resulting high figure of merit (F_V′ = 4.5 µC/m² K, F_d′ = 228 µC/m² K) make the sandwich structure films good candidate for pyroelectric thin-film devices.     

Nonlinear optical properties of laser deposited CuO thin films

In this work we investigate the third-order optical nonlinearities in CuO films by Z-scan method using a femtosecond laser (800 nm, 50 fs, 200 Hz). Single-phase CuO thin films have been obtained using pulsed laser deposition technique. The structure properties, surface image, optical transmittance and reflectance of the films were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and UV-vis spectroscopy. The Z-scan results show that laser-deposited CuO films exhibit large nonlinear refractive coefficient, n₂ = − 3.96 × 10^− 17 m²/W, and nonlinear absorption coefficient, β = − 1.69 × 10^− 10 m/W, respectively.     

Thermoelectric properties and thermoelectric devices of free-standing GaN and epitaxial GaN layer

We studied the thermoelectric properties of free-standing GaN (fr-GaN) and epitaxial GaN layer (epi-GaN), and furthermore, we have fabricated thermoelectric devices using these materials. For fr-GaN, the maximum power factor was 7.7 × 10^− 4 W/m K² at 373 K, and for epi-GaN layer, the maximum power factor was 9.4 × 10^− 4 W/m K² at 373 K. The devices fabricated are (a) fr-GaN and chromel of 4 pairs, and (b) epi-GaN and chromel of 3 pairs. The maximum output power and the open output voltage were (a) 3.35 × 10^− 6 W and 2.76 × 10^− 2 V at ΔT = 153 K, and (b) 1.21 × 10^− 7 W and 1.71 × 10^− 2 V at ΔT = 153 K, respectively.     

Low power CW optical limiting properties of bis(2-aminopyridinium)-succinate-succinic acid (2APS) single crystal

The increase in the usage of low power CW lasers in various applications needs for the design of optical limiters with low thresholds. The optical limiting properties and nonlinear refractive index (n₂ = −2.4189 × 10⁻⁸ cm²/W) of transparent organic crystal bis(2-aminopyridinium)-succinate-succinic acid (2APS) single crystal using continuous wave He-Ne laser excitation following Z-scan method have been evaluated. The sample exhibited negative (defocusing) nonlinearity. This thermally induced defocusing nature of 2APS crystal can be used to design the low power optical limiters. As the origin for this nonlinearity is thermal, a complete thermal transport properties such as thermal diffusivity (α_s = 5.97 ± 0.03 × 10⁻³ cm²/s), thermal effusivity (e_s = 1.94 ± 0.02 × 10⁻² J/cm²-K-s^1/2), thermal conductivity (k_s = (4. 66 ± 0.04) × 10⁻³ W/cm-K)) and specific heat capacity (C_ps = (5.61 ± 0.05) × 10⁻¹ J/g-K) of the material were studied following the photopyro electric (PPE) technique.     

Magnetic and electrical properties of Ba2CrMo1 − xWxO6 double perovskite

Effects of the partial substitution of W^5+,6+ for Mo^5+,6+ on the structural and physical properties of Ba₂CrMoO₆ have been investigated. Polycrystalline Ba₂CrMo_1 − xW_xO₆samples have been prepared by sol-gel method in a stream of 5% Ar/H₂ gas at various sintering temperatures. Rietveld analysis of X-ray diffraction patterns shows a partial disorder of Mo/W and Cr on the B sites of the double perovskite, which plays a dominant role in the structural and magnetic properties of these compounds. The symmetry is cubic (Fm3?m) for all samples, and no phase transition was detected for Ba₂CrMo_1 − xW_xO₆. The Curie temperature T_C has been analyzed by two methods: a linear extrapolation of M(T) to zero magnetization and the thermodynamic model. The experimental results indicate that T_C decrease from 335 K (x = 0) to 285 K (x = 0.5) with increasing W substitution independently of the method used to obtain T_C. A systematic decrease in saturation magnetization, M_s with increasing W substitution has been observed in this solid solution series. This decrease of magnetization arises from the disorder at the Cr and Mo/W sites. Electrical properties change as well strongly along the series.     

Nanoparticle formation by the debris produced by femtosecond laser ablation of silicon in ambient air

The debris produced by femtosecond laser ablation (180 fs, 775 nm, 1 kHz) of Si in ambient air is deposited around the ablated craters in a circular zone with diameters between ~ 40 and 300 μm for laser fluences (F) in the region F = 0.2-8 J/cm². The debris consists of nanoparticles. The mean height of the nanoparticles increases with laser fluence (from ~ 70 to 500 nm for fluences in the range F = 0.25-4.38 J/cm²) but at high fluences (F = 8 J/cm²) becomes equal to ~ 170 nm. The average horizontal dimension of the nanoparticles increases with laser fluence. Their average vertical dimension increases in proportion to their average horizontal dimension, but at high fluences becomes much smaller than their corresponding average horizontal dimension. The nanoparticles were found to be single crystals with d spacing of 1.71 ± 0.08 Å (corresponding to {311}).     

Properties of screen-printed Ho-Ba-Cu-O films on YSZ substrates

High-T_c screen-printed Ho-Ba-Cu-O films were prepared on YSZ substrates by a melt processing method. The films were fired at T_s = 1000-1050 °C for 5 min and cooled to 450 °C by two steps in flowing O₂. The maximum critical current density J_c (77 K, 0 T) of 2.0 × 10³ A cm^− 2 was only attained under much limited firing conditions; T_s = 1020 °C and cooled to 800 °C at a cooling rate of 400 °C h^− 1.     

Ferroelectric properties of lithia-doped (Bi0.95Na0.75K0.20)0.5Ba0.05TiO3 ceramics

Lead-free piezoelectric (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics have been prepared by conventional process for different lithium substitutions. The SEM images show that the ceramics are well sintered at 1428 K. Dielectric and ferroelectric measurements have been performed. With the increasing of lithium substitution, the Curie temperature of the (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics shifts from 570 K to 620 K, but the maximum value of the dielectric constant decreases from 6700 to 4700 correspondingly. A relatively larger remanent polarization of 36.8 μC/cm² has been found in the x = 0.05 sample. The coercive field decreases as the lithium substitution amount increases. An optimized d₃₃ = 194 × 10^− 12 C/N and a relative dielectric constant ε_r = 1510 have been obtained in (Bi_0.95Na_0.75K_0.15Li_0.05)_0.5Ba_0.05TiO₃.     

Adsorption of single Ag and Cu atoms on regular and defective MgO(0 0 1) substrates: an ab initio study

The DFT slab calculations were performed for Ag and Cu atoms adsorbed on both regular and defective MgO(0 0 1) substrates. Both metal atoms and surface O vacancies (F_s centers) were distributed uniformly with a concentration of one Ag, Cu or F_s per 2×2 surface supercell. Surface O²⁻ ions are energetically more preferable for metal-atom adsorption on a regular substrate as compared to Mg²⁺ ions. The nature of the interaction between Ag or Cu adatoms and a defectless MgO substrate is physisorption (despite the difference in the adsorption energies: 0.62 vs. 0.39 eV per Cu and Ag adatom, respectively). Above the F_s centers, metal atoms are bounded much stronger when compared with regular O²⁻ sites (2.4  vs. 2.1 eV per Cu and Ag adatoms, respectively). This is accompanied by a substantial charge transfer towards each adatom (Δq_Cu=0.41e and Δq_Ag=0.32e) as well as a formation of partly covalent Me-F_s bonds across the interface (Mulliken bond populations p_Cu-Fs=0.25e and p_Ag-Fs=0.33e).     

Ionization energies of the non-stoichiometric LinFn−1 (n = 3, 4, 6) clusters

Ionization energies of non-stoichiometric Li_nF_n−1 (n = 3, 4, 6) clusters determined by a thermal ionization mass spectrometry (TIMS) were 4.2 ± 0.2 eV for Li₃F₂, 4.3 ± 0.2 eV for Li₄F₃ and 4.1 ± 0.2 eV for Li₆F₅. The ionization energy of Li₆F₅ cluster was obtained experimentally for the first time. The ionization energies of Li₃F₂ and Li₄F₃ are in correlation with the results obtained by photoionization time-of-flight mass spectrometry. The determined ionization energies are comparable with theoretical ionization energies calculated by ab initio method. The theoretical predictions supported that the most stable isomers of a non-stoichiometric cluster Li_nF_n−1 (n = 3 and n = 4) in which the excess electron localizes on a specific site have a “segregated” electronic structure composed of the metallic part and ionic part.     

Dielectric properties of parylene AF4 as low-k material for microelectronic applications

Using dielectric spectroscopy analysis, three relaxation mechanisms have been identified in 60% semi-crystalline parylene AF4 ([-F₂C-C₆H₄-CF₂-]_n) fluoropolymers. At high temperature, fluorine species located at the electrode/polymer interface involve a space-charge polarization. β-Process assigned to local molecular motions of the C-F dipoles and the γ-relaxation due to local fluctuations of the F₂C-C₆H₄ groups are also evidenced at intermediate and cryogenic temperatures respectively. Space-charge fluorine F⁻, β and γ relaxations obey an Arrhenius law against temperature with activation energy E_a (F⁻) = 1.26 eV, E_a (β) = 0.59 eV and E_a (γ) = 0.29 eV.     

Synthesis, crystal structure and infrared study of LiEr(PO3)4

A single-crystal X-ray diffraction analysis has been performed on LiEr(PO₃)₄ prepared by the flux method. The compound crystallizes in the monoclinic system with space group C2/c and cell parameters: a = 16.262(2), b = 7.032(1), c = 9.549(2) Å and β = 125.95(1)°. The crystal structure was refined based on 1272 independent reflections with I > 2σ(I). Final values of the reliability factors were improven considerably: R(F²) = 0.0180 and wR(F²) = 0.0490. The LiEr(PO₃)₄ structure is characterized by infinite chains (PO₃)_n, extending parallel to the b direction. The ErO₈ dodecahedra and LiO₄ tetrahedra alternate on two-fold axes in the middle of four (PO₃)_n chains. The vibrational study by infrared absorption spectroscopy is reported.     

An investigation on palladium sulphide (PdS) thin films as a photovoltaic material

Polycrystalline PdS thin films with tetragonal structure have been grown by direct sulphuration of Pd layers. They are formed by crystallites of size ∼ 50 nm. As-grown PdS films show a Seebeck coefficient, S = − 250 ± 30 μV/K, which indicates an n-type conductivity. Electrical resistivity of the samples, measured by the four contact probe, is (6.0 ± 0.6) × 10^− 2 Ω·cm. Hall effect measurements, confirms n-type conductivity with a negative carrier density n = (8.0 ± 2.0) × 10¹⁸ cm^− 3 and electron mobility μ of (20 ± 2) cm²/V s. Band gap energy (E_g) and absorption coefficient (α) are determined from the optical transmission and reflectance of the films. A direct transition with energy gap E_g = (1.60 ± 0.01) eV is obtained. Optical absorption coefficient in the range of photon energies hν > 2.0 eV is higher than 10⁵ cm^− 1. All these properties make PdS thin films a good alternative material for solar applications.     

Crystal structure, thermal expansion and electrical properties of a new compound Al0.33DyGe2

A new ternary compound Al_0.33DyGe₂ has been synthesized and studied from 298 K to773 K by means of X-ray powder diffraction technique. The crystal structural refinement of Al_0.33DyGe₂ has been performed by using the Rietveld method. The ternary compound Al_0.33DyGe₂ crystallizes in the orthorhombic of the defect CeNiSi₂-type structure (space group Cmcm, a = 0.41018(2)nm, b = 1.62323(6)nm, c = 0.39463(1)nm, Z = 4 and D_calc = 8.004 g/cm³). The average thermal expansion coefficients α_a, α_b and α_c of Al_0.33DyGe₂ are 1.96 × 10^− 5 K^− 1, 0.93 × 10^− 5 K^− 1 and 1.42 × 10^− 5 K^− 1, respectively. The bulk thermal expansion coefficient α_V is 4.31 × 10^− 5 K^− 1. The resistivity is observed to fall from 387 to 308 µΩ cm between room temperature and 25 K.     

Improved performance of fluorinated copper phthalocyanine thin film transistors using an organic pn junction: Effect of copper phthalocyanine film thickness

We have investigated the effect of film thickness of copper phthalocyanine (CuPc) on improving fluorinated copper phthalocyanine (F₁₆CuPc) thin film transistor (TFT) performance with an organic pn junction. Electron field-effect mobility is exponentially enhanced up to 2.0 × 10^− 2 cm² V^− 1 s^− 1 with increasing of CuPc film thickness, and then unchanged when the CuPc thickness is over the saturation thickness (3 monolayers). The charge carrier density at the interface of F₁₆CuPc/CuPc decreases the total TFT resistance, which leads to the increase of mobility. Threshold voltage is suppressed with increasing CuPc films. On the other hand, larger current on/off ratio is obtained when islanded CuPc films are formed on the surface of F₁₆CuPc films. Therefore, employing an organic pn junction is an effective and simple method to fabricate high performance of n-channel transistors for practical applications.     

A new organically templated monodimensional mixed valence (Fe/Fe) phosphite: (C4H12N2)[FeFe(HPO3)2F3]: Solvothermal synthesis, crystal structure, spectroscopic and magnetic properties

The organically templated (C₄H₁₂N₂)[Fe^IIFe^III(HPO₃)₂F₃] compound has been synthesized under mild solvothermal conditions. The crystal structure has been determined from X-ray single-crystal diffraction data. The compound crystallizes in the P2₁/n monoclinic space group, with the unit-cell parameters a = 12.935(1), b = 6.4476(7), c = 15.693(2) Å, β = 105.630(9)° and Z = 4. The crystal structure consists of [Fe^IIFe^III(HPO₃)₂F₃]²⁻ chains formed by a central chain built of [Fe(2)O₄F₂] edge-sharing octahedra, and two side chains formed by alternating [Fe(1)O₃F₃] octahedra and [HP(1)O₃] tetrahedra. The piperazinium cations are placed between the chains linked by ionic and hydrogen interactions. The IR and Raman spectra show the existence of two phosphite crystallographically independent. From the diffuse reflectance spectrum the D_q parameter for the iron(II) cations has been calculated (D_q = 820 cm⁻¹). The Mössbauer spectrum in the paramagnetic state shows the simultaneous presence of Fe²⁺ and Fe³⁺. The magnetic measurements indicate the existence of antiferromagnetic interactions.     

Pyroelectric Ta-modified LiNbO3 thin films and devices for thermal infrared detection

High-performance pyroelectric infrared (IR) detectors have been fabricated using tantalate-doped lithium niobate LiNb_1 − xTa_xO₃ (abbreviated as LNT, with x = 0-1.0) thin films deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by diol-based sol-gel processing, in which, tantalate (Ta) is adopted as dopant in lithium niobate. The randomly oriented LNT thin film exhibits a relatively small dielectric constant and a large pyroelectric coefficient. The pyroelectric characteristics of detectors with various tantalate contents, as a function of modulation frequency, were investigated. It was found that LiNb_0.8Ta_0.2O₃ had the largest voltage responsivity of 7020 (V/W) at 70 Hz, and a specific detectivity (D^?) of 7.76 × 10⁷ cm Hz^1/2/W at 200 Hz. These results indicate that the LNT thin film with x = 0.20 is most suitable for application as high-performance pyroelectric thin-film detectors.     

Ohmic contacts and n-type doping on TixCr2 − xO3 films and the temperature dependence of their transport properties

A procedure to dope n-type Cr_2 − xTi_xO₃ thin films is proposed. Besides doping the material, at the same time the method forms ohmic contacts on Ti_xCr_2 − xO₃ films. It consists on the deposition of 10 nm Ti and 50 nm Au, followed by thermal annealing at 1000 °C for 20 min in N₂ atmosphere. Ohmic contacts were formed on three samples with different composition: x = 0.17, 0.41 and 1.07 in a van der Pauw geometry for Hall effect measurements. These measurements are done between 35 K and 373 K. All samples showed n-type nature, with a charge carrier density (n) on the order of 10²⁰ cm^− 3, decreasing as x increased. As a function of temperature, n shows a minimum around 150 K, while the mobilities have an almost constant value of 11, 28 and 7 cm²V^− 1 s^− 1 for x = 0.17, 0.41 and 1.07, respectively.     

Synthesis and crystal structure of triphosphate RbPrHP3O10

Crystals of RbPrHP₃O₁₀ have been grown by the flux technique and characterized by single-crystal X-ray diffraction. RbPrHP₃O₁₀ crystallizes in the triclinic space group with lattice parameters: a = 7.0655(5), b = 7.7791(4), c = 8.6828(6) Å, α = 74.074(3), β = 74.270(3), γ = 82.865(2)°, V = 441.09(5) ų, Z = 2. The crystal structure has been solved yielding a final R(F²) = 0.0443 and R_w(F²) = 0.1426 for 1955 independent reflections (F_o² ≥ 2σ(F_o²)). The structure of RbPrHP₃O₁₀ consists of PrO₈ polyhedra and P₃O₁₀⁵⁻ groups sharing oxygen atoms to form a two-dimensional framework; the PrO₈ polyhedra form infinite chains by edge-sharing. Each Rb⁺ ion is bonded to 10 oxygen atoms, these ions are located between chains formed of (HP₃O₁₀)⁴⁻. The energies of the vibrational modes of the crystal were obtained from measurements of the infrared spectrum.