Observation of orientational ordering processes of 4-n-pentyl-4′-cyanobiphenyl molecules during evaporation onto polyimide Langmuir-Blodgett films by optical polarized absorption measurements

Orientational ordering processes of 4-n-pentyl-4′-cyanobiphenyl (5CB) molecules were examined, in situ, using optical polarized absorption technique whilst 5CB molecules were evaporated onto synthetic silica substrates coated with n-layer Kapton-type polyimide Langmuir-Blodgett film (n = 3, 5, 7, and 11). It was found that the second orientational order parameter S₂ of 5CBs deposited onto the 3-layer polyimide substrate is lower than S₂ of 5CB deposited onto the 5-, 7-, and 11-layer polyimide-coated substrates when the amount of the deposited LC molecules is 10 times as much as that of a single monolayer. The results were discussed in terms of anchoring phenomena.     

Study on electrical conduction of viologen derivatives using scanning tunneling microscopy

The electrical conduction of self-assembled monolayers (SAMs) made from viologen derivatives was measured using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) with a focus on the molecular structural effect on the electrical conduction. For viologen derivative SAMs, resistances through the monolayers increased exponentially with increases in molecular length when the decay constants of transconductance β were ca. 0.35 to 0.48 nm^− 1. The estimated monolayer resistances of the viologen derivatives such as N-methyl-N′-(10-mercaptodecyl)-4,4′-bipyridinium (VC₁₀SH), N-methyl-N′-di(8-mercaptooctyl)-4,4′-bipyridinium (HSC₈VC₈SH), and N-methyl-N′-di(10-mercaptodecyl)-4,4′-bipyridinium (HSC₁₀VC₁₀SH) SAMs were 1.3 × 10⁸ Ω, 3.6 × 10⁹ Ω, and 3.8 × 10⁹ Ω, respectively.     

Polar Langmuir-Blodgett films

An optical and electro-optical technique is described for measuring the orientational order parameters $\text{P}\text{(}\text{cos}{}^{\mathrm{n}}\text{θ)}$ of Langmuir-Blodgett films (where θ is the angle between the long molecular axis and the normal to the film plane and $\text{P}$_n are the Legendre polynomials for n = 1, 2, 3, 4). Using these data the orientational distribution function $\text{F}$ (θ) was constructed.Polar multilayers $\text{(}\text{cos}\text{θ}\text{,}\text{cos}{}^3\text{θ}\text{≠ 0)}$ of the X and Z types were obtained from amphiphilic azony compounds. These layers were shown to have a macroscopic electrical polarization P = 10^-6 C cm^-2 and, as a result, they exhibit pyroelectric and piezoelectric properties as well as the linear electro-optical (Stark) effect.     

An angular distribution function for the sputter-depositing atoms and general equations describing the initial thickness profile of a thin film deposited inside a via and trench by sputtering

We developed an angular distribution function that describes the distribution of directional sputter-depositing atoms: g(θ) = (k sin²θ + k⁻¹cos²θ)⁻², where k is a directionality factor that determines the directionality of depositing atoms and θ is the incident angle of a depositing atom. The thickness profiles of the sputtered films deposited inside a vertical trench were simulated using the ballistic transport reaction model in conjunction with the angular distribution function, g(θ). The simulated thickness profile agreed well with the experimentally measured thickness profile. General equations that describe the thickness profile of sputter-deposited films inside vias and trenches were derived. The initial film thickness profile could be predicted by substituting the directionality factor, k, and the geometric parameters of the via or trench in the general equations. An optimum directionality factor, defined as the directionality factor that maximizes the sidewall coverage of a vertical pattern, was obtained by solving the general equations. An ideally-tapered via that maximizes sidewall coverage was identified by optimizing the directionality factors and aspect ratios of the vias.     

Synchrotron X-ray diffraction evidences of the amorphization/dissolution of the second phase particles (SPPs) in neutron irradiated Zircaloy-4

X-ray diffraction diagrams of neutron irradiated Zircaloy-4 were obtained at the Brazilian Synchrotron Laboratory (LNLS) with the aim to obtain bulk information about the amorphization process in which the Zircaloy-4 second phase particles (SPPs) undergoes due to neutron irradiation. Owing to the low concentration of the SPPs in the alloy (∼ 0.4 V%), no data regarding to the bulk were obtained until now. The synchrotron experiences allowed to detect five of the more intense lines of the phase C14 (SPPs structure) in unirradited Zircaloy-4: <110>_θ, <103>_θ, <112>_θ, <201>_θ and <004>_θ in the 34° < 2θ < 45° Bragg angle range and others of minor intensity. The diagrams of the samples irradiated at moderate doses (10²⁰ n/cm²) show these lines even in the as received samples. In contrast, none of these lines are observed for high fluency samples (∼ 10²² neutrons/cm²), confirming in the bulk what is known by TEM in thin films. In addition, in similar high fluency samples annealed 24 h or 72 h at 600 °C the intensity rises just at the 2θ range where the C14 lines were observed, showing a wide peak. That peak is interpreted as a result of the superposition of unresolved diffraction lines corresponding to the Zircaloy SPPs which are in a reconstitution process of crystallization.     

Growth of epitaxial Cu on MgO(001) by magnetron sputter deposition

100-nm-thick Cu layers were grown on MgO(001) substrates by ultra-high vacuum magnetron sputter deposition at substrate temperatures T_s ranging from 40 to 300 °C. X-ray diffraction ω−2θ scans, ω-rocking curves, and pole figures show that layers grown at T_s = 40 and 100 °C are complete single crystals with a cube-on-cube epitaxial relationship with the substrate: (001)_Cu||(001)_MgO with [100]_Cu||[100]_MgO. In contrast, T_s ≥ 200 °C leads to polycrystalline Cu layers with 001, 203, and 1¯75-oriented grains. The transition from a single- to a polycrystalline microstructure with increasing T_s is attributed to temperature-induced mass transport that allows Cu nuclei to sample a larger orientational space and find lower energy (and/or lower lattice mismatch) configurations. The large Cu- to-MgO lattice mismatch of 14% is relieved by 7 × 7 Cu unit cells occupying 6 × 6 MgO cells. In addition, for T_s ≥ 200 °C, the 001-oriented grains relax by tilting by 4° or 15° about 〈110〉 or 〈100〉 axes, respectively, while the 203 and 1¯75-oriented grains exhibit complex epitaxial relationships with the substrate: (203)_Cu||(001)_MgO with [010]_Cu||[110]_MgO and [302¯]_Cu||[11¯0]_MgO; and (1¯75)_Cu||(001)_MgO with [211¯]_Cu||[100]_MgO and [43¯5]_Cu||[010]_MgO. The surface roughness, as determined by X-ray reflectivity, increases with growth temperature. The smoothest layers are grown at 40 °C and exhibit an rms surface and buried interface roughness of 0.7 and 1.4 nm, respectively.     

Structural, thermal, magnetic and electrical studies of the iron oxophosphate Rb7Fe7(PO4)8O2·2H2O

A new iron oxophosphate of composition Rb₇Fe₇(PO₄)₈O₂·2H₂O has been synthesized and studied by X-ray diffraction, TG and DTA analysis, magnetic susceptibility, neutron diffraction, Mössbauer spectroscopy and ionic conductivity. This compound crystallizes in the monoclinic system with the P2₁/c space group and the unit cell parameters a = 8.224(8) Å, b = 22.162(6) Å, c = 9.962(6) Å and β = 109.41(8)°. Its structure is built up from Fe₇O₃₂ clusters of edge- and corner-sharing FeO₅ and FeO₆ polyhedra. Neighboring clusters are connected by the phosphate tetrahedra to form a three-dimensional framework. The Rb⁺ cations and the water molecules are occupying intersecting tunnels parallel to a and c. The presence of water molecules was confirmed by TG and DTA analysis. The magnetic susceptibility measurements have shown the existence of antiferromagnetic ordering below 22 K with a weak ferromagnetic component. Additionally, these measurements show evidence for a strong magnetic frustration characterized by |θ/T_N| ≈ 12. Powder neutron diffraction study confirms the presence of a long range antiferromagnetic order coupled to a weak ferromagnetic component along the b-axis. The strongly reduced magnetic moments extracted from the refinement support the existence of a magnetically frustrated ground state. The Mössbauer spectroscopy results confirmed the presence of only Fe³⁺ ions in both five and six coordination. The ionic conductivity measurements led to activation energy of 0.81 eV, a value that agrees with the obtained for other rubidium phosphates.     

Improvement of electrostrictive properties of a polyether-based polyurethane elastomer filled with conductive carbon black

The electrostrictive properties of a polyether-based polyurethane elastomer and its corresponding composites filled with conductive carbon black (CB) were studied by measuring the thickness strain S_Z induced by external electric fields E. For films with thicknesses of approximately 50 μm, the apparent electrostrictive coefficient M was measured at low electric fields, E ? 4 V/μm, and different CB contents (up to a volume fraction of 2%). Dielectric measurements in AC mode were performed in order to determine the percolation threshold f_c, which was 1.25 v%. This optimal volume fraction yielded a remarkable threefold increase in M, associated with an increase of the dielectric constant by a factor 7, in comparison with pure PU. This enhancement of the electric field-induced strain and apparent electrostriction was mainly triggered by an increase of the dielectric constant, even if the intrinsic electrostriction coefficient Q was decreased. The nanocomposites thus seem to be very attractive for low-frequency electromechanical applications. Above f_c, their conductivity was raised and their electrostrictive activity lost. Finally, there is a good agreement between the experimentally determined dependence on the CB content of the M coefficient and the theoretical estimation calculated from dielectric and mechanical measurements.     

Controllable synthesis and magnetic property of BiMn2O5 crystals

Uniform submicron BiMn₂O₅ particles were prepared via a facile one-step hydrothermal route at low temperature. Bi(NO₃)₃, MnCl₂·4H₂O and KMnO₄ were used as starting materials; KOH as a pH adjustor and also as a mineralizer. Single-crystalline orthorhombic BiMn₂O₅ sample with controllable morphology was obtained. The microstructure strongly depends on the molar ratio of the starting materials, KOH concentration and reaction temperature. X-ray photoelectron spectroscopy shows the existence of Mn⁴⁺ state. Magnetic measurement indicates Néel temperature T_N at 44 K. The susceptibility above T_N obeys the Curie-Weiss law, χ = C/(T − θ), with θ = −350 K. The effective paramagnetic moment μ_eff = 4.66 μ_B/Mn, demonstrating the coexistence of mixed Mn³⁺ and Mn⁴⁺ valences.     

Enhanced luminescence from lanthanide complex by silver nanoparticles

Photoluminescence of europium complex, Eu(III)DPA, where DPA is dipicolinic acid, in the presence of 5 nm-diameter Ag nanoparticles, was studied. Pronounced enhancement of Eu(III) luminescence was observed in the complex solution containing Ag nanoparticle with a concentration below 3.0 × 10¹³/ml. The dependencies of emission intensity of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transition on the concentration of Ag nanoparticles are quite different, the enhancement of electric dipole transition is stronger than that of magnetic dipole transition. There is no significant change in fluorescence lifetime in enhanced luminescence. The enhancement was attributed to a strong coupling between the radiation transition and surface plasmon resonance.     

Materials with layered structures XIII: electrical and optical properties of layered chalcogenides in the system CoIn2S4-CoIn2Se4

The system CoIn₂S_4xSe_4(1−x) has been investigated by X-ray powder methods on samples quenched at 700 °C. The spinel type phase has a phase width of 1≥x>0.9. A new layered compound is formed for 0.9>x>0.45 which crystallizes with the α-FeGa₂S₄-type with a=392.6 pm and c=1270.3 pm (x=0.5) for the hexagonal cell. Platelike crystals of the layered phase are obtained by transport reactions with iodine in a temperature gradient 750→700 °C. The band gaps of these crystals measured by optical absorption vary from 1.2 to 1.4 eV. The electrical conductivities of the crystals are found in the order of 10⁻⁵ Ω⁻¹ cm⁻¹.     

AFM surface analysis of ZnO layers prepared by pulsed laser deposition at different oxygen pressures

Polycrystalline thin films of zinc oxide were deposited by pulsed laser deposition onto silicon substrates at different oxygen partial pressures in the range of 1-35 Pa. For ablation of the sintered zinc oxide target a pulsed Nd:YAG laser was used. Other processing parameters such as laser pulse energy, pulse repetition rate, substrate temperature and deposition pressure were identical. The effect of oxygen pressure on the structural properties of the films was systematically studied by using atomic force microscopy. The surface morphology, average roughness S_a, root mean square S_q, and mean size of grains on selected places with 2 × 2 μm² area of prepared samples were evaluated. Detailed structural analysis confirmed that partial oxygen pressure leads to the modification of surface morphology. Mean grain size in height and lateral direction decreases with raising oxygen pressure from 1 to 5 Pa while the further increase of oxygen pressure from 5 to 35 Pa results in grain size enlargement. The zinc oxide film formed at oxygen partial pressure 5 Pa shows smallest values of evaluated parameters (S_a = 0.6 nm, S_q = 0.7 nm and mean size of grains 50 nm).     

Structural study and electronic band structure investigations of the solid solution NaxCu1 − xIn5S8 and its impact on the Cu(In,Ga)Se2/In2S3 interface of solar cells

The present work reports investigations on the new In₂S₃ containing Cu and/or Na compounds, which are expected to be formed at the Cu(In,Ga)Se₂/In₂S₃ interface. The knowledge of these materials properties is very important in order to better understand the operation of the devices based on these junction partners.It has been observed that a solid solution Na_xCu_1 − xIn₅S₈ exists from CuIn₅S₈ (x = 0) to NaIn₅S₈ (x = 1) with a spinel-like structure. The single crystal structure determination shows that indium, copper and sodium atoms are statistically distributed on the tetrahedral sites.XPS investigations on the CuIn₅S₈, Na_0.5Cu_0.5In₅S₈ and NaIn₅S₈ compounds combined with the band gap changes reported in a previous work show that these variations are mainly due to valence band maximum shift; it is moved downward when x increases from 0 to 1. These observations are confirmed by the electron structure calculations based on the density functional theory, which additionally demonstrate that the pure sodium compound has direct gap whereas the copper-containing compounds have indirect gaps.     

Subsolidus phase equilibria and the Li5Nd4FeO10 phase in the Li2O-Nd2O3-Fe2O3 system

A survey of the subsolidus phase equilibria in the system Li₂O-Nd₂O₃-Fe₂O₃ was made at subsolidus temperatures in the range 1000-1050 °C. A ternary phase was identified. The phase is centered on Li₅Nd₄FeO₁₀, with a cubic lattice a = 11.9494 Å. The compound melts incongruently at 1105 °C. The magnetic susceptibility was measured in the temperature range 4-300 K. The compound is paramagnetic in the temperature range 150-300 K and follows the Curie-Weiss law. At about T_N = 10 K, a long-range magnetic ordering is observed.     

Effect of A-site cation size on magnetic and charge-ordering properties of Ln0.5Sr0.5Mn0.9Cu0.1O3 (Ln = La,Pr, Nd,or Ho)

The structural, magnetic, and electrical properties of Ln_0.5Sr_0.5Mn_0.9Cu_0.1O₃ (Ln = La, Pr, Nd, or Ho) perovskite manganites have been investigated to explore the influence of A-site cation radius (〈r_A〉) and the A-site cation size-disorder (σ²) on the various interdependent phenomena such as ferromagnetism (FM), phase separation (PS), and charge ordering (CO). The temperature dependence magnetization (M–T) curve of La-based sample shows four distinct points at ∼269 K, 255 K, 200 K, and 148 K corresponding to strong FM, cluster glass (CG), weak FM, and charged ordered antiferromagnetic (COAFM) transitions, respectively. Our investigation shows that Neel temperatures (T_N) increases, whereas Curie (T_C) and irreversibility temperatures (T_irr) decrease with decreasing 〈r_A〉, i.e., with increasing σ². Furthermore, the value of the magnetization decreases and resistivity increases with decreasing 〈r_A〉. All samples exhibit insulating behavior in the temperature range 77–300 K and above 110 K the electronic conduction mechanism has been described within the framework of the variable range hopping (VRH) model.     

Large magnetocaloric effect in Ti-modified La0.70Sr0.30MnO3 perovskite

The effect of Ti substitution on the magnetocaloric effect of the La_0.70Sr_0.30MnO₃ perovskite was investigated. The magnetic entropy change (− ΔS_M) was deduced by two methods: a Maxwell relation and the Landau theory. The magnetocaloric data displays a large value of the magnetic entropy change (− ΔS_M) near the Curie temperature (T_C = 210 K), which increases when increasing the applied magnetic field. A good agreement is found between the experimental (− ΔS_M) and the one estimated by Landau theory. The relative cooling power values vary from 49 to 288 J kg^− 1 upon variation of the applied magnetic field at 1 and 5 T. Under 5 T, the relative cooling power value for La_0.70Sr_0.30Mn_0.90Ti_0.10O₃ is about 70% of the conventional refrigerant Gd material. As a result, the herein reported compound can be considered as a promising material in magnetic refrigeration technology.     

Crystallography of magnetron sputtered TiN coatings on steel substrates

Structure formation processes in TiN coatings deposited by reactive CFUBMS on steel substrates have been investigated by X-ray diffraction experiments in symmetric Bragg-Brentano (B-B) and grazing incidence asymmetric Bragg diffraction (GIABD) modes and by SEM. The results show that the deposits with thicknesses of 500 and 4000 nm are built-up of polycrystalline stoichiometric TiN, in addition to which, some negligible amount of Ti-O and Ti-N-O phases have also been observed predominantly at their surfaces. In the thinner 500 nm films only columnar crystallites with {1 1 1}, {2 0 0} and {2 2 0} crystallographic planes parallel to the surface were formed. The share of the micro-volumes belonging to the 〈1 1 1〉 out-of-plane texture component varied between 70% and 80% depending on the target current (I_d) used (4 or 8 A in the present experiments). During the more advanced stages of growth the 〈1 1 1〉 texture weakens and new texture components appear; the process being more pronounced when the application has been performed at higher I_d values. The obtained crystallographic texture results for the thinner films and their changes during the more advanced stages of the coatings formation are discussed with particular consideration of the crystallography of the TiN lattice and the anisotropy of its elastic parameters. Based on a precise estimation of the interplanar distances, d_{〈u v w〉}, corresponding to the main texture components of the investigated films in the direction along the surface macro-normal, it has been revealed that the elastic strain, ε_{〈u v w〉}, caused by the compressive residual macro-stresses acting parallel to the film surfaces and the corresponding elastic stored energy, U_{〈u v w〉}, values obey the following relationships: ε_{〈1 1 1〉}>ε_{〈2 2 0〉}> ε_{〈2 0 0〉} and U_{〈1 1 1〉}>U_{〈2 2 0〉}>U_{〈2 0 0〉}, respectively. The observed ε_{〈u v w〉} and U_{〈u v w〉} anisotropy is found to be more pronounced in the thinner coatings and is such that, at more advanced stages of growth, it would be expected to favour the transition from 〈1 1 1〉 to 〈2 0 0〉 out-of-plane preferred orientation. However, the experimental results do not confirm this expectation, which points out that the texture-formation at these stages is not governed solely by the minimization of the stored elastic energy, but is a rather complicated process depending on a larger number of factors, some of which are discussed in the paper.     

Photoluminescence, photoacoustic and Raman spectra of zinc oxide films grown by LP-MOCVD using diethylzinc and water as precursors

Polycrystalline zinc oxide (ZnO) films were grown on alkali-free glass substrates by low pressure metalorganic chemical vapor deposition (LP-MOCVD) using diethylzinc (DEZn) and water (H₂O) as precursors. Photoluminescence (PL) spectra of the films were composed of the near-band-edge (NBE) emission at ~ 380 nm and the orange band (OB) emission at ~ 600 nm. Variations of the intensity ratio of the NBE emission to the OB emission (I_NBE/I_OB) and the photon energy of the knee on the photoacoustic spectrum (PA edge) as a function of substrate temperature (T_S) could be divided into two regions at the boundary temperature (T_B). Below T_B, the NBE emission exhibited the tail extending to the higher energy than the bandgap of ZnO and the I_NBE/I_OB value became smaller with increasing T_S. In addition, the PA edge shifted towards lower energies with increasing T_S. Concerning the Raman results, these tendencies seem to have some relations with the decrease in concentration of the secondary phase Zn(OH)₂ with increasing T_S. Above T_B, however, the I_NBE/I_OB became larger and the PA edge shifted towards higher energies with increasing T_S, which is probably due to the removal of defect levels related to the excess oxygen atoms.     

Porphyrin based hybrid borate glasses: Structure and photophysical investigation

Freebase tetra phenyl porphyrin (H₂TPP) and its derivatives in different concentration ranges (0.5–2.0 mg per 12 g of boric acid) were incorporated into borate glass matrix by melt quenching technique at 230 °C. The formed glasses were stable and in green colour. The optical absorption and emission properties are different from that observed in solutions. The absorption spectrum shows a two line pattern Soret band at 435–454 nm and Q-band at 665–701 nm. The emission spectrum shows strong S₂ → S₀ emission at 490–520 nm region and S₁ → S₀ emission at 725–810 nm. The time resolved fluorescence decay of S₁ → S₀ emission shows three exponential decay. For example, in the case of 2 mg of H₂THP doped glass the lifetimes were found to be τ₁ = 0.511 ns (26.7%), τ₂ = 10 ps (64.68%) and τ₃ = 3.965 ns (8.62%). These unusual photophysical properties were found to arise from different structural motifs of porphyrin in the glass. These structures were further modeled through reactions of porphyrin with BF₃O(C₂H₅) in solution and DFT calculations.     

A promising red phosphor MgMoO4:Eu for white light emitting diodes

Motivated by the need for new red phosphors for solid-state lighting applications Eu³⁺-doped MgMoO₄ was prepared by solid-state reaction and its excitation and emission spectra were measured at room temperature. In addition, the effects of firing temperature and Eu³⁺ doping concentration on the PL intensities were also investigated. Compared with Y₂O₂S:0.05Eu³⁺, the obtained Mg_0.80MoO₄:Eu³⁺_0.20 phosphor shows a stronger excitation band near 400 nm and intensely red-emission lines at 616 nm correspond to the forced electric dipole ⁵D₀ → ⁷F₂ transitions on Eu³⁺ under 394 nm light excitation. The CIE chromaticity coordinates (x = 0.651, y = 0.348) of Mg_0.80MoO₄:Eu³⁺_0.20 close to the NTSC (National Television Standard Committee) standard values, and therefore may find application on near UV InGaN chip-based white light emitting diodes.