X-ray diffraction, vibrational and photoluminescence studies of the self-organized quantum well crystal H3N(CH2)6NH3PbBr4

We have prepared new semiconductor H₃N(CH₂)₆NH₃PbBr₄ crystals which are self-assembled organic-inorganic hybrid materials. The grown crystals have been studied by X-ray diffraction, infrared absorption and Raman spectroscopy scattering. We found that the title compound, abbreviated 2C₆PbBr₄, crystallizes in a two-dimensional (2D) structure with a P2₁/a space group. In the inorganic semiconductor sub-lattice, the corner sharing PbBr₆ octahedra form infinite 2D chains. The organic C₆H₁₈N₂⁺ ions form the insulator barriers between the inorganic semiconductor layers. Such a packing leads to a self-assembled multiple quantum well structure. Raman and infrared spectra of the title compound were recorded in the 50-500 and 400-4000 cm⁻¹ frequency regions, respectively. The assignment of the observed Raman lines was performed by comparison with the homologous compounds. Transmission measurements on thin films of 2C₆PbBr₄, obtained by the spin coating method, revealed a strong absorption peak at 380 nm. Luminescence measurements showed an emission line at 402 nm associated with radiative recombinations of excitons confined within the PbBr₆ layers. The electron-hole binding energy is estimated at 180 meV.     

Synthesis, structural and optical properties of a novel bilayered organic-inorganic perovskite C5Pb2I5

Single crystals of [C₅H₁₁NH₃]Pb₂I₅, abbreviated C₅Pb₂I₅, have been prepared. This compound is a new member of the family of the bilayered organic-inorganic lead-iodide based perovskites. Its crystal structure has been determined by X-ray diffraction. The inorganic sub-lattice consists of periodic bilayers of iodoplumbate octahedra. Each PbI₆ octahedra exhibits both edge- and corner-sharing with adjacent octahedra. The vibrational properties of this compound have been studied by Raman scattering spectroscopy. Optical absorption, photoluminescence and diffuse reflectance measurements have been performed. The room-temperature bandgap and free exciton absorption bands are observed at 2.46 and 2.23 eV, respectively. The exciton binding energy is 230 meV which is the largest value ever reported till date for the bilayered PbI based perovskites. Calculations assuming Wannier-type quasi-two-dimensional excitons and taking into account the image potential of the exciton charges showed that nearly 64% of the exciton binding energy is due to the dielectric confinement effect.     

Synthesis and characterization of a new monophosphate [2,5-(CH3)2C6H3NH3]H2PO4

Chemical preparation, calorimetric studies, crystal structure and spectroscopic investigations are given for a new noncentrosymmetric organic cation monophosphate [2,5-(CH₃)₂C₆H₃NH₃]H₂PO₄. This compound is orthorhombic P2₁2₁2₁ with the following unit-cell parameters: a=5.872(4), b=20.984(3), c=8.465(1) Å, Z=4, V=1043.0(5) Å³ and D_x=1.396 g cm⁻³. Crystal structure has been solved and refined to R=0.048 using 2526 independent reflections. Structure can be described as an inorganic layer parallel to (a,b) planes between which organic groups [2,5-(CH₃)₂C₆H₃NH₃]⁺ are located. Multiple hydrogen bonds connecting the different entities of compound thrust upon three-dimensional network a noncentrosymmetric configuration.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4)

A new compound, K₄(SO₄)(HSO₄)₂(H₃AsO₄) was synthesized from water solution of KHSO₄/K₃H(SO₄)₂/H₃AsO₄. This compound crystallizes in the triclinic system with space group P1¯ and cell parameters: a=8.9076(2) Å, b=10.1258(2) Å, c=10.6785(3) Å; α=72.5250(14)°, β=66.3990(13)°, γ=65.5159(13)°, V=792.74(3) Å³, Z=2 and ρ_cal=2.466 g cm⁻³. The refinement of 3760 observed reflections (I>2σ(I)) leads to R₁=0.0394 and wR₂=0.0755. The structure is characterized by SO₄²⁻, HSO₄⁻ and H₃AsO₄ tetrahedra connected by hydrogen bridge to form two types of dimer (H(16)S(3)O₄⁻?S(1)O₄²⁻ and H(12)S(2)O₄⁻?H₃AsO₄). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)-H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H₃AsO₄ group to build the chain S(1)O₄?H₃AsO₄ which are parallel to the “a” direction. The potassium cations are coordinated by eight oxygen atoms with K-O distance ranging from 2.678(2) to 3.354(2) Å.Crystals of K₄(SO₄)(HSO₄)₂(H₃AsO₄) undergo one endothermic peak at 436 K. This transition detected by differential scanning calorimetry (DSC) is also analyzed by dielectric and conductivity measurements using the impedance spectroscopy techniques. The obtained results show that this transition is protonic by nature.     

Ultrafast motion of liquids C2H4Cl2 and C2H4Br2 studied with a femtosecond laser

Transient optical Kerr effect of liquids C₂H₄Cl₂ and C₂H₄Br₂ is investigated, for the first time to our knowledge, with a femtosecond (fs) probe laser delayed with respect to a coherent fs pump laser. Coherent coupling and electronic Kerr signals are observed around zero delay when pump and probe overlap. Persisting after the pump-probe overlap are Kerr signals arising from the torsional and other intramolecular vibrations of the trans and gauche conformations; Kerr signals arising from the intermolecular motion are also observed. Vibrational quantum interference is only observed in liquid C₂H₄Br₂ and the related beats data are fitted with the torsional vibrations, 91 cm⁻¹ (gauche) and 132 cm⁻¹ (trans), and the CCBr angle-bending vibrations, 231 cm⁻¹ (gauche) and 190 cm⁻¹ (trans), with dephasing times, 0.45 ps, 0.45 ps, 2 ps, and 1.5 ps, respectively. These vibrational frequencies agree with those obtained in the frequency-domain. That no vibrational mode is observed for C₂H₄Cl₂ might be attributed to ineffective Raman-pumping. Kerr signals observed after the pump-probe overlap are Fourier transformed to give the spectra of the intermolecular motion and the vibrational spectrum, which agrees with the one observed in the infrared absorption and/or Raman scattering heretofore.     

Phase transitions and molecular motions in [Zn(NH3)4](BF4)2 studied by nuclear magnetic resonance, infrared and Raman spectroscopy

Middle infrared absorption, Raman scattering and proton magnetic resonance relaxation measurements were performed for [Zn(NH₃)₄](BF₄) in order to establish relationship between the observed phase transitions and reorientational motions of the NH₃ ligands and BF₄⁻ anions. The temperature dependence of spin-lattice relaxation time (T₁(¹H)) and of the full width at half maximum (FWHM) of the bands connected with ρ_r(NH₃), ν₂(BF₄) and ν₄(BF₄) modes in the infrared and in the Raman spectra have shown that in the high temperature phase of [Zn(NH₃)₄](BF₄)₂ all molecular groups perform the following stochastic reorientational motions: fast (τ_R≈10⁻¹² s) 120° flips of NH₃ ligands about three-fold axis, fast isotropic reorientation of BF₄⁻ anions and slow (τ_R≈10⁻⁴ s) isotropic reorientation (“tumbling”) of the whole [Zn(NH₃)₄]²⁺ cation. Mean values of the activation energies for uniaxial reorientation of NH₃ and isotropic reorientation of BF₄⁻ at phases I and II are ca. 3 kJ mol⁻¹ and ca. 5 kJ mol⁻¹, respectively. At phases III and IV the activation energies values for uniaxial reorientation of both NH₃ and of BF₄⁻ equal to ca. 7 kJ mol⁻¹. Nearly the same values of the activation energies, as well as of the reorientational correlation times, at phases III and IV well explain existence of the coupling between reorientational motions of NH₃ and BF₄⁻. Splitting some of the infrared bands at T_C2=117 K suggests reducing of crystal symmetry at this phase transition. Sudden narrowing of the bands connected with ν₂(BF₄), ν₄(BF₄) and ρ_r(NH₃) modes at T_C3=101 K implies slowing down (τ_R?10⁻¹⁰ s) of the fast uniaxial reorientational motions of the BF₄⁻ anions and NH₃ ligands at this phase transition.     

Crystal structure and thermal behavior of two new organic monosulfates NH3(CH2)5NH3SO4 1.5H2O and NH3(CH2)9NH3SO4 H2O

The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH₃(CH₂)₅NH₃SO₄ 1.5H₂O (DAP-S) and NH₃(CH₂)₉NH₃SO₄·H₂O (DAN-S). DAP-S is monoclinic P2₁/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å³; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P2₁/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å³ and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal.     

Coherent Raman spectra of the ν1 mode of BF3 and BF3

High resolution (0.001 cm⁻¹) coherent anti-Stokes Raman spectroscopy (CARS) was used to directly examine the ν₁ symmetric stretching mode of the planar symmetric D_3h molecules ¹⁰BF₃ and ¹¹BF₃. Simulations of the spectra were done using ν₁ rovibrational parameters deduced from published infrared hot-band and difference-band studies and the close similarity to the observed CARS spectra confirms the validity of the infrared constants. No significant perturbations by Fermi resonance or Coriolis interactions with nearby states are observed, in marked contrast to the case of sulfur trioxide, a similar D_3h molecule recently studied. In the harmonic approximation, the ¹⁰BF₃ and ¹¹BF₃ν₁ Q-branches would be identical since the isotopic substitution is at the center of mass but, interestingly, the ν₁ stretching frequency for ¹¹BF₃ is found to be 0.198 cm⁻¹higher than for the lighter ¹⁰BF₃ isotopomer. This counterintuitive result is reproduced almost exactly (0.200 cm⁻¹) by ab initio calculations (B3LYP/cc-pVTZ) that included evaluation of cubic and quartic force constants and x_ij anharmonicity constants. The ab initio computations also predict to within 1% the ΔB, ΔC changes in the rotational constants in going from the ground state to the v₁ = 1 vibrational level. The results illustrate nicely the complementary interplay of modern infrared, Raman, and ab initio methods in obtaining and analyzing rovibrational spectra.     

Structure and phase transitions in chloroantimonate(V) crystals: [(C2H5)3NH]SbCl6 and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl

New triethylammonium salts: [(C₂H₅)₃NH]SbCl₆ (TCA) and [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl (TCAT) have been synthesized. The compounds crystallise in monoclinic symmetry: space groups P2₁/n and P2₁/c, for TCA at 293 K and TCAT at 100 K, respectively. The crystal structure of [(C₂H₅)₃NH]SbCl₆ consists of discrete ionic pairs—triethylammonium cations and hexachloroantimonate anions—linked via the bifurcated N-H?Cl hydrogen bonds. The crystal structure of [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl is composed of three symmetrically independent triethylammonium cations, chlorine anion and two symmetrically independent hexachloroantimonate anions. TCA undergoes a structural phase transition at 336 K (on heating) into the orthorhombic C222 space group, whereas TCAT reveals a structural phase transition at 332 K. The phase transitions are of the first order type. TCA shows a ferroelastic domain structure below 336 K. Differential scanning calorimetry, dilatometric, dielectric dispersion and Raman scattering measurements have been used to study the phase transition mechanisms in these triethylammonium salts.     

Investigation of spectroscopic properties and thermal stability of Er/Yb-co-doped Bi2O3-B2O3-Ga2O3 glasses

A series of Er³⁺/Yb³⁺-co-doped 60Bi₂O₃-(40−x) B₂O₃ -xGa₂O₃ (BBGA x=0, 4, 8, 12, 16 mol%) glasses have been prepared. The absorption spectra, emission spectra, fluorescence lifetime of Er³⁺:⁴I_13/2 level and thermal stability were measured and investigated. Three Judd-Ofelt intensity parameters Ω_t (t=2,4,6) (Ω₂=(4.67-5.93)×10⁻²⁰ cm², Ω₄=(1.50-1.81)×10⁻²⁰ cm², Ω₆=(0.92-1.17)×10⁻²⁰ cm²) of Er³⁺ ions were calculated by Judd-Ofelt theory. It is found that the Ω₆ first increases with the increase of Ga₂O₃ content from 0 to 8 mol% and then decreases, which is mainly affected by the number of non-bridging oxygen ions of the glass network. The high peak of stimulated emission cross-section () of Er³⁺: ⁴I_13/2→⁴I_15/2 transition were obtained according to McCumber theory and broad full width at half maximum (FWHM=69-76 nm) of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions were measured. The results indicate that these new BBGA glasses can be used as a candidate host material for potential broadband optical amplifiers.     

Structural characterization,thermal, vibrational properties and molecular motions in perovskite-type diaminopropanetetrachlorocadmate NH3(CH2)3NH3CdCl4 crystal

The work presents a detailed analysis of the sequencing of the structural phase transitions in NH₃(CH₂)₃NH₃CdCl₄ crystal by differential scanning calorimetry (DSC), X-ray, infrared, far infrared and Raman spectroscopy. DSC studies have shown that in analyzed crystal occurring one reversible continuous phase transition at 375/374 K (on heating/cooling). Observed in Nujol and Fluorolube mulls in the wide temperature range between 296 K and 413 K spectral changes through the structural phase transition can be attributed to an onset of motion of cations. An assignment of some bands due to internal modes has been also proposed.     

Laser synthesis of thin layers of In4Se3, In4Te3 and modification of their structure and characteristics

Trends of structural modifications and phase composition occurring in In₄Se₃ thin films and In₄Se₃-In₄Te₃ epitaxial heterojunctions under laser irradiations have been investigated. Dynamics of the layer structure modification, depending on laser modes, i.e. pulse duration τ = 2-4 ms, irradiation intensity I₀ = 10-50 kW/cm², number of pulses N = 5-50, was studied by electron microscopy. An increase in laser influence promotes enlargement of the layer grains and transformation of their polycrystalline structure towards higher degree of stoichiometry. As a result of laser solid restructuring heterojunctions of In₄Se₃-In₄Te₃, being photosensitive within 1.0-2.0 μm and showing fast time of response, have been obtained. Laser modification of structure enables one to optimize electrical and optical properties of functional elements on the base of thin films and layers of In₄Se₃, In₄Te₃, widely used as infrared detectors and filters.     

Yb and Yb-Eu luminescent properties of the Li2Lu5O4(BO3)3 phase

The luminescence of Yb³⁺ in the oxyborate Li₂Lu₅O₄(BO₃)₃ is reported. At low temperature, in addition to the usual ytterbium infrared emission, this phosphor presents an emission in the ultraviolet () which corresponds to the transition from the charge transfer state (O-Yb) to the 4f levels of Yb³⁺. The temperature quenching Tq50% is equal to 120 K. The infrared emission studied at room temperature is located between 950 and 1100 nm.Europium emission quenching in the Li₂Yb₅O₄(BO₃)₃ phase is related to Eu→Yb transfer by cross-relaxation. The reverse Yb→Eu transfer by cooperative sensitization is highlighted in the codoped Li₂Lu₅O₄(BO₃)₃ compound.     

Continuous-wave dual-wavelength operation at 1062 and 1338 nm in Nd:YAl3(BO3)4 and observation of yellow laser light generation at 592 nm by their self-sum-frequency-mixing

We report simultaneous oscillation in continuous wave at 1062 and 1337 nm in a Nd³⁺:YAl₃(BO₃)₄ nonlinear crystal associated to the infrared laser channels ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2 of Nd³⁺. Generation of yellow laser light at 592 nm produced by Type I self-sum-frequency-mixing of both fundamental infrared laser waves is observed under non-optimal phase matching conditions.     

Enhanced white light emission in Er/Tm/Yb/Li codoped Y2O3 nanocrystals

Er/Tm/Yb codoped Y₂O₃ nanocrystals and Er/Tm/Yb/Li codoped Y₂O₃ nanocrystals have been synthesized by sol-gel method, bright white light emission has been observed at 976 nm excitation. The blue, green, and red emissions, respectively, arise from the transitions ¹G₄ → ³H₆ of Tm³⁺, ²H_11/2/⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺ ion. Moreover, after doping Li⁺ ions into Er/Tm/Yb codoped Y₂O₃ nanocrystals, the white light emission increase greatly. CIE coordinate of Er/Tm/Yb/Li codoped Y₂O₃ nanocrystals is X = 0.32 and Y = 0.36 at 10 W/cm² excitation, which is very close to the standard equal energy white light illuminate (X = 0.33, Y = 0.33).     

Current transport mechanism in Al/Si3N4/p-Si (MIS) Schottky barrier diodes at low temperatures

The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of metal-insulator-semiconductor (Al/Si₃N₄/p-Si) Schottky barrier diodes (SBDs) were measured in the temperature range of 80-300 K. By using the thermionic emission (TE) theory, the zero-bias barrier height Φ_B0 calculated from I-V characteristics was found to increase with increasing temperature. Such temperature dependence is an obvious disagreement with the negative temperature coefficient of the barrier height calculated from C-V characteristics. Also, the ideality factor decreases with increasing temperature, and especially the activation energy plot is nonlinear at low temperatures. Such behaviour is attributed to Schottky barrier inhomogeneties by assuming a Gaussian distribution of barrier heights (BHs) at interface. We attempted to draw a Φ_B0 versus q/2kT plot to obtain evidence of a Gaussian distribution of the BHs, and the values of Φ_Bo = 0.826 eV and α_o = 0.091 V for the mean barrier height and standard deviation at zero-bias, respectively, have been obtained from this plot. Thus, a modified ln(I_o/T²) − q²σ_o²/2(kT)² versus q/kT plot gives Φ_B0 and Richardson constant A^* as 0.820 eV and 30.273 A/cm² K², respectively, without using the temperature coefficient of the barrier height. This value of the Richardson constant 30.273 A/cm² K² is very close to the theoretical value of 32 A/cm² K² for p-type Si. Hence, it has been concluded that the temperature dependence of the forward I-V characteristics of the Al/Si₃N₄/p-Si Schottky barrier diodes can be successfully explained on the basis of TE mechanism with a Gaussian distribution of the barrier heights. In addition, the temperature dependence of energy distribution of interface state density (N_SS) profiles was determined from the forward I-V measurements by taking into account the bias dependence of the effective barrier height and ideality factor.     

Effect of a LaSrCoO3 buffer layer on Pb1 − xLaxTi1 − x/4O3 films studied by polarized Raman spectroscopy

A confocal Raman investigation of Pb_1 − xLa_xTi_1 − x/4O₃ (PLT) thin films grown by RF magnetron sputtering on PbO_x/Pt/Ti/SiO₂/Si substrates with an intermediate LaSrCoO₃ (LSCO) layer was performed. The influence of the LaSrCoO₃ buffer layer was analyzed taking advantage of the observed Raman spectral band variation, which varied according to different manufacturing procedures. In the presence of a LSCO layer, the A1(1TO) Raman mode, which was indicative of tetragonal distortion, was pronouncedly enhanced, and a slight deviation from the (0 0 1) plane of the film was observed from the angular dependence of the polarized Raman spectral intensity. Furthermore, the spectral band variation as well as the residual stress along the in-depth direction was measured in the film from cross-sectional spectral line scans. This latter measurement showed a relaxation of the lattice mismatch in the presence of LSCO and PbO layers.     

Antiferromagnetic resonance in ferroborate NdFe3(BO3)4

The AFMR spectra of the NdFe₃(BO₃)₄ crystal are measured in a wide range of frequencies and temperatures. It is found that by the type of its magnetic anisotropy the compound is an “easy-plane” antiferromagnet with a weak anisotropy in the basal plane. The effective magnetic parameters are determined: anisotropy fields H_a1=1.14 kOe and H_a2=60 kOe and magnetic excitation gaps Δν₁=101.9 GHz and Δν₂=23.8 GHz. It is shown that commensurate-incommensurate phase transition causes a shift in resonance field and a considerable change in absorption line width.At temperatures below 4.2 K nonlinear regimes of AFMR excitation at low microwave power levels are observed.