Low-temperature Spectra Research Articles

The manifestation of excitons in low-temperature luminescence spectra of solid solutions of zinc and nickel oxides

The X-ray luminescence spectra of Zn1–xNixO solid solutions of a number of compositions, including nickel oxide (x = 1), were studied in the vicinity of I1 and I2 lines with energies of 3.339 and 3.393 eV. A strong temperature dependence of these lines, similar to that which occurred during observation of donor and acceptor excitons of 3d impurities in compounds II–VI:3d, as well as a change in the intensity ratio with an increase in temperature and different decay kinetics, were found earlier, which made it possible to link the origin of I1 and I2 lines with radiative decay of excitons of various natures. Based on the results obtained in the work, it was confirmed that the band gap width of these compounds does not depend on the composition of the solutions (i.e., on the concentration x), and a set of nickel states, both d and s types, is found in NiO near the conduction band bottom. Based on the analysis of the electron energy spectrum calculated for the studied Zn1–xNixO solid solutions, it was established that one of the observed lines of the X-ray luminescence spectrum is associated with an exciton of s–p type, and the other one—with an exciton of p–d type.The X-ray luminescence spectra of Zn1–xNixO solid solutions of a number of compositions, including nickel oxide (x = 1), were studied in the vicinity of I1 and I2 lines with energies of 3.339 and 3.393 eV. A strong temperature dependence of these lines, similar to that which occurred during observation of donor and acceptor excitons of 3d impurities in compounds II–VI:3d, as well as a change in the intensity ratio with an increase in temperature and different decay kinetics, were found earlier, which made it possible to link the origin of I1 and I2 lines with radiative decay of excitons of various natures. Based on the results obtained in the work, it was confirmed that the band gap width of these compounds does not depend on the composition of the solutions (i.e., on the concentration x), and a set of nickel states, both d and s types, is found in NiO near the conduction band bottom. Based on the analysis of the electron energy spectrum calculated for the studied Zn1–xNixO solid solutions, it was establi...

DFT and Mössbauer Spectroscopy Study of a FeTe0.5Se0.5 Single Crystal

The iron based superconductor FeSe0.5Te0.5 has been studied applying Mossbauer spectroscopy and ab initio density functional theory calculations of hyperfine parameters of iron nuclei. It has been shown that the presence of interstitial iron atoms leads to dividing of iron atoms in the compound into three groups with different hyperfine parameters. The experimental room-temperature Mossbauer spectrum has been well described within a three-group model, based on the results of ab initio calculations. The low-temperature Mossbauer spectrum has been described by a set of magnetic sextets, characterized by a distribution of hyperfine fields on 57Fe nuclei. The latter is compatible with an incommensurate spin-density wave phase, which coexists with superconductivity as it was observed also in other iron-based superconductors.

Optical Sensing Properties of ZnO Nanoparticles Prepared by Spray Pyrolysis.

We studied the optical sensing properties of ZnO nanoparticles prepared by spray pyrolysis. To investigate their optical sensing performance, we incubated peptides on ZnO nanoparticles. The photoluminescence (PL) peak intensity of peptides on the ZnO nanoparticles was higher than that of peptides on the ZnO film or on the glass plate. This observed PL enhancement is attributed to the optical confinement of ZnO nanoparticles. The low-temperature spectra displayed a strong exciton emission peak with multiple sidebands, attributed to the bound exciton and its longitudinal optical phonon sidebands. The strong exciton emission is thought to be the combined effect of optical confinement due to the nanoparticle geometry, reduction of defect emission by thermal annealing, and reduction of non-radiative relaxation at low temperatures.

Conversion mechanism of conductivity and properties of nitrogen implanted ZnO single crystals induced by post-annealing

In this paper, the conversion mechanism of conductivity and properties of nitrogen (70 keV, 1 × 1016 cm−2) implanted ZnO single crystals induced by post-annealing in a temperature ranging from 500 °C to 800 °C have been investigated by Hall, Raman, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and low temperature photoluminescence (PL) measurements. The results indicate that the sample converts to p-type conductivity as annealed at 650 °C. For the as-implanted sample, different type of nitrogen (N) local states are detected, including substitutional N atom and N2 molecules on O site [No and (N2)o], N–O, N–H, and N–C complexes. However, their thermal stabilities display significant difference upon thermal annealing. Differing from the initial ZnO bulk, the low-temperature PL spectrum of the post-annealed N implanted ZnO sample consists of two dominant peaks located at 3.105 eV and 3.220 eV, respectively. The former is attributed to radiative electron transition from the conduction band to the Zn vacancy (VZn) acceptor level, and the latter is assigned to recombination of the donor–acceptor pair (DAP). Our results suggest that the VZn together with the No acceptors are responsible for p-type conductivity of N doped ZnO.

Люминесценция кристаллов ZnSe<Al> : Yb при 4.2 K

AbstractIt is established that the doping of ZnSe:Al crystals with ytterbium from the vapor phase brings about the appearance of two new bands in the low-temperature luminescence spectra. Emission in the energy range 1.15–1.35 eV is defined by transitions in Yb^3+ ions, and the edge band is defined by the LO-phonon-assisted transitions of free electrons to acceptor levels of uncontrollable Li impurities.

Фотолюминесценция кристаллов Cu-=SUB=-2-=/SUB=-O различного происхождения

The low-temperature luminescence spectra of Cu2O crystals grown by the different methods (copper oxidation, induction melting, magnetron sputtering, hydrothermal synthesis) and of natural crystal are compared. It was concluded that the high quality of natural crystals and crystals grown by the hydrothermal method is due to the low temperature process of their formation which minimizes the strains.

Узкозонные гетероструктуры InAs-=SUB=-1-y-=/SUB=-Sb-=SUB=-y-=/SUB=-/InAsSbP (y=0.09-0.16) для спектрального диапазона 4-6 μm, полученные методом МОГФЭ

AbstractAsymmetric n -InAs/InAs_(1 – _ y )Sb_ y / p -InAsSbP heterostructures with a narrow-gap active layer and a composition range y = 0.09–0.16 were grown by vapor phase epitaxy from metalorganic compounds. Room-temperature electroluminescence was observed at a wavelength of up to λ = 5.1 μm at a spectral maximum. The study of low-temperature electroluminescence spectra provided the possibility to establish the existence of two radiative recombination channels caused by the nature of the InAsSb/InAsSbP heterointerface. The effect produced by the chemistry of the active layer on the composition of the grown barrier layer and the formation of the InAsSb/InAsSbP heterojunction with an increase in the antimony content in the InAsSb solid solution was demonstrated.

Спектрально-люминесцентные свойства разнолигандных комплексных соединений европия(III) с n-метилбензойной кислотой

Abstract The spectral-luminescent properties of europium(III) complexes with p -methylbenzoic acid and nitrogen- and phosphorus-containing neutral ligands of compositions [Eu( p -MBA)_3 · D]_2 · x H_2O and [Eu( p ‑MBA)_3 · 2H_2O]_ n , where p -MBA is the anion of p -methylbenzoic acid and D is 1,10-phenanthroline (phen), 2,2' dipyridyl (dipy), triphenylphosphine oxide (tppo) ( х = 0), hexamethylphosphortriamide (hmpa) ( x = 1), or benzotriazole (bt) ( x = 2) are studied. The electronic absorption spectra, the luminescence excitation spectra, and the Stark structure of ^5 D _0−^7 F _ j ( j = 0–2) electronic transitions in low-temperature luminescence spectra of complex europium(III) p -methylbenzoates are analyzed. It is found that the excitation energy transfer to europium ions occurs from the levels of both p -methylbenzoic acid and neutral ligands.

Одиночные центры и димеры примесных ионов гольмия в кристаллах синтетического форстерита: спектроскопия высокого разрешения

AbstractHigh-resolution low-temperature absorption spectra in the region of the ^5 I _8 → ^5 I _7, 6, 5, 4, ^5 F _5 transitions in impurity Ho^3+ ions in synthetic forsterite crystals (Mg_2SiO_4) are recorded. Comparison of the observed shape of the spectral lines with the hyperfine structure calculated using crystal-field theory makes it possible to attribute the spectral lines to single centers and dimers for the first time.

Photoabsorption markers of pressure-induced phase changes in small mercury clusters. A case study on Hg8.

Photoabsorption spectra of Hg8 have been calculated at various cluster temperatures and external pressures. A diatomics-in-molecules method has been used to model cluster electronic structure and classical isothermal–isobaric Monte Carlo simulations have been employed for sampling representative cluster configurations. Contributions of different structural isomers of the Hg8 cluster have been analyzed and related to structural transitions in the cluster, particularly those induced by an increased pressure.

Large exciton binding energy, high photoluminescence quantum yield and improved photostability of organo-metal halide hybrid perovskite quantum dots grown on a mesoporous titanium dioxide template

Herein, we demonstrated a novel synthetic route to grow size-tunable hybrid perovskite (CH3NH3PbI3 and CH3NH3PbBr3) quantum dots (QDs) using a Fluorine-doped TiO2 (F-TiO2) mesoporous template and these QDs exhibit large exciton binding energy, high photoluminescence quantum yield and improved photostability. The pore size in F-TiO2 template is tuned by varying the HF molar concentration during its solvothermal growth and size of the perovskite QDs embedded in F-TiO2 pores is tuned in the range 1.7–5.1 nm, as revealed from the TEM analysis. A systematic blue-shift in UV–visible absorption edge, as well as photoluminescence (PL) spectrum, is observed with the reduced size of the perovskite QDs due to strong quantum confinement. The CH3NH3PbI3 QD with average size ∼1.7 nm exhibits ∼47 nm blue shift in the PL spectra, ∼43 fold enhancement in PL intensity and ∼25% PL quantum yield (QY). On the other hand, CH3NH3PbBr3 QD of similar size exhibits dramatically enhanced (∼124 times) PL emission with narrow line width and a PLQY of ∼57%, which is significant for the template-assisted growth of perovskite QDs film. The quantitative analysis of the PL emission energy vs QD size shows an excellent fit with the Brus equation confirming the strong quantum confinement effect in the perovskite QDs. Analysis of low-temperature PL spectra reveals very high exciton binding energy (162–272 meV) for the QDs as compared to the bulk film (32 meV) due to the high effective dielectric constant, and high electron-hole recombination probability in the QDs, which is consistent with the extremely high PLQY and stable emission from the QDs. The blue shift of the PL peak with increasing temperature is explained on the basis of localization effect. Time-resolved PL analysis for both the perovskite QDs reveals faster life time compared to their bulk counterparts, confirming the significant radiative recombination of carriers in the QDs at the room temperature. The CH3NH3PbBr3 QDs embedded in porous F-TiO2 template maintain its initial PL intensity up to several hours (≥10 h) under the UV laser exposure (18mW), while that of the bulk film decreases to <67%. Thus, template grown hybrid perovskite QDs exhibiting high photostability and very high PLQY demonstrated here are promising for the next generation optoelectronic applications.

Effect of Bridging Ligands on Magnetic Behavior in Dinuclear Dysprosium Cores Supported by Polyoxometalates.

A family of dinuclear dysprosium cores bridged by different ligands within a polyoxometalates (POMs) framework, (TBA)8.5H1.5[(PW11O39)2Dy2X2(H2O)2]·6H2O (X = OH (1), F (2), OAc (3); TBA = tetra- n-butylammonium), was successfully synthesized and structurally characterized. Magnetic studies indicate that the bridging ligands can significantly affect the magnetic behaviors, with 1 and 3 showing antiferromagnetic coupling and 2 bridged by fluoride ions showing ferromagnetic interaction. 1 and 2 behaved as single-molecule magnets (SMMs) with the thermally activated energy barrier of 98(5) and 74(6) cm-1 under zero dc filed, respectively, whereas no SMM behavior was observed for 3 bridged by two μ-η1:η2-acetato ligands. Notably, the low-temperature fluorescence spectra of 1-3 provide valuable information on the energy levels, which are consistent with the anisotropic barriers determined by magnetic measurements. These results offer an insight into the magneto-optical correlation. Furthermore, the effective energy barrier of 1 reaches a breakthrough among all POM-based SMMs.

Formation of Tm-Y centers in CaF2-YF3:Tm3+ solid-solution crystals

The spectroscopic properties of Tm3+ optical centers at the 3F4-3H6 two-micron transition in CaF2-YF3:Tm3+ solid-solution crystals are studied by the selective laser excitation technique. The low-temperature (14 K, 77 K) time-resolved fluorescence spectra and lifetimes of several observed optical centers of thulium ions are measured under selective excitation at the 3H6-3H5 transition. Four optical centers in the CaF2-YF3:Tm3+ crystal are identified, namely, a new long-lifetime (3F4 level lifetime longer than 100 ms) optical center similar to a center observed in CaF2:Tm3+ crystals and three spectrally different clustered centers, one of which has an analogue in the CaF2:Tm3+ crystal and two others are new complex Tm-Y clusters.

L-Stercobilin-HCl and d-Urobilin-HCl. Analysis of Their Chiroptical and Conformational Properties by VCD, ECD, and CPL Experiments and MD and DFT Calculations.

Vibrational circular dichroism (CD) and IR spectra of dichloromethane solutions of l-stercobilin and d-urobilin hydrochlorides have been recorded in the mid-IR region. The spectra are best interpreted by combining molecular dynamics calculations and density functional theory (DFT) calculations within the quantum mechanics/molecular mechanics ONIOM-type framework, and the combined predicted results are better and more informative than the more standard analysis provided by DFT calculations. The same approach also sheds light on the Cotton effect sign inversion of room temperature versus low-temperature electronic CD spectra of the same compounds in methanol-glycerol solution. Finally, circularly polarized luminescence spectra for l-stercobilin in chloroform solution provide information on the excited-state geometry of this molecule.

First-principles calculations and Raman scattering evidence for local symmetry lowering in rhombohedral ilmenite: temperature- and pressure-dependent studies

ATiO3-type materials may exist in two different crystalline forms: the perovskite and ilmenite. While many papers have devoted their attention to evaluating the structural properties of the perovskite phase, the structural stability of the ilmenite one still remains unsolved. Here, we present our results based on the lattice dynamics and first-principles calculations (density functional theory) of the CdTiO3 ilmenite phase, which are confronted with experimental data obtained through micro Raman spectroscopy that is a very good tool to probe the local crystal structure. Additional Raman bands, which are not foreseen from group-theory for the ilmenite rhombohedral structure, appeared in both low temperature (under vacuum condition) and high-pressure (at room temperature) spectra. The behavior can be explained by considering the local loss of inversion symmetry operation which reduces the overall space group from () to (). Our results can also be extended to other ilmenite-type compositions.

Growth of ZnSe Nanowires and their Photoluminescence Spectra

Zinc selenide (ZnSe) nanowires were grown on Si and fused quartz substrates by a simple vapor transport method of heating the ZnSe powder at 1100 °C in a tube of the furnace. The obtained yellow colored product has indicated to be the high density of ZnSe nanowires with diameters ranging from 50 to 200 nm. Low-temperature photoluminescence spectra for ZnSe nanowires show near band-edge emissions. The free excitonic emissions were observed at ~2.8 eV.

Heterostructures with InAs/AlAs Quantum Wells and Quantum Dots Grown on GaAs/Si Hybrid Substrates

Heterostructures with InAs/AlAs quantum dots are grown on GaAs/Si hybrid substrates. The experimentally observed low-temperature (5–80 K) photoluminescence spectra of InAs/AlAs/GaAs/Si heterostructures exhibit bands defined by excitonic recombination in quantum dots and a wetting layer, i.e., a thin quantum well lying at the base of the array of quantum dots. Temperature quenching of the photoluminescence of quantum dots occurs due to the direct trapping of charge carriers at defects localized in the AlAs matrix, in the vicinity of the quantum dots.

Tuning the Anomeric Effect in Sulfamide with Superacids.

This study shows that the anomeric effect (negative hyperconjugation) that arises in sulfamide, as a result of the relatively short S-N bonds, can be tuned by the utilization of superacidic media. Sulfamide was reacted in binary superacidic systems XF/MF5 (M=As, Sb; X=H, D) and HF/BF3 . The colorless salts formed, [X2 NSO2 NX3 ]+ [MF6 ]- and [H2 NSO2 NH3 ]+ [BF4 ]- were characterized by low-temperature vibrational spectroscopy. In the case of [H2 NSO2 NH3 ]+ [BF4 ]- , a single crystal X-ray diffraction study was performed. The salt crystallizes in the monoclinic space group P21 /c with four formula units per unit cell. An exclusive N,N'-diprotonation was observed in the superacidic system HF/SbF5 when using several equivalents of the Lewis acid. Low-temperature vibrational spectra as well as a single-crystal X-ray structure of [H3 NSO2 NH3 ]2+ 2 [SbF6 ]- ⋅2 HF are reported. The salt crystallizes in the orthorhombic space group Pna21 with four formula units per unit cell. Upon mono- or diprotonation of sulfamide, remarkable structural changes of the sulfur-nitrogen bond lengths were observed. Herein, these changes are discussed together with quantum chemical calculations.

Surface-Radiative Modes and Longitudinal Excitons in the Spectra of Exciton–Polariton Luminescence

Low-temperature (T = 2 K) spectra of exciton–polariton luminescence in the vicinity of the exciton resonance frequency An = 1 of CdS crystals are studied with allowance for the damping ℏΓ of mechanical excitons. The results of numerical calculations of the partial and interference contributions of bulk and surface-radiative spectral modes in the geometry of the radiation of additional waves into a vacuum with s and p polarizations are analyzed. It is shown that the contributions of purely longitudinal excitons and their interference with polaritons of the upper dispersion branch in luminescence in the vicinity of longitudinal frequency ωL are small (∼10−30%). Nevertheless, these contributions should be taken into account to achieve a quantitative agreement with the experiment; they determine the formation of an additional AL. line under oblique radiation.

Nanoscopic spectroscopy of potential barriers formed around V-pits in InGaN/GaN multiple quantum wells on moderate temperature GaN pit expansion layers

We used photoluminescence (PL) spectroscopy combined with scanning near-field optical microscopy to directly observe the potential barriers in InGaN/GaN multiple quantum wells (MQWs) on a GaN layer grown under moderate temperatures (MT-GaN) as the V-pit expansion layer. Area-averaged PL spectra revealed shoulders on the higher-energy side of the InGaN/GaN MQW emission (HE emission) even at room temperature (RT), as well as at low temperature. These HE emission regions, which possessed island-like shapes in the PL intensity maps at RT, corresponded to the dark regions in the PL intensity maps of GaN emission. Low-temperature local PL spectra acquired at the measurement points where HE emissions were observed revealed multiple HE emission peaks at two distinct energies. Most of the regions where the HE emission occurred corresponded to the dark regions of GaN emission and their outer circumference. These experimental observations demonstrate the formation of potential barriers around threading dislocations in the InGaN/GaN MQWs on an MT-GaN layer as the pit expansion layer. The energy difference between the HE emissions and the MQW emissions on the MT-GaN layer tended to increase with increasing V-pit diameter and was much larger than the energy difference for an InGaN/GaN superlattice structure. This was rationalized by the greater change in transition energy for thinner MQWs on V-pits than for planar MQWs, when the thickness in the MQWs on MT-GaN decreases owing to strain effects.