Low-temperature Spectra Research Articles

Fluctuations in the Electron–Phonon Coupling of a Single Chromoprotein

Sequences of high resolution low-temperature emission spectra from individual light-harvesting 2 (LH2) complexes from Rhodopseudomonas acidophila reveal a much larger variety of the emission profiles than previously observed. The results provide direct evidence for substantial variations in electron–phonon coupling and concomitantly of exciton (de)localization within single pigment-protein complexes.

Photo-induced current transient spectroscopy and photoluminescence studies of defects in AgGa0.6In0.4Se2

Defect levels in semi-insulating AgGa0.6In0.4Se2 crystals grown by the horizontal Bridgman technique and annealed in vacuum at 750 °C for 20 days have been characterized using photo-induced current transient spectroscopy (PICTS) and low-temperature photoluminescence (PL). PICTS measurements revealed four electron-related defects located at (0.027 ± 0.008), (0.051 ± 0.005), (0.31 ± 0.01) and (0.98 ± 0.01) eV and two hole-related defects at (0.040 ± 0.012) and (0.73 ± 0.02) eV. Free exciton (FE) and donor–acceptor pairs emissions were observed in low-temperature PL spectra. The defect levels measured from PICTS are consistent with those from PL. The compositional uniformity of the crystals was investigated by employing micro-PL mapping of FE at 77 K. The temperature coefficients of the band-gap energies were estimated and discussed.

Electronic structure of Ce3+ multicenters in yttrium aluminum garnets

Low temperature, infrared, and visible-ultraviolet absorption spectra of yttrium aluminum garnet (YAG) bulk crystals and epitaxial layers doped with Ce are presented. In the region of intra-configurational 4f–4f transitions, the spectra of the bulk YAG crystals exhibit existence of at least two different Ce3+ related centers, a major one associated with Ce in regular positions substituting yttrium and also additional center, due to so called antisite positions in the garnet host, i.e., ions in the Al positions. Crystal field analysis based on exchange charge model exhibit excellent agreement with the experimental data for major Ce3+ center.

Magnetism of ultrathin Fe films in MgO/Fe/MgO in epitaxial structures probed by nuclear resonant scattering of synchrotron radiation

Nuclear resonant scattering (NRS) of synchrotron radiation was used to investigate the magnetic anisotropy of iron films in MgO/Fe(t)/MgO(001) structures for t = (4–10) Å. The low-temperature NRS spectra were analyzed using a static magnetization model involving two interface-like components and a bulk-like component. We confirmed the existence of perpendicular magnetic anisotropy in MgO/Fe/MgO structures at 10 K with an increasing in-plane component of the magnetization for t > 8 Å over the entire thickness of the Fe film. The evolution of the magnetic structure with increasing temperature was studied for an Fe film thickness of 8.8 Å, and the temperature dependence of superparamagnetic fluctuations with characteristic frequencies ranging over tens of MHz was interpreted in terms of a spin reorientation transition. We showed that interfacial magnetic moments are less sensitive to thermal excitations than the magnetic moments in the film center, which was attributed to the spin pinning at the interface.

High Excitation Energy Quenching in Fucoxanthin Chlorophyll a/c-Binding Protein Complexes from the Diatom Chaetoceros gracilis

The fucoxanthin chlorophyll (Chl) a/c-binding protein (FCP) is responsible for excellent light-harvesting strategies that enable survival in fluctuating light conditions. Here, we report the light-harvesting and quenching states of two FCP complexes, FCP-A and FCP-B/C, isolated from the diatom Chaetoceros gracilis. Pigment analysis revealed that FCP-A is enriched in Chl c, whereas FCP-B/C is enriched in diadinoxanthin, reflecting differences in low-temperature steady-state absorption and fluorescence spectra of each FCP complex. Time-resolved fluorescence spectra were measured at 77 K, and the characteristic lifetimes were determined using global fitting analysis of the spectra. Tens of picosecond (ps) components revealed energy transfer to low-energy Chl a from Chls a and c, whereas the other components showed only fluorescence decay components with no concomitant rise components. The normalized amplitudes of hundreds of picosecond components were relatively 30% in the total fluorescence, whereas those of longest-lived components were 60%. The hundreds of picosecond components were assigned as excitation energy quenching, whereas the longest-lived components were assigned as fluorescence from the final energy traps. These results suggest that 30% of FCP complex forming quenching state and the other 60% of FCP complex forming light-harvesting state exist heterogeneously in each FCP fraction under continuous low-light condition.

Competition between Hydrogen Bonds and Lewis Acid-Base Interactions in the Equilibria between Bis(pentafluorophenyl)borinic Acid and Pyridine: Insights from NMR, Diffractometric and Computational Studies

Abstract 1H and 19F NMR spectroscopy, X-ray diffractometry and DFT computations have been used for investigating the interaction between the Lewis base pyridine and bis(pentafluorophenyl)borinic acid Ar2BOH (1, Ar=C6F5). Previous studies showed that the latter species in solution exists as an equilibrium mixture of the monomer (1 m ) and a cyclic trimer (1 t ), in variable ratios, and that the trimer is stabilized in the presence of Lewis bases, by the formation of strong 1 t ···L hydrogen bonds. In the present case, upon addition of 0.33 equivalents of pyridine, low-temperature NMR spectra showed the formation of deprotonated 1 t (anion 3), strongly hydrogen-bonded to the pyridinium cation Hpy+. The presence of this cation was confirmed by the high value of 1JHN (88 Hz). DFT computations confirmed the higher stability of the O-···H–N+ limit form over the neutral O–H···N one. Variable temperature NMR spectra showed that the Hpy+·3 ion pair has high conformational freedom. At temperatures higher than 260 K, 3 underwent reversible partial fragmentation to give 1 m and the Lewis acid-base covalent adducts between pyridine and either 1 m (2 py ), or its anhydride (5 py ). The fragmentation was perfectly reversible on lowering the temperature. The adduct 2 py became the only species in solution in the presence of 1 equivalent of pyridine, showing that the Lewis basicity of pyridine plays the major role, at variance with what previously observed with other bases. Hydrogen bond interactions, however, promote the supramolecular organization of 2 py in the form of a cyclic tetramer, as revealed by X-ray single crystal diffractometric analysis. The formation of the deprotonated trimer 3 was previously observed in the reaction of 1 with 1,8-bis(dimethylamino)naphtalene (DMAN). However the stepwise dearylation processes, which were dominant in the reactions with DMAN, have a very marginal role in the reaction with pyridine, due to its lower Brønsted basicity. For Supplementary Material see online version. Supporting information available: details of the NMR spectroscopic characterization (Figs. S1–S4), computed chemical shifts (Table S1), details of the X-ray diffractometric analysis, and tables of geometrical parameters (Tables S2–S3).

Effect of surface scattering of carriers in the photoconductivity spectra of CdS

The effect caused by electron scattering at a semiconductor surface is revealed in the low-temperature (77 K) photoconductivity spectra of second-group CdS crystals subjected to the influence of an external enriching transverse electric field.

Structural and dynamical properties of Bridgman-grown CdSexTe1−x(0<x≤0.35) ternary alloys

Measurements of the Raman scattering and extended x-ray-absorption fine-structure (EXAFS) spectroscopy are reported on a series of Bridgman-grown zinc-blende CdTe${}_{1\ensuremath{-}x}$Se${}_{x}$ (0.35 \ensuremath{\ge} $x$ > 0.05) ternary alloys to empathize their lattice dynamical and structural properties. Low-temperature Raman spectra have revealed the classic CdTe-like (TO${}_{1}$, LO${}_{1}$) and CdSe-like (TO${}_{2}$, LO${}_{2}$) pairs of optical phonons. The composition-dependent peak positions of the LO${}_{2}$ modes exhibited shifts towards the higher-energy side, while those of the LO${}_{1}$ phonon frequencies have unveiled the slight redshifts. Detailed analyses of EXAFS data by using the first-principles bond orbital model have enabled us to estimate both the lattice relaxations and nearest-neighbor radial force constants around the Se/Te atoms in the CdTe/CdSe matrix. These results are methodically integrated in the ``average $t$-matrix'' formalism within the Green's-function theory for defining the impurity perturbations to comprehend the composition-dependent optical phonons in CdTe${}_{1\ensuremath{-}x}$Se${}_{x}$ alloys. Based on our comprehensive calculations of impurity modes in the low-composition regime $x\ensuremath{\rightarrow}$ 0, we have assigned the weak phonon feature observed near \ensuremath{\sim}175 cm${}^{\ensuremath{-}1}$ in the low-temperature infrared reflectivity spectroscopy study to a Se${}_{\mathrm{Te}}$ localized vibrational mode.

Localized excitons mediate defect emission in ZnO powders

A series of continuous-wave spectroscopic measurements elucidates the mechanism responsible for the technologically important green emission from deep-level traps in ZnO:Zn powders. Analysis of low-temperature photoluminescence (PL) and PL excitation spectra for bound excitons compared to the temperature-dependent behavior of the green emission reveals a deep correlation between green PL and specific donor-bound excitons. Direct excitation of these bound excitons produces highly efficient green emission from near-surface defects. When normalized by the measured external quantum efficiency, the integrated PL for both excitonic and green emission features grows identically with excitation intensity, confirming the strong connection between green emission and excitons. The implications of these findings are used to circumscribe operational characteristics of doped ZnO-based white light phosphors whose quantum efficiency is almost twice as large when the bound excitons are directly excited.

Thermally activated luminescence in InN nanowires

The photoluminescence from InN nanowires is known to decrease in magnitude with increasing temperature up to 300K. Here it is shown that the luminescence reappears at higher temperatures, and continues to increase in intensity with increasing temperature up to 600K. The high temperature luminescence has similar features as the low temperature spectrum, however the magnitude of the high temperature peaks show activated temperature dependence not observed at low temperatures. This suggests that the high temperature luminescence is due to the thermal excitation of charge from long-lived trap states into the conduction band where it then relaxes radiatively.

Structure and vibrational spectra of gauche- and trans-conformers of ethanol: Nonempirical anharmonic calculations and FTIR spectra in argon matrices

Low-temperature infrared absorption spectra are obtained for ethanol isolated in an argon matrix at temperatures of 20–45 K range for ratios of the numbers of the molecules being studied to the numbers of matrix atoms of 1:1000 and 1:2000. A preliminary interpretation of the spectra is obtained on the basis of the temperature variations in the spectra and published data. The structure of the ethanol conformers, rotational constants, and internal rotation barriers of the methyl and hydroxyl groups are calculated in the B3LYP/cc-pVQZ approximation. The harmonic and anharmonic IR spectra of the gauche- and trans-conformers are calculated in the same approximation. The force fields of the two conformers and the distributions of the potential energy of the normal vibrations are calculated and compared for a general set of dependent coordinates. Anharmonicity effects are taken into account by introducing spectroscopic masses for the hydrogen atoms when calculating the normal vibrations in the harmonic approximation.

Microstructure and point defects in CdTe nanowires for photovoltaic applications

Defects in Au-catalysed CdTe nanowires vapour–liquid–solid-grown on polycrystalline underlayers have been critically evaluated. Their low-temperature photoluminescence spectra were dominated by excitonic emission with rarely observed above-gap emission also being recorded. While acceptor bound exciton lines due to monovalent metallic impurities (Ag, Cu or Na) were seen, only deeper, donor–acceptor-pair emission could be attributed to the Au contamination that is expected from the catalyst. Annealing under nitrogen acted to enhance the single crystal-like PL emission, whilst oxidizing and reducing anneals of the type that is used in solar cell device processing caused it to degrade. The incidence of stacking faults, polytypes and twins was related only to the growth axes of the wires (〈111〉 50%, 〈112〉 30% and 〈110〉 20%), and was not influenced by annealing. The potential electrical activity of the point and extended defects, and the suitability of these nanowire materials (including processing steps) for solar cell applications, is discussed. Overall they have a quality that is superior to that of thin polycrystalline films, although questions remain about recombination due to Au.

The site-selective excitation and the dynamical electron–lattice interaction on the luminescence of YBO3: Sb3+

The phosphor of YBO3: Sb3+ was synthesized by solid state reaction. Its crystal structure was examined by using X-ray diffraction analysis. The morphology and particle size were characterized with a scanning electron microscopy. The properties of YBO3: Sb3+ luminescence was systematically studied by exciting borate host, the P-1(1) and P-3(1) levels of Sb3+ at the C-i and C-1 sites with 168, 203, 218, 252 and 262 nm, respectively, measured on the synchrotron radiation instruments. The different configurations of emission spectra excited with variant wavelengths show the character of wavelength-selective excitation, which was attributed to different occupations of the Sb3+ site in the YBO3 crystal lattice. Moreover, strong electron-lattice interaction (i.e., the Jahn-Teller effect) between Sb3+ and YBO3 host was discriminated by comparing low temperature and room temperature spectra. The site-selective luminescence was confirmed by YBO3 activated with multiple Sb3+ concentrations. The spectral assignment was verified with fluorescence lifetime achieved by using the Time-Correlated Single-Photon Counting method. (C) 2013 Elsevier Inc. All rights reserved.

Investigation of electrical and optical properties of Ge–Ga–As–S glasses doped with rare-earth ions

Bulk glasses Ge250Ga17As83S650 were prepared by direct synthesis from high purity (6N) elements S, Ga, Ge, As and doped by rare earth (RE) elements — Pr, Dy, Nd, Sm or Ho. The DC and AC electrical conductivity were measured, and activation energies of relevant conductivity process were obtained from Arrhenius plots of its temperature dependence. The real part of complex permittivity as a function of temperature and frequency was deduced from AC conductivity data. Room temperature optical transmission and low temperature photoluminescence (PL) spectra were further investigated. The major feature in low-temperature PL spectra is the presence of the broad band luminescence of the base glass and relatively sharp 4f–4f radiative transitions due to the presence of RE3+ ions. When the broad band luminescence spectrally overlaps with 4f–4f transitions of RE3+ ions we can, in some cases, also observe the direct evidence of the energy transfer between the host glass and respective RE dopants which is manifested by the re-absorption of the base glass luminescence due to 4f–4f up-transitions.

Optical properties of ZnO powder prepared by using a proteic sol-gel process

We have studied the optical properties of ZnO powder synthesized by using a proteic sol-gel process with coconut water as the precursor. The energy dispersive X-ray spectrometer and X-ray diffraction results show high purity of the synthesized ZnO powder. From the low-temperature (12 K) and power-dependent PL spectra, the donor-bound exciton, the acceptor-bound exciton, the donor-to-acceptor pair (DAP), and the phonon-replica of the DAP transition have been observed at 3.38, 3.34, 3.26, and 3.19 eV, respectively. The free exciton emission (∼3.3 eV) is also observed at 300 K in the temperature-dependent PL spectra.

Stabilization of a spike in excitonic light reflection spectra of CdSe crystals subjected to low-energy electron bombardment

In the low-temperature (4.2 K) excitonic light reflection spectra of CdSe crystals subjected to the low-energy (several keV) electron irradiation, an effect of the stop of a “rotation” at the longitudinal exciton frequency is found. The connection of this effect with Fermi level pinning on the surface of the semiconductor is assumed.

Electronic Structure and Spectroscopic Properties of Mononuclear Manganese(III) Schiff Base Complexes: A Systematic Study on [Mn(acen)X] Complexes by EPR, UV/vis, and MCD Spectroscopy (X = Hal, NCS)

The manganese(III) Schiff base complexes [Mn(acen)X] (H2acen: N,N'-ethylenebis(acetylacetone)imine, X: I(-), Br(-), Cl(-), NCS(-)) are considered as model systems for a combined study of the electronic structure using vibrational, UV/vis absorption, parallel-mode electron paramagnetic resonance (EPR) and low-temperature magnetic circular dichroism (MCD) spectroscopy. By variation of the co-ligand X, the influence of the axial ligand field within a given square-pyramidal coordination geometry on the UV/vis, EPR, and MCD spectra of the title compounds is investigated. Between 25000 and 35000 cm(-1), the low-temperature MCD spectra are dominated by two very intense, oppositely signed pseudo-A terms, referred to as "double pseudo-A terms", which change their signs within the [Mn(acen)X] series dependent on the axial ligand X. Based on molecular orbital (MO) and symmetry considerations, these features are assigned to π(n.b.)(s, a) → yz, z(2) ligand-to-metal charge transfer transitions. The individual MCD signs are directly determined from the calculated MOs of the [Mn(acen)X] complexes. The observed sign change is explained by an inversion of symmetry among the π(n.b.)(s, a) donor orbitals which leads to an interchange of the positive and negative pseudo-A terms constituting the "double pseudo-A term".

Luminescence of CdS crystals due to near-surface potential fluctuations

The nature of the wide emission band observed in the low-temperature (T ≤ 60 K) near-edge photoluminescence spectra of CdS crystals placed in water and irradiated by a He-Cd laser has been investigated. The spectral and temporal characteristics of the band and its dependence on temperature, excitation intensity, and transverse electric field are considered. The relationship between the band and the radiative recombination of carriers localized on near-surface potential fluctuations, which are due to the defect-forming character of the joint effect of laser irradiation and water on near-surface layer of semiconductor is grounded.

Structural and optical properties of silicon layers with InSb and InAs nanocrystals formed by ion-beam synthesis

We have studied the formation of InSb and InAs precipitates with sizes of several nanometers in Si and SiO2/Si by means of implantation of (Sb+In) or (As+In) ions with energies from 170 to 350keV and fluencies from 2.8 to 3.5×1016cm−2 at 500°C and subsequent annealing at 1050–1100°C for 3–30min. RBS, TEM/TED, RS and PL techniques were employed to characterize the implanted layers. A broad band in the region of 1.2–1.6μm has been registered in the low-temperature PL spectra of both (Sb+In) and (As+In) implanted and annealed silicon crystals. It was shown that structural and optical properties of oxidized silicon crystals strongly depend on type of implanted species in silicon crystals.

Long-wavelength electroluminescence of InGaAs-capped type-II GaSb/GaAs quantum-rings at room temperature

The room-temperature photoluminescence (PL) and electroluminescence (EL) exceeding 1.3μm are observed for InGaAs-capped GaSb quantum-ring (QR) structures. With increasing In composition, the emission wavelength would shift from 1.18 to 1.31μm. The InGaAs-capped GaSb QRs also exhibit a higher injection current and stronger EL intensity at the same applied voltage. With the observation of low-temperature PL spectra, the transition mechanisms of the standard and InGaAs-capped QRs are further investigated.