Low-temperature Spectra Research Articles

IR Spectroscopy and ab initio calculations of imidazophenazine and its derivatives in a low-temperature argon matrix

IR Spectroscopy is used for the first time to study the absorption spectra (400–1700cm−1 range) of imidazo-[4,5-d]-phenazine (F1) and its derivatives 2-methylamidazo-[4,5-d]-phenazine (F2), 2-fluorodimethylimidazo-[4,5-d]-phenazine (F3), and 1,2,3-triazolo-[4,5-d]-phenazine (F4) in a low-temperature argon matrix (10K). The spectra of these compounds are analyzed using computed frequencies and intensities of harmonic vibrations obtained by the quantum-mechanical DFT method (B3LYP∕6-31++G** and M05-2X∕6-31++G**). The low-temperature IR spectra of the compounds F1–F4 are in good agreement with the computed spectra. The spectra obtained contain bands which are common to all compounds F1–F4 as well as bands which are characteristic to each individual compound. The strongest transformations in the IR spectra of the F2–F4 compounds relative to the F1 spectrum are observed for the compound F4 where they are due to the substitution of a nitrogen atom for a carbon atom in the imidazole ring. A comparative analysis of the spectra of the isolated molecules with the spectra of polycrystalline samples in KBr pellets is performed. The structures, interaction energies, and the harmonic frequencies and their intensities are calculated for hydrogen-bonded and stack dimers of these compounds. The computed dimer spectra describe well the IR spectra of the compounds F1–F4 in KBr pellets.

Temperature dependence of the OH-stretching frequencies in topaz-OH

Hydrogen bonding in topaz-OH, Al2SiO4(OH)2, was investigated by IR spectroscopic analysis of the temperature dependence of the OH-stretching frequencies. Low-temperature spectra ranging from −196 to −160°C prove the existence of four non-equivalent H-positions in the crystal structure from the occurrence of four different OH-bands. With increasing temperature, these bands merge first, above −160°C, into two OH-bands and then above 400°C into one asymmetric broad band. Shifting of the OH-bands is caused by thermally induced hydrogen order–disorder. Low temperature fixes the protons in their positions; increasing temperature induces proton movement and allows switching between the different positions. Autocorrelation analysis of the IR spectra reveals two phase transitions, one at about −155°C from P1 to Pbn2 1 characterized as static–dynamic change and the second at about 380°C from Pbn2 1 to Pbnm caused by disordering of the protons. The increasing symmetry with temperature is due to advanced proton movement and dynamical averages over the proton distribution densities.

Understanding of the Binding Interface between PsaC and the PsaA/PsaB Heterodimer in Photosystem I

The PsaC subunit of Photosystem I (PS I) is tightly bound to the PsaA/PsaB heterodimer via an extensive network of ionic and hydrogen bonds. To improve our understanding of the design of the PsaC-PsaA/PsaB binding interface, variants of PsaC were generated, each lacking a key binding contact with the PsaA/PsaB heterodimer. The characteristics of the reconstituted, variant PS I complexes were monitored by time-resolved optical spectroscopy, low-temperature EPR spectroscopy, and electron transfer throughput measurements. In the absence of the ionic bond forming contacts R52(C) or R65(C), a markedly slower charge recombination occurs between P(700)(+) and [F(A)/F(B)](-). The addition of PsaD leads to the restoration of native recombination kinetics in a fraction of the PS I complexes reconstituted with R52A(C), but not with R65A(C). Contrary to expectation, the absence of Y80(C), which forms two symmetry-breaking H-bonds with PsaB, does not significantly affect the binding of PsaC as judged by the rate of charge recombination between P(700)(+) and [F(A)/F(B)](-). However, the removal of the entire C-terminus results in a dramatic decrease in the rate of charge recombination. Low-temperature EPR spectra of the variant PS I complexes indicate that the magnetic environments of F(A) and F(B) are altered when compared to that of native PS I. The slowing of the rate of charge recombination in the variant PS I complexes could be due to an increase in the distance between F(X) and F(A)/F(B) as the result of non-native binding or to an altered reduction potential of the iron-sulfur clusters, which would result in a different rate of thermalization up the electron acceptor chain. The most significant finding is that the variant PS I complexes support lower rates of light-induced flavodoxin reduction and that the rates deteriorate rapidly on exposure to dioxygen due to the degradation of F(A) and F(B). We suggest that the extensive set of ionic bonds and H-bonds between PsaC and the PsaA/PsaB heterodimer has evolved to ensure an exceedingly tight binding interface, thereby rendering the [4Fe-4S] clusters in PsaC inaccessible to dioxygen at the onset of oxygenic photosynthesis.

TUNING MAGNETIC DISORDER IN DILUTED MAGNETIC SEMICONDUCTORS USING HIGH FIELDS TO 89 TESLA

We describe recent and ongoing studies at the National High Magnetic Field Laboratory at Los Alamos using the new "100 Tesla Multi-Shot Magnet", which is presently delivering fields up to ~89 T during its commissioning. We discuss the first experiments performed in this magnet system, wherein the linewidth of low-temperature photoluminescence spectra was used to directly reveal the degree of magnetic alloy disorder 'seen' by excitons in single Zn 0.80 Cd 0.22 Mn 0.08 Se quantum wells. The magnetic potential landscape in II-VI diluted magnetic semiconductors (DMS) is typically smoothed when the embedded Mn 2+ spins align in an applied field. However, an important (but heretofore untested) prediction of current models of compositional disorder is that magnetic alloy fluctuations in many DMS compounds should increase again in very large magnetic fields approaching 100 T. We observed precisely this increase above ~70 T, in agreement with a simple model of magnetic alloy disorder.

Luminescence properties in the visible of Dy:YAG/YAG planar waveguides

In this work, we investigate visible emission properties of dysprosium-doped yttrium aluminum garnet (YAG) waveguides prepared by the liquid phase epitaxy (LPE) method, which allowed obtaining samples of activator concentrations ranging from 0.2 at% up to ca. 18 at%. This unique set of Dy:YAG/YAG waveguides has been carefully examined by means of highly resolved laser spectroscopy to explore the luminescence properties in the visible (yellow–blue) part of spectrum. In particular, the low-temperature absorption spectra have been recorded and analyzed, giving a more detailed information on energy levels’ positions in these crystals. The concentration-dependant emission spectra and fluorescence dynamics profiles have been collected under direct excitation, enabling analysis of multi-ion processes responsible for concentration quenching. This, in turn, enabled optimization of activator concentration with respect to yellow emission efficiency. Additionally, the possible IR to visible up-conversion pathways have been discussed, giving a starting point for further investigations.

Theoretical Description of the Structure and Magnetic Properties of Nitroxide−Cu(II)−Nitroxide Spin Triads by Means of Multiconfigurational Ab Initio Calculations

The structural, electronic and magnetic properties of two different models of the heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides Cu(hfac)2L(R) have been studied by means of multiconfigurational perturbation theory based on a CASSCF (complete active space self-consistent field) wave function, i.e. the CASPT2 method. Our calculations reveal the presence of two minima in the electronic energy curve along the Cu-O(L) bond, separated by only 6 kcal/mol, and corresponding to the X-ray structures of the CuO6 centers in Cu(hfac)2L(Pr) at 115 and 293 K, respectively. The two energetic minima are characterized by a different electronic structure, thus giving rise to a different three-spin exchange coupling and explaining the thermally induced spin transitions in this family of compounds. The concomitant variations in the magnetic properties, i.e. g factors and magnetic moments mu(eff)(T) were calculated and compared with the experimental data of Cu(hfac)2L(Pr). Even if the correspondence is only qualitative, our calculations provide a convincing explanation of the observed magnetic peculiarities. In particular, at low temperatures, the predicted ground-state is 2A(u), well separated from the 2A(g), 4A(u) states and therefore exclusively populated. Its calculated g factors, g(parallel) = 1.848, g(perpendicular) = 1.965, 1.974, qualitatively correspond to the observed g < 2 signals in the low-temperature EPR spectra. The previously assumed formal spin assignment >N-O*-Cu-*O-N < for these linear spin triads is challenged by our calculations, pointing instead to a more important role of the end-standing NO in the exchange interactions with Cu(II).

Effect of Different Substrate Temperature on Phosphorus-Doped ZnO Thin Films Prepared by PLD on Sapphire Substrates

Phosphorus-doped ZnO (ZnO:P) thin films are deposited on a c-plane sapphire in oxygen at 350°C, 450°C, 550°C and 650°C, respectively, by pulsed laser deposition (PLD), then all the ZnO:P samples are annealed at 650°C in oxygen with a pressure of 1 × 105 Pa. X-ray diffraction measurements indicate that the crystalline quality of the ZnO:P thin films is improved with the increasing substrate temperature from 350°C to 550°C. With a further increase of the deposition temperature, the crystalline quality of the ZnO:P sample is degraded. The measurements of low-temperature photoluminescence spectra demonstrate that the samples deposited at the substrate temperatures of 350°C and 450°C show a strong acceptor-bound exciton (A°X) emission. The electrical properties of ZnO:P films strongly depend on the deposition temperature. The ZnO:P samples deposited at 350°C and 450°C exhibit p-type conductivity. The p-type ZnO:P film deposited at 450°C shows a resistivity of 1.846 Ω·cm and a relatively high hole concentration of 5.100 × 1017 cm−3 at room temperature.

Two-gap superconductivity in the Fe-1111 superconductor LaFeAsO1−x Fx: A point-contact Andreev-reflection study

Abstract Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superconductor LaFeAsO0.9F0.1 with resistive T con ∼ 27 K. The observation of two pairs of peaks in the low-temperature Andreev-reflection spectra clearly indicates the presence of two order parameters. The behavior of the two gaps as a function of temperature, obtained by fitting the conductance curves by means of the generalized two-band Blonder-Tinkham-Klapwijk model, shows some anomalies. A theoretical analysis performed within the two-band Eliashberg theory with a generic electron-boson coupling can reproduce the low-temperature value of the two gaps but generally fails in giving a satisfactory fit of their overall temperature dependence, indicating the rich and complex physics of these newly discovered superconductors.

Cryogenic Neon Matrix-isolation FTIR Spectroscopy of Evaporated Ionic Liquids: Geometrical Structure of Cation−Anion 1:1 Pair in the Gas Phase

Low-temperature infrared spectra of thermally evaporated ionic liquids, 1-ethyl- and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and bis(trifluoromethanesulfonyl)amide have been measured in a cryogenic Ne matrix. The experimental IR spectrum of bis(trifluoromethanesulfonyl)amide can be reproduced theoretically by not B3LYP/6-31G* but MP2/6-31G* calculation, which suggests that the vibrational analysis for ionic liquids composed of bis(trifluoromethanesulfonyl)imide anion would be more successfully performed using the MP2 calculation. By comparison of the matrix-isolation spectra of the ionic liquids with the MP2 calculation, their geometrical structures in the gas phase are determined to be of C(2-position)-H(+)...N(-) interaction structure, which corresponds to the geometry of the energetically second-lowest ion-pair structure. The present study may provide a valuable clue to understand a vaporization mechanism of ionic liquid.

Physical nature of the monomeric fluorescence of the ortho-bromobenzophenone crystal

The time-resolved low-temperature phosphorescence spectra of the ortho-bromobenzophenone (2-BrBP) crystal are investigated at a temperature of 52K. The spectra show that the emission of one series of vibrational bands of the phosphorescence decays in time, while the emission of another series of vibrational bands, which is identical but shifted to longer wavelengths, first rises and then decays. This behavior of the spectrum at the given temperature is explained by the overcoming of an energy barrier between two conformational states of the excited ortho-bromobenzophenone molecule. Besides the phosphorescence of the two series of vibrational bands, the time-resolved spectrum of the ortho-bromobenzophenone crystal at the given temperature exhibits excimer emission, which decays over a time ≈175μs.

Fabrication and properties of B–N codoped p-type ZnO thin films

A p-type B–N codoped ZnO film was grown on quartz by magnetron sputtering and post-annealing techniques. It has room-temperature resistivity of 2.3 Ω cm, Hall mobility of 11 cm2 V−1 s−1 and carrier concentration of 1.2 × 1017 cm−3, better than the electrical properties of the N-doped p-type ZnO. The ZnO homojunction fabricated by deposition of an undoped n-type ZnO layer on the B–N codoped p-type ZnO layer showed clear p–n diode characteristics. Differing from the N-doped ZnO, the low-temperature photoluminescence spectrum of the codoped ZnO film consists of two dominant peaks located at 3.096 eV and 3.251 eV, respectively. The former is due to radiative electron transition from the conduction band to the Zn vacancy acceptor level, and the latter due to recombination of the donor–acceptor pair. The mechanism of p-type conductivity was discussed in this work.

Determining exciton coherence from the photoluminescence spectral line shape in poly(3-hexylthiophene) thin films

The photoluminescence (PL) spectral line shape of regioregular poly(3-hexylthiophene) thin films is analyzed using a model which treats the polymer pi-stacks as H-aggregates with exciton-vibrational coupling and spatially correlated site disorder. The Stokes shift, linewidth, and relative vibronic peak intensities in the low-temperature PL spectrum (T=10 K) are accurately reproduced, allowing the coherence function corresponding to the lowest energy (emitting) exciton to be determined from the ratio of the 0-0 to 0-1 peak intensities. The exciton migration length is determined from the N-dependent Stokes shift, where N is the number of segments comprising the stack. Based on the temperature dependence of the PL spectrum it is concluded that emission arises from a low concentration of aggregates which are more disordered than the dominant species responsible for absorption. The emissive aggregates are characterized by shorter average conjugation lengths and hence greater exciton bandwidths. The coherence length of the emitting exciton is estimated to be only three lattice spacings ( approximately 1.1 nm) along the pi-stacking direction. By contrast, the exciton migration length for incoherent hopping between coherent domains is estimated to be approximately 15 nm.

Defects instability under IR laser treatment in p-CdZnTe crystals

With the purpose of defect transformation and obtaining material which is suitable for some optoelectronics applications, undoped р-CdZnTe crystals were treated by IR laser radiation. For crystal properties characterization their low-temperature photoluminescence spectra before and after IR treatment were investigated. It was shown that a fine photoluminescence spectra structure, stimulated by IR laser treatment, have been generated. Observed after the IR laser treatment the transformations in р-CdZnTe photoluminescence spectra are connected with activation of background impurities and intrinsic components in inclusions, existing in untreated crystals. The background impurities, which are activated by IR laser radiation, do not produce deep levels with sufficient density of states in the gap for a high degree of compensation to be realized.

A STUDY ON THE INDIUM-INDUCED VARIATIONS IN PHOTOLUMINESCENCE PROPERTIES OF INDIUM-DOPED ZINCOXIDE NANORODS

In doped ZnO nanorods ( ZnO : In NRs) were grown on Si (111) substrates by using AuGe as a catalyst. In-concentration was controlled as high as 0.33 at.%. The length L to diameter D relationship of NRs revealed that the growth mechanism should be explained in terms of not a simple vapor–liquid–solid (VLS) mechanism but a diffusion-induced growth model. In-induced changes have been investigated from the viewpoint of photoluminescence (PL) property. At room temperature, an increase of extrinsic carrier concentration and interaction with excitons were estimated from the blueshift of peak position and decrease of luminescence intensity. Also, narrow linewidth of low-temperature PL spectra characterized the high structural quality of ZnO : In NRs. We have assigned several PL lines and confirmed the assignment by using excitation power and temperature dependence PL. Considerable evidence of crystal quality degradation was not observed from both measurements; however, we could observe that a defect and donor-bound-exciton complex dominate the PL spectrum. ZnO : In NRs shows good optical quality within our experimental range, and this result strongly supports the feasibility of In -doping to enhance the electrical property of ZnO -based nanostructure.

Photoluminescence study of Si-doped a-plane GaN grown by MOVPE

Si-doped a-plane GaN films with different doping concentrations were grown by metal-organic vapor phase epitaxy. A mirrorlike surface without pits or anisotropic stripes was observed by optical microscopy. Detailed optical properties of the samples were characterized by temperature- and excitation-intensity-dependent PL measurements. A series of emission peaks at 3.487, 3.440, 3.375–3.350, 3.290 and 3.197 eV were observed in the low-temperature PL spectra of all samples. The origin of these emissions is discussed in detail.

Modeling optical spectra and Van Vleck paramagnetism in Er3+:YAlO3

Low-temperature absorption spectra are reported for Er3+:YAlO3 (YAP) between 1700 and 350 nm. The low-temperature vacuum ultraviolet absorption spectra are also reported between 400 and 190 nm. A total of 134 experimental energy (Stark) levels representing 30 multiplets with energies below 44 000 cm−1 were modeled using a parametrized Hamiltonian defined to operate within the 4f11 electronic configuration of the Er3+ ions substituting for Y3+ in YAP, an orthorhombically distorted perovskite. The Y3+ sites have low symmetry (CS). The crystal-field energy-level parameters were determined through use of a Monte Carlo method in which 14 independent parameters are given random starting values, which are optimized using standard least-squares fitting between calculated and experimental levels. The best solution obtained has a standard deviation of 6.92 cm−1 (rms error of 6.09 cm−1). In the presence of a magnetic field the Er3+ ions occupy two magnetically inequivalent sites. As an independent check of the crystal-field modeling results, crystal-field wave functions for the I415/2 ground-state manifold of Er3+(4f11) were used to calculate the orientation-dependent anisotropic magnetic susceptibility as a function of temperature over the Curie–Weiss region. The calculated susceptibilities along the a-, b-, and c-axes of the crystal are in excellent agreement with experimental values reported as part of the present study. Van Vleck paramagnetism must be included in the calculations in order to achieve agreement. The calculated angle φ (39.6°) associated with the magnetic moment of the Er3+ sublattice along the a- and b-axes is in good agreement with the corresponding angle φ reported for Er3+ in the orthoferrite structures.

Free and bound exciton fine structures in AlN epilayers grown by low-pressure metalorganic vapor phase epitaxy

Exciton fine structures were observed in partially polarized optical reflectance and cathodoluminescence (CL) spectra of AlN epilayers grown by low-pressure metalorganic vapor phase epitaxy on (0001) Al2O3 substrates. A few free and four bound exciton lines were clearly resolved in the low-temperature CL spectra of the lowest threading dislocation density (∼1×108 cm−2) AlN film. From the energy difference between the ground-state and the first excited states, the hydrogenic A-exciton binding energy in the present compressively strained (Δa/a≈−1.68%) AlN was estimated to be approximately 51 meV.

1H-NMR study of the idle-spin magnet Cu3(OH)4SO4

1H-NMR study of a spin-1/2 triple-chain system Cu3(OH)4SO4, in which the realization of an `idle-spin' state has been suggested recently from neutron experiments, were carried out in the temperature range down to 1.4 K. In the experiments under about 1 T, we found a clear anomaly of the nuclear magnetic relaxation rate and drastic change of the spectrum at around TN = 5.4 K. NMR spectrum below TN has a characteristic shape of an antiferromagnetic ordered state. This low-temperature spectrum is consistently reproduced by the calculation of local fields at proton sites on the basis of the idel-spin state. Further, TN decreased as the applied field is increased up to 6.9 T.

Structural and luminescent properties of ZnO:P films produced by annealing ZnP2 wafers in atomic oxygen

The epitaxial ZnO:P films are fabricated by annealing the ZnP2 wafers in atomic oxygen (oxygen radicals). The properties of the films are studied by X-ray diffraction analysis, atomic force microscopy, Auger spectroscopy, and photoluminescence measurements. In the X-ray diffraction spectra of the samples, the (002) peak is observed, suggesting that the ZnO:P films are oriented along the c axis. The Auger spectroscopy data show that the ZnO films contain phosphorus. The low-temperature photoluminescence spectrum observed for the ZnO: P films exhibits a 3.356 eV peak presumably corresponding to excitons bound at neutral acceptors and a peak at 3.306 eV (free electron)-acceptor transitions associated with the PO level. The phosphorus related acceptor level is localized at 128 meV above the top of the valence band. The origin of the 3.312 eV band related to recombination at donor-acceptor pairs is discussed.

Hot-Wall MOCVD for Highly Efficient and Uniform Growth of AlN

We demonstrated successful growth of AlN at a temperature of 1200 °C in a set of hot-wall MOCVD systems with the possibility of straightforward scaling up the process on larger wafer areas to meet the demand of device technologies. We outlined several aspects of the carefully optimized design and process parameters with relevance to achievement of a high overall growth rate (1 and up to 2 μm/h), efficiency, and uniformity, which to a great extent depends on how consumption of growth-limiting species by gas-phase adduct formation can actively be prevented. Mixing of the precursors upstream from the deposition area facilitates uniform epitaxial growth, while the greater uniformity of substrate temperature inherent to the hot-wall reactor and rotation of the wafer are of fundamental importance for layer-growth uniformity. The AlN layer thickness can be controlled with an accuracy of ±1.3% on 2 in. wafers. The low-temperature cathodoluminescence spectrum of the AlN epitaxial material is strongly dominated by the intense near band-gap deep UV emission at about 208 nm.