Low-temperature Spectra Research Articles

Specific features of the low-temperature conductivity and photoconductivity of CuInSe2-ZnIn2Se4 alloys

n-type CuInSe2-ZnIn2Se4 alloy single crystals are grown by the horizontal variant of the Bridgman method. The slight temperature dependence of the conductivity, high electron concentration, and the low photoconductivity of single crystals containing a low (5–10 mol %) fraction of ZnIn2Se4 are indicative of the nearly degenerate state of the crystals. It is established that, in the CuInSe2-ZnIn2Se4 single crystals containing 15 and 20 mol % of ZnIn2Se4, the hopping mechanism of conductivity is dominant at temperatures of T ∼ 27–110 K. At T ≥ 110 K, hopping conductivity gives way to activated conductivity. It is found that the specific feature of the low-temperature (27–77 K) photoconductivity spectrum of single crystals with ∼15 and 20 mol % of ZnIn2Se4 is a single narrow peak at a wavelength of λmax = 1190–1160 nm.

Properties of Silicon for Photoluminescence

For more than five decades, silicon has dominated the semiconductor industry that supports memory devices, ICs, photovoltaic devices, etc. Photoluminescence (PL) is an attractive silicon characterization technique because it is contactless and provides information on bulk impurities, defects, surface states, optical properties, and doping concentration. It can provide high resolution spectra, generally with the sample at low temperature and room-temperature spectra. The photoluminescence properties of silicon at low temperature are reviewed and discussed in this study. In this paper, silicon bulk PL spectra are shown in multiple peak positions at low temperature. They correspond with various impurities such as In, Al, and Be, phonon interactions, for example, acoustical phonons and optical phonons, different exciton binding energies for boron and phosphorus, dislocation related PL emission peak lines, and oxygen related thermal donor PL emissions.

Magnetic Coupling and Single-Ion Anisotropy in Surface-Supported Mn-Based Metal–Organic Networks

The electronic and magnetic properties of Mn coordinated to 1,2,4,5-tetracyanobenzene (TCNB) in the Mn-TCNB 2D metal-ligand networks have been investigated by combining scanning tunneling microscopy and X-ray magnetic circular dichroism (XMCD) performed at low temperature (3 K). When formed on Au(111) and Ag(111) substrates the Mn-TCNB networks display similar geometric structures. Magnetization curves reveal ferromagnetic (FM) coupling of the Mn sites with similar single-ion anisotropy energies, but different coupling constants. Low-temperature XMCD spectra show that the local environment of the Mn centers differs appreciably for the two substrates. Multiplet structure calculations were used to derive the corresponding ligand field parameters confirming an in-plane uniaxial anisotropy. The observed interatomic coupling is discussed in terms of superexchange as well as substrate-mediated magnetic interactions.

Low-temperature high-resolution absorption spectrum of 14NH3 in the ν1+ν3 band region (1.51 μm)

Jet-cooled spectra of 14NH3 and 15NH3 in natural abundance were recorded using cavity ring-down (CRDS, 6584–6670 cm−1) and cavity enhanced absorption (CEAS, 6530–6700 cm−1) spectroscopy. Line broadening effects in the CRDS spectrum allowed lines with J″-values between 0 and 3 to be identified. Intensity ratios in 14NH3 between the jet-cooled CRDS and literature room-temperature data from Sung et al. (J. Quant. Spectrosc. Radiat. Transfer, 113 (2012), 1066) further assisted the line assignments. Ground state combination differences were extensively used to support the assignments, providing reliable values for J, K and inversion symmetry of the ground state vibrational levels. CEAS data helped in this respect for the lowest J lines, some of which are saturated in the CRDS spectrum. Further information on a/s doublets arose from the observed spectral structures. Thirty-two transitions of 14NH3 were assigned in this way and a limited but significant number (19) of changes in the assignments results, compared to Sung et al. or to Cacciani et al. (J. Quant. Spectrosc. Radiat. Transfer, 113 (2012), 1084). Sixteen known and 25 new low-J transitions were identified for 15NH3 in the CRDS spectrum but the much scarcer literature information did not allow for any more refined assignment. The present line position measurements improve on literature values published for 15NH3 and on some line positions for 14NH3.

Information on quantum states pervades the visible spectrum of the ubiquitous Au144(SR)60 gold nanocluster.

Absorption spectra of very small metal clusters exhibit individual peaks that reflect the discreteness of their localized electronic states. With increasing size, these states develop into bands and the discrete absorption peaks give way to smooth spectra with, at most, a broad localized surface-plasmon resonance band. The widely accepted view over the last decades has been that clusters of more than a few dozen atoms are large enough to have necessarily smooth spectra. Here we show through theory and experiment that for the ubiquitous thiolate cluster compound Au144(SR)60 this view has to be revised: clearly visible individual peaks pervade the full near-IR, VIS and near-UV ranges of low-temperature spectra, conveying information on quantum states in the cluster. The peaks develop well reproducibly with decreasing temperature, thereby highlighting the importance of temperature effects. Calculations using time-dependent density-functional theory indicate the contributions of different parts of the cluster-ligand compound to the spectra.

Is Bi2+ Responsible for the Red-Orange Emission of Bismuth-Doped SrB4O7?

Red-orange luminescence from bismuth-doped SrB4O7 has previously been reported and assigned to 6p-6p transitions of divalent Bi. To provide support for this assignment and for the stability of this unusual valence state of Bi, we report here results from low-temperature luminescence spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and wave function based ab initio calculations. Low-temperature luminescence spectra reveal zero-phonon lines (ZPLs) in excitation and emission spectra, allowing an accurate determination of the energies for the electronic transitions. The influence of the Bi concentration on the emission intensity is shown to be small, and only a small increase of the red-orange emission is observed upon raising the nominal Bi concentration from 0.02% (200 ppm) to 2%. This result indicates that only a very low concentration of Bi2+ can be incorporated in SrB4O7. This observation is supported by EPR experiments, which do not show a signal that can be assigned to Bi2+, and by XANES experiments showing that most Bi is in the trivalent state. An upper limit of the Bi2+ concentration is estimated to be 20 ppm. Ab initio calculations on the (BiO9)16– cluster embedded in SrB4O7 give energies for excited states that are close to the experimentally observed energies. Also, the luminescence lifetime for the red-orange emission (∼12 μs) is consistent with the lifetime for 6p-6p emission calculated for the Bi2+ emission (3.5 μs). Equivalent ab initio calculations for Bi2+ luminescence are very far from the experimental results, providing independent evidence and additional support for the interpretation of stable Bi2+ species being responsible for the red-orange luminescence. The calculations provide a new interpretation of the third excitation band, which is not due to a 2S1/2 state of the 6s27s configuration of Bi2+, as previously assumed, but is due to a state with important characters of 6s6p2–4P (63%) and doublets of the 6s6p2, 6s26d, and 6s26p configurations; its higher intensity is due to its character of parity-allowed 6s → 6p and 6p → 6d excitations.

P-type Sodium-doped Zinc Oxide Nanowire Arrays Grown by High-pressure Pulsed Laser Deposition

Sodium-doped ZnO (ZnO:Na) nanowire arrays were grown with high-pressure pulsed laser deposition (HP-PLD). The influence of growth pressure and thickness of gold catalyst layer on the growth of ZnO:Na nanowires were systemically studied. It is found that c-orientated ZnO nanowire arrays grow on single crystal silicon substrates under the optimized condition, e.g. gold catalyst layer’s thickness of 4.2 nm, growth pressure of 3.33×10 Pa and growth temperature of 875 °C. X-ray diffraction pattern and X-ray photoelectron spectroscope analyses indicate that Na is introduced into ZnO nanowires successfully. Optical fingerprints of sodium-related acceptors in the low-temperature (15 K) photoluminescence spectrum are observed, such as neutral acceptor-bound exciton emission (3.356 eV, AX), free-electron to neutral-acceptor emission (3.312 eV, (e, A)), and donor-to-acceptor pair emission (3.233 eV, DAP). ZnO:Na nanowire arrays grown on ZnO:Al/sapphire substrates form the pn junction. The corresponding I-V curve measurements exhibit a clear rectifying behavior of pn homojunction, which further indicates that such ZnO:Na nanowire is of p-type conductivity. 156 无 机 材 料 学 报 第 29卷

Benzyl Alcohol Oxidation on Pd(111): Aromatic Binding Effects on Alcohol Reactivity

To investigate how surface oxygen participates in the reaction of important aromatic oxygenates, the surface chemistry of benzyl alcohol (PhCH2OH) and benzaldehyde (PhCHO) has been studied on oxygen-precovered Pd(111) (O/Pd(111)) using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On both Pd(111) and O/Pd(111), TPD using isotopically labeled benzyl alcohol and low-temperature HREEL spectra show that the oxidation of benzyl alcohol proceeds through a benzyl alkoxide (PhCH2O-) intermediate to adsorbed benzaldehyde so that the sequence of bond scission is O-H followed by C(α)-H. In the presence of surface O, some benzaldehyde desorbs from the surface below 300 K, consistent with the presence of a weakly adsorbed η(1) aldehyde state that is bound to the surface through its oxygen lone pair. Benzaldehyde also reacts with surface oxygen to produce benzoate (PhCOO-). Shifts in the OCO stretching frequency suggest that the benzoate orientation changes as the surface becomes less crowded, consistent with a strong interaction between the phenyl group and the surface. Adsorbed benzaldehyde and benzoate undergo decomposition to CO and CO2, respectively, as well as benzene. Deoxygenation of benzyl alcohol to toluene occurs at high coverages of benzyl alcohol when the relative surface O coverage is low. Experiments conducted on (18)O/Pd(111) reveal exchange occurring between surface O and the benzaldehyde and benzoate intermediates. This exchange has not been reported for other alcohols, suggesting that aromatic binding effects strongly influence alcohol oxidation on Pd.

Structure and Properties of 2,3,3,3-Tetrafluoropropene (HFO-1234yf)

The novel refrigerant 2,3,3,3-tetrafluoropropene (HFO-1234yf) is discussed as a replacement for 1,1,1,2-tetrafluoroethane (R-134a) due to its low global warming potential. Although it may therefore have great potential for use on a large scale, no structural data and only a fewvibrational data of the compound are known.We describe low-temperature X-ray structure analysis, vibrational spectra and combustion properties of 2,3,3,3-tetrafluoropropene. The experimental data are compared with the results of theoretical calculations. Furthermore the basic combustion properties have been investigated.

Invited Presentation: Photoluminescence of Carbon Nanotubes at Room and Cryogenic Temperatures

Semiconducting carbon nanotubes exhibit excitonic transitions in the near-infrared. The past decades have established an extensive understanding of carbon nanotubes photophysics mainly based on room-temperature studies. More recently, cryogenic spectroscopy has identified non-classical emission from individual nanotubes which can be accompanied by spectrally narrow emission. We will present our latest results obtained with photoluminescence spectroscopy of single carbon nanotubes both at room and cryogenic temperatures and discuss the dependence of low-temperature spectra on transverse electric field.

Optical properties of heavy lanthanide maleates in solution and crystal form

Monocrystals of Ln(C4O4H3)3·8H2O (Ln=Eu3+, Ho3+, Er3+) were grown and spectroscopic investigations were performed to understand their photophysical properties. It was found that all the complexes were isostructural with neodymium maleate, Nd(C4O4H3)3·8H2O, whose crystal structure had been determined previously. Electronic absorption and emission spectra at low temperatures for crystals of the lanthanide maleates (with the splitting of the levels) were allowed for the stated lanthanide ions, with a low symmetry environment, in these systems. The intensities of the electronic lines and the Judd–Ofelt parameters were calculated and compared to those obtained for light lanthanides (i.e. Nd(III) and Sm(III)) maleates. The variation of the spectroscopic parameters (e.g. β, δ) and their correlation with the nature of the Ln3+–L bond are discussed. The nephelauxetic ratio β and Sinha’s parameter δ suggested an increase in the ‘degree of covalency’ of the metal–ligand coordination bond from the Eu3+ crystal to the Ho3+ crystal and to the Er3+ crystal of the title compounds. The –CC– double bond in the carboxylate ions determines the distribution of the charge and the character of the Ln–O coordination bonds, which is very important when considering the potential applications of lanthanide carboxylates. In the low-temperature absorption and emission spectra, relatively strong vibronic lines occur, promoted by both ligand (L) and Ln–L localised vibrational modes. In the vibronic coupling, the role of π electrons is not negligible. The emission from the 5D1 level was detected for the europium maleate in spite of the presence of water molecules in the inner coordination sphere of the lanthanide maleate crystals. On the basis of the europium maleate-excited spectra, a ligand-to-metal charge transfer (LMCT) transition was determined at 345nm.

Cd-rich and Te-rich low-temperature photoluminescence in cadmium telluride

Low-temperature photoluminescence emission spectra were measured in cadmium telluride (CdTe) samples in which composition was varied to promote either Cd or Te-rich stoichiometry. The ability to monitor stoichiometry is important, since it has been shown to impact carrier recombination. Te-rich samples show transitions corresponding to acceptor-bound excitons (∼1.58 eV) and free-electron to acceptor transitions (∼1.547 eV). In addition to acceptor-bound excitons, Cd-rich samples show transitions assigned to donor-bound excitons (1.591 eV) and Te vacancies at 1.552 eV. Photoluminescence is a noninvasive way to monitor stoichiometric shifts induced by post-deposition anneals in polycrystalline CdTe thin films deposited by close-spaced sublimation.

Spectroscopic characteristics of praseodymium-doped cubic double sodium-yttrium fluoride crystals Na0.4Y0.6F2.2:Pr3+. Intensities of optical transitions and luminescence kinetics

Using the Bridgman-Stockbarger technique, we have grown a series of cubic crystals Na0.4Y0.6F2.2:Pr3+ (NYF:Pr3+) with a content of praseodymium in the range of 0.04–9 at %. We have determined the composition of crystals, evaluated their optical quality, and found the incorporation coefficient of Pr3+ ions into the Na0.4Y0.6F2.2 matrix (K Pr ∼ 0.9). We have examined optical spectra of NaYF:Pr3+ crystals at room and low (7 K) temperatures in the range of 200–2500 nm. The low-temperature absorption spectra of NYF:Pr3+ crystals have been shown to consist of broad weakly structured bands. Based on the analysis of low-temperature absorption spectra, the structure of the Stark splitting of praseodymium levels has been represented in terms of a model of “quasi-centers,” which are characterized by an inhomogeneous broadening of Stark components. From experimental absorption cross-section spectra at T = 300 K, we have calculated oscillator strengths for transitions from the ground state 3 H 4 to excited multiplets 3 H 5, 3 H 6, 3 F j (j = 2, 3, 4), 1 G 4, 1 D 2, and (3 P j ,1 I 6) (j = 0, 1, 2). Using the Judd-Ofelt method, we have determined intensity parameters Ω t and found that Ω2 = 0, Ω4 = 4.4 × 10−20, and Ω6 = 2.28 × 10−20 cm2. With these values, we have calculated the probabilities of radiative transitions, the branching coefficients, and the lifetimes of the radiative levels 1 D 2 and 3 P 0. The probabilities of multiphonon nonradiative transitions in NYF:Pr3+ crystals have been estimated. Using the method of kinetic spectroscopy with selective excitation, we have investigated the luminescence decay kinetics of praseodymium from the 3 P 0 and 1 D 2 levels upon their selective resonant excitation by nanosecond laser pulses. The inference has been made that Na0.4Y0.6F2.2:Pr3+ crystals are processable; admit doping by praseodymium in high concentrations; and, with respect to all their radiative characteristics, can be potentially considered as active media for converters of optical radiation and solid-state continuously tunable lasers in the visible range.

Bismuth-ion doped Cu(In,Ga)Se2 thin films: Preparation, microstructures, and electrical properties

The influence of doping with bismuth ions in the properties of synthesized Cu(In,Ga)Se2 films was investigated via the solution coating process followed by a selenization step. The incorporation of bismuth ions in Cu(In,Ga)Se2 significantly increased the size of the grains in the obtained films. The intermediate compound of bismuth copper oxide was thought to react with selenium vapor to form copper bismuth selenide, which acted as a flux agent at elevated temperatures. The low-temperature photoluminescence spectra of bismuth-containing Cu(In,Ga)Se2 films revealed attenuated intensity of the peak that was attributed to the donor–acceptor pair transition following the incorporation of bismuth. The donor–acceptor transition is caused by the selenium deficiency. Doping with bismuth ions reduced the amounts of the selenium deficiencies in the films. The incorporation of bismuth ions increased the open-circuit voltage and the fill factor of solar cells by increasing the carrier concentration and reducing the resistivity. The efficiency of solar cells increased from 4.37 to 6.29% as bismuth-ion doping f was increased from 0 to 1.0mol%, revealing that the properties of Cu(In,Ga)Se2 absorber layers were effectively improved via the addition of bismuth ions.

Spectroscopic characterization of CaNb2O6 single crystal doped with samarium ions

Spectroscopic measurements of the calcium niobate CaNb2O6 single crystal doped with samarium ions were performed. Polarized absorption and emission spectra as well as luminescence decay curves of this material were recorded as a function of temperature in the 5–300K temperature region. Analysis of low temperature spectra made it possible to determine energies of crystal field components of Sm3+ multiplets involved in the excitation and luminescence phenomena. Number of crystal field levels derived implies that Sm3+ ions in the CaNb2O6 host are accommodated in several different sites. Radiative lifetime of the 4G5/2 metastable level determined based on the Judd–Ofelt analysis of room temperature absorption spectra amounts to 777μs, a value close to that of luminescence lifetime determined from luminescence decay curve. Energy transfer between the NbO6 group and samarium ions was observed in wide temperature range. Intense luminescence related to the 4G5/2→6H7/2 and 4G5/2→6H9/2 transitions centered at 610nm and 660nm, respectively, combined with strong absorption band near 404nm that matches perfectly the pump light provided by commercial InGaN/GaN diode lasers points at a potential of CaNb2O6:Sm3+ for the design of all-solid-state visible lasers.

Optical and luminescence spectroscopy studies of electronic structure of Li6GdB3O9 single crystals

This article presents the study of electronic structure of Li6GdB3O9 single crystals and radiative relaxation of electronic excitations in them. The investigation was performed by the means of low-temperature optical and luminescence far-ultraviolet spectroscopy upon excitation by synchrotron radiation. On the basis of the low-temperature (T=10 and 30K) spectra of the reflection, recorded in the present research and the dispersions of the complex optical functions of εˆ(E),nˆ(E) and μ(E), calculated in the framework of the oscillator model, we have determined the parameters of the electronic structure of the Li6GdB3O9 crystals as follows. The value of the minimum energy for the interband transitions in the boron–oxygen framework is Eg=9.42eV, the energy position of the first excitonic peak in the excitation spectra for anionic excitons is En=1=7.46eV, the minimum threshold energy for excitation of excitons in linear chains of the Gd3+ cations is Ec=6.80eV. The excitation spectra of an intrinsic luminescence of Li6GdB3O9 crystals, recorded at 10K in the range of the optical charge-transfer transitions O–Gd have a band with the maximum at ECT=6.57eV.

The extraordinary fluxionality of Ru6(μ6-C)(μ-CO)(CO)16

The 13C NMR spectrum of Ru6(μ6-C)(μ-CO)(CO)16 (1) in solution at −125°C indicates that the CO ligands of the equatorial Ru4(μ6-CO)(CO)10 unit are undergoing ultra-fast exchange estimated to be ≳3×107s−1. The variable temperature solution IR spectra of 1 shows bands that are attributed to the Ru4(μ6-CO)(CO)10 unit at −58°C that broaden and collapse almost to the baseline in the spectrum at +70°C. On the other hand, two of the three strong CO stretches of the Ru(CO)3 units of 1 observed in the low temperature spectra coalesce to a sharp signal typical of a C3vfac-L3M(CO)3 grouping. These results are interpreted in terms of the carbonyls of the equatorial Ru4(μ-CO)(CO)10 unit undergoing exchange on the IR timescale (~1012s−1) at room temperature, consistent with DFT calculations.

Solution structures of lithium amino alkoxides used in highly enantioselective 1,2-additions.

Lithium ephedrates and norcarane-derived lithium amino alkoxides used to effect highly enantioselective 1,2-additions on large scales have been characterized in toluene and tetrahydrofuran. The method of continuous variations in conjunction with 6Li NMR spectroscopy reveals that the lithium amino alkoxides are tetrameric. In each case, low-temperature 6Li NMR spectra show stereoisomerically pure homoaggregates displaying resonances consistent with an S4-symmetric cubic core rather than the alternative D2d core. These assignments are supported by density functional theory computations and conform to X-ray crystal structures. Slow aggregate exchanges are discussed in the context of amino alkoxides as chiral auxiliaries.

Influence of exciton decay on the polariton luminescence spectra of CdTe crystal

The polariton luminescence spectra of CdTe crystals have been numerically calculated with allowance for the decay of mechanical excitons and compared with the known experimental low-temperature photoluminescence spectra of these crystals. The mechanism of formation of polariton luminescence in dependence of the optical parameters of crystal for the exciton resonance An=1 is discussed.

Photoluminescence studies of ZnO films fabricated by using a combination of a hydrothermal method and plasma-assisted molecular beam epitaxy regrowth

ZnO films were deposited on Si (100) substrates by using a two-step growth process. In the first step, ZnO nanorods were grown by using the hydrothermal method at 140 °C for 5 min. In the second step, a ZnO amorphous layer was deposited on the ZnO nanorods by spin-coating. After completion of the growth process, the films were annealed at 800 °C for 10 min, and a ZnO active layer was deposited on top of the amorphous layer by using plasma-assisted molecular beam epitaxy. Further, temperature-dependent photoluminescence (PL) measurement were conducted to study the optical properties of the prepared films. In the low-temperature PL spectra, emission peaks in the near-band-edge region were observed at 3.370, 3.362, 3.347, 3.329, 3.317, 3.288, 3.263, 3.219, 3.191, and 3.116 eV; these peaks were attributed to free excitons, neutral donor bound excitons, neutral acceptor donor excitons, two electron satellites, and donor acceptor pairs, respectively. These peaks were red-shifted, and their intensity decreased with increasing temperature. The binding energy of the donor was calculated as 43.1 meV by using the Haynes rule. Further, the value α and β, factors in the equation for the energy of localized excitons of donors and acceptors were obtained as 0.73 meV and 750 K, respectively, by fitting the free exciton (FX) peak according to Varshni’s equation. The full width at half-maximum of PL for the films was about 95.1 meV at room temperature; moreover, the following values were obtained for the films by using theoretical equations: the background impurity broadening, Γ0 = 62 meV, the parameter describing exciton-LO phonon interaction, Γ LO = 80 meV, LO phonon energy, ħω LO = 72 meV, and, the coupling strength of an exciton-acoustic phonon interaction, γ ph = 0.087 meV/K Furthermore, the activation energy was about 60.1 meV.