Low-temperature Spectra Research Articles

Photoluminescence and X-Ray Photoelectron Spectroscopy of p-Type Phosphorus-Doped ZnO Films Prepared by MOCVD

Reproducible p-type phosphorus-doped ZnO (p-ZnO:P) films are prepared on semi-insulating InP substrates by metal-organic chemical vapour deposition technology. The electrical properties of these films show a hole concentration of 9.02 × 10 17 cm−3, a mobility of 1.05 cm2/Vs, and a resistivity of 6.6 Ω · cm. Obvious acceptor-bound-exciton-related emission and P-induced zinc vacancy (VZn) emission are observed by low-temperature photoluminescence spectra of the films, and the acceptor binding energy is estimated to be about 125 meV. The local chemical bonding environments of the phosphorus atoms in the ZnO are also identified by x-ray photoelectron spectra. Our results show direct experimental evidence that PZn–2VZn shallow acceptor complex most likely contributes to the p-type conductivity of ZnO:P films.

Modeling of band shapes in the low-temperature molecular liquid spectra affected by resonance dipole–dipole interaction

The band shape theory of the vibrational transitions accompanied by an excitation of a strongly IR-active fundamental is elaborated for low-temperature molecular liquids with an account for strong intermolecular resonance dipole–dipole interactions. The developed model of a liquid uses the regular (fcc structure) crystal whose structure is then destroyed by introducing random, normally distributed displacements of particle positions, by assigning arbitrary molecular orientations as well as a varied number of randomly distributed vacancies. This model was applied to several molecular liquids ( CF 4 , SiF 4 , SF 6 , CF 3 Cl, and CF 3 Br ) , those having triply degenerate modes were studied more in detail.

Structural and photoluminescence of Mn-doped ZnO single-crystalline nanorods grown via solvothermal method

Mn-doped ZnO nanorods (composition: Mn 0.046Zn 0.954O) were grown by a simple solvothermal technique. The morphological, structural and optical properties of the as-prepared nanorods were investigated by means of transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and photoluminescence. The results showed that the sample had rod-like morphology and that the preferential growth direction was along the c-axis. The doping of Mn 2+ ions has significant influences on the optical properties of ZnO nanorods. Low-temperature photoluminescence spectra of Mn-doped ZnO nanorods exhibit a strong near band-edge emission at 370.8 nm and a weak deep-level emission around 525 nm, indicating the concentrations of the defects responsible for deep-level emissions are reduced on doping. The dynamic process of the UV emission for Mn-doped ZnO nanorods was discussed.

Magnetic Circular Dichroism Study of a Dicobalt(II) Complex with Mixed 5- and 6-Coordination: A Spectroscopic Model for Dicobalt(II) Hydrolases

The magnetic circular dichroism (MCD) study of [Co(2)(mu-OH)(mu-Ph(4)DBA)(TMEDA)(2)(OTf)], in which Ph(4)DBA is the dinucleating bis(carboxylate) ligand dibenzofuran-4,6-bis(diphenylacetate) and TMEDA is N,N,N',N'-tetramethylethylenediamine, is presented. This complex serves as an excellent spectroscopic model for a number of dicobalt(II) enzymes and proteins that have both the mu-hydroxo, mu-carboxylato bridging and asymmetric 6- and 5-coordination. The low-temperature MCD spectrum of the model complex shows bands at 490, 504, and 934 nm arising from d-d transitions on the 6-coordinate Co(II) and bands at 471, 522, 572, 594, and 638 nm arising from d-d transitions on the 5-coordinate Co(II). The most intense MCD bands are at 504 and 572 nm for 6- and 5-coordinate Co(II), respectively, and these two bands are found in the MCD spectra of dicobalt(II)-substituted methionine aminopeptidase from Escherichia coli (CoCoMetAP), glycerophosphodiesterase from Enterobacter aerogenes (CoCoGpdQ), aminopeptidase from Aeromonas proteolytica (CoCoAAP), and myohemerythrin from Themiste zostericola (CoCoMyoHry). These dicobalt(II)-substituted proteins are known to have one 5- and one 6-coordinate Co(II) bridged by one or two carboxylates and either a water or a hydroxide. The uncertainty of the bridging water's state of protonation is problematic, as this is a likely candidate for the attacking nucleophile in the dimetallohydrolases. Analysis of the variable-temperature variable-field (VTVH) MCD data determined that the Co(II) ions in the model complex are ferromagnetically coupled with a J of 3.0 cm(-1). A comparison of all dicobalt(II) complexes and dicobalt(II)-substituted protein active sites with the mu-hydroxo/aqua, mu-carboxylato bridging motif reveals that J is either zero or negative (antiferromagnetic) in the mu-aqua systems and positive (ferromagnetic) in the mu-hydroxo systems. It was also determined that the Co(II) ions in CoCoAAP and CoCoMyoHry are ferromagnetically coupled, each with a J of 3.4 cm(-1), which suggests that these ions have a mu-hydroxo bridging ligand.

Identification of the QY Excitation of the Primary Electron Acceptor of Photosystem II: CD Determination of Its Coupling Environment

Low-temperature absorption and CD spectra, measured simultaneously, are reported from Photosystem II (PS II) reduced with sodium dithionite. Spectra were obtained using PS II core complexes before and after photoaccumulation of Pheo(D1)(-), the anion of the primary acceptor. For plant PS II, Pheo(D1)(-) was generated under conditions in which the primary plastoquinone was present as an anion (Q(A)(-)) and as a modified species taken to be the neutral doubly reduced hydroquinone (Q(A)H(2)). The bleaches observed upon Pheo(D1)(-) formation in the presence of Q(A)(-) are shifted to the blue compared those in the presence of Q(A)H(2). This is attributed to the influence of the charge on Q(A)(-), and this effect mirrors the well-known electrochromic effect of Q(A)(-) on the neutral pigments. The absorption bleaches induced by Pheo(D1) reduction are species dependent. Structured changes of the CD in the 680-690 nm spectral region are seen upon photoaccumulation of Pheo(D1)(-) in PS II from plant, Synechocystis and Thermosynechococcus vulcanus. These CD changes are shown to be consistent with the overall electronic assignments of Raszewski et al. [Raszewski et al. Biophys. J. 2008, 95, 105], which place the dominant Pheo(D1) excitation near 672 nm. CD changes associated with Pheo(D1) reduction are modeled to arise from the shift and intensity changes of two CD features: one predominately of Chl(D1) character, the other predominately Pheo(D2) in character. The assignments are also shown to account for the Q(Y) absorption changes in samples where the quinone is its charged (Q(A)(-)) and neutral (Q(A)H(2)) states.

Optical and transport properties of [formula omitted] single quantum well structure with digital alloy barriers

Influence of the digital alloy nanostructuring of barriers in In 0.49Ga 0.51P/In 0.49(Ga 0.6Al 0.4) 0.51P single quantum well structure on its optical and transport properties has been theoretically investigated. Calculated band structure, low-temperature absorption spectra, room-temperature material gain spectra and capture/escape times have been compared for two kinds of structures with monolithic and nanostructured barriers. It has been shown that the optical characteristics are not affected so dramatically as transport properties. The design of digital alloys has a strong influence on the capture and escape times. We found that both the gain and absorption spectra in digital alloy structure are almost insensible to embedding of the digital alloy barriers, but the escape time can be more than two times reduced depending on barrier width of the digital alloy.

Observation of quantum confinement in ZnO nanorods fabricated using a two-temperature growth method

We observed a quantum confinement effect in vertically well-aligned ultrafine ZnO nanorods using polarized excitation photoluminescence measurements. Room-temperature and low-temperature photoluminescence spectra revealed that free excitons were confined in the nanorods. The magnitude of the energy shift due to the quantum confinement in the ultrafine ZnO nanorods was 6 meV at room temperature, which corresponded to the luminescence from ZnO nanorods 12.8 nm in diameter. The diameter estimated from the spectra was comparable to the value measured from SEM images.

Syntheses, Structures, and Dynamic Properties of Dicobalt π Complexes of Diplatinum Polyynediyl Complexes

Reactions of Co2(CO)6(μ-Ph2PCH2PPh2) and polyynediyl complexes trans,trans-(C6F5)(p-tol3P)2Pt(C≡C)mPt)(Pp-tol3)2(C6F5) (PtC2mPt) are conducted in refluxing toluene. With PtC8Pt, a 1:1 adduct, trans,trans-(C6F5)(p-tol3P)2PtC≡CC2[Co2(CO)4(μ-PPh2CH2PPh2)](C≡C)2Pt(Pp-tol3)2(C6F5) (1), is isolated in 53% yield after silica gel chromatography and crystallographically characterized. The dicobalt moiety binds to a PtC≡CC≡C linkage remote from the bulky platinum endgroups, with the P−Pt−P axis of the (p-tol3P)2PtC≡CC(Co)2 moiety nearly parallel to the cobalt−cobalt bond and that of the (p-tol3P)2PPtC≡CC≡CC(Co)2 moiety roughly perpendicular. The 31P and 1H NMR spectra of 1 evidence dynamic behavior. At the low-temperature limit, three 31P signals are observed (2:1:1), corresponding to the p-tol3P ligands on one endgroup and those that have cis/trans relationships to the cobalt−cobalt bond on the other. Low-temperature 31P and 1H NMR spectra show multiple Ph2PCH2PPh2 signals, which are analyzed. In the case of PtC12...

Electro-optical characterization of Ge–Se–Te glasses

Chalcogenide bulk glasses Ge 20Se 80− x Te x for xϵ(0,15) have been prepared by systematic replacement of Se by Te. Selected glasses have been doped with Ho, Er and Pr, and samples have been characterized by transmission spectroscopy, measurements of dc electrical conductivity and low-temperature photoluminescence. Absorption coefficients have been derived from measured transmittance and estimated reflectance. Arrhenius plots of dc electrical conductivity, in the measured temperature range 300–460 K, are characterized by single activation energies roughly equal to the half of the optical gap. Activation energies deduced from Arrhenius plots reveal a systematic decrease with increasing Te content. Similarly, both absorption and low-temperature photoluminescence spectra reveal shifts of absorption edge and/or dominant luminescence band to longer wavelength due to Te → Se substitution. Samples doped with Ho and Er exhibit a strong luminescence at 1200 and 1540 nm due to 5I 6 → 5I 8 and 4I 13/2 → 4I 15/2 transitions of Ho 3+ and Er 3+ ions, respectively. Pr doped samples exhibit only a relatively weak luminescence peak at 1590 nm, which we tentatively assign to 3F 3 → 3H 4 transition of Pr 3+ ions. Absorption of the base glass luminescence at 1460 and 1520 nm has been observed at low temperature on samples doped with Pr and Er, respectively.

Anomalous magnetic ordering induced spin–phonon coupling in BiFeO 3 thin films

Low-temperature magnetic properties and Raman spectra of epitaxial BiFeO 3 (BFO) thin films, grown on (111) SrTiO 3 substrates, have been studied. For all three prominent A 1 -symmetry Raman modes (138, 170, and 214 cm −1), there was a good linear correlation between the mode-frequency softening and the square of the in-plane magnetization in the temperature range between 80 and 300 K. These observations were ascribed to the spin–phonon coupling below the Néel temperature ( T N = 643 K ). The observed anomalous enhancements in Raman intensity below 200 K reveal spin reorientation transition due to change in the magnetic spin ordering.

Photoluminescence of highly excited AlN: Biexcitons and exciton-exciton scattering

Low-temperature photoluminescence spectra of nominally undoped high quality AlN layers on SiC and Al2O3 substrates are reported. Under high excitation conditions, we observe several bands that increase superlinearly with the excitation density. Based on temperature and excitation level dependences recorded on different samples, we identify a band 36 meV below the free A-exciton transition as due to exciton-exciton scattering (P2 band) and a second band down-shifted from the A-exciton transition by 27 meV as due to biexciton recombination. The combined data yield an exciton binding energy of 48 meV.

Growth of (1 0 1¯ 3¯ ) semipolar GaN on m-plane sapphire by hydride vapor phase epitaxy

The crystalline, surface, and optical properties of the (1 0 1¯ 3¯) semipolar GaN directly grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) were investigated. It was found that the increase of V/III ratio led to high quality (1 0 1¯ 3¯) oriented GaN epilayers with a morphology that may have been produced by step-flow growth and with minor evidence of anisotropic crystalline structure. After etching in the mixed acids, the inclined pyramids dominated the GaN surface with a density of 2×10 5 cm −2, revealing the N-polarity characteristic. In the low-temperature PL spectra, weak BSF-related emission at 3.44 eV could be observed as a shoulder of donor-bound exciton lines for the epilayer at high V/III ratio, which was indicative of obvious reduction of BSFs density. In comparison with other defect related emissions, a different quenching behavior was found for the 3.29 eV emission, characterized by the temperature-dependent PL measurement.

Intramolecular Charge Transfer in 5-Methoxy-2-(2-pyridyl)thiazole-Derived Fluorescent Molecules with Different Acceptor or Donor Substituents

On the basis of the rational derivation of 5-methoxy-2-(2-pyridyl)thiazole (2-MPT), we synthesized a new series of charge-transfer-based fluorescent molecules bearing different electronic donors or acceptors. The substituents range from strong electronic donors (e.g., amino and hydroxyl groups) to weak donating groups (e.g., proton and methyl groups) and electronic acceptors (e.g., pyridine ring). Through systematic investigation on the substituent-/polarity-dependent spectra (including room-temperature absorption, room-/low-temperature steady-state fluorescence spectra, and transient fluorescence lifetime characterization) and theoretical calculations, the emission properties of MPT derivatives are found to be governed by the rotation of the substituent around the triple bond axis, which produces distinct intramolecular charge transfer processes in either the twisted or planar excited states. The interconversion of excited-state geometry triggered by local interactions in polar solvents may produce a bathochromic shift of approximately 100 nm in fluorescence spectra. The solution state may also affect the ground- and excited-state conformation and hence results in the solvent-frozen-point sensitive fluorescence for some of the as-prepared molecules.

Deep levels in high resistivity GaN detected by thermally stimulated luminescence and first-principles calculations

Thermally stimulated luminescence spectroscopy has been applied to study the deep centres in unintentionally doped high resistivity GaN epilayers grown by the metal organic chemical vapour deposition method on c-sapphire substrates. Two trap states with activation energies of 0.12 and 0.62 eV are evaluated from two luminescence peaks at 141.9 and 294.7 K in the luminescence curve. Our spectroscopy measurement, in combination with more accurate first-principles studies, provided insights into the microscopic origin of these levels. Our investigations suggest that the lower level at 0.12 eV might originate from CN, which behaves as a hole trap state; the deeper level at 0.62 eV can be correlated with VGa that corresponds to the yellow luminescence band observed in low-temperature photoluminescence spectra.

Impurity scattering induced excitonic polariton damping and its influence on the reflectance spectra of GaN epilayers

The low-temperature reflectance spectra of a number of GaN epilayers with various impurity/defect concentrations and Hall mobility are systematically investigated in this paper. The interacting excitonic polariton model, considering both excitons A and B, is employed to reproduce the measured reflectance spectra of the GaN films, leading to the determination of the damping coefficients of the propagating polaritons in the films. It is shown that the impurity scattering plays a dominant role in the damping of the excitonic polaritons in the GaN epilayers with higher impurity concentrations at low temperatures, which is additionally supported by the photoluminescence and Hall measurements.

Nitrogen function of aluminum-nitride codoped ZnO films deposited using cosputter system

AlN codoped ZnO films were deposited on sapphire substrates at low temperature using a cosputter system under various N2/(N2 + Ar) flow ratios. To investigate the nitrogen function, the ratio of nitrogen ambient was varied during cosputtering. AlN codoped ZnO films with various crystallographic structures and bonding configurations were measured. With an adequate nitrogen atmosphere deposition condition and postannealing temperature at 450 °C, the p-type conductive behaviors of AlN codoped ZnO films were achieved due to the formation of Zn–N bonds. According to the low-temperature photoluminescence spectra, the binding energy (EA) of 0.16 eV for N acceptors can be calculated. Using time-resolved photoluminescence measurement, the carrier lifetime in AlN codoped ZnO films increases due to the reduction of oxygen vacancies caused by the occupation of adequate nitrogen atoms.

Increase in the rate of photosynthetic linear electron transport in cyanobacteruim Synechocystis sp. PCC 6803 lacking phycobilisomes

Compensating changes in the pigment apparatus of photosynthesis that resulted from a complete loss of phycobilisomes (PBS) were investigated in the cells of a PAL mutant of cyanobacterium Synechocystis sp. PCC 6803. The ratio PBS/chlorophyll calculated on the basis of the intensity of bands in the action spectra of photosynthetic activity of two photosystems in the wild strain was 1: 70 for PSII and 1: 300 for PSI. Taking into consideration the number of chlorophyll molecules per reaction center in each photosystem, these ratios could be interpreted as association of PBS with dimers of PSII and trimers of PSI as well as greater dependence of PSII as compared with PSI on light absorption by PBS. The ratio PSI/PSII determined by photochemical cross-section of the reactions of two photosystems was 3.5: 1.0 for wild strain of Synechocystis sp. PCC 6803 and 0.7: 1.0 for the PAL mutant. A fivefold increase in the relative content of PSII in pigment apparatus corresponds to a 5-fold increase in the intensity of bands at 685 and 695 nm as related to the band of PSI at 726 nm recorded in low-temperature fluorescence spectrum of the PAL mutant. Inhibition of PSII with diuron resulted in a pronounced stimulation of chlorophyll fluorescence in the PAL mutant as compared to the wild strain of Synechocystis sp. PCC 6803; these data suggested an activation of electron transfer between PSII and PSI in the mutant cells. Thus, the lack of PBS in the mutant strain of Synechocystis sp. PCC 6803 was compensated for by the higher relative content of PSII in the pigment apparatus of photosynthesis and by a rise in the rate of linear electron transport.

Optical Two-Dimensional Fourier Transform Spectroscopy of Semiconductor Quantum Wells

Coherent light-matter interactions of direct-gap semiconductor nanostructures provide a great test system for fundamental research into quantum electronics and many-body physics. The understanding gained from studying these interactions can facilitate the design of optoelectronic devices. Recently, we have used optical two-dimensional Fourier-transform spectroscopy to explore coherent light-matter interactions in semiconductor quantum wells. Using three laser pulses to generate a four-wave-mixing signal, we acquire spectra by tracking the phase of the signal with respect to two time axes and then Fourier transforming them. In this Account, we show several two-dimensional projections and demonstrate techniques to isolate different contributions to the coherent response of semiconductors. The low-temperature spectrum of semiconductor quantum wells is dominated by excitons, which are electron-hole pairs bound through Coulombic interactions. Excitons are sensitive to their electronic and structural environment, which influences their optical resonance energies and line widths. In near perfect quantum wells, a small fluctuation of the quantum well thickness leads to spatial localization of the center-of-mass wave function of the excitons and inhomogeneous broadening of the optical resonance. The inhomogeneous broadening often masks the homogeneous line widths associated with the scattering of the excitons. In addition to forming excitons, Coulombic correlations also form excitonic molecules, called biexcitons. Therefore, the coherent response of the quantum wells encompasses the intra-action and interaction of both excitons and biexcitons in the presence of inhomogeneous broadening. Transient four-wave-mixing studies combined with microscopic theories have determined that many-body interactions dominate the strong coherent response from quantum wells. Although the numerous competing interactions cannot be easily separated in either the spectral or temporal domains, they can be separated using two-dimensional Fourier transform spectroscopy. The most common two-dimensional Fourier spectra are S(I)(omega(tau),T,omega(t)) in which the second time period is held fixed. The result is a spectrum that unfolds congested one-dimensional spectra, separates excitonic pathways, and shows which excitons are coherently coupled. This method also separates the biexciton contributions and isolates the homogeneous and inhomogeneous line widths. For semiconductor excitons, the line shape in the real part of the spectrum is sensitive to the many-body interactions, which we can suppress by exploiting polarization selection rules. In an alternative two-dimensional projection, S(I)(tau,omega(Tau),omega(t)), the nonradiative Raman coherent interactions are isolated. Finally, we show S(III)(tau,omega(Tau),omega(t)) spectra that isolate the two-quantum coherences associated with the biexciton. These spectra reveal previously unobserved many-body correlations.

Energy levels and emission parameters of theDy3+ ion dopedinto the YPO4 host lattice

Single crystals of Dy3+-doped YPO4 havebeen grown from Pb2P2O7 flux and investigated by optical spectroscopy techniques. The energy level scheme of theactive ion has been deduced from the low temperature spectra and reproduced by means ofa crystal-field calculation. The room temperature absorption spectra have been analysed inthe framework of the Judd–Ofelt approach, and the results of this analysis havebeen applied in a discussion concerning the spectral composition of the visibleluminescence.

A small methanoato ligand in the structural differentiation of copper(II) complexes

Several copper(II) methanoato complexes, namely mononuclear [Cu(O 2CH) 2(2-mpy) 2] ( 1) (2-mpy = 2-methylpyridine), binuclear [Cu 2(μ-O 2CH) 4(2-mpy) 2] ( 2), and the polynuclear {[Cu(μ-O 2CH) 2(2-mpy) 2][Cu 2(μ-O 2CH) 4]} n ( 3) and {Na 2[Cu(μ-O 2CH) 2(O 2CH) 2][Cu 2(μ-O 2CH) 4]} n ( 4), have been synthesized. The mononuclear complex 1 is formed by two asymmetric chelate methanoate anions and two 2-methylpyridine molecules, giving a highly distorted ‘elongated octahedral’ coordination sphere. Complex 1 decomposes outside the mother-liquid, transforming into a regular isolated binuclear paddle-wheel complex 2 with four intra-binuclear bridging methanoates and two axial 2-mpy ligands. The polynuclear complex 3 is formed of alternate mononuclear and binuclear building blocks resembling the central cores of 1 and 2, but with significant differences, especially for the methanoates of the mononuclear units. The oxygen atom of the mononuclear unit in the octahedral axial position in 3 is simultaneously coordinated to the axial position of the binuclear paddle-wheel central core, thus enabling a chain type of structure. A chain of alternate mononuclear and binuclear building blocks, as in the neutral compound 3, are found as well in the ionic polymeric compound 4, though two types of bridges are found in 4, while there is only one type in 3. Namely, the axial position of the octahedral mononuclear unit in 4 is occupied by the methanoate oxygen atom that is already a part of the binuclear paddle-wheel unit, while one equatorial methanoate from the mononuclear unit serves as a triatomic bridge to the axial position of the binuclear building block. A very strong antiferromagnetic interaction is found for all the complexes with the paddle-wheel building blocks [Cu 2(μ-O 2CH) 4] 2– 4 (−2 J = 444–482 cm −1), attributed to the methanoate intra-binuclear bridges. On the other hand, this strong antiferromagnetism, found already at room temperature, reduces the intensity of the EPR S = 1 spin signals reported for the isolated paddle-wheel complex 2. For the polymeric 3, only the spin S = ½ signals are found in the EPR spectra, and they are assigned to the mononuclear building blocks. No signals with a clear origin are however seen in the room temperature EPR spectrum of the polymeric analogue 4, only the S = ½ signals in the low temperature spectra. This feature is suggested to be due to a specific influence between the adjacent S = 1 (binuclear) and S = ½ (mononuclear) species via their bridges.