Additional Absorption Band Research Articles

Surface-engineered bacterial cellulose as template for crystallization of calcium phosphate

Bacterial cellulose (BC), produced by Acetobacter xylinum, and cotton linters as reference were surface modified by ozone-induced graft polymerization of acrylic acid and used as a template for crystallization of calcium phosphate. The grafting was verified using attenuated total reflection-infrared radiation (ATR-IR) and electron spectroscopy for chemical analysis (ESCA). ATR-IR revealed an additional absorption band at 1700 cm−1, corresponding to the carbonyl group in polyacrylic acid. ESCA figures show, apart from the characteristic peaks for cellulose, additional peaks at 285 eV and 289 eV that correspond to groups in acrylic acid. The grafting yield is higher on cotton linters compared with BC, which most likely has to do with differences in crystallinity and reactivity of the different cellulose materials. No morphology difference directly caused by grafting could be seen with scanning electron microscopy (SEM), which might indicate that acrylic acid was grafted as a thin film on the surface of the cellulose micro fibrils. Calcium phosphate was formed on the surface-modified cellulose by first pre-soaking the materials in a saturated Ca(OH)2 and later in simulated body fluid (SBF). The atomic ratio of calcium phosphate was determined by ESCA to be about 1.5 for the different materials. Energy dispersive spectroscopy (EDS) was used to map and verify that the crystals were calcium phosphate. Secondary ion mass spectroscopy (SIMS) was also used to verify the presence of calcium phosphate complex onto BC. SEM images showed the difference in dimension, distribution and morphology of the crystals depending on the materials. Smaller and a greater number of crystals were obtained on the surface-modified BC and larger and fewer crystals on surface-modified cotton linters. Structural and grafting differences between the celluloses may lead to differences in nucleation sites and possibly differences in the morphology of the Ca-P crystals. The BC–calcium phosphate composite is expected to be useful as a scaffold for bone tissue regeneration.

Structure and spectroscopic properties of (AA′)(BB′)O3mixed-perovskite crystals

Five different mixed-perovskite (AA′)(BB′)O3single crystals were grown, where A = La, Nd; A′ = Sr; B = Al, Ga; and B′ = Ta, Nb. The as-grown crystals were yellowish/brownish. After annealing in air, the coloration was more intense. Annealing in a reducing atmosphere decreased coloration. The crystals were investigated by transmission spectroscopy, electron spectroscopy for chemical analysis (ESCA), and single-crystal x-ray diffraction. Additional broad absorption bands in the transmission spectra were observed for the as-grown samples. They are in line with the changes of the shape of O(1s) ESCA peaks. Redundant interstitial oxygen ions were recognized as the reason for the crystal coloration. All structures were solved and refined in different space groups of the regular system. Some of the unit cells have a doubled lattice constant: (i) lanthanum strontium gallium niobate,Pm3m,3.9323(5) Å, at 100 K, 3.9270(5) Å; (ii) neodymium strontium aluminum tantalate,Pm3m,3.8353(4) Å; (iii) lanthanum strontium aluminum tantalate,Pn3m,7.720(1) Å, annealed in reducing atmosphere, 7.708(1) Å; (iv) neodymium strontium aluminum niobate,Fm3m,7.744(4) Å.

The synthesis and electronic absorption spectra of 3-phenyl-3(4-pyrrolidino-2-substituted phenyl)-3 H-naphtho[2,1- b]pyrans: further exploration of the ortho substituent effect

Introduction of a substituent into a sterically demanding 2-position of a 3-(4-pyrrolidinophenyl) ring of a 3,3-diaryl-3 H-naphtho[2,1- b]pyran results in the generation of an additional short wavelength absorption band leading to organic photochromes that appear as dull shades of orange and red.

Color centers and charge state recharge in γ-irradiated Yb:YAP

Gamma-ray irradiation induced color centers and charge state recharge of impurity and doped ion in 10 at.% Yb:YAP have been studied. The change in the additional absorption (AA) spectra is mainly related to the charge exchange of the impurity Fe2+, Fe3+ and Yb3+ ions. Two impurity color center bands at 255 and 313 nm were attributed to Fe3+ and Fe2+ ions, respectively. The broad AA band centered at 385 nm may be associated with the cation vacancies and F-type center. The transition Yb3+ -> Yb2+ takes place in the process of gamma-irradiation. Oxygen annealing and gamma-ray irradiation lead to an opposite effect on the absorption properties of the Yb:YAP crystal. In the air annealing process, the transition Fe2+ -> Fe3+ and Yb2+ -> Yb3+ take place and the color centers responsible for the 385 nm band was destroyed. (c) 2005 Elsevier B.V. All rights reserved.

A TDDFT study of the low-lying excitation energies of polycyclic cinnolines and their carbocyclic analogues

The low-lying excited states of 21 compounds of polycyclic cinnoline, monoaza-hydrocarbon and their corresponding polycyclic aromatic hydrocarbon (PAH) analogues have been investigated in the framework of time-dependent density functional theory (TDDFT). The gradient corrected BLYP, hybrid B3LYP and B3P86 functionals together with 6-31G (d) basis set have been used. The hybrid-type B3LYP and B3P86 systematically overestimate the excitation energies for states with dominating ionic character (corresponding to group III band), with a mean absolute deviation (MAD) of 0.33 eV (B3LYP) and 0.34 eV (B3P86), respectively. However, they can accurately predict the excitation energies for states with covalent character (corresponding to group II and I bands). The MAD for group II (B3LYP: 0.05 eV; B3P86: 0.05 eV) and I bands (B3LYP: 0.12 eV; B3P86: 0.12 eV) are significantly smaller. The BLYP outperforms B3LYP and B3P86 for group III band (MAD: 0.09 eV), but has a worse performance for group II (MAD: 0.15 eV) and I (MAD: 0.13 eV) bands. Comparison of the lowest-lying excited states for polycyclic cinnolines with those of the corresponding PAH analogues, the first excited states of polycyclic cinnolines mainly result from n→π 0* transitions. Therefore, in non-polar solvents, the spectra of some polycyclic cinnolines exhibit an additional absorption band at longer wavelengths.

Electrochemical and optical characterization of p- and n-doped poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene

We study electrochemical p- and n-type doping in the well-known light-emitting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). Doping reactions are characterized using cyclic voltammetry. Optical measurements including photoluminescence and UV/Vis/NIR transmission were performed on doped samples. We find that oxidation in MEH-PPV is a highly reversible reaction resulting in stable freestanding doped films, while the reduced form is unstable and the reaction irreversible. We discuss the dependence of doping reactions on scan rate, film thickness, salt type and concentration, and working electrode type. We observe the development of two additional broad absorption bands in both lightly and heavily doped films accompanied by a slight blueshift in the primary optical transition, suggesting bipolaron band formation. Finally we find that both p and n dopings result in extremely sensitive photoluminescence quenching. We propose a physical model for understanding electrochemical doping in MEH-PPV and the implications this has on the development of such technologies as polymer light-emitting electrochemical cells, electrochromic devices, actuators, and sensors.

Temperature Dependence of Quasilocal Energy Levels in PbSe Doped with Thallium and Superstoichiometric Plumbum

Spectral dependences of the absorption coefficient in p-PbSe doped with thallium and superstoichiometric plumbum impurity are investigated within the temperature interval 96–600 K. Additional absorption bands, which are associated with the presence of thallium and chalcogen vacancies are revealed and identified. The energies of optical ionization of thallium and chalcogen vacancy are estimated. These are shown to increase with the temperature, while the energy of thermal ionization of thallium, according to the data on the transport phenomena studies, is decreased with increasing temperature. Probable reasons for different signs at the temperature-dependent rate of variation in the energy of optical and thermal ionization of thallium in plumbum chalcogenides are discussed.

Effect of surface temperature on the concentration of C60/C70 molecules in dc arc discharge fullerene generator

Synthesis of C60, C70 and higher fullerenes was carried out at Pakistan Institute of Nuclear Science and Technology (PINSTECH) using dc arc discharge technique. High-performance liquid chromatography (HPLC) and UV–vis spectroscopy are employed to characterize the fullerenes. The toluene extracted fullerene solutions of the raw soot collected from four different regions (A–D) of dc arc discharge fullerene generator were compared with pure C60 toluene solution. Due to poor solubility of C60 in HPLC grade methanol, different fractions of methanol as a mobile phase with toluene were tried. The mobile phase composition of 80:20 methanol:toluene was found an optimal ratio to achieve the best separation factor of 1.246 between C60 and C70 fullerenes. Quantitative concentration is calculated from the HPLC and UV–vis spectra. The molarity of C60 carbon molecules for all the regions is calculated from the HPLC spectra by comparing the spectra of known molar solution of C60. The maximum concentration of C70 is found to be 21% in the region B that is nearer the arc zone and at a higher temperature than other regions. Both the techniques confirm the presence of C60, C70 and higher fullerenes in all the four regions with variation in concentration of C60 and C70. UV–vis spectra of the region C show the additional absorption bands at 285 and 305nm.

Spectroelectrochemical Properties of Some Polymers of Polythiophene Series

The spectroelectrochemical behavior in situ of some polymers of polythiophene series, polybithiophene (PBT) and poly-3-phenylthiophene (P3PhT), is studied in the processes of anodic and cathodic doping. It is confirmed that in the case of PBT and P3PhT, the cathodic evolution of the absorption spectra, which is characteristic of the cathodic doping process, occurs in solutions of tetrabutylammonium salts, but not in solutions of lithium salts. It is shown that polymers of a more complicated structure than that of PBT, e.g. P3PhT, feature additional absorption bands, as compared to PBT, which demonstrates a single absorption band. Moreover, in the case of P3PhT, two color transitions can be observed by sight in the process of doping-undoping, unlike the single transition observed in the case of PBT and most other electron-conducting polymers. On the basis of quantum-mechanical calculations, an assumption is made that such a behavior of P3PhT is due to the decrease of the local symmetry of polythiophene chain in the presence of a bulky substituent, which causes the activation of the transitions in the electronic spectra that are forbidden in more symmetrical structures.

Two‐Photon Absorption of Bis[4‐(N,N‐diphenylamino)phenylethynyl]arenes

Attractive chromophores for photoscience: Two-photon (TP) absorption (TPA) of ethynyl chromophores 2–5 (see picture) was studied using femtosecond excitation between 710–950 nm. Compounds 2–5 possess higher excitation energies for the TP-excited than for the one-photon (OP) excited states. Chromophore 2 exhibits an additional slight absorption band at the S0→S1 transition. This is explained by a vibronic coupling mechanism.

First-Principles Studies on the Electronic Structures andOptical Properties for the PbWO4 Crystal Containing VPb2−

The electronic structures, dielectric functions,complex refractive indices and absorption spectra for a perfectPbWO4 (PWO) crystal and the PWO crystals containing lead vacancyVPb2− have been calculated using a full-potential(linearized) augmented plane-wave (LAPW) + local orbitals (LO) methodwith the lattice structure optimized. The peaks of the absorptionspectra corresponding to the electronic transitions have been studied.The calculated results indicate that the absorption band of the perfect PWOcrystal does not occur in the visible region. However, the PWO crystalcontaining VPb2− has two additional absorption bands inthis region. The two bands can be well decomposed into fourgaussian-shape bands peaking at 350 nm, 405 nm, 550 nm and 670 nm,respectively, which coincide well with the 350 nm, 420 nm, 550 nm and680 nm absorption bands measured in PWO crystals. Therefore, it can beconcluded that the 350 nm, 420 nm, 550 nm and 680 nm absorption bands arerelated to the existence of VPb2− in the PWO crystal.

Light-Induced Changes in the Photomodulated Absorption Spectra of Quantum-Dimensional CdSe∕Dielectric Structures

The effect of additional illumination on the photomodulated absorption spectra of quantum-dimensional CdSe/dielectric structures has been studied. Using this method, it is possible to reveal additional absorption bands inside the bandgap of semiconductor nanoparticles.

Temperature dependence of polarized absorption bands in p-type CdGeAs2

The temperature and polarization behaviors of four absorption bands in p-type CdGeAs2 bulk crystals have been studied from 5 to 300 K. One band, peaking near 0.22 eV (5.5 μm), is the V2→V1 intervalence band transition, and its intensity taken with E‖c is about 2.8 times larger than that taken with E⊥c. Three additional absorption bands can be separately resolved below 200 K. A broad band peaking near 0.38 eV is present only with E‖c. A second broad band peaking near 0.52 eV is detected using E⊥c. These two bands are assigned to transitions from the top two valence bands to a deep acceptor. Infrared photoluminescence studies of a 0.35 eV emission confirm the presence of the deep acceptor. A third absorption band peaking near 0.56 eV is detected using E‖c. This band is assigned to transitions from a shallow 120 meV acceptor to shallow donor states and/or conduction band states. At room temperature, the normally observed broad absorption feature extending from the band edge to beyond 0.2 eV is a result of the superposition of the four bands. These absorption bands presently limit the use of p-type CdGeAs2 crystals as nonlinear optical materials in infrared high-power laser systems.

Roton-Rotation Coupling of Acetylene inH4e

Rotational absorption spectra of acetylene in superfluid 4He calculated using a path-integral correlation function approach are seen to result in an anomalously large distortion constant in addition to a reduced rotational constant, with values in excellent agreement with recent experiments. Semianalytic treatment of the dynamics with a combined correlated basis function-diffusion Monte Carlo method reveals that this anomalous behavior is due to strong coupling of the higher rotational states of the molecule with the roton and maxon excitations of 4He, and the associated divergence of the 4He density of states in this region.

Structural changes and valence states in the MgCr2O4?FeCr2O4 solid solution series

The influence on the structure of Fe2+ → Mg substitution was studied in synthetic single crystals belonging to the MgCr2O4–FeCr2O4 series produced by flux growth at 900–1200 °C in controlled atmosphere. Samples were analyzed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption-, infrared- and Mossbauer spectroscopy. The Mossbauer data show that iron occurs almost exclusively as IVFe2+. Only minor Fe3+ (<0.005 apfu) was observed in samples with very low total Fe. Optical absorption spectra show that chromium with few exceptions is present as a trivalent cation at the octahedral site. Additional absorption bands attributable to Cr2+ and Cr3+ at the tetrahedral site are evident in spectra of end-member magnesiochromite and solid-solution crystals with low ferrous contents. Structural parameters a0, u and T–O increase with chromite content, while the M–O bond distance remains nearly constant, with an average value equal to 1.995(1) A corresponding to the Cr3+ octahedral bond distance. The ideal trend between cell parameter, T–O bond length and Fe2+ content (apfu) is described by the following linear relations: a0=8.3325(5) + 0.0443(8)Fe2+ (A) and T–O=1.9645(6) + 0.033(1)Fe2+ (A) Consequently, Fe2+ and Mg tetrahedral bond lengths are equal to 1.998(1) A and 1.965(1) A, respectively.

X-ray and optical studies of ytterbium-doped gallium garnets

Single crystals of yttrium gallium and lutetium gallium garnets doped with ytterbium were grown from the melt by Czochralski technique. Comparison is made between the lattice spacing of ceramic and single crystals to estimate the degree of octahedral ytterbium substitution. When heated to around 1200°C under reducing atmosphere the yttrium gallium garnet crystal acquires violet color due to appearance of additional absorption bands. Displacements of absorption bands as a function of the average radius on the dodecahedral site resemble those observed in aluminum garnets and generally considered as associated with divalent ytterbium. Basing on X-ray calibration, the peak positions of induced absorption bands can be used to estimate the dodecahedral concentration of ytterbium in the crystal. Charge transfer luminescence properties were studied in LuGG:Yb crystals.

Nonlinear properties of phototropic media on the basis of CuxSe nanoparticles in quartz glass

Energy and kinetic characteristics of the induced transparency in quartz sol-gel glasses containing copper selenide nanoparticles of different stoichiometry are studied. The dependence of the nonlinear optical properties of the glass samples on the chemical composition of copper selenide particles, which gives rise to an additional absorption band in the near-infrared region and determines its spectral position, is established. It is found that the time of relaxation of the induced transparency increases and the peak absorption cross section decreases as the absorption maximum shifts to the low-energy spectral region.

In situ and ex situ FTIR characterization of a cyanate adlayer on Cu(1 1 1)

Cyanate adlayers were investigated with in situ and ex situ infrared reflection absorption spectroscopy (IRRAS) on a Cu(1 1 1)-electrode. The cyanate layers were adsorbed in 5 mM KOCN + 0.5 mM H 2SO 4 solution and measured in cyanate-free as well as in cyanate-containing sulphuric acid. The in situ IR spectra exhibit an absorption band near 2270 cm −1 of N-bound cyanate that partly appears bipolar and also shows a potential dependent frequency shift between 30 and 80 cm −1 V −1 depending on the surrounding electrolyte and the immersion potential. Further analysis of the frequency and intensity of the band as a function of the electrode potential during the IR measurements points to cyanate desorption below 0.1 V and a rearrangement of the remaining molecules from on top positions at high surface coverages to bridge or hollow adsorption sites at lower coverages. Simultaneously parts of the remaining adsorbate lie down flat on the surface as the potential and therewith the coverage decreases. Two additional absorption bands at 2169 and 2220 cm −1 are detected which are assigned to solved cyanate and a copper complex in the thin layer solution. After transfer into UHV, the system was studied by means of ex situ IRRAS and Auger electron spectroscopy (AES). The ex situ IR spectra display a similar band at around 2250 cm −1 as the in situ spectra with a clear dependence on the emersion potential. This indicates the successful transfer of the electrochemical double layer including the potential dependent occupation of different adsorption sites. AES data show that K + ions are replaced by H + when rinsing the cyanate-covered surface with dilute sulphuric acid in the electrochemical cell.

Photoluminescence Excitation Spectroscopy Studies of Anodically Etched and Oxidized Porous Zn

Photoluminescence excitation (PLE) spectroscopy studies were performed for anodically etched porous Zn, which exhibited a PL in the blue/violet spectral range peaking at 420nm (2.95eV), and oxidzed porous Zn at 380℃ for 10min and 12 h. A broad absorption band was observed at 4.07eV (305 nm), 3.49 (355 nm) for anodically etched porous Zn. In contrast, both the oxidized porous Zn and sintered ZnO exhibited an almost identical one broad absorption band at 3.85eV (322 nm), when PLE spectra were measured at 378 nm (3.28eV). The oxidized porous Zn and sintered ZnO, which displayed both UV and green luminescence band, showed an additional absorption band at 389 nm (3.19eV) and 467 nm (2.66eV). In contrast, no significant absorption band was detected for a 10-min oxidized porous Zn, which only displayed one UV luminescence void of deep-level luminescence. These absorption bands determined by PLE studies enabled a clear understanding of an emission mechanism for the UV and green luminescence from ZnO.

Phonon anomalies in trilayer high- Tc cuprate superconductors

We present an extension of the model proposed recently to account for dramatic changes below T c (anomalies) of some c-axis polarized infrared-active phonons in bilayer cuprate superconductors, that applies to trilayer high- T c compounds. We discuss several types of phonon anomalies that can occur in these systems and demonstrate that our model is capable of explaining the spectral changes occurring upon entering the superconducting state in the trilayer compound Tl 2Ba 2Ca 2Cu 3O 10. The low-temperature spectra of this compound obtained by Zetterer and coworkers display an additional broad absorption band, similar to the one observed in underdoped YBa 2Cu 3O 7− δ and Bi 2Sr 2CaCu 2O 8. In addition, three phonon modes are strongly anomalous. We attribute the absorption band to the transverse Josephson plasma resonance, similar to that of the bilayer compounds. The phonon anomalies are shown to result from a modification of the local fields induced by the formation of the resonance. The spectral changes in Tl 2Ba 2Ca 2Cu 3O 10 are compared with those occurring in Bi 2Sr 2Ca 2Cu 3O 10, reported recently by Boris and coworkers.