Narrow Results

Novel water-soluble tetraarylporphyrin-labelled polymers have been synthesized by the reaction of bromoalkyl-containing poly(N-isopropylacrylamides) with 5-(4-pyridyl)-10,15,20-tri(4-methoxyphenyl) porphyrin. Corresponding Zn-porphyrin-containing polymers have also been prepared. Phase transition diagrams show that aqueous polymer solutions exhibit phase separation upon heating. Absorption spectra of the porphyrin-labelled polymers have been recorded in water and organic solvents. It is found that the absorption bands in aqueous polymer solutions are broadened and shifted bathochromically, the peak intensity of the Soret band in the polymer spectra being strongly lowered in water. Fluorescence properties of the polymers are briefly reported.

In this work, a detailed study of the structural, electronic and absorption properties of crystalline 2,6-dimethyl-4-(diphenylmethylene)-2,5-cyclohexadienone with $\alpha$ form ($\alpha$-DDCD) in the pressure range of 0–250GPa is performed by density-functional theory (DFT) calculations. The particular analysis of the variation tendencies of the lattice constants, bond lengths and bond angles under different pressures shows that there occur complex transformations in $\alpha$-DDCD under compression. In addition, it can be see that the $b$-direction is much stiffer than the $a$- and $c$-axes in the structure of $\alpha$-DDCD, suggesting the compressible crystal of $\alpha$-DDCD has anisotropy. Then, by analyzing the bandgap and density of states (DOS) of $\alpha$-DDCD, it is found that the crystal undergoes a phase transformation from semiconductor to metal at 90GPa and it becomes more sensitive under compression. Besides, in the pressure range 110–170GPa, repeated transformations between metal and semiconductor occur four times, suggesting the structural instability of $\alpha$-DDCD in this pressure range. Finally, the relatively high optical activity with the pressure increases of $\alpha$-DDCD is seen from the absorption spectra, and two obvious structural transformations are also observed at 130GPa and 140GPa, respectively.

In this work, we use density functional theory (DFT) calculations to study the structural, electronic and absorption properties of crystalline 2-benzylidene-1-indanone (signed as 2-BI) in the pressure range of 0–300GPa. The detailed analysis of the variation tendencies of the lattice constants, bond lengths and bond angles with increasing pressures shows that there occur several transformations in 2-BI under different pressures. In addition, it can be see that the $a$- and $c$-axis are much stiffer than the $b$-axis in the structure of 2-BI, suggesting the crystal is anisotropic. Then, the analysis of the band gap and DOS (PDOS) of 2-BI indicate that its electronic character has changed at 120GPa into metal phase, but then transfer into excellent insulator at 230GPa. Moreover, the relatively high optical activity with the increasing pressure of 2-BI is seen from the absorption spectra, and three obvious structural transformations are also observed at 60, 120 and 250GPa, respectively.

In this work, a detailed study of the structural, electronic and optical absorption properties of crystalline benzoic acid in the pressure range of 0–300GPa is performed by density functional theory (DFT) calculations. We found that occur complex transformations in benzoic acid under compression occurs, by analyzing the variation tendencies of the lattice constants, bond lengths and bond angles under different pressures. In the pressure range 0–280GPa, repeated formations and disconnections of hydrogen bonds between H1(P1) atom and O1(P1), O2(P4-$x$-$y$-$z$) atoms occur several times, and a new eight-atom ring (benzoic acid dimer) forms at 100GPa and 280GPa. Then, by analyzing the band gap and density of states (DOS) of benzoic acid, it is found that the crystal undergoes a phase transformation from insulator to semiconductor at 240GPa and it even becomes metal phase at 280GPa. In addition, the relatively high optical activity with the pressure increases of benzoic acid is seen from the absorption spectra, and three obvious structural transformations are also observed at 110, 240 and 290GPa, respectively.

In this work, a detailed study of the structural, electronic and absorption properties of crystalline 1-ethyl-1,4-dihydro-7-methyl-4-oxo-1,8-naphthyridine-3-carboxylic acid (nalidixic acid) in the pressure range 0–300GPa is performed by density functional theory (DFT) calculations. The detail analysis of the variation tendencies of the lattice constants, bond lengths and bond angles with increasing pressures shows that complex transformations occur in nalidixic acid under compression. In addition, it can be see that the $a$- and $c$-axes are much stiffer than the $b$-direction in the structure of nalidixic acid, suggesting the crystal is anisotropic. In the pressure range 90–250GPa, repeated formations and disconnections of covalent bonds between C6 (P1 or P4) and O1 (P4 or P1) occur several times, and a new eight-atom ring forms at 90, 160, 190 and 230GPa, respectively. Then, the analysis of the bandgap and density of states (DOS) of nalidixic acid indicates that its electronic character changes at 230GPa into an excellent insulator, but the electron transition is much easier at several pressure regions for the bandgap closing to 0eV. Moreover, as the pressure increases relatively high optical activity of nalidixic acid is seen from the absorption spectra, and two obvious structural transformations are also observed at 200 and 230GPa, respectively.

In this paper, the structural, electronic and absorption properties of 2,2′-iminobis (acetamide oxime) (IBO) under pressure of 0–300GPa are calculated by the density functional theory (DFT) calculations. Analysis of the variation trend of lattice constant, bond length and bond angle of IBO under compression conditions, shows there are complex transformations under different pressure. In addition, it is found that the structure of IBO in the $a$-axis is stiffer than that along the $b$- and $c$-axes, which indicates that the crystal has anisotropic compressibility. By analyzing the band structure and the density of states of IBO, it is seen that at 120GPa, the electronic structure of IBO changes into metallic system, and becomes more sensitive under compression conditions. The transition between metal and semiconductor occurs again at 150Gpa. Finally, at 180GPa, the crystal transforms into metal again. The three obvious phase transitions indicate that the structure of IBO becomes more unstable with the increase of pressure. The absorption spectra show that with the increase of pressure, the optical activity of IBO crystal grows higher, and three obvious structural transitions are, respectively, observed at 120, 150 and 180GPa.

In this work, the structural, electronic and absorption properties of 2-methyl-2H-naphtho-[1,8-de]triazine in the pressure ranges of 0–250GPa are studied in detail (hereinafter referred to as 2-methyl crystal). Density functional theory (DFT) is used to calculate the lattice constants, bond lengths and bond angles of 2-methyl under different pressures. The results show that the crystals undergo complex transformations under compression, and the major structural transformations occur at pressures of 90GPa and 210GPa with repeated formations and disconnections. In addition, the $a$- and $c$-directions of the 2-methyl are stiffer than the $b$-direction, which indicates that the compressibility of the crystal is anisotropic. From the specific analysis of the bandgaps of 2-methyl, we can know that the crystal is converted from semiconductor to metal at 90GPa. The absorption spectrum of the crystal also indicates that 2-methyl has a relatively high optical activity with the increasing pressure.

Te atoms were loaded into nanochannels of zeolite AFI single crystals. The polarized absorption and Raman scattering spectra were measured. The absorption spectra of Te loaded AFI showed large anisotropic properties. The absorption spectra of E//c deeply depended on the loading density of Te atoms. In low Te loading density, Te was adsorbed into the nanochannels in a dimer form whose absorption peak appeared at ca. 3.5 eV. In high loading density, Te form one dimensional nanowires inside the channels. Nanowire formation causes large anisotropic absorption and changes in the spectral shape.

The optical properties of metal-free phtalocyanine (H₂Pc) in thin film form is investigated. X-ray diffractograms of the (H₂Pc) powder show that it has a α-polycrystalline form with a monoclinic structure. The thermal evaporation of (H₂Pc) powder leads to α-polycrystalline films, oriented preferentially to the (001) plane. After annealing at 623 K for two hours, a mixture of α- and β-phases is formed on an amorphous background. The optical properties of (H₂Pc) thin films have been studied using spectrophotometeric measurements of transmittance and reflectance in the range 200–2200 nm for a different film thickness. The refractive index and the absorption index show independence on the film thickness. The refractive index shows anomalous dispersion in the region of the fundamental absorption edge. The absorption measurements show the characteristic splitting of the Q-band and ΔQ is obtained as 0.22 eV. The absorption coefficient in the absorption region reveals indirect transitions. The fundamental and the onset energy gap are determined as 2.74 eV and 1.55 eV, respectively.

The fundamental absorption edge of SnO₂ amorphous thin films has been investigated. It has been observed that the optical energy gap decreases with the increase in film thickness, substrate temperature and post deposition annealing. The results are analysed by assuming optical absorption by non-direct transition. The decrease in optical band gap with increase in film thickness may be interpreted in terms of the incorporation of oxygen vacancies in the SnO₂ lattice. The decrease in optical energy due to the increase in substrate temperature may be ascribed to the release of trapped electrons by thermal energy or by the outward diffusion of the oxygen-ion vacancies, which are quite mobile even at low temperatures. The decrease in optical band gap due to annealing may be due to the formation of tin species of lower oxidation state.

Zinc oxide (ZnO) thin films were deposited onto glass substrates by d.c. magnetron sputtering. The structural analysis, by X-ray diffraction and atomic force microscopy, indicate that the studied films are polycrystalline and have a wurtzite (hexagonal) structure. The film crystallites are preferentially oriented with (002) planes parallel to the substrates.

The mechanism of electronic transport is explained in terms of Seto's model elaborated for polycrystalline semiconducting films (crystallite boundary trapping theory). Some parameters of used model (impurity concentration, density and energy of the trapping states, etc.) have been calculated. The optical bandgap (E_g0 = 3.28–3.37 eV) was determined from absorption spectra.

Bandgap tailoring of $\beta$-Si₃N₄ is performed by single and co-doping by using density functional theory (DFT) of PBE functional and plane-wave pseudopotential method. The results reveal that a direct bandgap transfers into an indirect one when single-doped with As element. Also, a considerate decrease of bandgap to 0.221 eV and 0.315 eV is present for Al–P and As–P co-doped systems, respectively, exhibiting a representative semiconductor property that is characteristic for a narrower bandgap. Compared with other doped systems, Al-doped system with formation energy of 2.67 eV is present for a more stable structure. From charge density difference (CDD) maps, it is found that the blue area between co-doped atoms increases, illustrating an enhancement of covalent property for Al–P and Al–As bonds. Moreover, a slightly obvious “Blue shift” phenomenon can be obtained in Al, Al–P and Al–As doped systems, indicating an enhanced capacity of responses to light, which contributes to the insight for broader applications with regard to photoelectric devices.

The effect of intrinsic point defects on the electronic structure and absorption spectra of ZnO was investigated by first-principle calculation. Among the intrinsic point defects in ZnO, oxygen vacancies $({\mathrm{\text{V}}}_{\mathrm{\text{O}}})$ and interstitial zinc $({\mathrm{\text{Zn}}}_i)$ have the lower formation energy and the more stable structure under zinc(Zn)-rich condition, whereas zinc vacancies $({\mathrm{\text{V}}}_{\mathrm{\text{Zn}}})$ and interstitial oxygen $({\mathrm{\text{O}}}_i)$ have the lower formation energy and the more stable structure under oxygen(O)-rich condition. The band gap of ${\mathrm{\text{Zn}}}_{36}{\mathrm{\text{Zn}}}_i{\mathrm{\text{O}}}_{36}$ becomes narrow and the absorption spectrum has a redshift. In the visible region, the photo-excited electron transition of ${\mathrm{\text{Zn}}}_{36}{\mathrm{\text{O}}}_{35}$ is graded from the valence band top to the impurity level and then to the conduction band bottom, showing the redshift of absorption spectrum of ${\mathrm{\text{Zn}}}_{36}{\mathrm{\text{O}}}_{35}$ and explaining the reason of ${\mathrm{\text{V}}}_{\mathrm{\text{O}}}$ forming a deep impurity levels in ZnO. Moreover, the impurity energy level of ${\mathrm{\text{Zn}}}_{36}{\mathrm{\text{O}}}_{35}$ coincides with the Fermi level, indicating the significant trap effect and the slow recombination of electrons and holes, which are conducive to the design and preparation of novel ZnO photocatalysts. The band gap of ${\mathrm{\text{Zn}}}_{35}{\mathrm{\text{O}}}_{36}$ and ${\mathrm{\text{Zn}}}_{36}{\mathrm{\text{O}}}_i{\mathrm{\text{O}}}_{36}$ broadened and the absorption spectrum showed blueshift, explaining the different values of the ZnO band gap width.

The 0.1 mol% europium (Eu³⁺) activated lanthanum oxychloride nanophosphor was prepared by the liquid-phase method. The X-ray diffraction analysis of the prepared compound exhibits the tetragonal structure having the particles size in the range of 18–21 nm. The band gap of Eu³⁺ doped LaOCl phosphor was found to be around 5.2 eV using absorption and diffuse reflectance measurements. The photoluminescence (PL) emission spectra show the bright emission in orange–red region from 580 nm to 630 nm. The most intense emission peak at 621 nm was due to transition in energy levels ⁵D₀ → ⁷F₂ of Eu³⁺ ions. The absorption and stimulated emission cross-section of the intense peak were found to be 4.135 × 10^-20 cm² and 8.9 × 10^-20 cm² respectively. The wide band gap and high stimulated emission cross-section of prepared nanophosphor makes its possible applications as red emitting phosphor in display devices and laser system.

Azo dye ethyl red (ER) doped methyl methacrylate polymer (PMMA) films were prepared by the sol-gel process and the spin coating method. The samples were 3% ER by weight and 33 μm thick. The absorption spectra of the samples, illuminated by He-Cd laser (441.6 nm, CW) and YAG laser (532 nm, CW) were measured and the change of nonlinear refractive index distribution curve of the ER-doped PMMA thin film induced by 532 nm laser was simulated using the Kramers–Kronig relation. Additionally, the transmission intensity mutual modulations of the two beams were studied. Our results suggest that ethyl red (ER) has potential applications in all-optical devices.

Multiferroic Bi_1-xSm_xFeO₃(x = 0.00, 0.05, 0.1, 0.15, 0.2) ceramics were prepared by conventional solid state reaction method. X-ray diffraction measurement was carried out to characterize the crystal structure and to detect the impurities existing in these ceramics. The substitution of rare earth Sm for Bi was found to decrease the impurity phase in BiFeO₃ ceramics. There is strong evidence that both lattice constants a and c of the unit cell become smaller as the Sm³⁺ content is increased. The effect of introducing Sm³⁺ is shown to decrease the optical band gap for doped sample Bi_1-xSm_xFeO₃. Additionally, the temperature-dependent Raman measurement performed for the lattice dynamics study of Bi_1-xSm_xFeO₃ samples reveals a strong band centered at around 1000–1300 cm^-1 which is associated with the resonant enhancement of two-phonon Raman scattering in the multiferroic Bi_1-xSm_xFeO₃ samples. This two-phonon signal is shown to broaden with increasing x. The Raman spectra at low wavenumbers are suggested to be related with magnon in this system.

A study has been carried out on the Cu doping and PVA capping induced optical property changes in ZnS : Cu nanocrystalline powders and thin film. For this study, ZnS : Cu nanopowders with Cu concentrations of 0.1%, 0.15%, 0.2%, 0.3% and 0.4% are synthesized by the wet chemical method. The polyvinyl alcohol (PVA)-capped ZnS thin film with 0.2% Cu concentration and various PVA concentrations are prepared by the spin-coating method. The microstructures of the samples are investigated by the X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM). The results show that the prepared samples belong to the wurtzite structure with the average particle size of about 3–7 nm. The optical properties of samples are studied by measuring absorption and photoluminescence (PL) spectra in the wavelength range from 300 nm to 900 nm at 300 K. It is shown that the luminescent intensity of ZnS : Cu nanopowders reaches the highest intensity for optimal Cu concentration of 0.2% with the corresponding values of its direct band gap estimated to be about 3.90 eV. While the PVA coating does not affect the microstructure of ZnS nanometerials, the PL spectra of the samples are found to be affected by the PVA concentration as well as the exciting power density. The influence of the polymer coating on the optical properties can be explained by the quantum confinement effect of ZnS nanoparticles in the PVA matrix.

The absorption spectra of semiconductor quantum dots (QDs) are expected to be a series of δ-function-like discrete lines due to the nature of the density of states. In a realistic III–V QD system, the absorption spectra is the superimposition of the contribution from each individual dot and the overall behavior is modeled by considering a Gaussian size distribution. In this paper, we study and present the dependence of the Gaussian nature of the absorption spectra of In_XGa_1-XN/GaN QD systems on the dot size distribution and some fundamental parameters such as bowing effect of the band gap, band offset ratio, and so on. It is observed that the absorption spectra depend strongly on the dot size distribution. The results presented helps to get a better insight of the optical properties of In_XGa_1-XN/GaN QD systems.

In the present work, conventional chemical co-precipitation method was employed for the preparation of Nickel (2, 4, 6, 8 and 10 at.% of Ni) doped CdS nanoparticles. The crystallite size and lattice parameters for each sample are determined from X-ray diffraction (XRD) analysis. From XRD patterns the broadening of the diffraction peaks indicates the nanostructure nature of the samples. Surface morphology of the samples was studied by scanning electron microscope (SEM). UV-Visible absorption spectra for prepared samples are monitored for the optical properties of quantum sized particles. All nanoparticles formed lie in the size quantization regime and exhibit good crystallinity. Optical properties have been observed via UV-Vis spectroscopy and Photoluminescence spectra (PL) spectra. Ni doping in CdS is found to modify the luminescence properties by creating shallow acceptor states.

Gold and silver nanoparticles (NPs) are physically synthesized using Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG)-pulsed laser ablation in liquid (PLAL) technique which is a rapid, simple and efficient one-step synthesis. The gold and silver colloidal solutions are separately prepared by 1064nm of pulsed laser ablation of metallic target (gold and silver) which is immersed in deionized water. Ultraviolet–visible spectroscopy (UV–Vis) analysis shows the absorption band of gold and silver NPs at $\sim 520$nm and $\sim 400$nm, respectively. The absorption spectra and color variations of gold and silver NPs at three different laser parameters (output laser energies, target distances from focal point and laser time exposures). High-resolution transmission electron microscopy shows the spherical shape of gold and silver NPs with 34nm and 33nm diameter of size, respectively, are reported. The aggregation and particle sizes of gold and silver NPs due to minimum energy (75mJ) and maximum energy (311mJ) are observed.