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Using UV-VIS absorption spectroscopy, photoluminescence (PL) and photoluminescence excitation (PLE), the photodegradation reactions of folic acid (FA) in phosphate buffer (PB) solutions were studied. Regardless of the PB solution’s pH, the UV-VIS spectra showed a gradual decrease in absorbance at 284 nm simultaneous with an increase in the absorbance of another band in the spectral range of 320–380 nm, which was downshifted under UV irradiation. The relative intensity of the FA PL band, situated in the spectral range 375–600 nm, was dependent on the pH of the PB solution. The FA PL intensity increased as increasing UV irradiation time up to 281 min. in PB solutions with pH values of 6.4 and 5.4. Under an emission wavelength of 500 nm, the position of the FA PLE spectrum changed as the PB solution pH varied from 7 to 5.4 and the irradiation time increased to 317 min. These changes were correlated with the formation of two photodegradation products, namely, pterine-6-carboxylic acid and p-amino-benzoyl-L-glutamic acid. According to UV-VIS spectroscopy and PL and PLE studies, the presence of various excipients in commercial pharmaceutical tablets does not affect the photodegradation of FA in PB solutions. Using IR spectroscopy, new evidences for the formation of the two photodegradation products of FA in PB solutions are shown.

Mixtures of nitrogen-doped titanium dioxide (TiO

This paper prepared composites under the free membranes form that are based on thermoplastic polymers of the type of polyurethane (TPU) and polyolefin (TPO), which are blended in the weight ratio of 2:1, and ceramic nanoparticles (CNs) such as BaSrTiO3 and SrTiO3. The structural, optical, and conductive properties of these new composite materials are reported. The X-ray diffraction studies highlight a cubic crystalline structure of these CNs. The main variations in the vibrational properties of the TPU:TPO blend induced by CNs consist of the following: (i) the increase in the intensity of the Raman line of 1616 cm−1, (ii) the down-shift of the IR band from 800 to 791 cm−1, (iii) the change of the ratio between the absorbance of IR bands localized in the spectral range 950–1200 cm−1, and (iv) the decrease in the absorbance of the IR band from 1221 cm−1. All these variations were correlated with a preferential adsorption of thermoplastic polymers on the CNs surface. A photoluminescence (PL) quenching process of thermoplastic polymers is demonstrated to occur in the presence of CNs. The anisotropic PL measurements have highlighted a change in the angle of the binding of the TPU:TPO blend, which varies from 23.7° to ≈49.3° and ≈53.4°, when the concentration of BaSrTiO3 and SrTiO3 CNs, respectively, is changed from 0 to 25 wt. %. Using dielectric spectroscopy, two mechanisms are invoked to take place in the case of the composites based on TPU:TPO blends and CNs, i.e., one regarding the type of the electrical conduction and another specifying the dielectric–dipolar relaxation processes.

Raman enhancement in a layered structure of BiI 3 was studied under optical excitation near the edge of the fundamental absorption band. This phenomenon simultaneously appears at a low temperature with the generation of the excitonic photoluminescence (PL) band, unequally across the Raman spectrum, i.e. in the Stokes and anti-Stokes Raman branches, being dependent on the superposition of the excitation laser line to the excitonic emission band. Performed with the use of different resonant laser light excitations, this effect is regarded as a signature of the exciton-phonon interaction, which is interpreted in terms of a stimulated Raman scattering process (SRS), resulting from the mixing inside of the sample to two optical fields, the pump laser light and the excitonic PL light.

Stimulated Raman scattering (SRS) is studied in polycrystalline CdS/TiO2 mixtures based on laser power and temperature dependences in Raman investigation. Our Raman analysis reveals several features of nonlinear behavior, such as nonlinear laser power dependencies or high enhancement of Raman lines at a low temperature due to the intrinsic nonlinear nature of CdS, a high light scattering inside the matter, and a trap-based diffusive medium in a grain-boundary system. In particular, building up experimental evidence of conductivity at various temperatures, it is shown that the samples exhibit thermally activated conductivity due to potential local barriers revealed by a considerable fraction of the grain-boundaries in a diffusive medium. Taking into account the defect-based conductivity, a double Poole–Frenkel barrier is established within the grain-boundaries of CdS/TiO2 mixtures, and their barrier energies are investigated considering the temperature-dependent dark current of the grain-boundary model. We thus demonstrate that diffusive medium-based defects in a grain-boundary model makes CdS/TiO2 mixtures behave efficiently in nonlinear processes such as SRS. These features are of particular interest for optical communication technology, and the fundamental application of the SRS effect is semiconductor optical amplifiers.

We demonstrate the anisotropic features of the A1g, B2g and A2g vibrational Raman modes of phosphorene by performing for the first time a comparison of two orthogonal polarized Raman scattering studies in edge and top view measuring configurations. Despite the orthogonality between the vibration modes of A1g and A2g, they exhibit intense and very weak Raman intensities under light polarized along and transverse to the armchair edge, respectively, while the B2g mode is virtually non-existent. The B2g mode significantly appears only in the top view configuration. The studies performed under different intensities of excited light in the top view configuration are detailed for the sample rotated at 0°, 40°, 80° and 130° about the incident laser beam. The studies reveal a quadratic opposite behavior of the A2g and B2g modes and an unusual angle dependence of the A1g vibrational mode. An unexpected behavior of the A2g Raman mode at higher intensities, which still persists for the A1g mode, is shown. The anomalous behavior of the A1g mode from the edge and top view and the A2g mode in the top view configuration is explained in terms of the layer stacking structures of phosphorene.

New composite materials based on single-walled carbon nanotubes effectively separated into semiconducting (S-SWNTs) and metallic tubes (M-SWNTs) and poly(ortho-phenylenevinylene) (POPV) were synthesized by the electrochemical polymerization of the α,α,α′,α′-tetrabromo-o-xylene monomer onto the surface of Au supports covered with carbon nanotubes films. Using FTIR spectroscopy, an increase of the quinoid structure weight is highlighted when a charge transfer is induced to take place between the two constituents of the POPV/S-SWNT composite material. In addition, a down-shift of the Raman line belonging to POPV (from 530 cm−1 to 515 cm−1), which was accompanied by an increase in relative peak intensity, is reported as a result of steric hindrance effects induced by the covalent bonding of POPV onto the S-SWNT surface. The non-covalent functionalization of M-SWNTs with POPV is demonstrated to occur by Raman scattering and FTIR spectroscopy. The photoluminescence (PL) spectrum of POPV exhibits two bands at 2.82 and 2.25 eV, which were assigned to the electronic emission transitions of macromolecular chains having 3 and 7–10 repeating units, respectively. We demonstrate that POPV photoluminescence quenching in the presence of M + S-SWNTs, M-SWNTs and S-SWNTs is achieved by tubes with chirality (14, 7), (12, 10) and (11, 9), respectively.

Mixtures of nitrogen-doped titanium dioxide (TiO2:N) with different concentrations of Ag and/or SiO2 particles (0.5, 1 and 2 wt.%) were prepared in solid state by mechanico-chemical interactions. Using UV-VIS spectroscopy, Raman scattering, photoluminescence (PL) and photoluminescence excitation (PLE), the influence of the particles on the host material is evaluated. UV-VIS spectroscopy studies indicate a TiO2:N band gap shift to the UV range with increasing concentrations of SiO2 and Ag particles. PL intensities decrease with increasing concentrations of Ag and/or SiO2 particles in the TiO2:N host matrix, which in turn could effectively restrict the electron and hole recombination. To explain these processes, the different de-excitation ways will be advanced, taking into account the energy levels diagram of TiO2:N/Ag, TiO2:N/SiO2 and TiO2:N/Ag/SiO2 systems. PLE spectra show a gradual decrease in their relative intensities after 165 minute of continuous irradiation due to photosensitivity of TiO2:N. The plasmonic effect of Ag particles in the TiO2:N/Ag system is highlighted for the first time by PLE studies.

The exciton-phonon interaction, considered as a stimulated Raman scattering process, is studied in different semiconductor mixtures: PbI2/TiO2, PbI2/Si and CdS/Si. Raman spectra recorded at excitation wavelengths of 514.5 and 488 nm for PbI2 and CdS, respectively, reveal a strong enhancement of the Raman lines peaked at 97 and 305 cm-1, evaluated by the ratio I TK/I 300 K between the relative intensities of the spectra recorded in the temperature range of 88-300 K. It is found that PbI2 and CdS exhibit a decrease in the Raman intensity modes with decreasing temperature, while in TiO2 and Si an increase in the Raman lines intensities peaked at 138 and 520 cm-1 is observed. This behavior can be explained by an energy transfer process from PbI2 or CdS towards TiO2 and Si. This explanation is supported by the schematic potential energy levels diagram obtained from the density of states, which is calculated using the density functional theory. According to this energy levels diagram, the electrons are expected to migrate directly from the conduction band (CB) energetic levels of the PbI2 and CdS towards the CB levels of TiO2 and Si.

The enhancement of the Raman scattering in Cs3Bi2I9 is evaluated by the ratio IT/I300 K between the relative intensities of the Raman line peaked at 146 cm−1, when the spectra are recorded in the temperature range of 88–300 K, as a signature of exciton–phonon interactions. In the resonant and nonresonant conditions, excitation wavelengths 476, 561, and 660 nm, respectively, are used in order to overlap with great accuracy the bands disclosed by diffuse reflection, photoconductivity (PC), photoluminescence (PL), and photoluminescence excitation (PLE) spectra. Based on the experimental analyses, the strength of exciton–phonon interaction is dependent on the defects in the crystal and the type–range interaction of the excitations in an independent Bi2I93− cluster. The noticeable PL band, attributed to excitons trapped on different stacking faults, manifests some defects in crystal that diminish the movement of excitons. This effect significantly decreases the overlaps of excitons with the phonons, resulting in a reduced exciton–phonon coupling.

Raman studies performed on different morphologies of CdS samples indicate an exciton-phonon interaction (EPI) that manifests at low temperatures by different enhancements in intensity of the Raman spectra in the Stokes and anti-Stokes branches. This effect, interpreted as stimulated Raman effect, is conditioned on the appearance and strength by the degree of overlap of the excitation laser light with the excitonic photoluminescence band. EPI is greatest in samples characterized by high light diffusing power, which means a long path of light into the sample and a great overlapping of the two optical fields, i.e., the pump laser and excitonic light.