Hybrid organic-inorganic FAPbX(3) perovskites (FA = NH2CHNH2+, X = Cl-, Br-, I-) are currently intensively investigated in solar cells. In this study, off-resonance Raman and far-IR absorption spectra of all single-halogen FAPbX(3), as well as of mixed-halogen FAPbBr(2)X derivatives, are reported and analyzed. Vibrations of the PbX6 octahedra and librations of the FA cation lay at frequencies below 250 cm(-1), while external and internal FA bands are identified above 200 cm(-1). An increase in the frequencies of most vibrational bands is observed upon substituting lighter and more electronegative halides for heavier ones due to strengthening of the electrostatic interaction between X- and the FA NH2+ groups. Variable-temperature Raman measurements are also carried out for FAPbBr(3) and FAPbCl(3) in the 77-400 K range. Upon cooling, the three Pb-X Raman vibrations of the cubic structure, split into eight separate bands, signaling the transformation to the tetragonal phase at similar to 240 K and similar to 200 K for FAPbBr(3) and FAPbCl(3), respectively. At even lower temperatures, a successive phase transformation to low symmetry orthorhombic phases is evidenced. These temperature-induced effects are accompanied by intense narrowing, soft-mode behavior, and/or sudden frequency shift of specific Raman bands, attributed to ordering of the FA cation.

Hybrid organic–inorganic FAPbX₃ perovskites (FA = NH₂CHNH₂⁺, X = Cl–, Br–, I–) are currently intensively investigated in solar cells. In this study, off-resonance Raman and far-IR absorption spectra of all single-halogen FAPbX₃, as well as of mixed-halogen FAPbBr₂X derivatives, are reported and analyzed. Vibrations of the PbX₆ octahedra and librations of the FA cation lay at frequencies below 250 cm–¹, while external and internal FA bands are identified above 200 cm–¹. An increase in the frequencies of most vibrational bands is observed upon substituting lighter and more electronegative halides for heavier ones due to strengthening of the electrostatic interaction between X– and the FA NH₂⁺ groups. Variable-temperature Raman measurements are also carried out for FAPbBr₃ and FAPbCl₃ in the 77–400 K range. Upon cooling, the three Pb–X Raman vibrations of the cubic structure, split into eight separate bands, signaling the transformation to the tetragonal phase at ∼240 K and ∼200 K for FAPbBr₃ and FAPbCl₃, respectively. At even lower temperatures, a successive phase transformation to low symmetry orthorhombic phases is evidenced. These temperature-induced effects are accompanied by intense narrowing, soft-mode behavior, and/or sudden frequency shift of specific Raman bands, attributed to ordering of the FA cation.

We report on the preparation, crystal structure and spectral properties of the trimethylsulfonium tin triiodide perovskite, (CH3)3SSnI3. The air-sensitive lead-free perovskite compound is prepared by reacting the (CH3)3SI and SnI2 solid precursors in evacuated silica tubes at 100°C. According to powder x-ray diffraction and Rietveld analysis, (CH3)3SSnI3 crystallizes at room temperature in hexagonal symmetry and forms a 1D network of face-sharing [SnI6] octahedra along the c axis. UV–Vis reflectance and photoluminescence spectroscopies reveal a direct energy band gap of 2.85 eV accompanied by a weak luminescence signal. Multi-temperature Raman spectroscopy reveals a fully reversible structural phase transition just below 0°C related to the reduction of the unit cell symmetry. Comparison with the widely studied Cs-, CH3NH3- and (NH2)2CH-based 3D-perovskites that are commonly used in third generation solar cells confirms the higher stability of (CH3)3SSnI3. This is attributed to the beneficial role of the bulky trimethylsulfonium group in the ASnI3 structure.

A simple and effective preparation method for the simultaneous reduction and functionalization of graphene oxide (rGO) by 2,4-diamino benzene sulfonic acid has been developed. The derivatives exhibit excellent conductivity and high dispersibility in various solvents. The successful preparation of rGO and the presence of the sulfonated aromatic diamine on rGO surface has been confirmed by infrared and X-ray photoelectron spectroscopy, while, the analysis by micro-Raman spectroscopy indicated that the reduction/functionalization alters the lattice structure of GO by the increment the defect density when the 2,4-diamino benzene sulfonic acid is used. Moreover, the study of the dried products by X-ray diffraction spectroscopy suggested the turbostratic restacking of the exfoliated rGO into graphite-like nanostructures. The obtained derivative of simultaneous reduction and functionalization of GO was used for the preparation of highly conductive water-based gravure ink, which in turn, was successfully applied in printing on various flexible substrates, demonstrating its great potentiality in graphene-based flexible and printed electronics applications.

TiO2 photocatalysis is an advanced process, employed worldwide for the oxidation of organic compounds, that leads to significant technological applications in the fields of health and environment. The use of the photo catalytic approach in reduction reactions seems very promising and can open new horizons for green chemistry synthesis. For this purpose, titanium dioxide nanotubes (TNTs) were developed in autoclave conditions using TiO2 P25 as a precursor material. Based on these nanotubular substrates, TiO2/CoFe2O4 (TCF) nanocomposites were further obtained by wet impregnation method. The materials were thoroughly characterized and their structural, textural, vibrational, optoelectronic and magnetic properties were determined. The composite materials combine absorbance in the visible optical range and high BET surface area values (similar to 100 m(2)/g), showing extremely high yield in the photocatalytic reduction of 4-nitrophenol (4-NP), exceeding 94% within short illumination time (only 35 min). The developed nanocomposites were successfully reused in consecutive photo catalytic experiments and were easily removed from the reaction medium using magnets. Both remarkable recycling ability and high-performance stability in the photocatalytic reduction of nitrophenol were observed, thus justifying the significant economic potential and industrial perspectives for this advanced reduction process.

We report on the synthesis, characterization, and optoelectronic properties of the novel trimethylsulfonium lead triiodide perovskite, (CH3)3SPbI3. At room temperature, the air-stable compound adopts a hexagonal crystal structure with a 1D network of face-sharing [PbI6] octahedra along the c axis. UV-vis reflectance spectroscopy on a pressed pellet revealed a band gap of 3.1 eV, in agreement with first-principles calculations, which show a small separation between direct and indirect band gaps. Electrical resistivity measurements on single crystals indicated that the compound behaves as a semiconductor. According to multi-temperature single-crystal X-ray diffraction, synchrotron powder X-ray diffraction, Raman spectroscopy, and differential scanning calorimetry, two fully reversible structural phase transitions occur at -5 and ca. -100 °C with reduction of the unit cell symmetry to monoclinic as temperature decreases. The role of the trimethylsulfonium cation regarding the chemical stability and optoelectronic properties of the new compound is discussed in comparison with APbI3 (A = Cs, methylammonium, and formamidinium cation), which are most commonly used in perovskite solar cells.

Photocatalysis is an attractive advanced treatment process that can be used for water purification from emerging contaminants, including hexavalent chromium (Cr(VI)) removal. Up to now, photocatalytic reduction of Cr(VI) has been investigated mostly using titania (TiO2) photocatalysts in acidic water solutions. In this work, copper (Cu) and cuprous oxide (Cu2O) nanoparticles (NPs) decorated TiO2-alginate beads were synthesized and studied in the photocatalytic reduction of Cr(VI) to Cr(III) under UV/Vis irradiation. The target was to overcome the drawback of pristine TiO2 which requires acidified solutions to achieve enhanced photocatalytic reduction of hexavalent chromium. Moreover, the spherical and uniform size of the photocatalytic beads ensures efficient mass transfer, addresses the problems of limited irradiation into slurries and facilitates separation of the catalyst after the photocatalytic treatment. The hexavalent chromium reduction efficiency of the Cu/Cu2O decorated TiO2/alginate beads was satisfactory at pH range 2-6 and practically did not depend on the treated solution acidity. Small quantities (10 g L-1) of the newly synthesized photocatalytic beads succeeded to remove all toxic loads from a 5 ppm Cr6+ solution in only 15 min under UV light irradiation. These low-cost developed and non-toxic photocatalysts seam greatly promising for Cr(VI) pollution cleanup. (C) 2016 Elsevier B.V. All rights reserved.

The nonradiative decay of majority electrons has been studied over a wide temperature range from 80 K to 600 K using the time-resolved free-carrier-absorption (FCA) technique. At high injection level of the highly-luminescent N-B codoped 6H-SiC epilayer, we revealed three main relaxation components of injected free electrons over ps-to-ms time ranges. By means of temperature dependency, two components can be ascribed to thermal activation of holes from a shallow (200 meV) and a deep (500 meV) acceptor. The third one, which has a hundred us-time scale, we attribute to minority hole recombination from the valance band into the electron trap (53 meV). This recombination channel seems to compete with the deep-acceptor (Boron) to-donor (Nitrogen) pair visible emission at and below 300 K.

We applied a number of time-resolved optical techniques for investigation of optical and photoelectrical properties of cubic SiC grown by different technologies on different substrates. The excess carriers were injected by a short laser pulse and their dynamics was monitored by free-carrier absorption, light-induced transient grating, and photoluminescence techniques in a wide excitation range. Combining an optical and electrical probe beam delay, we found that free carrier lifetimes in differently grown layers vary from few ns up to 20 μs. Temperature dependences of carrier diffusivity and lifetime revealed a pronounced carrier trapping in thin sublimation grown layers. In free-standing layers and thick sublimation layers, the ambipolar mobility was found the highest (120 cm 2 /Vs at room temperature). A linear correlation between the room-temperature band edge emission and carrier lifetime in differently grown layers was attributed to defect density, strongly dependent on the used growth conditions.

We characterized optical and photoelectrical properties of undoped and Ga-doped ZnO layers differently grown on sapphire substrates by using complementary optical methods. Different stimulated emission threshold values for ZnO epitaxial layers grown by pulsed laser deposition and MBE methods were attributed to crystalline quality of the layers and the growth method used. Different carrier lifetimes in various ZnO epitaxial layers are explained by defect-related and intrinsic mechanisms of recombination.

Free carrier absorption (FCA) and picosecond light-induced transient grating (LITG) techniques were applied to study the photoelectrical properties of 3C-SiC(111) homoepitaxial layers grown by CVD method on VLS (vapour-liquid-solid) grown seeds. The thickness of the CVD layers was ~10.5 µm with non-intentional type doping of n (~ 1017 cm-3) or p (<1015 cm-3). The carrier lifetime and the diffusion coefficient were measured as the function of the sample temperature, the injected excess carrier density at different growth parameters. At room temperature the ambipolar diffusion coefficient was Da=2.5-3 cm2/s, while the lifetime was in the range of 12-18 ns. The best structural and electrical properties were obtained for a CVD layer grown at high, 1600 °C temperature.

We investigated non-equilibrium carrier dynamics in ~20μm thick 3C-SiC layers, grown by sublimation epitaxy directly on 6H-SiC substrate or buffered by a 3C seed layer. Differential transmission and light-induced transient grating techniques were applied to determine the ambipolar diffusion coefficient, carrier lifetime, and thermal activation energy of defects. The temperature dependences of ambipolar mobility and lifetime in 80-700 K range revealed the carrier scattering processes as well the impact of defects on the recombination rate, thus indicating slightly improved photoelectrical parameters of the homoepitaxially grown 3C layer. The determined thermal activation energies of 35 and 57 meV were attributed to the nitrogen impurity.

Residual stress and carrier lifetime variation have been measured in free‐standing n‐type 3C‐SiC wafer grown on undulated Si substrate. We identify extended regions of residual stress that lie parallel to epilayer surfaces. The opposite polarity of stress is identified toward the interface and toward the top surface. Integrated carrier lifetime has been determined by random defect density distribution which is enhanced in the areas of double‐positioning boundary defects. It is shown that carrier lifetimes are severely reduced by the presence of residual stress towards epilayer surfaces. In this way lifetime depth‐distribution can be mistakenly attributed to enhanced surface recombination.

Thin 3C-SiC(111) epilayers grown on 6H-SiC(0001) substrate by VLS and CVD procedures were studied by low temperature photoluminescence (LTPL) and nonlinear optical techniques at room and low temperatures. Free carrier density ((0.3-7)×1017 cm-3) and nitrogen concentration (4×1016 cm-3) in the layers were determined from Raman and LTPL data. Investigation of non-equilibrium carrier dynamics by using transient grating and free carrier absorption techniques provided an ambipolar diffusion coefficient Da (~2.5 cm2/s) and carrier lifetime τR (2-4 ns) values at room temperature. The temperature dependences of Da and τR in 40-300 K range revealed the scattering processes in high density plasma as well the impact of defects.

Photoelectric properties of 3C sublimation-grown epitaxial layers with different structural quality were studied by using time-resolved picosecond transient grating and free carrier absorption techniques. The layer quality was described by a parameter L-TW which gives the total length of twin boundaries in a layer. Optical measurements of diffusion coefficients and carrier lifetimes in wide excess carrier density (N >10(18) cm(-3)) and temperature range (10 K to 300 K) revealed the twin defect density dependent ambipolar mobility value at RT as well as essentially different temperature dependences of mobility of the layers. The larger value of absorption cross section in more defective layer at 1064 nm wavelength pointed out to contribution of defect-assisted absorption, which gradually vanished after the filling defect states by free carriers.

An alternative approach based on non-equilibrium free-carrier density measurements was used to characterize the fundamental absorption edge of 3C-SiC at room and 77 K temperatures. At 77 K temperature the extracted absorption edge compared well to the previous literature data revealing characteristic thresholds due to the phonon emission assisted transitions. At room temperature the absorption tail due to the phonon absorption assisted transition was revealed up to the value of 0.01 cm-1 exceeding the previous 5 cm-1 limit induced by unintentional sample doping.