Low Temperature Sol-Gel Preparation of Nanocrystalline TiO2 Thin Films

TiO₂ nanocrystalline thin films with varying degree of porosity have been prepared using a low temperature method. TiO₂ films of the anatase form have been obtained by using a polyethylene glycol (PEG) modified sol-gel method. Densification and crystallization of the films was found to result from the thermal treatment of the dip coated films in boiling water. The films have been characterized by Raman, XRD, FTIR, AFM and optical methods. Highly transparent films with transmission in excess of 85% and porosity as high as 58% are formed predominantly of anatase crystallites of dimensions of the order of 5 nm. Initial results on lithium intercalation into these films resulting in an efficient optical modulation in the visible and near infrared regions demonstrate a good potential of these films for electrochromic applications.     

Raman spectroscopy in AB2X4 pseudoternary layered compounds

We have synthesized several pseudoternary layered compounds by cation or anion cross substitution in ternary AB₂X₄ compounds. Here we report on the low frequency Raman spectra obtained from Zn_xCd_1-xIn₂S₄, Zn(In_xGa_1-x)₂S₄ and ZnIn₂(S_xSe_1-x)₄ single crystals. Within these systems we have identified five compositionally or dynamically different phases. Each of these phases may be characterized by its peculiar low frequency Raman spectrum which is connected to the dynamics of the layers in the unit cell. Abrupt structural/dynamical changes are observed as a function of composition between different phases. Within each phase compositional changes cause only smooth and small spectral variations.     

Raman spectroscopy of CaM2+Ge2O6 (M2+ = Mg,Mn, Fe,Co, Ni,Zn) clinopyroxenes

The Raman spectra of Ge‐clinopyroxenes CaM²⁺Ge₂O₆ (M²⁺ = Mg, Mn, Fe, Co, Ni, Zn), general formula M2M1T₂O₆, are reported for the first time. Their spectral features are discussed in comparison with corresponding Si‐pyroxenes. The vibrational wavenumbers of germanates may be roughly obtained by a scale factor of about ~0.8 by those of the corresponding silicates, due to the Ge‐Si mass difference. The main peaks in the germanate Raman spectra at ~850 and ~540 cm⁻¹ may be related to Ge‐O tetrahedral stretching and chain bending, respectively; minor peaks between 200 and 400 cm⁻¹ are ascribed to bending and stretching of the non‐tetrahedral cations. Within Ge‐pyroxenes, possible correlations between crystallographic parameters and the vibrational wavenumbers are investigated. The main stretching mode at ~850 cm⁻¹ shows wavenumber changes with M²⁺ substitutions, but no simple correlation can be found with M²⁺ cation mass or size. On the other hand, the chain bending wavenumber linearly decreases with increasing ionic radius of the M²⁺ cation: the expansion of the M1 polyhedron reduces the chain kinking angle and the Ge‐Ge distances correspondingly increase. Copyright © 2015 John Wiley & Sons, Ltd.     

Applications of Raman spectroscopy to gemology

Being nondestructive and requiring short measurement times, a low amount of material, and no sample preparation, Raman spectroscopy is used for routine investigation in the study of gemstone inclusions and treatments and for the characterization of mounted gems. In this work, a review of the use of laboratory Raman and micro-Raman spectrometers and of portable Raman systems in the gemology field is given, focusing on gem identification and on the evaluation of the composition, provenance, and genesis of gems. Many examples are shown of the use of Raman spectroscopy as a tool for the identification of imitations, synthetic gems, and enhancement treatments in natural gemstones. Some recent developments are described, with particular attention being given to the semiprecious stone jade and to two important organic materials used in jewelry, i.e., pearls and corals.     

Sol-Gel Preparation and Raman Characterization of CdTiO3

Cadmium titanate (CdTiO₃) has been prepared by a room-temperature sol-gel technique. The powders were characterized by means of X-Ray Diffraction. Samples treated at 800°C give rise only to the ilmenite-like phase, but, after a treatment at 1100°C for about four hours, the orthorhombic perovskite is the only crystalline phase observed. The Raman spectrum of the perovskite phase is reported for the first time and that of the ilmenite phase is compared with the spectrum of CdTiO₃ bulk crystals. The effects of the starting stoichiometry on the purity of the final phases, relative to the presence of TiO₂ (anatase and rutile) or CdO, are investigated.     

Polarization analysis of the Raman spectrum of As2S3 crystals

We present the first complete series of polarized Raman spectra of orpiment crystals and the corresponding possible symmetry assignments of the phonon peaks within the layer model structure. We observe new peaks and a low frequency mode which could be a rigid layer vibration. We discuss briefly on the possible implications of some inconsistencies between the expected and the observed results.     

Raman microspectrometric investigation of wall paintings in S. Giovanni Evangelista Abbey in Parma: a comparison between two artists of the 16th century

Micro-Raman spectroscopy, combined with gas chromatography and ultra-violet fluorescence photography, was used to study some wall paintings in the S. Giovanni Evangelista Abbey in Parma, Italy. The restoration of some painted chapels enabled a comparison between two painters of the 16th century: Parmigianino (Girolamo Francesco Maria Mazzola, 1503-1540) and Michelangelo Anselmi (1492?-1556?). Micro-Raman spectroscopy determined the palette used by the artists, leading to the identification of different white, yellow, red, brown, green, blue and black pigments. Some pigments are evidence of later restorations. Gas chromatography combined with mass spectroscopy revealed the presence of organic binding media and enabled to distinguish between fresco and secco paintings.     

Photoinduced effects in hybrid sol-gel materials

Hybrid silica-based sol-gel films containing Disperse Red 1 (DR1), carbazole units (CBZ) and 2,4,7-trinitro-9-fluorenone (TNF) at different concentrations show reversible photoinduced birefringence. Polarization sensitive holographic gratings have been produced and characterized. Dichroism measurements give information on the orientation mechanisms and on the aggregation of the dye molecules. The alignment of the DR1 molecules by corona poling is verified by second harmonic generation (SHG) and a moving grating technique is used to investigate an asymmetric energy exchange in a two-beam coupling experiment, typical of photorefractive gratings.     

Study of Anatase to Rutile Phase Transition in Nanocrystalline Titania Films

Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel dip-coating process. The effect of acetylacetone, diethanolamine and polyethylene glycol on the structure of the heat-treated titania films was examined by Raman and FTIR spectroscopy and X-ray diffraction. The effect the complexing agents have on the anatase to rutile phase transition during the heat treatment process is studied. The understanding of this effect is expected to enhance our capacity to tailor the composition and morphology of films and thus their properties. The Raman and the infrared spectra of nanocrystalline titania films and the changes induced by the heat treatment were also investigated. We have also studied the size of the crystallites in TiO₂ films and its dependence on the type of complexing agent used.