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The GaAs-like longitudinal-optical (LO) phonon frequency in hydrogenated GaAsN (x = 0.01) layers - with different H doses and similar low-energy irradiation conditions - was investigated by micro-Raman measurements in different scattering geometries and compared with those of epitaxial GaAs and as-grown GaAsN reference samples. A relaxation of the GaAs selection rules was observed, to be explained mainly on the basis of the biaxial strain affecting the layers. The evolution of the LO phonon frequency with increasing hydrogen dose was found to heavily depend on light polarization, thus suggesting that a linear relation between strain and the frequency of the GaAs-like LO phonon mode should be applied with some caution. Moreover, photoreflectance measurements in fully passivated samples of identical N concentration show that the blueshift of the GaAs-like LO frequency, characteristic of the hydrogenated structures, is dose-dependent and strictly related to the strain induced by the specific type of the dominant N-H complexes. A comparison of photoreflectance results with the finite element method calculations confirms that this dependence on the H dose is due to the gradual replacement of the N-2H complexes responsible for the electronic passivation of N with N-3H complexes, which are well known to induce an additional and sizeable lattice expansion.

Raman scattering is an effective tool for the investigation of the strain state of crystalline solids. In this brief review, we show how the analysis of the GaAs-like longitudinal optical phonon frequency allowed to map the strain behavior across interfaces in planar heterostructures consisting of GaAsN wires embedded in GaAsN:H matrices. Moreover, we recently showed how the evolution of the longitudinal optical frequency with increasing H dose strongly depends on polarization geometry. In a specific geometry, we observed a relaxation of the GaAs selection rules. We also present new results which demonstrate how laser irradiation intensity–even at low levels–may affect the line shape of the GaAs-like spectral features in GaAsN hydrogenated materials.

The last decade has come across an increasing demand for theranostic biocompatible nanodevices possessing the double ability of diagnosis and therapy. In this work, we report the design, synthesis and step-by-step characterization of rationally coated gold nanostars (GNSs) for the SERS imaging and photothermal therapy of HeLa cancer cells. The nanodevices were realized by synthesizing GNSs with a seed growth approach, coating them with a controlled mixture of thiols composed of a Raman reporter and a polyethylene glycol with a terminal amino group, and then reacting these amino groups with folic acid (FA), in order to impart selectivity towards cancer cells which overexpress folate receptors on their membranes. After a complete characterization, we demonstrate that these FA-functionalized GNSs (FA-GNSs) are able to bind selectively to the membranes of HeLa cells, acting as SERS tags and allowing SERS imaging. Moreover, we demonstrate that once bound to HeLa cell membranes, FA-GNSs exhibit photothermal effect which can be exploited to kill the same cells in vitro using laser irradiation in the NIR at a very low and safe irradiance. We thus demonstrate that the FA-GNSs designed following the described approach are an efficient prototype of theranostic nanodevices.

Microstructural modifications, induced by femtosecond laser irradiation on Lithium Niobate (LN) crystals were investigated by means of optical microscopy and micro-Raman spectroscopy. Commercial z-cut congruent LN substrates were irradiated with focused ultra-short laser pulses at 810 nm. At the focus region either structural changes or material removal were observed by varying the irradiation conditions. The mapping of Raman spectrum revealed the insurgence of Raman modes, which are forbidden in the used scattering geometry. This phenomenon increases gradually by approaching the ablation region. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present results from Raman scattering, EPR and static magnetization measurements in CaCu 3 Ti 4 O 12 , a perovskite-related material characterized by a colossal dielectric permittivity e 0 . The investigated samples were pure and partially substituted in the octahedral Ti sites by other transition metals. The results from the pure material are interpreted in terms of the presence of delocalized Copper-holes. Substitutions in the Ti sites drastically change the magnetic properties and, most evidently in the case of Cr, also affect the Raman spectra. These effects correlate with the known suppression of the colossal e 0 after the same kind of substitutions.

It is shown that doping the highly polarizable KTaO3 perovskite simultaneously by Li and Nb (K1−xLixTa1−yNbyO3, KLTN) gives rise to new impressive dipole ordering effects, very unusual physical properties and strong responses with respect to doping parameters. Besides, KLTN has outstanding perspectives of application in electrically controlled holographic and compositionally graded pyroelectric devices.

KTaO 3 with small amount ( x < 0.01) of Li and simultaneously diluted by Nb (K 1 m x Li x Ta y Nb 1 m y O 3 , KLTN) reveals original dipole ordering structural effects. Small Nb co-dopings ( y < 0.007) enhance the trend of the Li + off-centers related subsystem towards cooperative ordering and order-disorder phase transitions. At 0.01 < y < 0.05 the intrigue features of displacive and order-disorder transitions reveal. The most interesting features are the appearance of soft-mode driven ferroelectric transitions accompanied by a Li + off-centers system freezing followed by a reentrant dipole glass-like phase formation, and the emergence of a narrow transition with giant magnitude of l '(T). Evidences of reentrant dipole glass formation were observed in KTa 0.981 Nb 0.019 O 3 too. A possible model developed is based on the Li-Nb polar clusters, local configuration instabilities and superparaelectricity conceptions.

Li admixture greatly enhances n-type photoconductivity of KTaO3 crystals. Thermally stimulated current experiments indicate that this phenomenon is related with the presence of hole-trapping levels with depths of 50–100 meV. This is in good agreement with recent energy structure calculations showing the existence of levels at these energies above the top of the VB, due to perturbed O2- p-states. The study of visible photoluminescence and thermo-luminescence shows that the hole-trapping centre also controls the emission process. We conclude in favour of radiative recombination involving O−Ta4+polaron pairs. Keywords: ferroelectric perovskites: photoconductivity

Raman observations in the region of the TO 1 soft mode were performed on samples of K 1-x Li x Ta 1-y Nb y O 3 , having x in the range 0.6 - 1.0 % mol. and y in the range 0.2 - 0.3 % mol. The analysis of temperature evolution and spectral shape allowed to evidence the spontaneous splitting of the TO 1 mode and its relative stiffness below 50 K, accompanied by an abrupt increase of first order scattering strengths. Such effects are not observed when Li or Nb are separately introduced in the KTaO 3 matrix, at the given concentrations.

Visible photoluminescence in K1−xLixTaO3 was found to have similar spectral features but a large increase of emission yield with respect to KTaO3. Studying the temperature dependence of the phenomena and its correlation with photoconductivity, we found direct support for a process in which the emission is due to pair recombination of trapped electron and hole centers, Ta4+ and O−, intrinsic to the matrix.

The temperature evolution of TO1 Raman spectra was investigated in single crystals of K1−xLixTa1−yNbyO3:Cu with different Li and Nb amounts. Auxiliary observations of TO4 spectra were also performed. In a sample with x = 0·002 and y = 0·025 and containing 190 ppm of Cu, the results indicate a clear-cut transition to an ordered phase around 55 K, in good agreement with previous findings from dielectric measurements in the same crystal. However, no evidence was found of a role of Cu in this structural modification. Samples with x = 0·01 and y = 0·003 revealed features of an order-disorder phase transition around 45 K, irrespective of the presence of Cu.

Experiments on photoconductivity and thermally stimulated currents were performed on Li-doped KTaO3 single crystals. The large enhancement of conductivity, caused by UV excitation at low temperature was found to correlate with a subsequent release of trapped charge between 30 and 40 K. No corresponding release was shown by pure KTaO3, consistently with a very low yield of photoconduction. It is concluded that the presence of hole traps, with the consequent quenching of electron-hole recombination, is at the origin of the photoconductive response of K1−xLixTaO3.

A study of blue-green photoluminescence of K1-x Li x TaO3 single crystals, under UV irradiation, was performed in the range 15–20 K. It was shown that the emission is strongly enhanced and thermally more stable with respect to pure KTaO3, up to ∼40 K. Above this point the yield of emission decreases abruptly with an Arrhenius-like behaviour controlled by an activation energy of 70–100 meV. After low temperature excitation, a thermoluminescence spectrally very similar to PL and giving rise to a glow peak around 30K is also observed. Correlations with recent results on photoconductivity and thermally stimulated currents are illustrated. An argument in favour of close-pair recombinations between Ta4+ and O− centers is proposed.

Observations of the soft-mode Raman spectrum, performed in poled and unpoled KTa0.976Nb0.024O3, show that the polar transition in this sample is still characterized by a substantial degree of order, in contrast to the frozen dipole-glass behaviour of a K0.984Li0.016TaO3 sample. Results of second-harmonic generation indicate that this ordering may scale up to fairly large dimensions, even in the spontaneous case.

Raman investigations have been performed on K0.984Li0.016TaO3 and KTa0.976Nb0.024O3 single crystals, in order to follow the peculiarities of non-ferroelectric phonons, which may be indicative of a different evolution of microscopic order during the polar transition. Specific features in the Nb case confirm a ferroelectric behaviour, whereas in the Li-doped sample the results are in agreement with a dipole glass transition.

GaAsN/GaAsN:H heterostructures were made by an in-plane selective hydrogen incorporation controlled by H-opaque metallic masks. The strain field and hydrogen distributions in GaAsN micro-sized wires thus obtained have been mapped by an all optical procedure that combines micro-Raman scattering and photoreflectance spectroscopy. The strain field is related to the formation of N-H complexes along the hydrogen diffusion profile with an ensuing expansion of the GaAsN lattice whose patterning generates an anisotropic stress in the sample growth plane. These results highlight a powerful non-invasive tool to simultaneously determine both the H diffusion profile and the related strain field distribution.

Low frequency Raman and quasi-elastic light scattering measurements were performed in KTa0.976Nb0.024O3. Near the transition temperature, data were analyzed with the help of a simple soft mode-relaxor coupling model to take into account the presence of the wide central component. At lower temperatures the soft-mode line hardens and splits, thus characterizing the ferroelectric transition. A distinctive feature is the uncomplete softening of the TO1 naked frequency, despite the second order character of the transition, resulting from birefringence. The observed phenomena could be interpreted in terms of a critical dynamics in which both the order-disorder and displacive aspects are present.

We report on Raman scattering experiments on InAs/Al(x)Ga(1-x)As quantum dot heterostructures with 0 0, we detected several lines originating from the Al(x)Ga(1-x)As alloy. These can be related to scattering from GaAs-like and AlAs-like phonons with q congruent to 0, and weaker scattering from disorder-activated phonons with q not equal 0. In particular, we identified a line at similar to 250 cm(-1) as due to disorder-activated longitudinal optical phonons in the alloy. This conclusion is different than the attribution of this line to scattering from dots and, consequently, we do not recognize the possibility of deriving any information about the actual composition of the dots from an analysis of this line as proposed by other authors.

We report on recent findings obtained through Raman scattering, second harmonic generation and linear birefringence on the polar phase of doped KTaO3. At low temperature, the polar coherence length ranges from a few nanometers to 20 nm for 1.6% and 6% Li doping, respectively and shows no criticality. Comparable figures are found for the case of 12% Na doping. The evolution of low energy Raman spectra and the behaviour under a poling field provides further evidence of phenomena which limit the growth and cooperative ordering of polar microregions.

Etude du rendement de generation de la seconde harmonique et de la birefringence. On en conclut qu'en dessous de la temperature de solidification T f , les deplacements polaires de KTaO 3 :Li sont correles sur des distances qui augmentent de facon continue avec la concentration en dopant

Low-frequency Raman scattering in the mixed perovskite KTaO3 (Na 12%) is shown to give information on the correlation length of the local order parameter which is in agreement with that obtained by combined measurements of linear birefringence and second harmonic generation. Accordingly, the behavior of this system is interpreted in terms of a frustrated structural transition, characterized by freezing of the polar domain dimension (300A) in the low temperature region.

The paper reports on the first crystal growth and Raman scattering studies in K0.992Rb0.008Ta0.96Nb0.04O3 (KRTN) and KTaO3:Rb (3% mol. in the melt), in which Rb+ ions (ionic radius 1.47 angstrom) substitute K+(1.33 angstrom). The experiments were performed in the range 10-300 K and focused on the behavior of TO1 and TO4 modes. The results confirm indications by previous dielectric permittivity data about the ferroelectric nature of the phase transition in KRTN. It appears that a small amount of Rb changes the character of the phase transition induced by Nb from mainly displacive to mainly order-disorder, whereas no evidence of ferroelectricity is revealed in the KTaO3:Rb sample.

The GaAs-like longitudinal optical phonon frequency in two hydrogenated GaAs1-N-x(x)/GaAs1-N-x(x):H microwire heterostructures-with similar N concentration, but different H dose and implantation conditions-has been investigated by micro-Raman mapping. In the case of GaAs0.991N0.009 wires embedded in barriers where GaAs-like properties are recovered through H irradiation, the phonon frequency in the barriers undergoes a blue shift with respect to the wires. In GaAs0.992N0.008 wires embedded in less hydrogenated barriers, the phonon frequency exhibits an opposite behavior (red shift). Strain, disorder, phonon localization effects induced by H-irradiation on the GaAs-like phonon frequency are discussed and related to different types of N-H complexes formed in the hydrogenated barriers. It is shown that the red (blue) character of the frequency shift is related to the dominant N-2H (N-3H) type of complexes. Moreover, for specific experimental conditions, an all-optical determination of the uniaxial strain field is obtained. This may improve the design of recently presented devices that exploit the correlation between uniaxial stress and the degree of polarization of photoluminescence. (C) 2014 AIP Publishing LLC.

Waveguides obtained by proton exchange in LiNbO3 were characterized by Secondary Ion Mass Spectrometry and m-lines technique. Additional micro-Raman investigations evidenced marked changes in the spectra of the waveguide with respect to the substrate. This suggests that micro-Raman spectroscopy may constitute a direct non-destructive method to determine the depth of the exchanged layer, with a limit which is set by the diffraction-limited spot size.

Second-harmonic light generation and birefringence data are reported for KTaO3:Na. They are evaluated in terms of an inhomogeneous lattice polarization. The correlation of polar orientation is shown to extend to hundreds of A in the paraelectric phase of KTaO3:Na. A sharp increase of the correlation length for 15% Na doping to 1500 A is associated to the polar transition at 12 K. From the low-temperature value of the correlation length, conclusions are drawn about the nature of the polar phase.

A new approach based on the study of distortion in the AO3 layer of ABO3 perovskites is proposed with the aim to discuss their ferroelectric properties. Rhombohedral distortions exist for either A-O average distances shorter than 2.7 A (space group Nr.161) or A-O distances larger than 3 Å (space group Nr.160). In the case of small distortions tetragonal or orthorhombic symmetries are preferred. Moreover, a relationship between the dimension of the B cation and the distortion in this type of structure has been established. Preliminary results show interesting correlations between distortions in the AO3 plane and the critical temperature of ferroelectric materials.