In this work we present detailed optical and electrical characterization results on silicon capac... more In this work we present detailed optical and electrical characterization results on silicon capacitive pressure sensing elements. The device fabrication technology is based on the wafer bonding technique. Using the micro-Raman technique, we investigate the influence of specific process steps as well as of the wafer bonding conditions—performed either in air or in nitrogen ambient—on the flatness and stress distribution
We study experimentally the impact of spherical nanoparticles on the orientational order paramete... more We study experimentally the impact of spherical nanoparticles on the orientational order parameters of a host nematic liquid crystal. We use spherical core-shell quantum dots that are surface functionalized to promote homeotropic anchoring on their interface with the liquid crystal host. We show experimentally that the orientational order may be strongly affected by the presence of spherical nanoparticles even at low concentrations. The orientational order of the composite system is probed by means of polarised micro-Raman spectroscopy and by optical birefringence measurements as function of temperature and concentration. Our data show that the orientational order depends on the concentration in a non linear way, and the existence of a crossover concentration χc ≈ 0.004 pw. It separates two different regimes exhibiting pure-liquid crystal like (χ < χc) and distorted-nematic ordering (χ > χc), respectively. In the latter phase the degree of ordering is lower with respect to the pure-liquid crystal nematic phase.
We study experimentally the impact of spherical nanoparticles on the orientational order paramete... more We study experimentally the impact of spherical nanoparticles on the orientational order parameters of a host nematic liquid crystal. We use spherical core-shell quantum dots that are surface functionalized to promote homeotropic anchoring on their interface with the liquid crystal host. We show experimentally that the orientational order may be strongly affected by the presence of spherical nanoparticles even at low concentrations. The orientational order of the composite system is probed by means of polarised micro-Raman spectroscopy and by optical birefringence measurements as function of temperature and concentration. Our data show that the orientational order depends on the concentration in a non linear way, and the existence of a crossover concentration χc ≈ 0.004 pw. It separates two different regimes exhibiting pure-liquid crystal like (χ < χc) and distorted-nematic ordering (χ > χc), respectively. In the latter phase the degree of ordering is lower with respect to the pure-liquid crystal nematic phase.
CdAl& single crystals with the defect chalcopyrite structure have been studied by Raman spectrosc... more CdAl& single crystals with the defect chalcopyrite structure have been studied by Raman spectroscopy at hydrostatic pressures up to 150 kbar. The Raman scattering spectra were found to undergo substantial changes around 60 and 100 kbar, due to an order-disorder transition in the cation sublattice, which occurs in two stages as predicted earlier. From the pressure dependence of optical phonon frequencies we obtained values for mode shift parameters. The irreversible disappearance of Raman scattering signals at pressures above 140 kbar was attributed to a phase transition from the adamantine structure to a higher symmetry rocksalttype structure.
ABSTRACT Experiments on atherosclerotic plaque diagnosis were carried out using laser induced flu... more ABSTRACT Experiments on atherosclerotic plaque diagnosis were carried out using laser induced fluorescence (LIF) spectroscopy on carotid plaque specimens. The excitation laser was a nitrogen laser, emitting pulses at a wavelength of 337 nm. Over 10 samples were examined in vitro and several spectra were obtained from each sample. Results were compared with conventional clinical techniques, such as histopathological diagnosis, which showed three areas of different composition on the pathological samples: fibrous tissue, lipid constituents and calcified plaque. An effort was made to distinguish the composition of the sample from the obtained spectra. Also, the results were compared with our previous work using longer excitation wavelengths. Spectral morphology of UV excited fluorescence reveals multi-peaks lineshapes, as a result of the superposition of different tissue chromophore signals. However, there was no observed specific wavelength where spectra corresponding to fibrous tissue, calcified tissue and lipid constituents have peaks.
We measured the room temperature far-infrared reflectivity and Raman scattering spectra of NaV 2 ... more We measured the room temperature far-infrared reflectivity and Raman scattering spectra of NaV 2 O 5 single crystals. The frequencies of infrared active modes are obtained by Kramers-Kronig analysis of reflectivity data. The assignation of the observed modes is given according to the lattice dynamical calculation based on the valence shell model. According to the factor group analysis of the P 2 1 mn space group, which assumes the existence of the V 4+ and V 5+ chains, the 15 A 1 and 7 B 1 modes can be expected in both ir and Raman scattering spectra from the (001) plane. Only eight Raman and six infrared modes of A 1 symmetry are clearly seen. In the case of B 1 symmetry, three B 1 modes are observed both in the Raman and in the ir reflectivity spectra. The frequencies of these ir and Raman modes differ significantly. Because of this, we concluded that the space group of the NaV 2 O 5 crystal structure cannot be P 2 1 mn (non-centrosymmetric), but the space group which includes the mutual exclusion between Raman and infrared activity (centrosymmetric space group). We have shown that the appropriate space group is P mmn, for which we found our experimental spectra in complete agreement with factor-group analysis. This means that V atoms are indistinguishable in the unit cell and in a mixed-valence state.
We have studied the interference pattern of multiple reflections from a uniaxial, optically activ... more We have studied the interference pattern of multiple reflections from a uniaxial, optically active and non-absorbing plane parallel plate under normal incidence. Due to birefringence, a beam of polarized light splits into an ordinary and an extraordinary component. Because of optical activity, the number of beams is doubled after each reflection. During crossing of the plate the phase difference between beams is increased by fixed amounts. The calculated results show that the interference pattern of the transmitted light is more complex than that of an isotropic parallel plate. It consists of strong fringes at large phase intervals and weak doublets in between, at nearly the same phase intervals as in the isotropic plate.
Films of Ge x Se y Te z (x≈33) with a thickness of 0.8 µm and four different compositions were pr... more Films of Ge x Se y Te z (x≈33) with a thickness of 0.8 µm and four different compositions were prepared by thermal evaporation of preliminary synthesized glasses. The glass and film compositions were determined by Energy-Dispersive X-ray Spectroscopy. X-ray diffraction measurements have confirmed the amorphous structure of both bulk and thin film samples. The temperature dependence of the dark conductivity has been measured in the range 20 -110 o C. No significant change of the room temperature conductivity (around 5x10 -7 S/cm) and the dark current activation energy (around 0.7 eV) have been observed with increasing z. Spectral photocurrent measurements have revealed that the increase of Te content results in: (i) a photoconductivity decrease and (ii) a red shift of the low-energy side of the spectrum which indicates an optical band gap decrease. Resistive chemical sensing tests carried out at room temperature with ammonia, acetone, ethanol or water vapours in air as carrying gas have shown that the Ge 32 Se 55 Te 13 films are sensitive to ammonia but not to water vapour and hence they are suitable for selective ammonia sensors operating at room temperature. The observed sensitivity is related to the specific lattice structure of the region nearest to the film surface.
Controlling the number of layers of graphene grown by chemical vapor deposition is crucial for la... more Controlling the number of layers of graphene grown by chemical vapor deposition is crucial for large scale graphene application. We propose here an etching process of graphene which can be applied immediately after growth to control the number of layers. We use nickel (Ni) foil at high temperature (T = 900 °C) to produce multilayer-AB-stacked-graphene (MLG). The etching process is based on annealing the samples in a hydrogen/argon atmosphere at a relatively low temperature (T = 450 °C) inside the growth chamber. The extent of etching is mainly controlled by the annealing process duration. Using Raman spectroscopy we demonstrate that the number of layers was reduced, changing from MLG to few-layer-AB-stacked-graphene and in some cases to randomly oriented few layer graphene near the substrate. Furthermore, our method offers the significant advantage that it does not introduce defects in the samples, maintaining their original high quality. This fact and the low temperature our method...
High resolution X-ray diffraction and Raman spectroscopy have been used to study GaAs epilayers g... more High resolution X-ray diffraction and Raman spectroscopy have been used to study GaAs epilayers grown on GaAs substrates by conventional molecular beam epitaxy and by atomic layer epitaxy, at growth temperatures ranging between 600 and 200°C. No scattering was observed by TO phonons, indicating high-quality crystallinity. Epilayers grown at 200°C are tetragonally strained with a relaxed lattice constant greater than that of GaAs. The level of residual strains depends on the type of growth. The LO phonon frequencies were downshifted compared to GaAs, due to volume expansion by As excess, misfit strains, and changes in the effective charge and reduced mass of the unit cell. An estimate for the As excess has been obtained for the epilayers grown at 200°C.
Physical review. B, Condensed matter, Jan 15, 1992
The second-order Raman-scattering of AlSb is studied at and below room temperature. The excitatio... more The second-order Raman-scattering of AlSb is studied at and below room temperature. The excitation energies used cover both the direct and the indirect gap regions of the material. The most important spectroscopic features are assigned to contributions from various critical points of the Brillouin zone. A comparison is made with partial (overtones, summation, and differences) and/or total two-phonon density of
In this work we present detailed optical and electrical characterization results on silicon capac... more In this work we present detailed optical and electrical characterization results on silicon capacitive pressure sensing elements. The device fabrication technology is based on the wafer bonding technique. Using the micro-Raman technique, we investigate the influence of specific process steps as well as of the wafer bonding conditions—performed either in air or in nitrogen ambient—on the flatness and stress distribution
We study experimentally the impact of spherical nanoparticles on the orientational order paramete... more We study experimentally the impact of spherical nanoparticles on the orientational order parameters of a host nematic liquid crystal. We use spherical core-shell quantum dots that are surface functionalized to promote homeotropic anchoring on their interface with the liquid crystal host. We show experimentally that the orientational order may be strongly affected by the presence of spherical nanoparticles even at low concentrations. The orientational order of the composite system is probed by means of polarised micro-Raman spectroscopy and by optical birefringence measurements as function of temperature and concentration. Our data show that the orientational order depends on the concentration in a non linear way, and the existence of a crossover concentration χc ≈ 0.004 pw. It separates two different regimes exhibiting pure-liquid crystal like (χ < χc) and distorted-nematic ordering (χ > χc), respectively. In the latter phase the degree of ordering is lower with respect to the pure-liquid crystal nematic phase.
We study experimentally the impact of spherical nanoparticles on the orientational order paramete... more We study experimentally the impact of spherical nanoparticles on the orientational order parameters of a host nematic liquid crystal. We use spherical core-shell quantum dots that are surface functionalized to promote homeotropic anchoring on their interface with the liquid crystal host. We show experimentally that the orientational order may be strongly affected by the presence of spherical nanoparticles even at low concentrations. The orientational order of the composite system is probed by means of polarised micro-Raman spectroscopy and by optical birefringence measurements as function of temperature and concentration. Our data show that the orientational order depends on the concentration in a non linear way, and the existence of a crossover concentration χc ≈ 0.004 pw. It separates two different regimes exhibiting pure-liquid crystal like (χ < χc) and distorted-nematic ordering (χ > χc), respectively. In the latter phase the degree of ordering is lower with respect to the pure-liquid crystal nematic phase.
CdAl& single crystals with the defect chalcopyrite structure have been studied by Raman spectrosc... more CdAl& single crystals with the defect chalcopyrite structure have been studied by Raman spectroscopy at hydrostatic pressures up to 150 kbar. The Raman scattering spectra were found to undergo substantial changes around 60 and 100 kbar, due to an order-disorder transition in the cation sublattice, which occurs in two stages as predicted earlier. From the pressure dependence of optical phonon frequencies we obtained values for mode shift parameters. The irreversible disappearance of Raman scattering signals at pressures above 140 kbar was attributed to a phase transition from the adamantine structure to a higher symmetry rocksalttype structure.
ABSTRACT Experiments on atherosclerotic plaque diagnosis were carried out using laser induced flu... more ABSTRACT Experiments on atherosclerotic plaque diagnosis were carried out using laser induced fluorescence (LIF) spectroscopy on carotid plaque specimens. The excitation laser was a nitrogen laser, emitting pulses at a wavelength of 337 nm. Over 10 samples were examined in vitro and several spectra were obtained from each sample. Results were compared with conventional clinical techniques, such as histopathological diagnosis, which showed three areas of different composition on the pathological samples: fibrous tissue, lipid constituents and calcified plaque. An effort was made to distinguish the composition of the sample from the obtained spectra. Also, the results were compared with our previous work using longer excitation wavelengths. Spectral morphology of UV excited fluorescence reveals multi-peaks lineshapes, as a result of the superposition of different tissue chromophore signals. However, there was no observed specific wavelength where spectra corresponding to fibrous tissue, calcified tissue and lipid constituents have peaks.
We measured the room temperature far-infrared reflectivity and Raman scattering spectra of NaV 2 ... more We measured the room temperature far-infrared reflectivity and Raman scattering spectra of NaV 2 O 5 single crystals. The frequencies of infrared active modes are obtained by Kramers-Kronig analysis of reflectivity data. The assignation of the observed modes is given according to the lattice dynamical calculation based on the valence shell model. According to the factor group analysis of the P 2 1 mn space group, which assumes the existence of the V 4+ and V 5+ chains, the 15 A 1 and 7 B 1 modes can be expected in both ir and Raman scattering spectra from the (001) plane. Only eight Raman and six infrared modes of A 1 symmetry are clearly seen. In the case of B 1 symmetry, three B 1 modes are observed both in the Raman and in the ir reflectivity spectra. The frequencies of these ir and Raman modes differ significantly. Because of this, we concluded that the space group of the NaV 2 O 5 crystal structure cannot be P 2 1 mn (non-centrosymmetric), but the space group which includes the mutual exclusion between Raman and infrared activity (centrosymmetric space group). We have shown that the appropriate space group is P mmn, for which we found our experimental spectra in complete agreement with factor-group analysis. This means that V atoms are indistinguishable in the unit cell and in a mixed-valence state.
We have studied the interference pattern of multiple reflections from a uniaxial, optically activ... more We have studied the interference pattern of multiple reflections from a uniaxial, optically active and non-absorbing plane parallel plate under normal incidence. Due to birefringence, a beam of polarized light splits into an ordinary and an extraordinary component. Because of optical activity, the number of beams is doubled after each reflection. During crossing of the plate the phase difference between beams is increased by fixed amounts. The calculated results show that the interference pattern of the transmitted light is more complex than that of an isotropic parallel plate. It consists of strong fringes at large phase intervals and weak doublets in between, at nearly the same phase intervals as in the isotropic plate.
Films of Ge x Se y Te z (x≈33) with a thickness of 0.8 µm and four different compositions were pr... more Films of Ge x Se y Te z (x≈33) with a thickness of 0.8 µm and four different compositions were prepared by thermal evaporation of preliminary synthesized glasses. The glass and film compositions were determined by Energy-Dispersive X-ray Spectroscopy. X-ray diffraction measurements have confirmed the amorphous structure of both bulk and thin film samples. The temperature dependence of the dark conductivity has been measured in the range 20 -110 o C. No significant change of the room temperature conductivity (around 5x10 -7 S/cm) and the dark current activation energy (around 0.7 eV) have been observed with increasing z. Spectral photocurrent measurements have revealed that the increase of Te content results in: (i) a photoconductivity decrease and (ii) a red shift of the low-energy side of the spectrum which indicates an optical band gap decrease. Resistive chemical sensing tests carried out at room temperature with ammonia, acetone, ethanol or water vapours in air as carrying gas have shown that the Ge 32 Se 55 Te 13 films are sensitive to ammonia but not to water vapour and hence they are suitable for selective ammonia sensors operating at room temperature. The observed sensitivity is related to the specific lattice structure of the region nearest to the film surface.
Controlling the number of layers of graphene grown by chemical vapor deposition is crucial for la... more Controlling the number of layers of graphene grown by chemical vapor deposition is crucial for large scale graphene application. We propose here an etching process of graphene which can be applied immediately after growth to control the number of layers. We use nickel (Ni) foil at high temperature (T = 900 °C) to produce multilayer-AB-stacked-graphene (MLG). The etching process is based on annealing the samples in a hydrogen/argon atmosphere at a relatively low temperature (T = 450 °C) inside the growth chamber. The extent of etching is mainly controlled by the annealing process duration. Using Raman spectroscopy we demonstrate that the number of layers was reduced, changing from MLG to few-layer-AB-stacked-graphene and in some cases to randomly oriented few layer graphene near the substrate. Furthermore, our method offers the significant advantage that it does not introduce defects in the samples, maintaining their original high quality. This fact and the low temperature our method...
High resolution X-ray diffraction and Raman spectroscopy have been used to study GaAs epilayers g... more High resolution X-ray diffraction and Raman spectroscopy have been used to study GaAs epilayers grown on GaAs substrates by conventional molecular beam epitaxy and by atomic layer epitaxy, at growth temperatures ranging between 600 and 200°C. No scattering was observed by TO phonons, indicating high-quality crystallinity. Epilayers grown at 200°C are tetragonally strained with a relaxed lattice constant greater than that of GaAs. The level of residual strains depends on the type of growth. The LO phonon frequencies were downshifted compared to GaAs, due to volume expansion by As excess, misfit strains, and changes in the effective charge and reduced mass of the unit cell. An estimate for the As excess has been obtained for the epilayers grown at 200°C.
Physical review. B, Condensed matter, Jan 15, 1992
The second-order Raman-scattering of AlSb is studied at and below room temperature. The excitatio... more The second-order Raman-scattering of AlSb is studied at and below room temperature. The excitation energies used cover both the direct and the indirect gap regions of the material. The most important spectroscopic features are assigned to contributions from various critical points of the Brillouin zone. A comparison is made with partial (overtones, summation, and differences) and/or total two-phonon density of
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Papers by Y. Raptis