D. Christofilos

Charge transport mechanisms and memory effects in amorphous TaNx thin films

Amorphous semiconducting materials have unique electrical properties that may be beneficial in nanoelectronics, such as low leakage current, charge memory effects, and hysteresis functionality. However, electrical characteristics between different or neighboring regions in the same amorphous nanostructure may differ greatly. In this work, the bulk and surface local charge carrier transport properties of a-TaNx amorphous thin films deposited in two different substrates are investigated by conductive atomic force microscopy. The nitride films are grown either on Au (100) or Si [100] substrates by pulsed laser deposition at 157 nm in nitrogen environment. For the a-TaNx films deposited on Au, it is found that they display a negligible leakage current until a high bias voltage is reached. On the contrary, a much lower threshold voltage for the leakage current and a lower total resistance is observed for the a-TaNx film deposited on the Si substrate. Furthermore, I-V characteristics of the a-TaNx film deposited on Au show significant hysteresis effects for both polarities of bias voltage, while for the film deposited on Si hysteresis, effects appear only for positive bias voltage, suggesting that with the usage of the appropriate substrate, the a-TaNx nanodomains may have potential use as charge memory devices.

Inducing bioactivity of dental ceramic/bioactive glass composites by Nd:YAG laser

OBJECTIVES Aims of this study were to investigate the optimal conditions of laser irradiation of a novel Bioactive Glass/Dental Ceramic-BP67 composite for acceleration of hydroxyapatite-HA formation and to assess cellular responses on the precipitated HA region. METHODS BP67 (Bioactive Glass: 33.3%, Dental Ceramic: 66.7%) was fabricated by the sol-gel method. A laser assisted biomimetic-LAB process was applied to BP67 sintered specimens immersed in 1.5-times concentrated simulated body fluid-1.5×-SBF. The effect of various energy densities of pulsed nanosecond Nd-YAG (1064nm) laser and irradiation exposure times (30min, 1 and 3h) were evaluated for HA precipitation. The HA film was characterized by FTIR, XRD, SEM and micro Raman techniques. ICP-AES was used for revealing changes in chemical composition of the 1.5×-SBF during irradiation. Cell viability and morphological characteristics of periodontal ligament fibroblasts-PDLFs, human gingival fibroblasts-HGFs and SAOS-2 osteoblasts on the HA surface were evaluated by MTT assays and SEM. RESULTS At optimal energy fluence of 1.52J/cm2 and irradiation time for 3h followed by immersion in 1.5×-SBF at 60°C, a dense HA layer was formed on laser-irradiated BP67 within 7 days. The resulting HA film was tightly bonded to the underlying substrate and had mineral composition similar to cementum. MTT assay showed a consistent reduction of cell proliferation on the HA layer in comparison to conventional control ceramic and BP67 for all 3 cell lines studied. SIGNIFICANCE These findings suggest LAB is an effective method for acceleration of HA formation on materials with low bioactivity, while cellular responses need further investigation.

Raman and photoluminescence mapping of InxGa1−xN (x ∼ 0.4) at high pressure: Optical determination of composition and stress

The pressure response of a polar wurtzite InxGa1−xN (x = 0.37) film epitaxially grown on a GaN/sapphire template was studied by means of combined Raman and photoluminescence (PL) mappings. The pressure slopes of the Raman peaks (∂ω/∂P ∼ 4.7 cm−1·GPa−1) of the studied alloy are indicative of its intermediate stiffness between the end members of the InxGa1−xN system. The data analysis suggests that in our experiments the obtained slopes have marginal contribution, if any, from the substrate. Furthermore, the similarity of the ambient pressure value of the PL peak energy (∼1.97 eV) and its pressure slope (∂EPL/∂P ∼ 30 meV·GPa−1) with those obtained by absorption measurements implies that PL can be used to follow the pressure evolution of the energy bandgap. Finally, we demonstrate that all-optical characterization of the composition and residual stress of InxGa1−xN samples is feasible.

Pressure Raman study of vibrational modes of glassy As2X3 (X: O, S)

Pressure effects on the vibrational modes of As2O3 glass have been investigated with the aid of Raman scattering. Emphasis was put on both the intramolecular vibrational modes at high energies as well as the low-energy excitations (Boson peak) of the glass. The pressure dependence of the spectral features of Raman bands was found to be significantly different compared with the corresponding features of the structurally similar As2S3. Striking dissimilarities were also observed for the pressure dependence of the Boson peak in these two glasses.

High pressure Raman study of type‐I collagen

The high pressure response of type‐I collagen from bovine Achilles tendon is investigated with micro‐Raman spectroscopy. Fluorinert™ and methanol‐ethanol mixtures were used as pressure transmitting media (PTM) in a diamond anvil cell. The Raman spectrum of collagen is dominated by three bands centred at approximately 1450, 1660 and 2930 cm−1, attributed to C–H deformation, C=O stretching of the peptide bond (amide‐I band) and C–H stretching modes respectively. Upon pressure increase, using Fluorinert™ as PTM, a shift towards higher frequencies of the C–H stretching and deformation peaks is observed. Contrary, the amide‐I band peaks are shifted to lower frequencies with moderate pressure slopes. On the other hand, when using the alcohol mixture as PTM, the amide‐I band exhibits more pronounced C=O bond softening, deduced from the shift to lower frequencies, suggesting a strengthening of the hydrogen bonds between glycine and proline residues of different collagen chains due to the presence of the polar alcohol molecules. Furthermore, some of the peaks exhibit abrupt changes in their pressure slopes at approximately 2 GPa, implying a variation in the compressibility of the collagen fibres. This could be attributed to a pitch change from 10/3 to 7/2, sliding of the tropocollagen molecules, twisting variation at the molecular level and/or elimination of the D‐gaps induced by kink compression. All spectral changes are reversible upon pressure release, which indicates that denaturation has not taken place. Finally, a minor lipid phase contamination was detected in some sample spots. Its pressure response is also monitored.

High-pressure Raman study of the Sm2.75C60 fulleride

The high-pressure response of the Sm2.75C60 fulleride is probed by Raman spectroscopy. The Raman spectrum of Sm2.75C60 and its pressure evolution differ significantly from those of pristine C60. The initial well-resolved Raman spectrum of the fulleride disappears above 4 GPa. This change is reversible; however, the original low-pressure Raman profiles are recovered only below 2 GPa. These observations are in line with the pressure-induced hysteretic phase transition of the compound observed earlier by X-ray diffraction and the proposed insulator to metal transition.

Optical Properties of Hydrofullerene C60H36 Underpressure

We report the hydrostatic pressure dependence, up to 13 GPa, of the Raman scattering and the Photoluminescence (PL) emission in hydrofiillerene C60H36 at room temperature, as well as the PL emission at selective temperatures down to 10 K. The Raman spectrum, compared to that of C60, contains additional lines due to the lowering of molecular symmetry from the presence of H. The photoluminescence studies show that the PL band consists of four peaks all of them exhibiting a blue pressure induced energy shift at rates of 2.3, 2.0, 0.3 and 5.1 meV/GPa, respectively. The pressure coefficients of the Raman frequencies show changes at~0.6 GPa and ~ 6 GPa, while the emission PL band show a change from blue to red at ~6 GPa. The first discontinuity may be related to a structural phase transition from the initial orientationally disordered cubic structure to an orientationally ordered tetragonal one and the second one, at ~6 GPa, to a pressure-induced hydrogen bonding to a carbon atom of a neighbouring hydrofullerene cage. Finally, our PL measurements do not imply any temperature induced phase transition of C60H36 down to 10 K.

A high pressure Raman study of terbium molybdate

Abstract Tb2(MoO4)3 has been studied by Raman spectroscopy under hydrostatic pressure up to 9 GPa at room temperature. The measurements reveal two phase transitions, one at around 2 GPa and another one above 5 GPa. The first phase transition is associated with an increase in the coordination number of Mo while the second is probably a transition to an amorphous phase in which only a wide band originating from Mo-O vibrations remains. This behaviour is irreversible as the Raman spectrum of the initial structure is not recovered at atmospheric pressure.