Protein dynamics on different timescales

Structure and dynamics determine the function of proteins. This contribution discusses two aspects of protein dynamics, the structural fluctuation and the structural relaxation connected with conformational changes. Myoglobin and haemoglobin were investigated. To cover a wide time range different experimental techniques had to be used. Moreover, measurements in a large temperature regime were used to separate contributions from different modes of motions. Phonon assisted Mössbauer effect using synchrotron radiation allowed the study of the harmonic vibrations which have characteristic times of 1 fs to 0.6 ps. They are present in the whole temperature range from cryogenic to room temperature. With a combination of neutron structure analysis and incoherent neutron scattering it was possible to distinguish three types of hydrogen mean square displacements which are present only above a characteristic temperature T_c: These are the backbone-like (slower than about 100 ps), methyl-like (partly slower partly faster than about 100 ps) and lysine-like (faster than about 100 ps) displacements. The exceptional high energy resolution of Mössbauer absorption on ⁵⁷Fe allowed the measurement of quasi diffusive modes of molecular segments which have characteristic times slower than 1 ns and are present only above T_c. Conformational changes from the ligated to the unligated structure of myoglobin and haemoglobin were investigated by creating a metastable intermediate and observing the relaxation into the equilibrium conformation. A metastable state was obtained by X-ray irradiation. Structural relaxation was investigated as a function of time and temperature using the Mössbauer hyperfine interactions as indicator. Furthermore it was possible to measure intermediates created by photolysis of a ligand with temperature dependent X-ray structure analysis or time dependent X-ray structure analysis with the Laue technique. It was shown that the quasi diffusive structural fluctuations above T_c strongly facilitate structural relaxations.     

The densities of mixed glass former 0.35 Na2O + 0.65 [xB2O3 + (1 − x)P2O5] glasses related to the atomic fractions and volumes of short range structures

The mixed glass former effect (MGFE) is defined as a non-linear and non-additive change in the ionic conductivity with changing glass former fraction at constant modifier composition between two binary glass forming compositions. In this study, mixed glass former (MGF) sodium borophosphate glasses, 0.35 Na₂O + 0.65 [xB₂O₃ + (1 ? x)P₂O₅], 0 ≤ x ≤ 1, which have been shown to have a strong positive MGFE, have been prepared and their physical properties, density and molar volume, have been examined as predictors of structural change. The density exhibits a strong positive non-linear and non-additive change in the density with x and a corresponding negative non-linear and non-additive change in the molar volume. In order to understand the structural origins of these changes, a model of the molar volume was created and best-fit to the experimentally determined molar volumes in order to determine the volumes of the short range order (SRO) structural units in these glasses, how these volume change from the molar volumes of the binary glasses, and how these volumes change across the range of x in the ternary glasses. The best-fit model was defined as the model that required the smallest changes in the volumes of the ternary phosphate and borate SRO structural groups from their values determined by the densities of the binary sodium phosphate and sodium borate glasses. In this best-fit molar volume model, it was found that the volumes of the various phosphate and borate SRO structural groups decreased by values ranging from a minimum value of ~ 1% for x = 0.1 and 0.9 to a maximum value of ~ 6% for the phosphate and ~ 9% for the borate SRO groups at the minimum in molar volume at x = 0.4. The free volume was found to have a negative deviation from linear which is unexpected given the positive deviation in ionic conductivity.     

Precise sound velocity measurement for liquid Se50Te50 under high pressure

We have measured the sound velocity in liquid Se₅₀Te₅₀ at 19.5, 32.6, 45.6, and 58.2 MHz simultaneously by means of an ultrasonic pulse transmission/echo method. By using a phase-sensitive-detection technique the relative error was reduced to less than 0.1%. The temperature dependence of sound velocity at 100 MPa exhibits a minimum at 600 ^oC and a maximum at 1035 ^oC, which are related to the large structural change accompanied by the semiconductor–metal (S–M) transition. In addition, sound dispersion has been observed at temperatures from 500 °C to 900 °C for the first time, where anomalous sound attenuation was previously reported. This result implies that a structural relaxation on nanosecond timescale takes place in the S–M transition region.     

Ultraviolet-assisted synthesis of organic/inorganic hybrid nanocomposites and their application for lifetime enhancement of light emitting diodes

Ultraviolet (UV) curable organic/inorganic hybrid nanocomposites with excellent gas barrier effect, good adhesion strength, high refractive indices, moderate hardness, and fast curing speed have been successfully quickly synthesized by UV irradiation and utilized for the encapsulation of light emitting diodes (LEDs). In addition, the experimental results reveal that lab-made nanocomposites effectively resist the penetration of oxygen and moisture in the air into the LEDs and thus enhance their lifetimes to achieve 18,300 h while those without encapsulation are only 2400 h.     

Transport properties near the magneto/structural transition of Tb5Si2Ge2

We report high resolution measurements of the electrical resistivity (ρ, dρ/dT) and thermopower (S, dS/dT) measurements near the magnetic and structural transition of the layered Tb₅Si₂Ge₂ compound, which are fairly close but not fully coupled. The analysis of the transport properties confirms a split magneto/structural transition, with T_S ～ 97 K and T_S ～ 107 K for the structural transition (on cooling and heating respectively; 1st-order transition). The magnetic transition occurs only at T_C ～ 112 K and without hysteresis (2nd-order transition). The magnetic critical behavior of resistivity is analyzed, obtaining an almost classical mean field exponent (α ～ 0.59) for T > T_C. For the structural phase, and below T_S, we obtain a rather different exponent (α ～ 1.06).     

Influence of TiO2 on proton conductivity in fuel cell electrolytes based on sol–gel derived P2O5–SiO2 glasses

P₂O₅–TiO₂–SiO₂ based glasses have been prepared by a sol–gel process. The glasses were characterized by structural, thermal, nitrogen adsorption–desorption and conductivity measurements. The structural formation has been confirmed by the FTIR and NMR analysis. The proton conductivity of the glasses increased linearly with increase in temperature. Glasses with an average pore size less than 2 nm showed higher values of proton conductivity in humid atmosphere. The conductivity value increased from 6.47 × 10⁻⁴ S/cm to 3.04 × 10⁻² S/cm at 70% RH in the temperature range 30–90 °C. We observed in fuel cell measurements that the performance of the E1 electrode is superior to that of the other electrodes at the same operating condition. The power density shows a similar pattern to current density.     

Fabrication of buried waveguides in planar silica films using a direct CW laser writing technique

A CW CO₂ laser ablation technique is used to form buried waveguides in planar silica films. It is shown that the refractive index of a silica thin film is reduced sufficiently adjacent to the laser processed region to allow the fabrication of low loss waveguides. The refractive index distribution of these structures is measured using the reflectance of a focussed spot from the surface of the films. The change in refractive index is measured to be of the order of the core cladding refractive index difference of typical single mode waveguides. The spatial resolution of the reflectance technique is 1.3 μm with a refractive index resolution of ±5 × 10^?4. Devices such as 1 × 2 and 1 × 4 multi-mode interference (MMI) splitters have also been demonstrated and shown to exhibit low transmission losses.     

Role of sol-gel networking and fluorine doping in the silica Urbach energy

We present the results of the analysis of the ultraviolet (UV) absorption edge of fluorine-modified sol-gel silica. UV transmission data, obtained by means of synchrotron radiation, have been analyzed in the spectral range 7.5–8.5 eV, with a spectral resolution of about 10 meV. Data on silica samples with different F content (from 0 to few 10^? 1 mol%) have been analyzed and compared with literature data on quartz and pure synthetic commercial silica. The analysis allows us to discriminate between the effects of the fluorine addition and those ascribable to structural peculiarities of the sol-gel networking. The estimated Urbach energy E_U(T = 0) ranges between 45 and 55 meV, higher that in crystalline quartz and lower than in commercial synthetic silica. The study of the temperature dependence of E_U(T) shows that the fluorine modification of the silica network causes the lowering of the static disorder and the widening of the energy gap. However, there is also a relevant effect of the production process, since sol-gel silica samples show lower E_U values with respect to other types of silica, quite independently of the fluorine content. The analysis of the Raman spectra however shows that the starting amount of fluorine-modified molecular precursor influences the network condensation process, independently of the final fluorine content into the matrix.     

Disordered matter under extreme conditions: X-ray diffraction,electron spectroscopy and electroresistance measurements

In the last decade, we have developed a set of experimental techniques for in-house X-ray diffraction measurements under high-temperature (up to about 2300 K) and high-pressure (up to 10 GPa and 1500 K, up to 100 GPa and 700 K) conditions, electron energy-loss and Auger measurements for surface electronic structure measurements at high-temperature (up to about 1800 K), and electrical resistance measurements covering both low-temperatures (2 K) and high-temperatures/high-pressures (1500 K, 7 GPa) conditions. In this paper we discuss some important technical features and possibilities of these new equipments and present novel data collected for phase transitions and structural modifications occurring in liquid and solid systems. In particular, we present new results about phase transitions and undercooling of bismuth under pressure, extreme undercooling and metastable states in gallium films, and surface phase transitions of Si at high-temperatures. The relevance of these experiments to the exploitation of the potential of equipments available at synchrotron radiation facilities is emphasized.     

Growth temperature induced effects in non-polar a-plane GaN on r-plane sapphire substrate by RF-MBE

Non-polar a-plane GaN films were grown on an r-plane sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE). The effect of growth temperature on structural, morphological and optical properties has been studied. The growth of non-polar a-plane (1 1 ?2 0) orientation of the GaN epilayers were confirmed by high resolution X-ray diffraction (HRXRD) study. The X-ray rocking curve (XRC) full width at half maximum of the (1 1 ?2 0) reflection shows in-plane anisotropic behavior and found to decrease with increase in growth temperature. The atomic force micrograph (AFM) shows island-like growth for the film grown at a lower temperature. Surface roughness has been decreased with increase in growth temperature. Room temperature photoluminescence shows near band edge emission at 3.434–3.442 eV. The film grown at 800 °C shows emission at 2.2 eV, which is attributed to yellow luminescence along with near band edge emission.     

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100 nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577 °C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10 nm has been observed.     

Effects of substrate orientation on aluminum grown on MgAl2O4 spinel using molecular beam epitaxy

Al thin films have been grown on single-crystal MgAl₂O₄ spinel substrates using solid source molecular beam epitaxy. The structural properties of Al layers were systematically investigated as a function of substrate orientation. X-ray diffraction reveals that Al layers are coherently grown on both (0 0 1)- and (1 1 1)-oriented spinel substrates. However, scanning electron microscopy and atomic force microscopy show that Al layers on (0 0 1) spinel substrates display smoother surface morphology than those grown on (1 1 1) spinel substrates. Additionally, electron backscatter diffraction and transmission electron microscopy demonstrate the presence of a high density of twin domain structures in Al thin films grown on (1 1 1) spinel substrates.     

Electric field induced structural modification and second order optical nonlinearity in potassium niobium silicate glass

Second harmonic generation properties have been studied in 23 K₂O · 27Nb₂O₅ · 50 SiO₂ glass subjected to thermal poling. The poling-induced optical nonlinearity, with χ⁽²⁾ = 3.8 pm/V, has been related to structural modifications within a surface layer of a few microns on the anode side, as evidenced by means of confocal micro-Raman mapping along the sample thickness. The data indicate that the structural changes result from a charge transport process that causes network modifications in an alkali depleted layer whose thickness is comparable with that of the non-linear region. The Raman data also indicate that in the alkali depleted layer the network polymerization degree increases as a consequence of ion migration. The origin of the nonlinearity and the mechanisms activated by poling are discussed. The mechanism of non-bridging-oxygen to bridging-oxygen bond switching is proposed to explain ion migration and the subsequent structural changes in the glass.     

Erbium activated HfO2 based glass–ceramics waveguides for photonics

(100 − x)SiO₂ − xHfO₂ (x = 10, 20, 30 mol) glass–ceramics planar waveguides activated by 0.3 mol% Er³⁺ ions were prepared by sol–gel route, using dip-coating deposition on v-SiO₂ substrates. High resolution transmission electron microscopy has shown that after an adapted heat treatment, the resulting materials show nanocrystalline structures. The glass–ceramics waveguides were characterized by m-line, Raman, losses measurements, and photoluminescence spectroscopy. Photoluminescence spectroscopy has demonstrated the embedding of erbium ions in the nanocrystals. The results are discussed with the aim of assessing the role of hafnia on the structural, optical and spectroscopic properties of erbium doped silica hafnia glass–ceramics planar waveguides.     

Improving the performance of amorphous and crystalline silicon heterojunction solar cells by monitoring surface passivation

The influence of thermal annealing on the crystalline silicon surface passivating properties of selected amorphous silicon containing layer stacks (including intrinsic and doped films), as well as the correlation with silicon heterojunction solar cell performance has been investigated. All samples have been isochronally annealed for 1 h in an N₂ ambient at temperatures between 150 °C and 300 °C in incremental steps of 15 °C. For intrinsic films and intrinsic/n-type stacks, an improvement in passivation quality is observed up to 255 °C and 270 °C, respectively, and a deterioration at higher temperatures. For intrinsic/n-type a-Si:H layer stacks, a maximum minority carrier lifetime of 13.3 ms at an injection level of 10¹⁵ cm^? 3 has been measured. In contrast, for intrinsic/p-type a-Si:H layer stacks, a deterioration in passivation is observed upon annealing over the whole temperature range. Comparing the lifetime values and trends for the different layer stacks to the performance of the corresponding cells, it is inferred that the intrinsic/p-layer stack is limiting device performance. Furthermore, thermal annealing of p-type layers should be avoided entirely. We therefore propose an adapted processing sequence, leading to a substantial improvement in efficiency to 16.7%, well above the efficiency of 15.8% obtained with the ‘standard’ processing sequence.     

The glass transition temperature of mixed glass former 0.35Na2O + 0.65[xB2O3 + (1 − x)P2O5] glasses

The mixed glass former effect (MGFE) is defined as the non-linear and non-additive change in the ionic conductivity with changing glass former fraction at constant modifier composition between two binary glass former compositions. In this study, sodium borophosphate glasses, 0.35Na₂O + 0.65[xB₂O₃ + (1 ? x)P₂O₅] with 0 ≤ x ≤ 1, have been prepared and their glass transition temperatures (T_g) have been examined as an alternative indicator of the MGFE and as an indicator of changes in the short range order (SRO) structural network units that could cause or contribute to the MGFE. The changes in T_g show a positive non-additive and non-linear trend over the changing glass former fraction, x. The increase in T_g is related to the increasing number of bridging oxygens (BO) in the glass samples, which is caused by the increase in the number of tetrahedral boron, B⁴, units in the SRO structure.     

Simulation of surface structural relaxation kinetics in silica glass accelerated by water vapor

It is known that surface structural relaxation of silica glass takes place more rapidly than bulk structural relaxation, especially in the presence of water vapor. The effect of water vapor pressure, heat-treatment temperature and initial fictive temperature on the surface structural relaxation kinetics in silica glasses was investigated by measuring the change of the surface fictive temperature determined from the IR reflection peak shift of silica structural bands. The superimposed component of bulk structural relaxation was subtracted from the measured surface structural relaxation data to isolate the true surface structural relaxation kinetics. The obtained surface structural relaxation data as a function of fictive temperature, heating temperature and water vapor pressure were simulated with a model based on the diffusion equation with time-dependent surface concentration. The simulation model was used to predict the surface structural relaxation kinetics of the optical fiber having a high fictive temperature of ～ 1650 °C at 950 °C under 355 torr of water vapor, and it was confirmed that the present model can simulate surface structural relaxation of the fiber reasonably well.     

Role of oxygen in structural properties of annealed CuAlO2 films

CuAlO₂ films were sputtered on quartz substrates at different oxygen partial pressures (OPP) and carried out the annealing at 900 °C for 5 h in N₂ ambient. The structural properties of these films have been studied in detail by X-ray diffraction, Raman spectroscopy, and atomic force microscopy. Annealed CuAlO₂ films are grown along the (0 0 1) preferential orientation. The film deposited at 20% OPP demonstrates the excellent crystalline behavior and the smallest electrical resistivity (41.8 Ω cm). At higher OPP, the crystalline behavior begins to degenerate up to the amorphous state at 60% OPP, and some micro-caves presented in the film surface become larger and deeper with the increase in OPP. We believe that the negative thermal expansion behavior associated with excess oxygen atoms is the primary responsibility for the change in structural properties.     

The transformation balance between two types of structural defects in silica glass in ion-irradiation processes

Structural modification in silica glass irradiated by Au ions was investigated by ultraviolet (UV), photoluminescence and Raman spectrum at the energies from 0.5 to 8 MeV with a fluence of 5 × 10¹³ cm^? 2. In this transit energy region, both nuclear energy loss and electronic energy loss are not negligible. It was found that both the formation of irradiation-induced intrinsic defects and structural transformation from irregular large ring structure (LRS) to small three- and four-member ring structures (SRS) are dominated by the nuclear energy loss. Furthermore, unlike the case of irradiation with β, γ or proton, the concentration of non-bridging oxygen hole center is much enhanced followed with a distinct peak appearing at 5.05 eV in the UV absorption spectra that is attributed to divalent Si. The results suggest that the structural modification in silica glass in the transit energy region is dominated by nuclear energy loss. A mutual transformation balance model among irradiation-induced intrinsic defects, LRS and SRS is established to interpret the identical variation tendencies of intrinsic defects and structural transformation with ion energy.     

FT-IR and Raman study of silver lead borate-based glasses

Glasses from the xMnO · (100−x)[3B₂O₃ · 0.9PbO · 0.1Ag₂O] system with 0 ⩽ x ⩽ 20 mol% have been prepared and studied by means of FT-IR absorption and Raman scattering. We interpreted the spectroscopic data in conjunction with the structural information obtained by X-ray diffraction and scanning electron microscopy (SEM). The X-ray patterns have showed homogenous glasses over the entire compositional range while the SEM pictures have detected metallic silver or Ag₂O clusters dispersed in the glass network. Acting as complementary spectroscopic techniques, both types of measurements, FT-IR and Raman, revealed that the network structure of the studied glasses is mainly based on BO₃ and BO₄ units placed in different structural groups, the BO₃ units being dominant. The influence of manganese-ion content (x), on the N_BO4/N_BO3 ratio evolution was investigated.