Photoinduced anisotropy in an azo-containing ionic liquid-crystalline polymer

Photoinduced anisotropy in an azobenzene ionic liquid-crystalline polymer was investigated through dichroism, birefringence and polarization holography. A dichroism degree of 1.58 and a birefringence value Δn ∼ 10⁻² were achieved in the polymer film at room temperature, and the polymer film was found to possess the characteristics of reversible and long-term optical storage. Particularly the stored birefringence could be enhanced to Δn ∼ 10⁻¹ by annealing the film, and it is attributed to the thermal self-organization of the molecules. Furthermore, linear- and circular-polarization holographic recordings were accomplished in the polymer film and pure polarization gratings were produced.     

Surface structure and solidification morphology of aluminum nanoclusters

Classical molecular dynamics simulation with embedded atom method potential had been performed to investigate the surface structure and solidification morphology of aluminum nanoclusters Al_n (n=256, 604, 1220 and 2048). It is found that Al cluster surfaces are comprised of (1 1 1) and (0 0 1) crystal planes. (1 1 0) crystal plane is not found on Al cluster surfaces in our simulation. On the surfaces of smaller Al clusters (n=256 and 604), (1 1 1) crystal planes are dominant. On larger Al clusters (n=1220 and 2048), (1 1 1) planes are still dominant but (0 0 1) planes cannot be neglected. Atomic density on cluster (1 1 1)/(0 0 1) surface is smaller/larger than the corresponding value on bulk surface. Computational analysis on total surface area and surface energies indicates that the total surface energy of an ideal Al nanocluster has the minimum value when (0 0 1) planes occupy 25% of the total surface area. We predict that a melted Al cluster will be a truncated octahedron after equilibrium solidification.     

Sudan IV dye based poly(alkyloxymethacrylate) films for optical data storage

The polymer poly{1-[2′-methyl-4′-(2″-methylphenylazo) phenylazo]-2-(m-methacryloyloxyoctyloxy}naphthalene, where m = 6, 8, 10, is synthesized by free radical addition polymerization method for holographic optical data storage. Characterization of the polymers is done by formation of the holographic grating. A study of the dependence of diffraction efficiency of the grating formed on various parameters is presented. Surface relief gratings on these polymer films are created upon exposure to argon ion laser beams at 514.5 nm without any subsequent processing steps. The surface structure of the relief gratings has been investigated by atomic force microscopy. The depth of surface relief in a typical case is found to be around 40 nm.     

Deconfined criticality, runaway flow in the two-component scalar electrodynamics and weak first-order superfluid-solid transitions

We perform a comparative Monte Carlo study of the easy-plane deconfined critical point (DCP) action and its short-range counterpart to reveal close similarities between the two models for intermediate and strong coupling regimes. For weak coupling, the structure of the phase diagram depends on the interaction range: while the short-range model features a tricritical point and a continuous U(1) × U(1) transition, the long-range DCP action is characterized by the runaway renormalization flow of coupling into a first (I) order phase transition. We develop a “numerical flowgram” method for high precision studies of the runaway effect, weakly I-order transitions, and polycritical points. We prove that the easy-plane DCP action is the field theory of a weakly I-order phase transition between the valence bond solid and the easy-plane antiferromagnet (or superfluid, in particle language) for any value of the weak coupling strength. Our analysis also solves the long standing problem of what is the ultimate fate of the runaway flow to strong coupling in the theory of scalar electrodynamics in three dimensions with U(1) × U(1) symmetry of quartic interactions.     

Hologram multiplexing in acrylamide hydrophilic photopolymers

Unslanted diffraction gratings are recorded in a 900 μm thick acrylamide photopolymer by means of peristrophic multiplexing. A solid state Nd:YAG (λ = 532 nm) laser is used as the recording beam, with a total incident intensity of 5 mW/cm², and a He-Ne laser as the reconstruction beam. The dye concentration in the photopolymer is optimized so that it does not limit the dynamic range. Nine holograms are recorded using constant exposure time scheduling and variable exposure time scheduling. From the results obtained it may be deduced that optimization of the dye allows us to work in the linear response region of the photopolymer and at the same time obtain high values of diffraction efficiency for each hologram. An exponential increase in exposure time as the number of holograms increases enables the values of diffraction efficiency to be homogenized with regard to the case of constant exposure scheduling. In this way, better use is made of the dynamic range of acrylamide hydrophilic photopolymer.     

Crystallinity effect in the textural properties of titania-TPA catalysts

Catalysts based in titania mixed with tungstophosphoric acid (TPA), H₃PW₁₂O₄₀, in various proportions (1, 15, 25 and 50 wt%) were obtained by the sol-gel method. The gels were prepared by hydrolysis and gellation of titanium n-butoxide with a TPA solution, using HNO₃ as a catalyst to obtain a pH 3. Fresh samples were thermally treated from 100 to 800 °C, in a stepwise increment of 100 °C during 20 h per step. Specific surface areas were calculated by the BET method from the nitrogen adsorption isotherms; it was found that the surface area increased with TPA content. The crystallization behavior was followed by powder X-ray diffraction. Crystallite size measurements showed that anatase remains nanocrystalline in the studied temperature range. From the X-ray data, it was clear that below 700 °C TPA is highly dispersed in an amorphous state.     

Statistical physics and fluctuations in ballistic non-equilibrium systems

In nonequilibrium systems in the ballistic transport regime, every point of the system contains particles arriving from different regions-each of them at different temperatures-and there are only few collisions, in such a way that equilibrium between the different populations will be reached very slowly. Here, we tentatively approach the local distribution function by a superposition of local-equilibrium distribution functions with different temperatures, corresponding to the different starting positions of the particles. In a second-order expansion, we find a distribution function which depends not only on the Hamiltonian H but also on H², and we study the additional contribution to energy fluctuations.     

Methods for calculating the desorption rate of molecules from a surface at non-zero coverage: Water on MgO(0 0 1)

We present calculations of the desorption rate of water molecules from MgO(0 0 1) at a range of coverages θ and temperatures T. Our aim is to demonstrate that this can be done without making uncontrollable statistical mechanical approximations, and we achieve this by using the potential of mean force method reported previously. As in our earlier work on desorption of isolated molecules, we use a classical interaction model. We find that correlations between adsorbed molecules greatly increase the simulation time needed to obtain good statistical accuracy, compared with the isolated molecule. The activation energy for desorption varies significantly with coverage. The calculations also yield the chemical potential of adsorbed molecules as a function of θ and T, from which we can deduce the critical temperature and coverage for phase separation of adsorbed molecules.     

Characteristics of surface nano-structural modifications in nitrogen ion implanted W as a function of temperature

The surface modifications of tungsten massive samples (0.5 mm foils) made by nitrogen ion (30 keV; 1 × 10¹⁸ N⁺ cm⁻²) implantation are studied by XRD, AFM, and SIMS. XRD patterns clearly showed WN₂ (0 1 8) (rhombohedral) very close to W (2 0 0) line. Crystallite sizes obtained from WN₂ (0 1 8) line, showed an increase with substrate temperature. AFM images showed the formation of grains on W samples, which grew in size with temperature. These morphological changes are similar to those observed for thin films by increasing substrate temperature (i.e. structure zone model (SZM)). Surface roughness variation with temperature, showed a decrease with increasing temperature. The density of implanted nitrogen ions, and the depth of nitrogen ion implantation in W are studied by SIMS. The results show a minimum for N⁺ density at a certain temperature consistent with XRD results (i.e. I_W (2 0 0)/I_W (2 1 1)). This minimum in XRD results is again similar to that obtained for different thin films by Savaloni et al. [Physica B, 349 (2004) 44; Vacuum, 77 (2005) 245] and Shi and Player [Vacuum, 49 (1998) 257].     

Surface nano-structural modifications and characteristics in nitrogen ion implanted W as a function of temperature and N energy

The surface modifications of tungsten massive samples (0.5 mm foils) made by nitrogen ion implantation are studied by SEM, XRD, AFM, and SIMS. Nitrogen ions in the energy range of 16-30 keV with a fluence of 1 × 10¹⁸ N⁺ cm⁻² were implanted in tungsten samples for 1600 s at different temperatures. XRD patterns clearly showed WN₂ (0 1 8) (rhombohedral) very close to W (2 0 0) line. Crystallite sizes (coherently diffracting domains) obtained from WN₂ (0 1 8) line, showed an increase with substrate temperature. AFM images showed the formation of grains on W samples, which grew in size with temperature. Similar morphological changes to that has been observed for thin films by increasing substrate temperature (i.e., structure zone model (SZM)), is obtained. The surface roughness variation with temperature generally showed a decrease with increasing temperature. The density of implanted nitrogen ions and the depth of nitrogen ion implantation in W studied by SIMS showed a minimum for N⁺ density as well as a minimum for penetration depth of N⁺ ions in W at certain temperatures, which are both consistent with XRD results (i.e., I_{W (2 0 0)}/I_{W (2 1 1)}) for W (bcc). Hence, showing a correlation between XRD and SIMS results.     

Formation of nanodots on oblique ion sputtered InP surfaces

Using a field emission gun based scanning electron microscopy, we report the formation of nanodots on the InP surfaces after bombardment by 100 keV Ar⁺ ions under off-normal ion incidence (30° and 60° with respect to the surface normal) condition in the fluence range of 1 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻². Nanodots start forming after a threshold fluence of about 1 × 10¹⁷ ions cm⁻². It is also seen that although the average dot diameter increases with fluence the average number of dots decreases with increasing fluence. Formation of such nanostructured features is attributed due to ion-beam sputtering. X-ray photoelectron spectroscopy analysis of the ion sputtered surface clearly shows In enrichment of the sputtered InP surface. The observation of growth of nanodots on the Ar⁺-ion sputtered InP surface under the present experimental condition matches well with the recent simulation results based on an atomistic model of sputter erosion.     

Simulation of composition modification in ZnSe by nanosecond radiation of excimer laser

A numerical simulation of the composition modification induced in ZnSe by nanosecond irradiation of the KrF excimer laser (λ = 248 nm, τ = 20 ns) has been carried out. Intensive evaporation of components has shown to results in the material surface cooling and forming a nonmonotone temperature profile with maximum temperature in semiconductor volume at the distance of ∼6 nm from the surface. As a result of evaporation and diffusion of components formation of the near-surface layer with nonstoichiometric composition takes place and enrichment of selenium reaches maximum value not on the surface, but in the semiconductor volume.     

Two-photon recording of microholograms in photopolymer materials with new cationic thioxanthone photoinitiators

We have demonstrated two-photon induced recording of the microholograms at an arbitrary point within thick (∼100 μm) photopolymer material using photoinitiators on a basis of new cationic thioxanthone derivatives. Such material provides high values of refractive index change Δn = 4.8 × 10⁻³, and holographic recording sensitivity S = 1.2 cm/J. A nanosecond laser pulse at a wavelength of 532 nm was used for recording. For the selective on the depth reading of the microholograms the method of collinear heterodyning was applied.     

Simulation of dynamics of phase transitions in CdTe by pulsed laser irradiation

Numerical simulation of melting and solidification processes induced in CdTe by nanosecond radiation of ruby laser (λ = 694 nm, τ = 20 and 80 ns) and KrF excimer laser (λ = 248 nm, τ = 20 ns) taking into account components diffusion in melt and their evaporation from the surface has been carried out. Cd atoms evaporation has shown to essentially affect the dynamics of phase transitions in the near-surface region. Thus, in the case of the influence of ruby laser irradiation intensive surface cooling results in the formation of nonmonotone temperature profile with maximum temperature in semiconductor volume at the distance of ∼20 nm from the surface. The melt formed under the surface extends both to the surface and to the semiconductor volume as well. As a result of cadmium telluride components evaporation and diffusion in the melt the near-surface region is enriched with tellurium. The obtained melting threshold value of irradiation energy density is in a reasonable agreement with experimental data.     

Modification of medical metals by ion implantation of copper

The effect of copper ion implantation on the antibacterial activity, wear performance and corrosion resistance of medical metals including 317 L of stainless steels, pure titanium, and Ti-Al-Nb alloy was studied in this work. The specimens were implanted with copper ions using a MEVVA source ion implanter with ion doses ranging from 0.5 × 10¹⁷ to 4 × 10¹⁷ ions/cm² at an energy of 80 keV. The antibacterial effect, wear rate, and inflexion potential were measured as a function of ion dose. The results obtained indicate that copper ion implantation improves the antibacterial effect and wear behaviour for all the three medical materials studied. However, corrosion resistance decreases after ion implantation of copper. Experimental results indicate that the antibacterial property and corrosion resistance should be balanced for medical titanium materials. The marked deteriorated corrosion resistance of 317 L suggests that copper implantation may not be an effective method of improving its antibacterial activity.     

Enhanced near field mediated nanohole fabrication on silicon substrate by femtosecond laser pulse

Investigation of the process of nanohole formation on silicon surface mediated with near electromagnetic field enhancement in vicinity of gold particles is described. Gold nanospheres with diameters of 40, 80 and 200 nm are used. Irradiation of the samples with laser pulse at fluences below the ablation threshold for native Si surface, results in a nanosized surface modification. The nanostructure formation is investigated for the fundamental (λ = 800 nm, 100 fs) and the second harmonic (λ = 400 nm, 250 fs) of the laser radiation generated by ultrashort Ti:sapphire laser system. The near electric field distribution is analyzed by an Finite Difference Time Domain (FDTD) simulation code. The properties of the produced morphological changes on the Si surface are found to depend strongly on the polarization and the wavelength of the laser irradiation. When the laser pulse is linearly polarized the produced nanohole shape is elongated in the E-direction of the polarization. The shape of the hole becomes symmetrical when the laser radiation is circularly polarized. The size of the ablated holes depends on the size of the gold particles, as the smallest holes are produced with the smallest particles. The variation of the laser fluence and the particle size gives possibility of fabricating structures with lateral dimensions ranging from 200 nm to below 40 nm. Explanation of the obtained results is given on the basis simulations of the near field properties using FDTD model and Mie's theory.     

Morphological, structural and adsorption features of oxide composites with silica and titania matrices

Morphological, structural, electronic, and adsorption characteristics of complex oxides such as fumed silica/alumina and silica/titania, fumed silica with deposited oxides of Mg, Ti, Mn, Ni, Cu, Zn and Zr, silica gel with grafted ZrO₂, sol-gel titania doped by 3d-metals (Cr, Fe, Mn, V) were compared using adsorption, TEM, AFM, XRD, XPS, Mössbauer and Raman spectroscopy data. It was shown that surface, volume, and phase compositions of oxides, particle size distributions (5 nm-3 μm), specific surface area (S_BET ∼ 50-500 m²/g), and porosity (V_P ∼ 0.1-2 cm³/g) affected by synthesis technique and subsequent treatment determine electronic structure (bandgap, valence band and core levels structure) of the materials, adsorption of molecules and metal ions as well as other characteristics.     

Influence of zinc ion implantation on surface nanomechanical performance and corrosion resistance of biomedical magnesium-calcium alloys

It is believed that magnesium and its alloys may find applications in biomedical fields as implants, bone fixation devices, and tissue engineering scaffolds. However, their corrosion rate must be controlled. In this study, biomedical magnesium-calcium (Mg-Ca) alloys were ion-implanted with zinc. The surface nanomechanical performance and corrosion behavior of the ion-implanted Mg-Ca alloys are determined. The results show that zinc ion implantation at a dose of 0.9 × 10¹⁷ ions/cm² significantly improves the surface hardness and modulus. However, the results on corrosion resistance reveal that zinc ion implantation degrades the corrosion behavior of Mg-Ca alloys. Thus, zinc is not a favorable element for the ion implantation treatment of biomedical Mg-Ca alloys.     

Structural,optical and dielectric properties of Ni substituted NdFeO3

Present study reports the structural, optical and dielectric properties of Ni substituted NdFe_1−xNi_xO₃ (0 ≤ x ≤ 0.5) compounds prepared through the ceramic method. X-ray diffraction (XRD) confirmed an orthorhombic crystal structure of all the samples. Both unit cell volume and grain size were found to decrease with an increase in Ni concentration. Morphological study by Scanning electron microscope (SEM) shows less porosity with Ni substitution in present system. From UV–vis spectroscopy, the optical band gap was found to increase with Ni doping. This observed behavior was explained on the basis of reduction in crystallite size, unit cell volume and its impact on the crystal field potential of the system after Ni substitution. The dielectric properties (?′ and tanδ) as a function of frequency or temperature, and the ac electrical conductivity (σ_ac) as a function of frequency have been studied. Hopping of charge carriers between Fe²⁺ → Fe³⁺ ions and Ni²⁺ → Ni³⁺ ions are held responsible for both electrical and dielectric dispersion in the system. Wide optical band gap and a very high dielectric constant of these materials promote them to be a suitable candidate for memory based devices in electronic industry.