Synthesis of wurtzite-phase ZnS nanocrystal and its optical properties

The wurtzite phase of ZnS nanocrystal has been prepared by annealing in 200-600 degrees C temperature range, its cubic phase of 2-3 nm size. prepared through soft chemical method. Results of isochronal experiments of 2 h at different temperatures indicate that visible transformation to wurtzite from cubic ZnS appears at a temperature of 400 degrees C, which is about three times smaller than that of bulk ZnS phase transition temperature. The phases, nanostructures, and optical absorption characteristics are obtained through X-ray diffraction. transmission electron microscopy, and UV-visible absorption spectroscopy. A stable and green photoluminescence emission peaked at 518 nm is observed from the 600 degrees C annealed samples, under ultraviolet light excitation.

Vibration-rotation pattern in acetylene. II. Introduction of Coriolis coupling in the global model and analysis of emission spectra of hot acetylene around 3 mu m

A high temperature source has been developed and coupled to a high resolution Fourier transform spectrometer to record emission spectra of acetylene around 3 mu m up to 1455 K under Doppler limited resolution (0.015 cm(-1)). The nu(3)-ground state (GS) and nu(2)+nu(4)+nu(5)(Sigma(+)(u) and Delta(u))-GS bands and 76 related hot bands, counting e and f parities separately, are assigned using semiautomatic methods based on a global model to reproduce all related vibration-rotation states. Significantly higher J-values than previously reported are observed for 40 known substates while 37 new e or f vibrational substates, up to about 6000 cm(-1), are identified and characterized by vibration-rotation parameters. The 3 811 new or improved data resulting from the analysis are merged into the database presented by Robert et al. [Mol. Phys. 106, 2581 (2008)], now including 15 562 lines accessing vibrational states up to 8600 cm(-1). A global model, updated as compared to the one in the previous paper, allows all lines in the database to be simultaneously fitted, successfully. The updates are discussed taking into account, in particular, the systematic inclusion of Coriolis interaction.

The role of triplet states in the emission mechanism of polymer light-emitting diodes

The blue emission of polyfluorene (PF)-based light-emitting diodes (LEDs) is known to degrade due to a low-energy green emission, which hitherto has been attributed to oxidative defects. By studying the electroluminescence (EL) from ethyl-hexyl substituted PF LEDs in the presence of oxygen and in an inert atmosphere, and by using trace quantities of paramagnetic impurities (PM) in the polymer, we show that the triplet states play a major role in the low-energy emission mechanism. Our time-dependent many-body studies show a large cross-section for the triplet formation in the EL process in the presence of PM, primarily due to electron-hole recombination processes.

Radio frequency emission in electron beam-plasma and beam-beam interaction

A linear excitation of electromagnetic modes at frequencies (n + ı89 in a plasma through which two electron beams are contra-streaming along the magnetic field is investigated. This may be a source of the observed {cote emissions at auroral latitudes.

Anomalous plasma heating by an intense laser beam

Heating of laser produced plasmas by an instability is investigated. For intense laser beams anomalous absorption is found. A comparison is made with the experiment.

Diffusion of localized optical excitations and trap emission in ruby

Using the critical percolation conductance method the energy-dependent diffusion coefficient associated with thermally assisted transfer of the R1 line excitation between single Cr3+ ions with strain-induced randomness has been calculated in the 4A2 to E(2E) transition energies. For localized states sufficiently far away from the mobility edge the energy transfer is dominated by dipolar interactions, while very close to the mobility edge it is determined by short-range exchange interactions. Using the above energy-dependent diffusion coefficient a macroscopic diffusion equation is solved for the rate of light emission by Cr3+ ion-pair traps to which single-ion excitations are transferred. The dipolar mechanism leads to good agreement with recent measurements of the pair emission rate by Koo et al. (Phys. Rev. Lett., vol.35, p.1669 (1975)) right up to the mobility edge.

Acoustic emission source location and damage detection in a metallic structure using a graph-theory-based geodesic pproach

A geodesic-based approach using Lamb waves is proposed to locate the acoustic emission (AE) source and damage in an isotropic metallic structure. In the case of the AE (passive) technique, the elastic waves take the shortest path from the source to the sensor array distributed in the structure. The geodesics are computed on the meshed surface of the structure using graph theory based on Dijkstra's algorithm. By propagating the waves in reverse virtually from these sensors along the geodesic path and by locating the first intersection point of these waves, one can get the AE source location. The same approach is extended for detection of damage in a structure. The wave response matrix of the given sensor configuration for the healthy and the damaged structure is obtained experimentally. The healthy and damage response matrix is compared and their difference gives the information about the reflection of waves from the damage. These waves are backpropagated from the sensors and the above method is used to locate the damage by finding the point where intersection of geodesics occurs. In this work, the geodesic approach is shown to be suitable to obtain a practicable source location solution in a more general set-up on any arbitrary surface containing finite discontinuities. Experiments were conducted on aluminum specimens of simple and complex geometry to validate this new method.

In vitro demonstration of the heavy-atom effect for photodynamic therapy

Photodynamic therapy (PDT) is an emerging treatment modality for a range of disease classes, both cancerous and noncancerous. This has brought about an active pursuit of new PDT agents that can be optimized for the unique set of photophysical characteristics that are required for a successful clinical agent. We now describe a totally new class of PDT agent, the BF2-chelated 3,5-diaryl-1H-pyrrol-2-yl-3,5-diarylpyrrol-2-ylideneamines (tetraarylazadipyrromethenes). Optimized synthetic procedures have been developed to facilitate the generation of an array of specifically substituted derivatives to demonstrate how control of key therapeutic parameters such as wavelength of maximum absorbance and singlet-oxygen generation can be achieved. Photosensitizer absorption maxima can be varied within the body's therapeutic window between 650 and 700 nm, with high extinction coefficients ranging from 75,000 to 85,000 M(-1) cm(-1). Photosensitizer singlet-oxygen generation level was modulated by the exploitation of the heavy-atom effect. An array of photosensitizers with and without bromine atom substituents gave rise to a series of compounds with varying singlet-oxygen generation profiles. X-ray structural evidence indicates that the substitution of the bromine atoms has not caused a planarity distortion of the photosensitizer. Comparative singlet-oxygen production levels of each photosensitizer versus two standards demonstrated a modulating effect on singlet-oxygen generation depending upon substituent patterns about the photosensitizer. Confocal laser scanning microscopy imaging of 18a in HeLa cervical carcinoma cells proved that the photosensitizer was exclusively localized to the cellular cytoplasm. In vitro light-induced toxicity assays in HeLa cervical carcinoma and MRC5-SV40 transformed fibroblast cancer cell lines confirmed that the heavy-atom effect is viable in a live cellular system and that it can be exploited to modulate assay efficacy. Direct comparison of the efficacy of the photosensitizers 18b and 19b, which only differ in molecular structure by the presence of two bromine atoms, illustrated an increase in efficacy of more than a 1000-fold in both cell lines. All photosensitizers have very low to nondeterminable dark toxicity in our assay system.

Spinodal Decomposition in Tellurite-Based Glasses Induced by Excimer Laser Irradiation

Tellurite-based glasses in the TeO2-K3Li2Nb5O15, TeO2-Ba5Li2Ti2Nb8O30, and V2Te2O9 were fabricated by the conventional melt-quenching technique. Amorphous and glassy characteristics of the as-quenched samples were established via the X-ray powder diffraction technique and differential thermal analysis, respectively. The as-quenched samples were irradiated by an excimer laser (248 nm). The effect of laser power, duration of irradiation, and the frequency of the laser pulses on the surface features of the above glasses were studied. The optical microscopic studies carried out on the above systems revealed the presence of quasi-periodic and periodic structures on their surfaces. The local compositional variations of these structures were confirmed by back-scattered electron imaging using scanning electron microscope accompanied by energy-dispersive X-ray analysis. These results were convincing enough to state that the glasses in the present investigations had undergone spinodal decomposition on laser irradiation. The incidence of the interconnected texture of two different phases was observed owing to the quenching effect produced by the heating and cooling cycle of the successive laser pulses. Ring- and line-shaped patterns were also observed, respectively, when the pulse frequency of the laser and the duration of irradiation were increased.

Acoustic emission source location in composite structure by Voronoi construction using geodesic curve evolution

Conventional analytical/numerical methods employing triangulation technique are suitable for locating acoustic emission (AE) source in a planar structure without structural discontinuities. But these methods cannot be extended to structures with complicated geometry, and, also, the problem gets compounded if the material of the structure is anisotropic warranting complex analytical velocity models. A geodesic approach using Voronoi construction is proposed in this work to locate the AE source in a composite structure. The approach is based on the fact that the wave takes minimum energy path to travel from the source to any other point in the connected domain. The geodesics are computed on the meshed surface of the structure using graph theory based on Dijkstra's algorithm. By propagating the waves in reverse virtually from these sensors along the geodesic path and by locating the first intersection point of these waves, one can get the AE source location. In this work, the geodesic approach is shown more suitable for a practicable source location solution in a composite structure with arbitrary surface containing finite discontinuities. Experiments have been conducted on composite plate specimens of simple and complex geometry to validate this method.

LbL Fabricated Poly(Styrene Sulfonate)/TiO2 Multilayer Thin Films for Environmental Applications

Fabrication of multilayer ultrathin composite films composed of nanosized titanium dioxide particles (P25, Degussa) and polyelectrolytes (PELs), such as poly(allyl amine hydrochloride) (PAH) and poly(styrene sulfonate sodium salt) (PSS), on glass substrates using the layer-by-layer (LbL) assembly technique and its potentia application for the photodegradation of rhodamine B under ultraviolet (UV) irradiation has been reported. The polyelectrolytes and TiO2 were deposited on glass substrates at pH 2.5 and the growth of the multilayers was studied using UV/vis speccrophotometer. Thicknes measurements of the films showed a linear increase in film thickness with increase in number of bilayers. The surface microstructure of the thin films was characterized by field emission scanning electron microscope. The ability of the catalysts immobilized by this technique was compared with TiO2 films prepared by drop casting and spin coating methods. Comparison has been made in terms of film stability and photodegradation of rhodamine B. Process variables such as the effect of surface area of the multilayers, umber of bilayers, and initial dye concentration on photodegradation of rhodamine B were studied. Degradation efficiency increased with increase in number of catalysts (total surface area) and bilayers. Kinetics analysis indicated that the photodegradation rates follow first order kinetics. Under maximum loading of TiO2, with five catalyst slides having 20 bilayers of polyelectrolyte/TiO2 on each, 100 mL of 10 mg/L dye solution could be degraded completely in 4 h. The same slides could be reused with the same efficiency for several cycles. This study demonstrates that nanoparticles can be used in wastewater treatment using a simple immobilization technique. This makes the process an attractive option for scale up.

Field Emission Properties of Carbon Nanotube Arrays with Defects and Impurities

It has been found experimentally that the results related to the collective field emission performance of carbon nanotube (CNT) arrays show variability. The emission performance depends on the electronic structure of CNTs (especially their tips). Due to limitations in the synthesis process, production of highly pure and defect free CNTs is very difficult. The presence of defects and impurities affects the electronic structure of CNTs. Therefore, it is essential to analyze the effect of defects on the electronic structure, and hence, the field emission current. In this paper, we develop a modeling approach for evaluating the effect of defects and impurities on the overall field emission performance of a CNT array. We employ a concept of effective stiffness degradation for segments of CNTs, which is due to structural defects. Then, we incorporate the vacancy defects and charge impurity effects in our Green's function based approach. Simulation results indicate decrease in average current due to the presence of such defects and impurities.

Induced higher-order aberrations after laser in situ keratomileusis (LASIK) performed with wavefront-guided IntraLase femtosecond laser in moderate to high astigmatism

Background Wavefront-guided Laser-assisted in situ keratomileusis (LASIK) is a widespread and effective surgical treatment for myopia and astigmatic correction but whether it induces higher-order aberrations remains controversial. The study was designed to evaluate the changes in higher-order aberrations after wavefront-guided ablation with IntraLase femtosecond laser in moderate to high astigmatism. Methods Twenty-three eyes of 15 patients with moderate to high astigmatism (mean cylinder, −3.22 ± 0.59 dioptres) aged between 19 and 35 years (mean age, 25.6 ± 4.9 years) were included in this prospective study. Subjects with cylinder ≥ 1.5 and ≤2.75 D were classified as moderate astigmatism while high astigmatism was ≥3.00 D. All patients underwent a femtosecond laser–enabled (150-kHz IntraLase iFS; Abbott Medical Optics Inc) wavefront-guided ablation. Uncorrected (UDVA), corrected (CDVA) distance visual acuity in logMAR, keratometry, central corneal thickness (CCT) and higher-order aberrations (HOAs) over a 6 mm pupil, were assessed before and 6 months, postoperatively. The relationship between postoperative change in HOA and preoperative mean spherical equivalent refraction, mean astigmatism, and postoperative CCT were tested. Results At the last follow-up, the mean UDVA was increased (P < 0.0001) but CDVA remained unchanged (P = 0.48) and no eyes lost ≥2 lines of CDVA. Mean spherical equivalent refraction was reduced (P < 0.0001) and was within ±0.50 D range in 61 % of eyes. The average corneal curvature was flatter by 4 D and CCT was reduced by 83 μm (P < 0.0001, for all), postoperatively. Coma aberrations remained unchanged (P = 0.07) while the change in trefoil (P = 0.047) postoperatively, was not clinically significant. The 4th order HOAs (spherical aberration and secondary astigmatism) and the HOA root mean square (RMS) increased from −0.18 ± 0.07 μm, 0.04 ± 0.03 μm and 0.47 ± 0.11 μm, preoperatively, to 0.33 ± 0.19 μm (P = 0.004), 0.21 ± 0.09 μm (P < 0.0001) and 0.77 ± 0.27 μm (P < 0.0001), six months postoperatively. The change in spherical aberration after the procedure increased with an increase in the degree of preoperative myopia. Conclusions Wavefront-guided IntraLASIK offers a safe and effective option for vision and visual function improvement in astigmatism. Although, reduction of HOA is possible in a few eyes, spherical-like aberrations are increased in majority of the treated eyes.

Season of birth and location of residence: Defining the role of ultraviolet radiation exposure on childhood temperament and behaviour

This thesis added new insight to research knowledge about the role that season and ultraviolet radiation (UV) exposure during pregnancy has on children's temperament and behaviours, using a nation-wide longitudinal study. It was found that young children born in summer months are likely to have problematic behaviours. The thesis also found that summer-born children are likely to receive lowest levels of UV exposure during the gestational period. Finally, this work showed that low gestational UV exposure is associated with an increased risk of behavioural problems in children.