Table of contents

We investigate the energy spread, ΔE_1/2, versus emission current, i, of the doubly charged ions emitted by a Au₇₇Ge₁₄Si₉ liquid metal alloy ion source. For the ionic species of main interest, namely Ge²⁺, two modes were found with regard to the dependence of ΔE_1/2 on i. In the first mode ΔE_1/2∝i^0.3, and in the second ΔE_1/2∝i. The difference is attributed to changes in the composition of the alloy during field evaporation. Au²⁺ and Si²⁺ were also found to obey a ΔE_1/2∝i^0.3 law; in the case of Si²⁺ however, there is a deviation from this law at high currents.

Fluorescence lifetime imaging is a rather new and effective tool that can be used to study complex biological samples, either at microscopic or macroscopic levels. The map of the fluorescence lifetime allows one to discriminate amongst different fluorophores and to achieve valuable insights into the behaviour of emitting molecules, leading to information like local pH, oxygen concentration in cells, etc. Moreover, the distribution in space of any fluorescent marker achievable with this technique can be exploited for diagnostic purposes in medicine. After a brief introduction on the motivations for applying fluorescence lifetime imaging in biology and medicine, the basic principles of this technique will be addressed. Then, the two possible implementations of fluorescence lifetime imaging (i.e. the frequency domain and the time domain methods) will be presented. For this purpose, special attention will be devoted to practical aspects of image acquisition and processing, especially for what concerns the time domain method. Then, the analysis of the state-of-the-art systems will include a brief discussion on new concepts that have recently been introduced in this research field. Finally, two interesting applications of fluorescence lifetime imaging will be presented. The former refers to skin tumour detection and has been successfully applied in a preliminary clinical trial, the latter regards DNA chips reading and has been tested only at laboratory level, yet it has produced promising results for its future implementation in commercial systems.

The ability to manipulate the electronic properties of semiconductor surfaces and interfaces is of significant technological importance. Band engineering relies on a fundamental understanding of the physics involved in manipulating charge transfer across junctions, at free surfaces, Schottky contacts or heterojunctions. In this paper, the development of new techniques to control and manipulate free surfaces and interface barriers related to the improvement of semiconductor quantum well laser operation are considered. In particular, surface passivation of atomic scale defects on the laser facet using ultra-thin Si layers deposited at 280\r{}C is discussed. The local passivation mechanism was observed directly using a combination of surface science techniques, mainly scanning tunnelling microscopy/spectroscopy (STM/S). Si was found to selectively bond to defects and return the corresponding electronic properties back to the flat band condition. Secondly, heterojunction modification to enhance carrier confinement within the active region of the device is considered using multi-quantum barriers based on theoretical modelling and experimental verification. Limitations of the techniques are also addressed. The use of STM and STS to investigate interface modification on the nanometre scale is highlighted as being relevant to future device fabrication and optimization based on nano-technology.

The structure and magnetic properties of Y_1-xDy_xMn₆Sn₆ (x = 0-1) compounds have been studied by x-ray diffraction and magnetization measurements. All studied samples crystallize in HfFe₆Ge₆-type structure (space group P6/mmm). The compound with x = 0 behaves as anti-ferromagnetic behaviour, and the compounds (x = 0.6-1) display ferrimagnetic behaviours in the whole magnetic ordering temperature range. The compounds (x = 0.2-0.4) exhibit a transition from ferrimagnetic to anti-ferromagnetic, then to ferrimagnetic state again with increasing temperature. The magnetization curves of compounds (x = 0.3-1) at 5 K almost saturate at 5 T, and the saturation magnetization M_s decreases with increasing Dy content. The compounds (x = 0-0.3) display field-induced metamagnetic transition, and the threshold field decreases with increasing Dy content. The unit-cell volume V of Y_1-xDy_xMn₆Sn₆ (x = 0-1) compounds decreases, and the ordering points increase with increasing Dy content.

The microstructure and magnetic properties of nanocomposite SmCo₅/Sm₂Co₁₇, Sm₂(CoFe)₁₇, Sm₂(CoFeZr)₁₇, Sm₂(CoFeCuZr)₁₇ powders prepared by intensive milling and subsequent annealing have been studied systematically. A small amount of Fe substitution for Co was found to slightly increase the saturation magnetization and maximum energy product of SmCo₅/Sm₂Co₁₇ powders. The coercivity is enhanced by Zr and especially Cu. The highest maximum energy product of 11.5 MGOe and coercivity of 2.4 T were obtained in 40% SmCo₅ + 60% Sm₂(Co_0.9Fe_0.1)₁₇ and 40% SmCo₅ + 60% Sm₂(Co_0.74Fe_0.1Cu_0.12Zr_0.04)₁₇ powders, respectively. All the hysteresis loops showed a smooth demagnetization curve. A few percent of Zr substitution proved to be effective in refining the microstructure and Cu increased the volume fraction of SmCo₅ phase in the mixture of two phase powders. Measurements of δM and recoil curves indicated that intergranular exchange coupling and exchange spring magnet behaviour were observed in the above samples.

This paper presents a new approach for the design of finite-length shim and gradient coils of the type that are used in magnetic resonance imaging applications. A cylindrical target region is located arbitrarily within the coil, and the exact geometry of the coil is treated without approximation. Target fields of any desired type may be specified, and may involve zonal and tesseral harmonics. The method presented here then designs the coil windings needed to produce the specified target field. In this new approach, there is complete freedom in the choice of target field, with no requirement to restrict the target field to any one spherical harmonic. The method is therefore able to design coils in which the region of interest is located asymmetrically with respect to the coil length. In addition, to control the accuracy of the fields produced by the shim coil, the design methodology presented here is also extended to show how the desired target field can be matched at two or more different co-axial target radii. The method is illustrated for the design of shim coil that involves high-order tesseral fields located asymmetrically in a finite-length coil.

Photorefractive diffraction gratings were studied in cells of homogeneously aligned nematic liquid crystal doped with buckminsterfullerene (C₆₀). These Raman-Nath gratings were induced by the interference modulation of two coherent optical beams in conjunction with an applied dc field across the sample thickness. The diffraction efficiency depends on the applied dc field. Regardless of the writing-beam power, the maximal efficiency is found to occur at ~0.8 kV cm^-1. This critical field value also determines three applied-dc-field regimes that lead to three distinct behaviours of the corresponding time-evolved diffraction-intensity curves.

The charge motion in medium on a finite space interval is considered. We analyse recent alternative attempts to interpret the radiation described by the Tamm formula as an interference of two instantaneous accelerations arising at the beginning and termination of motion. Exact solution of the Tamm problem in the time representation shows that in some time interval, only the bremsstrahlung shock wave associated with the beginning of motion and the Cherenkov shock wave exist, and there is no bremsstrahlung shock wave associated with the end of motion. This proves that in the time representation the Cherenkov radiation is not necessarily related to the interference of initial and final bremsstrahlung shock waves. In the spectral representation, we consider the motion consisting of accelerated, decelerated, and uniform parts. Analytic formulae are obtained that describe radiation intensities for this motion. Approximating the instantaneous acceleration in the original Tamm problem by the acceleration on a finite path and then tending its length to zero, we prove that the radiation intensity produced on the accelerated part of the charge trajectory also tends to zero (despite the infinite value of acceleration in this limit). This means that in the original Tamm problem the instantaneous acceleration and deceleration do not contribute to the radiation intensity (as it is usually believed). It seems that only the combined consideration of the Tamm problem in the time and spectral representations shows that the above-mentioned alternative interpretation of the Cherenkov radiation fails.

Calculated net emission coefficients for thermal argon plasma contaminated with iron vapour are presented in this paper. These data were computed using a detailed theoretical analysis that accounts for line emission, free-bound emission, and free-free emission. All important line-broadening mechanisms are included and the effects of emission and absorption on the net emission coefficient are accounted for on a line-by-line basis. Results for iron mole fractions ranging from 0 to 100% are given for a number of different path lengths and plasma temperatures at a plasma pressure of 1 atm. Comparisons to previously published data are also made.

The density of atomic oxygen in pulsed microwave excited oxygen plasmas has been measured by means of time-resolved two-photon laser-induced fluorescence. The plasma source is a slot antenna type that is operated at 2.45 GHz. Calibration of the measurements has been performed by the use of a two-photon transition of xenon in order to obtain absolute densities of atomic oxygen in the downstream zone. Besides the influence of pulse frequency on the time-averaged density, we also studied pulsed plasmas at higher pressures, which show a significant delay between the onset of microwave power and the rise of the atomic oxygen density. By changing pulse frequency, an optimum frequency is obtained which leads to a gain in the measured time-averaged atomic oxygen density as compared to cw plasmas. This effect is compared to the properties of deposited films generated in a pulsed oxygen/hexamethyldisiloxane process plasma. The results are discussed in the framework of a volume-averaged model for oxygen plasmas, which takes into account molecular and atomic oxygen, metastable oxygen, and positive and negative oxygen ions.

The breakdown delay time and temporal evolutions of the current and voltage for homogeneous photo-triggered discharges were studied in mixtures of neon with C₂H₄ or C₃H₆ at 1 bar total pressure. Experimental results were compared to predictions of a fully self-consistent modelling of the discharge. Over the whole range of initial reduced electric fields (5<E/N<35 Td) and hydrocarbon concentrations (0.1-10%), a very good agreement was obtained between measurements and computations. This provides an overall validation of the data used in the model and particularly of the electron-molecule collision cross-sections, for which a complete set is proposed for the first time in the case of propene. Moreover, a detailed analysis of the ionization processes is given, which provides insights into the dependence of the breakdown delay times with respect to the hydrocarbon concentration and the initial reduced electric field values.

The transition layer, or sheath, located at the interface between the electrode and the electric arc plasma in circuit-breaker arcs is modelled and integrated into software for the simulation of arcs. The sheath model includes the equation of the continuity of electrons, the generalized Ohm's law and the equation of conservation of total energy. The latter equation takes into account Joule heating, the radiation from the arc and phenomena on the surface of the electrode such as thermionic and radiative cooling by thermal emission. The resulting arc model can predict electrode and arc temperatures simultaneously.

Polymer dispersed ferroelectric liquid crystal (PDFLC) films are composite materials of a ferroelectric liquid crystal dispersed in the form of micron-sized droplets in a polymer matrix. In this study PDFLC films with a novel ferroelectric mixture were prepared by a polymerization-induced phase separation technique. The uniform orientation of the ferroelectric liquid crystal droplets was obtained by shearing during the phase separation process. The electro-optic characteristics of the films were analysed. Experimental results on response times, contrast ratio and spontaneous polarization were presented. Since these films exhibit high electro-optic quality, they have the potential of use in a wide range of display devices.

Polycrystalline BaTiO₃/SrTiO₃ (BT/ST) multilayer thin films were fabricated via pulsed-laser deposition onto Pt/Ti/SiO₂/Si substrates. The dielectric constant of the multilayer films was obviously enhanced and the dielectric loss was almost the same as that of the pure BaTiO₃ and SrTiO₃ thin films. A dielectric constant of 721 at 10 kHz was observed for a stacking periodicity of 60 nm at room temperature and the corresponding dielectric loss was maintained below 0.04. The study indicates that there are some differences between the multilayer films and the pure films in electrical properties. The mechanism of dielectric enhancement of the multilayer thin films was discussed.

Thin films of tellurium oxide were prepared by thermal evaporation. The effects of preparation conditions and post-deposition vacuum annealing on the optical constants of the thin films were studied. Substantial changes in the optical constants, density, structure and stoichiometry were observed following changes in the preparation conditions and annealing. The majority of the films were found to be deficient in oxygen. The presence of metallic Te was detected in films deposited on heated substrates and in all the films that were annealed. All the samples showed some degree of absorption at photon energies below the band gap. One explanation for this absorption could be oxygen deficiency and the presence of metallic Te.

Nanocomposite films consisting of Fe nanocrystals (NCs) embedded in an amorphous Al₂O₃ matrix have been fabricated by alternate pulsed laser deposition in vacuum. The size, shape and distribution of the Fe NCs have been analysed by high resolution electron microscopy. The thermal stability of the films has been studied by in situ annealing in the electron microscope, and the chemical state of the Fe has been investigated by electron energy loss spectroscopy. The nucleation and growth of the Fe NCs has been studied as a function of the number of pulses. Nucleation is homogenous over the surface of the Al₂O₃ at the beginning of deposition. The distribution of the NCs is uniform and their in-plane shape can be approximated by ellipses for a small number of pulses on the Fe target. As the Fe content increases some adjacent NCs coalesce resulting in a sudden increase in dimensions and in-plane aspect ratio. Isothermal annealing treatments for up to 160 min show that the Fe NCs are stable up to 400\r{}C, with morphological changes occurring at higher temperatures. The as-prepared Fe NCs were found to be α-Fe and no changes of oxidation state of Fe were observed after annealing.

Silicon-based Ba_xSr_1-xTiO₃ (BST) thin films have been prepared by a sol-gel method with rapid thermal annealing (RTA) processes. Phase structure of the films has been investigated by x-ray diffraction. Atomic force microscopy studies reveal a dense and smooth surface of the sol-gel prepared films. Microstructure and electrical properties of the BST films can be affected by the substrate and the annealing process. RTA method is found to be very efficient to improve the electrical properties of the films. Dielectric constant and dielectric loss of the BST films at 100 kHz are 230 and 0.02, respectively. Leakage current density of the BST capacitors is 1.6×10^-7A cm^-2 at 3 V.

Crack growth resistance curves for the non-linear fracture parameters K_R, J_R and R were measured for unirradiated PGA and IM1-24 graphites that are used as moderators in British Magnox and AGR nuclear reactors respectively. All the curves show an initial rising part, followed by a plateau region where the measured parameter is independent of crack length. J_R and R decreased at large crack lengths. The initial rising curves were attributed to development of crack bridges in the wake of the crack front, while, in the plateau region, the crack bridging zone and the frontal process zone, ahead of the crack tip, reached steady state values. The decreases at large crack lengths were attributed to interaction of the frontal zone with the specimen end face. Microscopical evidence for graphite fragments acting as crack bridges showed that they were much smaller than filler particles, indicating that the graphite fragments are broken down during crack propagation. There was also evidence for friction points in the crack wake zone and shear cracking of some larger fragments. Inspection of K_R curves showed that crack bridging contributed ~0.4 MPa m^0.5 to the fracture toughness of the graphites. An analysis of J_R and R curves showed that the development of the crack bridging zone in the rising part of the curves contributed ~20% to the total work of fracture. Energies absorbed during development of crack bridges and steady state crack propagation were greater for PGA than for IM1-24 graphite. These differences reflect the greater extent of irreversible processes occurring during cracking in the coarser microtexture of PGA graphite.

This paper describes a polarization detection method to compress a large dynamic range of laser returned signals from water surface and underwater in an airborne laser bathymetry system. The experimental results show that the method can compress the dynamic range of returned signals by more than one order of magnitude. The laser reflection on the water surface and underwater backscatter is considerably reduced without significantly affecting the laser returned signals from the underwater target.

Motivated by the lack of efficient detection techniques for metal-free trinitrotoluene (TNT) containing landmines, ¹H-¹⁴N cross-relaxation experiments on TNT have been performed using field-cycling spectroscopy in an electronically switchable volume coil. Using ¹H NMR detection (indirect method), the ¹⁴N quadrupole spectra in several small (about 0.5 g) TNT samples of different producers are determined. The experiment is considered as a step toward a double resonance TNT landmine detection scheme using surface coils.