No description available.

No description available.

Photoelectron diffraction pattern is one of a number of surface-structure probes to knowledge of the atomic positions, identities and bonding mechanisms within few layers of the surface. The high-energy forward scattering version of photoelectron diffraction called X-Ray photoelectron diffraction (XPD). XPD features can be described theoretically by a simple single-scattering cluster model (SSC), which used either spherical wave (SW) or plane wave (PW) approximation. X-Ray photoelectron diffraction offers a direct method of determining interatomic bond directions in the surface region of a solid. Single Scattering Cluster theory with Plane Wave approximation (SSC-PW) is simple and reliable mean of interpreting XPD data, but it tends to overestimate the peak heights. The plane wave approximation developed by Lee and Fadley, but this developing is appropriate for photoemission from a s-core orbital. Parry modified this theory which takes account of the core sub-shell having non-zero orbital angular momentum <I>l<SUB>c</SUB>. </I>At typical XPD energies, <I>k </I>is large enough to make many partial waves phase shifts significant and this has encouraged use instead of scattering amplitudes as in Fadley's theory. This implies ignoring all terms in each Hankel function expansion, apart from the leading one <I>e<SUP>ikr</SUP>/r. </I>The corrections become progressively more important with increasing <I>l</I>. We are investigating, in this thesis, the importance of including, order by order, successive non-asymptotic terms in each Hankel function expansion and to see if thereby a successful theory can be set up without having to modify mean free path, and possibly without the complication of Debye-Waller factors. The new technique needs phase shifts to describe all the scattering processes that arise. In this thesis, we give a description of our fitting method and tabulate the phase shifts in an appendix. Trial calculations on simple systems (C-C, Cu-Cu, Ni-Ni and Si-Si) have been made and compared with predictions of Fadley theory. This work include application to some simple well defined cases <I>e.g.</I> photoemission from monolayer of Cu on Ni substrate and sandwich structure. We compared the results with experiment results. Photoemission from single crystals of elements (Cu and Si), and binary compounds (NaCl, LiF, ZnSe and GaAs) has been studied by our new technique and comparison with another experiment data and theoretical calculations has been made.

No description available.

No description available.

No description available.

No description available.

The main purpose of the work described in this thesis was to develop techniques for producing boules of ZnSe and ZnS(_x)Se(_1-x) suitable for research purposes. This was accomplished by extending the method that Clark and Woods((^1)) used for CdS to the higher temperatures needed to grow ZnS and ZnS(_x)Se(_1-x) in the range x = 0-0.6. In this system the capsule is connected to a reservoir of one of the components via a narrow orifice to maintain constant growth conditions. The system has been examined theoretically in an attempt to learn more about the actual conditions of growth within the capsule. It was concluded that growth occurs close to stoichiometry with(the ratio of P(_se2)/P(_zn) at the growth face) approximately 0.194 or 1.12 according to the element in the reservoir. Particular emphasis was placed upon the incorporation of manganese into the zinc selenide lattice. Concentrations of the order 300 p.p.m. were obtained when the element was added to the charge and MnCl(_2) was placed in the reservoir. Higher levels of manganese (~1%) were obtained using chemical vapour transport with iodine as the transport agent. Boules of solid solutions, within the range of compositions from ZnSe to ZnSe(_0.4)S(_0.6) were examined using a transmission electron microscope. The dominant crystallographic defects were found to change from thin twins to stacking faults as the amount of ZnS was increased. The origin of the defects was probably post growth stress. Finally, the anomalous photovoltaic effect was discovered in ZnSe needle crystals, and was explained qualitatively in terms of asymmetrical barriers along the polar axis of the crystal.

Magnetostriction measurements have been made on a thin film uniaxial garnet Tb(_2)Lu(_1)Fe(_5)O(_12) a double crystal diffractometer. A value of the magnetostriction constant ʎ (_111)was determined from an analysis of the measurements which employed a least squares technique. The value obtained, ʎ (_111)= (8.12-1.00)xl0(^-6), was in agreement with an expected value obtained by a linear weighting of the pure Rare Earth Iron garnet measurements of another author. Small (<10(^-5)) magnetostriction measurements in materials possessing only 180 magnetic domain walls, where an applied magnetic field is needed, have not previously been reported. The measurement given here made use of the fine resolving power of the double crystal diffractometer, and of a specially constructed electromagnet. A description of these two instruments has also been given. A review of magnetostriction measurements using single crystal X ray diffraction techniques points out the main limitation here of a comparatively low resolving power. Also, without an applied magnetic field, magnetostriction measurements by the X ray diffraction technique can only be made in crystals which have magnetic domain walls other than of the 180 type. Measurements made on different parts of the sample, and even at different parts of the rocking curve, showed considerable variation. This was attributed to local strains in the crystals used. A topographic study of the samples, using the double crystal diffractometer, revealed some of the strains which can be responsible for the local variation in magnetic anisotropy. Such an "in situ" study displays the power of this particular measurement technique over the commonly used electrical resistance strain gauge method. In the latter, various defects in the crystal structure may not be rrcvt-Mied during the course of measurement. Variations in individual magneto striction measurements may be related almost simultaneously to the defects in the crystal structure when the X ray double diffraction technique is used.

Molecular-based crystals provide faces offering specific chemical groups, ordered in a spatially well-defined array, dependent of the crystal space group. These surfaces therefore offer potential to be exploited as templates for directing material binding in order to construct regular arrays of technologically useful nanomaterials, e.g. preformed nanoparticles, metal clusters formed insitu. The dimensions typically associated with molecular crystalline materials (i.e. unit cell, spacing between repeating chemical groups in crystal structure) are typically on the angstrom scale. This offers potential for the construction of patterned arrays of materials with resolutions that may exceed the capabilities of current conventional lithographic techniques which can typically produce feature sizes down to 10 nm (for 2015, ITRS). Towards this, a series bis-pyridyl derivatives containing metal binding sites, separated by a range of spacing groups (differing in length, rigidity and aromaticity), have been prepared and characterized. Single crystal X-ray diffraction has been used to determine the molecular orientation with respect to each crystal face, and hence assess the usefulness as a substrate for metal deposition. A study into how to further influence the crystal structure, and therefore optimize the likelihood of producing surfaces with the desired binding site pattern, was conducted using a crystal engineering approach to generate polymorphs of previously synthesized compounds by varying the crystallization conditions of solvent and temperature. Models for metal binding have been prepared in the form of ligand-metal complexes and have been studied by X-ray crystallography. Reactions involving silver or zinc salts with the prepared bis-pyridyl ligands afford complexes which identify the likely metal binding sites(s) of the organic compounds and also provide details of the effect of binding with respect to ligand conformation. This information is used to predict if there will be a strong interaction between metal ions and the organic single crystal surfaces, where iii minimal changes in geometry are desired, so the pre-grown crystal substrates are left unaffected by metal deposition. AFM (atomic force microscopy) and XPS (X-ray photoelectron spectroscopy) have been used to study a range of phosphine crystal surfaces before and after metal deposition. AFM provides topological analysis of surfaces and XPS provides chemical analysis. AFM experiments on the different bare crystal faces show a range of rough, smooth and stepped surfaces. It has also been shown here that XPS experiments can be performed on different faces of an organic single crystal, with potential to distinguish between faces based on peak intensity. Similar AFM and XPS experiments have also been performed on crystals after gold nanoparticle deposition and with deposition of gold ions in different oxidation states. These experiments show clear preferential binding to specific crystal faces in line with predictions made from viewing their crystal structures.