Single crystal

About: Single crystal is a research topic. Over the lifetime, 59617 publications have been published within this topic receiving 870828 citations.

Reactive molecular beam epitaxy of aluminium nitride

Abstract: Single‐crystal films of AlN have been fabricated on sapphire and silicon substrates by reactive molecular beam epitaxy (MBE) at 1000°–1200°C. The crystal quality of the films was examined by the reflection high‐energy electron diffraction technique and the measurement of cathodoluminescence. The results suggest that the epitaxial AlN films prepared by reactive MBE have good quality as compared with bulk single crystal. Nonlinear optical coefficients and the electromechanical coupling constant of surface acoustic waves were also measured. It was shown that the AlN films have unique characteristics and are promising for nonlinear optical and surface acoustic wave devices.

Surface composition and structure of Co3O4(110) and the effect of impurity segregation

Abstract: The Co3O4(110) single crystal surface has been characterized by low energy electron diffraction (LEED), Auger electron spectroscopy, and x-ray photoelectron spectroscopy (XPS). LEED analysis of the clean Co3O4(110) spinel surface shows a well-ordered pattern with sharp diffraction features. The XPS spectra are consistent with stoichiometric Co3O4 as determined by the concentration ratio of oxygen to cobalt (CO/CCo) and spectral peak shape. In particular, the cobalt 2p XPS spectra are characteristic of the spinel structure with Co3+ occupying octahedral sites and Co2+ in tetrahedral sites within the lattice. During prolonged heating at 630 K, bulk impurities of K, Ca, Na, and Cu segregated to the surface. Sodium desorbed from the surface as NaOH at 825 K, potassium and calcium were only removed by sputtering since no desorption from the surface was detected for temperatures up to 1000 K. Copper also disappeared upon heating above 700 K, most likely by desorbing although the possibility of diffusion back into the bulk could not be eliminated. The appearance of copper impurities correlated with Co3O4(110) surface reduction to CoO, and the surface could not be fully reoxidized even upon extended oxygen annealing as long as the copper impurity remained on the surface. Upon removal of the Cu from the near-surface region, the surface was easily reoxidized to Co3O4 by O2.

Elastic Organic Crystals of a Fluorescent π-Conjugated Molecule.

Abstract: An elastic organic crystal of a π-conjugated molecule has been fabricated. A large fluorescent single crystal of 1,4-bis[2-(4-methylthienyl)]-2,3,5,6-tetrafluorobenzene (over 1 cm long) exhibited a fibril lamella morphology based on slip-stacked molecular wires, and it was found to be a remarkably elastic crystalline material. The straight crystal was capable of bending more than 180° under applied stress and then quickly reverted to its original shape upon relaxation. In addition, the fluorescence quantum yield of the crystal was about twice that of the compound in THF solution. Mechanical bending-relaxation resulted in reversible change of the morphology and fluorescence. This research offers a more general approach to flexible crystals as a promising new family of organic semiconducting materials.

A study on orientation effects in polyethylene in the light of crystalline texture

Abstract: This paper, first of a series, maps out the changes in molecular orientation occurring when doubly-oriented polyethylene is heat relaxed as a preliminary to further work on the relation of molecular orientation and crystalline texture. While earlier investigations have dealt with individual aspects of this field, the present results have now brought the previous, in some respects diverging views, into a single, unified scheme. Drawing followed by rolling produces a twinned, double texture which has been shown to result from a single texture (single crystal analogue) present under pressure, in agreement with an earlier hypothesis. The same single crystal analogue is produced by slight annealing. Further annealing leads to progressive molecular tilt around the b crystal axis and finally to a texture which is the macroscopic analogue of a solution grown crystal platelet. Beyond this stage, randomisation around b sets in, resulting in a texture analogue of a spherulitic fibril (Point's fibre “N”). Some orientation in the non-crystalline position of the material is also indicated, which, however, decays faster than that of the crystals.