A surface anisotropy has been shown previously to be induced in thin films of photoreactive coumarin side-chain polymers by polarized UV illumination. Consequently, the resultant cross-linked polymer layers can be used as photoalignment layers for liquid crystal displays. Homogeneous alignment of a nematic liquid crystal in contact with a layer of a model coumarin side chain polymer is obtained with the director parallel or perpendicular to the UV polarization axis depending on the incident fluence. Spectroscopic analysis of the alignment layer now confirms that both photodegradation and cross-linking occur with different dependencies on fluence. Low UV fluences give parallel photoalignment and high cross-linking reactivity. However, the residual, unreacted polymer side chains show negligible anisotropy because of their freedom to move in an isotropic fashion. Hence, parallel liquid crystal alignment is attributed to a steric interaction between the liquid crystal and syndimerized side chains of the cross-linked polymer. A switch of the photoalignment direction accompanies the subsequent development of anisotropy of the intact, unreacted polymer side chains. The side-chain anisotropy and hence perpendicular liquid crystal alignment is ascribed to photodegradation rather than cross-linking.

Polarized electroluminescence from a uniformly aligned nematic network, formed by photopolymerization of a LC monofluorene with diene end‐groups (see Figure), is reported. It is shown that macroscopic orientation of the chromophore is achieved with a photoalignment layer doped to allow hole transport. Thus standard photolithography can be used to make polarized, patterned, multilayer organic electroluminescent displays.

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We report an investigation of the incorporation mechanism of iron (Fe) as a dopant in GaN grown by MOVPE. A series of Fe doped GaN structures were studied by secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM) and X-ray photo-electron spectroscopy (XPS). A model is presented which describes the SIMS concentration profiles with excellent agreement, and, it is shown that Fe incorporation into GaN occurs via a surface segregation mechanism. This model is supported by direct measurement of a highly Fe-rich layer on the surface of Fe doped GaN by XPS. Furthermore, we find that the presence of Fe on the GaN surface promotes a transition from 2D to 3D GaN growth which is confirmed by AFM measurements of RMS roughness with increasing Fe flux.

Ill this paper, a range of polyrnethacrylate derivatives of hydroxycoumarin are investigated as photomduced alignment layers for liquid crystals, and their performance is compared with poiynyi cinnamate. For all the coumarin-containing polymers, the liquid crystal alignment direction is parallel to the polarization direction of the incident UV light at low fluences. At a critical fluence threshold, a sharp change to perpenthcular alignment is found. Molecular models and spectroscopy are used to explain this phenomenon as well as the fact that only perpendicular alignment is observed for polyvinyl cinnarnate. The azimuthal anchoring energies ofthe alignment layers are measured and values greater than 6 x io-5 j in2 are found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. Exposure of the coumarin-containing alignment layers at oblique incidence gives pretilted alignment with pretilt angles up to 5.1°.