3D imaging and simulation of the polarisation distribution in molecular crystals

Citation
A. Quintel et al., 3D imaging and simulation of the polarisation distribution in molecular crystals, MOLEC CRYST, 338, 2000, pp. 243
Citations number
17
Categorie Soggetti
Physical Chemistry/Chemical Physics
Volume
338
Year of publication
2000
Database
ISI
SICI code
Abstract
The spatial polarisation distribution in inhomogeneously polar molecular cr ystals has been imaged by scanning pyroelectric tomography and simulated by a Markov process. The experimental technique combines scanning pyroelectri c microscopy (SPEM) with layerwise thinning of crystals. The SPEM probes th e local spontaneous polarisation by its temperature dependence (pyroelectri c effect). A focused and intensity modulated laser beam scans the surface o f a polar sample and induces temperature changes in a volume depending on t he laser spot size and the thermal diffusion length lambda(th) Since high l ateral resolution of the SPEM is only available for small values of lambda( th) depth information to a resolution of similar to 10 mu m is achieved by repeated scanning and stepwise thinning of crystals. A layer by layer techn ique can provide 3D imaging of the polar ordering with a lateral resolution of similar to 20 mu m at a probed layer thickness of similar to 10 mu m. A pplied to perhydrotriphenylene (PI-ITP) co-crystallised with 1-(4-nitrophen yl)piperazine (NPP) two conical macro-domains of opposite and nearly consta nt polarisation were found. The SPEM results are in good agreement with a h omogeneous Markov chain model driving dipolar molecules into a parallel sta te within channels of PHTP. The new tomographic view reveals that the oppos ing cones are partially overlapping at the tips. Lateral growth processes a re assumed to be responsible for this. A theoretical section covers aspects of the convergency into the polar state and the length of polar chains dis cussed in terms of intermolecular interaction energies.