Influence of structural variations on high-resolution electron microscopy images of poly[1,6-di(N-carbazolyl)2,4-hexadiyne] nanocrystals

Abstract

High-resolution electron microscopy (HREM) was used to obtain information about structural variations in poly[1,6-di(N-carbazolyl)-2,4-hexadiyne) (polyDCHD) nanocrystals. Several particles in different crystallographic zones were imaged with high spatial resolution of up to 0.19nm, resulting in image information on a subunit-cell level (a = 1.74nm, b = 1.29nm and c = 0.49nm; β = 108.3°, P2₁/a; Z = 2). The average nanocrystal had a nominally rectangular shape and a size of about 40 nm × 80 nm with the major growth direction along the polymer chains (001). Most of the HREM images correspond to the [100] or [120] zone. In a few cases, it was also possible to image crystals oriented in the [001] zone with the polymer chain axis normal to the carbon substrate. Image interpretation was done by comparison with multislice HREM image simulations based on an approximation for the crystal structure and imaging conditions. These simulations performed for polyDCHD indicate that the HREM image is sensitive to small changes in the molecular geometry. A parameter to describe these changes in terms of intensity variations ΔI in the diffraction pattern was determined. Pendellösung plots show that depending on the zone imaged a change in the atomic positions of the order of a hundredth of a nanometre results in intensity variations ΔI of up to 70%, leading us to expect that experimental images are strongly influenced by small amounts of disorder. The relationship between disorder and expected changes in dynamic electron diffraction has been evaluated using a variety of models for the crystal structure obtained by molecular dynamics simulations at different temperatures. The influence of disorder on dynamic diffraction and HREM images becomes especially important for the analysis of defects or near surfaces, where distortions are expected. Nevertheless, no significant change in lattice spacing was observed near the surface of the nanocrystals. Image processing using local fast Fourier transform comparison and Fourier filtering was used to gain detailed information about the geometry of small angle grain boundaries in these polyDCHD nanocrystals, for example a triple junction with a misorientation of 3°.