Alexander S. Eggeman

Measurement of molecular motion in organic semiconductors by thermal diffuse electron scattering

Many of the remarkable electrical and optical properties of organic semiconductors are governed by the interaction of electronic excitations with intra- and intermolecular vibrational modes. However, in specific systems this interaction is not understood in detail at a molecular level and this has been due, at least in part, to the lack of easy-to-use and widely available experimental probes of the structural dynamics. Here we demonstrate that thermal diffuse scattering in electron diffraction patterns from organic semiconductors, such as 6,13-bistriisopropyl-silylethynyl pentacene, allows the dominant lattice vibrational modes to be probed directly. The amplitude and direction of the dominant molecular motions were determined by comparison of the diffuse scattering with simulations and molecular dynamics calculations. Our widely applicable approach enables a much deeper understanding of the structural dynamics in a broad range of organic semiconductors.

Reducing dynamic disorder in small-molecule organic semiconductors by suppressing large-amplitude thermal motions

Thermal vibrations and the dynamic disorder they create can detrimentally affect the transport properties of van der Waals bonded molecular semiconductors. The low-energy nature of these vibrations makes it difficult to access them experimentally, which is why we still lack clear molecular design rules to control and reduce dynamic disorder. In this study we discuss the promising organic semiconductors rubrene, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothio-phene and 2,9-di-decyl-dinaphtho-[2,3-b:20,30-f]-thieno-[3,2-b]-thiophene in terms of an exceptionally low degree of dynamic disorder. In particular, we analyse diffuse scattering in transmission electron microscopy, to show that small molecules that have their side chains attached along the long axis of their conjugated core are better encapsulated in their crystal structure, which helps reduce large-amplitude thermal motions. Our work provides a general strategy for the design of new classes of very high mobility organic semiconductors with a low degree of dynamic disorder.

Is precession electron diffraction kinematical? Part I:: "Phase-scrambling" multislice simulations

Results from multislice simulations are presented which demonstrate that diffracted intensities obtained using precession electron diffraction are less sensitive to the phases of structure factors compared to electron diffraction intensities recorded without precession. Since kinematical diffraction intensities depend only on the moduli of the structure factors, this result supports previous research indicating that the application of precession leads to electron diffraction intensities becoming more kinematical in nature.

Scanning precession electron tomography for three-dimensional nanoscale orientation imaging and crystallographic analysis

Three-dimensional (3D) reconstructions from electron tomography provide important morphological, compositional, optical and electro-magnetic information across a wide range of materials and devices. Precession electron diffraction, in combination with scanning transmission electron microscopy, can be used to elucidate the local orientation of crystalline materials. Here we show, using the example of a Ni-base superalloy, that combining these techniques and extending them to three dimensions, to produce scanning precession electron tomography, enables the 3D orientation of nanoscale sub-volumes to be determined and provides a one-to-one correspondence between 3D real space and 3D reciprocal space for almost any polycrystalline or multi-phase material.

Dislocation electron tomography and precession electron diffraction – minimising the effects of dynamical interactions in real and reciprocal space

Two techniques that explore how minimising the effects of dynamical interactions can lead to an improved understanding of underlying structural information in real and reciprocal space are reviewed and extended. In this special issue dedicated to David Cockayne, the techniques of dislocation tomography and precession electron diffraction described in this paper echo very strongly Davids interests in weak-beam imaging of dislocations and structure determination using kinematic diffraction. The weak-beam dark-field (WBDF) technique has been extended to three dimensions to visualise networks of dislocations in a number of materials systems. Some of the issues that arise from using this technique are explored. It is shown how the overall reconstruction can be improved through careful image processing. Using STEM medium-angle annular dark field (MAADF) imaging many of the artefacts seen in the WBDF technique are minimised, with a significant increase in the ease of acquisition and processing the data. Recent developments in precession electron diffraction and the desire to better understand and optimise the technique are reported. In particular, the idea of ‘intensity ordering’ is explored as a means of judging the likely success of structure determination from precession data.

Precession electron diffraction - a topical review

This topical review highlights progress made recently in the development and application of precession electron diffraction (PED) and its scanning variant for the determination of unknown crystal structures and the mapping of orientations at the nanoscale.

Refining structures against reflection rank: an alternative metric for electron crystallography.

A new metric is proposed to improve the fidelity of structures refined against precession electron diffraction data. The inherent dynamical nature of electron diffraction ensures that direct refinement of recorded intensities against structure-factor amplitudes can be prone to systematic errors. Here it is shown that the relative intensity of precessed reflections, their rank, can be used as an alternative metric for refinement. Experimental data from erbium pyrogermanate show that applying precession reduces the dynamical transfer of intensity between reflections and hence stabilizes their rank, enabling accurate and reliable structural refinements. This approach is then applied successfully to an unknown structure of an oxygen-deficient bismuth manganite resulting in a refined structural model that is similar to a calcium analogue.

Is precession electron diffraction kinematical? Part II A practical method to determine the optimum precession angle

A series of experiments was undertaken to investigate the kinematical nature of precession electron diffraction data and to gauge the optimum precession angle for a particular system. Kinematically forbidden reflections in silicon were used to show how a large precession angle is needed to minimise multi-beam conditions for specific reflections and so reduce the contribution from dynamical diffraction. Small precession angles were shown to be detrimental to the kinematical nature of some low-order reflections. By varying precession angles, precession electron diffraction data for erbium pyrogermanate were used to investigate the effect of dynamical diffraction on the output from structure solution algorithms. A good correlation was noted between the precession angle at which the rate of change of relative intensities is small and the angle at which the recovered structure factor phases matched the theoretical kinematical structure factor phases.

Symmetry-modified charge flipping

The charge-flipping algorithm has been adapted to allow symmetry constraints to be included during the solution of structures from diffraction data. Rather than impose symmetry at the start of the algorithm, which is known to cause the process to stagnate, it is shown that the algorithm must be allowed to find an intermediate stable solution first. Although care is needed when using this modified algorithm, the improvement in the fidelity of the structure solution is considerable.

Precession Electron Diffraction

Abstract Precession electron diffraction is considered a key technique available to electron microscopists to elucidate the structure of a wide variety of crystals. Although originally envisaged as a method to produce integrated intensities that could be used for structure solution and refinement, precession electron diffraction has proved much more versatile, enabling symmetry determination, texture analysis, and even measurements of bonding charge densities. In this report, we explain the mechanisms by which precession can improve the quality of electron diffraction data and how these benefits can be applied to different solution algorithms, including novel charge-flipping and maximum entropy methods, and to acquire other crystallographic information about a material. Such improvements have allowed the wide use of precession electron diffraction to solve a range of inorganic and organic crystal structures. Indeed, many X-ray crystallographers are now exploring the advantages of precession electron diffraction for solving crystal structures that are not amenable to conventional X-ray methods.