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Abstract: Background incl. aims: Conjugated polymers are an important class of organic light emitting diodes (OLEDs) and organic solar cells (OSCs). These materials are predominantly semi-crystalline or amorphous with intricate molecular packing and mixed variety of structural orders and disorders [1]. The susceptibility of these materials to ‘burn-in degradation’ [2] can induce blend-demixing and photo-induced ordering/disordering [3], thereby resulting in the performance losses of the devices [4]. Controlling this performance degradation during operation necessitates an understanding in changes in chemical structures and structural disorders at the nanoscale – the length scale commensurate with the transport of charge carriers. Yet direct nanoscale characterisation is limited for polymer semiconductors and their associated devices due to the irreversible changes in these materials structure when exposed to high-energy ion and electron beam conditions [5]. Here, we advance the structural characterisation of polymer semiconductors, whether in the form of free-standing films or cross-sectioned lamella, using low-dose four-dimesion scanning transmission electron microscopy (4D-STEM), enabling the analysis of the molecular packing, crystallinity, and atomic arrangement in the polymer semiconductors in response to temperature and ion milling-induced damage. Methods: Low-dose 4D-STEM analysis was conducted using established nanobeam scanning electron diffraction alignment at electron Physical Science Imaging Centre (ePSIC), Diamond Light Source. In particular, Merlin-Medipix detector and <1 mrad convergence semi-angle with 1-2 pA in probe current at 300 kV were used to minimize radiolytic damage. We obtained data at a range of camera lengths to enable both mapping of crystalline domains from Bragg scattering as well as reciprocal space (variance measures) and real space electron Pair Distribution Function (ePDF) analysis of disordered and amorphous regions. The materials under examination were free-standing polymer films (F8:F8BT, 1:1), prepared by spin-coating onto PDOT:PSS/ITO/Glass substrates. Subsequently, the multi-layered sample was submerged in deionized water, and the F8:F8BT films were floated onto carbon support films for 4D- STEM analysis. Additionally, we developed cryo-Focused Ion Beam (cryo-FIB) protocols to facilitate the structural examination of the cross-sectioned device model, Glass/ITO/PDOT:PSS/F8:F8BT (1:1). Results: The developed techniques reveal the formation of nano-crystalline domains in the F8:F8BT films after heat treatment. These domains are attributed to the crystallisation of F8 polymers, as evidenced by indexing some diffraction patterns aligning along the zone axis. Additionally, ePDF analysis allows us to characterise the atomic structures in amorphous areas with varying contrast. The analysis indicates that there were no chemical changes in the F8:F8BT blends induced by temperature. However, partial phase segregation occurred, as also supported by low-dose EELS analysis. We extended these analyses to a cross-sectioned device model prepared by cryo-FIB, and the findings demonstrate that our cryo-FIB protocol preserves the crystalinity of the polymer blends. ePDF shows that cryo-FIB milling does not alter the chemical structures of the films, i.e. intramolecular structure, but does affect the intermolecular arrangement. Conclusion: The developed electron microscopy techniques enable the characterisation of microstructures and nanoscale atomic arrangements in beam-sensitive polymer semiconductors, paving a pathway for examining phase segregation and chemical changes resulting from the burn-in degradation. By doing so, effective strategies can be developed to minimise structural degradation in polymer semiconductors, thereby preventing performance losses during operation.

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Abstract: The induction of structural distortion in a controlled manner through tilt engineering emerges as a potent method to finely tune the physical characteristics of Prussian blue analogues. Notably, this distortion can be chemically induced by filling their pores with cations that can interact with the cyanide ligands. With this objective in mind, we optimized the synthetic protocol to produce the stimuli-responsive Prussian blue analogue AxMn[Fe(CN)6] with A = K+, Rb+, and Cs+, to tune its stimuli-responsive behavior by exchanging the cation inside pores. Our crystallographic analyses reveal that the smaller the cation, the more pronounced the structural distortion, with a notable 20-degree Fe-CN bending when filling the cavities with K+, 10 degrees with Rb+, and 2 degrees with Cs+. Moreover, this controlled distortion offers a means to switch on/off its stimuli-responsive behavior, while modifying its magnetic response. Thereby empowering the manipulation of the PBA's physical properties through cationic exchange.

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Abstract: In conclusion, this work demonstrates the powerful nature of SED, combined with 3D-ED, to explore sub-crystal nanostructures and defects in beam-sensitive soft materials.

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Abstract: This work reports the thermal and electron beam stabilities of a series of isostructural metal-organic frameworks (MOFs) of type MFM-300(M), where M = Al, Ga, In, or Cr. MFM-300(Cr) was most electron beam stable, having an unusually high critical electron fluence of 1111 e-·Å-2 while the Group 13 element MOFs were found to be less stable. Within Group 13, MFM-300(Al) had the highest critical electron fluence of 330 e-·Å-2, compared to 189 e-·Å-2 and 147 e-·Å-2 for the Ga and In MOFs respectively. For all four MOFs, electron beam-induced structural degradation was independent of crystal size and was highly anisotropic, with the one-dimensional pore channels being the most stable, although the length and width of the channels decreased during electron beam irradiation. Notably, MFM-300(Cr) was found to retain crystallinity while shrinking up to 10%. Thermal stability was studied using in situ synchrotron X-ray diffraction at elevated temperature which revealed critical temperatures for crystal degradation to be 605, 570, 490 and 480°C for Al, Cr, Ga, and In, respectively. The pore channel diameters contracted by ~0.5% on desorption of solvent species but thermal degradation at higher temperatures was isotropic. The observed electron stabilities were found to scale with the relative inertness of the cations and correlate well to the measured lifetime of the materials when used as photocatalysts.

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Abstract: While indium selenide (InxSey) semiconductors have desirable properties that offer potential applications in photovoltaic and optoelectronic devices, these materials exhibit complex polymorphism, and different phases have different physical and chemical properties. Previous research has shown that confining indium selenide to two dimensions significantly alters its physical properties and could make it suitable for high-quality semiconducting components in future devices. In2Se3 nanowires (measuring between 40 and 200 nm wide) maintained the intricate phase-change behaviour, high photosensitivity and rapid photoresponse, but lack some of the exciting properties of single-layer InxSey. However, creating 2D sheets of indium selenide is challenging, typically requiring liquid-phase or mechanical exfoliation. In work recently published in ACS Nano, researchers from the University of Nottingham synthesised InxSey in single-walled carbon nanotubes (SWCNTs), which offer a unique environment for the templated growth of ultrathin nanomaterials, and demonstrated that aberration-corrected transmission electron microscopy (AC-TEM) allows identification of the phase of the encapsulated material. Their study demonstrates a robust method for synthesising two different phases of ultrathin InxSey nanoribbons, offering the potential for future production of bespoke and versatile nanoelectronic devices.