Mapping structure and morphology of amorphous organic thin films by 4D-STEM pair distribution function analysis

Microscopy, 2019, 301–309 doi: 10.1093/jmicro/dfz 015 Advance Access Publication Date: 28 March 2019 Article Mapping structure and morphology of amorphous organic thin films by 4D-STEM pair distribution function analysis Xiaoke Mu1, Andrey Mazilkin1, Christian Sprau2, Alexander Colsmann2, and Christian Kübel1,3,* 1 Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany, 2Light Technology Institute, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany, and 3Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany * To whom correspondence should be addressed. E-mail: Received 26 October 2018; Editorial Decision 4 February 2019; Accepted 24 February 2019 4D-STEM-PDF is implemented as a new approach to image the phase distribution in organic nanocomposites including information on the short- and medium range order in each phase enabling atomic level structural analysis. Abstract Imaging the phase distribution of amorphous or partially crystalline organic materials at the nanoscale and analyzing the local atomic structure of individual phases has been a long-time challenge. We propose a new approach for imaging the phase distribution and for analyzing the local structure of organic materials based on scanning transmission electron diffraction (4D-STEM) pair distribution function analysis (PDF). We show that electron diffraction based PDF analysis can be used to characterize the short- and medium-range order in aperiodically packed organic molecules. Moreover, we show that 4D-STEM-PDF does not only provide local structural information with a resolution of a few nanometers, but can also be used to image the phase distribution of organic composites. The distinct and thickness independent contrast of the phase image is generated by utilizing the structural difference between the different types of molecules and taking advantage of the dose efficiency due to use of the full scattering signal. Therefore, this approach is particularly interesting for imaging unstained organic or polymer composites without distinct valence states for electron energy loss spectroscopy. We explore the possibilities of this new approach using [6,6]-phenyl-C61- butyric acid methyl ester (PC61BM) and poly(3-hexylthiophene-2,5-diyl) (P3HT) as the archetypical and best-investigated semiconductor blend used in organic solar cells, compare our phase distribution with virtual dark-field analysis and validate our approach by electron energy loss spectroscopy. Key words: pair/radial distribution function (PDF/RDF), 4D-STEM, polymer blends, phase mapping, local structure analysis © The Author(s) 2019. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: 301 Microscopy, 2019 , Vol. 68 , No. 4 302 Introduction Materials This work uses the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) (Fig. 1a, top right) and the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) (Fig. 1a, top left) as model system to demonstrate 4D-STEM-PDF of organic blends. PC61BM and P3HT form bulkheterojunctions, which are widely used as light-harvesting semiconductors in organic solar cells [52,53] and, therefore, allow comparison with previous studies in the literature. HRTEM and EFTEM characterizations have been performed previously on this system [28,54]. In our measurements, neat P3HT and PC61BM films were used as reference samples and a blend of both semiconductors was used for the 4D-STEMPDF analysis. These organic thin films were prepared on glass substrates that were cleaned by ultrasonication in acetone and 2-propanol for 15 min. Afterwards, a sacrificial layer of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) was prepared by filtering Clevios P VP AI 4083 (PTFE 0.45 μm), diluting it with ethanol 1:3 by volume and spin coating a thin layer at 4000 rpm, which was subsequently Properties and functionality of organic composite materials are directly related to their molecular geometry, the molecular packing and the morphology of the phase distribution [1–3]. Knowledge of this information is therefore fundamental for polymer processing and device design. This requires characterization techniques with high spatial resolution. (Scanning) transmission electron microscopy ((S)TEM) has been an essential tool, intensively used to image soft materials for decades [4–8]. Most commonly, at the nanoscale, the morphology of organic composites, block copolymers and biological specimens has been studied using selective staining to create mass contrast for the different components [9–13]. However, artifacts can be introduced by staining [14,15], and finding suitable stains can be difficult for some material combinations. Highresolution TEM (HRTEM) has been used for lattice imaging of crystalline and partially crystalline organic materials [5,16–19] and can provide detailed structural information, e.g. on crystal defects [19–21]. The main challenge here is the electron beam sensitivity of the organic materials, where damage due to radiolysis [4,22,23,24] limits the achievable resolution and contrast severely. More recently, electron diffraction techniques have seen a significant development, combining electron beam precession with 3D nanodiffraction to solve the crystal structure of small organic crystals [25]. However, a large number of soft materials are aperiodic or even amorphous, limiting HRTEM and diffraction techniques. Electron energy loss spectroscopy (EELS) techniques, e.g. 4D-STEM-EELS and energy filtered TEM (EFTEM), especially focusing on the low-loss region, are increasingly used to image the phase distribution of soft materials where molecules/polymers possess distinct differences in their valence electron configurations [26–28]. Interpretation of the low-loss signal, relating it to the valence electron state and the plasmon resonance, is usually challenging. Moreover, the image contrast is commonly affected by thickness variations in the specimen [29]. Structural characterization of amorphous organic materials has been carried out by pair distribution function (PDF) analysis obtained from X-ray and neutron diffraction [30,31]. The PDF describes the population of atomic pairs as a function of interatomic distances r and therefore characterizes the atomic configuration of the material. Obtaining PDFs from electron diffraction (ePDF) has been introduced in references [32,33] for characterizing polycrystalline metals and glasses and has been further extended to nanoparticles in catalysis [34] and battery materials [35,36]. Application of ePDF to organic nanocrystals has recently been verified to provide identical information as an X-ray based PDF analysis [37]. The current spatial limit of ePDF is set by conventional diffraction, when using a broad beam with tens or hundreds of nano (...truncated)