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

Unveiling the Local Atomic Arrangements in the Shear Band Regions of Metallic Glass

“…We utilized atomic pair distribution function (PDF) as a structure descriptor and combined it with 4D-STEM to map the PDF locally at the nanoscale (STEM-PDF), [28,29] followed by independent component analysis (ICA) [30] to overcome the aforementioned challenges. By further correlation with APT results, we provide an explicit view of the SB and SBAZ, targeting the missing information of bonding, coordination environment, atomic packing geometry, and chemistry, visualizing their nanoscale distribution.…”

“…[32,33] STEM-PDF, schematically shown in Figure S1, Supporting Information, utilizes the 4D-STEM data to visualize the local structure information with nanometer spatial resolution. [28,29] The obtained STEM-PDF array was analyzed using ICA, which was recently applied to STEM-PDF for characterizing a variety of amorphous composites. [34,35] ICA was developed for "blind source separation" minimizing the mutual information between the estimated source signals using a statistical analysis of the data.…”

Calibration-less quantitative 4D-STEM imaging of amorphous samples

“…In this case, no special calibration is needed. 4D datasets captured by the 2D STEM detectors are often used for many imaging and analytical techniques including thickness measurement in the field of material science and diffracting samples [3] or less frequently used for analysis of amorphous samples [4].…”

4D-STEM analysis of an amorphous-crystalline polymer blend: combined nanocrystalline and RDF mapping.

“…Nanoscale morphology of multiphase polymer systems and blends, particularly at and near interfaces, is critical to understanding interphase adhesion mechanisms and their impact on bulk mechanical properties [1][2] . To visualize this morphology over a representative sampling, a fast high-resolution technique for mapping both crystalline and amorphous domains with a large field of view is needed.…”