"3D ED was the missing piece we needed to complete the electron microscopy toolbox," said Prof. Erdmann Spiecker, head of the Institute of Micro- and Nanostructure Research and the Center for Nanoanalysis and Electron Microscopy (CENEM) at FAU, who led the project.
At the heart of the work is a direct comparison of electron and X-ray data from identical samples. "It is striking how well the data agree, despite the fundamentally different nature of electrons and X-rays," said Irene Kraus, who carried out the 3D ED experiments as part of her doctoral research. The measurement geometries differ markedly: X-rays probe the sample at shallow angles in reflection, while electrons pass through it. "In 3D ED, we tilt the sample step by step, allowing us to reconstruct the average molecular order in three dimensions - much like tomography."
A key challenge in the work is radiation damage. Organic solar cells are extremely beam-sensitive, and even low doses can disrupt their delicate molecular order. This is particularly critical for electrons, which interact strongly with matter and deposit energy locally. "At first glance, electrons may seem unsuitable for such materials," said Dr. Mingjian Wu, senior scientist at CENEM, who co-supervised the project. "But with careful dose control, we can extract structural information before damage occurs." By optimizing the electron dose and developing tailored acquisition strategies, the team was able to reliably probe even highly sensitive nanocrystalline structures while preserving the material.
The results do not diminish the role of X-ray techniques. Rather, the methods are complementary. X-rays require minimal sample preparation and are particularly well suited for in situ studies of structural evolution during processing. Electron microscopy now uniquely combines averaged structural information with local imaging, diffraction imaging and chemical analysis in a single instrument. "This is what we call multimodal microscopy," explained Spiecker. "It allows us to directly link different types of information - from molecular texture to local order and composition - within one experiment."
The research is connected to two DFG-funded initiatives hosted at FAU: Research Training Group RTG 3103 "Correlative Materials Microscopy" (CorMic), a doctoral program comprising 13 PhD projects, and Collaborative Research Center CRC 1719 "Next-generation printed semiconductors: Atomic-level engineering via molecular surface chemistry" (ChemPrint). Both programs focus on advancing multimodal and correlative microscopy of functional materials.
Research Report:3D electron diffraction - the missing slice completing nanoscale analysis of organic solar cells in TEM
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