Liquid Phase Transmission Electron Microscopy: A Window into the Early Stages of Complex Material Formation

ABSTRACT Within the framework of Professor Helmut Cölfen's remarkable career and his invaluable contributions to the understanding of nucleation and growth processes, the development of Liquid‐Phase Transmission Electron Microscopy (LPTEM) now enables the direct observation of events occurring in solution during these critical stages of nanomaterial formation. By revisiting some of Professor Cölfen's key studies in this field, along with more recent investigations realized by applying this characterization tool on complex systems, this review aims to bridge his foundational contributions with the latest advancements in instrumentation and methodology, highlighting how this technique deepens our understanding of nucleation and growth in increasingly sophisticated nanosystems and offers unprecedented insights into dynamic nanoscale transformations. We focus our review on studies of complex materials such as oxides, biominerals, and organic–inorganic crystals, for which mechanistic understanding remains limited and still challenging. The review also highlights the main limitations of LPTEM, outlines the current state of the field, and discusses some future challenges and perspectives.