Metal Nanocrystal Formation during Liquid Phase Transmission Electron Microscopy: Thermodynamics and Kinetics of Precursor Conversion, Nucleation, and Growth

Colloidal nanoparticle synthesis involves a complex combination of physical and chemical processes that transform a liquid phase metal precursor into nanoparticles, each containing thousands to millions of atoms. Liquid phase transmission electron microscopy (LP-TEM) has enabled unprecedented atomic and nanoscale insights into mechanisms for colloidal nanoparticle formation and promises to unravel many of these complex processes. Despite intense sustained interest in this area, practical translation of LP-TEM mechanistic insights to improve upon and discover new nanomaterial synthesis approaches remains a major unmet challenge. One underlying reason for this is a poor fundamental understanding of how nanocrystal formation during LP-TEM compares to conventional flask-based synthesis. In this perspective, we discuss the fundamental thermodynamic and kinetic driving forces for metal nanocrystal formation during LP-TEM and compare them with established mechanisms for flask-based synthesis. The roles of electron beam induced solution chemistry and nanoscale solute transport phenomena in mediating precursor reduction, nanocrystal nucleation, and growth will be discussed. The recent discovery that the liquid cell enclosure membrane surface chemistry has a significant impact on nanocrystal nucleation mechanisms during LP-TEM will be highlighted. Increasingly quantitative, statistically relevant, and reproducible LP-TEM experiments together with rigorous comparisons to flask-based chemistry are expected to provide new insights into nanocrystal formation mechanisms that are directly relevant to flask-based synthesis.