Visualizing Ligand-Mediated Bimetallic Nanocrystal Formation Pathways with in Situ Liquid-Phase Transmission Electron Microscopy Synthesis.

Colloidal synthesis of alloyed multimetallic nanocrystals with precise composition control remains a challenge and a critical missing link in theory-driven rational design of functional nanomaterials. Liquid-phase transmission electron microscopy (LP-TEM) enables direct visualization of nanocrystal formation mechanisms that can inform discovery of design rules for nanocrystal synthesis, but it remains unclear whether the salient flask synthesis chemistry is preserved under electron beam irradiation during LP-TEM. Here, we demonstrate controlled in situ LP-TEM synthesis of alloyed AuCu nanocrystals while maintaining the molecular structure of electron beam sensitive metal thiolate precursor complexes. Ex situ flask synthesis experiments formed alloyed nanocrystals containing on average 70 atomic% Au using heteronuclear metal thiolate complexes as a precursor, while gold-rich alloys with nearly no copper formed in their absence. Systematic dose rate-controlled in situ LP-TEM synthesis experiments established a range of electron beam synthesis conditions that formed alloyed AuCu nanocrystals that had statistically indistinguishable alloy composition, aggregation state, and particle size distribution shape compared to ex situ flask synthesis, indicating the flask synthesis chemistry was preserved under these conditions. Reaction kinetic simulations of radical-ligand reactions revealed that polymer capping ligands acted as effective hydroxyl radical scavengers during LP-TEM synthesis and prevented oxidation of metal thiolate complexes at low dose rates. Our results revealed a key role of the capping ligands aside from their well-known functions, which was to prevent copper oxidation and facilitate formation of prenucleation cluster intermediates via formation of metal thiolate complexes. This work demonstrates that complex ion precursor chemistry can be maintained during LP-TEM imaging, enabling probing nonclassical nanocrystal formation mechanisms with LP-TEM under reaction conditions representative of ex situ flask synthesis.