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submitted on 15.10.2020 and posted on 16.10.2020by Mei Wang, Asher Leff, Yue Li, Taylor Woehl
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 directly
visualizing nanocrystal formation mechanisms that can inform discovery of design rules for
colloidal multimetallic nanocrystal synthesis, but it remains unclear whether the salient chemistry
of the flask synthesis is preserved in the extreme electron beam radiation environment during LPTEM. Here we demonstrate controlled in situ LP-TEM synthesis of alloyed AuCu nanoparticles
while maintaining the molecular structure of electron beam sensitive metal thiolate precursor
complexes. Ex situ flask synthesis experiments showed that nearly equimolar AuCu alloys formed
from heteronuclear metal thiolate complexes, while gold-rich alloys 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 nanoparticles with similar alloy
composition, random alloy structure, and particle size distribution shape as those from ex situ flask
synthesis, indicating metal thiolate complexes were 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 metal thiolate
oxidation at low dose rates. In situ synthesis experiments and ex situ atomic scale imaging revealed
that a key role of metal thiolate complexes was to prevent copper atom oxidation and facilitate
formation of prenucleation cluster intermediates. This work demonstrates that complex ion
precursor chemistry can be maintained during LP-TEM imaging, enabling probing nanocrystal
formation mechanisms with LP-TEM under reaction conditions representative of ex situ flask