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Abstract

In this article, we report on complex nanochemistry and transport phenomena associated with silver nanocrystal formation by electron beam induced growth and liquid cell electron microscopy (LCEM). We synthesized silver nanocrystals using scanning transmission electron microscopy (STEM) electron beam induced synthesis and systematically varied the electron dose rate, a parameter thought to regulate nanocrystal formation kinetics via the rate of metal precursor reduction. Rationally modifying the solution chemistry with tert-butanol to scavenge radical oxidizing species established a strongly reducing environment and enabled repeatable LCEM experiments. Interestingly, nanocrystal growth rate decreased with increasing electron dose rate despite the predicted increase in reductant concentration. We present evidence that this counterintuitive trend stems from increased oxidizing radical concentration and radical recombination at high magnifications, which together decrease rate of precursor reduction. Nucleation rate was proportional only to imaging magnification, which we rationalized on the basis of local radical accumulation at high magnification causing increased supersaturation and rapid nucleation kinetics. Radiation chemistry and reactant diffusion scaling models yielded new scaling laws that quantitatively explained the observed effects of electron dose rate on nucleation and growth kinetics of metal nanocrystals. Finally, we introduce a new reaction kinetic model that enables unraveling nucleation and growth kinetics to probe nucleation kinetics occurring at subnanometer length scales, which are typically not accessible with LCEM. Our systematic investigation of metal nanocrystal formation kinetics with LCEM indicates that the intricacies of radiation chemistry and reactant transport must be accounted for to effectively harness radical scavengers and electron beam induced growth to systematically probe metal nanocrystal formation kinetics. We expect the empirical trends, scaling laws, and reaction kinetic model presented here will be indispensable tools for in situ electron microscopists and materials chemists alike when designing, analyzing, and interpreting LCEM metallic nanocrystal formation data.

Impact Statement

Systematic study of electron beam induced growth of silver nanocrystals. The dose rate was controlled by beam current and magnification and tert butanol was added to the solution to scavenge hydroxyl radicals and oxygen gas, thus leading to an increased chemically reducing environment. The nucleation and growth rates of the nanocrystals did not increase with electron dose rate, rather the opposite occurred which is the opposite of what is traditionally expected in liquid phase EM. These results indicates that the nucleation and growth reactions are controlled by complex radiation chemistry and kinetic parameters.