Science of Shape-Selective Nanocrystal Synthesis

Kristen A. Fichthorn, PI, The Pennsylvania State University

A significant challenge in the development of functional nanomaterials is understanding the growth of colloidal metal nanocrystals.  From a practical perspective, a knowledge of how to selectively synthesize desired metal nanocrystal sizes and shapes will be important in achieving a sustainable energy future. The science of shape-selective nanocrystal synthesis has been advancing at a rapid pace, with numerous recent reports of syntheses of various beneficial nanocrystal morphologies. Despite many successes, it is still difficult to achieve high, selective yields in most synthesis protocols.  Many aspects of these complex syntheses remain poorly understood, which impairs the development of improved synthetic methods. The proposed research targets several key gaps in the fundamental understanding of these syntheses (1) Understanding thermodynamic shapes of nanocrystal seeds using replica-exchange molecular dynamics (REMD) simulations and machine learning; (2) Understanding the hand-off between thermodynamics and kinetics as nanocrystal seeds grow and achieve kinetic shapes using accelerated molecular dynamics simulations based on hyperdynamics (HD); (3) Understanding the growth of kinetic crystal morphologies, including crystals with high-index surfaces, using kinetic Monte Carlo (kMC) simulations based on quantum density-functional theory (DFT); and (4) Advancing ab initio grand-canonical Monte Carlo (AIGCMC) simulations based on quantum DFT to understand the role of solvent and halides/halogens in controlling nanocrystal morphology.

The proposed REMD and HD studies will yield important fundamental insight into minimum free-energy shapes of nanocrystal seeds and how these shapes evolve kinetically as nanocrystals grow – a topic of utmost importance in understanding and controlling nanocrystal syntheses. The proposed kMC studies will reveal the interplay between kinetics and thermodynamics that promotes kinetic nanocrystal shapes – including those with high-index facets.  A legacy of the kMC studies will be a software package called NanoGrower that can be used by the crystal-growth community to test hypotheses regarding particular kinetic mechanisms and how these contribute to the formation of crystal morphology.  The AIGCMC studies will push the envelope in understanding liquid-solid interfaces and their influence on crystal morphology.  The AIGCMC studies are seen as an important investment that will enable future studies of surface diffusion near the liquid-solid interface and interparticle forces that depend sensitively on the structure of fluid in the inter-particle gap.  Our ongoing research features several collaborations with experimental groups and we expect future studies to exploit these collaborations at the cutting edge of nanocrystal synthesis science.