Probing monocrystalline and multiply-twinned nanocrystal growth from thermodynamic and kinetic perspectives
Open Access
- Author:
- Qi, Xin
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 11, 2018
- Committee Members:
- Kristen Ann Fichthorn, Dissertation Advisor/Co-Advisor
Kristen Ann Fichthorn, Committee Chair/Co-Chair
Michael John Janik, Committee Member
Adrianus C Van Duin, Committee Member
Seong Han Kim, Outside Member - Keywords:
- Ag
nanocrystal growth
MD simulation
Theory
thermodynamics
kinetics - Abstract:
- Metal nanocrystals are beautiful and powerful. While they attract great attentions for their exceptional structural-dependent performance in an array of modern technologies, how they are born and why they can grow into various shapes still largely remain mysterious. The lack of comprehensive fundamental understanding in their growth mechanism inevitably obstructs the perfection of the process, specifically in increasing the selectivity and yield of certain structures and sizes, as well as elevating the production capacity to a manufacturing level. In synthesizing metal nanocrystals, the solution-phase polyol syntheses outper- form other protocols and are applicable to a variety of metal species. Chemical additives are often added in these processes to prevent aggregation and, at the same time, to promote the selection of a specific shape. Considering the chemical complexity and the length/time scale of these systems, it is particularly challenging to unravel the formation and growth mechanisms via experimental means. Compu- tational methods, on the other hand, can be notably useful in this regard, as they are able to probe atomistic details, and by integrating theories, they can be used to calculate properties that are difficult to measure. This dissertation elucidates several important aspects in Ag nanocrystal growth using classical all-atom molecular dynamics (MD) simulation. The work included in this dissertation covers the thermodynamic and kinetic ramification from chemical additives in monocrystalline Ag nanocrystals, as well as the 1-dimensional (1D) growth origin of the five-fold-twinned Ag nanowire. To evaluate the thermodynamic influence, we develop a method that calculates the facet-specific interfacial free energy γsl for multi-component solid-liquid interface described by complex force fields. We use this method to assess how polyvinylpyrroli- done (PVP), an effective structure-directing agent (SDA) for Ag(100)-faceted shapes, alters the interfacial free energy for Ag surfaces solvated in ethylene glycol upon adsorption, and further extend the study to characterize a general impact from additives on Ag surfaces at a range of binding energies. We also examine the facet-selective solution-phase deposition on to crystal surfaces regulated by PVP and show how it influences the steady-state kinetic shapes. In contrast to monocrystalline, multiply-twinned nanocrystals bear stress and strain. Interestingly, we find that the stress and strain in a 5-fold-twinned Ag nanowire can alter the surface atomistic structure and affect adatom distribution, and such alternations can bring unique surface atom diffusion that triggers the 1D expansion. Altogether, we report on our progress in probing monocrystalline and multiply- twinned nanocrystal growth from thermodynamic and kinetic perspectives and outlook for future research directions.