Dissertation Proposal: Molecular Insight into How Nanocrystals are Grown to Specific Shapes

Project Summary

Research objective

Intellectual Merit

Broader Impacts

Introduction

Nanoscale materials have the potential to solve many of today’s biggest problems such as peak oil, the global water crisis, and the burden of cancer. Faraday’s discovery of colloidal ruby gold producing different colored solutions in 1857 (Faraday 1857, Thompson 2007) has inspired generations of nanoscale science. Controlled synthesis of nanocrystals, namely quantum dots, was invented by Bawendi et al. in 1993 (Hakimi 1993, Murray 2000). They paved the way for the utilization of well-defined nanocrystals in various fields ranging from heterogeneous catalysis (Astruc 2008, Astruc 2006) to photovoltaics (Atwater 2010), DNA sequencing (McNally 2010), batteries (Panniello 2014), hydrogen storage (Jena 2011, Ramos-Castillo 2015), and cancer therapeutics (Jain 2010, Kim 2010).

Nanocrystals can be grown to specific sizes and shapes, but the question of what is the growth controlling mechanism remains elusive. This is important because numerous properties of metal nanocrystals are found to depend on their size (Roduner 2006) and shape (Xia 2008). In catalysis for instance, tetrahedral Pt nanocrystals are more active than spherical and cubic Pt nanocrystals as the catalyst for electron-transfer reactions (Narayanan 2005). The controlling mechanism for various systems have been studied using both experimental and computational techniques. Experimentalists have employed techniques such as in situ transmission electron microscopy (Liao 2014, Woehl 2014), X-ray photoelectron spectroscopy (Gao 2004, Park 2014, Huang 1996, Kedia 2012, Bonet 2000), and variation of reaction parameters (Personick 2013, Xia 2012, Zeng 2010, Zhang 1996, Chang 2011, Zhu 2011) to elucidate the controlling mechanism. Theorists address this important question using techniques such as density functional theory (Kilin 2008, Al-Saidi 2012, Saidi 2013, Zhang 2008) and molecular dynamics (Zhou 2014).

However, previous work in the literature have not yet adequately addressed the mechanism of shape control in the solution phase. Despite much excellent experimental and theoretical work, there is no quantitative evidence in the facet selective adsorption of structure-directing agents and their role in shape control. Without such evidence, we are left with an incomplete description of the shape control mechanism that creates the condition for ill-informed reaction engineering for scale-up. To date, only a few syntheses have been scaled up to the gram-scale, and yet they still have poor quality control (Jana 2005, Lohse 2013). This study will remedy this gap in the literature by elucidating the role of structure-directing agents using molecular dynamics simulation, in which explicit solvent is computationally feasible and observations in the atomic resolution can be made. Using enhanced sampling methods, I will provide quantitative evidence that will support or refute the facet selective adsorption hypothesis and further examine its implications.