(449f) Surface Chemistry of Cdse: Implications for Nanocrystalline Growth

Rempel, J. Y., Massachusetts Institute of Technology
Trout, B. L., Massachusetts Institute of Technology
Bawendi, M. G., Massachusetts Institute of Technology
Jensen, K. F., Massachusetts Institute of Technology

Synthesis of novel colloidal semiconductor nanocrystals of diverse shapes and sizes has proven to be a fruitful area of research over the past decade. However, despite great efforts directed toward the development of adaptable chemistries, the exact reaction mechanisms leading to crystal growth in these systems are still poorly understood. In the present work, we probe mechanistic details of CdSe growth, one of the most frequently studied and well-characterized nanocrystal systems. We employ first principles calculations within the framework of density functional theory to quantitatively examine surface chemistry of CdSe. We consider the homoepitaxy reactions on several bulk terminated relaxed and reconstructed wurtzite CdSe surfaces and demonstrate that addition reactions are favored on the Se terminated (000-1) surface. We further examine the effect of ligand binding on crystal growth by exploring chemisorption of model amine, phosphine, oxide, and carboxylic and phosphinic acid ligands and show that these ligands exhibit a range of affinities and selectivities for different facets of CdSe. Finally, we relate our findings to the experimental observations, in particular, nanocrystal morphology and shape anisotropy. We show that incorporation of the non-bulky phosphinic acid type ligands with high affinity and high selectivity for both the non-polar (11-20) and Cd terminated (0001) surfaces would enhance unidirectional growth on the (000-1) surface leading to generation of high aspect ratio nanocrystals.