Density functional theory study of ligand binding on CdSe (0001), (000(1)over-bar), and (11(2)over-bar-0) single crystal relaxed and reconstructed surfaces: Implications for nanocrystalline growth

To gain a better understanding of the influence of ligand-surface interactions on nanocrystalline growth, periodic density functional theory calculations were employed in the study of the binding of organic ligands on the relaxed nonpolar (11 (2) over bar0) and polar Se terminated (000 (1) over bar) surfaces and the relaxed and vacancy and adatom reconstructed Cd terminated (0001) surface. We examined chemisorption properties of phosphine, amine, phosphine oxide, carboxylic acid, and phosphinic acid model ligands, including preferred binding sites and geometries, vibrational frequencies, and binding energetics, and compared findings to intrinsic growth via addition of CdSe molecules or Cd and Se atoms. Our results indicate that binding of the ligands is preferred in the electron-poor 1-fold sites on all surfaces, with secondary coordination of the acidic ligands through the hydroxyl hydrogen to the electron-rich surface sites. In general ligand adsorption directly obstructs binding sites for growth species on the (11 (2) over bar0) surface and only indirectly on the two polar surfaces. The order of binding affinities on the (11 (2) over bar0) and (0001) surfaces is PH3 < OPH3 < HCOOH < NH3 < OPH2OH and that on the (000 (1) over bar) surface is OPH3 approximate to HCOOH < OPH2OH < NH3 < PH3. Our findings corroborate the experimental observation that incorporation of the nonbulky phosphinic acid- type ligands with high affinity and high selectivity for both the (11 (2) over bar0) and (0001) surfaces strongly enhances unidirectional growth on the (000 (1) over bar) surface, while incorporation of either bulky ligands or ligands with moderate affinity does not. Higher affinity of all traditionally used ligands for the (11 (2) over bar0) surface compared to the (0001) surface also suggests that new ligands should be engineered to achieve the synthesis of novel shapes that require preferential growth on the (11 (2) over bar0) surface.