Properties of the CdSe(0001), (0001̄), and (112̄0) Single Crystal Surfaces:  Relaxation, Reconstruction, and Adatom and Admolecule Adsorption

Abstract

Details of the chemical mechanism underlying the growth of colloidal semiconductor nanocrystals remain poorly understood. To provide insight into the subject, we have preformed a comprehensive study of the polar (0001) and (0001) and nonpolar (1120) wurtzite CdSe surfaces that are exposed during crystal growth using first-principles density functional theory (DFT-GGA) calculations. Stabilization of these surfaces by relaxation and reconstruction was considered. Two particular reconstructions of the polar surfaces were examined: vacancy formation on a 2 x 2 unit cell and addition of Se and Cd atoms on the (0001) and (0001) surfaces, respectively. Calculation results indicate that the (1120) is the most stable surface when compared to the two polar surfaces. Furthermore, reconstructions of the (0001) surface are energetically favored when compared to reconstructions of the (0001) facet. Adsorption of Cd and Se atoms and the CdSe molecule on the three relaxed surfaces and two reconstructed (0001) surfaces were also investigated. Several binding sites were considered to determine the most stable binding geometries and energetics. Atomic species preferentially bind in either 2-fold or 3-fold sites, while the CdSe molecule binds parallel to the surface on all of the considered surfaces. Vibrational frequencies of the adspecies were calculated for the most stable binding configurations and were included in the zero point energy correction. Diffusion barriers for the atomic and molecular species were estimated where possible to be between 0.2 and 0.4 eV on the three relaxed surfaces. Thermochemistry of the CdSe molecule binding and dissociation was also investigated. On all considered surfaces, dissociation is preferred to desorption with dissociation only exothermic on the (0001) surface. Comparison of the three relaxed and two reconstructed surfaces indicates that CdSe molecule binding and dissociation is thermodynamically favored on the (0001) surface. This implies that under a reaction-controlled regime, the rate of homoepitaxy would be faster on the (0001) Se terminated surface than on the (0001) and (1120) surfaces, making the (0001) surface of a nanocrystal the primary direction of growth.

High resolution STEM of quantum dots and quantum wires

“…Based on the atomic resolution images, the flat ends of the bulletshaped particles were identified to be Cd-terminated (0 0 1) facets and the opposite end Se-terminated (0 01) facets. The results are in agreement with the results of first principles calculations that the rate of growth in CdSe particles is highest at the Se-terminated (0 01) surfaces (Rempel et al, 2005). Replacing the ZnS shell over CdSe with a gradient CdS/ZnS shell was shown to increase the quantum yield from 34% to 84%.…”

In Situ Transmission Electron Microscopy of Cadmium Selenide Nanorod Sublimation

“…First-principles calculations of the relative surface energies of ligand-passivated facets show that the polar {0001} facets of a bare CdSe nanorod with a relaxed and reconstructed surface have a higher surface energy compared to the nonpolar sides. (31)(32)(33) Coordinating the wurtzite CdSe surfaces with methyl phosphonic acid-a ligand similar to the species in our system-further increased the stability of the nonpolar sides relative to the tips; the former were shown to be most stable upon ligand passivation. (32)(33)(34)(35)(36) In view of the factors above that can influence the kinetic barrier to sublimation, we first propose an idealized mechanism for the observed phase transition behavior.…”

“…(31)(32)(33) Coordinating the wurtzite CdSe surfaces with methyl phosphonic acid-a ligand similar to the species in our system-further increased the stability of the nonpolar sides relative to the tips; the former were shown to be most stable upon ligand passivation. (32)(33)(34)(35)(36) In view of the factors above that can influence the kinetic barrier to sublimation, we first propose an idealized mechanism for the observed phase transition behavior. Sublimation initiates when a CdSe unit positioned at a high-energy surface site on the nanorod end undergoes charge transfer and desorbs.…”

Double-heterojunction nanorods

“…The growth of the CdSe tips leads to a new absorption feature near 585 nm and the corresponding PL from CdSe band edge. The double-peaked PL spectrum of CdS/CdSe nanorod heterostructures arises from a bimodal size distribution of CdSe presumably owing to the differentiation of growth rates on the two ends of the seed CdS nanorod 30,40,41 (see Supplementary Fig. 4).…”