Effects of shape and composition on the properties of CdS nanocrystals

Using first-principles electronic structure calculations, we studied the relative stability between hexagonal wurtzite (WZ) and cubic zinc-blende (ZB) structures and the electronic properties of the CdS nanocrystals of varying shape and surface composition. Our study shows substantial enhancement of the stability of the WZ phase over that of the ZB phase upon moving from a spherical to a cylindrical shape. Our study provides microscopic understanding of the dominant stability of the WZ phase in cylindrically shaped nanocrystals. The calculated band gaps also show interesting variation as a function of varying shape and surface composition.

Figure 1

Calculated cohesive energies for the unpassivated (bare) stoichiometric (top panel), Cd-rich nonstoichiometric (middle panel), and S-rich nonstoichiometric (bottom panel) nanocrystals as a function of varying aspect ratio. The isolated data points shown in the top panel are for the infinite rod.

Figure 2

Calculated surface energies for the unpassivated (bare) stoichiometric (top panel), Cd-rich nonstoichiometric (middle panel), and S-rich nonstoichiometric (bottom panel) nanocrystals as a function of varying aspect ratio. The isolated data points shown in the top panel are for the infinite rod. The difference in dangling bonds between the ZB and WZ structures are shown in the insets of top panel (for the stoichiometric systems, I–IV) and middle panel (for nonstoichiometric systems, I–IV).

Figure 3

Cohesive energies for ZB and WZ structures plotted as a function of varying aspect ratio for stoichiometric (top panel) as well as Cd-rich (middle panel) and S-rich (bottom panel) H-passivated CdS nanocrystals.

Figure 4

Structurally relaxed cross-sectional view of the passivated S-rich nanocrystals (system II) in both WZ and ZB phases. Large, green (medium, yellow) balls represent Cd (S) atoms, and the small, cyan colored balls represent H atoms.

Figure 5

Shift in the band gap as a function of varying aspect ratios for the minimum energy phases of the stoichiometric, Cd-rich, and S-rich nanocrystals. The isolated data point for the stoichiometric infinite rod is shown in the left panel. The total number of atoms and the degree of nonstoichiometry (only in case of nonstoichiometric systems) are also marked for each data point. For Cd-rich system III and IV nanocrystals, the symmetry of the minimum energy structure changes between passivated and unpassivated cases. The data points corresponding to the minimum energy structure of the unpassivated cases are also shown as open symbols.