Dependence of Tetrahedral Bond Lengths on the Surface Passivation and Stacking-Fault Density of CdSe Nanocrystals

We have investigated the surface effect of colloidally prepared CdSe nanocrystals (NCs) with the size range of 23-40 Å on their structural properties by changing the organic capping ligands. The TOPO/HDA-passivated NCs reveal a size-dependent behavior involving an elongated axial bondR⁽¹⁾of an atomic tetrahedron and a shrunken equatorial bondsR⁽²⁾. After treatment of the NCs with pyridine, the bond lengthR⁽¹⁾decreases significantly whereasR⁽²⁾remains unchanged relative to the TOPO/HDA-passivated NCs, suggesting that a tensile stress along the [001] direction is contributed from the surface modification. In addition, we find that the expansion ratio of the pyridine-treated NCs along the c axis depends strongly on the density of stacking faults, which provides an evidence for the relaxation of atomic positions near the interface of stacking faults.