Surface Stoichiometry of CdSe Nanocrystals Determined by Rutherford Backscattering Spectroscopy

Abstract

Rutherford backscattering spectroscopy has been applied to study the surface stoichiometry of CdSe nanocrystals prepared by the high temperature pyrolysis of organometallics in trioctylphosphine oxide (TOPO). The diameter of the nanocrystals was varied from 22 to 56 Å. For all nanocrystal sizes we find the nanocrystals are Cd rich with an average Cd: Se ratio of 1.2 ± 0.1. The Cd: Se stoichiometry is independent of the Cd: Se starting ratio used for the nanocrystal synthesis, indicating the excess Cd is not associated with the initial abundance of Cd but is an intrinsic property of nanocrystals prepared by this method. The surface coverage of the passivating TOPO ligands has also been determined and is larger than reported in previous X-ray photoelectron spectroscopy (XPS) studies of Bowen Katari et al. [1] The origin and structural implications of nonstoichiometric nanocrystals are discussed.