Chemical Solution Deposited Copper-doped CdSe Quantum Dot Films

Abstract

Copper doped CdSe quantum dots (QDs) and larger nanocrystals were deposited by chemical solution deposition at two different temperatures (5°C and 80°C) and their morphological, optical and surface properties compared with similarly deposited undoped CdSe. Crystal sizes and structure were investigated by X-ray diffraction and transmission electron microscopy. All the samples exhibited cubic (sphalerite) structure except for the 80°C Cu-doped ones which were substantially hexagonal (wurtzite). For the 5°C samples, the crystal sizes were similar for the doped and undoped samples (ca. 3.4 nm). For the 80°C samples, the crystal size of the doped CdSe was much larger (84 nm) than the undoped one (6.3 nm) and extensive twinning occurred. Optical measurements on the doped samples showed the presence of surface states due to the copper seen as an increase in absorbance in the long wavelength region. Xray photoemission spectroscopy similarly showed tails in the valence band spectra of the doped samples. These tails were dominant in the 5°C samples and less so for the 80°C ones. XPS also showed that the Cu was present predominantly in the univalent state. The copper was found to be located on the QD surface for the 2–3 nm QDs and distributed both on the surface and in the bulk (or at twin boundaries) for the larger nanoparticles.