Anisotropic etching of semiconductor nanocrystals

The size and shape of CdSe nanorods, CdSe tetrapods, and CdS nanowires were tailored by chemical and photochemical etchings in chloromethane solvents. Nanocrystals were synthesized by colloidal growth, and their sizes and shapes were visualized by transmission electron microscopy before and after etching. Crystal structures were confirmed by X-ray diffraction, and optical properties were monitored during etching. Chemical etching with tributylphosphine preferentially shortens the length of nanocrystals, whereas photochemical etching with both primary alkylamine and tributylphosphine reduces the diameter more than the length. The surface of etched nanocrystals was characterized by X-ray photoelectron spectroscopy, and the etching products dissolved in solvent were analyzed by matrix-free laser desorption ionization mass spectrometry. Spectroscopic results suggest that the chloride ion is the active species: Chloride ions are generated either by chemical activation of chloromethane solvents with tributylphosphine or by photoinduced electron transfer from nanocrystals to chloromethane solvents adsorbed on the nanocrystal surface. Tributylphosphine increases the rate of both chemical and photochemical etching, whereas primary alkylamine inhibits chemical etching. Both chemical and photochemical etching processes can be combined together with organic-based colloidal synthesis to anisotropically reshape the quantum-size and aspect ratio of nanocrystals.