Surface-dependent, ligand-mediated photochemical etching of CdSe nanoplatelets

Photochemical etching of CdSe nanoplatelets was studied to establish a relationship between the nanocrystal surface and the photochemical activity of an exciton. Nanoplatelets were synthesized in a mixture of octylamine and oleylamine for the wurtzite (W) lattice or in octadecene containing oleic acid for the zinc-blende (ZB) lattice. For photochemical etching, nanoplatelets were dispersed in chloroform containing oleylamine and tributylphosphine in the absence or presence of oleic acid and then irradiated with light at the band-edge absorption maxima. Etching phenomena were characterized using UV-vis absorption spectroscopy and transmission electron microscopy. The absorption spectra of both W and ZB CdSe nanoplatelets showed that the exciton was confined in one dimension along the thickness. However, the two nanoplatelets presented different etching kinetics and erosion patterns. The rate of etching for W CdSe nanoplatelets was much faster than that for ZB nanoplatelets. Small holes were uniformly perforated on the planar surface of W nanoplatelets, whereas the corners and edges of ZB nanoplatelets were massively eroded without a significant perforation on the planar surface. This suggests that the amine-passivated surface of trivalent cadmium atoms on CdSe nanoplatelets is photochemically active, but the carboxylate-passivated surface of divalent cadmium atoms is not. Hence, the ligand, which induces the growth of W or ZB CdSe nanoplatelets, mediates the surface-dependent photochemical etching. This result implies that an electron-hole pair can be extracted from the planar surface of amine-passivated W nanoplatelets but from the corners and edges of carboxylate-passivated ZB nanoplatelets.