Metal-Halide-Ligated Cadmium Selenide Quantum Belts by Facile Surface Exchange

Amine-ligated {CdSe[n-octylamine]_0.53±0.06} quantum belts undergo instantaneous, reversible exchange of the L-type amine ligation for Z-type ligation provided by the neutral metal halides MX₂ (M = Cd, Zn; X = Cl, Br, I). The CdX₂-ligated quantum belts are determined to have the compositions {CdSe[CdCl₂]_1.23}, {CdSe[CdBr₂]_1.03}, and {CdSe[CdI₂]_0.42}, corresponding to two, two, and one CdX₂ monolayer shells, respectively. Exchange from L-type to Z-type MX₂ ligation results in large shifts of the quantum-belt absorption (extinction) features to lower energy, which are 252, 151, and 157 meV for CdCl₂, CdBr₂, and CdI₂ ligation, respectively. These spectral shifts, which are due to changes in the strain states of the quantum belts (QBs), and extension of the CdSe crystal lattice by surface coordination of the ligand Cd atoms, are rapidly and completely reversed by back exchange to n-octylamine ligation. Similar, reversible spectral shifts are observed upon L-type to Z-type exchange with ZnX₂ ligation, although the shifts are smaller in magnitude. The results demonstrate the facility, reversibility, and generality of L-type for Z-type surface exchanges in wurtzite CdSe quantum belts, and that neutral metal halides may function as Z-type ligands in semiconductor nanocrystals.