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

Abstract

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.

Facile Exchange of Tightly Bonded L-Type Oleylamine and Diphenylphosphine Ligands on Copper Indium Diselenide Nanocrystals Mediated by Molecular Iodine

“…We propose that I 2 acts as a Z-type ligand in which the I adatom forms a dative bond with the nanocrystal surface. This mechanism is consistent with reports indicating that Z-type and L-type ligands can participate in facile ligand exchange 51,52 and the observed binding of I 2 to CuInSe 2 surfaces. 60 These results support the findings that Ltype and Z-type ligands can participate in facile exchange and indicate that Z-type ligands can even induce exchange between two L-type ligands.…”

“…Z-Type ligands act as Lewis acids and coordinate with the nanocrystal surface by accepting two electrons to form a dative bond, as in the case of Cd­(oleate) 2 . Typically, ligands must be of the same type to readily exchange; however, charge neutral L-type and Z-type ligands have also been found to readily exchange in some cases. , …”

Delocalized Surface Electronic States on Polar Facets of Semiconductor Nanocrystals

“…Colloidal semiconductor nanocrystals with their sizes in the quantum-confinement regime have attracted extensive attention in recent years because of their size-dependent optical properties and solution processability . Rapid advancement of their synthetic chemistry − has made the size- and shape-controlled semiconductor nanocrystals widely available. At present, their unpredictable and often irreproducible surface defects − are becoming the main roadblock for realizing their full potential in fundamental research and industrial applications.…”

Surface Chemistry Impact on the Light Absorption by Colloidal Quantum Dots

“…X‐ray diffraction patterns and electron microscopy imaging can provide evidence that the QD structure and morphology, respectively, do not significantly alter upon ligand exchange (Figures 3a and 3b). Indeed, ligand‐induced structural and morphological transformations are common to lead halide perovskite nanocrystals, [42,43] whereas ligand‐induced strain may largely contribute to change the optical properties of the highly anisotropic quantum wires and wells [44,45] . The solution phase ligand exchange reactions can be conveniently monitored by NMR spectroscopy probing those resonances characteristic of the replaced and the replacing ligands.…”

“…Indeed, ligand‐induced structural and morphological transformations are common to lead halide perovskite nanocrystals,[ 42 , 43 ] whereas ligand‐induced strain may largely contribute to change the optical properties of the highly anisotropic quantum wires and wells. [ 44 , 45 ] The solution phase ligand exchange reactions can be conveniently monitored by NMR spectroscopy probing those resonances characteristic of the replaced and the replacing ligands. Ligands coordinated to the QDs in properly purified samples usually show broadened, downfield shifted resonances compared to those of the same unbound ligands, thus consenting a quantitative evaluation of the effective displacement of the ligands coming from the synthetic procedure.…”