We studied the formation of zinc selenide (ZnSe) from zinc chloride (ZnCl2) and trioctylphosphine selenide (TOP=Se) in oleylamine, a chemistry originally proposed to grow ZnSe shells around InP core quantum dots. By monitoring the formation of ZnSe in reactions with and without InP seeds by quantitative absorbance and nuclear magnetic resonance (NMR) spectroscopy, we observe that the ZnSe formation rate is independent of the presence of InP cores. Similar to the seeded growth of CdSe and CdS, this observation supports a ZnSe growth mechanism through the inclusion of reactive ZnSe monomers that form homogeneously in the solution. Furthermore, by combining NMR and mass spectrometry, we identified the dominant reaction products of the ZnSe formation reaction as oleylammonium chloride and amino-substitutions of TOP, i.e., iminophosphoranes (TOP=NR), aminophosphonium chloride salts [TOP(NHR)Cl], and bis(amino)phosphoranes [TOP(NHR)2]. Based on the acquired results, we outline a reaction scheme that involves the complexation of TOP=Se by ZnCl2, followed by the nucleophilic addition of oleylamine onto the Lewis acid activated P-Se bond, thereby eliminating ZnSe monomers and forming amino-substitutions of TOP. Our work highlights the central role of oleylamine, acting as both the nucleophile and Brønsted base, in the transformation of metal halides and alkylphosphine chalcogenides into metal chalcogenides.

The fine structure of exciton states in colloidal quantum dots (QDs) results from the compound effect of anisotropy and electron-hole exchange. By means of single-dot photoluminescence spectroscopy, we show that the emission of photoexcited InP/ZnSe QDs originates from radiative recombination of such fine structure exciton states. Depending on the excitation power, we identify a bright exciton doublet, a trion singlet, and a biexciton doublet line that all show pronounced polarization. Fluorescence line narrowing spectra of an ensemble of InP/ZnSe QDs in magnetic fields demonstrate that the bright exciton effectively consists of three states. The Zeeman splitting of these states is well described by an isotropic exciton model, where the fine structure is dominated by electron-hole exchange and shape anisotropy leads to only a minor splitting of the = 1 triplet. We argue that excitons in InP-based QDs are nearly isotropic because the particular ratio of light and heavy hole masses in InP makes the exciton fine structure insensitive to shape anisotropy.

Colloidal InP core nanocrystals are taking over CdSe-based nanocrystals, notably in optoelectronic applications. Despite their use in commercial devices, such as display screens, the optical properties of InP nanocrystals and especially their relation to the exciton fine structures remain poorly understood. In this work, we show that the ensemble magneto-optical properties of InP-based core/shell nanocrystals investigated in strong magnetic fields up to 30 T are strikingly different from other colloidal nanostructures. Notably, the mixing of the lowest spin-forbidden dark exciton state with the nearest spin-allowed bright state does not occur up to the highest magnetic fields applied. This lack of mixing in an ensemble of nanocrystals suggests an anisotropy tolerance of InP nanocrystals. This striking property allowed us to unveil the slow spin dynamics between Zeeman sublevels (up to 400 ns at 15 T). Furthermore, we show that the unexpected magnetic-field-induced lengthening of the dark exciton lifetime results from the hyperfine interaction between the spin of the electron in the dark exciton with the nuclear magnetic moments. Our results demonstrate the richness of the spin physics in InP quantum dots and stress the large potential of InP nanostructures for spin-based applications.