Homogeneously-Alloyed CdTeSe Single-Sized Nanocrystals with Bandgap Photoluminescence

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Abstract

Homogeneously alloyed ternary CdTeSe magic-sized nanocrystals (MSNs) with bandgap emission were synthesized in 1-octadecene (ODE) via a noninjection one-pot approach featuring synthetic reproducibility and large-scale capability. The noninjection approach used cadmium acetate dihydrate (Cd(OAc)2·2H2O), elemental selenium, and elemental tellurium as Cd, Se, and Te sources, respectively. The growth of the CdTeSe nanocrystals was carried out at temperatures from 120 to 200 °C for several hours in a reaction flask containing the reactants together with a long-chain fatty acid as capping ligands and ODE as the reaction medium. During synthesis, the CdTeSe nanocrystals exhibited persistent bandgap and did not grow in size anymore after their formation, either with longer growth periods or higher reaction temperature. Also, they exhibit an absorption doublet with 288 meV energy difference between the two peaks; the first excitonic transition peak is at 520 nm and bandgap photoemission peak at 524 nm with full width at half-maximum (fwhm) of ca. 20 nm. Both the growth pattern and the optical properties suggest that they are magic-sized and thus termed as single-sized. Nuclear magnetic resonance (NMR), with sensitivity to local environment, provided valuable information regarding the structure and composition of the nanocrystals. Solid-state 13C cross polarization/magic angle spinning (CP/MAS) NMR spectra showed that the carboxylate capping ligand is firmly attached to the crystal, and 1H decoupled 113Cd MAS spectra, with and without CP, distinguished between the surface Cd species and the Cd inside a nanocrystal. The 113Cd NMR results also confirmed, unambiguously, that the nanocrystals are homogeneously alloyed ternary CdTeSe, with 113Cd resonances located between those of CdTe and CdSe nanocrystals indicating a stoichiometry of approximately 1Se:1Te ratio throughout the whole nanocrystal. XRD supported that they are ternary-alloyed CdTeSe rather than binary CdTe or CdSe, with a wurtzite crystal structure. In addition, both energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) supported an approximate stoichiometric ratio of 1Se:1Te of the CdTeSe nanocrystals.

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