Aqueous synthesis of alloyed CdSexTe1-x nanocrystals

Alloyed composition of semiconductor nanocrystals opens up additional perspectives in engineering materials with different electronic and optical properties. We demonstrate a successful synthesis of water soluble alloyed CdSexTe1-x and CdSxTe1-x nanocrystals stabilized with short-chain thiols by simultaneous use of selenium (or sulfur) and tellurium precursors. Alloyed CdSexTe1-x particles were studied in details and were compared with bare CdSe and CdTe nanocrystals synthesized under similar conditions; different molar ratios of Se to Te precursors ranging from 1/9 to 1/1 were tested. The resulting nanocrystals were characterized by UV-vis and photoluminescence spectroscopy, powder X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray analysis. By changing the size and the composition of CdSexTe1-x nanocrystals, the spectral tunability of the absorption onset between 550-690 nm has been demonstrated. Alloyed nanocrystals showed band-edge emission with an intensity reduced by an order of magnitude as compared to the bare CdTe nanoparticles. The growth of alloyed nanocrystals was faster than that for the bare CdSe and CdTe particles, leading to formation of bigger nanocrystals at given reaction times. The reactivity of Se precursor (NaHSe) was found to be higher than that for Te (NaHTe) at the reaction conditions employed, resulting in a sufficiently increased amount of Se incorporated into alloyed nanocrystals, as compared to the ratios of Se/Te precursor used. This was confirmed by EDX measurements and has been qualitatively demonstrated by comparison of experimental data on optical bandgaps of CdSexTe1-x NCs of certain experimentally determined sizes with calculated dependences of the bandgap of alloyed NCs on their size and composition, taking in to account the optical bowing effect.