Ultrafast Fluorescence Upconversion of Alloyed III-V/II-VI Core/Shell Quantum Dots
Freymeyer, Nathaniel Johnson
Colloid quantum dots (QDs) are semiconductor nanocrystals that can be tuned to emit specific wavelengths of light based upon their size. Maintaining long-term brightness of QDs is crucial towards their applications. Achieving this brightness requires effective surface passivation. Ultrafast fluorescence upconversion spectroscopy allows these ensemble charge carriers and trapping dynamics to be monitored. The obtained femtosecond decay constants and relative amplitudes show the effectiveness of surface passivation and allow direct comparisons to what has been previously observed for other QD systems. InP QDs are being developed as an alternative to those that contain the heavy metal cadmium with bright and stable InP/ZnSe being the most promising. Because of their poor stability in air, the ZnSe shell is often added prior to purification resulting in indium incorporation in the ZnSe shell. Since the effect of this indium alloy is unknown, two distinct interfaces in thick-shell InP/ZnSe III-V/II-VI QDs were synthesized and investigated: alloy and core/shell. The ultrafast carrier dynamics showed that indium incorporation into the shell was detrimental towards charge carrier recombination due to the charge imbalance caused by replacing a Zn2+ in the ZnSe shell with an In3+. To fully confine the charge carriers to the InP core, the extent of indium alloying needs to be minimized, and the surface requires further passivation.