Radiative decay of surface-trapped carriers and quantum yield in CdSe nanocrystal quantum dots

The emission dynamics in nanocrystal quantum dots (NCQDs) is very sensitive to the surface conditions because a big portion of total atoms in NCQDs exists at surfaces. Two sets of CdSe NCQDs forming the wurtzite structure but having different surface conditions are prepared by using different solvents such as octadecene (ODE) and hexadecylamine (HDA) in the synthesis. Compared to CdSe NCQDs prepared with ODE, CdSe NCQDs prepared with HDA have higher quantum yield (QY) owing to the surface passivation which reduces surface energy traps. Time-resolved photoluminescence spectra exhibit a fast decay of initially populated elecron–hole (e–h) recombination and a slow decay of surface-trapped carriers’ radiative relaxation (surface-related emission). The relative intensity of surface-related emission to initially populated e–h recombination is studied in the two sets of CdSe NCQDs, and higher quantum yielding CdSe NCQDs have the stronger surface-related emission in both sets. It is found that the radiative decay of surface-trapped carrier is important in the enhancement of the optical emission of NCQDs. The comparison of surface-related emissions in the two sets of CdSe NCQDs demonstrates that the surface passivated-NCQDs achieve high optical efficiency.

► CdSe NCQDs having different surface conditions are prepared.
► surface-related emission intensity to initially populated e-h recombination intensity.
► Higher quantum yielding CdSe NCQDs have the stronger surface-related emission.