Radiative\nLifetimes of Zincblende CdSe/CdS Quantum\nDots

Abstract

Recent synthetic advances have made\navailable very monodisperse\nzincblende CdSe/CdS quantum dots having near-unity photoluminescence\nquantum yields. Because of the absence of nonradiative decay pathways,\naccurate values of the radiative lifetimes can be obtained from time-resolved\nPL measurements. Radiative lifetimes can also be obtained from the\nEinstein relations, using the static absorption spectra and the relative\nthermal populations in the angular momentum sublevels. One of the\ninputs into these calculations is the shell thickness, and it is useful\nto be able to determine shell thickness from spectroscopic measurements.\nWe use an empirically corrected effective mass model to produce a\n“map” of exciton wavelength as a function of core size\nand shell thickness. These calculations use an elastic continuum model\nand the known lattice and elastic constants to include the effect\nof lattice strain on the band gap energy. The map is in agreement\nwith the known CdSe sizing curve and with the shell thicknesses of\nzincblende core/shell particles obtained from TEM images. If selenium–sulfur\ndiffusion is included and lattice strain is omitted from the calculation\nthen the resulting map is appropriate for wurtzite CdSe/CdS quantum\ndots synthesized at high temperatures, and this map is very similar\nto one previously reported (J. Am. Chem. Soc. 2009, 131, 14299). Radiative lifetimes determined\nfrom time-resolved measurements are compared to values obtained from\nthe Einstein relations, and found to be in excellent agreement. For\na specific core size (2.64 nm diameter, in the present case), radiative\nlifetimes are found to decrease with increasing shell thickness. This\nis similar to the size dependence of one-component CdSe quantum dots\nand in contrast to the size dependence in type-II quantum dots.