Formation of nanoscale spatially indirect excitons: Evolution of the type-II optical character of CdTe/CdSe heteronanocrystals

Abstract

In this work, the evolution of the optical properties of nanoscale spatially indirect excitons as a function of the size, shape, and composition of the heteronanostructure is investigated, using colloidal CdTe/CdSe heteronanocrystals ͑2.6 nm diameter CdTe core and increasing CdSe volume fraction͒ as a model system. Emphasis is given to quantitative aspects such as the absorption cross section of the lowest-energy exciton transition ͑ SS ͒, Stokes shift, linewidths, and the exciton radiative lifetime. The hole wave function remains confined to the CdTe core while the electron wave function is initially delocalized over the whole heteronanocrystal ͑type-I 1/2 regime͒, and gradually localizes in the CdSe segment as the growth proceeds, until the spatially indirect exciton transition becomes the lowest-energy transition ͑type-II regime͒. This results in a progressive shift of the optical transitions to lower energies, accompanied by a decrease in the oscillator strengths at emission energies and an increase in the exciton radiative lifetimes. The onset of the type-II regime is characterized by the loss of structure of the lowest-energy absorption band, accompanied by a simultaneous increase in the Stokes shift values and transition linewidths. This can be understood by considering the dispersion of the hole and electron states in k space. The SS values decrease rapidly in the type-I 1/2 regime but only slightly in the type-II regime. This shows that the indirect exciton formation leads primarily to redistribution of the oscillator strength of the lowest-energy transition over a wider frequency range. The total absorption cross section per ion-pair unit ͑i.e., integrated over all the exciton transitions͒ remains essentially constant during the heteronanocrystal growth, demonstrating that SS is redistributed from higher-energy transitions of both the CdTe and the CdSe segments, in response to the reduction in the electron-hole wavefunction overlap. Two radiative decay rates are observed and ascribed to exciton states with different degrees of localization of the electron wave function ͑an upper state with a faster decay rate and a lower state with a slower decay rate͒. The results presented here provide fundamental insights into nanoscale spatially indirect exciton transitions, highlighting the crucial role of a number of parameters ͑viz., electron-hole spatial correlation, exciton dispersion and exciton degeneracy, shape effects, and electronic coupling͒.

Type-II Colloidal Quantum Wells: CdSe/CdTe Core/Crown Heteronanoplatelets

“…Previously, two similar decay channels were observed in CdTe/CdSe based colloidal quantum dots. 36 The origin of the coexistence is still under debate, however, these two long lifetime components clearly indicate that there are two distinct exciton states having Type-II properties with varying oscillator strength. A possible explanation might be the emissive mixed hole states (i.e., electron/light-hole and electron/heavy-hole) in these Type-II NPLs as supported by the broadening of the emission fwhm.…”

Temperature dependent radiative and non-radiative recombination dynamics in CdSe–CdTe and CdTe–CdSe type II hetero nanoplatelets

“…[16][17][18][19][20][21][22] Recent studies on the synthesis of type II CdSe-CdTe and CdTe-CdSe core-lateral shell nanoplatelets have investigated their morphology, the type II band offsets as well as the prolongation of the PL decay time with respect to core only nanoplatelets at room temperature. [23][24][25] In this report we concentrate on the temperature dependent photoluminescence (PL) decay dynamics and show that the observed luminescence decay is related to spatially indirect excitons, having a significantly prolonged lifetime with respect to core only nanoplates.…”

The Different Nature of Band Edge Absorption and Emission in Colloidal PbSe/CdSe Core/Shell Quantum Dots

“…2 Similar effects, combined with a decrease in the Auger recombination rate were observed for CdTe/CdSe QDs. 3,4 For CdSe/CdTe heteronanocrystals 5,6 and ZnTe/ZnSe QDs, 7 long radiative lifetimes, indicative of charge separation, and a strong redshift of the exciton transitions were reported.…”