Extinction Coefficients, Oscillator Strengths, and Radiative Lifetimes of CdSe, CdTe, and CdTe/CdSe Nanocrystals

Abstract

In this paper we critically examine the literature and provide new data on fundamental optical properties of II–VI quantum dots (QDs). Specifically, we examine the integrated extinction coefficients and radiative lifetimes of different sizes of CdSe and CdTe QDs and different shell thicknesses in CdTe/CdSe core/shell QDs. We have synthesized particles having very high quantum yields and find that the measured radiative lifetimes are considerably longer and have a very different size dependence than what has been previously reported. In a simple two-level system the integrated extinction coefficients (or oscillator strengths) are related to the radiative lifetimes through the Einstein relations. The situation is more complicated in the case of II–VI QDs because of the thermal accessibility of dark states resulting from the valence band fine structure. There are significant but not equal populations in both bright and dark sublevels of the 1Se-1S3/2 exciton and in the dark 1Se-1P3/2 exciton. These Boltzmann populations depend on the QD size and shape. We find that in all three cases, quantitative or semiquantitative agreement between the measured radiative lifetimes and values calculated from the integrated extinction coefficients is obtained only if Boltzmann populations in all of the thermally accessible bright and dark states are considered. We also find that the shell thickness dependence of the radiative lifetimes of the CdTe/CdSe core/shell particles can be quantitatively understood in terms of overlap of calculated electron and hole wave functions. The results and analyses presented here clarify several discrepancies in the literature.

Effect of Correlation and Dielectric Confinement on 1S1/2(e)nS3/2(h) Excitons in CdTe/CdSe and CdSe/CdTe Type-II Quantum Dots

“…Dashed lines show upper and lower limits on the exciton energies resulting from an uncertainty of 1 ML in the nominal core radii. constant hole confinement in the h/e heterostructure.We also find good agreement between the oscillator strength obtained by Gong et al42 from the absorption spectra and our calculations. Calculating the oscillator strength f n of the 1S 1X,DCI , we find that f 1 + f 2 ∼ constant (insetFigure 5), confirming the validity of relevant excitonic wave functions too.…”

“…Figure 5 shows the 1S 1/2 (e) 1S 3/2 (h) and 1S 21 We see good quantitative agreement between the calculated exciton energies and the experimental data, with the data lying in the channels defined by an uncertainty of ±1 ML width in the core size. It should be noted that the results of Oron et al 21 were obtained on zinc-blende NC structures, in addition to those of Cai et al 44 The papers by Gong et al 42 and Ma et al 43 do not explicitly state the crystal structures of the core/shell nanoparticles, although Ma et al 43 note that their core/shell NCs gave absorption and photoluminescence spectra very similar to those of Cai and co-workers. 44 Our calculations accurately reproduce the 0.25 eV energy separation between the 1S 1/2 (e) 1S 3/2 (h) and 1S 1/2 (e) 2S 3/2 (h) excitons that is nearly independent of shell thickness.…”

“…44 Our calculations accurately reproduce the 0.25 eV energy separation between the 1S 1/2 (e) 1S 3/2 (h) and 1S 1/2 (e) 2S 3/2 (h) excitons that is nearly independent of shell thickness. 42 This constant energy separation is characteristic of changing electron confinement III.C. Correlation Energies.…”

“…Comparison with Experiment. The 1S 1/2 (e) nS 3/2 (h) (n = 1, 2) states are the two lowest energy excitons observed in the absorption spectra of colloidal CdTe/CdSe nanocrystals (NCs), 21,39,42 making them the most important for understanding the near band-edge absorption characteristics of such nanoparticles. Figure 5 shows the 1S 1/2 (e) 1S 3/2 (h) and 1S 21 We see good quantitative agreement between the calculated exciton energies and the experimental data, with the data lying in the channels defined by an uncertainty of ±1 ML width in the core size.…”

Strain release in metastable CdSe/CdS quantum dots

“…[1][2][3][4] The trapping of the charge carriers on surface defect sites results in non-radiative recombination pathways and therefore photoluminescence QYs that are below unity. [5] In CdSe/CdS quantum dots -so called "quasi-type-II" -the conduction band offset is quite small (approximately 0 to 0.30 eV) while there is a considerable valence band offset (0.6 -0.7 eV). The result is that photogenerated holes are localized in the CdSe core and decoupled from surface trapping processes.…”