The Role of Intrinsic and Surface States on the Emission Properties of Colloidal CdSe and CdSe/ZnS Quantum Dots
© to the authors 2007
Received: 11 May 2007
Accepted: 4 September 2007
Published: 22 September 2007
Time Resolved Photoluminescence (TRPL) measurements on the picosecond time scale (temporal resolution of 17 ps) on colloidal CdSe and CdSe/ZnS Quantum Dots (QDs) were performed. Transient PL spectra reveal three emission peaks with different lifetimes (60 ps, 460 ps and 9–10 ns, from the bluest to the reddest peak). By considering the characteristic decay times and by comparing the energetic separations among the states with those theoretically expected, we attribute the two higher energy peaks to ± 1 U and ± 1 L bright states of the fine structure picture of spherical CdSe QDs, and the third one to surface states emission. We show that the contribution of surface emission to the PL results to be different for the two samples studied (67% in the CdSe QDs and 32% in CdSe/ZnS QDs), confirming the decisive role of the ZnS shell in the improvement of the surface passivation.
Colloidal II-VI highly luminescent nanocrystals are important both in fundamental studies, due to their peculiar optical properties, and in technological applications such as diodes, lasers, photovoltaic cells. In the last years, great improvement in the Quantum Yield (QY) has been obtained by optimizing the inorganic surface passivation techniques . The knowledge of the dependence of radiative and nonradiative processes on the QDs structure, with particular attention on the role of surface states in the carrier relaxation upon laser excitation, is fundamental in order to make improvement on the QD QY. To this aim, we have performed TRPL measurements on the picosecond time scale on colloidal CdSe core and CdSe/ZnS core/shell QDs in a temperature range from 15 to 300 K. We show that in the first 2 ns the PL arises from three states with different lifetimes. By considering typical decay times and the energetic separations among the states extracted from the transient spectra, we conclude that the two peaks at higher energies can be assigned to emission from the lowest intrinsic bright states ± 1 U and ± 1 L of the fine structure of spherical CdSe QDs, whereas the low energy peak is due to emission from surface states. Moreover, we found that, in a low temperature range (15–60 K), an interplay among the states occurs. In particular, we had evidence for thermal filling of ± 1 U and ± 1 L states, fed by surface states.
We have prepared CdSe cores (4.5 nm in diameter) following the method described in ref. , and we have grown the ZnS shell by using the approach described in ref. . The QDs have been deposited by drop casting from chloroform solution on Si–SiO2 substrates. For each sample we performed TRPL measurements in the temperature range of 15–300 K in steps of 10 K. The QDs were excited by the second harmonic (397 nm) of a Ti:sapphire laser (pulse duration of 80 fs, repetition rate of 80 MHz). The sample emission was dispersed by a spectrograph (0.35 m focal length) and detected by a streak camera (temporal resolution of 17 ps). All the measurements were performed at low excitation density, in order to overcome multiexciton generation.
Results and Discussion
Best fit values oft 1,t 2,t 3and of the relative weightsA 1,A 2,A 3for the two samples at 15 K
62 ± 4
490 ± 11
10 ± 1
0.034 ± 0.001
0.29 ± 0.03
0.67 ± 0.02
61 ± 1
450 ± 10
9.5 ± 0.7
0.199 ± 0.003
0.474 ± 0.002
0.326 ± 0.005
In summary, we have demonstrated that the PL of CdSe core and CdSe/ZnS core/shell QDs in the first 2 ns arises from the intrinsic bright ± 1 U and ± 1 L states with lifetime of about 60 ps and 450 ps, respectively, and from surface states with lifetime of 9–10 ns. The contribution of surface states to the PL is considerably reduced after inorganic passivation of the CdSe core QDs.
We would like to thank Paolo Cazzato for valuable technical assistance. This work was supported by the European projects SA-NANO (contract n. 013698) and by the Italian Ministry of research (contract n. RBIN048TSE).
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