4D06

Ultrafast Charge Transfer Dynamics and Auger
Recombination of CdTe/CdS Core/Shell Quantum Dots
（Kwansei Gakuin Univ.1, Jilin Univ.2）Li Wang1, Yumei Tian2, Naoto Tamai1【Introduction】
Core/shell semiconductor quantum dots (QDs) have potential applications in solar cell,bio-imaging and QD-based laser because of their unique optical properties.1 For type I
core/shell QDs, their quantum yields are much higher than the core because of theconfinement of the charge carriers in the cores and the surface passivation of the shells. Fortype II, the band edges can be tailored from blue to near-IR wavelengths and the lifetime ofcharge carriers becomes longer due to the indirect recombination and the charge separation ofthe core/shell QDs. The exciton dynamics of CdTe/CdS core/shell QDs have been discussed
in terms of the change of energy structure from type-I to type-II.2 However, little has beenreported on Auger recombination of CdTe/CdS core/shell QDs. In the present study, Auger
recombination and the change of energy structure of CdTe/CdS core/shell QDs with different
size of CdTe core are discussed according to the experimental results of time-resolved
luminescence and transient absorption (TA) spectra.
【Experimental】
Aqueous soluble CdTe/CdS core/shell QDs were hydrothermally synthesized with successive
ionic layer adsorption and reaction method. Steady-state absorption, luminescence, XRD
spectra and TEM were used to confirm the formation of CdS shells on CdTe cores.
Time-resolved luminescence dynamics were measured by a time-correlated single-photon
counting system. TA spectra
101.0

core/shell QDs.

0.5

【Results and discussion】

0.0

Steady-state

absorptionand1(a)Intensity / Counts

first exciton states of core andNorm. OD

were performed with different
excitation wavelengths at theCdTe
CdTeCdS1
CdTeCdS2
CdTeCdS3

20(b)CdTe/CdS 3

0.1

CdTe/CdS 2

0.01

CdTe/CdS 1
CdTe core400

500 600 700
Wavelength / nm

800

0100
200
Time / ns

300

luminescence spectra of CdTe Fig. 1 Steady-state absorption and luminescence spectra of CdTe
core and CdTe/CdS core/shell QDs (a) and the luminescence decaysand CdTe/CdS core/shell QDs are of CdTe core (2.2 nm) and CdTe/CdS core/shell QDs (b).

Fig. 2 The decay profiles corresponding to 1S states of CdTe core and CdTe/CdS core/shell QDs (a); thecharge separation scheme of CdTe/CdS core/shell QDs with 400 nm shell excitation (b); Auger recombination
dynamics of CdTe/CdS core/shell QDs (c).

shown in Fig. 1a. Additional peaks around 400 nm for core/shell QDs represent the formationof CdS shells. Moreover, the 1S states were red-shifted and finally became much featurelesswith the increasing of shell thickness. The luminescence spectra were also red-shifted andbroadened. In Fig. 1b, the luminescence lifetime of core/shell QDs increased with the increaseof the CdS shell thickness, which was explained in terms of the modification of surface
trapping and the delocalization of electrons in type-II core/shell QDs. With 400 nm excitation,
the decay profiles at 1S states of CdTe core and CdTe/CdS core/shell QDs were plotted in Fig.
2a. The exciton lifetime, fitted with two exponential components, of core/shell QDs became
longer than that of CdTe QDs, due to the surface passivation and charge separation. Under theshell excitation of CdTe/CdS QDs with thicker shells, the charge separation dynamics areschematically illustrated in the Fig. 2b. Type-I CdTe QDs converted to type-II CdTe/CdS
heterostructures, which was characterized by spatial separation of charge carriers, elongatedluminescence lifetimes and significant red-shifted spectra. In addition, Auger recombination
lifetime of CdTe/CdS core/shell QDs (~50 ps) became much longer as compared with that ofCdTe core (~4 ps), as shown in Fig. 2c. These results are in a good agreement with theprevious reports in our group.3,4References(1) Smith, A. M.; Mohs, A. M.; Nie, S., Nat. Nanotechnol. 2009, 4, 56-63.
(2) Yan, Y. R.; Chen, G.; Van Patten, P. G., J. Phys. Chem. C 2011, 115, 22717-22728.
(3) Kobayashi, Y.; Pan, L. Y.; Tamai, N., J. Phys. Chem. C 2009, 113, 11783-11789.
(4) Sagarzazu, G.; Kobayashi, Y.; Murase, N.; Yang, P.; Tamai, N., Phys. Chem. Chem. Phys. 2011, 13,3227-3230.