Article pubs.acs.org/JPCC
Ultra Long-Lived Radiative Trap States in CdSe Quantum Dots Mohamed Abdellah,†,‡,¶ Khadga J. Karki,*,†,¶ Nils Lenngren,† Kaibo Zheng,† Torbjörn Pascher,† Arkady Yartsev,† and Tõnu Pullerits*,† †
Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden Department of Chemistry, Qena Faculty of Science, South Valley University, Qena 83523, Egypt
‡
S Supporting Information *
ABSTRACT: Surface states and traps play an important role in the photophysics of colloidal quantum dots. These states typically lead to large red-shifted photoluminescence. We have used steady-state and time-resolved spectroscopic techniques to investigate the nature of the traps and their lifetimes in colloidal CdSe quantum dots. We conclude that at least two different types of traps contribute to the photoluminescence. The trapping is more pronounced at higher excitation energies compared to the band edge excitation. Hole trapping is dominant in CdSe quantum dots. The time-resolved photoluminescence and pump−probe measurements show that the trapped holes live for longer than tens of microseconds at room temperature.
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INTRODUCTION The optical and optoelectronic properties of quantum dots (QDs) are governed by quantum confinement. This means that delocalization of an exciton is limited by the physical size of the QD to a volume that is smaller than the exciton coherence domain in bulk. The radiative recombination of the electron− hole pair leads to photoluminescence1,2 (PL) close to the band edge. A significant amount of research has been done to understand and enhance the band-edge PL by controlling the size and composition of the QDs.3 Not all the carriers (electrons and holes) in the excitons undergo recombination. Some of them get trapped by defects and surface states. The carrier localization by the surface states has mostly been regarded as a nuisance in the application of QDs:4 it affects exciton decay dynamics,5 reduces PL,6 and hinders charge transport.7 Consequently, investigation of surface effects of QDs over the past decade has mainly focused on reducing the trapping.2,8,9 Nevertheless, some recent studies10,11 have challenged the traditional negative perspective on localized carriers in QDs. These works show that surface localization of the carriers can be controlled to shape (broaden) the spectrum and enhance the PL11,12 from the QDs. Such a controlled PL could be used in broadband light-emitting devices13 and sensing applications.14 Moreover, as surface atoms constitute a significant fraction of a QD, these systems are interesting in understanding the overall localization and delocalization dynamics of charge carriers in highly confined systems. The surface states of QDs may be classified as extrinsic states;15 their structure and properties are characterized not just by the surface atoms but also by the defects and adsorbates (capping ligands) on the surface. Such surfaces are difficult to characterize structurally. On the other hand, charge carriers © 2014 American Chemical Society
localized on these states give distinct red-shifted PL. Since the early studies of colloidal QDs,16 the PL from the localized carriers have been used to model properties of the surface states as well as the process of carrier localization.17−20 The assumptions used in different models of the types of surface traps on QDs vary widely.19−21 Early work by Chestnoy et al.19 assumes that trapping of both electrons and holes occur at different sites of the QD and the recombination due to radiative tunneling is attributed to red-shifted PL. The temperature dependence of the PL is explained by assuming that the phonon modes also influence the tunneling process. The work by Jones et al. uses classical Marcus electron-transfer theory to explain the effect of trapping on the PL at the band edge. However, this work neglects the existence of red-shifted PL. The most recent model proposed by Mooney et al.,20 which is a simplification of the previous model proposed by Chestnoy et al.,19 describes carrier localization as a charge-transfer process using semiclassical Marcus theory.20 In this model a single surface state (trap) potential at ΔG0 lower than the delocalized state is shifted along the configuration coordinate by Huang− Rhys parameter S. The two parameters are used to describe the width and red shift of the surface PL with respect to the core (band-edge) PL. On the basis of our steady-state and timeresolved PL and pump−probe experiments, we argue that though the model is useful in simplifying the general mechanism of carrier localization in QDs, a single trap state does not fully explain our results. We describe the discrepancies in this paper and also point out the implications of our results Received: July 1, 2014 Revised: August 26, 2014 Published: August 26, 2014 21682
dx.doi.org/10.1021/jp506536h | J. Phys. Chem. C 2014, 118, 21682−21686
The Journal of Physical Chemistry C
Article
on the nature of coupling between the trapped and delocalized charge carriers.
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EXPERIMENTAL SECTION In this work we used CdSe QDs with different sizes and CdSe/ ZnS gradient core/shell QDs (CSQDs) with 1.3 nm shell thickness. For oleic acid-capped QDs, we used the same procedure as in our previous work.22 Briefly, CdO (0.51 g) was dissolved in 1-octadecene solution (80 mL) and oleic acid (6 mL) at 150 °C under N2. To obtain different sizes of QDs, the injection temperature was varied from 190 to 280 °C. Asprepared QDs were purified twice using solvent extraction mixture (hexane−acetone), centrifuged, and redissolved in hexane. Gradient OA−CSQDs were prepared according to Bae et al.,23,24 and recently reproduced in our work.25 Briefly, both Cd2+ and Zn2+ oleate in 1-octadecene solution were heated to 325 °C under N2. Then, Se2− and S2− in trioctylphosphine (TOP) solution were swiftly injected into the cation precursor solution. To obtain CSQDs with 1.3 nm shell thickness, the growth process was terminated after 3 min by cooling the mixture using an ice bath. As-prepared CSQDs were purified twice using the same manner as above and finally redissolved in hexane as well. Absorption and emission spectra were recorded using an Agilent 845x spectrophotometer and Spex 1681 fluorescence spectrometer, respectively. Time-resolved PL measurements were performed using a time-correlated single-photon counting device (PicoQuant). A pulsed diode laser, triggered externally at 400 kHz, was used to excite the sample at 438 nm. The pulse duration of the laser was about 200 ps. The PL from the sample at different wavelengths was selected using long-pass filters to block out the band-edge emission and the excitation light. The emitted photons were focused onto a fast avalanche photodiode (SPAD, Micro Photon Device). The response time of the photodiode was
