Article pubs.acs.org/JPCC
Charge Separation by Indirect Bandgap Transitions in CdS/ZnSe Type-II Core/Shell Quantum Dots Sandeep Verma,* Sreejith Kaniyankandy, and Hirendra N. Ghosh* Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India S Supporting Information *
ABSTRACT: Femtosecond time-resolved absorption and picosecond time-resolved emission studies have been carried out to study the indirect type exciton of CdS/ZnSe core/shell quantum dots (QDs). The CdS/ZnSe core/shell QD samples are synthesized with increasing thickness of ZnSe shell on CdS core QDs. In these CdS/ZnSe core/shell samples, a new energy band lower than the energy gap of both the CdS core and ZnSe shell has been observed and attributed to indirect bandgap transitions from the valence band of the ZnSe shell to the conduction band of the CdS core. The transient PL studies have revealed that the indirect type exciton, e(CdS)/h(ZnSe) due to photoexcitation of this lowenergy band, endures less carrier trapping than selective excitation of the CdS core and charge transfer in the staggered photoexcited state. Femtosecond transient absorption studies have revealed that carrier trapping is as fast as 100 fs and interfacial charge recombination slows down with increasing ZnSe shell thickness on the CdS QD in CdS/ ZnSe core/shell QDs.
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INTRODUCTION There has been a continuous surge of interest in spectral and electronic properties of size-controlled semiconductor quantum dots (QDs) for their wide range of applications, including solar cells,1−3 light-emitting diodes,4,5 biolabeling,6,7 and lasing.8,9 In the past two decades, the size-dependent optical properties were largely investigated in II−VI semiconductor QDs mainly because of their ease of synthesis in aqueous and nonaqueous media.10,11 The QD research which was initially spurred by discrete energy spectra is now focused on carrier dynamics because of its paramount implications in optoelectronic devices such as intersublevel lasers12 and detectors.13 In this regard, various time-resolved techniques such as pump−probe transient absorption (TA) and up-conversion photoluminescence (PL) have been used in this field to understand different carrier relaxation pathways such as electron−phonon cooling, Auger cooling, trapping at surface defect states, charge recombination, etc.14−24 The majority of these experimental findings have been rationalized by the degree of carrier confinement in semiconductor QDs. The bandgap engineering of using two different QDs in core/shell geometry has been found to be very effective in controlling electron-hole (e-h) confinements.25−32 In such heterostructures, the carrier (e/h) confinement is governed by band alignment of core and shell semiconductors. Therefore, core/shell QDs may possess new electro-optical properties which were otherwise not accessible in single QDs. In type-I core/shell QDs, a narrow bandgap semiconductor is encapsulated in a wide bandgap semiconductor, for example, CdSe/ CdS,27 CdSe/ZnS,29 CdSe/ZnSe,30 etc. The inorganic capping of shell passivates the core surface, which yields high PL and stability, making type-I core/shell materials potentially useful © XXXX American Chemical Society
for biological tagging and light-emitting devices. In type-I core/ shell QDs, the photoexcited electrons and holes are confined primarily in the core region, producing e/h lifetimes identical to those of core QDs unless the charge carriers are trapped at surface states. However, photovoltaic applications, such as solar cells, low-threshold lasing,31 etc., require a smaller carrier overlap for long lifetimes. For such applications, type-II core/ shell QDs25,26 are more suitable because of their staggered band alignment which spatially separates the photoexcited charge carrier between core and shell QDs, giving an increased lifetime to the indirect exciton state. The type-II indirect type exciton has been reported earlier in CdS/ZnSe,32,33 CdTe/CdSe,34,35 CdSe/ZnTe,34,36 etc., core/shell QDs. Charge separation and recombination in type-II core/shell heterostructures have been investigated over a wide range of time scales using various spectroscopic techniques.34−42 Earlier, Bawendi and co-workers34 have used nanosecond time-resolved emission spectroscopy to determine long radiative lifetimes of spatially separated charge carriers (e/h) in CdTe/CdSe and CdSe/ZnTe core/shell QDs. Klimov and co-workers have reported different radiative decay rates of CdSe/ZnSe core/ shell QDs upon changing the carrier confinement from the type-I to the type-II regime.37 Later, Klimov and co-workers used transient absorption spectroscopy to explore spectral and dynamic properties of optical gain in type-II CdS/ZnSe core/ shell QDs.31,38 Time-resolved studies of this system were further carried out by Zamkov and co-workers43−46 using different heterostructures, such as ZnSe/CdS/ZnSe nanobarReceived: January 1, 2013 Revised: May 6, 2013
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dx.doi.org/10.1021/jp400014j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
ZnSe shell by the successive ionic layer adsorption and reaction (SILAR) method.48 None of the semiconductor materials comprising the heterostructure absorbs light in the visible region, so the charge carrier dynamics are studied by photoexciting CdS/ZnSe samples in the visible region using picosecond emission and femtosecond transient absorption spectroscopy. The effect of indirect bandgap transition on the charge carrier dynamics of the CdS/ZnSe system is described in this work.
bells and (ZnSe/CdS)CdS nanorods. Furthermore, Ivanov and Achermann47 have measured biexciton decay in the type-II localization regime of this system. In most of these transient studies, the pump wavelength was resonantly tuned with a narrower gap semiconductor, resulting in direct exciton states in that part of the core/shell heterostructure. The indirect type excitons were produced subsequently by carrier (e/h) transfer across the interface, so charge separation also consists of the carrier relaxation dynamics of occupying the lowest-energy state within the band offset. The intraband carrier relaxation can be intercepted by carrier trapping at interfacial defect states, which may be caused by lattice mismatch25,27 between core and shell QDs. This type of carrier trapping is expected to be less when charge separation is produced by indirect bandgap transition in type-II core/shell QDs. To explore any such possibility, it is important to perform time-resolved studies of indirect bandgap transitions in type-II core/shell QDs. In this work, we describe the indirect type exciton and carrier trapping dynamics in CdS/ZnSe core/shell QDs. The quaternary CdS/ZnSe heterostructure comprises a tolerable lattice mismatch of ∼2.7% and large conduction and valence band offsets.26 Scheme 1 shows type-II band alignment of the
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EXPERIMENTAL SECTION Preparation. Materials. Cadmium oxide (≥99%), zinc oxide (≥99%), Se powder (≥98%), oleic acid (OA; ≥90%), and octadecene (ODE; ≥90%) were purchased from Aldrich and used as received without further purification. Ethyl alcohol, chloroform, and n-hexane were used as received from SD Fine Chemicals. Synthesis of CdS QDs. Oleic-capped CdS QDs were synthesized by following the previously reported method49 of high-temperature pyrolysis of cadmium-oleate and sulfuroctadecene (ODE) precursors in hot noncoordinating solvent ODE. In brief, 1 mmol of cadmium oxide, 3 mmol of OA, and 50 mL of ODE were heated to 280 °C in an inert atmosphere (Ar purging) until a transparent solution appeared; 0.5 mmol of S dissolved in ODE was swiftly injected into this hot solution, and the mixture was allowed to cool to 250 °C. Ultrasmall CdS QDs were prepared by arresting the crystal growth at low temperature (40 ns lifetime). However, Demchenko and Wang52 have proposed a spin-forbidden, rather than an orbitalforbidden, nature of the dark exciton state of ultrasmall CdS QDs (40 ns) is perhaps due to the existence of surface states49,51,57 in ultrasmall CdS QDs. These surface states trap the charge carrier (e/h) in a few tens of picoseconds20,21 and decrease the overlap58 with other carriers (h/e) in CdS QDs. As a result, trapped charge carriers can exhibit a lifetime (radiative recombination) comparable to that of the optically passive dark exciton state. In the present study, the multiexponential decay profile of CdS QDs can be described by such radiative and nonradiative decay channels. The short decay component (∼0.8 ns; Table 1) is assigned to the optically active 1Se−1S3/2 exciton state. This assignment is supported by emission kinetics measured at peak wavelengths of the trap state emission (500 nm), which comprises a large decay with a subnanosecond time constant (∼71%; Supporting Information). The longer components, ∼8 and ∼50 ns, can be assigned to surface-trapped charge carriers and the dark exciton state in CdS QD, respectively. The PL kinetics of CdS/ZnSe-1, -2, and -3 core/shell QD samples show a significant increase of average lifetimes (Figure 4; Table 1). The average PL decay lifetime, ⟨τ⟩, of the CdS/ ZnSe-3 core/shell sample is observed to be 4.8 times greater than that of core CdS QD (Table 1). The long PL lifetime of the CdS/ZnSe-3 core/shell QD sample is confirmed further using a 445 nm excitation source in TCSPC measurements (Figure 4d). The PL dynamics measured after 445 nm photoexcitation is observed to be slower than the one obtained after 375 nm photoexcitation. Because 445 nm photoexcitation is capable of creating only indirect type excitons in CdS/ZnSe-3 D
dx.doi.org/10.1021/jp400014j | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
heterostructures, the increased PL lifetime is attributed to spatial charge separation in type-II CdS/ZnSe heterostructures. Figure 5 shows the kinetics of the trap state emission (610 nm) of a CdS/ZnSe-3 sample measured separately after
Figure 5. Transient emission kinetics of CdS/ZnSe-3 core/shell monitored at 610 nm after photoexcitation at 375 (a) and 445 nm (b).
Figure 6. TA spectra of CdS/ZnSe-3 core/shell QDs recorded 500 fs, 1 ps, 3 ps, 5 ps, 10 ps, and 50 ps after photoexcitation at 400 nm (top panel). Comparison of TA spectra of (a) CdS/ZnSe-1, (b) CdS/ZnSe2, and (c) CdS/ZnSe-3 core/shell at 5 ps delay time (bottom panel).
photoexcitation at 375 and 445 nm. The trap state emission kinetics exhibits a substantial decay on a shorter time scale (
