ZnTe Type II Core−Shell

ARTICLE pubs.acs.org/JPCC

Ultrafast Hole Transfer in CdSe/ZnTe Type II Core-Shell Nanostructure Sreejith Kaniyankandy,† S. Rawalekar, Sandeep Verma, and Hirendra N. Ghosh* Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai, 400094 India

bS Supporting Information ABSTRACT: We have synthesized thiol-capped CdSe/ZnTe quantum dot core-shell nanostructures by colloidal methods, have characterized them by steady-state absorption and photoluminescence (PL) spectroscopy and further confirmed by high resolution transmission electron microscopy and X-ray diffraction measurements. Clear red shift on shell formation was observed in optical absorption and photoluminescence studies. Timeresolved emission studies indicate longer emission lifetime of CdSe/ZnTe core-shell as compared to CdSe QD material where in both cases only CdSe gets excited, which indicates spatial charge separation in type-II core-shell. Ultrafast photoinduced charge transfer dynamics in type-II CdSe/ZnTe donor-acceptor core-shell were studied in real-time using femtosecond broadband pump-probe spectroscopy. Our transient absorption studies suggests that on photoexcitation core-shell hole transfer from CdSe core to ZnTe shell takes place in pulse-width limited time scale as evidenced by an increase in cooling dynamics of the charge carriers from 150 fs for CdSe to 300 fs for thickest CdSe/ZnTe core-shell. Increase in cooling dynamics in core-shell has been explained due to decoupling of electron and hole in photoexcited core-shell. Trapping dynamics play a major role in the excited dynamics of the photoexcited charge carriers of quantum dot materials. Bleach recovery kinetics of the photoexcited QD materials fitted multi-exponentially where 2.5 ps (first component) has been attributed to the electron trapping dynamics and the longer components (30-50 ps and >400 ps) attributed to the charge recombination dynamics.

1. INTRODUCTION Charge recombination in quantum dots (QDs) is an area of current ongoing research due to its important roles in QD-based devices, such as solar cells and light-emitting diodes.1 Concept and recent experimental reports on multiple exciton generation (MEG) by one absorbed photon in some QDs2 suggest that the efficiency of QD-based solar cells can be increased dramatically.3 However, the main hurdle arises due to exciton-exciton annihilation process in photoexcited QD that occurs on the 10 to 100 ps time scale.4 So it is necessary to dissociate the exciton by ultrafast charge transfer to electron donors and acceptors before the annihilation process. There are few reports available in the literature on ultrafast exciton (bound electron-hole pair) dissociation in QDs by electron transfer (ET) or by hole transfer to adsorbed acceptors, however, mechanism and rate of charge transfer are still poorly understood.5-9 As we discussed in early paragraph that for better application of QDs efficient charge separation still remains a challenge. Semiconductor core-shell nanostructure is one of the best possible ways for charge separation that can be synthesized by combining two or more semiconductors with proper lattice mismatch between them. Recently, considerable progress has been made in the synthesis of type II hetero junction nanostructures10 r 2010 American Chemical Society

that spatially separate photoexcited electrons and holes in different parts of a composite QD. These core-shell nanocrystals are made up of two semiconductor materials with a particular alignment of conduction and valence band edges at the interface, which creates a steplike radial potential favoring the localization of one of the carriers in the core of a QD and the other one in the shell. Such a structure is called a staggered band-aligned heterostructure or a type II core-shell.11 These nanostructures are important from the point of view of photovoltaic device application, as an improved charge separation enables slower recombination times due to reduced electron-hole (e-h) wave function overlap. Second the absorption related to direct excitation from one semiconductor can occur, somewhat weakly analogous to an “indirect band gap”, leading to an extension of optical response to the red side of the electromagnetic spectrum, thus facilitating in covering red region of the electromagnetic spectrum. However, the performance of such a heterostructure could be hampered due to formation of a defect interface between the core and shell. So it is necessary to understand how fast the electron or hole can Received: August 10, 2010 Revised: November 20, 2010 Published: December 27, 2010 1428

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The Journal of Physical Chemistry C be separated in core-shell nanostructures and also to understand the effect of a defect state on the charge transfer dynamics of type II core-shell. Considerable research efforts have been devoted to charge transfer dynamics of type II semiconductor QDs.10-15 Bawendi and co-workers10c reported the time-resolved emission in CdSe/ ZnTe and CdTe/CdSe where they have shown that charge recombination time in CdSe/ZnTe can be increased from 9.6 to 57 ns. In our earlier investigation, we have also investigated ultrafast charge transfer dynamics in CdTe/CdS core-shell using femtosecond time-resolved absorption spectroscopy. Chen and co-workers11a have reported spatial separation of charge carrier by changing the size of the core and the shell in CdSe/ZnTe. Feldman and co-workers16reported charge separation and tunneling behavior between CdTe and CdSe QD material with the formation of core-shell structures. Banin and co-workers17 reported photophysical properties of ZnSe QD within CdS nanorods and demonstrate the charge separation. Most of the above studies have clarified the issue of charge separation clearly; however, much more needs to be understood for a device application. For example, on UV excitation certain materials like PbSe, CdTe, PbTe, and so forth13 show the ability to generate multiexcitons; these multiexcitons need to be separated by e-h energy transfer before they annihilate into a hot exciton, and therefore an ultrafast charge separation is required. This indirectly means that we should decrease the interaction between the carriers before they annihilate by Auger recombination13 due to e-h mediated by coulomb interaction. Lower electron-hole wave function overlap and repulsive biexciton interaction on formation of a type II heterostructure enables in achieving slower e-h energy transfer. In fact, this information has been incorporated in creating a better quantum dot by forming a thick shell as described in previously reported results.10a,14 Klimov et al.14 has shown several orders of magnitude increase in the biexciton recombination by the formation of a thick shell. Additionally. Sionnest et al.10a have exploited the multiple shell configurations to achieve nanoseconds order cooling time. So it is very important know how fast the electrons or holes are migrated to another semiconductor and also how fast the photoexcited carriers relaxed in different QD materials. To understand charge transfer and carrier cooling dynamics in early time scale we have carried out femtosecond transient absorption studies in tailor-made CdSe/ZnTe core-shell with different thickness. We have synthesized the core-shell material by adopting eco-friendly colloidal method in water that much less hazardous as compared to organometallic synthesis by arrested precipitation at high temperature.10c,11a We have chosen CdSe/ZnTe core-shell where lattice mismatch between core and shell is less than 1% ,which ensures less defect (cleaner) surface and minimizes nonradiative relaxation via interface defects or charged excitons. Core-shell materials have been characterized by steady-state optical absorption and emission studies and also in high resolution transmission electron microscopy (HRTEM) technique. To unravel the charge transfer dynamics, we have carried out transient absorption studies in ultrafast time scale detecting the transients in visible to near-IR region by exciting both CdSe core and CdSe/ZnTe core-shell material at 400 nm laser light. Detail mechanism of charge transfer and carrier cooling mechanism has been discussed.

2. EXPERIMENTAL SECTION a. Materials. Cadmium chloride (g99%), Te powder (g98%), Se powder (>98%), 1-mercaptopropionic acid (g99%), and

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NaBH4 were purchased from Aldrich and used as received without further purification. Isopropyl alcohol was used as received from S. D. Fine Chemicals. b. Synthesis of CdSe QDs. Thiol-capped CdSe QD was prepared by the following previously reported method.18 In brief, cadmium chloride (cadmium precursor) was dissolved in N2 purged nanopure water followed by addition of 1-mercaptopropionic acid (MPA). The solution pH was adjusted to 9-10 using 1 M NaOH. NaHSe (selenium precursor) solution was prepared fresh by reducing Se with NaBH4 with Se/NaBH4 = 1:2 in water maintained at 273 K for 1 h. NaHSe was added to cadmium precursor solution with vigorous stirring. The ratio between Cd, MPA, and Se was maintained at 1:2.5:0.5, respectively. The above solution was refluxed for six hours. The reaction mixture was allowed to cool naturally and was concentrated to one/third of its original volume by using rotary evaporator. QDs were precipitated by adding isopropyl alcohol as the nonsolvent. Supernatant was discarded and the precipitate was redissolved in nanopure water and finally reprecipitated. The cleaning was complete by a three time dissolution and reprecipitation procedure to remove unreacted precursors. These samples were dried in ambient condition for further use. c. Shell Growth. For shell formation, the concentration of the precursor for formation of 2.5 nm shell was calculated according to reported methods. The 10-6 M MPA-capped CdSe was dissolved in N2 purged water. To this solution, zinc nitrate hexahydrate and MPA solution in 1:2.5 molar ratio was added and the pH was adjusted to 9-10. NaHTe (tellurium precursor) solution was prepared fresh by reducing Te with NaBH4 with Te/NaBH4 = 1:2 in N2 purged water maintained at 273 K for 8 h. This was added to the above prepared QD solution with Zn precursor with vigorous stirring. The Zn/MPA/Te molar ratio was maintained at 1:1.5:0.5. The solution was refluxed for 6 h for the growth of shell. To obtain shells of different shell thickness the samples were eluted at different time (0.5, 1, 2, and 6 h). The cleaning procedure was the same as described above for the CdSe QDs. The four samples are named as CdSe/ZnTe1, CdSe/ ZnTe2, CdSe/ZnTe3, and CdSe/ZnTe4 with increasing shell thickness order which were eluted at 0.5, 1, 2, and 6 h, respectively. d. TCSPC Studies. Time-resolved fluorescence measurements were carried out using a diode laser-based spectrofluorometer from IBH (U.K.). The instrument works on the principle of time-correlated single-photon counting (TCSPC). In the present work, 453 nm LED was used as the excitation light sources and a TBX4 detection module (IBH) coupled with a special Hamamatsu PMT was used for fluorescence detection. e. Femtosecond Transient Absorption Studies. The femtosecond tunable visible spectrometer has been developed based on a multipass amplified femtosecond Ti:sapphire laser system supplied by Thales, France. The pulses of 20 fs duration and 4 nJ energy per pulse at 800 nm obtained from a self-mode-locked Ti-Sapphire laser oscillator (Synergy 20, Femtolaser, Austria) were amplified in a regenerative and two-pass amplifier pumped by a 20 W DPSS laser (Jade) to generate 40 fs laser pulses of about 1.2 mJ energy at a repetition rate of 1 kHz. The 800 nm output pulse from the multipass amplifier is split into two parts to generate pump and probe pulses. In the present investigation, we have used frequency doubled 400 nm as excitation sources. To generate pump pulses at 400 nm, one part of 800 nm with 200 μJ/pulse is frequency doubled in BBO crystals. To generate visible probe pulses, about 3 μJ of the 800 nm beam is focused 1429

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Figure 1. (A) HRTEM image of CdSe/ZnTe quantum dot core-shell. (Inset: Selected electron diffraction pattern of the corresponding particles). (B) Same picture in higher resolution showing single particle.

onto a 1.5 mm thick sapphire window. The intensity of the 800 nm beam is adjusted by iris size and ND filters to obtain a stable white light continuum in the 400 nm to over 1000 nm region. The probe pulses are split into the signal and reference beams and are detected by two matched photodiodes with variable gain. We have kept the spot sizes of the pump beam and probe beam at the crossing point around 500 and 300 μm, respectively. The noise level of the white light is about ∼0.5% with occasional spikes due to oscillator fluctuation. We have noticed that most laser noise is low-frequency noise and can be eliminated by comparing the adjacent probe laser pulses (pump blocked vs unblocked using a mechanical chopper). The typical noise in the measured absorbance change is about 400 ps (38%). Here, the 155 fs time constant can be attributed to state filling to 1S excitonic level. The bleach recovery time constants suggest the charge recombination time constants between photogenerated electrons and holes. We have also monitored decay kinetics of excited state absorption (ESA) of photoexcited CdSe at 690 nm (Figure 6, inset) that can be fitted multiexponentially with time constants 1.3 ps (30%), 10 ps (45%), and >200 ps (25%). It is interesting to see that growth time of the signal can be fitted single exponentially with pulse-width limited time constant (400 ps (37%) for CdSe/ZnTe3 core-shell, and τ1 = 3 ps (21.5%), τ2 = 50 ps (32.5%), and τ3 = >400 ps (46%) for CdSe/ZnTe4 core-shell. It clearly hints the multiexponential nature of bleach recovery telling us that the charge carrier dynamics is governed by more than one process. The multiexponential nature of the dynamics can come from multiple sized QD in addition to trapping dynamics of trap states. If the dynamics is dominated by only one lifetime contribution, it could come from single dot contribution. However, in the present case we have a long component that contributes quite substantially (>30%). Since under the present excitation conditions N , 1, we can conclude with certainty that biexciton and Stark effect contribution are negligible in the present studies. Furthermore, we can rule out size distribution as the cause of the multiexponential nature as the position of the bleach wavelength does not change with time even up to 80 ps. So, the multiexponential dynamics can only come from different processes and traps are the most likely candidate. Comparison of bleach recovery kinetics from CdSe core to CdSe/ZnTe core-shell with different thickness reveal that the contribution of first component (τ1) reduces from ∼45 to 21.5% for CdSe core to CdSe/ZnTe4 core-shell. τ1 contribution can be attributed to electron traps at the surface of CdSe, as this component still remains in the core-shell samples also. Further evidence for this assignment also comes from the fact that CdSe surface is truncated by MPA by formation of Cd-S thiolate linkage. It is well-known that thiolate linkages act as hole traps and if the hole traps are the reason for that contribution we can expect the contribution to completely disappear on the formation of shell. Therefore we can conclude that the decrease in the contribution in the core-shell samples is due to annihilation of Cd related dangling bonds that act as electron traps at the surface. Now, let us discuss the dynamics associated with the τ2 and τ3 components. We can clearly see that both amplitude and contribution of the τ2 and τ3 components increases with the thickness of the shell. This observation clearly suggest us that τ2 and τ3 components might be due to charge recombination indicating better charge separation on the formation of the shell. This observation was also supported by time-resolved photoluminescence studies as discussed in an earlier section.

4. CONCLUSION We have synthesized thiol-capped CdSe QD and CdSe/ZnTe type II core-shell nano heterostructure that was characterized by steady-state absorption, steady-state emission study, and highresolution transmission electron microscopy. CdSe QD shows

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emission purely due to surface state, however on formation of ZnTe shell surface state emission drastically reduces with the appearance of exciton emission. Steady-state emission studies indicate red shift in the peak position of excitonic emission with shell thickness. Time-resolved emission kinetics suggest that average radiative lifetime of CdSe-ZnTe core-shell increases by 4-fold as compared to CdSe core, which clearly suggests spatial separation electron and hole in core-shell. To understand both charge transfer and carrier cooling dynamics, we have carried out femtosecond time-resolved absorption studies in the visible region. Our ultrafast studies suggest that in photoexcitation of CdSe-ZnTe core-shell, initially electron and holes are generated in the CdSe core and holes are migrated to ZnTe shell in pulse-width time scale followed by electron cooling in CdSe core that is greatly affected by the thickness of ZnTe shell. Our studies suggest that trapping dynamics plays a major role in the relaxation dynamics of photoexcited charge carriers in both core and core-shell materials. Trap states influence the overall performance of the devices constructed with quantum dots and core-shell nanostructure material. Our studies suggest that involvement of trap states adversely affect the charge carrier dynamics of quantum dot materials, which is very important in technological application viewpoint.

’ ASSOCIATED CONTENT

bS

Supporting Information. Nanosecond time-resolved emission data and femtosecond time-resolved bleach kinetics (both growth (τg) and recovery time constants of (τ1, τ2, and τ3)) of CdSe QD and CdSe/ZnTe core-shell with different thickness. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 00-91-22-2550-5151. † E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Raghav, Ashutosh Rath, and Dr. P. V. Satyam, Institute of Physics, Bhubaneswar for TEM measurements. We thank Dr. S. K. Sarkar and Dr. T. Mukherjee of Bhabha Atomic Research Centre for their encouragement. ’ REFERENCES (1) (a) Nozik, A. Ann. Rev. Phys. Rev. 1996, 52, 193. (b) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425–2427. (b) Steckel, J. S.; Snee, P.; Coe-Sullivan, S.; Zimmer, J. P.; Halpert, J. E.; Anikeeva, P.; Kim, L.-A.; Bulovic, V.; Bawendi, M. G. Angew. Chem., Int. Ed. 2006, 45, 5796. (c) Efros, Al. L.; Efros, A. Sov. Phys. Sem. 1982, 16, 772. (d) Brus, L. J. Chem. Phys. 1983, 79, 5566. (2) (a) Agranovich, V. M.; Klimov, V. I. Nat. Phys. 2005, 1, 189. (b) Luther, J. M.; Beard, M. C.; Song, Q.; Law, M.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2007, 7, 1779. (c) Schaller, R. D.; Sykora, M.; Pietryga, J. M.; Klimov, V. I. Nano Lett. 2006, 6, 424. (3) Nozik, A. J. Physica E 2002, 14, 115. (4) Klimov, V. I. Annu. Rev. Phys. Chem. 2007, 58, 635. (5) (a) Burda, C.; Green, T. C.; Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 1783. (b) Logunov, S.; Green, T.; Marguet, S.; El-Sayed, M. A. J. Phys. Chem. A 1998, 102, 5652. (6) Boulesbaa, A.; Issac, A.; Stockwell, D.; Huang, Z.; Huang, J.; Guo, J.; Lian, T. J. Am. Chem. Soc. 2007, 129, 15132. 1434

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