Spectral Switching of Type-II Quantum Dots by Charging - The

Wang , C.; Shim , M.; Guyot-Sionnest , P. Science 2001, 291, 2390 .... A. M.; Lee , J.; Nakayama , A.; Parker , J. A.; Mihaljevic , T.; Laurence , R. ...
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2009, 113, 6320–6323 Published on Web 03/31/2009

Spectral Switching of Type-II Quantum Dots by Charging Jiwon Bang, Bonghwan Chon, Nayoun Won, Jutaek Nam, Taiha Joo, and Sungjee Kim* Department of Chemistry, Pohang UniVersity of Science & Technology, San 31, Hyojadong, Namgu, Pohang 790-784, South Korea ReceiVed: January 19, 2009; ReVised Manuscript ReceiVed: March 19, 2009

We report photoluminescence (PL) spectral switching of quantum dots (QDs) by chemically controlling transfers of electrons in and out of the type-II heterostructures. When electron charged, CdTe/CdSe (core/shell) QDs show huge PL blue shifts (up to ∼100 nm) whereas PL of CdSe/CdTe (core/shell) QDs red-shifts. We demonstrate reversible spectral switching of type-II QDs by repeated charging and neutralization processes. The PL spectral shifts are due to the interactions between injected spectrator electrons and type-II character excitons. Electron charged CdTe/CdSe QDs show PL blue shifts because of the strong repulsions between the shell-localized excitonic electrons and injected spectrator electrons. It is opposite for CdSe/CdTe QDs, where injected electrons in surface states attract the holes in the shells. We investigate the interactions of spectrator electrons and excitons as varying the dimensions of type-II QDs. We also compare the type-II QDs with type-I QDs such as CdSe, CdTe, and CdTeSe alloyed QDs. We showcase unique optical properties of charged type-II QDs, which have many potential applications such as electro-optic modulators. The properties of charged semiconductor quantum dots (QDs) attract great interest for their potential applications such as electro-optical devices.1-7 Reversible fluorescence quenching of CdSe QDs has been demonstrated by charging.5,6 Charged QDs show significantly reduced threshold for stimulated emissions, promising applications toward efficient lasers.8 However, the studies on charged QDs have been limited to type-I QDs. On the other hand, type-II QDs are being extensively explored for potential applications including biological tagging,9 lasers,10-12 and solar cells.13-15 Type-II QDs can have electron-hole pairs that are spatially separated, and their effective band gaps are heavily governed by their band offsets of the cores and shells.16-22 Spatially separated excitons in type-II QDs show unique properties such as slow Auger and radiative decay rates,22 blue shifts of the biexciton energy and single exciton optical gains.10-12 Charging type-I QDs typically results in changes in photoluminescence (PL) intensities.1,23 Herein, we report reversible PL spectral switching of type-II CdTe/CdSe (core/shell, C/S) and CdSe/CdTe (C/S) QDs by charging. Unique PL properties of the charged type-II QDs are attributed to the novel type-II characteristic carrier separations and their interactions with spectrator electrons. Preparations of colloidal type-II CdTe/CdSe (C/S) and CdSe/ CdTe (C/S) QDs and electron transfers in and out of the QDs are carried out by similar methods reported previously.16,19,23,24 (See the Supporting Information (SI).) As-prepared QDs are dispersed in anhydrous hexanes and electrons are injected into the QDs using sodium particles under inert atmosphere. This charging process is known to be slow owing to the low mobility of QDs and sodium particles.1 Figure 1a shows PL wavelength shift of a CdTe/CdSe (2.3 nm radius core/1.8 nm thickness shell) QD sample during electron injection for 8 days. As electrons * To whom correspondence should be addressed. Tel: 82-54-279-2108. Fax: 82-54-279-1498. E-mail: [email protected].

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being accumulated on QDs over time, the PL continuously shifts to the blue. The spectral shift reaches as large as ∼100 nm. Figure 1b shows the absorption spectra of CdTe/CdSe (C/S) QDs in initial uncharged state and when electron charged for 8 days. Attenuation of absorption transitions near the band edge is observed. The attenuation of absorption transition is neither predominant nor concurrent with the PL shift. This suggests that injected electrons fill surface states of QDs first and occupy delocalized states over the shells. Similar behaviors have been also observed for type-I QDs.1,23 As control experiments, CdSe QDs and CdTe QDs are charged over a month and the PL shifts are found within a few nm presumably due to surface changes (Figure S1). Huge spectral shift of type-II QDs can be explained based on Coulombic interactions between injected spectrator electrons and generated excitons.26 CdTe/CdSe (C/S) QDs have lower potentials for electrons in CdSe shells than in CdTe cores.16-19 As a result, electrons are more localized in the shells whereas holes mostly reside in the cores. The chemically injected electrons are either trapped on surface states or localized in the shells due to the type-II character. When the injected spectrator electrons interact with excitons, electron-electron repulsions are expected to be stronger than the electron-hole attractions. This results in the progressive blue shift of the excitonic peak during the charging process. As the charging proceeds, the number of spectrator electrons should increase. In addition, initially surface trapped electrons can move inside the shells. Figure 1c shows the change of PL quantum yield (QY) during the charging process. Type-II CdTe/CdSe (C/S) QDs typically show the PL QY ∼ 5%. The left open square in figure 1c represent the PL QY of CdTe/CdSe (C/S) QD sample before the charging process. As charging proceeds, PL continuously decreases by Auger processes and generations of quenching surface sites. The open square over the arrow represents the regained PL QY after neutralization on day 8. Neutralization,  2009 American Chemical Society

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Figure 1. (a) Normalized PL spectra of a CdTe/CdSe (C/S) QD sample in initial uncharged state (black) and when electron charged for 2 (red), 5 (green), and 8 days (blue). (b) Absorption spectra of CdTe/CdSe (C/S) QDs in initial uncharged state (black) and when electron charged for 8 days (blue). (c) PL intensity change during the charging process. Uncharged and charged QD states are represented by open and closed squares, respectively. The arrow on day 8 denotes neutralization step.

Figure 2. (a) Normalized PL spectra, (b) absorption spectra, and (c) normalized PL decays of CdTe QDs (green), and CdTe/CdSe (C/S) QDs in initial uncharged state (black), in electron charged state (blue) and after neutralization (red). (d) Repeated PL spectral switching of a CdTe/CdSe (C/S) QD sample by multiple charging/neutralization processes. Open and closed circles denote emission wavelengths of a CdTe/CdSe (C/S) QD sample in uncharged and charged states, respectively. Solid and broken arrows represent charging and neutralization processes, respectively.

or removal of injected electrons from QDs, is made by following previously reported method of intentional exposure of the samples to air.1,23 The number of injected electrons may be correlated with the degree of spectral shift or the PL QY change. Unfortunately, our experimental setup does not allow quantification of injected electrons. It has been reported that electrochemical cells can be fabricated to study the number of electrons transferred in and out of QDs.6 Further research is required in this direction. Upon neutralizing charged type-II QDs, the PL returns back to the initial spectral position. Type-II QDs can show PL spectral switching by charging/neutralization. Figure 2a demonstrates the reversibility of the spectral switching. Initially 766 nm (PL peak) emitting CdTe/CdSe (C/S) QD sample is electron charged for 3 days. The PL blue shifts to 733 nm, and returns back to 768 nm upon neutralization. Slight tailing of the PL spectrum is accompanied by the neutralization due to little aggregations. It is noted that the PL spectrum of charged QDs does not overlap with that of bare CdTe QDs. The bare CdTe QDs are the cores for the CdSe shell deposition. Along with the reversible PL spectral switching, reversible absorption bleaching is observed as in Figure 2b. The absorption of CdTe/CdSe (C/S) QD becomes attenuated near the band edge when electron charged for 3 days. The injected spectrator electrons can occupy the conduction levels and reduce the oscillator strength of the band edge transitions.1,23,25 Upon neutralization of the charged CdTe/ CdSe (C/S) QD sample, attenuated band edge absorption is

almost recovered. Figure 2c shows the PL decays of the 4 samples that are used for Figure 2, panels a and b. Intensityweighted average lifetimes28 are obtained using biexponential and triexponential fittings. (See the SI for time-resolved PL measurement setup and the data analysis.) When fitted by biexponential decays, bare CdTe QD, initial CdTe/CdSe (C/S) QD before charging, charged CdTe/CdSe (C/S) QD and neutralized CdTe/CdSe (C/S) QD samples show the lifetimes of 6.3, 34, 8.7, and 68 ns, respectively (Table S1). In case of triexponential fitting, the lifetimes are 11 ns, 42 ns, 16 and 82 ns, respectively with the same order (Table S1). PL QYs of the initial CdTe/CdSe (C/S) QD, charged CdTe/CdSe (C/S) QD, and neutralized CdTe/CdSe (C/S) QD samples are 2.5%, 0.03%, and 7.0%, respectively. The lifetimes for bare CdTe QD and initial CdTe/CdSe (C/S) QD samples are similar to the previously reported lifetimes.16-18,29 Charged state CdTe/CdSe (C/ S) QD shows dramatically shortened lifetime when compared to initially uncharged or neutralized state. When biexponential (triexponential) decay fitting is used, the lifetime reduces from 34 ns (42 ns) to 8.7 ns (16 ns) as the QD sample becomes electron charged. The decrease of lifetime upon charging is particularly dominant for slow component of the decay (τ2 for biexponential and τ3 for triexponential fitting in Table S1). The slow component lifetime decreases from 37 ns (47 ns) to 10 ns (24 ns). On the other hand, changes of fast components are not as dramatic as the slow component. The lifetime τ1 (τ1, τ2) in biexponential (triexponential) fitting changes from 0.67 ns (0.19

6322 J. Phys. Chem. C, Vol. 113, No. 16, 2009 ns, 2.6 ns) to 0.55 ns (0.23 ns, 2.8 ns) from initial uncharged to charged state. Quatro-exponential fitting is also used for the liftetime analysis, where similar lifetime behaviors are observed (Table S1). We speculate that the dominant nonradiative channels become quite different for the cases when the quantum dots are neutral in uncharged state and when electron charged. When quantum dots are excited at a low excitation intensity (less than 1 e-h pair produced per dot), nonradiative decay channels are thought mainly generated from surface trap sites and internal defect sites. When electrons are injected into quantum dots, Auger processes between injected spectrator electrons and exciton or surface(or interface)-trapped hole may become dominant. Auger processes of charged quantum dots are expected to be very efficient. Obviously, we cannot rule out the possibility that the charging process generates efficient surface-oriented nonradiative channels. Unfortunately, there have been little quantitative studies for surface oriented nonradiative decay kinetics and Auger processes for type-II quantum dots. We estimate the Auger lifetime of CdTe/CdSe (C/S) QDs to be longer than type-I QDs with similar size.22 This may explain the increase of τ1 (from 0.19 to 0.23 ns) and τ2 (from 2.6 to 2.8 ns) in triexponential fitting and τ1 (from 0.088 to 0.15 ns) and τ2 (from 0.83 to 1.1 ns) in quatro-exponential fitting (Table S1). It is not feasible to directly correlate the lifetime kinetics with QY in our case. However, nonradiative decay channels of QD including Auger process are thought to be faster than radiative decays.17,22 Although the low QY of charged QD sample severely limits interpretations on radiative channels from the lifetime data, one can assume that the changes in slowest component are heavily dependent on the radiative channel. When electrons are injected into CdTe/CdSe (C/S) QDs, the excitonic electron-hole pair overlap should increase as the electron-electron repulsions in the shells dominate. Therefore, the exciton becomes less spatially separated and can show fast decays.22 Long-lived exciton dynamics of charged type-II QD becomes similar to that of type-I bare CdTe QD. This can be well visualized when the PL decay spectra are normalized by the area (Figure S2). Type-II characteristic slow decay of CdTe/ CdSe QD reappears upon neutralization. However, samples often show significant PL brightening after neutralization that is accompanied by prolonged PL decays on both fast and slow components (Figure S3). The origin of QY increase is not clear, yet presumably due to oxidative surface reconstructions which remove Se0 quenching sites.6,30 Figure 2d demonstrates the reversible switching capability of type-II QD by repeated charging/neutralization process. Initially ∼840 nm emitting CdTe/CdSe (C/S) QD sample is electron charged until it blue-shifts to ∼760 nm (the first solid arrow in Figure 2d). The spectral shift is comparable to the PL bandwidth. In the second and fifth cycle, the neutralization is made intentionally partial so as to show the modulating capability. The emission wavelength can be flexibly tuned by controlling the degree of charging. The switching can be made as many times until QDs lose the colloidal stability. Typically QDs can survive up to 10 cycles of repeated charging/ neutralization processes in solution. Solid state QD charging devices are expected to overcome the colloidal stability limitation for electro-optic modulating applications.4-6 In Figure 3a, CdTe/CdSe (C/S) QDs with identical cores and shells with different thicknesses are prepared. (See Figure S4 for TEM images.) PL peak appears at longer wavelengths for the QDs with thicker shells because of the enhanced type-II characteristics. They are charged for 5 days using an identical condition including the amount of charging agent per QD. QDs

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Figure 3. (a) Normalized PL spectra of CdTe/CdSe (C/S) QD samples in neutral states (open squares) and in their charged states after charging for 5 days (closed squares). The QD samples have identical cores of CdTe (2.3 nm radius) and different CdSe shell thicknesses of 0.5 nm (blue) 1.4 nm (green) and 1.8 nm (red). (b) Normalized PL spectra of alloyed CdTeSe QD samples with different size in their neutral states (open squares) and after charging for 5 days (closed squares).

Figure 4. Normalized PL spectra of CdSe/CdTe (C/S) QDs in initial uncharged state (black), in electron charged state (blue) and after neutralization (red).

with thicker shells, hence more type-II character QDs, show larger spectral shift by charging. The QD sample with the thickest shell shows 80 meV blue shift, while the other two show 33 and 29 meV in the order of decreasing shell thickness. Actually, injected electrons per QD should be smaller for the thicker shell QDs. The charging process is governed by diffusion, and larger QDs should be less susceptible to charging because of the low mobility.1 Size-dependent electron repulsion effect is not considered. Type-II character dependent spectral shift is observed consistently from the charging day 1. It is believed that the number of injected electrons per QD is not large enough to significantly consider the repulsion effect. For comparison, alloyed CdTeSe QDs with homogeneous internal structures are prepared by following previous reports.31 (See SI for the synthetic procedure and Figure S5 for optical properties and TEM image). Figure 3b shows PL spectra of different size CdTeSe alloy QDs. Though alloyed QDs emits at similar wavelengths as the type-II QDs, they only show insignificant spectral shifts by charging. CdSe/CdTe (C/S) QDs are prepared to investigate the case of spatially reversed electron-hole localizations. As shown in Figure 4, initially 775 nm (PL peak) emitting CdSe/CdTe (C/ S) QD sample shows red-shifted PL at 798 nm upon charging for 2 days, and returns back to 773 nm upon neutralization. The PL red-shift makes a sharp contrast with blue-shift of the counterpart CdTe/CdSe (C/S) QD sample shown in Figure 2a. CdSe/CdTe (C/S) QDs have electrons mostly confined in cores and holes that reside mostly in shells. Electrons on surface states of CdTe shells should attract excitonic holes in the shells more strongly than they repel electrons in the cores. As charging proceeds, injected electrons accumulate on the CdTe shell surface states and the PL continuously red-shifts. However, CdSe/CdTe (C/S) QDs quickly lose fluorescence for longer charging processes. They seem to easily create nonradiative channels via surface hole traps, especially in combination with

Letters the injected electrons. In this case, holes are localized in the shells, and injected electrons are supposed to fill the cores via surface states. It seems that the PL completely quenches before the injected electrons reach the cores. The surface charged electrons seem to enhance type-II characteristics by attracting excitonic holes, which results in the red-shift upon charging. Lifetime measurements should corroborate our analysis, which is unfortunately still limited by the very low PL QYs. In conclusion, we have demonstrated that QD spectral switching can be well engineered by controlling the transfers of electrons in and out of type-II QD heterostructures. Huge and reversible spectral shifts up or down by charging/ neutralization processes clearly showcase the unique properties of typeII QDs and their application potentials. Acknowledgment. This work was supported by KOSEF grant funded by the Korea government (MOST) (M1070300103608M0300-03610, KRF-2008-331-C00140, M1075502000307N5502-00310, R0A-2008-000-20114-0(2008)), Ministry of Health & Welfare (A060660) and by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-005-J13102). Supporting Information Available: Detail experimental procedure, UV-vis spectra, PL spectra, PL decay data, and TEM figure. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shim, M.; Guyot-Sionnest, P. Nature (London) 2000, 407, 987. (2) Wang, C.; Shim, M.; Guyot-Sionnest, P. Science 2001, 291, 2390. (3) Yu, D.; Wang, C.; Guyot-Sionnest, P. Science 2003, 300, 1277. (4) Guyot-Sionnest, P.; Wang, C. J. Phys. Chem. B 2003, 107, 7355. (5) Woo, W.-K.; Shimizu, K. T.; Jarosz, M. V.; Neuhauser, R. G.; Leatherdale, C. A.; Rubner, M. A.; Bawendi, M. G. AdV. Mater. 2002, 14, 1068. (6) Jha, P. P.; Guyot-Sionnest, P. J. Phys. Chem. C 2007, 111, 15440. (7) Gooding, A. K.; Go´mez, D. E.; Mulvaney, P. ACS Nano 2008, 2, 669.

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6323 (8) Wang, C.; Wehrenberg, B. L.; Woo, C. Y.; Guyot-Sionnest, P. J. Phys.Chem. B 2004, 108, 9027. (9) Kim, S.; Lim, Y. T.; Soltesz, E. G.; De Grand, A. M.; Lee, J.; Nakayama, A.; Parker, J. A.; Mihaljevic, T.; Laurence, R. G.; Dor, D. M.; Cohn, L. H.; Bawendi, M. G.; Frangioni, J. V. Nat. Biotechnol. 2004, 22, 93. (10) Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I.; Mc Guire, J. A.; Piryatinski, A. Nature (London) 2007, 447, 441. (11) Nanda, J.; Ivanov, S. A.; Achermann, M.; Bezel, I.; Piryatinski, A.; Klimov, V. I. J. Phys. Chem. C 2007, 111, 15382. (12) Ivanov, S. A.; Nanda, J.; Piryatinski, A.; Achermann, M.; Balet, L. P.; Bezel, I. V.; Anikeeva, P. O.; Tretiak, S.; Klimov, V. I. J. Phys. Chem. B 2004, 108, 10625. (13) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462. (14) Zhong, H.; Zhou, Y.; Yang, Y.; Yang, C.; Li, Y. J. Phys. Chem. C 2007, 111, 6538. (15) Wang, K.; Chen, J.; Zhou, W.; Zhang, Y.; Yan, Y.; Pern, J.; Mascarenhas, A. AdV. Mater. 2008, 20, 3248. (16) Kim, S.; Fisher, B.; Eisler, H. J.; Bawendi, M. G. J. Am. Chem. Soc. 2003, 125, 11466. (17) Chin, P. T. K.; de Mello Donega´, C.; van Bave, S. S.; Meskers, S. C. J.; Sommerdijk, N. A. J. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2007, 129, 14880. (18) Li, J. J.; Tsay, J. M.; Michalet, X.; Weiss, S. Chem. Phys. 2005, 318, 82. (19) Chen, C. Y.; Cheng, C. T.; Lai, C. W.; Hu, Y. H.; Chou, P. T.; Chou, Y. H.; Chiu, H. T. Small 2005, 1, 1215. (20) Dorfs, D.; Franzl, T.; Osovsky, R.; Brumer, M.; Lifshitz, E.; Klar, T. A.; Eychmu¨ller, A. Small 2008, 4, 1148. (21) Dorfs, D.; Salant, A.; Popov, I.; Banin, U. Small 2008, 4, 1319. (22) Oron, D.; Kazes, M.; Banin, U. Phys. ReV. B 2007, 75, 035330. (23) Shim, M.; Wang, C.; Guyot-Sionnest, P. J. Phys. Chem. B 2001, 105, 2369. (24) Scho¨ps, O.; Le Thomas, N.; Woggon, U.; Artemyev, M. V. J. Phys. Chem. B 2006, 110, 2074. (25) An, J. M.; Franceschetti, A.; Zunger, A. Phys. ReV. B 2007, 76, 161310. (26) Franceschetti, A.; Zunger, A. Phys. ReV. B 2000, 62, R16287. (27) Troparevsky, M. C.; Franceschetti, A. Appl. Phys. Lett. 2005, 87, 263115. (28) James, D. R.; Liu, Y. S.; Demayo, P.; Ware, W. R. Chem. Phys. Lett. 1985, 120, 460. (29) Wuister, S. F.; van Driel, F.; Meijerink, A. Phys. Chem. Chem. Phys. 2003, 5, 1253. (30) Myung, N.; Bae, Y.; Bard, A. J. Nano Lett. 2003, 3, 747. (31) Bailey, R. E.; Nie, S. J. Am. Chem. Soc. 2003, 125, 7100.

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