pubs.acs.org/NanoLett
Multiexcitonic Dual Emission in CdSe/CdS Tetrapods and Nanorods Andrey A. Lutich,† Christian Mauser,† Enrico Da Como,*,† Jing Huang,‡ Aleksandar Vaneski,§ Dmitri V. Talapin,‡ Andrey L. Rogach,§ and Jochen Feldmann† †
Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-Maximilians-Universita¨t, D-80799, Munich, Germany, ‡ Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States, and § Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong ABSTRACT CdSe/CdS semiconductor nanocrystal heterostructures are currently of high interest for the peculiar electronic structure offering unique optical properties. Here, we show that nanorods and tetrapods made of such material combination enable efficient multiexcitonic emission, when the volume of the nanoparticle is maximized. This condition is fulfilled by tetrapods with an arm length of 55 nm and results in a dual emission with comparable intensities from the CdS arms and CdSe core. The relative intensities of the dual emission, originating from exciton phase-space filling and reduced Auger recombination, can be effectively modulated by the photon fluence of the pump laser. The results, obtained under steady-state detection conditions, highlight the properties of tetrapods as multiexciton dual-color emitters. KEYWORDS Multiexcitons, nanocrystals, tetrapods, Auger recombination, dual emission
L
ight sources at the nanoscale have several applications in different areas ranging from information technology to life sciences. Indeed, they can be implemented in quantum computation schemes1 and nanophotonic circuits2 or used in optical sensing,3 bioimaging,4 and superresolution optical microscopy.5 Particularly interesting is the design of nanostructures capable of emitting light with high stability and efficiency. In the last years, colloidal semiconductor nanocrystals (NCs)6 have reached a high level of material quality, resulting in nonblinking quantum dots,7-9 polarized light sources,10-12 and single photon emitters.13 Importantly, colloidal synthesis allows for creation of many different NC shapes, potentially leading to an a priori design of the optical properties. Besides the well-known spherical NCs (quantum dots) and elongated NCs (quantum rods), high-quality tetrapods and hyperbranched structures have been realized.14-16 When considering the optical properties of nanostructures and in particular their photoluminescence (PL), it becomes interesting to have multiple states capable of efficient light emission. In imaging, ratiometric techniques17 and fluorescence microscopy18 often require more than one emissive electronic state. Fine control over the intensity ratio of the two emission bands is desirable, but usually faces the challenge of having one state much less efficient in light emission or less photostable. In this respect, semiconductor NCs are known to have a high degree of photostability with respect to some organic dyes and offer the possibility to have multiple emitting states with different colors by means of heterostructures.19
In addition, semiconductor NCs have emerged as an interesting class of materials for optoelectronic applications. Indeed,opticallypumpedlasers,20,21 light-emittingdiodes,22,23 light converters,22 and solar cells24-26 have been demonstrated. Despite the general interest in creating heterostructures with several emitting states,19 the role of particle shape and size in controlling the emission properties in terms of color and intensity has not yet been properly addressed. Here, we demonstrate for the first time that CdSe/CdS nanocrystal heterostructures with the shape of tetrapods can show equally intense dual emission from CdS and CdSe states in steady-state conditions. Because of the multiexcitonic nature of the dual emission process, its intensity is influenced by nonradiative multiexciton interactions such as Auger recombination.27,28 Tetrapods of large size (55 nm long CdS arms), having a larger volume, with respect to nanorods with a CdS arm of similar length, can accommodate multiexcitons and exhibit less severe nonradiative processes leading to efficient dual emission. Our results, comparing tetrapods and nanorods, investigate the role of particle shape and dimension in the dual emission and give insights on Auger recombination in these nanostructures. We have comprehensively studied three CdSe/CdS NC heterostructured samples differing in particle shape and dimensions: CdSe/CdS tetrapods with 55 nm long CdS arms denoted as T1, CdSe/CdS tetrapods with 28 nm long CdS arms denoted as T2, and asymmetric CdSe/CdS nanorods with a CdSe core located close to one end of a 60 nm long CdS arm, denoted as NR. Both tetrapods and nanorods were synthesized according to the method described in ref 16. The size of the CdSe core was the same for all samples studied (∼4 nm). In terms of length of the CdS arms, NR are
* Corresponding author,
[email protected]. Received for review: 08/9/2010 Published on Web: 10/22/2010 © 2010 American Chemical Society
4646
DOI: 10.1021/nl1028057 | Nano Lett. 2010, 10, 4646–4650
Figure 1 shows PL spectra of three CdSe/CdS samples at different pump fluencies. In all heterostructures light is primarily absorbed in CdS arms and charge carriers recombine after relaxation into the CdSe core. In Figure 1a, the PL spectrum of tetrapods with 55 nm long arms (T1) shows two emission peaks corresponding to the optical gap of CdSe (1.9 eV) and CdS (2.6 eV), whose relative intensity strongly depends on excitation fluence. The excitation fluences at 400 nm (3.1 eV) are listed in the respective frames. At low excitation levels, the emission spectra of CdSe/CdS tetrapods show one band at 2.05 eV corresponding to the emission from CdSe cores. However, at high pump fluence (∼640 µJ/ cm2) the intensities of the two peaks of T1 are almost equal, a rather unexpected behavior for a nanostructure where emission is predicted to originate only from the low energy state residing in the CdSe core.11,16,29 Parts b and c of Figure 1 show PL spectra obtained from the same experiments performed on tetrapods with 28 nm long arms (T2) and CdSe/CdS nanorods (NR), respectively. It appears that dual emission with equal intensity can be achieved only in tetrapods with large size (cf. Figure 1a). In contrast to the CdS emission intensity, the light fluence dependence of the CdSe PL is similar for all three samples studied. To better visualize the excitation fluence dependence of the PL intensity from the two emission bands in T1, we plot in Figure 2 the integrated CdSe (panel a) and CdS (panel b) PL intensity indicated as red and green stars, respectively. As reference samples, we also present the emission of spherical CdSe NCs (red dots) and of CdS-only nanorods (green bars in panel b). The PL intensity of the CdSe cores in T1 increases linearly and starts to show a sublinear behavior for pump fluences above 10 µJ/cm2. This sublinear increase in intensity takes place over a large interval of pump fluencies up to 300 µJ/cm2. Such a behavior reflects the fact that CdSe/CdS tetrapods have a much larger absorption cross section in comparison to spherical CdSe NCs at 3.1 eV and multiexciton Auger processes set already in at low pumping fluences. Signatures for multiexcitons within the core are apparent in the high energy tail of the CdSe emission in Figure 1a, in agreement with previous reports on nanorods.30 The fact that biexcitonic emission appears at high energy indicates repulsive interactions in the biexcitonic state.30,31 Tetrapods with shorter arms, T2, have the same dependence, but with a slightly different threshold for the sublinear intensity increase (not shown). For the fluence dependence of the CdS emission of T1 we observe a linear dependence up to ∼60 µJ/cm2 (inset in Figure 2b), which then turns to a superlinear increase in striking contrast to that seen for the CdSe emission. This observation is unexpected and is not in line with the general trend of sublinear PL intensity increase or saturation observed for most common semiconductor NCs. Saturation is indeed observed for CdS-only nanorods (Figure 2), which show a PL intensity saturating at a pump fluence of ∼200 µJ/cm2 (green bars). Note that these CdS-only nanorods also exhibit an initial
FIGURE 1. PL spectra at different pump fluencies (listed in each frame) for (a) T1 CdSe/CdS tetrapods (55 nm long arms), (b) T2 CdSe/ CdS tetrapods (28 nm long arms), and (c) NR CdSe/CdS nanorods (60 nm long arm). Schematic drawings of these three samples, together with their representative TEM images, are presented as insets.
comparable with T1. Schematic drawings of these three samples are presented as insets in Figure 1, together with their representative transmission electron microscopy (TEM) images, obtained with a JEOL microscope operated at 100 kV. For the optical studies the nanocrystals were diluted in toluene to provide an optical density of ∼0.2 at 3.1 eV in a 1 cm cuvette. PL and absorption of the solutions of NCs at different pump intensities were measured in the same conditions. Optical excitation was provided by 150 fs laser pulses with 3.1 eV photon energy and 100 kHz repetition rate. The laser beam was focused down to a 200 µm spot in the center of 1 cm quartz cuvette with the sample solution. Reliable measurements of the beam profile were operated inside the cuvette filled with the solvent in order to take into account thermal lens effects that may arise at high laser fluence. The measured spatial beam profile was fitted by Gaussian, and a beam diameter of 200 µm was extracted. The PL was collected in 90° geometry by a condenser lens and focused to the entrance slit of a spectrometer. Absorption of the solution of NCs at the excitation energy of 3.1 eV at different pump fluencies was calculated as 1 - I/I0, with I the transmitted intensity and I0 the intensity of the incident beam. No degradation of the sample was observed in the fluence ranges investigated in our experiments. © 2010 American Chemical Society
4647
DOI: 10.1021/nl1028057 | Nano Lett. 2010, 10, 4646-–4650
FIGURE 2. Excitation pump fluence dependence of the PL intensity from CdSe cores (a) and CdS arms (b) of the CdSe/CdS tetrapods, sample T1. For comparison, the PL fluence dependences of spherical CdSe NCs (red circles in a) and CdS-only nanorods (green bars in b) are plotted. (c) Electron and hole wave functions along one arm of the CdSe/CdS heterostructure calculated with an effective mass approach. Wave functions involved in the CdSe emission are shown in red, and those involved in the CdS emission are shown in green. The CdSe valence band shows eleven subband confined levels for holes and the first level spreading in the CdS. (d) Color contour plots of electron and hole wave function distributions in tetrapods for different subband levels.
linear increase similar to the one seen for the CdS arm emission in T1 (see inset Figure 2b). To gain insights on the nature of the dual emission and the spatial localization of the electronic states involved, we illustrate in Figure 2c the subband levels for the conduction and valence band of a CdSe/CdS core/arm heterostructure, calculated with an effective mass approach taking into account Coulomb effects.11,32 Such a theoretical approach is known to have limitations for high-energy subband levels but provides a reasonable description of the band-edge levels involved in recombination, which are considered here.33 Note that the schematic representation is simplified along one spatial coordinate and is valid for the sample NR, while it has to be repeated four times in space for T1 and T2 where it extends in three dimensions. Figure 2d further reports the same wave functions in color contour plots for a better visualization of the three-dimensional characteristics of tetrapods. These calculations clearly show how the CdSe/CdS energy landscape can be suitable to accommodate excitons (electron-hole pairs) in the spatially separated CdSe and CdS regions of the same nanoparticle. The PL intensity of NCs at high photoexcitation fluence is known to be limited by multiexcitonic processes. In elongated structures like nanorods Auger processes can be less severe and involve excitons rather than carriers, i.e., two quasi-particles process (exciton-exciton) rather than three (electron-hole and charge).27 We expect that our dual color emission, originating from multiexcitonic configurations (Figure 2c,d) should be influenced by Auger processes as well. The electron and hole wave function localization illustrated in parts c and d of Figure 2 suggests that our multiexcitonic levels are spatially separated, offering particle © 2010 American Chemical Society
dimension and shape as material parameters for limiting Auger recombination and promoting dual emission. To provide a quantitative description of dual emission, we evaluated the average number of excitons per nanoparticle (see ref 34). In brief, after estimating the absorption cross sections of the different NCs, σ, and the photon fluence, φ, the average number of excitons 〈N〉 was obtained from the formula 〈N〉 ) σφ. Figure 3a shows how this number increases as a function of the pump fluence for the samples T1 (black squares), T2 (green dots), and NR (blue triangles). The saturation in the number of excitons is due to exciton phase-space filling, after a certain value the absorption transition starts to be bleached.34 While NR exhibits saturation in the average number of excitons at 200 µJ/cm2 pumping fluence, T1 shows a linear increase for almost the whole range of fluences investigated. This clearly shows how T1 tetrapods can accommodate a larger number of excitons because of their larger size. According to the calculated energy level scheme of Figure 2c, the CdSe valence band has several double degenerate levels to accommodate holes, while in the conduction band CdSe and CdS levels are mixed constituting a common ladder of levels for the electrons.35,36 Here, we recall that CdSe exciton emission is the result of an ultrafast hole localization into the core, which drags the initially delocalized electron wave function toward the core.37 Within the core, multiexcitons can coexist as long as their repulsive interaction does not overcome the driving force for the Coulomb coupled localization.37 Therefore, CdS emission is expected to appear once the core levels are occupied with a high density of excitons. This configuration should create an exciton blockade where the electrons and holes, primarily 4648
DOI: 10.1021/nl1028057 | Nano Lett. 2010, 10, 4646-–4650
FIGURE 3. (a) Average number of excitons per NC as a function of pump fluence and (b) PL intensity of CdS arms as a function of the average number of excitons per particle measured for CdSe/CdS tetrapods and nanorods, as indicated in frame (a).
photogenerated in CdS, because of the large cross section at the photon energy 3.1 eV, are not subjected to a driving force for relaxing into the core and eventually experience efficient radiative recombination. In addition, repulsive interactions between excitons and modifications of the Coulomb potential due to many-body effects are expected to play a role. The dual emission reported here originates from a very different process with respect to that previously observed by Battaglia et al. in core/shell/shell NCs.19 Indeed, the observation of dual emission in core/shell/shell structures relies on the energy transfer from the spherical core to the outer two-dimensional shell. The relative intensity of the two emission lines can be varied with the inner shell thickness and requires a certain degree of decoupling between the two emitting units. In our experiments it is the mixing of energy levels between the strongly coupled CdSe and CdS part of the NC heterostructure, which enables the exciton blocking effect. The results of calculations presented in Figure 2d show that holes in CdS are separated from the band edge of CdSe by eleven quantized levels confined in the CdSe core. It is interesting to relate this number with the threshold of the superlinear increase in CdS emission of T1 reported in Figure 2b. This takes place for an average number of excitons above thirty as estimated by comparing the pump fluence of 60 µJ/cm2 with the number of excitons obtained from Figure 3a. Considering that each of the eleven core levels is double degenerate and exciton population shows Poissonian statistics,38 it is conceivable that the superlinear behavior in CdS emission is a result of core phase-space filling. This, however, is not apparent for the T2 sample, since this sample does not show an intense CdS PL as further discussed below (see Figure 1b). Therefore, an interpretation of the data in Figure 1 merely considering the number of excitons per particle and phase space filling is not sufficient to describe the observed intensities in dual emission. The comparison of T1 and T2 clearly indicates that the particle shape is not the only effect controlling the CdS emission but also the particle volume plays an important role. To understand the role of Auger recombination on the dual emission properties of our samples, we plot in Figure 3b the PL intensity of the CdS arms as a function of the average number of excitons per particle. The T1 sample © 2010 American Chemical Society
(black squares) starts to emit already for a low number of excitons, and in the whole range of fluencies investigated it shows the highest PL intensity when compared to T2 and NR. Here, it is important to remember that the number of excitons per particle follows a Poissonian distribution and therefore it is not surprising that a few tetrapods might have a large number of excitons approaching twenty-two. Essentially the purpose of Figure 3b is to demonstrate that with the same average number of excitons per particle, the T1 sample has a more efficient CdS emission. As a consequence of the larger volume in T1, Auger recombination is less severe with respect to the other NCs. In general we suggest that large tetrapods can offer efficient dual emission as a consequence of reduced Auger recombination. In conclusion, we have demonstrated efficient dual emission from NC heterostructures consisting of spherical CdSe cores and CdS arms. We have investigated the role of the particle shape (nanorods vs tetrapods) in their ability to provide dual emission. Tetrapods are more efficient light absorbers and can accommodate a large number of excitons before reaching optically induced transparency (bleaching) conditions. However, in terms of the emission properties the particle shape has little effect on the efficiency of multiexcitonic emission and the volume of the particle dominates. The particle volume has a strong influence on Auger nonradiative processes and controls the efficiency of the multiexcitonic dual emission. These results demonstrate the fine control of the dual emission intensity of luminescent NCs with possible applications in biolabeling and light-emitting devices. In addition, they give valuable information for the understanding of Auger processes with respect to optical gain39 and blinking phenomena in NCs.7,8 Acknowledgment. We thank S. Niedermaier and A. Helfrich for technical assistance. We are grateful to the DFG for financial support via the “Nanosystems Initiative Munich (NIM)” and the LMUexcellent program. We acknowledge the financial assistance of EU through the research training network ICARUS. A.L.R. acknowledges support from City University of Hong Kong (project 7002551). 4649
DOI: 10.1021/nl1028057 | Nano Lett. 2010, 10, 4646-–4650
REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
(20) Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I.; McGuire, J. A.; Piryatinski, A. Nature 2007, 447, 441. (21) Garcı´a-Santamarı´a, F.; Chen, Y.; Vela, J.; Schaller, R. D.; Hollingsworth, J. A.; Klimov, V. I. Nano Lett. 2009, 9, 3482. (22) Chanyawadee, S.; Lagoudakis, P. G.; Harley, R. T.; Charlton, M. D. B.; Talapin, D. V.; Huang, H. W.; Lin, C. H. Adv. Mater. 2010, 22, 602. (23) Rogach, A. L.; Gaponik, N.; Lupton, J. M.; Bertoni, C.; Gallardo, D. E.; Dunn, S.; Pira, N. L.; Paderi, M.; Repetto, P.; Romanov, S. G.; O’Dwyer, C.; Torres, C. M. S.; Eychmuller, A. Angew. Chem., Int. Ed. 2008, 47, 6538. (24) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (25) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737. (26) Gross, D.; Mora-Sero, I.; Dittrich, T.; Belaidi, A.; Mauser, C.; Houtepen, A. J.; Da Como, E.; Rogach, A. L.; Feldmann, J. J. Am. Chem. Soc. 2010, 132, 5981. (27) Htoon, H.; Hollingsworth, J. A.; Dickerson, R.; Klimov, V. I. Phys. Rev. Lett. 2003, 91, 227401. (28) Zavelani-Rossi, M.; Lupo, M. G.; Tassone, F.; Manna, L.; Lanzani, G. Nano Lett. 2010, 10, 3142. (29) Carbone, L.; Nobile, C.; De Giorgi, M.; Sala, F. D.; Morello, G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I. R.; Nadasan, M.; Silvestre, A. F.; Chiodo, L.; Kudera, S.; Cingolani, R.; Krahne, R.; Manna, L. Nano Lett. 2007, 7, 2942. (30) Sitt, A.; Della Sala, F.; Menagen, G.; Banin, U. Nano Lett. 2009, 9, 3470. (31) Htoon, H.; Malko, A. V.; Bussian, D.; Vela, J.; Chen, Y.; Hollingsworth, J. A.; Klimov, V. I. Nano Lett. 2010, 10, 2401. (32) Mu¨ller, J.; Lupton, J. M.; Lagoudakis, P. G.; Schindler, F.; Koeppe, R.; Rogach, A. L.; Feldmann, J.; Talapin, D. V.; Weller, H. Nano Lett. 2005, 5, 2044. (33) Wang, L. W.; Zunger, A. J. Phys. Chem. B 1998, 102, 6449. (34) Klimov, V. I. J. Phys. Chem. B 2000, 104, 6112. (35) Pandey, A.; Guyot-Sionnest, P. J. Chem. Phys. 2007, 127. (36) van Embden, J.; Jasieniak, J.; Mulvaney, P. J. Am. Chem. Soc. 2009, 131, 14299. (37) Mauser, C.; Da Como, E.; Baldauf, J.; Rogach, A. L.; Huang, J.; Talapin, D. V.; Feldmann, J. Phys. Rev. B 2010, 82, 081306. (38) Klimov, V. I.; Mikhailovsky, A. A.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Science 2000, 287, 1011. (39) Saba, M.; Minniberger, S.; Quochi, F.; Roither, J.; Marceddu, M.; Gocalinska, A.; Kovalenko, M. V.; Talapin, D. V.; Heiss, W.; Mura, A.; Bongiovanni, G. Adv. Mater. 2009, 21, 4942.
Strauf, S.; Stoltz, N. G.; Rakher, M. T.; Coldren, L. A.; Petroff, P. M.; Bouwmeester, D. Nat. Photonics 2007, 1, 704. Laucht, A.; Hofbauer, F.; Hauke, N.; Angele, J.; Stobbe, S.; Kaniber, M.; Bohm, G.; Lodahl, P.; Amann, M. C.; Finley, J. J. New J. Phys. 2009, 11, No. 023034. Xing, Y.; Chaudry, Q.; Shen, C.; Kong, K. Y.; Zhau, H. E.; Wchung, L.; Petros, J. A.; O’Regan, R. M.; Yezhelyev, M. V.; Simons, J. W.; Wang, M. D.; Nie, S. Nat. Protoc. 2007, 2, 1152. Bharali, D. J.; Lucey, D. W.; Jayakumar, H.; Pudavar, H. E.; Prasad, P. N. J. Am. Chem. Soc. 2005, 127, 11364. Harke, B.; Ullal, C. K.; Keller, J.; Hell, S. W. Nano Lett. 2008, 8, 1309. Rogach, A. L. Semiconductor Nanocrystals Quantum Dots; SpringerWienNewYork, 2008. Chen, Y.; Vela, J.; Htoon, H.; Casson, J. L.; Werder, D. J.; Bussian, D. A.; Klimov, V. I.; Hollingsworth, J. A. J. Am. Chem. Soc. 2008, 130, 5026. Osovsky, R.; Cheskis, D.; Kloper, V.; Sashchiuk, A.; Kroner, M.; Lifshitz, E. Phys. Rev. Lett. 2009, 102, 197401. Wang, X. Y.; Ren, X. F.; Kahen, K.; Hahn, M. A.; Rajeswaran, M.; Maccagnano-Zacher, S.; Silcox, J.; Cragg, G. E.; Efros, A. L.; Krauss, T. D. Nature 2009, 459, 686. Hikmet, R. A. M.; Chin, P. T. K.; Talapin, D. V.; Weller, H. Adv. Mater. 2005, 17, 1436. Mauser, C.; Limmer, T.; Da Como, E.; Becker, K.; Rogach, A. L.; Feldmann, J.; Talapin, D. V. Phys. Rev. B 2008, 77, 153303. Talapin, D. V.; Koeppe, R.; Gotzinger, S.; Kornowski, A.; Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H. Nano Lett. 2003, 3, 1677. Fisher, B.; Caruge, J. M.; Zehnder, D.; Bawendi, M. Phys. Rev. Lett. 2005, 94. Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J. B.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190. Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P. Nano Lett. 2007, 7, 2951. Taki, M.; Wolford, J. L.; O’Halloran, T. V. J. Am. Chem. Soc. 2004, 126, 712. Klar, T. A.; Jakobs, S.; Dyba, M.; Egner, A.; Hell, S. W. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 8206. Battaglia, D.; Blackman, B.; Peng, X. G. J. Am. Chem. Soc. 2005, 127, 10889.
© 2010 American Chemical Society
4650
DOI: 10.1021/nl1028057 | Nano Lett. 2010, 10, 4646-–4650
