J. Phys. Chem. C 2007, 111, 11811-11815
11811
Size- and Temperature-Dependent Emission Properties of Zinc-blende CdTe Nanocrystals in Ionic Liquid Yoshiyuki Nonoguchi, Takuya Nakashima,* and Tsuyoshi Kawai* Graduate School of Materials Science, Nara Institute of Science and Technology, NAIST, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan ReceiVed: April 24, 2007; In Final Form: May 18, 2007
Size- and temperature-dependence of photoluminescence from CdTe nanocrystals has been studied in a roomtemperature ionic liquid. With temperature decreasing from room temperature to 14 K, the photoluminescence intensity increases considerably and the emission quantum yield reaches almost 0.95 below 100 K. The emission profile below 150 K can be deconvoluted into two Gaussian peaks; one is an excitonic band and the other is a trap-related one. The temperature dependence of each band is characterized with regard to its intensity and energy. The energy splitting in the exciton fine structure of the zinc-blende CdTe nanocrystal is clearly demonstrated, and its degree can be estimated to be 5 and 13 meV for the larger (3.0 nm) and the smaller (1.9 nm) CdTe nanocrystals. The existence of the fine structure in the trap emission state and its size-dependent nature are also proposed.
Introduction In the past decade, semiconductor nanocrystals (NCs) have been widely studied as a zero-dimensional nanostructure, “quantum dot”, in which excitons are confined in all three dimensions. In the semiconductor NCs whose size is smaller than the bulk exciton Bohr radius, the electron and hole are individually confined and their properties are independently quantized.1 The quantum confinement effect is closely related to the size of NCs and contributes to the width of band gap, leading to the capability of controlling the optical properties with size and hence synthetic conditions. Recent progress in the controlled synthesis of semiconductor NCs with high quality explores their potential applications for bio-imaging probes,2,3 light-emitting diodes (LEDs),4 and dye-sensitized solar cells.5,6 CdTe NCs, one of the emissive II-VI semiconductor nanocrystals, are briefly and safely synthesized by means of the aqueous synthetic approach.7 The thiol-capped CdTe NCs, however, usually exhibit low stability and low emission quantum efficiency, probably because of the adsorption-desorption equilibrium of thiol molecules to the surface of NCs in water, resulting in the formation of some surface defects. These defects should promote the nonradiative transition from the photoexcited states in NCs. A number of modified synthetic methods8,9 and post-preparative treatments10,11 for improving the semiconductor NCs have been explored in the past decade. We have recently succeeded in the improvement of the emission intensity and the photodurability of CdTe NCs by introducing them into ionic liquids.12,13 Cationic CdTe NCs were easily extracted into hydrophobic ionic liquids and stabilized certainly by the effective solvation with ionic liquid molecules. There have been a number of studies on the recombination dynamics of wurtzite CdSe NCs, the most popular II-VI semiconductor NCs.14-28 The splitting of exciton energy level into “dark” exciton and “bright” exciton has been reported for * To whom correspondence should be addressed. E-mail:
[email protected] (T.K.);
[email protected] (T.N.). Tel: +81-743-726170. Fax: +81-743-72-6179.
the CdSe NCs, especially referred to as the exciton fine structure. The exchange interaction is responsible for the energy splitting and the forbidden-transition nature of the “dark” state, which is one of the specific natures of the quantum-confined system. In these studies, optical properties of CdSe NCs have been characterized at low temperature below 15 K, because the energy difference between the splitting states is on the order of a few millielectronvolts. Unlike the studies on the wurtzite CdSe NCs, however, low-temperature properties of zinc-blende type CdTe NCs have been almost unexplored. Organic ligand-capped NCs in aqueous solution are unstable and not so emissive at low temperature, which is reported as the “temperature antiquenching” phenomenon.29,30 This effect makes it difficult to understand the optical properties of the CdTe NCs at low temperature. On the other hand, for wurtzite CdSe NCs, core-shell nanocrystals such as CdSe/ZnS NCs and CdSe/CdS NCs are successfully synthesized and are extensively discussed from a viewpoint of the exciton fine structure, since they are still emissive even at low temperature. Recently, Woggon et al. have investigated the low-temperature emission properties of CdTe/ CdS core/shell NCs synthesized by the standard high-temperature reaction in organic solvent.31 They have demonstrated the slight decreases in both the intensity and the lifetime of CdTe/ CdS core/shell NCs emission from 25 to 15 K. A low-laying forbidden exciton state has been concluded to be responsible for this weak quenching effect, taking enhanced electron-hole exchange interaction in the confined nanocrystal systems into consideration. In spite of their advanced demonstrations, there might be some aspects of the emission properties of CdTe NCs at low temperature that remain unclear. For example, the overall quantum efficiency of NCs used in that study is rather low, indicating that undefined quenching process may take place even below 15 K and that only a part of NCs are emissive. The degree of energy splitting in the exciton fine structure and its size dependence have never been clarified, whereas they have already been demonstrated for CdSe NCs. Finally, since the core/shell structure should include considerable lattice distortion at the interface which might also affect the exciton states,32 it is worth
10.1021/jp073152q CCC: $37.00 © 2007 American Chemical Society Published on Web 07/24/2007
11812 J. Phys. Chem. C, Vol. 111, No. 32, 2007 while to study the low-temperature emission properties of “bare” CdTe NCs with high emission efficiency. In the present work, we investigated the emission properties of organic ligand capped CdTe NCs in ionic liquid at low temperature and found that the NCs are stable and exhibit markedly high emission intensity below 100 K. We describe herein the size- and temperature-dependent emission properties of zinc-blende CdTe NCs in ionic liquid at temperatures between 14 K and room temperature for the first time. The temperature dependence of the emission profile enables us to propose a fine structure of the exciton states and a complicated trap-related state in CdTe NCs. Experimental Section Synthesis of CdTe NCs. Thiocholine bromide (TCB)-capped CdTe NCs were synthesized by following the reported procedure with some modifications.7 Briefly, clear NaHTe solution was prepared by reacting 63 mg (0.49 mmol) of Te and 88 mg (2.3 mmol) of NaBH4 in 2 mL of water at 0 °C. Then, 0.24 mL of the 0.5 M NaHTe solution was added to 10 mL of water dissolving 29 mg of CdCl(2.5H2O) (0.13 mmol) and 61 mg of TCB (0.31 mmol). The solution was then subjected to a reflux. All the procedure was performed under nitrogen atmosphere. The mean diameter of the CdTe NCs formed with the reflux periods of 30 and 300 min were determined by transmission electron microscopy (JOEL, JEM-2200FS) to be about 1.9 and 3.0 nm, respectively. These samples will be depicted as NCs (1.9 nm) and NCs (3.0 nm), respectively. Measurement of Emission Spectra of CdTe Nanocrystals. Careful attention was given to all the sample preparations. TCBcapped CdTe NCs were extracted into 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (bmimTFSI) by a simple liquid-liquid-phase transfer technique. The sample solution was dehydrated, degassed, and tightly sealed in a quartz cuvette (1 mm). The concentration of NCs solution was low enough to prevent both the concentration quenching and energy transfer among NCs. In the measurement of emission spectra, the sample was excited by nitrogen laser (337 nm, ∆t ∼ 0.6 ns, 20 Hz, Usho, KEC-160). The emission profile was measured with an N2-cooled CCD spectrograph system (Acton Pro PS300i, Prinstom Instruments, Spec-10). The power of the laser irradiation was optimized to prevent the Auger process, the stimulated emission, and the destructions of NCs. Samples were cooled in a circulating He gas flow cryostat (IWATANI, D105, CA112). All the measurements were conducted after leaving for at least 30 min in thermal equilibrium at each temperature. The NCs (1.9 nm) and NCs (3.0 nm) exhibited emission quantum yields of 0.20 and 0.45, respectively, in ionic liquid at 290 K. In these evaluations, perylenebis(hexylheptyl)carboxydiimide in chloroform was used as the standard sample (φem ) 0.99) and relative integrated emission intensities were corrected by the optical density and by the square of the refractive index of medium. Results and Discussion Low-temperature emission properties of the CdTe NCs were studied in water and the ionic liquid bmimTFSI. As shown in Figure 1, the CdTe NCs exhibit significant photoluminescence emission under the UV light irradiation at room temperature both in water and in bmimTFSI. The photoluminescence emission was significantly enhanced in the ionic liquid at low temperature, while it was almost completely quenched in water after chilled in liquid N2. It should be noted that the lowtemperature photoluminescence in water exhibits low reproduc-
Nonoguchi et al.
Figure 1. Cooling effect on the emission of CdTe NCs (left) in water and (right) in bmimTFSI at (upper) room temperature and (lower) 77 K. These samples are illuminated with UV light (λex ) 365 nm). At 77 K, the emission intensity in bmimTFSI is rather high while that intensity is entirely quenched in water.
ibility and in some cases33 moderate emission intensity can be observed. The result observed in water is not contradictory to the previous reports on the “temperature antiquenching effect” of the NCs.29,30 The formation of water crystals tends to disturb the order of capping molecules on the surface of NCs, resulting in the quenching of photoluminescence intensity by the formation of surface defect states. As reported, CdTe NCs in ionic liquid exhibit significant durability.12 Ionic liquid molecules would effectively solvate the CdTe NCs having dense positive charges on their surface and suppress the electrostatic repulsions between the neighboring capping molecules, which substantially make the NCs surface stable. Upon cooling the ionic liquid in liquid N2, it forms a stable amorphous state34 and microscopic crystallization does not take major place, which would result in almost no antiquenching effect upon cooling. We then measured the temperature dependence of emission properties of the CdTe NCs in bmimTFSI. Figure 2a shows typical temperature-dependent emission spectra of NCs (1.9 nm) in bmimTFSI. As temperature decreased, the emission intensity increased and a characteristic side band appeared in addition to the major excitonic emission peak. This broad side band at lower energy can be attributed to the localized excited state. The emission spectra were deconvoluted into two Gaussian curves, as shown in Figure 2b. One is sharp and has a higher energy corresponding to an excitonic emission, and the other is rather broad and has a lower energy corresponding to the emission from the trap state. As shown in Figure 3, intensity of both the excitonic and trap emission increased markedly at low temperature. This effect is attributed to the suppression of phononcoupled thermal quenching in NCs. The relative values of the emission quantum yields were evaluated on the basis of those, 0.20 for NCs (1.9 nm) and 0.45 for NCs (3.0 nm), at room temperature, and we are now expecting less than 5% of undesired error for evaluating the emission quantum yield at all temperatures. They were almost constant below 100 K, and the sum of the emission quantum yields reached to φem > 0.95 in each sample, even if the plots had a little fluctuation or error in their values. This result indicates that there was almost no intrinsic “dark” nanocrystal in both samples. In Figure 4a, the peak energy of the excitonic emission from CdTe NCs in bmimTFSI shifted to higher energy upon cooling from 190 to 40 K for NCs (3.0 nm) and to 60 K for NCs (1.9 nm). Similar phenomena have been also observed for other semiconductor NCs,35-37 which can be assigned to the shrinking in the lattice constant at low temperature. It is worth noting that, in the temperature range from 40 to 20 K for NCs (3.0 nm) and from 60 to 40 K for NCs (1.9 nm), the peak energy shifted slightly to lower energy. These energy shifts are approximately estimated to be 5 meV for NCs (3.0 nm) and 13 meV for NCs (1.9 nm). Since these changes can be observed
Recombination Dynamics of CdTe NCs in Ionic Liquid
J. Phys. Chem. C, Vol. 111, No. 32, 2007 11813
Figure 2. (a) Temperature dependence of emission spectra of CdTe NCs (1.9 nm) in bmimTFSI and (b) deconvolution of the obtained spectrum. The spectrum is successfully fitted to two Gaussian curves.
Figure 3. Temperature dependence of relative excitonic and trap emission quantum yields of CdTe NCs (a) 3.0 nm and (b) 1.9 nm in bmimTFSI.
Figure 4. Size and temperature dependence of peak energies of (a) excitonic and (b) trap emissions from CdTe NCs in bmimTFSI.
reversibly upon cooling-heating cycles between 14 and 100 K and the degree of energy shift shows good reproducibility, these shifts seem to be an intrinsic property of the CdTe NCs. If a certain particle is smaller than the exciton Bohr radius, then the excited electron and hole are tightly confined.38 In zincblende CdTe NCs which have a relatively large exciton Bohr radius (aB ∼ 7.3 nm),39 the conduction and valence bands are built by the s orbitals of Cd and the p orbitals of Te, respectively.14,40 The orbital angular momentum is coupled to the spin angular momentum, resulting in the splitting of p orbitals into the topmost J ) 3/2 and the spin-off J ) 1/2. These two levels are considered to mix weakly with each other because of relatively large spin-orbit interaction in CdTe. Then the topmost states are regarded as the valence band. The 8-fold degenerate exciton states, seS3/2, split into two groups by the electron-hole exchange interaction for zinc-blende CdTe NCs. Two groups of “degenerate” emissive states have been reported for zinc-blende CdTe NCs.14 One is composed of allowed higher energy states (0U, (1U) and the other is composed of forbidden
lower energy states (0L, (1L, (2L). Woggon et al. have reported the exciton fine structure of zinc-blende CdTe NCs with CdTe/ CdS core/shell NCs.31 They also observed characteristic temperature dependence of emission intensity between 25 and 15 K similar to that of the present study. The emission intensity decreases slightly from 40 to 20 K for the NCs (3.0 nm) and from 60 to 40 K for the NCs (1.9 nm), which can be attributed to the change in the population between the two splitting energy levels. Since the CdTe NCs used in the present study are smaller than the samples used by Woggon et al., they exhibit larger exciton splitting. This larger spin-spin interaction makes it possible to figure out the exciton fine structure. Figures 2 and 3 show that not only the excitonic emission but also the trap emission are bright below 100 K and their emission quantum efficiencies are about 0.85 and 0.15, respectively. The formation of trap states has been attributed to distortions, disorders, and/or destructions of the surface structures of NCs. In the case of CdSe NCs, the emission from the deep trap states has been reported to be observed in the IR
11814 J. Phys. Chem. C, Vol. 111, No. 32, 2007 region.41 In the present work, “shallow” trap emissions are clearly observed at low temperature. Their peak energies are estimated to be about 2.15 eV which is relatively close to the excitonic emission peak energies. The trap emission peak energy exhibits complicated behavior at low temperature. As shown in Figure 4b, both NCs (1.9 nm) and NCs (3.0 nm) show overall red shift in the trap-related emission energy with cooling from 150 to 60 K. The NCs (1.9 nm) show a clear high-energy shift in the emission energy from 60 to 50 K, which roughly corresponds to the temperature range showing the energy shift in the excitonic emission as shown in Figure 4a. It should be also noted that the NCs (1.9 nm) show a clear decrease in the emission quantum yield of the trap emission at this temperature range (Figure 3b). The origin of these complicated temperature dependence of the trap-related emission behavior is not clear at the present stage. There might be a certain fine structure or inhomogeneity in the shallow trap states, which would be responsible for the change in the emission energy at low temperature. In the case of NCs (3.0 nm), the variation in the emission energy is considerably smaller than that in the NCs (1.9 nm). Again, there could be two or more possible explanations for this size dependence. For example, the shallow trap states could have a certain spatial distribution or have inhomogeneity in their own nature in particle-by-particle. The NCs (3.0 nm) might have a more improved and more uniform surface structure than that of the NCs (1.9 nm) of smaller emission quantum yield at room temperature. The density of surface defect states should be higher in the smaller particles because of their high surfaceto-volume ratio, leading to the high inhomogeneity in their quality. For the wurtzite CdSe NCs, ultrafast dynamics of an electron and a hole have been well studied,41 in which the various trap states including surface states and interface states have been proposed. Trap states in the zinc-blende CdS NCs have also also explored,42 but the details such as the trap recombination dynamics and trap-phonon coupling still remain unclear. For our CdTe NCs, the trap-related transition is not visible at room temperature, while it is bright (emission quantum yield of more than 0.1) and has long lifetime of more than 100 ns at 14 K (see Supporting information, S4). These results suggest, at least, that the trap state is thermally sensitive and the transition to a ground state is partially forbidden. The details are under investigation in our laboratory. Conclusion We investigated the size and temperature dependence of photoluminescence from zinc-blende CdTe NCs in an ionic liquid. Above 100 K, the emission efficiency depends on the thermally induced relaxation. Below 100 K, the emission quantum yield is estimated to be more than 0.95, which is independent of the size of NCs. The present results suggest that the emission is no longer affected by thermally assisted relaxation and this system is an ideal model to observe intrinsic properties of NCs including the exciton fine structure and the relationship between excitonic and trap emission at low temperature. Acknowledgment. The authors thank Mr. Yasuo Okajima and Mr. Hirotoshi Furusho, NAIST technical staffs, for their technical support regarding spectroscopic measurements and TEM observations. The authors also thank Prof. A. Yamamoto (NAIST) for helpful discussions. This work was partially
Nonoguchi et al. supported by Grant-in-Aids for Scientific Research (KAKENHI) on Priority Areas, “Ionic Liquid” (No. 18045022) and “Molecular Nano Dynamics” (No. 17034042) from Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Supporting Information Available: TEM observations of CdTe NCs, steady-state photoluminescence spectra of CdTe NCs in bmimTFSI, additional data of the emission from CdTe NCs in ionic liquid, and some comments on photoluminescence decay dynamics of CdTe NCs in bmimTFSI are given. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933-937. (2) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538-544. (3) Clarke, S. J.; Hollmann, C. A.; Zhang, Z.; Suffern, D.; Bradforth, S. E.; Dimitrijevic, N. M.; Minarik, W. G.; Nedeau, J. L. Nat. Mater. 2006, 5, 409-417. (4) Coe, S.; Woo, W-K.; Bawendi, M.; Bulovic´, V. Nature 2002, 420, 800-803. (5) Klimov, V. I. J. Phys. Chem. B 2006, 110, 16827-16845. (6) Plass, R.; Pelet, S.; Krueger, J.; Gra¨tzel, M.; Bach, U. J. Phys. Chem. B 2002, 106, 7578-7580. (7) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177-7185. (8) A typical review is the following: Donega´, C. M.; Liljeroth, P.; Vanmaekelbergh, D. Small 2005, 1, 1152-1162. (9) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664-670. (10) Bao, H.; Gong, Y.; Li, Z.; Gao, M. Chem. Mater. 2004, 16, 38533859. (11) Myung, N.; Bae, Y.; Bard, A. J. Nano Lett. 2003, 3, 747-749. (12) Nakashima, T.; Kawai, T. Chem. Commun. 2005, 1643-1645. (13) (a) Nakashima, T.; Sakakibara, T.; Kawai, T. Chem. Lett. 2005, 34, 1410-1411. (b) Nakashima, T.; Sakashita, M.; Nonoguchi, Y.; Kawai, T. Macromolecules, in press. (14) Efros, A. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. Phys. ReV. B 1996, 54, 4843-4856. (15) Alivisatos, A. P.; Harris, T. D.; Carrol, P. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1989, 90, 3463-3468. (16) Shiang, J. J.; Goldstein, A. N.; Alivisatos, A. P. J. Chem. Phys. 1990, 92, 3232-3233. (17) Schoenlein, R. W.; Mittleman, D. M.; Shiang, J. J.; Alivisatos, A. P.; Shank, C. V. Phys. ReV. Lett. 1993, 70, 1014-1017. (18) Woggon, U.; Gaponenko, S.; Langbein, W.; Uhrig, A.; Klingshirn, C. Phys. ReV. B 1993, 47, 3684-3689. (19) Donega´, C. M.; Bode, M.; Meijerink, A. Phys. ReV. B. 2006, 74, 085320. (20) Crooker, S. A.; Barrick, T.; Hollingsworth, J. A.; Klimov, V. I. Appl. Phys. Lett. 2003, 82, 2793-2795. (21) Lifshitz, E.; Dag, I.; Litvitn, I. D.; Hodes, G. J. Phys. Chem. B 1998, 102, 9245-9250. (22) Xu, S.; Mikhailovsky, A. A.; Hollingsworth, J. A.; Klimov, V. I. Phys. ReV. B 2002, 65, 045319. (23) Huxter, V. M.; Kovalevskij, V.; Scholes, G. D. J. Phys. Chem. B 2005, 109, 20060-20063. (24) Furis, M.; Htoon, H.; Petruska, M. A.; Klimov, V. I.; Barrick, T.; Crooker, S. A. Phys. ReV. B 2006, 74, 241313. (25) Labeau, O.; Tamarat, P.; Lounis, B. Phys. ReV. Lett. 2003, 90, 257404. (26) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G. J. Chem. Phys. 1997, 106, 9869-9882. (27) Wang, H.; Donega´, C. M.; Meijerink, A.; Glasbeek, M. J. Phys. Chem. B 2006, 110, 733-737. (28) Kraus, R. M.; Lagoudakis, P. G.; Rogach, A. L.; Talapin, D. V.; Weller, H.; Lupton, J. M.; Feldmann, J. Phys. ReV. Lett. 2007, 98, 017401. (29) Wuister, S. F.; Donega´, C. M.; Meijerink, A. J. Am. Chem. Soc. 2004, 126, 10397-10402. (30) Wuister, S. F.; Houselt, A.; Donega´, C. M.; Vanmaekelbergh, D.; Meijerink, A. Angew. Chem., Int. Ed. 2004, 43, 3029-3033. (31) Scho¨ps, O.; Le Thomas, N.; Woggon, U.; Altemyev, M. V. J. Phys. Chem. B 2006, 110, 2074-2079. (32) Jones, M.; Nedeljkovic, J.; Ellingson, R. J.; Nozik, A. J.; Rumbles, G. J. Phys. Chem. B 2003, 107, 11346-11352.
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J. Phys. Chem. C, Vol. 111, No. 32, 2007 11815 (39) Esch, V.; Fluegel, B.; Khitrova, G.; Gibbs, H. M.; Jiajin, X.; Kang, K.; Koch, W.; Liu, L. C.; Risbud, S. H.; Peyghambarian, N. Phys. ReV. B 1990, 42, 7450-7453. (40) Masumoto, Y.; Sonobe, K. Phys. ReV. B 1997, 56, 9723-9737. (41) Burda, C.; Link, S.; Mohamed, M.; El-Sayed, M. J. Phys. Chem. B 2001, 105, 12286-12292. (42) There are a number of reports about trap states in zinc-blende CdS NCs. One of the early reports is the following paper: Ha¨sselbarth, A.; Eychmu¨ller, A; Weller, H. Chem. Phys. Lett. 1993, 203, 271-276.
