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Langmuir 2008, 24, 9682-9685
Temperature-Modulated Photoluminescence of Quantum Dots Yi Hou,† Jing Ye,†,§ Zhou Gui,‡ and Guangzhao Zhang*,† Hefei National Laboratory for Physical Sciences at Microscale and State Key Laboratory of Fire Science, Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, China, and Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin N.T., Hong Kong ReceiVed January 29, 2008. ReVised Manuscript ReceiVed May 31, 2008 Cadmium sulfide (CdS) quantum dots (QDs) grafted with thermoresponsive poly(N-isopropylacrylamide) chains have been prepared. As the temperature increases, PNIPAM chains shrink and aggregate so that the QDs exhibit enhanced fluorescence emission. At a temperature around the lower critical solution temperature (LCST) of PNIPAM, the fluorescence exhibits a maximum intensity. Our experiments reveal that the fluorescence emission is determined by the interactions between QDs as a function of the interdot distance. The optical interdot distance for the maximum luminescence intensity is ∼10 nm. The chain length of PNIPAM also has an effect on the luminescence. Short PNIPAM chains are difficult to associate, leading to a large interdot distance, so that the luminescence intensity changes slightly with temperature.
Introduction Luminescent semiconductor quantum dots (QDs) have received increasing attention in physics, chemistry, and engineering. Because the electronic structure is dependent on quantum confinement effects in the three spatial dimensions, QDs have unique luminescence that is sensitive to surface properties.1–6 Namely, QDs usually exhibit high emission quantum yields, tenable emission wavelengths, and high chemical and photostability.7–14 Accordingly, QDs have found applications in the development of optical or optoelectronic devices such as optical switches, light-emitting diodes, and lasers.15–20 However, as artificial atoms between the molecular and bulk forms of matter, * To whom correspondence should be addressed. E-mail: gzzhang@ ustc.edu.cn. † Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China. ‡ State Key Laboratory of Fire Science, University of Science and Technology of China. § The Chinese University of Hong Kong. (1) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. ReV. Phys. Chem. 1998, 49, 371. (2) Kulakovich, O.; Strekal, N.; Yaroshevich, A.; Maskevich, S.; Gaponenko, S.; Nabiev, I.; Woggon, U.; Artemyev, M. Nano Lett. 2002, 2, 1449. (3) Gueroui, Z.; Libchaber, A. Phys. ReV. Lett. 2004, 93, 166108. (4) Eckel, R.; Walhorn, V.; Pelargus, C.; Martini, J.; Enderlein, J.; Nann, T.; Anselmetti, D.; Ros, R. Small 2007, 3, 44. (5) Franceschetti, A.; Zunger, A. Phys. ReV. B 2001, 63, 153304. (6) Lee, J.; Govorov, A. O.; Kotov, N. A. Angew. Chem., Int. Ed. 2005, 44, 7439. (7) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (8) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (9) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (10) Hao, E.; Sun, H.; Zhou, Z.; Liu, J.; Yang, B.; Shen, J. Chem. Mater. 1999, 11, 3096. (11) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183. (12) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207. (13) Qu, L.; Peng, A.; Peng, X. Nano Lett. 2001, 1, 333. (14) Katsikas, L.; Eychmu¨ller, A.; Giersig, M.; Weller, H. Chem. Phys. Lett. 1990, 172, 201. (15) Han, M.; Gao, X.; Su, J.; Nie, S. Nat. Biotechnol. 2001, 19, 631. (16) Moffitt, M.; Vali, H.; Eisenberg, A. Chem. Mater. 1998, 10, 1021. (17) Tong, X.; Zhao, Y. J. Am. Chem. Soc. 2007, 129, 6372. (18) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676. (19) Lee, J.; Govorov, A. O.; Kotov, N. A. Nano Lett. 2005, 5, 2063. (20) Govorov, A. O.; Bryant, G. W.; Zhang, W.; Skeini, T.; Lee, J.; Kotov, N. A.; Slocik, J. M.; Naik, R. R. Nano Lett. 2006, 6, 984.
QDs are useful for us to understand fundamental issues about the field-matter interaction.21–24 It is established that the emission of light by molecules and atoms involves enhancement and quenching processes, which relate to the local environment and nonradiative energy transfer, respectively.22 Theoretically, it is predicted that the dipole-dipole interaction that decays with particle separation plays an important role in luminescence.2,25 Experimentally, the luminescence enhancement of QDs spaced by noble metal nanoparticles has been observed.2,20 Such an enhancement has been attributed to the locally enhanced electromagnetic field as a result of the plasmon resonance of the spacer. However, luminescence quenching due to nonradiative electron or energy transfer has been attributed to the short-range dipole interaction in QD assemblies25,26 or a complex of QDs and gold nanoparticles.3 Note that the introduction of gold nanoparticles often makes the interactions in the system complex. In the present study, we have prepared nanostructures with a poly(N-isopropylacrylamide) (PNIPAM) as the corona and cadmium sulfide (CdS) QDs as the core. Because PNIPAM has a lower critical solution temperature (LCST) at ∼32 °C,27 the distance between QDs can be adjusted by controlling the temperature. The aggregation and luminescence of QDs grafted with PNIPAM in aqueous solution have been characterized by laser light scattering (LLS) and fluorescence spectroscopy. Our aim is to understand the inderdot distance dependence of luminescence emission of QDs.
Experimental Section Materials. N-Isopropylacrylamide (NIPAM, courtesy of Kohjin Co., Ltd.) was purified by recrystallization in a benzene/n-hexane mixture. 4,4′-Azobis(isobutyronitrile) (AIBN) from Acros was purified by recrystallization from methanol. Tetrahydrofuran (THF) (21) Gersen, H.; Garcı´a-Parajo´, M. F.; Novotny, L.; Veerman, J. A.; Kuipers, L.; van Hulst, N. F. Phys. ReV. Lett. 2000, 85, 5312. (22) Anger, P.; Bharadwaj, P.; Novotny, L. Phys. ReV. Lett. 2006, 96, 113002. (23) Yang, M.; Li, S. S. Phys. ReV. B 2006, 74, 073402. (24) Tamura, H.; Shiraishi, K.; Takayanagi, H. Jpn. J. Appl. Phys. 2004, 43, 691. (25) 25 Crooker, S. A.; Hollingsworth, J. A.; Tretiak, S.; Klimov, V. I. Phys. ReV. Lett. 2002, 89, 186802. (26) Micic, O. I.; Jones, K. M.; Cahill, A.; Nozik, A. J. J. Phys. Chem. B 1998, 102, 9791. (27) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163.
10.1021/la800312u CCC: $40.75 2008 American Chemical Society Published on Web 07/19/2008
Temperature-Modulated Photoluminescence of QDs
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was distilled from a purple sodium benzophenone ketyl solution prior to use. Other regents were all used as received. Sample Preparation. CdS QDs were prepared via the reaction of sodium sulfide with cadmium acetate in aqueous solution. In a typical experiment, after 20 mL of 3.5 × 10-2 g/mL cadmium acetate aqueous solution in a 100 mL round-bottomed flask equipped with a condenser was heated to 40 °C with vigorous stirring, 20 mL of 1.0 × 10-2 g/mL Na2S was added. The mixture was further stirred at 40 °C for 24 h. Then, it was cooled to room temperature under stirring. The mixture was filtered through a 0.8 µm Millipore membrane filter, yielding the CdS QDs. PNIPAM was synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization in THF with cyanoisopropyl dithiobenzoate as the chain-transfer agent and AIBN as the initiator. The number-average molar mass (Mn) and polydispersity index (Mw/ Mn) were determined by size exclusion chromatography (Waters 1515) using monodisperse polystyrene as the standard and THF as the eluent with a flow rate of 1.0 mL/min. For samples PNIPAM1, PNIPAM2, and PNIPAM3, the Mw values are 3.38 × 104, 5.90 × 103, and 1.80 × 103 g/mol, and Mw/Mn values are 1.09, 1.07, and 1.20, respectively. PNIPAM terminated with thiol end groups (PNIPAM-SH) was prepared by the reduction of dithioester-terminated PNIPAM with NaBH4. In a typical experiment, after 0.03 g of dithioester-terminated PNIPAM was dissolved in 30 mL of deionized water, NaBH4 aqueous solution with a NaBH4/PNIPAM molar ratio of 40/1 was introduced under vigorous stirring. The reaction mixture was stirred at room temperature for 1 week to ensure that dithioester terminal groups were completely reduced to yield PNIPAM-SH. Then, the PNIPAMSH solution was added dropwise to freshly prepared CdS QDs solution in water under ultrasonification. The mixture was stirred for another three days. The solution was centrifuged at 12 000 rpm for 1 h at 25 °C, and the supernatant was removed. Such a procedure was repeated three times. As a result, the unattached PNIPAM chains were removed, yielding QDs grafted with PNIPAM chains (PNIPAMQDs). PNIPAM-QDs were redispersed in deionized water at a concentration of 1 × 10-5 g/mL for LLS and fluorescence measurements. Laser Light Scattering (LLS). A commercial spectrometer (ALV/ DLS/SLS-5022F) equipped with multi-τ digital time correlation (ALV5000) and a cylindrical 22 mW UNIPHASE He-Ne laser (λ0 ) 632 nm) as the light source was used. In static LLS,28 we can obtain Mw and the z-average root-mean-square radius of gyration (〈Rg〉) of particles in a dilute solution from the angular dependence of the excess absolute scattering intensity or Rayleigh ratio Rvv(q) by a Zimm plot as follows,
(
KC Rvv(q)
)
≈ c f0
1 1 1 + 〈Rg2 〉q2 Mw 3
(
)
(q〈Rg2〉1⁄2 < 1)
(1)
where K ) 4π2n2(dn/dC)2/(NAλ04) and q ) (4πn/λ0) sin(θ/2) with NA, dn/dC, n, and λ0 being Avogadro’s number, the specific refractive index increment, the solvent refractive index, and the wavelength of the laser light in vacuum, respectively; A2 is the second virial coefficient. For large aggregates, the following Guinier plot instead of the Zimm plot was used.29
(
KC Rvv(q)
)
≈ c f0
1 1 exp 〈Rg2 〉zq2 Mw 3
(
)
(q〈Rg2〉1⁄2 > 1)
(2)
In dynamic LLS,30 the measured intensity-intensity time correlation function G(2)(q, t) in the self-beating mode can transfer into line-width distribution G(Γ) by Laplace inversion. For a pure diffusive relaxation, G(Γ) is related to the translational diffusion coefficient D by (Γ/q2)c f0, q f0 f D and further to the hydrodynamic radius Rh (28) Chu, B. Laser Light Scattering, 2nd ed, Academic Press, New York, 1991. (29) Guinier, A.; Fournet, G. Small-angel Scattering of X-ray John Wiley, New York, 1955. (30) Berne, B. J.; Pecora, R. Dynamic Light Scattering Plenum Press, New York, 1976.
Figure 1. Temperature dependence of the fluorescence emission intensity (I) of PNIPAM1-QDs, where the emission wavelength (λ) is 425 nm. The inset shows the fluorescence emission spectra at 25 and 37 °C.
via the Stokes-Einstein equation Rh ) kBT/6πη0D, where kB, T, and η0 are the Boltzmann constant, absolute temperature, and solvent viscosity, respectively. The solution was was allowed to stand for 2 to 3 h at each temperature so that the system was at equilibrium. The precision of the temperature is (0.05 °C. Fluorescence Measurements. Fluorescence spectra were recorded by the use of an LS55 luminescence spectrometer (Perkin-Elmer). The temperature of the water-jacketed cell holder was controlled by a heating bath. The slit widths were set at 15 nm for excitation and 9 nm for emission. Transmission Electron Microscopy (TEM). A drop of a dilute aqueous dispersion of PNIPAM-QDs with a concentration of 1.0 × 10-5 g/mL was deposited onto a carbon-coated copper mesh grid at 43 °C. Then, the morphology of the aggregates was observed on a Hitachi 800 TEM at an acceleration voltage of 200 kV. The morphology of individual CdS QDs without PNIPAM shells was observed on a JEOL2100 high-resolution TEM operating at an acceleration voltage of 200 kV at 25 °C. The diameter of the QDs is ∼4.6 nm.
Results and Discussion Figure 1 shows the temperature dependence of the luminescence emission intensity (I) of PNIPAM1-QDs at the emission wavelength λ ) 425 nm. The inset shows two typical fluorescence emission spectra. The intensity of PNIPAM1-QDs slightly varies at a temperature below the LCST (∼32 °C) since the QDs are dispersed in the solution as individuals, and they separate far away. As temperature increases up to the LCST, the intensity sharply increases. This is because the QDs form aggregates and they approach each other closer and closer. It is interesting to note that the intensity exhibits a maximum (Imax) at ∼37 °C. Namely, the intensity drops to constant at a temperature above ∼37 °C. It has been reported that the photoluminescence intensity of QDs coated with small molecules decreases when they form aggregates due to nonradiative energy transfer22,31 In the present study, each CdS QD was protected with thicker PNIPAM shell. Namely, the separation between CdS QDs in an aggregate is much larger than that for QDs coated with small molecules, which makes the nonradiative energy transfer impossible.31 Note that the emission scattering due to the aggregation in the system might also lead the measured fluoresence to increase. However, a recent study indicates that the emission scattering has a slight effect on the fluorescence intensity even in turbid media.32 The present system is a dilute solution and the aggregation of PNIPAM-QDs does not cause any turbidity, so the luminescence (31) Biju, V.; Itoh, T.; Baba, Y.; Ishikawa, M. J. Phys. Chem. B 2006, 110, 26068. (32) Mu¨ller, M. G.; Georgakoudi, I.; Zhang, Q. G.; Wu, J.; Feld, M. S. Appl. Opt. 2001, 40, 4633.
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Figure 2. Temperature dependence of Mw of PNIPAM1-QDs in water. The inset shows the hydrodynamic radius distributions f(Rh) at 25 and 37 °C, respectively.
Figure 4. TEM image of PNIPAM1-QD aggregates at 43 °C.
Figure 3. Temperature dependences of the average radius of gyration 〈Rg〉 and average hydrodynamic radius 〈Rh〉 of PNIPAM1-QDs in water. The inset shows 〈Rg〉/〈Rh〉.
enhancement cannot be attributed to emission scattering. To understand this phenomenon, we have examined the QD aggregation. Figure 2 shows that Mw of PNIPAM1-QDs slightly changes at a temperature below the LCST of PNIPAM. As the temperature increases up to the LCST, Mw significantly increases, indicating aggregation. The inset shows the temperature dependence of the hydrodynamic radius distribution f(Rh). At 25 and 37 °C, the average hydrodynamic radii (〈Rh〉) of PNIPAM1-QDs are ∼7.4 and 165 nm, respectively. In addition, the light-scattering intensity at 25 °C is much lower than that at 37 °C (not shown). These facts further indicate that the QDs are individual particles at a temperature below the LCST but form aggregates at temperatures above the LCST. Note that the aggregates are stable even at a temperature above 40 °C. Figure 3 shows the temperature dependence of 〈Rh〉 and 〈Rg〉 of PNIAPM1-QDs in water upon heating. The small 〈Rh〉 and 〈Rg〉 at a temperature below the LCST indicate the individual QDs. The sharp increases in 〈Rh〉 and 〈Rg〉 at ∼32 °C further reveal the aggregation of the QDs. At temperatures above the LCST, 〈Rh〉 and 〈Rg〉 tend to be constant, indicating the formation of stable PNIPAM1-QDs aggregates. The inset shows the temperature dependence of 〈Rg〉/〈Rh〉. It is well known that 〈Rg〉/ 〈Rh〉 can describe the structure of the scattering object. For uniform nondraining spheres, hyperbranched clusters, and random coils, 〈Rg〉/〈Rh〉 is ∼0.774, 1.0-1.2, and 1.5-1.8, respectively.33 At temperature lower than the LCST, 〈Rg〉 of individual PNIPAM1QD is too small to be detected, so we were unable to obtain the 〈Rg〉/〈Rh〉 value. At temperatures above the LCST, 〈Rg〉/〈Rh〉 is ∼0.67-0.73, indicating that the aggregates are spherical. The (33) Brown, W.; Burchard, W. Light Scattering Principles and DeVelopment Clarendon Press, Oxford, 1996.
Figure 5. Interdot distance (l) dependence of the fluorescence emission intensity of PNIPAM1-QDs.
TEM image in Figure 4 further indicates the spherical structure. In addition, the size measured by TEM is consistent with that from LLS. With the average molar mass (Mw,q) of the individual PNIPAMQDs and the average molar mass (Mw) and hydrodynamic radius (〈Rh〉) of the aggregates, we are able to estimate the average interdot distance (l) between two QDs by using l ) 2r - d, where r is obtained from r ) 〈Rh〉(Mw,q/Mw)1/3. Obviously, as the temperature increases, the interdot distance decreases. Figure 5 shows that the emission intensity of PNIPAM1-QDs exhibits its maximum at ∼10 nm. Namely, as the interdot distance decreases, the intensity increases at l > ∼10 nm but drops at l < ∼10 nm. The interdot distance dependence of the photoluminescence clearly indicates the continuous transition from fluorescence enhancement to quenching. Note that the luminescence of QDs without grafted PNIPAM chains is not temperature-dependent (not shown). Therefore, either photoluminescence enhancement or quenching arises from the interdot interactions.34,35 The maximum intensity implies at least two types of interaction between the QDs, which counteract each other. This is similar to the situations in atoms, molecules, and colloids. Artemyev et al.2 have investigated the luminescence of quantum dots in the presence of gold colloids and found that the (34) Wuister, S. F.; de Mello Donega´, C.; Meijerink, A. J. Am. Chem. Soc. 2004, 126, 10397. (35) Wuister, S. F.; Koole, R.; de Mello Donega´, C.; Meijerink, A. J. Phys. Chem. B 2005, 109, 5504.
Temperature-Modulated Photoluminescence of QDs
luminescence exhibits its maximum at ∼11 nm. They suggested that the enhanced electromagnetic field produced by the gold nanoparticles leads to the luminescence enhancement of QDs via increasing the excitation. The emission enhancement of metal-semiconductor nanoparticle assemblies due to the interparticle Coulomb interaction has also been observed.20,36 In the present system, there are no other particles except QDs. The enhancement definitely comes from the interaction between QDs. Actually, the electron or hole trapped on the QD surface can lead to a local electric field.37,38 As the interdot distance decreases, the Coulomb interaction between QDs increases, causing the local electric field around QDs to intensify, which induces a more effective QD excitation.2,22 Therefore, the locally enhanced electric field should be responsible for the photoluminescence enhancement. Note that the coherent super-radiance phenomenon or the Dicke effect might occur in an ensemble of identical emitters at small distances. In that case, a superemitter develops with an emission rate that is much higher than that of an individual emitter.39 However, such a phenomenon usually happens when the temperature is lower than that of liquid helium.40,41 In the present study, the temperature is far higher, so the Dicke effect cannot be taken into consideration. It has been suggested that electrons would transfer between neighboring dots without radiation as a result of the strong dipole-dipole interaction in close-packed arrays.25,26,42–47 At higher temperature, the aggregates become more compact, and the interdot distance is shorter. When the separation is less than 10 nm, the dipolar interactions between QDs play important roles. Accordingly, the electrons are expected to tunnel through the barriers and transfer in the QD aggregates. As a result, the fluorescence emission would be quenched.3 Note that nonradiative energy transfer may happen at very short interdot distances. We have also studied the effect of the length of PNIPAM chains on luminescence. Figure 6 shows the temperature dependence of the fluorescence intensity of PNIPAM2-QDs and PNIPAM3-QDs. In comparison with PNIPAM1-QDs, PNIPAM2QDs and PNIPAM3-QDs are much shorter. Clearly, the fluorescence intensity of either PNIPAM2-QDs or PNIPAM3QDs slightly changes with temperature. It is known that the scattering intensity is sensitive to aggregation. The inset shows the temperature dependence of the excess scattering intensity (Rvv(θ)/KC). Because Rvv(θ)/KC is proportional to Mw of the aggregates on the basis of eqs 1 and 2, it is sensitive to aggregation. Clearly, neither PNIPAM2-QDs nor PNIPAM3-QDs form aggregates even at temperatures higher than the LCST of PNIPAM. This is probably because PNIPAM2 or PNIPAM3 chains on CdS QD surfaces are too short to overlap. Note that the presence of polymer can sometimes cause the photoluminescence of QDs to increase as a result of surface passivation.48 (36) Matsumoto, Y.; Kanemoto, R.; Ishii, S.; Itoh, T.; Nakanishi, S.; Ishikawa, M.; Biju, V. J. Phys. Chem. C 2008, 112, 1345. (37) Norris, D.; Sacra, A.; Murray, C.; Bawendi, M. Phys. ReV. Lett. 1994, 72, 2612. (38) Klimov, V. I. J. Phys. Chem. B 2000, 104, 6112. (39) Dicke, R. H. Phys. ReV. 1954, 93, 99. (40) DeVoe, R. G.; Brewer, R. G. Phys. ReV. Lett. 1996, 18, 2049. (41) Blick, R. H.; Pfannkuche, D.; Haug, R. J.; Klitzing, K. V.; Eberl, K. Phys. ReV. Lett. 1998, 80, 4032. (42) Biju, V.; Makita, Y.; Sonoda, A.; Yokoyama, H.; Baba, Y.; Ishikawa, M. J. Phys. Chem. B 2005, 109, 13899. (43) Artemyev, M. V.; Bibik, A. I.; Gurinovich, L. I.; Gaponenko, S. V.; Woggon, U. Phys. ReV. B 1999, 60, 1504. (44) Ginger, D. S.; Greenham, N. C. Phys. ReV. B 1999, 59, 10622. (45) Schedelbeck, G.; Wegschneider, W.; Bichler, M.; Abstreiter, G. Science 1997, 278, 1792. (46) Do¨llefeld, H.; Weller, H.; Eychmu¨ller, A. J. Phys. Chem. B 2002, 106, 5604. (47) Do¨llefeld, H.; Weller, H.; Eychmu¨ller, A. Nano Lett. 2001, 1, 267.
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Figure 6. Temperature dependence of the fluorescence emission intensity (I) of PNIPAM2-QDs and PNIPAM3-QDs, where the emission wavelength (λ) is 425 nm. The inset shows the temperature dependence of the excess scattering intensity (Rvv(θ)/KC) of PNIPAM2-QDs and PNIPAM3-QDs.
Figure 7. Schematic illustration of the aggregation of PNIPAM-QDs.
In the present study, PNIPAM1-QDs, PNIPAM2-QDs, and PNIPAM3-QDs have similar structures; that is, they are all grafted with PNIPAM chains. The only difference is in the PNIPAM length. If the luminescence enhancement arose from the surface passivation effect, dielectric effect, or photoinduced effect, then all QDs should exhibit a luminescence enhancement. The fact that only QDs with the longest PNIPAM chains show such an enhancement indicates that the luminescence enhancement is not induced by the above effects. Instead, it is determined by the interdot interactions as a function of the interdot distance. Figure 7 illustrates the temperature-modulated aggregation and photoluminescence of PNIPAM-QDs.
Conclusions We have prepared cadmium sulfide quantum dots grafted with thermally responsive poly(N-isopropylacrylamide) chains and have investigated the temperature dependence of photoluminescence via fluorescence and laser light scattering. Our studies reveal that the fluorescence enhancement and quenching originate from the dipole-dipole interaction and a long-range interaction between QDs. The interactions vary with interdot distance. The results are helpful in understanding photophysical processes and molecular interactions. Acknowledgment. The financial support of the National Distinguished Young Investigator Fund (20474060) and the Ministry of Science and Technology of China (2007CB936401) is acknowledged. LA800312U (48) Biju, V.; Kanemoto, R.; Matsumoto, Y.; Ishii, S.; Nakanishi, S.; Itoh, T.; Baba, Y.; Ishikawa, M. J. Phys. Chem. C 2007, 111, 7924.
