Characterization of Covalently Immobilized Q-CdS Particles on Au

The immobilized Q-CdS particles were stable against tip scanning in scanning tunneling microscopy (STM). Tunneling spectroscopy (TS) of a single parti...
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Langmuir 1997, 13, 742-746

Characterization of Covalently Immobilized Q-CdS Particles on Au(111) by Scanning Tunneling Microscopy and Tunneling Spectroscopy with High Reproducibility Masahide Miyake,† Hajime Matsumoto,† Matsuhiko Nishizawa,† Takao Sakata,‡ Hirotaro Mori,‡ Susumu Kuwabata,† and Hiroshi Yoneyama*,† Department of Applied Chemistry, Faculty of Engineering, and Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Yamada-oka 2-1, Suita, Osaka 565, Japan Received July 16, 1996. In Final Form: November 26, 1996X Size-quantized CdS particles (Q-CdS) capped with 2-aminoethanethiol and 2-mercaptoethanesulfonate were prepared by means of the AOT/heptane inverse micelles method. The resulting CdS particles were covalently immobilized in a high dispersion on an Au(111) surface coated previously with a self-assembled monolayer of 3,3′-dithiobis(succinimidyl propionate). The immobilized Q-CdS particles were stable against tip scanning in scanning tunneling microscopy (STM). Tunneling spectroscopy (TS) of a single particle whose size was determined from a STM image allowed successfully the determination of the band-gap energy of the size-quantized particle, and the band-gap value obtained was in agreement with that predicted from the tight-binding approximation.

Introduction Semiconductor microcrystals (Q-particles) having the diameter smaller than 10 nm exhibit so-called quantum size effects due to confinement of charge carriers in a small space.1-3 Their photophysical and photochemical properties have been intensively studied1-15 because of intrinsic interest as well as their applicability to nonlinear optical devices10-12 and wet solar-cells.3,13,15,16 Recently, it has been found from studies on photoelectrochemical reactions on Q-particles that the size quantization favors enhancements in the rate of photoinduced reactions and/or the reaction selectivities depending on the size of Qparticle.3,6,9,16-20 The use of monodispersed Q-particles is desired in studies on the effect of size quantization. †

Department of Applied Chemistry. Research Center for Ultra-High Voltage Electron Microscopy. X Abstract published in Advance ACS Abstracts, January 15, 1997. ‡

(1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Kamat, P. V. Chem. Rev. 1993, 93, 267. (3) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (4) Haase, M.; Weller, H.; Henglein, A. J. Phys. Chem. 1988, 92, 4706. (5) Nosaka, Y. J. Phys. Chem. 1991, 95, 5054. (6) (a) Duonghong, D.; Ramsden, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1982, 104, 2977. (b) Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. 1982, 86, 2964. (7) Rossetti, R.; Brus, L. J. Phys. Chem. 1982, 86, 4470. (8) Zhang, J. Z.; O’Neil, R. H.; Roberti, T. W. J. Phys. Chem. 1994, 98, 3859. (9) (a) Matsumoto, H.; Uchida, H.; Matsunaga, T.; Tanaka, K.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem. 1994, 98, 11549. (b) Torimoto, T.; Maeda, K.; Maenaka, J.; Yoneyama, H. J. Phys. Chem. 1994, 98, 13658. (10) Chemla, D. S.; Miller, D. A. B. J. Opt. Soc. Am. B 1985, 2, 1156. (11) Schmitt-Rink, S.; Miller, D. A. B.; Chemla, D. S. Phys. Rev. B 1987, 35, 8113. (12) Uchida, H.; Matsunaga, T.; Yoneyama, H.; Sakata, T.; Mori, H.; Sasaki, T. Chem. Mater. 1993, 5, 716. (13) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (14) Reed, M. A. Sci. Am. 1993, 118. (15) (a) Mansur, H. S.; Grieser, F.; Marychurch, M. S.; Biggs, S.; Urquhart, R. S.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1995, 91, 665. (b) Mansur, H. S.; Grieser, F. J. Chem. Soc., Faraday Trans. 1995, 91, 3399. (16) Kamat, P. V. Chemtech 1995, 22. (17) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (18) (a) Hoffman, A. J.; Mills, G.; Yee, H.; Hoffmann, M. R. J. Phys. Chem. 1992, 96, 5546. (b) Hoffman, A. J.; Yee, H.; Mills, G.; Hoffmann, M. R. J. Phys. Chem. 1992, 96, 5540.

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However, since Q-particles used in various experiments had distributions in their size more or less, only qualitative information on the size quantization effect has been obtained in a rigid sense. Several kinds of attempts have been made so far to synthesize monodispersed microcrystals,21-23 but there has been still a size distribution in the prepared Q-particles. Scanning tunneling microscopy (STM) is a tool which allows observations of materials with resolution of an atomic level in vacuum, air, and solution,24-32 and tunneling spectroscopy (TS) is useful as a means of determination of electronic structures of metals and semiconductors.32-35 The electronic structure of a single particle can also be determined if the tip of a scanning tunneling microscope is fixed just on the upper part of the particle. So far, the TS studies have been carried out for Q-CdSe supported by a dioctadecyldimethylammonium (19) (a) Inoue, H.; Moriwaki, H.; Maeda, K.; Yoneyama, H. J. Photochem. Photobiol., A 1995, 86, 191. (b) Inoue, H.; Nakamura, R.; Yoneyama, H. Chem. Lett. 1994, 1227. (20) (a) Anpo, M.; Shima, T.; Kodama, S.; Kubokawa, Y. J. Phys. Chem. 1987, 91, 4305. (b) Anpo, M.; Chiba, K. J. Mol. Catal. 1992, 74, 207. (21) Murray, C. B.; Noriss, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (22) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmu¨ller, A.; Weller, H. J. J. Phys. Chem. 1994, 98, 7665. (23) Rajh, T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1993, 97, 11999. (24) Zen, J.-M.; Fan, F.-R. F.; Chen, G.; Bard, A. J. Langmuir 1989, 5, 1355. (25) Kim, Y.; McCarley, R. L.; Bard, A. J. Langmuir 1993, 9, 1941. (26) Zhao, X. K.; Xu, S.; Fendler, J. H. J. Phys. Chem. 1991, 95, 3716. (27) Gilbert, S. E.; Kennedy, J. H. J. Electrochem. Soc. 1988, 135, 2385. (28) Itaya, K.; Tomita, E. Chem. Lett. 1989, 285. (29) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794. (30) Facci, P.; Erokhin, V.; Tronin, A.; Nicolini, C. J. Phys. Chem. 1994, 98, 13323. (31) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995, 267, 367. (32) (a) Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1990, 94, 3761. (b) Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1991, 95, 1969. (c) Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1993, 97, 1431. (33) Uosaki, K.; Ye, S.; Sekine, N. Bull. Chem. Soc. Jpn. 1996, 69, 275. (34) Yang, R.; Symryl, W. H.; Evans, D. F.; Hendrickson, W. A. J. Phys. Chem. 1992, 96, 1428. (35) Hamers, R. J. In Scanning Tunneling Microscopy. I. General Principles and Applications to Clean and Adsorbate-Covered Surfaces; Gu¨nthrodt, H.-J., Wiesendanger, R., Eds.; Springer-Verlag: Berlin Heidelberg, 1992; Chapter 5.

© 1997 American Chemical Society

Covalently Immobilized Q-CdS Particles on Au(111)

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Scheme 1. Schematic Diagram of the Preparation of a Monolayer of Surface-Modified Q-CdS

bromide monolayer,36 for Q-CdS immobilized on a selfassembled monolayer of dithiol,37 and for electrodeposited Q-CdSe dots.38 In all these cases, a large amount of Q-particles were immobilized on flat substrates to give such a high surface density that it was not possible to directly correlate information obtained by TS with one single particle of a particular size. Rather, the size of the sample particle was estimated from the information obtained by TS. No study has been published on the successful determination of the band-gap of a semiconductor Q-particle of a known size by TS. The purpose of the present study was to accomplish this using Q-CdS chemically bound to a Au substrate in high dispersion. It was already reported that surface-modified Q-CdS particles dispersed on a self-assembled monolayer of dithiol were unstable against tip scanning in STM measurements,37 indicating the importance of stable immobilization of Q-particles for STM experiments. In the present study, the stable immobilization of Q-CdS particles onto a Au substrate was achieved by using the technique reported by Hodes for immobilization of gold particles onto Q-CdS,39 and the band-gap of Q-CdS particles of known size was successfully determined for the first time. Experimental Section Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) of reagent grade (Nakarai Tesque) and other chemicals of reagent grade (Wako Pure Chemical Industry) were used in the present study. Commercially available CdS powder particles (Nakarai Tesque) were used as a bulk CdS sample. Their average particle diameter was ca. 1 µm. Q-CdS was prepared in AOT/heptane inverse micelles.40 The inverse micelles were prepared by adding 2.0 cm3 of distilled water containing 7.0 g of AOT into 100 cm3 of heptane. Aqueous solutions of 1.0 M Cd(ClO4)2 (0.24 cm3) and 1.0 M Na2S (0.16 cm3) were, respectively, added to 60 and 40 cm3 aliquots of the prepared inverse micelle solution. After each solution was stirred individually for 1 h, they were mixed together and stirred for another 1 h, resulting in formation of Q-CdS in the inverse micelles. The surfaces of the resulting Q-CdS were modified both with 2-aminoethanethiol and with 2-mercaptoethanesulfonate. The modification with the latter compound was essential to dissolve the resulting particles into water in which immobilization of Q-CdS particles to a self-assembled monolayer (SAM) of 3,3′dithiobis(succinimidyl propionate) was conducted. Aqueous solutions of 0.32 M 2-aminoethanethiol (0.17 cm3) and 0.32 M 2-mercaptoethanesulfonate (0.33 cm3) were added to 100 cm3 of the inverse micelles solution containing Q-CdS and stirred for 1 day under N2 atmosphere, resulting in thiol-capped Q-CdS. Pyridine was then added to destroy the AOT inverse micelles. After drying under vacuum, the thiol-capped Q-CdS was washed

successively with pyridine, n-heptane, petroleum ether, 1-butanol, acetone, and methanol. Elemental analysis of the prepared Q-CdS revealed that the ratio of the surface concentration of the amino group to the sulfonate groups of the surface modifier was 0.4. Observations of Q-CdS particles were performed using a Hitachi H-9000 transmission electron microscope (TEM) at an operating voltage of 300 kV for samples prepared by the colloidal particles onto amorphous carbon overlayered on a Cu grid. Au(111) facets on Au balls of 1-2 mm diameter, prepared using the published procedures,29 were used as an electrode substrate and immersed in a DMSO solution containing 5 mM 3,3′-dithiobis(succinimidyl propionate) for overnight to form a self-assembled monolayer. After rinsing with water, the modified Au was immersed in 5 × 10-3 g/cm3 of surface-modified Q-CdS colloids for overnight to immobilize the Q-CdS particles through amido bonding of the amino groups of Q-CdS with the succinimidyl groups of the self-assembled monolayer (see Scheme 1).41 The immobilization of bulk CdS particles to the Au electrode substrate was made using the same technique after modification of the particle surface with both 2-aminoethanethiol and 2-mercaptoethanesulfonate. STM observations and TS measurements were performed using a STM apparatus (Nanoscope III, Digital Instruments, Inc., Santa Barbara, CA). The STM image was obtained by the constant current mode using a Pt-Ir tip with the bias voltage at 1.5 V (bias is that of the sample vs tip) and the tunneling current at 0.15 nA. On TS measurements, the distance between the tip and samples was first set at the bias voltage of 1.5 V and the tunneling current of 0.15 nA. After cutting the feedback control circuit, the tunneling currents were measured as a function of the bias voltage applied to the sample substrate against the tip. The sampling period of 100 µs was chosen for taking one I-V curve.

Results and Discussion Characterization and Immobilization of Q-CdS Particles. Figure 1 shows typical absorption spectra of 0.67 g L-1 of the surface-modified Q-CdS particles dispersed in water (a) and bulk CdS particles dispersed (36) Zhao, X. K.; McCormick, L. D.; Fendler, J. H. Langmuir 1991, 7, 1255. (37) Ogawa, S.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1995, 99, 11182. (38) Alperson, B.; Cohen, S.; Rubinstein, I.; Hodes, G. Phys. Rev. B 1995, 52, 17017. (39) Hodes, G. Isr. J. Chem. 1993, 33, 47. (40) (a) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, T. D.; Kortan, R.; Muller, A. J.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J. Am. Chem. Soc. 1988, 110, 3046. (b) 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. (41) Katz, E. Y. J. Electroanal. Chem. 1990, 291, 257.

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Figure 1. Absorption spectra of Q-CdS colloids in water (a) and bulk CdS particle colloids in pyridine (b). The amount of CdS particles was 0.67 g/L for both cases.

Figure 2. STM image of Au(111) surface modified with 3,3′dithiobis(succinimidyl propionate). Bias voltage, 1.5 V; current set point, 0.15 nA.

in pyridine (b). The absorption edge of the Q-CdS colloid was blue-shifted from that of the bulk CdS colloid, indicating an enlargement of the band-gap energy of Q-CdS caused by the quantum size effect.1-3 If the bandgap energy (Eg) of Q-CdS was estimated from the exciton peak observed at 410 nm, it was 3.0 eV, being 0.6 eV larger than the Eg of bulk CdS (2.4 eV2). However, it is recognized from the broad exciton peak observed that the prepared Q-CdS had a relatively large size distribution. Figure 2 shows a typical STM image of the Au facet covered with a self-assembled monolayer of 3,3′-dithiobis(succinimidyl propionate). The characteristic feature of the Au(111) plane, at which the monoatomic step lines are intersecting at an angle of 60°, is seen. Figure 3a is a STM image of the CdS-immobilized electrode obtained at the initial scanning. A number of protrusions having the size range between 2 and 3 nm are observed due to the immobilized Q-CdS particles. We obtained the image with a bias voltage of 1.5 V, which is thought to be large enough to induce electron tunneling, as explained later. Bard et al.37 have recently utilized STM to observe Q-CdS particles physically immobilized on a self-assembled monolayer of dithiol on Au, but particles were pushed to the outside of the area by repeating the tip scanning several times. On the other hand, the image shown in

Figure 3. STM images of Q-CdS particles immobilized on Au(111) obtained at the initial scan (a) and at the fifth scan (b). Bias voltage, 1.5 V; current set point, 0.15 nA.

Figure 3a was not changed at all by repeating the tip scanning 5 times or more, as shown in Figure 3b, indicating that the immobilized particles were very stable against tip scanning as a result of being covalently bound to the Au surface. It has been shown that STM can be used as a tool to determine the dimensions of metal particles31 and semiconductor particles37,42,43 immobilized on flat surfaces. To evaluate the accurate size of an individual particle, we focused the STM image on a single Q-CdS particle and determined its diameter from the cross-sectional view of the focused image. We conducted such a size-determination for 60 Q-CdS particles immobilized on the Au facets and obtained a histogram of the size distribution shown in Figure 4. In this figure, the size distribution obtained (42) Coury, J. E.; Pitts, E. C.; Shorrosh, R.; Felton, R. H.; Bottomley, L. A. J. Vac. Sci. Technol., B 1995, 13, 1167. (43) Tian, Y.; Fendler, J. H. Chem. Mater. 1996, 8, 969.

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Figure 4. Size distribution of Q-CdS particle obtained from STM images (filled bars) and from TEM images (shaded bars).

from high-resolution TEM pictures of particles before the immobilization is also shown. The average diameter of Q-CdS particles estimated by STM was 2.8 nm with a standard deviation of (0.32 nm, and that estimated by TEM was 2.6 nm with a standard deviation of (0.43 nm, giving fairly good accordance with each other. Band-Gap Estimation for a Single Q-CdS Particle by Tunneling Spectroscopy. The stable immobilization of Q-CdS allowed the determination of the exact bandgap energy of Q-CdS with high reproducibility by means of TS. To determine the band-gap energy of one particle of known diameter with confidence, the immobilization of Q-CdS particles in a highly dispersed state is desired. We realized such immobilization conditions by shortening the immersion time of the 3,3′-dithiobis(succinimidyl propionate)-modified Au substrate in the surface-modified Q-CdS aqueous colloid from overnight to 1 h. The STM image of the resulting sample is shown in Figure 5a. First, attempts were made to perform TS measurements of one particle given in a square of Figure 5a, which are given by Figure 5b. Prior to the determination of the band-gap energy, the size of this particle was determined to be 3.1 nm by employing the STM measurements as described above. Then the tip was positioned at the center of the particle, and the apparatus was changed into the TS mode. Figure 6 shows an obtained I-V curve for the Q-particle (a) and that for a bulk CdS particle (b). These I-V curves are different from that obtained for the Q-particle-free 3,3′-dithiobis(succinimidyl propionate)-modified Au substrate, where a symmetrical exponential curve characteristic of metals was obtained, indicating that the I-V profiles given in Figure 6 are indebted to the immobilized CdS particles. This means that there are electronic interactions between the immobilized CdS particles and the Au electrode substrate and between the CdS particle and the tip. Electron tunneling without any serious resistance may be a major process for such electronic interaction. The Au electrode substrate and the tip do not have any significant electronic interaction between them, because they are separated by the immobilized CdS particle. As shown in Figure 6, the I-V curve obtained for the Q-CdS particle was nearly symmetric against the potential axis, while that for the bulk CdS particle showed rectification, and no appreciable currents appeared with positive bias. Let it be discussed first why such a difference appeared between the I-V curves of the Q-CdS and the bulk CdS particle. There must be a fixed potential barrier between the immobilized CdS particle and the tip, which was polarized at a fixed potential during the course of TS measurements, because CdS makes a rectifying junction with noble

Figure 5. (a) STM image of Q-CdS particles on a Au(111) substrate prepared by immersing the Au substrate coated with a monolayer of 3,3′-dithiobis(succinimidyl propionate) into the surface-modified CdS particle aqueous colloid for 1 h, and (b) enlargement of one particle image given in a square of part a.

metals44 such as Pt, Pd, and Au which have large work functions. Accordingly it is unlikely that electrons are transferred from the tip to the conduction band of the immobilized CdS particle. Appreciable currents should start to flow if the potential of the Au substrate becomes more negative than that of the conduction band and more positive than that of the valence band of the immobilized CdS particle. In the former case electrons are transferred from the Au substrate to the tip through the conduction band of the immobilized CdS particle, and in the latter electrons are transferred from the tip to the Au substrate through the valence band of the CdS particle. Such a situation is certainly seen for the Q-CdS particle. However, the rectification in the I-V curve appeared at the bulk CdS particle. The difference in the I-V behaviors (44) Drillson, L. J. Phys. Rev. Lett. 1978, 40, 260.

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Figure 6. Current-voltage curves obtained by TS measurements for a Q-CdS particle (a) and a bulk CdS particle (b) immobilized on Au(111).

Figure 7. Differential conductance (dI/dV) spectra obtained from I-V curves of a 3.1 nm Q-CdS particle. The solid curve and dashed curve were taken at different times for the same particle (15 s interval). The length of the arrow shows the band-gap energy determined from these spectra.

between the Q-CdS and the bulk CdS particle seems to result from the contribution of space charge effects which are remarkable at the bulk CdS but not at the Q-CdS, whose size is too small for any remarkable space charge layer to be built up.45 When the potential of the Au substrate is made more positive than that of the conduction band of the bulk CdS particle, electrons are transferred from the particle surface to the Au substrate, making an upward bending of the band at the CdS particle surface facing the Au substrate. Such band bending becomes more marked with increasing positive shifts of the potential of the Au substrate. The space charge layer formed in this way does inhibit the electron flow from the immobilized CdS particle to the Au substrate, due to the electric field of the space charge layer. In this way, no appreciable currents appeared with polarization up to 3 V. The band-gap of Q-CdS particles can be determined on the basis of a potential region where no appreciable current flow is observed.37,38 To determine precisely the bandgap value, differential conductance (dI/dV) spectra were obtained for a 3.1 nm CdS particle. Results are given in Figure 7 for two different measurement times. The value of tunneling currents was varied a little with time as recognized from the figure, due probably to some fluctuation of the immobilized Q-CdS particle and/or to changes in conditions of the particle, the STM tip, and so on, whereas the potential region where almost null conductance appeared was not influenced by the measurement times and was eventually invariant. As shown in the (45) Albery, W. J.; Bartlett, N. J. Electrochem. Soc. 1984, 131, 315. (46) Liver, N.; Nitzan, A. J. Phys. Chem. 1992, 96, 3366.

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Figure 8. Band-gap energies of Q-CdS particles determined by TS as a function of the size of the particles determined by STM. The dashed curve and solid curve are theoretically derived ones based on tight-binding approximations with and without taking into consideration of the Coulombic interaction energy, respectively.47

figure, remarkable fluctuations in dI/dV are seen for sample voltages above 1.4 V and below -1.5 V. We assumed that the band-gap is equal to the potential difference between these two threshold voltages, beyond which significant fluctuations of dI/dV appeared. In this way, the band-gap of 2.9 eV was obtained for Q-CdS of 3.1 nm. The TS measurements were carried out for other Q-CdS particles of different sizes of 2.2, 2.6, and 3.6 nm, and their band-gap energies were determined using the same techniques. Figure 8 shows the plots of Eg values obtained as a function of particle sizes. The results obtained here are in good agreement with the theoretical predictions made by tight-binding approximations,47 reported by Lippens and Lannoo for Q-CdS, where no Coulombic interaction of electrons and holes was taken into consideration. A theoretically predicted relation which takes the Coulombic interaction into account47 is also given in Figure 8 by a dotted curve. Apparently the dotted curve deviates a little from the present experimental results. The dotted curve must be useful in cases where Eg values are determined from absorption spectra of Q-CdS particles.9a,37,38,48-50 In the measurements of absorption spectra, electrons and holes are generated when noticeable absorption occurs, and photogenerated electrons and holes must interact Coulombically with each other. Then, the use of the tight-binding approximations including the Coulombic interaction is rationalized. Anyway, the present TS studies have made the first success in determining Eg of Q-CdS as a function of the particle size by chemically immobilizing the particles onto a Au electrode substrate. Acknowledgment. This work was supported by Grant-in-Aid for Scientific Research No. 07455340 and for Priority Area No. 06239110 from the Ministry of Education, Science, Culture and Sports. LA960702V

(47) Lippens, P. E.; Lannoo, M. Phys. Rev. B 1989, 39, 10935. (48) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136. (49) (a) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (b) Bowers, C. R.; Pietrass, E.; Barash, E.; Pines, A.; Grubbs, R. K.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 9400. (50) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706.