J. Phys. Chem. 1996, 100, 4551-4555
4551
Photophysical Properties of ZnS Nanoclusters with Spatially Localized Mn2+ Kelly Sooklal, Brian S. Cullum, S. Michael Angel, and Catherine J. Murphy* Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: August 15, 1995; In Final Form: December 14, 1995X
The synthesis and photophysical characterization of nanometer-size ZnS with and without Mn2+ are reported. Without Mn2+, the ZnS nanoclusters emit in the blue upon ultraviolet excitation. ZnS doped with 1-5% Mn2+ (presumably in Zn2+ sites) yields the orange emission observed for bulk Zn:Mn phosphors but with greatly reduced emissive lifetimes. ZnS with surface-bound Mn2+, in contrast, emits in the ultraviolet with even shorter lifetimes. Thus, the physical location of Mn2+ in the ZnS nanocluster determines its optical properties.
Introduction Semiconductor nanoclusters (“quantum dots”) comprise a burgeoning area of materials science that has great potential for many optoelectronic applications.1-4 The extremely small size of these clusters results in quantum confinement of the photogenerated electron-hole pair, resulting in a blue shift in the absorption spectrum.1-4 The most well-studied semiconductor quantum dots are those of CdS and CdSe,1-21 which in the bulk have band gap energies (Eg) of ∼2.4 eV for CdS and ∼1.7 eV for CdSe.1 ZnS is also a semiconductor (Eg ∼3.6 eV) and is commercially used as a phosphor and also in thin-film electroluminescent devices, especially if doped with Mn2+.22-24 There have been far fewer studies of ZnS nanoclusters, most likely because their bulk band gap is already in the ultraviolet. Here we report a simple aqueous solution synthesis of very small ZnS particles with and without Mn2+. Colloidal solutions of ZnS without Mn2+ are photoluminescent in the blue, as is bulk material, although the absorption edge does blue shift as predicted for quantum confinement. We find that the addition of Mn2+ to the outside of preformed ZnS particles results in radically different photophysics than if the Mn2+ is introduced to the inside of ZnS nanoclusters, presumably in Zn2+ sites. These results emphasize the critical role the surface plays in the optical properties of these materials. Materials and Methods Na2S (Alfa), NaOH (Mallinckrodt), Zn(NO3)2‚6H2O (Aldrich), and Mn(NO3)2‚6H2O (Aldrich) were used as received. Deionized and purified water (Continental Water Systems) was used in all experiments. Electronic absorption spectra were collected with a Perkin Elmer 559A UV-vis spectrophotometer. Steady-state luminescence spectra were acquired with a SLM-Aminco 8100 spectrofluorometer, with excitation at 270 nm and 4 nm resolution. Transmission electron microscopy was carried out on a Hitachi H-8000 electron microscope; samples were prepared by placing a drop of the solution onto a nitrocellulosecopper grid and drying in a 70 °C oven for ∼1 h. The atomic composition of films was determined on a Hitachi S-2500∆ scanning electron microscope by X-ray energy-dispersive analysis. * To whom correspondence should be addressed. Email: murphy@ psc.psc.sc.edu. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4551$12.00/0
Emission lifetimes were acquired with a Q-switched Nd:YAG laser system (Quantel International model 580-20). The 266 nm harmonic was passed through a 266 nm band-pass filter (Acton Research Corp. Model 265-S-1D) and up through the bottom of the quartz cuvette containing the sample. The signal was collected by a Spex monochromator (model 1681), with slits set at 500 µm and a spectral resolution of 1.8 nm. The signal was detected by a Hammamatsu R2949 PMT and sent to a LeCroy 9350L digital sampling oscilloscope. To determine the lifetime of each sample, 1000 waveforms were averaged on the oscilloscope. The average waveform was then deconvoluted using Andre’s fast Fourier transform (FFT) method25 and fitted to single- or double-exponential decays. Electron paramagnetic resonance spectra were collected on a Varian E Line Century Series spectrometer operating at 9.420 GHz. The magnetic field was swept from 0 to 5000 G over a 4 min period at room temperature. Samples were placed in a quartz tube centered in the resonant cavity operating in the TE102 mode. Microwave power was adjusted from 40 to 70 mW depending on sample response. Synthesis of Colloidal ZnS. In a typical procedure, 5.9 mg (20 mmol) of Zn(NO3)2‚6H2O was added to 100 mL of degassed water. The pH was adjusted to 10.3 with 0.10 M NaOH (aqueous). Then 1.5 mg (19 mmol) of Na2S was added to the basic mixture, followed by rinsing with 2 mL of water. The solution was stirred for 10 min to ensure complete reaction. Synthesis of Mn2+-Activated ZnS. To 5 mL of 2.0 × 10-4 M ZnS solution as prepared above were added aliquots of a concentrated aqueous stock solution of Mn(NO3)2‚6H2O. Approximately 8-fold excess Mn2+ was required to achieve maximum emission enhancements. Synthesis of ZnS Doped with Mn2+ (ZnS:Mn). Aqueous stock solutions (2.0 mM) of Zn(NO3)2‚6H2O, Mn(NO3)2‚ 6H2O, and Na2S were freshly prepared and degassed before use. Zn2+ and Mn2+ solutions were mixed (0, 1, 2, 3, 4, and 5% Mn) for a total volume of 25.00 mL. The pH was adjusted to 10.3 as for ZnS alone, and 35 mL of the Na2S solution was added with stirring. Results ZnS. Preparation of ZnS nanoclusters is readily achieved in basic aqueous solution.26,27 Transmission electron microscopy (TEM) of dried films of 2.0 × 10-4 M ZnS solutions, 8 h after preparation, showed that particles were 51 Å ((20%) in diameter (average of 10 particles per image; Figure 1). X-ray electron-dispersive analysis of these films confirmed that the © 1996 American Chemical Society
4552 J. Phys. Chem., Vol. 100, No. 11, 1996
Sooklal et al.
Figure 3. Luminescence spectra of 0.2 mM ZnS as a function of time. (A) is taken after 1 h, (B) 2 days, (C) 8 days, and (D) 26 days.
TABLE 1: Photophysical Properties of ZnS Nanoclusters
Figure 1. Transmission electron micrograph of ∼50 Å ZnS particles. The length of the superimposed bar in the photograph corresponds to 670 Å.
absorption max (nm) (shoulder)
size by TEM (Å) (diameter, (20%)
calcd size (Å)a
τ at 435 nm (ns)
2.0 × 10-4
264
51
26
6.9 × 10-4
264
52
26
2.0 × 10-3 (solid prep) 2.0 × 10-3 (solution prep)
292
b
34
292
b
34
179.9 ( 1.3 16.3 ( 0.6 107.9 ( 0.5 23.6 ( 0.1 398.5 ( 2.7 63.0 ( 1.6 397.7 ( 2.5 63.6 ( 0.7
concentration (M)
a Size was calculated from the absorption spectrum using the effective mass model of Brus (see ref 4), assuming 0.25me as the effective mass of the electron in ZnS, 0.59me as the effective mass of the hole, and a dielectric constant of 8.3. b At this concentration, extensive aggregation of dried-down powders precluded meaningful measurements.
Figure 2. Absorption spectra of 0.2 mM ZnS as a function of time. The shoulder at A occurs at 264 nm after 1 h, B at 280 nm (19 h), C at 288 nm (4 days), and D at 288 nm (18 days).
particles were ZnS (atom % found: 46% S, 54% Zn). The absorption spectrum of freshly prepared nanoclusters at this concentration showed an onset at ∼320 nm, a shoulder at 260 nm, and a steep absorption edge at ∼250 nm, fully consistent with the quantum confinement effect1 (Figure 2). Freshly prepared nanoclusters were photoluminescent in the blue, with emission λmax ∼435 nm upon excitation at 270 nm, similar to what has been observed for large, micron-sized particles.28 The “solid” synthetic procedure (in which solid starting materials were dissolved together) resulted in products with higher photoluminescent intensities than those of the “solution” synthetic procedure (in which the reactants were aqueous stock solutions). This blue emission, for bulk ZnS, has been termed “self-activated” and is due to sulfur vacancies in the lattice; these vacancies produce localized donor sites which are ionized at room temperature to populate the conduction band.28 Thus, in the bulk the emission appears to result from band-gap or near-band-gap recombination. However, pure band-gap emission in our 50 Å ZnS nanoclusters would be in the ultraviolet, and we therefore suggest, by analogy with photophysical studies of CdS nanoclusters, that the emission is due to shallow traps acting as recombination centers for photogenerated charge carriers.14 The absorption and luminescence spectra of our ∼50 Å ZnS particles in basic aqueous solution change as a function of time (Figures 2 and 3), especially after 8 h. The red shift in the
absorption spectrum is most readily interpreted as nanocluster growth. Although TEM photomicrographs of dried films of aged solutions did indeed show particles up to 175 Å in diameter, a correspondence between larger diameter in the TEM and red shift in the absorption spectrum was not uniformly observed. One possible reason is that the preparation of particles for TEM, in which they are concentrated to dryness, causes them to aggregate, as has been seen by others.29 The aging behavior of colloidal ZnS solutions in methanol, where red shifts in the absorption spectra were uncorrelated with changes in particle size as judged by TEM, has previously been interpreted as a slow annealing of surface defects.29 A combination of both of these processes may contribute to the aging behavior we observe. Our lifetime measurements were therefore performed on freshlyprepared solutions containing particles whose size should be most closely reflected in the absorption spectra (see calculations in Table 1). The emission lifetimes of ZnS at different concentrations are given in Table 1. Biexponential fits to the emission decay yielded lifetime components on the order of 102 and 101 ns (Figure 4). Because there is a size distribution in our samples and the surface is certainly heterogeneous, we hesitate to interpret good fits to biexponential decays to mean that there are only two kinds of nanoclusters or two kinds of sites on nanoclusters. Other workers who have examined the photophysics of aqueous colloidal ZnS (∼40 Å diameter) have fit their time-resolved photoluminescence decays to the sum of three or four exponentials and obtain average lifetimes of 5-20 ns, including a long-lived (nearly microsecond) component.26 Given the likely nonuniform spatial distribution of electron traps and hole traps (from nanocluster to nanocluster) that serve as luminescence recombination centers, multiexponential behavior is not surprising.26 In micron-sized ZnS, Bard has observed
Properties of ZnS Nanoclusters
J. Phys. Chem., Vol. 100, No. 11, 1996 4553
Figure 6. Emission intensity of 0.2 mM ZnS as a function of pH.
Figure 4. Time-resolved luminescence decays upon excitation at 266 nm for 2.0 mM ZnS (top, monitored at 435 nm), 2.0 mM ZnS doped with 2% Mn2+ (middle, monitored at 591 nm), and 2.0 mM ZnS activated with excess Mn2+ (bottom, monitored at 390 nm). The biexponential fits (dashed lines) are superimposed upon the deconvoluted decays (solid lines). The insets in all three panels are the residuals.
Figure 5. Absorption shoulder energy of 0.2 mM ZnS as a function of pH.
multiexponential behavior on the nanosecond time scale for the blue emission.28 Taken together, these results suggest that particle size does not drastically affect radiative recombination rates in pure ZnS. We examined the effect of pH on ZnS nanocluster optical properties. We found that acidifying the original solution down to pH 3 shifted the shoulder in the absorption spectrum originally at ∼260 nm to longer wavelengths (Figure 5). This suggests that the nanoclusters are losing their probable Zn2+‚‚‚OHoverlayer and perhaps are aggregating into larger clusters. The emission spectrum was also sensitive to pH. While the wavelength maximum of emission was always ∼435 nm, the intensity was highest for pH ∼6 (Figure 6). This is to be contrasted with what Henglein has observed for CdS colloidal particles,8 where maximum emission was found to be at pH 9 or higher. This might be a reflection of the more acidic nature of aqueous Zn2+ compared to that of Cd2+. The pH dependence
Figure 7. Emission spectra of ZnS doped with 1-5% Mn2+.
we observe is very similar to what Bard has seen for the orange Mn2+-based emission in micron-sized ZnS:Mn particles.28 This suggests that the blue ZnS-based emission and the orange Mn2+based emission (see below) are intimately connected. ZnS:Mn. Bulk ZnS doped with Mn2+ is used extensively as a phosphor, yielding orange emission (λmax ∼585 nm) from the 4T1-6A1 transition of Mn2+.24,30 EXAFS and other techniques support the notion that Mn2+ substitutes for Zn2+ in the ZnS lattice.30 We found that both the 435 nm blue emission of ZnS and this orange Mn2+ emission were present at up to 2% Mn2+ doping in our ∼50 Å ZnS nanoclusters; the highest intensity emission at 435 nm was observed for 1% doping, while the highest emission intensity at 585 nm was observed at 2% doping (Figure 7). Both the blue and orange emission lifetimes were relatively insensitive to Mn2+ doping level (Table 2). We do not understand why 0.69 mM ∼50 Å ZnS particles would have luminescence lifetimes of ∼10 and ∼100 ns at 435 nm while 2.0 mM nanoclusters of apparently the same size have lifetimes of ∼60 and ∼400 ns at this wavelength (Table 2), but clearly the presence of Mn2+ in the lattice does not affect the lifetimes for a given batch. Mn2+-Activated ZnS. The addition of Mn2+ to the outside of preformed ZnS nanoclusters in basic aqueous solution did not have a significant effect on the absorption spectrum compared to free ZnS but had a remarkable effect on the photoluminescence (Figure 8). A peak at ∼350 nm grew in, with maximum intensity for an ∼8-fold excess of Mn2+ compared to Zn2+. The blue 435 nm emission was quenched considerably, both in intensity and lifetime, and also appeared to shift to ∼390 nm. Significantly, no Mn2+-based emission from the 4T1-6A1 transition was observed; we also did not observe any of this orange emission when Mn2+-activated ZnS solutions were allowed to age for ∼7 days at room temperature.
4554 J. Phys. Chem., Vol. 100, No. 11, 1996
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TABLE 2: Photophysical Properties of ZnS Nanoclusters with Mn2+ ZnS conc (M)
calcd size (Å)a
0% ZnS:Mn
6.9 × 10-4
26
1% ZnS:Mn
6.9 × 10-4
34
2% ZnS:Mn
6.9 × 10-4
35
5% ZnS:Mn
6.9 × 10-4
35
Mn2+-activated ZnS
6.9 × 10-4
29
0% ZnS:Mn
2.0 × 10-3
34
1% ZnS:Mn
2.0 ×
10-3
34
2% ZnS:Mn
2.0 × 10-3
35
5% ZnS:Mn
2.0 × 10-3
35
Mn2+-activated ZnS
2.0 × 10-3
34
sample
a
τ at 350 nm (ns)
13.0 ( 0.3
12.9 ( 0.2
τ at 390 nm (ns)
23.1 ( 0.2 5.5 ( 1.1
108.8 ( 0.5 4.2 ( 0.1
τ at 435 nm (ns)
τ at 590 nm (ns)
107.9 ( 0.5 23.6 ( 0.1 104.6 ( 0.2 13.0 ( 0.1 103.8 ( 0.6 9.3 ( 0.5 102.5 ( 0.5 7.7 ( 0.1
81.7 ( 0.2 6.5 ( 0.1 93.9 ( 3.1 6.4 ( 0.4 90.9 ( 2.1 4.5 ( 0.2
397.7 ( 2.5 63.6 ( 0.7 373.5 ( 1.8 85.0 ( 2.2 369.5 ( 2.2 55.0 ( 0.6 358.8 ( 1.4 57.3 ( 0.1
86.1 ( 0.4 6.5 ( 0.1 49.6 ( 0.5 3.9 ( 0.3 55.6 ( 0.9 4.7 ( 0.2
Size was calculated from the absorption spectrum using the effective mass model of Brus (see ref 4 and Table 1).
Figure 8. Activation of ZnS colloids with Mn2+, as followed by steadystate photoluminescence spectroscopy. (A) ZnS (0.2 mM, 5 mL) before activation. Addition of (B) 100 µL, (C) 310 µL, and (D) 810 µL of 0.2 mM Mn(NO3)2.
Discussion ZnS doped with Mn2+ is used extensively as a phosphor and in electroluminescent devices.22-24 In theory, spatially organized doping of ZnS with Mn2+ could result in improved photoluminescent and electroluminescent device efficiency.31 Under typical device preparation conditions, however, the elevated temperatures that promote crystallinity also cause any nonrandomly distributed Mn2+ ions to diffuse into random Zn2+ lattice sites.31 We examined these issues from the standpoint of nanocluster particles by introducing Mn2+ on the inside, and then solely on the outside, of ZnS particles.
We observed orange emission from ∼50 Å Mn2+-doped ZnS nanoclusters (that is, Mn2+ was introduced on the inside of the nanocluster). This Mn2+-based orange emission has been
observed in doped ZnS:Mn nanoclusters by others and is due to energy transfer from ZnS states to the 4T1-6A1 Mn2+-based transition.30 For 30 Å ZnS:Mn, these workers reported 20.5 and 3.7 ns components for the ∼590 nm emission lifetime, a dramatic shortening of the 1.8 ms lifetime observed in the bulk.30 The greatly shortened lifetime in the nanocluster is thought to be due to improved mixing of ZnS s and p states with the d states of the Mn2+ ion, relaxing the spin-forbidden nature of the Mn2+ transition.30 Our nanoclusters, somewhat larger, had longer lifetimes (Table 2), likely indicating that increased particle size results in decreased efficiency of such state mixing. Interestingly, our blue 435 nm emission lifetimes did not change significantly upon Mn2+ doping, and the corresponding intensity of the blue emission did not decrease linearly in response to increased orange emission intensity. This result does not agree with other work on micron-sized ZnS:Mn particles, in which the intensity of the orange emission increased at the expense of the blue emission.28 Remarkably, we observed ultraviolet emission for Mn2+activated ZnS nanoclusters (that is, Mn2+ introduced on the outside of the ZnS). The original blue emission itself shifts into the purple, and its lifetime is drastically shortened (Table 2). This result implies a substantial reorganization of the emitting states of these clusters that is promoted by surfacebound Mn2+ and is dramatically different from lattice-bound (“inside”) Mn2+:
We32 and others8 have seen similar behavior for CdS quantum dots activated with Cd2+, which has been interpreted as the passivation of sites of radiationless recombination.8 This seems to be a general feature of these quantum dots systems; the
Properties of ZnS Nanoclusters addition of divalent metal ions to the outside of nanoclusters quenches the red-shifted defect emission and efficiently promotes near-band-gap emission.33 We examined the EPR spectra of ZnS:Mn and Mn2+-activated ZnS colloidal solutions, in the hopes of distinguishing between the local environments of the Mn2+ as further support for our “inside” and “outside” concept. However, under our conditions (dilute aqueous solutions, room temperature), we were unable to obtain reproducible signals for either sample. Because the photophysical properties of ZnS nanoclusters with Mn2+ inside (doped) versus Mn2+ outside (activated) were so different, we thought that we could use the optical signatures of the Mn2+ sites to provide information on particle aggregation and/or Mn2+ diffusion into the nanocluster lattice. That is, if Mn2+ on the outside was to diffuse into the nanocrystallite and reside in the lattice, we might expect the ultraviolet emission to be reduced and orange emission to appear. Or, more likely at room temperature, if Mn2+ on the outside became buried on the inside as nanoclusters aggregated and grew, we also might expect the ultraviolet emission to be reduced and orange emission to appear. Aging studies (over a period of days at room temperature) with doped and activated ZnS-Mn2+ solutions, however, showed no shift in any of the emission peaks. Aggregation of our nanoclusters, as judged by precipitation from solution, typically occurred within 2 weeks, without any obvious spectral changes. Thus, the Mn2+ ion does not seem to be a good probe of Ostwald ripening in ZnS nanoclusters. Conclusions We have found that the location of Mn2+ in 50 Å ZnS nanoclusters profoundly affects the photophysics of the entire particle. Mn2+ substituted for Zn2+ in the ZnS lattice leads to the well-known Mn2+ (4T1-6A1)-based emission in the orange, with emission lifetimes that are best fit by a biexponential decay and are intermediate between those found for big (micron) nanoclusters and smaller nanoclusters. The addition of Mn2+ to the outside of preformed ZnS nanoclusters apparently stabilizes near-band-gap emission in the ultraviolet with even shorter lifetimes. Further studies are underway to examine the effect Mn2+ location has on the electroluminescent properties of thin films of these nanocrystals. Acknowledgment. C.J.M. and S.M.A. thank the University of South Carolina for funding. C.J.M. also thanks the National Science Foundation for a CAREER Award. We thank Dr. Dana Dunkelberger of the USC Electron Microscopy Center for experimental assistance. References and Notes (1) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (2) Weller, H. AdV. Mater. 1993, 5, 88.
J. Phys. Chem., Vol. 100, No. 11, 1996 4555 (3) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (4) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (5) Andres, R. P.; Averback, R. S.; Brown, W. L.; Brus, L. E.; Goddard, W. A., III; Kaldor, A.; Louie, S. G.; Moscovits, M.; Peercy, P. S.; Riley, S. J.; Siegel, R. W.; Spaepen, F.; Wang, Y. J. Mater. Res. 1989, 4, 704. (6) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 969. (7) Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. J. Phys. Chem. 1986, 90, 6074. (8) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (9) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 111, 530. (10) Haase, M.; Alivisatos, A. P. J. Phys. Chem. 1992, 96, 6756. (11) Yuan, Y.; Fendler, J. H.; Cabasso, I. J. Phys. Chem. 1992, 4, 312. (12) Hoffman, A. J.; Mills, G.; Yee, H.; Hoffmann, M. R. J. Phys. Chem. 1992, 96, 5546. (13) Chandler, R. R.; Coffer, J. L.; Atherton, S. J.; Snowden, P. T. J. Phys. Chem. 1992, 96, 2713. (14) Hasselbarth, A.; Eychmuller, A.; Weller, H. Chem. Phys. Lett. 1993, 203, 271. (15) Zhang, J. Z.; O’Neil, R. H.; Roberti, T. W.; McGowen, J. L.; Evans, J. E. Chem. Phys. Lett. 1994, 218, 479. (16) Choi, K. M.; Shea, K. J. J. Phys. Chem. 1994, 98, 3207. (17) Steigerwald, M. L.; Alivisatos, A. P.; Gibson, J. M.; Harris, T. D.; Kortan, R.; Thayer, A. M.; Duncan, T. M.; Douglass, D. C.; Brus, L. E. J. Am. Chem. Soc. 1988, 110, 3046. (18) Alivisatos, A. P.; Harris, A. L.; Levinos, N. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1988, 89, 4001. (19) Bawendi, M. G.; Wilson, W. L.; Rothberg, L.; Carroll, P. J.; Jedju, T. M.; Steigerwald, M. L.; Brus, L. E. Phys. ReV. Lett. 1990, 65, 1623. (20) Colvin, V. L.; Alivisatos, A. P. J. Chem. Phys. 1992, 97, 730. (21) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (22) ZnS phosphor: Garlick, G. F. J.; Gibson, A. F. J. Opt. Soc. Am. 1949, 39, 935. (23) Ohring, M. The Materials Science of Thin Films; Academic: San Diego, 1992. (24) McClean, I. P.; Thomas, C. B. Semicond. Sci. Technol. 1992, 7, 1394. (25) Andre, J. C.; Vincent, L. M.; O’Conner, D.; Ware, W. R. J. Phys. Chem. 1979, 83, 2285. (26) Dunstan, D. E.; Hagfeldt, A.; Almgren, M.; Siegbahn, H. O. G.; Mukhtar, E. J. Phys. Chem. 1990, 94, 6797. (27) ZnO is also a semiconductor that has been synthesized as quantum dots from Zn(II) and hydroxide precursors: Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826. In order to make sure that our synthetic procedure produces no ZnO, we eliminated the sodium sulfide in our preparation. On the basis of absorption and emission spectra, we do not form any semiconductor nanoclusters unless sulfide ion is present. (28) Becker, W. G.; Bard, A. J. J. Phys. Chem. 1983, 87, 4888. (29) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 553. (30) Bhargava, R. N.; Gallagher, D.; Hong, X.; Nurmikko, A. Phys. ReV. Lett. 1994, 72, 416. (31) Hunter, A.; Kitai, A. H. J. Appl. Phys. 1987, 62, 4244. (32) Mahtab, R.; Rogers, J. P.; Murphy, C. J. J. Am. Chem. Soc. 1995, 117, 9099. (33) Sooklal, K.; Murphy, C. J. Unpublished results on Cd2+, Zn2+, and Mn2+ activation of ZnS quantum dots.
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