Aggregation Kinetics of Dendrimer-Stabilized CdS Nanoclusters

Langmuir 2000, 16, 2621-2626

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Aggregation Kinetics of Dendrimer-Stabilized CdS Nanoclusters Leo H. Hanus,† Kelly Sooklal,‡ Catherine J. Murphy,‡ and Harry J. Ploehn*,§ Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716; Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208; and Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208 Received October 26, 1998. In Final Form: September 27, 1999

Precipitation of CdS in Starburst PAMAM dendrimer solutions offers a viable method of preparing CdS nanoclusters with long-term optical stability. Photoluminescence and UV-visible spectroscopy data show that the dendrimer stabilizes the precipitated CdS as nanoclusters with characteristic optoelectronic properties not seen in bulk CdS. However, photon correlation spectroscopy (PCS) indicates power-law growth of aggregates composed of dendrimer-stabilized nanoclusters. Temperature and reactant concentration are dominant variables controlling the aggregation kinetics. Power-law growth of the aggregate diameter and the intensity count rate suggest that the aggregation kinetics are diffusion-limited, at least at higher temperatures and reactant concentrations. The termination of the aggregation process and the stability of the final aggregates are confirmed by the absence of bulk CdS precipitate several months after synthesis. Photoluminescence and UV-visible spectroscopy link the optical properties of the aggregates to those of the CdS nanoclusters formed immediately after the initiation of the synthesis process. This substantiates that the CdS nanoclusters remain as distinct entities within the dendrimer aggregates, presumably stabilized against further coalescence by the dendrimers.

Introduction 1,2

3,4

Cadmium sulfide and metal nanoclusters have unique chemical, physical, and optoelectronic properties that make them potentially useful in microelectronic,5 catalytic,6 and bioanalytical7 applications. The interest in these applications has created an impetus for developing synthesis methods capable of creating colloidally stable nanosized particles. Many approaches2,5,8 have been attempted, including synthesis in anionic polymers,9 membranes,10 micelles,11 porous glass,12 and zeolites,13 but the results have been mixed with respect to control of particle size and optoelectronic properties. * Corresponding author: (803) 777-7307 telephone, (803) 7778265 fax, [email protected] e-mail. † University of Delaware. ‡ Department of Chemistry and Biochemistry, University of South Carolina. § Department of Chemical Engineering, University of South Carolina. (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Weller, H. Adv. Mater. 1993, 5 (2), 88. (3) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, T. A. Langmuir 1996, 12, 2, 1172. (4) Dagani, R. Chem. Eng. News 1999, February 8, 33. (5) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (6) Bo¨nnemann, H.; Braun, G.; Brijoux, W.; Brinkmann R.; Schulze Tilling, A.; Seevogel K.; Siepen K. J. Organomet. Chem. 1996, 520, 143. (7) Mahtab, R.; Rogers, J. P.; Singleton, C. P.; Murphy, C. J. J. Am. Chem. Soc. 1996, 118, 8, 7028. (8) Sookal, K.; Hanus, L. H.; Ploehn, H. J.; Murphy, C. J. Adv. Mater. 1998, 10 (April), 1083. (9) Smotkin, E. S.; Brown, R. M.; Rahenberg, L. K.; Salomon, K.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, E. S.; White, J. M. J. Phys. Chem. 1990, 94, 7543. (10) Zhao, X. K.; Barai, S.; Orlando, R.; Fendler, J. H. J. Am. Chem. Soc. 1988, 110, 1012. (11) Pileni, M. P. Adv. Colloid Interface Sci. 1993, 46, 139. (12) Mathieu, H.; Richard, T.; Allegre, J.; Lefebvre, P.; Arnaud, G.; Granier, W.; Boudes, L.; Mrc, J. L.; Pradel, A.; Ribes, M. J. Appl. Phys. 1995, 77, 287. (13) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 11, 350.

We have explored a new synthesis route: arrested precipitation14 of CdS nanoclusters in solutions containing polyamidoamine (PAMAM) dendrimers15,16 (Aldrich Starburst). There has been considerable interest in the general properties of these dendrimers,16,17,18 especially in their use as stabilizers in the synthesis of metal nanoclusters.4,19,20,21 Our previous publication8 reported the effect of dendrimer functionality (-NH2 vs -COOH), concentration, and architecture, as well as solvent type (H2O vs MeOH) and pH, on the size and optoelectronic properties of the product CdS nanoclusters. Data from photoluminescence and UV-visible spectroscopy suggests that the synthesis mechanism of CdS nanoclusters is similar to that of dendrimer-stabilized metal nanoclusters.4,19,20,21 Addition of Cd2+ ions to dendrimer solution produces a Cd-dendrimer complex. Reaction with S2- ions produces CdS nanoclusters. The absence of precipitate and stability of the optical properties suggest that the dendrimers provide the nanoclusters with long-term stability against coalescence into bulk CdS. Despite the apparent stability of the nanoclusters against coalescence, photon correlation spectroscopy (PCS) indicates the presence of larger particles that grow over a period of hours to days. We believe that these larger particles are weak aggregates of dendrimer-stabilized CdS nanoclusters that form due to the clusters’ Brownian motion and mutual van der Waals attraction. Data from a recent small-angle X-ray and neutron scattering (SAXS (14) Fendler, J. H.; De´ka´ny, I. Nanoparticles in Solids and Solutions; Kluwer: Amsterdam, 1996; Vol. 18, pp 131-153. (15) Dendritech: http://www.mmi.org/mmi/dendritech. (16) Matthews, O. A.; Shipway, A. N.; Stoddart, J. F. Prog. Polym. Sci. 1998, 23, 1. (17) Dagani, R. Chem. Eng. News 1996, June 3, 30. (18) Boris, D.; Rubinstein, M. Macromolecules 1996, 29, 7251. (19) Service, R. F. Science 1999, 283, 165. (20) Tomalia, D. A.; Balogh, L. J. Am. Chem. Soc. 1998, 120, 7355. (21) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877.

10.1021/la981505u CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000

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and SANS) study22 of dendrimer-stabilized CuS in D2O supports this view. For the dendrimer-CuS synthesis product after 10 h, scattered intensity vs scattering vector data (I vs q) from SAXS (Figure 6 in ref 22) for has two distinct linear regions with slopes of approximately -2 (low q) and -4 (high q) with the crossover point at q ≈ 0.05 Å-1. This crossover corresponds to a single particle radius of gyration of a ) q-1 ≈ 20 Å, equal to that of the dendrimer. The slope in the low q region corresponds to aggregates having a fractal dimension df near 2, based on23 I ∝ S(q) ∝ q-df. The slope in the high q region confirms Porod behavior [I ∝ P(q) ∝ q-4].24 These results suggest that static or dynamic light scattering may provide useful information about the fractal structure of supermolecular aggregates formed by the association of dendrimers carrying metal nanoclusters. This paper reports the use of PCS to characterize the Brownian aggregation kinetics25,26 of dendrimer-stabilized CdS nanoclusters by tracking the apparent aggregate size as a function of time, temperature, and reactant concentration. Photoluminescence and UV-visible spectroscopy yield the optoelectronic properties of the CdS-dendrimer aggregates over the same time period. Theory Brownian Aggregation. Colloidal aggregation requires two elements: a net interparticle attractive force and a flow or Brownian motion that brings particles into proximity. An aggregate consists of two or more particles associated in this way. Experimental studies and computer simulations27,28,29 indicate that Brownian aggregates have a fractal structure characterized by a noninteger fractal exponent. The aggregate mass M and number of single particles per aggregate N are proportional to the aggregate radius of gyration Rg through

M∝N∝

( ) Rg a

df

(1)

where a is the radius of a single constituent particle and df is the fractal dimension of the aggregate (1 e df e 3). Two universal limiting regimes of Brownian aggregation have been identified: diffusion-limited colloidal aggregation (DLCA), and reaction-limited colloidal aggregation (RLCA). In DLCA,27 strong attraction binds particles as soon as their Brownian motion brings them into contact. Since particles are more likely to be incorporated into the periphery of a growing cluster under DLCA conditions, the fractal dimension df has a relatively low value of about 1.8.30 In contrast, RLCA occurs when a repulsive barrier limits the rate of aggregation. Since particles have a greater chance of reaching the interior of a growing cluster in this case, df for RLCA has a larger value of about 2.1. (22) Tan, N. C. B.; Balogh, L.; Trevino, S. F.; Tomalia, D. A.; Lin, J. S. Polymer 1999, 40, 2537. (23) Schaefer, D. W.; Martin, J. E.; Wiltzius, P.; Cannell, D. S. Phys. Rev. Lett. 1984, 52, 2371. (24) Amal, R.; Raper, J. A.; Waite, T. D. J. Colloid Interface Sci. 1990, 140(1), 158. (25) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University: New York, 1989; Chapter 8. (26) Kyriakidis, A. S.; Yiatsios, S. G.; Karabelas, A. J. J. Colloid Interface Sci. 1997, 195, 299. (27) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin P. Nature 1989, 339, 360. (28) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Klein, R.; Ball, R. C.; Meakin P. J. Phys.: Condens. Matter 1990, 2, 3093. (29) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin P. Phys. Rev. A 1990, 41, 2005. (30) The java applet at http://polymer.bu.edu/java provides an information picture of DLCA aggregate growth.

For DLCA and RLCA universality to hold, aggregate formation must be irreversible29 (i.e., the aggregates must not restructure internally once formed). Photon Correlation Spectroscopy. Brownian aggregation kinetics and aggregate structure have been investigated using photon correlation spectroscopy (PCS). Generally, the aggregates’ hydrodynamic radius R h PCS is measured as a function of time t. DLCA produces powerh PCS) ∝ log(t)], while RLCA law aggregate growth28 [log(R h PCS) ∝ t]. For yields exponential aggregate growth29 [log(R DLCA, regression of the double-log plot produces a slope and intercept related to df and the aggregation rate, respectively. Several factors complicate the extraction of these parameters from PCS data, including conversion of the measured, intensity-weighted R h PCS to a number-weighting (equivalent to a correction for particle polydispersity), correction for the effects of particle rotation, and conversion of R h PCS to Rg. Lin et al.28,29 constructed master curves for performing these corrections, showing experimentally and confirming theoretically the universal behavior of DLCA and RLCA. This work shows that the proportionality h PCS , 1 or between R h PCS and Rg is constant as long as qR qR h PCS . 1 (q representing the magnitude of the scattering vector). Consequently, PCS may be used to characterize the aggregation kinetics by simply plotting R h PCS as a function of time, using R h PCS as a surrogate for Rg. However, caution must be exercised because the conversion between h PCS = 1. R h PCS and Rg is not constant when qR For DLCA kinetics, the concentration of aggregates C at time t can be modeled using Smoluchowski theory,25,31,32 leading to

C ) C0

1 1 ) t kSC0t 1+ 1+ tB 2W

(2)

where C0 ) 3φ0/4πa3 is the initial single particle (monomer) concentration, kS ) 8kBT/3η is the aggregation rate constant, kB is Boltzmann’s constant, T is absolute temperature, η is the solvent viscosity, W is the stability ratio,25 and tB ) W/2kSC0 ) 6ηW/8kBTC0 ) ηπa3W/φkBT is the Brownian aggregation time.31 Since C0/C equals the number of monomers N in an aggregate, eqs 1 and 2 can be used to relate Rg to the aggregation kinetics, giving

N)

( )

kSC0t Rg C0 )1+ ) C 2W a

df

(3)

For t/tB . 1 (i.e., a large number of particles within the aggregate), we have

(

) ( )

kSC0t Rg ) 1+ a 2W

1/df

≈

kSC0t 2W

1/df

(4)

and

log

( )

( )

()

Rg kSC0t t 1 1 ≈ log ≈ log a df 2W df tB

(5)

Thus, a plot of log(Rg) vs log(t) yields a slope of 1/df and an intercept of log(a) - 1/df log(tB). The kinetic rate constant kS/2W can be determined from the latter. The plot suggested by eq 5 requires Rg data, but PCS produces values of the intensity-weighted average radius, (31) Zhang, J.; Buffle, J. J. Colloid Interface Sci. 1995, 174, 500. (32) Virden, J. W.; Berg, J. C. J. Colloid Interface Sci. 1992, 149(2), 528.

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R h PCS. More specifically, single exponential or quadratic cumulant fits of the intensity autocorrelation function measured by PCS yield values of the intensity-average decay constant Γ h . The effective diffusion coefficient follows from Γ h ) q2Deff, where q ) 4πnλ sin(θ/2) is the scattering vector, n is the solvent refractive index, θ is the scattering angle, and λ is the laser wavelength in a vacuum. Finally, the Stokes-Einstein equation gives

R h PCS )

kBT 6πηDeff

)

q2kBT 6πηΓ h

)

(2θ)

πkSn2 sin2 λ2Γ h

(6)

As mentioned earlier, for small and large values of qR h PCS, the R h PCS to Rg conversion factors are constant. Experimental Section Synthesis Procedures. Details of the synthesis procedures and stock solutions used to synthesize CdS particles in dendrimer solution were described in more detail previously.8 A stock solution of amine dendrimer (Aldrich, generation-4.0, #41 2449) was prepared by diluting 2 mL of the as-received solution (10 wt % in methanol) in 100 mL of ice-cold methanol (Fisher, A412, absolute, certified ACS). The Cd2+ stock solution (2.0 mM) consisted of 0.062 g of Cd(NO3)2‚4H2O (Aldrich, #24 751-0) diluted in 100 mL of methanol. The S2- stock solution (2.0 mM) consisted of 0.0156 g of anhydrous Na2S (Alfa Aesar, #11664) diluted in 100 mL of methanol. The samples labeled as “5L” were prepared by adding a 2.5 mL aliquot of the Cd2+ stock solution to 10.0 mL of the dendrimer solution, followed by addition of a 2.5 mL aliquot of the S2- stock solution. The samples denoted as “10L” were prepared the same way except that 5.0 mL aliquots of Cd2+ and S2- stock solutions were used. In contrast to these “one-shot” syntheses, the samples labeled “10LI” were prepared using an incremental addition procedure similar to that described previously.8 In the incremental procedure, 0.5 mL aliquots of Cd2+ and S2- stock solutions were added alternately to 10.0 mL of dendrimer stock solution. A total of 10 additions (each consisting of one Cd2+ aliquot followed by one S2- aliquot, followed by brief mixing) were made at 2 min intervals. Photon Correlation Spectroscopy. A Brookhaven Instruments33 light scattering system with BI-200SM goniometer, BI9000AT digital correlator, and Lexel-95A argon-ion laser was used for the PCS measurements. The system was operated at a scattering angle of 90°, a PMT pinhole of 400 µm, a laser wavelength of 514.5 nm, and a laser intensity of between 50 and 100 mW. The synthesis reactions and analysis were carried out inside the sample cell (BI-RC27) of the instrument. Reactions were conducted at controlled temperatures of 0, 10, and 20 °C with methanol as the suspending solvent in all cases. Autocorrelation functions (ACFs) were recorded every minute for 2 h beginning immediately after the addition of S2- to the reaction mixture. The ACFs were fit with a single exponential (SE) model using nonlinear regression to determine Γ h and thus R h PCS. For the modeling approach used, the baseline of the ACF was treated as a fitting parameter rather than a measured value. In the sizing calculations, we used methanol viscosity values of 0.820, 0.686, and 0.586 cP (at 0, 10, and 20 °C, respectively), and a refractive index of approximately 1.326 (based on a curve fit of the data in ref 34). Average particle sizes in the dendrimer stock solution and the final products (days after synthesis) were determined from averages of five to 10 measurements, each representing 5 min of data collection at 20 °C. UV-Visible and Photoluminescence Spectroscopy. Electronic absorption spectra were acquired using a Perkin-Elmer Lambda 14 ultraviolet-visible spectrophotometer. Steady-state (33) Brookhaven Instruments Corporation (www.bic.com): Holtsville, New York, 1993. (34) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics; 77th ed.; CRC Press: New York, 1996.

photoluminescence spectra were obtained with an SLM-Amino 8100 spectrofluorometer with excitation at 340 nm and 4 nm resolution.

Results and Discussion Aggregation Mechanism. Figure 1 displays the time dependence of the average aggregate diameter and intensity count rate (ICR) obtained from PCS measurements of the 5L and 10L dendrimer-stabilized CdS syntheses at reaction temperatures of 0, 10, and 20 °C. For comparison, the average PCS-measured diameter of the generation-4.0 PAMAM dendrimer in its stock solution (methanol) was 4.5 ( 1.0 nm. The known molecular structure15 of this dendrimer also leads to a diameter of 4.5 nm. The results in Figure 1 indicate that aggregate growth is a strong function of reaction temperature with a weaker dependence on the reactant concentrations. At a fixed point in time (e.g., 120 min), the aggregate size increases with increasing T, decreasing η, and increasing C0. Assuming DLCA, eq 5 with tB ) 6ηW/8kBTC0 shows that these observations are all consistent with Brownian aggregation: at fixed t, Rg should increase with 1/tB and thus with increasing T, increasing C0, and decreasing η. As T and C0 increase [e.g., for 5L(20 °C), 10L(10 °C), and 10L(20 °C)], the aggregate diameter begins to exhibit power law growth. This can be clearly seen in the developing linearity of the plots in Figure 2 (power-law plots of the data in Figure 1) with increasing T and C0. The linearity of these plots, along with the increase in Rg (at fixed t) with increasing T, C0, and 1/η, suggests that the aggregation of dendrimer-stabilized CdS follows DLCA kinetics. For the lower values of T and C0 [e.g., for 5L(0 °C), 5L(10 °C), and 10L(0 °C)], the aggregation kinetics are slow and the light-scattering data manifest significant fluctuations. Although the causes of the fluctuations are uncertain, the intensity count rate (ICR, Figure 1) of the detected light offers a possible explanation. For the slow aggregation conditions (i.e., lower T and C0), the ICR remains low and unchanged from t ) 0 because the small primary particles scatter most of the light. Autocorrelation functions based on relatively low ICRs are more susceptible to statistical errors, leading to apparent fluctuations in the sizing results. The ICR increases when significant numbers of aggregates begin forming because the scattered intensity is directly proportional to the square of particle volume and thus Rg6. At the same time, the aggregate diameter curves become smoother. The mode of addition of Cd2+ and S2- does have an influence on aggregate growth. Figure 3 shows particle diameter and ICR vs time (on a log-log plot) for “incremental” synthesis 10LI(0 °C) and “one-shot” synthesis 10L(0 °C). For the 10LI synthesis, data acquired during the first 20 min should be discounted due to spurious scatter created by mixing turbulence. After the mixing period ends, the 10LI sample manifests powerlaw growth in particle diameter and ICR, indicative of DLCA kinetics. Thus, the aggregation mechanism (DLCA) does not depend on how the reactants are added. However, at the same elapsed time, the ICR and aggregate diameter in the 10LI sample are significantly greater than in the 10L sample. This may be due to enhancement of the aggregation rate by shear flow associated with repeated mixing of the 10LI sample. Despite this difference in the aggregation rate, the product of “one-shot” and “incremental” synthesis procedures have similar optical properties (Table 1) as discussed below. Fractal Dimensions. The slopes of the power-law plots of R h PCS and ICR (Figure 2) produce aggregate fractal

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Figure 1. Aggregate diameter and intensity count rate for the 5L and 10L samples as a function of temperature and time.

Figure 2. Power-law plots of the aggregate diameter (lower three curves) and intensity count rate (upper three curves) as functions of time for samples 5L(20 °C), 10L(10 °C), and 10L(20 °C). Table 1. Physical Characteristics of Dendrimer-Stabilized CdS Nanocluster Aggregates Measured Several Days after Synthesis temp (°C)

emission observed?

intensity at λ ) 460 nm

diameter (nm)

5L

0 10 20

yes yes yes

1400 855

90 ( 63 162 ( 42 12 ( 0.3

10L

0 10 20

yes yes yes

1500 700 600

77 ( 1 237 ( 3 75 ( 1

10LI

0 20

yes yes

400

208 ( 4

sample

dimensions (via eq 5) that are much greater than the df ) 1.8 expected for DLCA. There are at least two reasons for this. First, the short experiment duration (2 h) may not have permitted the growth of aggregates large enough

Figure 3. Power-law plots of the aggregate diameter (+) and intensity count rate (s) as a function of time for the “one-shot” (10L) and “incremental” (10LI) samples in methanol at 0 °C.

to show self-similar fractal structure. Assuming dendrimer and aggregate diameters of 4.5 and 20 nm, and 60 vol % packing of dendrimers in the aggregate, we estimate N ) 52. The actual value of N is probably less [according to eq 3] if df ) 1.8. Second, for 4.5 nm < 2R h PCS < 20 nm and the scattering conditions used in our experiments (q ) 0.023 h PCS < 0.23. Since qR h PCS is not nm-1), we have 0.05 < qR that much less than 1, corrections between R h PCS and Rg may be significant. A better estimate of df could be obtained by increasing the experiment duration or accelerating the aggregation process by increasing T or C0. This would increase qR h PCS. Alternately, small angle light scattering or SAXS could be used to reduce q and thus qR h PCS. Figure 3 shows that the aggregates in the 10LI(0 °C) synthesis become relatively large after 2 h. Assuming an aggregate diameter of 100 nm, 60% packing of dendrimers implies an average aggregation number of 6600. Unfortunately, qR h PCS ) 1.15 in this case, so corrections between R h PCS and Rg are expected to be significant. Nevertheless,

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Figure 5. UV-visible absorbance for sample 10L(0 °C) and photoluminescent emission for sample 5L(20 °C).

roughly 2.8 nm. The observed shapes of the absorption and photoluminescent emission spectra for the other samples listed in Table 1 imply that similar CdS cluster sizes are expected. In contrast, PCS measurements show that the aggregates in solution have much larger diameters. This provides a clear indication that the optical properties of these materials depend on the presence of small CdS nanoclusters and that these clusters are not free in solution but are bound in supermolecular dendrimer aggregates having much larger dimensions. Conclusions Figure 4. Photoluminescent intensity (s) and aggregate diameter (+) as a function of time for 10L samples synthesized at 0 °C and 10 °C.

assuming a rotational correction factor of Rg/R h PCS based on that for colloidal gold,28 the slope of the power-law plot for 10LI(0 °C) in Figure 3 yields df ) 1.95 ( 0.01. This result suggests that this technique yields reasonable fractal dimensions, although more careful analysis to correct for rotational effects28,29 is certainly merited. Optical Properties. In contrast with the relatively slow aggregation observed via light scattering, photoluminescence measurements reveal that the samples manifest strong photoluminescent emission as soon as the Cd2+ and S2- solutions are mixed. Figure 4 shows the photoluminescent intensity (at 460 nm) as a function of time for samples 10L(0 °C) and 10L(10 °C). Both samples fluoresced strongly from the beginning of the syntheses due the immediate formation of CdS nanoclusters. The photoluminescence remained strong during the subsequent aggregation process. These observations indicate that the formation of the nanoclusters is fast and effectively independent of the subsequent dendrimer aggregation process. Table 1 shows various properties of the CdS-dendrimer aggregates measured several days after synthesis. None of the samples showed any indication of precipitation of bulk CdS. All of the solutions manifested strong photoluminescent emission at 460 nm. Figure 5 shows typical UV-visible absorbance and photoluminescent emission spectra for two different samples. These spectra are essentially identical to those reported previously.8 From the location of the UV-visible absorption edge, the method of Brus35 provides an indication of the size of the nanoclusters responsible for the optical absorption. For the 10L(0 °C) sample, the similarity of the UV-visible absorbance spectrum (Figure 5) to others observed previously8 indicates that the CdS clusters have diameters of (35) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183.

Evidence from photoluminescence and UV-visible spectra support the hypothesis that precipitation of CdS in the presence of generation-4.0 PAMAM dendrimers produces CdS nanoclusters with stable optical properties. In the absence of the dendrimers, the mixing of the Cd2+ and S2- solutions leads to precipitation of bulk CdS that does not manifest photoluminescence.8 In contrast, PCS data show that these dendrimer-stabilized nanoclusters slowly associate to produce aggregates. At higher temperatures and reactant concentrations, aggregate diameter and ICR increase as a power of time, suggesting diffusion-limited colloidal aggregation (DLCA). However, our light scattering technique did not yield realistic values of the aggregate fractal dimension. Unlike other synthesis approaches,2,5 the dendrimer-stabilized CdS nanocluster aggregates remain in solution despite the formation of micron sized flocs. In fact, no precipitate has formed in our dendrimer-CdS solutions after several months of storage at -10 °C. Furthermore, the solutions have continued to manifest strong photoluminescence over this period. The aggregation behavior and resulting properties of the dendrimer-CdS aggregates can be rationalized by postulating that rapid nucleation and growth of CdS nanoclusters is followed by slow aggregation. Although we have no evidence that CdS nucleation begins within the dendrimers rather than in solution, studies of the similar synthesis of Cu nanoclusters19,20,21 suggest that nucleation occurs within the dendrimers. Regardless, the dendrimers stabilize the newly formed CdS nanoclusters, blocking cluster-cluster aggregation that would otherwise lead to bulk precipitation of CdS. Previous work8 has shown that the dendrimer and reactant concentrations determine the size and level of defects in the CdS nanoclusters and thus the ultimate optoelectronic properties of the material. Modification of these conditions can be used to tailor the material’s optoelectronic properties. Although the dendrimers stabilize the individual nanoclusters, residual attraction between the clusters results in slow aggregation of their dendrimer hosts. Fortunately,

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the final aggregates, apparently consisting of distinct CdS nanoclusters dispersed in a dendrimer matrix, remain suspended in solution for months with no significant change in optoelectronic properties. The results in this paper suggest that dynamic light scattering studies may provide useful information about the supermolecular structure of the dendrimer stabilized CdS synthesis product. Experiments should utilize either longer aggregation times or higher reactant concentrations to produce larger aggregates. Alternately, smaller scattering angles should be used to make qR h PCS much less than one. Two techniques not utilized here may provide additional information. The SAXS results of Tan et al.22

Hanus et al.

suggest that static light scattering may be able to provide aggregate structural information that can better differentiate between DLCA or RLCA. Finally, confocal microscopy offers a possible way of determining whether the strongly fluorescent CdS nanoclusters remain as distinct entities within the dendrimer aggregate matrix. Acknowledgment. We thank the National Science Foundation for funding through Grants CTS-9596071 and EPS-9630167 LA981505U