Synthesis of Size-Monodisperse CdS Nanocrystals Using

Stucky, G. D.; Mac Dougall, J. E. Science 1990, 247, 669. ...... West, A. R. Solid State Chemistry and Its Applications; John Wiley and Sons: New York...
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J. Phys. Chem. 1996, 100, 346-351

Synthesis of Size-Monodisperse CdS Nanocrystals Using Phosphatidylcholine Vesicles as True Reaction Compartments Brian A. Korgel and Harold G. Monbouquette* Chemical Engineering Department, 5531 Boelter Hall, Box 951592, UniVersity of California, Los Angeles, Los Angeles, CA 90095-1592 ReceiVed: July 26, 1995; In Final Form: September 25, 1995X

Phosphatidylcholine vesicles provide reaction compartments for synthesis of size-quantized CdS nanocrystals of dimension predicted to within 2.5 Å on the basis of initial encapsulated CdCl2 concentration and vesicle diameter. Vesicle formation by detergent dialysis of phosphatidylcholine/hexylglucoside mixed micelles yields highly monodisperse lipid capsules within which monodisperse CdS nanoparticles are precipitated with sulfide. Size-quantized CdS nanocrystals, with diameters ranging from 20 to 60 Å, have been produced with typical standard deviations about the mean diameter of (8%, as measured by transmission electron microscopy. Spectrophotometric and photoluminescence spectra are consistent with highly crystalline, monodisperse particles with few core or surface defects. Measured exciton energies show excellent agreement with data in the literature. The empirical pseudopotential model presented by Krishna and Friesner for a cubic CdS lattice, correcting for experimentally measured lattice contractions, best fits the exciton energy versus particle diameter data.

Introduction When the characteristic dimension of semiconductor nanocrystals is comparable to or smaller than their bulk exciton diameter, they exhibit size-dependent optoelectronic properties due to quantum confinement of electrons, the most well-known example of which is the shift in the absorption spectra to shorter wavelengths with decreasing particle size.1-9 Ideally, these materials could be custom-designed for specific photocatalytic applications and for use in optoelectronic devices by controlling the average particle size.9-12 Usage of such particles in theorized computing devices would require construction of arrays of billions of nanocrystals with precisely controlled size and crystalline properties.9 However, this most demanding of potential applications of size-quantized semiconductor particles will require nanocrystal preparations with a standard deviation about the mean particle diameter of less than 1%.13 Wet chemical preparations with this degree of size monodispersity have not yet been achieved. Other optoelectronic and photocatalytic technologies do not require such tight size distributions.9-12 Additional particle qualities, such as the core crystallinity and structure, the surface structure and composition, and the shape, also impact optoelectronic properties and must also be considered in determining the success of any synthesis procedure.14-19 Current lithographic technology cannot achieve the resolution necessary to synthesize size-quantized II-VI materials, such as CdS and CdSe, since their exciton diameters are less than 100 Å.4,9 Synthesis methods that employ polymers, glasses, zeolites, inverse micelles, capping molecules, or coordinating solvents to arrest or control particle growth remain the only means to produce size-quantized II-VI semiconductor nanoparticles. Unfortunately these approaches have limitations as well; polymers and glasses yield size-polydisperse materials,19,20 and zeolites limit nanocrystal diameters to pore dimensions, which typically are in the vicinity of 15 Å.4,21 Usage of stabilizing agents, which cap particles and thereby control crystal * Author to whom correspondence should be addressed. Telephone: 310825-8946. Fax: 310-206-4107. E-mail: [email protected]. X Abstract published in AdVance ACS Abstracts, December 1, 1995.

0022-3654/96/20100-0346$12.00/0

growth, has led to the most promising of current technologies for the production of large quantities of monodisperse nanocrystals that may be suitable for some important optoelectronic device applications. Numerous studies in the literature exist in which surfactants,22-29,73 capping molecules,14,29-36,42-45,90 and coordinating solvents15,16 have been used to control particle nucleation and growth and the final particle size. The development of subsequent processing steps, such as gel electrophoresis,37,38 capillary zone electrophoresis,45 chromatography,25,39,40 sizeselective precipitation,14,16,36 and high-temperature annealing,14,26,27,29 has helped to further narrow the size distribution and/or improve particle quality in many cases. The tightest size distributions reported to date have had standard deviations about the mean particle diameter of approximately (5%.14-16,29,36,41,45 These stabilizing agent synthesis methods, however, are only qualitatively understood and rely on trial and error to formulate optimum reactant concentrations.8 In some cases, the optical properties of nanocrystals must be monitored and reaction conditions optimized during the growth process in order to obtain particles of the highest quality with the tightest size distribution.15,16 The stabilizer molecules and the reaction temperature also contribute to determining the nanocrystal shape15,16 and structure,14,26 yet again this relationship is only qualitatively understood. These capping methods, which rely on surface modification to control growth, also heavily influence nanocrystal characteristics, such as photoluminescence, since the particle surface contributes much to these material properties.5,7,19,33,46-54,89 In many cases, the capping moieties are in equilibrium with free stabilizer molecules in solution.17,29 Since most stabilizers, such as thiophenol and mercaptoacetic acid, are toxic, such capped nanocrystals may be unsuitable for some applications. For these reasons, alternative synthesis methods are desirable. Growth of nanocrystals in surfactant bilayer vesicles (e.g., liposomes) may provide a more rationally based method to produce particles of predetermined size, shape, and crystallinity. Some living organisms synthesize inorganic crystalline materials at moderate temperature with precise size, shape, and structure, © 1996 American Chemical Society

Synthesis of Size-Monodisperse CdS Nanocrystals using vesicles as nanoreactors.55,74 Due to very low permeabilities to polar molecules and charged species, the vesicles serve as compartments within which reaction conditions such as pH and reactant concentrations can be controlled precisely.55,65,66,74 In contrast, inverse micelles control particle synthesis only partially through reaction compartmentalization as the micelles rapidly exchange contents with one another.56 The vesicle wall surfactants also may play an invaluable role in crystal formation by controlling nucleation, growth rates, and morphology.55,74 Nanocrystals of widely varying chemistry have been synthesized using surfactant vesicles, illustrating the versatility of the approach.57-63,68-70,75-83 However, size distributions have been disappointing, typically with standard deviations about the mean particle diameter of (25% or more.59,60,63 Broad size distributions in these studies mostly stem from use of vesicles formed by sonication, which yields vesicles with a broad size distribution.64-67 Due to the high-energy input employed for vesicle formation by sonication, this process also can lead to both oxidative and hydrolytic degradation of the lipid, which can result in vesicles that are leaky to the species used in particle synthesis.65,66 “Unexpected [nanocrystal UV-visible absorption] spectra”,71 unpredictable average particle size and size distributions60 and particle diameters that do not correlate well with vesicle size,68,69 are all indications that the sonicated vesicles used in these studies were not performing as closed, monodisperse reaction compartments. In order to mimic the biomineralization processes found in nature, the first requirement is stable, size-monodisperse vesicles. We previously have shown that phosphatidylcholine (PC) vesicles produced by detergent removal are highly size monodisperse and virtually impermeable to Cd2+.72 The point of departure in this study is the usage of such stable, monodisperse PC vesicles for the production of monodisperse, crystalline CdS nanoparticles of a size predicted on the basis of vesicle dimension and initial encapsulated cadmium concentration. True reaction compartmentalization is achieved in this system. Experimental Section Vesicle Preparation. Small unilamellar L-R-phosphatidylcholine (PC) (egg lecithin, Avanti Polar Lipids, Alabaster, Alabama) vesicles were prepared by detergent dialysis using the nonionic detergent, n-hexyl-β-D-glucopyranoside (HXG) (Sigma Chemical Co., St. Louis, MO).65,66,84 A solution consisting of PC (80 mg) and HXG in alcohol at a 0.05 lipid/ detergent mole ratio was dried for at least 4 h using a rotary evaporator at 40 °C. The dried film was rehydrated with CdCl2 (Aldrich Chemical Co., Inc., Milwaukee, WI) solution to give a final lipid concentration of 20 mg/mL in a mixed detergent and lipid micelle dispersion. The CdCl2 concentration was varied depending upon the target particle diameter. This lipid/ detergent mixture was dialyzed using a dialysis bag with a molecular weight cutoff of 6000-8000 (Spectrum Medical Industries, Los Angeles, CA). The vesicle diameter was very sensitive to the CdCl2 concentration and the dialysis rate. Faster dialysis and higher CdCl2 led to smaller vesicles. Vesicle Size Determination. The average vesicle diameter and approximate degree of size monodispersity were determined from static scattered light intensities obtained using the Dawn B/F laser photometer (Wyatt Technology, Inc., Santa Barbara, CA).87,88 In order to eliminate osmotic swelling or shrinkage effects on the vesicle size, light scattering data were obtained prior to desalting and nanocrystal formation using scintillation vials containing 10 mL of CdCl2 solution at the same concentration as that of the vesicle formation dialyzate. Typically, 400

J. Phys. Chem., Vol. 100, No. 1, 1996 347 µL of vesicle dispersion (∼8 mg of PC) was added to the scintillation vial prior to data collection. Ten different scans were averaged with approximately a 45° rotation of the vial between each scan to average out any scattering artifacts due to vial nonuniformities. The laser is a vertically polarized 5 mW He-Ne laser with an in Vacuo wavelength of 632.8 nm. Intensities were measured by fixed detectors at 15 scattering angles from 25.67° to 128.66°. CdS Formation. Prior to crystal growth, the vesicle dispersion was passed through either a cation exchange (Amberlite IR-120, Aldrich Chemical Co., Inc., Milwaukee, WI) or a gel filtration column (Spectra Gel AcA 202, Spectrum Medical Industries, Los Angeles, CA) to remove all cations external to the vesicles. The cation exchange column was found to desalt the vesicle dispersions with the most effectiveness and was used for most particle formation experiments. Immediately after desalting, ammonium sulfide, (NH4)2S (Aldrich Chemical Co., Inc., Milwaukee, WI), was added to the vesicle dispersion with rapid stirring to a final sulfide concentration of 10% w/v.85,86 CdS nanocrystals were stable in the vesicle dispersion for months, and the size distribution did not broaden with time. Elemental Composition Analysis. Subsequent to crystal growth, elemental analysis was performed to confirm the composition of the material using scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX). Prior to EDX, the vesicle/nanocrystal dispersion was passed through an anion exchange column (Amberlite 120-IRA, Aldrich Chemical Co., Inc., Milwaukee, WI) to remove any unreacted sulfide. The vesicle/nanocrystal dispersion was then dried in a rotary evaporator at 40 °C for at least 4 h to obtain a lipid film containing the CdS nanocrystals. The lipid/nanocrystal film was placed on an aluminum SEM pedestal with a conductive adhesive and sputter-coated with a 20-nm-thick Au/Pd film. A Kevex 2003 X-ray detector (Foster City, CA) recorded the EDX measurements in conjunction with an AMRAY 1100 SEM (Bedford, MA). The SEM-EDX measurements confirmed the presence of both cadmium and sulfur in the sample preparations. UV-Visible Absorption Spectroscopy. The absorption spectra were measured using a Beckman DU-65 spectrophotometer (Beckman Instruments, Fullerton, CA) with a scan rate of 500 nm/min. Nanocrystal spectra were measured immediately following sulfide incubation. Prior to data acquisition, the dispersion was passed through an anion exchange column to remove any unreacted sulfide. This step removed any residual yellow color due to the ammonium sulfide. It was necessary to add 20 mg of the detergent n-octyl-β-D-glucopyranoside dissolved in 200 µL of H2O to the CdS/vesicle dispersion to eliminate light scattering by the vesicles, which significantly affects measured nanocrystal absorbances. Nanocrystal agglomeration immediately after micellization was not observed, although if the dispersion was allowed to sit for several hours, the size distribution was observed to broaden. Photoluminescence Measurements. Photoluminescence measurements were performed using a SPEX-Fluorolog spectrometer with a 150 W Hg-Xe lamp. PC does not fluoresce, and further sample preparation, such as vesicle micellization, was not necessary to obtain accurate data. The spectra were corrected using the standard photoluminescence spectrum of quinine sulfate. Transmission Electron Microscopy (TEM). TEM was performed at the Center for High Resolution Electron Microscopy at Arizona State University (ASU) on the JEM 4000EX electron microscope (400 kV with (1.7 Å resolution). Samples were prepared on carbon-coated 400-mesh copper grids prepared at ASU. It was not necessary to isolate the nanocrystals from

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Korgel and Monbouquette TABLE 1: dhp and σ Measured by Averaging the Crystal Diameters of Approximately 100 CdS Nanocrystals Imaged Using TEM and dp,calc Calculated Using the Vesicle Inside Diameter (ID), the Encapsulated Cd2+ Concentration, and Eq 1 vesicle ID (nm) 93.5 47.4 47.5 42.6

Figure 1. TEM image of two CdS nanocrystals. Lattice fringes are clearly present with few core defects. The particles are oriented differently on the grid, with the particle on the upper left displaying the spacing between the {220} planes, whereas the particle on the lower right is lying on the {100} plane. The particle on the left measures approximately 57 Å by 52 Å.

the vesicles in order to resolve lattice fringes. Typically, a droplet of CdS nanocrystals in vesicles was placed on the grid for approximately 30 s to 1 min and then wicked off with a Kim-wipe. This procedure provided good sample coverage with very little nanocrystal agglomeration on the grid. Results and Discussion In order to evaluate CdS nanocrystal synthesis in detergentdialyzed PC vesicles, TEM images of the particles and UVvisible absorption and photoluminescence spectra were analyzed. The average particle diameters, size distributions, and particle quality were determined using TEM. Average particle diameters measured using TEM were compared to the particle diameters predicted from the vesicle diameter and the encapsulated Cd concentration. The degree and type of core crystallinity and the particle shape were also determined from the TEM images. UV-visible absorption and photoluminescence spectra provide complementary information about the size distribution and particle quality. We compare our data with other studies in the literature. This combined data is compared to several theoretical predictions of the size dependence of the exciton energy. Nanocrystal Size and Structure Determination. TEM images of the nanocrystals provide a wealth of information about the size and structure of the material. Shown in Figure 1 is a typical TEM image of representative CdS nanoparticles. Average particle diameters, dhp, with standard deviations, σ, were determined from large fields of approximately 100 particles. Although significant uncertainty exists in these measurements

Cd2+ conc (mM)

dhp (Å)

σ (%)

dp,calc (Å)

6.6 20.0 6.6 3.3

56.9 41.4 29.0 22.0

8.9 7.6 8.1 8.5

54.4 39.9 27.6 19.7

due to orientational variations on the grid, fuzzy particle boundaries, and nonspherical particles,14,16 Katari et al.15 and Vossmeyer et al.14 found that average particle diameters for CdSe and CdS nanocrystals, respectively, determined from TEM were comparable to those from small-angle X-ray scattering measurements to within (2 Å. The nanocrystals generated in our study were slightly prolate, with the average ratio of the long and short diameters being 1.15. This particle shape is similar to that of CdSe nanocrystals generated in the coordinating solvent trioctylphosphine (TOP) at relatively high temperatures16 and CdS nanocrystals grown at high temperature in the presence of 1-thioglycerol.14 Table 1 lists the dhp values of the long axis and the σ values measured for four different CdS nanocrystal preparations. The average particle diameter of the sample in Figure 1 is 56.9 Å with σ equal to (8.9%. In all preparations, large agglomerates of particles never were found. The size distribution standard deviations for these samples all were less than (10%. Although other groups have claimed to have achieved size distributions with σ ∼ (5% by other colloidal synthesis methods,14,16,29 our samples are not necessarily more polydisperse, since much of the deviation in the size distribution measurements results from the difficulties associated with the use of TEM as mentioned above.14,16 Shown in Figure 1 are CdS nanocrystals with clearly resolved lattice fringes that were synthesized using PC vesicles. This indicates good crystallinity. Although there are numerous TEM images of nanocrystals synthesized in surfactant vesicles in the literature,57-63,76,77,80,82,83 lattice fringes have only been observed in a few large Ag2O crystals58,59,82,83 and never for CdS nanocrystals synthesized in vesicles. The two particles imaged in Figure 1 lie with different orientations on the grid. Since CdS nanocrystals can take on either cubic or hexagonal structures depending upon the synthesis method,14 careful measurement of the nanocrystal lattice parameters is necessary to reveal the core structure. The particle in the upper left of Figure 1 shows a ladder-like pattern with 2.1 Å between each lattice, whereas the other particle shows a netlike pattern with lattice spacings of 1.9 and 1.7 Å. Both cubic (sphalerite) and hexagonal (wurtzite) CdS contain lattice spacings of such magnitudes. Nanocrystals lying with different orientations elsewhere on the TEM image (not shown) give different lattice parameters that reveal the core structure. This sample contained particles displaying lattice spacings of 3.4 Å, which is the lattice spacing between the {200} planes for cubic CdS.14,95,96 A corresponding d-spacing for hexagonal CdS does not exist. A lattice spacing of 3.6 Å, which is found only for hexagonal CdS14,95,96 was not found in any of our samples, indicating that cubic CdS was synthesized for all crystal sizes in our study. From this analysis it can be determined that the particle in the upper left of Figure 1 is oriented with the cubic {220} plane perpendicular to the grid and that the particle in the lower right is laying on the cubic {100} plane. The TEM images also revealed that most of the nanocrystals contained slight defects or dislocations such as the particle in the lower right of Figure

Synthesis of Size-Monodisperse CdS Nanocrystals

J. Phys. Chem., Vol. 100, No. 1, 1996 349

Figure 2. Typical scattering behavior of a phosphatidylcholine vesicle dispersion prior to desalting and CdS formation. The plot shows the variation of the relative scattered intensity with scattering angle. The solid curve corresponds to the best fit of the Rayleigh-Gans-Debye approximation for isotropic hollow spheres to the experimental data assuming monodisperse vesicles with a wall thickness of 3.7 nm.87,88 On the basis of the RGD fit the vesicles have an estimated diameter of (b) 59 nm, (9) 64 nm, and (2) 73 nm.

1. However, there were many that did not show defects, such as the particle in the upper left. A lattice contraction of approximately 3% was measured for the smallest particles (dhp ) 22.0 Å), which also has been observed in a number of CdS nanocrystal studies.29,91-93 Nanocrystal Diameter Prediction. Table 1 compares the measured dhp for four different CdS nanocrystal preparations with predicted particle diameters, dp,calc, based upon the average vesicle diameters and the encapsulated cation concentrations. dp,calc was calculated using a shell approximation from Lippens and Lannoo94 relating the particle diameter to the number of cadmium atoms, NCd, in the crystal:

[ ]

dp,calc ) a

3NCd 2π

Figure 3. Room temperature optical absorption spectra of four different CdS nanocrystal preparations using detergent-dialyzed phosphatidylcholine vesicles. The particle size in parentheses is a predicted diameter based on encapsulated Cd2+ concentration and vesicle diameter (see text).

1/3

(1)

where a is the lattice constant for the material. In this work, the value of a for cubic CdS, 5.832 Å,22,95 was used. Since the lattice parameters for CdS decrease only by approximately 3%, contractions in the lattice parameter were ignored in the calculations. The number of Cd atoms in the crystal was calculated from the cadmium concentration in the dialyzate and the inner vesicle diameter as determined using static light scattering measurements assuming that the vesicles are spherical with a bilayer thickness of 3.7 nm.87,88,97 The only two variables necessary for particle size determination are the cadmium concentration in the dialyzate and the vesicle inner diameter. Average vesicle diameters were determined from static light scattering intensity measurements. Figure 2 shows sample fits of the Raleigh-Gans-Debye scattering approximation for hollow spheres to the scattering data.87,88 This analysis is very sensitive to vesicle diameter, which is necessary for accurate calculation of the particle diameter. Comparison of measured and predicted particle diameters in Table 1 shows agreement to within 2.5 Å. Since the dp,calc calculation assumes spherical nanoparticles whereas they are observed to be prolate with the average longer axis given by dhp, dp,calc should be slightly smaller than dhp, as is shown for all cases. The data therefore indicate that the average nanocrystal diameter can be accurately predicted before synthesis, eliminating the necessity for labor-intensive TEM analysis of every sample for an estimate of the average particle diameter. CdS Nanocrystal Electronic Properties. Figure 3 shows UV-visible absorption spectra for four different CdS nanocrystal preparations using detergent-dialyzed PC vesicles. The particle diameter listed in parentheses is the predicted diameter; the others were measured using TEM. The absorption maxi-

Figure 4. Room temperature UV-visible absorption and corrected emission spectra for 24 Å CdS nanocrystals. An exciton peak occurs in the absorption spectrum at 2.89 eV and in the emission spectrum at 2.75 eV. No deep trap photoluminescence is detected. The excitation wavelength was 360 nm.

mum corresponding to the exciton energy shifts to shorter wavelengths with decreasing particle diameter, indicating quantum confined electronic behavior.1-8,94,98 This shift is predictable upon the basis of the calculation of dp,calc prior to CdS formation. The exciton peaks are relatively sharp, indicating a tight size distribution and good crystal quality,3,14-16,29,36,93 as expected from the TEM analyses of the samples. In Figure 4 the absorption spectrum of a 24 Å diameter sample is compared to the corrected room temperature photoluminescence spectrum. This photoluminescence peak results from a bound excitonic transition.8,48-54 The peak width and the energetic shift from the absorbance peak are an indication of the size distribution and sample quality.16,53,54 The peak is shifted by only 140 meV, and the photoluminescence line width is similar to the absorbance line width. Interestingly, emission at higher wavelengths, or lower energies, which have been associated with deep trap states due to surface or core defects,16,48,51-53 is not evident in these samples, indicating that detergent-dialyzed PC vesicles yield high-quality, size-monodisperse CdS nanocrystals. In Figure 5 the CdS nanocrystal diameters determined by TEM and predicted using eq 1 are plotted against their corresponding exciton energies from their UV-visible absorption spectra. The exciton energies all follow the same monotonically increasing trend with decreasing particle diameter. Also

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Figure 5. Consolidated plot of CdS nanocrystal exciton energy versus particle diameter from this study (9, dhp; b, dp,calc) and from the literature: ], Vossmeyer et al.;14 0, Weller;7 O, Wang and Herron;91 4, Matsumoto et al.;38 +, Herron et al.93 Curves correspond to the effective mass approximation (EMA) assuming an infinite potential well at the particle surface99-102 (-‚ ‚ ‚-), the EMA from Schmidt and Weller103 (- - -), tight-binding calculations by Lippens and Lannoo94 (-‚-‚), and empirical pseudopotential calculations by Krishna and Friesner98 for a corrected cubic CdS lattice (s) and an uncorrected hexagonal CdS lattice (‚ ‚ ‚).

plotted in Figure 5 are the exciton energies for CdS nanocrystals synthesized using other methods in the literature.7,14,38,91,93 Our data correspond well with those of others except for those of Weller7 and Vossmeyer et al.14 for particles with diameters less than 28 Å. The following comparison of the experimental data with theoretical predictions of the exciton size dependence in the literature explains this discrepancy. The curves plotted in Figure 5 are from five different theoretical predictions in the literature.94,98-103 The effective mass approximation,99-102 which calculates the exciton energy as a combination of kinetic energy increase analogous to the particle-in-a-box quantum mechanical model for an electron and a hole, and a Coulombic energy decrease due to electron-hole attraction, provides a qualitative understanding of the quantum confinement effect and serves as an upper bound for the exciton energy. However, due to the assumption of an infinite potential well at the surface, the model is not quantitatively accurate.94,98,103 In light of these shortcomings, Schmidt and Weller103 derived an effective mass approximation by assuming a different potential boundary at the particle surface. Their model agrees reasonably with the data from Weller.7 A tightbinding approach taken by Lippens and Lannoo94 fits the data from Vossmeyer et al.14 fairly well. However, most of the data from the literature, including our data, for nanocrystals smaller than 30 Å significantly deviate from the tight-binding curve. This deviation was first observed by Wang and Herron;91 however, researchers have continued to rely erroneously on this model to determine average diameters from UV-visible absorption spectra. The most accurate and revealing model of the data is the empirical pseudopotential method (EPM) used by Krishna and Friesner.98 The curve for cubic CdS, correcting for experimentally determined lattice contractions with decreasing particle size,91 fits our data and the data from the literature quite well. The EPM for hexagonal CdS (assuming no lattice contractions) agrees with the data from Weller7 and Vossmeyer et al.14 Vossmeyer et al.14 recently reported that different core CdS nanocrystal structures (i.e., cubic vs hexagonal) were obtained depending upon the capping agent and the growth temperature used. X-ray diffraction analysis of the samples from Vossmeyer et al.14 plotted in Figure 5 was indeed hexagonal. Such behavior has also been observed for CdSe nanocrystals, which, if grown at room temperature in inverse micelles, produce cubic CdSe,26 but growth at higher temperature yields hexagonal CdSe.15,16,26 This is a very interesting result, since the redox potentials are related to the conduction and valence

Korgel and Monbouquette band energies,104 meaning that the CdS nanocrystal core structure heavily influences the photocatalytic properties of the particles. Although the EPM for cubic CdS accounting for lattice contractions gives satisfactory agreement with the data, there is one other point worth mentioning. For the cubic CdS nanocrystals less than 13 Å in diameter in Figure 5, significant deviation between the model and the data exists. This deviation most likely results from the use of lattice contractions determined from the shift in the {111} peak in the X-ray diffraction data.91 These data can be misleading, since a deviation from perfectly spherical particles will also manifest itself as a shift in this peak.16 An accurate measure of the lattice contractions with particle size would probably yield a better fit for these particles. Conclusions Detergent-dialysis PC vesicles provide a convenient and effective means of nanocrystal synthesis. On the basis of the vesicle diameter and the encapsulated cation concentration, nanocrystals can be synthesized with diameters predicted to within 2.5 Å with a narrow size distribution. These results constitute the first unequivocal demonstration of true reaction compartmentalization for semiconductor nanoparticle synthesis. TEM analysis shows these particles to be cubic and nearly spherical with a high degree of core crystallinity and with few defects. UV-visible absorption and photoluminescence spectra are consistent with a narrow size distribution and excellent particle quality as well. The exciton energies of the nanocrystals shift to higher energies with smaller particle diameters, as predicted by Krishna and Friesner.98 References and Notes (1) Berry, C. R. Phys. ReV. 1967, 161, 848-851. (2) Brus, L. E. J. Phys. Chem. 1986, 90, 2555-2560. (3) Steigerwald, M. L.; Brus, L. E. Annu. ReV. Mater. Sci. 1988, 19, 471. (4) Stucky, G. D.; Mac Dougall, J. E. Science 1990, 247, 669. (5) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525-532. (6) Brus, L. E. Appl. Phys. A 1991, 53, 465-474. (7) Weller, H. AdV. Mater. 1993, 5, 88. (8) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (9) Reed, M. Sci. Am. 1993, 118. (10) Henglein, A. Top. Curr. Chem. 1988, 143, 113-180. (11) Corcoran, E. Sci. Am. 1990, 122-131. (12) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (13) Alivisatos, A. P.; Harris, A. L.; Levinos, N. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1988, 89, 4001. (14) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemsiddine, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665-7673. (15) Katari, J.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109. (16) Murray, C. B.; Norris, D.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (17) Sachleben, J. R.; Wooten, E. R.; Emsley, L.; Pines, A.; Colvin, V. L.; Alivisatos, A. P. Chem. Phys. Lett. 1992, 198, 431-436. (18) Becerra, L. R.; Murray, C. B.; Griffin, R. G.; Bawendi, M. G. J. Chem. Phys. 1994, 100, 3297-3300. (19) Nirmal, M.; Murray, C. B.; Bawendi, M. G. Phys. ReV. B 1994, 50, 2293-2300. (20) Persans, P. D.; Tu, A.; Wu, Y.-J.; Lewis, M. J. Opt. Soc. Am. B 1989, 6, 818. (21) Wang, Y.; Herron, N. Res. Chem. Intermed. 1991, 15, 17. (22) Meyer, M.; Wallberg, C.; Kurihara, K.; Fendler, J. H. J. Chem. Soc., Chem. Commun. 1984, 90. (23) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299-302. (24) Fendler, J. H. Chem. ReV. 1987, 87, 877-899. (25) 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, 3046-3050. (26) Bawendi, M. G.; Kortan, A.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1989, 91, 7282.

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