J. Phys. Chem. 1995,99, 17410-17416
17410
Solid Solution of CdYZnl,S Nanosize Particles Made in Reverse Micelles J. Cizeron and M. P. Piled* Laboratoire SRSI, URA CNRS 1662, Universitk P et M Curie (Paris VI), BP 52, 4 Place Jussieu, 75231 Paris Cedex 05, France, and C.E.A.- C.E. Saclay, DRECAM.-S.C.M, 91 191 Gif sur Yvette, Cedex, France Received: February 7, 1995; In Final Form: August 3, 1 9 9 P
The formation of nanosized particles of a Cd,Znl-,S solid solution is described with equimolar or in the presence of an excess of sulfide ions. The Cd,Znl-,S crystallites are characterized by an absorption spectrum and excitonic peak which are red shifted by increasing the size of the particles. The particles are well crystallized with a lattice parameter depending on the composition, y. The size of the Cd,Znl-,S particle is governed by the size of the water droplets and does not depend on the composition, y. In the presence of an excess of cations, CdS particles are preferentially formed because of a faster nucleation of CdS than ZnS.
1. Introduction The use of dispersed media to synthesize microparticles in situ has made considerable progress in the last few years. Reverse micelles,’ Langmuir-Blodgett films; zeolite? vesicles? glass matrixes and sol-gel method.5 Organometallic techniques6 have been also used to produce nanosized particles. Semiconductor nanocrystals present quantum confinement effects, called Q-dots.’ We use water in oil (w/o) microemulsions also called reverse micelles to synthesize mixed composition semiconductor particles. The ternary system sodium bis(2-ethylhexyl)sulfoccinate (AOT)/water/isooctaneshows a large zone of its phase diagram where the reverse micellar phase (L2) exists. This ternary system, alkane/AOT/water, presents enormous advantages: these are spheroidal aggregates8where water is readily solubilized in the polar core, forming a so-called “water pool”, characterized by the ratio of water-to-surfactant concentration (w = [HZO]/[AOT]]. In isooctane solution, the maximum amount of bound water in the micelles corresponds to a watersurfactant molar ratio about 10. Above a water content of 15, the water pool radius, R,, is linearly dependent on the water content [R, (A) = 1 . 5 ~ 1 .Another ~ important property of reverse micelles is their dynamic behavior. They can exchange the content of their water pools by a collision process.8 Such a process makes chemical reaction or precipitation between compounds solubilized in different droplets possible. In previous papers, we have shown that reverse micelles are used as microreactors to control the size of cadmium sulfide semiconductorsI0and silver or copper metallic particles.” In all cases, the particle size is controlled by the size of water droplets, average number of reactant per micelle and intennicellar potential. Ternary alloys are usually obtained at high temperatures. Several media such as dispersed media,I3 formamide,14reverse micelles,I5 and vesiclesI3 has been used to form Cd,Znl -,S ternary alloys at room temperature. In most of the cases the formation of Cd,Znl-,S has been demonstrated from the optical properties of the particles. Recently Fedorov et a l . I 2 demonstrated that crystals of Cd,Znl-,S alloys are thermodynamically stable at room temperature in the zinc blende structure.
* Address all correspondence to this author. @
Abstract published in Advance ACS Abstracts, September 1, 1995.
0022-3654/95/2099-17410$09.00/0
In the present paper we show by electron diffraction, microanalysis, and high-resolution transmission electron microscopy that reverse micelles can be used to make nanocrystallites of a Cd,ZnI-,S ternary alloy with the zinc-blende structure. The size of the droplet controls the size of the particles whatever the composition, y. These CdJnl-,S ternary alloys are formed when the synthesis is performed in the presence of an excess of sulfide ion or for an equimolecular ratio of cations over sulfide ion. In the presence of an excess of cations, CdS particles containing a few percent of zinc are formed. 11. Experimental Section
II.1. Products. Isooctane and sodium bis(2-ethylhexy1)sulfosuccinate were obtained from Fluka and Sigma, respectively. Pyridine, sodium sulfate, heptane, dodecanethiol, and benzenethiol were obtained from Merck. All these products were used without further purification. The syntheses of zinc and cadmium bis(Zethylhexy1)sulfosuccinate, Zn(A0T)z and Cd(AOT)z, have been described previously.’6 II.2. Equipment. Absorption spectra were recorded either on UVIKON 931 or Hewlett-Packard HP8452A spectrophotometers. Microanalyses (EDS) were obtained with a LINK AN10000 apparatus coupled with a JEOL JEMlOOCXII electron microscope. A drop of coated particles redispersed in heptane or pyridine (see section II.3) was evaporated on a carbon grid, and the electron micrographs were obtained with a JEOL (Jem 1OOCX2) electron microscope. II.3. Synthesis and Extraction of Nanosize Particles. Colloidal CdS particles are prepared by mixing three micellar solutions having the same water content {w = [HzO]/[AOT]), one containing the sulfide ions (Na2S) and the two others containing separately Zn(A0T)Z and Cd(AOT)2. Various volumes of these micellar solutions are such that the ratio x = ([Cd2+] [Zn2+])/[S2-]is kept constant. Due to the dynamic properties of reverse micelles, reaction occurs in a few seconds. The overall (2-ethylhexy1)sulfosuccinate concentration is kept constant and equal to 0.1 M. The formation of colloidal particles is followed by UVvisible spectroscopy. Immediately after mixing the solution, an absorption spectrum in the 300-500 nm range is recorded. With time, the absorption spectrum is progressively shifted to
+
0 1995 American Chemical Society
Solid Solution of Cd;Znl-,S
Nanosize Particles
ZnS
260 300 340 380 420
J. Phys. Chem., Vol. 99, No. 48, 1995 17411
CdS
ZnS
CdS
300 340 380 420 460
Wavelength (nm)
Figure 1. Absorption spectra of CdS and ZnS at various water content (w = [water]/[AOT] = 2.5, 5 , and 7.5).
the red, indicating an increase in the particle sizes. On benzene thiol or dodecane thiol addition, the absorption spectrum remains unchanged for several hours, indicating that the thio derivatives prevent the growth of the particles. According to the literature," this is due to a chemical reaction at the interface between cations and thio derivatives. The coated particles precipitate by alcohol addition to the micellar solution. The powder is washed with ethanol and redispersed in pyridine or heptane.
Wavelength (nm)
Figure 2. Absorption spectra of extracted particles of CdS and ZnS from the micelles (their initial spectra are shown in Figure 1) and dispersed in heptane.
C&.33Zno.csS
1.6 1.2
Cdo,trZno.3S
: w=2.5
W=2.5
111. Results 111.1. Change in the Absorption Spectra of Nanocrystallites with the Water Content and Composition. Cadmium and zinc sulfide suspensions are characterized by an absorption spectrum in the visible range. In the case of small particles, a quantum size effect is observed due to the perturbation of the electronic structure of the semiconductor with the change in particle size. For CdS and ZnS semiconductors,as the diameter of the particles approaches the excitonic diameter, their electronic properties start to change. This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. This phenomenon, well described in the l i t e r a t ~ r e , ' - ~ ~ ' ~ occurs * ' ~ - ~as~ the crystallite size is similar to or below the excitonic diameter. Figure 1 shows, for an equimolar ratio of cations over sulfide ion (x = [Cd2+]/[S2-] and [Zn2+]/[S2-] = l), the absorption spectra of CdS and ZnS obtained at various water contents. The CdS and ZnS particles have been extracted, as described above, from the micellar solution. Figure 2 shows that the shape and the onset of the absorption spectra of coated particles are similar to those obtained in reverse micelles. This indicates that the extraction process does not change the size of the particles. In both cases (ZnS and CdS), a red shift in the absorption spectra with increasing the water content is observed. This phenomenon has been largely described in the literature'-6 and attributed to an increase in the particle size with increasing water content. Instead of mixing a micellar solution containing sulfide ions with a solution containing separately either Zn(A0T)Z or Cd(AOT)2, both reactants (Zn(A0T)Z and Cd(AOT)2) are in the same micellar solution.
1.2
1
t
260 300 340 380 420 300 340 380 420 460 Wavelength (nm)
Figure 3. Absorption spectra of colloidal particles obtained by changing the micellar composition of Zn(A0T)Z and Cd(A0T)Z = 0.33 and 0.66) and various water content (w = [water]/[AOT] = 2.5, 5 , and 7.5).
Figure 3 shows the absorption spectra of these solutions obtained for various water contents, w ,and compositions, y 0, = [Cd(AOT)z]/([Cd(AOT)2] [Zn(AOT)2])). For a given composition, the absorption spectrum is red shifted by increasing the water content. At fixed water content, the absorption spectrum is red shifted by increasing the composition, y . These changes in the absorption spectra with the composition are in good agreement with the data given in the l i t e r a t ~ r e . ' ~ , ' ~ Figure 4 gives the absorption spectra obtained after extraction of the particles. An unchanged value in the absorption onset
+
17412 J. Phys. Chem.. Vol. 99, No. 48, 1995 Cd..,,Zn...S
Cizeron and Pileni
Cd..,.Znl.,S
Ws"elsn*UI
1.111)
Figure 4. Absorption spectra of the panicles synthesized with y = 0.33 and y = 0.66 extracted from the micellar solutions (their initial spectra are shown in Figure 3) and dispersed in heptane.
Figure 6. Electron microscopy pattern
0 1 colloidal panicles ohtained
by using three different compositions ( y = 0.5. 0.66. and 0.75) and at various water content (w = 2.5 and 7.5).
0
20
40
60
80
100
% ICdll(lCdl+IZnl) initial
Figure 5. Variation of the absorption onset of colloidal panicles obtained by changing the composition, y . for different water content (w = 2.5, 5. 7.5. 10. and 20). and in the shape of the absorption spectrum compared to that observed in micellar solutions can be observed. Figure 5 shows the change in the absorption onset with composition and various water contents. From w = 5 to 10 and at a given composition, y. a smooth shift in the absorption spectra with changing water content is observed. At higher water content (up to w = 20) no further changes in the absorption onset have been observed. Similar behavior has been observed previously with CdSto and ZnS particles. At w = 2.5, the absorption onsets show a plateau with the cation composition, which is reached for a composition equal to 20%. This could be due to the very small size of the clusters. 111.2. Analysis of the Nanocrystallites. Figure 6 shows, by transmission electron microscopy, an increase in the panicle size with increasing water content. However, an unchanged value in the average size of the panicles with cation composition is observed. This is confirmed in Figure 7 where the histograms of the electron microscopy pattern are given. There is a very
low polydispersity in size (less than 10% in diameter). Similar behavior is observed for all compositions. This indicates that the size of the particles does not change with composition and increases with the water content. High-resolution transmission electron microscopy, shown in Figure 8, indicates the formation of well-crystallized particles. Assuming that the particles keep a zinc blende structure and the observed spacing is due to 11 I high atomic density orientation, the lattice parameter has been measured for various compositions. The interreticular distances have been deduced from measurement of the distance between several fringes divided by their number. To make sure that the change in the interreticular distance is significant, this measurement has been performed on a large number of particles. The interreticular distance presented in Figure 8 is an average of the values measured as described above. The dispersion of the measure is less than 0.1 A, which corresponds to 3% uncertainty. As a consequence, the 10% variation of lattice parameter observed can be attributed to the change in the composition of the singlephase solid. The electron diffraction pattern given in Figure 9 confirms a zinc blende structure type with an intermediate lattice parameter between ZnS (5.4 A) and CdS (5.8 8, ). The particles show a linear increase in the lattice parameter with cadmium composition. The dispersion in the data is due to 2% uncertainty measurements. This confirms the data obtained in Figure 8 by transmission electron diffraction. To be certain that the composition of Cd(A0T)Z and Zn(AOT)* in reverse micelles is the same as that in the particles, microanalyses have been performed on extracted particles obtained from various compositions and at various water
Solid Solution of Cd,Zn,-$
Nanosize Particles
J. Phys. Chem., Val. 99, No. 48, 1995 17413
2--
5.8 5.6
5 LI
c
i5.4 v W
5 5.2
3
5
0
20
40
60
80
loo
[CdY([Cdl+[Zn]) initial Figure 9. Electron diffraction pattern Cdo.rZno.sS panicles made in reverse micelles at w = IO and variation of the lattice with composition estimated from electron diffraction.
Figure 7. Histogram of particles observed by given in Figure 4.
electmn
microscopy
i5.6p‘ 4 I
!
6
-$5.8
-
25
50
75
too
lCdl/llCdl+lZnl)%
Figure 10. Measured by EDS cationic composition of solid phase extracted as function of reacts concentration for syntheses using the ratio x = ([Cd(AOT)?I+ [Zn(AOT)~I)/[S‘-1= 1. (A) On redispersed w = 2.5. (0) w = 5, (A)w = IO. (B)On nanredispersed powders: (0) w = 2.5. (0) w = 7.5. powers: (0)
n
0
I.
n
c 5.4
.-=e 2
0
._
5.2
I1
~
5
0
20
40
60
80
100
[Cdl/(ICdl+IZnl)Initial Figure 8. High-resolution electron microscopy of Cda.sZna.sSparticles made in reverse micelles at w = I O and variation of lattice with composition estimated from high resolution microscopy.
relation between absorption onset and diameter using the formula20
2dh2
-+1
I
E(d)=T [me* mh*]
3.572e2 Ed
0.124e4
L+-
~ [ r n , * m’,*]
contents. Figure IO shows that most of the reactants participate in the formation of panicles. However it can be noticed, for a ratio x (x = [Cd2+] [Zn2+]/[S2-]) equal to I, that the solid phase is slightly richer in cadmium ions than the initial solution. 111.3. Quantum Size Effect in Semiconductor Solid Solution. In a first approximation a simple “electron-hole in a box” model can quantify the blue shift with the size ~ariation.’~ This model is called the effective mass model and gives a
+
where d. e,mh, and E are the diameter of panicle, the effective masses of electron and hole, and dielectric constant respectively. Assuming that (i) the variation of the bulk bandgap energyI9 with composition is given by the relationship Eg = 2.5 0 . 5 9 ~ 0.619 and (ii) the effective mass of electron and hole linearly depend on the relative composition, the absorption onset can be calculated for various particle diameters.
+
+
17414 J. Phys. Chem., Vol. 99, No. 48, 1995
Cizeron and Pileni
1
40
0.8
I W=2.5 Xd.5)
35 30 25 20
*z
-1 Y L
15 ,35
b 0.11
‘I
--5
0.6
0.4 0.2
30 25 20 15
300 340 380 420 460 340 380 420 460
250
Absorption onset (nm)
Figure 11. HUMO-LUMO transition energy of Cd,Znl-,S crystallites as a function of size compared with the prediction of the effective mass approximation. (A) Comparison between three theoretical curves for various compositions. (B-D) Comparison between experimental data and calculation.for respectively y = 0.25, y = 0.5 and y = 0.75.
To perform such calculation, for the effective mass of electron and hole, we used me- = 0,18 and mh+ = 0.53 for CdS, me- = 0.42 and mh+ = 0.61 for ZnS. Figure 11A shows the result of the calculation for various compositions, y . For very small particles (20 A diameter), Figure 11A shows a very small change of the absorption onset with composition. Figure 11 compares, at various y compositions, the observed HOMO-LUMO gap as a function of particle size with the prediction of a simple effective mass approximation (eq 1). Figure 9B-D shows, for a given composition of reactants 0,= 0.25, y = 0.5, y = 0.75), that the size predicted from the absorption onset is overestimated compared to that obtained by TEM. Similar behavior has been observed for CdS and CdSe There is a rather large distribution in absorption onset which could be due to slight change in the composition. Nevertheless experimental data clearly show quantum size effect curves shifted to high energy with increasing composition in cadmium. 111.4. Influence of the Relative Ratio of Cationic Ions to Sulfide ([Zn2+ Cd2+]/[S2-]). Absorption spectra obtained at various x values (x = ([Zn2+ Cd2+]/[S2-]) with a given composition 0,= 0.5) and various water contents (w = [water]/ [AOT]) are given in Figure 12. There is a shift in the absorption spectra with increasing w and x . Figure 13 shows an increase in the absorption onset with composition, y. At fixed water content, w = 10, however, such behavior depends on the x values: (i) At x = 0.5, the absorption onset increases with the cation composition. Microanalysis indicates a similar composition of extracted particles to that used to make the particles. (ii) At x = 1, as shown above, microanalysis indicates the presence of both cations (Zn2+ and Cd2+) in the solid phase. However a slight increase in cadmium compared to reactant composition is observed. (iii) At x values equal or up to 2, the behavior of the absorption onset with composition depends on the x value (Figure 13). At low cadmium composition, a red shift in the absorption onset is observed. At higher cadmium composition,
+
+
300
350
300 350
400
400 450
Wavelength (nm)
Figure 12. Absorption spectra of C&.sZno.sSparticles made at various water content w, and ratio of the cations over sulfide concentrations x = 0.5 and 2. 420 ,
. I
400
P9
380
300, 0
I
, 20
I
, 40
-
x=10
I
60
80
100
ICdlNCdl+Enl) % initial Figure 13. Variation of the absorption onset with the initial composition used to make CdyZnl-,S at various x values.
the absorption onset does not change with composition keeping a similar value to that obtained for pure CdS particles obtained under the same conditions. The plateau is reached at lower y composition when x increases from 2 to 10. Microanalysis has been performed for the particles prepared at x values equal or up to 2 . Values inserted in Figure 13 give the percentage of cadmium (relative to total cation) in the particles for the corresponding sample. It is clear that the appearance of an absorption onset plateau corresponds to the formation of CdS particles containing a small amount of zinc. From Figure 13, we observe that the absorption onset plateau is reached roughly for a ratio of [Cd2+]/[S2-] equal to unity. This c o n f i i s the preferential formation of CdS particles. The appearance of the absorption due to colloidal particles is followed by “stopped flow” at 300 nm. A very fast absorption is observed immediately after mixing the micellar solutions. This fast process (less than 20 ms) is followed by a very slow increase in the optical density.
Solid Solution of Cd,Znl-,S Nanosize Particles
IV. Discussion At fixed composition, y , for the value of the ratio x equal to 1 (x = ([Cd2+] [Zn2+])/[S2-]), the absorption spectrum of colloidal particles (Figures 3 and 4)obtained, by mixing micellar solutions containing zinc and cadmium ions, is never the sum of the absorption spectra corresponding to the formation of a mixture of ZnS and CdS particles. The optical density of the excitonic peak is higher than that obtained with CdS for the same concentration. Such changes in the absorption spectra with composition cannot be attributed to a size effect. As matter of fact, Figure 6 shows an unchanged value of the particle size with composition. This indicates that the absorption spectrum observed, at a given composition, is due to the formation of a solid solution of Cd,Znl-,S. This confirms the data obtained previ~usly.~~,’~ The formation of a solid solution of Cd,Znl -,S is c o n f i i e d by the variation of the lattice of crystallites with composition observed by high resolution TEM and electron diffraction (Figures 9 and 10). The formation of a solid solution is in good agreement with the Cd,Znl -,S phase diagram obtained by ionexchange methodsI2 for microsized particles. However, the coprecipitation reaction described is a nonequilibrium process (supersaturation process) and the properties of the nanocrystallites are not similar to the bulk solid. A tentative explanation could be the following: for particles having a diameter below 4 nm, the structure of CdS is zinc blende typeL0whereas above 4 nm it is wurtzite, as in the bulk phase. The size of the particles, obtained in the present paper, is lower than 4 nm. Therefore, in such diameter ranges, both crystallites (ZnS and CdS) have same structure (zinc blende type) which could favor the formation of the Cd,ZnI-,S solid phase. Figure 7 shows a very low size distribution (less than 10%). This indicates that the particle growth rate constant strongly decreases in reverse micelles compared to that obtained in homogeneous solution. This effect could be attributed to steric constraint as in zeolites3 or to chemical poisoning of cluster surface due to the surfactant. This is confirmed by the fact that the size of the particle does not change with the composition. Hence the size of the particle is controlled by the size of the reverse micelles, that is to say by the water content, w. Because of the low polydispersity (less than 10%) given in Figure 7, the mechanism of the formation of the crystallites is not similar to that described by Smoluchowski.22 According to Thomas et the growth of the particles in reverse micelles could be expressed by replacing the cluster-cluster growth rate constant by the intermicellar exchange rate constant. Such a mechanism implies the formation of polydispersed clusters which would induce bigger particles. The precipitation of either CdS and ZnS or Cd,Znl-,S is very fast (about a few milliseconds) with the appearance of a well-defined excitonic peak indicating formation of particles having a very low size distribution. Hence, the exchange micellar process is not the only parameter that controls the growth of the particles. Assuming that the effective mass of electron and hole linearly depends on the relative composition, a relation between absorption onset and diameter of the particle is deduced from the effective mass model. From a comparison between the theoretical calculation and the experimental results, there is, for various compositions, a similar behavior (red shift in the absorption spectra with the increase in the particle size). However, the diameters calculated are overestimated. For syntheses made in the presence of an excess of cations, the behavior strongly differs from that described above. As a matter of fact, Figure 13 shows the change in the absorption onset with composition. At relatively low y values, the
+
J. Phys. Chem., Vol. 99, No. 48, 1995 17415 absorption onset reaches a value similar to that obtained for pure CdS particles. The microanalysis performed on extracted particles indicates that only very small amounts of zinc ions are incorporated in the particles. This can be explained by the fact that the solubility product of CdS is lower than that of ZnS. Hence in the presence of an excess of cadmium, CdS is formed preferentially. The cadmium composition needed to reach the absorption onset of CdS decreases with the increase in the amount of cations which confirms a preferential formation of CdS. The fast increase in the optical density observed at 300 nm by “stopped flow” indicates that the nucleation process is very fast. Several reactions can be taken into account: nCd2+
+ nS2- - (CdS),
kZn2+
nucleation of CdS
+ (k 4-n)S2- + nCd2+ - Cd,Zn,S,+,
(1)
(3)
Because of the preferential foxmation of (CdS),, reaction 1 is faster than 2 and 3. Hence reactions 2 and 3 can be neglected. The difference in the rate constant between reactions 1 and 2 can be attributed to the low value of the solubility constant of CdS compared to ZnS. The growth of the particles is very slow compared to nucleation. This can be attributed to several processes. The various reactions following the growth of the particles are
Zn2+ -t- S2- + (CdS), Cd2+ + s2- + (Cds),
-
-
Cd,ZnS,+,
(CdS),,,
growth
(4)
with reactive ions
(5)
Experimentally it is observed that Cd2+reacts selectively with S2-. In such a case, the Zn2+ concentration is large compared with the concentration of (CdS),. This indicates that the rate constant of reaction 4 is very small. Reaction 5 is very slow because most of the sulfide ions already react with Cd2+ and the remaining sulfide concentration is very low. Concerning reaction 6, each cluster is located inside the water pool. The cluster concentration is very low compared to the micellar concentration. Hence most of the micellar exchange processes will involve empty droplets, and the probability of micellar exchange between filled droplets is very low. Then the very low increase in the growth of the particles can be explained by the fact that reactions 4-6 are either very slow or have a very low probability.
V. Conclusion From the data given in this paper we concluded that there is formation of solid solutions of Cd,Znl-,S crystallites when the synthesis is made with equimolar or the presence of an excess of sulfide ions. Absorption spectra of Cd,Znl-,S crystallites were recorded. They are red shifted by increasing the size of the particles or by increasing the composition in cadmium,y. The size of the Cd,Znl-,S particles depends not on the composition, y , but on the water content. In the presence of an excess of cations, CdS particles are preferentially formed because of a faster nucleation of CdS than ZnS.
17416 J. Phys. Chem., Vol. 99,No. 48, 1995
Acknowledgment. We thank the N.A.T.O. Collaborative Research Grand for their support (CRG 941221). References and Notes (1) Pileni, M. P. J. Phys. Chem. 1993,97,6961. Meyer. M.; Wallberg, C.; Kurihada, K.; Fendler, J. H. J. Chem. SOC., Chem. Commyn. 1984.90. Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986,125,299. Dknhauser,T.; O’Neil, M.; Johansson, K.; Whitten, D.; McLendon, G.J. Phys. Chem. 1986, 90, 6074. Lianos, P.; Thomas, J. K. J. Colloid InterSace Sci. 1987, 117, 50. Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. Atkinson, P. J.; Grimson, M. J.; Heenan, R. K.; Howe, A. M.; Robinson, B. H. J. Chem. SOC.,Chem. Commun. 1989, 1807. Petit, C.; Lixon, P.; Pileni, M. P. J. Phys.Chem. 1990, 94, 1598. Korlan, 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. Towey T. H.; Khan-Lodl, A,; Robinson, B. H. J. Chem. SOC.,Faraday Trans. 1990, 86, 3757. Chandler, R. R.; Coffer, J. L. J. Phys. Chem. 1991, 95, 4. Motte, L.; Petit, C.; Boulanger, L.; Lixon, P.; Pileni, M. P. Langmuir 1992, 8, 1049. Pileni, M. P.; Motte, L.; Petit,C. Chem. Mater. 1992, 4, 338. Pileni, M. P.; Lisieki, I.; Motte, L.; Petit, C.; Cizeron, J.; Moumen, N.; Lixon, P. Prog. Colloid Polym. Sci. 1993, 93, 1. Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. Lisieki, I.; Pileni, M. P. J. Am. Chem. SOC. 1993, 115, 3887. (2) Asaolu, I. A.; Blott, B. H.; Khan,W. I.; Melville, D.Thin Solid Films 1983, 99, 263. Zhao, X. K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (3) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G.D.; Cox, D. E.; Moller, K.; Bein, T. J. Am. Chem. SOC.1989, 111, 530. (4) Watzke, H. J.; Fendler, J. H. J. Phys. Chem. 1985,89, 854. Tricot, Y. M.; Fendler, J. H. J. Phys. Chem. 1986, 90, 3669. Tricot, Y. M; Manassen, J. J. Phys. Chem. 1988,92,5239. Youn, H. C.; Baral, S.;Fendler, J. H. J. Phys. Chem. 1988, 92, 6320. (5) Osakada, K.; Taniguchi, A,; Kubota, E.; Dev, S.; Tanaka, K.; Kubota, K.; Yamamoto, T. Chem. Mater. 1992, 4, 562. (6) Murray, C. B.; Nons, D. J.; Bawendi, M. G.J. Am. Chem. SOC. 1993, 115, 8706. (7) (a) Brus, L. E. J. Chem. Phys. 1983, 79,5566. Rossetti, R; Ellison, J. L.; Bigson, J. M.; BNS, L. E. J. Chem. Phys. 1984, 80,4464. Nozik, A. J.; Williams, Ferd.; Nenadocic, M. T.; Rajh, T.; Micic, 0. I. J. Phys. Chem. 1985,89, 397-399. Bawendi, M. G ;Steigerwald, M. L.; Brus, L. E. Annu. Rev. Phys. Chem. 1990,41,477. Henglein, A. Chem. Rev. 1989, 89, 1861. Henglein, A.; Kurmer, A.; Jondre, E.; Weller, H Chem. Phys. Lett. 1986, 132, 133. Wang, Y.; Herron, N. Phys. Rev. B 1990, 41, 6079. (b) Wang,
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