J. Phys. Chem. 1995,99, 16425-16429
16425
Self-Assembled Monolayer of Nanosized Particles Differing by Their Sizes L. MotteJJ F. Billoudet; and M. P. Pileni**?** Universit6 Pierre et Marie Curie, Laboratoire S. R. S. I., BP 52, Bat 74, 4 place Jussieu, 75005 Paris, France, and C.E.N. Saclay, DRECAM-SCM, Bat 522, 91191 Gif sur Yvette, France Received: July 5, 1995@
Reverse micelles are used to control the size of silver sulfide particles. After these particles are coated with dodecanethiol, they are extracted from micelles and dispersed in heptane solution. The size of the particles is determined by transmission electron microscopy and small-angle X-ray scattering. On a solid support, a self organization of the nanosized particles in a hexagonal network is observed. The self assembly is obtained for various particle sizes (from 3 to 6 nm) and solid supports. The network takes place on a long-range domain which is usually larger than 5 x lo5 nm2, By leaving a solid support in the colloidal solution, multilayers of nanoparticles are observed, indicating that the self assembly is mainly due to Van der Waals and dispersion forces.
Introduction In materials physics and chemistry, preparation an artificially engineered semiconductor structure of nanosized particles with new properties is a major goal. The electronic and optical properties of “quantum dots” or semiconductor crystallites that are small in comparison to the bulk electron delocalization length (1-10 nm) are the subject of investigation.’ The ability to assemble molecules into well defined two- and three-dimensional spatial configurations is a major goal in the field of selfassembled monolayers. Such assemblies can then be used to build up more complex structuresin three dimensions? enabling chemists to engineer complex organic structures on top of macroscopic surfaces. Because of the limitations in the ability to control the cluster environment, several experiments (such as ones to study transport properties, tunneling effect, etc.) have not been performed. Currently the nanocrystals can be isolated as a powder, dispersed in a solvent, placed in inorganic glasses or polymers for optical experiments, and deposited by evaporation on graphite for scanning tunneling microscopy imaging. Organized assemblies of crystallites should show interesting physical properties. Difficulties in producing and manipulating nearly monodisperse nanometer size crystallites of arbitrary diameter have prevented the fabrication of such well defined two- or three-dimensional structures. As a recent exception is special case of single crystals of identical 1.5 nm CdS cluster^.^ Self assembly techniques have also been used to deposit submonolayers of nanocrystallites on a surface. Mulvaney et aL4 show formation of a network formed with gold particles by electrodeposition. Bawendi et al.5 used a Langmuir-Blodgett technique to deposit submonolayers of CdSe nanocrystallites on a surface. Alivisatos et chemically bound nanocrystals on gold and aluminum surfaces. By using lithographic methods, Heitman et al.’ made large quantum dots and showed that electronic behavior can arise from interdot collective interactions. Silver sulfide particles, AgzS, could be used as a photosensitizer for photographic purposes.8 In this paper the formation of a monolayer organized in a hexagonal network over a long distance range without any
* To whom correspondence should be addressed at the UniversitC Pierre et Marie Curie. UniversitC Pierre et Marie Curie. C.E.N. Saclay. Abstract published in Advance ACS Abstracts, September 1, 1995.
*
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0022-365419512099-16425$09.00/0
extemal forces is demonstrated. The monolayers are obtained for various particle sizes having a diameter equal to 3, 4, and 6 nm. The self assembly takes place on various solid supports. Layers on three dimensions can be obtained when the support is left for some time in the presence of the solution containing the nanocrystals. Hence a three-dimensional crystal is obtained. Experimental Section Products. Sodium bis(2-ethylhexyl)sulfosuccinate (usually called NaAOT), sodium sulfide (NazS), and dodecanethiol were obtained from Sigma, Janssen, and Merck, respectively. The solvents, such as isooctane, heptane, and ethanol, were purchased from Fluka. Silver bis(2-ethylhexyl)sulfosuccinate (AgAOT) was prepared as described previo~sly.~ Equipment. Optical absorption spectra were collected at room temperature on a UVIKON 93 1 spectrophotometer using a 1 cm quartz cuvette. Transmission electron microscopy (TEM) was performed by using a Jeol electron microscope, model JEM lOOCX 11. A drop of the solution containing the particles was evaporated on a carbon grid. The small-angle X-ray scattering (SAXS) experiments were performed at L.U.R.E., Orsay, France, on the D22 diffractometer. X-ray diffraction (XRD) measurements were performed with a STOESIEMENS powder diffractometer; copper Ka radiation, , I= 1.542 A, was used. Small-Angle X-ray Scattering Treatment. The scattered intensity I(q) is
where 4 is the wave vector and is equal to 4n(sin 8)lA (28 is the diffusion angle) and P(q) and S(q) are the form factor and the structure factor, respectively. The form factor gives the shape of the aggregates, and the structure factor takes into account interactions between the aggregates. The particle volume fraction, estimated from their average size and assuming all the silver ions reacted, is found to be equal to 0.01%. At such a low particle volume fraction, aggregate interactions can be neglected and S(q) is assumed to be equal to 1. The electron density of capped Ag2S crystallites is higher than that of the particle surface formed by a dodecanethiol (Cl2H25SH)layer, and the scatter is due to spherical shells. The form factor is described by an inhomogeneous sphere model: 0 1995 American Chemical Society
16426 J. Phys. Chem., Vol. 99, No. 44, 1995
Motte et al.
with
where VAgZS and rAg2S are the volume and the radius of the particle without a layer and UIayer and qayer are the volume and the radius of the particle surrounded by the layer formed by dodecanethiol (C12HaSH) at the particle surface. The electronic densities are fixed as @layer = 0.3 e - ~ i i - ~@Ag2S , = 2.62 and ,os= 0.2 e-*A-3. The thickness of the layer is defined as nayer - rAg2S = 4 A. The polydispersity in size has been estimated either by using the size distribution determined by TEM or by assuming a Gaussian distribution. In the first case, the sum of the form factors due to the various sizes is compared to the scattering data. In the latter case, a root mean square deviation, a, from the mean particle radius, (rAgzS), is deduced as previously described.IO From the Porod plot, I(q)g' vs q, the characteristic diameter, Dc, is related to the first maximum and minimum of this representation by the following relationship:
14.5 nm
15 23 32 4.1 49 58 67 75 DIAMETER (nm)
H
Figure 1. Histograms and TEM of particles made in reverse micelles at water contents equal to 5, 10, and 20. 2
A
D,(nm) = 0.54/qmax = 0.9/qmi,,
(3)
1.5 1
From the Guinier plot, ln(I(q)) vs q2, the gyration diameter, D,, is deduced.
0.5
Synthesis of Various Sizes of ABS Nanocrystallites The quaternary system NaAOT- AgAOT-water-isooctane shows behavior similar to that demonstrated by the system without silver derivatives." The water droplet diameter, &, increases linearly with the water content12 (& (nm) = 0.30~). The ability of reverse micelles to exchange their water content is used to make nanosized silver sulfide particles. As has been observed previously, for other semiconductors or metallic particle^,^^ the size of the Ag2S particles is controlled by the water content of reverse mi~el1es.I~ Preparation of Ag2S nanocrystallites is achieved by mixing a 0.1 M NaAOT reverse micellar solution containing AgAOT with a sodium sulfide (Na2S) aqueous solution. The water content is fixed by the amount of aqueous solution containing sodium sulfide. The concentrations of AgAOT and Na2S are 4 x M. For water contents equal to 5, 10, and 20, the electron microscopy pattern and histograms show an increase in the average size from 2, 3, and 6 nm, respectively (Figure 1). The polydispersity in size is found to be equal to 30%. Dodecanethiol is added to the reverse micellar system containing Ag2S nanocrystallites. The micellar solution is then evaporated, and the resulting powder is washed with ethanol to remove AOT surfactant. Only the biggest particles are extracted from the micellar solutions. Later in the text it will be demonstrated that the average size of particles extracted from a micellar solution having a water content equal to 5, 10, and 20 is 3, 4, and 6 nm, respectively. These coated particles are called in the text A, B,and C,respectively. The X-ray and electron diffraction made on coated particles (A, B, and C) show a monoclinic structure, as is obtained in the bulk phase15 (called the /?Ag2S phase). Analysis of the relative amount of silver and sulfur is not possible because sulfur derivatives are provided by the sulfide ions participating in formation of the particle and by the
E I ,5 1
0.5
250 350 450 550 650 750
WAVELENGTH (nm)
Figure 2. Absorption spectra of particles A, B, and C dispersed in heptane.
dodecanethiol derivative bound to the interface. Furthermore, syntheses are performed in the presence of an excess of sulfide ions, which could induce an excess of sulfide ions at the interface. The particles A, B, and C are dispersed in heptane with formation of an optically clear solution. Results The absorption spectra of particles A, B,and C dispersed in heptane do not drastically change with particle size (Figure 2). The existence of tails makes difficult a precise determination of the band gap. To determine the band gap, the absorption
Self-Assembled Monolayer of Nanosized Particles
J. Phys. Chem., Vol. 99, No. 44, 1995 16427
TABLE 1: Band Gap Energy (E (eV)) Deduced from the Absorption Spectrum of Silver Suffide and Average Diameter Determined by TEM (&EM) of Particles A, B, and C Dispersed in Heptane A B
C
2.05 2.00 1.91
0.15 >
3.2 4 5.8
data are fitted with the following equation:I6
ahv = (hv - Eg)2 where u, hv, and Eg are the absorption coefficient, the photon energy, and the band gap, respectively. No drastic changes in the band gap of particles A, B, and C dispersed in heptane are observed (Table 1). The absorption spectra of particles made in reverse micelles and observed by TEM (Figure 1) are similar to those given in Figure 2. This indicates that the coating of particles does not change the absorption spectrum of the particles. SAXS experiments are performed on particles A, B, and C dispersed in heptane. The similarity in the radii, deduced from Guinier and Porod plots, indicates the scatter of spherical particles. The Porod plots obtained from A, B, and C show (Figure 3) a shift of the maximum and minimum to lower q values, indicating an increase in the average size of the particles (Table 2). The well defined maximum and minimum indicate a low polydispersity. From simulations and comparison to experimental data, the polydispersity in size is found to be equal to 14% (Table 2). A drop of the previous solutions (containing particles A, B, and C dispersed in heptane) is evaporated on carbon grids. TEM patterns (Figure 4) show that particles A are smaller than B, which are smaller than C. Histograms (Figure 5 ) indicate that the average size of particles A, B, and C is 3, 4, and 6 nm, respectively. The polydispersity in size is equal to 14%. These values are in good agreement with those deduced from SAXS (Table 2). Figure 4 shows formation of a monolayer made of nanosized particles organized in a hexagonal network. This is observed for all the particles used (A, B, and C). Whatever the particle size is, the same average interparticle distance is observed. It is close to the length of the hydrocarbon tail of the dodecanethiol derivative. From Figure 4 the average distance between particles is equal to 1.8 nm. The length of the dodecanethiol tail, L, can be evaluated from the empirical equation given by Bain et al.:I7
L (nm)= 0.25
+ 0.127n
where n is the number of CH2 groups (n = 12). The value calculated is found to be equal to 1.77 nm, which is very close to the average distance determined by TEM (Figure 4). The silver sulfide particles are well crystallized. Figure 6 shows the interreticular distance of the lattices obtained from high-resolution electron microscopy. The interreticular distance is found to be equal to 0.26 nm and is attributed to (-1 2 1) lattices planes. Comparison of histograms before (Figure 1) and after (Figure 4) extraction of particles from reverse micelles indicates a strong drop in the size distribution from 30% to 14%. Hence the extraction process selects the size of the particles (3, 4, and 6 nm). When a solid support such as various TEM grids or silicium wafers is left in heptane solutions containing A, B, or C, large domains of self assemblies are observed (Figure 7). The surface
0
0.2
0.1
0.3
q(A-1) Figure 3. Porod representations obtained from SAXS experiments on particles A, B, and C dispersed in heptane: (0)experimental data; (-) simulated curve.
TABLE 2: Average Radius of Particles A, B, and C Deduced from SAXS Analysis and TEMu D, (nm)
A B
c
max
min
D,(nm)
a(%)
4.3 4.6 5.5
4.4 5.8
4 4.4 5.5
14 14 14
DTEM(~~) 3.2 4 5.8
a D,, D,, &-EM, and u are the diameters of particles obtained from the Porod plot, from simulation, and from TEM,and polydispersity, respectively.
of such a network is larger than lo5 nm2and does not drastically change with the solid support used. The self assembly is observed for particles differing in size. From differences in the electron microscopic contrast and preliminary results obtained by grazing incidence small-angle X-ray scattering (GISAXS), multilayers of nanoparticles are formed. Each layer is arranged in a hexagonal network, as is shown in Figure 4. The superposition of the layers seems to be arranged in a face-centered cubic self-assembly. Discussion The absorption spectra of colloidal Ag2S particles differing in size do not drastically change. However a blue shift compared to the optical band edge of bulk silver sulfide, which is well-known to be at 1240 nm (1 eV), is obtained. The small change in the absorption spectra with the particle size is rather surprising. As a matter of fact, a change in the electronic properties is observed when the diameter of the particles approaches the excitonic diameter. This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. For nanocrystals having a direct gap absorption, a sharp absorption onset with multiple discrete features consistent with quantum confinement is observed.'s For indirect transition in nanocrystals, the electronic absorption shows no discrete features in the visible-IR region. However, for particles having an indirect transition, such as PbS or CdSe
16428 J. Phys. Chem., Vol. 99, No. 44, 1995
Motte et al. a"
I
AI
2.3 2.9 3.5 4.1 4.65 . 2 5.8 6.4 DIAMETER (nm)
H 145A
Figure 5. Histograms deduced from TEM experiments on particles A, B, and C.
Figure 4. Electron microscopy patterns of particles A, B,and C. submitted to high pressure,19a blue shift compared to the bulk phase with a decrease in the particle sizes is observed. In the case of silver sulfide a direct and an indirect transition take place. So, whatever the transition is, we would expect a large change in the energy band gap with a change in particle size. Because of the similarity of the Ag2S absorption spectra of particles observed before and after extraction from reverse micelles, this phenomenon cannot be attributed either to polydispersity in size or to an enhancement of the excitonic absorption by a change in the particle surface as described by Wang.20 As a matter of fact, the polydispersity in size decreases from 30% to 14% after extraction from micelles. Furthermore, addition of dodecanethiol induces strong changes in the interfacial surface, which favors the extraction process of the particles from reverse micelles. Such small changes in the absorption spectrum with a change in size cannot be easily explained. However this cannot be attributed to impurities. As a matter of fact, preliminary results indicate a shift in the excitation fluorescence spectrum, at 2 K, with a change in particle size. Figure 4 shows formation of a monolayer organized in a hexagonal network made with particles differing in size. Opposite to what has already been observed$-7 no external forces have to be applied to induce formation of self assemblies. The formation of a monolayer is observed by using various solid supports. This indicates the monolayer formation is not due to specific interactions with the solid surface. This phenomenon could be due to several factors acting simultaneously, such as (i) the low polydispersity in the particle size favoring the self organization of particles and (ii) the interactions between particles being strong enough to induce such organization. The best defined network is obtained for particles having the largest sizes (Figure 7). This can be attributed to an increase in Van der Waals interactions with size. Vrij et a1.21show, by SAXS experiments performed on silica spheres coated by octadecyl chains suspended in benzene, a strong increase in the attraction between the solute particles when the particle volume fraction increases. Similar attractive interactions can be taken into account for silver sulfide particles coated with dodecanethiol. By evaporation of the solvent
Figure 6. High-resolution electron microscopy pattern of particle C.
Figure 7. Electron microscopy pattern of a multilayer made with particles C.
containing the particles coated on the surface, the particle volume fraction increases, inducing an increase in the interaction between particles. The formation of multilayers confirms that such self assembly is mainly due to Van der Waals and dispersion forces.
Conclusion Reverse micelles have been used to control the size of silver sulfide particles. By coating the particles with a dodecanethiol
Self-Assembled Monolayer of Nanosized Particles derivative, the particles are extracted from micellar solution. Hence it has been possible to make powder nanosized particles which can be dispersed in heptane solution. The size of the particles has been determined by S A X S and TEM. A drop of such a solution containing a given size of particles is left on a solid support. This induces formation of a monolayer made of nanosized particles and organized in a hexagonal network. When a solid support is left in a colloidal solution, a self assembly organized in multilayers in the face centered cubic phase is observed. The various assemblies having the same organization differ by the size of the particles and do not depend on the solid support used.
References and Notes (1) Miller, D. A. B.; Chemla, D. S.; Schmitt-Rink, S. In Optical Nonlinearities and Instabilitv in semiconductors; Haw, - H., Ed.; Academic Press: Orlando, FL, 1988. (21 Ulman, A.: Tillman, N. Lannmuir 1989, 5, 1418. (3) Herron, N.; Calabrese, J. C i Farneth, W. E.; Wang, Y. Science 1993, 259, 1369. (4) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. ( 5 ) Dabbousi, B. 0.; Murray, C. B.; Rubner, M. F.; Bawendi, M. G. Chem. Mater. 1994, 6, 216. (6) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J . Am. Chem. Soc. 1992, 114, 5221. (7) Heitman, D.; Kotthaus, J. P. Phys. Today 1993, 46, 56.
J. Phys. Chem., Vol. 99, No. 44, 1995 16429 (8) De Rycke, G. L.; Henderickx, F. European Patent Application No. 892026 13.9, 1990. (9) Petit, C.; Lixon, P.; Pileni, M. P. J . Phys. Chem. 1993, 97, 12974. (10) Pitrk, F.; Regnault, C.; Pileni, M. P. Lungmuir 1993, 9, 2855. (11) Structure and reactivity in reverse micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989. (12) Pileni, M. P.; Zemb, T.; Petit, C. Chem. Phys. Lett. 1985, 118, 414. (13) Pileni, M. P. J . Phys. Chem. 1993, 97, 6961. Motte, L.; Petit, C.; Boulanger, L.; Pileni, M. P. Langmuir 1992, 8, 1049. Pileni, M. P.; Motte, L.; Petit, C. Chem. Mater. 1992, 4, 338. (14) Motte, L.; Billoudet, F.; Pileni, M. P. J . Mater. Sci., in press. (15) Lundolt Bornstein New Series Ilk Madelung, O., Ed.; SpringerVerlag: Berlin Heidelberg, 1983; Vol. 17e, p 156. (16) Scanlon, W. W. Phys. Rev. 1958, 109, 47. (17) Bain, C. D.; Evall, J.; Whitesides, G. M. J . Am. Chem. SOC.1989, 111, 7155. (18) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D. E.; Moller, K.; Bein, T. J . Am. Chem. SOC. 1989, 111, 530. Brus, L. E. J . Chem. Phys. 1983, 79, 5566. (19) Tolbert, S. H.; Herhold, A. M.; Johnson, C. S.; Alivisatos, A. P. Phys. Rev. Lett. 1994, 73, 3266. (20) Wang, Y.; Herron, N. J . Phys. Chem. 1991, 95, 525. Wang, Y.; Suna, A.; McHygh, J.; Hilinski, E. F.; Lucas, P. A,; Johnson, R. D. J . Chem. Phys. 1990, 92, 6927. (21) Vnj, A.; Jansen, J. W.; Dhont, J. K. G.; Pathmamanoharan, C.; Kops-Werkhoven, M. M.; Fijnaut, H. M. Faraday Discuss. Chem. SOC.1983, 76, 19.
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