1372
Langmuir 2001, 17, 1372-1380
“Supracrystals” Made of Nanocrystals. 2. Growth on HOPG Substrate A. Courty,† O. Araspin,‡ C. Fermon,§ and M. P. Pileni*,† Laboratoire L.M.2.N, URA CNRS 1662, Universite´ P. et M. Curie (Paris VI), BP 52, 4 place Jussieu, 75252 Paris Cedex 05, France, CEA-Saclay, DSM-DRECAM-SCM, 91191 Gif-sur-Yvette Cedex, France, and CEA-Saclay, DSM-DRECAM-SPEC, 91191 Gif-sur-Yvette Cedex, France Received August 17, 2000. In Final Form: November 2, 2000
Silver nanoparticles in 3D superlattices with heights and widths of several tens and hundreds of micrometers, respectively, have been fabricated, and the experimental parameters controlling their size and shape have been clearly identified. These parameters are the substrate temperature during the solvent evaporation, the initial concentration of the colloidal solution, and the deposition procedures. Scanning electron microscopy and small-angle X-ray diffraction are used to characterize the supracrystal phases.
I. Introduction Formation of two- or three-dimensional nanocrystal superlattices is of great interest because it provides a new horizon to study collective physical behavior resulting from interaction between neighboring particles. In most cases, arrays are spontaneously formed by evaporating, in air and at room temperature, the solvent on a substrate like amorphous carbon, highly oriented pyrolytic graphite (HOPG), or mica. A wide variety of 3D superlattices made of metal and semiconductor nanocrystals have been obtained.1-5 Little is known about the experimental parameters that make it possible to control the size and the morphology of these nanocrystal aggregates. In this paper, we report changes in the structure, size, and shape of 3D superlattices made of 5 nm silver nanocrystals and coated with dodecanethiol. Various parameters such as substrate temperature during solvent evaporation and initial colloidal solution concentration influence strongly the nucleation and growth of the 3D superlattices. II. Experimental Section Materials. Sodium di(2-ethyl-hexyl) sulfosuccinate Na(AOT) was purchased from Sigma. Isooctane, hexane, and pyridine were from Fluka. Hydrazine and dodecanethiol were obtained from Prolabo (France) and Janssen Chemicals, respectively. The materials were not purified any further. Silver di(2-ethyl-hexyl) sulfosuccinate, Ag(AOT), was prepared as described previously.6 The HOPG substrate was obtained from Carbon Loraine (France). * To whom correspondence should be addressed. † Laboratoire S.R.S.I, URA CNRS 1662, Universite ´ P. et M. Curie (Paris VI). ‡ CEA-Saclay, DSM-DRECAM-SCM. § CEA-Saclay, DSM-DRECAM-SPEC. (1) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (2) Motte, L.; Billoudet, F.; Lacaze, E.; Douin, J.; Pileni, M. P. J. Phys. Chem. 1997, 101, 138. (3) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950. (4) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Phys. Chem. 1998, 49, 371. (5) Harfenist, S. A.; Wang, Z. L.; Whetten, R. L.; Vezmar, I.; Alvarez, M. M. Adv. Mater. 1997, 9, 817. (6) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2026.
Apparatus. Scanning Electron Microscopy (SEM). A JSM 840A instrument was used. With the substrate tilted at an angle R for the scan, the height (H) of the aggregates was determined from H ) h/sin R, where h is the height measured from the image and the scale bar is taken into account. X-ray Reflectivity. X-ray reflectivity measurements were performed with a homemade θ/2θ diffractometer using nickelfiltered Cu KR (λ ) 1.54 Å) radiation from a rotating anode source. The data were computer controlled and optimized with variable measuring times, attenuators, and power of the X-ray generator for different q vectors. The spectrometer resolution is 0.01 Å-1. Substrate Temperature Control. A commercial thermocouplebased module from the company Comptoire Electronique du Languedoc in France was used to control the substrate temperature. The temperature can vary between -20 and +80 °C. The temperature of the module is maintained in this range by a homemade water circulation and voltage regulator.
III. Results and Discussion The 5 nm silver nanocrystals were dispersed in hexane (see Appendix, the synthesis mode) and their concentration was fixed at 3 × 10-6 M (excepted when otherwise specified in the text). The substrate temperature is controlled by a thermocouple-based module. Energy-dispersive X-ray measurements were performed on the top of the large aggregates obtained at various temperatures. In all the samples, the aggregates consist of silver, carbon, and sulfur atoms. This is fully consistent with a crystal made of silver nanosized particles coated by dodecanethiol. At the end of the experiment, the coverage sample is washed with hexane and the nanoparticles are redispersed in the solvent. The absorption spectrum of the latter solution is similar to that used to cover the substrate (free particles in hexane), indicating that the aggregates correspond to self-assembled silver nanocrystals. For any of the procedures, a large coverage of the substrate by nanocrystals is observed. SEM is used to visualize the aggregate formed and thus to give information about the changes in their size and also in their morphology with the deposition procedure used. The data presented below are highly reproducible. Two procedures were used to deposit the nanocrystals on a substrate: (i) Two drops (2 × 10 µL) of a silver nanocrystal solution are deposited on the HOPG substrate. The evaporation process takes place under air.
10.1021/la001181l CCC: $20.00 © 2001 American Chemical Society Published on Web 02/07/2001
Supracrystals Made of Nanocrystals
Langmuir, Vol. 17, No. 5, 2001 1373
Figure 1. SEM patterns at various magnifications obtained from the deposition of two drops (2 × 10 µL) of a highly concentrated silver colloidal solution (3 × 10-6 M) on HOPG substrate at different temperatures: T ) 10 °C (A), T ) 23 °C (B), and T ) 33 °C (C). The insets show a high magnification of some of the aggregates present in A, B, and C.
(ii) The HOPG substrate is horizontally immersed in 200 µL of a silver nanocrystal solution at a fixed (3 × 10-6
M) concentration (excepted when otherwise mentioned in the text). This occurs in a hexane vapor atmosphere. The
1374
Langmuir, Vol. 17, No. 5, 2001
Courty et al.
Figure 2. SEM patterns at low magnification obtained from HOPG substrate immersed in 200 µL of a highly concentrated silver colloidal solution (3 × 10-6 M) and dried in a hexane vapor atmosphere. The different substrate temperatures are T ) 0 °C (A), T ) 5 °C (B), T ) 10 °C (C), T ) 25 °C (D), T ) 35 °C (E), and T ) 50 °C (F).
evaporation time is then greatly reduced compared to that taking place under air (95 min instead of 9 h). Procedure i. By using procedure i, the shape and size of the aggregates differ with the substrate temperature. However, two specific behaviors are observed: (a) Low Substrate Temperature (0 e T e 10 °C). At 10 °C, long ribbons are formed with the appearance of small cracks (Figure 1A). The average height is 3 µm. Between the ribbons, small aggregates are deposited on a film made with a height of around 150 nm, made of few layers of nanocrystals. The aggregates deposited on the film are rather inhomogeneous in shape but are well faceted with an average size and width of 5 µm (insets of Figure 1A). (b) High Temperature (23 e T e 50 °C). At the substrate temperature of 23 °C (Figure 1B) and 33 °C (Figure 1C), the long ribbons observed at low temperature disappear. Only the film of nanocrystals on which aggregates are deposited remains. The average size of a well-faceted aggregate is larger than that observed at 10 °C, with an average width and height of 10 µm. To explain such a change in behavior with substrate temperature, the nonequilibrium regime has to be taken into account. Because of evaporation under air, a temperature gradient is created across a film of volatile solvent (hexane) deposited on the substrate. The increase in the substrate temperature induces an increase in the gradient with temperature disturbances at the liquid interface. Convective flows are then induced. Similar instabilities have been recently observed in our laboratory,7 where it
is demonstrated that the self-organization of nanocrystals drastically changes with evaporation rate. Procedure ii. By using procedure ii, it is shown that variation of the substrate temperature plays an important role in the size and shape of the aggregates. Figure 2 shows a large coverage with more matter in the center than in the border. However, the behavior of aggregate shape markedly differs with the temperature of the substrate. In the following, the results are presented in two temperature ranges: (a) Low Substrate Temperature. From 0 to 10 °C, a rough surface is observed (Figure 3). At 0 °C, some discontinuities in the deposition are seen (Figure 3A) with a height varying from 100 to 500 nm. At higher temperatures (5 and 10 °C), Figure 3B,C shows compact islands formed by the stacking of several layers of nanocrystals with the appearance of defects and a very rough surface. Holes are observed (see black spots on SEM patterns). Their areas are not well-defined and vary from 10 to 250 µm2. With the sample tilted (45°), their heights are found to be around 3 µm. Small-angle X-ray diffraction measurements of the three samples (0, 5, and 10 °C) show a peak without any secondorder peak (Figure 4). This indicates stacking of the monolayers made of nanocrystals with large defects. On increasing the temperature from 0 °C (Figure 4A) to 10 °C (Figure 4B), the first peak intensity increases, (7) Maillard, M.; Motte, L.; Ngo, T.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 11871.
Supracrystals Made of Nanocrystals
Langmuir, Vol. 17, No. 5, 2001 1375
Figure 3. SEM patterns at high magnification and at zero tilt obtained from HOPG substrate at low temperatures: T ) 0 °C (A), T ) 5 °C (B), and T ) 10 °C (C), immersed in 200 µL of a highly concentrated silver colloidal solution (3 × 10-6 M) and dried in a hexane vapor atmosphere. The insets in A, B, and C are at a tilt of 45°.
indicating a decrease of defects. However, the secondorder peak is still not observed. The average peak position
does not change with substrate temperature, suggesting that the average distance, d, between two nanocrystal
1376
Langmuir, Vol. 17, No. 5, 2001
Courty et al.
it can be applied for the surface, where σs, σd, and s0 are the surface and the diameter standard deviations and the mean surface, respectively, with σs ) 2σd. The mean surface is deduced from s0 ) πd2. The surface distribution is obtained from a simulation of
( ) ln2
p(s) )
Figure 4. X-ray reflectograms of silver nanocrystals in 3D superlattices on HOPG substrate. The 3D superlattices have been obtained for different substrate temperatures during solvent evaporation: T ) 0 °C (A), T ) 10 °C (B), and T ) 35 °C (C). In (C), the solid lines with squares are simulations as explained in the text.
monolayers is around 6.5 nm. This distance is large compared to that deduced by assuming a compact stacking of monolayers (5.6 nm). From these data, it is concluded that aggregates are made of nanocrystal layers with a rather low crystallinity. This is explained by a slow diffusion of nanocrystals during the evaporation time. They are locked during the solvent evaporation, and particle-particle attractive interactions are rather low. (b) High Substrate Temperature. At 25, 35, and 50 °C, the SEM patterns markedly differ (Figure 2D-F). Obviously, the amount of material is much larger in the center than in the border of the aggregates (the border behaves as was observed at lower nanocrystal concentrations, see below). Small cracks are seen in the center of the substrate. At high substrate temperature (50 °C), large rings appear (Figure 2F). This is due to convection processes induced by the difference in temperature between the substrate (50 °C) and the hexane vapor (room temperature). Figure 5 is a high magnification of the substrate centers showing that the silver nanocrystal aggregates are well separated by an average distance of 10 µm. The aggregate-surface distribution is determined for each substrate temperature (T ) 25, 35, and 50 °C) by measuring the surface of 300 to 500 aggregates from SEM patterns. Usually, the lognormal distribution is used for the diameter.8,9 However, (8) Kaiser, R.; Miskolczy, G. J. Appl. Phys. 1970, 41, 1064.
s s0
1 exp 2σs2 dσsx2π
The best fit of the aggregate surface distribution (Figure 6) is obtained for a given value of s0 and σd. Table 1 shows a low size distribution (calculated on a millimeter scale). The average surface, s0, remains on the same order of magnitude for any substrate temperature. The average aggregate height is determined by tilting the sample (45°), and it increases with increasing substrate temperature (Table 1). The broken edges and the roughness of the aggregate surface are a consequence of the thermal stress. X-ray reflectivity measurements performed on the sample prepared by drying the substrate at 35 °C show large silver nanocrystal aggregates of rather homogeneous shapes. Two superlattice peaks with widths (∆q ) 0.1 Å-1) much larger than the spectrometer resolution (0.01 Å-1) are observed (Figure 4C). No secondary peaks are obtained at high q values (q > 0.25 Å-1). The ratio between the first-order and second-order intensities (I1/I2 ) 25) agrees with the calculated profile given for a compact structure. However, the large width of the peaks is rather surprising for a compact structure. A short correlation length cannot explain this, as this would give only 1-2 monolayers for that length. Let us assume that each peak is due to a distribution of periods. The position of the peaks gives the period d. A simple model based on a minimal distance of 1.8 nm between particles and a varying angle θ of tetrahedrons (Figure 7) is in agreement with the experimental data (Figure 4C). For the simulation, six different configurations corresponding to six various periods (d ) 6.55, 6.1, 5.7, 5.2, 4.7, and 4.2 nm) are considered (Figure 7 gives two examples of such configurations). The mean period, deduced from the mean position of the peaks, is 5.6 nm. This agrees with the theoretical distance for a compact face-centered cubic (fcc) or hexagonal close-packed (hcp) structure with a 6.8 nm distance between particle centers. The coherence of each period is larger than five layers. In addition, the simulation gives a roughness of 0.4 nm, consistent with the polydispersity of the particles (around 13%). The two superlattice peaks are thus composed of several peaks corresponding to different periods d. The contributions of the different peaks have no great significance as the absorption of X-rays plays an important role. However, the shape of the first peak is not regular. From this, it is deduced that the smallest periods, 4.7 and 4.2, are dominant (50%) compared to the larger ones. These period variations are attributed to differential solvent evaporation. The residual solvent not yet evaporated drives away the silver nanocrystals that induce an increase in the particle-particle distance and variable periods. This is in agreement with data obtained by scanning tunneling microscopy under vacuum where monolayers of nanocrystals were visualized only after heating the substrate, which induces the solvent desorption.10 The residual solvent takes a long time to evaporate because the aggregates are very thick (around 15 µm). It is thus more easily evaporated near the surface (9) Granquist, C. G.; Buhrmann, R. A. J. Appl. Phys. 1976, 47, 2200. (10) Taleb, A.; Silly, F.; Gusev, A. O.; Charra, F.; Pileni, M. P. Adv. Mater. 2000, 9, 633.
Supracrystals Made of Nanocrystals
Langmuir, Vol. 17, No. 5, 2001 1377
Figure 5. SEM patterns at high magnification and at zero tilt obtained from HOPG substrate at high temperatures: T ) 25 °C (A), T ) 35 °C (B), and T ) 50 °C (C), immersed in 200 µL of a highly concentrated silver colloidal solution (3 × 10-6 M) and dried in a hexane vapor atmosphere. The insets in A, B, and C are at a 45° tilt.
of the aggregates. Moreover, X-ray diffraction measurements have been performed on one- and five-month-old samples.11 They show a decrease of the mean distance
between particles with aging and a large damage in the superlattice structure. This confirms that residual solvent remains bound to the alkyl chains and perturbs the
1378
Langmuir, Vol. 17, No. 5, 2001
Courty et al. Table 2. Surface and Height Distributions of Silver Nanocrystal Aggregates for Different Nanocrystal Concentrationsa nanocrystal concentration (µm2)
s0 h (µm) a
Figure 6. Surface histograms of silver nanocrystal aggregates obtained for different substrate temperatures during solvent evaporation: T ) 25 °C (A), T ) 35 °C (B), and T ) 50 °C (C).
Figure 7. Sketch showing the stacking of silver nanocrystal monolayers for two different configurations: (a) d ) 5.7 nm and a ) b ) 6.8 nm and (b) d ) 4.7 nm, a ) 6.8 nm, and b ) 8.7 nm. Table 1. Surface and Height Distributions of Silver Nanocrystal Aggregates for Different Substrate Temperatures during Solvent Evaporationa substrate temperature s0 (µm2) σd h (µm) a
25 °C
35 °C
50 °C
2100 0.3 10
2700 0.4 15
1700 0.3 25
The nanocrystal concentration is fixed at 3 × 10-6 M.
superlattice structure. In addition, it takes several months to evaporate. Nevertheless, it is difficult to characterize more precisely the effect of solvent evaporation on the silver nanocrystal organization. The solvent evaporation (11) Courty, A.; Fermon, C.; Pileni, M. P. Adv. Mater., in press.
10-7 M
3 × 10-7 M
3 × 10-6 M
50 0.5
100 3
2700 15
The substrate temperature is fixed at 35 °C.
is indeed too long to be following by differential scanning calorimetry measurements. The accuracy related to the device cannot be stable for a long period of time because the aging is observed only after one month. From the data described above, it is concluded that the substrate temperature plays an important role in the size, shape, and stacking of supra-aggregates. In these experimental conditions (ii), the major processes are the nanocrystal diffusion and the particle-particle interactions. As a matter of fact, the system is in a “quasi” equilibrium state (hexane vapor atmosphere) with a similar evaporation rate. The increase in the supraaggregate height and in the stacking monolayer order with increasing substrate temperature is attributed to an increase in nanocrystal diffusion that favors the particleparticle attractive interactions. To confirm this claim, similar experiments were performed under air and hexane vapor with the substrate temperature kept at 35 °C. Under hexane vapor, well-defined edges are obtained (Figure 8A,B). Under air evaporation (Figure 8C,D), the surface defects increase and the average height of the supraaggregate drastically decreases (5 µm instead of 15 µm). Furthermore, the edges are not as well-defined as those obtained under hexane vapor. Under air evaporation, the system reaches a nonequilibrium regime with a decrease of the evaporation time, which is 95 min instead of 9 h under hexane vapor. This creates a temperature gradient across the film. Infinitesimal temperature disturbances at the liquid interface are amplified by the temperature gradient, and convective flows are induced, driven by the surface tension. The decrease in the evaporation time induces an increase in the temperature gradient and in the Marangoni number and then an increase in the instabilities. By reducing the evaporation rate, the system equilibrates faster than the heat loss by the evaporation process. Under such conditions, instabilities disappear and temperature gradient and convective flows do not perturb attractive interactions between particles. At this point, one question arises: does the initial nanocrystal concentration play an important role in the shape and size of nanocrystals? The temperature is kept at 35 °C. The morphology of the aggregates does not drastically change on decreasing by a factor of 10 the nanocrystal concentration (3.10-7 M) (Figure 9A). However, the average surface decreases with decreasing nanocrystal concentration (Table 2). A further decrease in the nanocrystal concentration (10-7 M) induces formation of smaller aggregates with a large variety of shapes (Figure 9B). These drastic changes are explained in terms of a large number of nucleation sites. The decrease in the initial concentration blocks the growth of each nucleation site in large supracrystals. Similar behaviors are observed at the border of the large aggregates formed at a high initial nanocrystal concentration (Figure 2E). This is well demonstrated by enhancing the border of the large aggregate (Figure 9C) where patterns similar to those obtained by using a low initial nanocrystal concentration (Figure 9B) are observed.
Supracrystals Made of Nanocrystals
Langmuir, Vol. 17, No. 5, 2001 1379
Figure 8. SEM patterns at a 45° tilt and at different magnifications obtained by drying the silver colloidal solution (3 × 10-6 M) on a hot substrate at 35 °C in a hexane vapor atmosphere (A and B) and in air (C and D).
Figure 9. SEM patterns of samples obtained after deposition of silver nanocrystal solution at different concentrations: (A) 3 × 10-7 M and (B) 10-7 M. (C) SEM patterns, at high magnification, of the border of the sample obtained after deposition of silver colloidal solution (3 × 10-6 M). In A, B, and C, the substrate temperature during solvent evaporation is 35 °C. The insets in A and B at a 45° tilt show a high magnification of the aggregates.
1380
Langmuir, Vol. 17, No. 5, 2001
IV. Conclusion The experimental parameters controlling the size and the morphology of the 3D silver nanocrystal superlattices have been clearly identified. We show that the substrate temperature during solvent evaporation and the initial colloidal solution concentration markedly influence the growth of the 3D superlattices. These data are highly reproducible. The crystalline structure of silver nanocrystal aggregates is demonstrated by small-angle X-ray reflectivity measurements. The aggregates formed for a substrate temperature of 35 °C appear to be made of the best compact stacking of silver nanocrystal monolayers. The mean distance between two monolayers corresponds to the theoretical distance for a fcc or hcp structure with a 6.8 nm distance between particle centers. “Supra” crystals of rather large size (several tens of micrometers in heights and in widths) can be thus obtained with a very high reproducibility. Moreover, a temperature effect on the aggregate crystalline structure has been shown. The range ordering indeed decreases with the substrate temperature during solvent evaporation. This result is consistent with a less important diffusion of the silver nanocrystals at a low temperature that does not favor their organization. Acknowledgment. We are grateful to Dr. O. Anitoff for his help with the temperature control apparatus. Appendix The synthesis of Ag nanosized particles in reverse micelles and control of the size and distribution were
Courty et al.
described previously.12 The silver nanoparticles are obtained by a reduction reaction between Ag+ ions and hydrazine in water in oil droplets that had an average diameter of 0.6 nm.13 (Ag)n nanoparticles are formed with an average diameter of 4.5 nm and 43% of polydispersity. Dodecanethiol (2 µL/cm3) is then added to the solution, and there is a selective reaction with the silver atoms at the interface of the particles. Ethanol is next added to this solution, inducing precipitation of the dodecanethiolcoated silver particles, and the precipitate is then dispersed in hexane. To reduce the size distribution, a size-selected precipitation process, as described in detail in a previous paper,12 is used: Pyridine is progressively added to a hexane solution containing the silver-coated particles. At a given volume of added pyridine (roughly 70%), the solution becomes cloudy and a precipitate appears, indicating agglomeration of the largest particles. The solution is centrifuged and an agglomerated fraction rich in large particles is collected, leaving the smallest particles in the supernatant. The precipitate, dispersed in hexane, forms a homogeneous clear solution. Silver nanoparticles (5 nm diameter) are thus obtained with a rather low size distribution (around 13%). The particles are highly stable. By deposition of two drops of a dilute solution on a cleaved carbon substrate, a monolayer made of nanoparticles with spontaneous hexagonal organization is observed.12 LA001181L (12) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950. (13) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961.
