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Ring Stain Effect at Room Temperature in Silver Nanoparticles Yields High Electrical Conductivity Shlomo Magdassi,*,† Michael Grouchko,† Dana Toker,‡ Alexander Kamyshny,† Isaac Balberg,‡ and Oded Millo*,‡ Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, and Racah Institute of Physics and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received April 6, 2005. In Final Form: June 21, 2005 We demonstrate that metallic rings formed spontaneously at room temperature via evaporation of aqueous drops containing silver nanoparticles (20-30 nm in diameter) exhibit high electrical conductivity (up to 15% of that for bulk silver). The mechanism underlying this self-assembly phenomena is the “ring stain effect”, where self-pinning is combined with capillary flow to form a ring consisting of close-packed metallic nanoparticles along the perimeter of a drying droplet. Our macroscopic and microscopic (applying conductive atomic force microscopy) transport measurements show that the conductivity of the ring, which has a metallic brightness, is orders of magnitude larger than that of corresponding aggregates developed without the ring formation, where high conductivity is known to appear only after annealing at high temperature.
Direct deposition of conductive patterns is of great importance for microfabrication of low cost electronic devices. Significant efforts are devoted toward the use of metallic nanoparticle dispersions for that task.1 However, room temperature deposition of such dispersions on a solid substrate or their application by ink-jet printing yield very poor electrical conductivity, even in the case where apparently dense nanoparticle aggregates are formed. Noticeable electrical conductance could be obtained so far in these systems only after sintering at elevated temperatures (about 300 °C), thus establishing good interparticle electrical contacts.1-3 In this letter, we present the first experimental evidence on obtaining high electrical conductivity (15% of that for bulk silver) by self-aggregation of metallic nanoparticles at room temperature, with no resort to post-annealing. This is achieved by spontaneous formation of metallic rings during evaporation of aqueous drops containing silver nanoparticles with an average size of 20-30 nm. Macroscopic transport measurements performed along with conductive atomic force microscopy (C-AFM), which provides information on the local conductance properties, have shown that the typical electrical conductivity of the rings is larger by orders of magnitude compared to that of corresponding silver nanoparticles aggregates where rings were not formed. The mechanism underlying this self-assembly phenomenon is the “ring stain effect”4,5 that results, in our case, in improved electrical contact between * Corresponding authors. E-mail:
[email protected];
[email protected]. † Institute of Chemistry and the Center for Nanoscience and Nanotechnology. ‡ Racah Institute of Physics and the Center for Nanoscience and Nanotechnology. (1) Fuller, S. B.; Eric, J. W.; Jacobson, J. M. J. Microelectromech. Syst. 2002, 11 (1), 54. (2) Magdassi, S.; Bassa, A.; Vinetsky, Y.; Kamyshny, A. Chem. Mater. 2003, 15, 2208. (3) Kamyshny, A.; Ben-Moshe, M.; Aviezer, S.; Magdassi, S. Macromol. Rapid Commun. 2005, 26, 281. (4) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (5) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62 (1), 756.
adjacent silver grains, probably due to the relatively large pressure exerted on them. The “ring stain effect” is observed, for example, when a coffee drop is spilled on a solid surface, and during drying of the liquid, a ring-like deposit along the perimeter is obtained4 due to migration of solute or particles to the edge of the drop. Ring formation was reported for several colloidal systems, when droplets of dispersions of nanoparticles are dried on a solid surface.6-10 The mechanism of ring formation has been explained in terms of correlations between wetting properties, surface tension, evaporation-driven convective flows,9,10 capillary flow and pinned contact line, as described by Deegan et al. in several publications.4,5 It was found that after all of the liquid is evaporated, a ring-shaped deposit is left on the substrate that contains almost all of the solute. As shown for a dispersion of colloidal polymeric particles,10 one of the essential conditions for ring formation to occur is the contact line pinning. The self-pinning is related by a power law to the initial concentration of the solution, and results from evaporation at the thin layer which exists at the outer part of the drop, thus leaving a semisolid layer, which causes an outward flow of the liquid.4,5 Recently, the application of rings consisting of polystyrene to functionalize biomaterial surfaces was demonstrated.6 In our present work we extend the application of the ring stain effect to conductive materials. We found that while applying this phenomenon to a dispersion of silver nanoparticles very dense metallic rings are formed at the perimeter of the drops after evaporation at proper conditions. Surprisingly, even though the melting point of bulk silver is 960 °C, these rings have electrical conductivity comparable to bulk silver even if formed at room temperature without post-sintering. It should be emphasized that our results clearly indicate that (at room (6) Sommer, A. P.; Frank, R.-F. Nano Lett. 2003, 3, 573. (7) Denkov, N. D.; Velev, O.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (8) Parisse, F.; Allain, C. J. Phys. II 1996, 6, 1111. (9) Ohara, P.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1078. (10) Sommer, A. P.; Ben-Moshe, M.; Magdassi, S. J. Phys. Chem. B 2004, 108, 8.
10.1021/la0509044 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/22/2005
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Figure 1. Optical microscope image of a silver ring (∼2 mm in diameter and ∼10 µm wide, the width is marked by the arrows). The brightness of the ring can be seen, as compared to the adjacent inner region that consists of a more dilute assembly of silver nanoparticles.
temperature) ring formation is a prerequisite for obtaining such large conductivity in the silver nanoparticle aggregates. Figure 1 presents an optical microscopy image of a ring formed by depositing a 3 µL drop of dispersion of nanoparticles (prepared while using polypyrrole as a stabilizer during the reduction of silver acetate by disodium tartrate, as described in ref 3) with a silver concentration of 0.04 wt % (average particle size of 20-30 nm) on glass. We note that the ring has metallic brightness such as that of a silver mirror, indicating a very close packing of the nanoparticles within the ring. Particular conditions are needed for the formation of such a ring structure (showing large conductivity, see below). The dispersion should be at a proper concentration range (between 0.02 and 0.08 wt %), the drops should be relatively small (less than 5 µL), and the evaporation should be rapid (the best results were obtained at low humidity, by placing CaCl2 as a drying agent into a Petri
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dish containing the glass plate with the evaporating droplet). Figure 2a-d provides a comprehensive view of the ring structure and the area enclosed by it with increasing magnification. The central part of the dried drop contains almost no particles, and their density increases toward the ring (Figure 2a). From the scanning electron microscopy (SEM) image in Figure 2b, it is evident that indeed most of the nanoparticles are condensed in the ring, but still some aggregates are deposited also close to the inner rim of the ring that is formed by the dried drop. Figure 2c,d portrays by high-resolution SEM (HR-SEM) the detailed structure of the ring. A top view at higher magnification (Figure 2c) and a cross sectional view (Figure 2d) of the ring itself show that the ring is indeed composed of tightly packed (but not sintered) silver nanoparticles. For the macroscopic electrical resistivity measurements of the ring we have attached on top of each ring two (1 mm distance on average) In-Ga amalgam contacts, thus forming two semicircles. Then, the ring was disconnected at one point, and the electrical resistance of the semicircle of the ring was measured as shown in Figure 3. The resistivity was calculated for each ring, taking into consideration the specific dimension of the ring. Typically, the width of the external ring was 11 µm and the height was 1.6 µm. These measurements, which were carried out on many rings, reveal that the rings are electrically conductive, having a resistivity of the order of 1.6((0.5) × 10-7 ohm-m (nine measurements were performed for two rings). This is only 1 order of magnitude higher than that of bulk silver, whereas the highest result is 7 times higher. That this finding is quite remarkable can be appreciated by comparison with the similar, 1 order of magnitude, variation found in granular metals11 for the same silver % volume, since there the nanograins are fused to each other (thus forming a continuous metallic network). Hence, noting that our ring configuration was achieved by deposition at room temperature, without any additional
Figure 2. Optical and SEM images of a ring formed on a glass substrate. (a) Optical microscope image of a 2 mm diameter ring formed by drying a drop of silver dispersion and (b) SEM top-view image of the same ring, showing also the adjacent inner area enclosed by the ring and the gradual decrease in the particle density toward the center of the ring. Panels c and d are cross-sectional view and top view, respectively, HR-SEM images of the ring.
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Figure 3. Scheme of the electrical resistivity measurement of a ring.
(say, heating) treatments, this finding is a priori unexpected. The above factor of 7 can be explained by two geometrical considerations. First, from Figure 2, we see that the silver particles are closely packed, which means that only 64% of the ring volume is “bulk” silver. Second, considering the limited contact area which appears in Figure 2d, only 25-30% of the surface of a given particle is in contact with its neighbors. Correspondingly, the above factor of 7 is not too surprising provided that one assumes that the contacts themselves do not contribute any extra resistance. In a control experiment, a thin layer of silver nanoparticles was obtained by spreading a wet layer (6 µm) of much more concentrated dispersion (40 wt %) on a glass surface by a coating bar and letting it dry. In such a coating process, no ring is formed, and the resistivity was 5.93((4) ohm-m, 108 times higher than that of bulk silver. In an additional experiment, we obtained a homogeneous layer of silver nanoparticles by smearing a 5 µl droplet of the 40 wt % dispersion on a glass surface, until almost dry, without formation of a ring. This silver layer consists of a nearly uniform coverage of nanoparticles (see, e.g., Figure 4c), and the resistivity was also very high, 1.6 × 10-3 ohm-m. Therefore, we attribute the high conductivity of the ring to the forcing of the particles to be in very close contact by the pressure exerted by the convective flow during evaporation of the liquid. It is also possible that the hydrodynamic flux pushes the particles toward the ring so that the small particles are able to enter the void spaces between the bigger particles. Thus, the small particles could obstruct the pores between the bigger particles which could produce more percolation paths. The low conductivity of the “uniform coverage” (deposit without ring formation) can be explained then as due to the scarcity of effectively conducting percolation paths. Here it is important to note that, due to the lower aforementioned interparticle pressure (compared with the ring case), the effective contact area between adjacent particles, formed via nanoscale metallic filaments,12,13 is significantly reduced, and consequently, the interparticle contact resistance (dominated by the density and width of these contacts) is largely increased in comparison with the case of the ring. Thus, even apparently geometrically connected paths may not effectively contribute to the conduction percolation network (see below). We have gained further insight into the conductance properties of these silver nanoparticle assemblies by performing local C-AFM measurements. Figure 4a,c presents large-area (8 × 8 µm2) topographic AFM images of a ring and of a “uniform coverage” sample (without the ring formation), respectively. The typical corrugation associated with the individual silver particles was about 15 nm (the insert in Figure 4a shows a small scale scan
Figure 4. C-AFM images (8 × 8 µm2) showing correlated topography and current images acquired simultaneously on silver particles deposited on glass substrate. (a) Topography image focusing on an area at the external rim of a silver ring. (b) Current image acquired at a bias voltage of 1 mV corresponding to the topographic image in panel a; color scale range: 25 nA. (c) Topography image of a “uniform coverage” sample prepared without the ring formation. (d) Current image acquired at a bias voltage of 1 V corresponding to the topographic image shown in panel c; color scale range: 25 nA. The inset in panel a (placed on the “glass region” of the main figure) shows an image focusing on a small area (100 × 100 nm2 and corrugation of about 15 nm), portraying individual silver particles.
where individual silver particles are resolved). Figure 4b,d shows the corresponding current maps obtained simultaneously using the C-AFM technique, as described in ref 14. From this figure, it is immediately apparent that bulkcomparable conductivity is obtained only if the ring is formed. The small abundance of “current islands”14 in the current map measured on the “uniform coverage” deposit, Figure 4d, reveals the scarcity of conduction percolation paths conjectured above, yielding high macroscopic resistance. In contrast, current maps measured on the ring repeatedly reveal very high density of “current islands”, as shown in Figure 4c. Interestingly, the surface density of silver particles in the “uniform coverage” sample was found to be comparable to that observed on the ring. Consequently, the small abundance of conduction percolation paths in the “uniform coverage” sample is probably due mainly to the effect of reduced interparticle pressure discussed above. Significant density of “current islands” appeared also in C-AFM images measured on areas enclosed by, but in the vicinity of the ring, as long as the corresponding silver aggregates were still connected to the ring. The number of current islands, as well as the current magnitude, reduced toward the center of the ring, consistent with the decrease of silver particle density shown in Figure 2b, as well as with the smaller pressure pushing adjacent particles toward each other. In summary, we have demonstrated that highly conductive aggregates of silver nanoparticles can be obtained by depositing dispersions under controlled conditions (11) Abeles, B. Appl. Solid State Sci. 1976, 6, 1. (12) Bushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (13) Landman, U.; Luedtke, W. D.; Gao, J. Langmuir 1996, 12, 4514. (14) Toker, D.; Azulay, D.; Shimoni, N.; Balberg, I.; Millo, O. Phys. Rev. B 2003, 68, 41403(R).
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leading to the formation of rings via the “ring stain effect”. Electrical conductivity smaller by only a factor of 7 with respect to that of bulk silver was achieved with no resort to post-annealing. Typically, the ring conductivity is 5 orders of magnitude larger than that of aggregates deposited without ring formation, although the surface density of silver nanoparticles appeared to be comparable. This result highlights the important role played by interparticle pressure in determining the contact resistance between metallic particles. Our findings are expected to yield technological applications, since by proper selec-
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tion of the metallic dispersion and method of droplets placement (e.g., via advanced ink-jet printing), unique conductive nanopatterns relevant to the microelectronic industry could be fabricated. Acknowledgment. This work was supported in part by grants from the Deutche-Israel Program (DIP), the Israel Science Foundation, the European Union SA-NANO STREP program. LA0509044
