Wetting and de-wetting transitions of small metal particles on substrates under electron irradiation

NOTE Wetting and De-Wetting Transitions of Small Metal Particles on Substrates under Electron Irradiation Small particles of most metals are seen to wet substrates onto which they are deposited. It was shown before (Ajayan, P. M., and Marks, L. D., Nature (London) 338, 139 (1989).) in the case of Au-MgO that, depending on the relationship between the interracial energies, it is possible for the particle to "sink" into the substrate. It is reported here that under a high dose of electron irradiation, metallic particles dewet from nonconducting or semiconducting substrates, remain passive on conducting ones, and coalesce with particles of the same material. This phenomenon is discussed as arising from an increase in the particle/substrate interfacial energy during electron irradiation, perhaps due to charge localization at the interface. © 1991AcademicPress,Inc. 1. INTRODUCTION The way small particles are attached to a substrate has important implications in industrial applications, such as heterogeneous catalysis, where the area and nature of the exposed facets control the reaction, and in the manufacture of electronic components, where contamination by dust particles is an important issue. The work of adhesion which determines the magnitude of the interracial energy of a two-phase system decides how strongly the two phases interact or how strongly they wet each other. A lower interfacial energy corresponds to stronger interaction between the phases. When a small particle is deposited on a substrate it assumes a particular shape, which reflects an equilibrium of forces between the particle and substrate surface energies and the particle/substrate interracial energy, and can be uniquely represented by a Wulffconstruction (2). But the shape and position of the construction with respect to the substrate plane can change as the above balance is disturbed; for example, through adsorption of gaseous species on the surface which will change the surface energy or by a change in the interaction between the particle and the substrate through electron transfer or chemical reactions as in the case of strong metal substrate interaction systems (3). It was shown earlier ( 1) that for a small particle in the size range of less than 10 rim, when the kinetic barriers for diffusion are rather small and the interracial stresses are minimum, the thermodynamically stable state is achieved when the particle sinks into the substrate material. Figure 1 (taken from Ref. ( 4 )) shows clear demonstration of the nature of particle-substrate interactions in the AuMgO system, initial sinking (a-c), and later decoupling (d-f) under extended irradiation with electrons. The images were observed using electrons accelerated to 200 KeV at a current density of ~ 5 amp/cm 2, in an atomic resolution electron microscope. Figure 2 (a) shows schematically the energy balance at the particle/substrate/vapor contact, the initial state at the time of deposition and Fig.

2(b), the thermodynamically stable state. Note that the effective force would act along the direction of net negative energy, forcing the particle to sink. In Fig. 2(b), at some critical value of the wetting angle 02, larger than 01, equilibrium is achieved. In other words the particle attains the equilibrium shape and position with the substrate such that forces due to the interfacial energies balance at the particle-substrate contact plane. Here we describe the general effect of electron irradiation of small metal particles placed on substrates of different electrical conductance. The observations from many systems suggest that during strong electron irradiation the particle always decouples from the nonmetallic substrates, such as oxides, but remains wetted to conducting ones. A plausible explanation for this is that during irradiation charges get trapped at the metal-nonmetal interface, raising the interfacial energy. 2. DE-WETTING OF PARTICLES UNDER ELECTRON IRRADIATION Once the particle has reached the stable configuration inside the substrate, only prolonged irradiation under moderate electron fluxes (1-10 amp/era 2) or high flux ( 10-100 amp/era 2) electron irradiation for shorter times (high dose of electrons) changes the balance. As seen in Fig. 1(d,e) prolonged irradiation slowly pushed the particle out of the substrate capsule. This correspondsto the scheme in Fig. 2(c) where the value of the wetting angle (0) decreases, disturbing the equilibrium position of the particle with respect to the substrate. The final state is the decoupled particle (Fig. l(f)) from the original encapsulation. The scheme in Fig. 2(d) shows the above situation, in which the interfacial energy of the Au/MgO is higher than the sum of the energies of Au and MgO (100) surfaces and hence formation of an interface is not favorable. This can be extended to other systems to suggest that decoupling occurs when the interfacial energy becomes large, exceeding the sum of the particle and substrate surface energies. In

281

Journal of Colloid and Interface Science, Vol. 147,No. I, November1991

0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. Allrightsofreproductionin any formreserved.

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a. INITIAL STATE

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FIG. 2. A Scheme of the different stages of the transition shown in Fig. I. 7A,, 7MgO, 7Au/M~Oare the energies of Au/vapor, MgO/vapor, and Au/MgO interfaces and 0~, 02, 03 are the wetting angles of Au on MgO at three different stages. S, V, and P denote substrate, vapor, and particle. The situation in (a) corresponds to Fig. l (a); (b) to Fig. 1(b,c); (c) to Fig. 1(d,e); and (d) to Fig. 1(f).

the decoupled state the particle becomes structurally unstable, fluctuating between different shapes, and appears spherical (the image being the time average of many fluctuating configurations) with the symmetric fresnel white fringe contrast around the particle showing that it remains de-wetted from the substrate. This fact is also deduced from the occasional translational motion of the particle on the substrate, either due to coulombic forces or from the momentum transfer from the beam to a loosely attached particle. We have observed the same kind of initial encapsulation (although the strength of the interaction varied with the substrate used, the strongest being for MgO and weakest for amorphous carbon) and decoupling under strong dec-

tron irradiation for some of the fcc metals (Rh, Au, Ag, Pt) on 7-A1203, amorphous SiO2, and MgO. For the amorphous carbon support the effect is weak, the latter being a better conductor than the oxides. For two nonconducting materials in contact (SiO2 formed by oxidizing silicon particles and placed on MgO cubes) a strong decoupling effect occurs during irradiation. Figure 3 shows observations of the phenomenon on three different substrates. Figure 3(a,b) shows rhodium particles supported on 7-A1203 decoupling under a strong electron beam. Initially, when the beam was weak (Fig. 3 (a)), the particles were seen to be encapsulated by alumina. Irradiation under high electron flux pushed the particles onto narrow necks grown from the substrate (Fig. 3 (b)), decreasing the sub-

FIG. 1. Experimental observation of wetting and de-wetting transition of small particles of gold supported on MgO substrates (taken from Ref. (4)). The images from l ( a ) - l ( f ) were observed at 0, 60, 120, 180, 220, and 240 min respectively. Journal of Colloid and Interface Science, Vol. 147, No. l, November 1991

284

NOTE

FIG. 3. (a) Initial distribution of rhodium particles on the surface of )'-A1203 substrate (real catalyst). (b) Decoupling of the particles from the alumina surface during strong irradiation with electrons by growing narrow necks. (c) Growth of pillars from MgO substrate after irradiating gold particles distributed on the surface. (d) Partially decoupled platinum particle shown floating on a neck grown from the silica layer over a silicon particle.

strate interactions. The same kind of phenomenon is observed for gold particles on MgO (Fig. 3(c)), where long pillars are grown, and platinum particles on silica (Fig. 3( d )). Once the particles are decoupled from the substrates they become structurally unstable (4) and the fluctuations in the decoupled state persist under very weak electron intensity ( <0.1 amp / cm 2), even when the probability of charging would be extremely small. Increasing the irradiation flux increases the frequency of fluctuations. It is observed, for the Au-MgO system, that the pillar formation has a capillary nature, the length varying inversely as the particle size (Fig. 3(c)). In contrast to the above observation, in systems where conducting substrates were used, such as in gold particles supported on a-Fe203 or graphite and gold particles on gold film, irradiation did not produce the decoupling effect and the particles remained inactive without undergoing translational motion or structural fluctuations. When particles of the same material are irradiated together, coalescence is observed irrespective of the conductivity of the material (gold particles, silica particles), but the rate of coalescence depended on the flux o f the electron beam Journal of Colloid and Interface Science. Vol. 147,No. 1, November1991

used. It should be noted that different specimen preparation methods were tried, but gave similar results (for example, gold was deposited on MgO by electron beam evaporation, laser evaporation, and from cluster solution). The observed effect can be explained as arising from a disturbance in the interfacial energy balance during irradiation. This can occur by three means: a decrease in the surface energy of the gold particle or of the substrate facet on which the particle is sitting, or an increase in the particle-substrate interracial energy. Since intense electron irradiation brings structural instability in small particles (5, 6) and the particle is in a kind of quasi-solid state, the surface energy may decrease. But it was shown (4), and in many other systems it is observed, that the structural instability occurs after the particle is decoupled and hence cannot be the cause for decoupling. The MgO surface steps are seen to order slightly during irradiation but the energy change due to this should be quite small. Moreover the process was also seen when small particles are supported on nonconducting amorphous substrates. We assume that the effect of temperature rise due to the electron beam is negligible. In fact if the beam induced inelastic events in-

NOTE creased the temperature it would have much higher impact for the nonconducting substrates (7) than the metal particles. This would then decrease the surface energy of the substrate forcing the particle to sink further. This was truly the case observed; when the Au-MgO system was heated up to 600°C, strong encapsulation was observed, forming thick layers of MgO around the gold particle. The most likely cause of this decoupling of the particle during irradiation seems to be an increase in the particle/ substrate interfacial energy. Strong electron irradiation may continuously create mobile charges (holes) inside the metal particles which get trapped at the interface when the substrate is nonconducting. Charge localization that occurs at the interface could increase the interracial energy due to strong electrostatic fields. Inhomogeneous strains may develop across the interface due to partial polarization of the electron gas, and this may contribute to a higher energy interface. An exact mechanism, though, is difficult to predict. When the substrate used is conducting, charges can easily get dissipated from the interfacial region to the bulk, thereby not causing the particle to de-wet. In the case of particles of the same material, there are no interfaces to trap charges and the enhanced diffusion due to charging in individual particles (8) provides the driving force for the particles to coalesce. ACKNOWLEDGMENTS One of the authors (PMA) acknowledges Professor L. D. Marks. The experimental work on Au-MgO was done when the author was a student at the Department of Materials Science & Engineering, Northwestern University, Evanston, IL, USA.

285 REFERENCES

1. Ajayan, P. M., and Marks, L. D., Nature (London) 338, 139 (1989). 2. Winterbottom, W. L,, Acta Metall. 15, 303 (1967). 3. Spencer, M. S., Nature (London) 323, 685 (1986). 4. Ajayan, P. M., and Marks, L. D., Phys. Rev. Lett. 63, 279 (1989). 5. Bovin, J. O., Wallenberg, R., and Smith, D. J., Nature (London) 317, 47 (1985). 6. Iijima, S., and Ichihashi, T., Phys. Rev. Lett. 56, 616 (1986). 7. Riemer, L., "Transmission Electron Microscopy" (P. W. Hawkes, Ed.), Springer Series in Optical Sciences, Vol. 36, p. 433. Springer-Verlag, Berlin, 1989. 8. Iijima, S., and P. M. Ajayan, Radiation Effects, in press. P. M. AJAYAN SUMIO IIJIMA1

Fundamental Research Laboratories NEC Corporation 34 Miyukigaoka Tsukuba-shi Ibaraki-ken 305 Japan Received January 14, 1991; accepted April 25, 1991

To whom correspondence should be addressed.

Journal of Colloid and Interface Science, Vol. 147, No. 1, November 1991