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Functional Nanostructures Through Nanosecond Laser Dewetting of Thin Metal Films

Posted on: Tuesday, 7 October 2008, 03:00 CDT

By Krishna, H Favazza, C; Gangopadhyay, A K; Kalyanaraman, R

Techniques for processing nanoscale metallic structures with spatial order and tunable physical characteristics, such as size and microstructure, are paramount to realizing applications in the areas of magnetism, optics, and sensing. This paper discusses how pulsed laser melting of ultrathin films can be a powerful but simple and cost-effective technique to fabricate functional nanostructures. Ultrathin metal films (1 nm to 1,000 nm) on inert substrates like SiO^sub 2^ are generally unstable, with their free energy resembling that of a spinodal system. Such films can spontaneously evolve into predictable nanomorphologies with well-defined length scales. This study reviews this laser-based experimental technique and provides examples of resulting robust nanostructures that can have applications in magnetism and optics. INTRODUCTION

The spontaneous formation of patterns of well-defined structures by selfassembly and/or self-organization is a fundamental and technologically relevant topic of ongoing research. Various materials systems show self-organizing characteristics: atoms rearrange in a predictable manner to form specific crystals, epitaxially strained thin films break into quantum dots,1 and biological systems show characteristic patterns and length scales.2 The processes involved in these systems possess some very attractive features from the perspective of manufacturing processes, namely, they are repeatable and cost-effective. Another important characteristic is that such spontaneous processes can encompass a wide range of length (and time) scales, from nanometers, such as from thin film dewetting,3 to hundreds of kilometers, as seen in geographical structures.4 Therefore, the study of self-organizing systems is of tremendous importance and will likely play an important role in the realization of various nano- and micro- technologies.

Metallic nanostructures on surfaces or embedded inside bulk materials have many applications. For instance, ordered nanopartlcle arrays on surfaces can be used in magnetic information storage, to guide light below the diffraction limit, and to catalyze growth of ordered arrays of carbon nanotubes.5-12 Likewise, metal nanostructures in the bulk can perform several electronic (and structural) functionalities, including making and enhancing spintronic materials,13 improving light-emitting characteristics of polymer semiconductors,14 and heterogeneously seeding novel magnetic materials.15 However, despite the potential of such metallic nanostructures, fabrication is severely limited by the inability to economically and reliably process nanomaterials with desired physical and electronic characteristics. This study shows that the spontaneous pattern formation or self-organization resulting from nanosecond (ns) pulsed laser melting and subsequent dewetting of the ultrathin films is a promising route to realize robust nanoscale structures.16-24

BACKGROUND AND THEORY

Dewetting is a widely observable physical phenomenon in which a continuous liquid film spontaneously breaks into droplets. The fundamental underlying reason for the formation of drops is that the droplet-surface system has lower energy than the continuous film- substrate system. What is of most relevance to current nanomanufacturing strategies is whether this spontaneous dewetting process can yield predictable nanomorphologies in a reliable and cost-effective manner. Based on the findings from studies on the dewetting of thin polymer films made over the past 50 years, it is evident that dewetting could be a very advantageous processing technique. These studies have uncovered several features that are relevant to nanomanufacturing strategies. The most important is that therrnodynamically unstable polymer film-substrate systems show evidence for spontaneous dewetting with well-defined intermediate and final morphologies and length scales. For film-substrate systems that show this effect, there is a strong resemblance between the shape of the free energy as a function of film thickness and the composition-dependent behavior in binary systems showing spinodal phase segregation (Figure 1).25 Hence, such systems are often referred to dewet by spinodal dewetting.26-29 A typical thin film- substrate spinodal dewetting system will have a thickness-dependent free energy per unit area G(h) given by:

The surface free energy (G^sub Surf^) describes the energy of a liquid-vapor or solid-vapor interface and a film in contact with vacuum is given by the appropriate surface tension gamma^sub f-v^ with units of energy/area. The interfacial free energy (G^sub Int^) describes the energy of the liquid-solid, liquid-liquid, or solid- solid interface and for a film on substrate is given by the interfacial tension gamma^sub f-s^. A typical external free energy (G^sub Ext^) is the gravitational energy G^sub Ext^ = 1/2 rhogh^sup 2^, where rho is the density of the film and g is the local acceleration due to gravity. Unlike the surface, interface, and gravitational energy terms, the volume free energy term (G^sub Vol^) can take several forms and is dictated by the system of interest. For instance, in epitaxial solid film-substrate systems the energy associated with lattice mismatch strain will contribute to the volume free energy.30-32 In the case of polymer films and metal films on inert amorphous substrates such as SiO^sub 2^ (of relevance to the experimental results to be discussed later in this article), the commonly observed volume free energy term arises from intermolecular dispersion forces. The atomic origin of this intermolecular force is the van der Waals interaction between non- polar atoms in which the interaction energy varies as 1/h^sup 6^, where h is the spacing between the particles. The extension of this point-like interaction energy to describe the free energy of interaction between planar interfaces is achieved by a pair-wise addition of the van der Waals interaction. This results in an energy- per-imit area expressed as G^sup Disp^^sub vol^ = A/h^sup 2^, where A is the Hamaker coefficient, which determines the sign and magnitude of interaction between the substrate-film and film-vacuum interface. The Hamaker coefficient, A, can be calculated through a relation of the frequency-dependent dielectric coefficients of the different media, which is summed over the entire frequency regime and is proportional to (epsilon^sub 1^ - epsilon^sub 2^)(epsilon^sub 2^ - epsilon^sub 3^), where epsilon^sub 1^, epsilon^sub 2^, and epsilon^sub 3^ are the dielectric functions of substrate, film, and vacuum, respectively.33,34 From the Hamaker relation, it is seen that the interaction free energy is attractive when the film has a larger (or smaller) dielectric than both the substrate and the surrounding ambient. In the case of polymer liquids and metal films on dielectric substrates like SiO^sub 2^ in a gaseous or vacuum medium, the free energy is attractive. More importantly, for films in the thickness regime of 1 nm to 100 nm, for which gravitational energy is negligible, the curvature of the free energy is negative (i.e., d^sup 2^G/dh^sup 2^ < 0), as shown in Figure 1. This unstable regime strongly resembles the free energy curvature found within the spinodal regime of binary phase segregating systems.

The second important finding of dewetting studies of unstable polymer films is that the progression from an initially smooth liquid film to the final droplet state occurs with intermediate states that have well-defined length scales and complex morphologies, including holes, cellular structures, polygonal features, and, eventually, particles.^sup 26-29^ In this instability regime, the formation of holes occurs spontaneously and characteristic length scales emerge because the subsequent dewetting dynamics are characterized by a narrow spread of preferential or fastestgrowing length scales.^sup 35,36^ As shown theoretically, the dynamic leads to selection of characteristic patterning length scales ⋁ that vary with film thickness as ⋁ [proportional to] h^sup 2^.^sup 35-39^ This behavior has been observed experimentally by several authors, thus verifying the existence of this spontaneous, self-organizing process for spinodal- like liquid films.^sup 26,39^

On the other hand, for films that are in a metastable energy regime, such as those with thickness >100 nm, dewetting begins by the formation of holes via homogeneous nucleation at random spatial locations (in the absence of impurities) and as a result, the dewetting morphologies lack controllable and predictable length scales.

The study of dewetting in high-melting-point metal films (the focus of this article), like ferromagnetic elements (Co, Fe, Ni) or the plasmon-active metals (Ag, Au, Cu) is a challenging task. High temperatures introduce additional effects such as interfacial interactions and metal in-diffusion, thus complicating dewetting.^sup 40,41^ Such issues were typically not present in the near-roomtemperature experiments on polymer films. In order to overcome this difficulty, alternate techniques to raise the temperature in a controllable but rapid manner such that the solid metal film can be melted and subsequently undergo morphological changes due to dewetting are needed. Two such techniques are ion irradiation^sup 42^ and pulsed laser heating.3,16,19 In the mid- 1990s, it was shown that when an ns laser pulse was used to melt thin metal films (gold, copper, and nickel) deposited on SiO^sub 2^ surfaces with a thin buffer layer of chromium metal, dewetting with characteristics similar to that for spinodally unstable polymer systems were observed.^sup 17,18^ In the past few years, tremendous progress has been made toward understanding this dewetting pattern formation in metal films following ns pulsed laser irradiation.^sup 20-24^ EXPERIMENTAL DETAILS AND RESULTS

The first step in the dewetting experiment is to deposit extremely smooth metal films with controlled thickness ranging from 1

The second step in die dewetting experiment is the controlled exposure of the film to multiple ns laser pulses. As has been shown by several studies, dewetting progression of the ultrathin metal films by ns pulses is best captured when the metal film is transformed into its molten phase.3,16,22 In order to achieve this (while simultaneously minimizing evaporation and ablation of the film material), a carefully controlled investigation of the critical laser energy to melt the film must be performed. Furthermore, since laser energy absorption and the subsequent heating is strongly film- thickness dependent, this optimization must, in principle, be performed for each film thickness. However, analytical and numerical techniques can now be used to accurately predict the film-thickness- dependent melt threshold energy, which greatly simplifies the selection of appropriate laser energy for experiments.^sup 3,24^ Following are several specific examples of dewetting of ultrathin metal thin films that exemplify this laser-induced self- organization process and its potential in realizing nanoscale functionalities and unique microstructures.

Evolution of the Dewetting Morphology

In Figure 2, the dewetting morphology is shown after various times, as measured by the number of laser pulses n, for an iron film on SiO^sub 2^. Spatially uniform pulses from an ultraviolet (UV) laser of wavelength 266 nm, with pulse width of 9 ns and repetition rate of 50 Hz were applied. The laser energy was above the melt threshold energy for the iron films. As visible in the figure, following a few pulses, the dewetting morphology was characterized by a cellular network of polygons (Figure 2a). As n increased, the metal receded to the edge of the holes, resulting in a network of coalescing polygonal features. Continued irradiation resulted in the formation of nanoparticles primarily at the junctions of the polygons, as evident from the location of the particles in Figure 2b and c. At every observed stage, a characteristic length scale is present, as evidenced by the annular form of the power spectrum of the spatial correlations in the intensity variation within each pattern. The power spectrum is shown in the inset of the corresponding images in Figure 2a to c. For patterns consisting of polygons, the characteristic length scale represents the average distance between the centers of the polygons; for the nanoparticles, it represents the average nearest neighbor or interparticle spacing. The short-range spatial order in the inter-panicle spacing is a strong indicator of dewetting in the spinodal-like system.

Quantitative Verification of Spinodal-like Dewetting

In order to confirm that the nanomorphology presented above does result from the dewetting of a spinodallike system, it was necessary to verify the film thickness h dependence of a characteristic length scale ⋁. As shown by several authors, the characteristic length ⋁ for spinodal-like dewetting varies with film thickness as surface tension of the metal and A is the Hamaker coefficient for the vacuummetal-substrate system. While it is possible to select different morphologies to measure this length scale, we have selected the final interparticle spacing to describe ⋁. In Figure 3 the behavior of ⋁ as a function of film thickness is shown for iron films on SiO^sub 2^. The best fit, shown in the figure by the dashed line, yields a ⋁ ~ h^sup 2^ dependence, hence confirming the spinodal-like dewetting. Additional confirmation is also obtained from the dependence of nanoparticle radius (open circles) on thickness. By mass conservation arguments it can be shown that the radius should vary as r [proportional to] h^sup 5/3^.^sup 26^ The experimental results for iron are shown in Figure 3 by closed circles and the fit to this data (shown by the dotted line) clearly evidences the h^sup 5/3^ trend. This latter result was also evidence that evaporation and/or ablation played an insignificant role in this laser-induced dewetting process. Similar such behavior has been observed for several metals, including cobalt and silver, suggesting that this self-organizing process is widely applicable to other metals.

Unusual Magnetic Anisotropy in Ferromagnetic Metals

One of the unique advantages of using ns laser processing is that the resulting thermal profile of a single pulse is characterized by extremely fast heating and cooling rates. For the case of 9 ns laser pulses incident on ultrathin metal films (1-20 nm) on SiO^sub 2^ substrates, this heating/cooling rate is of the order ~10^sup 10^ K/ s. 3/19 Thus the dewetting nanomorphologies are exposed to a thermal cycle of large magnitude (i.e., room temperature to melting point) in addition to extremely fast heating/cooling. This highly non- equilibrium processing can result in unique microstructures as well as electronic properties. The authors recently discovered that ferromagnetic metal nanoparticles prepared by this ns dewetting show unique and unusual particle size-dependent magnetic anisotropy.^sup 44^ Figure 4a shows AFM morphology information of a self-organized array of hemispherical cobalt nanoparticles alongside its magnetic information (shown in Figure 4b), as measured by magnetic force microscopy (MFM) in zero applied field. The cobalt nanoarray was formed by laser-induced self-organization using approximately 10,000 pulses from a 9 ns, 266 nm neodymium-doped yttrium aluminum garnet laser. The magnetic domains of these nanomagnets were analyzed in great detail.^sup 44^

The difference in the magnetization directions is clearly evidenced by the different black/white contrast for each, as seen in Figure 4b, and in fact the directions can be estimated from the nature of these contrasts.^sup 44^ Three particles marked by arrows 1, 2, and 3 correspond to particles of size 45 nm, 80 nm, and 112 nm, respectively, in the AFM (see Figure 4a) and the corresponding MFM (see Figure 4b). These particles respectively show magnetization directions of 0[degrees], 45[degrees], and 90[degrees] with respect to the substrate plane. Particles show single domain (SD) until they reach 180 nm in diameter and then multi-domain. Interestingly, the magnetization direction of each SD magnet is dependent on its particle size. Previous studies have shown that as the particle size increases, the magnetization direction orients toward an out-of- plane orientation, (i.e., away from the substrate plane). This result was not consistent with the shape anisotropy considerations, which predicted an in-plane magnetization for all particles.^sup 44^ The explanation for this unusual behavior is that the nanoparticles have a residual biaxial tensile strain due to the large thermal expansion mismatch between the metal and the SiO^sub 2^ substrate. Further, since polycrystalline ferromagnetic elements have negative magnetostriction constants,^sup 45^ the biaxial strain-induced magnetoelastic anisotropy favors an out-of-plane magnetization.

On the other hand, the size-dependent behavior is most likely a result of size-dependent strain relaxation observed in nanocrystalline materials. As observed by Z. Shan et al.,^sup 46^ smaller nickel grains release strain at a higher rate than larger grains, implying that smaller grains will be in a more relaxed state. Therefore, it is likely that in the case of these pulsed- laser-produced nanoparticles, the preference for inplane orientation of the smaller grains comes from a lower residual biaxial strain.

Nanoparticle Nanocomposites from Dewetting of Immiscible Bilayer Metal Films

A nanoparticle nanocomposite, such as a nanoparticle containing individual grains of two or more immiscible elements, could be very useful in realizing multi-functional sensing tasks in small length scales. For example, if strong plasmon-active metals like Ag, Au, and Cu are combined with ferromagnetic metals like Co, Fe, and Ni, useful devices could be realized from the simultaneous magnetic and plasmonic functions that will be available. Figure 5 shows the morphology and microstructure results for ns laser dewetting of bilayer Ag/Co films on SiO^sub 2^. Once again spatially uniform laser pulses from a 266 nm, 9 ns pulsed laser was applied with laser energies above the melt threshold energy. As shown in the inset of Figure 5a, the nanoparticles that eventually form have spatial shortrange order, which is strong evidence for dewetting with a characteristic length scale. More importantly, transmission electron microscopy (TEM) of these nanoparticles (see Figure 5b) shows that the particles have a granular microstructure and the individual grains belong to pure silver or cobalt. This evidence was obtained from electron diffraction data taken from individual particles (such as that shown in Figure 5c). As shown previously, the characteristic length scale and nanoparticle size can be controlled by manipulating the thickness of the individual metal films in the bilayer, thus providing a means to manipulate the optical and magnetic property of this nanoscale nanocomposite.^sup 47^ CONCLUSION

Nanomanufacturing processes that utilize spontaneous self- organizing strategies to create controllable morphologies are envisioned to be very important for rapid advancement of nanotechnologies. Typically, ultrathin metal films on an inert surface can often be found in an unstable spinodal-like free energy state. When such films are melted by ns laser pulses, they spontaneously dewet. The resulting nanoscale patterns have a characteristic length scale that can be repeatedly obtained. Thus, the dewetting process provides a simple way to nanomanufacture metal nanostructures.

In addition, the observation that iron nanoparticle spacing formed by laserinduced dewetting follows a parabolic dependence on the film thickness is strong quantitative evidence for a spinodal- like dewetting system. The combination of self-organization and the ns laser processing also results in unique nanomaterials. In the case of ferromagnetic materials like cobalt, the nanoparticles show unusual size-dependent anisotropy that is attributed to the large residual thermal strain following the laser processing.

As a final example of the potential capabilities of ns laser dewetting, the technique has been applied to immiscible bilayer films, such as Ag/Co. The resulting nanoparticles are in the form of nanoscale nanocomposites (i.e., mixtures of pure cobalt and silver grains) within individual nanoparticles. Such nanocomposites could have novel applications by simultaneously showing strong magnetic and plasmonic properties in the nanoscale.

ACKNOWLEDGEMENT

The authors acknowledge support by the Center for Materials Innovation, McDonnell Academy Global Energy and Environment Partnership and the National Science Foundation through CAREER grant DM1-0449258 and grant DMR-0805258.

How would you...

...describe the overall significance of this paper?

It provides a succinct summary of the fundamentals of spontaneous dewetling and provides examples of the resulting spatially ordered nanostructures.

...describe this work to a materials science and engineering professional with no experience in your technical specialty?

This work utilizes nanosecond laser melting of thin films that are in an unstable free energy state in order to harness the dewetting morphological instability. This dewetting instability results in repeatable and predictable nanostructures.

...describe this work to a layperson?

This work provides an example of how a spontaneous process (dewetting) could be harnessed to create nanomaterials with well- defined properties. Consequently, a cost-effective nanomanufacturing process can be envisioned.

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H. Krishna, C. Favazza, and A.K. Gangopadhyay are with the Department of Physics and the Center for Materials Innovation, Washington University, St Louts, MO 63130; R. Kalyanaraman is now with the Department of Materials Science and Engineering and the Department of Chemical and Blomolecular Engineering, University of Tennessee, Knoxville, TN 37996. Prof. Kalyanaraman can be reached at ramkl@utk.edu.

Copyright Minerals, Metals & Materials Society Sep 2008

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