Morphology transitions in bilayer spinodal dewetting systems

S. Yadavali, H. Krishna, and R. Kalyanaraman

Phys. Rev. B 85, 235446 – Published 22 June 2012

Abstract

In spontaneous pattern formation by spinodal dewetting, attractive intermolecular forces overcome surface tension and cause an ultrathin liquid film on a low energy substrate to produce ordered structures. Spinodal dewetting in single-layer film on a substrate is usually manifested by an early stage surface deformation and a highly nonlinear ripening stage that results in characteristic morphologies, typically bicontinuous- or holelike states. Here we have experimentally constructed the dewetting morphology phase diagrams for a bilayer (Ag, Co) liquid film system on SiO $_{2}$ . Nanosecond pulsed laser melting was used to initiate and foster the dewetting as a function of film thickness and arrangement. The early stage ripening morphology was observed by scanning electron microscopy from which the phase diagrams were constructed. Unlike single-layer films, which only show one morphology transition between the bicontinuous to hole states as the film thickness is increased, the bilayer system can have multiple transitions. We have utilized the thickness-dependent free energy curvature approach [Sharma and Khanna, Phys. Rev. Lett. 81, 3463 (1998)] to analyze the phase diagram. The location of the multiple transitions cannot be predicted from the curvature minima, as was the case for single-layer films. Nevertheless, despite the complexity from multiple interacting forces and different surface deformation mode in bilayer systems, the phase diagram can be completely generated by knowledge of the free energy curvature of the respective single-layer films. These results can permit improved modeling of the nonlinear dynamics in naturally driven self-organized phenomenon and help design nanomaterials for advanced applications.

Received 5 March 2012

DOI:https://doi.org/10.1103/PhysRevB.85.235446

Authors & Affiliations

S. Yadavali¹, H. Krishna², and R. Kalyanaraman^1,3,4

¹Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA
²Department of Physics, Washington University in St. Louis, Missouri 63130, USA
³Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA
⁴Sustainable Energy Education & Research Center, University of Tennessee, Knoxville, Tennessee 37996, USA

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Vol. 85, Iss. 23 — 15 June 2012

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Images

Figure 1
(a) Schematic description of the geometry and forces in a bilayer dewetting system consisting of two liquids layers on the substrate. Also shown are the two possible types of deformation modes: bending (B) and squeezing (S). The primary interaction forces include the Hamaker coefficients A $_{i, j}$ 's between the various pairs of interfaces and the interfacial tension at the film-vacuum and film-film interfaces. (b) The free energy of the Co/SiO $_{2}$ system ( $Δ G$ ), its curvature ( $Δ^{2} G$ ), and the third derivative ( $Δ^{3} G$ ) are shown. The transition thickness $h_{T}$ corresponds to the minima in the curvature or the zero in the third derivative.[11] (c) Experimentally constructed morphology phase diagram for a single-layer system of Co/SiO $_{2}$ , which is identical to the bilayer system of Co/Co on SiO $_{2}$ . The dashed curves are drawn at the location of the morphology transition points for single-layer Co. The thickness of these lines represent the experimental uncertainty in measurement of the transition thickness. Various regions consisting of either bicontinuous (BC) or hole morphologies are shown.Reuse & Permissions
Figure 2
SEM images of the early stage spinodal dewetting morphologies in bilayer dewetting systems. The film thickness from (a) to (h) correspond as follows: (a) Co(4 m)/Ag(5 nm) corresponds to case 1, (b) Ag(5 nm)/Co(3 nm) corresponds to case 2, (c) Co(6 nm)/Ag(5 nm) corresponds to case 3, (d) Ag(4 nm)/Co(5 nm) corresponds to case 4, (e) Ag(6 nm)/Co(5 nm) corresponds to case 5, (f) Ag(12 nm)/Co(5 nm) corresponds to case 6, (g) Co(7 nm)/Ag(12 nm) corresponds to case 7, and (h) Co(5 nm)/Ag(8 nm) corresponds to case 8. All cases are described in Table .Reuse & Permissions
Figure 3
(a) Experimentally generated morphology phase diagram of AgCo system $(h_{T, 2} > h_{T, 1})$ . (b) Phase diagram of CoAg system $(h_{T, 2} < h_{T, 1})$ . (c) Free energy curvature of bilayer AgCo(5 nm) where the red and blue lines correspond to individual Co and Ag transitions. (d) Free energy curvature of CoAg(5 nm) where red and blue lines correspond to individual Co and Ag transitions. The width of the experimentally observed transition lines in (a) and (b) corresponds to the uncertainty in film thickness measurements.Reuse & Permissions
Figure 4
Second ( $Δ^{2} G$ ) and third derivative ( $Δ^{3} G$ ) of the free energy for the AgCo bilayer system for varying values of the bottom Co layer thickness of 5, 7, and 10 nm calculated from Eqs. (7) and (8). The minima in the $Δ^{2} G$ for each case corresponds to the zero in the $Δ^{3} G$ (shown by the vertical dashed line for each case). The location of the zero in $Δ^{3} G$ represents the location of the first transition point in the bilayer dewetting morphology.Reuse & Permissions

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