Modeling dewetting of ultra-thin solid films

We review some models for the dynamics of dewetting of ultra-thin solid films. We discuss the similarities and the differences between faceted and non-faceted systems. The faceting of the dewetting rim leads to corrections in the velocity of dewetting fronts both in flat and axisymmetric geometries. The faceting of the edge of the dewetting rim leads to a strong anisotropy of the dewetting instability. Faceting also induces novel dewetting regimes such as layer-by-layer dewetting, and monolayer dewetting.     

Persistent droplet motion in liquid-liquid dewetting

When a nonvolatile liquid film dewets from a partly compatible liquid substrate, the advancing dewetting front leaves behind droplets formed through a Rayleigh instability mechanism at its rim. We have found that these droplets continue to move in the direction of the dewetting front for extended periods (of order one day) with an initial droplet velocity varying linearly with the droplet size, and a displacement varying logarithmically with time. We attribute this persistent motion to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front.     

Thermal noise influences fluid flow in thin films during spinodal dewetting

Experiments on dewetting thin polymer films confirm the theoretical prediction that thermal noise can strongly influence characteristic time scales of fluid flow and cause coarsening of typical length scales. Comparing the experiments with deterministic simulations, we show that the Navier-Stokes equation has to be extended by a conserved bulk noise term to accomplish the observed spectrum of capillary waves. Because of thermal fluctuations the spectrum changes from an exponential to a power law decay for large wave vectors. Also the time evolution of the typical wave vector of unstable perturbations exhibits noise-induced coarsening that is absent in deterministic hydrodynamic flow.     

Pseudomorphism in ultra-thin metal films

Current models of the processes preceding nucleation and epitaxial growth of metal thin films on metallic substrates are reviewed briefly, with particular reference to field emission and field ion microscope studies of metallic adsorbates on tungsten. It is concluded that many aspects of the phenomena of pseudomorphism, and of crystal nucleation at high adsorbate coverages, can be understood in terms of the structural features of the substrate surface and the strength of the adsorbate-substrate interaction.     

Spinodal-like dewetting of thermodynamically-stable thin polymer films

Energetic considerations indicate that long-range Van der Waals forces stabilize thin polystyrene (PS) films against height fluctuations on silicon substrates. Nevertheless, we report here on the amplification of capillary waves of specific wavelengths for 15 nm thick PS films on silicon, ultimately leading to dewetting in a spinodal-like process. However, the temporal dependence of the wavelength of the growing instability does not agree with the classical spinodal dewetting mechanism. Therefore, this phenomenon is ascribed to the existence of structural forces resulting either from the restructuring of the films or from density variations within the films during annealing, in accordance with recent theoretical treatments. The process is shown not to be limited to polystyrene films, which indicates the generality of our findings.Received: 1 August 2003PACS: 68.15. + e Liquid thin films - 47.20.-k Hydrodynamic stability - 47.20.Ma Interfacial instability - 68.08.-p Liquid-solid interfaces     

薄膜去湿不稳定性的热力学分析

对边界滑移条件下薄膜去湿不稳定性用热力学方法进行了研究, 即在求解的过程中带入边界滑移条件, 得到描述的薄膜去湿不稳定性的特征长度的统一形式, 并比较了忽略Marangoni效应情况下的薄膜 去湿不稳定性和考虑Marangoni效应情况下的去湿不稳定性.     

Spinodal dewetting of thin liquid films at soft interfaces

We study theoretically the behavior of nanoscopic liquid films L (thickness e) intercalated between a solid S and a rubber R (elastic modulus μ). Thickness modulations involve a healing length , which results from a competition between elastic and disjoining pressure. With van der Waals interactions, , where a is a molecular size and h₀ the rubber capillary length ( , interfacial tension). If the Hamaker constant of the intercalated liquid is negative, the film dewets by amplification of peristaltic fluctuations (“spinodal dewetting”). The typical size of the contacts is predicted to scale like for films of thicknesses . The rise time of the fastest mode, predicted to scale like , should be very sensitive to the film thickness. Received 11 February 2000 and Received in final form 22 May 2000     

Slip vs. viscoelasticity in dewetting thin films

Ultrathin polymer films on non-wettable substrates display dynamic features which have been attributed to either viscoelastic or slip effects. Here we show that in the weak- and strong-slip regime, effects of viscoelastic relaxation are either absent or essentially indistinguishable from slip effects. Strong slip modifies the fastest unstable mode in a rupturing thin film, which questions the standard approach to reconstruct the effective interface potential from dewetting experiments.     

Compressed exponential form for disordered domain wall motion in ultra-thin Au/Co/Au ferromagnetic films

Magnetization reversal in ultra-thin Au/Co/Au films deposited on single crystal silicon (1 0 0) was investigated using Kerr microscopy. In the considered ultra-thin Co films, with a thickness between 0.7 and 1 nm, the coercivity and magnetic anisotropy decrease with decrease in cobalt layer thickness and the magnetization reversal dynamics is dominated by disordered domain wall motion. An analysis of the observed magnetization reversal dynamics is proposed, starting from the Fatuzzo-Labrune model. We show that the relaxation curves of these samples are well described by a function obtained by a technical transformation of Fatuzzo-Labrune model in the regime dominated by domain wall motion.     

New slip regimes and the shape of dewetting thin liquid films

We compare the flow behavior of liquid polymer films on silicon wafers coated with either octadecyl-(OTS) or dodecyltrichlorosilane (DTS). Our experiments show that dewetting on DTS is significantly faster than on OTS. We argue that this is tied to the difference in the solid/liquid friction. As the film dewets, the profile of the rim advancing into the undisturbed film is monotonically decaying on DTS but has an oscillatory structure on OTS. For the first time we can describe this transition in terms of a lubrication model with a Navier-slip condition for the flow of a viscous Newtonian liquid.     

Influence of surface cleaning on dewetting of thin polystyrene films

Thin polystyrene (PS) films on top of silicon substrates are a frequently investigated model system in the framework of unstable films. However, with respect to stability the various experiments yielded contradictory results. Focussing on the influence of preparation conditions such as the surface cleaning solves these contradictions. By applying different surface cleans the PS film can be changed from a stable homogeneous one into a completely dewetted one. In addition to the type of clean applied, the time between cleaning the surface and spin-coating the polymeric layer on top turned out to be an important experimental parameter.Received: 1 August 2003PACS: 68.55.-a Thin film structure and morphology - 68.15. + e Liquid thin films - 68.47.Mn Polymer surfaces     

Templating of thin films induced by dewetting on patterned surfaces

The instability, dynamics, and morphological transitions of patterns in thin liquid films on chemically heterogeneous striped surfaces are investigated based on 3D nonlinear simulations. The film breakup is suppressed on some potentially destabilizing nonwettable sites when their spacing is below a characteristic length scale of the instability, lambda(h). The thin film pattern replicates the substrate surface energy pattern closely only when (i) the periodicity of substrate pattern lies between lambda(h) and 2lambda(h), and (ii) the stripe width is within a range bounded by a lower critical length, below which no heterogeneous rupture occurs, and an upper transition length above which complex morphological features unlike the substrate pattern are formed.     

Grazing incidence X-ray studies of ultra-thin Lumogen films

Lumogen^® Yellow S0790 films have been produced on silicon wafer substrates via physical vapour deposition (PVD) and spin-coating (SC) methods. These coatings were characterised with X-ray reflectometry (XRR) and grazing incidence X-ray diffraction (GIXD) techniques. The results show that ultra-thin (less than 12 nm) PVD films coat amorphously, with crystallinity becoming increasingly apparent with increasing film thickness. In contrast, measurements of ultra-thin (less than ∼2 nm) spin-coated films reveal a second, apparently stable crystalline structure.     

Nucleation-limited dewetting of ice films on Pt(111)

Quantifying dewetting phenomena at the microscopic level is the key to deciphering how a balance between kinetic and equilibrium effects determines ice-film morphology on Pt(111). Overcoming the difficulty of imaging nominally insulating ice multilayers with scanning tunneling microscopy allowed us to track the dewetting process. The results show that the rate at which new layers nucleate, and not surface diffusion, determines how fast individual crystallite shapes equilibrate. Applying nucleation theory to measured growth rates versus crystallite size, we obtain new bounds on the energetics both of step formation on ice and of the Pt-ice interface.     

Room-temperature deposition of crystalline patterned ZnO films by confined dewetting lithography

In this work patterned ZnO films were prepared at room-temperature by deposition of ∼5 nm size ZnO nanoparticles using confined dewetting lithography, a process which induces their assembly, by drying a drop of ZnO colloidal dispersion between a floating template and the substrate. Crystalline ZnO nanoparticles exhibit a strong visible (525 nm) light emission upon UV excitation (λ = 350 nm). The resulting films were characterized by scanning electron microscopy (SEM) and atomic force microscope (AFM). The method described herein presents a simple and low cost method to prepare crystalline ZnO films with geometric patterns without additional annealing. Such transparent conducting films are attractive for applications like light emitting diodes (LEDs). As the process is carried out at room temperature, the patterned crystalline ZnO films can even be deposited on flexible substrates.     

Dynamic instability of strongly confined thin polymer films in spinodal dewetting

We present a general criterion for determining the dynamic instability of a thin polymer film that is strongly confined to a substrate. When such a polymer film is heated above its glass transition temperature, it dewets from the substrate by a spinodal dewetting scenario, in which the onset of the instability is governed by dispersion force and residual thermal stress. It turns out that the thermal stress plays an important role when there is a special interaction between a polymer and a substrate.     

About the depth sensitivity of second-harmonic radiation in ultra-thin metal films

Second-harmonic generation (SHG) results for Ni and Co films on Cu (001) have been reinvestigated regarding the depth sensitivity of this technique for thin films. We find that tangential components of the nonlinear susceptibility are much more sensitive to real film properties than normal components, which are confined to surfaces and interfaces. In consequence, for SHG experiments on ultra-thin metal films, polarization combinations should be favored that possess only tangential susceptibility components, even though the yield is weaker. Additional phase measurements are necessary to obtain full information about the film. Received: 20 January 2002 / Published online: 6 June 2002