Instability Of Thin Films Research Articles

Influence of the central mode and soft phonon on the microwave dielectric loss near the strain-induced ferroelectric phase transitions inSrn+1TinO3n+1

Recently, Lee et al. [Nature (London) 502, 532 (2013)] used \ensuremath{\sim}1% tensile strain to induce a ferroelectric instability in thin films of $\mathrm{S}{{\mathrm{r}}_{n}}_{+1}\mathrm{T}{\mathrm{i}}_{n}{\mathrm{O}}_{3n+1}(n=1\ensuremath{-}6)$ phases. They showed that the Curie temperature ${T}_{C}$ gradually increased with $n$, reaching $180\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $\mathrm{S}{\mathrm{r}}_{7}\mathrm{T}{\mathrm{i}}_{6}{\mathrm{O}}_{19}(n=6)$. The permittivity of this $(n=6)$ sample could also be tuned significantly by the application of an electric field with exceptionally low dielectric loss at 300 K, rivaling all known tunable microwave dielectrics. Here, we present microwave (MW), terahertz, and infrared spectra of strained $\mathrm{S}{{\mathrm{r}}_{n}}_{+1}\mathrm{T}{\mathrm{i}}_{n}{\mathrm{O}}_{3n+1}$ thin films deposited on (110) $\mathrm{DySc}{\mathrm{O}}_{3}$. Near the ferroelectric phase transitions, we observe the splitting and shifting of phonon and central mode frequencies, demonstrating the change of crystal symmetry below ${T}_{C}$. Moreover, our spectra reveal that the central mode contribution dominates MW loss. In the $\mathrm{S}{\mathrm{r}}_{7}\mathrm{T}{\mathrm{i}}_{6}{\mathrm{O}}_{19}$ thin film, the central mode vanishes at 300 K, explaining its low MW loss. Finally, we discuss the origin and general conditions for the appearance of central modes near ferroelectric phase transitions.

Distinguishing mechanisms of morphological instabilities in phase change materials during switching

The process of dewetting of a thin film from a solid substrate is important for its scientific and technological relevance, but can be difficult to observe experimentally. We report on an experimental method that may be used to investigate morphological changes, including dewetting, during laser heat treatment of alloys used for phase change memory devices. We have used nanosecond-scale time-resolved imaging to differentiate between competing thin film instabilities in GeTe, a chalcogenide-based phase change material. It is shown that in the absence of nucleated dewetting, thin films of phase change alloys may be unstable, but that nucleated dewetting can lead to a more disrupted final state of the thin film.

Evolution of homopolymer thin-film instability on surface-anchored diblock copolymers varying in composition.

The stability of molecularly thin polymer films deposited on various material substrates is of critical importance to many contemporary nanotechnologies involving functional coatings and nano/micropatterned surfaces, in which case the causes responsible for film destabilization must be fully understood. Previous experimental studies report that factors such as film thickness and polymer molecular weight play significant roles in governing the rate, as well as mechanism, of destabilization. Complementary theoretical predictions reveal that surface heterogeneities can likewise induce (and regulate the process of) destabilization. In this study, we investigate the destabilization rate and mechanism of homopolystyrene (PS) films differing in thickness on top of poly(styrene-b-methyl methacrylate) (SM) diblock copolymer monolayers varying in chemical composition anchored to flat silica-like substrates to examine the effect of surface constitution on PS stability. Copolymers with a long M block consistently promote PS dewetting by nucleation and growth, wherein the linear dewetting rate decreases monotonically with increasing PS molecular weight, film thickness, and S fraction in the SM copolymer. In analogous studies involving a copolymer with a relatively short M block, however, PS dewetting proceeds instead by spinodal dewetting that evolves gradually into nucleation and growth as the film thickness is increased.

Structural instability and electrical properties in epitaxial Er2O3-stabilized Bi2O3 thin films

Bismuth oxide based materials exhibit the highest oxygen ion conductivities, making them of great interest for use in energy conversion devices such as solid oxide fuel cells. However, these materials exhibit chemical and thermal instabilities and understanding and their stabilization is an actively pursued research goal. In this study, we investigate the structural and electrical properties of erbium oxide stabilized bismuth oxide (Er0.4Bi1.6O3−δ) as thin films. These are deposited by pulsed laser deposition onto several single crystal substrates (MgO, Al2O3 and SrTiO3). The films show new forms of instabilities, both upon aging treatments in air and even under conductivity measurements, with remarkable changes in the film composition and microstructure.

Interfacial instability of thin ferrofluid films under a magnetic field

AbstractWe study magnetically induced interfacial instability of a thin ferrofluid film subjected to an applied uniform magnetic field and covered by a non-magnetizable passive gas. Governing equations are derived using the long-wave approximation of the coupled static Maxwell and Stokes equations. The contact angle is imposed via a disjoining/conjoining pressure model. Numerical simulations show the patterning resulting from unstable perturbations and dewetting of the ferrofluid film. We find that the subtle competition between the applied field and the van der Waals induced dewetting determines the appearance of satellite droplets. The results suggest a new route for generating self-assembled ferrofluid droplets from a thin film using an external magnetic field. An axisymmetric droplet on a surface is also studied, and we demonstrate the deformation of the droplet into a spiked cone, in agreement with experimental findings.

Polymer thin film instability from a patterned edge

Patterning of polymer thin films with nanoscale features is a critical step in nanoscale technologies. Here, we investigate the dewetting process from both a mechanically patterned edge (ME) and a hole edge of a thin polymer film, as a means to assess the influence of residual stresses induced during patterning on stability. At low temperature in which hole nucleation is not observed dewetting can still proceed from the ME. Under conditions in which dewetting proceeded from both edges that from the ME exhibited a crossover to a dramatically reduced dewetting velocity. These results suggest mechanical stress concentrated along the ME remarkably depresses the dewetting process by decreasing the rim growth rate.

3D finite element modeling for instabilities in thin films on soft substrates

Spatial pattern formation in stiff thin films on compliant substrates is investigated based on a nonlinear 3D finite element model. Typical post-bifurcation patterns include 1D sinusoidal, checkerboard and herringbone shapes, with possible spatial modulations, boundary effects and localizations. The post-buckling behavior often leads to intricate response curves with several secondary bifurcations that were rarely studied and only in the case of periodic cells. The proposed finite element procedure allows accurately describing these bifurcation portraits by taking into account the effect of boundary conditions. It relies on the Asymptotic Numerical Method (ANM) that offers considerable advantages to get a robust path-following technique and to detect multiple bifurcations. The occurrence and evolution of sinusoidal, checkerboard and herringbone patterns will be highlighted.

Macromol. Rapid Commun. 5/2014

Front Cover: How to suppress the instability of liquid thin films on solid surfaces? Crystalline polymers (low weight percentage) dissolved in the liquid may produce in situ a network of the crystalline fibrils during the coating process. The network forms a liquid gel and prevents dewetting of the liquid. This strategy can be used to generate micropatterns of divserse materials such as polymers, metal oxides, and metals. This process allows fabrication of residual layer-free micropatterns, which has long been a challenge in soft lithography. Further details can be found in the article by J. Park , M. Park , and U. Jeong* on page 560.

Suppressing Instability of Liquid Thin Films by a Fibril Network and its Application to Micropatterning without a Residual Layer

This study proposes a method to coat thin films of non-volatile solvents on substrates. A small amount of crystalline polymer dissolved in solvents forms a network of crystalline fibrils during the coating process. The network suppresses dewetting of the solvent liquid and helps the liquid film sustaining on the substrate. This strategy can be used in soft lithography to generate micropatterns of diverse materials without having a residual layer. This process does not request etching for achieving residual layer-free micropatterns, which has been a long challenge in soft lithography. As examples, we demonstrate micropatterns of polymer hydrogels and metal oxides (ZnO, In2O3).

The Drainage of Thin Liquid Films on an Inclined Solid Surface

In this paper, we consider the thinning process of an inclined thin liquid film over a solid boundary with an inclination angle to the horizontal in gravity driven flow. Throughout this work, we assumed that the fluid thickness is constant far behind the front and we neglect the thickness of the film at the beginning of the motion. The equation of the film thickness is obtained analytically, using the similarity method by which we can isolate the explicit time dependence and then the shape of the film will depend on one variable only. The solution of the governing equations of the film thickness is obtained numerically by the Rung-Kutta method with the aid of Mat lab(ode45). We present here some of the theoretical aspects of the instability development in an inclined thin liquid films on a solid surface in two dimensional coordinate system with an inclination angle to the horizontal . There are different types of phenomena that can occur, such as drainage, details of rupture, non-Newtonian surface properties in moving contract lines in thin liquid films (1). These phenomena can help to describe the physical processes that occur in our real world. (2,3) have studied the case of contact line instabilities of thin liquid films but with constant flux configuration and also they considered some global models of a moving contact lines. (4) studied the thin liquid films flowing down the inverted substrate in three dimensional flow. (5) investigated the dynamics of an inclined thin liquid films of variable thickness in steady and unsteady cases and when the film is stationary and uniform. (6) considered the stability of thin liquid films and sessile droplets. The stability of the contact line of thin fluid film flows with constant flux configuration is considered by (7). (8) considered the spreading of thin liquid films with small surface tension in the case when the flow is unsteady. The Non-linear analysis of creeping flow on the inclined permeable substrate plane subjected to an electric field was considered by (9). In this paper we investigate the drainage of the inclined thin liquid films where the gravity and other forces such as viscous and surface tension forces have a significant effect on the flow of the film. We use the similarity method by which we can isolate the explicit time dependence and then the shape of the film will depend on one variable only. The solution of the governing equations of the liquid film thickness is obtained numerically.

Rayleigh Instability in Polymer Thin Films Coated in the Nanopores of Anodic Aluminum Oxide Templates

We study the Rayleigh instability of polystyrene (PS) thin films coated in the nanopores of anodic aluminum oxide (AAO) templates. After thermal annealing, the surface of the PS thin films undulates and the nanostructures transform from nanotubes to Rayleigh-instability-induced nanostructures (short nanorods with encapsulated air bubbles). With longer annealing times, the nanostructures further transform to nanorods with longer lengths. PS samples with two different molecular weights (24 and 100 kg/mol) are used, and their instability transformation processes are compared. The morphology diagrams of the nanostructures at different stages are also constructed to elucidate the mechanism of the morphology transformation.

Nonlinear Evolution of the Travelling Waves in Roll Coating Flows of Thin Viscoelastic Polymer Falling Films

Roll coating is widely used to apply a thin liquid film to a continuous, flexible substrate. It is known that macroscopic instabilities of the film system can result in a non-homogenous film growth to fluid flow. The influence of the Rossby number, the viscoelastic parameter, and the roller radius on the nonlinear hydrodynamic stability of a thin viscoelastic polymer fluid film coating flow down a rotating vertical roller is investigated. In contrast to most previous studies, the solution scheme employed in this study is based on a numerical approximation approach rather than an analytical method. The size of the explosive supercritical instability region increases significantly as the roller rotates. It is shown that the stability of the liquid film is enhanced by reducing the viscoelastic effect or decreasing the speed of the rotating roller. At higher values of the Reynolds number, the tendency of the rotation effect to prompt thin-film instability increases with an increasing roller radius.

Instabilities of Teflon AF thin films in alumina nanochannels and adhesion of bi-level Teflon AF nanopillars

In this paper, a novel replica-molding technique for fabrication of bi-level Teflon AF nanopillars, as an electrostatic-based dry adhesive, is reported. The technique reported herein relies on the concurrent heating and cooling of the Teflon AF melt which filled vertically aligned alumina nanochannels as the mold. Unlike conventional polymer infiltration methods which consist of filling the mold by only heating the polymer above its glass transition temperature, in the current method, the polymer melt was also simultaneously cooled down during the infiltration process. Concurrent cooling of the Teflon AF melt allowed control over the interfacial instabilities of the polymer thin film, which formed ahead of the polymer melt upon its infiltration into the alumina nanochannels. By doing so, the geometrical properties of the peculiar fluffy nanostructure which was subsequently developed—after removal of the mold—on top of the extremely high aspect-ratio Teflon AF nanopillars (200 nm in diameter, ∼25 μm tall) were modified. The height of the base nanopillars was measured and the structural properties (i.e., surface area fraction and roughness) of the fluffy nanostructure terminating the base nanopillars at the tip were quantified. Next, the effects of the topographical properties of the bi-level Teflon AF nanopillars on their adhesion, in both the normal and shear directions, were investigated. Tribological results were discussed in detail to clarify the contribution of the structural properties of the fabricated dry adhesive toward its remarkable adhesion and friction forces generated via contact electrification. It is worthwhile to mention that bi-level Teflon AF nanopillars with these specific structural properties have generated enhanced adhesion and friction strengths, up to ∼2.1 and 13 N cm−2, respectively.

Instability of hydrophobic and viscoelastic polymer thin films in water at room temperature

The instability of a polyisoprene (PI) thin film on a silicon substrate at room temperature in an aqueous environment was investigated by atomic force microscopy and optical microscopy. The instability mechanism changes from spinodal dewetting to hole nucleation with increasing film thickness, with the transitional thickness found to be around 46–50 nm. For PI films ≥50 nm, the dewetting was observed to proceed via successive stages of hole nucleation and growth, hole coalescence, cellular pattern formation and droplet formation. There is also a slowing down in the rate of the PI dewetting process and an increase in the pattern size as the film thickness is increased. In those films with observable holes, we also observed the coexistence of fine cellular cracking that is on a much smaller scale of hundreds of nanometres and extends only a few nanometres in depth from the film surface.

Ferroelectric and structural instability of (Pb,Ca)TiO3 thin films prepared in an oxygen atmosphere and deposited on LSCO thin films which act as a buffer layer

Structural, microstructural and ferroelectric properties of Pb0.90Ca0.10TiO3 (PCT10) thin films deposited using La0.50Sr0.50CoO3 (LSCO) thin films which serve only as a buffer layer were compared with properties of the thin films grown using a platinum-coated silicon substrate. LSCO and PCT10 thin films were grown using the chemical solution deposition method and heat-treated in an oxygen atmosphere at 700°C and 650°C in a tube oven, respectively. X-ray diffraction (XRD) and Raman spectroscopy results showed that PCT10 thin films deposited directly on a platinum-coated silicon substrate exhibit a strong tetragonal character while thin films with the LSCO buffer layer displayed a smaller tetragonal character. Surface morphology observations by atomic force microscopy (AFM) revealed that PCT10 thin films with a LSCO buffer layer had a smoother surface and smaller grain size compared with thin films grown on a platinum-coated silicon substrate. Additionally, the capacitance versus voltage curves and hysteresis loop measurement indicated that the degree of polarization decreased for PCT10 thin films on a LSCO buffer layer compared with PCT10 thin films deposited directly on a platinum-coated silicon substrate. This phenomenon can be described as the smaller shift off-center of Ti atoms along the c-direction 〈001〉 inside the TiO6 octahedron unit due to the reduction of lattice parameters. Remnant polarization (Pr) values are about 30μC/cm2 and 12μC/cm2 for PCT10/Pt and PCT10/LSCO thin films, respectively. Results showed that the LSCO buffer layer strongly influenced the structural, microstructural and ferroelectric properties of PCT10 thin films.

Domain-wall flexing instability and propagation in thin ferromagnetic films

We investigate field-driven domain-wall dynamics in thin ferromagnetic layers in the precessional regime using an analytical model that goes beyond the rigid wall approximation and takes into account the flexing modes of domain walls. This model allows us to turn on and off the stray field from domains adjacent to the domain wall in order to elucidate its role. We determine the eigenfrequencies of the flexing modes. The domain-wall flexing instabilities are shown to exist even without the stray field. The amplitude of the flexing modes shows a maximum when the magnetization precession frequency is equal to their eigenfrequency. At maximum amplitude of the flexing modes, the domain-wall velocity exhibits bumps even without the stray field. The stray field further enhances the velocity and broadens the bumps. This study is completed by micromagnetic simulations that show excellent agreement with the analytical model. The role of the various spatial modes in the velocity enhancement is clarified. Additional fractional frequencies appear in the time-dependent amplitude of the flexing modes, in analogy with the behavior of parametric oscillators.

Instability of Liquid Cu Films on a SiO2 Substrate

We study the instability of nanometric Cu thin films on SiO2 substrates. The metal is melted by means of laser pulses for some tens of nanoseconds, and during the liquid lifetime, the free surface destabilizes, leading to the formation of holes at first and then in later stages of the instability to metal drops on the substrate. By analyzing the Fourier transforms of the SEM (scanning electron microscope) images obtained at different stages of the metal film evolution, we determine the emerging length scales at relevant stages of the instability development. The results are then discussed within the framework of a long-wave model. We find that the results may differ whether early or final stages of the instability are considered. On the basis of the interpretation of the experimental results, we discuss the influence of the parameters describing the interaction of the liquid metal with the solid substrate. By considering both the dependence of dominant length scales on the film thickness and the measured contact angle, we isolate a model which predicts well the trends found in the experimental data.

Electrohydrodynamic instabilities in thin viscoelastic films – AC and DC fields

Electrohydrodynamic instabilities in thin liquid films are a promising route for the self-assembly of well-defined topographical features on the surfaces of materials. Here, we study the effect of viscoelasticity on these instabilities under the influence of AC and DC electric fields. Viscoelasticity is incorporated via a Jeffreys model, and both perfect and leaky dielectric materials are considered. In the case of DC fields, asymptotic methods are employed to shed light on the nature of a singularity that arises when solvent viscosity is neglected (i.e., the Maxwell-fluid limit). In the case of AC fields, we apply a numerical procedure based on Floquet theory to determine the maximum growth rate and corresponding wave number as a function of the oscillation amplitude and frequency. Elasticity is found to increase both the maximum growth rate and corresponding wave number, with the effects being most pronounced when the oscillation period is comparable to the fluid relaxation time.

The effect of the thermal conductivity and thickness of the wall on the nonlinear instability of a thin film flowing down an incline

The nonlinear thermal instability of a thin liquid film falling down a heated wall is investigated. In particular, the heat conductivity and the thickness of the wall are taken into account. It is found that these two effects are represented by only one parameter which is the ratio of the nondimensional thickness of the wall and the nondimensional heat conductivity of the wall, that is d/Qc. The longwave linear stability is described in a general form with respect to a wide range of values of this parameter in order to understand the behavior of the thin film. In the nonlinear case, the thin film instability is investigated in space and time for two examples of time dependent perturbations. The first one is at a perturbation frequency of 0.5 and the second one is at 2.5. The Reynolds numbers corresponding to the isothermal maximum growth rate are used and it is shown that they are located at important places of the k vs. R plane, where k is the wave number and R is the Reynolds number. It is found the important result that, for any fixed Marangoni number Ma, the increase of the parameter d/Qc stabilizes the flow and at the same time decreases the nonlinear amplitude of the perturbations.

Thermodynamic analyses of dewetting instability in thin films

The dewetting instabilities in thin films under the boundary slip condition are studied using the thermodynamic approach. The general form of characteristic dewetting length scale is obtained from. The dewetting instability in thin film without Marangoni effect and that with Marangoni effect are compared.