Buckling Of Film Research Articles

Facile Fabrication of Ordered Crystalline‐Semiconductor Microstructures on Compliant Substrates

Buckling and wrinkling of thin films on a compliant material has proved to be a resource in several applications, such as flexible electronics, thin‐film metrology and fabrication of tunable optical components. A versatile approach for the fabrication of two‐dimensional and linear arrays of buckled structures is demonstrated here using a stiff material, in the form of a nanomembrane, on a compliant substrate. The novelty of the fabrication process is that the substrates are strained by isotropic volume expansion in solvents. This work illustrates in detail the potential of our technology to fabricate ordered arrays of 3D structures on large‐area compliant substrates, with important implications for a large number of fields. Furthermore, this paper discusses the interesting interface chemistry and mechanics leading to controllable and reproducible fabrication of our 3D structures.

A simplified analysis on buckling of stressed and pressurized thin films on substrates

This paper is concerned with effect of mismatched pressure on buckling of stressed thin films on a semi-infinite rigid substrate. Analytical approximate solutions are established in explicit form by using total potential energy of the system and the Rayleigh–Ritz’s method, and their stabilities have also been determined. The dependences of the critical stress and film-center deflection on the mismatched pressure have been formulated. Results are compared with exact or numerical shooting solutions, and excellent agreements are observed for a large range of film-center deflection. These expressions are brief and can easily be used to derive the effects of various parameters on mechanical behavior of films.

Hydrogen microscopy – Distribution of hydrogen in buckled niobium hydrogen thin films

Hydrogen absorption in thin metal films clamped to rigid substrates results in mechanical stress that changes the hydrogen's chemical potential by ΔμH(σ) = −1.124σ kJ/molH for σ measured in [GPa]. In this paper we show that local stress relaxation by the detachment of niobium hydrogen thin films from the substrate affects the chemical potential on the local scale: using coincident proton–proton scattering at a proton microprobe, the hydrogen concentration is determined with μm resolution, revealing that hydrogen is not homogenously distributed in the film. The local hydrogen solubility of the film changes with its local stress state, mapping the buckled film fraction. In niobium hydrogen thin films loaded up to nominal concentrations in the two-phase coexistence region, the clamped film fraction remains in the solid solution phase, while the buckles represent the hydride phase. These results are compared to a simple model taking the stress impact on the chemical potential into account.

Creation of freestanding wrinkled nano-films with desired deformation properties by controlling the surface morphology of a sacrificial layer

Various wrinkle patterns can be formed due to the buckling of a stiff thin film on a compliant substrate. However, most wrinkled films previously reported were fixed on a large deformable substrate and thereby the potential deformability of the film was mechanically constrained by the substrate. In this study, we developed a technique for forming various wrinkled structures on the surface of a sacrificial resin layer. Since the sacrificial layer can be subsequently removed with a solvent, freestanding wrinkled films are created using the sacrificial layer. We found that a wrinkled structure is formed on the surface of the layer by applying a compressive strain to the resin layer at the appropriate moment during the hardening process. The wrinkle pattern depends on the curing time and the timing of the straining in two in-plane orthogonal directions. In addition to conventional stripe and labyrinth patterns by simple uniaxial and equi-biaxial strains, respectively, it was found that independent biaxial strains induce interesting structures, such as an orthogonally ordered wrinkle pattern and a nonsymmetrical buckling structure, in which the stripe array produced by the first straining remains and many finer wrinkles appear in each stripe by the second straining in the orthogonal direction. We conducted tensile experiments for 300-nm-thick freestanding Cu films having these wrinkled structures. The wrinkled nano-films have a variety of mechanical properties: the stripe structure has extremely high deformability (more than 10% strain) and reversibility, the labyrinth structure shows planar isotropic deformation, and the nonsymmetrical buckling structure has an anisotropic modulus and strength. Finite element analysis on the wrinkle structures revealed that the local stress concentration dominates the fracture limits.

Controlled Morphology of Thin Film Silicon Integrated with Environmentally Responsive Hydrogels

Environmentally responsive hydrogels hold multiple important applications. However, the functionality of these materials alone is often limited in comparison to other materials like silicon; thus, there is a need to integrate soft and hard materials for the advancement of environmentally sensitive materials. Here we demonstrate the capability of integrating a thermoresponsive hydrogel, poly(N-isopropylacrylamide), with thin film silicon ribbons, enabling the stiff silicon ribbons to become adaptive and drivable by the soft environmentally sensitive substrate. This integration provides a means of mechanical buckling of the thin silicon film due to changes in environmental stimuli (e.g., temperature, pH). We also investigate how advanced lithographic techniques can be used to generate patterned deformation on the aforementioned integrated structures. Furthermore, we explore multilayer hybrid hydrogel structures formed by the integration of different types of hydrogels that have tunable curvatures under the influence of different stimuli. Silicon thin film integration on such tunable curvature substrates reveal characteristic reversible buckling of the thin film in the presence of multiple stimuli. These results open new opportunities for developing stretchable and intelligent devices for multiple applications.

Control and manipulation of microfluidic flow via elastic deformations

We utilize elastic deformations via mechanical actuation to control and direct fluid flow within a flexible microfluidic device. The device consists of a microchannel with a flexible arch prepared by the buckling of a thin elastic film. The deflection of the arch can be predicted and controlled using the classical theory of Euler buckling. The fluid flow rate is then controlled by coupling the elastic deformation of the arch to the gap within the microchannel, and the results compared well with analytical predictions from a perturbation calculation and numerical simulations. We demonstrate that placement of these flexible valves in series enables directed flow towards regions of externally applied mechanical stress. The simplicity of the experimental approach provides a general design for advanced functionality in portable microfluidics, self-healing devices, and in situ diagnostics.

Interface step-induced thin-film delamination and buckling

Atomistic simulations based on experimental observations provide the first evidence that the interface delamination of a thin film from its substrate may start from interface steps. Buckling of the film after interface gliding from both edges of its delaminated part is also observed. In the framework of the Föppl–von Kármán theory of thin plates, the expression of the critical strain beyond which the film buckles has been then analytically determined as a function of the step height and gliding displacements. Both numerical and analytical results confirm that the formation of blisters is favoured in the neighbourhood of interfacial imperfections.

Inhomogeneous large deformation study of temperature-sensitive hydrogel

In this paper, inhomogeneous deformation of a temperature-sensitive hydrogel has been studied and analyzed under arbitrary geometric and boundary conditions. We present the governing equations and equilibrium conditions of an isothermal process based on the monophase gel field theory of hydrogel via a variational approach. We have adopted and implemented an explicit form of energy for temperature-sensitive hydrogel in a three-dimensional finite element method (FEM) using a user-supply subroutine in ABAQUS. For verification purpose, a few numerical results obtained by the proposed approach are compared with existing experimental data and analytical solutions. They are all in good agreement. We also provide several examples to show the possible applications of the proposed method to explain various complex phenomena, including the bifurcation, buckling of membrane, buckling of thin film on compliant substrate and the opening and closure of flowers.

Micro-scale delaminating and buckling of thin film on soft substrate

In this paper, a simple process is suggested to estimate the interfacial toughness of the material system ‘aluminum film/soft PDMS substrate’. The specimen, i.e. the aluminum film deposited on the soft polydimethylsiloxane (PDMS) substrate, is subject to a tensile load, and delaminating and buckling of aluminum film are observed in the perpendicular direction to the tensile strain. With the aid of the buckling blisters, the interfacial toughness of the material system is estimated. Large deformation is considered during the buckling of the thin film, and the interfacial toughness is deduced from a fracture theory. Besides, the evolution from one single blister to three blisters and then four blisters is observed in situ under microscope. This simplified method has potential applications to flexible electronics in which interfacial toughness of the metal film/soft substrate must be well controlled.

Detailed modelling of delamination buckling of thin films under global tension

Tensile specimens of metal films on compliant substrates are widely used for determining interfacial properties. These properties are identified by the comparison of experimentally observed delamination buckling and a mathematical model which contains the interface properties as parameters. The current two-dimensional models for delamination buckling are not able to capture the complex stress and deformation states arising in the considered uniaxial tension test in a satisfying way. Therefore, three-dimensional models are developed in a multi-scale approach. It is shown that, for the considered uniaxial tension test, the buckling and associated delamination process are initiated and driven by interfacial shear in addition to compressive stresses in the film. The proposed model is able to reproduce all important experimentally observed phenomena, like cracking stress of the film, film strip curvature and formation of triangular buckles. Combined with experimental data, the developed computational model is found to be effective in determining interface strength properties.

Artificially lattice-mismatched graphene/metal interface: Graphene/Ni/Ir(111)

We report the structural and electronic properties of an artificial graphene/Ni(111) system obtained by the intercalation of a monoatomic layer of Ni in graphene/Ir(111). Upon intercalation, Ni grows epitaxially on Ir(111), resulting in a lattice mismatched graphene/Ni system. By performing Scanning Tunneling Microscopy (STM) measurements and Density Functional Theory (DFT) calculations, we show that the intercalated Ni layer leads to a pronounced buckling of the graphene film. At the same time an enhanced interaction is measured by Angle-Resolved Photo-Emission Spectroscopy (ARPES), showing a clear transition from a nearly-undisturbed to a strongly-hybridized graphene $\pi$-band. A comparison of the intercalation-like graphene system with flat graphene on bulk Ni(111), and mildly corrugated graphene on Ir(111), allows to disentangle the two key properties which lead to the observed increased interaction, namely lattice matching and electronic interaction. Although the latter determines the strength of the hybridization, we find an important influence of the local carbon configuration resulting from the lattice mismatch.

Surface Effects on the Mechanical Behavior of Buckled Thin Film

The buckling of thin films with natural nonlinearity can provide a useful tool in many applications. In the present paper, the mechanical properties of controllable buckling of thin films are investigated by accounting for both geometric nonlinearity and surface effects at nanoscale. The effects of surface elasticity and residual surface tension on both static and dynamic behaviors of buckled thin films are discussed based on the surface-layer-based model. The dynamic design strategy for buckled thin films as interconnects in flexible electronics is proposed to avoid resonance in a given noise environment based on the above analysis. Further discussion shows that the thermal and piezoelectric effects on mechanical behavior of buckled thin film are equivalent to that of residual surface tension.

The importance of interfacial adhesion in the buckling-based mechanical characterization of materials

The effect of interfacial adhesion on the buckling-based mechanical characterization of materials has been elucidated and practical guides for the correct measurement of the modulus by the buckling method have been given accordingly. When a stiff film on a compliant substrate is compressed, the film buckles into well-ordered sinusoidal surface undulations, over a large area, for the case of good interfacial adhesion and the resulting modulus value agrees well with that observed via the microtensile test. On the other hand, the modulus value obtained from the buckling measurement deviates significantly from that of the microtensile method for the case of poor interfacial adhesion. The nanoscale interface debonds in the poor adhesion case and led to the reduction of effective sample dimension, and this finite width effect has been found to be the reason for the observed discrepancy. Furthermore, simple ways to avoid or reduce the adhesion-related artefacts in the buckling method have been demonstrated. Although the well-known PS/PDMS system has been used in the present work, due to its simplicity to control the interfacial adhesion simply by changing the mixing ratio of PDMS substrate, we believe that the present results should bear the same importance in any other buckling system. Therefore, great care should be taken when one uses or reports the mechanical properties of a material obtained from the buckling method, as well as in studies of physics and mechanics for a given buckling system.

1422 円孔を有するゲル薄膜に生じる膨潤誘起座屈の有限要素解析

1422 円孔を有するゲル薄膜に生じる膨潤誘起座屈の有限要素解析

Design of Ordered Wrinkled Patterns with Dynamically Tuned Properties

The formation of patterned surfaces is a common tool to engineer materials. The capability to design and reproduce detailed features is a key factor to fulfill requirements for functional surfaces. Generation of wrinkles via buckling of a stiff film on a compliant surface is an inexpensive, easy and reliable method to yield a patterned surface. The wrinkling method has been exploited in a wide variety of areas, including photovoltaics, microfluidics, adhesion, and anti-fouling systems. Here we show the ability to obtain deterministically ordered herringbone patterns. In a biaxially pre-stretched PDMS sample a thin film of a stiff coating is deposited by initiated chemical vapor deposition (iCVD). iCVD is a solventfree technique that yields a conformal thin coating on virtually any substrate, giving a controllable thickness and tunable structural, mechanical, thermal, wetting, and swelling properties. Sequential release of the film-substrate system shows the transition from 1-D ripples to an ordered herringbone pattern. Wrinkle features can be controlled adjusting the film thickness, the initial load and the release process. Moreover, the surface topography can be dynamically tuned by applying a controlled mechanical stimulus. These properties make these materials excellent candidates for flexible applications.

Guided wrinkling in swollen, pre-patterned photoresist thin films with a crosslinking gradient

A new thin film buckling system was developed from photosensitive SU-8 thin films doped with UV light absorbing dye, resulting in a depth-wise gradient modulus after photocrosslinking. When the film was swollen by an organic solvent, a wide range of wrinkling patterns was obtained, including lamella, peanut and semi-hexagonal patterns. Both the morphology and wavelength were found to be dependent on the original film thickness. By leveraging the thermoplasticity of SU-8, we imprinted one-dimensional (1-D) patterns on the dyed SU-8 with variable pitch and height from 1 μm to 20 μm and 15 nm to 2 μm, respectively. We then swelled the patterned films and investigated the interactions between the intrinsic buckling waves (both size and morphology) and the pre-patterns. As the pre-pattern pitch decreased, the swollen film in the patterned region evolved from isotropic wrinkles to out-of-phase, anisotropic waves, which further became in-phase when the pre-pattern pitch was smaller than the intrinsic wrinkle wavelength. For the latter, the aligned wrinkle morphology varied dramatically when the pre-pattern height decreased: from perpendicular to the pre-pattern wavevector to dual orientation with one set of wrinkles remained perpendicularly ordered and the other set of local buckling patterns aligned in parallel to the pre-pattern, and finally back to isotropic ones. Since the pre-patterns of different size and shape could be readily prepared, the combination of physical confinement together with controlled swelling in a graded thin film offers a new approach to access a wide range of controllable hierarchical patterns.

Buckle-driven delamination of hydrophobic micro-, nano-, and heterostructured membranes without a sacrificial layer

A fabrication method, based on thin film buckling, is demonstrated to form unique membranes that can be used for applications in optics and biosensing. This method should be applicable to a variety of material systems, which, along with its simplicity and compatibility with different film architectures, allows for widespread implementation.

Capillary buckling of a thin film adhering to a sphere

We present a combined theoretical and experimental study of the buckling of a thin film wrapped around a sphere under the action of capillary forces. A rigid sphere is coated with a wetting liquid, and then wrapped by a thin film having an approximately cylindrical shape. The equilibrium of the film is governed by the competing effects of elasticity and capillarity: elasticity tends to keep the film developable while capillarity tends to curve it in both directions so as to maximize the area of contact with the sphere. In the experiments, the region of contact between the film and the sphere has cylindrical symmetry when the sphere radius is small, but destabilizes to a non-symmetric, wrinkled configuration when the radius is larger than a critical value. We combine the Donnell equations for near-cylindrical shells to include a unilateral constraint with the impenetrable sphere, and the capillary forces acting along a moving edge. A non-linear solution describing the axisymmetric configuration of the film is derived. A linear stability analysis is then presented, which successfully captures the wrinkling instability, the symmetry of the unstable mode, the instability threshold and the critical wavelength. The motion of the free boundary at the edge of the region of contact, which has an effect on the instability, is treated without any approximation.

Structural Studies of Nanocrystalline Thin Pd Films Electrochemically Doped with Hydrogen

Hydrogen absorption in Pd causes a significant volume expansion. In free-standing bulk Pd, the hydrogen-induced volume expansion is isotropic. However, the situation becomes more complicated in thin Pd films. Contrary to bulk samples, thin films are clamped to an elastically stiff substrate, which prevents in-plane expansion. Hence, the volume expansion of a thin film is strongly anisotropic because it expands in the out-of-plane direction only. Internal stresses introduced by absorbed hydrogen may become so high that detachment of a film from the substrate is energetically favorable and buckles of various morphologies are formed. In the present work, we studied hydrogen-induced buckling in a nanocrystalline thin Pd film deposited on a sapphire substrate. Slow positron implantation spectroscopy (SPIS) was employed as a principal tool for the characterization of defects and investigation of defect interactions with hydrogen. SPIS studies were combined with X-ray diffraction and direct observations of buckling by light microscopy. It was found that buckling of thin Pd film occurs at hydrogen concentrations xH > 0.1 and is accompanied by a strong increase of dislocation density.

Asymptotic analysis of nonlinear micro-film buckling

The buckling design of micro-films has various potential applications to engineering. The substrate prestrain, interconnector buckling amplitude and critical strain are important parameters for the buckling design. In the presented analysis, the buckled film shape was described approximately by a trigonometric function and the buckled film amplitude was obtained by minimizing the total strain energy. However, this method only generates the first-order approximate solution for the nonlinear buckling. In the present paper, an asymptotic analysis based on the rigorous nonlinear differential equation for the buckled micro-film deformations is proposed to obtain more accurate relationship of the buckling amplitude and critical strain to prestrain. The obtained results reveal the nonlinear relation and are significant to accurate buckling design of micro-films.