Dewetting Of Films Research Articles

Dewetting-driven self-assembly of web-like silver nanowire networked film for highly transparent conductors

Silver nanowire (AgNW) networked films have received much attention as transparent conducting materials owing to their excellent conductivity, high transmittance, and moderate cost. In addition, AgNWs can be easily prepared as dispersions in liquids, enabling solution-based processing. Nevertheless, the fabrication of highly transparent AgNW networked electrodes remains challenging owing to the high percolation threshold of AgNWs. In this study, web-like AgNW networked films were fabricated via the dewetting-driven self-assembly of AgNWs using meniscus-dragging deposition. The dewetting of liquid thin films containing AgNWs was finely tuned by adjusting the ethylene glycol content of an AgNW–isopropyl alcohol dispersion and the surface energy of the coating substrate. The obtained AgNW networked electrodes with self-assembled web-like structures had a significantly lower percolation threshold (0.26 μg cm−2) than randomly networked AgNW electrodes (2.53 μg cm−2), resulting in an outstanding combination of sheet resistance and optical transparency (38 Ω sq−1 at T = 96%). This large scalable one-step coating strategy for metal mesh thin films can advance the development of next-generation transparent conducting electrodes.

Effects of fluid film properties on fouling in biphasic flow systems

Flow chemistry has the potential to enhance the large-scale accessibility of precision-engineered nanomaterials. While reactor fouling presented a major challenge in precision control, the liquid–liquid segmented flow method has been demonstrated to reduce or even eliminate it. However, guidelines for properly selecting materials and designing parameters in such systems are currently lacking. For these reasons, the present study investigated two biphasic flow systems for continuous gold nanoparticle (AuNP) synthesis. Visual observations showed no evidence of fouling in the silicone oil–water flow system, but some localized fouling was observed in the heptane-water flow system. The results indicated the silicone oil–water system gave a 50% lower polydispersity index and a 25% higher reaction yield due to fouling mitigation. We investigated two important factors of the continuous phase for fouling reduction: liquid film continuity and film thickness. Film continuity was evaluated by temperature-controlled contact angle measurements and the film dewetting velocity; And film thickness was estimated using two different mathematical models. It was concluded that the formation of a continuous oil film of thickness higher than the maximum surface asperities, as was the case only for the silicone oil–water flow system, is critical to eliminate fouling in continuous AuNP synthesis.

Phase-Field Simulation of Liquid-Vapor Equilibrium and Evaporation of Fluid Mixtures.

In solution processing of thin films, the material layer is deposited from a solution composed of several solutes and solvents. The final morphology and hence the properties of the film often depend on the time needed for the evaporation of the solvents. This is typically the case for organic photoactive or electronic layers. Therefore, it is important to be able to predict the evaporation kinetics of such mixtures. We propose here a new phase-field model for the simulation of evaporating fluid mixtures and simulate their evaporation kinetics. Similar to the Hertz-Knudsen theory, the local liquid-vapor (LV) equilibrium is assumed to be reached at the film surface and evaporation is driven by diffusion away from this gas layer. In the situation where the evaporation is purely driven by the LV equilibrium, the simulations match the behavior expected theoretically from the free energy: for evaporation of pure solvents, the evaporation rate is constant and proportional to the vapor pressure. For mixtures, the evaporation rate is in general strongly time-dependent because of the changing composition of the film. Nevertheless, for highly nonideal mixtures, such as poorly compatible fluids or polymer solutions, the evaporation rate becomes almost constant in the limit of low Biot numbers. The results of the simulation have been successfully compared to experiments on a polystyrene-toluene mixture. The model allows to take into account deformations of the liquid-vapor interface and, therefore, to simulate film roughness or dewetting.

Fabrication of metal-dielectric nanoparticles from a bi-layer gold-silicon film by femtosecond laser-induced dewetting

Today the world demand for the creation of highly efficient nanoscale white light sources is growing. It happens because energy-efficient information and communication systems are being developed, in which optical signals are replacing electrical signals. For the fabrication of such devices, creating efficient nanoscale white light sources with high efficiency is very acute. Such structures obtained by current methods have a common disadvantages: a small spectral width and low efficiency. Here we demonstrate the development of metal-dielectric structures exploiting the single-step and lithography-free laser-induced dewetting of bi-layer gold silicon films and study their broadband photoluminescence.

Impact-induced hole growth and liquid film dewetting on superhydrophobic surfaces

Wetting and dewetting phenomena occur widely in the fields of coating, anti-icing, and microfluidics. While liquid wetting via hole collapse has been intensively researched, liquid film dewetting, especially that induced by hole growth, has rarely been studied. This paper describes a combined experimental and theoretical investigation of metastable liquid film dewetting on superhydrophobic surfaces induced by dry hole growth. Experiments show that dry holes can form upon droplet impact, and these holes mainly exhibit growth, stability, or collapse depending on their initial size. Only the growth behavior can induce liquid film dewetting. Theoretical analysis further clarifies that the hole behavior is a result of competition between the capillary force and hydrostatic pressure, and the scale of the dewetting area is controlled by the Young–Laplace equation and affected by the shape of the superhydrophobic surface. The quantitative relationship between the dewetting velocity and the liquid film thickness is also established. These results deepen our understanding of liquid film dewetting on superhydrophobic surfaces and present fresh insights into related engineering applications.

Solvent Processing and Ionic Liquid-Enabled Long-Range Vertical Ordering in Block Copolymer Films with Enhanced Film Stability

Rapid and reliable processing methods for forming ordered block copolymer (BCP) materials with low defect density in a thin film geometry are required for many nanotechnology applications. Vertically aligned BCP structures, in particular, have applications ranging from nanolithography for electronics and photonics applications to nanoporous membranes for water remediation and novel batteries for flexible electronics. However, the attainment of a nearly complete vertical orientational order of the BCP-ordered phase remains challenging. Solvent-based techniques, such as direct immersion annealing (DIA) and solvent vapor annealing (SVA), have immense potential for these applications of BCP films, as it allows for a ‘tuning’ of their thermodynamic, structural, and chain mobility-driven kinetic properties. We first demonstrate that DIA, using a judicious choice of binary solvent mixtures, along with a relatively hydrophobic ionic liquid (IL), induces the rapid vertical ordering in polystyrene-b-poly(methyl methacrylate) (PS-PMMA) in lamellae-forming block copolymer films. The IL in a binary solvent mixture with toluene and heptane apparently gives rise to a near-neutral solvent for the BCP for itself and the boundary that is useful for attaining the vertical microstructure within the BCP film. Next, we show that an IL can moreover suppress the dewetting of the PS-PMMA films to achieve long-range order using SVA in cylindrical films after long annealing times. Both vertical and horizontal morphologies are attained in these films by selecting different solvent conditions. Attaining enhanced vertical and horizontal BCP structures with long-range defect-free order by tuning solvent quality and using additives like ILs can render them useful for many nanotech applications.

Rapid fabrication and interface structure of highly faceted epitaxial Ni-Au solid solution nanoparticles on sapphire

Supersaturated Ni-Au solid solution particles were synthesized by rapid solid-state dewetting of bilayer thin films deposited onto c-plane sapphire single-crystals. Rapid thermal annealing above the miscibility gap of the Ni-Au system followed by quenching to room temperature resulted in textured and faceted submicron-sized particles as a function of alloying content in the range of 0–28 at% Au. Morphologically, the observed kinetic crystal shapes are confined by close-packed planes; in addition, high-index facets are identified as a function of alloying content by TEM cross-sectioning and equilibrium crystal shape simulations. All samples exhibit a distinct 〈111〉 out-of-plane as well as in-plane texture along densely packed directions. Lattice parameters extracted from independent orthogonal X-ray and electron diffraction techniques prove the formation of a solid solution without tetragonal distortion imposed by the sapphire substrate. At the particle-substrate interface of highly alloyed particles segregation of Au atoms as well as dislocations in stand-off position are found. These observations are in-line with a semi-coherent interface, where Au segregation is triggered by the reduction of the overall strain energy due to: (i) a lower shear modulus on the particle side of the interface, (ii) the shifting of misfit dislocations in stand-off position further away from the stiffer substrate and (iii) a reduction of intrinsic misfit dislocation strain energy on the tensile side. In addition, the mechanical properties of pure and alloyed particles were characterized by in situ compression experiments in the SEM. Typical force-displacement data of defect-free single-crystals were obtained, reaching the theoretical strength of Ni for particles smaller than 400 nm. Alloying changes the mechanical response from an intermittent and discrete plastic flow behavior into a homogeneous deformation regime at large compressive strain.

Dewetting of non-polar thin lubricating films underneath polar liquid drops on slippery surfaces

HypothesisThe stability of thin lubricating fluid-coated slippery surfaces depends on the surface energy of the underlying solid surface. High energy solid surfaces coated with thin lubricating oil lead to the dewetting of the oil films upon depositing aqueous drops on them. Hence such surfaces are very suitable to investigate dewetting of thick films (thickness > 500 nm), which otherwise is not possible using a conventional dewetting system. ExperimentsLubricating films of different thicknesses are coated on hydrophilic solid surfaces, and glycerol drops are deposited on them. Fluorescence imaging of lubricating films and macroscopic wetting behavior of glycerol drops are analyzed to understand the dewetting phenomenon. FindingsUnderneath lubricating films undergo initial thinning and subsequently dewet. The dewetting dynamics during hole nucleation and growth and the final pattern of the dewetted oil droplets depend strongly on the thickness of the lubricating films. Ultrathin films dewet spontaneously via homogeneous nucleation, whereas thicker films dewet via heterogeneous nucleation. During dewetting, the apparent contact angle and radius of glycerol drops follow universal scaling behavior.

Lattice Boltzmann simulations of stochastic thin film dewetting.

We study numerically the effect of thermal fluctuations and of variable fluid-substrate interactions on the spontaneous dewetting of thin liquid films. To this aim, we use a recently developed lattice Boltzmann method for thin liquid film flows, equipped with a properly devised stochastic term. While it is known that thermal fluctuations yield shorter rupture times, we show that this is a general feature of hydrophilic substrates, irrespective of the contact angle θ. The ratio between deterministic and stochastic rupture times, though, decreases with θ. Finally, we discuss the case of fluctuating thin film dewetting on chemically patterned substrates and its dependence on the form of the wettability gradients.

Phase separation and dewetting of polymer dispersed liquid crystal (PDLC) thin films on flat and patterned substrates

The morphology, phase separation, instability and dewetting of Polymer dispersed liquid crystal (PDLC) thin film comprising of nematic liquid crystal, 4-n-pentyl-4ʹ-cyanobiphenyl (5CB) and Polystyrene (PS) is reported in this paper. Thin film of PS-5CB PDLC is spin coated on flat and topographically patterned silica substrates from a common solvent (toluene). Films with two different thickness h ≈ 35 nm and h ≈ 85 nm are used in the study. The composition of PDLC is chosen as RB (PS: 5CB) = 1:2 and 1:1. On flat substrate, the thinner films (h ≈ 35 nm) dewet at room temperature (T ≈ 25 °C) over time, forming isolated droplets for both RB. In contrast, the h ≈ 85 nm thick films are stable at room temperature though they exhibit gradual phase evolution of the 5CB domains with time. These films dewet only when they are subject to thermal annealing at T ≈ 70 °C right after spin coating and results in an isotropic collection of nearly equal sized droplets, each of which comprises of an inner polymer core surrounded by a 5CB ring. Finally, we show that the instability patterns as well as the phase separated LC domains can be aligned by casting the films on topographically patterned substrate. While the 35 nm thin film results in spin dewetted array of aligned droplets along the substrates grooves, the 85 nm thick film remains continuous and undergoes progressive phase evolution, resulting in alternating domain of phase separated LC (within the grooves) and PS domains align along the contours of the substrate patterns.

An energy-stable parametric finite element method for anisotropic surface diffusion

We propose an energy-stable parametric finite element method (ES-PFEM) to discretize the motion of a closed curve under surface diffusion with an anisotropic surface energy γ(θ) – anisotropic surface diffusion – in two dimensions, while θ is the angle between the outward unit normal vector and the vertical axis. By introducing a positive definite surface energy (density) matrix G(θ), we present a new and simple variational formulation for the anisotropic surface diffusion and prove that it satisfies area/mass conservation and energy dissipation. The variational problem is discretized in space by the parametric finite element method and area/mass conservation and energy dissipation are established for the semi-discretization. Then the problem is further discretized in time by a (semi-implicit) backward Euler method so that only a linear system is to be solved at each time step for the full-discretization and thus it is efficient. We establish well-posedness of the full-discretization and identify some simple conditions on γ(θ) such that the full-discretization keeps energy dissipation and thus it is unconditionally energy-stable. Finally the ES-PFEM is applied to simulate solid-state dewetting of thin films with anisotropic surface energies, i.e. the motion of an open curve under anisotropic surface diffusion with proper boundary conditions at the two triple points moving along the horizontal substrate. Numerical results are reported to demonstrate the efficiency and accuracy as well as energy dissipation of the proposed ES-PFEM.

Correlation of physical aging and glass transition temperatures in ultrathin polystyrene films supported on [formula omitted

This study examined the effects of the aging temperature (Tage) on structural relaxation in ultrathin-supported polystyrene (PS) films with thickness ~0.2Rg. X-ray reflectivity and differential scanning calorimetry were used to evaluate the glass transition temperature (Tg) of ultrathin PS and bulk PS, respectively. The ultrathin films in their confined state showed anomalous thickness relaxation behaviors that could be classified into three types: (1) Tage≤60°C, a slow increase in thickness with time and a decrease in relaxation magnitude with temperature; (2) 70°C≤Tage≤110°C, a slow decrease in thickness with time and an increase in relaxation magnitude with temperature; (3) Tage>110°C, an increase in thickness with time due to dewetting of PS films deposited on SiO2. Such an interesting relaxation behavior can arise from the enhanced dynamics at the surface and reduced mobility at the interface inherent in ultrathin-supported films. Two distinct Tgs peculiar to ultrathin films were confirmed. The lower Tg has not been reported for supported PS films. The relaxation summarized above revealed a strong correlation with the Tg values. The de Gennes modified sliding motion model was applied to explain the correlation with relaxation dynamics.

Grafted Nanoparticle Surface Wetting during Phase Separation in Polymer Nanocomposite Films.

Wetting of polymer-grafted nanoparticles (NPs) in a polymer nanocomposite (PNC) film is driven by a difference in surface energy between components as well as bulk thermodynamics, namely, the value of the interaction parameter, χ. The interplay between these contributions is investigated in a PNC containing 25 wt % polymethyl methacrylate (PMMA)-grafted silica NPs (PMMA-NPs) in poly(styrene-ran-acrylonitrile) (SAN) upon annealing above the lower critical solution temperature (LCST, 160 °C). Atomic force microscopy (AFM) studies show that the areal density of particles increases rapidly and then approaches 80% of that expected for random close-packed hard spheres. A slightly greater areal density is observed at 190 °C compared to 170 °C. The PMMA-NPs are also shown to prevent dewetting of PNC films under conditions where the analogous polymer blend is unstable. Transmission electron microscopy (TEM) imaging shows that PMMA-NPs symmetrically wet both interfaces and form columns that span the free surface and substrate interface. Using grazing-incidence Rutherford backscattering spectrometry (GI-RBS), the PMMA-NP surface excess (Z*) initially increases rapidly with time and then approaches a constant value at longer times. Consistent with the areal density, Z* is slightly greater at deeper quench depths, which is attributed to the more unfavorable interactions between the PMMA brush and SAN segments. The Z* values at early times are used to determine the PMMA-NP diffusion coefficients, which are significantly larger than theoretical predictions. These studies provide insights into the interplay between wetting and phase separation in PNCs and can be utilized in nanotechnology applications where surface-dependent properties, such as wettability, durability, and friction, are important.

Fabrication of polycrystalline silicon thin films by gold-induced crystallization of amorphous silicon suboxide

In this work, polycrystalline silicon (poly-Si) thin films were synthesized using Au-induced crystallization of amorphous silicon suboxide for the first time. The structure and elemental composition of the substrate/Au/a-SiO0.4 stacked structure annealed at 500–700°C were investigated by transmission electron microscopy (TEM), Raman, and energy-dispersive X-ray spectroscopy (EDX). TEM and Raman methods confirmed the formation of a poly-Si thin film in the bottom layer (on the substrate) as a result of annealing. At the same time, TEM and EDX studies showed the formation of a thin (barrier) SiO2 layer with a thickness of ~5 nm located between the poly-Si thin film and the upper layer consisting of Au, Si, and O. Based on the results, a mechanism for the growth of poly-Si thin films is proposed which has features (dewetting of Au films and formation of a barrier SiO2 layer) that are not characteristic of the well-known layer exchange process.

Sensitive Biosensing Using Plasmonic Enhancement of Fluorescence by Rapid Thermal Annealed Silver Nanostructures

Although immunofluorescence assays have been used for decades, they are not traditionally capable of quantifying biomarkers at ng/ml levels. Plasmonic enhancement of fluorescence using metallic surface nanostructures has exhibited potential in amplifying the fluorescence signal, which could reduce limits of detection and increase sensitivity. However, current methods for fabricating metallic nanostructures, such as e-beam nanolithography, colloidal lithography, and colloidal self-assembly, require complicated processes and have various drawbacks. In this work, we describe a nanostructure fabrication process we have developed based on the dewetting of thin silver films by rapid thermal annealing, which is suitable for large areas. The nanostructures were then coated with a thin silica film to protect them and also to control the distance between the metallic surface and fluorophores, an important parameter when tuning metal enhancement of fluorescence. Antibody-antigen immunoassays utilizing immunoglobulin G were applied to evaluate the fluorescence enhancement from these nanostructures. A nearly 19-fold increase in the fluorescence intensity was achieved with an optimized structure. Calibration of the resulting plasmonically enhanced immunofluorescence sensor showed that it is nearly 13 times more sensitive than the non-enhanced version and was capable of quantification of a biomarker at ~ 1 ng/ml levels. A model nanostructure was constructed for which finite difference time domain simulations were in good agreement with experimental results, allowing for optimization of process conditions for the further generation of nanostructures.

Retarded solid state dewetting of thin bismuth films with oxide capping layer

We present a study of the impeding influence of a capping, native oxide layer on the solid state dewetting of thin bismuth films on silicon(111) in vacuum. We study the temperature dependence of the film thickness and strain of the thin films through the analysis of crystal truncation rods of clean and capped bismuth films. This analysis reveals a dewetting temperature difference of 40°C between capped and uncapped films. The results are supported by scanning electron microscopy and x-ray photoelectron spectroscopy experiments. Furthermore, a model for the retarding effect of the oxide layer and the final shape of the thin film is presented.

Pulsed laser-induced dewetting and thermal dewetting of Ag thin films for the fabrication of Ag nanoparticles

Two different dewetting methods, namely pulsed laser-induced dewetting (PLiD)—a liquid-state dewetting process and thermal dewetting (TD)—a solid-state dewetting process, have been systematically explored for Ag thin films (1.9–19.8 nm) on Si substrates for the fabrication of Ag nanoparticles (NPs) and the understanding of dewetting mechanisms. The effect of laser fluence and irradiation time in PLiD and temperature and duration in TD were investigated. A comparison of the produced Ag NP size distributions using the two methods of PLiD and TD has shown that both produce Ag NPs of similar size with better size uniformity for thinner films (<6 nm), whereas TD produced bigger Ag NPs for thicker films (≥8–10 nm) as compared to PLiD. As the film thickness increases, the Ag NP size distributions from both PLiD and TD show a deviation from the unimodal distributions, leading to a bimodal distribution. The PLiD process is governed by the mechanism of nucleation and growth of holes due to the formation of many nano-islands from the Volmer−Weber growth of thin films during the sputtering process. The investigation of thickness-dependent NP size in TD leads to the understanding of void initiation due to pore nucleation at the film-substrate interface. Furthermore, the linear dependence of NP size on thickness in TD provides direct evidence of fingering instability, which leads to the branched growth of voids.

Controlling thermal-induced dewetting of As20Se80 thin films for integrated photonics applications

As the use of photonics circuits expands, the optical quality and performance of integrated components in the microscale become a major concern. Aiming to improve the performance while reducing the time processing, new microfabrication approaches are being investigated. The dewetting of glassy thin films have been recently proposed as an alternative for nano and microfabrication of chalcogenide optical components. Besides being the best materials for light transmission in the infrared region, chalcogenide glasses possess a flexible molecular structure that allows using a cheap and simple molding process. Here we investigate the thermal-induced dewetting of chalcogenide As20Se80 thin films, by studying the influence of temperature, atmosphere, and heating rate on the formation of self-assembled microstructures. We found that thin films between 150 and 700 nm dewet via structural relaxation, similarly to liquid agglomeration, and produce solid microstructures with the same composition and molecular structure as the initial film. By controlling the glass viscosity and the kinetics of the nucleation process it was possible to adjust the distribution and size of glassy microstructures. Additionally, we combine the dewetting process with standard photolithography and by avoiding the capillary instabilities, we are capable to obtain waveguides with the smooth and symmetric surfaces required for optical applications in the microscale size.

Toward Exotic Silicon Doping with a Low Thermal Budget and Flexible Profile Control by Liquid-Phase Epitaxy.

We investigate semiconductor p-n junction formation by liquid-phase epitaxy (LPE) using metallic pastes incorporating traditional and nontraditional dopants. The LPE technique enables us to control the shape of doping profiles with a low thermal budget through the choice of solvent, total amount of solvent deposited, and process temperature. We focus here on the Al-B, Zn-P, and Sn-Ga chemistries to dope silicon regions using the chemicophysical properties of a low-eutectic-temperature metallic solvent acting as a matrix for the dissolution of a high concentration of a dopant. Additionally, we developed a capping method enabling doping across a large surface area wafer with a tunable thickness well below 1 μm without film dewetting. In good agreement with thermodynamic simulation of the LPE process, we demonstrate B- and Al-doped regions with a sheet resistance ranging from less than 10 to 300 Ω/sq between 650 and 800 °C, which is significantly lower than the typical temperatures of gas-phase doping processes. Comprehensive electrical simulations suggest that LPE p-n junctions with a low carrier recombination activity can be fabricated via the reduction of surface doping concentration and improved surface recombination velocity. Our investigation of exotic LPE chemistries suggests that emitter saturation currents below 50 fA/cm2 could be achieved at doping concentrations relevant to solar cells.

Hyperbolic Metamaterials via Hierarchical Block Copolymer Nanostructures (Advanced Optical Materials 7/2021)

The dewetting of lamellar phase block copolymer films over topographically defined substrates generates hierarchically aligned hyperbolic metamaterials (HMM) with in-plane optical axis. Strong reduction in the fluorescence lifetime was observed in nanodiamonds coupled to the Au/air HMM. The analyzed system exhibits hyperbolic behavior in a broad wavelength range in the visible spectrum. For further details, see article number 2001933 by Federico Ferrarese Lupi and co-workers.