Various Silver Nanostructures on Sapphire Using Plasmon Self-Assembly and Dewetting of Thin Films

Silver(Ag) nanostructures demonstrate outstanding optical, electrical, magnetic, and catalytic properties and are utilized in photonic, energy, sensors, and biomedical devices.The target application and the performance can be inherently tuned by control of configuration, shape, and size of Ag nanostructures. In this work, we demonstrate the systematical fabrication of various configurations of Ag nanostructures on sapphire(0001) by controlling the Ag deposition thickness at different annealing environments in a plasma ion coater. In particular, the evolution of Ag particles(between 2 and 20 nm),irregular nanoclusters(between 30 and 60 nm), and nanocluster networks(between 80 and 200 nm) are found be depended on the thickness of Ag thin film. The results were systematically analyzed and explained based on the solid-state dewetting,surface diffusion, Volmer–Weber growth model, coalescence,and surface energy minimization mechanism. The growth behavior of Ag nanostructures is remarkably differentiated at higher annealing temperature(750 ℃) due to the sublimation and temperature-dependent characteristic of dewetting process.In addition, Raman and reflectance spectra analyses reveal that optical properties of Ag nanostructures depend on their morphology.

Influence of Solid-liquid Interaction and Temperature on the Dynamic Dewetting of a Thin Polymer Film:A Molecular Dynamics Study

Coarse-grained molecular dynamics simulations were carried out to investigate the dewetting behavior of a polymer thin film on partial wetting solid surface at the early stage of the dewetting process. Spontaneous dewetting is initiated by removing a band of strip from both the ends of the liquid polymer film which has achieved equilibrium. The solid-liquid interaction and temperature were varied to show their influence on the dewetting dynamics during dewetting as well as the shape evolution of the liquid polymer film. As is consistent with the results obtained in previous researches, the liquid film recedes at a constant speed initially with different solid-liquid couplings and tempe- ratures. Furthermore, smaller coupling parameters or higher temperatures tend to accelerate the recession speed of the liquid film and shorten the constant-speed recession duration. Obvious rims were not always observed. Both coupling parameter and temperature can influence the emergence of the rims.

Effect of an electric field on dewetting transition of nitrogen-water system

We investigate the influence of an external electric field on the dewetting behavior of nitrogen-water systems between two hydrophobic plates using molecular dynamics simulations. It is found that the critical distance of dewetting increases obviously with the electric field strength, indicating that the effective range of hydrophobic attraction is extended. The mechanism behind this interesting phenomenon is related to the rearrangement of hydrogen bond networks between water molecules induced by the external electric field. Changes in the hydrogen bond networks and in the dipole orientation of the water molecules result in the redistribution of the neutral nitrogen molecules, especially in the region close to the hydrophobic plates. Our findings may be helpful for understanding the effects of the electric field on the long-range hydrophobic interactions.

Molecular dynamics of dewetting of ultra-thin water films on solid substrate

Molecular dynamics simulation is applied to study the instability and rupture process of ultra-thin water films on a solid substrate. Results show the small disturbance of the film will develop linearly due to the spinodal instability, whereas the interaction between solid and liquid has less influences on the initial growth. Then the rupture occurs and the rim recedes with a dynamic contact angle. The radius of the rim. varies with time as the square root of t, which is consistent with the macroscopic theory available. Stronger interaction between solid and liquid will postpone rupture time decline the dynamic contact angle and raise the density of water near the interface between solid and liquid.

Influences of oscillatory structural forces on dewetting of nanoparticle-laden ultrathin films

To understand the influences of nanoparticles on dewetting in ultra-thin films, both linear stability the- ory and numerical simulations are performed in the present study, with the consideration of oscillatory structural （OS） forces. Long scale approximation is utilized to simplify the hydrodynamic and diffusion equations to a nonlinear system for film thickness and nanoparticle concentration. Results show that the presence of nanoparticles generally suppresses the dewetting process. Two physical mechanisms responsi- ble for this phenomenon are addressed in the present study. When the oscillatory structural forces are relatively smaller, the essential feature of film evolution is similar to the case of particle-free flow. The reduction of the linear growth rate and the postponement of film rupturing can be attributed to the increment of the viscosity due to the presence of nanoparti- cles. On the other hand, when the intensity of the OS forces becomes stronger, the stepwise thinning of film can be ob- served which prevents the film from rupture. Numerical sim- ulations indicate that this phenomenon is caused by the ex- istence of a stable zone due to the oscillatory nature of the structural forces. Another interesting finding is that the non- uniformity of the distribution of nanoparticle concentration might destabilize a spinodally stable film, and trigger the oc- currence of film dewetting.

AN ENERGY-STABLE PARAMETRICFINITE ELEMENT METHOD FOR SIMULATING SOLID-STATE DEWETTING PROBLEMS INTHREE DIMENSIONS

We propose an accurate and energy-stable parametric finite element method for solving the sharp-interface continuum model of solid-state dewetting in three-dimensional space.The model describes the motion of the film/vapor interface with contact line migration and is governed by the surface diffusion equation with proper boundary conditions at the contact line.We present a weak formulation for the problem,in which the contact angle condition is weakly enforced.By using piecewise linear elements in space and backward Euler method in time,we then discretize the formulation to obtain a parametric finite element approximation,where the interface and its contact line are evolved simultaneously.The resulting numerical method is shown to be well-posed and unconditionally energystable.Furthermore,the numerical method is generalized to the case of anisotropic surface energies in the Riemannian metric form.Numerical results are reported to show the convergence and efficiency of the proposed numerical method as well as the anisotropic effects on the morphological evolution of thin films in solid-state dewetting.

Durable, superhydrophobic, antireflection, and low haze glass surfaces using scalable metal dewetting nanostructuring

In this paper we report a multifunctional nanostructured surface on glass that, for the first time, combines a wide range of optical, wetting and durability properties, including low omnidirectional reflectivity, low haze, high transmission, superhydrophobicity, oleophobicity, and high mechanical resistance. Nanostructures have been fabricated on a glass surface by reactive ion etching through a nanomask, which is formed by dewetting ultrathin metal films （〈 10 nm thickness） subjected to rapid thermal annealing （RTA）. The nanostructures strongly reduce the initial surface reflectivity （-4%）, to less than 0.4% in the 390--800 nm wavelength range while keeping the haze at low values （〈 0.9%）. The corresponding water contact angle （0c） is -24.5~, while that on a flat surface is -43.5~. The hydrophilic wetting nanostructure can be changed into a superhydrophobic and oleophobic surface by applying a fluorosilane coating, which achieves contact angles for water and oil of -156.3~ and -116.2~, respectively. The multicomponent composition of the substrate （Coming~ glass） enables ion exchange through the surface, so that the nanopillars＇ mechanical robustness increases, as is demonstrated by the negligible changes in surface morphology and optical performance after 5,000-run wipe test. The geometry of the nanoparticles forming the nanomask depends on the metal material, initial metal thickness and RTA parameters. In particular we show that by simply changing the initial thickness of continuous Cu films we can tailor the metal nanoparticles＇ surface density and size. The developed surface nanostructuring does not require expensive lithography, thus it can be controlled and implemented on an industrial scale, which is crucial for applications.

Dewetting Behavior of Hydrogen Bonded Polymer Complex Film under Hydrothermal Condition

Hydrogen-bonded polymer complex films with the thickness ranging from 50 nm to 2400 nm were prepared by layer-by-layer （LbL） assembly of poly（2-ethyl-2-oxazoline） （PEOX） and poly（acrylic acid） （PAA）. The dewetting behavior of PEOX/PAA films under hydrothermal condition was investigated. It was found that the dewetting occurred at solid-liquid interface, and the typical morphologies such as holes, irregular cellular structure, and droplets were observed. Atomic force microscopy （AFM） revealed the initial rupture of the film. Microscopic Raman and infrared （IR） imaging demonstrated that the PEOX and PAA chains remained association during the dewetting process.

Fabrication of Polymersomes with Controllable Morphologies through Dewetting w/o/w Double Emulsion Droplets

In this work, monodisperse giant polymersomes are fabricated by dewetting of water-in-oil-in-water double emulsion droplets which are assembled by amphiphilic block copolymer molecules in a microfluidic device. The dewetting process can be tuned by solvation between solvent and amphiphilic block copolymer to get polymersomes with controllable morphology. Good solvent （chloroform and toluene） hinders dewetting process of double emulsion droplets and gets acornlike polymersomes or patched polymersomes. On the other hand, poor solvent （hexane） accelerates the dewetting process and achieves complete separation of inner water phase from oil phase to form complete bilayer polymersomes. In addition, twin polymersomes with bilayer membrane structure are formed by this facile method. The formation mechanism for different polymersomes is discussed in detail.

Dewetting Kinetics of Thin Polymer Films with Different Architectures:Effect of Polymer Adsorption

We have investigated the influence of the adsorption process on the dewetting behavior of the linear polystyrene film （LPS）, the 3-arm star polystyrene film （3SPS） and the ring polystyrene film （RPS） on the silanized Si substrate. Results show that the adsorption process greatly influences the dewetting behavior of the thin polymer films. On the silanized Si substrate, the 3SPS chains exhibit stronger adsorption compared with the LPS chains and RPS chains; as a result, the wetting layer forms more easily. For LPS films, with the decrease of annealing temperature, the kinetics of polymer film changes from exponential behavior to slip dewetting. As a comparison, the stability of 3SPS and RPS films switches from slip dewetting to unusual dewetting kinetic behavior. The adsorbed nanodroplets on the solid substrate play an important role in the dewetting kinetics by reducing the driving force of dewetting and increase the resistant force of dewetting. Additionally, Brownian dynamics （BD） simulation shows that the absolute values of adsorption energy （ε） gradually increase from linear polymer （-0.3896） to ring polymer （-0.4033） and to star polymer （-0.4264）, which is consistent with the results of our adsorption experiments.

Phase Separation, Wetting and Dewetting in PS/PVME Blend Thin Films:Dependence on Film Thickness and Composition Ratio

The effects of film thickness and composition ratio on the morphology evolution of polystyrene （PS）/poly（vinyl methyl ether） （PVME） blend thin films were investigated. Diverse morphology evolutions including droplet-matrix structure, hole emergence, bicontinuous structure formation, percolation-to-droplet transition could be observed under annealing in two-phase region, depending on film thickness and composition ratio. The mechanism for these morphology variations was related to the complex effects of phase separation, dewetting and preferential wetting. The comparison between the thickness of bottom PVME layer and the twice of gyration radius 2Rg（PVME） played a dominant role in morphology control. Only when the PS/PVME film had specific film thickness and compositional symmetry, phase separation and dewetting could happen in sequence.

Single-material Solvent-sensitive Fluorescent Actuator from Carbon Dots Inverse Opals Based on Gradient Dewetting

A novel solvent-sensitive fluorescent actuator with reversibility has been obtained from carbon dots （CDs） inverse opals, which is prepared via infiltrating CDs solution into the interstice of colloidal crystal template, thermal polymerization of CDs materials and removing the colloidal template. The as-prepared CDs inverse opal actuator shows a bending angle of 75° in 10.2 s, bending rate of 7.35 （°）.s-1. In particular, the fluorescence intensity of the films varies during the actuating process. The actuating behavior is attributed to the inhomogeneous swelling/shrinking of the film, which originates from the gradient dewetting by solvent evaporation and hydrogen-bonding interaction between the solvent molecules and oxygen/hydrogen ions of CDs side chain. The CDs inverse opal actuator has the advantages of quick response, good repeatability and strong fluorescence, which gives an important insight into the design and manufacture of novel and advanced solvent-actuators.

Slow Down Dewetting in Polymer Films by Isocyanate-treated Graphite Oxide

Isocyanate-treated graphite oxides （iGOs） were well-dispersed into the polystyrene （PS） thin films and formed a novel network structure. With control in fabrication, an iGOs-web layer was horizontally embedded near the surface of the films and thus formed a composite slightly doped by iGOs. This work demonstrated that the iGOs network can remarkably depress the dewetting process in the polymer matrix of the composite, while dewetting often leads to rupture of polymer films and is considered as a major practical limit in using polymeric materials above their glass transition temperatures （Tg）. Via annealing the 50-120 nm thick composite and associated neat PS films at temperatures ranging from 35℃ to 70 ℃ above Tg, surface morphology evolution of the films was monitored by atomic force microscopy （AFM）. The iGOs-doped PS exhibited excellent thermal stability, i.e., the number of dewetting holes was greatly reduced and the long-term hole growth was fairly restricted. In contrast, the neat PS film showed serious surface fluctuation and a final rupture induced by ordinary dewetting. The method developed in this work may pave a road to reinforce thin polymer films and enhance their thermal stability, in order to meet requirements by technological advances.

Role of Wetting Front in Dewetting of Liquid Solder Drop on Cu Thin Films

The thermodynamic conditions for dewetting of a liquid solder drop on copper thin films were examined under a hot-stage optical microscope in a flowing protective atmosphere.Dewetting of liquid solder was found to depend strongly on the copper film thickness and preceded by spalling of Cu 6 Sn 5 intermetallic compounds.However,the loss of interfacial bonding by spalling was not sufficient to cause immediate dewetting of solder drops if the wetting tip was still strongly bonded to the copper film.By introducing a pinning force on the wetting front,a sufficient condition was found from a force balance analysis for dewetting of the liquid solder drop,in general agreement with the experimental results.

EFFICIENT LINEAR SCHEMES WITH UNCONDITIONAL ENERGY STABILITY FOR THE PHASE FIELD MODEL OF SOLID-STATE DEWETTING PROBLEMS

In this paper,we study linearly first and second order in time,uniquely solvable and unconditionally energy stable numerical schemes to approximate the phase field model of solid-state dewetting problems based on the novel"scalar auxiliary variable"(SAV)approach,a new developed efficient and accurate method for a large class of gradient flows.The schemes are based on the first order Euler method and the second order backward differential formulas(BDF2)for time discretization,and finite element methods for space discretization.The proposed schemes are proved to be unconditionally stable and the discrete equations are uniquely solvable for all time steps.Various numerical experiments are presented to validate the stability and accuracy of the proposed schemes.

Wafer-level site-controlled growth of silicon nanowires by Cu pattern dewetting

An approach for the wafer-level synthesis of size- and site-controlled amorphous silicon nanowires （α-SiNWs） is presented in this paper. Microscale Cu pattern arrays are precisely defined on SiO2 films with the help of photolithography and wet etching. Due to dewetting, Cu atoms shrink to the center of patterns during the annealing process, and react with the SiO2 film to open a diffusion channel for Si atoms to the substrate, α-SiNWs finally grow at the center of Cu patterns, and can be tuned by varying critical factors such as Cu pattern volume, SiO2 thickness, and annealing time. This offers a simple way to synthesize and accurately position a SiNW array on a large area.

A θ-L APPROACH FOR SOLVING SOLID-STATE DEWETTING PROBLEMS

We propose a θ-L approach for solving a sharp-interface model about simulating solid-state dewetting of thin films with isotropic/weakly anisotropic surface energies.The sharp-interface model is governed by surface diffusion and contact line migration.For solving the model,traditional numerical methods usually suffer from the severe stability constraint and/or the mesh distribution trouble.In the θ-L approach,we introduce a useful tangential velocity along the evolving interface and utilize a new set of variables(i.e.,the tangential angle 6 and the total length L of the interface curve),so that it not only could reduce the stiffness resulted from the surface tension,but also could ensure the mesh equidistri-bution property during the evolution.Furthermore,it can achieve second-order accuracy when implemented by a semi-implicit linear finite element method.Numerical results are reported to demonstrate that the proposed θ-L approach is efficient and accurate.

Anisotropic dewetting polydimethylsiloxane surface fabricated using ultrashort laser pulses

Anisotropic dewetting polydimethylsiloxane （PDMS） surfaces, which consist of groove-like micro/ nanostructures （so-called hierarchical structures）, are fabricated using an ultrashort pulsed laser. The contact angles （CAs） are measured parallel to the microgrooves, which are always larger than those measured perpendicular to the microgrooves, exhibiting a superhydrophobic anisotropy of approximately 4°on these fabricated PDMS surfaces at optimized parameters. These pulsed-laser irradiated surfaces exhibit enhanced hydrophobicity with CAs that increase from 116°to 156°while preserving the anisotropic dewetting. Additionally, the wettability of the surfaces with different morphologies is investigated. The temporal evolution of the wettability of the pulsed-laser irradiated PDMS surface is also observed within the first few hours after pulsed laser irradiation.

Split-Ring Structured All-Inorganic Perovskite Photodetector Arrays for Masterly Internet of Things

Photodetectors with long detection distances and fast response are important media in constructing a non-contact human-machine interface for the Masterly Internet of Things(MIT).All-inorganic perovskites have excellent optoelectronic performance with high moisture and oxygen resistance,making them one of the promising candidates for high-performance photodetectors,but a simple,low-cost and reliable fabrication technology is urgently needed.Here,a dual-function laser etching method is developed to complete both the lyophilic split-ring structure and electrode patterning.This novel split-ring structure can capture the perovskite precursor droplet efficiently and achieve the uniform and compact deposition of CsPbBr3 films.Furthermore,our devices based on laterally conducting split-ring structured photodetectors possess outstanding performance,including the maximum responsivity of 1.44×105 mA W^(−1),a response time of 150μs in 1.5 kHz and one-unit area<4×10-2 mm2.Based on these split-ring photodetector arrays,we realized three-dimensional gesture detection with up to 100 mm distance detection and up to 600 mm s^(−1) speed detection,for low-cost,integrative,and non-contact human-machine interfaces.Finally,we applied this MIT to wearable and flexible digital gesture recognition watch panel,safe and comfortable central controller integrated on the car screen,and remote control of the robot,demonstrating the broad potential applications.

RUPTURING OF POLYMER FILMS WITH RUBBING-INDUCED SURFACE DEFECTS

It has been a long-standing question whether dewetting of polymer film from non-wettable substrate surfaceswherein the bicontinuous morphology never forms in the dewetting film is due to spinodal instability or heterogeneousnucleation. In this experiment, we use a simple method to make the distinction through introduction of topographical defectsof the films by rubbing the sample surface with a rayon cloth. Spinodal dewetting is identified for those films that dewet by acharateristic wavevector, q, independent of the density of rubbing-induced defects. Heterogeneous nucleation, on the otherhand, is identified for those with q increasing with increasing density of defects. Our result shows that PS films on oxidecoated silicon with thickness less than ≈ 13 nm are dominated by spinodal dewetting, but the thicker films are dominated bynucleation dewetting. We also confirm that spinodal dewetting does not necessarily lead to a bicontinuous morphology in thedewetting film, contrary to the classic theory of Cahn.