Nanoscale Wrinkles Research Articles

폴리우레탄 아크릴레이트 수지를 이용한 부분 경화 표면에서의 나노크기 주름

폴리우레탄 아크릴레이트 수지를 이용한 부분 경화 표면에서의 나노크기 주름

Controllable Wrinkling Patterns on Chitosan Microspheres Generated from Self-Assembling Metal Nanoparticles.

Materials with surface wrinkles at a micro/nanoscale possess extraordinary fascinating properties, and various techniques have been employed to create controllable wrinkles. Herein, natural polysaccharide was used to construct the surface wrinkled microsphere with controllable wrinkling patterns. A robust microsphere with an average size of about 55 μm fabricated from chitosan in alkali/urea aqueous solution was swelled and then coated orderly by introducing rigid silver nanoparticles (Ag NPs) with an average size of about 5 nm as the shell onto the surface through electrostatic layer-by-layer (LBL) self-assembly followed by deswelling, resulting in a surface wrinkled microsphere. The significant difference in the swelling behaviors between the stiff Ag shell and swelled chitosan microsphere could generate enough driving forces to form 3D micro- and nanoscale wrinkling surface topography. The surface wrinkled microspheres exhibited the hierarchically porous structure and hydrophobicity, and the topographical patterns could be adjusted by controlling the thickness of the Ag NP layer to achieve the sizes of wrinkling ranging from 60 to 300 nm. It was demonstrated that the wrinkled microspheres were superior as 3D surface-enhanced Raman spectroscopy (SERS) substrates, in which the wrinkled structure with spatial periodicity was proved to be effective for enhancing the SERS signal. The microsphere with controllable wrinkled surface topography could be applied to be a miniature 3D device, which promises potential technological applications in various areas.

Probing nanoscale defects and wrinkles in MoS2 by tip-enhanced Raman spectroscopic imaging

Nanoscale inhomogeneities, such as defects and wrinkles, in atomic layers of transition-metal dichalcogenide (TMDC) semiconductor materials deteriorate the remarkable physical and optoelectronic properties of these materials, precluding their use in optoelectronic devices. Such inhomogeneities can be investigated using vibrational spectroscopic analysis methods, such as Raman spectroscopy, because the deformations in a sample could be identified by the changes in the Raman vibrational energies of the sample. However, it has been challenging to characterize structures localized on the nanoscale in TMDC layers, because such characterization requires a nanoscale high spatial resolution. Here, we present tip-enhanced Raman spectroscopy (TERS) of molybdenum disulfide (MoS2) in the form of monolayers and a few layers to investigate nanoscale inhomogeneities. TERS enables Raman analysis with the nanoscale spatial resolution, using the near-field light generated at a metallic nano-tip through plasmon resonance. TERS imaging, which demonstrates spatially varying Raman spectral features on the nanoscale, allows one to elucidate the existence of nanoscale inhomogeneities. Using this method, defects and wrinkles within MoS2 layers were characterized with a spatial resolution better than 20 nm. This study provides important insights into unique optical and electronic properties of TMDCs for the development of future optoelectronic devices.

Durable Superhydrophobic/Superoleophilic Graphene-Based Foam for High-Efficiency Oil Spill Cleanups and Recovery.

One of the most pervasive environmental issues is the frequent oil spillsthat happen during oil production and transportation processes. This global challenge calls for emerging materials that can effectively separate oil pollutants from water. Here, a facile and feasible method to fabricate a high-performance oil absorbent (ODA-rGO@MF) through the coating of octadecylamine-grafted reduced graphene oxide (ODA-rGO) on the skeleton of commercial melamine foam (MF) is reported. The resultant foam not only possesses the hierarchical pore structure, superior compressibility, and mechanical stability of the original foam but also exhibits high water repellency, superhydrophobicity, and durability under harsh conditions, such as in strongly acidic (alkaline) and salty solutions. The superhydrophobicity of ODA-rGO@MF was created by employing the microarchitecture of MF, forming the rough rGO nanoscale wrinkles on the foam skeletons and further reducing the surface energy by octadecylamine (ODA). These fascinating characteristics make the functionalized foam an excellent oil absorbent with a high absorption capacity (44-111 times its own weight) and extraordinary recyclability. More importantly, the inexpensive materials and low-cost fabrication technique make it possible for large-scale production. Thus, the as-prepared foam has the potential to be used as a promising absorbent for oil-spill cleanups.

Two-stage wrinkling of Al films deposited on polymer substrates

This study reveals two-stage wrinkling behavior of Al films deposited on polydimethylsiloxane (PDMS) substrates by magnetron sputtering at a high temperature. The hierarchical structures with nano-micro dual scale wrinkles were created in Al films. The micro-scale wrinkles originated from thermal contraction, with the wavelength increasing with increasing film thickness, while nano-scale wrinkles, with an almost unchanged wavelength, are induced by the expansion of the mixed layer on the PDMS surface. The results provide a facile method to produce the hierarchical wrinkled surfaces for functionality.

Hierarchical structures on nickel-titanium fabricated by ultrasonic nanocrystal surface modification

Hierarchical structures on metallic implants can enhance the interaction between cells and implants and thus increase their biocompatibility. However, it is difficult to directly fabricate hierarchical structures on metallic implants. In this study, we used a simple one-step method, ultrasonic nanocrystal surface modification (UNSM), to fabricate hierarchical surface structures on a nickel-titanium (NiTi) alloy. During UNSM, a tungsten carbide ball hits metal surfaces at ultrasonic frequency. The overlapping of the ultrasonic strikes generates hierarchical structures with microscale grooves and embedded nanoscale wrinkles. Cell culture experiments showed that cells adhere better and grow more prolifically on the UNSM-treated samples. Compared with the untreated samples, the UNSM-treated samples have higher corrosion resistance. In addition, the surface hardness increased from 243 Hv to 296 Hv and the scratch hardness increased by 22%. Overall, the improved biocompatibility, higher corrosion resistance, and enhanced mechanical properties demonstrate that UNSM is a simple and effective method to process metallic implant materials.

Universal deformation pathways and flexural hardening of nanoscale 2D-material standing folds

In the present work, we use atomic force microscopy nanomanipulation of 2D-material standing folds to investigate their mechanical deformation. Using graphene, h-BN and talc nanoscale wrinkles as testbeds, universal force–strain pathways are clearly uncovered and well-accounted for by an analytical model. Such universality further enables the investigation of each fold bending stiffness κ as a function of its characteristic height h0. We observe a more than tenfold increase of κ as h0 increases in the 10–100 nm range, with power-law behaviors of κ versus h0 with exponents larger than unity for the three materials. This implies anomalous scaling of the mechanical responses of nano-objects made from these materials.

Wrinkling and Periodic Folding of Graphene Oxide Monolayers by Langmuir-Blodgett Compression.

Crumples, wrinkles, and other three-dimensional topographical features in graphene oxide (GO) have been of recent interest as these features have improved material performance for a variety of applications. However, wrinkling of monolayer GO films has yet to be reported. Herein, we demonstrate wrinkling and folding of monolayer GO using the Langmuir-Blodgett technique for the first time. First, cetyltrimethylammonium bromide (CTAB) and GO are deposited on the air-water interface and uniaxially compressed to form a monolayer. CTAB enhances in-plane rigidity of the monolayer through hydrophobic tail aggregation, preventing GO-GO in-plane sliding behavior. Overcompression of the GO monolayer results in the out-of-plane periodic nanoscale wrinkling and in turn generates folds that are stable during deposition onto a substrate and GO chemical reduction. Furthermore, we investigate one potential application of this material by constructing a 3D electrode of the stacked nanofolded GO-CTAB layers that exhibits superior volumetric capacitance compared to commercial devices and comparable volumetric capacitance compared to high-performing recently reported devices. The high volumetric capacitance is attributed to the electrolyte-accessible channels generated by the nanofolds which are similar in size to the hydrated ions.

Controlling nested wrinkle morphology through the boundary effect on narrow-band thin films

We describe the formation of nested wrinkles created by the thermal mismatch between a narrow-band thin film and a compliant substrate. When a film is described as “narrow-band”, it literally means that the film band width is much shorter than its length; more precisely, it means that the width is comparable with the wavelength of the wrinkles. A silicon mask was used during film sputtering to create narrow-band films on poly (dimethylsiloxane) substrate, thus creating regular boundaries to steer local stresses and control wrinkle morphology. Disordered nano-scale wrinkles were found nested within highly ordered micro-scale sinusoidal wrinkles. The formation of nested wrinkles was explained through the amplitude and wavelength saturation of nano-scale wrinkles. The disordered morphology of nano-scale wrinkles and the highly ordered morphology of micro-scale wrinkles were explained by using the boundary effect.

Temperature-induced strain release via rugae on the nanometer and micrometer scale in graphene monolayer

Wrinkles are one possible rugae phase that can be observed in two-dimensional crystals. Those topographic features, with dimensions ranging from nanometers to micrometers, can be identified in single-layer graphene transferred onto various substrates. When the temperature of the sample is varied, the different thermal expansion coefficients of graphene and its supporting substrate lead to reconstruction of the graphene's topography, which results in spatially inhomogeneous strain and doping. Using Raman spectral mapping, we investigated in situ the temperature dependence (50–300 K) of topographic features in a graphene monolayer grown by chemical vapor deposition (CVD) and transferred onto a Si/SiO2 substrate. We find that the temperature-induced strain follows the temperature variation of the cubic lattice parameter of the Si substrate. Furthermore, the temperature-induced strain has an unambiguous relation to the topographic reconstruction of the graphene monolayer. We focused on the behavior of two types of rugae: 1) large wrinkles with dimensions that are larger than the laser spot size and can be traced directly and 2) nanoscale wrinkles and ripples, which can be identified by changes in the parameters of the principal Raman active bands of the graphene monolayer. In addition, we observed that temperature-induced reconstructions are accompanied by local variation of doping.

Controlled fabrication of nanoscale wrinkle structure by fluorocarbon plasma for highly transparent triboelectric nanogenerator

In this paper, we report a novel nanoscale wrinkle-structure fabrication process using fluorocarbon plasma on poly(dimethylsiloxane) (PDMS) and Solaris membranes. Wrinkles with wavelengths of hundreds of nanometers were obtained on these two materials, showing that the fabrication process was universally applicable. By varying the plasma-treating time, the wavelength of the wrinkle structure could be controlled. Highly transparent membranes with wrinkle patterns were obtained when the plasma-treating time was <125 s. The transmittances of these membranes were >90% in the visible region, making it difficult to distinguish them from a flat membrane. The deposited fluorocarbon polymer also dramatically reduced the surface energy, which allowed us to replicate the wrinkle pattern with high precision onto other membranes without any surfactant coating. The combined advantages of high electron affinity and high transparency enabled the fabricated membrane to improve the performance of a triboelectric nanogenerator. This nanoscale, single-step, and universal wrinkle-pattern fabrication process, with the functionality of high transparency and ultra-low surface energy, shows an attractive potential for future applications in micro- and nanodevices, especially in transparent energy harvesters.

Controllable Fabrication of Large-Area Wrinkled Graphene on a Solution Surface.

It is unavoidable to form wrinkles, which are folds or creases in a material, in graphene, whenever the graphene is prepared by micromechanical exfoliation from graphite or chemical vapor deposition (CVD). However, the controllable formation and structures of graphene with nanoscale wrinkles remains a big challenge. Here, we report a liquid-phase shrink method to controllably fabricate large-area wrinkled graphene (WG). The CVD-prepared graphene self-shrinks into a WG on an ethanol solution surface. By modifying the concentration of the ethanol solution, we can easily and efficiently obtain WG with a uniform distribution of wrinkles with different heights. The WG shows high stretchability and can withstand more than 100% tensile strain and up to 720° twist. Furthermore, electromechanical response sensors based on double-layer stacking of WG show ultrahigh sensitivity. This simple, effective, and environmentally friendly liquid-phase shrink method will pave a way for the controllable formation of WG, which is an ideal candidate for application in highly stretchable and highly sensitive electronic devices.

An evaluation method for nanoscale wrinkle

In this paper, a spectrum-based wrinkling analysis method via two-dimensional Fourier transformation is proposed aiming to solve the difficulty of nanoscale wrinkle evaluation. It evaluates the wrinkle characteristics including wrinkling wavelength and direction simply using a single wrinkling image. Based on this method, the evaluation results of nanoscale wrinkle characteristics show agreement with the open experimental results within an error of 6%. It is also verified to be appropriate for the macro wrinkle evaluation without scale limitations. The spectrum-based wrinkling analysis is an effective method for nanoscale evaluation, which contributes to reveal the mechanism of nanoscale wrinkling.

Design of a Compact Biaxial Tensile Stage for Fabrication and Tuning of Complex Micro- and Nano-scale Wrinkle Patterns

Wrinkling of thin films is a strain-driven process that enables scalable and low-cost fabrication of periodic micro- and nano-scale patterns. In the past, single-period sinusoidal wrinkles have been applied for thin-film metrology and microfluidics applications. However, real-world adoption of this process beyond these specific applications is limited by the inability to predictively fabricate a variety of complex functional patterns. This is primarily due to the inability of current tools and techniques to provide the means for applying large, accurate, and nonequal biaxial strains. For example, the existing biaxial tensile stages are inappropriate because they are too large to fit within the vacuum chambers that are required for thin-film deposition/growth during wrinkling. Herein, we have designed a compact biaxial tensile stage that enables (i) applying large and accurate strains to elastomeric films and (ii) in situ visualization of wrinkle formation. This stage enables one to stretch a 37.5 mm long film by 33.5% with a strain resolution of 0.027% and maintains a registration accuracy of 7 μm over repeated registrations of the stage to a custom-assembled vision system. Herein, we also demonstrate the utility of the stage in (i) studying the wrinkling process and (ii) fabricating complex wrinkled patterns that are inaccessible via other techniques. Specifically, we demonstrate that (i) spatial nonuniformity in the patterns is limited to 6.5%, (ii) one-dimensional (1D) single-period wrinkles of nominal period 2.3 μm transition into the period-doubled mode when the compressive strain due to prestretch release of plasma-oxidized polydimethylsiloxane (PDMS) film exceeds ∼18%, and (iii) asymmetric two-dimensional (2D) wrinkles can be fabricated by tuning the strain state and/or the actuation path, i.e., the strain history. Thus, this tensile stage opens up the design space for fabricating and tuning complex wrinkled patterns and enables extracting empirical process knowledge via in situ visualization of wrinkle formation.

Nano-scale surface wrinkling in chiral liquid crystals and plant-based plywoods.

We present theoretical scaling and computational analysis of nanostructured free surfaces formed in chiral liquid crystals (LC) and plant-based twisted plywoods. A nemato-capillary model is used to derive a generalized equation that governs the shape of cholesteric free surfaces. It is shown that the shape equation includes three distinct contributions to the capillary pressure: area dilation, area rotation, and director curvature. To analyse the origin of periodic reliefs in plywood surfaces, these three pressure contributions and corresponding surface energies are systematically investigated. It is found that for weak homeotropic surface anchoring, the nano-wrinkling is driven by the director curvature pressure mechanism. Consequently, the model predicts that for a planar surface with a uniform tangential helix vector, no surface nano-scale wrinkling can be observed because the director curvature pressure is zero. Scaling is used to derive the explicit relation between the wrinkling's amplitude to the wavelength ratio as a function of the anisotropic surface tension, which is then validated with experimental values. These new findings can be used to characterize plant-based twisted plywoods, as well as to inspire the design of biomimetic chiro-optical devices.

Mechanics of nanoscale wrinkling of graphene on a non-developable surface

As a two-dimensional crystal, the graphene sheet is often used with substrate materials because the freestanding graphene tends to corrugate and can hardly display its extraordinary properties in devices. However, the substrate is rarely perfectly-flat, but has microscopic roughness. Whether or not the graphene sheet can conform fully to a substrate with nano- and micro-roughness is essential for the performance of the graphene-based devices. In this paper, a theoretical model is developed to predict the morphology of a monolayer graphene sheet attaching to the substrate with microscopic non-developable roughness by an energy-based analysis. The final graphene morphology is revealed to result from the competition between two energy terms: the adhesion energy between graphene and substrate, and the strain energy stored in the graphene due to the deformations. Thus, by accounting for these two parts of energy, the critical condition to predict the morphology conversion from full conformation to wrinkling is established, which agrees well with the results of molecular dynamics simulations. This study has significant meanings for design and fabrication of high quality nanostructured coatings on substrates with complex surfaces, and can offer a guide for designing new functional graphene electronical devices such as nano-sensors and nano-switches as well.

Hyperthermal hydrogen induced cross-linking and fabrication of nano-wrinkle patterns in ultrathin polymer films

This paper presents an efficient and simple processing method for the fabrication and regulation of nano-scale wrinkle patterns. The stiff film is created through cross-linking the surface of ultrathin polymer film within twenty nanometers by using the hyperthermal hydrogen induced cross-linking (HHIC) method, and then the bilayer is heated above the glass transition temperature of the polymer before quenched to room temperature. In this way, wrinkle patterns with wavelength ranging from 150 to 300nm are prepared. And a morphology transition between the pattern of single points (convex bumps) and the pattern of ridges (labyrinth) is observed. The formation of wrinkle patterns is systematically investigated as functions of the thickness of original ultrathin film, the time of HHIC treatment, the annealing temperature and the chemical structure of polymers. Furthermore, nano-patterned silicon wafer is used as a template to create multiscale, anisotropic wrinkle patterns.

Random lasing action in a polydimethylsiloxane wrinkle induced disordered structure

This paper presents a chip-scale random lasing action utilizing polydimethylsiloxane (PDMS) wrinkles with random periods as disordered medium. Nanoscale wrinkles with long range disorder structures are formed on the oxidized surface of a PDMS slab and confirmed by atomic force microscopy. Light multiply scattered at each PDMS wrinkle-dye interfaces is optically amplified in the presence of pump gain. The shift of laser emission wavelength when pumping at different regions indicates the randomness of the winkle period. In addition, a relatively low threshold of about 27 μJ/mm2 is realized, which is comparable with traditional optofluidic dye laser. This is due to the unique sinusoidal Bragg-grating-like random structure. Contrast to conventional microfluidic dye laser that inevitably requires the accurate design and implementation of microcavity to provide optical feedback, the convenience in both fabrication and operation makes PDMS wrinkle based random laser a promising underlying element in lab-on-a-chip systems and integrated microfluidic networks.

Polymer Nanowrinkles with Continuously Tunable Wavelengths

This paper describes a parallel method to generate polymer nanowrinkles over large areas with wavelengths that were continuously tuned down to 30 nm. Reactive ion etching using fluorinated gases was used to chemically treat thermoplastic polystyrene films, which resulted in a stiff skin layer. Upon heating, the treated thermoplastic, microscale, and nanoscale wrinkles were formed. We used variable-angle spectroscopic ellipsometry to characterize the thickness of the skin layer; this thickness could then be used to predict and control the nanowrinkle wavelength. Because the properties of these nanotextured polymer surfaces can be tuned over a large range of wrinkle wavelengths, they are promising for a broad range of applications, especially those that require large-area and uniform surface patterning.

Synthesis of Monolayer Graphene Having a Negligible Amount of Wrinkles by Stress Relaxation

For the chemical vapor deposition (CVD) of graphene, the grain growth of the catalyst metal and thereby surface roughening are unavoidable during the high temperature annealing for the graphene synthesis. Considering that nanoscale wrinkles and poor uniformity of synthesized graphene originate from the roughened metal surface, improving surface flatness of metal thin films is one of the key factors to synthesize high quality graphene. Here, we introduce a new method for graphene synthesis for fewer wrinkle formation on a catalyst metal. The method utilizes a reduced graphene oxide (rGO) interfacial layer between the metal film and the wafer substrate. The rGO interlayer releases the residual stress of the metal thin film and thereby suppresses stress-induced metal grain growth. This technique makes it possible to use much thinner nickel films, leading to a dramatic suppression of RMS roughness (~3 nm) of the metal surface even after high temperature annealing. It also endows excellent control of the graphene thickness due to the reduced amount of total carbon in the thin nickel film. The synthesized graphene layer having negligible amount of wrinkles exhibits excellent thickness uniformity (91% coverage of monolayer) and very high carrier mobility of ~15,000 cm(2)/V·s.