Yushuang Cui

Crack-free controlled wrinkling of a bilayer film with a gradient interface

We report a crack-free controlled wrinkling method based on a bilayer film system. A liquid UV-curable film is solidified on a uniaxially pre-stretched PDMS elastic sheet by UV-exposure. Subsequently, the sheet is released back to its initial non-stretched state, which results in spontaneous formation of grating wrinkles perpendicular to the stretching direction. An interface of gradient interpenetrating polymer networks (IPN) is considered to be formed between the stiff UV-cured film and the elastic support, which is practically beneficial for preventing crack formation and film delamination during the strain relaxation process. The periodicity of the gratings is tuned by controlling the thickness of the UV-cured polymer film and the amplitude of pre-strain of the elastic sheet. The imprinting results demonstrate that these self-formed wrinkles can serve as a mold to duplicate gratings by nanoimprint lithography. Furthermore, metal gratings are successfully fabricated from the wrinkling molds.

A New Strategy of Lithography Based on Phase Separation of Polymer Blends

Herein, we propose a new strategy of maskless lithographic approach to fabricate micro/nano-porous structures by phase separation of polystyrene (PS)/Polyethylene glycol (PEG) immiscible polymer blend. Its simple process only involves a spin coating of polymer blend followed by a development with deionized water rinse to remove PEG moiety, which provides an extremely facile, low-cost, easily accessible nanofabrication method to obtain the porous structures with wafer-scale. By controlling the weight ratio of PS/PEG polymer blend, its concentration and the spin-coating speed, the structural parameters of the porous nanostructure could be effectively tuned. These micro/nano porous structures could be converted into versatile functional nanostructures in combination with follow-up conventional chemical and physical nanofabrication techniques. As demonstrations of perceived potential applications using our developed phase separation lithography, we fabricate wafer-scale pure dielectric (silicon)-based two-dimensional nanostructures with high broadband absorption on silicon wafers due to their great light trapping ability, which could be expected for promising applications in the fields of photovoltaic devices and thermal emitters with very good performances, and Ag nanodot arrays which possess a surface enhanced Raman scattering (SERS) enhancement factor up to 1.64 × 108 with high uniformity across over an entire wafer.

P(VDF-TrFE-CFE) terpolymer thin-film for high performance nonvolatile memory

Vinylidene fluoride-trifluoroethylene-chlorofluoroethylene terpolymer, P(VDF-TrFE-CFE), with small amount of CFE is utilized for thin-film nonvolatile memory. Polarization switching voltage for a 50 nm-thick film can be as low as 1 V, and is well suited for integrated driving electronics. The writing-erasing procedure is completely reversible. High signal-to-noise and high capability for data storage are observed in this memory system. Polarization state of the terpolymer is rather stable, making it applicable for memory devices. Polarization switching behavior in the terpolymer can be ascribed to reduced polar domain size with respect to the P(VDF-TrFE) copolymer, and energy cost of domain wall motion during electrically polarization switching decreases.

Phase Separation of Silicon-Containing Polymer/Polystyrene Blends in Spin-Coated Films

In this Article, two readily available polymers that contain silicon and have different surface tensions, polydimethylsiloxane (PDMS) and polyphenylsilsequioxane (PPSQ), were used to produce polymer blends with polystyrene (PS). Spin-coated thin films of the polymer blends were treated by O2 reactive-ion etching (RIE). The PS constituent was selectively removed by O2 RIE, whereas the silicon-containing phase remained because of the high etching resistance of silicon. This selective removal of PS substantially enhanced the contrast of the phase separation morphologies for better scanning electron microscope (SEM) and atomic force microscope (AFM) measurements. We investigated the effects of the silicon-containing constituents, polymer blend composition, concentration of the polymer blend solution, surface tension of the substrate, and the spin-coating speed on the ultimate morphologies of phase separation. The average domain size, ranging from 100 nm to 10 μm, was tuned through an interplay of these factors. In addition, the polymer blend film was formed on a pure organic layer, through which the aspect ratio of the phase separation morphologies was further amplified by a selective etching process. The formed nanostructures are compatible with existing nanofabrication techniques for pattern transfer onto substrates.

A new approach to rubber reinforcement

In this paper, we report the highly efficient reinforcement of dynamically cross-linked silane-modified phenol formaldehyde resin (SMPF) in ethylene-propylene diene terpolymer rubber (EPDM). Only 5 phr of SMPF is able to greatly improve the tensile strength of EPDM from 5 MPa to 28 MPa, which is 55% higher than that of EPDM filled with 30 phr of carbon black N330, and it still retains a high elongation of more than 600%. The scanning electron microscopy and X-ray diffraction results reveal the reinforcement mechanism of EPDM. SMPF is uniformly dispersed in the EPDM matrix by a conventional Banbury mixer and is dynamically cross-linked to form a greatly expanded hard phase comprising the interpenetrating polymer networks of EPDM and SMPF during the mixing process, which leads to a high modulus. The high tensile strength is attributed to the strain-induced crystallization of EPDM at high strain according to the X-ray diffraction patterns of the stretched EPDM–SMPF composites. These results reveal the high efficient reinforcement of the thermoset resin in rubber, demonstrate a new mechanism of rubber reinforcement and suggest a new direction for rubber reinforcement.

High resolution soft mold for UV-curing nanoimprint lithography using an oxygen insensitive degradable material

Soft nanoimprint lithography has been developed to overcome the disadvantages of conventional nanoimprint lithography based on rigid molds. Hybrid nanoimprint-soft lithography mold is an efficient strategy to improve the resolution of soft nanoimprint because a rigid UV-curable material is used as the structural layer. In this paper, the authors design a novel UV-curable material for hybrid soft mold fabrication, which is degradable under mild acidic conditions and insensitive to oxygen during photopolymerization. The material comprises an acid-degradable cross-linker, 2,10-diacryloyloxymethyl-1,4,9,12-tetraoxaspiro[4.2.4.2] tetradecane, and an acyrlated polysiloxane, poly[(mercaptopropyl)methylsiloxane]. Oxygen sensitivity of acrylate groups during UV curing is avoided due to the cross-linking mechanism based on thiol-ene chemistry. The cured material can be decomposed into linear chains through the cleavage of acid-labile ketal links and dissolved in organic solvent when heated in an acidic solution. Th...

Fabrication of metallic patterns on highly curved substrates via nanoimprint lithography in association with an etch-in process

In this paper, we demonstrate the fabrication of metallic patterns on both planar and highly curved substrates via nanoimprint lithography in association with an etch-in process. First, the resist pattern is fabricated on the surface of a metal coated substrate via UV-curing nanoimprint lithography using a hybrid soft nanoimprint mold and the double transfer method. After the resist pattern was etched through by reactive ion etching (RIE), the sample is immersed in an etchant to fabricate metallic patterns using the resist pattern as the etching mask. An ammonium cerium nitrate/acetic acid system is used as the etchant to fabricate Cr patterns and the Fe3+/thiourea etching system is used to fabricate Au patterns. High quality 70 nm deep Cr gratings and 80 nm deep Au gratings with 550 nm pitch and 275 nm linewidth are obtained. Additionally, the linewidth of Cr patterns can be tuned from 275 nm to 95 nm by changing the etching time. Furthermore, a 550 nm pitch Cr grating is patterned on the cylindrical surface of a 125 μm-diameter optical fiber and a surface relief fiber Bragg grating with a groove depth of 330 nm is fabricated via RIE using the Cr mask. This method is universally used for fabricating metallic patterns on different substrates and it greatly widens the process window compared to the lift-off process.

Wafer-scale fabrication of metal nanoring and nanocrescent arrays from nanoimprinted nanopillar arrays

Abstract. An approach was presented to fabricate wafer-scale nanoring structures based on nanopillar array templates fabricated by nanoimprint lithography. This fabrication method combined UV-curing nanolithography technology, metal deposition, and an etching process, which made it possible to tune the geometric properties of nanorings: height, diameter, and linewidth for various materials, such as Au and Ni. Nanoring arrays showed potential applications in many fields, including memory storage, biosensing, and optical devices. The optical measurement of Au nanorings (d=380  nm) showed its strong transmission resonances at the wavelength of 2.1  μm. A modified version of this fabrication method by depositing Ni in a controlled angle as a sacrificial layer was also utilized to create nanocrescent arrays. This modified method could easily tune the width of crescents through the nickel deposition angles and nanopillar template heights. The large-area gold nanocrescent arrays showed strong polarization-dependent transmission bands. Plasmonic crescent structures were expected to apply in infrared metamaterial and chemical sensing.

A high-performance and low cost SERS substrate of plasmonic nanopillars on plastic film fabricated by nanoimprint lithography with AAO template

As a powerful spectroscopy technique, surface-enhanced Raman scattering (SERS) can provide non-destructive and sensitive characterization down to a single molecular level. Aiming to the main challenges of high-performance SERS-active substrates for their real-world applications involving the ultra-sensitive and reproducible signals detection and signal uniformity with large-area, herein, a facile and reliable strategy based on combination of thermal imprinting polycarbonate (PC) film with porous anodic aluminum oxide (AAO) mold and E-beam evaporation of gold is provided to fabricate a high-quality SERS-active substrate consisting of ultra-dense hot-spots with large-area uniformity. Two kinds of sub-10 nm gaps were obtained, including the nanogaps between the neighboring gold coated PC-nanopillars and those between gold on the top of the nanopillars and that on the base, which actually build up a three-dimensional (3D) hot-spot network for high-performance SERS detection. The effect of structural parameter...

Wafer Scale Fabrication of Dense and High Aspect Ratio Sub-50 nm Nanopillars from Phase Separation of Cross-Linkable Polysiloxane/Polystyrene Blend

We demonstrated a simple and effective approach to fabricate dense and high aspect ratio sub-50 nm pillars based on phase separation of a polymer blend composed of a cross-linkable polysiloxane and polystyrene (PS). In order to obtain the phase-separated domains with nanoscale size, a liquid prepolymer of cross-linkable polysiloxane was employed as one moiety for increasing the miscibility of the polymer blend. After phase separation via spin-coating, the dispersed domains of liquid polysiloxane with sub-50 nm size could be solidified by UV exposure. The solidified polysiloxane domains took the role of etching mask for formation of high aspect ratio nanopillars by O2 reactive ion etching (RIE). The aspect ratio of the nanopillars could be further amplified by introduction of a polymer transfer layer underneath the polymer blend film. The effects of spin speeds, the weight ratio of the polysiloxane/PS blend, and the concentration of polysiloxane/PS blend in toluene on the characters of the nanopillars were investigated. The gold-coated nanopillar arrays exhibited a high Raman scattering enhancement factor in the range of 108-109 with high uniformity across over the wafer scale sample. A superhydrophobic surface could be realized by coating a self-assembled monolayers (SAM) of fluoroalkyltrichlorosilane on the nanopillar arrays. Sub-50 nm silicon nanowires (SiNWs) with high aspect ratio of about 1000 were achieved by using the nanopillars as etching mask through a metal-assisted chemical etching process. They showed an ultralow reflectance of approximately 0.1% for wavelengths ranging from 200 to 800 nm.