Superhydrophobic hierarchical arrays fabricated by a scalable colloidal lithography approach.

Here we report an unconventional colloidal lithography approach for fabricating a variety of periodic polymer nanostructures with tunable geometries and hydrophobic properties. Wafer-sized, double-layer, non-close-packed silica colloidal crystal embedded in a polymer matrix is first assembled by a scalable spin-coating technology. The unusual non-close-packed crystal structure combined with a thin polymer film separating the top and the bottom colloidal layers render great versatility in templating periodic nanostructures, including arrays of nanovoids, nanorings, and hierarchical nanovoids. These different geometries result in varied fractions of entrapped air in between the templated nanostructures, which in turn lead to different apparent water contact angles. Superhydrophobic surfaces with >150° water contact angles and <5° contact angle hysteresis are achieved on fluorosilane-modified polymer hierarchical nanovoid arrays with large fractions of entrapped air. The experimental contact angle measurements are complemented with theoretical predictions using the Cassie's model to gain insights into the fundamental microstructure-dewetting property relationships. The experimental and theoretical contact angles follow the same trends as determined by the unique hierarchical structures of the templated periodic arrays.

Outstanding surface plasmon resonance performance enabled by templated oxide gratings.

Here we report a simple and scalable soft-lithography-based templating technology for fabricating Au-covered oxide (titania and zirconia) gratings by using DVDs as a structural template. The resulting plasmonic gratings simultaneously exhibit very high surface plasmon resonance (SPR) sensitivity (up to ∼940 nm per refractive index unit, nm per RIU) and figure of merit (FOM, up to 62.5). The effects of thermal annealing of the templated oxide gratings on their final plasmonic properties have been systematically investigated by both experiments and finite-difference time-domain (FDTD) simulations. Higher SPR sensitivities and slightly reduced FOMs have been observed for the annealed gratings. Additionally, the amplitude of the SPR dips gradually decreases with increasing annealing temperatures. Scanning electron microscopy and X-ray diffraction show that the annealing process enlarges the crystal domain sizes of the oxides and smoothens the final plasmonic grating surface. Systematic FDTD simulations reveal that the SPR properties (e.g., dip amplitude) of Au-covered oxide gratings are significantly affected by the deformation of the track-pitch structure caused by thermal annealing, agreeing with the experimental results. The outstanding SPR performance combined with the high thermal stability of the crystalline oxides could make the templated plasmonic gratings a promising SPR platform for many important sensing applications, such as in situ probing heterogeneous catalytic reactions under realistic conditions.

Sensitive surface plasmon resonance enabled by templated periodic arrays of gold nanodonuts.

Here we report a simple and scalable colloidal lithography technology for fabricating periodic arrays of gold nanodonuts for sensitive surface plasmon resonance (SPR) analysis. This new bottom-up approach leverages a unique polymer wetting layer between a self-assembled, non-close-packed monolayer silica colloidal crystal and a silicon substrate to template ordered gold nanodonuts with tunable geometries over wafer-sized areas. The processes involved in this templating nanofabrication approach, including spin coating, oxygen plasma etching, and metal sputtering, are all compatible with standard microfabrication technologies. Specular reflection measurements reveal that the efficient electromagnetic coupling of the incident light with the tunable SPR modes of the templated gold nanodonut arrays enables good spectral tunability. Bulk refractive index sensing experiments show that a high SPR sensitivity of ∼758 nm per refractive index unit, which outperforms many plasmonic nanostructures fabricated by both top-down and bottom-up approaches, can be achieved using the templated gold nanodonut arrays. Numerical finite-difference time-domain simulations have also been performed to complement the optical characterization and the theoretical results match well with the experimental measurements.

Self-dispersed crumpled graphene balls in oil for friction and wear reduction.

Ultrafine particles are often used as lubricant additives because they are capable of entering tribological contacts to reduce friction and protect surfaces from wear. They tend to be more stable than molecular additives under high thermal and mechanical stresses during rubbing. It is highly desirable for these particles to remain well dispersed in oil without relying on molecular ligands. Borrowing from the analogy that pieces of paper that are crumpled do not readily stick to each other (unlike flat sheets), we expect that ultrafine particles resembling miniaturized crumpled paper balls should self-disperse in oil and could act like nanoscale ball bearings to reduce friction and wear. Here we report the use of crumpled graphene balls as a high-performance additive that can significantly improve the lubrication properties of polyalphaolefin base oil. The tribological performance of crumpled graphene balls is only weakly dependent on their concentration in oil and readily exceeds that of other carbon additives such as graphite, reduced graphene oxide, and carbon black. Notably, polyalphaolefin base oil with only 0.01-0.1 wt % of crumpled graphene balls outperforms a fully formulated commercial lubricant in terms of friction and wear reduction.

High-Yield Spreading of Water-Miscible Solvents on Water for Langmuir-Blodgett Assembly.

Langmuir-Blodgett (LB) assembly is a classical molecular thin-film processing technique, in which the material is spread onto water surface from a volatile, water-immiscible solvent to create floating monolayers that can be later transferred to solid substrates. LB has also been applied to prepare colloidal thin films with an unparalleled level of microstructural control and thickness, which has enabled the discovery of many exciting collective properties of nanoparticles and the construction of bulk nanostructured materials. To maximize the benefits of LB assembly, the nanoparticles should be well dispersed in both the spreading solvent and on water. This is quite challenging since colloids usually need contrasting surface properties in order to be stable in the water-hating organic solvents and on water surface. In addition, many organic and polymeric nanostructures dissolve in those organic solvents and cannot be processed directly. Using water-liking spreading solvents can avoid this dilemma. However, spreading of water-miscible solvents on water surface is fundamentally challenging due to extensive mixing, which results in significant material loss. Here we report a conceptually simple strategy and a general technique that allows nearly exclusive spreading of such solvents on water surface using electrospray. Since the volume of these aerosolized droplets is reduced by many orders of magnitude, they are readily depleted during the initial spreading step before any significant mixing could occur. The new strategy drastically reduces the burden of material processing prior to assembly and broadens the scope of LB assembly to previously hard-to-process materials. It also avoids the use of toxic volatile organic spreading solvents, improves the reproducibility, and can be readily automated, making LB assembly a more robust tool for colloidal assembly and thin-film fabrication.

Self-assembled biomimetic superhydrophobic hierarchical arrays.

Here, we report a simple and inexpensive bottom-up technology for fabricating superhydrophobic coatings with hierarchical micro-/nano-structures, which are inspired by the binary periodic structure found on the superhydrophobic compound eyes of some insects (e.g., mosquitoes and moths). Binary colloidal arrays consisting of exemplary large (4 and 30 µm) and small (300 nm) silica spheres are first assembled by a scalable Langmuir-Blodgett (LB) technology in a layer-by-layer manner. After surface modification with fluorosilanes, the self-assembled hierarchical particle arrays become superhydrophobic with an apparent water contact angle (CA) larger than 150°. The throughput of the resulting superhydrophobic coatings with hierarchical structures can be significantly improved by templating the binary periodic structures of the LB-assembled colloidal arrays into UV-curable fluoropolymers by a soft lithography approach. Superhydrophobic perfluoroether acrylate hierarchical arrays with large CAs and small CA hysteresis can be faithfully replicated onto various substrates. Both experiments and theoretical calculations based on the Cassie's dewetting model demonstrate the importance of the hierarchical structure in achieving the final superhydrophobic surface states.

Large-scale fabrication of nanodimple arrays for surface-enhanced Raman scattering.

Here we report a simple bottom-up technology for scalably fabricating gold nanodimple arrays with tunable nanostructures for surface-enhanced Raman scattering (SERS). Double-layer silica colloidal crystal-polymer nanocomposites with an unusual non-close-packed structure created using a spin-coating technique are utilized as structural templates. A variety of nanodimple structures, including simple monolayer voids, nanoring-like nanodimples, and binary-void nanodimples, can be reproducibly templated over wafer-sized areas by simply controlling the conditions during an oxygen plasma etching process. Normal-incidence specular reflection measurements show that the resulting gold nanodimple arrays exhibit tunable surface plasmon properties. The efficient electromagnetic coupling between neighboring nanodimples and inner-outer walls of nanoring-like nanodimples leads to high SERS enhancement factors (>10(8)). Numerical simulations based on a finite-difference time-domain model complement the experimental measurements, showing the spatial distribution of electromagnetic "hot spots" surrounding the periodic gold nanodimple arrays.

Self-assembled nanoparticle antiglare coatings.

Here we report a simple and scalable bottom-up technology for assembling close-packed nanoparticle monolayers on both sides of a glass substrate as high-quality antiglare coatings. Optical measurements show that monolayer coatings consisting of 110 nm silica nanoparticles can significantly reduce optical reflectance and enhance specular transmittance of the glass substrate for a broad range of visible wavelengths. Both experiments and numerical simulations reveal that the antiglare properties of the self-assembled colloidal monolayers are significantly affected by the size of the colloidal particles.

High surface plasmon resonance sensitivity enabled by optical disks.

We report a systematic, experimental, and theoretical investigation on the surface plasmon resonance (SPR) sensing using optical disks with different track pitches, including Blu-ray disk (BD), digital versatile disk (DVD), and compact disk (CD). Optical reflection measurements indicate that CD and DVD exhibit much higher SPR sensitivity than BD. Both experiments and finite-difference time-domain simulations reveal that the SPR sensitivity is significantly affected by the diffraction order of the SPR peaks and higher diffraction order results in lower sensitivity. Numerical simulations also show that very high sensitivity (∼1600 nm per refractive index unit) is achievable by CDs.