Difu Zhu

Morphology and wettability control of silicon cone arrays using colloidal lithography.

In this paper we present a simple method to fabricate ordered silicon cone arrays with controllable morphologies on a silicon substrate using reactive ion etching with two-dimensional silica colloidal crystals as masks. The etching process and the morphologies of the obtained structure are quantified. Unlike works reported previously, we show that the surface roughness of the obtained silicon cone arrays can be adjusted by controlling the etching duration, which is proved to be of importance in tailoring the behavior of water droplets when being used as antireflection coatings with superhydrophobicity. Moreover, this strategy is compatible with the methods we have established on controlling the arrangement of colloidal spheres, and thus silicon cone arrays with tunable periodicities, different lattice structures, and various patterns can be prepared. The obtained silicon cone arrays with strips can be used as hydrophobic substrates with anisotropic dewetting just like the leaves of rice. It is found that by adjusting the strip width with and without silicon cones, the water droplets can transform from isotropic dewetting to anisotropic dewetting.

Modulating Two-Dimensional Non-Close-Packed Colloidal Crystal Arrays by Deformable Soft Lithography

We report a simple method to fabricate two-dimensional (2D) periodic non-close-packed (ncp) arrays of colloidal microspheres with controllable lattice spacing, lattice structure, and pattern arrangement. This method combines soft lithography technique with controlled deformation of polydimethylsiloxane (PDMS) elastomer to convert 2D hexagonal close-packed (hcp) silica microsphere arrays into ncp ones. Self-assembled 2D hcp microsphere arrays were transferred onto the surface of PDMS stamps using the lift-up technique, and then their lattice spacing and lattice structure could be adjusted by solvent swelling or mechanical stretching of the PDMS stamps. Followed by a modified microcontact printing (microcp) technique, the as-prepared 2D ncp microsphere arrays were transferred onto a flat substrate coated with a thin film of poly(vinyl alcohol) (PVA). After removing the PVA film by calcination, the ncp arrays that fell on the substrate without being disturbed could be lifted up, deformed, and transferred again by another PDMS stamp; therefore, the lattice feature could be changed step by step. Combining isotropic solvent swelling and anisotropic mechanical stretching, it is possible to change hcp colloidal arrays into full dimensional ncp ones in all five 2D Bravais lattices. This deformable soft lithography-based lift-up process can also generate patterned ncp arrays of colloidal crystals, including one-dimensional (1D) microsphere arrays with designed structures. This method affords opportunities and spaces for fabrication of novel and complex structures of 1D and 2D ncp colloidal crystal arrays, and these as-prepared structures can be used as molds for colloidal lithography or prototype models for optical materials.

Controlled Fabrication of Fluorescent Barcode Nanorods

We report a novel technique for generating polymer fluorescent barcode nanorods by reactive ion etching of polymer multilayer films using nonclose-packed (ncp) colloidal microsphere arrays as masks. The fluorescent polymer multilayer films were spin-coated on a substrate, and ncp microsphere arrays were transferred onto these films. The exposed polymers were then etched away selectively, leaving color-encoded nanorods with well-preserved fluorescent properties. By modifying the spin-coating procedure, the amount of polymer in each layer could be tuned freely, which determined the relative fluorescence intensity of the barcode nanorods. These nanorod arrays can be detached from the substrate to form dispersions of coding materials. Moreover, the shape of the nanorods is controllable according to the different etching speeds of various materials, which also endows the nanorods with shape-encoded characters. This method offers opportunities for the fabrication of novel fluorescent barcodes which can be used for detecting and tracking applications.

Mimicking the Rice Leaf—From Ordered Binary Structures to Anisotropic Wettability

In this paper, we report a method to fabricate a series of surfaces with large-area ordered binary arrays by controllable dewetting. The binary structure arrays consist of an ordered-stripe array and droplet-row array. In order to expand the system, polystyrene (PS) and poly(methyl methacrylate) (PMMA) are introduced in this experiment for investigation in detail. Through adjustment of the polymer solution concentration and the modified underlying pattern on substrate, the surface topographies can be controlled simply. Accordingly, three types of topographies with ordered binary arrays have been obtained by thermal annealing. These unique surfaces mimic the natural rice leaf structurally, which also displays anisotropic wettability for water droplet as natural surfaces. This method points out a new way for the manufacture of functional surfaces.

A universal approach to fabricate ordered colloidal crystals arrays based on electrostatic self-assembly.

We present a novel and simple method to fabricate two-dimensional (2D) poly(styrene sulfate) (PSS, negatively charged) colloidal crystals on a positively charged substrate. Our strategy contains two separate steps: one is the three-dimensional (3D) assembly of PSS particles in ethanol, and the other is electrostatic adsorption in water. First, 3D assembly in ethanol phase eliminates electrostatic attractions between colloids and the substrate. As a result, high-quality colloidal crystals are easily generated, for electrostatic attractions are unfavorable for the movement of colloidal particles during convective self-assembly. Subsequently, top layers of colloidal spheres are washed away in the water phase, whereas well-packed PSS colloids that are in contact with the substrate are tightly linked due to electrostatic interactions, resulting in the formation of ordered arrays of 2D colloidal spheres. Cycling these processes leads to the layer-by-layer assembly of 3D colloidal crystals with controllable layers. In addition, this strategy can be extended to the fabrication of patterned 2D colloidal crystals on patterned polyelectrolyte surfaces, not only on planar substrates but also on nonplanar substrates. This straightforward method may open up new possibilities for practical use of colloidal crystals of excellent quality, various patterns, and controllable fashions.

Morphology-controlled two-dimensional elliptical hemisphere arrays fabricated by a colloidal crystal based micromolding method

In this paper, we demonstrate a facile modified micromolding method to fabricate morphology-controlled elliptical hemisphere arrays (EHAs) by using stretched poly(dimethylsiloxane) (PDMS) nanowell arrays as molds. The PDMS nanowell arrays were fabricated via casting PDMS prepolymer onto two-dimensional (2D) non-close-packed (ncp) colloidal sphere arrays. By varying the stretching direction, stretching force, size of the colloidal spheres used and other experimental conditions in the fabrication process, we can control the shape, aspect ratio and size of the resulting microstructures. Moreover, our method does not involve any costly micromanufacture technique and can be applied to a great many materials, such as oil soluble polymers (e.g. polystyrene (PS)), water soluble polymers (e.g. poly(vinyl pyrrolidone) (PVP)), cross-linked polymers (e.g. photopolymerizable resin) and a variety of composites (e.g. polymer/nanoparticle composite). The anisotropic wetting properties of these EHAs were demonstrated. Potential application of the EHAs is to provide a model for the fundamental research of anisotropic surfaces and a template or mask for the fabrication of anisotropic surface patterns for potential applications of shape-dependent optical and magnetic devices.

Fabricating a binary pattern of ordered two-dimensional luminescent (mdppy)BF arrays by dewetting

We report a convenient method for the fabrication of ordered, binary patterned 2D (mdppy)BF arrays on the surface of gold substrates. Ordered arrays of water droplets formed in the hydrophilic regions of patterned self-assembled monolayers (SAMs) which consisted of isolated hydrophobic circles surrounded by a continuous hydrophilic region. Subsequent dewetting of (mdppy)BF solution in chloroform led to the formation of an ordered array of rings in the hydrophilic regions and an ordered array of circles in the hydrophobic regions. The former resulted from a surface-directed and concentration-controlled dewetting process, while the latter was a result of the selective wetting process. Dewetting structures could be controlled by changing the experimental conditions, such as the concentration of the (mdppy)BF solution and the water condensation process.

Fabrication of Heterogeneous Double-Ring-Like Structure Arrays by Combination of Colloidal Lithography and Controllable Dewetting

We report a novel technique for fabricating the heterogeneous double-ring-like structural array by colloidal lithography and two-step dewetting process. First, the 2D non-closed-packed (ncp) silica sphere arrays were obtained by lift-up lithography. Then, the ncp sphere array transferred onto the Rhodamine B (RB)@poly(vinyl alcohol) (PVA) film was used for the mask during reactive ion etching (RIE) process. Sequentially, the substrate with RB@PVA ring-like structure arrays under the silica sphere was dip-coated from poly(N-vinylcarbazole) (PVK) chloroform solution with certain concentration. Due to the presence of ordered 2D sphere arrays, the two-step dewetting behavior happened on top of the sphere and the silicon wafer between adjacent spheres, respectively. After removing the silica sphere arrays by hydrofluoric acid, the RB@PVA/PVK heterogeneous double-ring-like structure array was exhibited on the substrate. We characterized this particular structure by SEM, AFM, and fluorescence spectrum, which prove that both the inner RB@PVA ring and outer PVK ring are independent without any reaction. Accordingly, this method could be extended to other materials owing to its universality. These unique structural arrays have potential application in optoelectronic devices, surface photocatalysis, and surface enhanced Raman scattering (SERS).

A Versatile Approach to Fabricate Ordered Heterogeneous Bull's-Eye-Like Microstructure Arrays

In this paper, ordered heterogeneous bulls-eye-like microstructure arrays were fabricated through a simple two-step method on gold substrate with patterned self-assemble monolayers (SAMs). First, we prepared ordered polymer dot arrays on the SAMs patterned gold substrate by SAMs-direct dewetting. Subsequently, by manipulating concentration-controlled dewetting process, ordered ring arrays were obtained on the dot arrays patterned surface under the protection of water droplets. Namely, ordered bulls-eye-like structure arrays were fabricated successfully. The mechanism of these two kinds of dewetting process has been investigated in detail. And due to these two steps were independent, different materials could be simply introduced to the current system. Therefore, ordered homogeneous and heterogeneous bulls-eye-like structure arrays such as poly(N-vinylcarbazole) (PVK) (dot)/PVK (ring), PVK/5,12-ditetradecylquinolino[2,3-b]acridine-7,14(5H,12H)-dione (DTQA), and PVK/Fe(3)O(4) nanoparticles were obtained. This straightforward method may open up new possibilities for practical use of microchips with binary heterogeneous structure arrays.