Showing papers by "Younan Xia published in 2001"

Template-assisted self-assembly: a practical route to complex aggregates of monodispersed colloids with well-defined sizes, shapes, and structures.

Abstract: This paper describes a strategy that combines physical templating and capillary forces to assemble monodispersed spherical colloids into uniform aggregates with well-controlled sizes, shapes, and structures. When an aqueous dispersion of colloidal particles was allowed to dewet from a solid surface that had been patterned with appropriate relief structures, the particles were trapped by the recessed regions and assembled into aggregates whose structures were determined by the geometric confinement provided by the templates. We have demonstrated the capability and feasibility of this approach by assembling polystyrene beads and silica colloids (> or =150 nm in diameter) into complex aggregates that include polygonal or polyhedral clusters, linear or zigzag chains, and circular rings. We have also been able to generate hybrid aggregates in the shape of HF or H2O molecules that are composed of polymer beads having different diameters, polymer beads labeled with different organic dyes, and a combination of polymeric and inorganic beads. These colloidal aggregates can serve as a useful model system to investigate the hydrodynamic and optical scattering properties of colloidal particles having nonspherical morphologies. They should also find use as the building blocks to generate hierarchically self-assembled systems that may exhibit interesting properties highly valuable to areas ranging from photonics to condensed matter physics.

Self‐Assembly Approaches to Three‐Dimensional Photonic Crystals

Abstract: A brief review of the historical development of photonic bandgap (PBG) materials is provided and the fabrication methods employed are discussed with emphasis on self-assembly processes. The factors influencing the generation of a complete bandgap, from both an experimental and a calculational standpoint are then presented and discussed. The Figure shows a diamond-like 3D periodic structure.

Method of forming articles including waveguides via capillary micromolding and microtransfer molding

Abstract: Chemically or biochemically active agents or other species are patterned on a substrate surface by providing a micromold having a contoured surface and forming, on a substrate surface, a chemically or biochemically active agent or fluid precursor of a structure. A chemically or biochemically active agent or fluid precursor also can be transferred from indentations in an applicator to a substrate surface. The substrate surface can be planar or non-planar. Fluid precursors of polymeric structures, inorganic ceramics and salts, and the like can be used to form patterned polymeric articles, inorganic salts and ceramics, reactive ion etch masks, etc. at the surface. The articles can be formed in a pattern including a portion having a lateral dimension of less than about 1 millimeter or smaller. The indentation pattern of the applicator can be used to transfer separate, distinct chemically or biochemically active agents or fluid precursors to separate, isolated regions of a substrate surface. Waveguide arrays, combinatorial chemical or biochemical libraries, etc. can be made. Differences in refractive index of waveguide and cladding can be created by subjecting the waveguide and cladding, made of identical prepolymeric material, to different polymerization or cross-linking conditions. Interferometers are defined by coupling arrays of waveguides, where coupling can be controlled by altering the difference in refractive index between cladding and waveguide at any desired location of the array. Alteration and refractive index can be created photochemically, chemically, or the like. Sensors also are disclosed, including biochemical sensors.

Three‐Dimensional Photonic Crystals with Non‐spherical Colloids as Building Blocks

Abstract: This paper discusses the potential use of non-spherical colloids (see Figure) as the building blocks in self-assembly to fabricate three-dimensional photonic crystals with bandgaps located in the optical regime. The methods of producing non-spherical colloids as monodisperse samples, and with well-defined shapes and tightly controlled dimensions are presented, and the methods employed to organize these non-spherical colloids into 3D crystalline lattices are discussed.

Synthesis and Characterization of Mesoscopic Hollow Spheres of Ceramic Materials with Functionalized Interior Surfaces

Abstract: Mesoscopic hollow spheres of ceramic materials whose interior surfaces were functionalized by nanoscopic objects were generated by templating sol−gel precursors against three-dimensional arrays of polystyrene beads whose surfaces had been derivatized with submonolayers of the nanoscopic objects.

Surface patterning and its application in wetting/dewetting studies

Abstract: Recent advances in microfabrication have allowed one to pattern the surface of a solid substrate with patches of different wettabilities on the micrometer-sized scale. These textured surfaces provide a well-characterized model system for studying the wetting and dewetting behaviors of liquids on heterogeneous surfaces. They also present a well-defined template to direct the self-organization of liquids on the surfaces of solid substrates, and to form patterned microstructures of various materials without using expensive, clean-room facilities. As demonstrated in a number of studies, the three-dimensional morphologies of the liquid microstructures could be easily controlled by changing the two-dimensional features patterned on the surface of a solid substrate. These demonstrations suggest that microfabrication based on surface patterning and selective wetting or dewetting will offer immediate advantages in applications such as fabrication of microreactor arrays and microfluidic devices, where a liquid (or solution) is the primary material to be patterned.

Growth of Large Crystals of Monodispersed Spherical Colloids in Fluidic Cells Fabricated Using Non-photolithographic Methods

Abstract: This paper describes a convenient approach to the fabrication of fluidic cells to be used for crystallizing spherical colloids into three-dimensionally periodic lattices over large areas. The major component of the fluidic cell was a rectangular gasket sandwiched between two glass substrates. Here we demonstrate that these gaskets could be simply cut out of commercial Mylar films. Three non-photolithographic methods were also demonstrated to create shadow channels between the Mylar film and two glass substrates: (i) by wiping (along one single direction) both sides of the Mylar film with a piece of soft paper (Kimwipes EX-L); (ii) by coating both surfaces of the Mylar film with polymer beads whose size was smaller than those to be packed in the cell; and (iii) by patterning the surface of the bottom glass substrate with an array of gold channels using a combination of microcontact printing and selective etching. When an aqueous dispersion of monodispersed spherical colloids was injected into this packing...

Photonic band-gap properties of opaline lattices of spherical colloids doped with various concentrations of smaller colloids

Abstract: Monodispersed polystyrene beads have been organized into highly ordered, three-dimensional (3D) lattices using a self-assembly procedure recently demonstrated by our group. Such a 3D periodic structure consisting of high and low dielectric regions exhibits a pseudo-band gap (or a stop band) in the optical regime, with the position of this gap mainly determined by the size of the polymer beads. Doping of this 3D crystalline lattice with polymer beads of a smaller size was found to have a profound influence on the order (and thus the photonic band-gap properties) of the lattice. When the concentration of the dopant reached a certain level, phase segregation occurred which led to the formation of samples with relatively smaller domain sizes. In accordance, the attenuation (or rejection ratio) of the stop band also decreased monotonically as the doping level was increased.

Assembly of monodispersed spherical colloids into one-dimensional aggregates characterized by well-controlled structures and lengths

Abstract: Liquid dewetting, physical confinement, and attractive capillary forces have been combined to assemble monodispersed spherical colloids into complex, zigzag aggregates characterized by well-defined internal structures and tightly controlled lengths.

Optimization of elastomeric phase masks for near-field photolithography

Abstract: Rigorous electromagnetic theory has been used to optimize elastomeric phase masks for generating sub-100-nm parallel lines by means of near-field photolithography J. A. Rogers et al., Appl. Phys. Lett. 70, 2658 (1997). In the near-field region, the scattering effect is so strong that the scalar theory is no longer adequate: A bright line was found adjacent to the dark line previously predicted by the scalar theory, and the widths of both lines were found to be insensitive to the refractive index of the photoresist. The simulation results are in good agreement with experimental studies, which showed that the bright and dark lines could be used to generate trenches and lines in a positive-tone photoresist by controlling the exposure time. Our simulations also indicate that parallel lines as small as 50 nm can be generated by adjusting the parameters of the phase mask.