San Ming Yang

The Race for the Photonic Chip: Colloidal Crystal Assembly in Silicon Wafers

This paper surveys recent developments in engineering physics approaches and self-assembly chemistry methodologies for creating 3D photonic crystals and how this has led to in-wafer patterned colloidal crystals. These materials are comprised of single crystal micrometer scale features of silica colloidal crystals that have controlled thickness, area, and orientation and are embedded within a single crystal silicon wafer. Two processes for growing opal-patterned chips are described. One is based upon microfluidic and the other spin coating driven self-assembly of colloidal silica micro-spheres within a lithographic patterned silicon wafer.

Opal Circuits of Light—Planarized Microphotonic Crystal Chips

We present a novel technique coined directed evaporation-induced self-assembly (DEISA) that enables the formation of planarized opal-based microphotonic crystal chips in which opal crystal shape, size, and orientation are under synthetic control. We provide detailed synthetic protocols that underpin the DEISA process and formulate directed self-assembly strategies that are suited for the fabrication of opal architectures with complex form and designed optical functionality. These developments bode well for the utilization of opal-based photonic crystals in microphotonic crystal devices and chips.

Formation of Hollow Helicoids in Mesoporous Silica: Supramolecular Origami

In the past, helical shapes in nature have inspired inventions such as the water screw for agriculture, the retaining screw for wine presses, and architectural designs for spiral staircases. Similarly, these days helix-shaped DNA, proteins and carbon nanotubes evoke great interest in biotechnology and nanotechnology. Also biomimetic synthesis of helical morphologies of calcium carbonate, barium sulfate, and silica provides insight into morphogenesis of mineralized spiral forms in biology and ideas for new opportunities in materials science. Herein we describe the synthesis of hollow helicoids made of hexagonal mesoporous silica, a remarkable topology in the materials world. They have a hierarchical architecture comprised of 5 nm diameter channels that coil in the form of a micrometerscale tubular spiral. A population analysis of helicoid shapes defines a surprisingly narrow distribution of pitch and flute widths, pitch angles, inside and outside diameters, and significantly an equal number of leftand right-handed forms. Evidence is presented that morphogenesis involves polymerization-induced differential contraction of a patch of hexagonal silicate liquid-crystal film formed at the air± water interface, which can fold into a hollow helicoid. A supramolecular Origami theoretical model explains the creation and observed narrow distribution of mesoporous silica, hollow helicoid shapes. Mesoporous silica hollow helicoids were prepared by using cetyltrimethylammonium chloride (CTACl) as the surfactant micellar template and tetraethylorthosilicate (TEOS) as the silica precursor. An aqueous solution of CTACl, hydrochloric acid and formamide was aged for 48 h before adding TEOS, and the material was formed after 3 days in a quiescent state. The use of formamide in the synthesis is intentional because upon acid hydrolysis it yields ammonium chloride and formic acid to give an ultimate solution ca. pH 1.9 and an ionic strength that favors hollow helicoid formation. This solution pH is notably higher than the one used in the synthesis of mesoporous silica curved shapes. Control experiments demonstrate that a high concentration of ammonium and formate ions is essential for the formation of mesoporous silica at a pH close to two, which borders on the isoelectric point of aqueous silica. We believe that a low acidity and high ionic strength medium favor a slow rate of silicification, and hence polymerization-induced differential contraction of silicate micelle rods in a patch of silicate liquid-crystal film formed at the air±water interface becomes influential in hollow helicoid formation. Powder X-ray diffraction (PXRD) patterns in Figure 1 clearly define as-synthesized and calcined

Towards the synthetic all-optical computer: science fiction or reality?

The global race for the optically integrated photonic chip is driven by the prospective that miniaturization of optical devices and enhanced chip functionality may revolutionize the manufacture of optical circuits, and the futuristic dream of the all-optical computer may come true. The aim of this article is to take a brief yet critical look at some developments in microsphere self-assembly of colloidal photonic crystals and their technological potential from the perspective of research results that have recently emerged from our materials chemistry group. The focus of the discussion centers on the provocative vision of the “colloidal photonic crystal micropolis”, Fig. 1, which depicts the direction in which the colloidal photonic crystal research of our materials chemistry group is heading. It is intended to bring to the forefront the pointed question of whether the most recent versions of colloidal photonic crystals and their integration on chips, developed in our laboratory, can rise to the stringent specifications of structural perfection and optical quality, functionality and complexity that will be demanded for photonic crystal optical devices and optical circuits touted for next generation all-optical chip and telecommunication technologies.

Morphokinetics: Growth of Mesoporous Silica Curved Shapes

52 O WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/0101-0052

Barium Titanate Inverted Opals—Synthesis, Characterization, and Optical Properties

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Origin of the red luminescence band in photoluminescence spectra of ZnSe nanowires

Barium titanate inverted opals with powder and film morphologies were synthesized from barium ethoxide and titanium isopropoxide in the interstitial spaces of a polystyrene opal. This procedure involves infiltration of precursors into the interstices of the polystyrene opal template followed by hydrolytic polycondensation of the precursors to amorphous barium titanate and removal of the polystyrene opal by solvent extraction or calcination. In-situ variable temperature powder X-ray diffraction and micro-Raman spectroscopy allow one to observe the thermally induced transformation of the as-synthesized amorphous barium titanate inverted opal to the nanocrystalline form. In this way, a nanocrystalline barium titanate inverted opal can be engineered as either the cubic or tetragonal polymorph. Control of this process is key to the practical realization of a room-temperature stable ferroelectric barium titanate inverted opal that can be thermally tuned through the ferroelectric–paraelectric transition around the Curie temperature. Optical characterization demonstrated photonic crystal behavior of the inverted barium titanate opals and results were in good agreement with photonic band structure calculations. The synthesis of optical quality ferroelectric barium titanate inverted opals provides an opportunity to electrically and optically engineer the photonic band structure and the possibility of developing tunable three-dimensional photonic crystal devices.

Opal chips: vectorial growth of colloidal crystal patterns inside silicon wafers

In this work, the origin of the deep level, defect related photoluminescence emission band in ZnSe is investigated. Using the dependence of the peak energy on excitation intensity, it was shown to originate from donor-acceptor pair recombination. The binding energy of the donor-acceptor pair was estimated to be 18±0.5meV and the shallow impurity Bohr radius was estimated to be 9.1±0.2nm. Using a postgrowth annealing treatment in a Zn atmosphere, the two species involved in the donor-acceptor pair recombination process were attributed to Zn vacancies and Zn interstitials.

Replicating the Structure of a Crosslinked Polyferrocenylsilane Inverse Opal in the Form of a Magnetic Ceramic

A simple, quick, reproducible and inexpensive method is described that combines self-assembly, micro-fluidics and soft lithography, to achieve a novel example of vectorial control of thickness, area, orientation and registry of patterned single crystal silica colloidal crystals in silicon wafers, coined opal chips, for potential applications in photonic chip and lab-on-chip technologies.

Colloidal photonic crystal microchannel array with periodically modulated thickness

Crosslinked polyferrocenylsilane inverse opals have been prepared by the thermal ring-opening polymerization of spirocyclic [1]silaferrocenophanes confined within the interstitial void spaces of silica crystal colloidal templates. These organometallic polymer inverse opals were converted to magnetic ceramic replicas through pyrolysis at 900 °C in high yields.