To fabricate quantum dot arrays with programmable periodicity, functionalized DNA origami nanotubes were developed. Selected DNA staple strands were biotin-labeled to form periodic binding sites for streptavidin-conjugated quantum dots. Successful formation of arrays with periods of 43 and 71 nm demonstrates precise, programmable, large-scale nanoparticle patterning; however, limitations in array periodicity were also observed. Statistical analysis of AFM images revealed evidence for steric hindrance or site bridging that limited the minimum array periodicity.

Following the work of John [1] and Yablonovitch, [2] the study of photonic crystals (PCs) has become an important area of research for applications in optoelectronics and electromagnetics, as well as chemical and biological sensors. Formation of a complete photonic bandgap (PBG) requires a three-dimensional (3D) periodic structure exhibiting high refractiveindex contrast. PCs based on infiltration of self-assembled opals are promising structures, [3±5] and a full PBG at infrared wavelengths has been produced in a silicon PC. [6,7] Also, transparent materials with indexes ranging from~1.4 (SiO 2 ) to 3.8 (Sb 2 S 3 ) have been used to form inverse opals with pseudophotonic bandgaps (PPBGs) and potentially full PBGs in the visible-wavelength range.[8±12] Two-dimensional (2D) and 3DPC structures are being extensively modeled, and these studies show that changes in the structures, such as shifting the distribution of dielectric material, can significantly improve PBG properties. The importance of the precise placement of the dielectric material is demonstrated by the inverted ªshellº structure, where the opal is infiltrated with a conformal shell-like coating, leaving small air pockets in the face-centered cubic (fcc) interstitial sites. For example, in a silicon inverse shell opal, the width of the PBG can be increased from 4.25 % to 8.6 %. [13] Similarly, the PBG width can also be increased to 9.6 % by formation of a non-close-packed structure. [14] Thus, the performance of these structures critically depends on precisely and accurately placed high-dielectric material, and the fabrication of these optimized structures will require a highly controllable infiltration method. Similarly, even for 2D and 3D photonic crystals not based on the opal architecture, highly controllable deposition methods will be imperative for maximizing desired photonic effects in real structures. Atomic layer deposition (ALD) allows formation of low porosity, conformal films with submonolayer control.[15] These features make ALD ideal for infiltration, and we have successfully used the technique to create ZnS/Mn inverse opals. [16,17] A similar layer-by-layer method has also been demonstrated to increase the mechanical stability and oxide-filling fraction of a SiO 2 opal using SiCl 4 and H 2 O precursors. [18] In this paper, we extend the ALD studies to TiO 2 , which has long been a candidate material for use in PCs because its refractive index (n) can exceed 2.8 and 2.65 (k = 500 nm) for the rutile and anatase phases, respectively. [3,19±21] Unfortunately, the growth methods used to date have resulted in infiltration filling fractions of, at most, 50 % of the available pore volume. In addition, these opals were infiltrated either by solution precipitation or by nanoparticle co-sedimentation, neither of which offers much precision in placement of the high-dielectric material. Infiltration by ALD holds promise for attaining inverse shell opals that exhibit filling fractions very close to the optimum 90 % of the pore volume. For this study, ...

A novel approach is presented for the large-scale fabrication of ordered TiO 2 nanobowl arrays. The process starts with a self-assembled monolayer of polystyrene spheres, which is used as a template for atomic layer deposition of a TiO 2 layer. After ion-milling, toluene-etching, and annealing of the TiO 2 -coated spheres, ordered arrays of nanostructured TiO 2 nanobowls have been fabricated. The nanobowls exhibit smooth interior and exterior surfaces and uniform sizes and thickness. The nanobowl arrays have been demonstrated to be useful for selecting spheres smaller than the inner diameter of the bowls. This approach can be extended to a wide range of coating materials and substrates (ceramics, metals, polymers, glasses) with controlled wall thickness and size.Monolayer self-assembly (MSA) of polystyrene (PS) submicron spheres on a flat substrate 1,2 is an effective and economical technique for fabricating patterns on a relatively large scale.3,4 The catalyst pattern created by MSA has been applied for growing aligned and spatial-distribution controlled carbon nanotubes 5 and oxide nanorods. 6 Atomic layer deposition (ALD), in which film growth is a cyclic, multistep process of alternating surface-limited chemical reactions, has been demonstrated to be a powerful technique for fabrication of high-quality and multifunctional thin films on various substrates. 7,8 A diversity of nanostructures can be synthesized using ALD owing to its wide operation temperature and precursor adaptability. For example, by controlling the thickness of the uniformly deposited films, a templateassisted ALD process has been applied to the fabrication of inverse opal structures 9,10 and quasi-one-dimensional (1D) nanostructures. 11,12 In this paper, we present a process that utilizes MSA and ALD for fabricating arrays of TiO 2 nanobowls. The TiO 2 nanobowls exhibit smooth surfaces and uniform size and thickness. The nanobowls may be used as ultra small containers for holding fluid of nanoscale volume, and are also demonstrated to be useful for the size selection of submicron spheres. The approach presented could be extended to a wide range of coating materials and substrates with controlled wall thickness and size.The experimental procedures are schematically illustrated in Figure 1. First, a monolayer of highly ordered PS spheres (505 nm in diameter) was self-assembled onto a sapphire substrate (5 mm × 5 mm) using a technique we reported previously 6 (Figure 1a). The substrate was placed at the center of an ALD chamber, which was kept at 80°C during the entire growth process. Then, pulses of TiCl 4 vapor and H 2 O vapor were introduced sequentially into the chamber under a vacuum of 4.5 × 10 -3 Torr. The pulse duration was 4 s for each precursor, and the pulses were separated by a N 2 purging gas for 10 s. A TiO 2 layer was slowly grown on the surfaces of the PS spheres and the substrate (Figure 1b). The growth was terminated after 200 pulse cycles, which produced a uniform amorphous TiO 2 layer ∼20 nm in thickness. The estima...

Coherent exciton delocalization in dye aggregate systems gives rise to a variety of intriguing optical phenomena, including J- and H-aggregate behavior and Davydov splitting. Systems that exhibit coherent exciton delocalization at room temperature are of interest for the development of artificial light-harvesting devices, colorimetric detection schemes, and quantum computers. Here, we report on a simple dye system templated by DNA that exhibits tunable optical properties. At low salt and DNA concentrations, a DNA duplex with two internally functionalized Cy5 dyes (i.e., dimer) persists and displays predominantly J-aggregate behavior. Increasing the salt and/or DNA concentrations was found to promote coupling between two of the DNA duplexes via branch migration, thus forming a four-armed junction (i.e., tetramer) with H-aggregate behavior. This H-tetramer aggregate exhibits a surprisingly large Davydov splitting in its absorbance spectrum that produces a visible color change of the solution from cyan to violet and gives clear evidence of coherent exciton delocalization.

Exciton delocalization in dye aggregate systems is a phenomenon that is revealed by spectral features, such as Davydov splitting, J- and H-aggregate behavior, and fluorescence suppression. Using DNA as an architectural template to assemble dye aggregates enables specific control of the aggregate size and dye type, proximal and precise positioning of the dyes within the aggregates, and a method for constructing large, modular two- and three-dimensional arrays. Here, we report on dye aggregates, organized via an immobile Holliday junction DNA template, that exhibit large Davydov splitting of the absorbance spectrum (125 nm, 397.5 meV), J- and H-aggregate behavior, and near-complete suppression of the fluorescence emission (∼97.6% suppression). Because of the unique optical properties of the aggregates, we have demonstrated that our dye aggregate system is a viable candidate as a sensitive absorbance and fluorescence optical reporter. DNA-templated aggregates exhibiting exciton delocalization may find application in optical detection and imaging, light-harvesting, photovoltaics, optical information processing, and quantum computing.

A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems.

A simple method of making reliable electrical contact to multiwalled carbon nanotubes is described. With these contacts, current in the mA range can be routinely passed through individual multiwalled nanotubes without adverse consequences, thus allowing their resistance to be measured using a common multimeter. The contacts are robust enough to withstand temperature excursions between room temperature and 77 K. I(V) data from different multiwalled nanotubes are presented and analyzed.

The demonstration of a practical technology for 3D optical microfabrication is a vital step in the development of photonic-crystal-based optical signal processing.[1] However, the extension of the optical methods that dominate integrated electronic circuit fabrication to three dimensions is a formidable materials-processing challenge: such a process must be capable not only of sub-micrometer pattern definition in three dimensions, but also of the transfer of this pattern into a homogeneous dielectric with an appropriately high refractive index. In a companion paper, [2] we show that two optical methods, holographic lithography [3] and direct two-photon laser writing, [4][5][6] can be combined to create a rapid and flexible method for the definition of photonic crystal device structures in photoresist. In this communication, we report a further essential step towards the creation of devices operating within a full photonic bandgap: we have used atomic layer deposition (ALD), itself an established semiconductor processing technique, to create high-index TiO 2 inverted replicas of holographically defined photonic crystals, followed by removal of the polymeric template by plasma etching. A range of techniques for 3D optical lithography has been demonstrated. A 3D photonic crystal structure can be written by holographic lithography, [3] which makes use of a periodic interference pattern generated by a multiple-beam interferometer to expose a thick layer of photoresist. 3D microstructures, both periodic and aperiodic, can also be generated by point-by-point exposure of the resist by two-photon absorption at a laser focus. [4][5][6][7] Two-photon laser writing is a serial process; point-by-point fabrication of a 3D photonic crystal is necessarily slower than holographic lithography, which is capable of defining the entire periodic structure in a single laser pulse.[3] The two techniques are complementary: two-photon laser writing can be used to modify a holographic exposure.[8]We have shown that, by imaging the distribution of photochemical change induced by holographic exposure, it is possible to align a subsequent two-photon exposure with the 3D photonic crystal lattice to achieve the precise registration that is required of a device structure embedded in a 3D photonic crystal. [2] This hybrid technique is rapid and flexible, but the polymeric resists used for 3D microfabrication have refractive indices n in the range 1.4-1.6, which is too low for most device applications. Devices based on waveguides and microcavities embedded within a photonic crystal [1] are designed to operate at frequencies within a complete (omnidirectional) photonic bandgap in order to suppress radiative loss; [9] to create a complete photonic bandgap, even in an optimized air-dielectric structure, a refractive contrast of at least 1.9 is necessary.