Hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form because of the difference in diffusion rates between two components in a diffusion couple. Starting with cobalt nanocrystals, we show that their reaction in solution with oxygen and either sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds. A simple extension of the process yielded platinum-cobalt oxide yolk-shell nanostructures, which may serve as nanoscale reactors in catalytic applications.

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Formation of hollow nanocrystals through the nanoscale Kirkendall effect

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Formation of Hollow Nanocrystals through the Nanoscale Kirkendall Effect Yadong Yin, Robert M. Rioux, Can K. Erdonmez, Steven Hughes, Gabor A. Somorjai, A. Paul Alivisatos* Department of Chemistry, University of California at Berkeley, and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. *To whom correspondence should be addressed. Email: alivis@uclink4.berkeley.edu Abstract We demonstrate that hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form due to the difference in diffusion rates between two components in a diffusion couple. Cobalt nanocrystals are chosen as a primary example to show that their reaction in solution with oxygen, sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of large numbers of compounds. A simple extension of this process yields platinum-cobalt oxide yolk-shell nanostructures which may serve as nanoscale reactors in catalytic applications.

Formation of hollow nanocrystals through the nanoscale kirkendall effect

We show a simple, robust, chemical route to the fabrication of ultrahigh-density arrays of nanopores with high aspect ratios using the equilibrium self-assembled morphology of asymmetric diblock copolymers. The dimensions and lateral density of the array are determined by segmental interactions and the copolymer molecular weight. Through direct current electrodeposition, we fabricated vertical arrays of nanowires with densities in excess of 1.9 x 10(11) wires per square centimeter. We found markedly enhanced coercivities with ferromagnetic cobalt nanowires that point toward a route to ultrahigh-density storage media. The copolymer approach described is practical, parallel, compatible with current lithographic processes, and amenable to multilayered device fabrication.

/pdf/ultrahigh-density-nanowire-arrays-grown-in-self-assembled-1mgxshd8uu.pdf

Ultrahigh-Density Nanowire Arrays Grown in Self-Assembled Diblock Copolymer Templates

Brownian motors: noisy transport far from equilibrium

Self-organized hexagonal pore arrays with a 50–420 nm interpore distance in anodic alumina have been obtained by anodizing aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays with distances as large as 420 nm were obtained under a constant anodic potential in phosphoric acid. By comparison of the ordered pore formation in the three types of electrolyte, it was found that the ordered pore arrays show a polycrystalline structure of a few micrometers in size. The interpore distance increases linearly with anodic potential, and the relationship obtained from disordered porous anodic alumina also fits for periodic pore arrangements. The best ordered periodic arrangements are observed when the volume expansion of the aluminum during oxidation is about 1.4 which is independent of the electrolyte. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighboring pores at the metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.

http://www-old.mpi-halle.mpg.de/mpi/publi/pdf/2438_98.pdf

Hexagonal pore arrays with a 50-420 nm interpore distance formed by self-organization in anodic alumina

The photonic band gap of a two-dimensional photonic crystal is continuously tuned using the temperature dependent refractive index of a liquid crystal. Liquid crystal $E7$ was infiltrated into the air pores of a macroporous silicon photonic crystal with a triangular lattice pitch of 1.58 $\ensuremath{\mu}$m and a band gap wavelength range of 3.3--5.7 \ensuremath{\mu}m. After infiltration, the band gap for the H polarized field shifted dramatically to 4.4--6.0 \ensuremath{\mu}m while that of the E-polarized field collapsed. As the sample was heated to the nematic-isotropic phase transition temperature of the liquid crystal $(59\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ the short-wavelength band edge of the H gap shifted by as much as 70 nm while the long-wavelength edge was constant within experimental error. Band structure calculations incorporating the temperature dependence of the liquid crystal birefringence can account for our results and also point to an escaped-radial alignment of the liquid crystal in the nematic phase.

Tunable two-dimensional photonic crystals using liquid crystal infiltration

washed three times with water, and dried over MgSO4. After removing the solvent, chromatography is carried out on the residue on neutral alox. First, unreacted compound 4 and monoadduct 8 are eluted with pentane, and then bis-adduct 5 with petrolether/ethyl acetate. (54 %, m.p.: 191 C). H NMR (CDCl3, ppm) 3.64 (s, 8 H, CH2), 4.33 (dd, 2 H, CH), 6.99 (dd, 2 H, CH), 7.14 (s, 8 H, aromatic); C NMR (CDCl3, ppm) 33.72, 54.79, 126.37, 129.22, 134.8, 139.74, 140.72. Compound 2: Compound 5 (0.8 g, 2.64 mmol) and chloranil (1.28 g, 5.28 mmol) are dissolved in toluene (150 mL), and heated for 1.5 h under reflux. After removing the solvent, the residue is purified via chromatography (neutral alox/toluene) (75 %, m.p.: 239 C). H NMR (CDCl3, ppm) 5.33 (dd, 2 H, CH), 7.06 (dd, 2 H, CH), 7.37 (AA¢BB¢, 4 H, aromatic), 7.71 (AA¢BB¢, 4 H, aromatic), 7.73(s, 4 H, aromatic); C NMR (CDCl3, ppm) 50.74, 121.71, 126, 127.92, 132.25, 138.73, 142.66. Compound 6a: A solution of compound 2 (100 mg, 0.32 mmol) and 2,3,4,5-tetrachlorothiophenedioxide (167 mg, 0.64 mmol) in CH2Cl2 (4 mL) is heated for 3 days at 70 C under a pressure of 6 kbar. After the reaction is complete, the solvent is removed under vacuum. Chromatography is carried out on the residue on silica gel with hexane/toluene 4/1 as eluent. The first fraction gives the desired product (75 %). The material can be further purified by recrystallization from dichloroethane/ethyl acetate. H NMR (CD2Cl2, ppm) 3.54 (s, 2 H, CH), 5.13 (s, 2 H, CH), 7.45 (AA¢BB¢, 4 H, aromatic), 7.80 (s, 2 H, aromatic), 7.82 (AA¢BB¢, 4 H, aromatic), 7.86 (s, 2 H, aromatic); C NMR (CD2Cl2, ppm) 48.08, 48.86, 122.91, 123.93, 124.29, 126.24, 126.35, 128.02, 128.07, 131.38, 132.91, 133.07, 137.47, 139.58. Compound 6b: Using the procedure described for the synthesis of compound 6a, compound 6b is obtained in a yield of 70 %. H NMR (CD2Cl2, ppm) 3.58 (s, 2 H, CH), 5.19 (s, 2 H, CH), 7.46 (AA¢¢BB¢, 4 H, aromatic), 7.82 (s, 2 H, aromatic), 7.84 (AA¢BB¢, 4 H, aromatic), 7.86 (s, 2 H, aromatic); C NMR (CD2Cl2, ppm) 49.74, 51.89, 120.49, 122.87, 123.96, 126.22, 126.33, 128.02, 128.08, 128.72, 132.90, 133.06, 137.51, 139.73.

/pdf/fabrication-and-microstructuring-of-hexagonally-ordered-two-3lypv8gllx.pdf

Fabrication and Microstructuring of Hexagonally Ordered Two‐Dimensional Nanopore Arrays in Anodic Alumina

Nanopore arrays with 6×108–5×1010 cm−2 pore densities were fabricated by self-organized anodization on aluminum. A two-step anodization process was used to oxidize aluminum in oxalic, sulfuric, and phosphoric acid solutions. Hexagonally ordered pore arrays were obtained within domains of a few micrometers, which are separated from neighboring domains with different orientation of the pore lattice by domain boundaries, i.e., the nanopore arrays show characteristics analogous to two-dimensional polycrystalline structure. The interpore distance can be controlled by changing the electrolyte and/or the applied voltage.

/pdf/polycrystalline-nanopore-arrays-with-hexagonal-ordering-on-1mufbfoekm.pdf

Polycrystalline nanopore arrays with hexagonal ordering on aluminum

We have fabricated macroporous silicon and proved its applicability as a two-dimensional photonic bandgap material in the near-infrared spectral range. Two different triangular lattices of circular air rods with lattice constants of 2.3 and 1.5 μm were etched at least 75 μm deep in an n-type silicon substrate by electrochemical pore formation in aqueous hydrofluoric acid. Photolithographic pre-patterning techniques and subsequent alkaline etching were used. In the case of the 1.5 μm lattice the photo-mask was modified so that series of etch pits were left out. These local perturbations of the initially regular lattice of air rods introduced photonic defects like waveguides. In the case of the 2.3 μm lattice we succeeded to micromechanically structure the macroporous layer to make 200 μm wide free-standing bars of porous material remain on the silicon substrate. These bars were used to measure the transmission of the photonic lattice dependent on the light polarization relative to the pore axes using FT infrared spectroscopy. The results excellently agree with theoretical calculations. The generation of KOH pits with lattice constants on a sub-μm length scale was demonstrated.

A. Birner

Papers

Hexagonal pore arrays with a 50-420 nm interpore distance formed by self-organization in anodic alumina

Tunable two-dimensional photonic crystals using liquid crystal infiltration

Fabrication and Microstructuring of Hexagonally Ordered Two‐Dimensional Nanopore Arrays in Anodic Alumina

Polycrystalline nanopore arrays with hexagonal ordering on aluminum

Macroporous Silicon: A Two‐Dimensional Photonic Bandgap Material Suitable for the Near‐Infrared Spectral Range