TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

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

Graphenes as potential material for electronics.

Brownian motors: noisy transport far from equilibrium

α‐Fe2O3 Nanotubes in Gas Sensor and Lithium‐Ion Battery Applications

We have developed a simple technique for the fabrication of polymer nanotubes with a monodisperse size distribution and uniform orientation. When either a polymer melt or solution is placed on a substrate with high surface energy, it will spread to form a thin film, known as a precursor film, similar to the behavior of low molar mass liquids ([1][1], [2][2]).

Similar wetting phenomena occur if porous templates are brought into contact with polymer solutions or melts: A thin surface film will cover the pore walls in the initial stages of wetting. This is because the cohesive driving forces for complete filling are much weaker than the adhesive forces. Wall wetting and complete filling of the pores thus take place on different time scales. The latter is prevented by thermal quenching in the case of melts or by solvent evaporation in the case of solutions, thus preserving a nanotube structure. If the template is of monodisperse size distribution, aligned or ordered, so are the nanotubes, and ordered polymer nanotube arrays can be obtained if the template is removed. Any melt-processible polymer, such as polytetrafluoroethylene (PTFE), blends, or multicomponent solutions can be formed into nanotubes with a wall thickness of a few tens of nanometers. Owing to its versatility, this approach should be a promising route toward functionalized polymer nanotubes.

We used ordered porous alumina and oxidized macroporous silicon templates with narrow pore size distribution ([3][3]). Extended regular pore arrays were prepared by lithography. The pores are well-defined, straight, with a smooth inner surface and with diameters DP between 300 and 900 nm. To process melts, we placed the polymer on a pore array at a temperature well above its glass transition temperature, in the case of amorphous polymers, or its melting point, in the case of partially crystalline polymers. The liquid polymer forms a thin wetting film covering the entire pore surface on a time scale ranging from a few minutes to half an hour. Polymer solutions were dropped on the templates at ambient conditions (fig. S1A) ([4][4]). The resulting nanotubes obtained from either method had wall thicknesses between 20 and 50 nm and lengths of up to 100 μm. Oligomers as well as polymers with molecular masses Mn up to several hundreds of thousands of grams per mol were processed. [Figure 1][5]depicts nanotubes formed from several polymers by melt-wetting. The tip of a polystyrene (PS) nanotube ( Mn ∼ 850,000 g/mol) formed in an alumina template was uncovered by etching the alumina substrate with aqueous potassium hydroxide ([Fig. 1][5]A) ([4][4]). [Figure 1][5]B shows the same sample after the complete removal of the template. [Figure 1][5]C depicts an array of aligned PTFE nanotubes obtained by wetting an alumina template and [Fig. 1][5]D a highly ordered array of polymethyl methacrylate (PMMA; Mn ∼ 80,000 g/mol) nanotubes prepared by wetting a macroporous silicon template. After selectively dissolving the template, the remaining nanotube array still exhibits its hexagonal long-range order.

![Figure 1][6] 

Figure 1 
Scanning electron micrographs of nanotubes obtained by melt-wetting. ( A ) Damaged tip of a PS nanotube ( Mn ∼ 850,000 g/mol) protruding from a porous alumina membrane. The substrate, on which the pore array was located, has been removed to uncover the tube tips. ( B ) Ordered array of tubes from the same PS sample after complete removal of the template. ( C ) Array of aligned PTFE tubes. ( D ) PMMA tubes with long-range hexagonal order obtained by wetting of a macroporous silicon pore array after complete removal of the template.



The wetting technique can be easily extended to prepare functionalized nanotubes, for example, palladium/polymer composite nanotubes. We first wetted the porous templates with a solution containing poly-l-lactide (PLLA) and palladium(II)acetate under ambient conditions. After evaporation of the solvent dichloromethane, a PLLA/palladium(II)acetate film covered the pore walls. The template was subsequently annealed in vacuum at temperatures of up to 300°C to degrade PLLA ([5][7]) and to reduce Pd. In a second wetting step, molten PS was added, so that Pd/PS composite tubes were formed (fig. S1B). Energy-dispersive x-ray microanalysis verified the presence of Pd (fig. S1C), and selected area electron diffraction of single composite tubes revealed that it was metallic with a typical crystallite size of 2 to 3 nm (fig. S1D). As demonstrated by this example, template-wetting should have an outstanding potential in providing customized nanotubes for a broad range of applications in nanoscience.

1. [↵][8]1. P. G. de Gennes
 , Rev. Mod. Phys. 57, 827 (1985).
 [OpenUrl][9][CrossRef][10][Web of Science][11]

2. [↵][12]S. F. Kistler, in Wettability , Surfactant Science Series , vol. 49, J. C. Berg, Ed. (Dekker, New York, 1993), chap. 6.
 

3. [↵][13]1. R. B. Wehrspohn, 2. J. Schilling
 , MRS Bull. 8, 623 (2001).
 [OpenUrl][14]

4. [↵][15]Materials and methods are available as supporting mate rial on Science Online.
 

5. [↵][16]1. M. Bognitzki 2. et al.
 , Adv. Mater. 12, 637 (2000).
 [OpenUrl][17]

6. Support from the Deutsche Forschungsgemeinschaft (WE 2637/1-1 and WE 496/19-1) and transmission electron microscopy investigations by C. Dietzsch are gratefully acknowledged. Supporting Online Material [www.sciencemag.org/cgi/content/full/296/5575/1997/DC1][18] Materials and Methods Fig. S1

 [1]: #ref-1
 [2]: #ref-2
 [3]: #ref-3
 [4]: #ref-4
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 [6]: pending:yes
 [7]: #ref-5
 [8]: #xref-ref-1-1 "View reference 1 in text"
 [9]: {openurl}?query=rft.jtitle%253DReviews%2Bof%2BModern%2BPhysics%26rft.stitle%253DRev.%2BMod.%2BPhys.%26rft.issn%253D1539-0756%26rft.volume%253D57%26rft.spage%253D827%26rft_id%253Dinfo%253Adoi%252F10.1103%252FRevModPhys.57.827%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx
 [10]: /lookup/external-ref?access_num=10.1103/RevModPhys.57.827&link_type=DOI
 [11]: /lookup/external-ref?access_num=A1985APC5700007&link_type=ISI
 [12]: #xref-ref-2-1 "View reference 2 in text"
 [13]: #xref-ref-3-1 "View reference 3 in text"
 [14]: {openurl}?query=rft.jtitle%253DMRS%2BBull.%26rft.volume%253D8%26rft.spage%253D623%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx
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 [17]: {openurl}?query=rft.jtitle%253DAdv.%2BMater.%26rft.volume%253D12%26rft.spage%253D637%26rft.atitle%253DADV%2BMATER%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx
 [18]: http://www.sciencemag.org/cgi/content/full/296/5575/1997/DC1

/pdf/polymer-nanotubes-by-wetting-of-ordered-porous-templates-2py17nlryz.pdf

Polymer nanotubes by wetting of ordered porous templates.

Applying metamaterial concepts to dielectric systems offers low losses compared with metallic structures. Here, silicon-based metamaterial and nanophotonic advances are reviewed. The prospect of creating metamaterials with optical properties greatly exceeding the parameter space accessible with natural materials has been inspiring intense research efforts in nanophotonics for more than a decade. Following an era of plasmonic metamaterials, low-loss dielectric nanostructures have recently moved into the focus of metamaterial-related research. This development was mainly triggered by the experimental observation of electric and magnetic multipolar Mie-type resonances in high-refractive-index dielectric nanoparticles. Silicon in particular has emerged as a popular material choice, due to not only its high refractive index and very low absorption losses in the telecom spectral range, but also its paramount technological relevance. This Review overviews recent progress on metamaterial-inspired silicon nanostructures, including Mie-resonant and off-resonant regimes.

Metamaterial-inspired silicon nanophotonics

A perfect two-dimensional porous alumina photonic crystal with 500 nm interpore distance was fabricated on an area of 4 cm2 via imprint methods and subsequent electrochemical anodization. By comparing measured reflectivity with theory, the refractive indices in the oxide layers were determined. The results indicate that the porous alumina structure is composed of a duplex oxide layer: an inner oxide layer consisting of pure alumina oxide of 50 nm in thickness, and an outer oxide layer of a nonuniform refractive index. We suggest that the nonuniform refractive index of the outer oxide arises from an inhomogeneous distribution of anion species concentrated in the intermediate part of the outer oxide.

/pdf/perfect-two-dimensional-porous-alumina-photonic-crystals-1140do1gwg.pdf

Perfect two-dimensional porous alumina photonic crystals with duplex oxide layers

Ultrafast tuning of the band edge of a two-dimensional silicon/air photonic crystal is demonstrated near a wavelength of 1.9 mm. Changes in the silicon refractive index are optically induced by injecting free carriers with 800 nm, 300 fs pulses. The rise time of the shift occurs on the time scale of the pulse width apart from a small component associated with carrier cooling; the recovery time is related to electron-hole recombination. The band edge is observed to shift linearly with pump beam fluence, with a shift in excess of 30 nm for a pump beam fluence of 2 mJ cm 22 . A nonuniform spectral shift is attributed to finite pump beam absorption depth effects.

/pdf/ultrafast-band-edge-tuning-of-a-two-dimensional-silicon-4jfxim8ab3.pdf

Ultrafast band-edge tuning of a two-dimensional silicon photonic crystal via free-carrier injection

The lack of a dipolar second-order susceptibility (χ(2)) in silicon due to the centrosymmetry of its diamond lattice usually inhibits efficient second-order nonlinear optical processes in the silicon bulk. Recently, the deposition of stressed silicon nitride layers and the corresponding inhomogeneous strain in silicon lead to the demonstration of second harmonic generation and electro-optic modulation in strained silicon waveguides. However, the respective impact of the stress/strain gradient and the involved interfaces is not clear. Here, the influence of the stress and the stressing silicon nitride layer using second harmonic generation measurements in transmission is investigated. The results show that the enhancement of the second-order nonlinearity arises from a constructive superposition of stress-induced and interface-related effects. Particularly, the stress gradient in silicon breaks the symmetry of the crystal lattice, while positive fixed charges at the silicon/silicon nitride interface are responsible for a pronounced electric-field-induced-second harmonic (EFISH) contribution. These results demonstrate the impact of external factors for the creation of an effective χ(2) in materials and open new perspectives for the use of second-order nonlinear optical processes in silicon photonics.

/pdf/second-order-optical-nonlinearity-in-silicon-waveguides-37i2kc0y34.pdf

Jörg Schilling

Papers

Polymer nanotubes by wetting of ordered porous templates.

Metamaterial-inspired silicon nanophotonics

Perfect two-dimensional porous alumina photonic crystals with duplex oxide layers

Ultrafast band-edge tuning of a two-dimensional silicon photonic crystal via free-carrier injection

Second‐Order Optical Nonlinearity in Silicon Waveguides: Inhomogeneous Stress and Interfaces