also be transferred from the highest occupied molecular orbital (HOMO) of the dye to the Fermi level of the dopant formate, so blocking electron±hole recombination 3 . Our results from emulsions sensitized by a carbocyanine thought to ful®ll the above criterion indicate that the sensitivity is indeed increased (Fig. 4), compared to emulsions without formate supposed to absorb the same quantity of photons. Depending on the surface coverage by dye molecules, these are in the form of either monomers or Jaggregates, which are two-dimensional arrays of edged-on adsorbed dye molecules with a large slip angle. The difference is about DlogI 3 t exp 0:9 for emulsions sensitized by dye monomers (l max 480 and 500 nm), excited at l exp > 477 nm; for emulsions sensitized by J-aggregates (additional l max at 552 nm), excited at l exp > 519 nm, we obtain DlogI 3 t exp 0:7. This enables us to assess the effect of aggregates only without coexisting monomers. The sensitivity enhancement that is systematically observed in the presence of formate ion therefore attests to ef®cient hole transfer to the dopant formate from the HOMO level of the dye, in both the monomer and the J-aggregate forms. This mechanism is comparable with the scavenging of the intrinsic hole created by direct AgBr excitation, and results in a marked inhibition of recombination. M

Photonic technology, using light instead of electrons as the information carrier, is increasingly replacing electronics in communication and information management systems. Microscopic light manipulation, for this purpose, is achievable through photonic bandgap materials, a special class of photonic crystals in which three-dimensional, periodic dielectric constant variations controllably prohibit electromagnetic propagation throughout a specified frequency band. This can result in the localization of photons, thus providing a mechanism for controlling and inhibiting spontaneous light emission that can be exploited for photonic device fabrication. In fact, carefully engineered line defects could act as waveguides connecting photonic devices in all-optical microchips, and infiltration of the photonic material with suitable liquid crystals might produce photonic bandgap structures (and hence light-flow patterns) fully tunable by an externally applied voltage. However, the realization of this technology requires a strategy for the efficient synthesis of high-quality, large-scale photonic crystals with photonic bandgaps at micrometre and sub-micrometre wavelengths, and with rationally designed line and point defects for optical circuitry. Here we describe single crystals of silicon inverse opal with a complete three-dimensional photonic bandgap centred on 1.46 microm, produced by growing silicon inside the voids of an opal template of dose-packed silica spheres that are connected by small 'necks' formed during sintering, followed by removal of the silica template. The synthesis method is simple and inexpensive, yielding photonic crystals of pure silicon that are easily integrated with existing silicon-based microelectronics.

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In our information-rich world, it is becoming increasingly important to develop technologies capable of displaying dynamic and changeable data, for reasons ranging from valueadded advertising to environmental sustainability. There is an intense drive at the moment towards paper-like displays, devices having a high reflectivity and contrast to provide viewability in a variety of environments, particularly in sunlight where emissive or backlit devices perform very poorly. The list of possible technologies is extensive, including electrophoretic, cholesteric liquid crystalline, electrochromic, electrodewetting, interferometric and more. Despite tremendous advances, the key drawback of all these existing display options relates to colour. As soon as an RGB (red, green and blue) colour filter or spatially modulated colour scheme is implemented, substantial light losses are inevitable even if the intrinsic reflectivity of the material is very good.We describe a reflective flat-panel display technology based on the electrical actuation of photonic crystals. These materials display non-bleachable structural colour, reflecting narrow bands of wavelengths tuned throughout the entire visible spectrum by expansion and contraction of the photonic-crystal lattice. The material is inherently bright in high-light environments, has electrical bistability, low operational voltage, can be integrated onto flexible substrates, and is unique among all display technologies in that a continuous range of colours can be accessed without the need for colour filters or optical elements.Photonic crystals (PCs) 1,2 , materials with a periodic modulation in refractive index, can be sources of exceptionally bright and brilliant reflected colours arising from coherent Bragg optical diffraction 3 . Exemplified by gemstone opals, threedimensional PCs are readily available by means of colloidal selfassembly, making them a fertile test-bed for investigating concepts based on tunable structural colour. Synthetic preparations for silica or polymer microspheres with low dispersity (,2%) are well-developed, and their self-assembly into a close-packed ordered structure leaves a void volume of 26% available for further material infiltration or modification 4,5 . Tuning of colloidal PC optical properties has been effected by the infilling of metals, insulators, semiconductors and polymers of all types, and by the inversion of such constructs through removal of the template spheres.Given their bright colours with potential tunability, it seems obvious at first glance to use PCs as the active materials in refreshable full-colour digital displays. The fact that this has not yet been achieved highlights the difficult set of requirements that must be met by any potential candidate, including electrical tunability, access to thin and homogeneous oriented films, large tuning range, relatively rapid response time, mechanical and cycling stability, low voltage/current requirements, and obviously the ability to implement the material into a feasible, practical, sca...