A wet-chemical method was employed to prepare zinc oxide nanocrystals having controlled morphology through thermal decomposition of a zinc precursor in self-assembled supramolecular structures in solvent under mild conditions. This solution method offers finer tailoring of the size and shape of the nanocrystals and is complementary to most reported physical methods. Understanding the morphological effects of pure or modified zinc oxide nanocrystals on photocatalytic activity is important in regard to enhanced solar energy capture and utilization but has been scarcely addressed in the past. The photocatalytic rate was found to have no dependence on ZnO particle size, but the shape factor seems to be of overriding importance. Hexagonal platelike nanocrystals were found to display at least 5 times higher activity than rod-shaped crystals, which clearly suggests that the polar (001) and (001) faces are more active surfaces than the nonpolar surfaces perpendicular to them.

Presbyopia is an age-related loss of accommodation of the human eye that manifests itself as inability to shift focus from distant to near objects. Assuming no refractive error, presbyopes have clear vision of distant objects; they require reading glasses for viewing near objects. Area-divided bifocal lenses are one example of a treatment for this problem. However, the field of view is limited in such eyeglasses, requiring the user to gaze down to accomplish near-vision tasks and in some cases causing dizziness and discomfort. Here, we report on previously undescribed switchable, flat, liquid-crystal diffractive lenses that can adaptively change their focusing power. The operation of these spectacle lenses is based on electrical control of the refractive index of a 5-m-thick layer of nematic liquid crystal using a circular array of photolithographically defined transparent electrodes. It operates with high transmission, low voltage (<2 V rms), fast response (<1 sec), diffraction efficiency > 90%, small aberrations, and a power-failure-safe configuration. These results represent significant advance in stateof-the-art liquid-crystal diffractive lenses for vision care and other applications. They have the potential of revolutionizing the field of presbyopia correction when combined with automatic adjustable focusing power.ophthalmic lens ͉ switchable lens ͉ vision correction T he use of nematic liquid crystals to implement switchable lenses has been proposed previously but had limited success for ophthalmic applications (1). Hybrid liquid-crystal refractive lenses incorporating convex and concave substrates have been demonstrated (2, 3). However, the large thickness of the liquidcrystal layers (Ͼ400 m) make their response and recovery times long and their transmission low because of optical scattering. To reduce the thickness of the active layer, surface relief Fresnel lens substrates have been proposed (4). However, in this geometry the lens is optically active in the electrically off-state, which is not desirable for ophthalmic applications where a loss of electrical power could suddenly result in near-vision correction during a critical distance vision task such as driving. In other approaches, thin uniform layers of liquid crystal were used, and refractive lenses were produced by the use of discrete electrodes (5), continuous highly resistive electrodes (6), or spatially distributed electric fields (microlenses) (7). However, in these lenses either the range of focal length or their small diameter made them unsuitable for ophthalmic applications. We employ a photolithographically patterned thin diffractive lens with large aperture, fast response time, and a power-failure-safe configuration to overcome these limitations. Although high-efficiency liquid-crystal-based diffractive devices have been demonstrated for beam-steering (8, 9), less effort was given to the development of switchable diffractive lenses. The diffraction efficiencies of the lenses achieved for imaging (10-15) and other applications (16, 17) we...

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