Plasmonic antenna effects on photochemical reactions.

Efficient solar energy conversion has been vigorously pursued since the 1970s, but its large-scale implementation hinges on the availability of high-efficiency modules. For maximum efficiency, it is important to absorb most of the incoming radiation, which necessitates both efficient photoexcitation and minimal electron-hole recombination. To date, researchers have primarily focused on the latter difficulty: finding a strategy to effectively separate photoinduced electrons and holes. Very few reports have been devoted to broadband sunlight absorption and photoexcitation. However, the currently available photovoltaic cells, such as amorphous silicon, and even single-crystal silicon and sensitized solar cells, cannot respond to the wide range of the solar spectrum. The photoelectric conversion characteristics of solar cells generally decrease in the infrared wavelength range. Thus, the fraction of the solar spectrum absorbed is relatively poor. In addition, the large mismatch between the diffraction limit of light and the absorption cross-section makes the probability of interactions between photons and cell materials quite low, which greatly limits photoexcitation efficiency. Therefore, there is a pressing need for research aimed at finding conditions that lead to highly efficient photoexcitation over a wide spectrum of sunlight, particularly in the visible to near-infrared wavelengths. As characterized in the emerging field of plasmonics, metallic nanostructures are endowed with optical antenna effects. These plasmonic antenna effects provide a promising platform for artificially sidestepping the diffraction limit of light and strongly enhancing absorption cross-sections. Moreover, they can efficiently excite photochemical reactions between photons and molecules close to an optical antenna through the local field enhancement. This technology has the potential to induce highly efficient photoexcitation between photons and molecules over a wide spectrum of sunlight, from visible to near-infrared wavelengths. In this Account, we describe our recent work in using metallic nanostructures to assist photochemical reactions for augmenting photoexcitation efficiency. These studies investigate the optical antenna effects of coupled plasmonic gold nanoblocks, which were fabricated with electron-beam lithography and a lift-off technique to afford high resolution and nanometric accuracy. The two-photon photoluminescence of gold and the resulting nonlinear photopolymerization on gold nanoblocks substantiate the existence of enhanced optical field domains. Local two-photon photochemical reactions due to weak incoherent light sources were identified. The optical antenna effects support the unprecedented realization of (i) direct photocarrier injection from the gold nanorods into TiO(2) and (ii) efficient and stable photocurrent generation in the absence of electron donors from visible (450 nm) to near-infrared (1300 nm) wavelengths.