Extraordinary Field Enhancement of TiO_2 Porous Layer up to 500-Fold

摘要

Titanium dioxide (TiO_2) is known as a very important material forphotocatalysts, the photoelectrode for hydrogen evolution reaction,and the porous layer of perovskite solar cells. Their properties as well asdevice performances such as photoconversion efficiencies are significantlyenhanced if integration of TiO_2 providing a large field enhancement oflight is made possible. However, the field enhancement has not beenrevealed for TiO_2 materials, and then increased device performances havenot been reported either so far. Here, extraordinary field enhancement ofa porous TiO_2 layer, mesoscopic film, prepared by a facile wet processin conjunction with ball milling and drop casting is shown. The fieldenhancement is investigated with respect to the fluorescence intensity ofa dye molecule and an enhancement factor (EF) of up to 500 is achieved,which corresponds to the largest EF for a semiconductor. Furthermore, EFis up to 30 000 after numerical corrections. The large EF is realized for aporous TiO_2 layer composed of a specific particle size of 550 nm, which isconsistent with the results of fluorescence intensity, scattering intensity,and two different theoretical calculations based on Mie scattering theory,with respect to particle size. A special advantage of the current system isthe mesoscopic porous layer giving many hot spots among TiO_2 particlesof the specific size.