Engineered nanoporous and nanostructured films

Nanoporous and nanostructured films and surfaces have been exploited by nature to spectacular effect. Plant leaves use nanostructured surfaces to shed water(1, 2) as shown in Fig. 1 and the Namib desert beetle uses a similar surface to collect water from dew(3,) (4,). Butterflies have fashioned nanostructured surfaces to attract mates, deter predators, and provide camouflage(5,) (6) (Fig. 2). Geckos(7,) (8), flies, and other insects(9) use nanostructured surfaces to adhere to walls (Fig. 3), and all cell membranes can be thought of as sophisticated nanoporous films(10). The development of nanoporous nanostructured thin films is relatively recent and has been driven by the need for low dielectric constant materials in the semiconductor industry(11-15), the need for low refractive index materials in the photonics industry(16-20), the need for nearly blackbody absorptivity in the solar cell industry(21), the need for nanoporous membranes in the gas separations industry(22), the need for superhydrophobic or superoleophobic materials to control wetting and spreading(23,) (24), and the general need for thin film catalytic and separations processes in the fuel cell(25,) (26), and biotechnology(27,) (28) fields. Since the range of applications is so vast and the materials and methods used to fabricate these materials is so broad, this article will focus primarily on nanoporous and nanostructured dielectric films for potential use in the microelectronics and photonics industries.open1134sciescopu

Internal high-reflectivity omni-directional reflectors

An internal high-reflectivity omni-directional reflector (ODR) for the visible spectrum is realized by the combination of total internal reflection using a low-refractive-index (low-n) material and reflection from a one-dimensional photonic crystal (1D PC). The low-n layer limits the range of angles in the 1D PC to values below the Brewster angle, thereby enabling high reflectivity and omni-directionality. This ODR is demonstrated using GaP as ambient, nanoporous SiO2 with a very low refractive index (n = 1.10), and a four-pair TiO2/SiO2 multilayer stack. The results indicate a two orders of magnitude lower angle-integrated transverse-electric-transverse-magnetic polarization averaged mirror loss of the ODR compared with conventional distributed Bragg reflectors and metal reflectors. This indicates the high potential of the internal ODRs for optoelectronic semiconductor devices, e. g., light-emitting diodes. (c) 2005 American Institute of Physics.open113949sciescopu