Microscopy Guided Design of Radial p-n Junction in Single TiO2 Nanotubes

Engineering one-dimensional nanostructures in the axial direction is of great importance to researchers in energy harvesting fields, as utilizing materials with such a unique configuration could potentially introduce significant efficiency improvements for use in future solar energy conversion, storage, and nanoelectronic devices [1-3]. In 2007, the Lewis group proposed an interesting solar cell configuration consisting of radial p-n junction nanorods that exhibited a considerably improved cell efficiency compared to traditional planar solar cells [4]. Such a configuration decouples the direction of incident light and the proximity of generated charge carriers to the p-n junction. Vertical alignment of many cylindrical junctions facilitates the transport of photon-generated minority carriers to the junctions, and therefore, greatly increases carrier collection efficiency, as well as enhanced tolerance to radiation damage, defects, and impurities. Further advantages are anticipated with nanotube structures for solar energy conversion based on recent theoretical calculations, which showed that nanohole arrays (aligned nanochannels in a Si matrix) provide better light absorption compared to nanowire arrays due to the high density of waveguide modes [5]. Anodic TiO2 nanotube arrays (NTA) have been studied extensively such that their radial physical properties can be tailored by incorporating an additional layer having different physical properties. These layers are either incorporated on the inner or outer walls of the nanotubes [6,7]; however, the synthesis of well-controlled and reproducible axial p-n junctions with desirable properties at a relatively low cost for NTAs remains a challenge.