No abstract

Ordered nanostructure arrays have attracted intensive attention because of their various applications. However, it is still a great challenge to achieve ordered nanostructure patterning over a large area (such as wafer-scale) by a technique that allows high throughput, large pattern area and low equipment costs. Here, through a unique design of the fabrication and transferring processes, we achieve a facile transferring of wafer-scale ultrathin alumina membranes (UTAMs) onto substrates without any twisting, folding, cracking and contamination. The most important in our method is fixing the UTAM onto the wafer-scale substrate before removing the backside Al and alumina barrier layer. It is also demonstrated that the thickness and surface smoothing of UTAMs play crucial roles in this transferring process. By using these perfectly transferred UTAMs as masks, various nanostructure patterning including nanoparticle, nanopore (nanomesh) and nanowire arrays are fabricated on wafer-scale substrates with tunable and uniform dimension. Because there are no requirements for UTAMs, substrates and materials to be deposited, the method presented here shall provide a cost-effective platform for the fabrication of ordered nanostructures on large substrates for various applications in nanotechnology.

Instead of investigating the quantum effect that influences the absorption band edge of TiO 2 nanostructures, herein we report that geometrical parameters can also be utilized to manipulate the optical band gap of the TiO 2 nanotube arrays. Hexagonal arrays of TiO 2 nanotubes with an excellent crystalline quality were fabricated by techniques combining anodic aluminum oxide templates and atomic layer deposition. Through absorption spectroscopic analysis we observed that the optical absorption band edge of the TiO 2 nanotube arrays exhibited a red shift as the diameter of the nanotube was tuned to be larger and the distance between two nanotubes became smaller accordingly, while the wall thickness of the nanotube was kept constant. Subsequent finite-difference time-domain simulations supported the observation from theoretical aspect and revealed a large near-field enhancement around the outer space of the nanotubes for the arrays with densely distributed nanotubes when the corresponding arrays were exposed to the illuminations. Thus, this paper provides a new perspective for the shift of the optical band gap, which is of great significance to the research in photoelectronics. ■ INTRODUCTIONThe unique optical properties of semiconductor nanostructures have been extensively studied in the aspect of quantum confinement effects: when the dimensions of semiconductors are put at or below the characteristic size like exciton Bohr radius, the band gap becomes larger in comparison with the relevant bulk material, followed by a blue shift of the optical absorption onset. Such phenomenon is caused by localization of electrons and holes in a confined space that results in observable quantization of the energy levels of the electrons. 1−3 In this case, the optical properties of nanomaterials present a series of features that the bulk materials do not possess and have been widely applied in biomedicine, 4−6 sensors, 7 photonics, 8,9 and photovoltaics. 8,10 However, other geometrical factors excluding the quantum effects that influence the optical band gap of semiconductors are rarely reported. 11,12 These days, the development of technologies for precisely controlling morphological parameters of nanostructures and the corresponding arrays enables us with sufficient approaches to turn back to reexamine the renowned optical band gap shift of semiconductors. Specifically, the technique based on anodic aluminum oxide (AAO) template has been proven to be one of the most popular methodologies to fabricate nanostructure arrays with perfectly manipulatable morphologies, owing to the advantages in low cost, easy fabrication, and high manipulation. 13−16 Accordingly, a large diversity of nanostructure arrays, including nanopores, nanodots, nanorods, nanotubes, and even the nanocones, has been realized. 13−22 In consideration of the attractive property of TiO 2 that has been broadly utilized in dye-sensitized solar cells, 23,24 photocatalysts, 25 and photonics, 26,27 in this paper, we focus on the optical band gap modulation of TiO 2 na...