TiO₂ nanotip arrays: anodic fabrication and field-emission properties

In contrast to the main-stream strategy of growing convex nanostructures upward from the substrates and using them as cold electron sources, it is illustrated in this article that growing concave nanostructures downward into substrates also results in configurations suitable for field emission. Well-ordered TiO₂ nanotube arrays were developed on the titanium foils in two-step anodizations. Simultaneously, arrays of sharp nanotips, which resembled the Spindt emitter arrays in appearance, also manifested themselves on the outmost surface of the foils. These nanotips were actually the remainder of the titanium foil surfaces that survived dissolution during anodization. Annealing transformed the amorphous TiO₂ nanotips into anatase crystals and further to rutile. Despite the lack of an overall large aspect ratio, the sharpness of these nanotips guaranteed sufficiently strong electric fields for electron extraction. As a result, field emission was readily obtained from the TiO₂ nanotip arrays, either before or after annealing. Photoelectron spectroscopy of the samples demonstrated that the majority of the emitted electrons came from local states in the band gap. Annealing at an appropriate temperature increased these local states and improved the field-emission capability of the samples.