Extending the functionality of anodicaily oxidized valve metal multilayers by tailoring their morphology, nanostructure and properties

摘要

Transition metal (including valve metal) oxides possess various useful properties such as supercon?ductivity, magneto-resistance, catalytic activity, electrochromism, gas sensitivity and many others. Nanostructuring of transition metal oxides is an active and competitive area of research because it offers unprecedented opportunities for the development of advanced materials and microsystem components at the nanoscale with improved performances or substantially enhanced properties. In recent years an original approach has been developed to grow 2-D arrays of metal-oxide nanostructures (hillocks, rods, cones etc.), anchored to dielectric or conducting substrates, via smart anodizing of a valve metal bilayer comprising a thin layer of aluminum superimposed on a layer of different metal [1 ]. In this approach, the upper Al layer is first converted into its nanoporous oxide, this being sequentially followed by an?odic oxidation of the underlying metal through the alumina barrier layer. In this situation the alumina layer is used not just as a porous mask but as part of the complex electrochemical system, included in a series of electrochemical and solid-state reactions Up to now, periodic arrays of dot-like oxide nanos?tructures have been successfully grown from the Al/Ta (aluminum-above-tantalum), Al/Ti and Al/W couple and already received several useful applications [2,3]. However, among the above metals, the Ta underlayer has proved to be the only metal that allows reanodizing to substantially higher voltages with its oxide self-directed along the alumina nanopores [2]. These tantala nanocolumns fill the pores length?ening proportionally to the applied voltage until dielectric breakdown interrupts their growth (1 kV at maximum). Thus, further progress with this technique is associated, first, with anodizing and analyti?cally examining more valve-metal couples, which include Nb, Zr, Mo and some of their binary alloys, and second, with applying high voltage reanodizing to possibly stretch up the oxides in the pores, striv?ing to engineer their surface morphologies and discover new functionalities.