Nanoprocessing of metastable nm-period multilayers

Abstract

Nanometer-period nickel-carbon multilayers were used as a medium for the fabrication of nanostructures by gap voltage manipulations in a scanning tunneling microscope. The written metallic structures were stable over at least several weeks. No traces of tip material were found in the processed areas. Two well-distinguished hillock-like nanostructure types were observed depending on the tip-sample separation, polarity and interaction time. Relatively slow local annealing under positive sample potential without a direct tip-sample contact resulted in the formation of nanostructures about 20 nm wide and a few nm high. Rapid melting followed by metal melt extrusion was observed if the tip contacted the sample during the nanostructure formation. These metal-like structures were tens of nm high and had a good electronic contrast to the initial carbon-coated surface. The formation of nanostructures was strongly dependent on the tip condition. Possible mechanisms of nanostructure formation are discussed.The temporal stability of metallic nanostructures is one of the most serious limitations of possible applications, for example for ultrahigh-density data storage or nanoelectronic applications. One had to perform the exciting experiments on formation of patterned atomic arrays or "quantum coralls" [1, 2] under liquid-helium temperatures in order to prevent atoms from surface migration for a reasonably long time. The works [3-7] on formation of golden nanostructures in ultrahigh vacuum (UHV) showed diffusional dissolution of written features on a time scale of minutes or hours.A possible solution for the stability problem was proposed in [8,9]. It was shown that metastable nm-period metal-carbon multilayers are a promising medium for stable nanoprocessing. In contrast to the bulk crystalline materials, multilayer thin films have the advantage that their physical properties can be continuously adjusted, for example by varying the material composition, the thickness of the layers, or the number of layer periods. It was recognized that because of internal stresses and forced intermixing of finite miscible materials at the interfaces the multilayers frequently occur in some frozen-in non-equilibrium metastable state [10]. This metastable state is kinetically stable under normal conditions, but upon application of sufficient activation energy the multilayers exhibit well-defined morphological or phase transitions with corresponding alteration of physical properties. The initial metastability opens unique possibilities to fabricate stable nanostructures through local annealing, since the written nanostructures are in a more favorable energetic state than the initial multilayer.In the last decade a considerable interest has grown in the nanostructure fabrication by various scanning probe techniques. To date the scanning tunneling microscope (STM) is the only instrument that can provide a nanometer-sized beam of low energy electrons (0-20 eV). These energies are comparable to the bonding energies of atoms in solids, but are still ...