gold-based nanostructures, namely solid AuAg alloy nanowires, nanoporous gold nanowires, and gold nanocones. The synthesis of these structures is based on ion- track technology and electrodeposition methods that were investigated and optimized.
The synthesis of AuAg nanowires with tailored composition and diameter was achieved by electrodeposition of AuAg alloy in cylindrical nanochannels of etched ion-track templates. The analysis of the electrochemical reduction of Au and Ag yields high current efficiencies. The potentiostatic electrodeposition at - 1.1 V vs. Ag/AgCl occurs close to the diffusion-limited regime. The composition of AuAg nanowires is thus only determined by the electrolyte composition. The deposition was shown to be independent of the channel size. All nanowires fabricated at such a potential consist of long single-crystalline sections. Au40Ag60
and Au60Ag40 nanowires were synthesized and subsequently analyzed by means
of high-resolution transmission electron microscopy coupled with energy dispersive X-ray spectroscopy with high spatial resolution showed clear evidence of surface segregation. Au60Ag40 nanowires were found to have a Au-rich surface
layer whereas Au40Ag60 nanowires rather form a Ag-rich surface layer. This
explains why Au-rich nanowires cannot be dealloyed and thus remain solid. Contrarily, dealloying of Ag-rich wires yields highly porous nanostructures made of a network of interconnected ligaments with small domains of residual silver. Crystalline defects seem neither to influence nor to be affected by dealloying. Our approach allows the fabrication of continuous porous gold nanowires with diameters as small as 85 nm. This size limit appears because nanowires with smaller initial diameter dealloy into single ligaments and tend to fragment. The fabrication of sensors based on the synthesis of such porous gold nanowires requires electric contacts and device integration. As a first step towards contacting
template-released nanowires with diameters as small as 45 nm, laser lithography based contacts were developed. In parallel, nanowires were placed on pre- fabricated chips and trenches. Successful dealloying was demonstrated for as- contacted nanowires. Preliminary characterization of electrical and thermal transport properties of solid and porous cylindrical nanowires are currently under test at the Hochschule RheinMain.
The synthesis of gold nanocones was achieved by electrodeposition from tip-to- base in conical-shaped nanochannels of etched ion-track membranes. Asymmetrical etching of ion tracks yields nanochannels with slightly tunable cone angles. The electrodeposition processes using a commercial Au electrolyte is shown to occur close to the diffusion limit. The reduction process of Au inside single conical nanochannels from the tip to the base provides clear identification of the three main electrodeposition steps: initiation, cone growth, and cap formation. This information permits controlled deposition of the cones and enables high efficiency of the electrodeposition as well as minimization of parasitic structures such as tubes or incomplete cone bases. By growing the cones from tip to base, high reproducibility of the synthesis process is achieved. Vertically aligned freestanding gold nanocones of homogeneous lengths and with sharp tips as small as 50 nm are successfully produced. Their mechanical stability is excellent, the risk of breaking cones is low, and the contact to the substrate is outstanding. Compared to nanocone arrays, the fabrication of a single nanocone is more challenging due to rather large scattering of the breakthrough time when etching membranes with an individual ion track. In addition, the unpredictable closing of single channels with very small opening is problematic. After the dissolution of the membrane, single nanocones are also more vulnerable to external harm and thus often suffer from bending of distortion.
The field emission properties of our nanocone arrays are promising. Voltage scans allow mapping of patterned nanocones arrays. For low cone densities, even single nanocone emitters could be identified. Field emission measurements from nanocone arrays follow the Fowler-Nordheim model, but large discrepancies of
enhancement factors and breakdown currents are observed. Optical and scanning electron micrographs reveal inhomogeneous contributions of the cones to the emission current as well as contributions of nanocones positioned far away (distances higher than 10 times the cone length). This effect is partly attributed to the non-planarity of the samples. Although the new growth technique developed here provides highly improved substrate contact, some of the nanocones are molten or even completely evaporated after field emission. For better integration in operating systems, the synthesis of arrays of Spindt-type cold emitter based on the electrodeposition of gold nanocones is proposed. Preliminary tests demonstrate proof of concept.
The application of solid and porous nanowires as well as nanocones requires a detailed physical and chemical characterization in particular, their electrical transport properties. The results achieved in this thesis will serve as a starting base for further studies.
The synthesis conditions developed in this work can be adapted to the fabrication of porous gold nanowire networks. It has already been demonstrated that interconnected nanowires provide sufficient mechanical stability to manipulate the 3D systems.78,79,240,242 Adding nanoporosity to such 3D-networks would
enable the fabrication of highly porous systems with bimodal porosity. Such structures are of interest for catalysis where the macroscopic porosity (tuned by the spacing of the nanowires) yields efficient fluidic flow through the device whereas the nanoporosity (obtained by dealloying) catalyzes chemical reactions. The influence of surface segregation of AuAg nanowires evidenced in this work can be further explored regarding mechanical properties and chemical activity of porous nanostructures. Deeper insight is necessary to better understand environmental factors on surface segregation and how the surface layer affects chemical, optical, or mechanical properties of nanowires. Also the time dependence of the segregation process is of great interest concerning the applicability of ultrathin nanowires in devices. Surface segregation of AuAg
nanowires also creates new opportunities. Considering the possibility of complete segregation of alloy nanowires by aging might enable the production of core-shell nanowires. Likewise, ultrathin nanowires or nanotubes could be produced by selective removal of one of the components of core-shell nanowires.
Our field emission studies of nanocone arrays shows that fundamental understanding of the process can only be achieved by analyzing the emission of single nanocones. The fabrication of single nanocones is challenging and needs to be optimized, in particular, regarding the etching process of single ion tracks and the manipulation of single nanochannels and nanocones samples.
The tip-to-base cone synthesis process demonstrated in this work with gold may also be applied to a large variety of materials like metals, semiconductors, and conductive polymers. Thus, arrays of vertically aligned freestanding nanocones with tunable geometry and densities open interesting new perspectives for many other applications.