Thomas Cornelius

Morphological evolution of au nanowires controlled by rayleigh instability

A sound knowledge and understanding of the thermal stability of nanowires is a prerequisite for the reliable implementation of nanowire-based devices. We investigate the morphology of Au nanowires annealed isothermally at different temperatures. During the processes, triggered by heating, the wires undergo various configurational changes to finally break up into chains of nanospheres at much lower than bulk melting temperatures due to capillary or so-called Rayleigh instability. The role of three parameters, namely, wire diameter, temperature, and annealing time, on the final morphology is investigated. Both the average sphere diameter and the mean spacing between adjacent spheres are larger than the values predicted for materials with isotropic surface energy. Possible reasons are discussed in the paper.

Resonances of individual metal nanowires in the infrared

With infrared spectroscopic microscopy using synchrotron light, the authors studied resonant light scattering from single metal nanowires with diameters in the 100nm range and with lengths of a few microns. The Au and Cu nanowires were electrochemically grown in polycarbonate etched ion-track membranes and transferred on infrared-transparent substrates. Significant antennalike plasmon resonances were observed in good agreement with exact light-scattering calculations. The resonances depend not only on length and diameter but also on the dielectric surrounding of the nanowire. The observed maximum extinction at resonance corresponds to an electromagnetic far-field enhancement by a factor of about 5.

The experimental investigation of thermal conductivity and the Wiedemann-Franz law for single metallic nanowires.

A new method for the measurement of thermal conductivity of electrically conducting single nanowires is presented. First experimental investigations are focused on the thermal conductivity of metallic Pt nanowires with a diameter of (typically) 100 nm and a length of 10 microm. Thermal conductivity data are compared with measurements of electrical conductivity in order to test the Wiedemann-Franz law for metallic nanowires. Compared to the bulk values at room temperature, electrical and thermal conductivities of the nanowire are decreased by a factor of 2.5 and 3.4, respectively. Consequently, the Lorenz number L = lambda/sigmaT = 1.82 x 10(-8) V(2) K(-2) of the nanowire is smaller than the bulk Lorenz number L(bulk) = (pi(2)/3)(k/e)(2) = 2.44 x 10(-8) V(2) K(-2) of metals. Furthermore, the temperature coefficient beta of electrical resistivity is also reduced compared to the bulk value. These decreases of lambda, sigma and beta can be attributed to size effects, mainly caused by grain boundary scattering of electrons.

Influence of crystallinity on the Rayleigh instability of gold nanowires

The influence of the crystalline structure of nanowires on their thermal instability has been systematically investigated. Both poly- and single-crystalline (SC) cylindrical nanowires with diameters 87 and 132 nm transform into chains of spheres during annealing at 600–700 °C. SC nanowires oriented along the 1 1 0 direction are found to be more stable, i.e. longer annealing times are needed for their complete transformation into sphere chains. Sphere size and spacing between adjacent spheres formed after decay are controlled by the crystallinity of the wires and both are larger in the case of SC nanowires.

Quantum size effects manifest in infrared spectra of single bismuth nanowires

Infrared transmission spectroscopy measurements on single bismuth nanowires of various diameters d are presented. The spectra show a strong absorption whose onset is blueshifted proportionally to 1∕d2. We ascribe the absorption to interband transitions. The blueshift results from quantum size effects since they lead to the d-dependent splitting of the energy bands and to a respective shift of energy gaps.

Electromagnetic Nanowire Resonances for Field-Enhanced Spectroscopy

Electromagnetic resonances of metal nanowires lead to strong enhancement of the near field of the particle. Antenna-like resonances that give the biggest enhancement are explained theoretically. The preparation of high-quality wires is introduced. Spectroscopic results for resonance curves are shown and discussed with respect to field enhancement. Surface-enhanced Raman scattering and surfaceenhanced infrared absorption are introduced focusing on nanowire-assisted configurations, and examples of these enhanced spectroscopies for molecules on resonant nanowires are shown.