Carmen M. Lilley

SURFACE EFFECT ON THE ELASTIC BEHAVIOR OF STATIC BENDING NANOWIRES

The surface effect from surface stress and surface elasticity on the elastic behavior of nanowires in static bending is incorporated into Euler-Bernoulli beam theory via the Young-Laplace equation. Explicit solutions are presented to study the dependence of the surface effect on the overall Youngs modulus of nanowires for three different boundary conditions: cantilever, simply supported, and fixed-fixed. The solutions indicate that the cantilever nanowires behave as softer materials when deflected while the other structures behave like stiffer materials as the nanowire cross-sectional size decreases for positive surface stresses. These solutions agree with size dependent nanowire overall Youngs moduli observed from static bending tests by other researchers. This study also discusses possible reasons for variations of nanowire overall Youngs moduli observed.

Surface stress effect on bending resonance of nanowires with different boundary conditions

The influence of surface stress on the resonance frequencies of bending nanowires was studied by incorporating the generalized Young–Laplace equation into Euler–Bernoulli beam theory. Theoretical solutions are presented for three different boundary conditions. The overall Young’s modulus was used to study the surface stress influenced mechanical behavior of bending nanowires and a comparison was made for the overall Young’s modulus calculated from nanowires in resonance and static bending. It was found that the overall Young’s modulus can be simply related to a nondimensional surface effect factor via empirical formulae.

Surface and size effects on the electrical properties of Cu nanowires

Copper nanowires were patterned with e-beam lithography and fabricated with a copper film deposited by e-beam evaporation. Various electrical properties of these nanowires (including resistivity, temperature coefficient of resistance, and failure current density) were characterized. It was experimentally found that surface and size have apparent effects on the electrical properties. Smaller values for the temperature coefficient of resistance and higher failure current density were found for Cu nanowires with decreasing wire width. The experimental finding of width dependent failure current density also agrees with finding for theoretical heat transfer of the nanowire and substrate system as calculated with the finite element method.

An in situ investigation of electromigration in Cu nanowires.

Electromigration in copper (Cu) nanowires deposited by electron beam evaporation has been investigated using both resistance measurement and the in situ scanning electron microscopy technique. During electromigration, voids formed at the cathode end while hillocks (or extrusions) grew close to the anode end. The failure lifetimes were measured for various applied current densities and the mean temperature in the wire was estimated. Electromigration activation energies of 1.06 eV and 0.94 eV were found for the wire widths of 90 nm and 141 nm, respectively. These results suggest that the mass transport of Cu during electromigration mainly occurs along the wire surfaces. Further investigations of the Auger electron spectrum show that both Cu atoms and the surface contaminants of carbon and oxygen migrate from cathode to anode under the electrical stressing.

Surface scattering effect on the electrical resistivity of single crystalline silver nanowires self-assembled on vicinal Si (001)

Fundamental questions as to the nature of electron surface scattering in nanoscale materials remain unanswered. In order to isolate the effects of surface scattering from grain boundary scattering, single crystalline trapezoidal silver (Ag) nanowires were self-assembled on vicinal silicon substrate. The well established kinetic theory to model electron surface scattering effects on the electrical resistivity of nanowires was extended to include trapezoidal geometries. The experimentally measured electrical resistivity for Ag nanowires was found to fit the theoretical resistivity for the case of electrons diffusely scattering from the nanowire surface.

Surface contamination effects on resistance of gold nanowires

Gold nanowires were patterned with e-beam lithography and fabricated with a gold film deposited by e-beam evaporation. The resistances of these wires were measured and found to be nonlinear with respect to surface area/volume. With x-ray photoelectron spectroscopy analysis, carbon and oxygen contaminants in the forms of C, C–O–C, and CO were found adsorbed on the gold surface. This contamination adsorbed on the surface may lead to increased resistance of nanowires.

Resonant frequency analysis of Timoshenko nanowires with surface stress for different boundary conditions

The influence of both surface and shear effects on the resonant frequency of nanowires (NWs) was studied by incorporating the Young-Laplace equation with the Timoshenko beam theory. Face-centered-cubic metal NWs were studied. A dimensional analysis of the resonant frequencies for fixed-fixed gold (100) NWs were compared to molecular dynamic simulations. Silver NWs with diameters from 10 nm–500 nm were modeled as a cantilever, simply supported and fixed-fixed system for aspect ratios from 2.5–20 to identify the shear, surface, and size effects on the resonant frequencies. The shear effect was found to have a larger significance than surface effects when the aspect ratios were small (i.e., <5) regardless of size for the diameters modeled. Finally, as the aspect ratio grows, the surface effect becomes significant for the smaller diameter NWs.

Electrical Failure Analysis of Au Nanowires

Au nanowires were patterned with electron beam (e-beam) lithography and fabricated with an Au film deposited by e-beam evaporation. Two failure analyses were performed: failure current density and electromigration. It was experimentally found that the failure current density increases for the smaller width wire. Size and surface effects on the failure current density were explored. Also, in situ electromigration studies on Au nanowires were performed to characterize the activation energy of Au nanowires with a SEM.

Comparison between bulk and nanoscale copper-silicide: Experimental studies on the crystallography, chemical, and oxidation of copper-silicide nanowires on Si(001)

Self-assembled copper-silicide (Cu-Si) nanowires were prepared by the evaporation of Cu onto Si(001) under high vacuum conditions. The Cu-Si nanowires were studied in situ by scanning electron microscopy. Crystallographic, structural, and chemical properties of the nanowires were investigated by transmission electron microscopy at cross-sections of these nanowires fabricated by focused ion beam. In addition, chemical of nanowires were also studied by scanning transmission electron microscopy. The morphology of Cu-Si nanowires due to oxidation at ambient conditions was investigated as well. Experimental results obtained from the Cu-Si nanowires were compared to bulk data reported previously by other authors.

Self assembled Cu nanowires on vicinal Si(001) by the E-beam evaporation method

This paper describes an experimental procedure for growing self assembled Cu nanowires on a 4° miscut Si(001) substrate with and without the native oxide layer by the e-beam evaporation method. The physical characteristics of the Cu nanowires were observed using an in situ and ex situ scanning electron microscope (SEM). The results are compared with Ag nanowires that were fabricated with the same procedure.