Electronic Material Properties - und Geowissenschaften ...

intensity I [a.u.]
intensity I [a.u.]
a)
111
200
200
220
220
311
30 40 50 60 70 80 90 100 110 120
b)
111
texture
311
texture
30 40 50 60 70 80 90 100 110 120
222
222
diffraction angle 2θ [°]
400
400
331 420
331 420
Fig. 2: X-ray diffractograms of Au nanowire ensembles of a) texture under ac deposition;
b) texture under dc deposition conditions; the dashed vertical lines with triangles atop
represent the intensity distribution of a polycrystalline and untextured sample.
To explain these experimental results, the broken bond model [4] is applied to the
anisotropy of single-crystalline or textured wires in combination with surface-energy
minimization growth tendencies: The energy of an atomic surface is considered to be
proportional to its broken-bond density. The constitution of the circular top and base
planes, as well as the cylindrical mantle surfaces depend on of the wire and can be
determined from crystallographic projections [5]. The evolution of the surface energy E
of textured fcc nanowires with increasing aspect ratio h/d, is shown in Fig. 3.
Up to an aspect ratio of 1 a texture represents the configuration of lowest energy.
At higher aspect ratios the orientation becomes the most favourable growth
direction. In TEM investigations for such oriented Au nanowires with strong textured large
elongated grains were observed. Under ac conditions, lattice sites of high reactivity, i.e.,
lattice defects or surfaces with high number of broken bonds are preferentially dissolved
during the anodic cycles of the alternating polarity. As {110} surfaces exhibit a high
number of broken bonds these lattice sites, and thus the texture, successively
disappear under ac conditions. In absence of {110} surfaces, a texture represents
the orientation of lowest energy for aspect ratios larger than~5 (see Fig. 3). In TEM
investigations a distinct texture in direction was only observed at the upper part of
the nanowires, i.e. the texture starts to develop at an advanced state of the
electrodeposition. X-Ray line profile analysis can be used to show that surface defects are
preferably dissolved compared to line defects (dislocations) und ac conditions. The
texture under dc and the texture under ac deposition conditions become stronger at
smaller diameters, as the transition aspect ratios are reached earlier.
Field emission properties of nanowire ensembles can be investigated in terms of field
emission scanning microscopy. Emitter densities of 4.4×10 4 cm -2 and 2.4×10 4 cm -2 were
determined for bare and Au-coated Cu nanowire ensembles of 10 6 cm -2 , respectively.
Emission of only a few percent of the nanowires was ascribed to mutual electrostatic
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