Ultrasmooth Gold Surfaces Prepared by Chemical Mechanical
Polishing for Applications in Nanoscience
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Abstract
For
over 20 years, template stripping has been the best method
for preparing ultrasmooth metal surfaces for studies of nanostructures.
However, the organic adhesives used in the template stripping method
are incompatible with many solvents, limiting the conditions that
may subsequently be used to prepare samples; in addition, the film
areas that can be reliably prepared are typically limited to ∼1
cm<sup>2</sup>. In this article, we present chemical–mechanical
polishing (CMP) as an adhesive-free, scalable method of preparing
ultrasmooth gold surfaces. In this process, a gold film is first deposited
by e-beam evaporation onto a 76-mm-diameter silicon wafer. The CMP
process removes ∼4 nm of gold from the tops of the grains comprising
the gold film to produce an ultrasmooth gold surface supported on
the silicon wafer. We measured root-mean-square (RMS) roughness values
using atomic force microscopy of 12 randomly sampled 1 μm ×
1 μm areas on the surface of the wafer and repeated the process
on 5 different CMP wafers. The average RMS roughness was 3.8 ±
0.5 Å, which is comparable to measured values for template-stripped
gold (3.7 ± 0.5 Å). We also compared the use of CMP and
template-stripped gold as bottom electrical contacts in molecular
electronic junctions formed from <i>n</i>-alkanethiolate
self-assembled monolayers as a sensitive test bed to detect differences
in the topography of the gold surfaces. We demonstrate that these
substrates produce statistically indistinguishable values for the
tunneling decay coefficient β, which is highly sensitive to
the gold surface topography