Abstract:
Scanning tunneling microscopy measurements of tunneling through molecules adsorbed on a surface have been
simulated using a standard empirical model based upon the Wentzel-Kramer-Brillouin method applied to tunneling
through a barrier. The Gaussian noise inherent in these experiments has been added to the model data using a Monte
Carlo technique. By generating multiple sets of current-voltage curves and fitting these to the model we have evaluated
how reliably barrier height can be determined as a function of noise level. The results suggest that for constant
percentage standard deviation in the noise greater than 5% the barrier height cannot be determined reliably. At this level,
the standard deviation in the estimate of the barrier height is about 10%. Weighted fits give more reliable estimates of
the barrier height. If the height of the tip above the molecule is known, so that the fit is only a single parameter the
barrier height can be determined reliably even at percentage noise levels as high as 20%. However, in this case
unweighted fits must be used otherwise the estimated value deviates by up to 15% from the true value. Data with
constant absolute noise give similar results. The effects of experimental resolution have been evaluated in a similar
manner and are shown to have a significant influence on the reliability. At a resolution of about 0.1% of full scale the
standard deviation in the estimate of barrier height is only about 2% but increases rapidly to 10% for a resolution of
about 1%.
