One of the main challenges of Molecular electronics is to understand and
control charge transfer through a reproducible single molecule contact
between electrodes. Most investigations of electron transport through
molecules have been performed in “blind” junction experiments, where the
molecular conformation and contact geometry cannot be probed. Therefore
large gaps in our knowledge remains since in molecular electronics the
atomic-scale structure of the entire junction including the leads is
important for its conductance properties.
Our goal is to study electrical transport through well-defined molecular
junction on semiconductor surfaces. Formation of molecular junctions using
organic molecules on semiconductor surfaces might lead to interesting
phenomena such as negative differential resistance [1].
In this contribution, we investigate formation of molecular junction
consisting of a single acetophenone molecule deposited on Si (100) surface
in upright position by means of simultaneous AFM/STM measurements and DFT
calculations. We used a modified UHV VT STM/AFM Omicron machine allowing
simultaneous acquisition of the current and forces with atomic resolution
using a tuning fork sensor [2]. At first we preformed simultaneous force
spectroscopy and current spectroscopy to identify and to determine the
position of the molecule. Here in addition we have an idea now how far the
tip is located from the molecule and enable us to have precise control of
contact formation. All this can be done only with the guide provided by DFT
calculations that gives us better insight into interaction mechanism between
probe and molecule. Afterwards, we record Kelvin-force spectroscopy, in
which the bias voltage is swept and both frequency shift and the current
through the junction are monitored as function of tiop-sample distance. In
addition, we performed DFT-based simulations, which allows us to get more
insight into ongoing processes along tip approach. Our approach combining
AFM/STM/KPFM and theoretical simulations provides complex information about
the charge states and charge transport through a single acetophenone
molecule on Si(100) surface.
[1] T. Rakshit et al. Nanoletters 4, 1803−1807 (2004).
[2] Z. Majzik et al, Beilstein J. Of Nanotech. 3, 249 (2012).
[3] A. R. Rocha et al. Phys. Rev. B 73, 085414 (2006).
[4] N. Fournier et al Phys. Rev. B 84, 035435 (2011).
