Version 2 2018-02-26, 19:49Version 2 2018-02-26, 19:49
Version 1 2018-02-22, 20:07Version 1 2018-02-22, 20:07
journal contribution
posted on 2018-02-07, 00:00authored byRobert C. Bruce, Lin You, Anja Förster, Sujitra Pookpanratana, Olivia Pomerenk, Han Ju Lee, Maria D. Marquez, Rashid Ghanbaripour, Oussama Zenasni, T. Randall Lee, Christina A. Hacker
Surface
dipoles are a powerful tool in interfacial modification
for improving device output via energy level matching. Fluorinated
alkanethiols show a strong promise for these applications as they
can generate large and tunable dipoles based on fluorine location
and chain length. Furthermore, these chains can be designed to possess
fluorocarbons solely along the backbone, enabling an “embedded”
configuration that generates a significant dipole effect from the
fluorines while maintaining surface chemistry to prevent deleterious
side effects from altered surface interactions. However, fluorine
substitution can modify other molecular electronic properties, and
it is important to consider the transport properties of these interfacial
modifiers so that knowledge can be used to tailor the optimal device
performance. In this paper, we report the transport properties of
self-assembled monolayers derived from a series of fluorinated alkanethiols,
both with and without the embedded dipole structure. Photoelectron
spectroscopy and Kelvin probe force microscopy show significant work
function modification from all fluorine-containing molecules compared
to purely hydrocarbon thiols. However, although embedded fluorocarbons
generate a smaller electrostatic effect than terminal fluorocarbons,
they yield higher tunneling currents across Au/monolayer/eutectic
gallium–indium junctions compared to both terminal fluorocarbon
and purely hydrocarbon alkanethiols. Computational studies show that
the location of the fluorine constituents modifies not only dipoles
and energy levels but also molecular orbitals, enabling the presence
of delocalized lowest unoccupied molecular orbital levels within the
alkanethiol backbone and, thereby, the appearance of larger tunneling
currents compared to other alkanethiols. Ultimately, we show that
fluorinated alkanethiols and the embedded dipole architecture are
both powerful tools, but they must be thoroughly analyzed for proper
utilization in a device setting.