posted on 2016-06-13, 20:19authored byTimothy
A. Su, Haixing Li, Rebekka
S. Klausen, Jonathan R. Widawsky, Arunabh Batra, Michael L. Steigerwald, Latha Venkataraman, Colin Nuckolls
While the single-molecule conductance
properties of π-conjugated
and σ-conjugated systems have been well-studied, little is known
regarding the conductance properties of mixed σ–π
backbone wires and the factors that control their transport properties.
Here we utilize a scanning tunneling microscope-based break-junction
technique to study a series of molecular wires with π–σ–π
backbone structures, where the π-moiety is an electrode-binding
thioanisole ring and the σ-moiety is a triatomic α–β–α
chain composed of C, Si, or Ge atoms. We find that the sequence and
composition of group 14 atoms in the α–β–α
chain dictates whether electronic communication between the aryl rings
is enhanced or suppressed. Placing heavy atoms at the α-position
decreases conductance, whereas placing them at the β-position
increases conductance: for example, the C–Ge–C sequence
is over 20 times more conductive than the Ge–C–Ge sequence.
Density functional theory calculations reveal that these conductance
trends arise from periodic trends (i.e., atomic size, polarizability,
and electronegativity) that differ from C to Si to Ge. The periodic
trends that control molecular conductance here are the same ones that
give rise to the α and β silicon effects from physical
organic chemistry. These findings outline a new molecular design concept
for tuning conductance in single-molecule electrical devices.