Improved
understanding of charge-transport in single molecules
is essential for harnessing the potential of molecules, e.g., as circuit
components at the ultimate size limit. However, interpretation and
analysis of the large, stochastic data sets produced by most quantum
transport experiments remain an ongoing challenge to discovering much-needed
structure–property relationships. Here, we introduce segment clustering, a novel unsupervised hypothesis generation
tool for investigating single molecule break junction distance–conductance
traces. In contrast to previous machine learning approaches for single
molecule data, segment clustering identifies groupings of similar pieces of traces instead of entire traces. This offers a new and advantageous perspective into data set structure
because it facilitates the identification of meaningful local trace
behaviors that may otherwise be obscured by random fluctuations over
longer distance scales. We illustrate the power and broad applicability
of this approach with two case studies that address common challenges
encountered in single molecule studies: First, segment clustering
is used to extract primary molecular features from a varying background
to increase the precision and robustness of conductance measurements,
enabling small changes in
conductance in response to molecular design to be identified with
confidence. Second, segment clustering is applied to a known data
mixture to qualitatively separate distinct molecular features in a
rigorous and unbiased manner. These examples demonstrate two powerful
ways in which segment clustering can aid in the development of structure–property
relationships in molecular quantum transport, an outstanding challenge
in the field of molecular electronics.
The rational design of single molecule electrical components requires a deep and predictive understanding of structure-function relationships. Here we explore the relationship between chemical substituents and the conductance of metal-single molecule-metal junctions, using functionalized oligophenylenevinylenes as a model system. Using a combination of mechanically controlled break-junction experiments and various levels of theory including non-equilibrium Green's functions, we demonstrate that the connection between gas-phase molecular electronic structure and in-junction molecular conductance is complicated by the involvement of multiple mutually correlated and opposing effects that contribute to energy level alignment in the junction. We propose that these opposing correlations represent powerful new "design principles," because their physical origins make them broadly applicable, and they are capable of predicting the direction and relative magnitude of observed conductance trends. In particular, we show that they are consistent with the observed conductance variability not just within our own experimental results, but also within disparate molecular series reported in literature and, crucially, with the trend in variability across these molecular series, which previous simple models fail to explain. The design principles introduced here can therefore aid in both screening and suggesting novel design strategies for maximizing conductance tunability in single-molecule systems.
Structure–function
relationships constitute an important
tool to investigate the fundamental principles of molecular electronics.
Most commonly, this involves identifying a potentially important molecular
structural element, followed by designing and synthesizing a set of
related organic molecules, and finally interpretation of their experimental
and/or computational quantum transport properties in the light of
this structural element. Though this has been extremely powerful in
many instances, we demonstrate here the common need for more nuanced
relationships even for relatively simple structures, using both experimental
and computational results for a series of stilbene derivatives as
a case study. In particular, we show that the presence of multiple
competing and subtle structural factors can combine in unexpected
ways to control quantum transport in these molecules. Our results
clarify the reasons for previous widely varying and often contradictory
reports on charge transport in stilbene derivatives and highlight
the need for refined multidimensional structure–property relationships
in single-molecule electronics.
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