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What is “New Physical Insight”? Suggestions for Transport Studies
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insight. A measurement of the conductance of a single molecule, or a calculation of the same, or even the two together, is not generally sufficient. Without a demonstration of WHY the transport properties are as they are, and more usefully an explanation of a chemical trend, the scope of the work is limited. As long as experimental and theoretical uncertainty precludes quantitative agreement as the gold standard, the precise value of a physical observable is not generally transferable. In this context, the “why” of transport properties is often more challenging than the “what”. When we probe deeper to understand the why, we end up back where we started. We must face the challenge that physical chemistry, and chemistry more generally, does not have a long tradition in these environments or observables, and thus there is an absence of chemical concepts or structure−property relationships to answer the question of why. So we must build these too as we journey on and enjoy the challenge of taking molecules into unexplored territory.
ransport studies have taken molecules, and thereby physical chemistry, into unexplored territory. The physical observables alone take us beyond traditional chemistry. Electrical and thermal conductance, thermopower, and shotnoise, to name a few, while well-known in physics, are not properties generally associated with molecules. When transport is measured or calculated, it is essential that the molecule (or monolayer or thin film) be contacted to conducting leads. At the limit of a single molecule, this represents a very unusual environment. No longer do the physical properties correspond to that of the molecule in solution, the gas phase, or the solid state. Now the molecule is physically or chemically bound in a heterogeneous environment, the details of which at the atomic scale are generally unknown (and possibly unknowable). As the system is expanded to a monolayer or thin film, the molecules may recover an environment akin to the solid state, but this may or may not correspond to the environment of the molecular crystal. In the context of the methods available today, and their inherent uncertainties, quantitative agreement between theory and experiment is not the gold standard it is in other fields. Essentially, it is generally impossible to know we have “the right answer for the right reasons”. Many of the challenges associated with reproducibly for both transport theory (across different theoretical implementations) and experiment (across different test beds in different laboratories), have been overcome and the field has generally converged to qualitative agreement on chemical trends. This standard is quite different from what one might expect in a more mature field but is the climate in which we ask the question: so what then is significant new physical insight? The answer, I would suggest, is 2-fold. Either (1) methods that offer a significant improvement over the state-of-the-art or (2) improved qualitative understanding of chemical trends in transport properties, with a particular focus on why the molecules/materials/junctions behave as they do. With the status of the field today, there is certainly scope for new theoretical and experimental methods to emerge that offer significant advantages and new avenues for progress. Techniques or analyses that improve reproducibility, provide atomic-scale information about the transport junction, provide information about the transport mechanism, or spectroscopically characterize the molecules within the junction all have the possibility to advance the field significantly. However, it is clear from progress to date that any one development is unlikely to be a “giant killer”. As with life, we do not get “something for nothing”, and different approaches to transport have their respective strengths and weaknesses. The ability of any new method to change the scope of the questions we can ask and provide complementary information to existing approaches will, in part, provide a measure of its significance. While the standard of qualitative agreement on chemical trends might seem like a low bar for the field, in some ways it increases the challenge of demonstrating significant physical © 2017 American Chemical Society
Gemma C. Solomon,* Senior Editor
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Nano-Science Center and Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark
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[email protected]. ORCID
Gemma C. Solomon: 0000-0002-2018-1529 Notes
The author declares no competing financial interest.
Published: July 13, 2017 14381
DOI: 10.1021/acs.jpcc.7b06249 J. Phys. Chem. C 2017, 121, 14381−14381
