Signatures of Room-Temperature Quantum Interference in Molecular Junctions

Abstract

Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS: During the past decade or so, research groups around the globe have sought to answer the question: "How does electricity flow through single molecules?" In seeking the answer to this question, a series of joint theory and experimental studies have demonstrated that electrons passing through singlemolecule junctions exhibit exquisite quantum interference (QI) effects, which have no classical analogues in conventional circuits. These signatures of QI appear even at room temperature and can be described by simple quantum circuit rules and a rather intuitive magic ratio theory. The latter describes the effect of varying the connectivity of electrodes to a molecular core and how electrical conductance can be controlled by the addition of heteroatoms to molecular cores. The former describes how individual moieties contribute to the overall conductance of a molecule and how the overall conductance can change when the connectivities between different moieties are varied. Related circuit rules have been derived and demonstrated, which describe the effects of connectivity on Seebeck coefficients of organic molecules. This simplicity arises because when a molecule is placed between two electrodes, charge transfer between the molecule and electrodes causes the molecular energy levels to adjust, such that the Fermi energy (E F ) of the electrodes lies within the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital. Consequently, when electrons of energy E F pass through a molecule, their phase is protected and transport takes place via phase-coherent tunneling. Remarkably, these effects have been scaled up to self-assembled monolayers of molecules, thereby creating two-dimensional materials, whose room temperature transport properties are controlled by QI. This leads to new molecular design strategies for increasing the on/off conductance ratio of molecular switches and to improving the performance of organic thermoelectric materials. In particular, destructive quantum interference has been shown to improve the Seebeck coefficient of organic molecules and increase their on/off ratio under the influence of electrochemical gating. The aim of this Account is to introduce the novice reader to these signatures of QI in molecules, many of which have been identified in joint studies involving our theory group in Lancaster University and experimental group in Bern University.

Manipulating Quantum Interference between σ and π Orbitals in Single-Molecule Junctions via Chemical Substitution and Environmental Control

“…Figure 2 demonstrates the experimentally determined conductance signatures of pyz-based molecules with linker groups (pyridine N, or SMe) which allow selective bridging of Au electrodes to either the para (1,4) or the meta (1,3) positions. 20,23,36 We observe that pyz, with linkers only at the para position, does not produce clearly identifiable molecular conductance signatures, as seen in the individual experiment traces (Figure 2 inset) and in the histogram (Figure 2) in green as predicted by our analysis above.…”

Metal Complex Molecular Junctions as Thermoelectric Devices

“…Quantum interference (QI), a quantum-mechanical phenomenon that interfere de Broglie waves when propagating through discrete molecular orbitals and create a combined wave, is one of the most important concepts in MJ. [48,49] Notably, QI can be constructive and destructive (CQI and DQI, respectively, see Figure 8c bottom). Given their antiresonance features, which lead to steep slope of the transmission function, DQI-exhibiting molecules have attracted substantial attention as thermoelectric materials.…”

“…Quantum interference (QI), a quantum‐mechanical phenomenon that interfere de Broglie waves when propagating through discrete molecular orbitals and create a combined wave, is one of the most important concepts in MJ [48,49] . Notably, QI can be constructive and destructive (CQI and DQI, respectively, see Figure 8c bottom).…”

Fine‐Tuning the Molecular Design for High‐Performance Molecular Diodes Based on Pyridyl Isomers

“…One kind of the smallest structural difference in chemistry is the isomer, and their different behaviors in charge transport are commonly unexpected [6] . For instance, as for the cis‐trans isomerism, W. Hong [7] et al .…”

“…One kind of the smallest structural difference in chemistry is the isomer, and their different behaviors in charge transport are commonly unexpected. [6] For instance, as for the cis-trans isomerism, W. Hong [7] et al employed C=N isomerization of azobenzene derivatives with distinct molecular conductance states in response to thermal-photo stimulation of dynamic acyl hydrazone bond, to prepare photoswitched molecular devices with on/off ratio � 10. For tautomer, as N. Tao [8] et al reported, the conductance of two tetraphenylbenzene isomers solely changing anchor points varied by over 10 times, which was attributed to the different injection point of electrons from the Au electrode.…”