Dipole-Modulated Charge Transport through PNP-Type Single-Molecule Junctions.

The PNP structure realized by energy band engineering is widely used in various electronic and optoelectronic devices. In this work, we succeed in constructing a PNP-type single-molecule junction and explore the intrinsic characteristics of the PNP structure at the single-molecule level. A back-to-back azulene molecule is designed with opposite ∼1.7 D dipole moments to create PNP-type single-molecule junctions. In combination with theoretical and experimental studies, it is found that the intrinsic dipole can effectively adjust single-molecule charge transport and the corresponding potential barriers. This energy band control and charge transport regulation at the single-molecule level improve deep understanding of molecular charge transport mechanisms and provide important insights into the development of high-performance functional molecular nanocircuits toward practical applications.

Accurate Single-Molecule Kinetic Isotope Effects.

An accurate single-molecule kinetic isotope effect (sm-KIE) was applied to circumvent the inherent limitation of conventional ensemble KIE by using graphene-molecule-graphene single-molecule junctions. In situ monitoring of the single-molecule reaction trajectories in real time with high temporal resolution has the capability to characterize the deeper information brought by KIE. The C-O bond cleavage and the C-C bond formation of the transition state (TS) were observed in the Claisen rearrangement through the secondary kinetic isotope effect, demonstrating the high detection sensitivity and accuracy of this method. More interestingly, this sm-KIE can be used to determine TS structures under different electric fields, revealing the multidimensional regulation of the TS. The detection and manipulation of the TS provide a broad perspective to understand and optimize chemical reactions and biomimetic progress.

Effect of H2O Adsorption on Negative Differential Conductance Behavior of Single Junction.

Large negative differential conductance (NDC) at lower bias regime is a very desirable functional property for single molecular device. Due to the non-conjugated segment separating two conjugated branches, the single thiolated arylethynylene molecule with 9,10-dihydroanthracene core (denoted as TADHA) presents excellent NDC behavior in lower bias regime. Based on the ab initio calculation and non-equilibrium Green's function formalism, the NDC behavior of TADHA molecular device and the H2O-molecule-adsorption effects are studied systematically. The numerical results show that the NDC behavior of TADHA molecular junction originates from the Stark effect of the applied bias which splits the degeneration of the highest occupied molecular orbital (HOMO) and HOMO-1. The H2O molecule adsorbed on the terminal sulphur atom strongly suppresses the conductance of TADHA molecular device and destroys the NDC behavior in the lower bias regime. Single or separated H2O molecules adsorbed on the backbone of TADHA molecule can depress the energy levels of molecular orbitals, but have little effects on the NDC behavior of the TADHA molecular junction. Aggregate of several H2O molecules adsorbed on one branch of TADHA molecule can dramatically enhance the conductance and NDC behavior of the molecular junction, and result in rectifier behavior.