Reversible Switching between Destructive and Constructive Quantum Interference Using Atomically Precise Chemical Gating of Single-Molecule Junctions.

Quantum interference (QI) plays an imperative role in the operation of molecular devices within the phase-coherent length, and it is vital to harness the patterns of QI, i.e., constructive and destructive interference.

Stable-radicals increase the conductance and Seebeck coefficient of graphene nanoconstrictions.

Nanoscale thermoelectricity is an attractive target technology, because it can convert ambient heat into electricity for powering embedded devices in the internet of things. We demonstrate that the thermoelectric performance of graphene nanoconstrictions can be significantly enhanced by the presence of stable radical adsorbates, because radical molecules adsorbed on the graphene nanoconstrictions create singly-occupied orbitals in the vicinity of Fermi energy. This in turn leads to sharp features in their transmission functions close to Fermi energy, which increases the electrical conductance and Seebeck coefficient of the nanoconstrictions.

Bias-Driven Conductance Increase with Length in Porphyrin Tapes.

A key goal in molecular electronics has been to find molecules that facilitate efficient charge transport over long distances. Normally, molecular wires become less conductive with increasing length. Here, we report a series of fused porphyrin oligomers for which the conductance increases substantially with length by >10-fold at a bias of 0.

Detecting Mechanochemical Atropisomerization within an STM Break Junction.

We have employed the scanning tunneling microscope break-junction technique to investigate the single-molecule conductance of a family of 5,15-diaryl porphyrins bearing thioacetyl (SAc) or methylsulfide (SMe) binding groups at the ortho position of the phenyl rings (S2 compounds). These ortho substituents lead to two atropisomers, cis and trans, for each compound, which do not interconvert in solution under ambient conditions; even at high temperatures, isomerization takes several hours (half-life 15 h at 140 °C for SAc in CClD). All the S2 compounds exhibit two conductance groups, and comparison with a monothiolated (S1) compound shows the higher group arises from a direct Au-porphyrin interaction.

High cross-plane thermoelectric performance of metallo-porphyrin molecular junctions.

We investigated the thermoelectric properties of flat-stacked 5,15-diphenylporphyrins containing divalent metal ions Ni, Co, Cu or Zn, which are strongly coordinated with the nitrogens of pyridyl coated gold electrodes. Changing metal atom has little effect on the thermal conductance due to the phonons. The room-temperature Seebeck coefficients of these junctions are rather high, ranging from 90 μV K for Cu, Ni and Zn-porphyrins to -16 μV K for Co-porphyrin.

High-performance thermoelectricity in edge-over-edge zinc-porphyrin molecular wires.

If high efficiency organic thermoelectric materials could be identified, then these would open the way to a range of energy harvesting technologies and Peltier coolers using flexible and transparent thin-film materials. We have compared the thermoelectric properties of three zinc porphyrin (ZnP) dimers and a ZnP monomer and found that the "edge-over-edge" dimer formed from stacked ZnP rings possesses a high electrical conductance, negligible phonon thermal conductance and a high Seebeck coefficient of the order of 300 μV K. These combine to yield a predicted room-temperature figure of merit of ZT ≈ 4, which is the highest room-temperature ZT ever reported for a single organic molecule.

Tuning the electrical conductance of metalloporphyrin supramolecular wires.

In contrast with conventional single-molecule junctions, in which the current flows parallel to the long axis or plane of a molecule, we investigate the transport properties of M(II)-5,15-diphenylporphyrin (M-DPP) single-molecule junctions (M=Co, Ni, Cu, or Zn divalent metal ions), in which the current flows perpendicular to the plane of the porphyrin. Novel STM-based conductance measurements combined with quantum transport calculations demonstrate that current-perpendicular-to-the-plane (CPP) junctions have three-orders-of-magnitude higher electrical conductances than their current-in-plane (CIP) counterparts, ranging from 2.10 G for Ni-DPP up to 8.

Terahertz emission in ladder plus Y-configurations in double quantum dot structure.

The ladder plus Y-double quantum dot (QD) structure has been proposed to improve the second-order nonlinear susceptibility (SONS) under applied electric field. For this purpose, the wetting layer (WL) and QD inhomogeneity contribution in SONS has been considered. In addition, the structure size effect, energy level separation, momentum matrix elements, and pump detuning have been examined, and show that the SONS is increased by using the WL, a low momentum matrix element of WL-QD transition in comparison with interdot transition while QD inhomogeneity reduces SONS to half.