Theory of molecular-scale transport in nanojunctions

It remains a major challenge to identify and exploit room-temperature quantum interference (QI) effects in charge transport through molecular systems at the angstromscale, although extensive and intensive research has been carried out by experimentalists and theoreticians. In this thesis, I investigate charge transport properties, thermoelectricity and solvent influences at the molecular scale by meansof density functional theory (DFT) and equilibrium Green’sfunction theory. The charge transport properties of halide perovskite quantumdots( QDs) are first investigated. It is demonstrated that room-temperature quantum interference (QI) is observed based on the fact that the conductance decays exponentially with the increasing distance between the twogold-goldtips and also there is a distinct conductance “jump”at the end of the sliding process. These findings open the way to new conceptual designs for perovskite-based molecular devices by exploiting QI effects. As for the property of exponentially attenuating electrical conductance with the length, molecular wires with low decaying factor � (�~�]^_ ) are of significance to realize themolecular electronics. Here we measured and calculated the single-molecule conductances of a series of cumulenes and cumulene analogues, where the number of consecutive C=C bonds in the core is n = 1,2,3 and 5. The [n] cumulenes with n =3 and 5 have almost the same conductance,and they are both more conductive than the alkene (n = 1). The lack of length dependence in the conductance of [3] cumulene and [5] cumulene is attributed to the strong decrease in HOMO-LUMO gap with increasing length. The conductance of the allene (n = 2) is much lower, due to its twisted geometry. Therefore, I suggest the cumulene series as a good candidate for high conductance molecular wires. Additionally, and also significantly, seeking materials for harvesting energy is an urgent task facing the serious global energy shortage. Herein, I investigated the electrical and thermoelectrical properties of glycine chains with and without cysteine terminal groups. The electrical conductance of (Gly)g, (Gly)gCys and Cys(Gly)gCys molecules (where Gly, Cys represent glycine and cysteine and n=1-3) was found to decay exponentially with length � as �]^_ (β~1.0 Å]k) .Furthermore, it is shown the (Gly)kC�� and Cys(Gly)kCys systems show good thermoelectrical performance ( high Seebeck coefficients ~ 0.2mV/K). With the contributions of both electrons and phonons taken into consideration, a high figure of merit ZT=0.8 is obtained for (Gly)kCys at room temperature, suggesting that peptide-based SAM junctions are promising candidates for thermoelectric energy harvesting. In the investigations of charge transports above, it is realized that the functionalities, reproducibility, stability of molecular junctions not only depend on the functional-molecular cores, but also on other effects such as connecting anchors and solvents. Therefore the conductances of single-molecule junctions with different anchoring groups in a variety of solvent environments are studied. It is found that the conductance of single-molecule junctions can be manipulated by nearly an order of magnitude by varying the solvent, and the solvent gating effect depends significantly on the choice of anchor group. My work suggests that the solvent-molecule interaction can provide significant solvent gating effect for the weakly coupled (-SMe anchor) single-molecule junction

Structure‐Independent Conductance of Thiophene‐Based Single‐Stacking Junctions

The experimental investigation of intermolecular charge transport in pi-conjugated materials is challenging. Herein, we describe the investigation of charge transport through intermolecular and intramolecular paths in single-molecule and single-stacking thiophene junctions by the mechanically controllable break junction (MCBJ) technique. We found that the ability for intermolecular charge transport through different single-stacking junctions was approximately independent of the molecular structure, which contrasts with the strong length dependence of conductance in single-molecule junctions with the same building blocks, and the dominant charge-transport path of molecules with two anchors transited from an intramolecular to an intermolecular path when the degree of conjugation increased. An increase in conjugation further led to higher binding probability owing to the variation in binding energies, as supported by DFT calculations

Phase-Coherent Charge Transport through a Porphyrin Nanoribbon

Quantum interference in nano-electronic devices could lead to reduced-energy computing and efficient thermoelectric energy harvesting. When devices are shrunk down to the molecular level it is still unclear to what extent electron transmission is phase coherent, as molecules usually act as scattering centres, without the possibility of showing particle-wave duality. Here we show electron transmission remains phase coherent in molecular porphyrin nanoribbons, synthesized with perfectly defined geometry, connected to graphene electrodes. The device acts as a graphene Fabry-P\'erot interferometer, allowing direct probing of the transport mechanisms throughout several regimes, including the Kondo one. Electrostatic gating allows measurement of the molecular conductance in multiple molecular oxidation states, demonstrating a thousand-fold increase of the current by interference, and unravelling molecular and graphene transport pathways. These results demonstrate a platform for the use of interferometric effects in single-molecule junctions, opening up new avenues for studying quantum coherence in molecular electronic and spintronic devices.Comment: 14 pages, 3 figure

Penaeid shrimp genome provides insights into benthic adaptation and frequent molting

Crustacea, the subphylum of Arthropoda which dominates the aquatic environment, is of major importance in ecology and fisheries. Here we report the genome sequence of the Pacific white shrimp Litopenaeus vannamei, covering similar to 1.66 Gb (scaffold N50 605.56 Kb) with 25,596 protein-coding genes and a high proportion of simple sequence repeats (>23.93%). The expansion of genes related to vision and locomotion is probably central to its benthic adaptation. Frequent molting of the shrimp may be explained by an intensified ecdysone signal pathway through gene expansion and positive selection. As an important aquaculture organism, L. vannamei has been subjected to high selection pressure during the past 30 years of breeding, and this has had a considerable impact on its genome. Decoding the L. vannamei genome not only provides an insight into the genetic underpinnings of specific biological processes, but also provides valuable information for enhancing crustacean aquaculture

Cross-plane transport in a single-molecule two-dimensional van der Waals heterojunction

Two-dimensional van der Waals heterostructures (2D-vdWHs) stacked from atomically thick 2D materials are predicted to be a diverse class of electronic materials with unique electronic properties. These properties can be further tuned by sandwiching monolayers of planar organic molecules between 2D materials to form molecular 2D-vdW heterojunctions (M-2D-vdWHs), in which electricity flows in a cross-plane way from one 2D layer to the other via a single molecular layer. Using a newly developed cross-plane break junction (XPBJ) technique, combined with density functional theory calculations, we show that M-2D-vdWHs can be created, and that cross-plane charge transport can be tuned by incorporating guest molecules. More importantly, the M-2D-vdWHs exhibit distinct cross-plane charge transport signatures, which differ from those of molecules undergoing in-plane charge transport

Single-Atom Control of Single-Molecule van der Waals Junctions with Semimetallic Transition Metal Dichalcogenide Electrodes

Electrodes play an essential role in controlling electrode-molecule coupling. However, conventional metal electrodes require linkers to anchor the molecule. Van der Waals interaction offers a versatile strategy to connect the electrode and molecule without anchor groups. Except for graphene, the potential of other materials as electrodes to fabricate van der Waals molecular junctions remains unexplored. Herein, we utilize semimetallic transition metal dichalcogenides (TMDCs) 1T'-WTe as electrodes to fabricate WTe /metalated tetraphenylporphyrin (M-TPP)/WTe junctions via van der Waals interaction. Compared with chemically bonded Au/M-TPP/Au junctions, the conductance of these M-TPP van der Waals molecular junctions is enhanced by ∼736%. More importantly, WTe /M-TPP/WTe junctions exhibit the tunable conductance from 10 to 10 (1.15 orders of magnitude) via single-atom control, recording the widest tunable range of conductance for M-TPP molecular junctions. Our work demonstrates the potential of two-dimensional TMDCs for constructing highly tunable and conductive molecular devices

Solvent-molecule interaction induced gating of charge transport through single-molecule junctions

To explore solvent gating of single-molecule electrical conductance due to solvent-molecule interactions, charge transport through single-molecule junctions with different anchoring groups in various solvent environments was measured by using the mechanically controllable break junction technique. We found that the conductance of single-molecule junctions can be tuned by nearly an order of magnitude by varying the polarity of solvent. Furthermore, gating efficiency due to solvent–molecule interactions was found to be dependent on the choice of the anchor group. Theoretical calculations revealed that the polar solvent shifted the molecular-orbital energies, based on the coupling strength of the anchor groups. For weakly coupled molecular junctions, the polar solvent–molecule interaction was observed to reduce the energy gap between the molecular orbital and the Fermi level of the electrode and shifted the molecular orbitals. This resulted in a more significant gating effect than that of the strongly coupled molecules. This study suggested that solvent–molecule interaction can significantly affect the charge transport through single-molecule junctions

Charge transport through single-molecule bilayer-graphene junctions with atomic thickness

The van der Waals interactions (vdW) between the π-conjugated molecules offer new opportunities for fabricating the heterojunction-based devices and investigating charge transport in heterojunctions with atomic thickness. In this work, we fabricate sandwiched single-molecule bilayer-graphene junctions via vdW interactions and characterize their electrical transport properties by employing the cross-plane break junction (XPBJ) technique. Experimental results show that the cross-plane charge transport through single-molecule junctions is determined by the size and layer number of molecular graphene in these junctions. Density functional theory (DFT) calculations reveal that the charge transport through the molecular graphene in these molecular junctions is sensitive to the angles between the graphene flake and peripheral mesityl groups, and those rotated groups can be used to tune the electrical conductance. This study provides new insight into cross-plane charge transport in atomically thin junctions and highlights the role of through-space interactions in vdW heterojunctions at the molecular scale