Single-Molecule Conductance Behavior of Molecular Bundles

Controlling the orientation of complex molecules in molecular junctions is crucial to their development into functional devices. To date, this has been achieved through the use of multipodal compounds (i.e., containing more than two anchoring groups), resulting in the formation of tri/tetrapodal compounds. While such compounds have greatly improved orientation control, this comes at the cost of lower surface coverage. In this study, we examine an alternative approach for generating multimodal compounds by binding multiple independent molecular wires together through metal coordination to form a molecular bundle. This was achieved by coordinating iron(II) and cobalt(II) to 5,5′-bis(methylthio)-2,2′-bipyridine (L1) and (methylenebis(4,1-phenylene))bis(1-(5-(methylthio)pyridin-2-yl)methanimine) (L2) to give two monometallic complexes, Fe-1 and Co-1, and two bimetallic helicates, Fe-2 and Co-2. Using XPS, all of the complexes were shown to bind to a gold surface in a fac fashion through three thiomethyl groups. Using single-molecule conductance and DFT calculations, each of the ligands was shown to conduct as an independent wire with no impact from the rest of the complex. These results suggest that this is a useful approach for controlling the geometry of junction formation without altering the conductance behavior of the individual molecular wires

Intermolecular coupling enhanced thermopower in single- molecule diketopyrrolopyrrole junctions

Sorting out organic molecules with high thermopower is essential for understanding molecular thermoelectrics. The intermolecular coupling offers a unique chance to enhance the thermopower by tuning the bandgap structure of molecular devices, but the investigation of intermolecular coupling in bulk materials remains challenging. Herein, we investigated the thermopower of diketopyrrolopyrrole (DPP) cored single-molecule junctions with different coupling strengths by varying the packing density of the self-assembled monolayers (SAM) using a customized scanning tunneling microscope break junction (STM-BJ) technique. We found that the thermopower of DPP molecules could be enhanced up to one order of magnitude with increasing packing density, suggesting that the thermopower increases with larger neighboring intermolecular interactions. The combined density functional theory (DFT) calculations revealed that the closely-packed configuration brings stronger intermolecular coupling and then reduces the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, leading to an enhanced thermopower. Our findings offer a new strategy for developing organic thermoelectric devices with high thermopower

Quantum Theory of Electronic and Thermal Transport through Nanostructures

It is essential for nano- and molecular-scale applications to explore and understand the electron and phonon transport characteristics of molecular junctions consisting of a scattering region such as a molecule connected to metallic electrodes. This thesis presents a series of studies into the electronic and thermoelectric properties of molecular junctions using theoretical methods described in chapters 2 and 3. Chapter 2 presents an introduction to the density functional theory (DFT). It is followed by an outline of transport theory in chapter 3, based on a Green’s function formalism. Recent studies of molecular thermoelectrics help to understand how atomic-scale structural modifications in junctions can affect the thermopower of molecular devices. This is illustrated in chapter 4, where I investigate the connectivity dependence on the thermoelectric properties of a series of thiophenediketopyrrolopyrrole (DPP) derivative molecules. For example. I find that molecules with connectivitites leading to destructive quantum interference (DQI) show significant conductance variations upon ring rotation. This DQI also leads to enhanced Seebeck coefficients, which can reach 500−700 μV/K. For the molecule with constructive quantum interference (CQI), I find that after including the contribution to the thermal conductance from phonons, the full figure of merit (ZT) for the CQI molecules could reach 1.5 at room temperature. Based on the DPP molecules, Chapter 5 presents a collaborative study with experimentalists at Xiamen University, China, of the effect of branching alkyl chains (isopentane, 3-methylheptane, and 9-methylnonadecane) on the geometrical changes such as pi-stacked distances and backbone dihedral angles. It is demonstrated that as the alkyl chain becomes longer the electrical conductance decreases due to an increase in the torsional angles between the aromatic rings. The relationship between the conductance and the torsion angle follows approximately T(E, )∝ cos6 . This indicates that the insulating side chain could be used to control single-molecule conductance, which is of significance for the design of future organic devices

Signatures of Topological States in Conjugated Macrocycles

Single-molecule electrical junctions possess a molecular core connected to source and drain electrodes via anchor groups, which feed and extract electricity from specific atoms within the core. As the distance between electrodes increases, the electrical conductance typically decreases, which is a feature shared by classical Ohmic conductors. Here we analyze the electrical conductance of cycloparaphenylene (CPP) macrocycles and demonstrate that they can exhibit a highly nonclassical increase in their electrical conductance as the distance between electrodes increases. We demonstrate that this is due to the topological nature of the de Broglie wave created by electrons injected into the macrocycle from the source. Although such topological states do not exist in isolated macrocycles, they are created when the molecule is in contact with the source. They are predicted to be a generic feature of conjugated macrocycles and open a new avenue to implementing highly nonclassical transport behavior in molecular junctions

Conformation and Quantum-Interference-Enhanced Thermoelectric Properties of Diphenyl Diketopyrrolopyrrole Derivatives

Manipulating the connectivity of external electrodes to central rings of carbon-based molecules in single molecule junctions is an effective route to tune their thermoelectrical properties. Here we investigate the connectivity dependence of the thermoelectric properties of a series of thiophene-diketopyrrolopyrrole (DPP) derivative molecules using density functional theory and tight-binding modeling, combined with quantum transport theory. We find a significant dependence of electrical conductance on the connectivity of the two thiophene rings attached to the DPP core. Interestingly, for connectivities corresponding to constructive quantum interference (CQI), different isomers obtained by rotating the thiophene rings possess the same electrical conductance while those corresponding to destructive quantum interference (DQI) show huge conductance variations upon ring rotation. Furthermore, we find that DQI connectivity leads to enhanced Seebeck coefficients, which can reach 500–700 μV/K. After including the contribution to the thermal conductance from phonons, the full figure of merit (ZT) for the CQI molecules could reach 1.5 at room temperature and it would further increase to 2 when temperature elevates to 400 K. Finally, we demonstrate that doping with tetracyanoquinodimethane can change the sign of the Seebeck coefficients by forming a charge-transfer system with the DPP

Surface Directed Growth of a Stable Free Radical Polymer Layer

Molecular radicals such as nitric oxide (NO) play a role in numerous important biological processes. NO, however, is inherently unstable, and there is considerable interest in stable radicals with analogous behaviour, such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), and their potential as biologically active surface coatings. Here we show that it is possible to grow stable TEMPO monolayers with an ordered arrangement and specific orientation of the nitroxide group. A combination of high-resolution atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations reveal that strong dipole-dipole interactions between neighbouring TEMPO molecules determine their orientation, resulting in an anti-parallel arrangement with upwards and downwards facing nitroxide groups. Surface coating is made possible using plasma polymerisation, a surface molecular engineering technique suitable for growing functional coatings of highly stable organic molecules. Typically, plasma polymerisation is considered a surface-independent process, with the plasma power and pressure dominating the deposited film\u27s properties. We therefore test this assumption and its impact on TEMPO layers by studying initial stage growth on multiple material surfaces. We show that whilst plasma polymer growth creates ordered layers on gold and graphite surfaces, there is substantial substrate-dependence with less ordered growth on other materials up to a film thickness of 30 nm, suggesting variation in molecular packing and retention. Beyond that thickness, films convergence to a flat, uniform, surface-agnostic structure. These findings establish the utility of plasma polymerisation as a method for ordered growth and demonstrate the ability to direct the orientation of TEMPO molecules and NO free radicals within a thin film

Single-Molecule Charge Transport Modulation Induced by Steric Effects of Side Alkyl Chains

Experimental investigation of the side chain effects on intramolecular charge transport in π-conjugated molecules is essential, but remains challenging. Herein, the dependence of intra-molecular conductance on the nature of branching alkyl chains is investigated through a combination of the scanning tunneling microscope break junction (STM-BJ) technique and density functional theory. Three thiophene-flanked diketopyrrolopyrrole (DPP) derivatives with different branching alkyl chains (isopentane, 3-methylheptane , and 9-methylnonadecane) are used with phenylthiomethyl groups as the anchoring groups. The results of single-molecule conductance measurements show that as the alkyl chain becomes longer, the torsional angles between the aromatic rings increase due to steric crowding, and therefore, the molecular conductance of DPP decreases due to reduction in conjugation. Both theoretical simulations and 1 H NMR spectra demonstrate that the planarity of the DPPs is directly reduced after introducing longer branching alkyl chains, which leads to the reduced conductance. This work indicates that the effect of insulating side chain on single-molecule conductance cannot be neglected, which should be considered for the design of future organic semiconducting materials

Single-Molecule Conductance Behavior of Molecular Bundles

Controlling the orientation of complex molecules in molecular junctions is crucial to their development into functional devices. To date, this has been achieved through the use of multipodal compounds (i.e., containing more than two anchoring groups), resulting in the formation of tri/tetrapodal compounds. While such compounds have greatly improved orientation control, this comes at the cost of lower surface coverage. In this study, we examine an alternative approach for generating multimodal compounds by binding multiple independent molecular wires together through metal coordination to form a molecular bundle. This was achieved by coordinating iron­(II) and cobalt­(II) to 5,5′-bis­(methylthio)-2,2′-bipyridine (L 1 ) and (methylenebis­(4,1-phenylene))­bis­(1-(5-(methylthio)­pyridin-2-yl)­methanimine) (L 2 ) to give two monometallic complexes, Fe-1 and Co-1, and two bimetallic helicates, Fe-2 and Co-2. Using XPS, all of the complexes were shown to bind to a gold surface in a fac fashion through three thiomethyl groups. Using single-molecule conductance and DFT calculations, each of the ligands was shown to conduct as an independent wire with no impact from the rest of the complex. These results suggest that this is a useful approach for controlling the geometry of junction formation without altering the conductance behavior of the individual molecular wires

Single-Molecule Conductance Behavior of Molecular Bundles

Controlling the orientation of complex molecules in molecular junctions is crucial to their development into functional devices. To date, this has been achieved through the use of multipodal compounds (i.e., containing more than two anchoring groups), resulting in the formation of tri/tetrapodal compounds. While such compounds have greatly improved orientation control, this comes at the cost of lower surface coverage. In this study, we examine an alternative approach for generating multimodal compounds by binding multiple independent molecular wires together through metal coordination to form a molecular bundle. This was achieved by coordinating iron(II) and cobalt(II) to 5,5′-bis(methylthio)-2,2′-bipyridine (L1) and (methylenebis(4,1-phenylene))bis(1-(5-(methylthio)pyridin-2-yl)methanimine) (L2) to give two monometallic complexes, Fe-1 and Co-1, and two bimetallic helicates, Fe-2 and Co-2. Using XPS, all of the complexes were shown to bind to a gold surface in a fac fashion through three thiomethyl groups. Using single-molecule conductance and DFT calculations, each of the ligands was shown to conduct as an independent wire with no impact from the rest of the complex. These results suggest that this is a useful approach for controlling the geometry of junction formation without altering the conductance behavior of the individual molecular wires