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Building electronic devices out of single molecules has been the ultimate goal of downscaling electric circuits. Understanding charge transport through single-molecule junctions is central to achieving this goal. To gain deeper insights into charge transport through single molecules, this dissertation centers on detailed experimental modulation and control of charge transport through single-molecule junctions using modified scanning probe microscope break-junction (SPM-BJ) techniques. First, I explored the effect of molecule-electrode contact interfaces. Using force-conductance cross-correlation analysis, I mapped out the correlation between conductance and force of modulated Au-octanedithiol-Au junctions measured with CAFM-BJ. The investigation of the conductance change during junction elongation showed a unique contact tunneling barrier of octanedithiol, which was interpreted by a newly developed contact barrier model. A systematic control of anchoring groups of benzene-based molecular junctions showed that current rectification occurred whenever asymmetric anchoring groups were introduced, which is mainly due to asymmetry in potential drop across the contacts. Second, I studied the impact of DNA's structural change on its conductance. The conductance of poly d(GC)4 DNA duplex was found to decrease by two orders of magnitude during a B- to Z-form structural transition, which is mainly attributed to the breaking of Ï0-Ï0 stacking between adjacent base pairs caused by the transition. Using stretch-hold mode STM-BJ technique, the structural transition was successfully monitored solely based on conductance measurements. Then, I attempted to modify the structure of DNA for functional I-V feature. A DNA-based molecular rectifier was for the first time constructed by site-specific intercalation of coralyne molecules into a custom-designed DNA duplex. Measured I-V curves of the resulting DNA-coralyne complex showed strong rectification with a rectification ratio of 15 at 1.1V. Based on NEGF-DFT calculations, this rectification is mainly caused by asymmetric coupling of the HOMO-1 level to the electrodes when an external bias is applied, an unprecedented rectification mechanism. Finally, Fermi level pinning of charge transfer resonances was investigated in junctions composed of terthiophene containing molecular wires. Taken together, these results not only provide new understanding of charge transport through molecules, they also opened new route for building functional molecular electronic devices.
Molecular electronics has attracted increasing interest in the past decades. Constructing metal/molecules/metal junctions is a basic step towards the investigation of molecular electronics. We have witnessed significant development in both experiment and theory in molecular junctions. This thesis focuses mainly on the study of charge transport through molecular junctions. Conducting polymers and copper filaments were electrochemically deposited with a scanning electrochemical microscope (SECM) configuration between a tip and a substrate electrode. In doing so, we have developed a new way to fabricate atomic contact and molecular junctions, and we have explored the possibility to control, protect and switch these systems.Firstly, SECM, where two microelectrodes are located face-to-face separated by a micrometric gap, has been successfully used for the fabrication of redox-gated conducting polymers junctions, such as PEDOT and PBT. Highly stable and reversible redox-gated nano-junctions were obtained with conductance in the 10-7-10-8 S range in their conducting states. These results, associated with the wire-like growth of the polymer, suggest that the conductance of the entire junction in the conductive state is governed by less than 20 to 100 oligomers.Secondly, to obtain the nano-junctions in a controllable way, a break junction strategy combined with the SECM set up is adopted. A nano-junction could be acquired by pulling the tip away from its initial position. And conductance traces showed that PEDOT junctions can be broken step by step before complete breakdown. Similarly as STM-BJ conductance steps were observed on a PEDOT molecular junction before break down by using SECM-BJ. SECM break junction technique proved to be an efficient way of molecular junction fabrication studies, especially for redox gated polymer molecular junctions. Moreover, a self-terminated strategy is found to be another way to obtain nano-junctions. An external resistance connected to the electrode plays an important role in controlling the size of conducting polymer junctions.PFTQ and PFETQ molecular junctions exhibit well-defined ambipolar transport properties. However, an unbalanced charge transport properties in n- and p- channel for these two polymer junctions was observed when the junctions are in the fiber device scale. In contrast, when molecular junction changes into nano-junction, a balanced n- and p-channel transport property is acquired. We propose that such effect is due to charge transport mechanism changing from diffusive (ohm's law) to ballistic (quantum theory) when the junction size is reduced from fiber devices to nanodevices.High stable Au NPs/ITO electrodes exhibit a well localized surface plasmon (LSP) behavior. These plasmonic substrates have been successfully used to trigger switching of molecular junctions under light irradiation, demonstrating that surface plasmon resonance can induce electrochemical reduction. Such conductance reduction can be attributed to the hot electrons plasmonically generated from gold nanoparticles trapped into the PEDOT junction, resulting in PEDOT being reduced and changed to an insulating state.Finally, copper metallic nanowires were generated using an electrochemical self-terminated method based on SECM configuration. The presence of a few atoms that control the electron transport highlights the formation of metallic nanowires between the asymmetric electrodes. Furthermore, a similar study was performed on mesoporous silica film on ITO used as a substrate electrode. The mesoporous silica films have vertically aligned channels with a diameter of about 3 nm and a thickness of 115 nm, which play a crucial role in protecting the copper filament.
Quantum tunnelling is one of the strangest phenomena in chemistry, where we see the wave nature of atoms acting in “impossible” ways. By letting molecules pass through the kinetic barrier instead of over it, this effect can lead to chemical reactions even close to the absolute zero, to atypical spectroscopic observations, to bizarre selectivity, or to colossal isotopic effects. Quantum mechanical tunnelling observations might be infrequent in chemistry, but it permeates through all its disciplines producing remarkable chemical outcomes. For that reason, the 21st century has seen a great increase in theoretical and experimental findings involving molecular tunnelling effects, as well as in novel techniques that permit their accurate predictions and analysis. Including experimental, computational and theoretical chapters, from the physical and organic to the biochemistry fields, from the applied to the academic arenas, this new book provides a broad and conceptual perspective on tunnelling reactions and how to study them. Quantum Tunnelling in Molecules is the obligatory stop for both the specialist and those new to this world.
Unique in its scope, this book comprehensively combines various synthesis strategies with applications for nanogap electrodes. Clearly divided into four parts, the monograph begins with an introduction to molecular electronics and electron transport in molecular junctions, before moving on to a whole section devoted to synthesis and characterization. The third part looks at applications with single molecules or self-assembled monolayers, and the whole is rounded off with a section on interesting phenomena observed using molecular-based devices.
Studying charge transport through single molecules is of great importance for unravelling charge transport mechanisms, investigating fundamentals of chemistry, and developing functional building blocks in molecular electronics. First, a study of the thermoelectric effect in single DNA molecules is reported. By varying the molecular length and sequence, the charge transport in DNA was tuned to either a hopping- or tunneling-dominated regimes. In the hopping regime, the thermoelectric effect is small and insensitive to the molecular length. Meanwhile, in the tunneling regime, the thermoelectric effect is large and sensitive to the length. These findings indicate that by varying its sequence and length, the thermoelectric effect in DNA can be controlled. The experimental results are then described in terms of hopping and tunneling charge transport models. Then, I showed that the electron transfer reaction of a single ferrocene molecule can be controlled with a mechanical force. I monitor the redox state of the molecule from its characteristic conductance, detect the switching events of the molecule from reduced to oxidized states with the force, and determine a negative shift of ~34 mV in the redox potential under force. The theoretical modeling is in good agreement with the observations, and reveals the role of the coupling between the electronic states and structure of the molecule. Finally, conclusions and perspectives were discussed to point out the implications of the above works and future studies that can be performed based on the findings.
Here, we use and develop first-principles methods based on density functional theory (DFT) and beyond to understand and predict charge transport phenomena in the novel class of nanostructured devices: molecular junctions. Molecular junctions, individual molecules contacted to two metallic leads, which can be systematically altered by modifying the chemistry of each component, serve as test beds for the study of transport at the nanoscale. To date, various experimental methods have been designed to reliably assemble and mea- sure transport properties of molecular junctions. Furthermore, theoretical methods built on DFT designed to yield quantitative agreement with these experiments for certain classes of molecular junctions have been developed. In order to gain insight into a broader range of molecular junctions and environmental effects associated with the surrounding solution, this dissertation will employ, explore and extend first-principles DFT calculations coupled with approximate self-energy corrections known to yield quantitative agreement with experiments for certain classes of molecular junctions. To start we examine molecular junctions in which the molecule is strongly hybridized with the leads: a challenging limit for the existing methodology. Using a physically motivated tight-binding model, we find that the experimental trends observed for such molecules can be explained by the presence of a so-called "gateway" state associated with the chemical bond that bridges the molecule and the lead. We discuss the ingredients of a self-energy corrected DFT based approach to quantitatively predict conductance in the presence of these hybridization effects. We also develop and apply an approach to account for the surrounding environment on the conductance, which has been predominantly ignored in past transport calculations due to computational complexity. Many experiments are performed in a solution of non-conducting molecules; far from benign, this solution is known to impact the measured conductance by as much as a factor of two. Here, we show that the dominant effect of the solution stems from nearby molecules binding to the lead surface surrounding the junction and altering the local electrostatics. This effect operates in much the same way adsorbates alter the work function of a surface. We develop a framework which implicitly includes the surrounding molecules through an electrostatic-based lattice model with parameters from DFT calculations, reducing the computational complexity of this problem while retaining predictive power. Our approach for computing environmental effects on charge transport in such junctions will pave the way for a better understanding of the physics of nanoscale devices, which are known to be highly sensitive to their surroundings.