Electron Transfer Reactivity Patterns at Chemically Modified Electrodes: Fundamentals and Application to the Optimization of Redox Recycling Amplification Systems

Electroanalytical chemistry is often utilized in chemical analysis and Fundamental studies. Important advances have been made in these areas since the advent of chemically modified electrodes: the coating of an electrode with a chemical film in order to impart desirable, and ideally, predictable properties. These procedures enable the exploitation of unique reactivity patterns. This dissertation presents studies that investigate novel reaction mechanisms at self-assembled monolayers on gold. In particular, a unique electrochemical current amplification scheme is detailed that relies on a selective electrode to enable a reactivity pattern that results in regeneration of the analyte (redox recycling). This regenerating reaction can occur up to 250 times for each analyte molecule, leading to a notable enhancement in the observed current. The requirements of electrode selectivity and the resulting amplification and detection limit improvements are described with respect to the heterogeneous and homogeneous electron transfer rates that characterize the system. These studies revealed that the heterogeneous electrolysis of the analyte should ideally be electrochemically reversible, while that for the regenerating agent should be held to a low level. Moreover, the homogeneous reaction that recycles the analyte should occur at a rapid rate. The physical selectivity mechanism is also detailed with respect to the properties of the electrode and redox probes utilized. It is shown that partitioning of the analyte into/onto the adlayer leads to the extraordinary selectivity of the alkanethiolate monolayer modified electrode. Collectively, these studies enable a thorough understanding of the complex electrode mechanism required for successful redox recycling amplification systems, Finally, in a separate (but related) study, the effect of the akyl chain length on the heterogeneous electron transfer behavior of solution-based redox probes is reported, where an odd-even oscillation (with respect to the number of methylene units in the alkyl chain) was observed. Characterization of the adlayers by infrared reflection spectroscopy, ellipsometry, and wetting revealed odd-even effects in the orientation of the terminal methyl group and hydrophobic character of the adlayers. Using these structural characterizations as a basis, several possible mechanisms that can account for the odd-even effect in the heterogeneous electron transfer rates of solution-based redox couples are discussed

A Tale of Two Sandys

Responses to Hurricane Sandy consistently cluster into two types according to how the issues have been defined and understood. On one hand, the crisis was seen as an extreme weather event that created physical and economic damage, and temporarily moved New York City away from its status quo. On the other hand, Hurricane Sandy exacerbated crises which existed before the storm, including poverty, lack of affordable housing, precarious or low employment, and unequal access to resources generally. A Tale of Two Sandys describes these two understandings of disaster and discuss their implications for response, recovery, and justice in New York City. The white paper is based on 74 interviews with policymakers, environmental groups, volunteer first responders, and residents affected by the storm; ethnographic observation; analysis of public reports from government, community-based organizations, and other groups; qualitative analysis of canvassing forms and data; and a review of the academic literature on disaster response. As a framing document, A Tale of Two Sandys selects certain case studies for their exemplary nature, including how different groups identified vulnerable populations, timelines for aid and recovery, a case study of housing and rebuilding, and finally, urban climate change politics. The primary purpose of A Take of Two Sandys is to propose a sophisticated, accurate, and useful way of understanding the inequalities entwined with Sandy’s aftermath and to enable ways to address them

Electronic Characteristics and Charge Transport Mechanisms for Large Area Aromatic Molecular Junctions

This paper reports the electron transport characteristics of carbon/molecule/Cu molecular junctions, where aromatic molecules (azobenzene or AB and nitroazobenzene or NAB) are employed as the molecular component. It is shown that these devices can be made with high yield (>90%), display excellent reproducibility, and can withstand at least 1.5 × 10 9 potential cycles and temperatures of at least 180°C. Transport mechanisms are investigated by analysis of current density/voltage (J-V) curves as a function of the molecular layer thickness and temperature. Results show that J decreases exponentially with thickness, giving a measured value for the low-bias attenuation factor ( ) of 2.5 ( 0.1 nm -1 for AB and NAB. In addition, it is shown that transport is not thermally activated over a wide range of temperatures (5-450 K) and that the appearance of a thermally "activated" region at higher temperatures can be accounted for by the effect of temperature on the distribution of electrons around the Fermi level of the contact(s). These results indicate that quantum mechanical tunneling is likely the mechanism for charge transport in these junctions. Although application of the Simmons tunneling model leads to transport parameters consistent with nonresonant tunneling, the parameters obtained from fitting experimental data indicate that the barrier height and/or shape, effective mass, and dielectric constant (ε) can all change with thickness. Experimental measurements of ε and density functional theory (DFT) calculations of molecular energy levels and polarizability support these conclusions. Finally, the implications of the transport mechanisms are discussed from the viewpoint of designing functional molecular electronic devices

Solid-State Protein Junctions:Cross-Laboratory Study Shows Preservation of Mechanism at Varying Electronic Coupling

Successful integration of proteins in solid-state electronics requires contacting them in a non-invasive fashion, with a solid conducting surface for immobilization as one such contact. The contacts can affect and even dominate the measured electronic transport. Often substrates, substrate treatments, protein immobilization, and device geometries differ between laboratories. Thus the question arises how far results from different laboratories and platforms are comparable and how to distinguish genuine protein electronic transport properties from platform-induced ones. We report a systematic comparison of electronic transport measurements between different laboratories, using all commonly used large-area schemes to contact a set of three proteins of largely different types. Altogether we study eight different combinations of molecular junction configurations, designed so that Ageo of junctions varies from 105 to 10−3 μm2. Although for the same protein, measured with similar device geometry, results compare reasonably well, there are significant differences in current densities (an intensive variable) between different device geometries. Likely, these originate in the critical contact-protein coupling (∼contact resistance), in addition to the actual number of proteins involved, because the effective junction contact area depends on the nanometric roughness of the electrodes and at times, even the proteins may increase this roughness. On the positive side, our results show that understanding what controls the coupling can make the coupling a design knob. In terms of extensive variables, such as temperature, our comparison unanimously shows the transport to be independent of temperature for all studied configurations and proteins. Our study places coupling and lack of temperature activation as key aspects to be considered in both modeling and practice of protein electronic transport experiments

Electron transfer reactivity patterns at chemically modified electrodes: fundamentals and application to the optimization of redox recycling amplification systems

Electroanalytical chemistry is often utilized in chemical analysis and fundamental studies. Important advances have been made in these areas since the advent of chemically modified electrodes: the coating of an electrode with a chemical film in order to impart desirable, and ideally, predictable properties. These procedures enable the exploitation of unique reactivity patterns. This dissertation presents studies that investigate novel reaction mechanisms at self-assembled monolayers on gold. In particular, a unique electrochemical current amplification scheme is detailed that relies on a selective electrode to enable a reactivity pattern that results in regeneration of the analyte (redox recycling). This regenerating reaction can occur up to 250 times for each analyte molecule, leading to a notable enhancement in the observed current. The requirements of electrode selectivity and the resulting amplification and detection limit improvements are described with respect to the heterogeneous and homogeneous electron transfer rates that characterize the system. These studies revealed that the heterogeneous electrolysis of the analyte should ideally be electrochemically reversible, while that for the regenerating agent should be held to a low level. Moreover, the homogeneous reaction that recycles the analyte should occur at a rapid rate. The physical selectivity mechanism is also detailed with respect to the properties of the electrode and redox probes utilized. It is shown that partitioning of the analyte into/onto the adlayer leads to the extraordinary selectivity of the alkanethiolate monolayer modified electrode. Collectively, these studies enable a thorough understanding of the complex electrode mechanism required for successful redox recycling amplification systems. Finally, in a separate (but related) study, the effect of the alkyl chain length on the heterogeneous electron transfer behavior of solution-based redox probes is reported, where an odd-even oscillation (with respect to the number of methylene units in the alkyl chain) was observed. Characterization of the adlayers by infrared reflection spectroscopy, ellipsometry, and wetting revealed odd-even effects in the orientation of the terminal methyl group and hydrophobic character of the adlayers. Using these structural characterizations as a basis, several possible mechanisms that can account for the odd-even effect in the heterogeneous electron transfer rates of solution-based redox couples are discussed.</p

Electron-beam evaporated silicon as a top contact for molecular electronic device fabrication

This paper discusses the electronic properties of molecular devices made using covalently bonded molecular layers on carbon surfaces with evaporated silicon top contacts. The Cu "top contact" of previously reported carbon/molecule/Cu devices was replaced with e-beam deposited Si in order to avoid Cu oxidation or electromigration, and provide further insight into electron transport mechanisms. The fabrication and characterization of the devices is detailed, including a spectroscopic assessment of the molecular layer integrity after top contact deposition. The electronic, optical, and structural properties of the evaporated Si films are assessed in order to determine the optical gap, work function, and film structure, and show that the electron beam evaporated Si films are amorphous and have suitable conductivity for molecular junction fabrication. The electronic characteristics of Si top contact molecular junctions made using different molecular layer structures and thicknesses are used to evaluate electron transport in these devices. Finally, carbon/molecule/silicon devices are compared to analogous carbon/molecule/metal junctions and the possible factors that control the conductance of molecular devices with differing contact materials are discussed. \ua9 the Owner Societies 2011.Peer reviewed: YesNRC publication: Ye

Internal photoemission in molecular junctions: Parameters for interfacial barrier determinations

The photocurrent spectra for large-area molecular junctions are reported, where partially transparent copper top contacts permit illumination by UV-vis light. The effect of variation of the molecular structure and thickness are discussed. Internal photoemission (IPE), a process involving optical excitation of hot carriers in the contacts followed by transport across internal system barriers, is dominant when the molecular component does not absorb light. The IPE spectrum contains information regarding energy level alignment within a complete, working molecular junction, with the photocurrent sign indicating transport through either the occupied or unoccupied molecular orbitals. At photon energies where the molecular layer absorbs, a secondary phenomenon is operative in addition to IPE. In order to distinguish IPE from this secondary mechanism, we show the effect of the source intensity as well as the thickness of the molecular layer on the observed photocurrent. Our results clearly show that the IPE mechanism can be differentiated from the secondary mechanism by the effects of variation of experimental parameters. We conclude that IPE can provide valuable information regarding interfacial energetics in intact, working molecular junctions, including clear discrimination of charge transport mediated by electrons through unoccupied system orbitals from that mediated by hole transport through occupied system orbitals.Peer reviewed: YesNRC publication: Ye

Light Emission as a Probe of Energy Losses in Molecular Junctions

Visible light emission was observed for molecular junctions containing 5–19 nm thick layers of aromatic molecules between carbon contacts and correlated with their current–voltage behaviors. Their emission was compared to that from Al/AlOx/Au tunnel junctions, which has been previously attributed to transport of carriers across the AlOx layer to yield “hot carriers” which emit light as they relax within the Au contact. The maximum emitted photon energy is equal to the applied bias for the case of coherent tunneling, and such behavior was observed for light emission from AlOx and thin (<5 nm) molecular junctions. For thicker films, the highest energy observed for emitted photons is less than <i>eV</i>_app and exhibits an energy loss that is strongly dependent on molecular layer structure and thickness. For the case of nitroazobenzene junctions, the energy loss is linear with the molecular layer thickness, with a slope of 0.31 eV/nm. Energy loss rules out coherent tunneling as a transport mechanism in the thicker films and provides a direct measure of the electron energy after it traverses the molecular layer. The transition from elastic transport in thin films to “lossy” transport in thick films confirms that electron hopping is involved in transport and may provide a means to distinguish between various hopping mechanisms, such as activated electron transport, variable range hopping, and Poole Frankel transport

Direct Optical Determination of Interfacial Transport Barriers in Molecular Tunnel Junctions

Molecular electronics seeks to build circuitry using organic components with at least one dimension in the nanoscale domain. Progress in the field has been inhibited by the difficulty in determining the energy levels of molecules after being perturbed by interactions with the conducting contacts. We measured the photocurrent spectra for large-area aliphatic and aromatic molecular tunnel junctions with partially transparent copper top contacts. Where no molecular absorption takes place, the photocurrent is dominated by internal photoemission, which exhibits energy thresholds corresponding to interfacial transport barriers, enabling their direct measurement in a functioning junction

Interpretation of Molecular Device Transport Calculation

The field of molecular electronics will benefit from rational design approaches based on a complete understanding of the electronic structure of molecule-based devices. However, many computational approaches that are used to study molecular-scale devices are based on methods that have deficiencies that must be understood in order for those methods to be useful to the modelling and experimental community. Density-functional theory based methods have some well-known pitfalls that limit their application to the study of electron transport in models of molecular junction devices. Some of the impacts of these deficiencies are highlighted in this work through the use of a graphene model system and a variety of simple hydrocarbon molecule. Self-interaction error in simple functionals built from the local density approximation and the generalized gradient approximation result in very large errors in predicted absolute and relative ionization potentials. This demonstrates that electron transmission spectra predicted using these functionals should be considered with caution. We also demonstrate that care must be taken with the use of finite models for electrodes.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author