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

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

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

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

Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions

In this work, we bridge the gap between short-range tunneling in molecular junctions and activated hopping in bulk organic films, and greatly extend the distance range of charge transport in molecular electronic devices. Three distinct transport mechanisms were observed for 4.5-22-nm-thick oligo(thiophene) layers between carbon contacts, with tunneling operative when d< 8 nm, activated hoppingwhen d> 16nmfor high temperatures and lowbias, and a third mechanism consistent with field-induced ionization of highest occupiedmolecular orbitals or interface states to generate charge carriers when d = 8-22 nm. Transport in the 8-22-nm range is weakly temperature dependent, with a field-dependent activation barrier that becomes negligible at moderate bias. We thus report here a unique, activationless transport mechanism, operative over 8-22-nm distanceswithout involving hopping,which severely limits carriermobility and device lifetime in organic semiconductors. Charge transport in molecular electronic junctions can thus be effective for transport distances significantly greater than the 1-5 nm associated with quantum-mechanical tunneling.Peer reviewed: YesNRC publication: Ye