Covalent Bonding of Organic Molecules to Cu and Al Alloy 2024 T3 Surfaces via Diazonium Ion Reduction

Cu surfaces and polished aluminum alloy 2024 T3 substrates were derivatized at open-circuit potential with aryl diazonium salts in both aprotic and aqueous media. Raman spectroscopy confirmed the presence of a derivatized film on the substrates before and after exposure to boiling water and sonication in acetone. Two different Cu substrate surfaces were prepared and used for X-ray photoelectron spectroscopy (XPS) analysis of the derivatization results. One surface was native oxide Cu, predominantly in the form of Cu_2O, and one surface was predominantly Cu^0. Results of the XPS analysis indicate the presence of both a Cu-O-C linkage and a Cu-C covalent bond between the aryl ring and the Cu substrate, and a high coverage of the organic layer. XPS results also indicate the formation multilayers on both types of Cu surfaces with different percentages of azo coupling within the multilayers on the two surfaces. Applications of a covalently bonded organic film on copper and alloy surfaces include adhesion promotion, corrosion protection, and possibly inhibition of oxygen reduction

Raman Spectroscopy of Monolayers Formed from Chromate Corrosion Inhibitor on Copper Surfaces

Surface enhanced Raman scattering (SERS) was used to observe interactions of dilute Cr^VI solutions with silver and copper surfaces in situ. Using silver as a model surface which supports strong SERS with a 514.5 nm laser, it was possible to observe Cr^III at the near monolayer level, and the spectra were compared to those from Cr^III oxyhydroxide species and Cr^III/Cr^VI mixed oxide. Similar experiments were conducted with Cu surfaces and 785 nm excitation. Upon exposure to Cr^VI solution, the characteristic Cu oxide Raman bands disappeared, and a Cr^III band increased in intensity over a period of ~20 h. The intensity of the Cr^III band on Cu became self-limiting after the formation of several Cr^III monolayers, as supported by chronoamperometry experiments. This Cr^III spectrum was stable after Cr^VI was removed from the solution provided the potential remained negative of –200 mV vs. Ag/AgCl. The results support the conclusion that Cr^VI is reductively adsorbed to Cu at the near neutral pH and open circuit potentials expected for Cu/Al alloys in field applications. The Cr^III film is stable and is a strong inhibitor of electron transfer in general and oxygen reduction in particular. An important mechanistic feature of Cr^III formation is the substitution lability of Cr^VI compared to Cr^III. The Cr^VI-O bond can be broken much more rapidly than the substitution inert Cr^III-O bond, making formation of Cr^III/Cr^VI mixed oxide kinetically favorable. Once reduced to Cr^III, however, the substitution inert oxyhydroxide film is much less labile. An important and central feature of Cr^VI as a corrosion inhibitor is its transformation via reductive adsorption from a mobile, substitution labile Cr^VI form to an insoluble, substitution inert Cr^III oxyhydroxide. Furthermore, Cr^VI reduction is likely to occur at cathodic sites previously responsible for oxygen reduction, which are then permanently blocked by a stable Cr^III film with a thickness of a few monolayers

Raman Spectroscopy for Chemical Analysis

Owing to its unique combination of high information content and ease of use, Raman spectroscopy, which uses different vibrational energy levels to excite molecules (as opposed to light spectra), has attracted much attention over the past fifteen years. This book covers all aspects of modern Raman spectroscopy, including its growing use in both the laboratory and industrial analysis.Includes bibliographical references and index.Owing to its unique combination of high information content and ease of use, Raman spectroscopy, which uses different vibrational energy levels to excite molecules (as opposed to light spectra), has attracted much attention over the past fifteen years. This book covers all aspects of modern Raman spectroscopy, including its growing use in both the laboratory and industrial analysis

Hubbard NEGF Analysis of Photocurrent in Nitroazobenzene Molecular Junction

We present combined experimental and theoretical study of photo-induced current in molecular junctions consisting of monolayers of nitroazobenzene oligomers chemisorbed on carbon surfaces and illuminated by UV-Vis light through a transparent electrode. Experimentally observed dependence of the photocurrent on light frequency, temperature and monolayer thickness is analyzed within first principles simulations employing the Hubbard NEGF diagrammatic technique. We reproduce qualitatively correct behavior and discuss mechanisms leading to characteristic behavior of dark and photo-induced currents in response to changes in bias, frequency of radiation, temperature and thickness of molecular layer.Comment: 22 pages, 4 figure

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

Analytical Challenges in Molecular Electronics

Peer reviewed: YesNRC publication: N

Carbon-Based Molecular Junctions for Practical Molecular Electronics

ConspectusThe field of molecular electronics has grown rapidly since its experimental realization in the late 1990s, with thousands of publications on how molecules can act as circuit components and the possibility of extending microelectronic miniaturization. Our research group developed molecular junctions (MJs) using conducting carbon electrodes and covalent bonding, which provide excellent temperature tolerance and operational lifetimes. A carbon-based MJ based on quantum mechanical tunneling for electronic music represents the world’s first commercial application of molecular electronics, with >3000 units currently in consumer hands. The all-carbon MJ consisting of aromatic molecules and oligomers between vapor-deposited carbon electrodes exploits covalent, C–C bonding which avoids the electromigration problem of metal contacts. The high bias and temperature stability as well as partial transparency of the all-carbon MJ permit a wide range of experiments to determine charge transport mechanisms and observe photoeffects to both characterize and stimulate operating MJs. As shown in the Conspectus figure, our group has reported a variety of electronic functions, many of which do not have analogs in conventional semiconductors. Much of the described research is oriented toward the rational design of electronic functions, in which electronic characteristics are determined by molecular structure.In addition to the fabrication of molecular electronic devices with sufficient stability and operating life for practical applications, our approach was directed at two principal questions: how do electrons move through molecules that are components of an electronic circuit, and what can we do with molecules that we cannot do with existing semiconductor technology? The central component is the molecular junction consisting of a 1–20+ nm layer of covalently bonded oligomers between two electrodes of conducting, mainly sp2-hybridized carbon. In addition to describing the unique junction structure and fabrication methods, this Account summarizes the valuable insights available from photons used both as probes of device structure and dynamics and as prods to stimulate resonant transport through molecular orbitals.Short-range (<5 nm) transport by tunneling and its properties are discussed separately from the longer-range transport (5–60 nm) which bridges the gap between tunneling and transport in widely studied organic semiconductors. Most molecular electronic studies deal with the <5 nm thickness range, where coherent tunneling is generally accepted as the dominant transport mechanism. However, the rational design of devices in this range by changing molecular structure is frustrated by electronic interactions with the conducting contacts, resulting in weak structural effects on electronic behavior. When the molecular layer thickness exceeds 5 nm, transport characteristics change completely since molecular orbitals become the conduits for transport. Incident photons can stimulate transport, with the observed photocurrent tracking the absorption spectrum of the molecular layer. Low-temperature, activationless transport of photogenerated carriers is possible for up to at least 60 nm, with characteristics completely distinct from coherent tunneling and from the hopping mechanisms proposed for organic semiconductors. The Account closes with examples of phenomena and applications enabled by molecular electronics which may augment conventional microelectronics with chemical functions such as redox charge storage, orbital transport, and energy-selective photodetection