Chemically transformed monolayers on acene thin films for improved metal/organic interfaces.

Anhydride terminated acene thin films were chemically transformed to thiol or carboxylic acid functionalities, groups heretofore incompatible with monolayer reactions. The molecular surface imparts large rate acceleration when imides are formed, while disfavored disulfides can be formed from the thiols. The modified surface imparts improved adhesion to top metal contacts in flexible/bendable applications.

Reaction induced morphology changes of tetracene and pentacene surfaces.

Morphology plays a critical role in determining the properties of solid-state molecular materials, yet fluctuates wildly as these materials undergo reaction. A prototypical system, a vapor-solid Diels-Alder reaction of tetracene and pentacene thin-films, is used to observe the evolution of morphology features as the reaction transitions from surface to bulk. The initial stages of reaction display little topographical change as measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and substrates are coated with a uniform layer of product 1-2 molecules thick, as determined by energy-dispersive X-ray (EDX) spectroscopy. The highly textured surfaces of late stage reactions are a result of aggregated products, as identified via EDX spectroscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS); areas of the surface in between product aggregates resemble the initial stages. The mechanism by which products aggregate into surface asperities requires the assistance of a facilitating media - in this case condensed vapor; simple thermally assisted surface diffusion was unable to generate these morphology changes. The combined data indicate that reactions of molecular solids, could be confined to the surface in the absence of condensate of the vapor phase reactant.

The Role of Thermal Activation and Molecular Structure on the Reaction of Molecular Surfaces.

Though surface modifications of organic thin films dramatically improve optoelectronic device performance, chemistry at organic surfaces presents new challenges that are not seen in conventional inorganic surfaces. This work demonstrates that the subsurface of pentacene remains highly accessible, even to large adsorbates, and that three distinct reaction regimes (surface, subsurface, and bulk) are accessed within the narrow thermal range of 30-75 °C. Progression of this transition is quantitatively measured via polarization modulation infrared reflection absorption spectroscopy, and atomic force microscopy is used to measure the thin-film morphology. Together, they reveal the close relationship between the extent of the reaction and the morphology changes. Finally, the reaction kinetics of the pentacene thin film is measured with a series of adsorbates that have different reactivity and diffusivity in the thin film. The results suggest that reaction kinetics in the thin film is controlled by both the reactivity and the adsorbate diffusivity in the thin-film lattice, which is very different than the traditional solution kinetics that is dominated by the chemical activation barriers. Combined, these experiments guide efforts toward rationally functionalizing the surfaces of organic semiconductors to enable the next generation of flexible devices.

Solid state and surface effects in thin-film molecular switches.

Thin-films of three dihydroindolizine molecular switches were monitored via polarization modulation infrared reflection absorption spectroscopy to quantify solid state and surface-based inhibition of switching as a function of irradiation time. In the solid state, the molecular switches diverged dramatically with flexible alkyl substituents resulting in switching rates up to three times that of switches containing rigid analogs. For thin-films, decreasing film thickness from 30 nm to approximately 4 molecules thick resulted in an increase in inhibition. This was found to be consistent across all molecules regardless of structure. Increased inhibition is isolated as a metal/molecule interaction, and its consistency across structure is suggestive of energy transfer to the surface. These molecular switches highlight the interplay between molecular design, electronic structure, and switching efficiency.

Substituent parameters impacting isomer composition and optical properties of dihydroindolizine molecular switches.

In an attempt to understand which factors influence constitutional isomer control of 6'- and 8'-substituted dihydroindolizines (DHIs), a series of asymmetric pyridines was condensed with dimethyl spiro[cycloprop[2]ene-1,9'-fluorene]-2,3-dicarboxylate. The substituents on the pyridial derivatives ranged from donating to withdrawing and demonstrated control over the isomeric ratios for all DHIs. Substituent control proved to be selective for the highly donating amino, which exclusively formed the 8' isomer. The same ratios were reproduced via photolytic experiments, which suggested that the condensation reaction is dominated by the product's thermodynamic stability. The electronic influences of the substituents extends beyond isomer control, as it impacts the DHIs' optical properties and electrocyclization (switching) rates to the spiro conformers. Our results allow us to predict the syntheses and properties of future 6'- or 8'-substituted DHIs, molecules that will be applied in understanding the role of the dipole vector orientation to work function switching.

Functionalization of organic semiconductor crystals via the Diels-Alder reaction.

A surface adlayer is generated on organic single crystals (tetracene and rubrene) using the site specific Diels-Alder reaction and a series of vapor phase dienophiles. X-ray photoelectron spectroscopy (XPS) confirms adsorption on the surfaces of tetracene and rubrene and mass spectrometry demonstrates the reaction's applicability to a range of dienophiles.

High-density amine-terminated monolayers formed on fluorinated CVD-grown graphene.

There has been considerable interest in chemically functionalizing graphene films to control their electronic properties, to enhance their binding to other molecules for sensing, and to strengthen their interfaces with matrices in a composite material. Most reports to date have largely focused on noncovalent methods or the use of graphene oxide. Here, we present a method to activate CVD-grown graphene sheets using fluorination followed by reaction with ethylenediamine (EDA) to form covalent bonds. Reacted graphene was characterized via X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy as well as measurements of electrical properties. The functionalization results in stable, densely packed layers, and the unbound amine of EDA was shown to be active toward subsequent chemical reactions.

Assembly of nanorods into designer superstructures: the role of templating, capillary forces, adhesion, and polymer hydration.

The assembly mechanism by which hundreds of thousands of two-segment gold-polypyrrole nanorods are assembled into kinetically controlled shape-directed superstructures is examined to predict the range of nanoparticle sizes and materials that can be utilized in their formation. Four processes are responsible for assembly: templating, capillary force assembly, adhesion, and polymer hydration. It is shown that templating, where rods are prepositioned for assembly, is scale invariant and that the energy-minimized state after this step is highly disordered. In addition, we predict that superstructures can be made independently from patterns of rods separated by a distance as small as six times the inter-rod spacing. Both modeling and experiment show that adhesion and polymer dehydration, which induces curvature in the superstructures, are applicable to other materials. However, the high surface energy and low elastic modulus of polypyrrole are advantageous toward generating three-dimensional structures, inducing curvature at gold/polypyrrole length ratios as large as 7:1.

Simultaneous measurements of electronic conduction and Raman response in molecular junctions.

Electronic conduction through single molecules is affected by the molecular electronic structure as well as by other information that is extremely difficult to assess, such as bonding geometry and chemical environment. The lack of an independent diagnostic technique has long hampered single-molecule conductance studies. We report simultaneous measurement of the conductance and the Raman spectra of nanoscale junctions used for single-molecule electronic experiments. Blinking and spectral diffusion in the Raman response of both p-mercaptoaniline and a fluorinated oligophenylyne ethynylene correlate in time with changes in the electronic conductance. Finite difference time domain calculations confirm that these correlations do not result from the conductance modifying the Raman enhancement. Therefore, these observations strongly imply that multimodal sensing of individual molecules is possible in these mass-producible nanostructures.

Actuation of self-assembled two-component rodlike nanostructures.

A model for predicting the effects of stimuli-induced contraction of the polypyrrole ends of two-segment gold-polypyrrole nanorods on their assembly into curved superstructures is presented. The model and experimental data presented here show that small changes (ca. 3%) in the diameter of the polypyrrole segment of each rod will induce dramatic changes (up to 20%) in the radii of the resulting superstructures, providing a convenient means for actuating their opening and closing. We show experimentally that this actuation can be affected via humidity, temperature, and light.

Electrochemical origin of voltage-controlled molecular conductance switching.

We have studied electron transport in bipyridyl-dinitro oligophenylene-ethynelene dithiol (BPDN) molecules both in an inert environment and in aqueous electrolyte under potential control, using scanning tunneling microscopy. Current-voltage (IV) data obtained in an inert environment were similar to previously reported results showing conductance switching near 1.6 V. Similar measurements taken in electrolyte under potential control showed a linear dependence of the bias for switching on the electrochemical potential. Extrapolation of the potentials to zero switching bias coincided with the potentials of redox processes on these molecules. Thus switching is caused by a change in the oxidation state of the molecules.

Effects of hydration on molecular junction transport.

The study of charge transport through increasingly complex small molecules will benefit from a detailed understanding of how contaminants from the environment affect molecular conduction. This should provide a clearer picture of the electronic characteristics of molecules by eliminating interference from adsorbed species. Here we use magnetically assembled microsphere junctions incorporating thiol monolayers to provide insight into changing electron transport characteristics resulting from exposure to air. Using this technique, current-voltage analysis and inelastic electron tunnelling spectroscopy (IETS) demonstrate that the primary interaction affecting molecular conduction is rapid hydration at the gold-sulphur contacts. We use IETS to present evidence for changing mechanisms of charge transport as a result of this interaction. The detrimental effects on molecular conduction discussed here are important for understanding electron transport through gold-thiol molecular junctions once exposed to atmospheric conditions.

Neutral complexes of first row transition metals bearing unbound thiocyanates and their assembly on metallic surfaces.

A series of transition metal coordination complexes designed to assemble on gold surfaces was synthesized, their electronic structure and transitions analyzed, and their magnetic properties studied. By taking advantage of recently developed thiocyanate assembly protocols, these molecules were then assembled onto a gold surface, without the need for an inert atmosphere, to give a loosely packed monolayer. The assembled molecules exhibit properties similar to that of the bulk molecules, indicating little change in molecular structure outside of chemisorption.

Molecular engineering and measurements to test hypothesized mechanisms in single molecule conductance switching.

Six customized phenylene-ethynylene-based oligomers have been studied for their electronic properties using scanning tunneling microscopy to test hypothesized mechanisms of stochastic conductance switching. Previously suggested mechanisms include functional group reduction, functional group rotation, backbone ring rotation, neighboring molecule interactions, bond fluctuations, and hybridization changes. Here, we test these hypotheses experimentally by varying the molecular designs of the switches; the ability of the molecules to switch via each hypothetical mechanism is selectively engineered into or out of each molecule. We conclude that hybridization changes at the molecule-surface interface are responsible for the switching we observe.

Spontaneous assembly of organic thiocyanates on gold sufaces. Alternative precursors for gold thiolate assemblies.

Thiolate self-assembly on gold has proven to be a valuable technique for assembling monolayers on a wide variety of substrates. However, the oxidative instability of the thiols, especially aromatic thiols and alpha,omega-dithiols, presents several difficulties. Shown here is that thiocyanates, easily synthesized stable thiol derivatives, can be directly assembled on gold surfaces with no auxiliary reagents required. Assembly is complete in 24 h and leaves a similar gold thiolate structure as seen in typical thiol self-assembled monolayers.