Charge transport at the nanoscale [electronic resource] : new materials and probes
Keywords: Materials Science and Engineering

Many biological phenomena and energy harvesting devices involve electron transport through organic molecules. Conductance measurements at the interface between inorganic electrodes and a single organic molecule or molecular monolayer, when combined with structural characterization, can yield precise understanding of molecular charge transport. However, difficulties such as interface instability, molecular structure fluctuations, and limited in-situ probe access have hampered progress. One of the major challenges has been ambiguity in the electron distribution and electrostatic potential within a molecular junction. The charge transport is known to be critically dependent on these parameters, yet experimental measurements have been lacking. We have developed an experimental method to measure these parameters using synchrotron X-ray reflectivity (XRR) combined with a soft lithographic technique to form robust large-area molecular junctions. High resolution electron distribution plots of a chlorophyll monolayer between two macroscopic electrodes were obtained. Using a lock-in technique to detect small changes in reflected intensity as a function of applied voltage, the electrostatic potential profile within the junction was measured. Many studies involving systematic variations of molecular length have yielded important insights into charge transport. More elaborate structural variations have not been as thoroughly explored due to lack of suitable materials. Diamondoids are a new class of carbon nanomaterial with rigid, well-defined sizes and shapes making them an attractive platform to explore the relationship between molecular structure and charge transport. We deposited a series of diamondoid thiol monolayers on gold and measured current-voltage (I-V) tunneling curves using conducting atomic force microscopy (AFM). One of the diamondoid isomers showed surprisingly efficient charge transport, making it appear more like a conjugated molecule despite its being a fully saturated hydrocarbon. Using ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) computations, along with in-depth structural characterization of the monolayers, we are able to explain this finding by enhanced intermolecular electronic coupling. Reinterpretations of certain results in the field of molecular electronics are suggested by our results. We also lay the groundwork for future electron tunneling studies through diamondoid molecular assemblies by characterizing the first diamondoid Langmuir films. Isothermal data, AFM, grazing incidence X-ray diffraction (GIXD), and interfacial stress rheometry (ISR) were used to characterize the thermodynamic, morphological, structural, and mechanical properties of diamondoid Langmuir and Langmuir-Blodgett films. This is the first study of a pure nanodiamond film at the air/water interface. Finally, we fabricate and characterize the performance and stability of a molecular electronic device. This device consists of a diamondoid siloxane monolayer deposited via solution or vapor phase on a silicon substrate. This composite material shows intense, stable monochromatic electron photoemission. Contact angle measurements, Fourier transform infrared (FTIR) spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and X-ray photoelectron spectroscopy (XPS) are used to characterize the structure and stability of the monolayers. Appendices discuss diamondoid molecular crystal growth and mechanical properties, a new method for X-ray reflectivity data analysis, air-free chemical attachment of monolayers, and the relevance of Landauer transport modeling to intermolecular charge transport as measure by transition voltage spectroscopy.