Date of Award

August 2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Wilfred T. Tysoe

Committee Members

Dennis Bennett, Peter Geissinger, Alan Schwabacher, Jorg Woehl

Keywords

Back-Gating Effect, Gold-Nanoparticle Arrays, Isocyanide- and Thiol-Terminated Aromatic Molecules, Molecular Electronics, Tuning the Gold Fermi Level, Tunneling Barrier

Abstract

In 1947, Bell Laboratories produced an amplifier design in which an electric field would enhance the flow of electrons near the surface of a layer of silicon, it was called the “point-contact transistor”, the world’s first semiconductor amplifier. In order to minimize the electronic circuit elements (device miniaturization), the ability to utilize single molecules that function as self-contained electronic devices (molecular electronics) has motivated researchers around the world. Molecular electronics investigates single molecules and collections of molecules assembled into electronic circuits. Currently, semiconductor devices are fabricated using a “top-down” approach that employs lithographic and etch techniques to pattern a substrate, but as feature sizes decrease, the top-down approach becomes challenging. As a result, circuits are synthesized using a “bottom-up” approach that builds small structures from molecules.

In 1959, Nobel Laureate Richard Feynman gave his famous lecture “There's Plenty of Room at the Bottom” to an American Physical Society meeting at Caltech, where he he discussed the use of single molecules or atoms to build nano-devices, sizes that cannot be built using traditional lithographic techniques.

In 1974, Aviram and Ratner postulated a very simple electronic device, based on the use of a single organic molecule, and by modulating HOMO and LUMO orbitals to produce a ‘molecular rectifier’.

In this work, a new strategy for building molecular electronics is investigated; by using different organic linkers that self-assemble and bridge gold nanoparticles in order to create electron-transfer pathways between them. This involves measuring the electrical properties and surface structures of isocyanide- and thiol-terminated aromatic molecules. It was found that diisocyanides and dithiols with one, two, and three benzene rings can form oligomeric chains linking between gold nanoparticle arrays on mica in order to decrease the tunneling barrier and enhance the conductivity. The results showed that these molecules generally form oligomeric bridges when dosed on gold-nanoparticle arrays, and that the tunneling barrier can be affected by the number of phenyl rings for each molecule where it was observed that the energy barrier increased when increasing the molecular length (number of benzene rings) of the linker molecule. For example, the slopes of plots of ln (film resistance versus 1⁄√T, denoted α versus ln (Ro), where Ro is the resistance of the initial nanoparticle array before dosing, for 1,4-benzenedithiol (BDT), 4,4′-biphenyldithiol (BPDT), and 4,4″-terphenyldithiol (TPDT) were analyzed to show that the height of the tunneling barrier increases with increasing number of benzene rings, which means that increased extent of conjugation does not necessarily lead to lower tunneling barrier. Biphenyl and terphenyl molecules can adopt a twisted ring configuration which appears to be observed for BPDT and TPDT. This distortion may decrease the conjugation by reducing the orbital overlap between adjacent aromatic rings. Consequently, the poly-phenyl dithiol molecules have higher tunneling barrier than benzene dithiol (Chapter 6).

Similar to dithiols, the tunneling barrier of diisocyanides increases when increasing the number of phenyl rings. Diisocyanides with one, two and three benzene rings can also adopt a twisted ring configuration. As a result, a possible reason for the increase in the height of the tunneling barrier as the number of benzene rings increases, is a reduced orbital overlap between adjacent benzene rings because of twisting. An alternative possibility is that increasing the number of phenyl rings increases the electron donation to the gold that leaves a partial positive charge on the isocyanide molecule which leads to a lowering the molecular orbital energies and moves the HOMO orbital away from the Fermi level, as evidenced by variations in the isocyanide stretching frequencies (Chapter 7).

The properties of an asymmetric molecule containing both isocyanide and sulfur groups are investigated by studying the surface structure and electrical properties by dosing gold films with 4,4′-disulfanediyldibenzoisonitrile (DBN) from solution. It is found that DBN has ability to form oligomers between gold nanoparticles and contributes to tuning the gold Fermi level and decrease the height of the electron tunneling barrier. The ATR-IR spectra provide information that is complementary to the electrical measurements and allow the surface structure of DBN molecules on the gold-nanoparticle array to be determined.

In Chapter 9, back-gating behavior was explored for three different linker molecules; an external field was applied using a back-gated device in order to modulate the conductivity of molecular wires that bridge gold nanoparticles in a granular thin film. It was found that the external field effect could both modulate the energy of the molecular orbitals (HOMO) of the molecular wire to influence their alignment with respect to the Fermi level of gold nanoparticles, as well as polarizing charge from the gold nanopartilces, thereby modulating the current through the molecular layer. The field effect-conductivity results showed a small increase in the conductivity with increasing gate potential of either sign. Electrical measurements were performed using 1,4-PDI-, 1,4-BDT- and 1,3-BDT-linked gold nanoparticles where the maximum sheet resistance changes were ~1.5 to 2.3%. At a sample temperature of ~110K, for what the sheet resistance changed by 4.5% under a negative bias.

These results show that it is possible to form molecules linkages between gold nanoelectrodes by attaching the molecules either using isocyanide or sulfur groups, suggesting that this is a promising strategy for eventually fabricating molecular electronic circuits.

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