Date of Award
Doctor of Philosophy
Wilfred T. Tysoe
Paul Lyman, Alan Schwabacher, Dennis Bennett, Jorg Woehl
Gold, Molecular Electronics, Molecular Wires, Self-assembly
The Surface Chemistry of Adatom Mediated Oligomers
of Aromatic Diisocyanides and Dithiols on Au(111) and Granular Films
John D. Kestell
The University of Wisconsin-Milwaukee
Under the supervision of Professor W. T. Tysoe
Two seminal events are thought by many to be the birth of nanotechnology and molecular electronics. In 1959, Nobel Laureate Richard Feynman gave his famous "There's Plenty of Room at the Bottom" lecture to an American Physical Society meeting at Caltech. The transistor had been invented by Bell Laboratories in 1947, and already the race was on for device miniaturization. Feynman's lecture focused on some of the more serious technological problems of reading and writing on the atomic scale, and in particular, how one manipulates single atoms and molecules to build useful nano-architectures. His radical new paradigm was that rather than work from the top down, literally carving devices out of bulk silicon (an approach that still dominates today), perhaps we could take a lesson from Mother Nature. By intelligent design of molecules, he argues, it might be possible to allow them to assemble themselves.
Later, in 1974, Avarim and Ratner proposed a radical idea. Silicon devices such as transistors rely on modulating bands in semiconductors to modulate charge through them. Avarim and Ratner proposed that with proper design, it might be possible to synthesize discreet molecules with essentially the same functionality. Analogous to band modulation in semiconductors, they argued that perhaps HOMO and LUMO orbitals could be modulated in a similar way to produce a sort of "molecular rectifier".
In an effort to understand new paradigms in fabrication and molecular electronics, approaches to self-assembly will be presented in this dissertation. The research picks up with the observation that when single-crystal Au(111) was exposed to 1,4-phenylene diisocyanide, long, oligomeric stuctures spontaneously formed. 1,4-phenylene diisocyanide is a well studied and published molecule which has been subjected to a range of single molecule conductivity measurements. The structures are composed of alternating -(Au--PDI)n- subunits, where "Au" is actually an adatom resting in a hollow site on the metal surface.
In this dissertation the structure of the 1,4-PDI oligomers will be discussed at length along with a proposed growth mechanism for the chains. To extend the chemistry further, another system was investigated. 1,4-benzene dithiol (1,4-BDT) is another well studied molecule. However there are large holes in the existing literature, and we report here for the first time that on Au(111), 1,4-BDT undergoes oligomerization chemistry as well. Similarly to 1,4-PDI, 1,4-BDT generates gold adatoms that seem critical in the growth of the oligomers, where either end of the molecule is covalently secured into the chain via RS-Au-SR type linkages.
To examine the electrical properties of these oligomers, granular gold films were prepared using thermal evaporation, and upon exposure to the "molecular wire materials", a dramatic reduction in sheet resistance across the film was observed. It will be demonstrated that this reduction is a result of the oligomers bridging the gap between the particles and not a morphological change in the particles themselves.
To further explore the requirements for the proposed self-assembly chemistry, other molecules were explored. 1,3-phenylene diisocyanide was shown to oligomerize, but due to the 1200 angle between -NC groups, the chains assembled into "zigzag" structures. Curiously, 1,3-BDT was not shown to self assemble and instead is seen as di-adatom monomers at all coverages. Finally, to extend the chemistry further, 1,3,5-phenylene triisocyanide was prepared. The predicted "honeycomb" structures were shown to spontaneously form on Au(111) and represents a viable strategy for the fabless fabrication of a well-defined array of small nanopores, once again templated by gold adatoms in linear -NC-Au-CN- configurations.
The self-assembly chemistry appears to be a rather general phenomenon, and with suitable synthetic modification a wide range of functional hybrid nanomaterials could be envisaged. Adatoms, particularly gold adatoms, are interesting in their own right as they possess higher reactivity than bulk or surface atoms do (a consequence of their low coordination number).
Kestell, John, "The Surface Chemistry of Adatom Mediated Oligomers of Aromatic Diisocyanides and Dithiols on Au(111) and Granular Films" (2014). Theses and Dissertations. 499.