Quantum imaging and spectroscopy of molecular diamondoids and topological nanostructures [electronic resource]
Keywords: Applied Physics

Identification and characterization of new and novel materials is one of the major challenges facing the continued advancement of digital electronics. Traditionally, the electronics industry has been dominated by silicon. However, as devices begin to approach atomic scales, state-of-the-art electronics will have to increasingly embrace new and complementary materials, some of which bear little resemblance to their silicon brethren. Molecular systems and crystals that contain novel fermionic states have the potential to rapidly and greatly impact the electronics industry, given the right advances in fabrication and performance. This thesis reports the use of scanning tunneling microscopy to explore a variety of new materials that exhibit novel---and potentially marketable---electronic properties, yet that can also be synthesized using relatively low-cost and straightforward techniques. The first material is the family of diamondoid molecules, which represent an exciting new direction in the field of nanoscale carbon. These molecules are carbon cages consisting of the smallest caged subunits of the diamond lattice, with surface bonds saturated by hydrogen. While theoretically known to be stable, diamondoids have been experimentally inaccessible due to synthesis roadblocks and lack of natural sources, until recently purified from crude oil. This advancement allows for potential access to the unique and extreme properties of diamond (rigidity, thermal conductivity, wide band-gap, and doping behavior, among others) in nanoscale and molecular devices. Of particular interest is the cross-over regime between the molecule-like behavior expected of the smaller diamondoids and the properties of macroscopic diamond. The first part of this dissertation explores the hierarchical nature of these molecules, investigated at the single-molecule level with scanning tunneling microscopy (STM). I will present structural data showing the quality of self-assembled monolayers (SAMs) composed of a series of thiolated diamondoids, and the variation that emerges as the number of diamondoid cages increases. I-V spectroscopy (combined with density functional calculations) allows us to determine the energy band line-ups of the molecular orbitals. The robustness of the SAMs and the insulating behavior implied by spectroscopy suggest that--at the few-eV energy scale typical of STM--diamondoid thiol SAMs may be useful as rigid decoupling layers, tunable by appropriate choice of cage structure. Moving beyond thiolated molecules (which have well-documented uses in the field of molecular electronics), we have begun exploring more exotic diamondoid-based derivates as novel nanoelectronic elements. Over the past few decades, new fields of research have emerged based on the sp2 molecular forms of carbon such as graphene, fullerenes, and nanotubes. Materials that sit at the intersection of the sp2 and sp3 bonding structures are an exciting new area for nanoscale science, combining the unique electronic properties of these two very different hybridizations. I will introduce hybrid molecules that fuse the sp2 and sp3 allotropes-in the form of C60 fullerenes and diamondoids-into one well-defined molecular system. These molecules were synthesized with the intention of creating diode-like elements for single- or few-molecule electronic devices. STM measurements on SAMs of these molecules indeed show evidence of unconventional rectifying behavior. These measurements represent (to our knowledge) the first purely hydrocarbon rectifier, and demonstrate the emerging diversity of electronic phenomena observed in diamondoid-based molecules. The second half of this thesis turns from molecular systems to a particular set of crystalline systems that exhibit electrons that behave as Dirac fermions. These particles are very different from the standard electrons in metals or semiconductors in that they propagate relativistically, despite being confined to a solid state crystal. STM has proven to be an indispensable tool in characterizing the signatures of such particles. These materials have a host of technological applications, so lowering the cost of synthesis is an important research direction. I therefore survey a growth techniques by using STM to look for Dirac fermions in graphene and topological insulator systems. As in the molecular systems introduced above, STM studies are important in these Dirac systems because their unique transport behavior depends critically on their nanoscale properties.