Electronic Properties of Molecular Silicon
Creators: Li, Haixing
Publisher: name: Columbia University
Year: 2017
Subjects: Chemistry, Physical and theoretical
Microphysics
Microchemistry
Silicon

description: This dissertation explores the electronic characteristics of silicon at the single molecule level. This idea is born as we enter the post-Moore’s law era when the exponential shrinking of conventional silicon microelectronics has begun to stall and an investigation of molecular materials is timely. Single-molecule electronic components have shown promising functionalities such as conductors, switches, and diodes, and single molecule junctions have become a widely used test-bed for probing electron transport properties at the molecular level. In this thesis, we use scanning tunneling microscope break junction method to create single molecule junctions with a variety of silicon molecular wires. Our results demonstrate electronic properties of silicon beyond it being a semiconductor in its bulk form.
      We begin this work in pursuit of an expanded understanding of low-k dielectric components with an experimental goal on determining the cause of its breakdown. Low-k dielectrics are beneficial as they enable faster switching speeds and lower heat dissipation, however, they tend to breakdown after prolonged usage under an applied voltage. At the atomic level, low-k dielectric breakdown involves bond rupture. To determine which bond breaks easily, we conduct experimental studies on the robustness of individual chemical bonds that are commonly found in low-k dielectrics. We subject the single molecule junctions to a high bias and investigate the breakdown phenomenon of individual Si-Si, Ge-Ge, Si-O, and Si-C bonds. Among these, Si-C proved to be significantly more durable than the others. To further prove our hypothesis that the Si-Si bond ruptures under the applied high bias, we design a two-path molecular structure consisting of a Si-Si bond in parallel with a naphthyl group. The broken junction shows conduction through the naphthyl pathway, strongly indicating that the Si-Si bond is breaking. This demonstrates a method for probing the bond cleavage under an electric field and provides insights to the weak links in low-k dielectrics.
      Next, we study the fundamental charge transport characteristics of single molecule junctions comprised of Si and Ge-based molecular wires, starting with the simplest form - linear atomic chains. We observe a slower decay of conductance with increasing length in the silanes and germanes than in alkanes, indicating that the electronic delocalization in the Si-Si and Ge-Ge -bonds is stronger than that of the well-studied C-C bonds. Furthermore, we demonstrate that this electronic delocalization in the Si-Si and Ge-Ge bonded backbones enables single-molecule conductance switching. This conductance switch, induced by a mechanical modulation, relies on the nature of the terminal groups and constitutes the first example of a stereoelectronic switch. We also study the molecular conductance of these silanes with metal contacts other than Au, which can potentially open up interesting avenues as metal varies in its electronic states and catalytic activities. We find that Ag electrodes enable higher conductance for thiol-terminated silanes than Au or Pt electrodes.
      The electrical properties of more complex silicon structures - silicon rings - were probed. We choose a five-membered silicon ring as a target system to investigate the effect of isomerism on single molecule conductance. We find that due to the flexibility of the ring, multiple conformations contribute to the spread in the measured conductance for each isomer. This provides us with a starting point to further compare the conductance of a variety of silicon rings. We find that most of the silicon rings are less conductive than their linear counterparts due to the suboptimal backbone conformation for electronic coupling. In particular, destructive quantum interference appears in one of the bicyclic structures and leads to an exceptionally low conductance. This is the first example of a destructive quantum interference feature ever observed experimentally in a π-bonded rather than a σ-bonded system.
      Finally, we investigate the impact of strain on molecular conductance of silanes. In one case, we introduce the strain using a silacyclobutane ring in the backbone. Unexpectedly, we find that ring strain enables a new Au-silacycle binding mode, resulting in a much higher conductance state. In another molecular design, we choose disilaacenaphthene in the backbone. This strained disilane is found to constitute an example of a direct Si-to-Au contact in single molecule circuits, thereby demonstrating a new binding motif that is valuable for designing high conducting molecular components. Taken together, this body of work provides important knowledge about the transport properties of silicon at the nano-scale, as well as insights on the design of silicon components for nanoelectronics. This work represents one step forward to create functional silicon molecular components.