Local Modifications of Interfaces and Electronic Properties on the Molecular Scale with Scanning Probe Microscopy

Abstract

The nanoscience field is shifting focus from characterizing and synthesizing novel nanomaterials, to incorporating nanomaterials into functional devices for photovoltaic, energy, and sensing applications. Furthermore, developing greener materials could reduce the use of expensive, toxic metals in many of these applications while potentially increasing their efficiency. Developing greener fabrication methods could reduce manufacturing costs, save time, and limit resource use. Nanoscale understanding, and control, of how materials interact is critical to rapidly engineer new devices. The research presented in this dissertation is dedicated to measuring local changes in electronic properties and fabricating nanoscale structures using scanning probe microscopy techniques. The first two chapters provide introductory information relevant for this dissertation. Chapter 1 includes pertinent information about the nanoscience field, as well as materials and applications that are related to research in this dissertation. Chapter 1 discusses graphene’s unique properties and its potential use in a wide variety of applications. Graphene’s electronic properties are typically modified through doping to tailor it to a specific application. Current literature demonstrates extensive examples of this doping phenomenon, but most are bulk measurements. This dissertation aims to locally map how dopants affect graphene’s electronic properties at the molecular scale. To do this, we use atomic force microscopy (AFM) techniques due to their versatility and ability to probe features on the nanoscale. Chapter 2 presents the theory and implementation of AFM-based techniques to map local surface topography. Kelvin probe force microscopy (KPFM) is used to spatially map local surface potential changes on samples. In Chapters 3 and 4, we proposed using azulene as a novel, tunable dopant for graphene. Azulene is a planar, conjugated ring system with complementary frontier molecular orbital density. Therefore, the energy of either azulene’s highest occupied molecular orbital or lowest unoccupied molecular orbital can be selectively tuned based on the location and type of substituent placed around the azulene backbone. This provides a framework for which to study how manipulating a physisorbed dopant’s energy level alignment with the substrate’s Fermi level affects local surface potential. In our experiments, we used highly ordered pyrolytic graphite (HOPG) as a model for how these molecules would interact with graphene. HOPG and graphene have similar chemical and electronic properties, but graphite substrates are more consistent and reproducible. Using graphite substrates allowed for reliable comparisons of molecules’ behavior across multiple samples. The beginning of Chapter 3 focuses on optimizing deposition techniques for azulene-based molecules on graphite. Vapor deposition proved to be the simplest and cleanest deposition method. This technique was subsequently applied to other substituted azulenes in Chapter 4. Noncontact atomic force microscopy (NCAFM) and Kelvin probe microscopy (KPFM) were conducted simultaneously on samples to obtain local maps of topography and surface potential. The remainder of Chapter 3 focuses on analysis for azulene and naphthalene on graphite, while Chapter 4 expands these techniques to other substituted azulenes. We were able to identify single molecules adsorbed to graphite’s terraces using topographical cross-sections, as well as resolve changes in the relative local contact potential differences with KPFM when different molecules were adsorbed on graphite. These distinct differences suggest that there is a critical relationship between the dopant’s molecular structure and the doping effect. However, this relationship is not straightforward. We found this doping effect to be localized to where the dopant is adsorbed on the substrate. While Chapters 3 and 4 center around probing the arrangement and properties of dopants on the molecular scale, Chapter 5 focuses on growing metal nanostructures with a defined size, shape, and orientation on a surface. Chapter 5 aims to develop a novel resist layer to increase the versatility of bottom-up metal nanostructure growth. Having control over the size, shape, and placement of metallic nanostructures on surfaces is integral for plasmonic sensors. Typical organic resist layers used in bottom-up metal nanostructure fabrication can only be used on certain substrates, require harsh reagents to remove from the surface, and cannot withstand the high temperatures required for certain metal deposition techniques. A thin layer of sodium chloride (NaCl) could behave as a versatile, robust resist layer that can withstand the high temperatures required for thermal evaporation or atomic layer deposition (ALD). Such a resist layer could easily be removed using water, instead of harsh reagents. We demonstrated that small NaCl nanoparticles could be manipulated with scanning probe lithography on a Si(111) surface to create recognizable patterns that could withstand gold-coating via thermal evaporation. Any NaCl nanoparticles not completely encased in gold were able to be removed, leaving a recognizable imprint in the deposited gold metal. A method for depositing a uniform NaCl layer needs to be pursued, but this proof of concept experiment has demonstrated its use as a versatile resist layer for bottom-up metal nanostructure fabrication. Chapter 6 summarizes key conclusions from this research and proposes future directions to expand what we’ve learned. Locally probing and controlling nanoscale interactions is crucial to understanding how heterostructures behave as nanomaterials are incorporated into more complicated devices. Increasing the flexibility of fabrication methods is important for scaling up nanodevice manufacturing. The research in this dissertation advances knowledge required for using nanomaterials for practical applications.