Understanding and engineering molecular interactions and electronic transport at 2D materials interfaces

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references.2D materials are defined as solids with strong in-plane chemical bonds but weak out-of-plane, van der Waals (vdW) interactions. In order to realize potential applications of 2D materials in the areas of optoelectronics, surface modification, and complex materials, there are many engineering challenges associated with understanding and engineering molecular interactions at 2D materials interfaces, which requires understanding and engineering multiscale physical phenomena. With this in mind, the goal of this thesis has been to combine continuum modeling, molecular dynamics (MD) simulations, chemical synthesis, and device fabrication to understand and engineer molecular interactions at 2D materials interfaces at different length scales. The three main topics considered include: (i) wetting behavior of graphene (micrometer scale), (ii) solution processing of graphene and graphene oxide (nanometer scale), and (iii) electronic modification in graphene and molybdenum disulfide (atomic scale). The first part of my thesis investigates the wetting behavior of graphene-coated surfaces. Based on the classical theory of van der Waals interactions, monolayer graphene acts like a "nonlinear translucent" barrier, transmitting about 30% of the original water-substrate interactions through it. The contact angle on a graphene-coated substrate is determined by both liquid-graphene and liquid- substrate interactions. This, in turn, results in different degrees of "wetting transparency". By combining theoretical analysis, MD simulations, and contact angle measurements, I show that monolayer graphene becomes more "transparent" to wetting on hydrophilic substrates and more "opaque" to wetting on hydrophobic substrates. The second part of my thesis develops a fundamental understanding and engineering strategies to disperse graphene and graphene oxide in a liquid phase. The mechanism of stabilization of liquid-phase exfoliated graphene sheets in polar solvents is investigated using potential of mean force (PMF) calculations and MD simulations. Along with a kinetic theory of colloid aggregation, the graphene sheets are predicted to aggregate based on thermodynamic arguments. Because of the different affinities of various solvents for the surface of graphene, efficient solvents can enhance the stability of the graphene sheets by: (i) reducing the depth of the vdW well, and (ii) increasing the energy barrier. Using the calculated PMF curves associated with different solvents, with only one adjustable parameter, the kinetic theory is able to predict the lifetimes of graphene sheets, including ranking the five solvents considered in terms of their ability to stabilize graphene. In addition, I present an advanced concept for the layer-controlled production of pristine large graphene dispersions. The use of ionic graphite intercalation compounds to produce Stage-2 and Stage-3 graphite intercalation compounds (GICs) are shown to be excellent precursors for the production of bilayer and tri-layer graphene dispersions. When combined with an on-chip separation method, a population of large area graphene flakes is produced, such that conventional photolithography is enabled for top-gate device fabrication. My present approach enables the only viable route at this time to produce AB stacked bi- and tri-layer graphene on arbitrary substrates on a large scale. Moreover, a comparative study that combines experiments and MD simulations is carried out to understand the effects of pH on the colloidal stability and surface activity of graphene oxide (GO) aqueous solutions. The reported pH-dependent behavior originates from the degree of deprotonation of the carboxyl groups at the edge of GO sheets. At low pH, the carboxyl groups are protonated, such that the GO sheets become less hydrophilic and form suspended GO aggregates. The GO aggregates formed at lower pH are found to be surface active and do not exhibit the salient critical-micelle-concentration (CMC) feature associated with the formation of surfactant micelles. At higher pH, the carboxyl groups are deprotonated and the strong hydrophilicity of the edge carboxyl groups pulls the GO sheets into bulk water, making GO behave like a regular salt dissolved in water. A series of surface tension measurements further suggests that GO does not behave like a conventional surfactant in both pH 1 and pH 14 aqueous solutions. The third part of my thesis develops engineering strategies to modify electronic characteristics of graphene using molecular adsorption, covalent functionalization, and a molybdenum disulfide (MoS₂) - graphene heterojunction. I investigate the effects of surfactant adsorbates on transport characteristics in graphene transistors. The surfactant adsorbates are found to: (i) transfer electrons to graphene, (ii) scatter carrier transport, and (iii) induce more electron-hole puddles on the SiO₂ substrate. The mechanism behind the unusually observed behaviors can be rationalized using a new theoretical model based on the self-consistent transport theory. I find that the change in transport characteristics is surfactant-dependent, and results from the interactions between the surfactant adsorbates, graphene, and the underlying SiO₂ . In addition, I demonstrate an efficient method to covalently functionalize monolayer and bilayer graphene (MLG and BLG) in a precise and controllable manner using electrochemical aryl diazonium chemistry. Using this method, for the first time, I study the transport characteristics of bottom-gated MLG and dual-gated BLG field effect transistor (FET) devices as a function of the degree of functionalization, which provides insight on the electronic transport in functionalized graphene. I show that the electronic transport in functionalized graphene is limited by the formation of electron-hole puddles and mid-gap states due to chemical functionalization. A more significant transport band gap can be created in functionalized BLG at a highly positive transverse electric displacement field. Moreover, I investigate charge transfer, photoluminescence, and gate-controlled electronic transport in the junction between two 2D materials - MoS₂ and graphene. Without applying any transverse electric fields, there is a significant number of electrons transferred from MoS₂ to graphene due to their work function difference. The charge transfer also results in the formation of a Schottky barrier at the interface, increasing interlayer impedance between the two materials. Despite the interlayer impedance, the quantum yield for MoS2 in the heterostructure is still considerably quenched, since the hot carriers generated in MoS₂ during photoexcitation can overcome the barrier readily, subsequently being collected by the adjacent graphene layer. I fabricate FET devices comprised of the MoS₂ - graphene heterostructure, and show that the interlayer impedance can be further manipulated by the gate and drain voltages, demonstrating a new type of FET device, which enables a controllable transition from NMOS digital to bipolar characteristics. I show that an on/off current ratio ~100 can be achieved without sacrificing the field-effect electron mobilities in graphene. This thesis advances our understanding on how to engineer molecular interactions at 2D materials interfaces. Specifically, I demonstrate that by combining continuum theory, MD simulations, chemical synthesis, and device fabrication, one can elucidate the multiscale physics underlying these interactions, and further propose new engineering approaches to overcome the associated challenges. There is ample opportunity and need for the combined theoretical and experimental studies in this emerging field to understand and design these nanoscale materials for various electronic, energy, and environmental applications. As reflected in this thesis, it is hoped that the interactive connections between theories and experiments, as well as the engineering innovations driven by multiscale understanding, will significantly facilitate the development of 2D materials commercialization.by Chih-Jen Shih.Ph. D