Browsing by Subject "Monolayer"
Now showing 1 - 3 of 3
- Results Per Page
- Sort Options
Item Electronic properties and electron-electron interaction effects in transition metal dichalcogenides(2018-08-09) Larentis, Stefano; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; MacDonald, Allan H; Register, Leonard F; Shi, LiTransition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter.Item Experimental and theoretical investigation of electrochemically synthesized AuPt dendrimer-encapsulated nanoparticles (DENs)(2020-06-22) Lapp, Aliya Siegel; Crooks, Richard M. (Richard McConnell); Henkelman, Graeme; Mullins, Charles B; Humphrey, Simon M; Roberts, Sean THerein, experiment and theory are combined to study the efficacy of synthesizing core@shell Au@Pt dendrimer-encapsulated nanoparticles (DENs) through electrochemical means. DENs are small (~1-2 nm), catalytically active nanoparticles (NPs) with precise sizes and compositions. These features assist pairing experiment with theory and catalytic interpretation. The small sizes of DENs can impart interesting physical and chemical properties that differ from bulk phase materials. Core@Pt shell NPs with monolayer (ML)-thick shells minimize the use of Pt, which is a key catalyst for various reactions but is expensive and scarce. Owing to the fact that the available techniques for Pt ML deposition were originally developed for bulk Au, their applicability to ~1.6 nm Au DEN cores is not trivial. In this dissertation, we explore several electrochemical strategies for synthesizing Au@Pt DENS: hydride-terminated (HT) Pt electrodeposition and underpotential deposition, followed by galvanic exchange with Pt (UPD/Pt GE) using two different UPD metals (Cu and Pb). Each of these techniques deposits a single Pt ML onto bulk Au surfaces. However, when they are applied to ~1.6 nm Au DENs, the AuPt NP structures that form are both dissimilar to one another and to the corresponding structures for bulk Au. The HT synthesis is found to lead to an alloy structure, whereas AuPt NP structure formed upon UPD/Pt GE depends on the choice of the UPD metal (Cu or Pb). More specifically, Cu UPD/Pt GE produces a core@shell structure, whereas an alloy structure is afforded by Pb UPD/Pt GE. These conclusions are supported by extensive experimental characterization and density functional theory (DFT) calculations. For the HT method, a core@shell structure can ultimately be obtained, but requires 3-5 total HT pulses. Due to the fact that catalysis tends to be highly structure sensitive, the results of these studies are important for rational design of catalysts. Indeed, we show that varying the number of HT pulses (from 1-10) can be used to tune electrocatalysis for formic acid oxidation (FAO).Item Template directed self-assembly of particle monolayers(2020-08-14) Zhu, Xilan; Bonnecaze, R. T. (Roger T.); Truskett, Thomas Michael, 1973-; Yu, Edward T; Willson, Carlton G; Lynd, Nathaniel AThe template directed self-assembly (TDSA) is a promising method for cost-efficient and robust fabrication of nanoscale features. In this dissertation, we study the TDSA of particle monolayers to assess the possibility of its application in manufacturing. The 2D directed self-assembly of micron-sized hard spherical particles within square confinements is studied theoretically and experimentally. Grand Canonical Monte Carlo simulations are used to predict the equilibrium packing structures of particles in square wells of specific dimensions. Spin coating and mechanical rubbing processes are used to direct the self-assembly of a monolayer of particles into the square wells. The precision of the graphoepitaxy decreases as template size increases. Both simulations and experiments show that ordered graphoepitaxy of square lattices vanishes with square templates of side length greater than five times the particle diameter. Missing particles and polydispersity of particles are two main causes disrupting the square packing structure in cells of three to five particle diameter side length. The phase diagram of a monolayer of soft particles described by the Daoud–Cotton model for star polymers is presented. Ground state calculations and grand canonical Monte Carlo simulations are used to determine the phase behavior as a function of the number of arms on the star and the areal coverage of the soft particles. The phase diagram exhibits rich behavior including reentrant melting and freezing and solid–solid transitions with triangular, stripe, honeycomb and kagome phases. These structures in 2D are analogous to the structures observed in 3D. The evolution of the structure factor with density is qualitatively similar to that measured in experiments for polymer grafted nanocrystals [Chen et al., Macromolecules, 2017, 50, 9636]. The formation of particle chains with long range order and orientational control is studied computationally. Monolayers of spherical particle usually self-assemble into isotropic structures. It has been found that some core-shell particles undergo anisotropic compression at elevated density when the particle interaction is piecewise. We expected these particles to self-assemble into aligned particle chains with controlled orientation under the guidance of some parallel confinements. Grand Canonical Monte Carlo simulations are used to predict the equilibrium packing structures of particle monolayers with parallel guidelines. Two different core-shell particles are studied. A graphoepitaxy behavior is observed and the formation of ordered particle chains is found for both particles at certain packing density with widely spaced templates. The orientation of formed particle chains can be controlled with the rigid template and the particle spacing is predicted. The self-assembled particle monolayer is an important alternative for photolithography in nanofabrication. The intolerable high defect rate has been the main challenge before the process can be used in the fabrication of high precision structures. The soft repulsive interaction is introduced for the quality enhancement of square packing structures formed from the template directed self-assembly of particle monolayers. An obvious reduction on the defect rate is observed with the star-polymer-like particles. Also, the soft interaction brings more flexibility in the template design for target structures and the tuning of particle spacing is possible.