Browsing by Subject "Transition metal dichalcogenide"
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Item Ab-initio electronic structure and quantum transport calculations on quasi-two-dimensional materials for beyond Si-CMOS devices(2013-05) Chang, Jiwon, active 2013; Banerjee, Sanjay; Register, Leonard F.Atomically two-dimensional (2-D) graphene, as well as the hexagonal boron nitride dielectric have been and are continuing to be widely investigated for the next generation nanoelectronic devices. More recently, other 2-D materials and electronic systems including the surface states of topological insulators (TIs) and monolayers of transition metal dichalcogenides (TMDs) have also attracted considerable interest. In this work I have focused on these latter two material systems on possible device applications. TIs are characterized by an insulating bulk band gap and metallic Dirac surface states which are spin-polarized. Here, the electronic structures of bulk and thin film TIs are studied using ab-initio density functional theory (DFT). Band inversion, an essential characteristic of TIs, is shown in the bulk band structures. Properties of TI surface bands in thin film such as the critical film thickness to induce a gap, the thickness dependent gap size, and the localization length of surface states are reported. Effects of crystalline dielectric materials on TI surface states are also addressed by ab-initio calculations. I discuss the sensitivity of Dirac point degeneracy and linear band dispersion of TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and TI. Additionally, this work presents research on exciton condensation in TI using a tight-binding model combined with self-consistent non-local Hartree-Fock mean-field theory. Possibility of exciton condensation in the TI Bi₂Se₃ thin film is assessed. Non-equilibrium Green's function (NEGF) simulations with the atomistic tight-binding (TB) Hamiltonian are carried out to explore the performance of metal-oxide-semiconductor field-effect-transistor (MOSFET) and tunnel field-effect-transistor (TFET) based on the Bi₂Se₃ TI thin film. How the high dielectric constant of Bi₂Se₃ affects the performance of MOSFET and TFET is presented. Bulk TMDs such as MoS₂, WS₂ and others are the van der Waals-bonded layered material, much like graphite, except monolayer (and Bulk) TMDs have a large band gap in-contrast to graphene (and graphite). Here, the performance of nanoscale monolayer MoS₂ n-channel MOSFETs are examined through NEGF simulations using an atomistic TB Hamiltonian. N- and p-channel MOSFETs of various monolayer TMDs are also compared by the same approach. I correlate the performance differences with the band structure differences. Finally, ab-initio calculations of adatom doping effects on the monolayer MoS₂ is shown. I discuss the most stable atomic configurations, the bonding type and the amount of charge transfer from adatom to the monolayer MoS₂.Item Pressure induced structure-property tuning of two dimensional materials(2015-05) Nayak Pradeep, Avinash; Akinwande, Deji; Lin, Jung-Fu; Wang, Yaguo; Wang, Zheng; Hall, Neal; Dodabalapur, AnanthControlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of two dimensional (2D) materials. Here we investigate the electronic and lattice vibrational dynamics of the distorted monolayer 1T-MoS₂ (1T') and the monolayer 2H-MoS₂ via a diamond anvil cell (DAC) and density functional theory (DFT) calculations. The direct optical band gap of the monolayer 2H-MoS₂ increases by 11.7% from 1.85 eV to 2.08 eV, which is the highest reported for a 2D transition metal dichalcogenide (TMD) material. DFT calculations reveal a subsequent decrease in the band gap with eventual metallization of the monolayer 2H-MoS₂, an overall complex structure-property relation due to the rich band structure of MoS₂. Remarkably, the metastable 1T'-MoS₂ metallic state remains invariant with pressure, with the J₂, A₁[subscript g], and E₂[subscript g] modes becoming dominant at high pressures. This substantial reversible tunability of the electronic and vibrational property of the MoS₂ family can be extended to other 2D TMDs. These results present an important advance toward controlling the band structure and optoelectronic properties of monolayer MoS₂ via pressure, which has vital implications for enhanced device applications.Item WSe₂-based devices and oxide structures(2014-05) Tan, Cheng; Akinwande, Deji; Ruoff, Rodney S.In this work the transition metal dichalcogenide WSe₂ is exfoliated and characterized. It is shown from the electrical measurements that the material can display both p-type conduction as well as ambipolar characteristics. WSe₂ flakes were also oxidized through thermal and laser treatments to produce oxide structures. The structures and oxidation processes are characterized and described in this thesis.