Ferdows Zahid

Electronic and thermoelectric properties of few-layer transition metal dichalcogenides

The electronic and thermoelectric properties of one to four monolayers of MoS2, MoSe2, WS2, and WSe2 are calculated. For few layer thicknesses, the near degeneracies of the conduction band K and Σ valleys and the valence band Γ and K valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within kBT of a valley minimum changes with layer thickness. In all cases, the maximum ZT coincides with the greatest near-degeneracy within kBT of the band edge that results in the sharpest turn-on of the density of modes. The thickness at which this maximum occurs is, in general, not a monolayer. The transition from few layers to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses.

Electrical Conduction through Molecules

Publisher Summary This chapter provides an intuitive description of the current-voltage (I-V) characteristics of molecules and then introduced several simple toy models that capture the basic physics. These toy models are also used to motivate the rigorous non-equilibrium Greens function (NEGF) theory and applied using the Hiickel Hamiltonian to a phenyl dithiol molecule coupled to gold contacts. With both the simple toy models and the NEGF-Hiickel calculations one can find that the three most important properties that determine the I-V characteristics of a molecule are: the position of the energy levels of the molecule relative to the Fermi energy of the contacts. Second, the broadening of the energy levels due to the coupling to the contacts determine the magnitude of the current through the molecule. Lastly, the charging of the molecule broadens the conductance peaks by the charging energy per electron U. This can explain many experimental observations without invoking any additional inelastic scattering. Also, the asymmetry of this charging effect can explain why symmetric molecules give asymmetric I-V when coupled unequally to the two contacts.

Thermoelectric properties of Bi2Te3 atomic quintuple thin films

Motivated by recent experimental realizations of quintuple atomic layer films of Bi2Te3, the thermoelectric figure of merit ZT of the quintuple layer is calculated and found to increase by a factor of 10 (ZT=7.15) compared to that of the bulk at room-temperature. The large enhancement in ZT results from the change in the distribution of the valence band density of modes brought about by the quantum confinement in the thin film. The theoretical model uses ab initio electronic structure calculations (VASP) with full quantum-mechanical structure relaxation combined with a Landauer formalism for the linear-response transport coefficients.

A generic tight-binding model for monolayer, bilayer and bulk MoS2

Molybdenum disulfide (MoS2) is a layered semiconductor which has become very important recently as an emerging electronic device material. Being an intrinsic semiconductor the two-dimensional MoS2 has major advantages as the channel material in field-effect transistors. In this work we determine the electronic structures of MoS2 with the highly accurate screened hybrid functional within the density functional theory (DFT) including the spin-orbit coupling. Using the DFT electronic structures as target, we have developed a single generic tight-binding (TB) model that accurately produces the electronic structures for three different forms of MoS2 - bulk, bilayer and monolayer. Our TB model is based on the Slater-Koster method with non-orthogonal sp3d5 orbitals, nearest-neighbor interactions and spin-orbit coupling. The TB model is useful for atomistic modeling of quantum transport in MoS2 based electronic devices.

A self-consistent transport model for molecular conduction based on extended Hückel theory with full three-dimensional electrostatics

We present a transport model for molecular conduction involving an extended Hückel theoretical treatment of the molecular chemistry combined with a nonequilibrium Greens function treatment of quantum transport. The self-consistent potential is approximated by CNDO (complete neglect of differential overlap) method and the electrostatic effects of metallic leads (bias and image charges) are included through a three-dimensional finite element method. This allows us to capture spatial details of the electrostatic potential profile, including effects of charging, screening, and complicated electrode configurations employing only a single adjustable parameter to locate the Fermi energy. As this model is based on semiempirical methods it is computationally inexpensive and flexible compared to ab initio models, yet at the same time it is able to capture salient qualitative features as well as several relevant quantitative details of transport. We apply our model to investigate recent experimental data on alkane dithiol molecules obtained in a nanopore setup. We also present a comparison study of single molecule transistors and identify electronic properties that control their performance.

Negative differential resistance in bilayer graphene nanoribbons

Lack of a bandgap is one of the significant challenges for application of graphene as the active element of an electronic device. A bandgap can be induced in bilayer graphene by application of a potential difference between the two layers. The simplest geometry for creating such a potential difference is two overlayed graphene nanoribbons independently contacted. Calculations, based on density functional theory and the nonequilibrium Green’s function formalism, show that transmission through such a structure is a strong function of applied bias. The simulated current voltage characteristics mimic the characteristics of resonant tunneling diode featuring negative differential resistance.

Structure and dielectric properties of amorphous high-κ oxides: HfO 2, ZrO 2, and their alloys

High-

Effect of O2 adsorption on electron scattering at Cu"001… surfaces

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Electronic and thermoelectric properties of van der Waals materials with ring-shaped valence bands

metal oxides are a class of materials playing an increasingly important role in modern device physics and technology. Here we report theoretical investigations of the properties of structural and lattice dielectric constants of bulk amorphous metal oxides by a combined approach of classical molecular dynamics (MD) - for structure evolution, and quantum mechanical first principles density function theory (DFT) - for electronic structure analysis. Using classical MD based on the Born-Mayer-Buckingham potential function within a melt and quench scheme, amorphous structures of high-

Oxygen vacancy filament formation in TiO2: A kinetic Monte Carlo study

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