Mahesh Neupane

Tin Disulfide—An Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on-off current ratios >10(6), as well as carrier mobilities up to 230 cm(2)/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-k (deionized water) top gate. SnS2 transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states.

Direct Bandgap Transition in Many-Layer MoS2 by Plasma-Induced Layer Decoupling

We report a robust method for engineering the optoelectronic properties of many-layer MoS2 using low-energy oxygen plasma treatment. Gas phase treatment of MoS2 with oxygen radicals generated in an upstream N2 -O2 plasma is shown to enhance the photoluminescence (PL) of many-layer, mechanically exfoliated MoS2 flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS2 from indirect to direct bandgap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS2 . Ab initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 many-layer MoS2 flakes (2-15 layers) tested, indicating that this method is robust and scalable. Monolayer MoS2 , while direct bandgap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct bandgap of monolayer MoS2 , without reducing sample thickness, and hence optical density.

Synthesis of Atomically Thin

Atomically thin molybdenum disulfide (MoS2) triangles and hexagrams were prepared by a two-step growth ambient pressure chemical vapor deposition (APCVD) process. Molybdenum Trioxide (MoO3) nanobelts, a few microns in length and width, were prepared using a hydrothermal technique and utilized as the starting material. High temperature treatment of the MoO3 nanobelts followed by a rigorous sulfurization via APCVD processing provided different morphologies of MoS2 monolayers and bilayer (BL) sheets. Triangle and hexagram morphologies were characterized using Raman spectroscopy, photoluminescence (PL) measurements, scanning electron microscopy and atomic force microscopy (AFM). The regrowth step in the CVD process was proven to be ideal in enlarging the grain size. PL and Raman spectroscopy and AFM results confirmed the presence of monolayer and BL regions in the regrowth growth process. Triangle and hexagram domains are observed to be cooperatively nucleating and coalescing together to form large-area layers. Furthermore, the electrical transport properties of the synthesized MoS2 layers were studied. Electron mobility based on back gated field effect transistors was measured to be approximately 0.02 cm2/V. S.

{\bf MoS}_{\bf 2}

The coherent, interlayer resistance of a misoriented, rotated interface between two stacks of AB graphite is determined for a variety of misorientation angles. The quantum-resistance of the ideal AB stack is on the order of 1 to 10 mΩ μm2. For small rotation angles, the coherent interlayer resistance exponentially approaches the ideal quantum resistance at energies away from the charge neutrality point. Over a range of intermediate angles, the resistance increases exponentially with cell size for minimum size unit cells. Larger cell sizes, of similar angles, may not follow this trend. The energy dependence of the interlayer transmission is described.

Triangles and Hexagrams and Their Electrical Transport Properties

The finite number, random placement, and discrete nature of the dopants in the source of an InAs nanowire tunnel field-effect transistor affect the drive current and the inverse subthreshold slope. The impact of source scattering is negligible, since the current is limited by the interband tunneling. The most significant effect of the discrete dopants is to create variations of the electric fields in the tunnel barrier, which cause variations in the current. The relative variation in the ON current decreases as the average doping density and/or nanowire diameter increases. Results from full self-consistent nonequilibrium Greens function calculations and semiclassical calculations are compared.

The coherent interlayer resistance of a single, rotated interface between two stacks of AB graphite

The effect of the Ge core size on the confinement energies, barrier heights, and hole lifetimes in spherical Ge/Si core-shell nanocrystals is studied using an atomistic, tight-binding model with an sp3 d5 s* basis including spin-orbit coupling. Nanocrystal diameters range from 11 nm to 17.5 nm with Ge core diameters ranging from 1 nm to 7.5 nm. With a Ge core diameter of ~4 nm, and a Si shell thickness of ≥3 nm, the thermionic barrier presented by the Si shell increases the hole lifetime by a factor of ~2×108 compared to the hole lifetime in an all-Si nanocrystal in SiO2. A retention lifetime of 10 years is obtained with a 3 nm Ge core and a 3 nm Si shell with a 3 nm SiO2 tunnel oxide.

Effect of Random, Discrete Source Dopant Distributions on Nanowire Tunnel FETs

The coherent, interlayer conductance of misoriented bilayer graphene ribbons is a strong function of the Fermi energy and magnetic field. Edge states can result in a large peak in the interlayer transmission at the charge neutrality point that is several orders of magnitude larger than the surrounding low-energy transmission. The coherent interlayer conductance is consistently asymmetric around the charge neutrality point for all structures with the value differing by up to 3 orders of magnitude at Ef = ±0.05 eV. The low-energy states exhibit a high magnetoconductance ratio, and the magnetoconductance ratio tends to increase as the width of the ribbons decrease. The maximum value for the 35 nm wide bilayer ribbons at 10 T is 15 000%. Non-equilibrium Greens function calculations of the interlayer transport properties are also supported by semi-analytical calculations based on Fermis Golden Rule.

Core size dependence of the confinement energies, barrier heights, and hole lifetimes in Ge-core/Si-shell nanocrystals

Ge/Si nanocrystals can serve as charge storage sites in a nanocrystal memory by providing a hole quantum-well in the Ge region. The electronic states of realistically shaped Ge/Si nanocrystals with crescent-shaped Ge-cores are calculated to determine the hole confinement energies, effective masses, barrier heights, and thermionic lifetimes. As the Ge crescent thickness increases from 1 nm to 3.5 nm, the hole confinement energy decreases from 0.52 to 0.28 eV, the barrier height to escape into the Si valence band increases from 0.25 to 0.51 eV, and the resulting thermionic hole lifetime increases from 10−9 to 10−5 s. The nanocrystals are modeled with an atomistic, 20-band sp3d5s* tight-binding model including spin-orbit coupling as implemented in NEMO3D. Geometry relaxation and strain are included using the valence-force-field model with Keating potentials.

Interlayer magnetoconductance of misoriented bilayer graphene ribbons

The electronic structure and optical properties of Ge-core/Si-shell nanocrystal or quantum dot (QD) are investigated using the atomistic tight binding method as implemented in NEMO3D. The thermionic lifetime that governs the hole leakage mechanism in the Ge/Si QD based laser, as a function of the Ge core size and strain, is also calculated by capturing the bound and extended eigenstates, well below the band edges. We also analyzed the eect of core size and strain on optical properties such as transition energies and transition rates between electron and hole states. Finally, a quantitative and qualitative analysis of the leakage current due to the hole leakage through the Ge-core/Si-shell QD laser, at dierent temperatures and Ge core sizes, is presented.

Electronic states of Ge/Si nanocrystals with crescent-shaped Ge-cores

Exciton condensation in an electron-hole bilayer system of monolayer transition metal dichalcogenides is analyzed at three different levels of theory to account for screening and quasiparticle renormalization. The large effective masses of the transition metal dichalcogenides place them in a strong coupling regime. In this regime, mean field (MF) theory with either an unscreened or screened interlayer interaction predicts a room temperature condensate. Interlayer and intralayer interactions renormalize the quasiparticle dispersion, and this effect is included in a GW approximation. The renormalization reverses the trends predicted from the unscreened or screened MF theories. In the strong coupling regime, intralayer interactions have a large impact on the magnitude of the order parameter and its functional dependencies on effective mass and carrier density.