Integrated Circuits and Logic Operations Based on Single-Layer MoS2

Logic circuits and the ability to amplify electrical signals form the functional backbone of electronics along with the possibility to integrate multiple elements on the same chip. The miniaturization of electronic circuits is expected to reach fundamental limits in the near future. Two-dimensional materials such as single-layer MoS2 represent the ultimate limit of miniaturization in the vertical dimension, are interesting as building blocks of low-power nanoelectronic devices, and are suitable for integration due to their planar geometry. Because they are less than 1 nm thin, 2D materials in transistors could also lead to reduced short channel effects and result in fabrication of smaller and more power-efficient transistors. Here, we report on the first integrated circuit based on a two-dimensional semiconductor MoS2. Our integrated circuits are capable of operating as inverters, converting logical “1” into logical “0”, with room-temperature voltage gain higher than 1, making them suitable for incorporation into digital circuits. We also show that electrical circuits composed of single-layer MoS2 transistors are capable of performing the NOR logic operation, the basis from which all logical operations and full digital functionality can be deduced

Atomic Scale Microstructure and Properties of Se-Deficient Two-Dimensional MoSe2

We study the atomic scale microstructure of non-stoichiometric two-dimensional(2D) transition metal dichalcogenide MoSe2-x, by employing aberration-corrected high-resolution transmission electron microscopy. We show that a Se-deficit in single layers of MoSe2 grown by molecular beam epitaxy gives rise to a dense network of mirror-twin-boundaries (MTBs) decorating the 2D-grains. With the use of density functional theory calculations, we further demonstrate that MTBs are thermodynamically stable structures in Se-deficient sheets. These line defects host spatially localized states with energies close to the valence band minimum, thus giving rise to enhanced conductance along straight MTBs. However, electronic transport calculations show that-the transmission of hole charge carriers across MTBs is strongly suppressed due to band bending effects. We further observe formation of MTBs during in situ removal of Se atoms by the electron beam of the microscope, thus confirming that MTBs appear due-to Se-deficit, and not coalescence of individual grains during growth. At a very high local Se-deficit, the 2D sheet becomes unstable and transforms to a nanowire. Our results on Se-deficient MoSe2 suggest routes toward engineering the properties of 2D transition Metal dichalcogenides by deviations from the stoichiometric composition

Light activated Cl₂ etching of GaAs and optical holographic pattern formation

The photochemical reaction of Cl₂ gas with (100) GaAs was studied in this work. Thermal effects due to light induced heating of the GaAs substrate were isolated from photochemical effects. Light induced heating was calculated and the corresponding photothermal Cl₂ etching of GaAs was accounted for. The temperature dependence of the photochemical etch rate was examined. The light-induced etch rate potentially follows an Arrhenius temperature dependence with an activation energy of 0.161 eV. Near 200°C the light-induced etch rate does not increase with increasing temperature. The photochemical etch rate was found to depend linearly on intensity. A possible explanation for the temperature dependence of the photochemical etch rates is presented. It is proposed that the desorption of Ga Cl₃ is the important mechanism in the photochemical Cl₂-GaAs etch. GaCl₃ is photodesorbed with illumination, which increases the rate of etching.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Surface evolution during gallium arsenide homoepitaxy with molecular beam epitaxy

GaAs grown with MBE is the basis for many useful optoelectric devices. Measurements are presented of the smoothing of patterned and randomly roughened GaAs surfaces during homoepitaxy over a large range of Ga flux, substrate temperatures, arsenic fluxes, and Bi surfactant. The bulk of these measurements were taken by in-situ elastic light scattering or ex-situ AFM. These measurements provide experimental support for a non-linear continuum growth model that has been derived analytically from basic atomic level phenomena that occur in epitaxial film growth. During epitaxial growth the smoothing is observed to change in nature as the surface amplitude decreases. One of the regimes of smoothing is associated with the linear smoothing coefficients from the physically based non-linear continuum growth equation. The temperature and growth rate dependence of the smoothing coefficients are presented and found to be in good agreement with predictions from the continuum growth model. A key parameter in the continuum growth equation, the density of atomic steps, is measured independently using AFM. The step density, which agrees with theoretical predictions, is used to compute smoothing coefficients and is shown to be in agreement with the light scattering measurements. Complex shapes are observed for epitaxial growth on patterned GaAs substrates. Two characteristic surface morphologies were observed. The first is characterized by downward V-shaped cusps and rounded mounds caused by non-linear smoothing. The second morphology is similar, however the symmetry of the surface structure was inverted. This surface morphology has not been previously observed in GaAs. Step edge attachment was found to be the driving mechanism that produced both of these morphologies. Bismuth is observed to act as a surfactant in GaAs homoepitaxy. While Bi assisted growth is found to decrease the overall surface roughness, it is also found to alter the characteristics of the surface morphology. Notably, roughness at low spatial frequency was increased with the addition of Bi, while at high spatial frequency roughness was decreased. Significant changes to the shape evolution of patterned substrate are also observed when Bi is added to GaAs epitaxial growth.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Atomic Scale Microstructure and Properties of Se-Deficient Two-Dimensional MoSe₂

We study the atomic scale microstructure of nonstoichiometric two-dimensional (2D) transition metal dichalcogenide MoSe_2–<i>x</i> by employing aberration-corrected high-resolution transmission electron microscopy. We show that a Se-deficit in single layers of MoSe₂ grown by molecular beam epitaxy gives rise to a dense network of mirror-twin-boundaries (MTBs) decorating the 2D-grains. With the use of density functional theory calculations, we further demonstrate that MTBs are thermodynamically stable structures in Se-deficient sheets. These line defects host spatially localized states with energies close to the valence band minimum, thus giving rise to enhanced conductance along straight MTBs. However, electronic transport calculations show that the transmission of hole charge carriers across MTBs is strongly suppressed due to band bending effects. We further observe formation of MTBs during <i>in situ</i> removal of Se atoms by the electron beam of the microscope, thus confirming that MTBs appear due to Se-deficit, and not coalescence of individual grains during growth. At a very high local Se-deficit, the 2D sheet becomes unstable and transforms to a nanowire. Our results on Se-deficient MoSe₂ suggest routes toward engineering the properties of 2D transition metal dichalcogenides by deviations from the stoichiometric composition

Atomic Scale Microstructure and Properties of Se-Deficient Two-Dimensional MoSe₂

We study the atomic scale microstructure of nonstoichiometric two-dimensional (2D) transition metal dichalcogenide MoSe_2–<i>x</i> by employing aberration-corrected high-resolution transmission electron microscopy. We show that a Se-deficit in single layers of MoSe₂ grown by molecular beam epitaxy gives rise to a dense network of mirror-twin-boundaries (MTBs) decorating the 2D-grains. With the use of density functional theory calculations, we further demonstrate that MTBs are thermodynamically stable structures in Se-deficient sheets. These line defects host spatially localized states with energies close to the valence band minimum, thus giving rise to enhanced conductance along straight MTBs. However, electronic transport calculations show that the transmission of hole charge carriers across MTBs is strongly suppressed due to band bending effects. We further observe formation of MTBs during <i>in situ</i> removal of Se atoms by the electron beam of the microscope, thus confirming that MTBs appear due to Se-deficit, and not coalescence of individual grains during growth. At a very high local Se-deficit, the 2D sheet becomes unstable and transforms to a nanowire. Our results on Se-deficient MoSe₂ suggest routes toward engineering the properties of 2D transition metal dichalcogenides by deviations from the stoichiometric composition

Creating Law at the Securities and Exchange Commission: The Lawyer as Prosecutor

Transition metal dichalcogenides (TMDCs), together with other two-dimensional (2D) materials, have attracted great interest due to the unique optical and electrical properties of atomically thin layers. In order to fulfill their potential, developing large-area growth and understanding the properties of TMDCs have become crucial. Here, we have used molecular beam epitaxy (MBE) to grow atomically thin MoSe₂ on GaAs(111)­B. No intermediate compounds were detected at the interface of as-grown films. Careful optimization of the growth temperature can result in the growth of highly aligned films with only two possible crystalline orientations due to broken inversion symmetry. As-grown films can be transferred onto insulating substrates, allowing their optical and electrical properties to be probed. By using polymer electrolyte gating, we have achieved ambipolar transport in MBE-grown MoSe₂. The temperature-dependent transport characteristics can be explained by the 2D variable-range hopping (2D-VRH) model, indicating that the transport is strongly limited by the disorder in the film