Electron and Hole Mobilities in Single-Layer WSe₂

Single-layer transition metal dichalcogenide WSe₂ has recently attracted a lot of attention because it is a 2D semiconductor with a direct band gap. Due to low doping levels, it is intrinsic and shows ambipolar transport. This opens up the possibility to realize devices with the Fermi level located in the valence band, where the spin/valley coupling is strong and leads to new and interesting physics. As a consequence of its intrinsically low doping, large Schottky barriers form between WSe₂ and metal contacts, which impede the injection of charges at low temperatures. Here, we report on the study of single-layer WSe₂ transistors with a polymer electrolyte gate (PEO:LiClO₄). Polymer electrolytes allow the charge carrier densities to be modulated to very high values, allowing the observation of both the electron- and the hole-doped regimes. Moreover, our ohmic contacts formed at low temperatures allow us to study the temperature dependence of electron and hole mobilities. At high electron densities, a re-entrant insulating regime is also observed, a feature which is absent at high hole densities

Nonvolatile Memory Cells Based on MoS₂/Graphene Heterostructures

Memory cells are an important building block of digital electronics. We combine here the unique electronic properties of semiconducting monolayer MoS₂ with the high conductivity of graphene to build a 2D heterostructure capable of information storage. MoS₂ acts as a channel in an intimate contact with graphene electrodes in a field-effect transistor geometry. Our prototypical all-2D transistor is further integrated with a multilayer graphene charge trapping layer into a device that can be operated as a nonvolatile memory cell. Because of its band gap and 2D nature, monolayer MoS₂ is highly sensitive to the presence of charges in the charge trapping layer, resulting in a factor of 10⁴ difference between memory program and erase states. The two-dimensional nature of both the contact and the channel can be harnessed for the fabrication of flexible nanoelectronic devices with large-scale integration

Ripples and Layers in Ultrathin MoS₂ Membranes

Single-layer molybdenum disulfide (MoS₂) is a newly emerging two-dimensional semiconductor with a potentially wide range of applications in the fields of nanoelectronics and energy harvesting. The fact that it can be exfoliated down to single-layer thickness makes MoS₂ interesting both for practical applications and for fundamental research, where the structure and crystalline order of ultrathin MoS₂ will have a strong influence on electronic, mechanical, and other properties. Here, we report on the transmission electron microscopy study of suspended single- and few-layer MoS₂ membranes with thicknesses previously determined using both optical identification and atomic force microscopy. Electron microscopy shows that monolayer MoS₂ displays long-range crystalline order, although surface roughening has been observed with ripples which can reach 1 nm in height, just as in the case of graphene, implying that similar mechanisms are responsible for the stability of both two-dimensional materials. The observed ripples could explain the degradation of mobility in MoS₂ due to exfoliation. We also find that symmetry breaking due to the reduction of the number of layers results in distinctive features in electron-beam diffraction patterns of single- and multilayer MoS₂, which could be used as a method for identifying single layers using only electron microscopy. The isolation of suspended single-layer MoS₂ membranes will improve our understanding of two-dimensional systems, their stability, and the interplay between their structures, morphologies, and electrical and mechanical properties

Suppressing Nucleation in Metal–Organic Chemical Vapor Deposition of MoS₂ Monolayers by Alkali Metal Halides

Toward the large-area deposition of MoS₂ layers, we employ metal–organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS₂ grown this way exhibits high crystallinity and optoelectronic quality comparable to single-crystal MoS₂ produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS₂ after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm² V^–1 s^–1 at cryogenic temperatures

MoS₂ Transistors Operating at Gigahertz Frequencies

The presence of a direct band gap− and an ultrathin form factor has caused a considerable interest in two-dimensional (2D) semiconductors from the transition metal dichalcogenides (TMD) family with molybdenum disulfide (MoS₂) being the most studied representative of this family of materials. While diverse electronic elements,, logic circuits,, and optoelectronic devices, have been demonstrated using ultrathin MoS₂, very little is known about their performance at high frequencies where commercial devices are expected to function. Here, we report on top-gated MoS₂ transistors operating in the gigahertz range of frequencies. Our devices show cutoff frequencies reaching 6 GHz. The presence of a band gap also gives rise to current saturation, allowing power and voltage gain, all in the gigahertz range. This shows that MoS₂ could be an interesting material for realizing high-speed amplifiers and logic circuits with device scaling expected to result in further improvement of performance. Our work represents the first step in the realization of high-frequency analog and digital circuits based on 2D semiconductors

Electrical Transport Properties of Single-Layer WS₂

We report on the fabrication of field-effect transistors based on single layers and bilayers of the semiconductor WS₂ and the investigation of their electronic transport properties. We find that the doping level strongly depends on the device environment and that long <i>in situ</i> annealing drastically improves the contact transparency, allowing four-terminal measurements to be performed and the pristine properties of the material to be recovered. Our devices show n-type behavior with a high room-temperature on/off current ratio of ∼10⁶. They show clear metallic behavior at high charge carrier densities and mobilities as high as ∼140 cm²/(V s) at low temperatures (above 300 cm²/(V s) in the case of bilayers). In the insulating regime, the devices exhibit variable-range hopping, with a localization length of about 2 nm that starts to increase as the Fermi level enters the conduction band. The promising electronic properties of WS₂, comparable to those of single-layer MoS₂ and WSe₂, together with its strong spin–orbit coupling, make it interesting for future applications in electronic, optical, and valleytronic devices

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

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

Intervalley Scattering of Interlayer Excitons in a MoS₂/MoSe₂/MoS₂ Heterostructure in High Magnetic Field

Degenerate extrema in the energy dispersion of charge carriers in solids, also referred to as valleys, can be regarded as a binary quantum degree of freedom, which can potentially be used to implement valleytronic concepts in van der Waals heterostructures based on transition metal dichalcogenides. Using magneto-photoluminescence spectroscopy, we achieve a deeper insight into the valley polarization and depolarization mechanisms of interlayer excitons formed across a MoS₂/MoSe₂/MoS₂ heterostructure. We account for the nontrivial behavior of the valley polarization as a function of the magnetic field by considering the interplay between exchange interaction and phonon-mediated intervalley scattering in a system consisting of Zeeman-split energy levels. Our results represent a crucial step toward the understanding of the properties of interlayer excitons with strong implications for the implementation of atomically thin valleytronic devices