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The Nernst effect, a transverse thermoelectric phenomenon, has attracted significant attention for its potential in energy conversion, thermoelectrics, and spintronics. However, achieving high performance and versatility at low temperatures remains elusive. Here, we demonstrate a large and electrically tunable Nernst effect by combining graphene's electrical properties with indium selenide's semiconducting nature in a field-effect geometry. Our results establish a novel platform for exploring and manipulating this thermoelectric effect, showcasing the first electrical tunability with an on/off ratio of 10^3. Moreover, photocurrent measurements reveal a stronger photo-Nernst signal in the Gr/InSe heterostructure compared to individual components. Remarkably, we observe a record-high Nernst coefficient of 66.4 {\mu}V K^(-1) T^(-1) at ultra-low temperatures and low magnetic fields, paving the way toward applications in quantum information and low-temperature emergent phenomena.

Two-dimensional flat-band systems have recently attracted considerable interest due to the rich physics unveiled by emergent phenomena and correlated electronic states at van Hove singularities. However, the difficulties in electrically detecting the flat band position in field-effect structures are slowing down the investigation of their properties. In this work, we employ Indium Selenide (InSe) as a flat-band system due to a van Hove singularity at the valence band edge in a few-layer form of the material without the requirement of a twist angle. We investigate tunneling photocurrents in gated few-layer InSe structures and relate them to ambipolar transport and photoluminescence measurements. We observe an appearance of a sharp change in tunneling mechanisms due to the presence of the van Hove singularity at the flat band. We further corroborate our findings by studying tunneling currents as a reliable probe for the flat-band position up to room temperature. Our results create an alternative approach to studying flat-band systems in heterostructures of 2D materials.

While most neuromorphic systems are based on nanoscale electronic devices, nature relies on ions for energy-efficient information processing. Therefore, finding memristive nanofluidic devices is a milestone toward realizing electrolytic computers mimicking the brain down to its basic principles of operation. Here, we present a nanofluidic device designed for circuit scale in-memory processing that combines single-digit nanometric confinement and large entrance asymmetry. Our fabrication process is scalable while the device operates at the second timescale with a conductance ratio in the range 10-60. In-operando optical microscopy unveils the origin of memory, arising from the reversible formation of liquid blisters modulating the device conductance. The combination of features of these mechano-ionic memristive switches permits assembling logic circuits composed of two interactive devices and an ohmic resistor. These results open the way to design multi-component ionic machinery, such as nanofluidic neural networks, and implementing brain-inspired ionic computations.

Transition-metal dichalcogenide bilayers exhibit a rich exciton landscape including layer-hybridized excitons, i.e. excitons which are of partly intra- and interlayer nature. In this work, we study hybrid exciton-exciton interactions in naturally stacked WSe$_2$ homobilayers. In these materials, the exciton landscape is electrically tunable such that the low-energy states can be rendered more or less interlayer-like depending on the strength of the external electric field. Based on a microscopic and material-specific many-particle theory, we reveal two intriguing interaction regimes: a low-dipole regime at small electric fields and a high-dipole regime at larger fields, involving interactions between hybrid excitons with a substantially different intra- and interlayer composition in the two regimes. While the low-dipole regime is characterized by weak inter-excitonic interactions between intralayer-like excitons, the high-dipole regime involves mostly interlayer-like excitons which display a strong dipole-dipole repulsion and give rise to large spectral blue-shifts and a highly anomalous diffusion. Overall, our microscopic study sheds light on the remarkable electrical tunability of hybrid exciton-exciton interactions in atomically thin semiconductors and can guide future experimental studies in this growing field of research., Comment: 8+8 pages, 4+2 figures

Intensive research is carried out on two-dimensional materials, in particular molybdenum disulfide, towards high-performance transistors for integrated circuits. Fabricating transistors with ohmic contacts is challenging due to the high Schottky barrier that severely limits the transistors' performance. Graphene-based heterostructures can be used in addition or as a substitute for unsuitable metals. We present lateral heterostructure transistors made of scalable chemical vapor-deposited molybdenum disulfide and chemical vapor-deposited graphene with low contact resistances of about 9 k${\Omega}$${\mu}$m and high on/off current ratios of 10${^8}$. We also present a theoretical model calibrated on our experiments showing further potential for scaling transistors and contact areas into the few nanometers range and the possibility of a strong performance enhancement by means of layer optimizations that would make transistors promising for use in future logic circuits., Comment: 39 pages

Led by the rise of the internet of things, the world is experiencing exponential growth of generated data. Data-driven algorithms such as signal processing and artificial neural networks are required to process and extract meaningful information from it. They are, however, seriously limited by the traditional von-Neuman architecture with physical separation between processing and memory, motivating the development of in-memory computing. This emerging architecture is gaining attention by promising more energy-efficient computing on edge devices. In the past few years, two-dimensional materials have entered the field as a material platform suitable for realizing efficient memory elements for in-memory architectures. Here, we report a large-scale integrated 32x32 vector-matrix multiplier with 1024 floating-gate field-effect transistors (FGFET) that use monolayer MoS2 as the channel material. In our wafer-scale fabrication process, we achieve a high yield and low device-to-device variability, which are prerequisites for practical applications. A statistical analysis shows the potential for multilevel and analog storage with a single programming pulse, allowing our accelerator to be programmed using an efficient open-loop programming scheme. Next, we demonstrate reliable, discrete signal processing in a highly parallel manner. Our findings set the grounds for creating the next generation of in-memory processors and neural network accelerators that can take advantage of the full benefits of semiconducting van der Waals materials for non-von Neuman computing.

Interactions between out-of-plane dipoles in bosonic gases enable the long-range propagation of excitons. The lack of direct control over collective dipolar properties has hitherto limited the degrees of tunability and the microscopic understanding of exciton transport. In this work, we modulate the layer hybridization and interplay between many-body interactions of excitons in a van der Waals heterostructure with an applied vertical electric field. By performing spatiotemporally resolved measurements supported by microscopic theory, we uncover the dipole-dependent properties and transport of excitons with different degrees of hybridization. Moreover, we find constant emission quantum yields of the transporting species as a function of excitation power with dominating radiative decay mechanisms over nonradiative ones, a fundamental requirement for efficient excitonic devices. Our findings provide a complete picture of the many-body effects in the transport of dilute exciton gases and have crucial implications for the study of emerging states of matter, such as Bose-Einstein condensation, as well as for optoelectronic applications based on exciton propagation.

Interactions among a collection of particles generate many-body effects in solids resulting in striking modifications of material properties. The heavy carrier mass that yields strong interactions and gate control of carrier density over a wide range, make two-dimensional semiconductors an exciting playground to explore many-body physics. The family of III-VI metal monochalcogenides emerges as a new platform for this purpose due to its excellent optical properties and the flat valence band dispersion with a Mexican-hat-like inversion. In this work, we present a complete study of charge-tunable excitons in few-layer InSe by photoluminescence spectroscopy. From the optical spectra, we establish that free excitons in InSe are more likely to be captured by ionized donors due to the large exciton Bohr radius, leading to the formation of bound exciton complexes. Surprisingly, a pronounced redshift of the exciton energy accompanied by a decrease of the exciton binding energy upon hole-doping reveals a significant band gap renormalization and dynamical screening induced by the presence of the Fermi reservoir. Our findings establish InSe as a reproducible and potentially manufacturable platform to explore electron correlation phenomena without the need for twist-angle engineering.

Large-area nanopore drilling is a major bottleneck in state-of-the-art nanoporous 2D membrane fabrication protocols. In addition, high-quality structural and statistical descriptions of as-fabricated porous membranes are key to predicting the corresponding membrane-wide permeation properties. In this work, we investigate Xe-ion focused ion beam as a tool for scalable, large-area nanopore fabrication on atomically thin, free-standing molybdenum disulphide. The presented irradiation protocol enables designing ultrathin membranes with tunable porosity and pore dimension, along with spatial uniformity across large-area substrates. Fabricated nanoporous membranes were characterized using scanning transmission electron microscopy imaging and the observed nanopore geometries were analyzed through a pore-edge detection script. We further demonstrated that the obtained structural and statistical data can be readily passed on to computational and analytical tools to predict the permeation properties at both individual pore and membrane-wide scales. As an example, membranes featuring angstrom-scale pores were investigated in terms of their emerging water and ion flow properties through extensive all-atom molecular dynamics simulations. We believe that the combination of experimental and analytical approaches presented here should yield accurate physics-based property estimates and thus potentially enable a true function-by-design approach to fabrication for applications such as osmotic power generation, desalination/filtration, as well as their strain-tunable versions.

Dipolar bosonic gases are currently the focus of intensive research due to their interesting many-body physics in the quantum regime. Their experimental embodiments range from Rydberg atoms to GaAs double quantum wells and van der Waals heterostructures built from transition metal dichalcogenides. Although quantum gases are very dilute, mutual interactions between particles could lead to exotic many-body phenomena such as Bose-Einstein condensation and high-temperature superfluidity. Here, we report the effect of repulsive dipolar interactions on the dynamics of interlayer excitons in the dilute regime. By using spatial and time-resolved photoluminescence imaging, we observe the dynamics of exciton transport, enabling a direct estimation of the exciton mobility. The presence of interactions significantly modifies the diffusive transport of excitons, effectively acting as a source of drift force and enhancing the diffusion coefficient by one order of magnitude. The repulsive dipolar interactions combined with the electrical control of interlayer excitons opens up appealing new perspectives for excitonic devices.

Hexagonal boron nitride (hBN) has emerged as a promising material platform for nanophotonics and quantum sensing, hosting optically-active defects with exceptional properties such as high brightness and large spectral tuning. However, precise control over deterministic spatial positioning of emitters in hBN remained elusive for a long time, limiting their proper correlative characterization and applications in hybrid devices. Recently, focused ion beam (FIB) systems proved to be useful to engineer several types of spatially-defined emitters with various structural and photophysical properties. Here we systematically explore the physical processes leading to the creation of optically-active defects in hBN using FIB, and find that beam-substrate interaction plays a key role in the formation of defects. These findings are confirmed using transmission electron microscopy that reveals local mechanical deterioration of the hBN layers and local amorphization of ion beam irradiated hBN. Additionally, we show that upon exposure to water, amorphized hBN undergoes a structural and optical transition between two defect types with distinctive emission properties. Moreover, using super-resolution optical microscopy combined with atomic force microscopy, we pinpoint the exact location of emitters within the defect sites, confirming the role of defected edges as primary sources of fluorescent emission. This lays the foundation for FIB-assisted engineering of optically-active defects in hBN with high spatial and spectral control for applications ranging from integrated photonics, to quantum sensing to nanofluidics., Comment: 11 pages, 5 figures

Transition metal dichalcogenides (TMDs) represent an entire new class of semiconducting 2D materials with exciting properties. Defects in 2D TMDs can crucially affect their physical and chemical properties. However, characterization of the presence and spatial distribution of defects is limited either in throughput or in resolution. Here, we demonstrate large area mapping of reactive sulfur-deficient defects in 2D-TMDs coupling single-molecule localization microscopy with fluorescence labeling using thiol chemistry. Our method, reminiscent of PAINT strategies, relies on the specific binding by reversible physisorption of fluorescent probes to sulfur-vacancies via a thiol group and their intermittent emission to apply localization of the labeled defects with a precision down to 15 nm. Tuning the distance between the fluorophore and the docking thiol site allows us to control F\"oster Resonance Energy Transfer (FRET) process and reveal large structural defects such as grain boundaries and line defects, due to the local irregular lattice structure. Our methodology provides a simple and fast alternative for large-scale mapping of non-radiative defects in 2D materials and paves the way for in-situ and spatially resolved monitoring of the interaction between chemical agent and the defects in 2D materials that has general implications for defect engineering in aqueous condition.

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 MoS$_2$/MoSe$_2$/MoS$_2$ heterostructure. We account for the non-trivial 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 towards the understanding of the properties of interlayer excitons, with strong implications for the implementation of atomically thin valleytronic devices., Comment: just accepted in Nano Letters, DOI: 10.1021/acs.nanolett.8b01484

Long-lived interlayer excitons with distinct spin-valley physics in van der Waals heterostructures based on transition metal dichalcogenides make them promising for information processing in next-generation devices. While the emission characteristics of interlayer excitons in different types of hetero stacks have been extensively studied, the manipulation of these excitons required to alter the valley-state or tune the emission energy and intensity is still lacking. Here, we demonstrate such control over interlayer excitons in MoSe2/WSe2 heterostructures. The encapsulation of our stack with h-BN ensures ultraclean interfaces, allowing us to resolve four separate narrow interlayer emission peaks. We observe two main interlayer transitions with opposite helicities under circularly polarized excitation, either conserving or inverting the polarization of incoming light. We further demonstrate control over the wavelength, intensity, and polarization of exciton emission by electrical and magnetic fields. Such ability to manipulate the interlayer excitons and their polarization could pave the way for novel excitonic and valleytronic device applications.

The observation of micron size spin relaxation makes graphene a promising material for applications in spintronics requiring long distance spin communication. However, spin dependent scatterings at the contact/graphene interfaces affect the spin injection efficiencies and hence prevent the material from achieving its full potential. While this major issue could be eliminated by nondestructive direct optical spin injection schemes, graphenes intrinsically low spin orbit coupling strength and optical absorption place an obstacle in their realization. We overcome this challenge by creating sharp artificial interfaces between graphene and WSe2 monolayers. Application of a circularly polarized light activates the spin polarized charge carriers in the WSe2 layer due to its spin coupled valley selective absorption. These carriers diffuse into the superjacent graphene layer, transport over a 3.5 um distance, and are finally detected electrically using BN/Co contacts in a non local geometry. Polarization dependent measurements confirm the spin origin of the non local signal.

The growth of single-layer MoS2 with chemical vapor deposition is an established method that can produce large-area and high quality samples. In this article, we investigate the geometrical and optical properties of hundreds of individual single-layer MoS2 crystallites grown on a highly-polished sapphire substrate. Most of the crystallites are oriented along the terraces of the sapphire substrate and have an area comprised between 10 {\mu}m2 and 60 {\mu}m2. Differential reflectance measurements performed on these crystallites show that the area of the MoS2 crystallites has an influence on the position and broadening of the B exciton while the orientation does not influence the A and B excitons of MoS2. These measurements demonstrate that differential reflectance measurements have the potential to be used to characterize the homogeneity of large area CVD grown samples., Comment: 10 pages, 5 figures

Monolayer transition metal dichalcogenides (TMDC) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDC. In this work we show that the optical quality of CVD-grown MoSe$_2$ is completely recovered if the material is sandwiched in MoS$_2$/MoSe$_2$/MoS$_2$ trilayer van der Waals heterostructures. We show by means of density-functional theory that this remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS$_2$ layers are donated to heal Se vacancy defects in the middle MoSe$_2$ layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe$_2$ without the need for resonant excitation. Our fabrication approach, relying solely on simple flake transfer technique, paves the way for the scalable production of large-area TMDC materials with excellent optical quality., Comment: Just accepted for publication in Nano Letters (http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b00904)

Optical spectroscopy techniques such as differential reflectance and transmittance have proven to be very powerful techniques to study 2D materials. However, a thorough description of the experimental setups needed to carry out these measurements is lacking in the literature. We describe a versatile optical microscope setup to carry out differential reflectance and transmittance spectroscopy in 2D materials with a lateral resolution of ~1 micron in the visible and near-infrared part of the spectrum. We demonstrate the potential of the presented setup to determine the number of layers of 2D materials and to characterize their fundamental optical properties such as excitonic resonances. We illustrate its performance by studying mechanically exfoliated and chemical vapor-deposited transition metal dichalcogenide samples., Comment: 5 main text figures + 1 table with all the part numbers to replicate the experimental setup + 4 supp. info. figures

The band structure of transition metal dichalcogenides (TMDCs) with valence band edges at different locations in the momentum space could be harnessed to build devices that operate relying on the valley degree of freedom. To realize such valleytronic devices, it is necessary to control and manipulate the charge density in these valleys, resulting in valley polarization. While this has been demonstrated using optical excitation, generation of valley polarization in electronic devices without optical excitation remains difficult. Here, we demonstrate spin injection from a ferromagnetic electrode into a heterojunction based on monolayers of WSe2 and MoS2 and lateral transport of spin-polarized holes within the WSe2 layer. The resulting valley polarization leads to circularly polarized light emission which can be tuned using an external magnetic field. This demonstration of spin injection and magnetoelectronic control over valley polarization provides a new opportunity for realizing combined spin and valleytronic devices based on spin-valley locking in semiconducting TMDCs., Comment: in Nano Letters (2016)

The interest in MoS2 for radio-frequency (RF) application has recently increased. However, little is known on the scaling behavior of transistors made from MoS2 for RF applications, which is important for establishing performance limits for electronic circuits based on 2D semiconductors on flexible and rigid substrates. Here, we present a systematic study of top-gated trilayer MoS2 RF transistors with gate lengths scaled down to 70 and 40 nm. In addition, by introducing edge-contacted injection of electrons in trilayer MoS2 devices, we decrease the contact resistance and as a result obtain the highest cutoff frequency of 6 GHz before the de-embedding procedure and 25 GHz after the de-embedding procedure., Comment: in Electron Devices Meeting (IEDM), 2015 IEEE International

Atomically thin rhenium disulphide (ReS2) is a member of the transition metal dichalcogenide (TMDC) family of materials characterized by weak interlayer coupling and a distorted 1T structure. Here, we report on the electrical transport study of mono- and multilayer ReS2 with polymer electrolyte gating. We find that the conductivity of monolayer ReS2 is completely suppressed at high carrier densities, an unusual feature unique to monolayers, making ReS2 the first example of such a material. While thicker flakes of ReS2 also exhibit a conductivity dome and an insulator-metal-insulator sequence, they do not show a complete conductivity suppression at high doping densities. Using dual-gated devices, we can distinguish the gate-induced doping from the electrostatic disorder induced by the polymer electrolyte itself. Theoretical calculations and a transport model indicate that the observed conductivity suppression can be explained by a combination of a narrow conduction band and Anderson localization due to electrolyte-induced disorder., Comment: Submitted version

To translate electrical into optical signals one uses the modulation of either the refractive index or the absorbance of a material by an electric field. Contemporary electroabsorption modulators (EAMs) employ the quantum confined Stark effect (QCSE), the field-induced red-shift and broadening of the strong excitonic absorption resonances characteristic of low-dimensional semiconductor structures. Here we show an unprecedentedly strong transverse electroabsorption (EA) signal in a monolayer of the two-dimensional semiconductor MoS2. The EA spectrum is dominated by an apparent linewidth broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary to the conventional QCSE, the signal increases linearly with the applied field strength and arises from a linear variation of the distance between the strongly overlapping exciton and trion resonances. The achievable modulation depths exceeding 0.1 dBnm-1 bear the scope for extremely compact, ultrafast, energy-efficient EAMs for integrated photonics, including on-chip optical communication.

Nanoelectromechanical systems (NEMS) constitute a class of devices lying at the interface between fundamental research and technological applications. Integrating novel materials such as graphene into NEMS allows studying their mechanical and electromechanical characteristics at the nanoscale and addressing fundamental questions such as electron-phonon interaction and bandgap engineering. In this work, we integrate single and bilayer graphene into NEMS and probe the interplay between their mechanical and electrical properties. We show that the deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. The proposed theoretical model suggests that these oscillations arise from quantum mechanical interference taking place due to the lateral displacement of graphene layers with respect to each other. Our work shows that bilayer graphene conceals unexpectedly rich and novel physics with promising potential in NEMS-based applications., Comment: First three authors contributed equally

The bandgap of MoS2 is highly strain-tunable which results in the modulation of its electrical conductivity and manifests itself as the piezoresistive effect while a piezoelectric effect was also observed in odd-layered MoS2 with broken inversion symmetry. This coupling between electrical and mechanical properties makes MoS2 a very promising material for nanoelectromechanical systems (NEMS). Here we incorporate monolayer, bilayer and trilayer MoS2 in a nanoelectromechanical membrane configuration. We detect strain-induced band gap tuning via electrical conductivity measurements and demonstrate the emergence of the piezoresistive effect in MoS2. Finite element method (FEM) simulations are used to quantify the band gap change and to obtain a comprehensive picture of the spatially varying bandgap profile on the membrane. The piezoresistive gauge factor is calculated to be -148 +/- 19, -224 +/- 19 and -43.5 +/- 11 for monolayer, bilayer and trilayer MoS2 respectively which is comparable to state-of-the-art silicon strain sensors and two orders of magnitude higher than in strain sensors based on suspended graphene. Controllable modulation of resistivity in 2D nanomaterials using strain-induced bandgap tuning offers a novel approach for implementing an important class of NEMS transducers, flexible and wearable electronics, tuneable photovoltaics and photodetection., Comment: 12 pages, 4 figures in Nano Letters (2015)

Avalanche photodiodes (APDs) are the semiconducting analogue of photomultiplier tubes offering very high internal current gain and fast response. APDs are interesting for a wide range of applications in communications1, laser ranging2, biological imaging3, and medical imaging4 where they offer speed and sensitivity superior to those of classical p-n junction-based photodetectors. The APD principle of operation is based on photocurrent multiplication through impact ionization in reverse-biased p-n junctions. APDs can either operate in proportional mode, where the bias voltage is below breakdown, or in Geiger mode, where the bias voltage is above breakdown. In proportional mode, the multiplication gain is finite, thus allowing for photon energy discrimination, while in Geiger mode of operation the multiplication gain is virtually infinite and a self-sustaining avalanche may be triggered, thus allowing detection of single photons5. Here, we demonstrate APDs based on vertically stacked monolayer MoS2 and p-Si, forming an abrupt p-n heterojunction. With this device, we demonstrate carrier multiplication exceeding 1000. Even though such multiplication factors in APDs are commonly accompanied by high noise, our devices show extremely low noise levels comparable with those in regular photodiodes. These heterostructures allow the realization of simple and inexpensive high-performance and low-noise photon counters based on transition metal dichalcogenides.

Semiconductor quantum dots have emerged as promising candidates for implementation of quantum information processing since they allow for a quantum interface between stationary spin qubits and propagating single photons. In the meanwhile, transition metal dichalcogenide (TMD) monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large band gap with degenerate valleys and non-zero Berry curvature. Here we report the observation of quantum dots in monolayer tungsten-diselenide with an energy that is 20 to 100 meV lower than that of two dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host TMD. The large $\sim$ 1 meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement most likely induced by impurities or defects in the host TMD. Electrical control in van der Waals heterostructures and robust spin-valley degree of freedom render TMD quantum dots promising for quantum information processing.

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 disulphide (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 two-dimensional semiconductors., Comment: Nano Letters (2014), Supplementary information available at http://dx.doi.org/10.1021/nl5028638

We report on the fabrication of field-effect transistors based on single 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 in-situ 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 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/Vs at low temperatures (above 300 cm2/Vs in the case of bi-layers). 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.

Single-layer transition metal dichalcogenide (TMD) 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 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., Comment: ACS Nano (2014)

A monolayer of a transition metal dichalcogenide (TMD) such as WSe$_2$ is a two-dimensional (2D) direct band-gap valley-semiconductor having an effective Honeycomb lattice structure with broken inversion symmetry. The inequivalent valleys in the Brillouin zone could be selectively addressed using circularly-polarized light fields, suggesting the possibility for magneto-optical measurement and manipulation of the valley pseudospin degree of freedom. Here we report such experiments that demonstrate the valley Zeeman effect -- strongly anisotropic lifting of the degeneracy of the valley pseudospin degree of freedom using an external magnetic field. While the valley-splitting measured using the exciton transition is consistent with the difference of the conduction and valence band orbital magnetic moments, the trion transition exhibits an unexpectedly large valley Zeeman effect which cannot be understood using an independent electron-hole picture. Instead, we find an explanation using the recently predicted large Berry curvature and the associated magnetic moment for the electron-hole exchange interaction modified trion dispersion. Our results raise the possibility of observing optical excitation induced valley Hall effect in monolayer TMDs or topological states of photons strongly coupled to trion excitations in a microcavity.

Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics and energy harvesting. Large-area growth methods are needed to open the way to the applications. While significant progress to this goal was made, control over lattice orientation during growth still remains a challenge. This is needed in order to minimize or even avoid the formation of grain boundaries which can be detrimental to electrical, optical and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the uniform growth of high-quality centimeter-scale continuous monolayer MoS2 with control over lattice orientation. Using transmission electron microscopy we show that the monolayer film is composed of coalescing single islands that share a predominant lattice orientation due to an epitaxial growth mechanism. Raman and photoluminescence spectra confirm the high quality of the grown material. Optical absorbance spectra acquired over large areas show new features in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment, we can easily transfer the grown material and fabricate field-effect transistors on SiO2 substrates showing mobility superior to the exfoliated material.

Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors (FETs). Their high mechanical flexibility, stability and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our pn diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of two-dimensional (2D) semiconductors and silicon., Comment: Submitted version

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 10000 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., Comment: Submitted version

Two-dimensional (2D) materials are a new class of materials with interesting physical properties and ranging from nanoelectronics to sensing and photonics. In addition to graphene, the most studied 2D material, monolayers of other layered materials such as semiconducting dichalcogenides MoS2 or WSe2 are gaining in importance as promising insulators and channel materials for field-effect transistors (FETs). The presence of a direct band gap in monolayer MoS2 due to quantum mechanical confinement, allows room-temperature field-effect transistors with an on/off ratio exceeding 108. The presence of high-k dielectrics in these devices enhanced their mobility, but the mechanisms are not well understood. Here, we report on electrical transport measurements on MoS2 FETs in different dielectric configurations. Mobility dependence on temperature shows clear evidence of the strong suppression of charge impurity scattering in dual-gate devices with a top-gate dielectric together with phonon scattering that shows a weaker than expected temperature dependence. High levels of doping achieved in dual-gate devices also allow the observation of a metal-insulator transition in monolayer MoS2. Our work opens up the way to further improvements in 2D semiconductor performance and introduces MoS2 as an interesting system for studying correlation effects in mesoscopic systems., Comment: Submitted January 11, 2013

In our previous paper, we reported on switchable monolayer MoS2 transistors with a high on-off ratio and we claim that dielectric screening can be used to increase the mobility of monolayer MoS2. We estimated its mobility using a method previously applied by Lemme et al. to top-gated graphene nanoribbons. We discuss here the comments raised by M. Fuhrer and J. Hone in their post 1301.4288 and give our own estimates of the possible errors in previous mobility measurements and their origins., Comment: Submitted

In this Letter we demonstrate the operation of an analog small-signal amplifier based on single-layer MoS2, a semiconducting analogue of graphene. Our device consists of two transistors integrated on the same piece of single-layer MoS2. The high intrinsic band gap of 1.8 eV allows MoS2-based amplifiers to operate with a room temperature gain of 4. The amplifier operation is demonstrated for the frequencies of input signal up to 2 kHz preserving the gain higher than 1. Our work shows that MoS2 can effectively amplify signals and that it could be used for advanced analog circuits based on two-dimensional materials., Comment: Submitted version of the manuscript