Search

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, which 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 nanoscale sensing, and to nanofluidics.

Machine learning and signal processing on the edge are poised to influence our everyday lives with devices that will learn and infer from data generated by smart sensors and other devices for the Internet of Things. The next leap toward ubiquitous electronics requires increased energy efficiency of processors for specialized data-driven applications. Here, we show how an in-memory processor fabricated using a two-dimensional materials platform can potentially outperform its silicon counterparts in both standard and nontraditional Von Neumann architectures for artificial neural networks. We have fabricated a flash memory array with a two-dimensional channel using wafer-scale MoS 2 . Simulations and experiments show that the device can be scaled down to sub-micrometer channel length without any significant impact on its memory performance and that in simulation a reasonable memory window still exists at sub-50 nm channel lengths. Each device conductance in our circuit can be tuned with a 4-bit precision by closed-loop programming. Using our physical circuit, we demonstrate seven-segment digit display classification with a 91.5% accuracy with training performed ex situ and transferred from a host. Further simulations project that at a system level, the large memory arrays can perform AlexNet classification with an upper limit of 50 000 TOpS/W, potentially outperforming neural network integrated circuits based on double-poly CMOS technology.

Metallic two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attracting great attention because of their interesting low-temperature properties such as superconductivity, magnetism, and charge density waves (CDW). However, further studies and practical applications are being slowed down by difficulties in synthesizing high-quality materials with a large grain size and well-determined thickness. In this work, we demonstrate epitaxial chemical vapor deposition (CVD) growth of 2D NbS 2 crystals on a sapphire substrate, with a thickness-dependent structural phase transition. NbS 2 crystals are epitaxially aligned by the underlying c-plane sapphire resulting in high-quality growth. The thickness of NbS 2 is well controlled by growth parameters to be between 1.5 and 10 nm with a large grain size of up to 500 μm. As the thickness increases, we observe in our NbS 2 a transition from a metallic 3R-polytype to a superconducting 2H-polytype, confirmed by Raman spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electrical transport measurements. A Berezinskii-Kosterlitz-Thouless (BKT) superconducting transition occurs in the CVD-grown 2H-phase NbS 2 below the transition temperature ( T c ) of 3 K. Our work demonstrates thickness and phase-controllable synthesis of high-quality superconducting 2D NbS 2 , which is imperative for its practical applications in next-generation TMDC-based electrical devices.

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (∼20 fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single-layer (1L)-MoS 2 , as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane A' 1 phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a time scale of few tens of fs. We observe an enhancement by almost 2 orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in the 1L-MoS 2 band structure induced by the A' 1 phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. These results explain the CP generation process in 1L-TMDs and demonstrate that CP excitation in 1L-MoS 2 can be described as a Raman-like scattering process.

The electric potential, electric field, and charge density of a monolayer of MoS 2 have been quantitatively measured at atomic-scale resolution. This has been performed by off-axis electron holography using a double aberration-corrected transmission electron microscope operated at 80 kV and a low electron beam current density. Using this low dose rate and acceleration voltage, the specimen damage is limited during imaging. In order to improve the sensitivity of the measurement, a series of holograms have been acquired. Instabilities of the microscope such as the drifts of the specimen, biprism, and optical aberrations during the acquisition have been corrected by data processing. Phase images of the MoS 2 monolayer have been acquired with a sensitivity of 2π/698 rad associated with a spatial resolution of 2.4 Å. The improvement in the signal-to-noise ratio allows the charge density to be directly calculated from the phase images using Poisson's equation. Density functional theory simulations of the potential and charge density of this MoS 2 monolayer were performed for comparison to the experiment. The experimental measurements and simulations are consistent with each other, and notably, the charge density in a sulfur monovacancy (V S ) site is shown.

Vertically stacked two-dimensional (2D) heterostructures composed of 2D semiconductors have attracted great attention. Most of these include hexagonal boron nitride (h-BN) as either a substrate, an encapsulant, or a tunnel barrier. However, reliable synthesis of large-area and epitaxial 2D heterostructures incorporating BN remains challenging. Here, we demonstrate the epitaxial growth of nominal monolayer (ML) MoSe 2 on h-BN/Rh(111) by molecular beam epitaxy, where the MoSe 2 /h-BN layer system can be transferred from the growth substrate onto SiO 2 . The valence band structure of ML MoSe 2 /h-BN/Rh(111) revealed by photoemission electron momentum microscopy ( kPEEM) shows that the valence band maximum located at the K point is 1.33 eV below the Fermi level ( E F ), whereas the energy difference between K and Γ points is determined to be 0.23 eV, demonstrating that the electronic properties, such as the direct band gap and the effective mass of ML MoSe 2 , are well preserved in MoSe 2 /h-BN heterostructures.

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 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.

The observation of micrometer 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, graphene's 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 WSe 2 monolayers. Application of circularly polarized light activates the spin-polarized charge carriers in the WSe 2 layer due to its spin-coupled valley-selective absorption. These carriers diffuse into the superjacent graphene layer, transport over a 3.5 μm distance, and are finally detected electrically using Co/h-BN contacts in a nonlocal geometry. Polarization-dependent measurements confirm the spin origin of the nonlocal signal. We also demonstrate that such signal is absent if graphene is contacted to bilayer WSe 2 where the inversion symmetry is restored.

Toward the large-area deposition of MoS 2 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 MoS 2 grown this way exhibits high crystallinity and optoelectronic quality comparable to single-crystal MoS 2 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 MoS 2 after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm 2 V -1 s -1 at cryogenic temperatures.

Monolayer transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDCs. 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.

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 MoSe 2 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 MoSe 2 . 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.

We present flexible photodetectors (PDs) for visible wavelengths fabricated by stacking centimeter-scale chemical vapor deposited (CVD) single layer graphene (SLG) and single layer CVD MoS2, both wet transferred onto a flexible polyethylene terephthalate substrate. The operation mechanism relies on injection of photoexcited electrons from MoS2 to the SLG channel. The external responsivity is 45.5A/W and the internal 570A/W at 642 nm. This is at least 2 orders of magnitude higher than bulk-semiconductor flexible membranes. The photoconductive gain is up to 4 × 10(5). The photocurrent is in the 0.1-100 μA range. The devices are semitransparent, with 8% absorptance at 642 nm, and are stable upon bending to a curvature of 1.4 cm. These capabilities and the low-voltage operation (<1 V) make them attractive for wearable applications.

Continuous tuning of material properties is highly desirable for a wide range of applications, with strain engineering being an interesting way of achieving it. The tuning range, however, is limited in conventional bulk materials that can suffer from plasticity and low fracture limit due to the presence of defects and dislocations. Atomically thin membranes such as MoS2 on the other hand exhibit high Young's modulus and fracture strength, which makes them viable candidates for modifying their properties via strain. The bandgap of MoS2 is highly strain-tunable, which results in the modulation of its electrical conductivity and manifests itself as the piezoresistive effect, whereas 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 2 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, tunable photovoltaics, and photodetection.

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 applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to electrical, optical, and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the growth of high-quality monolayer MoS2 with control over lattice orientation. We show that the monolayer film is composed of coalescing single islands with limited numbers of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance 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 via van der Waals interaction, we can easily transfer the grown material and fabricate devices. Local potential mapping along channels in field-effect transistors shows that the single-crystal MoS2 grains in our film are well connected, with interfaces that do not degrade the electrical conductivity. This is also confirmed by the relatively large and length-independent mobility in devices with a channel length reaching 80 μm.

We study the atomic scale microstructure of nonstoichiometric 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.

CONSPECTUS: Atomic crystals of two-dimensional materials consisting of single sheets extracted from layered materials are gaining increasing attention. The most well-known material from this group is graphene, a single layer of graphite that can be extracted from the bulk material or grown on a suitable substrate. Its discovery has given rise to intense research effort culminating in the 2010 Nobel Prize in physics awarded to Andre Geim and Konstantin Novoselov. Graphene however represents only the proverbial tip of the iceberg, and increasing attention of researchers is now turning towards the veritable zoo of so-called "other 2D materials". They have properties complementary to graphene, which in its pristine form lacks a bandgap: MoS2, for example, is a semiconductor, while NbSe2 is a superconductor. They could hold the key to important practical applications and new scientific discoveries in the two-dimensional limit. This family of materials has been studied since the 1960s, but most of the research focused on their tribological applications: MoS2 is best known today as a high-performance dry lubricant for ultrahigh-vacuum applications and in car engines. The realization that single layers of MoS2 and related materials could also be used in functional electronic devices where they could offer advantages compared with silicon or graphene created a renewed interest in these materials. MoS2 is currently gaining the most attention because the material is easily available in the form of a mineral, molybdenite, but other 2D transition metal dichalcogenide (TMD) semiconductors are expected to have qualitatively similar properties. In this Account, we describe recent progress in the area of single-layer MoS2-based devices for electronic circuits. We will start with MoS2 transistors, which showed for the first time that devices based on MoS2 and related TMDs could have electrical properties on the same level as other, more established semiconducting materials. This allowed rapid progress in this area and was followed by demonstrations of basic digital circuits and transistors operating in the technologically relevant gigahertz range of frequencies, showing that the mobility of MoS2 and TMD materials is sufficiently high to allow device operation at such high frequencies. Monolayer MoS2 and other TMDs are also direct band gap semiconductors making them interesting for realizing optoelectronic devices. These range from simple phototransistors showing high sensitivity and low noise, to light emitting diodes and solar cells. All the electronic and optoelectronic properties of MoS2 and TMDs are accompanied by interesting mechanical properties with monolayer MoS2 being as stiff as steel and 30× stronger. This makes it especially interesting in the context of flexible electronics where it could combine the high degree of mechanical flexibility commonly associated with organic semiconductors with high levels of electrical performance. All these results show that MoS2 and TMDs are promising materials for electronic and optoelectronic applications.

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.

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 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 a high room-temperature on/off current ratio of ∼10(6). They show clear metallic behavior at high charge carrier densities and mobilities as high as ∼140 cm(2)/(V s) at low temperatures (above 300 cm(2)/(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.

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.

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. 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 p-n 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 2D semiconductors and silicon.

Atomically thin nanopore membranes are considered to be a promising approach to achieve single base resolution with the ultimate aim of rapid and cheap DNA sequencing. Molybdenum disulfide (MoS2) is newly emerging as a material complementary to graphene due to its semiconductive nature and other interesting physical properties that can enable a wide range of potential sensing and nanoelectronics applications. Here, we demonstrate that monolayer or few-layer thick exfoliated MoS2 with subnanometer thickness can be transferred and suspended on a predesigned location on the 20 nm thick SiNx membranes. Nanopores in MoS2 are further sculpted with variable sizes using a transmission electron microscope (TEM) to drill through suspended portions of the MoS2 membrane. Various types of double-stranded (ds) DNA with different lengths and conformations are translocated through such a novel architecture, showing improved sensitivity (signal-to-noise ratio>10) compared to the conventional silicon nitride (SiNx) nanopores with tens of nanometers thickness. Unlike graphene nanopores, no special surface treatment is needed to avoid hydrophobic interaction between DNA and the surface. Our results imply that MoS2 membranes with nanopore can complement graphene nanopore membranes and offer potentially better performance in transverse detection.

Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100-320 K are reported and analyzed. The linear temperature coefficients of the in-plane E2g 1 and the out-of-plane A1g modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity κ = (34.5(4) W/mK at room temperature agrees well with the first principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical.

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 10(4) 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.

Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si₃N₄ substrate-supported monolayer and few-layer MoS₂ 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electron-hole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirect-direct band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS₂. Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS₂ along with controlling their dimensions.

Two-dimensional (2D) materials such as monolayer molybdenum disulfide (MoS(2)) are extremely interesting for integration in nanoelectronic devices where they represent the ultimate limit of miniaturization in the vertical direction. Thanks to the presence of a band gap and subnanometer thickness, monolayer MoS(2) can be used for the fabrication of transistors exhibiting extremely high on/off ratios and very low power dissipation. Here, we report on the development of 2D MoS(2) transistors with improved performance due to enhanced electrostatic control. Our devices show currents in the 100 μA/μm range and transconductance exceeding 20 μS/μm as well as current saturation. We also record electrical breakdown of our devices and find that MoS(2) can support very high current densities, exceeding the current-carrying capacity of copper by a factor of 50. Our results push the performance limit of MoS(2) and open the way to their use in low-power and low-cost analog and radio frequency circuits.

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 MoS(2) 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 MoS(2). 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 MoS(2) transistors are capable of performing the NOR logic operation, the basis from which all logical operations and full digital functionality can be deduced.

We report on measurements of the stiffness and breaking strength of monolayer MoS(2), a new semiconducting analogue of graphene. Single and bilayer MoS(2) is exfoliated from bulk and transferred to a substrate containing an array of microfabricated circular holes. The resulting suspended, free-standing membranes are deformed and eventually broken using an atomic force microscope. We find that the in-plane stiffness of monolayer MoS(2) is 180 ± 60 Nm(-1), corresponding to an effective Young's modulus of 270 ± 100 GPa, which is comparable to that of steel. Breaking occurs at an effective strain between 6 and 11% with the average breaking strength of 15 ± 3 Nm(-1) (23 GPa). The strength of strongest monolayer membranes is 11% of its Young's modulus, corresponding to the upper theoretical limit which indicates that the material can be highly crystalline and almost defect-free. Our results show that monolayer MoS(2) could be suitable for a variety of applications such as reinforcing elements in composites and for fabrication of flexible electronic devices.

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.

We used AFM to investigate the interaction of polyelectrolytes such as ssDNA and dsDNA molecules with graphene as a substrate. Graphene is an appropriate substrate due to its planarity, relatively large surfaces that are detectable via an optical microscope, and straightforward identification of the number of layers. We observe that in the absence of the screening ions deposited ssDNA will bind only to the graphene and not to the SiO(2) substrate, confirming that the binding energy is mainly due to the π-π stacking interaction. Furthermore, deposited ssDNA will map the graphene underlying structure. We also quantify the π-π stacking interaction by correlating the amount of deposited DNA with the graphene layer thickness. Our findings agree with reported electrostatic force microscopy (EFM) measurements. Finally, we inspected the suitability of using a graphene as a substrate for DNA origami-based nanostructures.