Integrated circuits and logic operations based on single-layer MoS2

Remarkable magnetism and ferromagnetic coupling in semi-sulfuretted transition-metal dichalcogenides

Motivated by recent investigations of semi-decorated two dimensional honeycomb structures, we demonstrated, via spin-polarized molecular-dynamics simulations and density-functional-theory calculations, that semi-sulfuretted transition-metal dichalcogenides of MX type (M = V, Nb, Ta; X = S, Se, Te) are stable and display remarkable magnetism. The unpaired d electron of the transition-metal atom arising from the breakage of the M-X bond is the mechanism behind the induction of the magnetism. The remarkable magnetism of the transition-metal atoms is caused by ferromagnetic coupling due to the competitive effects of through-bond interactions and through-space interactions. This implies the existence of an infinite ferromagnetic sheet with structural integrity and magnetic homogeneity. The estimated Curie temperatures suggest that the ferromagnetism can be achieved above room temperature in the VS, VSe, VTe, NbTe and TaTe sheets. Depending on the species of the M and X atoms, the MX sheet can be a magnetic metal, magnetic semiconductor or half-metal. Furthermore, in contrary to the recently reported semi-hydrogenated and semi-fluorinated layered materials consisting of B, C, N, etc., the MX sheets with many unpaired d electrons can offer a much stronger spin polarization and possess a more stable ferromagnetic coupling, which is critical for practical nanoscale device applications.

Raman spectroscopy study of lattice vibration and crystallographic orientation of monolayer MoS2 under uniaxial strain

The false-color (3D type) image of the intensity of the Raman spectra of monolayer MoS2 versus both peak positions and polar angles is plotted. It shows that the strongest E2g (1+) and E2g (1-) peaks appear at different angles, reflected as the alternation of the maxima of the intensity within the frequency range of the E2g (1) mode, which is the consequence of the crystallographic orientation relevant to the strain direction as predicted by theoretical analysis.

Direct laser-patterned micro-supercapacitors from paintable MoS2 films

Micrometer-sized electrochemical capacitors have recently attracted attention due to their possible applications in micro-electronic devices. Here, a new approach to large-scale fabrication of high-capacitance, two-dimensional MoS2 film-based micro-supercapacitors is demonstrated via simple and low-cost spray painting of MoS2 nanosheets on Si/SiO2 chip and subsequent laser patterning. The obtained micro-supercapacitors are well defined by ten interdigitated electrodes (five electrodes per polarity) with 4.5 mm length, 820 μm wide for each electrode, 200 μm spacing between two electrodes and the thickness of electrode is ∼0.45 μm. The optimum MoS2 -based micro-supercapacitor exhibits excellent electrochemical performance for energy storage with aqueous electrolytes, with a high area capacitance of 8 mF cm(-2) (volumetric capacitance of 178 F cm(-3) ) and excellent cyclic performance, superior to reported graphene-based micro-supercapacitors. This strategy could provide a good opportunity to develop various micro-/nanosized energy storage devices to satisfy the requirements of portable, flexible, and transparent micro-electronic devices.

Mobility engineering and a metal-insulator transition in monolayer MoS₂

Two-dimensional (2D) materials are a new class of materials with interesting physical properties and applications 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 channel materials for field-effect transistors (FETs). The presence of a direct bandgap in monolayer MoS₂ due to quantum-mechanical confinement allows room-temperature FETs with an on/off ratio exceeding 10(8). The presence of high- κ 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. The dependence of mobility on temperature shows clear evidence of the strong suppression of charged-impurity scattering in dual-gate devices with a top-gate dielectric. At the same time, phonon scattering 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 due to strong electron-electron interactions. 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.

Thickness-dependent interfacial Coulomb scattering in atomically thin field-effect transistors

Two-dimensional semiconductors are structurally ideal channel materials for the ultimate atomic electronics after silicon era. A long-standing puzzle is the low carrier mobility (μ) in them as compared with corresponding bulk structures, which constitutes the main hurdle for realizing high-performance devices. To address this issue, we perform a combined experimental and theoretical study on atomically thin MoS2 field effect transistors with varying the number of MoS2 layers (NLs). Experimentally, an intimate μ-NL relation is observed with a 10-fold degradation in μ for extremely thinned monolayer channels. To accurately describe the carrier scattering process and shed light on the origin of the thinning-induced mobility degradation, a generalized Coulomb scattering model is developed with strictly considering device configurative conditions, that is, asymmetric dielectric environments and lopsided carrier distribution. We reveal that the carrier scattering from interfacial Coulomb impurities (e.g., chemical residues, gaseous adsorbates, and surface dangling bonds) is greatly intensified in extremely thinned channels, resulting from shortened interaction distance between impurities and carriers. Such a pronounced factor may surpass lattice phonons and serve as dominant scatterers. This understanding offers new insight into the thickness induced scattering intensity, highlights the critical role of surface quality in electrical transport, and would lead to rational performance improvement strategies for future atomic electronics.

Joined edges in MoS2: metallic and half-metallic wires

We present the results of first-principles electronic structure investigations, using spin-polarized density-functional-theory, of the electronic and geometric structures of the joined edges formed between two domains of single-layer MoS2. We find joined-edge defects to be metallic or half-metallic wires in the considered models. More interestingly, we find the signature of magnetism in the joined-edge defect formed between two Mo-terminated (1010) edges. Analysis of the geometric structure leads us to the conclusion that certain joined-edge defects undergo (2 × 1) reconstruction.

Few-layered CoHPO4 · 3H2O ultrathin nanosheets for high performance of electrode materials for supercapacitors

Ultrathin cobalt phosphate (CoHPO4 · 3H2O) nanosheets are successfully synthesized by a one pot hydrothermal method. Novel CoHPO4 · 3H2O ultrathin nanosheets are assembled for constructing the electrodes of supercapacitors. Benefiting from the nanostructures, the as-prepared electrode shows a specific capacitance of 413 F g(-1), and no obvious decay even after 3000 charge-discharge cycles. Such a quasi-two-dimensional material is a new kind of supercapacitor electrode material with high performance.

Ultrasensitive photodetectors based on monolayer MoS2

Two-dimensional materials are an emerging class of new materials with a wide range of electrical properties and potential practical applications. Although graphene is the most well-studied two-dimensional material, single layers of other materials, such as insulating BN (ref. 2) and semiconducting MoS2 (refs 3, 4) or WSe2 (refs 5, 6), are gaining increasing attention as promising gate insulators and channel materials for field-effect transistors. Because monolayer MoS2 is a direct-bandgap semiconductor due to quantum-mechanical confinement, it could be suitable for applications in optoelectronic devices where the direct bandgap would allow a high absorption coefficient and efficient electron-hole pair generation under photoexcitation. Here, we demonstrate ultrasensitive monolayer MoS2 phototransistors with improved device mobility and ON current. Our devices show a maximum external photoresponsivity of 880 A W(-1) at a wavelength of 561 nm and a photoresponse in the 400-680 nm range. With recent developments in large-scale production techniques such as liquid-scale exfoliation and chemical vapour deposition-like growth, MoS2 shows important potential for applications in MoS2-based integrated optoelectronic circuits, light sensing, biomedical imaging, video recording and spectroscopy.

Visualization and quantification of transition metal atomic mixing in Mo1-xWxS2 single layers

The alloying behaviour of materials is a well-known problem in all kinds of compounds. Revealing the heteroatomic distributions in two-dimensional crystals is particularly critical for their practical use as nano-devices. Here we obtain statistics of the homo- and heteroatomic coordinates in single-layered Mo_1−xW_xS₂ from the atomically resolved scanning transmission electron microscope images and successfully quantify the degree of alloying for the transition metal elements (Mo or W). The results reveal the random alloying of this mixed dichalcogenide system throughout the chemical compositions (x=0 to 1). Such a direct route to gain an insight into the alloying degree on individual atom basis will find broad applications in characterizing low-dimensional heterocompounds and become an important complement to the existing theoretical methods.

Understanding the influence of disorder on the properties of two-dimensional materials is of increasing importance, given the interest in these compounds for electronic applications. Using a scanning transmission electron microscope, Dumcenco et al. quantify the atomic mixing in two-dimensional films of Mo_1–xW_xS₂.

Few-Layer MoS2 with high broadband Photogain and fast optical switching for use in harsh environments

Few-layered MoS2 as Schottky metal-semiconductor-metal photodetectors (MSM PDs) for use in harsh environments makes its debut as two-dimensional (2D) optoelectronics with high broadband gain (up to 13.3), high detectivity (up to ~10(10) cm Hz(1/2)/W), fast photoresponse (rise time of ~70 μs and fall time of ~110 μs), and high thermal stability (at a working temperature of up to 200 °C). Ultrahigh responsivity (0.57 A/W) of few-layer MoS2 at 532 nm is due to the high optical absorption (~10% despite being less than 2 nm in thickness) and a high photogain, which sets up a new record that was not achievable in 2D nanomaterials previously. This study opens avenues to develop 2D nanomaterial-based optoelectronics for harsh environments in imaging techniques and light-wave communications as well as in future memory storage and optoelectronic circuits.

Nanoscale Multilayer Transition-Metal Dichalcogenide Heterostructures: Band Gap Modulation by Interfacial Strain and Spontaneous Polarization

Using density functional theory calculations, we unveil intriguing electronic properties of nanoscale multilayer transition-metal dichalcogenide (TMDC) heterostructures, (MoX2)n(MoY2)m (X, Y = S, Se or Te). Our results show that the structural stability and electronic band structure of the TMDC heterostructures depend sensitively on the choice of constituent components and their relative thickness. In particular, the electronic band gap can be tuned over a wide range by the intrinsic mismatch strain and spontaneous electrical polarization at the interface of the heterostructures, which suggests desirable design strategies for TMDC-based devices with an easily adjustable band gap. These interfacial effects also make the electronic properties more susceptible to the influence of a bias electric field, which can induce sensitive and considerable changes in the band gap and even produce a semiconductor-metal transition at relatively low electric fields. Such effective electronic band gap engineering via a combination of internal (i.e., the composition and layer thickness) and external (i.e., a bias field) control makes the TMDC-based heterostructures promising candidates for applications in a variety of nanodevices.

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

Degenerate n-doping of few-layer transition metal dichalcogenides by potassium

We report here the first degenerate n-doping of few-layer MoS2 and WSe2 semiconductors by surface charge transfer using potassium. High-electron sheet densities of ~1.0 × 10(13) cm(-2) and 2.5 × 10(12) cm(-2) for MoS2 and WSe2 are obtained, respectively. In addition, top-gated WSe2 and MoS2 n-FETs with selective K doping at the metal source/drain contacts are fabricated and shown to exhibit low contact resistances. Uniquely, WSe2 n-FETs are reported for the first time, exhibiting an electron mobility of ~110 cm(2)/V·s, which is comparable to the hole mobility of previously reported p-FETs using the same material. Ab initio simulations were performed to understand K doping of MoS2 and WSe2 in comparison with graphene. The results here demonstrate the need of degenerate doping of few-layer chalcogenides to improve the contact resistances and further realize high performance and complementary channel electronics.

Role of metal contacts in designing high-performance monolayer n-type WSe2 field effect transistors

This work presents a systematic study toward the design and first demonstration of high-performance n-type monolayer tungsten diselenide (WSe2) field effect transistors (FET) by selecting the contact metal based on understanding the physics of contact between metal and monolayer WSe2. Device measurements supported by ab initio density functional theory (DFT) calculations indicate that the d-orbitals of the contact metal play a key role in forming low resistance ohmic contacts with monolayer WSe2. On the basis of this understanding, indium (In) leads to small ohmic contact resistance with WSe2 and consequently, back-gated In-WSe2 FETs attained a record ON-current of 210 μA/μm, which is the highest value achieved in any monolayer transition-metal dichalcogenide- (TMD) based FET to date. An electron mobility of 142 cm(2)/V·s (with an ON/OFF current ratio exceeding 10(6)) is also achieved with In-WSe2 FETs at room temperature. This is the highest electron mobility reported for any back gated monolayer TMD material till date. The performance of n-type monolayer WSe2 FET was further improved by Al2O3 deposition on top of WSe2 to suppress the Coulomb scattering. Under the high-κ dielectric environment, electron mobility of Ag-WSe2 FET reached ~202 cm(2)/V·s with an ON/OFF ratio of over 10(6) and a high ON-current of 205 μA/μm. In tandem with a recent report of p-type monolayer WSe2 FET ( Fang , H . et al. Nano Lett. 2012 , 12 , ( 7 ), 3788 - 3792 ), this demonstration of a high-performance n-type monolayer WSe2 FET corroborates the superb potential of WSe2 for complementary digital logic applications.

Progress, challenges, and opportunities in two-dimensional materials beyond graphene

Graphene's success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications.

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 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.

Solvatochromic effect on the photoluminescence of MoS₂ monolayers

The effect of surrounding solvents on the photoluminescence (PL) of MoS2 monolayers on Si/SiO2 substrates is studied. A redshift (up to -60 meV) is observed for MoS2 monolayers with nonhalogenated solvent surroundings. A blueshift (up to 60 meV) and intensity increase (2-50 times) are observed for monolayers with halogenated solvent surroundings.

Vapor-solid growth of high optical quality MoS₂ monolayers with near-unity valley polarization

Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin direct-gap semiconductors with potential applications in nanoelectronics, optoelectronics, and electrochemical sensing. Recent theoretical and experimental efforts suggest that they are ideal systems for exploiting the valley degrees of freedom of Bloch electrons. For example, Dirac valley polarization has been demonstrated in mechanically exfoliated monolayer MoS2 samples by polarization-resolved photoluminescence, although polarization has rarely been seen at room temperature. Here we report a new method for synthesizing high optical quality monolayer MoS2 single crystals up to 25 μm in size on a variety of standard insulating substrates (SiO2, sapphire, and glass) using a catalyst-free vapor-solid growth mechanism. The technique is simple and reliable, and the optical quality of the crystals is extremely high, as demonstrated by the fact that the valley polarization approaches unity at 30 K and persists at 35% even at room temperature, suggesting a virtual absence of defects. This will allow greatly improved optoelectronic TMDC monolayer devices to be fabricated and studied routinely.

Memory devices using a mixture of MoS₂ and graphene oxide as the active layer

A mixed film consisting of 2D MoS₂ and graphene oxide (GO) nanosheets is used to fabricate memory devices. The conductive MoS₂ component in the MoS₂-GO film increases the film conductivity, thus facilitating oxygen migration in GO. The MoS₂-GO film-based device exhibits rewritable, nonvolatile, electrical bistable switching with low switching voltage (≤ 1.5 V) and high ON/OFF current ratio (≈ 10²).

Vertically stacked multi-heterostructures of layered materials for logic transistors and complementary inverters

Graphene has attracted considerable interest for future electronics, but the absence of a bandgap limits its direct applicability in transistors and logic devices. Recently, other layered materials such as molybdenum disulphide (MoS(2)) have been investigated to address this challenge. Here, we report the vertical integration of multi-heterostructures of layered materials for the fabrication of a new generation of vertical field-effect transistors (VFETs) with a room temperature on-off ratio > 10(3) and a high current density of up to 5,000 A cm(-2). An n-channel VFET is created by sandwiching few-layer MoS(2) as the semiconducting channel between a monolayer graphene sheet and a metal thin film. This approach offers a general strategy for the vertical integration of p- and n-channel transistors for high-performance logic applications. As an example, we demonstrate a complementary inverter with a larger-than-unity voltage gain by vertically stacking graphene, Bi(2)Sr(2)Co(2)O(8) (p-channel), graphene, MoS(2) (n-channel) and a metal thin film in sequence. The ability to simultaneously achieve a high on-off ratio, a high current density and a logic function in such vertically stacked multi-heterostructures can open up possibilities for three-dimensional integration in future electronics.

Exciton dynamics in suspended monolayer and few-layer MoS₂ 2D crystals

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.

Evolution of electronic structure in atomically thin sheets of WS2 and WSe2

Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS(2)) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS(2) opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS(2) and WSe(2) that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS(2) and WSe(2), respectively. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepared by a chemical vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe(2).

High performance multilayer MoS2 transistors with scandium contacts

While there has been growing interest in two-dimensional (2-D) crystals other than graphene, evaluating their potential usefulness for electronic applications is still in its infancy due to the lack of a complete picture of their performance potential. The focus of this article is on contacts. We demonstrate that through a proper understanding and design of source/drain contacts and the right choice of number of MoS(2) layers the excellent intrinsic properties of this 2-D material can be harvested. Using scandium contacts on 10-nm-thick exfoliated MoS(2) flakes that are covered by a 15 nm Al(2)O(3) film, high effective mobilities of 700 cm(2)/(V s) are achieved at room temperature. This breakthrough is largely attributed to the fact that we succeeded in eliminating contact resistance effects that limited the device performance in the past unrecognized. In fact, the apparent linear dependence of current on drain voltage had mislead researchers to believe that a truly Ohmic contact had already been achieved, a misconception that we also elucidate in the present article.

Predicting dislocations and grain boundaries in two-dimensional metal-disulfides from the first principles

Guided by the principles of dislocation theory, we use the first-principles calculations to determine the structure and properties of dislocations and grain boundaries (GB) in single-layer transition metal disulfides MS(2) (M = Mo or W). In sharp contrast to other two-dimensional materials (truly planar graphene and h-BN), here the edge dislocations extend in third dimension, forming concave dreidel-shaped polyhedra. They include different number of homoelemental bonds and, by reacting with vacancies, interstitials, and atom substitutions, yield families of the derivative cores for each Burgers vector. The overall structures of GB are controlled by both local-chemical and far-field mechanical energies and display different combinations of dislocation cores. Further, we find two distinct electronic behaviors of GB. Typically, their localized deep-level states act as sinks for carriers but at large 60°-tilt the GB become metallic. The analysis shows how the versatile GB in MS(2) (if carefully engineered) should enable new developments for electronic and opto-electronic applications.

Breakdown of high-performance monolayer MoS2 transistors

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.

Tensile strain switched ferromagnetism in layered NbS2 and NbSe2

Developing approaches to effectively induce and control the magnetic states is critical to the use of magnetic nanostructures in quantum information devices but is still challenging. Here we have demonstrated, by employing the density functional theory calculations, the existence of infinite magnetic sheets with structural integrity and magnetic homogeneity. Examination of a series of transition metal dichalcogenides shows that the biaxial tensile strained NbS(2) and NbSe(2) structures can be magnetized with a ferromagnetic character due to the competitive effects of through-bond interaction and through-space interaction. The estimated Curie temperatures (387 and 542 K under the 10% strain for NbS(2) and NbSe(2) structures, respectively) suggest that the unique ferromagnetic character can be achieved above room temperature. The self-exchange of population between 4d orbitals of the Nb atom that leads to exchange splitting is the mechanism behind the transition of the spin moment. The induced magnetic moments can be significantly enhanced by the tensile strain, even giving rise to a half-metallic character with a strong spin polarization around the Fermi level. Given the recent progress in achieving the desired strain on two-dimensional nanostructures, such as graphene and a BN layer, in a controlled way, we believe that our calculated results are suitable for experimental verification and implementation, opening a new path to explore the spintronics in pristine two-dimensional nanostructures.

Preparation of MoS₂-polyvinylpyrrolidone nanocomposites for flexible nonvolatile rewritable memory devices with reduced graphene oxide electrodes

A facile method for exfoliation and dispersion of molybdenum disulfide (MoS2) with the aid of polyvinylpyrrolidone (PVP) is proposed. The resultant PVP-coated MoS2 nanosheets, i.e., MoS2-PVP nanocomposites, are well dispersed in the low-boiling ethanol solvent, facilitating their thin film preparation and the device fabrication by solution processing technique. As a proof of concept, a flexible memory diode with the configuration of reduced graphene oxide (rGO)/MoS2-PVP/Al exhibited a typical bistable electrical switching and nonvolatile rewritable memory effect with the function of flash. These experimental results prove that the electrical transition is due to the charge trapping and detrapping behavior of MoS2 in the PVP dielectric material. This study paves a way of employing two-dimensional nanomaterials as both functional materials and conducting electrodes for the future flexible data storage.

Graphene: an emerging electronic material

Graphene, a single layer of carbon atoms in a honeycomb lattice, offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. Its unique form factor and exceptional physical properties have the potential to enable an entirely new generation of technologies beyond the limits of conventional materials. The extraordinarily high carrier mobility and saturation velocity can enable a fast switching speed for radio-frequency analog circuits. Unadulterated graphene is a semi-metal, incapable of a true off-state, which typically precludes its applications in digital logic electronics without bandgap engineering. The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already shown the numerous advantages and novel functionality that graphene provides.

Evidence for interlayer coupling and moiré periodic potentials in twisted bilayer graphene

We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calculations. We observe two noninteracting cones near the Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moiré superlattice formed by the misorientation of the two graphene layers. We attribute the emergence of these minigaps to a periodic potential induced by the moiré. These anticrossing features point to coupling between the two graphene sheets, mediated by moiré periodic potentials.

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides

The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs have been studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS(2), MoSe(2), WS(2) and WSe(2) have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. We review the historical development of TMDCs, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.

Wafer-scale MoS2 thin layers prepared by MoO3 sulfurization

Atomically thin molybdenum disulfide (MoS(2)) layers have attracted great interest due to their direct-gap property and potential applications in optoelectronics and energy harvesting. Meanwhile, they are extremely bendable, promising for applications in flexible electronics. However, the synthetic approach to obtain large-area MoS(2) atomic thin layers is still lacking. Here we report that wafer-scale MoS(2) thin layers can be obtained using MoO(3) thin films as a starting material followed by a two-step thermal process, reduction of MoO(3) at 500 °C in hydrogen and sulfurization at 1000 °C in the presence of sulfur. Spectroscopic, optical and electrical characterizations reveal that these films are polycrystalline and with semiconductor properties. The obtained MoS(2) films are uniform in thickness and easily transferable to arbitrary substrates, which make such films suitable for flexible electronics or optoelectronics.

Tuning Magnetism and Electronic Phase Transitions by Strain and Electric Field in Zigzag MoS2 Nanoribbons

Effective modulation of physical properties via external control may open various potential nanoelectronic applications of single-layer MoS2 nanoribbons (MoS2NRs). We show by first-principles calculations that the magnetic and electronic properties of zigzag MoS2NRs exhibit sensitive response to applied strain and electric field. Tensile strain in the zigzag direction produces reversible modulation of magnetic moments and electronic phase transitions among metallic, half-metallic, and semiconducting states, which stem from the energy-level shifts induced by an internal electric polarization and the competing covalent/ionic interactions. A simultaneously applied electric field further enhances or suppresses the strain-induced modulations depending on the direction of the electric field relative to the internal polarization. These findings suggest a robust and efficient approach to modulating the properties of MoS2NRs by a combination of strain engineering and electric field tuning.

Integrated circuits based on bilayer MoS₂ transistors

Two-dimensional (2D) materials, such as molybdenum disulfide (MoS(2)), have been shown to exhibit excellent electrical and optical properties. The semiconducting nature of MoS(2) allows it to overcome the shortcomings of zero-bandgap graphene, while still sharing many of graphene's advantages for electronic and optoelectronic applications. Discrete electronic and optoelectronic components, such as field-effect transistors, sensors, and photodetectors made from few-layer MoS(2) show promising performance as potential substitute of Si in conventional electronics and of organic and amorphous Si semiconductors in ubiquitous systems and display applications. An important next step is the fabrication of fully integrated multistage circuits and logic building blocks on MoS(2) to demonstrate its capability for complex digital logic and high-frequency ac applications. This paper demonstrates an inverter, a NAND gate, a static random access memory, and a five-stage ring oscillator based on a direct-coupled transistor logic technology. The circuits comprise between 2 to 12 transistors seamlessly integrated side-by-side on a single sheet of bilayer MoS(2). Both enhancement-mode and depletion-mode transistors were fabricated thanks to the use of gate metals with different work functions.

Coherent atomic and electronic heterostructures of single-layer MoS2

Nanoscale heterostructures with quantum dots, nanowires, and nanosheets have opened up new routes toward advanced functionalities and implementation of novel electronic and photonic devices in reduced dimensions. Coherent and passivated heterointerfaces between electronically dissimilar materials can be typically achieved through composition or doping modulation as in GaAs/AlGaAs and Si/NiSi or heteroepitaxy of lattice matched but chemically distinct compounds. Here we report that single layers of chemically exfoliated MoS(2) consist of electronically dissimilar polymorphs that are lattice matched such that they form chemically homogeneous atomic and electronic heterostructures. High resolution scanning transmission electron microscope (STEM) imaging reveals the coexistence of metallic and semiconducting phases within the chemically homogeneous two-dimensional (2D) MoS(2) nanosheets. These results suggest potential for exploiting molecular scale electronic device designs in atomically thin 2D layers.

Anisotropic photoresponse properties of single micrometer-sized GeSe nanosheet

Micrometer-sized single-crystal GeSe nanosheets have been synthesized by a solution method. The single GeSe nanosheet exhibits novel anisotropic photoresponse properties in two photodetectors based on individual nanosheet. The on/off switching ratio of the photodetector perpendicular to the nanosheet is 3.5 times higher than that parallel to the nanosheet.

A molybdenum disulfide/carbon nanotube heterogeneous complementary inverter

We report a simple, bottom-up/top-down approach for integrating drastically different nanoscale building blocks to form a heterogeneous complementary inverter circuit based on layered molybdenum disulfide and carbon nanotube (CNT) bundles. The fabricated CNT/MoS(2) inverter is composed of n-type molybdenum disulfide (MOS(2)) and p-type CNT transistors, with a high voltage gain of 1.3. The CNT channels are fabricated using directed assembly while the layered molybdenum disulfide channels are fabricated by mechanical exfoliation. This bottom-up fabrication approach for integrating various nanoscale elements with unique characteristics provides an alternative cost-effective methodology to complementary metal-oxide-semiconductors, laying the foundation for the realization of high performance logic circuits.

MoS₂ nanosheet phototransistors with thickness-modulated optical energy gap

We report on the fabrication of top-gate phototransistors based on a few-layered MoS(2) nanosheet with a transparent gate electrode. Our devices with triple MoS(2) layers exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection. The varied functionalities are attributed to energy gap modulation by the number of MoS(2) layers. The photoelectric probing on working transistors with the nanosheets demonstrates that single-layer MoS(2) has a significant energy bandgap of 1.8 eV, while those of double- and triple-layer MoS(2) reduce to 1.65 and 1.35 eV, respectively.

GaS and GaSe ultrathin layer transistors

Room-temperature, bottom-gate, field-effect transistor characteristics of 2D ultrathin layer GaS and GaSe prepared from the bulk crystals using a micromechanical cleavage technique are reported. The transistors based on active GaS and GaSe ultrathin layers demonstrate typical n-and p-type conductance transistor operation along with a good ON/OFF ratio and electron differential mobility.

Hysteresis in single-layer MoS2 field effect transistors

Field effect transistors using ultrathin molybdenum disulfide (MoS(2)) have recently been experimentally demonstrated, which show promising potential for advanced electronics. However, large variations like hysteresis, presumably due to extrinsic/environmental effects, are often observed in MoS(2) devices measured under ambient environment. Here, we report the origin of their hysteretic and transient behaviors and suggest that hysteresis of MoS(2) field effect transistors is largely due to absorption of moisture on the surface and intensified by high photosensitivity of MoS(2). Uniform encapsulation of MoS(2) transistor structures with silicon nitride grown by plasma-enhanced chemical vapor deposition is effective in minimizing the hysteresis, while the device mobility is improved by over 1 order of magnitude.