MoS2 Devices Research Articles

High-performance photodetector and its optoelectronic mechanism of MoS2/WS2 vertical heterostructure

Two-dimensional heterostructures based on transition metal dichalcogenides (TMDCs) exhibit enhanced electrical and optoelectrical properties, which are promising for next-generation optoelectronics devices. In this work, we demonstrate the preparation of MoS2/WS2 vertical heterostructure and investigate their photoresponse properties by fabricating n-type photodetectors with titanium (Ti) as source-drain electrode. Uniform MoS2 and WS2 films are synthesized on SiO2/Si substrates by chemical vapor deposition (CVD) method respectively, and then MoS2/WS2 vertical heterostructures are achieved via transferring MoS2 onto WS2 films. The WS2- and MoS2-based devices exhibit on/off ratio and electron mobility of 106, 0.03 cm2V-1S-1 and > 106, 2.6 cm2V-1S-1 at Vds = 6 V. Notably, the MoS2/WS2 heterostructure photodetector achieves on/off ratio of near 108 and electron mobility of 12.6 cm2V-1S-1, which is higher than that of pure WS2 and MoS2 devices. The photodetector based on MoS2/WS2 heterostructure shows enhanced optoelectronic performance with photoresponsivity of 298 A/W and specific detectivity of 2.38 × 1011 Jones under 405 nm laser with the power density of 0.09 mW/cm2. Simultaneously, fast photoresponse speed of 9 ms is obtained owing to the speed of separating and recombining electron-hole pairs. The photodetectors exhibit excellent stability and repeatability to laser, which provides more possibilities for future electronic and optoelectronic applications.

High‐performance symmetric supercapacitor based on molybdenum disulfide/poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) composite electrodes deposited by spray‐coating

Here we present the study of supercapacitors based on molybdenum disulfide and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (MoS2/PEDOT:PSS) composite electrodes deposited by spray-coating. To produce the MoS2/PEDOT:PSS composite ink, dispersions of hydrothermally synthesized MoS2 and PEDOT:PSS were mixed in different weight proportions. Electrodes containing only PEDOT:PSS and MoS2 were also fabricated for comparison. The composite films were deposited by transferring the as-prepared inks to an airbrush coupled to a modified 3D printer base with in-line displacement. The films were characterized in terms of the scanning electron microscope (SEM), Raman spectroscopy, and sheet resistance. The electrochemical properties of supercapacitors were evaluated by cyclic voltammetry (CV), galvanostatic charge/discharges (GCD) techniques, impedance spectroscopy (IS), and their correlated parameters. The maximum specific capacitance of the MoS2-based supercapacitor was estimated to be 145.9 F g−1 at 1 mV s−1, while the 3:1 MoS2/PEDOT:PSS weight ratio supercapacitor exhibited a specific capacitance of 131.1 F g−1. Despite the slightly lower CSP, the 3:1 device provided a better overall performance, with a remarkably higher energy density of 11.2 Wh kg−1 and a maximum power density of 1020 W kg−1. It also displayed better long-term cycle stability, retaining 92% of its initial capacitance after 1000 CV cycles (against 54% of MoS2 devices). In summary, we demonstrated the study and production of a high-performance supercapacitor with spray-deposited MoS2/PEDOT:PSS composite electrodes, in which the production can be easily scalable for mass production and practical applications.

Two-Dimensional Metallic Vanadium Ditelluride as a High-Performance Electrode Material.

Two-dimensional (2D) metallic transition-metal dichalcogenides (MTMDCs) are considered as ideal electrode materials for enhancing the device performances of 2D semiconducting transition-metal dichalcogenides, due to their similar atomic structures and complementary electronic properties. Vanadium ditelluride (VTe2) behaves as a fascinating material in MTMDCs family, presenting room-temperature ferromagnetism, charge density waves order, and topological property. However, its practical applications in universal electrode/energy-related fields remain unexplored. Herein, we achieved the direct synthesis of ultrathin, large-domain, and thickness-tunable 1T-VTe2 nanosheets on an easily available mica substrate by chemical vapor deposition (CVD). We further uncover that the CVD-derived 1T-VTe2 can serve as a high-performance electrode material thanks to its ultrahigh conductivity. Accordingly, a 6 times higher field-effect mobility (∼47.5 cm2 V-1 s-1) was achieved in 1T-VTe2-contacted monolayer MoS2 devices than that using a conventional Ti/Au electrode (∼8.1 cm2 V-1 s-1). Moreover, the CVD-synthesized 1T-VTe2 nanosheets are revealed to present excellent electrocatalytic activity for hydrogen evolution reaction. These results should propel the direct application of CVD-grown 2D MTMDCs as high-performance electrode materials in all 2D materials related devices.

Electronic Properties of Multilayer MoS2 Field Effect Transistor with Unique Irradiation Resistance

Due to their small size and low power consumption, two-dimensional (2D) MoS2 devices have emerged as attractive candidates for next-generation nanoelectronics. However, in some particular working e...

Self-aligned-TiSi2 bottom contact with APM cleaning and post-annealing for sputtered-MoS2 film

Electrical contact characteristics between self-aligned titanium silicide (TiSi2) and sputtered-molybdenum disulfide (MoS2) films were newly demonstrated. In contrast with metal contacts, the surface of the TiSi2 bottom-contact was cleaned by using an ammonia and hydrogen peroxide mixture (APM) before MoS2 deposition, because the TiSi2 film has high oxidation resistance. In order to extract the contact resistance, a transmission line model device was fabricated. A two-order reduction in the contact resistance was achieved by a post-annealing at 650 °C in forming gas ambient (3% H2 in N2). This reduction was attributed to selective titanium diffusion from bottom contact into the MoS2 film. The TiSi2 contact is thus speculated as a candidate for a practical contact material in MoS2 devices.

A new strategy to improve the performance of MoS2-based 2D photodetector by synergism of colloidal CuInS2 quantum dots and surface plasma resonance of noble metal nanoparticles

Owing to their extraordinary optoelectronic properties, 2D transition metal dichalcogenides (TMDs) are proved to be promising materials for photodetectors. However, they suffered from poor absorptivity due to their atomically thin profile. Herein, CuInS2 quantum dots (QDs) and plasmonic Au nanoparticles (AuNPs) were adopted to achieve a MoS2-CuInS2-AuNPs 2D–0D hybrid by simultaneous deposition of CuInS2 QDs and AuNPs on the MoS2 nanosheet. In this configuration, MoS2 is used as a high mobility carrier transport channel, while CuInS2 QDs act as an efficient light absorber, and the AuNPs serve as a light sensitizer to enhance the local electric field around MoS2 and CuInS2 QDs via surface plasmon resonance (SPR). With such synergism of CuInS2 QDs and Au NPs, the highest responsivity of 16.65AW−1 and D* as high as 2.27×1012Jones are achieved for the MoS2-CuInS2-AuNPs hybrid devices under 635 nm laser, which is two orders of magnitude higher than the pristine MoS2 devices and outperformed most of the other MoS2-based photodetectors. Finally, the synergistic effect of the CuInS2 QDs and Au nanoparticles is illustrated by Finite-Difference Time-domain(FDTD) simulation. The underlying synergism strategy for such a 2D–0D configuration may also be applied to other photo-electric devices.

High-Performance Broadband Photodetector Based on Monolayer MoS2 Hybridized with Environment-Friendly CuInSe2 Quantum Dots.

Monolayer MoS2, a direct bandgap transition metal dichalcogenide (TMD), has attracted worldwide attention in electronics and optoelectronics. However, the performance of photodetectors based on monolayer MoS2 is restricted to a weak optical absorption, narrow absorption range, and persistent photoconductance. Herein, benefiting from an easy solution process, high light absorption coefficient, and wide absorption range, environment-friendly CuInSe2 quantum dots (QDs) are hybridized with monolayer MoS2 for high-performance broadband photodetectors. Owing to the favorable type-II energy band alignment of MoS2/CuInSe2-QDs, the hybrid photodetector exhibits a broadband photoresponse from the ultraviolet to near-infrared region, with an ultrahigh photoresponsivity of 74.8 A/W at 1064 nm, and compared with those of the pristine MoS2 device, the photoresponsivity and specific detectivity in the ultraviolet-visible region were enhanced by about 30 and 20 times, respectively. Furthermore, the formed depletion region at the MoS2/CuInSe2-QDs interface can significantly increase the photoresponse speed, and the accumulated holes in the QD side induce a strong photogating effect to improve the photoresponsive characteristics of the hybrid photodetector. Our work opens up opportunities for fabricating high-performance monolayer TMD-based broadband photodetectors.

The Roll-to-Roll Deposition of 2D Nanoflake Films of Semiconducting Tmds for Large Area Solar-to-Chemical Energy Conversion

The exceptional semiconducting properties of exfoliated transition metal dichalcogenide (TMDs) nanosheets have long inspired their use in photoelectrochemical applications; in particular, photoelectrodes for solar-to-chemical energy conversion. Indeed, past demonstrations using non-exfoliated, bulk material gave solar-to-hydrogen efficiencies greater than 7%.1 In taking advantage of the exfoliable nature of these materials, nano-TMDs not only provide a new range of tunable optoelectronic properties, but also create a direct path toward ultrathin, flexible devices. However, a major challenge in commercializing TMD-based devices is their large-scale production.2 Our group has recently addressed this issue by developing methods for solution-based fabrication of nanoflake TMD thin films for large area solar energy conversion applications.3 In this presentation we highlight our results employing WSe2 for solar water splitting and report the further advancement of our preparation methods toward a continuous roll-to-roll deposition of TMD-based thin films from nanoflake dispersions using a liquid-liquid interfacial self-assembly technique.3,4 We demonstrate reproducible printing of 100mm wide TMD flake films on plastic substrates at 10 mm s-1 with nanoflake loadings of 35 mg m-2.4 Importantly we show that these large-scale films of semiconducting TMDs are robust and capable of solar-to-chemical energy conversion comparable to that of their batch-scale counterparts.4 In addition, we address the effects of processing conditions on TMD flake quality and optoelectronic device performance. We find that the type of exfoliation and related parameters have a significant impact on the defect density within the flakes, which effects the harvesting of photogenerated charge carriers. We observe this for MoS2 and WSe2-based devices, suggesting that this is a fundamental limitation of liquid exfoliated TMDs. To overcome this limitation we present distinct defect mitigation strategies that can drastically improve the solar energy conversion ability of TMD nanoflake films.5 These techniques lead to state-of -the-art performance of solution-processed WSe2 nanoflake films for photoelectrochemical H2 production with internal quantum efficiency of over 50% and 1 sun photocurrent density over 4 mA cm-2.5

Sub-10 nm patterning of few-layer MoS2 and MoSe2 nanolectronic devices by oxidation scanning probe lithography

The properties of 2D materials devices are very sensitive to the physical, chemical and structural interactions that might happen during processing. Low-invasive patterning methods are required to fabricate devices at the nanoscale. Here we developed a process that combines oxidation scanning probe lithography (o-SPL) and oxygen plasma to fabricate nanoribbon field-effect transistors and nano-constrictions on few-layer MoS2 and MoSe2. The oxygen plasma has a double role in this process. First, it forms a thin, uniform oxide layer on top of the flake surface to enable o-SPL nanopatterning with full control of shape and size. Second, the oxide layer thins down the flake. Both plasma-based and o-SPL oxides are soluble in deionized H2O, which enabled etching and the definition of electrically isolated nano-constrictions and nanoribbons. The accuracy and robustness of the process was applied to pattern sub-10 nm wide constrictions and nanoribbon transistors.

In Situ Study of Molecular Doping of Chlorine on MoS2 Field Effect Transistor Device in Ultrahigh Vacuum Conditions.

We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS2 channel when the channel part is exposed to Cl2 gas. The gas exposure and the electrical measurement of the MoS2 FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS2 FET can provide and understand the surface properties. With the Cl2 gas exposure to the channel, the plot of the drain current versus the gate voltage (Id–Vg curve) shifts monotonically toward the positive direction of Vg, suggesting that the adsorbate acts as an electron acceptor. The Id–Vg shifts are numerically estimated by measuring the onset of Id (threshold voltage, Vth) and the mobility as a function of the dosing amounts of the Cl2 gas. The behaviors of both the Vth shift and the mobility with the Cl2 dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS2 surface up to the monolayer coverage. The charge transfer makes the Vth shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS2 channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl2 molecule. The monotonic shift of Vth with the coverage suggests that the MoS2 device sensor has a good sensitivity to detect 10–3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.

First Principle Study of Adsorption Behavior of PF5 Gas Molecule on S and Mo Vacancy MoS2 Monolayer

The molybdenum disulfide monolayer (MoS2) is gaining more attention due to its attractive electronic property, and it is extensively used in different electronic applications. The presence of vacancies on the MoS2 monolayer leads to an increase in the conductivity of the material. In this work, we have investigated the adsorption behavior and estimated the gas-sensing properties of the S-vacancy (VS) and the Mo-vacancy (VMo) MoS2 monolayers and the two-probe MoS2 devices with phosphorus pentafluoride (PF5) gas molecule. To explore the sensing and electronic properties of VS and VMo MoS2 towards PF5 gas adsorption, the adsorption distance, adsorption energy, charge transfer, band structure, and density of the states have been analyzed using density functional theory in combination with Non-Equilibrium Green's Function. The results show that the PF5 gas molecule is allowed to adsorb on the S- and Mo-vacancy MoS2 monolayers through van der Waals interaction. The PF5 gas molecule shows adsorption distances of 3.3274 A and 2.8673 A, adsorption energies of − 0.1640 eV and − 0.3489 eV, and charge transfers of − 0.025 Q (e) and − 0.053 Q (e) on the VS/VMo MoS2 monolayers. To study the electron transport properties, the device density of the states, the transmission spectra, and the current–voltage characteristics of the VS/VMo MoS2 two-probe devices have been analyzed. The results predicted that the Mo-vacancy MoS2 device shows relatively more adsorption towards the PF5 gas molecule when compared with the VS MoS2 device.

Graphene based Van der Waals contacts on MoS2 field effect transistors

Device performance of two dimensional (2D) material based field effect transistors is severely limited by the relatively high contact resistance encountered at the contact-channel interface. Metal-graphene hybrid contacts have been previously used to improve the contact resistance of devices based on thick exfoliated materials. Here we report a novel 2D FET fabrication process entailing the transfer of metal-graphene hybrid contacts on top of 3 monolayer-thick chemical vapor deposition (CVD) MoS2, enabling a lithography free contacting strategy, with respect to MoS2. Three different metal-graphene stacks consisting of Ni, Pd and Ru, have been fabricated, transferred onto MoS2 and characterized extensively using electrical and physical characterization techniques. We find strong correlation between the measured electrical characteristics and physical characterization of the contact interface. From Raman spectra measurement, maximum charge transfer of 1.7 × 1013 cm−2 is observed between graphene and Ru, leading to an improved contact resistance for MoS2 devices with Ru-Gr contacts. Ru-Gr contact shows the lowest contact resistance of 9.34 kΩ · µm among the three metal-graphene contact stacks reported in this article. This contact resistance is also the best among reported CVD grown graphene contacted MoS2 devices. Using more than 400 devices, we study the impact of the different metal-graphene contacts on other electrical parameters such as hysteresis, sub-threshold swing and threshold voltage. The metal-graphene contact stack transfer technique represents a technologically relevant contacting approach which can be further up-scaled to larger wafer areas.

Plasmonic Au Nanoparticles Sensitized MoS₂ for Bifunctional NO₂ and Light Sensing

The possibility to synergise two-dimensional (2D) materials with 0D nanoparticles has sparked a surge in high performance futuristic electronic devices. Here, we decorated plasmonic Au nanoparticles on surface of chemical vapor deposition (CVD) grown 2D MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanosheet and demonstrated bifunctional sensing behaviour within a single device. The plasmonic Au nanoparticles functionalized MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> device showed about ~5 times higher sensitivity to NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> than that of pristine MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at room temperature. The enhanced gas sensing performance was attributed to a combination of Schottky barriers modulation at Au/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanointerfaces and catalytic effects upon exposing the gas analyte. In addition, the device also exhibited enhanced photoresponse with a high photo-responsivity of ~17.6 A/W and a moderate detectivity of ~6.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> Jones due to enhanced local plasmonic effects. Finally, photons and gas molecules are detected in sequence, which proved that only a single Au-MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> device exhibited remarkable bifunctional sensing characteristics. Such excellent bifunctional sensing ability of a single Au-MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> device paves the way to integrate the 2D material with plasmonic nanostructures for developing an advanced multifunctional sensor.

Optically tunable charge carrier injection in monolayer MoS2

We report on excitation laser power-dependent Raman and photoluminescence (PL) measurements on gold nanoparticles–monolayer MoS2 hybrid nanostructures. Excitation of localized surface plasmon resonances in gold nanoparticles and their subsequent non-radiative relaxation inject charge carriers (electrons) into the adjacent monolayer MoS2. Due to the non-centrosymmetric nature of monolayer MoS2, the localized electron doping induces lattice compression via inverse piezoelectric response. The resultant lattice distortion manifests as a shift as well as broadening of $${A}_{1g}(\Gamma )$$ and $${E}_{2g}^{1}\left(\Gamma \right)$$ MoS2 Raman modes. A splitting of $${E}_{2g}^{1}\left(\Gamma \right)$$ mode is also observed. The PL spectra reveal power-dependent enhancement of trion feature, which again is a signature of an increasing electron doping. The observed effects are confirmed to be doping related as they are absent in monolayer MoS2 without gold nanoparticles. Our observations reveal that charge carrier injection is effectively controlled by varying the excitation laser power, which may aid in optically tuning physical response of MoS2 hybrid nanostructures and devices.

Low‐Damaged Layer‐by‐Layer Etching of Large‐Area Molybdenum Disulfide Films via Mild Plasma Treatment

AbstractControlling the number of layers of 2D transition metal dichalcogenide (TMDC) film is indispensable for optoelectronic applications because of their layer‐dependent optical features. However, most of methodologies on controlling TMDC layers induce undesirable defect formation and are performed on TMDC flakes, obtained from mechanical exfoliation. These limitations make practical TMDC‐based optoelectronic applications more difficult. Herein, layer‐by‐layer etching methodology of molybdenum disulfide (MoS2) films is developed using mild sulfur hexafluoride (SF6) plasma treatment. It is confirmed that the number of layers is successfully controlled from trilayer to monolayer by increasing plasma treatment time, while preserving overall film quality. Such low‐damaged layer‐by‐layer etching is attributed to this mild plasma etching system to only introduce the chemical reaction between ionized fluorine and MoS2, without physical damage to MoS2. Electrical properties of layer‐controlled MoS2 devices are also investigated, achieving comparatively high field‐effect mobilities of 16.2, 8.84, and 2.87 cm2 V−1 s−1 for tri‐, bi‐, and monolayer MoS2, respectively. This finding provides the possibility of realizing layer‐by‐layer engineering of 2D TMDCs.

Tuning Electrical Conductance in Bilayer MoS2 through Defect-Mediated Interlayer Chemical Bonding.

Interlayer interaction could substantially affect the electrical transport in transition metal dichalcogenides, serving as an effective way to control the device performance. However, it is still challenging to utilize interlayer interaction in weakly interlayer-coupled materials such as pristine MoS2 to realize layer-dependent tunable transport behavior. Here, we demonstrate that, by substitutional doping of vanadium atoms in the Mo sites of the MoS2 lattice, the vanadium-doped monolayer MoS2 device exhibits an ambipolar field effect characteristic, while its bilayer device demonstrates a heavy p-type field effect feature, in sharp contrast to the pristine monolayer and bilayer MoS2 devices, both of which show similar n-type electrical transport behaviors. Moreover, the electrical conductance of the doped bilayer MoS2 device is drastically enhanced with respect to that of the doped monolayer MoS2 device. Employing first-principle calculations, we reveal that such striking behaviors arise from the presence of electrical transport networks associated with the enhanced interlayer hybridization of S-3pz orbitals between adjacent layers activated by vanadium dopants in the bilayer MoS2, which is nevertheless absent in its monolayer counterpart. Our work highlights that the effect of dopant not only is confined in the in-plane electrical transport behavior but also could be used to activate out-of-plane interaction between adjacent layers in tailoring the electrical transport of the bilayer transitional metal dichalcogenides, which may bring different applications in electronic and optoelectronic devices.

Plasmonic-tape-attached multilayered MoS2 film for near-infrared photodetection

Molybdenum disulfide has been intensively studied as a promising material for photodetector applications because of its excellent electrical and optical properties. We report a multilayer MoS2 film attached with a plasmonic tape for near-infrared (NIR) detection. MoS2 flakes are chemically exfoliated and transferred onto a polymer substrate, and silver nanoparticles (AgNPs) dewetted thermally on a substrate are transferred onto a Scotch tape. The Scotch tape with AgNPs is attached directly and simply onto the MoS2 flakes. Consequently, the NIR photoresponse of the MoS2 device is critically enhanced. The proposed tape transfer method enables the formation of plasmonic structures on arbitrary substrates, such as a polymer substrate, without requiring a high-temperature process. The performance of AgNPs-MoS2 photodetectors is approximately four times higher than that of bare MoS2 devices.

A highly sensitive wearable flexible strain sensor based on polycrystalline MoS2 thin film

The present study investigates the piezoresistive properties of polycrystalline MoS2 film for strain-sensing applications. The gauge factor (GF) of the flexible MoS2 device (MoS2/PET) has been calculated to be 102 ± 5 in the stress range from ~7 MPa to ~14 MPa. In addition, to improve the sensing stress range, the flexible strain sensors are encapsulated by SU-8. The effect of encapsulation layer thickness is reflected in the GF, which is attributed to the shifting of the neutral axis. However, the calculated GF is constant in the higher stress range, 80 ± 2 and 12 ± 1 for 2 μm and 10 μm thick SU-8, respectively. Herein, we report a cost-effective and scalable approach to fabricate large-area polycrystalline MoS2-based flexible sensors for a wider stress range. The encapsulated devices remained undistorted and intact for stress values beyond 14 MPa. Further, we demonstrate the durability of the fabricated sensors with body movements, such as hand gestures, for all the three types of strain sensor.

Laser annealing towards high-performance monolayer MoS2 and WSe2 field effect transistors

The transition metal dichalcogenides (TMDCs) have been intensively investigated as promising nanoelectronic and optoelectronic materials. However, the pervasive adsorbates on the surfaces of monolayer TMDCs, including oxygen and water molecules from the ambient environment, tend to degrade the device performance, thus hindering specific applications. In this work, we report the effect of laser irradiation on the transport and photoresponse of monolayer MoS2 and WSe2 devices, and this laser annealing process is demonstrated as a straightforward approach to remove physically adsorbed contaminants. Compared to vacuum pumping and in situ thermal annealing treatments, the field-effect transistors after laser annealing show a more than one order of magnitude higher on-state current, and no apparent degradation of device performance at low temperatures. The mobility of the monolayer WSe2 devices can be enhanced by three to four times, and for single-layered MoS2 devices with the commonly used SiO2 as the back-gate, the mobility increases by 20 times, reaching . The efficient cleaning effect of laser annealing is also supported by the reduction of channel and contact resistance revealed by a transmission line experiment. Further, an enhanced photocurrent, by a factor of ten, has been obtained in the laser annealed device. These findings pave the way for high-performance monolayer TMDC-based electronic and optoelectronic devices with a clean surface and intrinsic properties.

The optimization of contact interface between metal/MoS2 FETs by oxygen plasma treatment

The quality of the contact is key for the performance enhancement of the MoS2 device. As the oxygen plasma treatment is developed in the fabrication of MoS2 field effect transistor (FET), we demonstrate that the tunneling layer (MoO3) is formed on the top of the MoS2 contact region and the process induced organic residues in the source and drain regions is effectively removed. The formation of MoO3 can degrade the Fermi level pinning effect at the MoS2/metal interface and lowering the Schottky barrier height. And the flatness of the interface has been greatly improved, with the roughness reduced from 0.53 to 0.166 nm. In this paper, we improve the device performance with an on-/off-current ratio increase by four orders of magnitude, a mobility increase by 10 times at room temperature and the 33-fold decrease in contact resistance. This research shows that oxygen plasma treatment is a promising method for the integration of MoS2 FET in the future.