17 research outputs found

    Large-Area Highly Conductive Transparent Two-Dimensional Ti<sub>2</sub>CT<sub><i>x</i></sub> Film

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    We report a simple and scalable method to fabricate homogeneous transparent conductive thin films (Ti<sub>2</sub>CT<sub><i>x</i></sub>, one of the MXene) by dip coating of an Al<sub>2</sub>O<sub>3</sub> substrate in a colloidal solution of large-area Ti<sub>2</sub>CT<sub><i>x</i></sub> thin flakes. Scanning electron microscopy and atomic force microscopy images exhibit the wafer-scale homogeneous Ti<sub>2</sub>CT<sub><i>x</i></sub> thin film (∼5 nm) covering the whole substrate. The sheet resistance is as low as 70 Ω/sq at 86% transmittance, which corresponds to the high figure of merit (FOM) of 40.7. Furthermore, the thickness of the film is tuned by a SF<sub>6</sub>+Ar plasma treatment, which etches Ti<sub>2</sub>CT<sub><i>x</i></sub> film layer by layer and removes the top oxidized layer without affecting the bottom layer of the Ti<sub>2</sub>CT<sub><i>x</i></sub> flake. The resistivity of plasma-treated Ti<sub>2</sub>CT<sub><i>x</i></sub> film is further decreased to 63 Ω/sq with an improved transmittance of 89% and FOM of 51.3, demonstrating the promise of Ti<sub>2</sub>CT<sub><i>x</i></sub> for future transparent conductive electrode application

    The Preparation of BN-Doped Atomic Layer Graphene via Plasma Treatment and Thermal Annealing

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    We report a new method for the codoping of boron and nitrogen in a monolayer graphene film. After the CVD synthesis of monolayer graphene, BN-doped graphene is prepared by performing power-controlled plasma treatment and thermal annealing with borazine. BN-doped graphene films with various doping levels, which were controlled by altering the plasma treatment power, were found with Raman and electrical measurements to investigate exhibit p-doping behavior. Transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy were used to demonstrate that the synthesized BN-doped graphene films have a sp<sup>2</sup> hybridized hexagonal structure. This approach to tuning the distribution and doping levels of boron and nitrogen in monolayer sp<sup>2</sup> hybridized BN-doped graphene is expected to be very useful for applications requiring large-area graphene with an opened band gap

    Multifunctional Homogeneous Lateral Black Phosphorus Junction Devices

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    We demonstrate a controllable doping technique of few-layer black phosphorus (BP) via surface charge transfer using an ionic liquid mixture of EMIM­(C<sub>6</sub>H<sub>11</sub>N<sub>2</sub><sup>+</sup>):TFSI­(C<sub>2</sub>F<sub>6</sub>NO<sub>4</sub>S<sub>2</sub><sup>–</sup>) [EMIM:TFSI, 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl) imide]. A wide range of hole carrier densities, from 10<sup>11</sup> cm<sup>–2</sup> (nondegenerate) to 10<sup>13</sup> cm<sup>–2</sup> (degenerate), can be obtained by controlling the weight percentage of the ionic liquid mixture. The doping method we proposed in this paper can be applied to make a multifunctional homogeneous lateral p–n junction device. By doping a fraction of the BP sample and by applying a gate voltage to the other fraction of the BP, we obtain homogeneous lateral p<sup>+</sup>–p, p<sup>+</sup>–n, p<sup>+</sup>–n<sup>+</sup> junction diodes in a single BP channel. The homogeneous lateral BP p<sup>+</sup>–p and p<sup>+</sup>–n junctions display ideal rectifying behavior and a much stronger photoresponse due to the built-in potential. Furthermore, at high positive gate voltages, the interband tunneling enables the homogeneous lateral p<sup>+</sup>–n<sup>+</sup> junction transistors to provide both a negative differential resistance (NDR) and a negative transconductance (NTC) in the current–voltage characteristics at room temperature. On the basis of our results, it is possible to build novel devices utilizing the large NDR and NTC in BP such as amplifiers, oscillators, and multivalued logic systems

    Highly Sensitive and Reusable Membraneless Field-Effect Transistor (FET)-Type Tungsten Diselenide (WSe<sub>2</sub>) Biosensors

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    In recent years when the demand for high-performance biosensors has been aroused, a field-effect transistor (FET)-type biosensor (BioFET) has attracted great interest because of its high sensitivity, label-free detection, fast detection speed, and miniaturization. However, the insulating membrane in the conventional BioFET, which is essential in preventing the surface dangling bonds of typical semiconductors from nonspecific bindings, has limited the sensitivity of biosensors. Here, we present a highly sensitive and reusable membraneless BioFET based on a defect-free van der Waals material, tungsten diselenide (WSe<sub>2</sub>). We intentionally generated a few surface defects that serve as extra binding sites for the bioreceptor immobilization through weak oxygen plasma treatment, consequently magnifying the sensitivity values to 2.87 × 10<sup>5</sup> A/A for 10 mM glucose. The WSe<sub>2</sub> BioFET also maintained its high sensitivity even after several cycles of rinsing and glucose application were repeated

    Defect-Free Copolymer Gate Dielectrics for Gating MoS<sub>2</sub> Transistors

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    In this study, the poly­(2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane-<i>co</i>-cyclohexyl methacrylate) [p­(V4D4-<i>co</i>-CHMA)] copolymer was developed for use as a gate dielectric in molybdenum disulfide (MoS<sub>2</sub>) field-effect transistors (FETs). The p­(V4D4-<i>co</i>-CHMA) copolymer was synthesized via the initiated chemical vapor deposition (<i>i</i>CVD) of two types of monomers: 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4D4) and cyclohexyl methacrylate (CHMA). Four vinyl groups of V4D4 monomers and cyclohexyl groups of CHMA monomers were introduced to enhance the electrical strength of gate dielectrics through the formation of a highly crosslinked network and to reduce the charge trap densities at the MoS<sub>2</sub>–dielectric interface, respectively. The <i>i</i>CVD-grown p­(V4D4-<i>co</i>-CHMA) copolymer films yielded a dielectric constant of 2.3 and a leakage current of 3.8 × 10<sup>–11</sup> A/cm<sup>2</sup> at 1 MV/cm. The resulting MoS<sub>2</sub> FETs with p­(V4D4-<i>co</i>-CHMA) gate dielectrics exhibited excellent electrical properties, including an electron mobility of 35.1 cm<sup>2</sup>/V s, a subthreshold swing of 0.2 V/dec, and an on–off current ratio of 2.6 × 10<sup>6</sup>. In addition, the environmental and operational stabilities of MoS<sub>2</sub> FETs with p­(V4D4-<i>co</i>-CHMA) top-gate dielectrics were superior to those of devices with SiO<sub>2</sub> back-gate dielectrics. The use of <i>i</i>CVD-grown copolymer gate dielectrics as demonstrated in this study provides a novel approach to realizing next-generation two-dimensional electronics

    Plasma-Treated Thickness-Controlled Two-Dimensional Black Phosphorus and Its Electronic Transport Properties

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    We report the preparation of thickness-controlled few-layer black phosphorus (BP) films through the modulated plasma treatment of BP flakes. Not only does the plasma treatment control the thickness of the BP film, it also removes the chemical degradation of the exposed oxidized BP surface, which results in enhanced field-effect transistor (FET) performance. Our fabricated BP FETs were passivated with poly(methyl methacrylate) (PMMA) immediately after the plasma etching process. With these techniques, a high field-effect mobility was achieved, 1150 cm<sup>2</sup>/(V s), with an <i>I</i><sub>on</sub>/<i>I</i><sub>off</sub> ratio of ∼10<sup>5</sup> at room temperature. Furthermore, a fabricated FET with plasma-treated few-layer BP that was passivated with PMMA was found to retain its <i>I</i>–<i>V</i> characteristics and thus to exhibit excellent environmental stability over several weeks

    Proton-Conductor-Gated MoS<sub>2</sub> Transistors with Room Temperature Electron Mobility of >100 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>

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    Room temperature electron mobility of >100 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> is achieved for a few-layer MoS<sub>2</sub> transistor by use of a polyanionic proton conductor as the top-gate dielectric of the device. The use of a proton conductor that inherently exhibits a cationic transport number close to 1 yields unipolar electron transport in the MoS<sub>2</sub> channel. The high mobility value is attributed to the effective formation of an electric double layer by the proton conductor, which facilitates electron injection into the MoS<sub>2</sub> channel, and to the effective screening of the charged impurities in the vicinity of the device channel. Through careful temperature-dependent transistor and capacitor measurements, we also confirm quenching of the phonon modes in the proton-conductor-gated MoS<sub>2</sub> channel, which should also contribute to the achieved high mobility. These devices are then used to assemble a simple resistive-load inverter logic circuit, which can be switched at high frequencies above 1 kHz

    Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo<sub>2</sub>C/MoS<sub>2</sub> Hybrid Structure <i>via</i> Chemical Conversion of Molybdenum Disulfide

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    The epitaxial synthesis of molybdenum carbide (Mo<sub>2</sub>C, a 2D MXene material) <i>via</i> chemical conversion of molybdenum disulfide (MoS<sub>2</sub>) with thermal annealing under CH<sub>4</sub> and H<sub>2</sub> is reported. The experimental results show that adjusting the thermal annealing period provides a fully converted metallic Mo<sub>2</sub>C from MoS<sub>2</sub> and an atomically sharp metallic/semiconducting hybrid structure <i>via</i> partial conversion of the semiconducting 2D material. Mo<sub>2</sub>C/MoS<sub>2</sub> hybrid junctions display a low contact resistance (1.2 kΩ·μm) and low Schottky barrier height (26 meV), indicating the material’s potential utility as a critical hybrid structural building block in future device applications. Density functional theory calculations are used to model the mechanisms by which Mo<sub>2</sub>C grows and forms a Mo<sub>2</sub>C/MoS<sub>2</sub> hybrid structure. The results show that Mo<sub>2</sub>C conversion is initiated at the MoS<sub>2</sub> edge and undergoes sequential hydrodesulfurization and carbide conversion steps, and an atomically sharp interface with MoS<sub>2</sub> forms through epitaxial growth of Mo<sub>2</sub>C. This work provides the area-controllable synthesis of a manufacturable MXene from a transition metal dichalcogenide material and the formation of a metal/semiconductor junction structure. The present results will be of critical importance for future 2D heterojunction structures and functional device applications

    Dye-Sensitized MoS<sub>2</sub> Photodetector with Enhanced Spectral Photoresponse

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    We fabricated dye-sensitized MoS<sub>2</sub> photodetectors that utilized a single-layer MoS<sub>2</sub> treated with rhodamine 6G (R6G) organic dye molecules (with an optical band gap of 2.38 eV or 521 nm). The proposed photodetector showed an enhanced performance with a broad spectral photoresponse and a high photoresponsivity compared with the properties of the pristine MoS<sub>2</sub> photodetectors. The R6G dye molecules deposited onto the MoS<sub>2</sub> layer increased the photocurrent by an order of magnitude due to charge transfer of the photoexcited electrons from the R6G molecules to the MoS<sub>2</sub> layer. Importantly, the photodetection response extended to the infrared (λ < 980 nm, which corresponded to about half the energy band gap of MoS<sub>2</sub>), thereby distinguishing the device performance from that of a pristine MoS<sub>2</sub> device, in which detection was only possible at wavelengths shorter than the band gap of MoS<sub>2</sub>, <i>i.e.</i>, λ < 681 nm. The resulting device exhibited a maximum photoresponsivity of 1.17 AW<sup>–1</sup>, a photodetectivity of 1.5 × 10<sup>7</sup> Jones, and a total effective quantum efficiency (EQE) of 280% at 520 nm. The device design described here presents a significant step toward high-performance 2D nanomaterial-based photodetector

    Catalytic Transparency of Hexagonal Boron Nitride on Copper for Chemical Vapor Deposition Growth of Large-Area and High-Quality Graphene

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    Graphene transferred onto h-BN has recently become a focus of research because of its excellent compatibility with large-area device applications. The requirements of scalability and clean fabrication, however, have not yet been satisfactorily addressed. The successful synthesis of graphene/h-BN on a Cu foil and DFT calculations for this system are reported, which demonstrate that a thin h-BN film on Cu foil is an excellent template for the growth of large-area and high-quality graphene. Such material can be grown on thin h-BN films that are less than 3 nm thick, as confirmed by optical microscopy and Raman spectroscopy. We have evaluated the catalytic growth mechanism and the limits on the CVD growth of high-quality and large-area graphene on h-BN film/Cu by performing Kelvin probe force microscopy and DFT calculations for various thicknesses of h-BN
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