Two-dimensional (2D) materials, consisting of atomically thin crystal layers bound by the van der Waals force, have attracted much interest because of their potential in diverse technologies, including electronics, optoelectronics and catalysis1-10. In particular, solution-processable 2D semiconductor (such as MoS2) nanosheets are attractive building blocks for large-area thin-film electronics. In contrast to conventional zero- and one-dimensional nanostructures (quantum dots and nanowires, respectively), which are typically plagued by surface dangling bonds and associated trapping states, 2D nanosheets have dangling-bond-free surfaces. Thin films created by stacking multiple nanosheets have atomically clean van der Waals interfaces and thus promise excellent charge transport11-15. However, preparing high-quality solution-processable 2D semiconductor nanosheets remains a challenge. For example, MoS2 nanosheets and thin films produced using lithium intercalation and exfoliation are plagued by the presence of the metallic 1T phase and poor electrical performance (mobilities of about 0.3 square centimetres per volt per second and on/off ratios of less than 10)2,12, and materials produced by liquid exfoliation exhibit an intrinsically broad thickness distribution, which leads to poor film quality and unsatisfactory thin-film electrical performance (mobilities of about 0.4 square centimetres per volt per second and on/off ratios of about 100)14,16,17. Here we report a general approach to preparing highly uniform, solution-processable, phase-pure semiconducting nanosheets, which involves the electrochemical intercalation of quaternary ammonium molecules (such as tetraheptylammonium bromide) into 2D crystals, followed by a mild sonication and exfoliation process. By precisely controlling the intercalation chemistry, we obtained phase-pure, semiconducting 2H-MoS2 nanosheets with a narrow thickness distribution. These nanosheets were then further processed into high-performance thin-film transistors, with room-temperature mobilities of about 10 square centimetres per volt per second and on/off ratios of 106 that greatly exceed those obtained for previous solution-processed MoS2 thin-film transistors. The scalable fabrication of large-area arrays of thin-film transistors enabled the construction of functional logic gates and computational circuits, including an inverter, NAND, NOR, AND and XOR gates, and a logic half-adder. We also applied our approach to other 2D materials, including WSe2, Bi2Se3, NbSe2, In2Se3, Sb2Te3 and black phosphorus, demonstrating its potential for generating versatile solution-processable 2D materials.

Solution-processable 2D semiconductors for high-performance large-area electronics

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS2, MoSe2, MoTe2, WS2 and WSe2, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1–2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

The development of an active, earth-abundant, and inexpensive catalyst for the oxygen evolution reaction (OER) is highly desirable but remains a great challenge. Here, by combining experiments and first-principles calculations, we demonstrate that MoS2 quantum dots (MSQDs) are efficient materials for the OER. We use a simple route for the synthesis of MSQDs from a single precursor in aqueous medium, avoiding the formation of unwanted carbon quantum dots (CQDs). The as-synthesized MSQDs exhibit higher OER activity with a lower Tafel slope in comparison to that for the state of the art catalyst IrO2/C. The potential cycling of the MSQDs activates the surface and improves the OER catalytic properties. Density functional theory calculations reveal that MSQD vertices are reactive and the vacancies at the edges also promote the reaction, which indicates that the small flakes with defects at the edges are efficient for the OER. The presence of CQDs affects the adsorption of reaction intermediates and dramaticall...

MoS2 Quantum Dots as Efficient Catalyst Materials for the Oxygen Evolution Reaction

A scalable and catalyst-free method to deposit stoichiometric molybdenum disulfide (MoS2) films over large areas is reported, with the maximum area limited by the size of the substrate holder. The method allows deposition of MoS2 layers on a wide range of substrates without any additional surface preparation, including single-crystal (sapphire and quartz), polycrystalline (HfO2), and amorphous (SiO2) substrates. The films are deposited using carefully designed MoS2 targets fabricated with excess sulfur and variable MoS2 and sulfur particle size. Uniform and layered MoS2 films as thin as two monolayers, with an electrical resistivity of 1.54 × 104 Ω cm–1, were achieved. The MoS2 stoichiometry was confirmed by high-resolution Rutherford backscattering spectrometry. With the method reported here, in situ graded MoS2 films ranging from ∼1 to 10 monolayers can be deposited.

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Large-Area Deposition of MoS2 by Pulsed Laser Deposition with In Situ Thickness Control

Unlike zero-dimensional quantum dots, one-dimensional nanowires/nanorods, and three-dimensional networks or even their bulk counterparts, the charge carriers in two-dimensional (2D) materials are confined along the thickness while being allowed to move along the plane. They have distinct characteristics like strong quantum confinement, tunable thickness, and high specific surface area, which makes them a promising candidate in a wide range of applications such as electronics, topological spintronic devices, energy storage, energy conversion, sensors, biomedicine, catalysis, and so on. After the discovery of the extraordinary properties of graphene, other graphene-like 2D materials have attracted a great deal of attention. Like graphene, to realize their potential applications, high efficiency and low cost industrial scale methods should be developed to produce high-quality 2D materials. The electrochemical methods usually performed under mild conditions are convenient, controllable, and suitable for mass production. In this review, we introduce the latest and most representative investigations on the fabrication of 2D monoelemental Xenes, 2D transition-metal dichalcogenides, and other important emerging 2D materials such as organic framework (MOF) nanosheets and MXenes through electrochemical exfoliation. The electrochemical exfoliation conditions of the bulk layered materials are discussed. The numerous factors which will affect the quality of the exfoliated 2D materials, the possible exfoliating mechanism and potential applications are summarized and discussed in detail. A summary of the discussion together with perspectives and challenges for the future of this emerging field is also provided in the last section.

Electrochemical exfoliation of graphene-like two-dimensional nanomaterials

We report a simple and selective solution method to prepare Molybdenum Disulfide (MoS2) thin films for functional thin film transistors (TFTs). The selective area solution-processed MoS2 grows on top and around the gold (Au) source and drain electrodes and in the channel area of the TFT. MoS2 thicknesses in the channel area are in the order of 11 nm. A mechanism for the selective growth is also proposed. The Au electrodes act not only as contact, but also as a catalytic surface for the hydrazine hydrate used in the reaction, which induces the selective growth of MoS2 on the Au surface and into the channel region. This one step process demonstrates functional TFTs with a carrier mobility of ∼0.4 cm2 V−1 s−1.

Fabrication of MoS2 thin film transistors via selective-area solution deposition methods

A method is presented to extract the series resistance, the threshold voltage, and the mobility enhancement factor of thin-film MOSFETs. This integration-based method, which only requires measuring the saturation drain current from a single test device, was tested using simulated and measured data of three different devices: amorphous indium–gallium–zinc oxide (IGZO), zinc oxide (ZnO), and polysilicon thin-film transistors (TFTs).

A New Integration-Based Procedure to Extract the Threshold Voltage, the Mobility Enhancement Factor, and the Series Resistance of Thin-Film MOSFETs

Performance and reliability comparison of oxide-based thin-film transistors fabricated at a maximum process temperature of 115 °C is presented. A fully patterned and passivated process was successfully evaluated and implemented using polycrystalline ZnO and amorphous indium–gallium–zinc–oxide (IGZO) as an active layer in glass substrates. Saturation mobilities of 14.2 and 9.0 cm $^{{2}}\cdot \text{V}^{-{1}}\cdot \text{s}^{-{1}}$ were obtained for ZnO and IGZO, respectively, with threshold voltages of 2.2 and 2.0 V, an ON/ OFF ratio $> {1} \times 10^{{8}}$ , and an ${I}_{{\text {off}}} A. The small mobility change in IGZO with gate voltage is due to its amorphous character. Longer variation is observed in polycrystalline ZnO due to its grain boundaries. Reliability studies show a threshold voltage shifting of 0.4 and 1.8 V for ZnO and IGZO devices, testing after 1200-s stress. Devices were successfully implemented in ZnO- and IGZO-based inverters using saturation and zero-drive structures, showing a maximum dc gain of 2.4- and 25-V/V, respectively.

https://par.nsf.gov/servlets/purl/10140341

Performance and Reliability Comparison of ZnO and IGZO Thin-Film Transistors and Inverters Fabricated at a Maximum Process Temperature of 115 °C

Thin-film transistors were fabricated using 45-nm thick ZnO deposited by pulsed laser deposition and 15-nm thick Al2O3 gate insulator deposited by atomic layer deposition. A 1-D model with a constant density of interface states above and below the conduction band edge is used to explain the current-voltage characteristic. This model does not create distortion of transconductance versus gate voltage at low gate voltage that can be created by exponential tail or Gaussian distributions of interface states. The “subthreshold” regime is at the top ZnO surface away from the bottom gate, but subthreshold swing is controlled by interface states at the ZnO/Al2O3 bottom interface. The observed increase in mobility with gate voltage is delayed by filling of the interface states above the conduction band edge. Interface state filling and a rapid increase in mobility with increased gate voltage produce an apparent large threshold voltage as extrapolated from a linear fit to current measured at large gate voltage. The difference between threshold voltage and turn-off voltage is an important figure of merit (FOM) that should be kept as small as possible. In this paper, FOM is 2.5 and 1.8 V for 0.1 and 6 V drain voltage, respectively.

Nanocrystalline ZnO TFTs Using 15-nm Thick Al 2 O 3 Gate Insulator: Experiment and Simulation

Two different parameter extraction methods are proposed in this article. First, a simple DC method is presented to extract the difference between the drain and source series resistance of MOSFETs. This method is valid for any three- or four-terminal MOSFET and it can be used in linear, triode or saturation region. Second, an integration-based method is proposed to extract the drain resistance and the source resistance of thin-film MOSFETs. Both methods were tested using simulated and measured data of two different devices: zinc oxide (ZnO) and polysilicon TFTs.

Rodolfo A. Rodriguez-Davila

Papers

Fabrication of MoS2 thin film transistors via selective-area solution deposition methods

A New Integration-Based Procedure to Extract the Threshold Voltage, the Mobility Enhancement Factor, and the Series Resistance of Thin-Film MOSFETs

Performance and Reliability Comparison of ZnO and IGZO Thin-Film Transistors and Inverters Fabricated at a Maximum Process Temperature of 115 °C

Nanocrystalline ZnO TFTs Using 15-nm Thick Al 2 O 3 Gate Insulator: Experiment and Simulation

On the DC extraction of the asymmetric parasitic source and drain resistances for MOSFETs