Solution-processed MoS2 Research Articles

Solution-processed MoS2 nanostructures and thermal oxidization on carbon fabric with enhanced thermopower factor for wearable thermoelectric application

Miniaturization of electronic devices has become the new phase of modern technology. Wearable Thermoelectric Generators (WTEG), being flexible, lightweight, self-sustained, and safe are the only replacement for conventional batteries for mobile applications. Molybdenum Disulfide (MoS2), a graphene akin compound having diverse transport behaviour and low thermal conductivity is one of the most preferable candidates for thermoelectric application. However, limited reports are available on MoS2 for Wearable thermoelectric applications. In this study, we have accomplished successful growth of MoS2 nanostructures on conductive carbon fabric via one step hydrothermal method and the effect of thermal oxidation of MoS2 has been studied at various annealing temperature. The structural and elemental analysis confirms the transformation of MoS2 into MoO3. Annealing creates an MoS2/MoO3 interface thus enhancing the carrier mobility resulting in enhancement of power factor (481 nW.m−1K−2) of the sample annealed at 200 °C (M1) by 1.29 times than that of pristine MoS2. The sample possessing high performance has been employed to fabricate a WTEG and its real time output has been measured. The design and operating conditions of the device is studied and optimized to obtain the output open circuit voltage as high as 6.4 mV. This paves a promising route to fabricate self- powered wearable biomedical devices.

Mixed-Dimensional van der Waals Heterostructure (2D ReS2/0D MoS2 Quantum Dots)-Based Broad Spectral Range with Ultrahigh-Responsive Photodetector.

The remarkable properties of two-dimensional (2D) materials have led to significant advancements in photodetection and optoelectronics research. Currently, there are many successful methods that are employed to improve the responsivity of photodetectors, but the limited spectral range of the device remains a limitation. This work demonstrates the development of a mixed-dimensional (2D/0D) hybrid photodetector device fabricated using chemical vapor deposition (CVD)-grown monolayer ReS2 and solution-processed MoS2 quantum dots (QDs). The mixed dimensionality of 2D (ReS2) and zero-dimensional (0D) MoS2 QDs assist in improving the spectral range of the device [ultraviolet (360 nm) to near-infrared (780 nm)]. Further, due to the work function difference between ReS2 and MoS2 QDs, the built-in electric field across the mixed-dimensional interface promotes effective charge separation and migration, resulting in improved responsivities of the device. The calculated responsivities of the fabricated photodetector are 5.4 × 102, 3.3 × 102, and 2.6 × 102 A/W when subjected to visible, UV, and NIR light illumination, which is remarkable when compared to the existing reports on broadband photodetection. The mixed-dimensionality heterostructure coupled with contact engineering paves the way for highly responsive broadband photodetectors for potential applications in security, healthcare, etc.

Solution-Processed Robust Multifunctional Memristor of 2D Layered Material Thin Film.

Memristors have gained significant attention recently due to their unique ability to exhibit functionalities for brain-inspired neuromorphic computing. Here, we demonstrate a high-performance multifunctional memristor using a thin film of liquid-phase exfoliated (LPE) 2D MoS2 pinched between two electrodes. Nanoscale inspection of a solution-processed MoS2 thin film using scanning electron and scanning probe microscopies revealed the high-quality and defect-free nature. Systematic current-voltage (I-V) characterizations depict a facile, nonvolatile resistive switching behavior of our 2D MoS2 thin film device with a current On/Off ratio of 103 and energy cost of only a few picojoules. Excellent performance metrics, including at least 103 cycle endurance, 104 s retention, and switching speed down to a few nanoseconds, reflect robust high-performance data storage capability. Charge carriers trapping and detrapping at the sulfur vacancy defect sites in MoS2 nanosheets mainly display the resistive switching property, supported by the impedance analysis and theoretical fitting results. Multifunctionality is leveraged through implementing two-input logic gate operations, edge computation, and crucial adaptive learning via a Pavlov's dogs experiment. Overall, our solution-processed MoS2 memristor has the potential for tremendous future opportunities in integrated circuits and different computing paradigms, including energy-efficient neuromorphic computing hardware in artificial intelligence.

Low-Cost Solution-Processed MoS2 Quantum Dots-Based Deep UV Photodetector for Monitoring Disinfection

Deep UV radiation is commonly used as a disinfectant with an appropriate dose for better efficacy. However, continuous monitoring of UV radiation is required to have appropriate dose exposure for proper disinfection. This article reports a low-cost solution-processed fabrication method of the UV photodetector (PD) with a device geometry of Al/MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> quantum dots (QDs)/Al on Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> substrate, which is sensitive to deep UV radiations ~275 nm, commonly used for water and surfaces disinfection technologies. Colloidal MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> QDs with an average size of 1.99 nm have been synthesized by an inexpensive hydrothermal method. The surface ligand of MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> QDs has been replaced by a shorter ligand 1, 2-ethanedithiol (EDT) to reduce dot-to-dot distance. The fabricated device shows high responsivity (21.62 mA/W), external quantum efficiency (EQE) (9.74%), and detectivity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.57\times 10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> Jones) under the deep UV irradiation of ~275 nm. The PD has a good photoresponse speed of rise and decay times of 173 and 92 ms, respectively.

Bottom-up water-based solution synthesis for a large MoS2 atomic layer for thin-film transistor applications

A bottom-up water-based solution-process method was developed for atomic layered MoS2 with a one-step annealing process and no sulfurization. The chosen MoS2 precursor is water soluble and was carefully formulated to obtain good coating properties on a silicon substrate. The coated precursor was annealed in a furnace one time to crystallize it. This method can obtain a large and uniform atomic layer of 2D MoS2 with 2H lattice structure. The number of atomic layers (4–7) was controlled through the precursor concentrations and showed good uniformity, which was confirmed by STEM and AFM. Four types of thin-film transistors (TFTs) were prepared from the solution-processed MoS2 on Al2O3 and SiO2 dielectric with either thermal evaporated Al or printed Ag source and drain electrodes. The best result shows an improved mobility of 8.5 cm2 V−1 s−1 and a reasonable on–off ratio of about 1.0 × 105 with solid output saturation.

One-Pot Synthesis of Chlorophyll-Assisted Exfoliated MoS2/WS2 Heterostructures via Liquid-Phase Exfoliation Method for Photocatalytic Hydrogen Production.

Developing strategies for producing hydrogen economically and in greener ways is still an unaccomplished goal. Photoelectrochemical (PEC) water splitting using photoelectrodes under neutral electrolyte conditions provides possibly one of the greenest routes to produce hydrogen. Here, we demonstrate that chlorophyll extracts can be used as an efficient exfoliant to exfoliate bulk MoS2 and WS2 to form a thin layer of a MoS2/WS2 heterostructure. Thin films of solution-processed MoS2 and WS2 nanosheets display photocurrent densities of −1 and −5 mA/cm2, respectively, and hydrogen evolution under simulated solar irradiation. The exfoliated WS2 is significantly more efficient than the exfoliated MoS2; however, the MoS2/WS2 heterostructure results in a 2500% increase in photocurrent densities compared to the individual constituents and over 12 h of PEC durability under a neutral electrolyte. Surprisingly, in real seawater, the MoS2/WS2 heterostructure exhibits stable hydrogen production after solar illumination for 12 h. The synthesis method showed, for the first time, how the MoS2/WS2 heterostructure can be used to produce hydrogen effectively. Our findings highlight the prospects for this heterostructure, which could be coupled with various processes towards improving PEC efficiency and applications.

Solution-processed MoS2 quantum dot/GaAs vertical heterostructure based self-powered photodetectors with superior detectivity

The characteristics of a novel 0D/3D heterojunction photodetector fabricated using solution-processed colloidal MoS2 quantum dots (QDs) on GaAs is presented. MoS2 QDs with a dimension of ∼2 nm, synthesized by a standard sono-chemical exfoliation process with 2D layers have been used for the purpose. The microscopic and spectroscopic studies confirmed the formation of semiconducting (2H phase) MoS2 QDs. The photodetectors were fabricated using n-GaAs substrates with two different doping concentrations resulting in n–n heterojunctions between n-type 0D MoS2 QDs and bulk n-GaAs. The devices fabricated using GaAs with a higher doping concentration, showed an increase in the reverse current of the order of ∼102 upon illumination, while the same with a lower doping concentration showed an increase of the order of ∼103. All the heterojunction photodetector devices show a broadband operation over the visible wavelength range of 400–950 nm, with a peak responsivity of the devices being observed at 500 nm. The peak responsivity and detectivity are found to be ∼400 mA W−1 and ∼4 × 1012 Jones, respectively, even without any external applied bias, which are useful for self-powered photodetection. The results indicate that colloidal MoS2/GaAs based hybrid heterostructures provide a platform for fabricating broadband photodetectors by using highly absorbing MoS2 QDs, which may show the pathway towards next-generation optoelectronic devices with superior detection properties.

Stretchable MoS2 Electromechanical Sensors with Ultrahigh Sensitivity and Large Detection Range for Skin-on Monitoring.

Although two-dimensional (2D) layered molybdenum disulfide (MoS2) has widespread electrical applications in catalysis, energy storage, display, and photodetection, very few reports are available to achieve MoS2 for electromechanical sensing. Here, we report a novel solution-processed MoS2 strain sensor by constructing nanojunctions between layered MoS2 nanosheets and high-conductivity silver nanofibers (AgNFs) inside an elastic film. Benefiting from the outstanding lubrication property of layered MoS2 nanosheets, these nanojunctions can be easily separated by strains, giving rise to excellent electromechanical response. The resulting MoS2 strain sensor for the first time exhibits ultrahigh sensitivity with a gauge factor of 3,300 in a large detection range over 10%. The pronounced strain-sensing ability, combined with fast response speed and good operational stability, enables the MoS2 sensor for real-time and skin-on monitorings of various physiological signals such as finger movements, pulse, and breath. Our results may pave the way to extend 2D materials in novel applications such as soft robotics and human-machine interfaces.

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

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.

Improvement by Channel Recess of Contact Resistance and Gate Control in Large-Scale Spin-Coated MoS2 MOSFETs

Solution-processed 2-D materials, being low temperature, low cost, and scalable, are attractive for future-generation thin-film and flexible transistors. However, it is challenging to dispense solution-processed 2-D material into a thin and continuous channel for effective and uniform gate control. In addition, a thick channel under the source and drain contacts is required to increase the transfer length and decrease the edge contact resistance. To overcome such a dilemma and to obtain the optimum combination of effective gate control and low contact resistance, channel recess was demonstrated for the first time on MoS2, so that the channel is thin in the gate region but thick in the source and drain regions. Specifically, channel recess by CHF3/O2 dry etching up to 60 s was performed on submicron buried-gate MOSFETs fabricated on 20-nm-thick spin-coated MoS2. It was found that the channel recess improved the current on/off ratio by 3 orders of magnitude while maintaining approximately the same contact resistance and peak transconductance as that of a uniformly 20-nm-thick channel. The resulted performance was among the best of all solution-processed MoS2 MOSFETs. The same channel recess technique can be used to improve the performance of MOSFETs made of other solution-processed 2-D materials.

MoS2/WS2 Heterojunction for Photoelectrochemical Water Oxidation

The solar-assisted oxidation of water is an essential half reaction for achieving a complete cycle of water splitting. The search of efficient photoanodes that can absorb light in the visible range is of paramount importance to enable cost-effective solar energy-conversion systems. Here, we demonstrate that atomically thin layers of MoS2 and WS2 can oxidize water to O2 under incident light. Thin films of solution-processed MoS2 and WS2 nanosheets display n-type positive photocurrent densities of 0.45 mA cm–2 and O2 evolution under simulated solar irradiation. WS2 is significantly more efficient than MoS2; however, bulk heterojunctions (B-HJs) of MoS2 and WS2 nanosheets results in a 10-fold increase in incident-photon-to-current-efficiency, compared to the individual constituents. This proves that charge carrier lifetime is tailorable in atomically thin crystals by creating heterojunctions of different compositions and architectures. Our results suggest that the MoS2 and WS2 nanosheets and their B-HJ blend are interesting photocatalytic systems for water oxidation, which can be coupled with different reduction processes for solar-fuel production.

Fabrication of a solution-processed, highly flexible few layer MoS2(n)–CuO (p) piezotronic diode on a paper substrate for an active analog frequency modulator and enhanced broadband photodetector

In this work, we demonstrate for the first time, a solution-processed MoS2(n)–CuO (p) piezotronic diode on a flexible paper substrate for an enhanced broadband photodetector and active analog frequency modulator by application of external mechanical strain.

ChemInform Abstract: Solution‐Processed Two‐Dimensional MoS2 Nanosheets: Preparation, Hybridization, and Applications

AbstractReview: [83 refs.

Solution‐Processed Two‐Dimensional MoS2 Nanosheets: Preparation, Hybridization, and Applications

As one member of the emerging class of ultrathin two-dimensional (2D) transition-metal dichalcogenide (TMD) nanomaterials, the ultra-thin MoS2 nanosheet has attracted increasing research interest as a result of its unique structure and fascinating properties. Solution-phase methods are promising for the scalable production, functionalization, hybridization of MoS2 nanosheets, thus enabling the widespread exploration of MoS2 -based nanomaterials for various promising applications. In this Review, an overview of the recent progress of solution-processed MoS2 nanosheets is presented, with the emphasis on their synthetic strategies, functionalization, hybridization, properties, and applications. Finally, the challenges and opportunities in this research area will be proposed.

Hybrid Flexible Resistive Random Access Memory-Gated Transistor for Novel Nonvolatile Data Storage.

Here, a single-device demonstration of novel hybrid architecture is reported to achieve programmable transistor nodes which have analogies to flash memory by incorporating a resistive switching random access memory (RRAM) device as a resistive switch gate for field effect transistor (FET) on a flexible substrate. A high performance flexible RRAM with a three-layered structure is fabricated by utilizing solution-processed MoS2 nanosheets sandwiched between poly(methyl methacrylate) polymer layers. Gate coupling with the pentacene-based transistor can be controlled by the RRAM memory state to produce a nonprogrammed state (inactive) and a programmed state (active) with a well-defined memory window. Compared to the reference flash memory device based on the MoS2 floating gate, the hybrid device presents robust access speed and retention ability. Furthermore, the hybrid RRAM-gated FET is used to build an integrated logic circuit and a wide logic window in inverter logic is achieved. The controllable, well-defined memory window, long retention time, and fast access speed of this novel hybrid device may open up new possibilities of realizing fully functional nonvolatile memory for high-performance flexible electronics.

MoS2 Nanosheet-Pd Nanoparticle Composite for Highly Sensitive Room Temperature Detection of Hydrogen.

Highly sensitive hydrogen detection at room temperature can be realized by employing solution-processed MoS2 nanosheet-Pd nanoparticle composite. A MoS2-Pd composite exhibits greater sensing performance than its graphene counterpart, indicating that solvent exfoliated MoS2 holds great promise for inexpensive and scalable fabrication of highly sensitive chemical sensors.

Photoelectrochemical properties of chemically exfoliated MoS2

Group 6 transition metal dichalcogenides (TMD) such as MoS2 are promising candidates for photocatalysis and photoelectrochemical applications. Despite their promise, scalable deposition of thin films remains a challenge for TMD-based photoanodes. Here we investigate the photoelectrochemical properties of ultra-thin films of chemically exfoliated MoS2 and its composites with TiO2 nanoparticles. We show that MoS2 monolayer films exhibit photoelectrochemical properties similar to bulk materials, generating photocurrent at excitation wavelengths above the direct band gap edge at ∼660 nm (∼1.9 eV). We also demonstrate that MoS2 monolayers adsorbed on TiO2 behave as effective photosensitizers. We find that in TiO2–MoS2 composite photoanodes, photoexcited hot electrons in MoS2 are able inject into TiO2 whilst holes are removed by the electrolyte so as to generate electrical current from incident light. Our results demonstrate the potential of solution-processed MoS2 monolayers for photoelectrochemical applications.

Efficient work-function engineering of solution-processed MoS2 thin-films for novel hole and electron transport layers leading to high-performance polymer solar cells

The work-function of MoS2 interfacial layers can be efficiently modulated by p- and n-doping treatments. As a result, the PCE of devices with a p-doped MoS2-based HTL is increased from ∼2.8 to ∼3.4%. Particularly, after n-doping the PCE was dramatically increased due to the change in work-function compared with un-doped MoS2 thin-films.

Photoconductivity of solution-processed MoS2 films

Solution-exfoliated MoS2 nano-platelets were formed into thin films by deposition onto a water surface followed by transfer to indium tin oxide coated glass. After drying, a gold electrode was evaporated on top to give a sandwich structure with quasi-Ohmic contacts. Illumination of this device with broadband light of ∼1 kW m−2 intensity gave a fourfold increase in conductivity. The photocurrent increased sub-linearly with intensity and exponentially with time indicating the presence of traps. The photo-responsively at low intensity was ∼10−4 A per W at 15 V. This work demonstrates the potential for liquid-exfoliated, inorganic nanosheets to be fabricated into low-cost optoelectronic devices.