Single‐Layer MoS2 Nanosheets with Amplified Photoacoustic Effect for Highly Sensitive Photoacoustic Imaging of Orthotopic Brain Tumors

Photoacoustic (PA) imaging, as a fast growing technology that combines the high contrast of light and large penetration depth of ultrasound, has demonstrated great potential for molecular imaging of cancer. However, PA molecular imaging of orthotopic brain tumors is still challenging, partially due to the limited options and insufficient sensitivity of available PA molecular probes. Here, the direct formation of single‐layer (S‐MoS₂), few‐layer (F‐MoS₂), and multi‐layer (M‐MoS₂) nanosheets by the albumin‐assisted exfoliation without further surface modifications is reported. It is demonstrated that the PA effect of the MoS₂ nanosheets is highly dependent on their layered nanostructures. Decreasing the number of nanosheet layers from M‐MoS₂ to S‐MoS₂ can both significantly enhance the near‐infrared light absorption and improve the elastic properties of the nanomaterial, resulting in greatly amplified PA effect. The in vitro experiments demonstrate that the prepared S‐MoS₂ with excellent biocompatibility can be efficiently internalized into U87 glioma cells, producing strong PA signals for highly sensitive detection of brain tumor cells, with a detection limit of ≈100 cells. Intravenous administration of S‐MoS₂ to both U87 subcutaneous and orthotopic tumor‐bearing mice shows highly efficient tumor retention and significantly enhanced PA contrast. Tumor tissue ≈1.5 mm below the skull can still be clearly visualized in vivo. Previous studies suggest that the fabricated S‐MoS₂ with amplified PA effect have high potential to serve as an efficient nanoplatform for sensitive PA molecular imaging and hold promising prospect for translational medicine.     

Phototransistors with Negative or Ambipolar Photoresponse Based on As‐Grown Heterostructures of Single‐Walled Carbon Nanotube and MoS2

A facile synthesis method for the heterostructures of single‐walled carbon nanotubes (SWCNTs) and few‐layer MoS₂ is reported. The heterostructures are realized by in situ chemical vapor deposition of MoS₂ on individual SWCNTs. Field effect transistors based on the heterostructures display different transfer characteristics depending on the formation of MoS₂ conduction channels along SWCNTs. Under light illumination, negative photoresponse originating from charge transfer from MoS₂ to SWCNT is observed while positive photoresponse is observed in MoS₂ conduction channels, leading to ambipolar photoresponse in devices with both SWCNT and MoS₂ channels. The heterostructure phototransistor, for negative photoresponse, exhibits high responsivity (100–1000 AW^?1) at low bias voltages (0.1 V) in the visible spectrum (500–700 nm) by combining high mobility conduction channel (SWCNT) with efficient light absorber (MoS₂).     

Ultraviolet‐Light‐Induced Reversible and Stable Carrier Modulation in MoS2 Field‐Effect Transistors

The tuning of charge carrier concentrations in semiconductor is necessary in order to approach high performance of the electronic and optoelectronic devices. It is demonstrated that the charge‐carrier density of single‐layer (SL), bilayer (BL), and few‐layer (FL) MoS₂ nanosheets can be finely and reversibly tuned with N₂ and O₂ gas in the presence of deep‐ultraviolet (DUV) light. After exposure to N₂ gas in the presence of DUV light, the threshold voltages of SL, BL, and FL MoS₂ field‐effect transistors (FETs) shift towards negative gate voltages. The exposure to N₂ gas in the presence of DUV light notably improves the drain‐to‐source current, carrier density, and charge‐carrier mobility for SL, BL, and FL MoS₂ FETs. Subsequently, the same devices are exposed to O₂ gas in the presence of DUV light for different periods and the electrical characteristics are completely recovered after a certain time. The doping by using the combination of N₂ and O₂ gas with DUV light provides a stable, effective, and facile approach for improving the performance of MoS₂ electronic devices.     

Three‐Dimensional Structures of MoS2 Nanosheets with Ultrahigh Hydrogen Evolution Reaction in Water Reduction

The hydrogen evolution reaction in an alkaline environment using a non‐precious catalyst with much greater efficiency represents a critical challenge in research. Here, a robust and highly active system for hydrogen evolution reaction in alkaline solution is reported by developing MoS₂ nanosheet arrays vertically aligned on graphene‐mediated 3D Ni networks. The catalytic activity of the 3D MoS₂ nanostructures is found to increase by 2 orders of magnitude as compared to the Ni networks without MoS₂. The MoS₂ nanosheets vertically grow on the surface of graphene by employing tetrakis(diethylaminodithiocarbomato)molybdate(IV) as the molybdenum and sulfur source in a chemical vapor deposition process. The few‐layer MoS₂ nanosheets on 3D graphene/nickel structure can maximize the exposure of their edge sites at the atomic scale and present a superior catalysis activity for hydrogen production. In addition, the backbone structure facilitates as an excellent electrode for charge transport. This precious‐metal‐free and highly efficient active system enables prospective opportunities for using alkaline solution in industrial applications.     

Nonvolatile Floating‐Gate Memories Based on Stacked Black Phosphorus–Boron Nitride–MoS2 Heterostructures

Research on van der Waals heterostructures based on stacked 2D atomic crystals is intense due to their prominent properties and potential applications for flexible transparent electronics and optoelectronics. Here, nonvolatile memory devices based on floating‐gate field‐effect transistors that are stacked with 2D materials are reported, where few‐layer black phosphorus acts as channel layer, hexagonal boron nitride as tunnel barrier layer, and MoS₂ as charge trapping layer. Because of the ambipolar behavior of black phosphorus, electrons and holes can be stored in the MoS₂ charge trapping layer. The heterostructures exhibit remarkable erase/program ratio and endurance performance, and can be developed for high‐performance type‐switching memories and reconfigurable inverter logic circuits, indicating that it is promising for application in memory devices completely based on 2D atomic crystals.     

Plasmonic‐Induced Photon Recycling in Metal Halide Perovskite Solar Cells

Organic–inorganic metal halide perovskite solar cells have emerged in the past few years to promise highly efficient photovoltaic devices at low costs. Here, temperature‐sensitive core–shell Ag@TiO₂ nanoparticles are successfully incorporated into perovskite solar cells through a low‐temperature processing route, boosting the measured device efficiencies up to 16.3%. Experimental evidence is shown and a theoretical model is developed which predicts that the presence of highly polarizable nanoparticles enhances the radiative decay of excitons and increases the reabsorption of emitted radiation, representing a novel photon recycling scheme. The work elucidates the complicated subtle interactions between light and matter in plasmonic photovoltaic composites. Photonic and plasmonic schemes such as this may help to move highly efficient perovskite solar cells closer to the theoretical limiting efficiencies.     

6‐Mercaptopurine‐Induced Fluorescence Quenching of Monolayer MoS2 Nanodots: Applications to Glutathione Sensing,Cellular Imaging,and Glutathione‐Stimulated Drug Delivery

Molybdenum disulfide (MoS₂) nanodots (NDs) with sulfur vacancies have been demonstrated to be suitable to conjugate thiolated molecules. However, thiol‐induced fluorescence quenching of MoS₂ NDs has been rarely explored. In this study, 6‐mercaptopurine (6‐MP) serves as an efficient quencher for the fluorescence of monolayer MoS₂ (M‐MoS₂) NDs. 6‐MP molecules are chemically adsorbed at the sulfur vacancy sites of the M‐MoS₂ NDs. The formed complexes trigger the efficient fluorescence quenching of the M‐MoS₂ NDs due to acceptor‐excited photoinduced electron transfer. The presence of glutathione (GSH) efficiently triggers the release of 6‐MP from the M‐MoS₂ NDs, thereby switching on the fluorescence of the M‐MoS₂ NDs. Thus, the 6‐MP‐M‐MoS₂ NDs are implemented as a platform for the sensitive and selective detection of GSH in erythrocytes and live cells. Additionally, thiolated doxorubicin (DOX‐SH)‐loaded M‐MoS₂ NDs (DOX‐SH/M‐MoS₂ NDs) serve as GSH‐responsive nanocarriers for DOX‐SH delivery. In vitro studies reveal that the DOX‐SH/M‐MoS₂ NDs exhibit efficient uptake by HeLa cells and greater cytotoxicity than free DOX‐SH and DOX. In vivo study shows that GSH is capable of triggering the release of DOX‐SH from M‐MoS₂ ND‐based nanomaterials in mice. It is revealed that the DOX‐SH/M‐MoS₂ NDs can be implemented for simultaneous drug delivery and fluorescence imaging.     

Piezo‐Phototronic Effect for Enhanced Flexible MoS2/WSe2 van der Waals Photodiodes

The recent discoveries of transition‐metal dichalcogenides (TMDs) as novel 2D electronic materials hold great promise to a rich variety of artificial van der Waals (vdWs) heterojunctions and superlattices. Moreover, most of the monolayer TMDs become intrinsically piezoelectric due to the lack of structural centrosymmetry, which offers them a new degree of freedom to interact with external mechanical stimuli. Here, fabrication of flexible vdWs p–n diode by vertically stacking monolayer n‐MoS₂ and a few‐layer p‐WSe₂ is achieved. Electrical measurement of the junction reveals excellent current rectification behavior with an ideality factor of 1.68 and photovoltaic response is realized. Performance modulation of the photodiode via piezo‐phototronic effect is also demonstrated. The optimized photoresponsivity increases by 86% when introducing a −0.62% compressive strain along MoS₂ armchair direction, which originates from realigned energy‐band profile at MoS₂/WSe₂ interface under strain‐induced piezoelectric polarization charges. This new coupling mode among piezoelectricity, semiconducting, and optical properties in 2D materials provides a new route to strain‐tunable vdWs heterojunctions and may enable the development of novel ultrathin optoelectronics.     

Molybdenum Disulfide‐Based Tubular Microengines: Toward Biomedical Applications

2D molybdenum disulfide (MoS₂) is herein explored as an advanced surface material in the fabrication of powerful tubular microengines. The new catalytic self‐propelled open‐tube bilayer microengines have been fabricated using a template electrodeposition and couple the unique properties of sp² hybridized MoS₂ with highly reactive inner granular Pt catalytic structures. The MoS₂/metal microengines display extremely efficient bubble propulsion, reflecting the granular structure of the inner catalytic platinum or gold layers (compared to the smooth metal surfaces of common micromotors). The efficient movement of functionalized MoS₂ micromotors can address challenges imposed by slow mass transport processes involved in various applications of MoS_2. The delocalized electron network of the MoS₂ outer layer facilitates π–π stacking interactions and endows the tubular microengines with a diverse array of capabilities. These are demonstrated here for efficient loading and release of the drug doxorubicin, and rapid and sensitive “OFF–ON” fluorescent detection of important nucleic acids (miRNA‐21) and proteins (thrombin) using microengines modified with dye‐labeled single‐stranded DNA and aptamer, respectively. Such coupling of the attractive capabilities of 2D‐MoS₂ nanosheets with rapidly moving microengines provides an opportunity to develop multifunctional micromachines for diverse biomedical applications ranging from efficient drug delivery to the detection of important bioanalytes.     

Dependence of Photocurrent Enhancements in Quantum Dot (QD)‐Sensitized MoS2 Devices on MoS2 Film Properties

This report demonstrates highly efficient nonradiative energy transfer (NRET) from alloyed CdSeS/ZnS semiconductor nanocrystal quantum dots (QDs) to MoS₂ films of varying layer thicknesses, including pristine monolayers, mixed monolayer/bilayer, polycrystalline bilayers, and bulk‐like thicknesses, with NRET efficiencies of over 90%. Large‐area MoS₂ films are grown on Si/SiO₂ substrates by chemical vapor deposition. Despite the ultrahigh NRET efficiencies there is no distinct increase in the MoS₂ photoluminescence intensity. However, by studying the optoelectronic properties of the MoS₂ devices before and after adding the QD sensitizing layer photocurrent enhancements as large as ≈14‐fold for pristine monolayer devices are observed, with enhancements on the order of ≈2‐fold for MoS₂ devices of mixed monolayer and bilayer thicknesses. For the polycrystalline bilayer and bulk‐like MoS₂ devices there is almost no increase in the photocurrent after adding the QDs. Industrially scalable techniques are specifically utilized to fabricate the samples studied in this report, demonstrating the viability of this hybrid structure for commercial photodetector or light harvesting applications.     

Novel and Enhanced Optoelectronic Performances of Multilayer MoS2–WS2 Heterostructure Transistors

Van der Waals heterostructures designed by assembling isolated two‐dimensional (2D) crystals have emerged as a new class of artificial materials with interesting and unusual physical properties. Here, the multilayer MoS₂–WS₂ heterostructures with different configurations are reported and their optoelectronic properties are studied. It is shown that the new heterostructured material possesses new functionalities and superior electrical and optoelectronic properties that far exceed the one for their constituents, MoS₂ or WS₂. The vertical transistor exhibits a novel rectifying and bipolar behavior, and can also act as photovoltaic cell and self‐driven photodetector with photo‐switching ratio exceeding 10³. The planar device also exhibits high field‐effect ON/OFF ratio (>10⁵), high electron mobility of 65 cm²/Vs, and high photo­responsivity of 1.42 A/W compared to that in isolated multilayer MoS₂ or WS₂ nanoflake transistors. The results suggest that formation of MoS₂–WS₂ heterostructures could significantly enhance the performance of optoelectronic devices, thus open up possibilities for future nanoelectronic, photovoltaic, and optoelectronic applications.     

Surface‐Shielding Nanostructures Derived from Self‐Assembled Block Copolymers Enable Reliable Plasma Doping for Few‐Layer Transition Metal Dichalcogenides

Precise modulation of electrical and optical properties of 2D transition metal dichalcogenides (TMDs) is required for their application to high‐performance devices. Although conventional plasma‐based doping methods have provided excellent controllability and reproducibility for bulk or relatively thick TMDs, the application of plasma doping for ultrathin few‐layer TMDs has been hindered by serious degradation of their properties. Herein, a reliable and universal doping route is reported for few‐layer TMDs by employing surface‐shielding nanostructures during a plasma‐doping process. It is shown that the surface‐protection oxidized polydimethylsiloxane nanostructures obtained from the sub‐20 nm self‐assembly of Si‐containing block copolymers can preserve the integrity of 2D TMDs and maintain high mobility while affording extensive control over the doping level. For example, the self‐assembled nanostructures form periodically arranged plasma‐blocking and plasma‐accepting nanoscale regions for realizing modulated plasma doping on few‐layer MoS₂, controlling the n‐doping level of few‐layer MoS₂ from 1.9 × 10¹¹ cm^?2 to 8.1 × 10¹¹ cm^?2 via the local generation of extra sulfur vacancies without compromising the carrier mobility.     

Enhanced Quality of Wafer‐Scale MoS2 Films by a Capping Layer Annealing Process

Wafer‐scale, single‐crystalline 2D semiconductors without grain boundaries and defects are needed for developing reliable next‐generation integrated 2D electronics. Unfortunately, few literature reports exist on the growth of 2D semiconductors with single‐crystalline structure at the wafer scale. It is shown that direct sulfurization of as‐deposited epitaxial MoO₂ films (especially, with thicknesses more than ≈5 nm) produces textured MoS₂ films. This texture is inherited from the high density of defects present in the as‐prepared epitaxial MoO₂ film. In order to eliminate the texture of the converted MoS₂ films, a new capping layer annealing process (CLAP) is introduced to improve the crystalline quality of as‐deposited MoO₂ films and minimize its defects. It is demonstrated that sulfurization of the CLAP‐treated MoO₂ films leads to the formation of single‐crystalline MoS₂ films, instead of textured films. It is shown that the single‐crystalline MoS₂ films exhibit field‐effect mobility of 6.3 cm² V^?1 s^?1, which is 15 times higher than that of textured MoS₂. These results can be attributed to the smaller concentration of defects in the single‐crystalline films.     

Multifunctional Lateral Transition‐Metal Disulfides Heterojunctions

The intrinsic spin‐dependent transport properties of two types of lateral VS₂|MoS₂ heterojunctions are systematically investigated using first‐principles calculations, and their various nanodevices with novel properties are designed. The lateral VS₂|MoS₂ heterojunction diodes show a perfect rectifying effect and are promising for the applications of Schottky diodes. A large spin‐polarization ratio is observed for the A‐type device and pure spin‐mediated current is then realized. The gate voltage significantly tunes the current and rectification ratio of their field‐effect transistors. In addition, they all demonstrate a sensitive photoresponse to blue light, and could be used as photodetector and photovoltaic device. Moreover, they generate an effective thermally driven current when a temperature gratitude appears between the two terminals, suggesting them as potential thermoelectric materials. Hence, the lateral VS₂|MoS₂ heterojunctions show a multifunctional nature and have various potential applications in spintronics, optoelectronics, and spin caloritronics.     

Ultrafast Plasmonic Hot Electron Transfer in Au Nanoantenna/MoS2 Heterostructures

2D transition metal dichalcogenides are becoming attractive materials for novel photoelectric and photovoltaic applications due to their excellent optoelectric properties and accessible optical bandgap in the near‐infrared to visible range. Devices utilizing 2D materials integrated with metal nanostructures have recently emerged as efficient schemes for hot electron‐based photodetection. Metal‐semiconductor heterostructures with low cost, simple procedure, and fast response time are crucial for the practical applications of optoelectric devices. In this paper, template‐based sputtering method is used first to fabricate Au nanoantenna (NA)/MoS₂ heterostructures with low cost, simple preparation, broad spectral response, and fast response time. Through the measurement of femtosecond pump‐probe spectroscopy, it is demonstrated that plasmon‐induced hot electron transfer takes place in the Au NA/MoS₂ heterostructure on the order of 200 fs with an injected electron density of about 5.6 × 10¹² cm^?2. Moreover, the pump‐power‐dependent photoluminescence spectra confirm that the exciton energy of MoS₂ can be enhanced, coupled, and reradiated by the Au NA. Such ultrafast plasmon‐induced hot electron transfer in the metal‐semiconductor heterostructure can enable novel 2D devices for light harvesting and photoelectric conversion.     

Atomically Thin MoS2: A Versatile Nongraphene 2D Material

Two‐dimensional inorganic materials are emerging as a premiere class of materials for fabricating modern electronic devices. The interest in 2D layered transition metal dichalcogenides is especially high. Particularly, 2D MoS₂ is being heavily researched due to its novel functionalities and its suitability for a wide range of electronic and optoelectronic applications. In this article, the progress in mono/few layer(s) MoS₂ research is reviewed by focusing primarily on the layer dependent evolution of crystal, phonon, and electronic structure. The review includes extensive detail into the methodologies adapted for single or few layer(s) MoS₂ growth. Further, the review covers the versatility of 2D MoS₂ for a broad range of device applications. Recent advancements in the field of van der Waals heterostructures are also highlighted at the end of the review.     

Tuning the Excitonic States in MoS2/Graphene van der Waals Heterostructures via Electrochemical Gating

The behavior of excitons in van der Waals (vdWs) heterostructures depends on electron–electron interactions and charge transfer at the hetero‐interface. However, what still remains to be unraveled is to which extent the carrier densities of both counterparts and the band alignment in the vdWs heterostructures determine the photoluminescence properties. Here, we systematically study the photoluminescence properties of monolayer MoS₂/graphene heterostructures by modulating the carrier densities and contact barrier at the interface via electrochemical gating. It is shown that the PL intensities of excitons can be tuned by more than two orders of magnitude, and a blue‐shift of the exciton peak of up to 40 meV is observed. By extracting the carrier density of MoS₂ using an electric potential distribution model, and the Schottky barrier using first‐principle calculations, we find that the controllable carrier density in MoS₂ plays a dominant role in the PL tuning at negative gate bias, whereas the interlayer relaxation of excitons induced by the Schottky barrier has a major contribution at positive gate bias. This is further verified by controlling the tunneling barrier and screening field across MoS₂ by inserting self‐assembled monolayers (SAMs) at the interface. These findings will benefit to better understand the effect of many‐body interactions and hetero‐interfaces on the optical and optoelectronic properties in vdWs heterostructures.     

Monolayer MoS2 Growth on Au Foils and On‐Site Domain Boundary Imaging

Controllable synthesis of large domain, high‐quality monolayer MoS₂ is the basic premise both for exploring some fundamental physical issues, and for engineering its applications in nanoelectronics, optoelectronics, etc. Herein, by introducing H₂ as carrier gas, the successful synthesis of large domain monolayer MoS₂ triangular flakes on Au foils, with the edge length approaching to 80 mm is reported. The growth process is proposed to be mediated by two competitive effects with H₂ acting as both a reduction promoter for efficient sulfurization of MoO₃ and an etching reagent of resulting MoS₂ flakes. By using low‐energy electron microscopy/diffraction, the crystal orientations and domain boundaries of MoS₂ flakes directly on Au foils for the first time are further identified. These on‐site and transfer‐free characterizations should shed light on the initial growth and the aggregation of MoS₂ on arbitrary substrates, further guiding the growth toward large domain flakes or monolayer films.     

Fluorescent Block Copolymer‐MoS2 Nanocomposites for Real‐Time Photothermal Heating and Imaging

Novel self‐monitoring photothermal (PT) agents are developed using optothermally responsive block copolymer‐MoS₂ composites (BCP‐MoS₂), which enable simultaneous PT heating and imaging of temperature profiles. In particular, upon near‐infrared light exposure, PT energy from MoS₂ successfully increases local temperature and induces thermally activated conformational transitions of the BCP on MoS₂. This leads to fluorescent signal changes caused by distance‐dependent Förster resonance energy transfer between the BCP and MoS₂. Importantly, it is demonstrated that the use of BCP‐MoS₂ for PT heating and optical mapping is fully reversible with excellent stability. The detailed mechanism of the responsive behavior of BCP‐MoS₂ is elucidated by measurements of time‐resolved fluorescence and dynamic light scattering. In addition, the BCP‐MoS₂ system is integrated into organogel matrices to demonstrate its potential as aportable, self‐monitoring PT system suitable for biological and environmental applications.     

High‐Concentration Aqueous Dispersions of MoS2

Molybdenum disulfide (MoS₂) nanosheets have been attracting increasing research interests due to their unique material properties. However, the lack of a reliable large‐scale production method impedes their practical applications. Here a facile, efficient, and scalable method for the fabrication of high‐concentration aqueous dispersion of MoS₂ nanosheets using combined grinding and sonication is reported. The 26.7 ± 0.7 mg/mL concentration achieved is the highest concentration in an aqueous solution reported up to now. Grinding generates pure shear forces to detach the MoS₂ layers from the bulk materials. Subsequent sonication further breaks larger crystallites into smaller crystallites, which promotes the dispersion of MoS₂ nanosheets in ethanol/water solutions. The exfoliation process establishes a new paradigm in the top‐down fabrication of 2D nanosheets in aqueous solution. In the meantime, MoS₂‐based sensing film produced using this approach has successfully demonstrated the feasibility of a low‐cost and efficient NH₃ gas sensor using inkjet printing as a viable method.