A phototransistor based on a chemical vapor deposited (CVD) MoS2 monolayer exhibits a high photoresponsivity (2200 A W−1) and an excellent photogain (5000). The presence of shallow traps contributes ...to the persistent photoconductivity. Ambient adsorbates act as p‐dopants to MoS2, decreasing the carrier mobility, photoresponsivity, and photogain.
A three‐dimensional Ni foam deposited with graphene layers on surfaces is used as a conducting solid support to load MoSx catalysts for electrocatalytic hydrogen evolution. The graphene sheets grown ...on Ni foams provide robust protection and efficiently increase the stability in acid. The superior performance of hydrogen evolution is attributed to the relatively high catalyst loading weight as well as its relatively low resistance.
The monolayer transition metal dichalcogenides have recently attracted much attention owing to their potential in valleytronics, flexible and low-power electronics, and optoelectronic devices. Recent ...reports have demonstrated the growth of large-size two-dimensional MoS2 layers by the sulfurization of molybdenum oxides. However, the growth of a transition metal selenide monolayer has still been a challenge. Here we report that the introduction of hydrogen in the reaction chamber helps to activate the selenization of WO3, where large-size WSe2 monolayer flakes or thin films can be successfully grown. The top-gated field-effect transistors based on WSe2 monolayers using ionic gels as the dielectrics exhibit ambipolar characteristics, where the hole and electron mobility values are up to 90 and 7 cm2/Vs, respectively. These films can be transferred onto arbitrary substrates, which may inspire research efforts to explore their properties and applications. The resistor-loaded inverter based on a WSe2 film, with a gain of ∼13, further demonstrates its applicability for logic-circuit integrations.
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the ...electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
It is critically important to characterize the band alignment in semiconductor heterojunctions (HJs) because it controls the electronic and optical properties. However, the well‐known Anderson's ...model usually fails to predict the band alignment in bulk HJ systems due to the presence of charge transfer at the interfacial bonding. Atomically thin 2D transition metal dichalcogenide materials have attracted much attention recently since the ultrathin HJs and devices can be easily built and they are promising for future electronics. The vertical HJs based on 2D materials can be constructed via van der Waals stacking regardless of the lattice mismatch between two materials. Despite the defect‐free characteristics of the junction interface, experimental evidence is still lacking on whether the simple Anderson rule can predict the band alignment of HJs. Here, the validity of Anderson's model is verified for the 2D heterojunction systems and the success of Anderson's model is attributed to the absence of dangling bonds (i.e., interface dipoles) at the van der Waal interface. The results from the work set a foundation allowing the use of powerful Anderson's rule to determine the band alignments of 2D HJs, which is beneficial to future electronic, photonic, and optoelectronic devices.
The band alignment of stacked 2D material heterojunctions is experimentally proven to follow the Anderson's model. Based on this discovery it is demonstrated that electron affinity and band gap values are sufficient to construct the band alignment of stacked 2D heterojunctions.
Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material ...molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 10(8). Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.
Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. ...Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of ∼20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.
MoSx is grown on crumpled graphene particles supported on carbon cloth substrates for the hydrogen evolution reaction. Modifying carbon cloth with crumpled graphene allows for higher loading levels ...of MoSx and thus significantly enhances its electrocatalytic activity. Measurements yield a current density of –220 mA cm−2 at an overpotential of 0.3 V (before iR correction) for the crumpled‐graphene‐modified carbon cloth.
The optical energy gap of as‐grown MoS2 flakes from chemical vapor deposition can be modulated from 1.86 eV (667 nm) to 1.57 eV (790 nm) by a vapor phase selenization process. This approach, ...replacing one chalcogen by another in the gas phase, is promising in modulating the optical and electronic properties of other transition metal dichalcogenide monolayers.
CoFe-based alloys and rare earth (RE) elements are among the most studied materials in applying magnetic devices to improve soft magnetic characteristics. A series of Co
Fe
Sm
films are deposited on ...a glass substrate via the sputtering technique, followed by an annealing process to investigate their effect on microstructural and optical properties of Co
Fe
Sm
films. In this study, the increase in the thickness of Co
Fe
Sm
films and annealing temperatures resulted in a smoother surface morphology. The 40 nm Co
Fe
Sm
films annealed 300 °C are expected to have good wear resistance and adhesive properties due to their high values of H/E ratio and surface energy. Optical transparency also increased due to the smoother surface of the Co
Fe
Sm
films.
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