Synthesis of a Hybrid Nanostructure of ZnO-Decorated MoS2 by Atomic Layer Deposition.

We introduce the synthesis of hybrid nanostructures comprised of ZnO nanocrystals (NCs) decorating nanosheets and nanowires (NWs) of MoS2 prepared by atomic layer deposition (ALD). The concentration, size, and surface-to-volume ratio of the ZnO NCs can be systematically engineered by controlling both the number of ZnO ALD cycles and the properties of the MoS2 substrates, which are prepared by sulfurizing ALD MoO3. Analysis of the chemical composition combined with electron microscopy and synchrotron X-ray techniques as a function of the number of ZnO ALD cycles, together with the results of quantum chemical calculations, help elucidate the ZnO growth mechanism and its dependence on the properties of the MoS2 substrate. The defect density and grain size of MoS2 nanosheets are controlled by the sulfurization temperature of ALD MoO3, and the ZnO NCs in turn nucleate selectively at defect sites on MoS2 surface and enlarge with increasing ALD cycle numbers. At higher ALD cycle numbers, the coalescence of ZnO NCs contributes to an increase in areal coverage and NC size. Additionally, the geometry of the hybrid structures can be tuned by changing the dimensionality of the MoS2, by employing vertical NWs of MoS2 as the substrate for ALD ZnO NCs, which leads to improvement of the relevant surface-to-volume ratio. Such materials are expected to find use in newly expanded applications, especially those such as sensors or photodevices based on a p-n heterojunction which relies on coupling transition-metal dichalcogenides with NCs.

Correction: Textile-based high-performance hydrogen evolution of low-temperature atomic layer deposition of cobalt sulfide.

Correction for 'Textile-based high-performance hydrogen evolution of low-temperature atomic layer deposition of cobalt sulfide' by Jusang Park, Hyungjun Kim et al., Nanoscale, 2019, 11, 844-850.

Textile-based high-performance hydrogen evolution of low-temperature atomic layer deposition of cobalt sulfide.

Hydrogen is an appealing green energy resource to meet increasing energy demands. To produce hydrogen using the hydrogen evolution reaction (HER), platinum, an expensive and scarce metal, is commonly used and plays a crucial role in maximizing catalytic performance. Transition metal chalcogenides, especially cobalt sulfides (CoSx), are considered an alternative to platinum because of their electrochemical properties, for example, low Tafel slopes and overpotentials. Here, we report a light weight, flexible textile-based HER catalyst through a low-temperature process using the atomic layer deposition (ALD) of CoSx. The electrochemical properties of HER catalysts were investigated and found to be impressive, with a low Tafel slope of 41 mV dec-1 and high exchange current density, demonstrating that these are one of the best characteristics among textile-based HER catalysts. The superb catalytic performances were attributed to the amorphous CoSx phase, confirmed by DFT calculations. This study demonstrates that the integration of HER catalysts with textiles allows the development of highly efficient hydrogen energy production systems.

High-Performance Gas Sensor Using a Large-Area WS2 xSe2-2 x Alloy for Low-Power Operation Wearable Applications.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have attracted considerable attention as promising building blocks for a new generation of gas-sensing devices because of their excellent electrical properties, superior response, flexibility, and low-power consumption. Owing to their large surface-to-volume ratio, various 2D TMDCs, such as MoS2, MoSe2, WS2, and WSe2, have exhibited excellent gas-sensing characteristics. However, exploration toward the enhancement of TMDC gas-sensing performance has not yet been intensively addressed. Here, we synthesized large-area uniform WS2 xSe2-2 x alloys for room-temperature gas sensors. As-synthesized WS2 xSe2-2 x alloys exhibit an elaborative composition control owing to their thermodynamically stable sulfurization process. Further, utilizing uniform WS2 xSe2-2 x alloys over a large area, we demonstrated improved NO2-sensing performance compared to WSe2 on the basis of an electronic sensitization mechanism. The WS0.96Se1.04 alloy gas sensor exhibits 2.4 times enhanced response for NO2 exposure. Further, we demonstrated a low-power wearable NO2-detecting wristband that operates at room temperature. Our results show that the proposed method is a promising strategy to improve 2D TMDC gas sensors and has a potential for applications in advanced gas-sensing devices.

Recovery Improvement for Large-Area Tungsten Diselenide Gas Sensors.

Semiconducting two-dimensional transition-metal dichalcogenides are considered promising gas-sensing materials because of their large surface-to-volume ratio, excellent electrical conductivity, and susceptible surfaces. However, enhancement of the recovery performance has not yet been intensively explored. In this study, a large-area uniform WSe2 is synthesized for use in a high-performance semiconductor gas sensor. At room temperature, the WSe2 gas sensor shows a significantly high response (4140%) to NO2 compared to the use of NH3, CO2, and acetone. This paper demonstrates improved recovery of the WSe2 gas sensor's NO2-sensing performance by utilizing external thermal energy. In addition, a novel strategy for improving the recovery of the WSe2 gas sensor is realized by reacting NH3 and adsorbed NO2 on the surface of WSe2: the NO2 molecules are spontaneously desorbed, and the recovery time is dramatically decreased (85 min → 43 s). It is expected that the fast recovery of the WSe2 gas sensor achieved here will be used to develop an environmental monitoring system platform.

Low-temperature synthesis of 2D MoS2 on a plastic substrate for a flexible gas sensor.

The efficient synthesis of two-dimensional molybdenum disulfide (2D MoS2) at low temperatures is essential for use in flexible devices. In this study, 2D MoS2 was grown directly at a low temperature of 200 °C on both hard (SiO2) and soft substrates (polyimide (PI)) using chemical vapor deposition (CVD) with Mo(CO)6 and H2S. We investigated the effect of the growth temperature and Mo concentration on the layered growth by Raman spectroscopy and microscopy. 2D MoS2 was grown by using low Mo concentration at a low temperature. Through optical microscopy, Raman spectroscopy, X-ray photoemission spectroscopy, photoluminescence, and transmission electron microscopy measurements, MoS2 produced by low-temperature CVD was determined to possess a layered structure with good uniformity, stoichiometry, and a controllable number of layers. Furthermore, we demonstrated the realization of a 2D MoS2-based flexible gas sensor on a PI substrate without any transfer processes, with competitive sensor performance and mechanical durability at room temperature. This fabrication process has potential for burgeoning flexible and wearable nanotechnology applications.

Catalytic chemical vapor deposition of large-area uniform two-dimensional molybdenum disulfide using sodium chloride.

The effective synthesis of atomically thin molybdenum disulfides (MoS2) of high quality and uniformity over a large area is essential for their use in electronic and optical devices. In this work, we synthesize MoS2 that exhibit a high quality and large area uniformity using chemical vapor deposition (CVD) with volatile S organic compound and NaCl catalysts. In the latter process, the NaCl enhances the growth rate (5 min for synthesis of monolayer MoS2) and purity of the synthesized MoS2. The optical microscopy, Raman spectroscopy, x-ray photoemission spectroscopy, photoluminescence, and transmission electron microscopy measurements indicate that the NaCl-CVD MoS2 has a large grain size, clear Raman shift, strong photoluminescence, good stoichiometry, and 6-fold coordination symmetry. Moreover, we demonstrate that the electron mobility (10.4 cm2 V-1 s-1) and on/off current ratio (3 × 107) of monolayer MoS2 measured using a field-effect transistor are comparable to those of previously reported MoS2 synthesized using CVD.

Highly stable 2D material (2DM) field-effect transistors (FETs) with wafer-scale multidyad encapsulation.

Field-effect transistors (FETs) composed of 2D materials (2DMs) such as transition-metal dichalcogenide (TMD) materials show unstable electrical characteristics in ambient air due to the high sensitivity of 2DMs to water adsorbates. In this work, in order to demonstrate the long-term retention of electrical characteristics of a TMD FET, a multidyad encapsulation method was applied to a MoS2 FET and thereby its durability was warranted for one month. It was well known that the multidyad encapsulation method was effective to mitigate high sensitivity to ambient air in light-emitting diodes (LEDs) composed of organic materials. However, there was no attempt to check the feasibility of such a multidyad encapsulation method for 2DM FETs. It is timely to investigate the water vapor transmission ratio (WVTR) required for long-term stability of 2DM FETs. The 2DM FETs were fabricated with MoS2 flakes by both an exfoliation method, that is desirable to attain high quality film, and a chemical vapor deposition (CVD) method, that is applicable to fabrication for a large-sized substrate. In order to eliminate other unwanted variables, the MoS2 FETs composed of exfoliated flakes were primarily investigated to assure the effectiveness of the encapsulation method. The encapsulation method uses multiple dyads comprised of a polymer layer by spin coating and an Al2O3 layer deposited by atomic layer deposition (ALD). The proposed method shows wafer-scale uniformity, high transparency, and protective barrier properties against adsorbates (WVTR of 8 × 10-6 g m-2 day-1) over one month.

Effect of Al2O3 Deposition on Performance of Top-Gated Monolayer MoS2-Based Field Effect Transistor.

Deposition of high-k dielectrics on two-dimensional MoS2 is an important process for successful application of the transition-metal dichalcogenides in electronic devices. Here, we show the effect of H2O reactant exposure on monolayer (1L) MoS2 during atomic layer deposition (ALD) of Al2O3. The results showed that the ALD-Al2O3 caused degradation of the performance of 1L MoS2 field effect transistors (FETs) owing to the formation of Mo-O bonding and trapping of H2O molecules at the Al2O3/MoS2 interface. Furthermore, we demonstrated that reduced duration of exposure to H2O reactant and postdeposition annealing were essential to the enhancement of the performance of top-gated 1L MoS2 FETs. The mobility and on/off current ratios were increased by factors of approximately 40 and 103, respectively, with reduced duration of exposure to H2O reactant and with postdeposition annealing.

Improvement of Gas-Sensing Performance of Large-Area Tungsten Disulfide Nanosheets by Surface Functionalization.

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) are promising gas-sensing materials due to their large surface-to-volume ratio. However, their poor gas-sensing performance resulting from the low response, incomplete recovery, and insufficient selectivity hinders the realization of high-performance 2D TMDC gas sensors. Here, we demonstrate the improvement of gas-sensing performance of large-area tungsten disulfide (WS2) nanosheets through surface functionalization using Ag nanowires (NWs). Large-area WS2 nanosheets were synthesized through atomic layer deposition of WO3 followed by sulfurization. The pristine WS2 gas sensors exhibited a significant response to acetone and NO2 but an incomplete recovery in the case of NO2 sensing. After AgNW functionalization, the WS2 gas sensor showed dramatically improved response (667%) and recovery upon NO2 exposure. Our results establish that the proposed method is a promising strategy to improve 2D TMDC gas sensors.

Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides.

This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 10(8). This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.

Controllable synthesis of molybdenum tungsten disulfide alloy for vertically composition-controlled multilayer.

The effective synthesis of two-dimensional transition metal dichalcogenides alloy is essential for successful application in electronic and optical devices based on a tunable band gap. Here we show a synthesis process for Mo1-xWxS2 alloy using sulfurization of super-cycle atomic layer deposition Mo1-xWxOy. Various spectroscopic and microscopic results indicate that the synthesized Mo1-xWxS2 alloys have complete mixing of Mo and W atoms and tunable band gap by systematically controlled composition and layer number. Based on this, we synthesize a vertically composition-controlled (VCC) Mo1-xWxS2 multilayer using five continuous super-cycles with different cycle ratios for each super-cycle. Angle-resolved X-ray photoemission spectroscopy, Raman and ultraviolet-visible spectrophotometer results reveal that a VCC Mo1-xWxS2 multilayer has different vertical composition and broadband light absorption with strong interlayer coupling within a VCC Mo1-xWxS2 multilayer. Further, we demonstrate that a VCC Mo1-xWxS2 multilayer photodetector generates three to four times greater photocurrent than MoS2- and WS2-based devices, owing to the broadband light absorption.

Layer-controlled, wafer-scale, and conformal synthesis of tungsten disulfide nanosheets using atomic layer deposition.

The synthesis of atomically thin transition-metal disulfides (MS2) with layer controllability and large-area uniformity is an essential requirement for their application in electronic and optical devices. In this work, we describe a process for the synthesis of WS2 nanosheets through the sulfurization of an atomic layer deposition (ALD) WO3 film with systematic layer controllability and wafer-level uniformity. The X-ray photoemission spectroscopy, Raman, and photoluminescence measurements exhibit that the ALD-based WS2 nanosheets have good stoichiometry, clear Raman shift, and bandgap dependence as a function of the number of layers. The electron mobility of the monolayer WS2 measured using a field-effect transistor (FET) with a high-k dielectric gate insulator is shown to be better than that of CVD-grown WS2, and the subthreshold swing is comparable to that of an exfoliated MoS2 FET device. Moreover, by utilizing the high conformality of the ALD process, we have developed a process for the fabrication of WS2 nanotubes.