The conventional synthesis of two-dimensional (2D) transition metal dichalcogenide (TMDC) heterostructures is low yielding and lack the heterojunction interface quality. The chemical vapor deposition (CVD) techniques have achieved highquality heterostructure interfaces but require a high synthesis temperature (>600 °C) and have a low yield of heterostructures. Therefore, the large scale and high interface quality of TMDC heterojunctions using low-temperature synthesis methods are in demand. Here, high-quality, wafer-scale MoS 2 and WS 2 heterostructures with 2D interfaces were prepared by a one-step sulfurization of the molybdenum (Mo) and tungsten (W) precursors via plasma-enhanced CVD at a relatively low temperature (150 °C). The 4 inch wafer-scale synthesis of the MoS 2 −WS 2 heterostructures was validated using various spectroscopic and microscopic techniques. Further, the photocurrent generation and photoswitching phenomenon of the so-obtained MoS 2 −WS 2 heterostructures were studied. The photodevice prepared by the MoS 2 −WS 2 heterostructures at 150 °C showed a photoresponsivity of 83.75 mA/W. The excellent photoresponse and faster photoswitching highlight the advantage of MoS 2 −WS 2 heterostructures toward advanced photodetectors.

A model is presented to describe particle growth in inductively coupled plasma. The model consists of plasma chemistry and a coagulation module that adopts a modified collision frequency function. The modified collision frequency function is modified by a collision correlation factor that reflects the repulsive force of the particle charge in plasma in order to describe the reduction of coagulation among medium size particles (around 100 nm). In this model, plasma state and concentration of nuclei are determined by a spatially averaged global model in the plasma chemistry module. Particle growth is calculated by a coagulation module. To verify the validity of the model, comparison analysis is performed between experimental data obtained with PBMS and models, some of which are modified by a collision correlation factor. The analysis is performed with respect to dependencies on synthesis time, plasma source power and chamber pressure. From the analysis, we confirm the validity of the model that adopts a modified collision frequency function for the plasma condition.

Recently two dimensional layered structures, such as graphene and Molybdenum disulfide (MoS2), have got attention because of their unique physical and chemical properties. It has been reported that the integration of carbon-based materials and metal oxide nanoparticle shows synergistic effects in electrochemical application. Graphene sheets are explored due to their large active surface area, which can be embedded by intercalation methods and high mobility, on/off ratio. However, graphene doesn’t have band-gap which adds a large leakage current and reduces dynamic range of sensor hence it is difficult to use in sensing applications. In contrast, MoS2 has a direct band gap whereas all other features are similar to graphene. Typically, MoS2 is composed of three atomic layers which are composed by a Mo layer between Surfer layers. These layers are stacked and held each other by weak van der Waals force so molecule can be embedded. These properties lead to fabrication of MoS2 nanosheet-based field effect biosensor for potential use in the bio-sensing applications. It is well known that for biosensor, selective, accurate and rapid detection methods are necessary. As an example significant work has been carried out for the detection of H2O2 using fluorescence, chemiluminescence, titrimetry, cell imaging, spectrophotometry and electrochemical methods. Among them, electrochemical method has great advantages including high sensitivity, selectivity, low cost and simple instrumentation. Until now, horseradish peroxidase (HRP), a heme enzyme, has been commonly used to contrast an efficient H2O2 biosensor. However, the preparation of H2O2 biosensor with high performance has proven challenging. For achieving it, MoS2 has been utilized to modify the surface of electrode and immobilize HRP because of high surface reaction activity and good catalytic efficiency. In the present work, the electrocatalytic activity of layered MoS2 toward the reduction of H2O2 with the help of HRP is demonstrated. The chemical vapor deposition (CVD) synthesized MoS2 thin films are transferred on gold (Au) electrode and was characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and high-resolution transmission microscopy (HR-TEM). IgG-HRP was then immobilized on MoS2 thin film layers. Here MoS2 work as an electrocatalyst between Au electrode and active centers of HRP. Cyclic voltammetry (CV) results demonstrated the fast electron transfer process between HRP and the Au electrode. Figure 1 show the CV plots of IgG-HRP modified MoS2 towards H2O2 for 10 to 100 mVs-1 scan rates. It is observed that no reduction peak for 10 mM PBS (curve a). However, reduction peak were observed (curve b, c, and d) as the scan rate increases from 10 to 100 mVs-1. This phenomenon can be attributed to electrochemical reaction of immobilized IgG-HRP on MoS2 Au electrode and H2O2. In conclusion, in the present work we successfully demonstrated MoS2 based electrochemical biosensor platform for the detection of H2O2. Further H2O2 concentration dependent evolution is in process. We believe that this work will be helpful for the research community to develop MoS2 based electrochemical biosensors.

The graphene has been spot lighted as next generation 2D materials with excellent electrical, mechanical and optical properties for various fields of application. Recently, development of the wafer scale 2D thin film materials synthesis process leads to commercialization for the wearable devices. However, the zero band-gap of graphene has limitations for ‘off current’ realization, which attribute to difficult in utilizing for semiconductor devices. On the other hands, among the transition metal dichalcogenides (TMSs), molybdenum sulfide (MoS2) one can tune the band-gap depending on the number of layers from the 1.2 eV for bulk state to 1.8 eV for monolayer. It is reported that the layer dependent properties of MoS2 play important role in phototransistor application. Meanwhile, number of layers for MoS2 thin film can be controlled by the sulfurization of metal molybdenum (Mo). This process depends on pre-deposited Mo thickness and reaction with dissociated H2S gas during chemical vapor deposition (CVD). Also, in addition to the controlled growth methods, various etching techniques have been investigated as like the laser thinning method, thermal ablation and thermal annealing at 650 oC. These methods were able to etch the MoS2 thin film layers without changing the crystalline quality and surface roughness. However, requiring a high temperature process and a long process time were remain as challenge. Recently, though Ar+ plasma has been investigated for the controlled layer through electrochemical etching of patterned MoS2 thin films, this cause the limitation for physical damage on the surface due to the physical bombardment of Ar+ ion. Also, chemical etching of MoS2 thin film using XeF2 gas. However, the etch rate was not a linear function of etch time due to heat dissipation during the chemical reaction indicating the difficulties in the precise layers control. Therefore, the effective process for thickness control in 2D semiconductor thin film materials, such as MoS2, is needed for various flexible device application. In this study, A few layered MoS2 thin films were synthesized by plasma enhanced CVD (PECVD) and followed by dry electrochemical etching with the help of CF4 inductively coupled plasma (ICP). First the MoS2 thin films were synthesized in the PECVD system by Mo sulfurization on SiO2/Si wafers at 300 oC with optimized plasma conditions. Next, the synthesized six-layer MoS2 thin films were etched using a separate ICP etching system using CF4 plasma at room temperature with etching time variables. The results from Raman spectroscopy and atomic force microscopy (AFM) showed that, one layer of MoS2 thin film can be etched during 20 sec. exposure of CF4 plasma after an initial incubation time of 20 sec. The X-ray photoelectron spectroscopy (XPS) data reveals that there are damages and contaminations to MoS2 thin films during CF4 plasma exposure and these can be recovered effectively during 10 min. exposure of H2S plasma in PECVD. From these results we demonstrated that, the control on layer numbers in MoS2 (2D materials) is a facile and promising method for fabricating devices with the plasma processes.