Flexible MoS2-Polyimide Electrode for Electrochemical Biosensors and Their Applications for the Highly Sensitive Quantification of Endocrine Hormones: PTH, T3, and T4.

Flexibile biosensors have a lot of applications in measuring the concentration of target bioanalytes. In combination with its flexibility, electrochemical sensors containing 2D materials have particular advantages such as enlarged area compatibility, transparency, and high scalability. A flexible biosensor was fabricated by direct synthesis of molybdenum disulfide (MoS2) on a polyimide (PI) substrate, which can be used as the working electrode in electrochemistry platforms. The direct formation of 2D-MoS2 on the PI was achieved using plasma-enhanced chemical vapor deposition (PE-CVD). Since the MoS2 provides higher electrical conductivity, the MoS2-Au-PI flexible sensor is able to provide highly sensitive detection of target proteins with a relatively fast response via cyclic voltammetry. To evaluate the high performance of the fabricated sensor, we selected the endocrine-related hormones parathyroid hormone (PTH), triiodothyronine (T3), and thyroxine (T4) as analytes because they are one of the most important markers for the determination of endocrinopathy, however, they are very difficult to quantify. The newly developed biosensor achieved highly sensitive detection of the hormones and could determine their location with high accuracy. In addition, we performed electrochemical measurements of hormones obtained from 30 clinical patients' sera with confirmed agreement and compared with the measurements performed with standard immunoassay equipment (E 170, Roche Diagnostics, Germany).

Wafer-Scale and Low-Temperature Growth of 1T-WS2 Film for Efficient and Stable Hydrogen Evolution Reaction.

The metallic 1T phase of WS2 (1T-WS2 ), which boosts the charge transfer between the electron source and active edge sites, can be used as an efficient electrocatalyst for the hydrogen evolution reaction (HER). As the semiconductor 2H phase of WS2 (2H-WS2 ) is inherently stable, methods for synthesizing 1T-WS2 are limited and complicated. Herein, a uniform wafer-scale 1T-WS2 film is prepared using a plasma-enhanced chemical vapor deposition (PE-CVD) system. The growth temperature is maintained at 150 °C enabling the direct synthesis of 1T-WS2 films on both rigid dielectric and flexible polymer substrates. Both the crystallinity and number of layers of the as-grown 1T-WS2 are verified by various spectroscopic and microscopic analyses. A distorted 1T structure with a 2a0 × a0 superlattice is observed using scanning transmission electron microscopy. An electrochemical analysis of the 1T-WS2 film demonstrates its similar catalytic activity and high durability as compared to those of previously reported untreated and planar 1T-WS2 films synthesized with CVD and hydrothermal methods. The 1T-WS2 does not transform to stable 2H-WS2 , even after a 700 h exposure to harsh catalytic conditions and 1000 cycles of HERs. This synthetic strategy can provide a facile method to synthesize uniform 1T-phase 2D materials for electrocatalysis applications.

Electron Release via Internal Polarization Fields for Optimal S-H Bonding States.

Transition metal dichalcogenides (TMDs) have received considerable attention as promising electrocatalysts for the hydrogen evolution reaction (HER), yet their potential is often constrained by the inertness of the basal planes arising from their poor hydrogen adsorption ability. Here, the relationship between the electronic structure of the WS2 basal plane and HER activity is systemically analyzed to establish a clear insight. The valance state of the sulfur atoms on the basal plane has been tuned to enhance hydrogen adsorption through sequential engineering processes, including direct phase transition and heterostructure that induces work function-difference-induced unidirectional electron transfer. Additionally, an innovative synthetic approach, harnessing the built-in internal polarization field at the W-graphene heterointerface, triggers the in-situ formation of sulfur vacancies in the bottom WSx (x < 2) layers. The resultant modulation of the valance state of the sulfur atom stabilizes the W-S bond, while destabilizing the S-H bond. The electronic structural changes are further amplified by the release and transfer of surplus electrons via sulfur vacancies, filling the valance state of W and S atoms. Consequently, this work provides a comprehensive understanding of the interplay between the electronic structure of the WS2 basal plane and the HER activity, focusing on optimizing S-H bonding state.

Synthesis of multiphase MoS2 heterostructures using temperature-controlled plasma-sulfurization for photodetector applications.

Two-dimensional (2D) materials exhibit outstanding performance in photodetectors because of their excellent optical and electronic properties. Specifically, 2D-MoS2, a transition metal dichalcogenide, is a prominent candidate for flexible and portable photodetectors based on its inherent phase-dependent tunable optical band gap properties. This research focused on creating high-performance photodetectors by carefully arranging out-of-plane 2D heterostructures. The process involved stacking different phases of MoS2 (1T and 2H) using controlled temperature during plasma-enhanced chemical vapor deposition. Among the various phase combinations, the best photocurrent response was obtained for the 1T/2H-MoS2 heterostructure, which exhibited an approximately two-fold higher photocurrent than the 2H/1T-MoS2 heterostructure and 2H/2H-MoS2 monostructure. The 1T/2H-MoS2 heterostructure exhibited a higher photoresponse than the monostructured MoS2 of the same thickness (1T/1T- and 2H/2H-MoS2, respectively). The effect of the stacking sequences of different phases was examined, and their photoperformances were investigated. This study demonstrates that phase engineering in 2D-MoS2 van der Waals heterostructures has significant potential for developing high-performance photodetectors.

Low-Temperature and Large-Scale Production of a Transition Metal Sulfide Vertical Heterostructure and Its Application for Photodetectors.

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 high-quality 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 MoS2 and WS2 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 MoS2-WS2 heterostructures was validated using various spectroscopic and microscopic techniques. Further, the photocurrent generation and photoswitching phenomenon of the so-obtained MoS2-WS2 heterostructures were studied. The photodevice prepared by the MoS2-WS2 heterostructures at 150 °C showed a photoresponsivity of 83.75 mA/W. The excellent photoresponse and faster photoswitching highlight the advantage of MoS2-WS2 heterostructures toward advanced photodetectors.

Low-Temperature Synthesis of Wafer-Scale MoS2-WS2 Vertical Heterostructures by Single-Step Penetrative Plasma Sulfurization.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their synergetic effects with other 2D materials, such as graphene and hexagonal boron nitride, in TMD-based heterostructures. Therefore, it is important to understand the physical properties of TMD-TMD vertical heterostructures for their applications in next-generation electronic devices. However, the conventional synthesis process of TMD-TMD heterostructures has some critical limitations, such as nonreproducibility and low yield. In this paper, we synthesize wafer-scale MoS2-WS2 vertical heterostructures (MWVHs) using plasma-enhanced chemical vapor deposition (PE-CVD) via penetrative single-step sulfurization discovered by time-dependent analysis. This method is available for fabricating uniform large-area vertical heterostructures (4 in.) at a low temperature (300 °C). MWVHs were characterized using various spectroscopic and microscopic techniques, which revealed their uniform nanoscale polycrystallinity and the presence of vertical layers of MoS2 and WS2. In addition, wafer-scale MWVHs diodes were fabricated and demonstrated uniform performance by current mapping. Furthermore, mode I fracture tests were performed using large double cantilever beam specimens to confirm the separation of the MWVHs from the SiO2/Si substrate. Therefore, this study proposes a synthesis mechanism for TMD-TMD heterostructures and provides a fundamental understanding of the interfacial properties of TMD-TMD vertical heterostructures.