Temperature dependent studies on centimeter-scale MoS2and vdW heterostructures.

Transition metal dichalcogenides is an emerging 2D semiconducting material group which has excellent physical properties in the ultimately scaled thickness dimension. Specifically, van der Waals heterostructures hold the great promise in further advancing both the fundamental scientific knowledge and practical technological applications of 2D materials. Although 2D materials have been extensively studied for various sensing applications, temperature sensing still remains relatively unexplored. In this work, we experimentally study the temperature-dependent Raman spectroscopy and electrical conductivity of molybdenum disulfide (MoS2) and its heterostructures with platinum dichalcogenides (PtSe2and PtTe2) to explore their potential to become the next-generation temperature sensor. It is found that the MoS2-PtX2heterostructure shows the great promise as the high-sensitivity temperature sensor.

Large-area vertically aligned 2D MoS2layers on TEMPO-cellulose nanofibers for biodegradable transient gas sensors.

Crystallographically anisotropic two-dimensional (2D) molybdenum disulfide (MoS2) with vertically aligned (VA) layers is attractive for electrochemical sensing owing to its surface-enriched dangling bonds coupled with extremely large mechanical deformability. In this study, we explored VA-2D MoS2layers integrated on cellulose nanofibers (CNFs) for detecting various volatile organic compound gases. Sensor devices employing VA-2D MoS2/CNFs exhibited excellent sensitivities for the tested gases of ethanol, methanol, ammonia, and acetone; e.g. a high response rate up to 83.39% for 100 ppm ethanol, significantly outperforming previously reported sensors employing horizontally aligned 2D MoS2layers. Furthermore, VA-2D MoS2/CNFs were identified to be completely dissolvable in buffer solutions such as phosphate-buffered saline solution and baking soda buffer solution without releasing toxic chemicals. This unusual combination of high sensitivity and excellent biodegradability inherent to VA-2D MoS2/CNFs offers unprecedented opportunities for exploring mechanically reconfigurable sensor technologies with bio-compatible transient characteristics.

Wafer-Scale Two-Dimensional MoS2 Layers Integrated on Cellulose Substrates Toward Environmentally Friendly Transient Electronic Devices.

We explored the feasibility of wafer-scale two-dimensional (2D) molybdenum disulfide (MoS2) layers toward futuristic environmentally friendly electronics that adopt biodegradable substrates. Large-area (> a few cm2) 2D MoS2 layers grown on silicon dioxide/silicon (SiO2/Si) wafers were delaminated and integrated onto a variety of cellulose-based substrates of various components and shapes in a controlled manner; examples of the substrates include planar papers, cylindrical natural rubbers, and 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibers. The integrated 2D layers were confirmed to well preserve their intrinsic structural and chemical integrity even on such exotic substrates. Proof-of-concept devices employing large-area 2D MoS2 layers/cellulose substrates were demonstrated for a variety of applications, including photodetectors, pressure sensors, and field-effect transistors. Furthermore, we demonstrated the complete "dissolution" of the integrated 2D MoS2 layers in a buffer solution composed of baking soda and deionized water, confirming their environmentally friendly transient characteristics. Moreover, the approaches to delaminate and integrate them do not demand any chemicals except for water, and their original substrates can be recycled for subsequent growths, ensuring excellent chemical benignity and process sustainability.

Centimeter-scale Green Integration of Layer-by-Layer 2D TMD vdW Heterostructures on Arbitrary Substrates by Water-Assisted Layer Transfer.

Two-dimensional (2D) transition metal dichalcogenide (2D TMD) layers present an unusually ideal combination of excellent opto-electrical properties and mechanical tolerance projecting high promise for a wide range of emerging applications, particularly in flexible and stretchable devices. The prerequisite for realizing such opportunities is to reliably integrate large-area 2D TMDs of well-defined dimensions on mechanically pliable materials with targeted functionalities by transferring them from rigid growth substrates. Conventional approaches to overcome this challenge have been limited as they often suffer from the non-scalable integration of 2D TMDs whose structural and chemical integrity are altered through toxic chemicals-involved processes. Herein, we report a generic and reliable strategy to achieve the layer-by-layer integration of large-area 2D TMDs and their heterostructure variations onto a variety of unconventional substrates. This new 2D layer integration method employs water only without involving any other chemicals, thus renders distinguishable advantages over conventional approaches in terms of material property preservation and integration size scalability. We have demonstrated the generality of this method by integrating a variety of 2D TMDs and their heterogeneously-assembled vertical layers on exotic substrates such as plastics and papers. Moreover, we have verified its technological versatility by demonstrating centimeter-scale 2D TMDs-based flexible photodetectors and pressure sensors which are difficult to fabricate with conventional approaches. Fundamental principles for the water-assisted spontaneous separation of 2D TMD layers are also discussed.

Three dimensionally-ordered 2D MoS2 vertical layers integrated on flexible substrates with stretch-tunable functionality and improved sensing capability.

The intrinsically anisotropic crystallinity of two-dimensional (2D) transition metal dichalcogenide (2D TMD) layers enables a variety of intriguing material properties which strongly depend on the physical orientation of constituent 2D layers. For instance, 2D TMDs with vertically-aligned layers exhibit numerous dangling bonds on their 2D layer edge sites predominantly exposed on the surface, projecting significantly improved physical and/or chemical adsorption capability compared to their horizontally-oriented 2D layer counterparts. Such property advantages can be further promoted as far as the material can be integrated onto unconventional substrates of tailored geometry/functionality, offering vast opportunities for a wide range of applications which demand enhanced surface area/reactivity and mechanical flexibility. Herein, we report a new form of 2D TMDs, i.e., three-dimensionally ordered 2D molybdenum disulfide (2D MoS2) with vertically-aligned layers integrated on elastomeric substrates and explore their tunable multi-functionalities and technological promise. We grew large-scale (>2 cm2) vertically-aligned 2D MoS2 layers using a three-dimensionally patterned silicon dioxide (SiO2) template and directly transferred/integrated them onto flexible polydimethylsiloxane (PDMS) substrates by taking advantage of the distinguishable water-wettability of 2D MoS2vs. SiO2. The excellent structural integrity of the integrated vertical 2D MoS2 layers was confirmed by extensive spectroscopy/microscopy characterization. In addition, the stretch-driven unique tunability of their optical and surface properties was also examined. Moreover, we applied this material for flexible humidity sensing and identified significantly improved (>10 times) sensitivity over conventionally-designed horizontal 2D MoS2 layers, further confirming their high potential for unconventional flexible technologies.

Centimeter-Scale Periodically Corrugated Few-Layer 2D MoS2 with Tensile Stretch-Driven Tunable Multifunctionalities.

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers exhibit superior optical, electrical, and structural properties unattainable in any traditional materials. Many of these properties are known to be controllable via external mechanical inputs, benefiting from their extremely small thickness coupled with large in-plane strain limits. However, realization of such mechanically driven tunability often demands highly complicated engineering of 2D TMD layer structures, which is difficult to achieve on a large wafer scale in a controlled manner. Herein, we explore centimeter-scale periodically corrugated 2D TMDs, particularly 2D molybdenum disulfide (MoS2), and report their mechanically tunable multifunctionalities. We developed a water-assisted process to homogeneously integrate few layers of 2D MoS2 on three-dimensionally corrugated elastomeric substrates on a large area (>2 cm2). The evolution of electrical, optical, and structural properties in these three-dimensionally corrugated 2D MoS2 layers was systematically studied under controlled tensile stretch. We identified that they present excellent electrical conductivity and photoresponsiveness as well as systematically tunable surface wettability and optical absorbance even under significant mechanical deformation. These novel three-dimensionally structured 2D materials are believed to offer exciting opportunities for large-scale, mechanically deformable devices of various form factors and unprecedented multifunctionalities.