Three dimensionally-ordered 2D MoS₂vertical layers integrated on flexible substrates with stretch-tunable functionality and improved sensing capability

Abstract

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

“…The work of adhesion, W, required for the spontaneous 2D layer separation is expressed as , where γ SW , γ WL , and γ SL are interfacial tension between substrate and water, water and 2D layer, substrate and 2D layer, respectively. In comparing the 2D MoS 2 layers of horizontal vs. vertical orientation, we note that vertically-aligned 2D MoS 2 layers exhibit significantly higher adhesion energy with respect to the SiO 2 surface owing to the exposure of energetically reactive 2D edge sites; i.e., order-of-magnitude higher binding energy of 2D edges with dangling bonds over 2D basal panes with saturated bonds 55,56 . Accordingly, the value of γ SL for vertically-aligned 2D MoS 2 layers should be much smaller than that for horizontally-aligned ones, which further justifies the observed difficulty with their spontaneous separation inside water.…”

Scalably Nanomanufactured Atomically Thin Materials‐Based Wearable Health Sensors

“…The as‐prepared humidity sensor shows an extremely high moisture sensitivity ( R dry / R humid ) up to 2357 (for 90% RH), which outperforms the existing stretchable humidity sensors. [ 722–726 ] The sensor can withstand up to 40% tensile strain without apparent performance degradation. In addition, the sensor exhibits reliable signals under dynamic RH changing when a saturated water moisture gas is periodically introduced.…”

Mechanically rollable photodetectors enabled by centimetre-scale 2D MoS2 layer/TOCN composites

“…Furthermore, the shapes of the UV-vis spectral profiles are consistent with previous observations with thickness-varying 2D MoS2 layers integrated on other optically transparent substrates. [43][44][45][46] Detailed optical characteristics of the integrated 2D MoS2 layers were further analysed by developing Tauc plots from the UV-vis spectra, 47,48 as shown in Fig. 2d.…”