Abstract
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.
Highlights
Factors such as wearable and stretchable devices
The process is carried out in following steps: (1) Deposition of transition metals on the surface of growth substrates (i.e., SiO2/Si) followed by their conversion to 2D TMD layers via chemical vapor deposition (CVD). (2) Immersion of the 2D TMDsgrown SiO2/Si substrates inside water followed by spontaneous 2D layer separation
Influence of 2D layer orientation: We have observed that the spontaneous delamination of 2D layers inside water happens with 2D TMDs obtained from the sulfurization of thin metal seeds. 2D TMDs obtained in this manner exhibit horizontally-aligned basal planes of well-resolved layers; for example, horizontally-aligned 2D MoS2 layers of ~7–8 nm thickness obtained from the sulfurization of ~2.5 nm thick Mo have been confirmed by atomic force microscopy (AFM) and cross-sectional transmission electron microscope (TEM) characterization (Supplementary Information, S4)
Summary
Factors such as wearable and stretchable devices. heterogeneously integrating 2D TMD layers of distinguishable yet tailored components has been predicted to enable even more exotic functionalities impossible with conventional thin film semiconductor growth technologies[3,13,14,15,16,17,18,19,20,21,22,23,24,25]. As 2D TMD layers exert weak van der Waals (vdW) attraction to underlying growth substrates, it is possible to individually assemble them in a layer-by-layer manner achieving targeted electronic structures, implying new venues for 2D heterojunction devices with tailored band offsets[22,23,24,25,26] Such atom thick semiconductor heterostructures have been technically challenging to integrate with conventional thin film growth strategies owing to their intrinsic lattice match constraint which imposes the crystallographic limitation for the choice of materials to be integrated. The technological versatility of this 2D layer integration method has been demonstrated by developing large-area 2D TMDs-based flexible photodetectors and pressure sensors on plastics and papers, respectively
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