Centimeter-Scale 2D van der Waals Vertical Heterostructures Integrated on Deformable Substrates Enabled by Gold Sacrificial Layer-Assisted Growth.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as molybdenum or tungsten disulfides (MoS2 or WS2) exhibit extremely large in-plane strain limits and unusual optical/electrical properties, offering unprecedented opportunities for flexible electronics/optoelectronics in new form factors. In order for them to be technologically viable building-blocks for such emerging technologies, it is critically demanded to grow/integrate them onto flexible or arbitrary-shaped substrates on a large wafer-scale compatible with the prevailing microelectronics processes. However, conventional approaches to assemble them on such unconventional substrates via mechanical exfoliations or coevaporation chemical growths have been limited to small-area transfers of 2D TMD layers with uncontrolled spatial homogeneity. Moreover, additional processes involving a prolonged exposure to strong chemical etchants have been required for the separation of as-grown 2D layers, which is detrimental to their material properties. Herein, we report a viable strategy to universally combine the centimeter-scale growth of various 2D TMD layers and their direct assemblies on mechanically deformable substrates. By exploring the water-assisted debonding of gold (Au) interfaced with silicon dioxide (SiO2), we demonstrate the direct growth, transfer, and integration of 2D TMD layers and heterostructures such as 2D MoS2 and 2D MoS2/WS2 vertical stacks on centimeter-scale plastic and metal foil substrates. We identify the dual function of the Au layer as a growth substrate as well as a sacrificial layer which facilitates 2D layer transfer. Furthermore, we demonstrate the versatility of this integration approach by fabricating centimeter-scale 2D MoS2/single walled carbon nanotube (SWNT) vertical heterojunctions which exhibit current rectification and photoresponse. This study opens a pathway to explore large-scale 2D TMD van der Waals layers as device building blocks for emerging mechanically deformable electronics/optoelectronics.

Artificial Neuron using Vertical MoS2/Graphene Threshold Switching Memristors.

With the ever-increasing demand for low power electronics, neuromorphic computing has garnered huge interest in recent times. Implementing neuromorphic computing in hardware will be a severe boost for applications involving complex processes such as image processing and pattern recognition. Artificial neurons form a critical part in neuromorphic circuits, and have been realized with complex complementary metal-oxide-semiconductor (CMOS) circuitry in the past. Recently, metal-insulator-transition materials have been used to realize artificial neurons. Although memristors have been implemented to realize synaptic behavior, not much work has been reported regarding the neuronal response achieved with these devices. In this work, we use the volatile threshold switching behavior of a vertical-MoS2/graphene van der Waals heterojunction system to produce the integrate-and-fire response of a neuron. We use large area chemical vapor deposited (CVD) graphene and MoS2, enabling large scale realization of these devices. These devices can emulate the most vital properties of a neuron, including the all or nothing spiking, the threshold driven spiking of the action potential, the post-firing refractory period of a neuron and strength modulated frequency response. These results show that the developed artificial neuron can play a crucial role in neuromorphic computing.

Uniform Vapor-Pressure-Based Chemical Vapor Deposition Growth of MoS2 Using MoO3 Thin Film as a Precursor for Coevaporation.

Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS2) by using thermally evaporated thin films of molybdenum trioxide (MoO3) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO3 thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS2 throughout the substrate. These high-quality crystals are as large as 95 µm and are characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. The bottom-gated field-effect transistors fabricated using the as-grown single crystals show n-type transistor behavior with a good on/off ratio of 106 under ambient conditions. Our results presented here address the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS2 for future semiconductor device applications.