**2. Fabrication of 2D-based memristors**

A dozen 2D materials have been investigated for non-volatile resistive switching, including transition metal sulfides (MS2, M = Mo, W, Re, Sn), transition metal selenides (MSe2, M = Mo, W, Re, Sn, Pt), a transition metal telluride (MoTe2), a TMD heterostructure (WS2/MoS2) and an insulator (h-BN). These selected 2D materials can be readily grown as mono or few layers with unambiguous characterization of material quality and thickness, using chemical vapor deposition (CVD) or metal– organic chemical vapor deposition (MOCVD) method [32, 33].

Two device structures were used for the 2D-based memory device fabrication. First is the typical crossbar device with the advantages of small-area capability and better probing condition. The schematic and optical image of MoS2 crossbar device are shown in **Figure 1a** and **b**. Most of the electrical measurements were performed on the crossbar devices. The other structure, the litho-free and transfer-free device, was fabricated based on the 2D materials directly on metal foils to avoid possible residues or contamination induced by lithography or transfer process (schematic shown in **Figure 1c**). The crossbar device fabrication started with bottom electrodes (BE) patterning by electron beam lithography and 2 nm Cr/60 nm Au metal stack deposition on an SiO2(285 nm)/Si substrate. Monolayer TMD was then transferred onto the fabricated substrate using a resist-free polydimethylsiloxane (PDMS) stamp transfer method. In this method, monolayer TMD was brought into conformal contact with PDMS. The substrate-TMD-PDMS system was subsequently soaked into diluted water. Since the original SiO2 substrate is hydrophilic, it is easy for water to diffuse into the TMD-substrate interface, which helps separate the two layers. Then, the PDMS-TMD film was brought into contact with the target substrate with BE on it. The PDMS stamp was peeled off to leave monolayer TMD films on the target substrate. CVD h-BN was transferred onto BE from the Ni foil

#### **Figure 1.**

*(a, b) schematic and optical image of MIM structure of TMD crossbar device. (c) Schematic of TMD litho-free and transfer-free device based on MoS2 grown on Au foil.*

substrate using another poly(methyl methacrylate) (PMMA)-assisted wet transfer method. A thin layer of PMMA was spin coated onto the h-BN/Ni and then the Ni was etched away in 0.5 M ammonia persulfate solution. The PMMA/h-BN was rinsed in DI water to remove any etchant by-product before lifting by the target substrate with BE. The PMMA was then removed by immersing in acetone. For crossbar devices, top electrodes (TE) was patterned by e-beam lithography and deposited by e-beam evaporation using the same fabrication process as BE. In lithofree and transfer-free device, metal foils were used as global BE, and the TE (60 nm Au) was deposited via a shadow mask.
