**1. Introduction**

Memristors have been studied for several decades and a large variety of materials has been utilized in memristors. One of the most representative and well-studied materials is metal oxide, which exhibits resistive switching phenomenon and has been widely used as the active layer in resistive random-access memory (RRAM). In the recent years, two dimensional materials have been discovered and developed rapidly as the most attractive novel materials. In 2004, the first two-dimensional material, graphene (consisting of a single layer of carbon atoms), was discovered by A. Geim and K. Novoselov [1]. Since then, the remarkable and diverse electronic, optical, mechanical and thermal properties have drawn much interest and inspired a large amount of 2D materials to be identified and analyzed, including transition metal dichalcogenides (TMDs), diatomic hexagonal boron nitride (h-BN), and monoatomic buckled crystals Xenes [2]. The 2D atomic sheets can be defined as atomically thin, layered crystalline solids, featuring intralayer covalent bonding and interlayer van der Waals bonding. These materials are recognized as twodimensional since they represent the thinnest unsupported crystalline materials that can be realized. Graphene has been utilized in electronics devices mainly as the conductive electrodes since it is a zero-gap semiconductor. The material exhibits high electron mobility at room temperature with reported values of more than 15000 cm2 V−1 s−1. Nevertheless, the zero-gap nature prevents its potential

applications in field effect transistors (FET). MoS2, a representative TMD material, has ~1.8 eV direct band gap in its monolayer form. Thus, it is suitable to be applied in FET devices [3]. In addition, hexagonal boron nitride (h-BN) also has drawn considerable attention among other 2D materials as a high band gap insulating material at ~5.9 eV, making it suitable for the production of ultrahigh mobility 2D heterostructures based on various types of 2D semiconductors [4]. Now the collection of 2D materials has been expanded to hundreds or expectedly thousands owing to more elemental and compound sheets uncovered [2, 5].

Non-volatile memory (NVM) has long been studied and developed by both academia and industry [6]. The most common non-volatile memory is flash memory [6, 7]. Although flash memory has advantages of fast read and write speed, low power consumption and less prone to damage compared with traditional hard disk drives, it has some drawbacks such as limited endurance and retention, high programming and erasing voltages, and the existing problems in small-area transistor structure like bias-temperature instability (BTI) or stress induced leakage current (SILC). In the search for the next-generation non-volatile memory, researchers have been working on various emerging alternatives, including ferroelectric random access memory (FeRAM), phase change memory (PCM), spin-transfer torque magnetic random access memory (STT-MRAM) and resistive random access memory (RRAM) [8, 9].

Among those emerging NVM, the RRAM devices show excellent endurance and retention compared with the commonly used flash memory, featuring lower power consumption, faster switching speed and better scalability [10]. The basic structure of a RRAM device is quite simple, basically a metal–insulator–metal (MIM) stacking. The conventional insulating material in RRAM is bulk metal oxides, such as SiO2, TiO2, or HfO2 [11–14]. As the most common switching mechanism, conductive filaments will be formed in the insulator with external electrical bias. Depending on the formation and rupture of the conductive filament, the device can be repeatably switched between a high-resistance state (HRS) and a low-resistance state (LRS) and sustained without power supply. This is commonly referred to as the non-volatile resistance switching (NVRS) or memristive phenomenon. Recently, extensive works have been done in the development of RRAM devices not only in NVM application but also in brain-inspired neuromorphic computing due to its analog-like multi-state switching behavior [15–17].

In the past few years, motivated by the rapid development on 2D materials, researchers have found that several 2D materials also exhibit memristive phenomenon, expanding the NVRS materials to a large collection of ultrathin layered crystalline films. As a zero-gap 2D material, graphene is not suitable for resistance switching devices. On the other hand, graphene oxide has been successfully proved as the active layer in memristors [18]. MoS2 is a representative 2D semiconductor, which has been found to show memristive effect in the form of 1 T phase [19]. In addition, Sangwan et al. reported that grain boundaries in monolayer MoS2 film can produce NVRS in planar (horizontal) structure [20]. Nevertheless, the planar structure without 3D stacking ability has the limitation of low integration density. Another example is h-BN, a representative 2D insulator, which has been demonstrated to show the resistive switching behavior in multilayer nanosheets [21]. However, the monolayer 2D materials were not reported to exhibit the effect in vertical MIM configuration.

In this chapter, the memristors based on 2D monolayers (primarily TMDs and h-BN) are presented and discussed [22–25]. The devices (collectively labeled as atomristors) feature forming-free bipolar and unipolar switching, with relatively low switching voltages down to <1 V and large on/off current ratio of more than 106 . Besides DC operation, the device can switch with fast switching speed by pulse *Memristors Based on 2D Monolayer Materials DOI: http://dx.doi.org/10.5772/intechopen.98331*

operation (< 15 ns). An atomic-resolution Dissociation-Diffusion-Adsorption model has been proposed attributing the enhanced conductance to metal atoms/ ions adsorption into intrinsic vacancies, a conductive-point mechanism supported by first-principle calculations and scanning tunneling microscopy (STM) characterizations [25, 26]. Besides voltage-sweep DC measurement, other characterization method like current sweeping and constant electric stress can be employed on the 2D-based memristors and illustrates more information in the resistive switching mechanisms [27, 28]. Benefit from the ultra-thin nature of the active layer, a novel application, RF switch, is realized based on the atomristors with operating frequencies covering the RF, 5G, and mm-wave bands and exhibits superior performance compared to those of existing solid-state switches [29–31]. The results discussed in this chapter have been organized and reproduced with permissions based on several representative publications in this field.
