Atomically thin two-dimensional (2D) materialssuch as transition metal dichalcogenide (TMD) monolayers and hexagonal boron nitride (hBN)and their van der Waals layered preparations have been actively researched to build electronic devices such as field-effect transistors, junction diodes, tunneling devices, and, more recently, memristors. Twodimensional material memristors built in lateral form, with horizontal placement of electrodes and the 2D material layers, have provided an intriguing window into the motions of ions along the atomically thin layers. On the other hand, 2D material memristors built in vertical form with top and bottom electrodes sandwiching 2D material layers may provide opportunities to explore the extreme of the memristive performance with the atomic-scale interelectrode distance. In particular, they may help push the switching voltages to a lower limit, which is an important pursuit in memristor research in general, given their roles in neuromorphic computing. In fact, recently Akinwande et al. performed a pioneering work to demonstrate a vertical memristor that sandwiches a single MoS 2 monolayer between two inert Au electrodes, but it could neither attain switching voltages below 1 V nor control the switching polarity, obtaining both unipolar and bipolar switching devices. Here, we report a vertical memristor that sandwiches two MoS 2 monolayers between an active Cu top electrode and an inert Au bottom electrode. Cu ions diffuse through the MoS 2 double layers to form atomic-scale filaments. The atomic-scale thickness, combined with the electrochemical metallization, lowers switching voltages down to 0.1−0.2 V, on par with the state of the art. Furthermore, our memristor achieves consistent bipolar and analogue switching, and thus exhibits the synapse-like learning behavior such as the spike-timing dependent plasticity (STDP), the very first STDP demonstration among all 2D-material-based vertical memristors. The demonstrated STDP with low switching voltages is promising not only for low-power neuromorphic computing, but also from the point of view that the voltage range approaches the biological action potentials, opening up a possibility for direct interfacing with mammalian neuronal networks.

Recently, as applications based on triboelectricity have expanded, understanding the triboelectric charging behavior of various materials has become essential. This study investigates the triboelectric charging behaviors of various 2D layered materials, including MoS , MoSe , WS , WSe , graphene, and graphene oxide in a triboelectric series using the concept of a triboelectric nanogenerator, and confirms the position of 2D materials in the triboelectric series. It is also demonstrated that the results are obviously related to the effective work functions. The charging polarity indicates the similar behavior regardless of the synthetic method and film thickness ranging from a few hundred nanometers (for chemically exfoliated and restacked films) to a few nanometers (for chemical vapor deposited films). Further, the triboelectric charging characteristics could be successfully modified via chemical doping. This study provides new insights to utilize 2D materials in triboelectric devices, allowing thin and flexible device fabrication.

2D semiconductors, especially transition metal dichalcogenide (TMD) monolayers, are extensively studied for electronic and optoelectronic applications. Beyond intensive studies on single transistors and photodetectors, the recent advent of large‐area synthesis of these atomically thin layers has paved the way for 2D integrated circuits, such as digital logic circuits and image sensors, achieving an integration level of ≈100 devices thus far. Here, a decisive advance in 2D integrated circuits is reported, where the device integration scale is increased by tenfold and the functional complexity of 2D electronics is propelled to an unprecedented level. Concretely, an analog optoelectronic processor inspired by biological vision is developed, where 32 × 32 = 1024 MoS2 photosensitive field‐effect transistors manifesting persistent photoconductivity (PPC) effects are arranged in a crossbar array. This optoelectronic processor with PPC memory mimics two core functions of human vision: it captures and stores an optical image into electrical data, like the eye and optic nerve chain, and then recognizes this electrical form of the captured image, like the brain, by executing analog in‐memory neural net computing. In the highlight demonstration, the MoS2 FET crossbar array optically images 1000 handwritten digits and electrically recognizes these imaged data with 94% accuracy.

For practical device applications, monolayer transition metal dichalcogenide (TMD) films must meet key industry needs for batch processing, including the high‐throughput, large‐scale production of high‐quality, spatially uniform materials, and reliable integration into devices. Here, high‐throughput growth, completed in 12 min, of 6‐inch wafer‐scale monolayer MoS2 and WS2 is reported, which is directly compatible with scalable batch processing and device integration. Specifically, a pulsed metal–organic chemical vapor deposition process is developed, where periodic interruption of the precursor supply drives vertical Ostwald ripening, which prevents secondary nucleation despite high precursor concentrations. The as‐grown TMD films show excellent spatial homogeneity and well‐stitched grain boundaries, enabling facile transfer to various target substrates without degradation. Using these films, batch fabrication of high‐performance field‐effect transistor (FET) arrays in wafer‐scale is demonstrated, and the FETs show remarkable uniformity. The high‐throughput production and wafer‐scale automatable transfer will facilitate the integration of TMDs into Si‐complementary metal‐oxide‐semiconductor platforms.

Metal-semiconductor junctions are indispensable in semiconductor devices, but they have recently become a major limiting factor precluding device performance improvement. Here, we report the modification of a metal/n-type Si Schottky contact barrier by the introduction of two-dimensional (2D) materials of either graphene or hexagonal boron nitride (h-BN) at the interface. We realized the lowest specific contact resistivities (ρ) of 3.30 nΩ cm (lightly doped n-type Si, ∼ 10/cm) and 1.47 nΩ cm (heavily doped n-type Si, ∼ 10/cm) via 2D material insertion are approaching the theoretical limit of 1.3 nΩ cm. We demonstrated the role of the 2D materials at the interface in achieving a low ρ value by the following mechanisms: (a) 2D materials effectively form dipoles at the metal-2D material (M/2D) interface, thereby reducing the metal work function and changing the pinning point, and (b) the fully metalized M/2D system shifts the pinning point toward the Si conduction band, thus decreasing the Schottky barrier. As a result, the fully metalized M/2D system using atomically thin and well-defined 2D materials shows a significantly reduced ρ. The proposed 2D material insertion technique can be used to obtain extremely low contact resistivities in metal/n-type Si systems and will help to achieve major performance improvements in semiconductor technologies.

In article number https://doi.org/10.1002/adma.201801210, Sang‐Woo Kim, Hyeon‐Jin Shin, and co‐workers report an investigation on the triboelectric charging behaviors of various two‐dimensional (2D) materials, including MoS2, MoSe2, WS2, WSe2, graphene, and graphene oxide by using the concept of the triboelectric nanogenerator (TENG). The relative charging polarities of the 2D materials against materials in the conventional triboelectric series are observed by analyzing the output signals obtained from TENGs of various combinations.

Notwithstanding its excellent properties such as high work function and low resistance, Ru has not been widely applied in the preparation of electrodes for various electronic devices. This is because of the occurrence of severe morphological degradation in the actual devices employing Ru. Herein, we investigated Ru chemistry for electrode application and the degradation mechanism of Ru during subsequent processes such as thin film deposition or thermal annealing. We revealed that subsurface oxygen induces Ru degradation owing to the alteration of Ru chemistry by the pretreatment under various gas ambient conditions and due to the growth behavior of TiO2 deposited via atomic layer deposition (ALD). The degradation of Ru is successfully ameliorated by conducting an appropriate pretreatment prior to ALD. The TiO2 thin film deposited on the pretreated Ru electrode exhibited a rutile-phased crystal structure and smooth surface morphology, thereby resulting in excellent electrical properties. This paper presents an important development in the application of Ru as the electrode that can facilitate the development of various next-generation electronic devices.

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