Artificial Neuron and Synapse Devices Based on 2D Materials.

Neuromorphic systems, which emulate neural functionalities of a human brain, are considered to be an attractive next-generation computing approach, with advantages of high energy efficiency and fast computing speed. After these neuromorphic systems are proposed, it is demonstrated that artificial synapses and neurons can mimic neural functions of biological synapses and neurons. However, since the neuromorphic functionalities are highly related to the surface properties of materials, bulk material-based neuromorphic devices suffer from uncontrollable defects at surfaces and strong scattering caused by dangling bonds. Therefore, 2D materials which have dangling-bond-free surfaces and excellent crystallinity have emerged as promising candidates for neuromorphic computing hardware. First, the fundamental synaptic behavior is reviewed, such as synaptic plasticity and learning rule, and requirements of artificial synapses to emulate biological synapses. In addition, an overview of recent advances on 2D materials-based synaptic devices is summarized by categorizing these into various working principles of artificial synapses. Second, the compulsory behavior and requirements of artificial neurons such as the all-or-nothing law and refractory periods to simulate a spike neural network are described, and the implementation of 2D materials-based artificial neurons to date is reviewed. Finally, future challenges and outlooks of 2D materials-based neuromorphic devices are discussed.

Selective p-Doping of 2D WSe2 via UV/Ozone Treatments and Its Application in Field-Effect Transistors.

Development of two-dimensional (2D) semiconductor devices with good Ohmic contact is essential to utilize their full potential for nanoelectronics applications. Among the methods that have been introduced to reduce the Schottky barrier in 2D material-based electronic devices, charge transfer doping has attracted significant interest because of its efficiency, simplicity, and compatibility with the microelectronic fabrication process. In this study, 2D WSe2-based field-effect transistors (FETs) were subjected to selective UV/ozone treatment to improve the Ohmic contact by forming WOX with a high work function, which induced hole doping in the neighboring WSe2 via electron transfer. The atomic force microscopy, cross-sectional transmission electron microscopy, and micro-Raman spectroscopy analyses confirmed the self-limiting formation of WOX while maintaining the crystallinity of the underlying WSe2. The channel layer of the back-gated 2D WSe2 FETs was encapsulated using 2D hexagonal boron nitride to prevent the UV/ozone-induced oxidation. By contrast, the regions that were in contact with the underlying metal electrodes were open, which allowed area-selective p-doping in the 2D WSe2. Our study demonstrated that the Ohmic-like behaviors obtained after area-selective UV/ozone treatment improved the electrical properties of the 2D WSe2-based FETs such as the field-effect mobility (improvement of 3-4 orders of magnitude) and current on/off ratio (improvement of five orders of magnitude), while maintaining the p-type normally-off characteristics. These results provide useful insights into an effective and facile method to reduce contact resistance in 2D semiconductor materials, thereby enhancing the electrical performances of 2D material-based electronic devices.

Ambipolar Charge Transport in Two-Dimensional WS2 Metal-Insulator-Semiconductor and Metal-Insulator-Semiconductor Field-Effect Transistors.

Two-dimensional (2D) materials with ambipolar transport characteristics have attracted considerable attention as post-complementary metal-oxide semiconductor (CMOS) materials. These materials allow for electron- or hole-dominant conduction to be achieved in a single channel of the field-effect transistors (FETs) without an extrinsic doping. In this study, all-2D metal-insulator-semiconductor (MIS)-based devices, which were composed of all-2D graphene, hexagonal boron nitride, and WS2, exhibited ambipolar and symmetrical transport characteristics with a low surface state density (Dit, min ≈ 7 × 1011 cm-2·eV-1). Hole- or electron-dominant inversion under the influence of electrostatic doping was obtained in a WS2-based 2D capacitor up to a frequency range of 1 MHz. n- and p-channel conductions with enhancement-mode operations were selectively realized in a single MISFET, which presented a current on/off ratio of >106 and high field-effect mobility (µe = 58-67 cm2/V·s and µh = 19-30 cm2/V·s). Furthermore, a monolithic CMOS-like logic inverter, which employed a single WS2 flake, exhibited a high gain of 78. These results can be used to reduce the footprints of the device architectures and simplify the device fabrication processes of next-generation CMOS integrated circuits.

Intimate Ohmic contact to two-dimensional WSe2 via thermal alloying.

The most important interface in semiconductor devices is the interface between the semiconductor and the first layer of the metal contact. However, the van der Waals (vdWs) gap in two-dimensional (2D) materials hindered the formation of an intimate contact between the 2D material and the metal electrode, limiting the device performances. We demonstrated a gapless Ohmic contact to 2D WSe2 by forming a Pt-W-Se alloy, which significantly improved the device performances (contact resistance, current on/off ratio, output current density, field-effect mobility, and hysteresis) of the 2D WSe2 field-effect transistor. The contact resistance to 2D WSe2 was reduced by more than seven orders of magnitude after thermal alloying. The disappearance of the vdW gap confirmed by scanning transmission electron microscopy enhanced the hole conduction and quenched the electron conduction. Our strategy of metallurgical alloying is effective to form a low-resistance stable Ohmic contact to WSe2, which paves the way for utilization of the full potential of 2D materials.

Two-Dimensionally Layered p-Black Phosphorus/n-MoS2/p-Black Phosphorus Heterojunctions.

Layered heterojunctions are widely applied as fundamental building blocks for semiconductor devices. For the construction of nanoelectronic and nanophotonic devices, the implementation of two-dimensional materials (2DMs) is essential. However, studies of junction devices composed of 2DMs are still largely focused on single p-n junction devices. In this study, we demonstrate a novel pnp double heterojunction fabricated by the vertical stacking of 2DMs (black phosphorus (BP) and MoS2) using dry-transfer techniques and the formation of high-quality p-n heterojunctions between the BP and MoS2 in the vertically stacked BP/MoS2/BP structure. The pnp double heterojunctions allowed us to modulate the output currents by controlling the input current. These results can be applied for the fabrication of advanced heterojunction devices composed of 2DMs for nano(opto)electronics.

Defect-engineered graphene chemical sensors with ultrahigh sensitivity.

We report defect-engineered graphene chemical sensors with ultrahigh sensitivity (e.g., 33% improvement in NO2 sensing and 614% improvement in NH3 sensing). A conventional reactive ion etching system was used to introduce the defects in a controlled manner. The sensitivity of graphene-based chemical sensors increased with increasing defect density until the vacancy-dominant region was reached. In addition, the mechanism of gas sensing was systematically investigated via experiments and density functional theory calculations, which indicated that the vacancy defect is a major contributing factor to the enhanced sensitivity. This study revealed that defect engineering in graphene has significant potential for fabricating ultra-sensitive graphene chemical sensors.