Anisotropic Carrier Mobility from 2H WSe2.

Transition metal dichalcogenides (TMDCs) with 2H phase are expected to be building blocks in next-generation electronics; however, they suffer from electrical anisotropy, which is the basics for multi-terminal artificial synaptic devices, digital inverters, and anisotropic memtransistors, which are highly desired in neuromorphic computing. Herein, the anisotropic carrier mobility from 2H WSe2 is reported, where the anisotropic degree of carrier mobility spans from 0.16 to 0.95 for various WSe2 field-effect transistors under a gate voltage of -60 V. Phonon scattering, impurity ions scattering, and defect scattering are excluded for anisotropic mobility. An intrinsic screening layer is proposed and confirmed by Z-contrast scanning transmission electron microscopy (STEM) imaging to respond to the electrical anisotropy. Seven types of intrinsic screening layers are created and calculated by density functional theory to evaluate the modulated electronic structures, effective masses, and scattering intensities, resulting in anisotropic mobility. The discovery of anisotropic carrier mobility from 2H WSe2 provides a degree of freedom for adjusting the physical properties of 2H TMDCs and fertile ground for exploring and integrating TMDC electronic transistors with better performance along the direction of high mobility.

High precision epidermal radio frequency antenna via nanofiber network for wireless stretchable multifunction electronics.

Recently, stretchable electronics combined with wireless technology have been crucial for realizing efficient human-machine interaction. Here, we demonstrate highly stretchable transparent wireless electronics composed of Ag nanofibers coils and functional electronic components for power transfer and information communication. Inspired by natural systems, various patterned Ag nanofibers electrodes with a net structure are fabricated via using lithography and wet etching. The device design is optimized by analyzing the quality factor and radio frequency properties of the coil, considering the effects of strain. Particularly, the wireless transmission efficiency of a five-turn coil drops by approximately only 50% at 10 MHz with the strain of 100%. Moreover, various complex functional wireless electronics are developed using near-field communication and frequency modulation technology for applications in content recognition and long-distance transmission (>1 m), respectively. In summary, the proposed device has considerable potential for applications in artificial electronic skins, human healthcare monitoring and soft robotics.

Lateral 2D WSe2 p-n Homojunction Formed by Efficient Charge-Carrier-Type Modulation for High-Performance Optoelectronics.

As unique building blocks for next-generation optoelectronics, high-quality 2D p-n junctions based on semiconducting transition metal dichalcogenides (TMDs) have attracted wide interest, which are urgent to be exploited. Herein, a novel and facile electron doping of WSe2 by cetyltrimethyl ammonium bromide (CTAB) is achieved for the first time to form a high-quality intramolecular p-n junction with superior optoelectronic properties. Efficient manipulation of charge carrier type and density in TMDs via electron transfer between Br- in CTAB and TMDs is proposed theoretically by density functional theory (DFT) calculations. Compared with the intrinsic WSe2 photodetector, the switching light ratio (Ilight /Idark ) of the p-n junction device can be enhanced by 103 , and the temporal response is also dramatically improved. The device possesses a responsivity of 30 A W-1 , with a specific detectivity of over 1011 Jones. In addition, the mechanism of charge transfer in CTAB-doped 2D WSe2 and WS2 are investigated by designing high-performance field effect transistors. Besides the scientific insight into the effective manipulation of 2D materials by chemical doping, this work presents a promising applicable approach toward next-generation photoelectronic devices with high efficiency.

Piezoelectricity in Multilayer Black Phosphorus for Piezotronics and Nanogenerators.

Recently, piezoelectric characteristics have been a research focus for 2D materials because of their broad potential applications. Black phosphorus (BP) is a monoelemental 2D material predicted to be piezoelectric because of its highly directional properties and non-centrosymmetric lattice structure. However, piezoelectricity is hardly reported in monoelemental materials owing to their lack of ionic polarization, but piezoelectric generation is consistent with the non-centrosymmetric structure of BP. Theoretical calculations of phosphorene have explained the origin of piezoelectric polarization among P atoms. However, the disappearance of piezoelectricity in multilayer 2D material generally arises from the opposite orientations of adjacent atomic layers, whereas this effect is limited in BP lattices due to their spring-shaped space structure. Here, the existence of in-plane piezoelectricity is experimentally reported for multilayer BP along the armchair direction. Current-voltage measurements demonstrate a piezotronic effect in this orientation, and cyclic compression and release of BP flakes show an intrinsic current output as large as 4 pA under a compressive strain of -0.72%. The discovery of piezoelectricity in multilayer BP can lead to further understanding of this mechanism in monoelemental materials.

Investigating the interlayer electron transport and its influence on the whole electric properties of black phosphorus.

Two-dimensional (2D) nanomaterials have attracted great attention in next generation electronic and optoelectronic technologies due to the unique layered structure and excellent physical and chemical properties. However, the mechanism of transmission along the vertical direction of 2D semiconductor materials has not been investigated. Here, we use first-principles calculations to explore the bandgap energies along different directions, and fabricate a vertical, a lateral and a mixture-structured black phosphorus field effect transistor (BPFET) to study the electrical characteristics along different directions under variable temperatures. The variable temperature test indicates that the mixture-structured device performs more like a lateral device, while the conductance along the vertical direction is hard to be tuned by temperature and electrical field. The unchanged conductance under electric field and variable temperatures allows the vertical device to act as a fixed resistor, promising possible application for the prospective electronic and optoelectronic devices.

In2O3 Nanowire Field-Effect Transistors with Sub-60 mV/dec Subthreshold Swing Stemming from Negative Capacitance and Their Logic Applications.

Heat dissipation is a key issue for scaling metal-oxide-semiconductor field-effect transistors (MOSFETs). The Boltzmann distribution of electrons imposes a physical limit on the subthreshold swing (SS), which impedes both the reduction of the switching energy and the further increase of the device density. The negative capacitance effect is proposed to rescue MOSFETs from this phenomenon called "Boltzmann tyranny". Herein, we report In2O3 nanowire (NW) transistors with SS values in the sub-60 mV/dec region, which utilize the ferroelectric P(VDF-TrFE) as the dielectric layer. An ultralow SS down to ∼10 mV/dec is observed and spans over 5 orders of magnitude in the drain current. Meanwhile, a high on/off ratio of more than 108 and a transconductance ( gm) of 2.3 µS are obtained simultaneously at Vd = 0.1 V. The results can be understood by the "voltage amplification" effect induced from the negative capacitance effect. Moreover, the steep slope FET-based inverters indicate a high voltage gain of 41.6. In addition to the NOR and NAND gates, the Schmitt trigger inverters containing only one steep slope FET are demonstrated. This work demonstrates an avenue for low-power circuit design with a steep SS.

Tunable Tribotronic Dual-Gate Logic Devices Based on 2D MoS2 and Black Phosphorus.

With the Moore's law hitting the bottleneck of scaling-down in size (below 10 nm), personalized and multifunctional electronics with an integration of 2D materials and self-powering technology emerge as a new direction of scientific research. Here, a tunable tribotronic dual-gate logic device based on a MoS2 field-effect transistor (FET), a black phosphorus FET and a sliding mode triboelectric nanogenerator (TENG) is reported. The triboelectric potential produced from the TENG can efficiently drive the transistors and logic devices without applying gate voltages. High performance tribotronic transistors are achieved with on/off ratio exceeding 106 and cutoff current below 1 pA µm-1 . Tunable electrical behaviors of the logic device are also realized, including tunable gains (improved to ≈13.8) and power consumptions (≈1 nW). This work offers an active, low-power-consuming, and universal approach to modulate semiconductor devices and logic circuits based on 2D materials with TENG, which can be used in microelectromechanical systems, human-machine interfacing, data processing and transmission.

Te-Doped Black Phosphorus Field-Effect Transistors.

Element doping allows manipulation of the electronic properties of 2D materials. Enhanced transport performances and ambient stability of black-phosphorus devices by Te doping are presented. This provides a facile route for achieving airstable black-phosphorus devices.

Enhanced stability of black phosphorus field-effect transistors with SiO2 passivation.

Few-layer black phosphorus (BP) has attracted much attention due to its high mobility and suitable band gap for potential applic5ations in optoelectronics and flexible devices. However, its instability under ambient conditions limits its practical applications. Our investigations indicate that by passivation of the mechanically exfoliated BP flakes with a SiO2 layer, the fabricated BP field-effect transistors (FETs) exhibit greatly enhanced environmental stability. Compared to the unpassivated BP devices, which show a fast drop of on/off current ratio by a factor of 10 after one week of ambient exposure, the SiO2-passivated BP devices display a high retained on/off current ratio of over 600 after one week of exposure, just a little lower than the initial value of 810. Our investigations provide an effective route to passivate the few-layer BPs for enhancement of their environmental stability.