Self-Reinforced Doping Strategy in the Multiscale PMIA Paper for High Mechanical Properties and Insulating Performance.

The poly(m-phenylene isophthalamide) (PMIA) paper has attracted extensive interests due to its ultrahigh mechanical properties as an ideal protective material for anti-impact damage applications. In the pursuit of additional properties, composites based on the PMIA matrix and various fillers are widely explored. However, additional improvements are frequently obtained at the expense of mechanical properties because of the serious interfacial compatibility brought by different components. In this study, a self-reinforced doping strategy is proposed by combining microscale PMIA fibers as the fillers and nanoscale PMIA fibers as the matrix to form a micronano paper. Without the limitation of the interfacial compatibility issues, the nanofibers are tightly aligned and adhered to the microfibers, enabling the in situ generation of hydrogen bonds at the interfaces. A compact interfacial structure is thus constructed with reduced porosity on the surface. It indicates that the microfibers have a positive impact on the improvement of mechanical properties. In our optimized sample with 5 wt % microfibers, the elastic modulus, tensile strength, and elongation are 1530 MPa, 24.8 MPa, and 5.3%, respectively, which are 142, 49.4, and 65% higher than those of the pristine nano-PMIA paper. In addition, the insulating performance is also improved, facilitating its further application extended to broad fields.

Mid-Infrared Optoelectronic Devices Based on Two-Dimensional Materials beyond Graphene: Status and Trends.

Since atomically thin two-dimensional (2D) graphene was successfully synthesized in 2004, it has garnered considerable interest due to its advanced properties. However, the weak optical absorption and zero bandgap strictly limit its further development in optoelectronic applications. In this regard, other 2D materials, including black phosphorus (BP), transition metal dichalcogenides (TMDCs), 2D Te nanoflakes, and so forth, possess advantage properties, such as tunable bandgap, high carrier mobility, ultra-broadband optical absorption, and response, enable 2D materials to hold great potential for next-generation optoelectronic devices, in particular, mid-infrared (MIR) band, which has attracted much attention due to its intensive applications, such as target acquisition, remote sensing, optical communication, and night vision. Motivated by this, this article will focus on the recent progress of semiconducting 2D materials in MIR optoelectronic devices that present a suitable category of 2D materials for light emission devices, modulators, and photodetectors in the MIR band. The challenges encountered and prospects are summarized at the end. We believe that milestone investigations of 2D materials beyond graphene-based MIR optoelectronic devices will emerge soon, and their positive contribution to the nano device commercialization is highly expected.

Tailoring Quantum Tunneling in a Vanadium-Doped WSe2/SnSe2 Heterostructure.

2D van der Waals layered heterostructures allow for a variety of energy band offsets, which help in developing valuable multifunctional devices. However, p-n diodes, which are typical and versatile, are still limited by the material choice due to the fixed band structures. Here, the vanadium dopant concentration is modulated in monolayer WSe2 via chemical vapor deposition to demonstrate tunable multifunctional quantum tunneling diodes by vertically stacking SnSe2 layers at room temperature. This is implemented by substituting tungsten atoms with vanadium atoms in WSe2 to provoke the p-type doping effect in order to efficiently modulate the Fermi level. The precise control of the vanadium doping concentration is the key to achieving the desired quantum tunneling diode behaviors by tuning the proper band alignment for charge transfer across the heterostructure. By constructing a p-n diode for p-type V-doped WSe2 and heavily degenerate n-type SnSe2, the type-II band alignment at low V-doping concentration is clearly shown, which evolves into the type-III broken-gap alignment at heavy V-doping concentration to reveal a variety of diode behaviors such as forward diode, backward diode, negative differential resistance, and ohmic resistance.

High-mobility junction field-effect transistor via graphene/MoS2 heterointerface.

Monolayer molybdenum disulfide (MoS2) possesses a desirable direct bandgap with moderate carrier mobility, whereas graphene (Gr) exhibits a zero bandgap and excellent carrier mobility. Numerous approaches have been suggested for concomitantly realizing high on/off current ratio and high carrier mobility in field-effect transistors, but little is known to date about the effect of two-dimensional layered materials. Herein, we propose a Gr/MoS2 heterojunction platform, i.e., junction field-effect transistor (JFET), that enhances the carrier mobility by a factor of ~ 10 (~ 100 cm2 V-1 s-1) compared to that of monolayer MoS2, while retaining a high on/off current ratio of ~ 108 at room temperature. The Fermi level of Gr can be tuned by the wide back-gate bias (VBG) to modulate the effective Schottky barrier height (SBH) at the Gr/MoS2 heterointerface from 528 meV (n-MoS2/p-Gr) to 116 meV (n-MoS2/n-Gr), consequently enhancing the carrier mobility. The double humps in the transconductance derivative profile clearly reveal the carrier transport mechanism of Gr/MoS2, where the barrier height is controlled by electrostatic doping.

Tunable Negative Differential Resistance in van der Waals Heterostructures at Room Temperature by Tailoring the Interface.

Vertically stacked two-dimensional van der Waals (vdW) heterostructures, used to obtain homogeneity and band steepness at interfaces, exhibit promising performance for band-to-band tunneling (BTBT) devices. Esaki tunnel diodes based on vdW heterostructures, however, yield poor current density and peak-to-valley ratio, inferior to those of three-dimensional materials. Here, we report the negative differential resistance (NDR) behavior in a WSe2/SnSe2 heterostructure system at room temperature and demonstrate that heterointerface control is one of the keys to achieving high device performance by constructing WSe2/SnSe2 heterostructures in inert gas environments. While devices fabricated in ambient conditions show poor device performance due to the observed oxidation layer at the interface, devices fabricated in inert gas exhibit extremely high peak current density up to 1460 mA/mm2, 3-4 orders of magnitude higher than reported vdW heterostructure-based tunnel diodes, with a peak-to-valley ratio of more than 4 at room temperature. Besides, Pd/WSe2 contact in our device possesses a much higher Schottky barrier than previously reported Cr/WSe2 contact in the WSe2/SnSe2 device, which suppresses the thermionic emission current to less than the BTBT current level, enabling the observation of NDR at room temperature. Diode behavior can be further modulated by controlling the electrostatic doping and the tunneling barrier as well.