PdSe2/2H-MoTe2 Heterojunction Self-Powered Photodetector: Broadband Photodetection and Linear/Circular Polarization Capability.

In this research, we introduce a PdSe2/2H-MoTe2 heterojunction photodetector that exhibits both broadband self-powered photodetection and linear/circular polarization detection capabilities. It has a broad spectral response range (covering 375-2200 nm) and reaches a peak sensitivity at 532 nm, exhibiting a notable responsivity of 7.3 × 103 A/W and a substantial specific detectivity of 8.5 × 1012 Jones. Even in the near-infrared region of 1310 nm, it still has a high responsivity of 20 A/W. The self-powered photodetection capabilities of the PdSe2/2H-MoTe2 heterojunction are equally impressive, covering a broad range from 375 to 1550 nm, with a responsivity of 243 mA/W, a specific detectivity of 6.46 × 1010 Jones, a fill factor of 0.8, and an external quantum efficiency of 56.73%. Finally, simultaneous implementation of linear/circular polarization detection on the PdSe2/2H-MoTe2 heterojunction provides a powerful solution for near-infrared full-Stokes polarization detectors with high integration, miniaturization, and portability.

Photogating-assisted tunneling boosts the responsivity and speed of heterogeneous WSe2/Ta2NiSe5 photodetectors.

Photogating effect is the dominant mechanism of most high-responsivity two-dimensional (2D) material photodetectors. However, the ultrahigh responsivities in those devices are intrinsically at the cost of very slow response speed. In this work, we report a WSe2/Ta2NiSe5 heterostructure detector whose photodetection gain and response speed can be enhanced simultaneously, overcoming the trade-off between responsivity and speed. We reveal that photogating-assisted tunneling synergistically allows photocarrier multiplication and carrier acceleration through tunneling under an electrical field. The photogating effect in our device features low-power consumption (in the order of nW) and shows a dependence on the polarization states of incident light, which can be further tuned by source-drain voltages, allowing for wavelength discrimination with just a two-electrode planar structure. Our findings offer more opportunities for the long-sought next-generation photodetectors with high responsivity, fast speed, polarization detection, and multi-color sensing, simultaneously.

Visible to mid-infrared giant in-plane optical anisotropy in ternary van der Waals crystals.

Birefringence is at the heart of photonic applications. Layered van der Waals materials inherently support considerable out-of-plane birefringence. However, funnelling light into their small nanoscale area parallel to its out-of-plane optical axis remains challenging. Thus far, the lack of large in-plane birefringence has been a major roadblock hindering their applications. Here, we introduce the presence of broadband, low-loss, giant birefringence in a biaxial van der Waals materials Ta2NiS5, spanning an ultrawide-band from visible to mid-infrared wavelengths of 0.3-16 µm. The in-plane birefringence Δn ≈ 2 and 0.5 in the visible and mid-infrared ranges is one of the highest among van der Waals materials known to date. Meanwhile, the real-space propagating waveguide modes in Ta2NiS5 show strong in-plane anisotropy with a long propagation length (>20 µm) in the mid-infrared range. Our work may promote next-generation broadband and ultracompact integrated photonics based on van der Waals materials.

Mechanistic understanding of the interfacial properties of metal-PtSe2 contacts.

With the advantages of a moderate band gap, high carrier mobility and good environmental stability, two-dimensional (2D) semiconductors show promising applications in next-generation electronics. However, the accustomed metal-2D semiconductor contact may lead to a strong Fermi level pinning (FLP) effect, which severely limits the practical performance of 2D electronics. Herein, the interfacial properties of the contacts between a promising 2D semiconductor, PtSe2, and a sequence of metal electrodes are systematically investigated. The strong interfacial interactions formed in all metal-PtSe2 contacts lead to chemical bonds and a significant interfacial dipole, resulting in a vertical Schottky barrier for Ag, Au, Pd and Pt-based systems and a lateral Schottky barrier for Al, Cu, Sc and Ti-based systems, with a strong FLP effect. Remarkably, the tunneling probability for most metal-PtSe2 is significantly high and the tunneling-specific resistivity is two orders of magnitude lower than that of the state-of-the-art contacts, demonstrating the high efficiency for electron injection from metals to PtSe2. Moreover, the introduction of h-BN as a buffer layer leads to a weakened FLP effect (S = 0.50) and the transformation into p-type Schottky contact for Pt-PtSe2 contacts. These results reveal the underlying mechanism of the interfacial properties of metal-PtSe2 contacts, which is useful for designing advanced 2D semiconductor-based electronics.

Strain engineering of the electronic and transport properties of monolayer tellurenyne.

Two-dimensional (2D) materials exhibiting quality electronic properties such as suitable band gap, giant Rashba effect and high carrier mobility are essential for promising applications in electronics and spintronics. Strain engineering has been recognized as an effective strategy to engineer the atomic and electronic properties of 2D materials. Herein, based on density functional theory, we demonstrate that the electronic properties of tellurenyne can be tuned well by using uniaxial strain. We find that tellurenyne retains the unique noncovalent bond structure and exhibits good stability under the uniaxial strain. Meanwhile, the band gap of tellurenyne can be tuned to a large scale (0.33-1.18 eV and 0.73-1.27 eV under the uniaxial strain along and perpendicular to the chain direction, respectively). Under 10% tension strain along the chain direction, the Rashba constant reaches 2.96 eV Å, belonging to giant Rashba systems. More importantly, the hole mobility of tellurenyne along the chain direction reaches 1.1 × 105 cm2 V-1 s-1 under 10% tension strain along the chain direction, which is one order of magnitude larger than that of phosphorene. Therefore, these remarkable electronic properties of tellurenyne engineered by using strain indicate its potential applications in electronics and spintronics.

Monolayer tellurenyne assembled with helical telluryne: structure and transport properties.

Two-dimensional (2D) crystals are candidate materials for electronics and spintronics, but their deficient carrier mobility, inappreciable spin-orbit coupling effect, and environmental instability have such limited applications. Herein, using density functional theory methods, we propose a novel 2D monolayer material, named tellurenyne, built with an atomic tellurium chain (named telluryne) via a noncovalent bond. The comparable electrostatic and van der Waals contributions to interchain binding enable tellurenyne to exhibit remarkable stabilities and transport properties. The carrier mobility of tellurenyne is even higher than phosphorene, with the largest anisotropy among all known systems. Importantly, by changing the phase orders of one-dimensional telluryne, one can switch the preferred carrier type and rotate the dominant direction of carrier transport by 90°. Additionally, tellurenyne is found to exhibit Rashba spin splitting with the coupling parameter of 2.13 eV Å, belonging to the giant Rashba systems. Therefore, this novel 2D material, tellurenyne, is promising for applications in electronics and spintronics.