Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p-n Diodes

Polarization-Resolved p-Se/n-WS2 Heterojunctions toward Application in Microcomputer System as Multivalued Signal Trigger

Polarization-sensitive photodetectors based on van der Waals heterojunctions (vdWH) have excellent polarization-resolved optoelectronic properties that can enable the applications in polarized light identification and imaging. With the development of optical microcomputer control systems (OMCS), it is crucial and energy efficient to adopt the self-powered and polarization-resolved signal-generators to optimize the circuit design of OMCS. In this work, the selenium (Se) flakes with in-plane anisotropy and p-type character are grown and incorporated with n-type tungsten disulfide (WS₂ ) to construct the type-II vdWH for polarization-sensitive and self-powered photodetectors. Under 405 nm monochrome laser with 1.33 mW cm^-2 power density, the photovoltaic device exhibits superior photodetection performance with the photoelectric conversion efficiency (PCE) of 3.6%, the responsivity (R) of 196 mA W^-1 and the external quantum efficiency (EQE) of about 60%. The strong in-plane anisotropy of Se crystal structure gives rise to the capability of polarized light detection with anisotropic photocurrent ratio of ≈2.2 under the 405 nm laser (13.71 mW cm^-2 ). Benefiting from the well polarization-sensitive and photovoltaic properties, the p-Se/n-WS₂ vdWH is successfully applied in the OMCS as multivalued signal trigger. This work develops the new anisotropic vdWH and demonstrates its feasibility for applications in logic circuits and control systems.

Alternative Lone-Pair ns2 -Cation-Based Semiconductors beyond Lead Halide Perovskites for Optoelectronic Applications

Lead halide perovskites have emerged in the last decade as advantageous high-performance optoelectronic semiconductors, and have undergone rapid development for diverse applications such as solar cells, light-emitting diodes , and photodetectors. While material instability and lead toxicity are still major concerns hindering their commercialization, they offer promising prospects and design principles for developing promising optoelectronic materials. The distinguished optoelectronic properties of lead halide perovskites stem from the Pb²⁺ cation with a lone-pair 6s² electronic configuration embedded in a mixed covalent-ionic bonding lattice. Herein, we summarize alternative Pb-free semiconductors containing lone-pair ns² cations, intending to offer insights for developing potential optoelectronic materials other than lead halide perovskites. We start with the physical underpinning of how the ns² cations within the material lattice allow for superior optoelectronic properties. We then review the emerging Pb-free semiconductors containing ns² cations in terms of structural dimensionality, which is crucial for optoelectronic performance. For each category of materials, the research progresses on crystal structures, electronic/optical properties, device applications, and recent efforts for performance enhancements are overviewed. Finally, the issues hindering the further developments of studied materials are surveyed along with possible strategies to overcome them, which also provides an outlook on the future research in this field.

Strong valley splitting ind0two-dimensional SnO induced by magnetic proximity effect

Strong magnetic interfacial coupling in van der Waals heterostructures is important for designing novel electronic devices. Besides the most studied transition metal dichalcogenides (TMDCs) materials, we demonstrate that the valley splitting can be activated in two-dimensional tetragonald⁰metal oxide, SnO, via the magnetic proximity effect by EuBrO. In SnO/EuBrO, the valley splitting of SnO can reach ∼46 meV, which is comparable to many TMDCs and equivalent to an external magnetic field of 800 T. In addition, the valley splitting can be further enhanced by adjusting interlayer distance and applying uniaxial strains. A design principle of new spintronic device based on this unique electronic structure of SnO/EuBrO has been proposed. Our findings indicate that SnO is a promising material for future valleytronics applications.

Reversible Half Wave Rectifier Based on 2D InSe/GeSe Heterostructure with Near-Broken Band Alignment

2D van der Waals heterostructures (vdWHs) offer tremendous opportunities in designing multifunctional electronic devices. Due to the ultrathin nature of 2D materials, the gate-induced change in charge density makes amplitude control possible, creating a new programmable unilateral rectifier. The study of 2D vdWHs-based reversible unilateral rectifier is lacking, although it can give rise to a new degree of freedom for modulating the output state. Here, a InSe/GeSe vdWH-FET is constructed as a gate-controllable half wave rectifier. The device exhibits stepless adjustment from forward to backward rectifying performance, leading to multiple operation states of output level. Near-broken band alignment in the InSe/GeSe vdWH-FET is a crucial feature for high-performance reversible rectifier, which is shown to have backward and forward rectification ratio of 1:38 and 963:1, respectively. Being further explored as a new bridge rectifier, the InSe/GeSe device has great potential in future gate-controllable alternating current/direct current convertor. These results indicate that 2D vdWHs with near-broken band alignment can offer a pathway to simplify the commutating circuit and regulating speed circuit.

Recent Advances in 2D Metal Monochalcogenides

The family of emerging low-symmetry and structural in-plane anisotropic two-dimensional (2D) materials has been expanding rapidly in recent years. As an important emerging anisotropic 2D material, the black phosphorene analog group IVA-VI metal monochalcogenides (MMCs) have been surged recently due to their distinctive crystalline symmetries, exotic in-plane anisotropic electronic and optical response, earth abundance, and environmentally friendly characteristics. In this article, the recent research advancements in the field of anisotropic 2D MMCs are reviewed. At first, the unique wavy crystal structures together with the optical and electronic properties of such materials are discussed. The Review continues with the various methods adopted for the synthesis of layered MMCs including micromechanical and liquid phase exfoliation as well as physical vapor deposition. The last part of the article focuses on the application of the structural anisotropic response of 2D MMCs in field effect transistors, photovoltaic cells nonlinear optics, and valleytronic devices. Besides presenting the significant research in the field of this emerging class of 2D materials, this Review also delineates the existing limitations and discusses emerging possibilities and future prospects.

Liquid-Phase Exfoliated GeSe Nanoflakes for Photoelectrochemical-Type Photodetectors and Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) systems represent powerful tools to convert electromagnetic radiation into chemical fuels and electricity. In this context, two-dimensional (2D) materials are attracting enormous interest as potential advanced photo(electro)catalysts and, recently, 2D group-IVA metal monochalcogenides have been theoretically predicted to be water splitting photocatalysts. In this work, we use density functional theory calculations to theoretically investigate the photocatalytic activity of single-/few-layer GeSe nanoflakes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in pH conditions ranging from 0 to 14. Our simulations show that GeSe nanoflakes with different thickness can be mixed in the form of nanoporous films to act as nanoscale tandem systems, in which the flakes, depending on their thickness, can operate as HER- and/or OER photocatalysts. On the basis of theoretical predictions, we report the first experimental characterization of the photo(electro)catalytic activity of single-/few-layer GeSe flakes in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H2SO4 (pH 0.3), 1 M KCl (pH 6.5), and 1 M KOH (pH 14). The films of the GeSe nanoflakes are fabricated by spray coating GeSe nanoflakes dispersion in 2-propanol obtained through liquid-phase exfoliation of synthesized orthorhombic (Pnma) GeSe bulk crystals. The PEC properties of the GeSe nanoflakes are used to design PEC-type photodetectors, reaching a responsivity of up to 0.32 AW-1 (external quantum efficiency of 86.3%) under 455 nm excitation wavelength in acidic electrolyte. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching that of self-powered commercial UV-Vis photodetectors. The obtained results inspire the use of 2D GeSe in proof-of-concept water photoelectrolysis cells.

Negative differential transconductance device with a stepped gate dielectric for multi-valued logic circuits

Multi-valued logic (MVL) technology is a promising approach for improving the data-handling capabilities and decreasing the power consumption of integrated circuits. This is especially attractive as conventional complementary metal-oxide-semiconductor technology is approaching its scaling and power density limits. Here, an ambipolar WSe₂ field-effect transistor with two or more negative-differential-transconductance (NDT) regions in its transfer characteristic (NDTFET) is proposed for MVL applications of various radices. The operation and charge carrier transport mechanism of the NDTFET are studied first by Kelvin probe force microscopy, electrical, and capacitance-voltage measurements. Next, strategies for increasing the number of NDT regions and engineering the NDTFET transfer characteristic are discussed. Finally, the extensibility and tunability of our concept are demonstrated by adapting NDTFETs as core devices for ternary, quaternary, and quinary MVL inverters through simulations, where only WSe₂ is employed as a channel material for all devices comprising the inverters. The MVL inverter operation principle and the mechanism of the multiple logic state formation are analyzed in detail. The proposed concept is practically verified by the fabrication of a ternary inverter.

Breaking symmetry in device design for self-driven 2D material based photodetectors

The advent of graphene and other two-dimensional (2D) materials offers great potential for optoelectronic applications. Various device structures and novel mechanisms have been proposed to realize photodetectors with unique detecting properties. In this minireview, we focus on a self-driven photodetector that has great potential for low-power or even powerless operation required in the internet of things and wearable electronics. To address the general principle of self-driven properties, we propose and elaborate the concept of symmetry breaking in 2D material based self-driven photodetectors. We discuss various mechanisms of breaking symmetry for self-driven photodetectors, including asymmetrical contact engineering, field-induced asymmetry, PN homojunctions, and PN heterostructures. Typical device examples based on these mechanisms are reviewed and compared. The performance of current self-driven photodetectors is critically assessed and future directions are discussed towards the target application fields.

A promising blue phosphorene/C2N van der Waals type-II heterojunction as a solar photocatalyst: a first-principles study

An appropriate band structure and effective carrier separation are very important for the performance of a solar photocatalyst. In this paper, based on first-principles calculations, it was predicted that blue phosphorene (BlueP) and a C2N monolayer can form a promising metal-free type-II heterojunction. The electronic structure of the BlueP/C2N heterojunction facilitated the overall water splitting reactions well. The projected band structure showed that the conduction band edge was contributed by C2N and the valence band edge was dominated by BlueP. Under the combination of the driving force of the band offset and the built-in electric field between the two layers, the photo-generated electrons and holes were transferred spontaneously to the conduction band of C2N and the valence band of BlueP, respectively. An effective carrier separation in the heterostructure was thus achieved. More notably, the obtained light absorption of the BlueP/C2N junction showed an obvious red-shift, which greatly extended the area of light adsorption to the visible light region. We further proposed that strain could also be used to modulate the band gap and the band edge positions of the heterojunction. Our results not only provide a theoretical design, but also reveal the fundamental separation mechanism of the photo-generated carriers in the BlueP/C2N heterojunction.

Chemically Tuned p- and n-Type WSe2 Monolayers with High Carrier Mobility for Advanced Electronics

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post-silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom-thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p- and n-type semiconductors is essential for various device applications, such as complementary metal-oxide-semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe₂ is demonstrated by chemical doping. Two different molecules, 4-nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe₂ field-effect transistors (FETs) to p- and n-type, respectively. Moreover, the chemically doped WSe₂ show increased effective carrier mobilities of 82 and 25 cm² V^-1 s^-1 for holes and electrons, respectively, which are much higher than those of the pristine WSe₂ . The doping effects are studied by photoluminescence, Raman, X-ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe₂ FETs are integrated into CMOS inverters, exhibiting extremely low power consumption (≈0.17 nW). Furthermore, a p-n junction within single WSe₂ grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe₂ will contribute to the development of TMDC-based advanced electronics.

High photoresponsivity and broadband photodetection with a band-engineered WSe2/SnSe2 heterostructure

van der Waals (vdW) heterostructures formed by stacking different two-dimensional layered materials have been demonstrated as a promising platform for next-generation photonic and optoelectronic devices due to their tailorable band-engineering properties. Here, we report a high photoresponsivity and broadband photodetector based on a WSe2/SnSe2 heterostructure. By properly biasing the heterostructure, its band structure changes from near-broken band alignment to type-III band alignment which enables high photoresponsivity from visible to telecommunication wavelengths. The highest photoresponsivity and detectivity at 532 nm are ∼588 A W-1 and 4.4 × 1010 Jones and those at 1550 nm are ∼80 A W-1 and 1.4 × 1010 Jones, which are superior to those of the current state-of-the-art layered transition metal dichalcogenides based photodetectors under similar measurement conditions. Our work not only provides a new method for designing high-performance broadband photodetectors but also enables a deep understanding of the band engineering technology in the vdW heterostructures possible for other applications, such as modulators and lasers.

Effects of stacking method and strain on the electronic properties of the few-layer group-IVA monochalcogenide heterojunctions

Group-IV monochalcogenides (GeSe, SnSe, GeS, SnS) are a class of promising monolayer materials for nanoelectronic applications. However, the GeSe monolayer is the only direct semiconductor in the group-IV monochalcogenides, which limits their application in nanoelectronic fields. Stacking is usually a good strategy to design two-dimensional (2D) materials with novel properties. Taking these monolayer monochalcogenides as basic building blocks, various van der Waals (vdW) heterojunctions can be constructed by different stacking methods. In this study, we systematically investigated the structures, stabilities and electronic properties of thirty-six few-layer group-IV monochalcogenide heterojunctions. All the vdW heterojunctions are proved to be stable. The degree of stability of the few-layer heterojunctions is found to increase with the number of layers. The band gap values of heterojunctions are dependent not only on the components, but also on the stacking order. Five novel 2D direct semiconductors (SnSe/GeSe, GeS/SnS, SnSe/GeSe/SnSe, SnS/GeSe/SnSe and SnS/GeSe/SnSe) are obtained. It's found that biaxial strain can not only tune the values of band gap, but also change the type of the 2D materials. The band gaps of the heterojunctions monotonically increase with the increasing strain and most few-layer heterojunctions transform between direct and indirect semiconductors under biaxial strain. Five heterojunctions (SnSe/GeSe, GeS/SnS, GeSe/SnSe/SnS, SnS/GeSe/SnSe and GeSe/SnS/GeS/SnSe) are found to remain as direct semiconductors under tensile strain (0–0.1). Since the band gaps of these heterojunctions are easy to control in a suitable range, they may have potential applications in nanoelectronic fields.

We studied the effect of stacking method and biaxial strain on the electronic properties of the few-layer group-IV monochalcogenides heterojunction.