Three-Dimensional Optical Anisotropy of Low-Symmetry Layered GeS

Two-Dimensional GeS/SnSe2 Tunneling Photodiode with Bidirectional Photoresponse and High Polarization Sensitivity

A two-dimensional (2D) broken-gap (type-III) p-n heterojunction has a unique charge transport mechanism because of nonoverlapping energy bands. In light of this, type-III band alignment can be used in tunneling field-effect transistors (TFETs) and Esaki diodes with tunable operation and low consumption by highlighting the advantages of tunneling mechanisms. In recent years, 2D tunneling photodiodes have gradually attracted attention for novel optoelectronic performance with a combination of strong light-matter interaction and tunable band alignment. However, an in-depth understanding of the tunneling mechanisms should be further investigated, especially for developing electronic and optoelectronic applications. Here, we report a type-III tunneling photodiode based on a 2D multilayered p-GeS/n+-SnSe2 heterostructure, which is first fabricated by the mechanical exfoliation and dry transfer method. Through the Simmons approximation, its various tunneling transport mechanisms dependent on bias and light are demonstrated as the origin of excellent bidirectional photoresponse performance. Moreover, compared to the traditional p-n photodiode, the device enables bidirectional photoresponse capability, including maximum responsivity values of 43 and 8.7 A/W at Vds = 1 and -1 V, respectively, with distinctive photoactive regions from the scanning photocurrent mapping. Noticeably, benefiting from the in-plane anisotropic structure of GeS, the device exhibits an enhanced photocurrent anisotropic ratio of 9, driven by the broader depletion region at Vds = -3 V under 635 nm irradiation. Above all, the results suggest that our designed architecture can be potentially applied to CMOS imaging sensors and polarization-sensitive photodetectors.

Stability of mechanically exfoliated layered monochalcogenides under ambient conditions

Monochalcogenides of groups III (GaS, GaSe) and VI (GeS, GeSe, SnS, and SnSe) are materials with interesting thickness-dependent characteristics, which have been applied in many areas. However, the stability of layered monochalcogenides (LMs) is a real problem in semiconductor devices that contain these materials. Therefore, it is an important issue that needs to be explored. This article presents a comprehensive study of the degradation mechanism in mechanically exfoliated monochalcogenides in ambient conditions using Raman and photoluminescence spectroscopy supported by structural methods. A higher stability (up to three weeks) was observed for GaS. The most reactive were Se-containing monochalcogenides. Surface protrusions appeared after the ambient exposure of GeSe was detected by scanning electron microscopy. In addition, the degradation of GeS and GeSe flakes was observed in the operando experiment in transmission electron microscopy. Additionally, the amorphization of the material progressed from the flake edges. The reported results and conclusions on the degradation of LMs are useful to understand surface oxidation, air stability, and to fabricate stable devices with monochalcogenides. The results indicate that LMs are more challenging for exfoliation and optical studies than transition metal dichalcogenides such as MoS2, MoSe2, WS2, or WSe2.

Broadband Polarization-Sensitive Photodetection of Magnetic Semiconducting MnTe Nanoribbons

2D materials with low symmetry are explored in recent years because of their anisotropic advantage in polarization-sensitive photodetection. Herein the controllably grown hexagonal magnetic semiconducting α-MnTe nanoribbons are reported with a highly anisotropic (100) surface and their high sensitivity to polarization in a broadband photodetection, whereas the hexagonal structure is highly symmetric. The outstanding photoresponse of α-MnTe nanoribbons occurs in a broadband range from ultraviolet (UV, 360 nm) to near infrared (NIR, 914 nm) with short response times of 46 ms (rise) and 37 ms (fall), excellent environmental stability, and repeatability. Furthermore, due to highly anisotropic (100) surface, the α-MnTe nanoribbons as photodetector exhibit attractive sensitivity to polarization and high dichroic ratios of up to 2.8 under light illumination of UV-to-NIR wavelengths. These results demonstrate that 2D magnetic semiconducting α-MnTe nanoribbons provide a promising platform to design the next-generation polarization-sensitive photodetectors in a broadband range.

Vertical Conductivity and Topography in Electrostrictive Germanium Sulfide Microribbon via Conductive Atomic Force Microscopy

Layered group IV monochalcogenides are two-dimensional (2D) semiconducting materials with unique crystal structures and novel physical properties. Here, we report the growth of single crystalline GeS microribbons using the chemical vapor transport process. By using conductive atomic force microscopy, we demonstrated that the conductive behavior in the vertical direction was mainly affected by the Schottky barriers between GeS and both electrodes. Furthermore, we found that the topographic and current heterogeneities were significantly different with and without illumination. The topographic deformation and current enhancement were also predicted by our density functional theory (DFT)-based calculations. Their local spatial correlation between the topographic height and current was established. By virtue of 2D fast Fourier transform power spectra, we constructed the holistic spatial correlation between the topographic and current heterogeneity that indicated the diminished correlation with illumination. These findings on layered GeS microribbons provide insights into the conductive and topographic behaviors in 2D materials.

Designing SnS/MoS2 van der Waals heterojunction for direct Z-scheme photocatalytic overall water-splitting by DFT investigation

Construction of direct Z-scheme photocatalytic heterojunctions with an internal electric field has been proposed as an outstanding method to achieve efficient utilization of solar energy for photocatalytic overall water-splitting. In this work, the properties of van der Waals (vdW) heterojunctions formed by group-IV mono-chalcogenides (MXs) (M = Ge, Sn; X = S, Se, Te) and MoS₂ are systematically studied by first-principles calculations, including the vdW binding energy, the direction of an internal electric field and the electronic structure. The results predict that GeS/MoS₂, GeSe/MoS₂ and SnS/MoS₂ vdW heterojunctions are potential direct Z-scheme water-splitting photocatalysts with appropriate band alignments, a wide light absorption range and low effective charge-carrier mass. Furthermore, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) activities of the heterojunctions as photocatalysts are predicted. The results indicate that SnS/MoS₂ with the Sn vacancy has a low Gibbs free energy of the HER (0.06 eV), and MoS₂ with the S edge can offer OER active sites. This study provides a theoretical basis for the further design and preparation of a new two-dimensional overall water-splitting photocatalyst, which is conducive to the development of efficient two-dimensional photocatalysts in the field of clean energy.

In-plane ferroelectricity in few-layered GeS and its van der Waals ferroelectric diodes

Two-dimensional ferroelectric semiconductors (2DFeSs) have been attracting extensive research attention on account of their unique properties and versatile applications in random-access memory, digital signal processors, and neuromorphic computing. Germanium sulfide (GeS) is predicted as a typical 2DFeS with a large spontaneous polarization of 484 pC m^-1. Furthermore, the moderate band gap equivalent to 1.63 eV of GeS provides it with significant potential to create a strong bulk photovoltage in the visible light range. However, the fabrication of chemically stable few-to-monolayer GeS has not been reported so far, owing to the strong interlayer force and high chemical reactivity of the surface. Herein we demonstrate a new method for fabricating high quality, air-stable, ultrathin GeS nanoflakes. The electrical characterization confirms the formation of few-layered GeS with a remarkable in-plane ferroelectric hysteresis, which is forbidden by the inversion symmetry in bulk GeS crystals. After applying a coercive field of about 18.1 kV cm^-1, a switchable shift current can also be observed in the polarized GeS nanoflakes under light irradiation. To further enhance the photoresponsivity, few-layered InSe was transferred onto the GeS nanoflakes to form van der Waals ferroelectric diodes. The interfacial perturbation breaking the inversion symmetry results in the enhancement of robust dipoles in the GeS side along the interface, which can be tuned by the in-plane electric field. Overall, this work opens the door for exploring the low-dimensional ferroelectric memory and energy conversion applications based on 2D GeS nanoflakes and provides a deeper understanding of the photovoltaic mechanism with in-plane 2D ferroelectric diodes.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

In-plane anisotropic 2D Ge-based binary materials for optoelectronic applications

In-plane anisotropic two-dimensional (2D) materials possess unique in-plane anisotropic physical properties arising from their low crystal lattice symmetry.

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.

Enhanced optical anisotropy via dimensional control in alkali-metal chalcogenides

Crystals with both large birefringence and wide transparent range are suitable for broad applications in the areas of optical communications, the laser industry and modulation of the light polarization requirement. In this work, to assist the design of urgently needed crystals with large birefringence in the infrared (IR) region, typical alkali-metal chalcogenides, KPSe6, Na2Ge2Se5, and Li2In2GeSe6 have been studied. They exhibit a hierarchical characteristic in the calculated birefringence by about 0.21, 0.11, and 0.04, respectively. To explore the origin of the birefringence difference, the polarizability anisotropy and the effect of electron distribution anisotropy are analyzed. The alkali-metal chalcogenides KPSe6, Na2Ge2Se5, and Li2In2GeSe6 feature infinite one-dimensional (1D) chains of [PSe6], 2D anionic framework of [Ge2Se5] layers and 3D [In2GeSe9] networks, respectively. It is found that the anionic group with low-dimensional configuration could enhance polarizability anisotropy and render large birefringence for the macroscopic structure. This provides evidence that a low-dimensionality configuration in the structure would be beneficial for the enhancement of optical anisotropy, which can motivate the exploration and design of novel IR birefringent materials.

In-plane anisotropic electronics based on low-symmetry 2D materials: progress and prospects

Low-symmetry layered materials such as black phosphorus (BP) have been revived recently due to their high intrinsic mobility and in-plane anisotropic properties, which can be used in anisotropic electronic and optoelectronic devices. Since the anisotropic properties have a close relationship with their anisotropic structural characters, especially for materials with low-symmetry, exploring new low-symmetry layered materials and investigating their anisotropic properties have inspired numerous research efforts. In this paper, we review the recent experimental progresses on low-symmetry layered materials and their corresponding anisotropic electrical transport, magneto-transport, optoelectronic, thermoelectric, ferroelectric, and piezoelectric properties. The boom of new low-symmetry layered materials with high anisotropy could open up considerable possibilities for next-generation anisotropic multifunctional electronic devices.

Effective Hexagonal Boron Nitride Passivation of Few-Layered InSe and GaSe to Enhance Their Electronic and Optical Properties

Indium selenide (InSe) and gallium selenide (GaSe), members of the III-VI chalcogenide family, are emerging two-dimensional (2D) semiconductors with appealing electronic properties. However, their devices are still lagging behind because of their sensitivity to air and device fabrication processes which induce structural damage and hamper their intrinsic properties. Thus, in order to obtain high-performance and stable devices, effective passivation of these air-sensitive materials is strongly required. Here, we demonstrate a hexagonal boron nitride (hBN)-based encapsulation technique, where 2D layers of InSe and GaSe are covered entirely between two layers of hBN. To fabricate devices out of fully encapsulated 2D layers, we employ the lithography-free via-contacting scheme. We find that hBN acts as an excellent encapsulant and a near-ideal substrate for InSe and GaSe by passivating them from the environment and isolating them from the charge disorder at the SiO₂ surface. As a result, the encapsulated InSe devices are of high quality and ambient-stable for a long time and show an improved two-terminal mobility of 30-120 cm² V^-1 s^-1 as compared to mere ∼1 cm² V^-1 s^-1 for unencapsulated devices. On employing this technique to GaSe, we obtain a strong and reproducible photoresponse. In contrast to previous studies, where either good performance or long-term stability was achieved, we demonstrate a combination of both in our devices. This work thus provides a systematic study of fully encapsulated devices based on InSe and GaSe, which has not been reported until now. We believe that this technique can open ways for fundamental studies as well as toward the integration of these materials in technological applications.