van der Waals epitaxial two-dimensional CdSxSe(1-x) semiconductor alloys with tunable-composition and application to flexible optoelectronics

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

High performance photodetector based on CdS/CdS0.42Se0.58nanobelts heterojunction

The ternary alloy CdSxSe1-xcombines the physical properties of CdS and CdSe, and its band gap can be adjusted by changing the element composition. The alloy has charming photoelectric properties as well as potential application value in photoelectric devices. In this work, the CdS/CdS0.42Se0.58nanobelt (NB) heterojunction device was prepared by chemical vapor deposition combined with a typical dry transfer technique. The heterojunction photodetector shows high light switching ratio of 6.79 × 104, large spectral responsivity of 1260 A W-1, high external quantum efficiency of 2.66 × 105% and large detectivity of 7.19 × 1015cm Hz1/2W-1under 590 nm illumination and 3 V bias. Its rise and decay time is about 45/90μs. The performance of the heterojunction photodetector was comparable or even better than that of other CdS(Se) based photodetector device. The results indicate that the CdS/CdS0.42Se0.58NB heterojunction possesses a promising potential application in high performance photodetectors.

Controlled Growth and Size-Dependent Magnetic Domain States of 2D γ-Fe2O3

Nonlayered two-dimensional (2D) magnets have attracted special attention, as many of them possess magnetic order above room temperature and enhanced chemical stability compared to most existing vdW magnets, which offers remarkable opportunities for developing compact spintronic devices. However, the growth of these materials is quite challenging due to the inherent three-dimensionally bonded nature, which hampers the study of their magnetism. Here, we demonstrate the controllable growth of air-stable pure γ-Fe2O3 nanoflakes by a confined-vdW epitaxial approach. The lateral size of the nanoflakes could be adjusted from hundreds of nanometers to tens of micrometers by precisely controlling the annealing time. Interestingly, a lateral-size-dependent magnetic domain configuration was observed. As the sizes continuously increase, the magnetic domain evolves from single domain to vortex and finally to multidomain. This work provides guidance for the controllable synthesis of 2D inverse spinel-type crystals and expands the range of magnetic vortex materials into magnetic semiconductors.

Epitaxial van der Waals contacts of 2D TaSe2-WSe2 metal-semiconductor heterostructures

The electronic contact between two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors and metal electrodes is a formidable challenge due to the undesired Schottky barrier, which severely limits the electrical performance of TMD devices and impedes the exploration of their unconventional physical properties and potential electronic applications. In this study, we report a two-step chemical vapor deposition (CVD) growth of 2D TaSe2-WSe2 metal-semiconductor heterostructures. Raman mapping confirms the precise spatial modulation of the as-grown 2D TaSe2-WSe2 heterostructures. Transmission electron microscopy (TEM) characterization reveals that this two-step method provides a high-quality and clean interface of the 2D TaSe2-WSe2 heterostructures. Meanwhile, the upper 1T-TaSe2 is formed heteroepitaxially on/around the pre-synthesized 2H-WSe2 monolayers, exhibiting an epitaxial relationship of (20-20)TaSe2//(20-20)WSe2 and [0001]TaSe2//[0001]WSe2. Furthermore, characterization studies using a Kelvin probe force microscope (KPFM) and electrical transport measurements present compelling evidence that the 2D metal-semiconductor heterostructures under investigation can improve the performance of electrical devices. These results bear substantial significance in augmenting the properties of field-effect transistors (FETs), leading to notable improvements in FET mobility and on/off ratio. Our study not only broadens the horizons of direct growth of high-quality 2D metal-semiconductor heterostructures but also sheds light on potential applications in future high-performance integrated circuits.

The Photodetectors Based on Lateral Monolayer MoS2/WS2 Heterojunctions

Monolayer transition metal dichalcogenides (TMDs) show promising potential for next-generation optoelectronics due to excellent light capturing and photodetection capabilities. Photodetectors, as important components of sensing, imaging and communication systems, are able to perceive and convert optical signals to electrical signals. Herein, the large-area and high-quality lateral monolayer MoS2/WS2 heterojunctions were synthesized via the one-step liquid-phase chemical vapor deposition approach. Systematic characterization measurements have verified good uniformity and sharp interfaces of the channel materials. As a result, the photodetectors enhanced by the photogating effect can deliver competitive performance, including responsivity of ~ 567.6 A/W and detectivity of ~ 7.17 × 1011 Jones. In addition, the 1/f noise obtained from the current power spectrum is not conductive to the development of photodetectors, which is considered as originating from charge carrier trapping/detrapping. Therefore, this work may contribute to efficient optoelectronic devices based on lateral monolayer TMD heterostructures.

Strain Engineering in 2D Material-Based Flexible Optoelectronics

Flexible optoelectronics, as promising components hold shape-adaptive features and dynamic strain response under strain engineering for various intelligent applications. 2D materials with atomically thin layers are ideal for flexible optoelectronics because of their high flexibility and strain sensitivity. However, how the strain affects the performance of 2D materials-based flexible optoelectronics is confused due to their hypersensitive features to external strain changes. It is necessary to establish an evaluation system to comprehend the influence of the external strain on the intrinsic properties of 2D materials and the photoresponse performance of their flexible optoelectronics. Here, a focused review of strain engineering in 2D materials-based flexible optoelectronics is provided. The first attention is on the mechanical properties and the strain-engineered electronic properties of 2D semiconductors. An evaluation system with relatively comprehensive parameters in functionality and service capability is summarized to develop 2D materials-based flexible optoelectronics in practical application. Based on the parameters, some strategies to improve the functionality and service capability are proposed. Finally, combining with strain engineering in future intelligence devices, the challenges and future perspective developing 2D materials-based flexible optoelectronics are expounded.

2D material broadband photodetectors

Our review provides a comprehensive overview of the latest evolution of broadband photodetectors (BBPDs) based on 2D materials (2DMs). We begin with BBPDs built on various 2DM channels, including narrow-bandgap 2DMs, 2D topological semimetals, 2D charge density wave compounds, and 2D heterojunctions. Then, we introduce defect-engineered 2DM BBPDs, including vacancy engineering, heteroatom incorporation, and interfacial engineering. Subsequently, we summarize 2DM based mixed-dimensional (0D-2D, 1D-2D, 2D-3D, and 0D-2D-3D) BBPDs. Finally, we provide several viewpoints for the future development of this burgeoning field.

PbS Quantum Dots/2D Nonlayered CdS xSe1- x Nanosheet Hybrid Nanostructure for High-Performance Broadband Photodetectors

Two-dimensional (2D) nonlayered nanomaterials have attracted extensive attention for electronic and optoelectronic applications recently because of their distinct properties. In this work, we first employed a facile one-step method to synthesize 2D nonlayered cadmium sulfide selenide (CdS _xSe_{1- x}, x = 0.33) nanosheets with a highly crystalline structure and then we introduced a generic spin-coating approach to fabricate hybrid nanomaterials composed of PbS quantum dots (QDs) and 2D CdS _xSe_{1- x} nanosheets and demonstrated their potential for high-performance broadband photodetectors. Compared with pure 2D CdS _xSe_{1- x} nanosheet photodetectors, the photoelectric performance of the PbS/CdS _xSe_{1- x} hybrid nanostructure is enhanced by 3 orders of magnitude under near-infrared (NIR) light illumination and maintains its performance in the visible (Vis) range. The photodetector exhibits a broadband response range from Vis to NIR with an ultrahigh light-to-dark current ratio (3.45 × 10⁶), a high spectral responsivity (1.45 × 10³ A/W), and high detectivity (1.05 × 10¹⁵ Jones). The proposed QDs/2D nonlayered hybrid nanostructure-based photodetector paves a promising way for next-generation high-performance broadband optoelectronic devices.

2D library beyond graphene and transition metal dichalcogenides: a focus on photodetection

Two-dimensional materials beyond graphene and TMDs can be promising candidates for wide-spectra photodetection.