Search

[Display omitted] Inspired by the great success of ultrathin two-dimensional (2D) layered crystals, more and more attention is being paid to preparing 2D nanostructures from non-layered materials. They can significantly enrich the 2D materials and 2D heterostructures family, extend their application prospects, and bring us distinct properties from their bulk counterparts due to the strong 2D confinement effect. However, the realization of 2D non-layered semiconductors with strong light-harvesting capability and the ability to construct high-performance 2D heterostructures is still a critical challenge. Herein, we successfully synthesized 2D PbSe semiconductors with a large lateral dimension and ultrathin thickness via van der Waals epitaxy. The fabricated 2D PbSe device exhibits good electrical conductivity and superior multi-wavelength photoresponse performance with high responsivity (∼103 A/W) and impressive detectivity (∼2 × 1011 Jones). Furthermore, we demonstrate that 2D PbSe nanosheets can serve as component units for constructing high-performance heterostructure devices. With our strategy, ultrahigh current on/off ratio (∼108) and rectification ratio (∼106), as well as high responsivity (∼3 × 103 A/W) and detectivity (∼7 × 1012 Jones), can be achieved in PbSe/MoS 2 back-gated transistors. These results indicate that 2D PbSe nanosheets and their heterostructures have tremendous applications potential in electrical and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Highlights from the article: Semiconductor heterostructures play a key role in the development of solid-state electronic and optoelectronic devices. However, with any new techniques, the early-stage studies related to heterostructure devices were mainly focused on the proposals of new device concepts. As a result, the device exhibits very large current modulation capacity, allowing us to demonstrate multiple high-performance heterostructure devices.

The interfacial coupling in van der Waals (vdWs) heterostructures can effectively modulate the device performance. In this study, we demonstrate a high performance infrared detector fabricated by direct epitaxial growth of Te nanowires onto a MoS2 monolayer. Such directly grown Te/MoS2 heterostructures show much stronger interfacial coupling than artificially transferred Te/MoS2 hybrids, as evidenced from their Raman and photoluminescence spectra. The strong vdWs interfacial coupling leads to a high performance infrared detector with both ultrahigh photoresponsivity (>103 A/W) and rapid response time (τrising = 15 ms) at the telecommunication wavelength of 1550 nm. The anti-bipolar and rectification behaviors observed in the strongly coupled grown Te-MoS2 heterojunction further confirm the effective interfacial coupling. In contrast, the weakly coupled transferred Te-MoS2 heterojunction that is obtained by artificial transfer exhibits negligible anti-bipolar behavior and slight rectification behavior. These findings indicate that the coupled vdWs hybrid structures have great potential for achieving high performance photodetectors. [ABSTRACT FROM AUTHOR]

Bio‐inspired machine visions have caused wide attentions due to the higher time/power efficiencies over the conventional architectures. Although bio‐mimic photo‐sensors and neuromorphic computing have been individually demonstrated, a complete monolithic vision system has rarely been studied. Here, a neuromorphic machine vision system (NMVS) integrating front‐end retinomorphic sensors and a back‐end convolutional neural network (CNN) based on a single ferroelectric‐semiconductor‐transistor (FST) device structure is reported. As a photo‐sensor, the FST shows a broadband (275–808 nm) retina‐like light adaption function with a large dynamic range of 20.3 stops, and as a unit of the CNN, the FST's weight can be linearly programmed. In total, the NMVS has a high recognition accuracy of 93.0% on a broadband‐dim‐image classification task, which is 20% higher than that of an incomplete system without the retinomorphic sensors. Because of the monolithic unit, the NVMS shows high feasibility for integrated bio‐inspired machine vision systems. [ABSTRACT FROM AUTHOR]

The magnetic anisotropy can stabilize long-range ferromagnetic order in pristine two-dimensional (2D) crystals, which inspires the research of fundamental physics and spintronic devices based on 2D ferromagnetic materials. 2D ferromagnetic materials with intriguing magnetic properties offer an excellent platform for studying magnetism at 2D limit and exploring advanced spintronic devices. As the dimensionality decreases to 2D scale, 2D ferromagnetic materials exhibit distinctive magnetic properties compared with their bulk counterparts. In this Perspective, the recent progress and prospects of 2D ferromagnetic materials in magnetism, manipulation, and device applications are highlighted. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) metal-oxidesemiconductor (CMOS) field-effect-transistors (FETs) with ferroelectric semiconducting α-In 2 Se 3 as the channel can achieve logic, in-memory computing, and optoelectrical logic and non-volatile computing functionalities in a single device. [Display omitted] Exploring materials with multiple properties who can endow a simple device with integrated functionalities has attracted enormous attention in the microelectronic field. One reason is the imperious demand for processors with continuously higher performance and totally new architecture. Combining ferroelectric with semiconducting properties is a promising solution. Here, we show that logic, in-memory computing, and optoelectrical logic and non-volatile computing functionalities can be integrated into a single transistor with ferroelectric semiconducting α-In 2 Se 3 as the channel. Two-input AND, OR, and non-volatile NOR and NAND logic operations with current on/off ratios reaching up to five orders, good endurance (1000 operation cycles), and fast operating speed (10 μs) are realized. In addition, optoelectrical OR logic and non-volatile implication (IMP) operations, as well as ternary-input optoelectrical logic and in-memory computing functions are achieved by introducing light as an additional input signal. Our work highlights the potential of integrating complex logic functions and new-type computing into a simple device based on emerging ferroelectric semiconductors. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) subthermionic field-effect transistors (FETs) with sub-5 nm gate lengths are demonstrated using ferroelectric (FE) van der Waals heterostructures (vdWHs). Overcoming the sub-5 nm gate length limit and decreasing the power dissipation are two main objects in the electronics research field. Besides advanced engineering techniques, considering new material systems may be helpful. Here, we demonstrate two-dimensional (2D) subthermionic field-effect transistors (FETs) with sub-5 nm gate lengths based on ferroelectric (FE) van der Waals heterostructures (vdWHs). The FE vdWHs are composed of graphene, MoS 2 , and CuInP 2 S 6 acting as 2D contacts, channels, and ferroelectric dielectric layers, respectively. We first show that the as-fabricated long-channel device exhibits nearly hysteresis-free subthermionic switching over three orders of magnitude of drain current at room temperature. Further, we fabricate short-channel subthermionic FETs using metallic carbon nanotubes as effective gate terminals. A typical device shows subthermionic switching over five-to-six orders of magnitude of drain current with a minimum subthreshold swing of 6.1 mV/dec at room temperature. Our results indicate that 2D materials system is promising for advanced highly-integrated energy-efficient electronic devices. [ABSTRACT FROM AUTHOR]

Controlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe2/MoS2 field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices. A new application of vdWHs to dynamically control and optimize the electronic and optoelectronic properties of 2D materials is demonstrated. The semivertical MoTe2/MoS2 structure allows for a desirable multifunctional integration of field effect transistors with an on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices exhibit strong capability of suppressing the widely observed trap states–related negative photoresponse effect. [ABSTRACT FROM AUTHOR]

Due to its high carrier mobility, superior air stability, and intriguing self-modulation doping effects, Bi2O2Se has shown great potential for applications in high performance field-effect transistors and infrared photodetectors. However, Bi2O2Se generally tends to form 2D micromorphology because of its native layered structure. Here, we developed a space-confined CVD method to realize the epitaxial growth of highly oriented 1D Bi2O2Se arrays. The controllable length and width of the vertical growth nanowires mainly distribute in the ranges of 30–42 μm and 120–160 nm, respectively. The orientation of Bi2O2Se nanowires was determined by the epitaxial relationship between Bi2O2Se {110} and mica {00n} planes. In addition, various morphologies of Bi2O2Se including 1D nanowires, 2D nanosheets, and rectangular nanosheets can be controllably synthesized by precisely modifying the growth temperature. The integrated detector based on the as-synthesized Bi2O2Se nanowire arrays demonstrated excellent performances in the spectrum from the UV to near-infrared regime. The responsivity, detectivity, external quantum efficiency, and response time can reach up to 722.2 A W−1, 5.64 × 1011 Jones, 189 000%, and 0.267 ms, respectively. These findings manifest that the oriented Bi2O2Se nanowire arrays have great prospects in ultrafast and near-infrared photodetection applications. [ABSTRACT FROM AUTHOR]

Two-dimensional layered materials (2DLMs) have attracted a tremendous amount of attention as photodetectors due to their fascinating features, including high potentials in new-generation electronic devices, wide coverage of bandgaps, ability to construct van der Waals heterostructures, extraordinary light–mass interaction, strong mechanical flexibility, and the capability of enabling synthesis of 2D nonlayered materials. Until now, most attention has been focused on the well-known graphene and transition metal dichalcogenides (TMDs). However, a growing number of functional materials (more than 5619) with novel optoelectronic and electronic properties are being re-discovered, thereby widening the horizon of 2D libraries. In addition to showing common features of 2DLMs, these new 2D members may bring new opportunities to their well-known analogues, like wider bandgap coverage, direct bandgaps independence with thickness, higher mechanical flexibility, and new photoresponse phenomena. The impressive results communicated so far testify that they have shown high potentials with photodetections covering THz, IR, visible, and UV ranges with comparable or even higher performances than well-known TMDs. Here, we give a comprehensive review on the state-of-the-art photodetections of two-dimensional materials beyond graphene and TMDs. The review is organized as follows: fundamentals of photoresponse first are discussed, followed by detailed photodetections of new 2D members including both layered and non-layered ones. After that, photodiodes and hybrid structures based on these new 2D materials are summarized. Then, the integration of these 2D materials with flexible substrates is reviewed. Finally, we conclude with the current research status of this area and offer our perspectives on future developments. We hope that, through reading this manuscript, readers will quickly have a comprehensive view on this research area. [ABSTRACT FROM AUTHOR]

Abstract: As an emerging 2D layered material, Bi2O2Se has shown great potential for applications in thermoelectric and electronics, due to its high carrier mobility, near‐ideal subthreshold swing, and high air‐stability. Although Bi2O2Se has a suitable band gap for infrared (IR) applications, its photoresponse properties have not been investigated. Here, high‐quality ultrathin Bi2O2Se sheets are synthesized via a low‐pressure chemical vapor deposition method. The thickness of 90% Bi2O2Se sheets is below 10 nm and lateral sizes mainly distribute in the range of 7–11 µm. In addition, it is found that triangular sheets largely lack “O” content, even only 0.2 for Bi2O0.2Se. The near‐IR photodetection performance of Bi2O2Se nanosheets is systematically studied by variable temperature measurements. The response time, responsivity, and detectivity can approach up to 2.8 ms, 6.5 A W−1, and 8.3 × 1011 Jones, respectively. Additionally, the critical performance parameters, including responsivity, rising time, and decay time, remain at almost the same level when the temperature is changed from 80 to 300 K. These phenomena are likely due to the fact that as‐grown ultrathin Bi2O2Se sheets have no surface trap states and shallow defect energy levels. The findings indicate ultrathin Bi2O2Se sheets have great potentials for future applications in ultrafast, flexible near‐IR optoelectronic devices. [ABSTRACT FROM AUTHOR]

Abstract: As an emerging 2D layered material, Bi2O2Se has shown great potential for applications in thermoelectric and electronics, due to its high carrier mobility, near‐ideal subthreshold swing, and high air‐stability. Although Bi2O2Se has a suitable band gap for infrared (IR) applications, its photoresponse properties have not been investigated. Here, high‐quality ultrathin Bi2O2Se sheets are synthesized via a low‐pressure chemical vapor deposition method. The thickness of 90% Bi2O2Se sheets is below 10 nm and lateral sizes mainly distribute in the range of 7–11 µm. In addition, it is found that triangular sheets largely lack “O” content, even only 0.2 for Bi2O0.2Se. The near‐IR photodetection performance of Bi2O2Se nanosheets is systematically studied by variable temperature measurements. The response time, responsivity, and detectivity can approach up to 2.8 ms, 6.5 A W−1, and 8.3 × 1011 Jones, respectively. Additionally, the critical performance parameters, including responsivity, rising time, and decay time, remain at almost the same level when the temperature is changed from 80 to 300 K. These phenomena are likely due to the fact that as‐grown ultrathin Bi2O2Se sheets have no surface trap states and shallow defect energy levels. The findings indicate ultrathin Bi2O2Se sheets have great potentials for future applications in ultrafast, flexible near‐IR optoelectronic devices. [ABSTRACT FROM AUTHOR]

Nonvolatile memories based on van der Waals heterostructures have been proved to be promising candidates for next‐generation data storage devices. However, little attention has been focused on the structure with separated floating and control gates (the floating gates and control gates distribute at the different side of the channels), which were recently predicted to be capable of further improving device performance. Here, nonvolatile multibit optoelectronic memories are demonstrated using MoS2, hexagonal boron nitride (h‐BN), and graphene in a top‐floating‐gated structure. With separated top graphene floating gate, the devices show a large memory window (≈95 V) via sweeping gate voltage from 80 to −80 V, a high on/off ratio (≈106) with an ultralow dark current (≈10−14 A), as well as excellent retention characteristic (≈104 s) and cyclic endurance. In addition, these devices can also be erased by a laser illumination with broadband spectrum after being electrically programmed. For the multilevel storage property, 7/6 stages controlled by different electrical operations, and 13/6/3 stages by different laser pulse illuminations are gained. The obtained results show a promising performance for nonvolatile optoelectronic memory using a top‐floating‐gated structure. [ABSTRACT FROM AUTHOR]

Barriers that charge carriers experience while injecting into channels play a crucial role on determining the device properties of van der Waals semiconductors (vdWS). Among various strategies to control these barriers, inserting a graphene layer underneath bulk metal may be a promising choice, which is still lacking experimental verification. Here, it is demonstrated that graphene/metal hybrid structures can form quasi‐van der Waals contacts (q‐vdWC) to ambipolar vdWS, combining the advantages of individual metal and graphene contacts together. A new analysis model is adopted to define the barriers and to extract the barrier heights in ambipolar vdWS. The devices with q‐vdWC show significantly reduced Schottky barrier heights and thermionic field emission activation energies, ability of screening the influence from substrate, and Fermi level unpinning effect. Furthermore, phototransistors with these special contacts exhibit enhanced performances. The proposed graphene/metal q‐vdWC may be an effective strategy to approach the Schottky–Mott limit for vdWS. [ABSTRACT FROM AUTHOR]