Defect-Type-Dependent Carrier Lifetimes in Monolayer WS2 Films

Two-dimensional molybdenum disulfide (MoS2), featuring unique optoelectronic properties, has attracted tremendous interest in developing novel photodetection devices. However, the limited light absorption and small carrier transport rate of the monolayer MoS2 result in low photoresponse, and the large band gap limits its detection range in the visible region. In this study, we propose a nanoslit array-MoS2 hybrid device architecture with enhanced and broadened photoresponse. The nanoslit array can localize free-space light to achieve strong interactions with MoS2, and acts as the channel to improve charge transport. As a result, the Au nanoslit array-MoS2 hybrid detector exhibits a nearly 100-fold increase in photocurrent compared to the pure MoS2 device. More importantly, the hybrid device can broaden the photoresponse to the optical communication band of 1550 nm which is lower than the band gap of MoS2, by efficiently utilizing the hot carriers generated by the Au nanoslits. The experimental results are supported by both theoretical analysis and numerical simulation. Since our demonstration leverages the engineering of the hybrid photodetectors with metal nanostructures rather than semiconductor materials, it should be universal and applicable to other devices for broadband, high-efficiency photoelectric conversion.

Enhancing and Broadening the Photoresponse of Monolayer MoS2 Based on Au Nanoslit Array.

We report on tungsten disulfide (WS2) flakes grown by chemical vapor deposition (CVD), which exhibit a flower-like surface structure above the primary few-layer flake with a triangular shape. The fine structure is only revealed in the mechanical, chemical, and electronic properties of the flake but not in the topography. The origin of this structure is the peculiar one-step growth during the CVD process that permits to control the sulfur concentration at any time. A high concentration of S at the onset of the deposition process leads to a rapid growth of the flake, resulting in tungsten vacancies. Reducing the sulfur concentration toward the end of the growth slows down the reaction and leads to sulfur vacancies. These microscale domains were studied by confocal- and tip-enhanced Raman spectroscopy revealing their chemical composition with high spatial resolution. A strong quenching of the photoluminescence in the tungsten-vacancy domains is observed. Atomic force microscope measurements, performed in intermittent contact mode, force modulation mode (including lateral force mode), and PeakForce quantitative nanomechanics mode, show that the mechanical properties of these domains differ. Within the tungsten-vacancy domains, the adhesion force is reduced, while the friction force increased. Kelvin probe force microscopy measurements show that the electronic properties of the flakes are modulated by these domains. The combined nanomechanical and nanospectroscopy measurements provide detailed insights on the inhomogeneous surface properties of the single WS2 flake, further highlighting how its multidomain properties can be finely tuned using CVD.

The Hidden Flower in WS2 Flakes: A Combined Nanomechanical and Tip-Enhanced Raman Exploration.

The discovery of novel large band gap two-dimensional (2D) materials with good stability and high carrier mobility will innovate the next generation of electronics and optoelectronics. A new allotrope of 2D violet phosphorus P11 was synthesized via a salt flux method in the presence of bismuth. Millimeter-sized crystals of violet-P11 were collected after removing the salt flux with DI water. From single-crystal X-ray diffraction, the crystal structure of violet-P11 was determined to be in the monoclinic space group C2/c (no. 15) with unit cell parameters of a = 9.166(6) Å, b = 9.121(6) Å, c = 21.803(14)Å, β = 97.638(17)°, and a unit cell volume of 1807(2) Å3. The structure differences between violet-P11, violet-P21, and fibrous-P21 are discussed. The violet-P11 crystals can be mechanically exfoliated down to a few layers (∼6 nm). Photoluminescence and Raman measurements reveal the thickness-dependent nature of violet-P11, and exfoliated violet-P11 flakes were stable in ambient air for at least 1 h, exhibiting moderate ambient stability. The bulk violet-P11 crystals exhibit excellent stability, being stable in ambient air for many days. The optical band gap of violet-P11 bulk crystals is 2.0(1) eV measured by UV-Vis and electron energy-loss spectroscopy measurements, in agreement with density functional theory calculations which predict that violet-P11 is a direct band gap semiconductor with band gaps of 1.8 and 1.9 eV for bulk and monolayer, respectively, and with a high carrier mobility. This band gap is the largest among the known single-element 2D layered bulk crystals and thus attractive for various optoelectronic devices.

Two-Dimensional Violet Phosphorus P11: A Large Band Gap Phosphorus Allotrope.

Defect repairing in two-dimensional transition metal dichalcogenides

Massive data processing with high computing efficiency and low operating power is required owing to the rapid development of artificial intelligence and information technology. However, the von Neumann structure computing system with the separated memory and processor can cause large energy consumption and a low running speed during massive data processing. Therefore, the brain‐inspired neuromorphic computing system is developed, that can provide hardware support for emulating biological synaptic functions and realizing highly intensive data processing with low power consumption. As a neuromorphic device, the optoelectronic synaptic device (OSD) is regarded as an ideal device to replace the von Neumann‐based computer because of its ultrafast signal transmission, large bandwidth, low energy consumption, and wireless communication. Owing to their unique optoelectronic property, metal halide perovskites (MHPs) have received growing attention as effective photosensitive materials in OSDs. Therefore, the review introduces the recent progress on OSDs based on MHPs (MHPs‐OSDs) including the structures and properties of MHPs, and the architectures and performance characteristics of MHPs‐OSDs. Furthermore, applications of MHPs‐OSDs are presented. Finally, the outlook and opportunity of MHPs‐OSDs are discussed.

Recent Developments of Optoelectronic Synaptic Devices Based on Metal Halide Perovskites

On-Chip Detection of Multiwavelength Surface Plasmon Polaritons Based on Plasmonic Demultiplexers

Monolayer transition‐metal dichalcogenides grown by chemical vapor deposition (CVD) always contain certain types of defects that dramatically affect their electronic and optical properties. For CVD‐grown hexagonal WS2 monolayer, complex photoluminescence (PL) patterns are commonly observed, but the defect‐related optical mechanisms are still not well understood. Here, by combining the optical and structural characterizations and ab initio calculations, the correlation between the patterned PL emission and the details of defects in CVD‐grown hexagonal WS2 monolayer are revealed. The temperature‐dependent PL spectra show the correlation between the defect‐trapped and band‐edge exciton emission. The high‐resolution scanning transmission electron microscopy identifies the positive correlation between the density of WSx‐vacancy and PL intensity. In the end, the ab initio calculations and molecule adsorption PL spectra show that the coexistence of p‐ and n‐doping effects, caused by the W and S complex vacancy, weakens the modulation of molecular adsorption on PL intensity. This work gives new insights into the origin of the inhomogeneous PL distribution in WS2 monolayer, which provides important guidance in the regulation of electronic and optical properties of transition‐metal dichalcogenides via defect engineering.

Revealing the Competition between Defect‐Trapped Exciton and Band‐Edge Exciton Photoluminescence in Monolayer Hexagonal WS2

Plasmonic demultiplexers hold promise for the realization of the subwavelength and high-splitting ratio dichroic splitter and have a wide range of applications from optical communication, and manipulation to ultrafast data treatment. However, this vision has not been realized for a long time due to lacking the suitable splitting structure design, which limits its further development of integrated photonic circuits. Here, we demonstrate a plasmonic demultiplexer with subwavelength feature size (0.54 µm) and broadband spectral (620-870 nm) range, and high-splitting ratio (17 dB in experiments and 20 dB in calculations). It consists of two adjacent Fabry-Perot cavities (covered by PMMA polymer) and coupling gratings, which are integrated with the Au waveguide. The relatively simple double cavities design of our device has a simple theoretical analysis and fabrication process. Our work has relevance for various optical applications, such as multiple wavelength photodetectors and optical multichannel interconnects.

Zhenyu Yang

Papers

Recent Developments of Optoelectronic Synaptic Devices Based on Metal Halide Perovskites

On-Chip Detection of Multiwavelength Surface Plasmon Polaritons Based on Plasmonic Demultiplexers

Revealing the Competition between Defect‐Trapped Exciton and Band‐Edge Exciton Photoluminescence in Monolayer Hexagonal WS2

Subwavelength dichroic demultiplexer based on double Fabry-Perot cavities.