huangpanqi

2D TaSe₂-MoSe₂ metal–semiconductor heterostructures are successfully achieved usin an edge-induced epitaxial growth mode. The unique contact potential and strong current rectification behavior will facilitate the development high-performance transition metal dichalcogenide-based electronic devices.

Abstract

Van der Waals (vdWs) heterostructures based on 2D metals and semiconductors have attracted considerable attention due to their excellent properties and great application potential in next-generation electronic and optoelectronic devices. To obtain such vdWs heterostructures, the conventional approach with artificial exfoliation and stacking of 2D metals onto 2D semiconductors in the vertical direction is still far from satisfactory, because of the low yield and impurity-involved transfer process. Here, two-step vapor deposition growth of 2D TaSe₂-MoSe₂ metal–semiconductor heterostructures is reported. Raman maps confirm the precise spatial modulation of the as-grown 2D TaSe₂-MoSe₂ heterostructures. Structural analysis reveals that the upper 1T-TaSe₂ is formed heteroepitaxially on/around the presynthesized 2H-MoSe₂ monolayers with an epitaxial relationship of (10-10)_TaSe2//(10-10)_MoSe2 and [0001]_TaSe2//[0001]_MoSe2. Based on the detailed characterizations of morphology, structure, and composition, an edge-induced growth mechanism is proposed to illustrate the formation process of the 2D heterostructures, confirmed by first-principle calculations. In addition, Kelvin probe force microscope characterizations and electrical transport measurements confirm that the 2D metal–semiconductor heterostructures exhibit typical rectification characteristics with a contact potential height of ≈431 mV. The direct growth of high-quality 2D metal–semiconductor heterostructures marks an important step toward high-performance integrated optoelectronic devices.

This article comprehensively reviews the progress of chemical vapor deposition growth, characterization, and electrical properties of graphene depending on layer number and twist angle. The characterization methods for measuring the layer number and twist angle are summarized. Electrical properties and applications of graphene, particularly magic-angle twist bilayer graphene, are briefly introduced. Outlooks and challenges are presented.

Abstract

Numerous studies conducted on the layered graphene family—including the monolayer, bilayer, trilayer, few-layer, and multilayer—draw plenty of attention to stacking modes and twist angles, which are extensively explored for its controlled growth, properties, and applications. This review provides a comprehensive overview of current challenges and opportunities for the chemical vapor deposition (CVD) growth, characterization, and electrical properties of graphene depending on the layer number and twist angles. Various state-of-the-art innovations using the CVD method, which incorporates graphene synthesis through the control of metal substrates, layer numbers, and twist angles, are presented. The underlying growth mechanisms are discussed in terms of the interactions among graphene substrates/layers and its dynamic process. The characterization methods for determining the layer number and twist angle of graphene layers are summarized. Furthermore, the electrical properties and applications of graphene, particularly magic-angle twist bilayer graphene, are briefly introduced. Finally, outlooks and perspectives for the engineering of CVD graphene layers are discussed.

Nature Electronics, Published online: 21 April 2022; doi:10.1038/s41928-022-00745-7

This Review examines the development of perovskite light-emitting diodes, exploring the key challenges involved in creating efficient and stable devices.

A neuromorphic optoelectronic floating-gate transistor based on multilayer graphene/h-BN/MoS₂ vdW heterostructure exhibits programmable synaptic plasticity due to the unique light-induced carrier tunneling through vdW heterostructure. Ultra-low energy consumption for the electrical response to light stimulation is also realized under a low V _ds at program state, demonstrating its great potential in building efficient artificial neural networks based on vdW heterostructures.

Abstract

Van der Waals (vdW) heterostructures provide a unique opportunity to develop various electronic and optoelectronic devices with specific functions by designing novel device structures, especially for bioinspired neuromorphic optoelectronic devices, which require the integration of nonvolatile memory and excellent optical responses. Here, we demonstrate a programmable optoelectronic synaptic floating-gate transistor based on multilayer graphene/h-BN/MoS₂ vdW heterostructures, where both plasticity emulation and modulation were successfully realized in a single device. The dynamic tunneling process of photogenerated carriers through the as-fabricated vdW heterostructures contributed to a large memory ratio (10⁵) between program and erase states. Our device can work as a functional or silent synapse by applying a program/erase voltage spike as a modulatory signal to determine the response to light stimulation, leading to a programmable operation in optoelectronic synaptic transistors. Moreover, an ultra-low energy consumption per light spike event (~2.5 fJ) was obtained in the program state owing to a suppressed noise current by program operation in our floating-gate transistor. This study proposes a feasible strategy to improve the functions of optoelectronic synaptic devices with ultra-low energy consumption based on vdW heterostructures designed for highly efficient artificial neural networks.

Common metal salts are used to realize the controlled phase transformation of transition metal dichalcogenides (TMDs) from the conventional thermodynamically stable 2H phase to unconventional metastable 1T′ phase. This work not only paves the way to prepare high-quality and high-purity unconventional metastable 1T′-TMDs for fundamental and practical investigations, but also greatly simplifies the procedure for the large-scale production of 1T′-TMDs.

Abstract

Phase engineering of nanomaterials (PEN) has demonstrated great potential in the fields of catalysis, electronics, energy storage and conversion, and condensed matter physics. Recently, transition metal dichalcogenides (TMDs) with unconventional metastable phases (e.g., 1T and 1T′) have attracted increasing research interest due to their unique and appealing physicochemical properties. However, there is still a lack of a simple, universal, and controlled method for the preparation of large-scale and high-purity unconventional-phase TMD crystals, restricting their further fundamental study and practical applications. Here, a facile, one-step salt-assisted general strategy is reported for the controlled phase transformation of commercially available TMDs with conventional 2H phase, yielding a large amount of metastable 1T′-phase TMDs, including WS₂, WSe₂, MoS₂, and MoSe₂. It is found that the easily accessible metal salts, such as K₂C₂O₄·H₂O, K₂CO₃, Na₂CO₃, Rb₂CO₃, Cs₂CO₃, KHCO₃, NaHCO₃, and NaC₂O₄, can be used to assist the 2H-to-1T′ phase transformation, greatly simplifying the synthetic process for producing metastable 1T′-TMDs. Importantly, this method can also be used to prepare 1T′-TMD alloys, such as 1T′-WS_{2 x} Se_{2(1− x )}. This newly developed strategy is robust and highly effective, which can also be used for the phase engineering of other materials with various polymorphs.

Performing multifield modulation of coupled localized laser writing and nonlocal moisture treatment on metal halide perovskites crystals enables the creation and manipulation of localized photoluminescence for optical encryption from 2D to 3D and 4D, such as reversible encryption–reading, repeatable erasing–refreshing, and even spatially and temporally resolved reading with high resolution.

Abstract

The ability to generate and manipulate photoluminescence (PL) with high spatial resolution has been of primary importance for applications in micro-optoelectronics, while the emerging metal halide perovskites offer novel material platforms where diverse photonic functionalities and fine structuring are constantly explored. Herein, micro-PL patterns consisting of highly luminescent CsPbBr₃ nanocrystals (NCs) in nonluminescent perovskite crystals are directly fabricated by focused femtosecond laser irradiation. Further modulation with a moisture field leads to the selective dissolution of the laser-destabilized perovskite structures as revealed by density functional theory simulations, thus allowing for facile control of the reversible PL from the recrystallization of moisture-induced CsPbBr₃ NCs. By leveraging the coupled laser writing and moisture modulation, multimodal information encryption is realized by reversible encryption–reading and repeatable erasing–refreshing. This optical storage mechanism is also extended to 3D and 4D by realizing spatially and temporally resolved optical encryption. The coupled multifield modulation on perovskite crystals can enable potential applications in optical storage and encryption, and offer a novel solution for the creation and manipulation of localized PL structures with high temporal and spatial resolutions.

In-plane polarization in MoS₂ is realized through contact with low-symmetric CrOCl. The emergence of asymmetric second harmonic generation pattern in MoS₂/CrOCl heterojunction indicates the variation of lattice symmetry in MoS₂, which stems from lattice-mismatch-induced uniaxial strain because of the strong interlayer interactions. More importantly, the strong linear polarization-sensitive photodetection is realized.

Abstract

2D materials with low-symmetry exhibit anisotropic physical properties, making them promising candidates for various applications. However, the lack of matured synthesis methods in anisotropic 2D materials is still the main obstacle to their future applications. Given the mature synthesis method of transition metal dichalcogenides (TMDCs), manipulating anisotropy in 2D TMDCs becomes a promising way to tune or trigger functional properties. Herein, for the first time, a van der Waals symmetry engineering is reported to introduce in-plane polarization in MoS₂ through contact with low-symmetric CrOCl. The emergence of asymmetric second harmonic generation pattern in MoS₂/CrOCl heterojunction indicates the variation of lattice symmetry in MoS₂. Furthermore, the theoretical simulation shows that such change stems from lattice-mismatch-induced uniaxial strain because of the strong interlayer interactions. The angle-dependent Raman and photoluminescence spectra further identify that the uniaxial strain gives rise to the in-plane polarization in MoS₂. In addition, the polarized MoS₂ exhibits excellent orientation-sensitive electrical characteristics with a conductance anisotropy ratio of ≈1.5. More importantly, the strong linear polarization-sensitive photodetection is realized, and the anisotropic ratio reached 1.25 with 532 nm. The results suggest that symmetric engineering potentially opens up a new field to endow high-symmetry 2D materials with anisotropic functionalities.

WSe₂/α-In₂Se₃ heterojunction structure is constructed to enhance the optical absorption and realize a p–n junction photodetector. The photodetector can work efficiently over a broadband spectrum (from 405 to 905 nm) and a rise/fall time of 110/120 µs is achieved. Consequently, the underwater wireless optical communication system is established under λ = 520 nm illumination, which is promising for practical applications.

Abstract

P–n junctions based on 2D materials can be achieved using a selective doping technique, while such a method is challenged by the complex fabrication process. Here, a facile van der Waals (vdWs) structured p–n heterojunction is demonstrated by simply transferring an n-type multilayer α-In₂Se₃ (direct bandgap) on a p-type ultra-thin WSe₂ nanosheet. The vdWs stacked photodetector with an improved type-II band alignment not only realizes a broadband spectral response from visible to near infrared (405–905 nm), but also operates well with a diode-like behavior. This behavior is further confirmed by the high-resolution scanning photocurrent mapping. As a result, the as-fabricated device exhibits a short response time (<120 µs) and a high responsivity of 1.84 A W⁻¹ under 520 nm laser illumination. Accordingly, an underwater optical communication system based on the WSe₂/α-In₂Se₃ p-n heterojunction photodetector is demonstrated, which is promising for next-generation high-performance and low-power detection applications.

Nature Electronics, Published online: 25 April 2022; doi:10.1038/s41928-022-00754-6

Few-nanometre-thick flakes of trigonal and monoclinic Cr5Te8 can be grown using chemical vapour deposition, with the monoclinic phase exhibiting an anomalous Hall conductivity of 650 Ω–1 cm–1 and anomalous Hall angle of 5%.

Advanced Functional Materials, Volume 32, Issue 17, April 25, 2022.

The photoluminescence quantum efficiency of a CaO:Eu NIR phosphor is significantly improved and stabilized at high temperature. By utilizing GeO₂ decomposition, the oxygen vacancies in the CaO lattice are effectively repaired. A record-high external quantum efficiency of 54.7% at 740 nm is obtained with a thermal stability greatly improved from 57% to 90% at 125 °C.

Abstract

Near-infrared (NIR) luminescence materials with broadband emissions are necessary for the development of light-emitting diodes (LEDs) based light sources. However, most known NIR-emitting materials are limited by their low external quantum efficiency. This work demonstrates how the photoluminescence quantum efficiency of europium-activated calcium oxide (CaO:Eu) NIR phosphor can be significantly improved and stabilized at operating temperatures of LEDs. A carbon paper wrapping technology is innovatively developed and used during the solid-state sintering to promote the reduction of Eu³⁺ into Eu²⁺. In parallel, the oxygen vacancies in the CaO lattice are repaired utilizing GeO₂ decomposition. Through this process, a record-high external quantum efficiency of 54.7% at 740 nm is obtained with a thermal stability greatly improved from 57% to 90% at 125 °C. The as-fabricated NIR-LEDs reach record photoelectric efficiency (100 mA@23.4%) and output power (100 mA @ 319.5 mW). This discovery of high-performance phosphors will open new research avenues for broadband NIR LED light sources in a variety of photonics applications.

The memristor based on 2D BiOI exhibits high-performance memristive behaviors with an ultralow SET voltage of ≈0.05 V, which is one order of magnitude lower than that of most reported memristors based on 2D materials. The memristor demonstrates electrical and light-induced synaptic plasticity eminently suitable for low-power optoelectronic synapses, which can be used to simulate the “learning experience” behaviors.

Abstract

Artificial optoelectronic synapses with both electrical and light-induced synaptic behaviors have recently been studied for applications in neuromorphic computing and artificial vision systems. However, the combination of visual perception and high-performance information processing capabilities still faces challenges. In this work, the authors demonstrate a memristor based on 2D bismuth oxyiodide (BiOI) nanosheets that can exhibit bipolar resistive switching (RS) performance as well as electrical and light-induced synaptic plasticity eminently suitable for low-power optoelectronic synapses. The fabricated memristor exhibits high-performance RS behaviors with a high ON/OFF ratio up to 10⁵, an ultralow SET voltage of ≈0.05 V which is one order of magnitude lower than that of most reported memristors based on 2D materials, and low power consumption. Furthermore, the memristor demonstrates not only electrical voltage-driven long-term potentiation, depression plasticity, and paired-pulse facilitation, but also light-induced short- and long-term plasticity. Moreover, the photonic synapse can be used to simulate the “learning experience” behaviors of human brain. Consequently, not only the memristor based on BiOI nanosheets shows ultra-low SET voltage and low-power consumption, but also the optoelectronic synapse provides new material and strategy to construct low-power retina-like vision sensors with functions of perceiving and processing information.

The compositional architecture of lanthanide-doped upconverting core-shell nanoparticles (UCNPs) strongly affects their suitability for Resonant Energy Transfer (RET) based sensing. The upconversion (UC) in lanthanides advantageously diminishes background signal, but concurrently complicates luminescence kinetics analysis. Newly suggested RET quantification methods exploit long luminescence risetimes of the donor and fluorescent acceptor molecules to understand the mechanisms behind and propose optimized donor NPs.

Abstract

Förster Resonance Energy Transfer (FRET) between single molecule donor ( D ) and acceptor ( A ) is well understood from a fundamental perspective and is widely applied in biology, biotechnology, medical diagnostics, and bio-imaging. Lanthanide doped upconverting nanoparticles (UCNPs) have demonstrated their suitability as alternative donor species. Nevertheless, while they solve most disadvantageous features of organic donor molecules, such as photo-bleaching, spectral cross-excitation, and emission bleed-through, the fundamental understanding and practical realizations of bioassays with UCNP donors remain challenging. Among others, the interaction between many donor ions (in donor UCNP) and many acceptors anchored on the NP surface and the upconversion itself within UCNPs, complicate the decay-based analysis of D - A interaction. In this work, the assessment of designed virtual core-shell NP (VNP) models leads to the new designs of UCNPs, such as …@Er, Yb@Er, Yb@YbEr, which are experimentally evaluated as donor NPs and compared to the simulations. Moreover, the luminescence rise and decay kinetics in UCNP donors upon RET is discussed in newly proposed disparity measurements. The presented studies help to understand the role of energy-transfer and energy migration between lanthanide ion dopants and how the architecture of core-shell UCNPs affects their performance as FRET donors to organic acceptor dyes.

A photoelectrochemical-type photosensor using monocrystalline p-GaN nanowires on the Si platform is built with two primary interfaces, at which the charge separation and transport determine the photoresponses. With rational Platinum decoration, the balance of the charge carriers is tuned, making the device exhibit either positive or negative photocurrent upon different light illumination.

Abstract

The carrier transport dynamics at the surface/interface of semiconductors determine the electronic and optical properties of devices. Thus, precise control of their dynamic processes while understanding the nature of these characteristics is crucial for modulating device functionalities. Here, a photoelectrochemical-type photosensor is built using monocrystalline p-GaN nanowires on the Si platform, which unambiguously exhibits either positive or negative photocurrent upon different light illumination. Such dual-polarity photocurrents are attributed to photogenerated-carrier-transport competition at two interfaces: the GaN/electrolyte and GaN/Si interface. Particularly, a rational Pt decoration successfully accelerates the carrier migration at the GaN/electrolyte interface that breaks the original balance of carrier transport. This mechanism is further elaborated by Kelvin-probe-force-microscopy characterization, which intuitively reveals the impact of Pt decoration on modulating nanowires’ surface band bending and the consequent carrier dynamics at the interfaces. These insights into the control of carrier dynamics shed light on achieving multi-functional PEC devices built upon simple semiconductor architectures.

Several new cleaning methods such as ozone cleaning, annealing, and acetone treatment are found to be ineffective for the complete removal of poly(methyl methacrylate) (PMMA) residues from wet-transferred monolayer MoS₂. In this context, a simple yet effective chemical treatment, 96 h of hot ethanol cleaning, is found to remove the PMMA residues completely. The ultraclean monolayer MoS₂ has shown improved field-effect transistor performance.

Abstract

The transfer of monolayer molybdenum disulfide (1L-MoS₂) onto any target substrates is inevitable for the next generation of optoelectronic devices such as flexible electronics. However, the existing post-transfer treatments are ineffective for the complete removal of poly(methyl methacrylate) (PMMA) polymer, which is usually used as carrier polymer in the wet-transfer method. The presence of PMMA residues seriously degrades the intrinsic properties of any 2D materials. Several new cleaning methods such as annealing, ozone cleaning, and acetone treatment adopted in this report are found to be ineffective for the complete removal of PMMA residues on the transferred 1L-MoS₂ film. A new chemical route is developed and demonstrated with a PMMA nonsolvent, ethanol, and ultraclean transferred 1L-MoS₂ is obtained after 96 h hot ethanol treatment. An interfacial diffusion model is proposed for the mechanism of PMMA removal from the 1L-MoS₂ surface. To observe the effect of cleaning process on electrical properties, 1L-MoS₂ field-effect transistor (FET) devices are fabricated. An enhancement of 80%–85% in the electron mobility is achieved for ultraclean 1L-MoS₂. One order improvement in the FET parameters such as ON/OFF ratio and the subthreshold slope is also observed. It is believed that this novel method can also be applicable for other 2D materials.

Moiré lattice in artificially stacked monolayers of two-dimensional (2D) materials effectively modulates the electronic structures of materials, which is widely highlighted. Formation of the electronic Moiré s...

The increasing interest in 2D ultrawide bandgap materials requires in-depth research on their crystal structures, physical properties, preparation, and potential applications. In this review, comprehensive progress of 2D ultrawide bandgap semiconductor materials is discussed. Proposed challenges need to be addressed in the future to unveil their potential applications.

Abstract

2D ultrawide bandgap (UWBG) semiconductors have aroused increasing interest in the field of high-power transparent electronic devices, deep-ultraviolet photodetectors, flexible electronic skins, and energy-efficient displays, owing to their intriguing physical properties. Compared with dominant narrow bandgap semiconductor material families, 2D UWBG semiconductors are less investigated but stand out because of their propensity for high optical transparency, tunable electrical conductivity, high mobility, and ultrahigh gate dielectrics. At the current stage of research, the most intensively investigated 2D UWBG semiconductors are metal oxides, metal chalcogenides, metal halides, and metal nitrides. This paper provides an up-to-date review of recent research progress on new 2D UWBG semiconductor materials and novel physical properties. The widespread applications, i.e., transistors, photodetector, touch screen, and inverter are summarized, which employ 2D UWBG semiconductors as either a passive or active layer. Finally, the existing challenges and opportunities of the enticing class of 2D UWBG semiconductors are highlighted.

The synthesis of the 2D SnP nanosheets on liquid tin (Sn) substrate via the atmospheric pressure chemical vapor deposition (APCVD) method is demonstrated here. HRTEM and SAED reveal high-quality and single crystalline structure of the SnP nanosheet. The temperature-dependent and angle-resolved polarization Raman spectra from an experimental perspective are performed. Furthermore, the 2D SnP FET devices show a typical n-type semiconductor characteristic.

Abstract

As an important metal phosphides material, 2D tin phosphides (SnP_x 0 <  x ≤ 3) have been theoretically predicted to have intriguing physicochemical properties and potential applications in electronics, optoelectronics, and energy fields. However, the synthesis of high-quality 2D SnP single crystal has not been reported due to the lack of efficiency and reliable growth method. Here, a facile atmospheric pressure chemical vapor deposition (APCVD) method is developed to realize the growth of high-quality 2D SnP nanosheets, by employing tin (Sn) foil as both liquid metal substrates and reaction precursor. Temperature-dependent and angle-resolved polarization Raman spectra observed Raman peaks located at 142.6, 303.3, and 444.2 cm^-1 are concluded to belong to A_1g mode, which are consistent with the theoretical calculation results. Moreover, the field-effect transistor (FET) devices based on SnP nanosheets show a typical n-type characteristic with an on/off ratio of 10³ at 200 K. SnP nanosheets also demonstrate excellent photoresponse performance under the illumination of 473, 532, and 639 nm lasers, which can be tuned by V _gs, V _ds, and light power density. It is believed that these findings can provide the first-hand experimental information for the future study of 2D SnP nanosheets.