Jiuxiang Dai

Dark and photoinduced charge transport in the metal (M)-intercalated 2D carbon nitride poly(heptazine imide) (M-PHI) is investigated, revealing predominantly ionic, humidity-assisted conductivity. A correlation between ionic conductivity and photocatalytic hydrogen evolution activity is revealed, which suggests efficient screening of photogenerated charges by mobile ions, leading to prolonged charge-carrier lifetimes.

Abstract

Carbon nitrides are among the most studied materials for photocatalysis; however, limitations arise from inefficient charge separation and transport within the material. Here, this aspect is addressed in the 2D carbon nitride poly(heptazine imide) (PHI) by investigating the influence of various counterions, such as M = Li⁺, Na⁺, K⁺, Cs⁺, Ba²⁺, NH₄ ⁺, and tetramethyl ammonium, on the material's conductivity and photocatalytic activity. These ions in the PHI pores affect the stacking of the 2D layers, which further influences the predominantly ionic conductivity in M-PHI. Na-containing PHI outperforms the other M-PHIs in various relative humidity (RH) environments (0–42%RH) in terms of conductivity, likely due to pore-channel geometry and size of the (hydrated) ion. With increasing RH, the ionic conductivity increases by 4–5 orders of magnitude (for Na-PHI up to 10^-5 S cm^-1 at 42%RH). At the same time, the highest photocatalytic hydrogen evolution rate is observed for Na-PHI, which is mirrored by increased photogenerated charge-carrier lifetimes, pointing to efficient charge-carrier stabilization by, e.g., mobile ions. These results indicate that also ionic conductivity is an important parameter that can influence the photocatalytic activity. Besides, RH-dependent ionic conductivity is of high interest for separators, membranes, or sensors.

Nature Communications, Published online: 07 December 2021; doi:10.1038/s41467-021-27418-x

The short exciton lifetime and strong exciton-exciton interaction in transition metal dichalcogenides limit the efficiency of exciton emission. Here, the authors show that exciton-exciton interaction in monolayer WS2 can be screened using proximal metal plates, leading to an improved quantum yield.

A 2D heterostructure-based memtransistor is designed to emulate the Bienenstock–Cooper–Munro (BCM) theory, since this structure not only induces spontaneous forgetting process but also offers another gate-tunable forgetting effect. BCM learning rule is perfectly demonstrated on this memtransistor using triplet-STDP. Furthermore, high-order spatiotemporal recognition is achieved in a feedforward neuron network based on the memtransistor.

Abstract

The Bienenstock, Cooper, and Munro (BCM) theory of synaptic plasticity is regarded as the most precise model of the synapse, and is more compatible with neuromorphic computing. However, the development in BCM synaptic modification is rather limited since the memristive devices used to emulate the BCM lack tunable forgetting rate. Compared with memristors, memtransistors provide another gate-tunable freedom degree, which will help to modulate the forgetting rate. In this work, the authors demonstrate a perfect BCM learning rule based on the 2D heterostructure memtransistor through using triplet-spike timing dependent plasticity model. Two critical characteristics of the BCM rule, sliding frequency threshold and enhanced depression effect, are perfectly presented due to their spontaneous/gate-assistant forgetting effect. The experimental results are extremely consistent with the BCM learning rule and suggest the potential application of 2D memtransistors in high-order spatiotemporal recognition.

Both n- and p-channel multivalue field-effect transistors (FETs) are fabricated using p-MoTe₂/n-MoS₂ heterostack channel architecture, where either p- or n-channel ternary value FET is reproducible by switching the stacking order. For a state-of-the-art device application, a quaternary NAND logic circuit is for the first time demonstrated by integrating two ternary n-channel FETs, and a complementary ternary inverter is also fabricated.

Abstract

Applications of 2D semiconductors have been extensively studied, much oriented to various electron devices. Recently, multivalue field-effect transistors (FETs) are also included among 2D-based electron device studies in consideration that multivalue FETs may resolve power consumption issues in future integrated circuits. Several n-channel devices are thus reported along with a few p-channel devices, while studies to achieve both n- and p-channel multivalue FETs are hardly found. Here, both n- and p-channel multivalue FETs are fabricated using p-MoTe₂/n-MoS₂ heterostack channel architecture, where either p- or n-channel ternary value FET is reproducible by switching the stacking order of p- and n-channel layer. The main ternary value mechanism originates from resonant tunneling type injection and channel inversion, which take place during device operation. For a state-of-the-art device application in 2D electronics, a quaternary NAND logic circuit is for the first time demonstrated by integrating two ternary n-channel FETs, and a complementary ternary inverter is also fabricated by integrating multivalue p-channel and plain n-channel FET.

Charge transfer properties in heterostructures formed by Bi₂O₂Se and transition metal dichalcogenide (TMD) monolayers of WS₂ and MoS₂ are comprehensively studied. The interfacial energy band alignments of both heterostructures are established via Kelvin probe force microscopy. Transient absorption measurements further reveal efficient interlayer charge transfer and formation of the interlayer excitons.

Abstract

Atomically thin bismuth oxyselenide (Bi₂O₂Se) exhibits attractive properties for electronic and optoelectronic applications, such as high charge-carrier mobility and good air stability. Recently, the development of Bi₂O₂Se-based heterostructures have attracted enormous interests with promising prospects for diverse device applications. Although the electrical properties of Bi₂O₂Se-based heterostructures have been widely studied, the interlayer charge transfer in these heterostructures remains elusive, despite its importance in harnessing their emergent functionalities. Here, a comprehensive experimental investigation on the interlayer charge transfer properties of two heterostructures formed by Bi₂O₂Se and representative transition metal dichalcogenides (namely, WS₂/Bi₂O₂Se and MoS₂/Bi₂O₂Se) is reported. Kelvin probe force microscopy is used to measure the work functions of the samples, which are further employed to establish type-II band alignment of both heterostructures. Photoluminescence quenching is observed in each heterostructure, suggesting high charge transfer efficiency. Time-resolved and layer-selective pump–probe measurements further prove the ultrafast interlayer charge transfer processes and formation of long-lived interlayer excitons. These results establish the feasibility of integrating 2D Bi₂O₂Se with other 2D semiconductors to fabricate heterostructures with novel charge transfer properties and provide insight for understanding the performance of optoelectronic devices based on such 2D heterostructures.

P(VDF-CTFE) is used as a ferroelectric gate to control the states of WS₂/Si junction and it achieves an enhanced infrared lateral photovoltiac effect. The polarization electric field not only broadens the range of absorption wavelength but also greatly promotes sensitivity and response speed. This ferroelectric-tuned WS₂/Si device has great potential application in high-performance, ultra-fast, super-sensitive near-infrared photodetectors.

Abstract

As one of the typical transition-metal dichalcogenides with distinct optical and electrical properties, WS₂ exhibits tremendous potential for optoelectronic devices. However, its inherent band gap range limits the application in the infrared region. To overcome this draw-back and improve the sensitivity, P(VDF-CTFE) is used as a ferroelectric gate to control the states of WS₂/Si junctions and achieve an enhanced infrared photodetection. The polarization electric field not only broadens the range of absorption wavelength (405–1550 nm) but also greatly promotes the sensitivity of lateral photovoltaic effect (LPE) (from 198.6 to 503.2 mV mm⁻¹). This phenomenon is attributed to the reduction of WS₂ band gap and the change of potential barrier at the interface of the junction. Meanwhile, the response speed is improved significantly due to the increase of carrier initial kinetic energy. This new scheme for ferroelectric tuned LPE opens up a way to realize high-sensitivity, ultrafast, and stable infrared photodetection.

This review introduces various 2D van der Waals layered materials and summarizes their wafer-scale growth processes, including strategies for mono-orientated film growth. Applications of these materials in integrated devices and new epitaxial technologies are discussed, as well as the required future research directions in this very important field.

Abstract

Wafer-scale growth has become a critical bottleneck for scaling up applications of van der Waal (vdW) layered 2D materials in high-end electronics and optoelectronics. Most vdW 2D materials are initially obtained through top-down synthesis methods, such as exfoliation, which can only prepare small flakes on a micrometer scale. Bottom-up growth can enable 2D flake growth over a large area. However, seamless merging of these flakes to form large-area continuous films with well-controlled layer thickness and lattice orientation is still a significant challenge. This review briefly introduces several vdW layered 2D materials covering their lattice structures, representative physical properties, and potential roles in large-scale applications. Then, several methods used to grow vdW layered 2D materials at the wafer scale are reviewed in depth. In particular, three strategies are summarized that enable 2D film growth with a single-crystalline structure over the whole wafer: growth of an isolated domain, growth of unidirectional domains, and conversion of oriented precursors. After that, the progress in using wafer-scale 2D materials in integrated devices and advanced epitaxy is reviewed. Finally, future directions in the growth and scaling of vdW layered 2D materials are discussed.

In this work, an effective van der Waals passivation method for 2D materials with inorganic molecular crystal Sb₂O₃ as the encapsulation layer is developed. The scalable encapsulation method, carried out through a complementary metal-oxide-semiconductor-compatible manufacturing process, opens unprecedented opportunities for 2D materials to be applied in optoelectronic devices toward chip-level development.

Abstract

Encapsulation is critical for devices to guarantee their stability and reliability. It becomes an even more essential requirement for devices based on 2D materials with atomic thinness and far inferior stability compared to their bulk counterparts. Here a general van der Waals (vdW) encapsulation method for 2D materials using Sb₂O₃ layer of inorganic molecular crystal fabricated via thermal evaporation deposition is reported. It is demonstrated that such a scalable encapsulation method not only maintains the intrinsic properties of typical air-susceptible 2D materials due to their vdW interactions but also remarkably improves their environmental stability. Specifically, the encapsulated black phosphorus (BP) exhibits greatly enhanced structural stability of over 80 days and more sustaining-electrical properties of 19 days, while the bare BP undergoes degradation within hours. Moreover, the encapsulation layer can be facilely removed by sublimation in vacuum without damaging the underlying materials. This scalable encapsulation method shows a promising pathway to effectively enhance the environmental stability of 2D materials, which may further boost their practical application in novel (opto)electronic devices.

An asymmetric split-gate phototransistor configuration, called the “asymmetry field-effect phototransistor” (AFEPT) is demonstrated. The structure allows for an effective modulation of the electric-field profile throughout the channel, as well as enhanced photocarrier transport, thereby providing a new platform for improving optoelectronic properties and probing photocarrier dynamics in intrinsic 2D material layers.

Abstract

The probing of fundamental photophysics is a key prerequisite for the construction of diverse optoelectronic devices and circuits. To date, though, photocarrier dynamics in 2D materials remains unclear, plagued primarily by two issues: a large exciton binding energy, and the lack of a suitable system that enables the manipulation of excitons. Here, a WSe₂-based phototransistor with an asymmetric split-gate configuration is demonstrated, which is named the “asymmetry field-effect phototransistor” (AFEPT). This structure allows for the effective modulation of the electric-field profile across the channel, thereby providing a standard device platform for exploring the photocarrier dynamics of the intrinsic WSe₂ layer. By controlling the electric field, this work the spatial evolution of the photocurrent is observed, notably with a strong signal over the entire WSe₂ channel. Using photocurrent and optical spectroscopy measurements, the physical origin of the novel photocurrent behavior is clarified and a room-temperature exciton binding energy of 210 meV is determined with the device. In the phototransistor geometry, lateral p–n junctions serve as a simultaneous pathway for both photogenerated electrons and holes, reducing their recombination rate and thus enhancing photodetection. The study establishes a new device platform for both fundamental studies and technological applications.

Active hydrogen evolution reaction (HER) at heterophase boundaries between semiconducting 2H and metallic 1T’ phases in large-scale MoTe₂ is reported. Despite the small area ratio of the 1D heterophase boundary to the basal plane, HER is improved up to a turnover frequency of 317 s⁻¹ at the 1D geometry. Kelvin probe force microscopy demonstrates a sharp band bending for the HER.

Abstract

The phase engineering of transition metal dichalcogenides (TMDs) is considered a promising strategy for promoting efficient catalysis, such as the hydrogen evolution reaction (HER). While theoretical studies predict the presence of catalytically active atomic sites at heterophase boundaries in TMDs, conventional bulk HER measurements are not able to precisely explore these 1D heterophase regions for HER. Here, one reports on active HER occurring at heterophase boundaries between the semiconducting 2H and metallic 1T’ phases in large-scale MoTe₂ grown via chemical vapor deposition. Microreactors are used to investigate the local HER at varying lengths of 1D heterophase boundaries, and the results are systematically compared with the HER performance at the pristine basal planes of MoTe₂. Despite the small area ratio between the 1D heterophase boundary and the open region for local HER, a clear improvement in HER is observed with a turnover frequency of 317 s^–1. The Kelvin probe force microscopy determines a surface potential difference of 50 mV across the heterophase boundary, which supports sharp band bending and local charge accumulation as the basis for the TMDs’ efficient electrochemical catalysis.

npj 2D Materials and Applications, Published online: 03 December 2021; doi:10.1038/s41699-021-00270-9

Wafer scale synthesis of organic semiconductor nanosheets for van der Waals heterojunction devices

This work presents the synthesis of a Bi₂Te₂Se/Bi₂O₂Se vertical heterojunction via one-step chemical vapor deposition. The hybrid photodetector based on the heterojunction demonstrates outstanding performance with a responsivity of 2.21 × 10³ A W^–1 at 532 nm, which can be further improved by field effect modulation.

Abstract

Bismuth oxyselenide (Bi₂O₂Se) has emerged as a promising candidate for electronic and optoelectronic applications due to its outstanding electron mobility and ambient stability. However, high dark current and relatively slow photoresponse that originate from high charge carrier concentration as well as bolometric effect in Bi₂O₂Se inhibit further improvement of Bi₂O₂Se based photodetectors. Here, a one-step van der Waals (vdW) epitaxy synthesis of Bi₂Te₂Se/Bi₂O₂Se vertical heterojunction with type-II band alignment and high-quality interface is demonstrated. The crystal quality and uniformity of the heterojunction are supported by Raman, transmission electron microscopy and energy dispersive spectroscopy results. A photodetector based on Bi₂Te₂Se/Bi₂O₂Se heterojunction demonstrates steady photoresponse over a large wavelength range (532–1456 nm), with a high specific responsivity of 2.21 × 10³ A W^–1 at 532 nm and fast response speed of 50 ms. Moreover, field effect regulation allows for further improvement of the photoresponse performance of the heterojunction field effect transistor device, where the responsivity can be increased to 3.34 × 10³ A W^–1 with a 60 V gate voltage. Overall, the one-step vdW epitaxy process is a promising and convenient route towards constructing high quality Bi₂O₂Se based heterojunction for improving its photodetection performance.

The transfer of graphene from growth substrates onto target substrates is highly needed for further applications of graphene. However, the as-transferred graphene films still suffer from the issues of cracks, surface contaminants, wrinkles, and unintentional doping. In the review, the focus is on these issues and corresponding solutions.

Abstract

Owing to the fascinating properties of graphene, fulfilling the promising characteristics of graphene in applications has ignited enormous scientific and industrial interest. Chemical vapor deposition (CVD) growth of graphene on metal substrates provides tantalizing opportunities for the large-area synthesis of graphene in a controllable manner. However, the tedious transfer of graphene from metal substrates onto desired substrates remains inevitable, and cracks of graphene membrane, transfer-induced doping, wrinkles as well as surface contamination can be incurred during the transfer, which highly degrade the performance of graphene. Furthermore, new issues can arise when moving to large-scale transfer at an industrial scale, thus cost-efficient and environment-friendly transfer techniques also become imperative. The aim of this review is to provide a comprehensive understanding of transfer-related issues and the corresponding experimental solutions and to provide an outlook for future transfer techniques of CVD graphene films on an industrial scale.

Sb gradient-doped SnO₂/Sb₂S₃ heterostructure for photoelectrochemical water-splitting is designed and fabricated by an integrated optimization strategy. Quadruple optimization effects of optical absorption matching, interface reinforcement, electrons mobility improvement, and carrier separation/transport acceleration are realized simultaneously by this integrated strategy. This integrated strategy can be applied to fabricate efficient heterojunction photoelectrodes with quality interface.

Abstract

In this study, an effective quadruple optimization integrated synergistic strategy is designed to fabricate quality Sb gradient-doped SnO₂/Sb₂S₃ heterostructure for an efficient photoelectrochemical (PEC) cell. The experimental results and theoretical calculations reveal that i) optical absorption matching is realized by combining the anti-reflection of SnO₂ and high light absorption ability of Sb₂S₃ in the visible region; ii) interface reinforcement is carried out by coordinating gradient-distributed Sb in SnO₂ with S in S-rich precursor of Sb₂S₃ for improving the Sb₂S₃ crystallization process and matching crystalline lattice of Sb:SnO₂ and Sb₂S₃; iii) ultrahigh electron mobility is achieved by making Sb gradient-doped SnO₂; iv) carrier separation and transport are accelerated by constructing type-II heterojunction with appropriate energy level alignment and forming a high-speed electron transport channel. All of above-mentioned optimization effects are integrated into a synergistic strategy for constructing the Sb:SnO₂/Sb₂S₃ photoanode, achieving a photocurrent density of 2.30 mA cm⁻², hydrogen generation rate of 30.03 µmol cm⁻² h⁻¹, and decent working stability. Notably, this method can also be used in other large-scale fabrication processes, such as drop-casting, spray-coating, blade-coating, printing, slot-die, etc. Moreover, this universal integrated strategy paves an avenue to fabricate efficient photoelectrodes with excellent photoelectrochemical performances.

Nature Nanotechnology, Published online: 02 December 2021; doi:10.1038/s41565-021-01023-x

Square-centimetre scale, multilayer superlattice structures based on atomically thin two-dimensional chalcogenide monolayers enable the realization of excitonic metamaterials.

Synthesis of new multidimensional superlattice heterostructure “GerMXene” from MXene and Xene nanosheets is reported. This protocol demonstrates the conversion of 2D transition-metal carbide MXene to a quantum-manipulated heterostructure with unique structural and material properties. GerMXene crystals possess high surface activity and electro-conductivity, suggesting its potential applications in multiple fields. Furthermore, GerMXene displays excellent bioactivity for tissue engineering and regenerative medicine.

Abstract

Integration of 2D structures into other low-dimensional materials results in the development of distinct van der Waals heterostructures (vdWHSs) with enhanced properties. However, obtaining 2D–1D–0D vdWHSs of technologically useful next generation materials, transition-metal carbide MXene and monoelemental Xene nanosheets in a single superlattice heterostructure is still challenging. Here, the fabrication of a new multidimensional superlattice heterostructure “GerMXene” from exfoliated M₃X₂T _x MXene and hydrogenated germanane (GeH) crystals, is reported. Direct experimental evidence for conversion of hydrothermally activated titanium carbide MXene (A-MXene) to GerMXene heterostructure through the rapid and spontaneous formation of titanium germanide (TiGe₂ and Ti₆Ge₅) bonds, is provided. The obtained GerMXene heterostructure possesses enhanced surface properties, aqueous dispersibility, and Dirac signature of embedded GeH nanosheets as well as quantum dots. GerMXene exhibits functional bioactivity, electrical conductivity, and negative surface charge, paving ways for its applications in biomedical field, electronics, and energy storage.

The progress on studying the interlayer interactions in one-dimensional (1D) van der Waals (vdW) moiré superlattices is reviewed, with highlighting the unique physics of 1D moiré systems. Important questions and future directions in this emerging field are also discussed in this perspective.

Abstract

Studying two-dimensional (2D) van der Waals (vdW) moiré superlattices and their interlayer interactions have received surging attention after recent discoveries of many new phases of matter that are highly tunable. Different atomistic registry between layers forming the inner and outer nanotubes can also form one-dimensional (1D) vdW moiré superlattices. In this review, experimental observations and theoretical perspectives related to interlayer interactions in 1D vdW moiré superlattices are summarized. The discussion focuses on double-walled carbon nanotubes (DWNTs), a model 1D vdW moiré system, and the authors highlight the new optical features emerging from the non-trivial strong interlayer coupling effect and the unique physics in 1D DWNTs. Future directions and questions in probing the intriguing physical phenomena in 1D vdW moiré superlattices such as, correlated physics in different 1D moiré systems beyond DWNTs are proposed and discussed.

Depleted Sb₂S₃ photoconductive detectors using TiO₂ insert layer between glass and Sb₂S₃ film are presented. The photodetectors (PDs) obtain a higher switching ratio because of the vertical deplete region, greatly enhancing responsivity and detectivity. The response time significantly decreases to 0.6 ms. The depleted Sb₂S₃ PD with the peak-to-valley ratio of 1.3 shows potentials for polarization detection.

Abstract

Antimony sulfide (Sb₂S₃) with high absorption coefficient reveals great potential for photodetectors (PDs) in various fields of military and national economy. A new type of depleted Sb₂S₃ thin film photoconductive detector by using titanium dioxide (TiO₂) interlayer with the structure of Au/(glass/TiO₂:)Sb₂S₃/Au is designed. Compared with normal Au/Sb₂S₃/Au devices, the depleted Sb₂S₃ PDs obtain a higher switching ratio of 90, greatly enhancing responsivity and specific detectivity. Owing to the electric field of the depletion region, the response time significantly decreases from 23.2 to 0.6 ms. In addition, the anisotropic Sb₂S₃ film with [120] dominated horizontal orientation is suitable for polarized light detection. The peak-to-valley ratio of the depleted Sb₂S₃ PD reaches 1.3 at polarized light. The results and methods of this study are expected to expand the application of PDs based on Sb₂S₃ films.