Two-Dimensionally Layered p-Black Phosphorus/n-MoS2/p-Black Phosphorus Heterojunctions

Heterogeneous Interface Engineering of 2D Black Phosphorus-Based Materials for Enhanced Photocatalytic Performance

Photocatalysis has garnered significant attention as a sustainable approach for energy conversion and environmental management. 2D black phosphorus (BP) has emerged as a highly promising semiconductor photocatalyst owing to its distinctive properties. However, inherent issues such as rapid recombination of photogenerated electrons and holes severely impede the photocatalytic efficacy of single BP. The construction/stacking mode of BP with other nanomaterials decreases the recombination rate of carriers and extend its functionalities. Herein, from the perspective of atomic interface and electronic interface, the enhancement mechanism of photocatalytic performance by heterogeneous interface engineering is discussed. Based on the intrinsic properties of BP and corresponding photocatalytic principles, the effects of diverse interface characteristics (point, linear, and planar interface) and charge transfer mechanisms (type I, type II, Z-scheme, and S-scheme heterojunctions) on photocatalysis are summarized systematically. The modulation of heterogeneous interfaces and rational regulation of charge transfer mechanisms can enhance charge migration between interfaces and even maximize redox capability. Furthermore, research progress of heterogeneous interface engineering based on BP is summarized and their prospects are looked ahead. It is anticipated that a novel concept would be presented for constructing superior BP-based photocatalysts and designing other 2D photocatalytic materials.

Precise Crystal Orientation Identification and Twist-Induced Giant Modulation of Optical Anisotropy in 1T'-ReS2

The ability to precisely identify crystal orientation as well as to nondestructively modulate optical anisotropy in atomically thin rhenium dichalcogenides is critical for the future development of polarization programmable optoelectronic devices, which remains challenging. Here, we report a modified polarized optical imaging (POI) method capable of simultaneously identifying in-plane (Re chain) and out-of-plane (c-axis) crystal orientations of the monolayer to few-layer ReS2, meanwhile, propose a nondestructive approach to modulate the optical anisotropy in ReS2 via twist stacking. The results show that parallel and near-cross POI are effective to independently identify the in-plane and out-of-plane crystal orientations, respectively, while regulating the twist angle allows for giant modulation of in-plane optical anisotropy from highly intrinsic anisotropy to complete optical isotropy in the stacked ReS2 bilayer (with either the same or opposite c-axes), as well modeled by linear electromagnetic theory. Overall, this study not only develops a simple optical method for precise crystal orientation identification but also offers an efficient light polarization control strategy, which is a big step toward the practical application of anisotropic van der Waals materials in the design of nanophotonic and optoelectronic devices.

AC Characteristics of van der Waals Bipolar Junction Transistors Using an MoS2/WSe2/MoS2 Heterostructure

Two-dimensional layered materials, characterized by their atomically thin thicknesses and surfaces that are free of dangling bonds, hold great promise for fabricating ultrathin, lightweight, and flexible bipolar junction transistors (BJTs). In this paper, a van der Waals (vdW) BJT was fabricated by vertically stacking MoS2, WSe2, and MoS2 flakes in sequence. The AC characteristics of the vdW BJT were studied for the first time, in which a maximum common emitter voltage gain of around 3.5 was observed. By investigating the time domain characteristics of the device under various operating frequencies, the frequency response of the device was summarized, which experimentally proved that the MoS2/WSe2/MoS2 BJT has voltage amplification capability in the 0-200 Hz region. In addition, the phase response of the device was also investigated. A phase inversion was observed in the low-frequency range. As the operating frequency increases, the relative phase between the input and output signals gradually shifts until it is in phase at frequencies exceeding 2.3 kHz. This work demonstrates the signal amplification applications of the vdW BJTs for neuromorphic computing and wearable healthcare devices.

Realization of High Current Gain for Van der Waals MoS2/WSe2/MoS2 Bipolar Junction Transistor

Two-dimensional (2D) materials have attracted great attention in the past few years and offer new opportunities for the development of high-performance and multifunctional bipolar junction transistors (BJTs). Here, a van der Waals BJT based on vertically stacked n+-MoS2/WSe2/MoS2 was demonstrated. The electrical performance of the device was investigated under common-base and common-emitter configurations, which show relatively large current gains of α ≈ 0.98 and β ≈ 225. In addition, the breakdown characteristics of the vertically stacked n+-MoS2/WSe2/MoS2 BJT were investigated. An open-emitter base-collector breakdown voltage (BVCBO) of 52.9 V and an open-base collector-emitter breakdown voltage (BVCEO) of 40.3 V were observed under a room-temperature condition. With the increase in the operating temperature, both BVCBO and BVCEO increased. This study demonstrates a promising way to obtain 2D-material-based BJT with high current gains and provides a deep insight into the breakdown characteristics of the device, which may promote the applications of van der Waals BJTs in the fields of integrated circuits.

Highly Sensitive and Selective DNA Sequencing Device Using Metal Adatom Adsorption on 2D Phosphorene

Two-dimensional (2D) material revolutionarily extends the technique capability of traditional nanopore/nanogap-based DNA sequencing devices. However, challenges associated with DNA sequencing on nanopores still remained in improving the sensitivity and specificity. Herein, by first-principles calculation, we theoretically studied the potential of transition-metal elements (Cr, Fe, Co, Ni, and Au) anchored on monolayer black phosphorene (BP) to act as all-electronic DNA sequencing devices. The spin-polarized band structures appeared in Cr-, Fe-, Co-, and Au-doped BP. Remarkably, the adsorption energy of nucleobases can be significantly enhanced on BP with Co, Fe, and Cr doping, which contribute to the enlarged current signal and lower noise levels. Furthermore, the order of nucleobases in terms of their adsorption energies onto the Cr@BP is C > A > G > T, which exhibits more distinct adsorption energies than Fe@BP or Co@BP. Therefore, Cr-doped BP is more effective to avoid ambiguity in recognizing various bases. We thus envisaged a possibility of a highly sensitive and selective DNA sequencing device based on phosphorene.

Narrow Directed Black Phosphorus Nanoribbons Produced by A Reformative Mechanical Exfoliation Approach

Black phosphorus nanoribbons (PNRs) are ideal candidates for constructing electronic and optoelectronic devices owing to their unique structure and high bandgap tunability. However, the preparation of high-quality narrow PNRs aligned along the same direction is very challenging. Here, a reformative mechanical exfoliation approach combining tape and polydimethylsiloxane (PDMS) exfoliations to fabricate high-quality, narrow, and directed PNRs with smooth edges for the first time is developed. In this method, partially-exfoliated PNRs are first formed on thick black phosphorus (BP) flakes via the tape exfoliation and further peeled off to obtain separated PNRs via the PDMS exfoliation. The prepared PNRs have widths from a dozen to hundreds of nanometers (down to 15 nm) and a mean length of 18 µm. It is found that the PNRs can align along a same direction and the length directions of directed PNRs are along the zigzag direction. The formation of PNRs is attributed to that the BP prefers to be unzipped along the zigzag direction and has an appropriate magnitude of interaction force with the PDMS substrate. The fabricated PNR/MoS₂ heterojunction diode and PNR field-effect transistor exhibit good device performance. This work provides a new pathway to achieve high-quality, narrow, and directed PNRs for electronic and optoelectronic applications.

Type-I PtS2/MoS2 van der Waals heterojunctions with tunable photovoltaic effects and high photosensitivity

Recent advances in two-dimensional (2D) materials play an essential role in boosting modern electronics and optoelectronics. Thus far, transition metal dichalcogenides (TMDs) as emerging members of 2D materials, and the van der Waals heterostructures (vdWHs) based on TMDs have been extensively investigated owing to their prominent capabilities and unique crystal structures. In this work, an original vdWH composed of molybdenum disulfide (MoS₂) and platinum disulfide (PtS₂) was comprehensively studied as a field-effect transistor (FET) and photodetector. A gate-tunable rectifying behavior was obtained, stemming from the band design of PtS₂/MoS₂ vdWH. Upon 685 nm laser illumination, it also exhibited a superior photodetection performance with a distinctly high photoresponsivity of 403 A W^-1, a comparable detectivity of 1.07 × 10¹¹ Jones, and an excellent external quantum efficiency of 7.32 × 10⁴%. More importantly, fast rise (24 ms) and decay (21 ms) times were obtained under 685 nm light illumination attributed to the unilateral depletion region structure. Further, the photovoltaic effect and photocurrent of the heterojunction could be modulated by a back gate voltage. All these results indicated that such 2D-TMD-based vdWHs provide a new idea for realizing high-performance electronic and optoelectronic devices.

Ab initioscreening of two-dimensional CuQxand AgQxchalcogenides

Two-dimensional (2D) chalcogenides have attracted great interest from the scientific community due to their intrinsic physical-chemical properties, which are suitable for several technological applications. However, most of the reported studies focused on particular compounds and composition, e.g., MoS₂, MoSe₂, WS₂, and WSe₂. Thus, there is an increased interest to extend our knowledge on 2D chalcogenides. Here, we report a density functional theory (DFT) screening of 2D coinage-metal chalcogenides (MQ_x), whereM= Cu, Ag,Q= S, Se, Te,x= 0.5, 1.0, 1.5, 2.0, with the aim to improve our atomistic understanding of the physical-chemical properties as a function of cation (M), anion (Q), and composition (x). Based on 258 DFT calculations, we selected a set of 22 stableMQ_xmonolayers based on phonons analyses, where we identified 9 semiconductors (7 AgQ_xand 2 CuQ_x), with band gaps from 0.07 eV up to 1.67 eV, while the remaining systems have a metallic character. Using all 258 systems, we found a logarithmic correlation between the average weighted bond lengths and effective coordination number of cations and anions. As expected, the monolayer cohesive energies increase with the radius of theQspecies (i.e., from S to Te). Furthermore, an increase in the anion size diminishes the work function for nearly allMQ_xmonolayers, which can be explained by the nature of the electronic states at the valence band maximum.

2D Molybdenum Sulfide-Based Materials for Photo-Excited Antibacterial Application

Bacterial infections have seriously threatened human health and the abuse of natural or artificial antibiotics leads to bacterial resistance, so development of a new generation of antibacterial agents and treatment methods is urgent. 2D molybdenum sulfide (MoS₂ ) has good biocompatibility, high specific surface area to facilitate surface modification and drug loading, adjustable energy bandgap, and high near-infrared photothermal conversion efficiency (PCE), so it is often used for antibacterial application through its photothermal or photodynamic effects. This review comprehensively summarizes and discusses the fabrication processes, structural characteristics, antibacterial performance, and the corresponding mechanisms of MoS₂ -based materials as well as their representative antibacterial applications. In addition, the outlooks on the remaining challenges that should be addressed in the field of MoS₂ are also proposed.

A Critical Review on Black Phosphorus-Based Photocatalytic CO2 Reduction Application

Energy shortages and greenhouse effects are two unavoidable problems that need to be solved. Photocatalytically converting CO₂ into a series of valuable chemicals is considered to be an effective means of solving the above dilemmas. Among these photocatalysts, the utilization of black phosphorus for CO₂ photocatalytic reduction deserves a lightspot not only for its excellent catalytic activity through different reaction routes, but also on account of the great preponderance of this relatively cheap catalyst. Herein, this review offers a summary of the recent advances in synthesis, structure, properties, and application for CO₂ photocatalytic reduction. In detail, the review starts from the basic principle of CO₂ photocatalytic reduction. In the following section, the synthesis, structure, and properties, as well as CO₂ photocatalytic reduction process of black phosphorus-based photocatalyst are discussed. In addition, some possible influencing factors and reaction mechanism are also summarized. Finally, a summary and the possible future perspectives of black phosphorus-based photocatalyst for CO₂ reduction are established.

Gate-Tunable Negative Differential Resistance Behaviors in a hBN-Encapsulated BP-MoS2 Heterojunction

Two-dimensional (2D) heterostructures show great potential in achieving negative differential resistance (NDR) effects by Esaki diodes and or resonant tunneling diodes. However, most of the reported Esaki diode-based NDR devices realized by bulk 2D films lack sufficient gate tunability, and the tuning of NDR behavior from appearing to vanishing remains elusive. Here, a gate-tunable NDR device is reported based on a vertically stacked black phosphorus (BP) and molybdenum disulfide (MoS₂) thin 2D heterojunction. At room temperature, a rectifying ratio of ∼6 orders of magnitude from a reverse rectifying diode to a forward rectifying diode by gate modulation is obtained. Through analyzing the temperature-dependent electrical properties, the tunneling mechanism at a certain gate voltage range is revealed. Moreover, the switchable and continuously gate-tunable NDR behavior is realized with a maximum peak-to-valley ratio of 1.23 at 77 K, as shown in the I_DS mappings by sweeping V_DS under different V_GS. In addition, a compact model for gate-tunable NDR behavior in 2D heterostructures is proposed. To our best knowledge, this is the first demonstration of NDR behavior in BP-MoS₂ heterostructures. Consequently, this work sheds light on the gate-tunable NDR devices and reconfigurable logic devices for realizing ternary and reconfigurable logic systems.

Design and tailoring of two-dimensional Schottky, PN and tunnelling junctions for electronics and optoelectronics

Owing to their superior carrier mobility, strong light-matter interactions, and flexibility at the atomically thin thickness, two-dimensional (2D) materials are attracting wide interest for application in electronic and optoelectronic devices, including rectifying diodes, transistors, memory, photodetectors, and light-emitting diodes. At the heart of these devices, Schottky, PN, and tunneling junctions are playing an essential role in defining device function. Intriguingly, the ultrathin thickness and unique van der Waals (vdW) interlayer coupling in 2D materials has rendered enormous opportunities for the design and tailoring of various 2D junctions, e.g. using Lego-like hetero-stacking, surface decoration, and field-effect modulation methods. Such flexibility has led to marvelous breakthroughs during the exploration of 2D electronics and optoelectronic devices. To advance further, it is imperative to provide an overview of existing strategies for the engineering of various 2D junctions for their integration in the future. Thus, in this review, we provide a comprehensive survey of previous efforts toward 2D Schottky, PN, and tunneling junctions, and the functional devices built from them. Though these junctions exhibit similar configurations, distinct strategies have been developed for their optimal figures of merit based on their working principles and functional purposes.

Construction of heterojunctions between ReS2 and twin crystal ZnxCd1−xS for boosting solar hydrogen evolution

Nanoflower-like ReS₂ anchoring on nanotwins Zn_xCd_1−xS greatly boosts photocatalytic hydrogen evolution rate with 31-times higher than pure phase P-ZCS.

Laser Writable Multifunctional van der Waals Heterostructures

Achieving multifunctional van der Waals nanoelectronic devices on one structure is essential for the integration of 2D materials; however, it involves complex architectural designs and manufacturing processes. Herein, a facile, fast, and versatile laser direct write micro/nanoprocessing to fabricate diode, NPN (PNP) bipolar junction transistor (BJT) simultaneously based on a pre-fabricated black phosphorus/molybdenum disulfide heterostructure is demonstrated. The PN junctions exhibit good diode rectification behavior. Due to different carrier concentrations of BP and MoS₂ , the NPN BJT, with a narrower base width, renders better performance than the PNP BJT. Furthermore, the current gain can be modulated efficiently through laser writing tunable base width W_B , which is consistent with the theoretical results. The maximum gain for NPN and PNP is found to be ≈41 (@W_B ≈600 nm) and ≈12 (@W_B ≈600 nm), respectively. In addition, this laser write processing technique also can be utilized to realize multifunctional WSe₂ /MoS₂ heterostructure device. The current work demonstrates a novel, cost-effective, and universal method to fabricate multifunctional nanoelectronic devices. The proposed approach exhibits promise for large-scale integrated circuits based on 2D heterostructures.

Electrochemical Strategy for Analyzing the Co-evolution of Cu2+ and •OH Levels at the Early Stages of Transgenic AD Mice

As more researchers have acknowledged that the aggregation of amyloid β (Aβ) peptides might only be a pathological phenomenon that appears during the course of Alzheimer's disease (AD), it is therefore of great significance to have a preclinical or an early clinical diagnosis. Cu²⁺ dyshomeostasis and oxidative stress, such as hydroxyl radical (^•OH), are found to be associated with peptide aggregations. However, we still do not know how the levels of Cu²⁺ and ^•OH are altered in the brain before massive Aβ plaques appear. Herein, we demonstrated the design and application of a sensitive electrochemical sensor to monitor Cu²⁺ and ^•OH simultaneously in one system without obvious cross-talk. The electrode was fabricated using black phosphorus-loaded Au (BP-Au) nanoparticles, which were then sequentially linked with DNA1, DNA2-labeled Au (Au-DNA2) nanoparticles, and methylene blue (MB). Cu²⁺ was first recognized and captured onto the sensor by BP with high selectivity and then produced a reduction current at around -0.01 V. The ^•OH quantification was established on the cleavage of the hybrid structure between DNA1 and BP-Au upon the appearance of ^•OH in the phosphate-buffered saline (PBS), leading to the depletion of the voltammetric response of MB around -0.25 V. Good linear correlations were obtained over concentrations of 0.5-127.5 μM for Cu²⁺ and 0.5-96.0 μM for ^•OH. Most importantly, the developed sensor was successfully applied to track the variations of the two species in brain tissues from APP/PS1 transgenic AD mice at the early stages before massive Aβ plaques appeared.

A gate-tunable symmetric bipolar junction transistor fabricated via femtosecond laser processing

Two-dimensional (2D) bipolar junction transistors (BJTs) with van der Waals heterostructures play an important role in the development of future nanoelectronics. Herein, a convenient method is introduced for fabricating a symmetric bipolar junction transistor (SBJT), constructed from black phosphorus and MoS₂, with femtosecond laser processing. This SBJT exhibits good bidirectional current amplification owing to its symmetric structure. We placed a top gate on one side of the SBJT to change the difference in the major carrier concentration between the emitter and collector in order to further investigate the effects of electrostatic doping on the device performance. The SBJT can also act as a gate-tunable phototransistor with good photodetectivity and photocurrent gain of β = ∼21. Scanning photocurrent images were used to determine the mechanism governing photocurrent amplification in the phototransistor. These results promote the development of the applications of multifunctional nanoelectronics based on 2D materials.

Super-Slippery Degraded Black Phosphorus/Silicon Dioxide Interface

The interfaces between two-dimensional (2D) materials and the silicon dioxide (SiO₂)/silicon (Si) substrate, generally considered as a solid-solid mechanical contact, have been especially emphasized for the structure design and the property optimization in microsystems and nanoengineering. The basic understanding of the interfacial structure and dynamics for 2D material-based systems still remains one of the inevitable challenges ahead. Here, an interfacial mobile water layer is indicated to insert into the interface of the degraded black phosphorus (BP) flake and the SiO₂/Si substrate owing to the induced hydroxyl groups during the ambient degradation. A super-slippery degraded BP/SiO₂ interface was observed with the interfacial shear stress (ISS) experimentally evaluated as low as 0.029 ± 0.004 MPa, being comparable to the ISS values of incommensurate rigid crystalline contacts. In-depth investigation of the interfacial structure through nuclear magnetic resonance spectroscopy and in situ X-ray photoelectron spectroscopy depth profiling revealed that the interfacial liquid water was responsible for the super-slippery BP/SiO₂ interface with extremely low shear stress. This finding clarifies the strong interactions between degraded BP and water molecules, which supports the potential wider applications of the few-layer BP nanomaterial in biological lubrication.

Multifunctional van der Waals Broken-Gap Heterojunction

The finite energy band-offset that appears between band structures of employed materials in a broken-gap heterojunction exhibits several interesting phenomena. Here, by employing a black phosphorus (BP)/rhenium disulfide (ReS₂ ) heterojunction, the tunability of the BP work function (Φ _BP ) with variation in flake thickness is exploited in order to demonstrate that a BP-based broken-gap heterojunction can manifest diverse current-transport characteristics such as gate tunable rectifying p-n junction diodes, Esaki diodes, backward-rectifying diodes, and nonrectifying devices as a consequence of diverse band-bending at the heterojunction. Diversity in band-bending near heterojunction is attributed to change in the Fermi level difference (Δ) between BP and ReS₂ sides as a consequence of Φ _BP modulation. No change in the current transport characteristics in several devices with fixed Δ also provides further evidence that current-transport is substantially impacted by band-bending at the heterojunction. Optoelectronic experiments on the Esaki diode and the p-n junction diode provide experimental evidence of band-bending diversity. Additionally, the p⁺ -n-p junction comprising BP (38 nm)/ReS₂ /BP(5.8 nm) demonstrates multifunctionality of binary and ternary inverters as well as exhibiting the behavior of a bipolar junction transistor with common-emitter current gain up to 50.

Electronic Transport through DNA Nucleotides in Atomically Thin Phosphorene Electrodes for Rapid DNA Sequencing

Rapid progresses in developing the fast, low-cost, and reliable methods for DNA sequencing are envisaged for development of personalized medicine. In this respect, nanotechnology has paved the role for the development of advanced DNA sequencing techniques including sequencing with solid-state nanopores or nanogaps. Herein, we have explored the application of a black phosphorene based nanogap-device for DNA sequencing. Using density-functional-theory based non-equilibrium Green's function approach, we have computed transverse transmission and current-voltage ( I- V) characteristics of all the four DNA nucleotides (deoxy adenosine monophosphate, deoxy guanidine monophosphate, deoxy thymidine monophosphate, and deoxy cytosine monophosphate) as functions of applied bias voltages. We deduce that it is in principle; possible to differentiate between all the four nucleotides by three sequencing runs at distinct applied bias voltages, i.e., at 0.2, 1.4, and 1.6 V, where individual identification of all the four nucleotides may be possible. Hence, we believe our study might be helpful for experimentalist towards the development of a phosphorene based nanodevice for DNA sequencing to diagnose critical diseases.