Coordination Polymer to Atomically Thin,Holey, Metal‐Oxide Nanosheets for Tuning Band Alignment

Holey 2D metal oxides have shown great promise as functional materials for energy storage and catalysts. Despite impressive performance, their processing is challenged by the requirement of templates plus capping agents or high temperatures; these materials also exhibit excessive thicknesses and low yields. The present work reports a metal‐based coordination polymer (MCP) strategy to synthesize polycrystalline, holey, metal oxide (MO) nanosheets with thicknesses as low as two‐unit cells. The process involves rapid exfoliation of bulk‐layered, MCPs (Ce‐, Ti‐, Zr‐based) into atomically thin MCPs at room temperature, followed by transformation into holey 2D MOs upon the removal of organic linkers in aqueous solution. Further, this work represents an extra step for decorating the holey nanosheets using precursors of transition metals to engineer their band alignments, establishing a route to optimize their photocatalysis. The work introduces a simple, high‐yield, room‐temperature, and template‐free approach to synthesize ultrathin holey nanosheets with high‐level functionalities.     

Scalable Functionalization of Optical Fibers Using Atomically Thin Semiconductors

Atomically thin transition metal dichalcogenides are highly promising for integrated optoelectronic and photonic systems due to their exciton-driven linear and nonlinear interactions with light. Integrating them into optical fibers yields novel opportunities in optical communication, remote sensing, and all-fiber optoelectronics. However, the scalable and reproducible deposition of high-quality monolayers on optical fibers is a challenge. Here, the chemical vapor deposition of monolayer MoS₂ and WS₂ crystals on the core of microstructured exposed-core optical fibers and their interaction with the fibers’ guided modes are reported. Two distinct application possibilities of 2D-functionalized waveguides to exemplify their potential are demonstrated. First, the excitonic 2D material photoluminescence is simultaneously excited and collected with the fiber modes, opening a novel route to remote sensing. Then it is shown that third-harmonic generation is modified by the highly localized nonlinear polarization of the monolayers, yielding a new avenue to tailor nonlinear optical processes in fibers. It is anticipated that the results may lead to significant advances in optical-fiber-based technologies.     

High‐Performance Photodiode Based on Atomically Thin WSe2/MoS2 Nanoscroll Integration

Self‐assembled structures of 2D materials with novel physical and chemical properties, such as the good electrical and optoelectrical performance in nanoscrolls, have attracted a lot of attention. However, high photoresponse speed as well as high responsivity cannot be achieved simultaneously in the nanoscrolls. Here, a photodiode consisting of single MoS₂ nanoscrolls and a p‐type WSe₂ is demonstrated and shows excellent photovoltaic characteristics with a large open‐circuit voltage of 0.18 V and high current intensity. Benefiting from the heterostructure, the dark current is suppressed resulting in an increased ratio of photocurrent to dark current (two orders of magnitude higher than the single MoS₂ nanoscroll device). Furthermore, it yields high responsivity of 0.3 A W^?1 (corresponding high external quantum efficiency of ≈75%) and fast response time of 5 ms, simultaneously. The response speed is increased by three orders of magnitude over the single MoS₂ nanoscroll device. In addition, broadband photoresponse up to near‐infrared could be achieved. This atomically thin WSe₂/MoS₂ nanoscroll integration not only overcomes the disadvantage of MoS₂ nanoscrolls, but also demonstrates a single nanoscroll‐based heterostructure with high performance, promising its potential in the future optoelectronic applications.     

Efficient All-Optical Plasmonic Modulators with Atomically Thin Van Der Waals Heterostructures

All-optical modulators are attracting significant attention due to their intrinsic perspective on high-speed, low-loss, and broadband performance, which are promising to replace their electrical counterparts for future information communication technology. However, high-power consumption and large footprint remain obstacles for the prevailing nonlinear optical methods due to the weak photon–photon interaction. Here, efficient all-optical mid-infrared plasmonic waveguide and free-space modulators in atomically thin graphene-MoS₂ heterostructures based on the ultrafast and efficient doping of graphene with the photogenerated carrier in the monolayer MoS₂ are reported. Plasmonic modulation of 44 cm⁻¹ is demonstrated by an LED with light intensity down to 0.15 mW cm⁻², which is four orders of magnitude smaller than the prevailing graphene nonlinear all-optical modulators (≈10³ mW cm⁻²). The ultrafast carrier transfer and recombination time of photogenerated carriers in the heterostructure may achieve ultrafast modulation of the graphene plasmon. The demonstration of the efficient all-optical mid-infrared plasmonic modulators, with chip-scale integrability and deep-sub wavelength light field confinement derived from the van der Waals heterostructures, may be an important step toward on-chip all-optical devices.     

Nonlinear All-Optical Coherent Generation and Read-Out of Valleys in Atomically Thin Semiconductors

With conventional electronics reaching performance and size boundaries, all-optical processes have emerged as ideal building blocks for high speed and low power consumption devices. A promising approach in this direction is provided by valleytronics in atomically thin semiconductors, where light-matter interaction allows to write, store, and read binary information into the two energetically degenerate but non-equivalent valleys. Here, nonlinear valleytronics in monolayer WSe₂ is investigated and show that an individual ultrashort pulse with a photon energy tuned to half of the optical band-gap can be used to simultaneously excite (by coherent optical Stark shift) and detect (by a rotation in the polarization of the emitted second harmonic) the valley population.     

Molecular Beam Epitaxy and Electronic Structure of Atomically Thin Oxyselenide Films

Atomically thin oxychalcogenides have been attracting intensive attention for their fascinating fundamental properties and application prospects. Bi₂O₂Se, a representative of layered oxychalcogenides, has emerged as an air‐stable high‐mobility 2D semiconductor that holds great promise for next‐generation electronics. The preparation and device fabrication of high‐quality Bi₂O₂Se crystals down to a few atomic layers remains a great challenge at present. Here, molecular beam epitaxy (MBE) of atomically thin Bi₂O₂Se films down to monolayer on SrTiO₃ (001) substrate is achieved by co‐evaporating Bi and Se precursors in oxygen atmosphere. The interfacial atomic arrangements of MBE‐grown Bi₂O₂Se/SrTiO₃ are unambiguously revealed, showing an atomically sharp interface and atom‐to‐atom alignment. Importantly, the electronic band structures of one‐unit‐cell (1‐UC) thick Bi₂O₂Se films are observed by angle‐resolved photoemission spectroscopy (ARPES), showing low effective mass of ≈0.15 m₀ and bandgap of ≈0.8 eV. These results may be constructive to the synthesis of other 2D oxychalcogenides and investigation of novel physical properties.     

Room-Temperature Synthesis of 2D Janus Crystals and their Heterostructures

Janus crystals represent an exciting class of 2D materials with different atomic species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an electric field and leads to a wealth of novel properties, such as large Rashba spin–orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe₂ and via plasma stripping followed thermal annealing of MoS₂. However, the high processing temperatures prevent growth of other Janus materials and their heterostructures. Here, a room-temperature technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low-energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room-temperature method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.     

Efficient and Layer‐Dependent Exciton Pumping across Atomically Thin Organic–Inorganic Type‐I Heterostructures

The fundamental light–matter interactions in monolayer transition metal dichalcogenides might be significantly engineered by hybridization with their organic counterparts, enabling intriguing optoelectronic applications. Here, atomically thin organic–inorganic (O–I) heterostructures, comprising monolayer MoSe₂ and mono‐/few‐layer single‐crystal pentacene samples, are fabricated. These heterostructures show type‐I band alignments, allowing efficient and layer‐dependent exciton pumping across the O–I interfaces. The interfacial exciton pumping has much higher efficiency (>86 times) than the photoexcitation process in MoSe₂, although the pentacene layer has much lower optical absorption than MoSe₂. This highly enhanced pumping efficiency is attributed to the high quantum yield in pentacene and the ultrafast energy transfer between the O–I interface. Furthermore, those organic counterparts significantly modulate the bindings of charged excitons in monolayer MoSe₂ via their precise dielectric environment engineering. The results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using atomically thin O–I heterostructures.     

Role of Charge Traps in the Performance of Atomically Thin Transistors

Transient currents in atomically thin MoTe₂ field‐effect transistors (FETs) are measured during cycles of pulses through the gate electrode. The curves of the transient currents are analyzed in light of a newly proposed model for charge‐trapping dynamics that renders a time‐dependent change in the threshold voltage as the dominant effect on the channel hysteretic behavior over emission currents from the charge traps. The proposed model is expected to be instrumental in understanding the fundamental physics that governs the performance of atomically thin FETs and is applicable to the entire class of atomically thin‐based devices. Hence, the model is vital to the intelligent design of fast and highly efficient optoelectronic devices.     

Unveiling the Fine Structural Distortion of Atomically Thin Bi2O2Se by Third-Harmonic Generation

Bismuth oxyselenide (Bi₂O₂Se), a new type of 2D material, has recently attracted increased attention due to its robust bandgap, stability under ambient conditions, and ultrahigh electron mobility. In such complex oxides, fine structural distortion tends to play a decisive role in determining the unique physical properties, such as the ferrorotational order, ferroelectricity, and magnetoelasticity. Therefore, an in-depth investigation of the fine structural symmetry of Bi₂O₂Se is necessary to exploit its potential applications. However, conventional techniques are either time consuming or requiring tedious sample treatment. Herein, a noninvasive and high-throughput approach is reported for characterizing the fine structural distortion in 2D centrosymmetric Bi₂O₂Se by polarization-dependent third-harmonic generation (THG). Unprecedentedly, the divergence between the experimental results and the theoretical prediction of the perpendicular component of polarization-dependent THG indicates a fine structural distortion, namely, a <1.4° rotation of the oxygen square in the tetragonal (Bi₂O₂) layers. This rotation breaks the intrinsic mirror symmetry of 2D Bi₂O₂Se, eventually reducing the symmetry from the D_4h to the C_4h point group. The results demonstrate that THG is highly sensitive to even fine symmetry variations, thereby showing its potential to uncover hidden phase transitions and interacting polarized sublattices in novel 2D material systems.     

The First 2D Homochiral Lead Iodide Perovskite Ferroelectrics: [R‐ and S‐1‐(4‐Chlorophenyl)ethylammonium]2PbI4

2D organic–inorganic lead iodide perovskites have recently received tremendous attention as promising light absorbers for solar cells, due to their excellent optoelectronic properties, structural tunability, and environmental stability. However, although great efforts have been made, no 2D lead iodide perovskites have been discovered as ferroelectrics, in which the ferroelectricity may improve the photovoltaic performance. Here, by incorporating homochiral cations, 2D lead iodide perovskite ferroelectrics [R‐1‐(4‐chlorophenyl)ethylammonium]₂PbI₄ and [S‐1‐(4‐chlorophenyl)ethylammonium]₂PbI₄ are successfully obtained. The vibrational circular dichroism spectra and crystal structural analysis reveal their homochirality. They both crystalize in a polar space group P1 at room temperature, and undergo a 422F1 type ferroelectric phase transition with transition temperature as high as 483 and 473.2 K, respectively, showing a multiaxial ferroelectric nature. They also possess semiconductor characteristics with a direct bandgap of 2.34 eV. Nevertheless, their racemic analogue adopts a centrosymmetric space group P2₁/c at room temperature, exhibiting no high‐temperature phase transition. The homochirality in 2D lead iodide perovskites facilitates crystallization in polar space groups. This finding indicates an effective way to design high‐performance 2D lead iodide perovskite ferroelectrics with great application prospects.     

Heterostructures Based on 2D Materials: A Versatile Platform for Efficient Catalysis

The unique structural and electronic properties of 2D materials, including the metal and metal‐free ones, have prompted intense exploration in the search for new catalysts. The construction of different heterostructures based on 2D materials offers great opportunities for boosting the catalytic activity in electo(photo)chemical reactions. Particularly, the merits resulting from the synergism of the constituent components and the fascinating properties at the interface are tremendously interesting. This scenario has now become the state‐of‐the‐art point in the development of active catalysts for assisting energy conversion reactions including water splitting and CO₂ reduction. Here, starting from the theoretical background of the fundamental concepts, the progressive developments in the design and applications of heterostructures based on 2D materials are traced. Furthermore, a personal perspective on the exploration of 2D heterostructures for further potential application in catalysis is offered.     

High‐Performance Photovoltaic Detector Based on MoTe2/MoS2 Van der Waals Heterostructure

Van der Waals heterostructures based on 2D layered materials have received wide attention for their multiple applications in optoelectronic devices, such as solar cells, light‐emitting devices, and photodiodes. In this work, high‐performance photovoltaic photodetectors based on MoTe₂/MoS₂ vertical heterojunctions are demonstrated by exfoliating‐restacking approach. The fundamental electric properties and band structures of the junction are revealed and analyzed. It is shown that this kind of photodetectors can operate under zero bias with high on/off ratio (>10⁵) and ultralow dark current (≈3 pA). Moreover, a fast response time of 60 µs and high photoresponsivity of 46 mA W^?1 are also attained at room temperature. The junctions based on 2D materials are expected to constitute the ultimate functional elements of nanoscale electronic and optoelectronic applications.     

Mobility Engineering in Vertical Field Effect Transistors Based on Van der Waals Heterostructures

Vertical integration of 2D layered materials to form van der Waals heterostructures (vdWHs) offers new functional electronic and optoelectronic devices. However, the mobility in vertical carrier transport in vdWHs of vertical field‐effect transistor (VFET) is not yet investigated in spite of the importance of mobility for the successful application of VFETs in integrated circuits. Here, the mobility in VFET of vdWHs under different drain biases, gate biases, and metal work functions is first investigated and engineered. The traps in WSe₂ are the main source of scattering, which influences the vertical mobility and three distinct transport mechanisms: Ohmic transport, trap‐limited transport, and space‐charge‐limited transport. The vertical mobility in VFET can be improved by suppressing the trap states by raising the Fermi level of WSe₂. This is achieved by increasing the injected carrier density by applying a high drain voltage, or decreasing the Schottky barrier at the graphene/WSe₂ and metal/WSe₂ junctions by applying a gate bias and reducing the metal work function, respectively. Consequently, the mobility in Mn vdWH at +50 V gate voltage is about 76 times higher than the initial mobility of Au vdWH. This work enables further improvements in the VFET for successful application in integrated circuits.     

Engineering the Phases and Heterostructures of Ultrathin Hybrid Perovskite Nanosheets

Low-dimensional perovskites have gained increasing attention recently, and engineering their material phases, structural patterning and interfacial properties is crucial for future perovskite-based applications. Here a phase and heterostructure engineering on ultrathin perovskites, through the reversible cation exchange of hybrid perovskites and efficient surface functionalization of low-dimensional materials, is demonstrated. Using PbI₂ as precursor and template, perovskite nanosheets of varying thickness and hexagonal shape on diverse substrates is obtained. Multiple phases, such as PbI₂, MAPbI₃ and FAPbI₃, can be flexibly designed and transformed as a single nanosheet. A perovskite nanosheet can be patterned using masks made of 2D materials, fabricating lateral heterostructures of perovskite and PbI₂. Perovskite-based vertical heterostructures show strong interfacial coupling with 2D materials. As a demonstration, monolayer MoS₂/MAPbI₃ stacks give a type-II heterojunction. The ability to combine the optically efficient perovskites with versatile 2D materials creates possibilities for new designs and functionalities.