Xingxing Zhang

2D atomically thin transition metal dichalcogenides (TMDs) show a remarkable light–matter interaction and atomic thickness. The possible maximum open‐circuit voltages, one of the most important photovoltaic parameters, of monolayer TMD‐based solar cells are quantified. The values determined from this work reveal the potential of atomically thin TMDs for high‐voltage, ultralight, flexible, and eye‐transparent future photovoltaic devices.

Abstract

One of the most fundamental parameters of any photovoltaic material is its quasi‐Fermi level splitting (∆µ) under illumination. This quantity represents the maximum open‐circuit voltage (V _oc) that a solar cell fabricated from that material can achieve. Herein, a contactless, nondestructive method to quantify this parameter for atomically thin 2D transition metal dichalcogenides (TMDs) is reported. The technique is applied to quantify the upper limits of V _oc that can possibly be achieved from monolayer WS₂, MoS₂, WSe₂, and MoSe₂‐based solar cells, and they are compared with state‐of‐the‐art perovskites. These results show that V _oc values of ≈1.4, ≈1.12, ≈1.06, and ≈0.93 V can be potentially achieved from solar cells fabricated from WS₂, MoS₂, WSe₂, and MoSe₂ monolayers at 1 Sun illumination, respectively. It is also observed that ∆µ is inhomogeneous across different regions of these monolayers. Moreover, it is attempted to engineer the observed ∆µ heterogeneity by electrically gating the TMD monolayers in a metal‐oxide‐semiconductor structure that effectively changes the doping level of the monolayers electrostatically and improves their ∆µ heterogeneity. The values of ∆µ determined from this work reveal the potential of atomically thin TMDs for high‐voltage, ultralight, flexible, and eye‐transparent future solar cells.

2D van der Waals (vdW) magnets, which present intrinsic ferromagnetic/antiferromagnetic ground states down to atomiclayer thicknesses, open a new horizon in materials science. Recent state‐of‐the‐art characterization and tuning of the magnetic properties of 2D vdW magnets are outlined. Future perspectives and emerging 2D vdW magnets are also discussed, to provide unprecedented opportunities in the fields of spintronics.

Abstract

2D van der Waals (vdW) magnets, which present intrinsic ferromagnetic/antiferromagnetic ground states at finite temperatures down to atomic‐layer thicknesses, open a new horizon in materials science and enable the potential development of new spin‐related applications. The layered structure of vdW magnets facilitates their atomic‐layer cleavability and magnetic anisotropy, which counteracts spin fluctuations, thereby providing an ideal platform for theoretically and experimentally exploring magnetic phase transitions in the 2D limit. With reduced dimensions, the susceptibility of 2D magnets to a large variety of external stimuli also makes them more promising than their bulk counterpart in various device applications. Here, the current status of characterization and tuning of the magnetic properties of 2D vdW magnets, particularly the atomic‐layer thickness, is presented. Various state‐of‐the‐art optical and electrical techniques have been applied to reveal the magnetic states of 2D vdW magnets. Other emerging 2D vdW magnets and future perspectives on the stacking strategy are also given; it is believed that they will excite more intensive research and provide unprecedented opportunities in the field of spintronics.

Few‐layer (2D) intercalation compounds present an attractive pathway to tune the structure, as well as the electronic, optical, and energy‐storage properties near the atomic limit. Recent advances in 2D intercalation are summarized in the historical context of bulk intercalation as well as scaling relationships to motivate research in synthesizing and characterizing 2D materials for optoelectronic and energy‐storage devices.

Abstract

Intercalation in few‐layer (2D) materials is a rapidly growing area of research to develop next‐generation energy‐storage and optoelectronic devices, including batteries, sensors, transistors, and electrically tunable displays. Identifying fundamental differences between intercalation in bulk and 2D materials will play a key role in developing functional devices. Herein, advances in few‐layer intercalation are addressed in the historical context of bulk intercalation. First, synthesis methods and structural properties are discussed, emphasizing electrochemical techniques, the mechanism of intercalation, and the formation of a solid‐electrolyte interphase. To address fundamental differences between bulk and 2D materials, scaling relationships describe how intercalation kinetics, structure, and electronic and optical properties depend on material thickness and lateral dimension. Here, diffusion rates, pseudocapacity, limits of staging, and electronic structure are compared for bulk and 2D materials. Next, the optoelectronic properties are summarized, focusing on charge transfer, conductivity, and electronic structure. For energy devices, opportunities also emerge to design van der Waals heterostructures with high capacities and excellent cycling performance. Initial studies of heterostructured electrodes are compared to state‐of‐the‐art battery materials. Finally, challenges and opportunities are presented for 2D materials in energy and optoelectronic applications, along with promising research directions in synthesis and characterization to engineer 2D materials for superior devices.

Two‐dimensional (W, Mo)(S, Se, Te)₂ atomic crystals offer a natural sizable and tunable bandgap, which makes them suitable candidates for semiconductor‐based optoelectronic and electronic device applications. Recent advances in controlled vapor‐phase growth of these 2D materials for heterojunction devices are reviewed, considering both experimental and theoretical aspects. Technical challenges to be overcome and future research directions are further discussed.

Abstract

An overview of recent developments in controlled vapor‐phase growth of 2D transition metal dichalcogenide (2D TMD) films is presented. Investigations of thin‐film formation mechanisms and strategies for realizing 2D TMD films with less‐defective large domains are of central importance because single‐crystal‐like 2D TMDs exhibit the most beneficial electronic and optoelectronic properties. The focus is on the role of the various growth parameters, including strategies for efficiently delivering the precursors, the selection and preparation of the substrate surface as a growth assistant, and the introduction of growth promoters (e.g., organic molecules and alkali metal halides) to facilitate the layered growth of (Mo, W)(S, Se, Te)₂ atomic crystals on inert substrates. Critical factors governing the thermodynamic and kinetic factors related to chemical reaction pathways and the growth mechanism are reviewed. With modification of classical nucleation theory, strategies for designing and growing various vertical/lateral TMD‐based heterostructures are discussed. Then, several pioneering techniques for facile observation of structural defects in TMDs, which substantially degrade the properties of macroscale TMDs, are introduced. Technical challenges to be overcome and future research directions in the vapor‐phase growth of 2D TMDs for heterojunction devices are discussed in light of recent advances in the field.

The growth mechanism of vertical heterostructures is understood in terms of nucleation and kinetics, where active clusters with a high diffusion barrier promote the nucleation on top of transition metal dichalcogenide templates to realize vertical heterostructures. Benefiting from the efficient control of the diffusion barrier of the active clusters, high‐quality vertical heterostructures with various configurations and compositions are successfully synthesized.

Abstract

The rational control of the nucleation and growth kinetics to enable the growth of 2D vertical heterostructure remains a great challenge. Here, an in‐depth study is provided toward understanding the growth mechanism of transition metal dichalcogenides (TMDCs) vertical heterostructures in terms of the nucleation and kinetics, where active clusters with a high diffusion barrier will induce the nucleation on top of the TMDC templates to realize vertical heterostructures. Based on this mechanism, in the experiment, through rational control of the metal/chalcogenide ratio in the vapor precursors, effective manipulation of the diffusion barrier of the active clusters and precise control of the heteroepitaxy direction are realized. In this way, a family of vertical TMDCs heterostructures is successfully designed. Optical studies and scanning transmission electron microscopy investigations exhibit that the resulting heterostructures possess atomic sharp interfaces without apparent alloying and defects. This study provides a deep understanding regarding the growth mechanism in terms of the nucleation and kinetics and the robust growth of 2D vertical heterostructures, defining a versatile material platform for fundamental studies and potential device applications.

An ethanol assisted transfer method for transferring two dimensional materials is favorable for the exploration of intrinsic properties. Various van der Waals heterostrustures, predefined patterns, suspended samples can be assembled based on this method. Furthermore, thermal conductivity at room‐temperature of 52 W m⁻¹ K⁻¹ for five layer MoS₂ is measured on a suspended microbridge device using this clean transfer method.

Abstract

As two dimensional materials (2D materials) demonstrate unique and diverse properties, clean transfer methods can serve a cornerstone for creative assembly of these 2D building blocks for both fundamental explorations and versatile applications. One of the major challenges for preserving the pristine properties of 2D materials during transfer and construction is to debond 2D materials from original substrates without inducing structural damage and external contamination. In this work, through both molecular dynamic studies and experimental demonstration, it is found that droplets of ethanol, a common and environmental friendly solvent, can be used to effectively reduce the adhesion energy between 2D materials and substrates, and therefore enable a clean transfer method for 2D mterials. Various assembled structures based on 2D building blocks, such as van der Waals heterostructures, predesigned artificial patterns, 2D materials on suspended devices are all demonstrated. Thermal conductivity measurements of MoS₂ nanosheets on a suspended microbridge device also confirm the successful application of suspended 2D transfer. It is expected that this ethanol assisted transfer method can enable clean assembly of 2D building blocks for construction of novel structures and suspended devices.

Photodetectors based on 2D materials with excellence performance are demonstrated in recent years. The fast development of this field, including many important progresses such as ultrahigh photoresponsivities, room temperature long wavelength infrared photodetections, images by graphene array devices, and polarization sensitive photodetections, are reviewed. The large number of 2D materials give great opportunities to explore novel high‐performance photodetectors.

Abstract

2D material based photodetectors have attracted many research projects due to their unique structures and excellent electronic and optoelectronic properties. These 2D materials, including semimetallic graphene, semiconducting black phosphorus, transition metal dichalcogenides, insulating hexagonal boron nitride, and their various heterostructures, show a wide distribution in bandgap values. To date, hundreds of photodetectors based on 2D materials have been reported. Here, a review of photodetectors based on 2D materials covering the detection spectrum from ultraviolet to infrared is presented. First, a brief insight into the detection mechanisms of 2D material photodetectors as well as introducing the figure‐of‐merits which are key factors for a reasonable comparison between different photodetectors is provided. Then, the recent progress on 2D material based photodetectors is reviewed. Particularly, the excellent performances such as broadband spectrum detection, ultrahigh photoresponsivity and sensitivity, fast response speed and high bandwidth, polarization‐sensitive detection are pointed out on the basis of the state‐of‐the‐art 2D photodetectors. Initial applications based on 2D material photodetectors are mentioned. Finally, an outlook is delivered, the challenges and future directions are discussed, and general advice for designing and realizing novel high‐performance photodetectors is given to provide a guideline for the future development of this fast‐developing field.

Transition metal ions (Mo and W) drive self‐organization of a metallo‐hydrogel (Mo‐gel and W‐gel) into a lamellar nanostructured soft material. Subsequent chalcogenization at moderate temperatures in a sulfur atmosphere (420 °C) yields large area transition metal dichalcogenides (TMDCs) (MoS₂ and WS₂) on versatile substrates. Thin film transition is directly printed using metallo‐hydrogel converted TMDC as channel material.

Abstract

Two dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted interest for their compelling nanoscale new properties and numerous potential applications including fast optoelectronic devices, ultrathin photovoltaics, and high‐performance catalysts. Large‐scale growth of uniform TMDC materials is essential for investigating their physics and for their integration into devices. However, the wafer scale deposition of TMDCs on arbitrary nonselective substrates is still beyond the current state‐of‐the‐art. In this article, a method to synthesize layered TMDCs (MoS₂ and WS₂) at the wafer‐scale by sulfurization of transition metal ions (Mo⁵⁺ and W⁶⁺) in a gelatin template (metallo‐hydrogel) is reported. This process is adaptable to versatile substrates, including amorphous silicon oxide, high‐temperature quartz, and silicon. Although the products are nominally few layer materials, direct band photoluminescent (≈1.8 eV), similar to single‐ or decoupled multilayer MoS₂ is observed. Finally, the solution‐based deposition enables contact printing of TMDC channels to be useable for device applications including thin film transistors with printed silver contacts using the same process.

Controlled synthesis of chemical vapor deposition‐WS₂ is achieved by sulfur‐mastery. The explicit control of thickness (layer number), morphology (triangle, truncated, and hexagon), and lateral dimension is achieved by precise control of feeding rate, optimized amount, and exposure time of sulfur precursors. Thickness‐dependent field effect transistor devices, patterned fluorescence, and integrated photodetector array are realized on the thickness‐, shape‐, and size‐controlled WS₂ flakes.

Abstract

Chemical vapor deposition (CVD) has been developed as the most promising method for the growth of transition metal dichalcogenides (TMDs). In this work, the key factor determining the growth of TMDs is ascertained. A straightforward method is devised to directly achieve a holistic control of thickness, shape, and size of WS₂ flakes via a single parameter control, namely, the status of the S‐precursor. The thickness‐dependent growth of WS₂ flakes from mono‐ to quad‐layers is achieved by precise control of the feeding rate of elemental S‐precursor. Moreover, the explicit control over amount and exposure time of S‐precursor determines the most optimum combination of these parameters to tune the shape of the crystals from triangular to hexagonal with appropriate size. Hence, the experimental findings provide a promising strategy to engineer the growth evolution of WS₂ atomic layers by fine tuning of the sulfur supply, paving a pathway to scalable electronic and photonic devices.

For the first time, continuous and homogeneous wafer‐scale monolayer MoS₂ films are grown by a two‐step process, with controllable excitonic and electronic properties. The excitonic and electronic performance of these films are superior to those of all previously reported two‐step methods. In fact, they are comparable to monolayer MoS₂ films deposited by chemical vapor deposition.

Abstract

To realize multifunctional devices at the wafer scale, the growth process of monolayer (ML) 2D semiconductors must meet two key requirements: 1) growth of continuous and homogeneous ML film at the wafer scale and 2) controllable tuning of the properties of the ML film. However, there is still no growth method available that fulfills both of these criteria. Here, the first report is presented on the preparation of continuous and uniform ML MoS₂ films through a two‐step process at the wafer scale. Unlike in previous ML MoS₂ film growth processes, the ML MoS₂ film can be uniformly modulated across the wafer in terms of material structure and composition, exciton state, and electronic transport performance. A significant result is that the high‐quality wafer‐scale ML MoS₂ films realize superior electronic performance compared to reported two‐step‐grown films, and it even matches or exceeds reported ML MoS₂ films prepared by other processes. The transistor performance of the optimized ML film achieves a field effect mobility of 10 to 30 cm² V⁻¹ s⁻¹, an on/off current ratio of about 10⁷, and hysteresis as low as 0.4 V.

Two-dimensional (2D) nanomaterials (sheet-like materials with few-atoms thickness and lateral size above 100 nm) have always aroused scientists’ interests since 2004 when Novoselov et al successfully exfoliated graphene from graphite using Scotch tape. It is some unique characters of 2D nanomaterials such as the confinement of electrons in two dimensions in the ultrathin region, strong in-plane covalent bond and atomic thickness, ultra-high specific surface area and exposed atoms that enable 2D nanomaterials to show excellent properties in electrics, catalysis and mechanics. Recently, amorphous materials (varied from crystal materials by atomic arrangement) have demonstrated high performance in mechanics, catalysis and magnetic owing to their unique long-range atomic disorder arrangements. Thus, the 2D amorphous nanomaterials inspire a new path to the study of high performance 2D materials. Herein, we summarize the recent progress in 2D amorphous nanomaterials, whose synt...

2D van der Waals (vdW) magnets, which present intrinsic ferromagnetic/antiferromagnetic ground states down to atomiclayer thicknesses, open a new horizon in materials science. Recent state‐of‐the‐art characterization and tuning of the magnetic properties of 2D vdW magnets are outlined. Future perspectives and emerging 2D vdW magnets are also discussed, to provide unprecedented opportunities in the fields of spintronics.

Abstract

2D van der Waals (vdW) magnets, which present intrinsic ferromagnetic/antiferromagnetic ground states at finite temperatures down to atomic‐layer thicknesses, open a new horizon in materials science and enable the potential development of new spin‐related applications. The layered structure of vdW magnets facilitates their atomic‐layer cleavability and magnetic anisotropy, which counteracts spin fluctuations, thereby providing an ideal platform for theoretically and experimentally exploring magnetic phase transitions in the 2D limit. With reduced dimensions, the susceptibility of 2D magnets to a large variety of external stimuli also makes them more promising than their bulk counterpart in various device applications. Here, the current status of characterization and tuning of the magnetic properties of 2D vdW magnets, particularly the atomic‐layer thickness, is presented. Various state‐of‐the‐art optical and electrical techniques have been applied to reveal the magnetic states of 2D vdW magnets. Other emerging 2D vdW magnets and future perspectives on the stacking strategy are also given; it is believed that they will excite more intensive research and provide unprecedented opportunities in the field of spintronics.

This review discusses 2D magnets with applications for low‐energy spintronics with an emphasis on experimental phenomena. Historical examples are discussed beginning in the 1960s. Recent progress using cleavable van der Waals materials in the period from 2016–2018 is highlighted. The theoretical mechanisms for improved magnetic performance are introduced and discussed against the background of experimental evidence.

Abstract

The recent discovery of 2D magnetic order in van der Waals materials has stimulated a renaissance in the field of atomically thin magnets. This has led to promising demonstrations of spintronic functionality such as tunneling magnetoresistance. The frantic pace of this emerging research, however, has also led to some confusion surrounding the underlying phenomena of phase transitions in 2D magnets. In fact, there is a rich history of experimental precedents beginning in the 1960s with quasi‐2D bulk magnets and progressing to the 1980s using atomically thin sheets of elemental metals. This review provides a holistic discussion of the current state of knowledge on the three distinct families of low‐dimensional magnets: quasi‐2D, ultrathin films, and van der Waals crystals. It highlights the unique opportunities presented by the latest implementation in van der Waals materials. By revisiting the fundamental insights from the field of low‐dimensional magnetism, this review highlights factors that can be used to enhance material performance. For example, the limits imposed on the critical temperature by the Mermin–Wagner theorem can be escaped in three separate ways: magnetocrystalline anisotropy, long‐range interactions, and shape anisotropy. Several recent experimental reports of atomically thin magnets with Curie temperatures above room temperature are highlighted.

An ethanol assisted transfer method for transferring two dimensional materials is favorable for the exploration of intrinsic properties. Various van der Waals heterostrustures, predefined patterns, suspended samples can be assembled based on this method. Furthermore, thermal conductivity at room‐temperature of 52 W m⁻¹ K⁻¹ for five layer MoS₂ is measured on a suspended microbridge device using this clean transfer method.

Abstract

As two dimensional materials (2D materials) demonstrate unique and diverse properties, clean transfer methods can serve a cornerstone for creative assembly of these 2D building blocks for both fundamental explorations and versatile applications. One of the major challenges for preserving the pristine properties of 2D materials during transfer and construction is to debond 2D materials from original substrates without inducing structural damage and external contamination. In this work, through both molecular dynamic studies and experimental demonstration, it is found that droplets of ethanol, a common and environmental friendly solvent, can be used to effectively reduce the adhesion energy between 2D materials and substrates, and therefore enable a clean transfer method for 2D mterials. Various assembled structures based on 2D building blocks, such as van der Waals heterostructures, predesigned artificial patterns, 2D materials on suspended devices are all demonstrated. Thermal conductivity measurements of MoS₂ nanosheets on a suspended microbridge device also confirm the successful application of suspended 2D transfer. It is expected that this ethanol assisted transfer method can enable clean assembly of 2D building blocks for construction of novel structures and suspended devices.

Transition metal ions (Mo and W) drive self‐organization of a metallo‐hydrogel (Mo‐gel and W‐gel) into a lamellar nanostructured soft material. Subsequent chalcogenization at moderate temperatures in a sulfur atmosphere (420 °C) yields large area transition metal dichalcogenides (TMDCs) (MoS₂ and WS₂) on versatile substrates. Thin film transition is directly printed using metallo‐hydrogel converted TMDC as channel material.

Abstract

Two dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted interest for their compelling nanoscale new properties and numerous potential applications including fast optoelectronic devices, ultrathin photovoltaics, and high‐performance catalysts. Large‐scale growth of uniform TMDC materials is essential for investigating their physics and for their integration into devices. However, the wafer scale deposition of TMDCs on arbitrary nonselective substrates is still beyond the current state‐of‐the‐art. In this article, a method to synthesize layered TMDCs (MoS₂ and WS₂) at the wafer‐scale by sulfurization of transition metal ions (Mo⁵⁺ and W⁶⁺) in a gelatin template (metallo‐hydrogel) is reported. This process is adaptable to versatile substrates, including amorphous silicon oxide, high‐temperature quartz, and silicon. Although the products are nominally few layer materials, direct band photoluminescent (≈1.8 eV), similar to single‐ or decoupled multilayer MoS₂ is observed. Finally, the solution‐based deposition enables contact printing of TMDC channels to be useable for device applications including thin film transistors with printed silver contacts using the same process.

A green and facile strategy is developed to generate few‐layer black phosphorus (BP): ice‐assisted exfoliation from bulk BP. A metal‐free photocatalyst is synthesized by combining BP and g‐C₃N₄, which protects BP from oxidation and leads to efficient and broadband activity as well as long‐term stability in H₂ evolution. Benefiting from formed NP bonds, the efficient charge transfer between them yields excellent photocatalytic performance.

Abstract

A 2D/2D heterojunction of black phosphorous (BP)/graphitic carbon nitride (g‐C₃N₄) is designed and synthesized for photocatalytic H₂ evolution. The ice‐assisted exfoliation method developed herein for preparing BP nanosheets from bulk BP, leads to high yield of few‐layer BP nanosheets (≈6 layers on average) with large lateral size at reduced duration and power for liquid exfoliation. The combination of BP with g‐C₃N₄ protects BP from oxidation and contributes to enhanced activity both under λ > 420 nm and λ > 475 nm light irradiation and to long‐term stability. The H₂ production rate of BP/g‐C₃N₄ (384.17 µmol g⁻¹ h⁻¹) is comparable to, and even surpasses that of the previously reported, precious metal‐loaded photocatalyst under λ > 420 nm light. The efficient charge transfer between BP and g‐C₃N₄ (likely due to formed NP bonds) and broadened photon absorption (supported both experimentally and theoretically) contribute to the excellent photocatalytic performance. The possible mechanisms of H₂ evolution under various forms of light irradiation is unveiled. This work presents a novel, facile method to prepare 2D nanomaterials and provides a successful paradigm for the design of metal‐free photocatalysts with improved charge‐carrier dynamics for renewable energy conversion.

Transition metal dichalcogenide nanodisks as high-index dielectric Mie nanoresonators

Transition metal dichalcogenide nanodisks as high-index dielectric Mie nanoresonators, Published online: 06 May 2019; doi:10.1038/s41565-019-0442-x

In individual tungsten diselenide nanodisks, excitonic and anapole modes can strongly couple to form a polariton.

2D semiconductors hosting strain-induced quantum emitters offer unique abilities to achieve scalable architectures for deterministic coupling to nanocavities and waveguides that are required to enable chip-based quantum information processing technologies. A severe drawback remains that exciton emission from quantum emitters in WSe 2 quenches beyond 30 K, which requires cryogenic cooling. Here we demonstrate an approach to increase the temperature survival of exciton quantum emitters in WSe 2 that is based on maximizing the emitter quantum yield. Utilizing optimized material growth that leads to reduced density of nonradiative defects as well as coupling of the exciton emission to plasmonic nanocavities modes, we achieve average quantum yields up to 44%, thermal activation energies up to 92 meV, and single photon emission signatures up to temperatures of 160 K. At these values non-cryogenic cooling with thermo-electric chips becomes already feasible, while qu...

Scanning thermal microscopy is used to spatially map the temperature distribution within monolayer transition metal dichalcogenide devices upon dissipating electrical power across lateral interfaces. These findings demonstrate that lateral MoS₂–WS₂ heterojunctions form well‐stitched interfaces that have a modest effect on localization of the dissipated heat, while grain boundaries of MoS₂ exhibit various defects and lead to significant non‐uniform heating.

Abstract

Lateral heterogeneities in atomically thin 2D materials such as in‐plane heterojunctions and grain boundaries (GBs) provide an extrinsic knob for manipulating the properties of nano‐ and optoelectronic devices and harvesting novel functionalities. However, these heterogeneities have the potential to adversely affect the performance and reliability of the 2D devices through the formation of nanoscopic hot‐spots. In this report, scanning thermal microscopy (SThM) is utilized to map the spatial distribution of the temperature rise within monolayer transition metal dichalcogenide (TMD) devices upon dissipating a high electrical power through a lateral interface. The results directly demonstrate that lateral heterojunctions between MoS₂ and WS₂ do not largely impact the distribution of heat dissipation, while GBs of MoS₂ appreciably localize heating in the device. High‐resolution scanning transmission electron microscopy reveals that the atomic structure is nearly flawless around heterojunctions but can be quite defective near GBs. The results suggest that the interfacial atomic structure plays a crucial role in enabling uniform charge transport without inducing localized heating. Establishing such structure–property‐processing correlation provides a better understanding of lateral heterogeneities in 2D TMD systems which is crucial in the design of future all‐2D electronic circuitry with enhanced functionalities, lifetime, and performance.

Ultra‐broadband photodetectors based on noble transition metal dichalcogenide, PdSe₂, with unique pentagonal atomic structure are demonstrated. Devices respond from visible to mid‐infrared (up to ≈4.05 µm) and efficient absorption beyond 8 µm is observed. The maximum photoresponsivity and photogain are 708 A W⁻¹ and 82 700%, respectively. Anisotropic photoresponse is also observed.

Abstract

Photodetection over a broad spectral range is crucial for optoelectronic applications such as sensing, imaging, and communication. Herein, a high‐performance ultra‐broadband photodetector based on PdSe₂ with unique pentagonal atomic structure is reported. The photodetector responds from visible to mid‐infrared range (up to ≈4.05 µm), and operates stably in ambient and at room temperature. It promises improved applications compared to conventional mid‐infrared photodetectors. The highest responsivity and external quantum efficiency achieved are 708 A W⁻¹ and 82 700%, respectively, at the wavelength of 1064 nm. Efficient optical absorption beyond 8 µm is observed, indicating that the photodetection range can extend to longer than 4.05 µm. Owing to the low crystalline symmetry of layered PdSe₂, anisotropic properties of the photodetectors are observed. This emerging material shows potential for future infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Ultrathin CoSe nanoplates with tunable structure phases are synthesized by a chemical vapor deposition route. Electrical transport studies reveal that both types of CoSe nanoplates show strong thickness‐tunable electrical properties, excellent breakdown current density, yet distinct conductance trends with the decreasing temperature. The tetragonal CoSe nanoplates show angle‐dependent magnetoresistance and weak antilocalization at lower field.

Abstract

Multiple structural phases in transition metal dichalcogenides have attracted considerable recent interest for their tunable chemical and electronic properties. Herein, a chemical vapor deposition route to ultrathin CoSe nanoplates with tunable structure phases is reported. By precisely tailoring the growth temperature, ultrathin 2D layered tetragonal CoSe nanoplates and nonlayered hexagonal CoSe nanoplates can be selectively prepared as square or hexagonal geometries, with thickness as thin as 2.3 and 3.7 nm, respectively. X‐ray diffraction, transmission electron microscopy, and selected area electron diffraction studies show that both types of nanoplates are high‐quality single crystals. Electrical transport studies reveal that both the tetragonal and hexagonal CoSe nanoplates show strong thickness‐tunable electrical properties and excellent breakdown current density. The 2D hexagonal CoSe nanoplates display metallic behavior with an excellent conductivity up to 6.6 × 10⁵ S m⁻¹ and an extraordinary breakdown current density up to 3.9 × 10⁷ A cm⁻², while the square tetragonal nanoplates show considerably lower conductivity up to 8.2 × 10⁴ S m⁻¹ with angle‐dependent magnetoresistance and weak antilocalization effect at lower field. This study offers a tunable material system for exploring multiphase 2D materials and their potential applications for electronic and magnetoelectronic devices.

Excitonic valley polarization in electrostatically doped monolayer WSe₂ is controlled by modulating the carrier density. Screening of long‐range e–h exchange interactions by the doped carriers suppresses valley relaxation for neutral excitons between the K and −K valleys.

Abstract

Enhancement and continuous control of the excitonic valley polarization in electrostatically doped monolayer WSe₂ are demonstrated. Under excitation with circularly polarized light, 20% valley polarization of excitons around the charge neutrality condition at 70 K is increased to 40% by modulating the electron/hole density up to 2 × 10¹² cm⁻². This increase originates from slow valley relaxation for neutral exciton between the K and −K valleys owing to screening of long‐range e–h exchange interactions by doped carriers. The gate‐dependences of the exciton valley polarization at various temperatures are reproduced by theoretical calculations, which holds potential for next‐generation valleytronic devices continuously controlled by an applied bias voltage.