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Double perovskite oxides with antiferromagnetic ground state have received much consideration as they exhibit small stray-field and ultra-fast spin dynamics, which is extremely convenient for nanoelectronic devices.

Intrinsic structural phase is a crucial foundation for the fundamental physical properties, and for creating innovative devices with unprecedented performances and unique functionalities. Long-range ferromagnetic orders of van der Waals CrI3 are strongly tied with interlayer stacking orders. However, the intrinsic structure of few-layer CrI3 still remains elusive; the predicted monoclinic phase has not yet been experimentally detected in bare few-layer CrI3. Here we uncover the intrinsic structure of few-layer CrI3 with interlayer antiferromagnetic coupling, which unambiguously show monoclinic stacking in both bare and hBN-encapsulated bilayer and tri-five-layer CrI3 throughout an entire temperature range from 300 to 10 K. An exotic spring damping effect from hBN encapsulation layers is experimentally observed in hBN/CrI3/hBN heterostructures, which partly hinders interlayer sliding of CrI3. This work demonstrates the intrinsic monoclinic crystal phase of few-layer CrI3 and associated correlation with magnetic orders, opening up numerous opportunities for creating magnetic texture by stacking design.

The giant light-matter interaction induced by van Hove singularities (vHSs) of twisted bilayer graphene (tBLG) is responsible for enhanced optical absorption and strong photoresponse. Here, we investigated the evolution of vHSs in tBLG under pressure by using Raman spectroscopy. Pressure not only induces a blue shift of the G/R band but also tunes the intensity of the G/R band. The blue shift of the G/R band is due to the reduction of the in-plane lattice constant, and the variation of the G/R band intensity is due to the vHSs' shift of tBLG. Moreover, the main band in the absorption spectrum of tBLG is attributed to multiple transitions from valence to conduction bands. Because the ratio of R to G band intensity increases under pressure and the origins of R and G bands are different, we claim that pressure enhances intervalley electron scattering. This study paves the way for pressure engineering of vHS and the corresponding photon-electron-phonon interaction in tBLG.

Out-of-plane mirror symmetry breaking has enabled pseudo Janus structures as a platform to explore the coupling between electric polarization, piezoelectricity, spin–orbit coupling, and Rashba splitting in a two-dimensional (2D) system.

Substitutionally doped tin dichalcogenides have potential for electric, photoelectric and thermoelectric applications.

Layered ferromagnets with strong magnetic anisotropy energy (MAE) have special applications in nanoscale memory elements in electronic circuits. Here, we report a strain tunability of perpendicular magnetic anisotropy in van der Waals (vdW) ferromagnets VI

Double perovskite oxides (DPO) with antiferromagnetic ground state have received much consideration as they exhibit small stray-field and ultra-fast spin dynamics, which is extremely convenient for high-density and high-frequency data storage devices. It is a well-established fact that strain can easily tune the physical properties of the materials; therefore, the electronic and magnetic properties of recently synthesized ordered ALaCuOsO

Over the years, there have been major advances in two-dimensional (2D) materials on account of their excellent and unique properties. Among the various strategies for 2D material fabrication, chemical vapor deposition (CVD) is considered as the most promising method to achieve large-area and high-quality 2D film growth. Furthermore, to realize the potential applications of 2D materials in different fields, the integration of 2D materials into functional devices is essential. However, the materials made by common CVD are randomly distributed on substrates, which is disadvantageous for fabricating arrays of devices. To solve this problem, a site-selective growth method was developed to meet the requirement of batch production for practical applications because it achieves control over the locations of products and benefits the subsequent direct integration. Herein, state-of-the-art methods for site-selective synthesis, including seeded growth and patterned growth, are reviewed. Then, the electronic and optoelectronic applications of the as-grown 2D materials are also reviewed. Finally, the remaining challenges and future prospects regarding site-selective methods and applications are discussed.

The development of stable and low-cost catalysts with high reactivity to replace Pt-based ones is the central focus but challenging for hydrogen evolution reaction (HER). The incorporation of single atoms into two-dimensional (2D) supports has been demonstrated as an effective strategy because of the highly active single atomic sites and extremely large surface area of two-dimensional materials. However, the doping of single atoms is normally performed on the surface suffering from low stability, especially in acidic media. Moreover, it is experimentally challenging to produce monolayered 2D materials with atomic doping. Here, we propose a strategy to incorporate single foreign Fe atoms to substitute W atoms in sandwiched two-dimensional WS2. Because of the charge transfer between the doped Fe atom and its neighboring S atoms on the surface, the proximate S atoms become active for HER. Our theoretical prediction is later verified experimentally, showing an enhanced catalytic reactivity of Fe-doped WS2 in HER with the Volmer-Heyrovsky mechanism involved. We refer to this strategy as proximity catalysis, which is expected to be extendable to more sandwiched two-dimensional materials as substrates and transition metals as dopants.

Two-dimensional (2D) magnetic CrI3 has received considerable research attention because of its intrinsic features, including insulation, Ising ferromagnetism, and stacking-order-dependent magnetism, as well as potential in spintronic applications. However, the current strategy for the production of ambient-unstable CrI3 thin layer is limited to mechanical exfoliation, which normally suffers from uncontrollable layer thickness, small size, and low yet unpredictable yield. Here, via a confined vapor epitaxy (CVE) method, we demonstrate the mass production of flower-like CrI3 monolayers on mica. Interestingly, we discovered the crucial role of K ions on the mica surface in determining the morphology of monolayer CrI3, reacting with precursors to form a KIx buffer layer. Meanwhile, the transport agent affects the thickness and size of the as-grown CrI3. Moreover, the Curie temperature of CrI3 is greatly affected by the interaction between CrI3 and the substrate. The monolayer CrI3 on mica could act as a magnetic substrate for valley Zeeman splitting enhancement of WSe2. We reckon our work represents a major advancement in the mass production of monolayer 2D CrI3 and anticipate that our growth strategy may be extended to other transition metal halides.

Lead-free metal halide perovskites CsCu2X3 (X = Cl, Br, I) with a high photoluminescence quantum yield are promising materials for optoelectronic devices. However, the origin of photoluminescence (PL) emission is still under debate, and the anomalous dependence of PL on pressure is unclear. Here, we systemically study the effects of high pressure on the structural, electronic, and optical properties of CsCu2I3 using a diamond anvil cell (DAC) and first-principles calculations. We argue that the ground state structure of CsCu2I3 belongs to the pnma phase rather than the cmcm phase under ambient conditions. There is a structural phase transition from the pnma to the cmcm phase for CsCu2I3 at ∼5 GPa. The optical band gap derivative from absorption spectra increases from 3.57 to 3.62 eV within a pressure range of 0 to 4.03 GPa, and it then decreases over 4.03 GPa. There are two major PL emissions peaks at 2.11 and 2.32 eV, which are attributed to the intrinsic defect related trap states in CsCu2I3. Interestingly, there is an anomalous dependence of both PL emissions on pressure, such that PL peaks show a blueshift and the PL intensity is enhanced from 0 to ∼4 GPa, with redshifting and decreasing at pressures above ∼4 GPa. The anomalous evolution of the two PL emissions also suggests a defect origin of emissions.

As an emerging ultrathin semiconductor material, Bi2O2Se exhibits prominent performances in electronics, optoelectronics, ultrafast optics, etc. However, until now, the in-plane growth of Bi2O2Se thin films is mostly fulfilled on atomically flat mica substrates with interfacial electrostatic forces setting obstacles for Bi2O2Se transfer to fabricate functional van der Waals heterostructures. In this work, controlled growth of inclined Bi2O2Se ultrathin films is realized with apparently reduced interfacial contact areas upon mica flakes. Consequently, the transfer of Bi2O2Se could be facile by overcoming weaker electrostatic interactions. From cross-sectional characterizations at the Bi2O2Se/mica interfaces, it is found that there are no oxide buffer layers in existence for both in-plane and inclined growths, while the un-neutralized charge density is apparently decreased for inclined films. By mechanical pressing, inclined Bi2O2Se could be transferred onto SiO2/Si substrates, and back-gated Bi2O2Se field effect transistors are fabricated, outperforming previously reported in-plane Bi2O2Se devices transferred with the assistance of corrosive acids and adhesive polymers. Furthermore, Bi2O2Se/graphene heterostructures are fulfilled by a probe tip to fabricate hybrid phototransistors with pristine interfaces, exhibiting highly efficient photoresponses. The results in this work demonstrate the potential of inclined Bi2O2Se to act as a building block for prospective van der Waals heterostructures.

CrI3, a two-dimensional layered material, has recently attracted a lot of research interest due to its exotic magnetic property. However, the synthesis of a CrI3 monolayer (ML) by vapor-phase deposition has not been achieved by current experimental endeavors, which require a better understanding of the vapor-phase growth mechanism involved. In this study, we theoretically simulate the growth of the CrI3 ML on the Si(111) surface by using a first-principles calculation. Our key finding is that an iodine buffer layer on the Si surface is crucial to the formation of the CrI3 ML by stabilizing the precursor and also reacting with the precursor. Moreover, our simulation reveals that the CrI2 cluster as the growth building unit, which can be formed by controlling the chemical potential of the I source, is preferred for the formation of CrI3 ML. We think that our work will provide insightful guidance for the experimental synthesis of CrI3 ML in the future.

Chromium tri-iodide (CrI3), a recently discovered two-dimensional ferromagnetic material for spintronics, has attracted great attention because of its intriguing physical properties, such as insula...

Valley pseudospin in two-dimensional (2D) transition-metal dichalcogenides (TMDs) allows optical control of spin-valley polarization and intervalley quantum coherence. Defect states in TMDs give rise to new exciton features and theoretically exhibit spin-valley polarization; however, experimental achievement of this phenomenon remains challenges. Here, we report unambiguous valley pseudospin of defect-bound localized excitons in CVD-grown monolayer MoS2; enhanced valley Zeeman splitting with an effective g-factor of -6.2 is observed. Our results reveal that all five d-orbitals and the increased effective electron mass contribute to the band shift of defect states, demonstrating a new physics of the magnetic responses of defect-bound localized excitons, strikingly different from that of A excitons. Our work paves the way for the manipulation of the spin-valley degrees of freedom through defects toward valleytronic devices.

The Schottky junction, composed of a rectifying metal–semiconductor interface, is an essential component for microelectronic and optoelectronic devices. However, due to the considerable reverse tunneling current, typical Schottky junctions cannot be widely applied in devices requiring high signal-to-noise ratios, such as photodetectors with high detectivity. Here, a van der Waals (vdW) Schottky junction is constructed by mechanically stacking a gold (Au) electrode onto a multilayer indium selenide (InSe) nanosheet, which shows an ultralow reverse current in sub-picoamperes and an excellent rectification ratio exceeding 106 at room temperature. The reverse current, which corresponds to the thermionic emission transport model, is independent of the applied reverse bias. As a result, the Au–InSe vdW Schottky junction device can function as an ultrasensitive photodetector with a photodetectivity over 2.4 × 1015 Jones, corresponding to a photoresponsivity of 853 A W−1 and a light on/off ratio exceeding 1 × 107. The work offers an idea for investigating electronic and optoelectronic devices with high signal-to-noise ratios based on vdW Schottky junctions.

Layered ferromagnets with high coercivity have special applications in nanoscale memory elements in electronic circuits, such as data storage. Therefore, searching for new hard ferromagnets and effectively tuning or enhancing the coercivity are the hottest topics in layered magnets today. Here, we report a strain tunability of perpendicular magnetic anisotropy in van der Waals (vdW) ferromagnets VI3 using magnetic circular dichroism measurements. For an unstrained flake, the M-H curve shows a rectangular-shaped hysteresis loop with perpendicular magnetic anisotropy and a large coercivity (up to 1.775 T at 10 K). Furthermore, the coercivity can be enhanced to a maximum of 2.6 T at 10 K under a 2.9% in-plane tensile strain. Our DFT calculations show that the magnetic anisotropy energy (MAE) can be dramatically increased after applying an in-plain tensile strain, which contributes to the enhancement of coercivity in the VI3 flake. Meanwhile, the strain tunability on the coercivity of CrI3, with a similar crystal structure, is limited. The main reason is the strong spin-orbital coupling in V3+ in VI6 octahedra in comparison with that in Cr3+. The strain tunability of coercivity in VI3 flakes highlights its potential for integration into vdW heterostructures, paving the way toward nanoscale spintronic devices and applications in the future.

Layered two-dimensional (2D) semiconductors have been widely exploited in photodetectors due to their excellent electronic and optoelectronic properties. To improve their performance, photogating, photoconductive, photovoltaic, photothermoelectric, and other effects have been used in phototransistors and photodiodes made with 2D semiconductors or hybrid structures. However, it is difficult to achieve the desired high responsivity and linear photoresponse simultaneously in a monopolar conduction channel or a p-n junction. Here we present dual-channel conduction with ambipolar multilayer WSe2 by employing the device concept of dual-gate phototransistor, where p-type and n-type channels are produced in the same semiconductor using opposite dual-gating. It is possible to tune the photoconductive gain using a vertical electric field, so that the gain is constant with respect to the light intensity-a linear photoresponse, with a high responsivity of ~2.5*10^4 A/W. Additionally, the 1/f noise of the device is kept at a low level under the opposite dual-gating due to the reduction of current and carrier fluctuation, resulting in a high detectivity of ~2*10^13 Jones in the linear photoresponse regime. The linear photoresponse and high performance of our dual-gate WSe2 phototransistor offer the possibility of achieving high-resolution and quantitative light detection with layered 2D semiconductors., Comment: 29 pages, 4 figures

Two-dimensional (2D) semiconducting monolayers such as transition metal dichalcogenides (TMDs) are promising channel materials to extend Moore's Law in advanced electronics. Synthetic TMD layers from chemical vapor deposition (CVD) are scalable for fabrication but notorious for their high defect densities. Therefore, innovative endeavors on growth reaction to enhance their quality are urgently needed. Here, we report that the hydroxide W species, an extremely pure vapor phase metal precursor form, is very efficient for sulfurization, leading to about one order of magnitude lower defect density compared to those from conventional CVD methods. The field-effect transistor (FET) devices based on the proposed growth reach a peak electron mobility ~200 cm

Despite increasing research interest in two-dimensional (2D) perovskites, the stability and spectral response range of the photodetectors based on 2D perovskites are yet far from satisfactory. In this work, a semiconductor heterojunction is constructed based on dimethyl itaconate (DI) treated (BA)2PbI4 microplates and CsPb(Brx/I1−x)3 nanocrystals. The DI treatment not only passivates the defects but also blocks the moisture, resulting in improved stability and suppressed defect trapping. Meanwhile, the type-II heterojunctions facilitate the separation of electron–hole pairs. As a result, the photodetector based on the DI-(BA)2PbI4/CsPb(Brx/I1−x)3 exhibits a responsivity up to 209 nA/mW and a detectivity up to 5.9 × 108 Jones with a broad spectral response range of 400–600 nm. Furthermore, after storage in the ambient environment for 10 days, the responsivity retains about 70%, which is significantly ameliorated compared to the photodetector based on bare (BA)2PbI4 (drops more than 90%). Therefore, this work demonstrates that surface passivation and 2D/three-dimensional heterojunctions are promising strategies to improve responsivity, broaden spectral response range, and enhance stability of photodetectors based on 2D perovskites.

Since the discovery of graphene, van der Waals (vdW) two-dimensional (2D) materials have attracted considerable attention for various potential applications. Recently, a Se-terminated bismuth oxychalcogenide, Bi2O2Se, has been fabricated using the vapor deposition method. Bi2O2Se is not a vdW 2D material, but the as-grown substance shows 2D characteristics. For example, Bi2O2Se exhibits layer number-dependent absorption spectra in experiments, but until now, there has been no reasonable explanation as to why. Here, we propose a 50% Se-passivation surface model, which elucidates the production of such spectra. Our model is also consistent with recent observations using scanning tunneling microscopy. Moreover, high-resolution transmission electron microcopy observations show a broken zipper-like structure in Bi2O2Se. We ascribe Bi2O2Se as a zipper 2D material, and we summarize the characteristics of zipper 2D materials while proposing the development of others. Zipper 2D materials not only are an important subset of 2D materials but also bridge the gap between vdW 2D materials and traditional 3D materials. Because they are a big family, including insulators, semiconductors, and magnetic metals, zipper 2D materials lend themselves to a plethora of applications.

Plenty of research efforts have been devoted to developing the anisotropic 2D architectures with intriguing electrochemical and optoelectronic properties. However, the scalable production of nonlay...

Low-dimensional organic–inorganic hybrid perovskites have triggered many fundamental research studies due to their intrinsic tunable photovoltaic properties, technologically relevant stability, and...

Recently, Bi2O2Se is discovered as a promising two-dimensional (2D) semiconductor for next generation electronics because of its stability in air, high Hall mobility and on/off ratio at room-temperature. Using first-principles calculations, we predict half-metallicity in Mn-doped monolayer Bi2O2Se. The magnetism enhancement from 4.0 to 6.0 µB and half-metal to magnetic semiconductor transition of monolayer Bi2O2Se are realizable by applying biaxial tensile strain. The symmetry reduction at the elastic-plastic transition is the key factor in controlling the magnetic coupling order between the Mn and the neighboring Se atoms. The results imply a possible way to achieve two-dimensional magnetic semiconductors that have great potential applications in spintronic devices.

Recently, single-layer CrI3, a member of the chromium trihalides CrX3 (where X = Cl, Br, or I), has been exfoliated and experimentally demonstrated as an atomically thin material suitable for two-dimensional spintronics. Valley splitting due to the magnetic proximity effect has been demonstrated in a WSe2/CrI3 van der Waals heterojunction. However, the understanding of the mechanisms behind the favorable performance of CrI3 is still limited. Here, we systematically study the carrier mobility and the intrinsic point defects in CrX3 and assess their influence on valley splitting in WSe2/CrI3 by first-principles calculations. The flat-band nature induces extremely large carrier mass and ultralow carrier mobility. In addition, intrinsic point defects-localized states in the middle of the band gap-show deep transition energy levels and act as carrier recombination centers, further lowering the carrier mobility. Moreover, vacancies in CrI3 can enhance ferromagnetism and valley splitting in a WSe2/CrI3 heterojunction, proving that chromium trihalides are excellent ferromagnetic insulators for spintronic and valleytronic applications.

Three new porphyrin dyes XW45 − XW47 have been synthesized employing a dialkoxy-wrapped phenothiazine donor, a tetraalkoxy-wrapped porphyrin π-framework, a benzothiadiazole (BTD)-based auxiliary acceptor, and an anchoring benzoic acid group. On the basis of our previously reported dye XW36 , XW45 was synthesized by introducing a BTD unit to broaden the absorption spectrum, further introducing a hexyl-substituent into the BTD unit afforded XW46 , and an additional fluorine atom was introduced to the carboxyphenyl acceptor to afford XW47 . As expected, the BTD unit obviously broadens and red-shifts the absorption threshold of XW45 − XW47 to ca . 750 nm. Dye-sensitized solar cells (DSSCs) were fabricated based on a cobalt electrolyte using chenodeoxycholic acid (CDCA) as the coadsorbent. Under full sun illumination, XW45 exhibits an efficiency of 9.73%, which is slightly lower than that of 10.19% obtained for the reference dye SM315 . By contrast, XW46 and XW47 show higher efficiencies than SM315 owing to the improved anti-aggregation ability associated with the hexyl group on the BTD unit and better ICT effect induced by the fluorine atom on the carboxyphenyl unit. Thus, XW47 exhibits the highest efficiency of 10.41% among the porphyrin dyes. Furthermore, PT-C6 was used as the cosensitizer to improve the light harvesting ability and efficiencies of the cells due to its broad absorption within 350–560 nm. Thus, high efficiencies of 10.32%, 11.38% and 10.90% were achieved for the cosensitized solar cells based on XW45 – XW47 , respectively, owing to the obviously enhanced photocurrent density ( J SC). In addition, under 30% full sun illumination, XW46 + PT-C6 exhibits a high efficiency of 13.08%. These results give an effective method for building high performance DSSCs through the cosensitization of porphyrin dyes containing a wrapped donor, a wrapped porphyrin framework and a properly substituted auxiliary benzothiadiazole unit.

The generation and manipulation of valley polarization in controllable ways are important for the valley-related physics and devices. In analogy to multiferroic materials with more than one ferromagnetic, ferroelectric, and ferroelastic orders, a new triferroic system with ferromagnetism, ferroelectricity, and ferrovalley is proposed, namely, the monolayer AgBiP

As one of the bismuth-based oxychalcogenide materials, Bi2O2Se ultrathin films have received intense research interest due to their high carrier mobility, narrow bandgaps, ultrafast intrinsic photoresponse and long-term ambient stability; they exhibit great potential in electronic and optoelectronic applications. However, the device performance of photodetectors based on metal/Bi2O2Se/metal structures has degraded due to the undesirable defects or contaminants from the electrode deposition or the sample transfer processes. In this work, highly efficient photodetectors based on Au/Bi2O2Se junctions were achieved with Au electrodes transferred under the assistance of a probe tip to avoid contaminants from traditional lighography methods. Furthermore, to improve the charge transfer efficiency, specifically by increasing the intensity of the electrical field at the Au/Bi2O2Se interface and along the Bi2O2Se channels, the device annealing temperature was optimized to narrow the van der Waals gap at the Au/Bi2O2Se interface and the device channel length was shortened to improve the overall device performance. Among all the devices, the maximum device photoresponsivity was 9.1 A W−1, and the device response time could approach 36 μs; moreover, the photodetectors featured broadband spectral responses from 360 nm to 1090 nm.

The development of stable and low-cost catalysts with high reactivity to replace Pt-based ones is the central focus but challenging for hydrogen evolution reaction (HER). The incorporation of single atoms into two-dimensional (2D) supports has been demonstrated as an effective strategy because of the highly active single atomic sites and extremely large surface area of two-dimensional materials. However, the doping of single atoms is normally performed on the surface suffering from low stability, especially in acidic media. Moreover, it is experimentally challenging to produce monolayered 2D materials with atomic doping. Here, we propose a strategy to incorporate single foreign Fe atoms to substitute W atoms in sandwiched two-dimensional WS

Two-dimensional (2D) magnetic CrI

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Lead-free metal halide perovskites CsCu

Next-generation high-performance biological and chemical sensors based on the emerging multitudinous two-dimensional (2D) layered materials have been attracting great attention in recent years. The performance of 2D biochemical sensors is strongly dependent on the structural defects, which provide indispensable active sites for sensitive and selective adsorption of analytes. However, achieving controllable defect engineering is still a big challenge. In the present work, we propose achieving superior biochemical sensor performance with high-surface-density grain boundaries (GBs), a kind of ubiquitous structural defects, in polycrystalline 2D thin films, which can be controllably synthesized. As a proof-of-concept, by utilizing the high-density GBs in monolayer (1L) WS2 films, we fabricated a series of surface plasmon resonance (SPR) sensors for mercury ion (Hg2+) detection. Our investigation has demonstrated substantial sensitivity enhancement of Hg2+ detection down to trace attomolar-level quantification (detection limit of 1 aM), which is ascribed to the abundance of active sites on high-density GBs. This work provides a promising avenue for the design of ultra-sensitive sensors toward commercialized products based on the GB-rich 2D layered materials.

As an emerging ultrathin semiconductor material, Bi

Preferentially exposed active edges are advantageous in the application of two-dimensional materials (2DMs) in high-performance biochemical sensors. Despite current progresses, controllable large-scale production of edge-rich 2DM films for practical applications remains a challenge. In this work, we report the controllable single-step growth of wafer-scale edge-rich WS2 films of high-density nanoscale multilayer WS2 islands and their application in sensitive, low detection limit biochemical sensors. The abundant edge active sites are shown to be crucial to their superior biochemical sensing performance. Investigations reveal that high-density edge S-active sites enable the highly sensitive attomolar-level Hg2+ detection as direct target probes via selective Hg-S bonding. Furthermore, 1 fM level selective detection of target viral nucleic acids (RdRp, ORF1ad) are achieved via functionalization of active edge sites with cDNA receptors. Our investigations provide a comprehensive method for application of similar edge-rich 2DM films in high-performance sensors for broad ranges of analytes. Data and materials availability The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Materials .