Xingxing Zhang

Realizing efficient spin‐orbit‐based switching requires the harnessing of both new materials and physics to obtain high charge‐to‐spin conversion efficiencies. The observation of spin‐orbit torque switching in bilayers consisting of a semimetallic film of 1T′‐MoTe₂ adjacent to permalloy is reported. Deterministic switching is achieved without external magnetic fields at room temperature with currents one order of magnitude smaller than those using heavy metals.

Abstract

The ability to switch magnetic elements by spin‐orbit‐induced torques has recently attracted much attention for a path toward high‐performance, nonvolatile memories with low power consumption. Realizing efficient spin‐orbit‐based switching requires the harnessing of both new materials and novel physics to obtain high charge‐to‐spin conversion efficiencies, thus making the choice of spin source crucial. Here, the observation of spin‐orbit torque switching in bilayers consisting of a semimetallic film of 1T′‐MoTe₂ adjacent to permalloy is reported. Deterministic switching is achieved without external magnetic fields at room temperature, and the switching occurs with currents one order of magnitude smaller than those typical in devices using the best‐performing heavy metals. The thickness‐dependence can be understood if the interfacial spin‐orbit contribution is considered in addition to the bulk spin Hall effect. Further threefold reduction in the switching current is demonstrated with resort to dumbbell‐shaped magnetic elements. These findings foretell exciting prospects of using MoTe₂ for low‐power semimetal‐material‐based spin devices.

In situ environmental transmission electron microscopy (TEM) measurements show that pit formation by thermal oxidation is preferentially initiated at the interface between adventitious carbon (C) nanoparticles and monolayer molybdenum disulfide (MoS₂), rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS₂ interface favors the sequential adsorption of oxygen atoms with facile kinetics.

Abstract

Forming pits on molybdenum disulfide (MoS₂) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS₂, and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS₂ monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS₂ monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS₂, rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS₂ interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS₂.

Rationally designed artificial chiral architectures have found broad applications in various fields. In article number https://doi.org/10.1002/adma.2020023562002356, Yanlei Hu, Dong Wu, and co‐workers describe how highly ordered chiral microstructures can be spontaneously assembled by the meniscus‐directed capillary force arising in an evaporating liquid. The chirality is facilitated by rationally breaking the intrinsic symmetry through cooperative control of the geometry and spatial topology of the pillar unit cells.

In article number https://doi.org/10.1002/adma.2020016562001656, Jyoti Katoch, Søran Ulstrup, and co‐workers apply nanoscale angle‐resolved photoemission (nanoARPES) to measure the superlattice van Hove singularity formed by the Dirac cones of twisted bilayer graphene. The photoemission experiment is performed on a device architecture that permits electrostatic tuning of the doping level, revealing the doping dependence of the singularity over a wide energy range.

A flexible large‐area light‐emitting device is realized by integrating a centimeter‐scale grown WS₂ monolayer into a p–n device architecture on conductive polymer foil. This flexible device demonstrates homogeneous red light emission from a 6 mm² area. Uniquely, the electroluminescence can be tuned over 30 meV simply by bending the devices, i.e., by applying a defined strain.

Abstract

Strong covalent in‐plane bonds and a tiny thickness in the nanometer range make two‐dimensional (2D) materials ideally suited for flexible electronic or optoelectronic applications. Despite this exciting perspective, only a few prototypes of such flexible devices—photodetectors and transistors—have been reported until now. The first large‐area flexible light‐emitting device (LED) based on 2D materials is realized by integrating a transition metal dichalcogenide (TMDC) monolayer synthesized by metal organic chemical vapor deposition (MOCVD) into a p–n architecture on conductive polymer foil. This flexible LED demonstrates homogeneous red light emission from a few square millimeter area in a scalable design. Uniquely, the electroluminescence can be tuned over 30 meV simply by bending the devices, i.e., by applying a defined strain. This approach combines the flexibility of organic semiconductor device concepts with the durability of inorganic semiconductor technology.

Supramolecular columns of nanoscale periodicity (≈4.75 nm) are aligned in a uniform direction over a large area by photothermal laser writing. The alignment direction of the supramolecular structures is determined by varying the laser heating conditions. Area‐selective alignment is also demonstrated by local laser writing, and planar structures of supramolecular materials along the laser scanning direction at the specified areas are generated.

Abstract

Controlling the orientation of highly periodic supramolecular structures of small feature size (<5 nm) is the first step for potential applications in optoelectronics, membranes, and template synthesis. A new method, namely, laser photothermal writing, is introduced to direct the orientation of supramolecular columns over a large area. Supramolecular columns consisting of taper‐shaped molecules with long aliphatic tail groups are aligned by a thermal gradient, which is induced by exposing a near‐infrared laser beam to a graphene photothermal conversion layer. Intriguingly, the orientation of the supramolecular columns can be controlled in a facile manner by varying the laser scanning velocity and power. In contrast to previous methodologies for aligning supramolecular structures, this laser photothermal mechanism allows the directional and continuous alignment of supramolecular structures over an arbitrary large area with the easy control of laser irradiation. Besides, the laser process also enables area‐selective orientation of the supramolecular structures for device‐oriented nanopatterning.

Orthotropic control of resist stencils and basal molds is developed to realize high‐resolution true triaxial fabrication of 3D metastructures. New strategies to modulate the basal mold with high precision are realized by a controlled annealing process. A new alignment technique is also introduced to stacking the 3D metastructures with high addressability.

Abstract

Metamaterials have gained much attention thanks to their extraordinary and intriguing optical properties beyond natural materials. However, universal high‐resolution fabrications of 3D micro/nanometastructures with high‐resolution remain a challenge. Here, a novel approach to fabricate sophisticated 3D micro/nanostructures with excellent robustness and precise controllability is demonstrated by simultaneously modulating of flexible resist stencils and basal molds. This method allows arbitrary manipulations of morphology, size, and orientation, as well as contact angles of the objects. Combined with a new alignment strategy of high‐resolution, previously inaccessible architectures are fabricated with ultrahigh precision, leading to an excellent spectra response from the fabricated metastructures. This method provides a new possibility to realize true 3D metamaterial fabrications featuring high‐resolution and direct‐compatibility with broad planar lithography platforms.

Single‐crystal SnSe fibers are achieved using thermal drawing and laser‐induced recrystallization. The resulting single‐crystal rock‐salt SnSe fibers possess high thermoelectric properties, enhancing the ZT value close to 2 at 860 K, while being highly flexible, ultralong, and mechanically stable. This simple and low‐cost approach engages the fiber‐shaped high‐performance single‐crystal materials in applications from 1D fiber devices to multidimensional wearable fabrics.

Abstract

Single‐crystal tin selenide (SnSe), a record holder of high‐performance thermoelectric materials, enables high‐efficient interconversion between heat and electricity for power generation or refrigeration. However, the rigid bulky SnSe cannot satisfy the applications for flexible and wearable devices. Here, a method is demonstrated to achieve ultralong single‐crystal SnSe wire with rock‐salt structure and high thermoelectric performance with diameters from micro‐ to nanoscale. This method starts from thermally drawing SnSe into a flexible fiber‐like substrate, which is polycrystalline, highly flexible, ultralong, and mechanically stable. Then a CO₂ laser is employed to recrystallize the SnSe core to single‐crystal over the entire fiber. Both theoretical and experimental studies demonstrate that the single‐crystal rock‐salt SnSe fibers possess high thermoelectric properties, significantly enhancing the ZT value to 2 at 862 K. This simple and low‐cost approach offers a promising path to engage the fiber‐shaped single‐crystal materials in applications from 1D fiber devices to multidimensional wearable fabrics.

Recent progress on the electrochemical intercalation of foreign species into atomically thin 2D materials and van der Waals heterostructures via versatile electrochemical platforms and their influence on the structural phase transition of materials and novel device applications is reviewed, and the future opportunities especially on the intercalation via solid ion conductor field effect transistors are discussed.

Abstract

In van der Waals (vdWs) materials and heterostructures, the interlayers are bonded by weak vdWs interactions due to the lack of dangling bonds. The vdWs gap at the homo‐ or heterointerface provides great freedom to enrich the tunability of electronic structures by external intercalation of foreign ions or atoms at the interface, leading to the discovery of new physics and functionalities. Herein, the recent progress on electrochemical intercalation of foreign species into atomically thin vdWs materials for structural phase transition and device applications is reviewed and future opportunities are discussed. First, several kinds of electrochemical intercalation platforms to achieve the intercalation in vdWs materials and heterostructures are introduced. Next, the in situ characterization of electrochemical intercalation dynamics by state‐of‐the‐art techniques is summarized, including optical techniques, scanning probe techniques, and electrical transport. Moreover, particular attention is paid on the experimentally reported phase transition and multifunctional applications of intercalated devices. Finally, future applications and challenges of intercalation in vdWs materials and heterostructures are proposed, including the intrinsic intercalation mechanism of solid ion conductors, exact identification of intercalated foreign species by near‐field optical techniques, and the tunability of intercalation kinetics for ultrafast switching.

A novel diode‐like selective enhancement of the carrier transport through a metal–insulator–semiconductor (MIS) contact is revealed through comparison with a conventional metal–semiconductor contact on a single monolayer MoS₂ triangular domain. The MIS contact exploits monolayer hexagonal boron nitride as an ultrathin decorating layer added between MoS₂ and the metal contact.

Abstract

2D semiconductors such as monolayer molybdenum disulfide (MoS₂) are promising material candidates for next‐generation nanoelectronics. However, there are fundamental challenges related to their metal–semiconductor (MS) contacts, which limit the performance potential for practical device applications. In this work, 2D monolayer hexagonal boron nitride (h‐BN) is exploited as an ultrathin decorating layer to form a metal–insulator–semiconductor (MIS) contact, and an innovative device architecture is designed as a platform to reveal a novel diode‐like selective enhancement of the carrier transport through the MIS contact. The contact resistance is significantly reduced when the electrons are transported from the semiconductor to the metal, but is barely affected when the electrons are transported oppositely. A concept of carrier collection barrier is proposed to interpret this intriguing phenomenon as well as a negative Schottky barrier height obtained from temperature‐dependent measurements, and the critical role of the collection barrier at the drain end is shown for the overall transistor performance.

Coexistence of a 2D potential well and robust ferroelectric polarization is achieved in layered‐perovskite Bi₂WO₆, leading to a large and stable photocurrent density in an artificially designed photosensitive Bi₂WO₆/SrTiO₃ heterostructure. Such a prototype device shows a great potential for applications in high‐efficiency photovoltaic cells and nonvolatile information processing with low power consumption.

Abstract

The coexistence of large conductivity and robust ferroelectricity is promising for high‐performance ferroelectric devices based on polarization‐controllable highly efficient carrier transport. Distinct from traditional perovskite ferroelectrics, Bi₂WO₆ with a layered structure shows a great potential to preserve its ferroelectricity under substantial electron doping. Herein, by artificial design of photosensitive heterostructures with desired band alignment, three orders of magnitude enhancement of the short‐circuit photocurrent is achieved in Bi₂WO₆/SrTiO₃ at room temperature. The microscopic mechanism of this large photocurrent originates from separated transport of electrons and holes in [WO₄]⁻² and [Bi₂O₂]⁺² layers respectively with a large in‐plane conductivity, which is understood by a combination of ab initio calculations and spectroscopic measurements. The layered electronic structure and appropriately designed band alignment in this layered ferroelectric heterostructure provide an opportunity to achieve high‐performance and nonvolatile switchable electronic devices.

The selenium vacancy in crystalline ReSe₂ enhances its electroactivity for both nitrogen reduction and hydrogen evolution. To restrict HER, selenium vacancy‐rich ReSe₂@carbonized bacterial cellulose (V_r‐ReSe₂@CBC) nanofibers are buried between two CBC layers, leading to boosted Faradaic efficiency and ammonia yield on an abrupt interface.

Abstract

Vacancy engineering has been proved repeatedly as an adoptable strategy to boost electrocatalysis, while its poor selectivity restricts the usage in nitrogen reduction reaction (NRR) as overwhelming competition from hydrogen evolution reaction (HER). Revealed by density functional theory calculations, the selenium vacancy in ReSe₂ crystal can enhance its electroactivity for both NRR and HER by shifting the d‐band from −4.42 to −4.19 eV. To restrict the HER, we report a novel method by burying selenium vacancy‐rich ReSe₂@carbonized bacterial cellulose (V_r‐ReSe₂@CBC) nanofibers between two CBC layers, leading to boosted Faradaic efficiency of 42.5 % and ammonia yield of 28.3 μg h⁻¹ cm⁻² at a potential of −0.25 V on an abrupt interface. As demonstrated by the nitrogen bubble adhesive force, superhydrophilic measurements, and COMSOL Multiphysics simulations, the hydrophobic and porous CBC layers can keep the internal V_r‐ReSe₂@CBC nanofibers away from water coverage, leaving more unoccupied active sites for the N₂ reduction (especially for the potential determining step of proton‐electron coupling and transferring processes as *NN → *NNH).

A rippled gate‐tunable ultrahigh sensitive and highly stretchable phototransistor is demonstrated. Taking advantage of synergistic effect of graphene's ultrahigh mobility, strong absorption of ZnO, and elasticity of polydimethylsiloxane, the rippled metallic‐nanowires/graphene/ZnO‐nanoparticle nanostack device shows back‐gate tunable, highly stable ultrahigh performance, comparable to its rigid counterpart, even under strain conditions far beyond the native stretchability of single‐layer graphene.

Abstract

Despite being one of the most robust materials with intriguing optoelectronic properties, the practical use of single‐layer graphene (SLG) in soft‐electronic technologies is limited due to its poor native stretchability, low absorption coefficient, poor on/off ratio, etc. To circumvent these difficulties, here, a rippled gate‐tunable ultrahigh responsivity nanostack phototransistor composed of SLG, semiconductor‐nanoparticles (NPs), and metallic‐nanowires (NWs) embedded in an elastic film is proposed. The unique electronic conductivity of SLG and high absorption strength of semiconductor‐NPs produce an ultrahigh photocurrent gain. The metallic NWs serve as an excellent stretchable gate electrode. The ripple structured nanomaterials surmount their native stretchability, providing strength and electromechanical stability to the composite. Combining all these unique features, highly stretchable and ultrasensitive phototransistors are created, which can be stretched up to 30% with high repeatability maintaining a photoresponsivity, photocurrent gain, and detectivity of ≈10⁶ A W⁻¹, 10⁷, and 10¹³ Jones, respectively, which are comparable with the same class of rigid devices. In addition, the device can be turned‐off by applying a suitable gate voltage, which is very convenient for photonic circuits. Moreover, the study can be extended to many other 2D systems, and therefore paves a crucial step for designing high‐performance soft optoelectronic devices for practical applications.

Pristine graphene flakes are 2D amphiphiles with well‐defined hydrophilic edges and hydrophobic basal plane surfaces, the interplay of which allows small flakes to be utilized as stabilizers. The interactions between flakes can be controlled by varying the flake size and the oil‐to‐water ratio. Pristine graphene flakes can stabilize water/oil emulsions even under high pressure, high temperature, and in saline solutions.

Abstract

The fundamental colloidal properties of pristine graphene flakes remain incompletely understood, with conflicting reports about their chemical character, hindering potential applications that could exploit the extraordinary electronic, thermal, and mechanical properties of graphene. Here, the true amphipathic nature of pristine graphene flakes is demonstrated through wet‐chemistry testing, optical microscopy, electron microscopy, and density functional theory, molecular dynamics, and Monte Carlo calculations, and it is shown how this fact paves the way for the formation of ultrastable water/oil emulsions. In contrast to commonly used graphene oxide flakes, pristine graphene flakes possess well‐defined hydrophobic and hydrophilic regions: the basal plane and edges, respectively, the interplay of which allows small flakes to be utilized as stabilizers with an amphipathic strength that depends on the edge‐to‐surface ratio. The interactions between flakes can be also controlled by varying the oil‐to‐water ratio. In addition, it is predicted that graphene flakes can be efficiently used as a new‐generation stabilizer that is active under high pressure, high temperature, and in saline solutions, greatly enhancing the efficiency and functionality of applications based on this material.

Ultrathin hybrid perovskite nanosheets are synthesized based on lead iodide templates. Multiple phases, such as PbI₂, MAPbI₃ and FAPbI₃, are flexibly designed and transformed as a single nanosheet through efficient cation exchange. By using other 2D materials as patterning masks and/or band alignment partner, artificial patterns, lateral, and vertical heterostructures of perovskite nanosheets are realized.

Abstract

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.

Microdevices based on rolled‐up nanomembranes are obtained at high yield and reproducibility utilizing a dry‐release scheme based on SF₆ plasma release. The high quality of the obtained electrical and optical devices is demonstrated by the simultaneous formation of high‐performance rolled‐up capacitors at wafer scale as well as by record‐breaking rolled‐up optical whispering‐gallery‐mode resonators.

Abstract

Mechanical strain formed at the interfaces of thin films has been widely applied to self‐assemble 3D microarchitectures. Among them, rolled‐up microtubes possess a unique 3D geometry beneficial for working as photonic, electromagnetic, energy storage, and sensing devices. However, the yield and quality of microtubular architectures are often limited by the wet‐release of lithographically patterned stacks of thin‐film structures. To address the drawbacks of conventionally used wet‐etching methods in self‐assembly techniques, here a dry‐release approach is developed to roll‐up both metallic and dielectric, as well as metallic/dielectric hybrid thin films for the fabrication of electronic and optical devices. A silicon thin film sacrificial layer on insulator is etched by dry fluorine chemistry, triggering self‐assembly of prestrained nanomembranes in a well‐controlled wafer scale fashion. More than 6000 integrated microcapacitors as well as hundreds of active microtubular optical cavities are obtained in a simultaneous self‐assembly process. The fabrication of wafer‐scale self‐assembled microdevices results in high yield, reproducibility, uniformity, and performance, which promise broad applications in microelectronics, photonics, and opto‐electronics.

Microscopy data of nanomaterials often contains rich yet complicated information that reflects the material properties, but is mostly overlooked by researchers. Deep learning is an ideal approach to finding these highly correlated and non‐linear features. As a case study, a neural network model called “2DMOINet” is trained for optical identification and characterization of exfoliated 2D materials.

Abstract

Advanced microscopy and/or spectroscopy tools play indispensable roles in nanoscience and nanotechnology research, as they provide rich information about material processes and properties. However, the interpretation of imaging data heavily relies on the “intuition” of experienced researchers. As a result, many of the deep graphical features obtained through these tools are often unused because of difficulties in processing the data and finding the correlations. Such challenges can be well addressed by deep learning. In this work, the optical characterization of 2D materials is used as a case study, and a neural‐network‐based algorithm is demonstrated for the material and thickness identification of 2D materials with high prediction accuracy and real‐time processing capability. Further analysis shows that the trained network can extract deep graphical features such as contrast, color, edges, shapes, flake sizes, and their distributions, based on which an ensemble approach is developed to predict the most relevant physical properties of 2D materials. Finally, a transfer learning technique is applied to adapt the pretrained network to other optical identification applications. This artificial‐intelligence‐based material characterization approach is a powerful tool that would speed up the preparation, initial characterization of 2D materials and other nanomaterials, and potentially accelerate new material discoveries.

A liquid metal establishes an ultrasmooth liquid–liquid interface that catalyses the dissociation of organic precursors into interfacial graphitic carbon films. The electrochemical synthesis of these 2D graphitic films is accomplished at room temperature (with only a small energy input, an onset voltage of 0.45 V) and they self‐exfoliate from the nonpolar surface of the liquid metal by applying higher voltages.

Abstract

Room‐temperature synthesis of 2D graphitic materials (2D‐GMs) remains an elusive aim, especially with electrochemical means. Here, it is shown that liquid metals render this possible as they offer catalytic activity and an ultrasmooth templating interface that promotes Frank–van der Merwe regime growth, while allowing facile exfoliation due to the absence of interfacial forces as a nonpolar liquid. The 2D‐GMs are formed at low onset potential and can be in situ doped depending on the choice of organic precursors and the electrochemical set‐up. The materials are tuned to exhibit porous or pinhole‐free morphologies and are engineered for their degree of oxidation and number of layers. The proposed liquid‐metal‐based room‐temperature electrochemical route can be expanded to many other 2D materials.

The strain‐engineered anisotropic optical and electrical properties in solution‐grown, sub‐millimeter‐size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral‐chain lattice. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain‐engineered functionalities.

Abstract

Atomically thin materials, leveraging their low‐dimensional geometries and superior mechanical properties, are amenable to exquisite strain manipulation with a broad tunability inaccessible to bulk or thin‐film materials. Such capability offers unexplored possibilities for probing intriguing physics and materials science in the 2D limit as well as enabling unprecedented device applications. Here, the strain‐engineered anisotropic optical and electrical properties in solution‐grown, sub‐millimeter‐size 2D Te are systematically investigated through designing and introducing a controlled buckled geometry in its intriguing chiral‐chain lattice. The observed Raman spectra reveal anisotropic lattice vibrations under the corresponding straining conditions. The feasibility of using buckled 2D Te for ultrastretchable strain sensors with a high gauge factor (≈380) is further explored. 2D Te is an emerging material boasting attractive characteristics for electronics, sensors, quantum devices, and optoelectronics. The results suggest the potential of 2D Te as a promising candidate for designing and implementing flexible and stretchable devices with strain‐engineered functionalities.

Enhanced SHG intensity with near‐unity polarization from inversion‐symmetry‐broken 2D TMDC atomic layers, spiral structures, and heterostructures are realized at room temperature. It is found that there is no significant effect of multilayer interlayer interaction on SHG polarization.

Abstract

With unique valley‐dependent optical and optoelectronic properties, 2D transition metal dichalcogenides (2D TMDCs) are promising materials for valleytronics. Second‐harmonic generation (SHG) in 2D TMDCs monolayers has shown valley‐dependent optical selection rules. However, SHG in monolayer TMDCs is generally weak; it is important to obtain materials with both strong SHG signals and a large degree of polarization. In the work, a variety of inversion‐symmetry‐breaking (3R‐like phase) TMDCs (WSe₂, WS₂, MoS₂) atomic layers, spiral structures, and heterostructures are prepared, and their SHG polarization is studied. Through circular‐polarization‐resolved SHG experiments, it is demonstrated that the SHG intensity is enhanced in thicker samples by breaking inversion symmetry while maintaining the degree of polarization close to unity at room temperature. By studying TMDCs with different twist angles and the spiral structures, it is found that there is no significant effect of multilayer interlayer interaction on valley‐dependent SHG. The realization of strong SHG with high degree of polarization may pave the way toward a new platform for nonlinear optical valleytronics devices based on 2D semiconductors.

A first‐principles study of the structure, bonding, and ferroelectricity in quasi 2D monochalcogenides (GeSe, GeTe, SnSe, SnTe) is carried out. It is found that a few bilayers are sufficient to recover bulk behavior in selenides, while tellurides deviate strongly from the bulk, even for thick models. These differences stem from the effects of depolarizing fields and the different bonding mechanisms.

Abstract

Extreme miniaturization is known to be detrimental for certain properties, such as ferroelectricity in perovskite oxide films below a critical thickness. Remarkably, few‐layer crystalline films of monochalcogenides display robust in‐plane ferroelectricity with potential applications in nanoelectronics. These applications critically depend on the electronic properties and the nature of bonding in the 2D limit. A fundamental open question is thus to what extent bulk properties persist in thin films. Here, this question is addressed by a first‐principles study of the structural, electronic, and ferroelectric properties of selected monochalcogenides (GeSe, GeTe, SnSe, and SnTe) as a function of film thickness up to 18 bilayers. While in selenides a few bilayers are sufficient to recover the bulk behavior, the Te‐based compounds deviate strongly from the bulk, irrespective of the slab thickness. These results are explained in terms of depolarizing fields in Te‐based slabs and the different nature of the chemical bond in selenides and tellurides. It is shown that GeTe and SnTe slabs inherit metavalent bonding of the bulk phase, despite structural and electronic properties being strongly modified in thin films. This understanding of the nature of bonding in few‐layers structures offers a powerful tool to tune materials properties for applications in information technology.

A novel piezoelectrocatalysis system is developed, which involves quartz microrods abundantly decorated with active‐edge‐site MoS₂ nanosheets to form a quartz microrods@few‐layered MoS₂ hierarchical heterostructure (QMSH). The quartz microrods in the QMSH present an internal bias to the MoS₂ nanosheets, thus yielding a piezoelectrocatalysis system. An efficient piezoelectrocatalytic hydrogen evolution reaction and decomposition of wastewater can be achieved simultaneously without light irradiation.

Abstract

Intense light attenuation in water/wastewater results in photocatalysts exhibiting a low quantum efficiency. This study develops a novel piezoelectrocatalysis system, which involves quartz microrods (MRs) abundantly decorated with active‐edge‐site MoS₂ nanosheets to form a quartz microrods@few‐layered MoS₂ hierarchical heterostructure (QMSH). Through theoretical calculations, it is found that the quartz MRs serve as a parallel‐plate capacitor, which is self‐powered to provide an internal electric field to the few‐layered MoS₂ nanosheets surrounding the quartz MR surfaces, and the piezoelectric potential (piezopotential) effectively facilitates redox reactions with the free carriers in MoS₂. The self‐powered quartz MRs in the QMSH present an internal bias to the MoS₂ nanosheets, thus yielding a piezoelectrocatalysis system. An efficient piezoelectrocatalytic hydrogen evolution reaction and decomposition of wastewater without light irradiation can be achieved simultaneously. The second‐order rate constant of the QMSH is ≈0.631 L mg⁻¹ min⁻¹, which is 650‐fold that of quartz MRs, indicating that the piezoelectric heterostructural catalysts display exceptionally high efficiency on piezoelectrocatalytic redox reactions rather than in the piezocatalytic process. The H₂‐production rate of QMSH catalysts approaches ≈6456 µmo1 g⁻¹ h⁻¹ and peaks at ≈16.8 mmol g⁻¹ in 8 h. The piezoelectrocatalytic process may be a promising method for treating industrial wastewater and producing clean energy.

“Janus‐like” 2D porous heterostructures are described for realizing ultrastable surface reactivity of chemiresistive 2D materials as proven by a chemical sensing case study and multiscale simulations. The few‐layered holey graphene oxide passivation layer effectively shields the black phosphorus and MXene from oxidative degradation, while allowing the selective diffusion of NO₂ molecules toward the underlying 2D sensing materials.

Abstract

2D black phosphorus (BP) and MXenes have triggered enormous research interest in catalysis, energy storage, and chemical sensing. Unfortunately, the low stability of these materials under practical operating conditions remains a critical bottleneck, particularly as they are prone to oxidization under moisture. In this work, the design and application of stable 2D heterostructures obtained from decorating BP and MXene (Ti₃C₂T _x ) with few‐layer holey graphene oxide (FHGO) membranes are presented. In the resulting heterostructured systems, FHGO serves as a multifunctional passivation layer that shields BP or MXene from oxidative degradation, while allowing the selective diffusion of target gas molecules through its micropores and toward the underlying 2D material. Through a case study of dilute NO₂ sensing, it is demonstrated that these heterostructures show a greatly enhanced sensing performance under humid conditions, where fast sensing speed and response are consistently observed, and high stability is impressively retained upon repetitive sensing cycles for 1000 min. These results corroborate the efficacy of material decoration with porous FHGO membranes and suggest that this is a generalizable strategy for reliable high‐performance applications of 2D materials.