A bright future for engineering piezoelectric 2D crystals

Lateral Piezoelectricity of Alzheimer's Aβ Aggregates

Alzheimer's disease (AD) is the most frequent neurodegenerative disorder in the elderly aged over 65. The extracellular accumulation of beta-amyloid (Aβ) aggregates in the brain is considered as the major event worsening the AD symptoms, but its underlying reason has remained unclear. Here the piezoelectric characteristics of Aβ aggregates are revealed. The vector piezoresponse force microscopy (PFM) analysis results exhibit that Aβ fibrils have spiraling piezoelectric domains along the length and a lateral piezoelectric constant of 44.1 pC N-1. Also, the continuous sideband Kelvin probe force microscopy (KPFM) images display that the increment of charge-induced surface potential on a single Aβ fibril is allowed to reach above +1700 mV in response to applied forces. These findings shed light on the peculiar mechano-electrical surface properties of pathological Aβ fibrils that exceed those of normal body components.

Piezoelectricity in wide bandgap semiconductor 2D crystal GaN nanosheets

Gallium nitride (GaN) exhibits various potential applications in optics and optoelectronics due to its outstanding physical characteristics, including a wide direct bandgap, strong deep-ultraviolet emission, and excellent electron transport properties. However, research on the piezoelectric and related properties of GaN nanosheets are scarce, as previous small-scale GaN investigations have mainly concentrated on nanowires and nanotubes. Here, we report a strategy for growing 2D GaN nanosheets using chemical vapor deposition on Ga/W liquid-phase substrates. Additionally, utilizing scanning probe techniques, it has been observed that 700 nm-thick GaN nanosheets demonstrate a piezoelectric constant of deff33 = 1.53 ± 0.21 pm V-1 and possess the capability to effectively modulate the Schottky barrier. The piezoelectric characteristics of 2D GaN are offering new options for innovative applications in various fields, including energy harvesting, electronics, sensing, and communications.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Piezoelectric, Thermoelectric, and Photocatalytic Water Splitting Properties of Janus Arsenic Chalcohalide Monolayers

In this study, we systematically investigate the piezoelectric, thermoelectric, and photocatalytic properties of novel two-dimensional Janus arsenic chalcohalide monolayers, AsXX' (X = S and Se and X' = Cl, Br, and I) using density functional theory. The positive phonon spectra and ab initio molecular dynamics simulation plots indicate these monolayers to be dynamically and thermally stable. The mechanical stability of these monolayers is confirmed by a nonzero elastic constant (C ij ), Young's modulus (Y 2D), and Poisson ratio (ν). These monolayers exhibit strong out-of-plane piezoelectric coefficients, making them candidate materials for piezoelectric devices. Our calculated results indicate that these monolayers have a low lattice thermal conductivity (κl) and high thermoelectric figure of merit (zT) up to 1.51 at 800 K. These monolayers have an indirect bandgap, high carrier mobility, and strong visible light absorption spectra. Furthermore, the AsSCl, AsSBr, and AsSeI monolayers exhibit appropriate band alignment for water splitting. The calculated value of the corrected solar-to-hydrogen conversion efficiency can reach up to 19%. The nonadiabatic molecular dynamics simulations reveal the prolonged electron-hole recombination rates of 1.52 0.98, and 0.67 ns for AsSCl, AsSBr, and AsSeI monolayers, respectively. Our findings demonstrate these monolayers to be potential candidates in energy-harvesting fields.

3D Nanoprinting of Heterogeneous Metal Oxides with High Shape Fidelity

3D nanoprinting can significantly enhance the performance of sensors, batteries, optoelectronic/microelectronic devices, etc. However, current 3D nanoprinting methods for metal oxides are suffering from three key issues including limited material applicability, serious shape distortion, and the difficulty of heterogeneous integration. This paper discovers a mechanism in which imidazole and acrylic acid synergistically coordinate with metal ions in water. Using the mechanism, this work develops a series of metal ion synergistic coordination water-soluble (MISCWS) resins for 3D nanoprinting of various metal oxides, including MnO2, Cr2O3, Co3O4, and ZnO, as well as heterogeneous structures of MnO2/NiO, Cr2O3/Al2O3, and ZnO/MgO. Besides, the synergistic coordination effect results in a 2.54-fold increase in inorganic mass fraction within the polymer, compared with previous works, which effectively mitigates the shape distortion of metal oxide microstructures. Based on this method, this work also demonstrates a 3D ZnO microsensor with a high sensitivity (1.113 million at 200 ppm NO2), surpassing the conventional 2D ZnO sensors by tenfold. The method yields high-fidelity 3D structures of heterogeneous metal oxides with nanoscale resolution, paving the way for applications such as sensing, micro-optics, energy storage, and microsystems.

Alternatives to Fluoropolymers for Motion-Based Energy Harvesting: Perspectives on Piezoelectricity, Triboelectricity, Ferroelectrets, and Flexoelectricity

Fluoropolymers, including polytetrafluoroethylene (PTFE, Teflon), polyvinylidene difluoride (PVDF), and fluorine kautschuk materials (FKMs, Viton) are critical polymers for applications ranging from non-stick coatings, corrosion resistant seals, semiconductor manufacturing, membranes, and energy harvesting technologies. However, the synthesis of these fluoropolymers requires the use of per- and polyfluorinated alkyl substances (PFAS) known colloquially as "forever chemicals," and as such there is a pressing need to develop alternative technologies that can serve the end-use of fluoropolymers without the environmental cost of using PFAS. Further, fluoropolymers themselves fall under the PFAS umbrella. Here, alternative mechanical-to-electrical energy harvesting polymers are reviewed and benchmarked against the leading fluoropolymer energy harvesters. These alternative technologies include nonfluoropolymer piezoelectric polymers, triboelectric nanogenerators (TENGs), ferroelectric elastomers, and flexoelectric polymers. A vision towards sustainable, non-fluoropolymer-based energy harvesting is provided.

Piezoelectric Biomaterials Inspired by Nature for Applications in Biomedicine and Nanotechnology

Bioelectricity provides electrostimulation to regulate cell/tissue behaviors and functions. In the human body, bioelectricity can be generated in electromechanically responsive tissues and organs, as well as biomolecular building blocks that exhibit piezoelectricity, with a phenomenon known as the piezoelectric effect. Inspired by natural bio-piezoelectric phenomenon, efforts have been devoted to exploiting high-performance synthetic piezoelectric biomaterials, including molecular materials, polymeric materials, ceramic materials, and composite materials. Notably, piezoelectric biomaterials polarize under mechanical strain and generate electrical potentials, which can be used to fabricate electronic devices. Herein, a review article is proposed to summarize the design and research progress of piezoelectric biomaterials and devices toward bionanotechnology. First, the functions of bioelectricity in regulating human electrophysiological activity from cellular to tissue level are introduced. Next, recent advances as well as structure-property relationship of various natural and synthetic piezoelectric biomaterials are provided in detail. In the following part, the applications of piezoelectric biomaterials in tissue engineering, drug delivery, biosensing, energy harvesting, and catalysis are systematically classified and discussed. Finally, the challenges and future prospects of piezoelectric biomaterials are presented. It is believed that this review will provide inspiration for the design and development of innovative piezoelectric biomaterials in the fields of biomedicine and nanotechnology.

Piezoelectric Materials and Pyroelectric Materials:High Efficient Catalysts for Photoelectrochemical Water Splitting

Directly transforming solar energy into chemical compounds via photocatalytic water splitting can continually producing hydrogen, regarded as one of the ultimate sustainable energy sources. The key point of achieving high photoelectrochemical (PEC) water splitting performance depends on the successful design and synthesis of high-efficient photocatalysts. However, the slow separation and fast recombination of photo generated charge carriers greatly limit the utilization of solar energy, resulting in low PEC water splitting efficiency. Recently, piezoelectric/pyroelectric effect assisted PEC water splitting brings new sight on improving charger separate and transfer behaviors. In this review, the recent advancements and state-of-the-art progress in piezoelectric/pyroelectric effect assisted PEC water splitting are summarized and discussed. Different types of photocatalysts are classified according to their chemical constitutions and the corresponding advantages of each type are also discussed. Furthermore, the progress of coupling piezoelectric effect and pyroelectric effect in one PEC water splitting system is also introduced. Finally, the prospects, critical challenges and promising strategies for the application of piezoelectric/pyroelectric materials in PEC water splitting are highlighted.

Piezoionic Elastomers by Phase and Interface Engineering for High-Performance Energy-Harvesting Ionotronics

Piezoionic materials play a pivotal role in energy-harvesting ionotronics. However, a persistent challenge lies in balancing the structural requirements for voltage generation, current conduction, and mechanical adaptability. The conventional approach of employing crystalline heterostructures for stress concentration and localized charge separation, while effective for voltage generation, often compromises the stretchability and long-range charge transport found in homogeneous quasisolid states. Herein, phase and interface engineering strategy is introduced to address this dilemma and a piezoionic elastomer is presented that seamlessly integrates ionic liquids and ionic plastic crystals, forming a finely tuned microphase-separated structure with an intermediate phase. This approach promotes charge separation via stress concentration among hard phases while leveraging the high ionic charge mobility in soft and intermediate phases. Impressively, the elastomer achieves an extraordinary piezoionic coefficient of about 6.0 mV kPa-1, a more than threefold improvement over current hydrogels and ionogels. The resulting power density of 1.3 µW cm-3 sets a new benchmark, exceeding that of state-of-the-art piezoionic gels. Notably, this elastomer combines outstanding stretchability, remarkable toughness, and rapid self-healing capability, underscoring its potential for real-world applications. This work may represent a stride toward mechanically robust energy harvesting systems and provide insights into ionotronic systems for human-machine interaction.

Ultrasound-Sensitive Intelligent Nanosystems: A Promising Strategy for the Treatment of Neurological Diseases

Neurological diseases are a major global health challenge, affecting hundreds of millions of people worldwide. Ultrasound therapy plays an irreplaceable role in the treatment of neurological diseases due to its noninvasive, highly focused, and strong tissue penetration capabilities. However, the complexity of brain and nervous system and the safety risks associated with prolonged exposure to ultrasound therapy severely limit the applicability of ultrasound therapy. Ultrasound-sensitive intelligent nanosystems (USINs) are a novel therapeutic strategy for neurological diseases that bring greater spatiotemporal controllability and improve safety to overcome these challenges. This review provides a detailed overview of therapeutic strategies and clinical advances of ultrasound in neurological diseases, focusing on the potential of USINs-based ultrasound in the treatment of neurological diseases. Based on the physical and chemical effects induced by ultrasound, rational design of USINs is a prerequisite for improving the efficacy of ultrasound therapy. Recent developments of ultrasound-sensitive nanocarriers and nanoagents are systemically reviewed. Finally, the challenges and developing prospects of USINs are discussed in depth, with a view to providing useful insights and guidance for efficient ultrasound treatment of neurological diseases.

Advancing piezoelectric 2D nanomaterials for applications in drug delivery systems and therapeutic approaches

Precision drug delivery and multimodal synergistic therapy are crucial in treating diverse ailments, such as cancer, tissue damage, and degenerative diseases. Electrodes that emit electric pulses have proven effective in enhancing molecule release and permeability in drug delivery systems. Moreover, the physiological electrical microenvironment plays a vital role in regulating biological functions and triggering action potentials in neural and muscular tissues. Due to their unique noncentrosymmetric structures, many 2D materials exhibit outstanding piezoelectric performance, generating positive and negative charges under mechanical forces. This ability facilitates precise drug targeting and ensures high stimulus responsiveness, thereby controlling cellular destinies. Additionally, the abundant active sites within piezoelectric 2D materials facilitate efficient catalysis through piezochemical coupling, offering multimodal synergistic therapeutic strategies. However, the full potential of piezoelectric 2D nanomaterials in drug delivery system design remains underexplored due to research gaps. In this context, the current applications of piezoelectric 2D materials in disease management are summarized in this review, and the development of drug delivery systems influenced by these materials is forecast.

Piezoelectric Applications of Low-Dimensional Composites and Porous Materials

Low-dimensional (LD) materials, with atomically thin anisotropic structures, exhibit remarkable physical and chemical properties, prominently featuring piezoelectricity resulting from the absence of centrosymmetry. This characteristic has led to diverse applications, including sensors, actuators, and micro- and nanoelectromechanical systems. While piezoelectric effects are observed across zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) LD materials, challenges such as effective charge separation and crystal structure imperfections limit their full potential. Addressing these issues requires innovative solutions, with the integration of LD materials with polymers, ceramics, metals, and other porous materials proving a key strategy to significantly enhance piezoelectric properties. This review comprehensively covers recent advances in synthesizing and characterizing piezoelectric composites based on LD materials and porous materials. The synergistic combination of LD materials with other substances, especially porous materials, demonstrates notable performance improvements, addressing inherent challenges. The review also explores future directions and challenges in developing these composite materials, highlighting potential applications across various technological domains.

Multipiezo Effect in Altermagnetic V2SeTeO Monolayer

Strain engineering has been used as an efficient method to modulate various properties of quantum materials and electronic devices. One may establish piezo effects based on a disciplined response to the strain in multifunctional nanosystems. Inspired by a recent theoretical proposal on the interesting piezomagnetism and C-paired valley polarization in the V2Se2O monolayer, we predict a stable altermagnetic Janus monolayer V2SeTeO using density functional theory calculations. It exhibits a novel "multipiezo" effect combining piezoelectricity, piezovalley, and piezomagnetism. Most interestingly, the valley polarization and the net magnetization under strain in V2SeTeO exceed these in V2Se2O, along with the additional large piezoelectric coefficient. The "multipiezo" effect makes Janus monolayer V2SeTeO as a tantalizing material for potential applications in nanoelectronics, optoelectronics, spintronics, and valleytronics.

Extendable piezo/ferroelectricity in nonstoichiometric 2D transition metal dichalcogenides

Engineering piezo/ferroelectricity in two-dimensional materials holds significant implications for advancing the manufacture of state-of-the-art multifunctional materials. The inborn nonstoichiometric propensity of two-dimensional transition metal dichalcogenides provides a spiffy ready-available solution for breaking inversion centrosymmetry, thereby conducing to circumvent size effect challenges in conventional perovskite oxide ferroelectrics. Here, we show the extendable and ubiquitous piezo/ferroelectricity within nonstoichiometric two-dimensional transition metal dichalcogenides that are predominantly centrosymmetric during standard stoichiometric cases. The emerged piezo/ferroelectric traits are aroused from the sliding of van der Waals layers and displacement of interlayer metal atoms triggered by the Frankel defects of heterogeneous interlayer native metal atom intercalation. We demonstrate two-dimensional chromium selenides nanogenerator and iron tellurides ferroelectric multilevel memristors as two representative applications. This innovative approach to engineering piezo/ferroelectricity in ultrathin transition metal dichalcogenides may provide a potential avenue to consolidate piezo/ferroelectricity with featured two-dimensional materials to fabricate multifunctional materials and distinguished multiferroic.

An Emerging Family of Piezocatalysts: 2D Piezoelectric Materials

Piezocatalysis is an emerging technique that holds great promise for the conversion of ubiquitous mechanical energy into electrochemical energy through piezoelectric effect. However, mechanical energies in natural environment (such as wind energy, water flow energy, and noise) are typically tiny, scattered, and featured with low frequency and low power. Therefore, a high response to these tiny mechanical energies is critical to achieving high piezocatalytic performance. In comparison to nanoparticles or 1D piezoelectric materials, 2D piezoelectric materials possess characteristics such as high flexibility, easy deformation, large surface area, and rich active sites, showing more promise in future for practical applications. In this review, state-of-the-art research progresses on 2D piezoelectric materials and their applications in piezocatalysis are provided. First, a detailed description of 2D piezoelectric materials are offered. Then a comprehensive summary of the piezocatalysis technique is presented and examines the piezocatalysis applications of 2D piezoelectric materials in various fields, including environmental remediation, small-molecule catalysis, and biomedicine. Finally, the main challenges and prospects of 2D piezoelectric materials and their applications in piezocatalysis are discussed. It is expected that this review can fuel the practical application of 2D piezoelectric materials in piezocatalysis.

Enhancing piezocatalytic H2O2 production through morphology control of graphitic carbon nitride

Piezocatalytic H₂O₂ production has attracted significant attention as a green alternative to traditional anthraquinone methods with heavy environmental pollution and high energy consumption. However, since the efficiency of piezocatalyst in producing H₂O₂ is poor, searching for a suitable method to improve the yield of H₂O₂ is of great interest. Herein, a series of graphitic carbon nitride (g-C₃N₄) with different morphologies (hollow nanotube, nanosheet and hollow nanosphere) are applied to enhance the piezocatalytic performance in yielding H₂O₂. The hollow nanotube g-C₃N₄ exhibited an outstanding H₂O₂ generation rate of 262 umol·g^-1·h^-1 without any co-catalyst, which is 1.5 and 6.2 times higher than nanosheets and hollow nanospheres, respectively. Piezoelectric response force microscopy, piezoelectrochemical tests, and Finite Element Simulation results revealed that the excellent piezocatalytic property of hollow nanotube g-C₃N₄ is mainly attributed to its larger piezoelectric coefficient, higher intrinsic carrier density, and stronger external stress absorption conversion. Furthermore, mechanism analysis indicated that piezocatalytic H₂O₂ production follows a two-step single-electro pathway, and the discovery of ¹O₂ furnishes a new insight into explore this mechanism. This study offers a new strategy for the eco-friendly manufacturing of H₂O₂ and a valuable guide for future research on morphological modulation in piezocatalysis.

Nonvolatile electro-mechanical coupling in two-dimensional lattices

Electro-mechanical coupling is of great interest for applications in sensors, actuators and energy harvesters. While the control of electrical charge by mechanical force has been studied extensively, reverse coupling is rarely explored, especially in two-dimensional (2D) lattices. Herein, we propose a novel mechanism for electro-mechanical coupling that realizes the electric field switching of the dimensions of a 2D lattice in a reversible and nonvolatile fashion, through the mediated strength of interlayer interactions in ferroelectric bilayer systems. Based on first-principles calculations, the validity of this mechanism is demonstrated in a series of real bilayer materials, i.e., MoS₂/ReIrGe₂S₆, Sb/In₂Se₃ and bilayer In₂Se₃. The interface differences due to polarization play a crucial role in realizing such nonvolatile electro-mechanical coupling, and the underlying physical origin is discussed. These explored phenomena and insights offer a novel avenue for the highly desired nonvolatile electric field control of mechanical strain.

Design, synthesis, and application of some two-dimensional materials

Two-dimensional (2D) materials are widely used as key components in the fields of energy conversion and storage, optoelectronics, catalysis, biomedicine, etc. To meet the practical needs, molecular structure design and aggregation process optimization have been systematically carried out. The intrinsic correlation between preparation methods and the characteristic properties is investigated. This review summarizes the recent research achievements of 2D materials in the aspect of molecular structure modification, aggregation regulation, characteristic properties, and device applications. The design strategies to fabricate functional 2D materials starting from precursor molecules are introduced in detail referring to organic synthetic chemistry and self-assembly technology. It provides important research ideas for the design and synthesis of related materials.

Graphene-based cardiac sensors and actuators

Graphene, a 2D carbon allotrope, is revolutionizing many biomedical applications due to its unique mechanical, electrical, thermal, and optical properties. When bioengineers realized that these properties could dramatically enhance the performance of cardiac sensors and actuators and may offer fundamentally novel technological capabilities, the field exploded with numerous studies developing new graphene-based systems and testing their limits. Here we will review the link between specific properties of graphene and mechanisms of action of cardiac sensors and actuators, analyze the performance of these systems from inaugural studies to the present, and offer future perspectives.

Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties

Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, respectively, according to the X-ray analysis. Unexpectedly, the electrochemical study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of g_abs and g_lum (3.6 × 10^-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its electric and magnetic dipole transition moments. Furthermore, spectroelectrochemical studies as well as the fluorescence spectroscopy support the aforementioned experimental findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.

Bond-Orbital-Resolved Piezoelectricity in Sp2-Hybridized Monolayer Semiconductors

Sp²-hybridized monolayer semiconductors (e.g., planar group III-V and IV-IV binary compounds) with inversion symmetry breaking (ISB) display piezoelectricity governed by their σ- and π-bond electrons. Here, we studied their bond-orbital-resolved electronic piezoelectricity (i.e., the σ- and π-piezoelectricity). We formulated a tight-binding piezoelectric model to reveal the different variations of σ- and π-piezoelectricity with the ISB strength (Δ). As Δ varied from positive to negative, the former decreased continuously, but the latter increased piecewise and jumped at Δ=0 due to the criticality of the π-electrons' ground-state geometry near this quantum phase-transition point. This led to a piezoelectricity predominated by the π-electrons for a small |Δ|. By constructing an analytical model, we clarified the microscopic mechanisms underlying the anomalous π-piezoelectricity and its subtle relations with the valley Hall effect. The validation of our models was justified by applying them to the typical sp² monolayers including hexagonal silicon carbide, Boron-X (X = N, P, As, Ab), and a BN-doped graphene superlattice.

Energy Interplay in Materials: Unlocking Next-Generation Synchronous Multisource Energy Conversion with Layered 2D Crystals

Layered 2D crystals have unique properties and rich chemical and electronic diversity, with over 6000 2D crystals known and, in principle, millions of different stacked hybrid 2D crystals accessible. This diversity provides unique combinations of properties that can profoundly affect the future of energy conversion and harvesting devices. Notably, this includes catalysts, photovoltaics, superconductors, solar-fuel generators, and piezoelectric devices that will receive broad commercial uptake in the near future. However, the unique properties of layered 2D crystals are not limited to individual applications and they can achieve exceptional performance in multiple energy conversion applications synchronously. This synchronous multisource energy conversion (SMEC) has yet to be fully realized but offers a real game-changer in how devices will be produced and utilized in the future. This perspective highlights the energy interplay in materials and its impact on energy conversion, how SMEC devices can be realized, particularly through layered 2D crystals, and provides a vision of the future of effective environmental energy harvesting devices with layered 2D crystals.

Large Vertical Piezoelectricity in a Janus Cr2I3F3 Monolayer

Two-dimensional (2D) materials have potential applications in nanoscale sensors and spintronic devices. Herein, motivated by experimental synthesis of a CrI3 monolayer possessing intrinsic magnetism and a Janus MoSSe monolayer with piezoelectricity, we propose a 2D Janus Cr2I3F3 monolayer as a multifunctional material exhibiting both piezoelectricity and ferromagnetism. Using density functional theory calculations, we systematically investigated the structural stability and the electronic, magnetic, and piezoelectric properties of the Janus Cr2I3F3 monolayer. We predicted that a vertical polarization of up to −0.155 × 10−10 C/m is induced in the Cr2I3F3 monolayer due to the breaking of symmetry. The origination mechanism of polarization was demonstrated in terms of a local dipole moment calculated by maximally localized Wannier functions. Meanwhile, it was found that a remarkable piezoelectric response can be produced under a uniaxial strain in the basal plane. The calculated piezoelectric coefficients of the Cr2I3F3 monolayer compare favorably with those of the frequently used bulk piezoelectric materials such as α–quartz and wurtzite AlN. Particularly, the e31 and d31 values of the Cr2I3F3 monolayer are nearly 10 times as large as that of Mo-based transition metal dichalcogenides. We also found that the magnitude of e31 mainly arises from the ionic contribution, while the electronic contribution can be nearly neglected. The considerable piezoelectric response combined with the intrinsic magnetism make the Janus Cr2I3F3 monolayer a potential candidate for novel multifunctional devices integrating both piezoelectric and spintronic applications.

Nanoampere-Level Piezoelectric Energy Harvesting Performance of Lithography-Free Centimeter-Scale MoS2 Monolayer Film Generators

2D transition-metal dichalcogenides have been reported to possess piezoelectricity due to their lack of inversion symmetry; thus, they are potentially applicable as electromechanical energy harvesters. Herein, the authors propose a lithography-free piezoelectric energy harvester composed of centimeter-scale MoS₂ monolayer films with an interdigitated electrode pattern that is enabled only by the large scale of the film. High-quality large-scale synthesis of the monolayer films is conducted by low-pressure chemical vapor deposition with the assistance of an unprecedented Na₂ S promoter. The extra sulfur supplied by Na₂ S critically passivates the sulfur vacancies. The energy harvester having a large active area of ≈18.3 mm² demonstrates an unexpectedly high piezoelectric energy harvesting performance of ≈400.4 mV and ≈40.7 nA under a bending strain of 0.57%, with the careful adjustment of side electrodes along the zigzag atomic arrays in the two dominant domain structure. Nanoampere-level harvesting has not yet been reported with any 2D material-based harvester.

Doped 2D SnS materials derived from liquid metal-solution for tunable optoelectronic devices

Gas-liquid reaction phenomena on liquid-metal solvents can be used to form intriguing 2D materials with large lateral dimensions, where the free energies of formation determine the final product. A vast selection of elements can be incorporated into the liquid metal-based nanostructures, offering a versatile platform for fabricating novel optoelectronic devices. While conventional doping techniques of semiconductors present several challenges for 2D materials. Liquid metals provide a facile route for obtaining doped 2D semiconductors. In this work, we successfully demonstrate that the doping of 2D SnS can be realized in a glove box containing a diluted H₂S gas. Low melting point elements such as Bi and In are alloyed with base liquid Sn in varying concentrations, resulting in the doping of 2D SnS layers incorporating Bi and In sulphides. Optoelectronic properties for photodetectors and piezoelectronics can be fine-tuned through the controlled introduction of selective migration doping. The structural modification of 2D SnS results in a 22.6% enhancement of the d₁₁ piezoelectric coefficient. In addition, photodetector response times have increased by several orders of magnitude. Doping methods using liquid metals have significantly changed the photodiode and piezoelectric device performances, providing a powerful approach to tune optoelectronic device outputs.