Flexo-photovoltaic effect in MoS2

Bulk photovoltaic effect in a two-dimensional ferroelectric semiconductor α-In2Se3

The bulk photovoltaic effect, arising from the separation of charge carriers driven by crystal symmetry, is an intriguing physical phenomenon that has been attracting broad interest in the field of photovoltaic applications due to its junction-free nature and potential to surpass the Shockley-Queisser limit. The photovoltaic applications of conventional ferroelectric materials with wide bandgaps (2.7-4 eV) are limited due to their low photocurrent densities and weak photovoltaic response in the visible light region. The emergence of two-dimensional ferroelectric semiconductors with coupled visible light absorption and spontaneous polarization characteristics is promising for the development of functional photoferroelectrics. Herein, we report the experimental demonstration of enhanced bulk photovoltaic response in a two-dimensional ferroelectric semiconductor α-In2Se3 under the excitation of visible light. The generated photovoltaic current density is nearly two orders of magnitude higher than that of conventional bulk ferroelectric materials. Our findings highlight the potential of two-dimensional ferroelectric semiconductor materials for bulk photovoltaic applications across a broad spectral region.

Ferroelectricity and Nonlinear Optical Responses in Two-Dimensional Distorted MX2Y (M = Cu, Ag, Au; X = Chalcogens; Y = Halogen) Monolayers

Two-dimensional (2D) materials with spontaneous polarization can exhibit large second-order nonlinear optical (NLO) effects. Here, we present a series of stable distorted MX2Y monolayers by using first-principles calculations and lattice vibration analysis. The structural distortion leads to a lower polar symmetry, giving rise to intrinsic ferroelectricity with a Curie point up to room temperature. We show that the bulk photovoltaic effect (BPVE) in polar MX2Y monolayers has an enhanced shift current, an order of magnitude larger than that in the undistorted nonpolar counterparts. Meanwhile, the second harmonic generation (SHG) susceptibility reaches up to the order of 106 pm2/V, superior to that of 2D conventional materials such as MoS2, h-BN, and GeS. Our study advances the research in 2D ferroelectric materials and would stimulate more efforts in developing optoelectronic devices based on NLO effects.

In-Plane Bulk Photovoltaic Effect in a MoSe2/NbOI2 Heterojunction for Efficient Polarization-Sensitive Self-Powered Photodetection

Two-dimensional ferroelectric materials can generate a bulk photovoltaic effect, making them highly promising for self-powered photodetectors. However, their practical application is limited by a weak photoresponse due to a weak transition strength and wide band gap. In this study, we construct a van der Waals heterojunction using NbOI2, which has significant in-plane polarization, with a highly absorbing MoSe2 layer. We observe ultrafast hole transfer from MoSe2 to NbOI2 within 0.4 ps and electron transfer in the opposite direction within 3.8 ps, facilitating efficient charge dissociation and extraction. Applying a direct current electric field poling modulates the ferroelectric domains in NbOI2, enhancing the bulk photovoltaic effect. This results in one of the highest responsivities for self-powered photodetectors (101.3 mA/W) at 0 V bias alongside excellent polarization sensitivity (∼7.58). This work advances the understanding of self-powering mechanisms via the bulk photovoltaic effect and proposes new strategies for future self-powered devices.

Strain-Reduced Inversion Symmetry in Ultrathin SnP2Se6 Crystals for Giant Bulk Piezophotovoltaic Generation

With the potential to surpass the Shockley-Queisser (S-Q) limitation for solar energy conversion, the bulk photovoltaic (BPV) effect, which is induced by the broken inversion symmetry of the lattice, presents prospects for future light-harvesting technologies. However, the development of BPV is largely limited by the low solar spectrum conversion efficiency of existing noncentrosymmetric materials with wide band gaps. This study reports that the strain-induced reduction of inversion symmetry can enhance the second-order nonlinear susceptibility (χ(2)) of SnP2Se6 crystals by an order of magnitude, which contributes to an extremely high value of 1.3 × 10-8 m·V-1 under 1550 nm excitation, and is high among two-dimensional (2D) crystals. More importantly, owing to the orientation-dependent reduction of lattice symmetry, the BPV generation induced by strain, referred to as the bulk piezophotovoltaic effect, is demonstrated in the SnP2Se6 crystal with strong in-plane anisotropy. The strain along the Se zigzag direction greatly facilitates the generation of the giant photocurrent covering an extended spectrum ranging from 400 to 1100 nm, resulting in leading-level values of the BPV coefficient among noncentrosymmetric crystals, while the BPV effect is barely modulated along the Se armchair direction even with a large strain of 0.57%. This study highlights the potential of the bulk piezophotovoltaic effects for energy conversion efficiency and offers a promising strategy for the design of next-generation light-harvesting devices.

Beyond visible: giant bulk photovoltaic effect for broadband neuromodulation

The giant bulk photovoltaic effect in tellurene nanomaterials has been harnessed to enable broadband infrared neuromodulation, expanding the potential for safe, non-invasive neural stimulation and highlighting the importance of material innovation in advancing infrared photonic applications.

Boosting bulk photovoltaic effect in transition metal dichalcogenide by edge semimetal contact

Oxide materials with a non-centrosymmetric structure exhibit bulk photovoltaic effect (BPVE) but with a low cell efficiency. Over the past few years, relatively larger BPVE coefficients have been reported for two-dimensional (2D) layers and stacks with asymmety-induced spontaneous polarization. Here, we report a crucial breakthrough in boosting the BPVE in 3R-MoS2 by adopting edge contact (EC) geometry using bismuth semimetal electrode. In clear contrast to the typically used top contact (TC) geometry, the EC metal which strongly adheres to the edges and the subtrates can induce a pronounced tensile strain to the 3R-MoS2, and the lateral contact geometry allows to completely access to in-plane polarization from underneath layers reachable by light, leading to >100 times of BPVE enhancement in photocurrent. We further design a 3R-MoS2/WSe2 heterojunction to demonstrate constructive coupling of BPVE with the conventional photovoltaic effect, indicating their potential in photodetectors and photovoltaic devices.

Tunable WSe2-MoSe2 Lateral Heterojunction Photodetector Based on Piezoelectric and Flexoelectric Effects

Two-dimensional transition metal dichalcogenides (TMDs) with piezoelectric effects are ideal materials for future wearable devices. While enhancing the piezoelectric performance by forming vertical heterojunctions, shortcomings such as contamination at the heterojunction interface and limited built-in electric field width have been noticed. In this work, a lateral heterojunction of monolayer WSe2-MoSe2 with type-II band alignment was employed to amplify the electromechanical optoelectronic efficiency. The considerable built-in field width (BFW) in the lateral heterojunction facilitates rapid separation of carriers. The lattice mismatch induced a flexoelectric effect during the lateral heterojunction growth. The flexoelectric and piezoelectric effects under external strain can regulate the photodetector performance of the device. Under the compressive strain of -0.93%, the photocurrent increased 9.1 times compared to the tensile strain of 0.47%. Flexoelectric effect can reduce the dark current under no external strain. This work reveals the roles of flexoelectric and piezoelectric effects in enhancing photoelectric conversion, suggesting lateral heterojunction devices may be applied in the field of flexible low-light detection.

Leveraging Femtosecond Laser Ablation for Tunable Near-Infrared Optical Properties in MoS2-Gold Nanocomposites

Transition metal dichalcogenides (TMDCs), particularly molybdenum disulfide (MoS2), have gained significant attention in the field of optoelectronics and photonics due to their unique electronic and optical properties. The integration of TMDCs with plasmonic materials allows to tailor the optical response and offers significant advantages for photonic applications. This study presents a novel approach to synthesize MoS2-Au nanocomposites utilizing femtosecond laser ablation in liquid to achieve tunable optical properties in the near-infrared (NIR) region. By adjusting ablation and fragmentation protocols, we successfully synthesize various core-shell and core-shell-satellite nanoparticle composites, such as MoS2/MoSxOy, MoSxOy/Au, and MoS2/MoSxOy/Au. UV-visible absorption spectroscopy unveils considerable changes in the optical response of the particles depending on the fabrication regime due to structural modifications. Hybrid nanoparticles exhibit enhanced photothermal properties when subjected to NIR-I laser irradiation, demonstrating potential benefits for selective photothermal therapy. Our findings underscore that the engineered nanocomposites not only facilitate green synthesis but also pave the way for tailored therapeutic applications, highlighting their role as promising candidates in the field of nanophotonics and cancer treatment.

Fundamentals of Flexoelectricity, Materials and Emerging Opportunities Toward Strain-Driven Nanocatalysts

Flexoelectricity, an intrinsic property observed in materials under nonuniform deformation, entails a coupling between polarization and strain gradients. Recent catalyst advancements have reignited interest in flexoelectricity, particularly at the nanoscale, where pronounced strain gradients promote robust flexoelectric effects. This paper comprehensively examines flexoelectricity, encompassing methodologies for precise measurement, elucidating its distinctions from related phenomena, and exploring its potential applications in augmenting catalytic properties. So far, the greatest potentials are based on lead strontium titanate (PST) and other metallic titanates such as titania (TiO2), strontium titanate (STO), barium strontium titanate (BST) sulfates (MoS2, ZnS) and halide perovskites (with archetype XPbI3). This review explores the promise of flexoelectric properties in addressing material and photocatalytic challenges, such as charge carrier recombination and ineffective surface charge separation. Additionally, it sheds light on the synergy with emerging paradigms like photo-flexo catalysis and synergistic flexo-piezo catalysis, specifically focusing on selective chemical transformations like green hydrogen production. Current limitations related to the usage of photoflexoelectricity for photocatalysis are mostly the stability of the used substance (susceptibility to photodegradation) or the voltage values, which represent the inferior potential for specific practical applications. This work underscores the indispensable role of flexoelectricity in catalysis and its capacity to steer future research and technological advancement.

Unveiling Intrinsic Bulk Photovoltaic Effect in Atomically Thin ReS2

The bulk photovoltaic effect (BPVE) offers a promising avenue to surpass the efficiency limitations of current solar cell technology. However, disentangling intrinsic and extrinsic contributions to photocurrent remains a significant challenge. Here, we fabricate high-quality, lateral devices based on atomically thin ReS2 with minimal contact resistance, providing an optimal platform for distinguishing intrinsic bulk photovoltaic signals from other extrinsic photocurrent contributions originating from interfacial effects. Our devices exhibit large bulk photovoltaic performance with intrinsic responsivities of ∼1 mA/W in the visible range, without the need for external tuning knobs such as strain engineering. Our experimental findings are supported by theoretical calculations. Furthermore, our approach can be extrapolated to investigate the intrinsic BPVE in other noncentrosymmetric van der Waals materials, paving the way for a new generation of efficient light-harvesting devices.

Strain-Engineered Ferroelectricity in 2H Bilayer MoS2

The exploration of two-dimensional (2D) materials exhibiting out-of-plane ferroelectric and piezoelectric properties through interlayer twist/translation or strain, known as sliding ferroelectricity, has become a focal point in the quest for low-power electronic devices, capitalizing on weak van der Waals interactions. Herein, we delve into the behavior of strained bilayer molybdenum disulfide (2L-MoS2) transferred onto a nanocone-patterned substrate. An intriguing observation is the emergence of unexpected vertical ferroelectricity in MoS2, irrespective of whether it was prepared using chemical vapor deposition or mechanical exfoliation from the bulk crystal. Such an observation underscores the versatility and reproducibility of the emerging ferroelectricity across different preparation methods. Furthermore, the piezoelectric coefficients recorded are exceptionally high, with the values of 37.54 and 24.80 pm V-1 for monolayer and bilayer MoS2, respectively, outperforming most currently discovered 2D piezoelectrics. The presence of room-temperature out-of-plane ferroelectricity in strained 2L-MoS2 is confirmed through first-principles calculations and piezoresponse force microscopy. This ferroelectric behavior can be attributed to the symmetry breaking and interlayer sliding within the strained 2L-MoS2 structure. Our findings not only deepen the understanding of ferroelectricity in 2D materials but also offer insights for the design of 2D ferroelectrics, thereby enabling diverse functionalities and applications in ferroelectricity.

Intertwined Flexoelectricity and Stacking Ferroelectricity in Marginally Twisted hBN Moiré Superlattice

Moiré superlattices in twisted van der Waals homo/heterostructures present a fascinating interplay between electronic and atomic structures, with potential applications in electronic and optoelectronic devices. Flexoelectricity, an electromechanical coupling between electric polarization and strain gradient, is intrinsic to these superlattices because of the lattice misfit strain at the atomic scale. However, due to its weak magnitude, the effect of flexoelectricity on moiré ferroelectricity has remained underexplored. Here, the role of flexoelectricity in shaping and modulating the moiré ferroelectric patterns in twisted hBN homojunction is unveiled. Enhanced flexoelectric effects induce unique stacking ferroelectric domains with hollow triangular structures. Interlayer bubbles influence domain shape and periodicity through local electric field modulation, and tip-stress enables the reversible manipulation of domain area and polarization direction. These findings highlight the impact of flexoelectric effects on moiré ferroelectricity, offering a new tuning knob for its manipulation.

Infrared optoelectronics in twisted black phosphorus

Electrons and holes, fundamental charge carriers in semiconductors, dominate optical transitions and detection processes. Twisted van der Waals (vdW) heterostructures offer an effective approach to manipulate radiation, separation, and collection processes of electron-hole pairs by creating an atomically sharp interface. Here, we demonstrate that twisted interfaces in vdW layered black phosphorus (BP), an infrared semiconductor with highly anisotropic crystalline structure and properties, can significantly alter both recombination and separation processes of electron-hole pairs. On the one hand, the twisted interface breaks the symmetry of optical transition states resulting in infrared light emission of originally symmetry-forbidden optical states along the zigzag direction. On the other hand, spontaneous electronic polarization/bulk photovoltaic effect is generated at the twisted interface enabling effective separation of electron-hole pairs without external voltage bias. This is supported by first-principles calculations and repeated experiments at various twisted angles from 0 to 90°. Importantly, these phenomena can be observed in twisted heterostructures with thickness beyond two-dimensional. Our results suggest that the engineering of vdW twisted interfaces is an effective strategy for manipulating the optoelectronic properties of materials and constructing functional devices.

Giant infrared bulk photovoltaic effect in tellurene for broad-spectrum neuromodulation

Given the surpassing of the Shockley-Quiesser efficiency limit in conventional p-n junction photovoltaic effect, bulk photovoltaic effect (BPVE) has garnered significant research interest. However, the BPVE primarily focuses on a narrow wavelength range, limiting its potential applications. Here we report a giant infrared bulk photovoltaic effect in tellurene (Te) for broad-spectrum neuromodulation. The generated photocurrent in uniformly illuminated Te excludes other photoelectric effects and is attributed to the BPVE. The bulk photovoltaic wavelength in Te spans a wide range from the ultraviolet (390 nm) to the mid-infrared (3.8 µm). Moreover, the photocurrent density of 70.4 A cm-2 under infrared light simulation outperforms that in previous ultraviolet and visible semiconductors as well as infrared semimetals. Te attached to the dendrites or somata of the cortical neurons successfully elicit action potentials under broad-spectrum light irradiation. This work lays the foundation for the further development of infrared BPVE in narrow bandgap materials.

Converse Flexoelectricity in van der Waals (vdW) Three-Dimensional Topological Insulator Nanoflakes

Low-dimensional van der Waals (vdW) three-dimensional (3D) topological insulators (TIs) have been overlooked, regarding their electromechanical properties. In this study, we experimentally investigate the electromechanical coupling of low-dimensional 3D TIs with a centrosymmetric crystal structure, where a binary compound, bismuth selenide (Bi2Se3), is taken as an example. Piezoresponse force microscopy (PFM) results of Bi2Se3 nanoflakes show that the material exhibits both out-of-plane and in-plane electromechanical responses. With careful analyses, the electromechanical responses are verified to arise from the converse flexoelectricity. The Bi2Se3 nanoflakes have a decreasing effective out-of-plane piezoelectric coefficient d 33 eff with the thickness increasing, with the d 33 eff value of ∼0.65 pm V-1 for the 37 nm-thick sample. The measured effective out-of-plane piezoelectric coefficient is mainly contributed by the flexoelectric coefficient, μ39, which is estimated to be approximately 0.13 nC m-1. The results can help to understand the flexoelectricity of low-dimensional vdW TIs with centrosymmetric crystal structures, which is crucial for the design of nanoelectromechanical devices and spintronics built by vdW TIs.

Intercalated Architecture of the Ca2A2Z5 Monolayer with High Electron Mobilities and High Power Conversion Efficiencies

The exploration of novel two-dimensional (2D) materials with a direct band gap and high mobility has attracted huge attention due to their potential application in electronic and optoelectronic devices. Here, we propose a feasible way to construct multiatomic monolayer Ca2A2Z5 (A = Al and Ga and Z = S, Se, and Te) by first-principles calculations. Our results indicated that the energies of α1-phase Ca2A2Z5 are slightly lower than those of experimentally synthesized α3-phase-like Ca2A2Z5 monolayers with excellent structural stability. Moreover, the α1- and α3-phase Ca2A2Z5 monolayers possess not only direct band gaps but also high electron mobilities (up to ∼103 cm2 V-1 s-1), demonstrating an intriguing range of visible light absorption. Importantly, α1- and α3-phase Ca2Ga2Se5 monolayers are good donor materials, and the corresponding Ca2Ga2Se5/ZrSe2 type-II heterostructures exhibit desirable power conversion efficiencies of 22.4% and 22.9%, respectively. Our findings provide a feasible way to explore new 2D materials and offer several Ca2A2Z5 candidate monolayers for the application of high-performance solar cells.

Ionic Photovoltaics-in-Memory in van der Waals Material

The photovoltaic effect is gaining growing attention in the optoelectronics field due to its low power consumption, sustainable nature, and high efficiency. However, the photovoltaic effects hitherto reported are hindered by the stringent band-alignment requirement or inversion symmetry-breaking, and are challenging for achieving multifunctional photovoltaic properties (such as reconfiguration, nonvolatility, and so on). Here, a novel ionic photovoltaic effect in centrosymmetric CdSb2Se3Br2 that can overcome these limitations is demonstrated. The photovoltaic effect displays significant anisotropy, with the photocurrent being most apparent along the CdBr2 chains while absent perpendicular to them. Additionally, the device shows electrically-induced nonvolatile photocurrent switching characteristics. The photovoltaic effect is attributed to the modulation of the built-in electric field through the migration of Br ions. Using these unique photovoltaic properties, a highly secure circuit with electrical and optical keys is successfully implemented. The findings not only broaden the understanding of the photovoltaic mechanism, but also provide a new material platform for the development of in-memory sensing and computing devices.

Mixed-Dimensional Integration of 3D-on-2D Heterostructures for Advanced Electronics

Two-dimensional (2D) materials have garnered significant attention due to their exceptional properties requisite for next-generation electronics, including ultrahigh carrier mobility, superior mechanical flexibility, and unusual optical characteristics. Despite their great potential, one of the major technical difficulties toward lab-to-fab transition exists in the seamless integration of 2D materials with classic material systems, typically composed of three-dimensional (3D) materials. Owing to the self-passivated nature of 2D surfaces, it is particularly challenging to achieve well-defined interfaces when forming 3D materials on 2D materials (3D-on-2D) heterostructures. Here, we comprehensively review recent progress in 3D-on-2D incorporation strategies, ranging from direct-growth- to layer-transfer-based approaches and from non-epitaxial to epitaxial integration methods. Their technological advances and obstacles are rigorously discussed to explore optimal, yet viable, integration strategies of 3D-on-2D heterostructures. We conclude with an outlook on mixed-dimensional integration processes, identifying key challenges in state-of-the-art technology and suggesting potential opportunities for future innovation.

Electrically Controlled High Sensitivity Strain Modulation in MoS2 Field-Effect Transistors via a Piezoelectric Thin Film on Silicon Substrates

Strain can modulate bandgap and carrier mobilities in two-dimensional (2D) materials. Conventional strain-application methodologies relying on flexible/patterned/nanoindented substrates are limited by low thermal tolerance, poor tunability, and/or scalability. Here, we leverage the converse piezoelectric effect to electrically generate and control strain transfer from a piezoelectric thin film to electromechanically coupled 2D MoS2. Electrical bias polarity change across the piezo film tunes the nature of strain transferred to MoS2 from compressive (∼0.23%) to tensile (∼0.14%) as verified through Raman and photoluminescence spectroscopies and substantiated by density functional theory calculations. The device architecture, on silicon substrate, integrates an MoS2 field-effect transistor on a metal-piezoelectric-metal stack enabling strain modulation of transistor drain current (130×), on/off ratio (150×), and mobility (1.19×) with high precision, reversibility, and resolution. Large, tunable tensile (1056) and compressive (-1498) strain gauge factors, electrical strain modulation, and high thermal tolerance promise facile integration with silicon-based CMOS and micro-electromechanical systems.

Surface photogalvanic effect in Ag2Te

The bulk photovoltaic effect (BPVE) in non-centrosymmetric materials has attracted significant attention in recent years due to its potential to surpass the Shockley-Queisser limit. Although these materials are strictly constrained by symmetry, progress has been made in artificially reducing symmetry to stimulate BPVE in wider systems. However, the complexity of these techniques has hindered their practical implementation. In this study, we demonstrate a large intrinsic photocurrent response in centrosymmetric topological insulator Ag2Te, attributed to the surface photogalvanic effect (SPGE), which is induced by symmetry reduction of the surface. Through diverse spatially-resolved measurements on specially designed devices, we directly observe that SPGE in Ag2Te arises from the difference between two opposite photocurrent flows generated from the top and bottom surfaces. Acting as an efficient SPGE material, Ag2Te demonstrates robust performance across a wide spectral range from visible to mid-infrared, making it promising for applications in solar cells and mid-infrared detectors. More importantly, SPGE generated on low-symmetric surfaces can potentially be found in various systems, thereby inspiring a broader range of choices for photovoltaic materials.

Enhanced Electromechanical Response Due to Inhomogeneous Strain in Monolayer MoS2

2D transition metal dichalcogenides (TMDs) exhibit exceptional resilience to mechanical deformation. Applied strain can have pronounced effects on properties such as the bandgaps and exciton dynamics of TMDs, via deformation potentials and electromechanical coupling. In this work, we use piezoresponse force microscopy to show that the inhomogeneous strain from nanobubbles produces dramatic, localized enhancements of the electromechanical response of monolayer MoS2. Nanobubbles with diameters under 100 nm consistently produce an increased piezoresponse that follows the features' topography, while larger bubbles exhibit a halo-like profile, with maximum piezoresponse near the periphery. We show that spatial filtering enables these effects to be eliminated in the quantitative determination of effective piezoelectric or flexoelectric coefficients. Numerical strain modeling reveals a correlation between the hydrostatic strain gradient and the effective piezoelectric coefficient in large MoS2 nanobubbles, suggesting a localized variation in electromechanical coupling due to symmetry reduction induced by inhomogeneous strain.

Novel two-dimensional HfSi2N4 monolayer with excellent bandgap modulation and electronic properties modulation

The bandgap modulation and electronic properties modulation of two-dimensional HfSi2N4 monolayer induced by strain, electric field and atomic adsorption are studied by first principles. The HfSi2N4 monolayer was found to be dynamically, thermally, and mechanically stable at equilibrium, and it is a direct semiconductor with a bandgap of 1.87 eV. The bandgap of the HfSi2N4 monolayer can be precisely modulated by strain. Under the action of strain, HfSi2N4 monolayer not only transforms from direct semiconductor to indirect semiconductor, but also improves the absorption of visible light. An external electric field in the 0-0.5 eV/Å range can also modulate the bandgap of HfSi2N4 monolayer from 1.87 eV to 0 eV, and most importantly, at an external electric field of 0.5 eV/Å, HfSi2N4 monolayer shows the characteristics of spin gapless semiconductor. The calculated adsorption energy shows that the structures of H, O and F atoms adsorbed by HfSi2N4 monolayer can all exist stably. The bandgap of the configuration after adsorption of O and F atoms is significantly reduced compared with that of HfSi2N4 monolayer. Furthermore, the HfSi2N4 monolayer after adsorption of H and F atoms is transformed into a magnetic semiconductor. METHOD: All calculations were performed using Vienna ab initial simulation package, The electronic structure, mechanical properties, electronic properties and other properties were carried out using generalized gradient approximation (GGA-PBE), supplemented by HSE06 and GGA + U. The total-energy and force convergence are less than 10-6 eV and 0.001 eV/Å, respectively. The vacuum on the z-axis is selected 20 Å. The vdW interactions were corrected using the Grimme scheme (DFT-D3).

Advancing Nanoelectronics Applications: Progress in Non-van der Waals 2D Materials

Extending the inventory of two-dimensional (2D) materials remains highly desirable, given their excellent properties and wide applications. Current studies on 2D materials mainly focus on the van der Waals (vdW) materials since the discovery of graphene, where properties of atomically thin layers have been found to be distinct from their bulk counterparts. Beyond vdW materials, there are abundant non-vdW materials that can also be thinned down to 2D forms, which are still in their early stage of exploration. In this review, we focus on the downscaling of non-vdW materials into 2D forms to enrich the 2D materials family. This underexplored group of 2D materials could show potential promise in many areas such as electronics, optics, and magnetics, as has happened in the vdW 2D materials. Hereby, we will focus our discussion on their electronic properties and applications of them. We aim to motivate and inspire fellow researchers in the 2D materials community to contribute to the development of 2D materials beyond the widely studied vdW layered materials for electronic device applications. We also give our insights into the challenges and opportunities to guide researchers who are desirous of working in this promising research area.

Dual polarization-enabled ultrafast bulk photovoltaic response in van der Waals heterostructures

The bulk photovoltaic effect (BPVE) originating from spontaneous charge polarizations can reach high conversion efficiency exceeding the Shockley-Queisser limit. Emerging van der Waals (vdW) heterostructures provide the ideal platform for BPVE due to interfacial interactions naturally breaking the crystal symmetries of the individual constituents and thus inducing charge polarizations. Here, we show an approach to obtain ultrafast BPVE by taking advantage of dual interfacial polarizations in vdW heterostructures. While the in-plane polarization gives rise to the BPVE in the overlayer, the charge carrier transfer assisted by the out-of-plane polarization further accelerates the interlayer electronic transport and enhances the BPVE. We illustrate the concept in MoS2/black phosphorus heterostructures, where the experimentally observed intrinsic BPVE response time achieves 26 ps, orders of magnitude faster than that of conventional non-centrosymmetric materials. Moreover, the heterostructure device possesses an extrinsic response time of approximately 2.2 ns and a bulk photovoltaic coefficient of 0.6 V-1, which is among the highest values for vdW BPV devices reported so far. Our study thus points to an effective way of designing ultrafast BPVE for high-speed photodetection.

Enormous and Tunable Bulk Charge/Spin Photovoltaic Effect in Piezoelectric Binary Materials T-IV-VI and T-V-V

Understanding the nonlinear response of light and materials is crucial for fundamental physics and next-generation electronic devices. In this work, we have investigated the second-order nonlinear bulk photovoltaic (BPV) and bulk spin photovoltaic (BSPV) effects in the piezoelectric binary materials T-IV-VI and T-V-V (IV = Ge, Sn; VI = S, Se; and V = P, As, Sb, Bi). The independent nonzero conductivity tensors of charge current are derived for these binaries through the symmetry analysis, along with the mechanism for generating pure spin current. These binaries, with their unique folded structure, exhibit significant charge and spin currents under illumination. Furthermore, we find that strain engineering can effectively modulate charge/spin currents by influencing charge density distribution and built-in electric field due to the piezoelectric effect. Our research suggests that the piezoelectric binary materials possess enormous and tunable charge/spin currents, underscoring their potential for applications in nonlinear flexible optoelectronics and spintronics.

Flexoelectric Engineering of Bulk Photovoltaic Photodetector

The bulk photovoltaic effect (BPVE) offers an interesting approach to generate a steady photocurrent in a single-phase material under homogeneous illumination, and it has been extensively investigated in ferroelectrics exhibiting spontaneous polarization that breaks inversion symmetry. Flexoelectricity breaks inversion symmetry via a strain gradient in the otherwise nonpolar materials, enabling manipulation of ferroelectric order without an electric field. Combining these two effects, we demonstrate active mechanical control of BPVE in suspended 2-dimensional CuInP2S6 (CIPS) that is ferroelectric yet sensitive to electric field, which enables practical photodetection with an order of magnitude enhancement in performance. The suspended CIPS exhibits a 20-fold increase in photocurrent, which can be continuously modulated by either mechanical force or light polarization. The flexoelectrically engineered photodetection device, activated by air pressure and without any optimization, possesses a responsivity of 2.45 × 10-2 A/W and a detectivity of 1.73 × 1011 jones, which are superior to those of ferroelectric-based photodetection and comparable to those of the commercial Si photodiode.

Strain-Prompted Giant Flexo-Photovoltaic Effect in Two-Dimensional Violet Phosphorene Nanosheets

As a second-order nonlinear optical phenomenon, the bulk photovoltaic (BPV) effect is expected to break through the Shockley-Queisser limit of thermodynamic photoelectron conversion and improve the energy conversion efficiency of photovoltaic cells. Here, we have successfully induced a strong flexo-photovoltaic (FPV) effect, a form of BPV effect, in strained violet phosphorene nanosheets (VPNS) by utilizing strain engineering at the h-BN nanoedge, which was first observed in nontransition metal dichalcogenide (TMD) systems. This BPV effect was found to originate from the disruption of inversion symmetry induced by uniaxial strain applied to VPNS at the h-BN nanoedge. We have revealed the intricate relationship between the bulk photovoltaic effect and strain gradients in VPNS through thickness-dependent photovoltaic response experiments. A bulk photovoltaic coefficient of up to 1.3 × 10-3 V-1 and a polarization extinction ratio of 21.6 have been achieved by systematically optimizing the height of the h-BN nanoedge and the thickness of VPNS, surpassing those of reported TMD materials (typically less than 3). Our results have revealed the fundamental relationship between the FPV effect and the strain gradients in low-dimensional materials and inspired further exploration of optoelectronic phenomena in strain-gradient engineered materials.

Giant In-Plane Flexoelectricity and Radial Polarization in Janus IV-VI Monolayers and Nanotubes

Nanotubes have established a new paradigm in nanoscience because of their atomically thin geometries and intriguing properties. However, because of their typical metastability compared to their 2D and 3D counterparts, it is still fundamentally challenging to synthesize nanotubes with controlled size. New strategies have been suggested for synthesizing nanotubes with a controlled geometry. One of these is considering Janus 2D layers, which can self-roll to form a nanotube. Herein, we study 412 nanotubes (along the armchair and zigzag directions) based on 36 Janus IV-VI compounds using density functional theory (DFT) calculations. By investigating the energy-radius relationship using structural models and Bayesian predictions, the most stable nanotubes show negative strain energies and radii below 20 Å, where curvature effects can play a significant role. The band structures show that the selected nanotubes exhibit sizable band gaps and size-dependent electronic properties. More strikingly, the flexoelectricity along the in-plane directions and radial directions in these nanotubes is significantly larger than that in other nanotubes and their 2D counterparts. This work opens up an avenue of structure-property relationships of Janus IV-VI nanotubes and demonstrates giant flexoelectricity in these nanotubes for future electronic and energy applications.

Piezo-photovoltaic Effect in Monolayer 2H-MoS2

Noncentrosymmetric bulk materials effectively convert light energy into electricity by making use of the bulk photovoltaic effect (BPVE). However, whether such an effect persists when reducing the thickness of materials down to atomic-scale remains to be revealed. Here, we show the piezo-photovoltaic effect in atomically thin two-dimensional materials, where the strain-induced polarization can generate photovoltaic outputs in the noncentrosymmetric mono- and few-layer 2H-MoS2 crystals. The photocurrent is enhanced by orders of magnitude when the MoS2 crystals experience an in-plane strain of about 0.2%, with photopower-dependent responsivity up to 0.1 A/W that rivals other state-of-the-art BPVE materials. In addition, studies on the spatial distributions of photocurrents on MoS2 with a controlled number of layers also allow us to disentangle various factors that couple the piezoelectricity and photovoltaics. Therefore, our results also provide insights into the mechanisms of the piezo-photovoltaic effect in two-dimensional materials with thicknesses at the atomic-scale limit.

Revealing Strong Flexoelectricity and Optoelectronic Coupling in 2D Ferroelectric CuInP2S6 Via Large Strain Gradient

The interplay between flexoelectric and optoelectronic characteristics provides a paradigm for studying emerging phenomena in various 2D materials. However, an effective way to induce a large and tunable strain gradient in 2D devices remains to be exploited. Herein, we propose a strategy to induce large flexoelectric effect in 2D ferroelectric CuInP2S6 by constructing a 1D-2D mixed-dimensional heterostructure. The strong flexoelectric effect is induced by enormous strain gradient up to 4.2 × 106 m-1 resulting from the underlying ZnO nanowires, which is further confirmed by the asymmetric coercive field and the red-shift in the absorption edge. The induced flexoelectric polarization efficiently boosts the self-powered photodetection performance. In addition, the improved photoresponse has a good correlation with the induced strain gradient, showing a consistent size-dependent flexoelectric effect. The mechanism of flexoelectric and optoelectronic coupling is proposed based on the Landau-Ginzburg-Devonshire double-well model, supported by density functional theory (DFT) calculations. This work provides a brand-new method to induce a strong flexoelectric effect in 2D materials, which is not restricted to crystal symmetry and thus offers unprecedented opportunities for state-of-the-art 2D devices.

Enhanced Piezoelectric Response at Nanoscale Vortex Structures in Ferroelectrics

The piezoelectric response is a measure of the sensitivity of a material's polarization to stress or its strain to an applied field. Using in operando X-ray Bragg coherent diffraction imaging, we observe that topological vortices are the source of a 5-fold enhancement of the piezoelectric response near the vortex core. The vortices form where several low-symmetry ferroelectric phases and phase boundaries coalesce. Unlike bulk ferroelectric solid solutions in which a large piezoelectric response is associated with coexisting phases in the proximity of the triple point, the largest responses for pure BaTiO3 at the nanoscale are in spatial regions of extremely small spontaneous polarization at vortex cores. The response decays inversely with polarization away from the vortex, analogous to the behavior in bulk ceramics as the cation compositions are varied away from the triple point. We use first-principles-based molecular dynamics to augment our observations, and our results suggest that nanoscale piezoelectric materials with a large piezoelectric response can be designed within a parameter space governed by vortex cores. Our findings have implications for the development of next-generation nanoscale piezoelectric materials.

A giant intrinsic photovoltaic effect in atomically thin ReS2

The photovoltaic (PV) effect in non-centrosymmetric materials consisting of a single component under homogeneous illumination can exceed the fundamental Shockley-Queisser limit compared to the traditional p-n junctions. Two-dimensional (2D) materials with a reduced dimensionality and smaller bandgap were predicated to be better candidates for the PV effect with high efficiency exceeding that of traditional ferroelectric perovskite oxides. Here, we report the giant intrinsic PV effect in atomically thin rhenium disulfide (ReS2) with centrosymmetry breaking. In graphene/ReS2/graphene sandwich structures, significant short-circuit currents (Isc) were observed with illumination over the visible spectral range, presenting the highest responsivity (110 mA W-1) and external quantum efficiency (25.7%) among those reported PV effects in 2D materials. This giant PV effect could be ascribed to the spontaneous-polarization induced depolarization field in even-number-layered ReS2 flakes benefiting from the distorted 1T lattice structure. Our results provide a new potential candidate material for the development of novel high-efficiency, miniaturized and easily integrated photodetectors and solar cells.

Giant intrinsic photovoltaic effect in one-dimensional van der Waals grain boundaries

The photovoltaic effect lies at the heart of eco-friendly energy harvesting. However, the conversion efficiency of traditional photovoltaic effect utilizing the built-in electric effect in p-n junctions is restricted by the Shockley-Queisser limit. Alternatively, intrinsic/bulk photovoltaic effect (IPVE/BPVE), a second-order nonlinear optoelectronic effect arising from the broken inversion symmetry of crystalline structure, can overcome this theoretical limit. Here, we uncover giant and robust IPVE in one-dimensional (1D) van der Waals (vdW) grain boundaries (GBs) in a layered semiconductor, ReS2. The IPVE-induced photocurrent densities in vdW GBs are among the highest reported values compared with all kinds of material platforms. Furthermore, the IPVE-induced photocurrent is gate-tunable with a polarization-independent component along the GBs, which is preferred for energy harvesting. The observed IPVE in vdW GBs demonstrates a promising mechanism for emerging optoelectronics applications.

Giant Flexoelectricity in Bent Semiconductor Thinfilm

We elucidate the flexoelectricity of semiconductors in the high strain gradient regime, the underlying mechanism of which is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high-strain-gradient-induced band gap closure. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film. Therefore, upon the band gap closure, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to a pronounced change of polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.

Disorder-induced bulk photovoltaic effect in a centrosymmetric van der Waals material

Sunlight is widely seen as one of the most abundant forms of renewable energy, with photovoltaic cells based on pn junctions being the most commonly used platform attempting to harness it. Unlike in conventional photovoltaic cells, the bulk photovoltaic effect (BPVE) allows for the generation of photocurrent and photovoltage in a single material without the need to engineer a pn junction and create a built-in electric field, thus offering a solution that can potentially exceed the Shockley-Queisser efficiency limit. However, it requires a material with no inversion symmetry and is therefore absent in centrosymmetric materials. Here, we demonstrate that breaking the inversion symmetry by structural disorder can induce BPVE in ultrathin PtSe2, a centrosymmetric semiconducting van der Waals material. Homogenous illumination of defective PtSe2 by linearly and circularly polarized light results in a photoresponse termed as linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE), which is mostly absent in the pristine crystal. First-principles calculations reveal that LPGE originates from Se vacancies that act as asymmetric scattering centers for the photo-generated electron-hole pairs. Our work emphasizes the importance of defects to induce photovoltaic functionality in centrosymmetric materials and shows how the range of materials suitable for light sensing and energy-harvesting applications can be extended.

Research Progress in Composite Materials for Photocatalytic Nitrogen Fixation

Ammonia is an essential component of modern chemical products and the building unit of natural life molecules. The Haber-Bosch (H-B) process is mainly used in the ammonia synthesis process in the industry. In this process, nitrogen and hydrogen react to produce ammonia with metal catalysts under high temperatures and pressure. However, the H-B process consumes a lot of energy and simultaneously emits greenhouse gases. In the "double carbon" effect, to promote the combination of photocatalytic technology and artificial nitrogen fixation, the development of green synthetic reactions has been widely discussed. Using an inexhaustible supply of sunlight as a power source, researchers have used photocatalysts to reduce nitrogen to ammonia, which is energy-dense and easy to store and transport. This process completes the conversion from light energy to chemical energy. At the same time, it achieves zero carbon emissions, reducing energy consumption and environmental pollution in industrial ammonia synthesis from the source. The application of photocatalytic technology in the nitrogen cycle has become one of the research hotspots in the new energy field. This article provides a classification of and an introduction to nitrogen-fixing photocatalysts reported in recent years and prospects the future development trends in this field.

Chirality dependent electromechanical properties of single-layer MoS2 under out-of-plane deformation: a DFT study

2D transition metal dichalcogenides (TMDs) demonstrate significant promise in logic circuits and optoelectronic devices because of their unique structures and excellent semiconductor properties. However, they inevitably undergo out-of-plane deformation during practical applications due to their ultra-thin structures. Recent experiments have shown that out-of-plane deformation significantly affects the electronic structures of 2D TMDs. However, the underlying physical mechanism is largely unknown. Therefore, it is critical to have a deeper understanding of out-of-plane deformation in 2D TMDs to optimize their applications in different fields. Currently, one of the most pressing matters that requires clarification is the chirality dependence of out-of-plane deformation in tuning the electromechanical properties of 2D TMDs. In this work, using single-layer MoS2 as a probe, we systematically investigate the effects of out-of-plane deformation along different chirality directions on the bond length, bending stiffness, electric polarization, and band structure of 2D TMDs by employing first-principles calculations based on density functional theory. Our results indicate that the bond length, bending energy, polarization strength, and band gap size of single-layer MoS2 are isotropic under out-of-plane deformation, while the band gap type is closely related to the direction of deformation. Our study will provide an essential theoretical basis for further revealing the structure-performance relationship of 2D TMDs.

Building Low-Cost, High-Performance Flexible Photodetector Based on Tetragonal Phase VO2 (A) Nanorod Networks

We present a straightforward and cost-effective method for the fabrication of flexible photodetectors, utilizing tetragonal phase VO2 (A) nanorod (NR) networks. The devices exhibit exceptional photosensitivity, reproducibility, and stability in ambient conditions. With a 2.0 V bias voltage, the device demonstrates a photocurrent switching gain of 1982% and 282% under irradiation with light at wavelengths of 532 nm and 980 nm, respectively. The devices show a fast photoelectric response with rise times of 1.8 s and 1.9 s and decay times of 1.2 s and 1.7 s for light at wavelengths of 532 nm and 980 nm, respectively. In addition, the device demonstrates exceptional flexibility across large-angle bending and maintains excellent mechanical stability, even after undergoing numerous extreme bending cycles. We discuss the electron transport process within the nanorod networks, and propose a mechanism for the modulation of the barrier height induced by light. These characteristics reveal that the fabricated devices hold the potential to serve as a high-performance flexible photodetector.

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2

In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional (2D) material, MoTe2, caused by the phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semimetallic 1T'-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 to 63 meV during phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semimetallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 μA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30%, respectively. Our findings are distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials and a cutting-edge approach to optimize the efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

Ultralow Tip-Force Driven Sizable-Area Domain Manipulation through Transverse Flexoelectricity

Deterministic control of ferroelectric domain is critical in the ferroelectric functional electronics. Ferroelectric polarization can be manipulated mechanically with a nano-tip through flexoelectricity. However, it usually occurs in a very localized area in ultrathin films, with possible permanent surface damage caused by a large tip-force. Here it is demonstrated that the deliberate engineering of transverse flexoelectricity offers a powerful tool for improving the mechanical domain switching. Sizable-area domain switching under an ultralow tip-force can be realized in suspended van der Waals ferroelectrics with the surface intact, due to the enhanced transverse flexoelectric field. The film thickness range for domain switching in suspended ferroelectrics is significantly improved by an order of magnitude to hundreds of nanometers, being far beyond the limited range of the substrate-supported ones. The experimental results and phase-field simulations further reveal the crucial role of the transverse flexoelectricity in the domain manipulation. This large-scale mechanical manipulation of ferroelectric domain provides opportunities for the flexoelectricity-based domain controls in emerging low-dimensional ferroelectrics and related devices.

Strong bulk photovoltaic effect in engineered edge-embedded van der Waals structures

Bulk photovoltaic effect (BPVE), a second-order nonlinear optical effect governed by the quantum geometric properties of materials, offers a promising approach to overcome the Shockley-Quiesser limit of traditional photovoltaic effect and further improve the efficiency of energy harvesting. Here, we propose an effective platform, the nano edges embedded in assembled van der Waals (vdW) homo- or hetero-structures with strong symmetry breaking, low dimensionality and abundant species, for BPVE investigations. The BPVE-induced photocurrents strongly depend on the orientation of edge-embedded structures and polarization of incident light. Reversed photocurrent polarity can be observed at left and right edge-embedded structures. Our work not only visualizes the unique optoelectronic effect in vdW nano edges, but also provides an effective strategy for achieving BPVE in engineered vdW structures.

Giant Bulk Electrophotovoltaic Effect in Heteronodal-Line Systems

The realization of a giant and continuously tunable second-order photocurrent is desired for many nonlinear optical (NLO) and optoelectronic applications, which remains a great challenge. Here, based on a two-band model, we propose a concept of the bulk electrophotovoltaic effect, that is, an out-of-plane external electric field (E_{ext}) that can continuously tune in-plane shift current along with its sign flip in a heteronodal-line (HNL) system. While strong linear optical transition around the nodal loop may potentially generate giant shift current, an E_{ext} can effectively control the radius of the nodal loop, which can continuously modulate the shift-vector components inside and outside the nodal loop holding opposite signs. This concept has been demonstrated in the HNL HSnN/MoS_{2} system using first-principles calculations. The HSnN/MoS_{2} heterobilayer can not only produce a shift-current conductivity with magnitude that is one to two orders larger than other reported systems, but it can also realize a giant bulk electrophotovoltaic effect. Our finding opens new routes to create and manipulate NLO responses in 2D materials.

Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

Towards two-dimensional van der Waals ferroelectrics

The discovery of ferroelectricity in two-dimensional (2D) van der Waals (vdW) materials has brought important functionalities to the 2D materials family, and may trigger a revolution in next-generation nanoelectronics and spintronics. In this Perspective, we briefly review recent progress in the field of 2D vdW ferroelectrics, focusing on the mechanisms that drive spontaneous polarization in 2D systems, unique properties brought about by the reduced lattice dimensionality and promising applications of 2D vdW ferroelectrics. We finish with an outlook for challenges that need to be addressed and our view on possible future research directions.

Observation of the photovoltaic effect in a van der Waals heterostructure

van der Waals (vdW) heterostructures, which can be assembled with various two-dimensional materials, provide a versatile platform for exploring emergent phenomena. Here, we report an observation of the photovoltaic effect in a WS₂/MoS₂ vdW heterostructure. Light excitation of WS₂/MoS₂ at a wavelength of 633 nm yields a photocurrent without applying bias voltages, and the excitation power dependence of the photocurrent shows characteristic crossover from a linear to square root dependence. Photocurrent mapping has clearly shown that the observed photovoltaic effect arises from the WS₂/MoS₂ region, not from Schottky junctions at electrode contacts. Kelvin probe microscopy observations show no slope in the electrostatic potential, excluding the possibility that the photocurrent originates from an unintentionally formed built-in potential.

Silicon flexoelectronic transistors

It is extraordinarily challenging to implement adaptive and seamless interactions between mechanical triggering and current silicon technology for tunable electronics, human-machine interfaces, and micro/nanoelectromechanical systems. Here, we report Si flexoelectronic transistors (SFTs) that can innovatively convert applied mechanical actuations into electrical control signals and achieve directly electromechanical function. Using the strain gradient-induced flexoelectric polarization field in Si as a "gate," the metal-semiconductor interfacial Schottky barriers' heights and the channel width of SFT can be substantially modulated, resulting in tunable electronic transports with specific characteristics. Such SFTs and corresponding perception system can not only create a high strain sensitivity but also identify where the mechanical force is applied. These findings provide an in-depth understanding about the mechanism of interface gating and channel width gating in flexoelectronics and develop highly sensitive silicon-based strain sensors, which has great potential to construct the next-generation silicon electromechanical nanodevices and nanosystems.

Revisiting the Ferroelectric Photovoltaic Properties of Vertical BiFeO3 Capacitors: A Comprehensive Study

The ferroelectric photovoltaic effect has been extensively studied for possible applications in energy conversion and photo-electrics. The reversible spontaneous polarization gives rise to a switchable photovoltaic behavior. However, despite its long history, the origin of the ferroelectric photovoltaic effect still lacks a full understanding since multiple mechanisms such as bulk and Schottky-barrier-related interface effects are involved. Herein, we report a comprehensive study on the photovoltaic response of BiFeO₃-based vertical heterostructures, using multiple strategies to clarify its origin. We found that, under white light illumination, polarization-modulated Schottky barrier at the interface is the dominating mechanism. By varying the top metal contacts, only the photovoltaic effect of the polarization downward state is strongly modulated, suggesting selective interface contribution in different polarization states. A Schottky-barrier-free device shows negligible photovoltaic effect, suggesting the lack of bulk photovoltaic effect in vertical heterostructures under white light illumination.

Recent Progress in Strain Engineering on Van der Waals 2D Materials: Tunable Electrical, Electrochemical, Magnetic, and Optical Properties

Strain engineering is a promising way to tune the electrical, electrochemical, magnetic, and optical properties of 2D materials, with the potential to achieve high-performance 2D-material-based devices ultimately. This review discusses the experimental and theoretical results from recent advances in the strain engineering of 2D materials. Some novel methods to induce strain are summarized and then the tunable electrical and optical/optoelectronic properties of 2D materials via strain engineering are highlighted, including particularly the previously less-discussed strain tuning of superconducting, magnetic, and electrochemical properties. Also, future perspectives of strain engineering are given for its potential applications in functional devices. The state of the survey presents the ever-increasing advantages and popularity of strain engineering for tuning properties of 2D materials. Suggestions and insights for further research and applications in optical, electronic, and spintronic devices are provided.

Liquid Precursor-Guided Phase Engineering of Single-Crystal VO2 Beams

The study of VO₂ flourishes due to its rich competing phases induced by slight stoichiometry variations. However, the vague mechanism of stoichiometry manipulation makes the precise phase engineering of VO₂ still challenging. Here, stoichiometry manipulation of single-crystal VO₂ beams in liquid-assisted growth is systematically studied. Contrary to previous experience, oxygen-rich VO₂ phases are abnormally synthesized under a reduced oxygen concentration, revealing the important function of liquid V₂ O₅ precursor: It submerges VO₂ crystals and stabilizes their stoichiometric phase (M1) by isolating them from the reactive atmosphere, while the uncovered crystals are oxidized by the growth atmosphere. By varying the thickness of liquid V₂ O₅ precursor and thus the exposure time of VO₂ to the atmosphere, various VO₂ phases (M1, T, and M2) can be selectively stabilized. Furthermore, this liquid precursor-guided growth can be used to spatially manages multiphase structures in single VO₂ beams, enriching their deformation modes for actuation applications.

An Air-Rechargeable Zn Battery Enabled by Organic-Inorganic Hybrid Cathode

Self-charging power systems collecting energy harvesting technology and batteries are attracting extensive attention. To solve the disadvantages of the traditional integrated system, such as highly dependent on energy supply and complex structure, an air-rechargeable Zn battery based on MoS2/PANI cathode is reported. Benefited from the excellent conductivity desolvation shield of PANI, the MoS2/PANI cathode exhibits ultra-high capacity (304.98 mAh g-1 in N2 and 351.25 mAh g-1 in air). In particular, this battery has the ability to collect, convert and store energy simultaneously by an air-rechargeable process of the spontaneous redox reaction between the discharged cathode and O2 from air. The air-rechargeable Zn batteries display a high open-circuit voltage (1.15 V), an unforgettable discharge capacity (316.09 mAh g-1 and the air-rechargeable depth is 89.99%) and good air-recharging stability (291.22 mAh g-1 after 50 air recharging/galvanostatic current discharge cycle). Most importantly, both our quasi-solid zinc ion batteries and batteries modules have excellent performance and practicability. This work will provide a promising research direction for the material design and device assembly of the next-generation self-powered system.