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2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl2 has a stronger ferroelectric order than NbOI2. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl2. STM imaging reveals the unreconstructed atomic structures of NbOX2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.
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Graphene oxide (GO) was initially developed to emulate graphene, but it was soon recognized as a functional material in its own right, addressing an application space that is not accessible to graphene and other carbon materials. Over the past decade, research on GO has made tremendous advances in material synthesis and property tailoring. These, in turn, have led to rapid progress in GO-based photonics, electronics and optoelectronics, paving the way for technological breakthroughs with exceptional performance. In this Review, we provide an overview of the optical, electrical and optoelectronic properties of GO and reduced GO on the basis of their chemical structures and fabrication approaches, together with their applications in key technologies such as solar energy harvesting, energy storage, medical diagnosis, image display and optical communications. We also discuss the challenges of this field, together with exciting opportunities for future technological advances.
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Hybrid organic-inorganic perovskites (HOIPs) are promising stimuli-responsive materials (SPMs) owing to their molecular softness and tailorable structural dimensionality. The design of mechanically responsive HOIPs requires an in-depth understanding of how lattice strain induces intermolecular rearrangement that impacts physical properties. While chirality transfer from an organic cation to an inorganic lattice is known to influence chiral-optical properties, its effect on strain-induced phase conversion has not been explored. As opposed to achiral or racemic organic cations, chiral organic cations can potentially afford a new dimension in strain-responsive structural change. Herein, we demonstrate that mechanical strain induces a solid phase crystal conversion in chiral halide pseudo-perovskite single crystals (R/S)-(FE)2CuCl4 (FE = (4-Fluorophenyl)ethylamine) from a 0D isolated CuCl4 tetrahedral to 1D corner-sharing CuFCl5 octahedral framework via the incorporation of Cu···F interaction and N-H···F hydrogen bonding. This strain-induced crystal-to-crystal conversion involves the connection of neighboring 0D CuCl4 tetrahedra via Cu2+-Cl--Cu2+ linkages as well as the incorporation of a F-terminated organic cation as one of the X atoms in BX6 octahedra, leading to a reduced band gap and paramagnetic-to-ferromagnetic conversion. Control experiments using nonchiral or racemic perovskite analogs show the absence of such solid phase conversion. To demonstrate pressure-sensitive properties, the 0D phase is dispersed in water-soluble poly(vinyl alcohol) (PVA) polymer, which can be applied to a large-scale pressure-induced array display on fibrous Spandex substrates via a screen-printing method.
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Transition-metal trihalides MX3 (M = Cr, Ru; X = Cl, Br, and I) belong to a family of novel two-dimensional (2D) magnets that can exhibit topological magnons and electromagnetic properties, thus affording great promises in next-generation spintronic devices. Rich magnetic ground states observed in the MX3 family are believed to be strongly correlated to the signature Kagome lattice and interlayer van der Waals coupling raised from distinct stacking orders. However, the intrinsic air instability of MX3 makes their direct atomic-scale analysis challenging. Therefore, information on the stacking-registry-dependent magnetism for MX3 remains elusive, which greatly hinders the engineering of desired phases. Here, we report a nondestructive transfer method and successfully realize an intact transfer of bilayer MX3, as evidenced by scanning transmission electron microscopy (STEM). After surveying hundreds of MX3 thin flakes, we provide a full spectrum of stacking orders in MX3 with atomic precision and calculated their associated magnetic ground states, unveiled by combined STEM and density functional theory (DFT). In addition to well-documented phases, we discover a new monoclinic C2/c phase in the antiferromagnetic (AFM) structure widely existing in MX3. Rich stacking polytypes, including C2/c, C2/m, R3Ì , P3112, etc., provide rich and distinct magnetic ground states in MX3. Besides, a high density of strain soliton boundaries is consistently found in all MX3, combined with likely inverted structures, allowing AFM to ferromagnetic (FM) transitions in most MX3. Therefore, our study sheds light on the structural basis of diverse magnetic orders in MX3, paving the way for modulating magnetic couplings via stacking engineering.
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Dynamic reconstruction of catalyst active sites is particularly important for metal oxide-catalyzed oxygen evolution reaction (OER). However, the mechanism of how vacancy-induced reconstruction aids OER remains ambiguous. Here, we use Co3O4 with Co or O vacancies to uncover the effects of different defects in the reconstruction process and the active motifs relevant to alkaline OER. Combining in situ characterization and theoretical calculations, we found that cobalt oxides are converted to an amorphous [Co(OH)6] intermediate state, and then the mismatched rates of *OH adsorption and deprotonation lead to irreversible catalyst reconstruction. The stronger *OH adsorption but weaker deprotonation induced by O defects provides the driving force for reconstruction, while Co defects favor dehydrogenation and reduce the reconstruction rate. Importantly, both O and Co defects trigger highly OER-active bridge Co sites in reconstructed catalysts, of which Co defects induce a short Co-Co distance (3.38 Å) under compressive lattice stress and show the best OER activity (η10 of 262 mV), superior to reconstructed oxygen-defected Co3O4-VO (η10 of 300 mV) and defect-free Co3O4 (η10 of 320 mV). This work highlights that engineering defect-dependent reconstruction may provide a rational route for electrocatalyst design in energy-related applications.
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Single-atom catalysts (SACs) show great potential for rechargeable Zn-air batteries (ZABs); however, scalable production of SACs from sustainable resources is difficult owing to poor control of the local coordination environment. Herein, lignosulfonate, a by-product of the papermaking industry, is utilized as a multifunctional bioligand for the mass production of SACs with highly active MN4 S sites (M represents Fe, Cu, and Co) via strong metalnitrogen/sulfur coordination. This effectively adjusts the charge distribution and promotes the catalytic performance, leading to highly durable and excellent performance in oxygen reduction and evolution reactions for ZABs. This study paves the way for the industrial production of cost-effective SACs in a sustainable manner.
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Multilayers consisting of alternating soft and hard layers offer enhanced toughness compared to all-hard structures. However, shear instability usually exists in physically sputtered multilayers because of deformation incompatibility among hard and soft layers. Here, we demonstrate that 2D hybrid organic-inorganic perovskites (HOIP) provide an interesting platform to study the stress-strain behavior of hard and soft layers undulating with molecular scale periodicity. We investigate the phonon vibrations and photoluminescence properties of Ruddlesden-Popper perovskites (RPPs) under compression using a diamond anvil cell. The organic spacer due to C4 alkyl chain in RPP buffers compressive stress by tilting (n = 1 RPP) or step-wise rotational isomerism (n = 2 RPP) during compression, where n is the number of inorganic layers. By examining the pressure threshold of the elastic recovery regime across n = 1-4 RPPs, we obtained molecular insights into the relationship between structure and deformation resistance in hybrid organic-inorganic perovskites.
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To realize the practical application of lithium-sulfur (Li-S) batteries, there is a need to inhibit uncontrolled Li deposition by facilitating Li-ion migration, and suppress the irreversible consumption of cathodes by preventing polysulfide shuttling. However, a permselective artifical membrane or interlayer which features fast ion transport but low polysulfide crossover is elusive. Here, we report the design and synthesis of a fluorinated covalent organic framework (4F-COF)-based membrane with a high permselectivity and increased battery lifespan. Combining density functional theory calculation, molecular dynamic simulation, and in situ Raman analysis, we demonstrate that fluorinated COF eliminates polysulfides shutting and dendritic lithium formation. Consequently, Li symmetrical cells demonstrate Li plating/stripping behaviors for 2000 h under 1 mA cm-2. More importantly, Li-S batteries based on the 4F-COF/PP separator achieve cycling retention of 82.3% over 1000 cycles at 2 C, rate performance of 568.0 mA h g-1 at 10 C, and an areal capacity of 7.60 mA h cm-2 with a high sulfur loading (â¼9 mg cm-2). This work demonstrates that functionalizing nanochannels in COFs can impart permselectivity for energy storage applications.
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It is important to understand the polymorph transition and crystal-amorphous phase transition in In2Se3 to tap the potential of this material for resistive memory storage. By monitoring layer-by-layer growth of ß-In2Se3 during molecular beam epitaxy (MBE), we are able to identify a cyclical order-disorder transition characterized by a periodic alternation between a glassy-like metastable subunit cell film consisting of n < 5 sublayers (nth layers = the number of subunit cell layers), and a highly crystalline ß-In2Se3 at n = 5 layers. The glassy phase shows an odd-even alternation between the indium-cluster layer (n = 1, 3) and an In-Se solid solution (n = 2, 4), which suggests the ability of In and Se atoms to diffuse, aggregate, and intermix. These dynamic natures of In and Se atoms contribute to a defect-driven memory resistive behavior in current-voltage sweeps that is different from the ferroelectric switching of α-In2Se3.
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Metal-free 2D phosphorus-based materials are emerging catalysts for ammonia (NH3 ) production through a sustainable electrochemical nitrogen reduction reaction route under ambient conditions. However, their efficiency and stability remain challenging due to the surface oxidization. Herein, a stable phosphorus-based electrocatalyst, silicon phosphide (SiP), is explored. Density functional theory calculations certify that the N2 activation can be realized on the zigzag Si sites with a dimeric end-on coordinated mode. Such sites also allow the subsequent protonation process via the alternating associative mechanism. As the proof-of-concept demonstration, both the crystalline and amorphous SiP nanosheets (denoted as C-SiP NSs and A-SiP NSs, respectively) are obtained through ultrasonic exfoliation processes, but only the crystalline one enables effective and stable electrocatalytic nitrogen reduction reaction, in terms of an NH3 yield rate of 16.12 µg h-1 mgcat. -1 and a Faradaic efficiency of 22.48% at -0.3 V versus reversible hydrogen electrode. The resistance to oxidization plays the decisive role in guaranteeing the NH3 electrosynthesis activity for C-SiP NSs. This surface stability endows C-SiP NSs with the capability to serve as appealing electrocatalysts for nitrogen reduction reactions and other promising applications.
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The aqueous zinc (Zn) battery is a safe and eco-friendly energy-storage system. However, the use of Zn metal anodes is impeded by uncontrolled Zn deposition behavior. Herein, we regulate the Zn-ion deposition process for dendrite-free Zn metal anodes using an aminosilane molecular layer with high zincophilic sites and narrow molecule channels. The aminosilane molecular layer causes Zn ions to undergo consecutive processes including being captured by the amine functional groups of aminosilane and diffusing through narrow intermolecular channels before electroplating, which induces partial dehydration of hydrated Zn ions and uniform Zn ion flux, promoting reversible Zn stripping/plating. Through this molecule-induced capture-diffusion-deposition procedure of Zn ions, smooth and compact Zn electrodeposited layers are obtained. Hence, the aminosilane-modified Zn anode has high Coulombic efficiency (â¼99.5%), long lifespan (â¼3000 h), and high capacity retention in full cells (88.4% for 600 cycles). This strategy not only has great potential for achieving dendrite-free Zn anodes in practical Zn batteries but also suggests an interface-modification principle at the molecular level for other alternative metallic anodes.
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The ability to create a robust and well-defined artificial atomic charge in graphene and understand its carrier-dependent electronic properties represents an important goal toward the development of graphene-based quantum devices. Herein, we devise a new pathway toward the atomically precise embodiment of point charges into a graphene lattice by posterior (N) ion implantation into a back-gated graphene device. The N dopant behaves as an in-plane proton-like charge manifested by formation of the characteristic resonance state in the conduction band. Scanning tunneling spectroscopy measurements at varied charge carrier densities reveal a giant energetic renormalization of the resonance state up to 220 meV with respect to the Dirac point, accompanied by the observation of gate-tunable long-range screening effects close to individual N dopants. Joint density functional theory and tight-binding calculations with modified perturbation potential corroborate experimental findings and highlight the short-range character of N-induced perturbation.
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Brain-inspired neuromorphic computing systems with the potential to drive the next wave of artificial intelligence demand a spectrum of critical components beyond simple characteristics. An emerging research trend is to achieve advanced functions with ultracompact neuromorphic devices. In this work, a single-transistor neuron is demonstrated that implements excitatory-inhibitory (E-I) spatiotemporal integration and a series of essential neuron behaviors. Neuronal oscillations, the fundamental mode of neuronal communication, that construct high-dimensional population code to achieve efficient computing in the brain, can also be demonstrated by the neuron transistors. The highly scalable E-I neuron can be the basic building block for implementing core neuronal circuit motifs and large-scale architectural plans to replicate energy-efficient neural computations, forming the foundation of future integrated neuromorphic systems.
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Valleytronics is a promising candidate to address low-energy signal transport on chip, leveraging the valley pseudospin of electrons as a new degree of freedom to encode, process and store information1-7. However, valley-carrier nanocircuitry is still elusive, because it essentially requires valley transport that overcomes three simultaneous challenges: high fidelity, high directionality and room-temperature operation. Here we experimentally demonstrate a nanophotonic circuit that can route valley indices of a WS2 monolayer unidirectionally via the chirality of photons. Two propagating modes are supported in the gap area of the circuit and interfere with each other to generate beating patterns, which exhibit complementary profiles for circular dipoles of different handedness. Based on the spin-dependent beating patterns, we showcase valley fidelity of chiral photons up to 98%, and the circulation directionality is measured to be 0.44 ± 0.04 at room temperature. The proposed nanocircuit can not only enable the construction of large-scale valleytronic networks but also serve as an interactive interface to integrate valleytronics3-5, spintronics8-10 and integrated photonics11-13, opening new possibilities for hybrid spin-valley-photon ecosystems at the nanoscale.
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Ferroelectric materials play an important role in a wide spectrum of semiconductor technologies and device applications. Two-dimensional (2D) van der Waals (vdW) ferroelectrics with surface-insensitive ferroelectricity that is significantly different from their traditional bulk counterparts have further inspired intensive interest. Integration of ferroelectrics into 2D-layered-material-based devices is expected to offer intriguing working principles and add desired functionalities for next-generation electronics. Herein, fundamental properties of ferroelectric materials that are compatible with 2D devices are introduced, followed by a critical review of recent advances on the integration of ferroelectrics into 2D devices. Representative device architectures and corresponding working mechanisms are discussed, such as ferroelectrics/2D semiconductor heterostructures, 2D ferroelectric tunnel junctions, and 2D ferroelectric diodes. By leveraging the favorable properties of ferroelectrics, a variety of functional 2D devices including ferroelectric-gated negative capacitance field-effect transistors, programmable devices, nonvolatile memories, and neuromorphic devices are highlighted, where the application of 2D vdW ferroelectrics is particularly emphasized. This review provides a comprehensive understanding of ferroelectrics-integrated 2D devices and discusses the challenges of applying them into commercial electronic circuits.
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Chirality generates spontaneous symmetry breaking and profoundly influences the topology, charge, and spin orders of materials. The chiral charge density wave (CDW) exhibits macroscopic chirality in the achiral crystal during the spontaneous electronic phase transitions. However, the mechanism of chiral CDW formation is shrouded in controversy. In this work, we report that two-dimensional H-phase TaS2 synthesized by molecular-beam epitaxy (MBE) shows a predominantly chiral CDW phase. Scanning tunneling microscopy (STM) imaging of the CDW reconstruction spots reveals a clockwise or anticlockwise intensity variation along the STM-imaged spots. First-principles calculations further show that the rotational symmetry of the momentum-dependent electron-phonon coupling is broken, giving rise to chirality. Our work provides new insights into the physical origin of the chiral charge-ordered states, shedding light on a general ordering rule in chiral CDWs.
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Organic-inorganic metal halide perovskite films have emerged as potential candidate materials for photoelectric devices because of their superior optoelectronic properties. The performance of these devices depends on the quality of perovskite films defined by their grain size, crystallinity, and absence of pinholes. While solution-based processing is the most cost-effective and scalable approach to producing these films, the impact of the process parameters on the film quality and nanoscale details of these processes are unknown. Specifically, it is unclear how perovskites grow from a liquid precursor to form solid-phase nanocrystals and how these nanocrystals arrange to form a uniform film. Here, using liquid-phase transmission electron microscopy (TEM), we show how perovskite nanocrystals nucleate from a precursor solution and then grow and coalesce to form a polycrystalline film. Furthermore, we show how additives, such as urea, can improve the film crystallinity by increasing perovskite solubility, which induces the dissolution and subsequent redeposition of smaller crystals onto larger grains. Our approach to studying the growth of perovskite films provides an important insight into improving the synthesis of perovskites and other technologically relevant crystalline films.
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Impulsive light excitation presents a powerful tool for investigating the interdependent structural and electronic responses in layered two-dimensional (2D) halide perovskites. However, detailed understanding of the nonlinear lattice dynamics in these soft hybrid materials remains limited. Here, we explicate the intrinsic strain propagation mechanisms in 2D perovskite single crystals using transient reflection spectroscopy. Ultrafast photoexcitation leads to the generation of strain pulses via thermoelastic (TE) stress and deformation potential (DP) interaction whence their detection proceed via Brillouin scattering. Using a two-temperature model together with strain wave propagation, we discern the TE and DP contributions in strain generation. Hot carrier cooling plays a dominant role in effecting the weak modulation amplitude. Out-of-plane lattice stiffness is reduced by the weak van der Waals bond between organic layers, resulting in a slow strain propagation velocity. Our findings inject fresh insights into the basic strain properties of layered perovskites critical for manipulating their functional properties for new applications.
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The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe2 crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T' MoTe2 single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe2 shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer Td phase MoTe2 under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer Td MoTe2 produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers.
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Central to the application of spintronic devices is the ability to manipulate spins by electric and magnetic fields, which relies on a large Landé g-factor. The self-intercalation of layered transitional metal dichalcogenides with native metal atoms can serve as a new strategy to enhance the g-factor by inducing ferromagnetic instability in the system via interlayer charge transfer. Here, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are performed to extract the g-factor and characterize the electronic structure of the self-intercalated phase of 2H-TaS2 . In Ta7 S12 , a sharp density of states (DOS) peak due to the Ta intercalant appears at the Fermi level, which satisfies the Stoner criteria for spontaneous ferromagnetism, leading to spin split states. The DOS peak shows sensitivity to magnetic field up to 1.85 mV T-1 , equivalent to an effective g-factor of ≈77. This work establishes self-intercalation as an approach for tuning the g-factor.
