Spin-Driven Ferroelectricity in Two-Dimensional Magnetic Heterostructures.

Achieving magnetic control of ferroelectricity or electric control of magnetism is usually challenging in material systems as their magnetism and ferroelectricity have distinct fundamental origins and are subject to different symmetry constraints. However, such control has significant promise for a wide range of device applications. In this work, we employ first-principles density functional theory calculations to demonstrate the emergence of spin-driven ferroelectricity in a vertically stacked two-dimensional (2D) van der Waals magnetic heterostructure, formed by two ferromagnetic (FM) CrBr3 layers separated by an antiferromagnetic (AFM) MnPSe3 layer, delicately designed to be structurally inversion symmetric but magnetically asymmetric. The spin-induced out-of-plane electric polarization of the entire heterostructure can be reversibly controlled by an external magnetic field. We further validate the effectiveness of this design strategy in several other lattice-matched FM/AFM/FM heterostructures, thereby providing a novel family of multiferroic systems based on 2D materials.

Observation of Electronic Strong Correlation in VTe_{2}-2sqrt[3]×2sqrt[3] Monolayer.

Strong electron correlation under two-dimensional limit is intensely studied in the transition metal dichalcogenides monolayers, mostly within their charge density wave (CDW) states that host a star of David period. Here, by using scanning tunneling microscopy and spectroscopy and density functional theory calculations with on-site Hubbard corrections, we study the VTe_{2} monolayer with a different 2sqrt[3]×2sqrt[3] CDW period. We find that the dimerization of neighboring Te-Te and V-V atoms occurs during the CDW transition, and that the strong correlation effect opens a Mott-like full gap at Fermi energy (E_{F}). We further demonstrate that such a Mott phenomenon is ascribed to the combination of the CDW transition and on-site Coulomb interactions. Our work provides a new platform for exploring Mott physics in 2D materials.

Near-Field Modulation of Differently Oriented Single Photon Emitters with A Plasmonic Probe.

Single photon emitters (SPEs) are critical components of photon-based quantum technology. Recently, the interaction between surface plasmons and emitters has attracted increasing attention because of its potential to improve the quality of single-photon sources through stronger light-matter interactions. In this work, we use a hybrid plasmonic probe composed of a fiber taper and silver nanowire to controllably modulate the radiation properties of SPEs with differently oriented polarization. For out-of-plane oriented SPEs such as single CdSe quantum dots, the radiation lifetime could be reduced by a factor as large as seven; for in-plane oriented SPEs such as hBN defect SPEs, the average modulation amplitude varied from 0.69 to 1.23, depending on the position of the probe. The experimental results were highly consistent with the simulations and theory. This work provides an efficient approach for optimizing the properties of SPEs for quantum photonic integration.

Accurate Single-Molecule Kinetic Isotope Effects.

An accurate single-molecule kinetic isotope effect (sm-KIE) was applied to circumvent the inherent limitation of conventional ensemble KIE by using graphene-molecule-graphene single-molecule junctions. In situ monitoring of the single-molecule reaction trajectories in real time with high temporal resolution has the capability to characterize the deeper information brought by KIE. The C-O bond cleavage and the C-C bond formation of the transition state (TS) were observed in the Claisen rearrangement through the secondary kinetic isotope effect, demonstrating the high detection sensitivity and accuracy of this method. More interestingly, this sm-KIE can be used to determine TS structures under different electric fields, revealing the multidimensional regulation of the TS. The detection and manipulation of the TS provide a broad perspective to understand and optimize chemical reactions and biomimetic progress.

Tuning the Electronic Structure of an α-Antimonene Monolayer through Interface Engineering.

The interfacial charge transfer from the substrate may influence the electronic structure of the epitaxial van der Waals (vdW) monolayers and, thus, their further technological applications. For instance, the freestanding Sb monolayer in the puckered honeycomb phase (α-antimonene), the structural analogue of black phosphorene, was predicted to be a semiconductor, but the epitaxial one behaves as a gapless semimetal when grown on the Td-WTe2 substrate. Here, we demonstrate that interface engineering can be applied to tune the interfacial charge transfer and, thus, the electron band of the epitaxial monolayer. As a result, the nearly freestanding (semiconducting) α-antimonene monolayer with a band gap of ∼170 meV was successfully obtained on the SnSe substrate. Furthermore, a semiconductor-semimetal crossover is observed in the bilayer α-antimonene. This study paves the way toward modifying the electron structure in two-dimensional vdW materials through interface engineering.

In-Situ Surface Reconstruction of InN Nanosheets for Efficient CO2 Electroreduction into Formate.

Probing and understanding the intrinsic active sites of electrocatalysts is crucial to unravel the underlying mechanism of CO2 electroreduction and provide a prospective for the rational design of high-performance electrocatalysts. However, their structure-activity relationships are not straightforward because electrocatalysts might reconstruct under realistic working conditions. Herein, we employ in-situ measurements to unveil the intrinsic origin of the InN nanosheets which served as an efficient electrocatalyst for CO2 reduction with a high faradaic efficiency of 95% for carbonaceous product. During the CO2 electroreduction, InN nanosheets reconstructed to form the In-rich surface. Density functional theory calculations revealed that the reconstruction of InN led to the redistribution of surface charge that significantly promoted the adsorption of HCOO* intermediates and thus benefited the formation of formate toward CO2 electroreduction. This work establishes a fundamental understanding on the mechanism associated with self-reconstruction of heterogeneous catalysts toward CO2 electroreduction.

Harmonizing the Electronic Structures of the Adsorbate and Catalysts for Efficient CO2 Reduction.

In CO2 electroreduction, the critical bottleneck lies in the CO2 activation which requires high overpotentials. CO2 activation is related to both the electronic structures of catalysts and those of adsorbates, thus an ideal catalyst should match its electronic structures with those of the adsorbate. Here, we harmonized the electronic structures of the adsorbate and Mn-doped In2S3 nanosheets for efficient CO2 reduction. The introduction of Mn dopants into In2S3 nanosheets enhanced both the Faradaic efficiency (FE) for carbonaceous products and current density (j). At -0.9 V vs RHE, Mn-doped In2S3 nanosheets exhibited a remarkable FE of 92% for carbonaceous product at a high j of 20.1 mA cm-2. Mechanistic studies revealed that Mn doping enabled the harmonic overlaps between the p orbitals of O atoms and d orbitals of Mn atoms near the conduction band edge of Mn-doped In2S3 nanosheets during the activation of CO2. Due to the unique electronic structures of the coadsorbed configurations, Mn-doped In2S3 nanosheets exhibited an energy barrier for CO2 activation into HCOO* lower than that over pristine In2S3 nanosheets.

Intercorrelated In-Plane and Out-of-Plane Ferroelectricity in Ultrathin Two-Dimensional Layered Semiconductor In2Se3.

Enriching the functionality of ferroelectric materials with visible-light sensitivity and multiaxial switching capability would open up new opportunities for their applications in advanced information storage with diverse signal manipulation functions. We report experimental observations of robust intralayer ferroelectricity in two-dimensional (2D) van der Waals layered α-In2Se3 ultrathin flakes at room temperature. Distinct from other 2D and conventional ferroelectrics, In2Se3 exhibits intrinsically intercorrelated out-of-plane and in-plane polarization, where the reversal of the out-of-plane polarization by a vertical electric field also induces the rotation of the in-plane polarization. On the basis of the in-plane switchable diode effect and the narrow bandgap (∼1.3 eV) of ferroelectric In2Se3, a prototypical nonvolatile memory device, which can be manipulated both by electric field and visible light illumination, is demonstrated for advancing data storage technologies.

Nickel Doping in Atomically Thin Tin Disulfide Nanosheets Enables Highly Efficient CO2 Reduction.

Engineering electronic properties by elemental doping is a direct strategy to design efficient catalysts towards CO2 electroreduction. Atomically thin SnS2 nanosheets were modified by Ni doping for efficient electroreduction of CO2 . The introduction of Ni into SnS2 nanosheets significantly enhanced the current density and Faradaic efficiency for carbonaceous product relative to pristine SnS2 nanosheets. When the Ni content was 5 atm %, the Ni-doped SnS2 nanosheets achieved a remarkable Faradaic efficiency of 93 % for carbonaceous product with a current density of 19.6 mA cm-2 at -0.9 V vs. RHE. A mechanistic study revealed that the Ni doping gave rise to a defect level and lowered the work function of SnS2 nanosheets, resulting in the promoted CO2 activation and thus improved performance in CO2 electroreduction.

Highly Efficient and Exceptionally Durable CO2 Photoreduction to Methanol over Freestanding Defective Single-Unit-Cell Bismuth Vanadate Layers.

Unearthing an ideal model for disclosing the role of defect sites in solar CO2 reduction remains a great challenge. Here, freestanding gram-scale single-unit-cell o-BiVO4 layers are successfully synthesized for the first time. Positron annihilation spectrometry and X-ray fluorescence unveil their distinct vanadium vacancy concentrations. Density functional calculations reveal that the introduction of vanadium vacancies brings a new defect level and higher hole concentration near Fermi level, resulting in increased photoabsorption and superior electronic conductivity. The higher surface photovoltage intensity of single-unit-cell o-BiVO4 layers with rich vanadium vacancies ensures their higher carriers separation efficiency, further confirmed by the increased carriers lifetime from 74.5 to 143.6 ns revealed by time-resolved fluorescence emission decay spectra. As a result, single-unit-cell o-BiVO4 layers with rich vanadium vacancies exhibit a high methanol formation rate up to 398.3 µmol g-1 h-1 and an apparent quantum efficiency of 5.96% at 350 nm, much larger than that of single-unit-cell o-BiVO4 layers with poor vanadium vacancies, and also the former's catalytic activity proceeds without deactivation even after 96 h. This highly efficient and spectrally stable CO2 photoconversion performances hold great promise for practical implementation of solar fuel production.

High-Temperature Quantum Anomalous Hall Effect in n-p Codoped Topological Insulators.

The quantum anomalous Hall effect (QAHE) is a fundamental quantum transport phenomenon that manifests as a quantized transverse conductance in response to a longitudinally applied electric field in the absence of an external magnetic field, and it promises to have immense application potential in future dissipationless quantum electronics. Here, we present a novel kinetic pathway to realize the QAHE at high temperatures by n-p codoping of three-dimensional topological insulators. We provide a proof-of-principle numerical demonstration of this approach using vanadium-iodine (V-I) codoped Sb_{2}Te_{3} and demonstrate that, strikingly, even at low concentrations of ∼2% V and ∼1% I, the system exhibits a quantized Hall conductance, the telltale hallmark of QAHE, at temperatures of at least ∼50 K, which is 3 orders of magnitude higher than the typical temperatures at which it has been realized to date. The underlying physical factor enabling this dramatic improvement is tied to the largely preserved intrinsic band gap of the host system upon compensated n-p codoping. The proposed approach is conceptually general and may shed new light in experimental realization of high-temperature QAHE.

Generation and the role of dislocations in single-crystalline phase-change In2Se3 nanowires under electrical pulses.

We report the observation of the generation of dislocations in single-crystal phase-change In2Se3 nanowires under electrical pulses and the impact of these dislocations on electrical properties. Particularly, we correlated the atomic-scale structural characteristics with local electrical resistance variations, by performing transmission electron microscopy and scanning Kelvin probe microscopy on the same nanowires. By coupling the experimental results with first-principles density functional theory calculations, we show that the immobile dislocations are generated via vacancy condensations. Importantly, these dislocations lead to several orders of magnitude increase in the electrical resistance, while maintaining the single crystallinity of the lattice. These results significantly advance the fundamental understanding of the structure-property relation in this phase-change material under transient electrical excitations. From a practical perspective, the significant increase in the electrical resistance, driven by the formation of dislocations, can be exploited as a new electronic state in the single-crystalline phase in this phase-change material.

Superior Electrical Conductivity in Hydrogenated Layered Ternary Chalcogenide Nanosheets for Flexible All-Solid-State Supercapacitors.

As the properties of ultrathin two-dimensional (2D) crystals are strongly related to their electronic structures, more and more attempts were carried out to tune their electronic structures to meet the high standards for the construction of next-generation smart electronics. Herein, for the first time, we show that the conductive nature of layered ternary chalcogenide with formula of Cu2 WS4 can be switched from semiconducting to metallic by hydrogen incorporation, accompanied by a high increase in electrical conductivity. In detail, the room-temperature electrical conductivity of hydrogenated-Cu2 WS4 nanosheet film was almost 10(10) times higher than that of pristine bulk sample with a value of about 2.9×10(4)  S m(-1) , which is among the best values for conductive 2D nanosheets. In addition, the metallicity in the hydrogenated-Cu2 WS4 is robust and can be retained under high-temperature treatment. The fabricated all-solid-state flexible supercapacitor based on the hydrogenated-Cu2 WS4 nanosheet film shows promising electrochemical performances with capacitance of 583.3 F cm(-3) at a current density of 0.31 A cm(-3) . This work not only offers a prototype material for the study of electronic structure regulation in 2D crystals, but also paves the way in searching for highly conductive electrodes.

Interface Engineering in Two-Dimensional Heterostructures: Towards an Advanced Catalyst for Ullmann Couplings.

The design of advanced catalysts for organic reactions is of profound significance. During such processes, electrophilicity and nucleophilicity play vital roles in the activation of chemical bonds and ultimately speed up organic reactions. Herein, we demonstrate a new way to regulate the electro- and nucleophilicity of catalysts for organic transformations. Interface engineering in two-dimensional heteronanostructures triggered electron transfer across the interface. The catalyst was thus rendered more electropositive, which led to superior performance in Ullmann reactions. In the presence of the engineered 2D Cu2 S/MoS2 heteronanostructure, the coupling of iodobenzene and para-chlorophenol gave the desired product in 92 % yield under mild conditions (100 °C). Furthermore, the catalyst exhibited excellent stability as well as high recyclability with a yield of 89 % after five cycles. We propose that interface engineering could be widely employed for the development of new catalysts for organic reactions.

Tuning the electronic and chemical properties of monolayer MoS2 adsorbed on transition metal substrates.

Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS(2) adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS(2) overlayer. We find that, for each of the metal substrates, the contact nature is of Schottky-barrier type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS(2) layer. The enhanced binding of hydrogen, by as much as ~0.4 eV, is attributed in part to a stronger H-S coupling enabled by the transferred charge from the substrate to the MoS(2) overlayer, and in part to a stronger MoS(2)-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS(2)-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.

Atomic-scale manipulation of polar domain boundaries in monolayer ferroelectric In2Se3.

Domain boundaries have been intensively investigated in bulk ferroelectric materials and two-dimensional materials. Many methods such as electrical, mechanical and optical approaches have been utilized to probe and manipulate domain boundaries. So far most research focuses on the initial and final states of domain boundaries before and after manipulation, while the microscopic understanding of the evolution of domain boundaries remains elusive. In this paper, we report controllable manipulation of the domain boundaries in two-dimensional ferroelectric In2Se3 with atomic precision using scanning tunneling microscopy. We show that the movements of the domain boundaries can be driven by the electric field from a scanning tunneling microscope tip and proceed by the collective shifting of atoms at the domain boundaries. Our density functional theory calculations reveal the energy path and evolution of the domain boundary movement. The results provide deep insight into domain boundaries in two-dimensional ferroelectric materials and will inspire inventive applications of these materials.

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.

Tailoring magnetic doping in the topological insulator Bi2Se3.

We theoretically investigate the possibility of establishing ferromagnetism in the topological insulator Bi2Se3 via magnetic doping of 3d transition metal elements. The formation energies, charge states, band structures, and magnetic properties of doped Bi2Se3 are studied using first-principles calculations within density functional theory. Our results show that Bi substitutional sites are energetically more favorable than interstitial sites for single impurities. Detailed electronic structure analysis reveals that Cr and Fe doped materials are still insulating in the bulk but the intrinsic band gap of Bi2Se3 is substantially reduced due to the strong hybridization between the d states of the dopants and the p states of the neighboring Se atoms. The calculated magnetic coupling suggests that Cr doped Bi2Se3 is possible to be both ferromagnetic and insulating, while Fe doped Bi2Se3 tends to be weakly antiferromagnetic.

Suppression of grain boundaries in graphene growth on superstructured Mn-Cu(111) surface.

As undesirable defects, grain boundaries (GBs) are widespread in epitaxial graphene using existing growth methods on metal substrates. Employing density functional theory calculations, we first identify that the misorientations of carbon islands nucleated on a Cu(111) surface lead to the formation of GBs as the islands coalesce. We then propose a two-step kinetic pathway to effectively suppress the formation of GBs. In the first step, large aromatic hydrocarbon molecules are deposited onto a sqrt[3]×sqrt[3] superstructured Cu-Mn alloyed surface to seed the initial carbon clusters of a single orientation; in the second step, the seeded islands are enlarged through normal chemical vapor deposition of methane to form a complete graphene sheet. The present approach promises to overcome a standing obstacle in large scale single-crystal graphene fabrication.

High Photoreactivity on a Reconstructed Anatase TiO2(001) Surface Predicted by Ab Initio Nonadiabatic Molecular Dynamics.

Anatase TiO2(001) surface with (4 × 1) reconstruction is proposed to be a highly active catalytic surface. In this work, using time-domain ab initio nonadiabatic molecular dynamics, we reveal that the ridge structure formed by anatase(001) surface reconstruction is the photoreactive site for hole migration and trapping. Moreover, the ridge structure is destroyed by low-coverage CH3OH adsorption, leading to the suppression of its high photoreactivity. However, when the CH3OH coverage is increased and intermolecular hydrogen bonds (H-bonds) form, the ridge structure and its high photoreactivity are restored. Furthermore, the hole trapping dynamics is strongly coherent with intermolecular proton transfer in structures with intermolecular H-bonds. Our study proves that anatase TiO2(001)-(4 × 1) is a highly photoreactive surface where the ridge is the photoreactive site for hole trapping, which is coherent with the proton transfer process.