Designing edge states from fractional polarization insulators.

We theoretically investigated disconnected dispersive edge states in an anisotropic honeycomb lattice without chiral symmetry. When both mirror and chiral symmetries are present, this system is defined by a topological quantity known as fractional polarization (FP) term and exhibits a bulk band gap, classifying it as an FP insulator. While the FP insulator accommodates robust, flat topological edge states (TES), it also offers the potential to engineer these edge states by deliberately disrupting a critical symmetry that safeguards the underlying topology. These symmetry-breaking terms allow the edge states to become dispersive and can generate differing configurations along the open boundaries. Furthermore, disconnected helical-like and chiral-like edge states analogous to TES seen in quantum spin and anomalous hall effect are achieved by the finite size effect, not possible from the symmetry-breaking terms alone. The demonstration of manipulating these edge states from an FP insulator can open up new avenues in constructing devices that utilize topological domain walls.

Strongly correlated multielectron bunches from interaction with quantum light.

Strongly correlated electron systems are a cornerstone of modern physics, being responsible for groundbreaking phenomena from superconducting magnets to quantum computing. In most cases, correlations in electrons arise exclusively because of Coulomb interactions. In this work, we reveal that free electrons interacting simultaneously with a light field can become highly correlated via mechanisms beyond Coulomb interactions. In the case of two electrons, the resulting Pearson correlation coefficient for the joint probability distribution of the output electron energies is enhanced by more than 13 orders of magnitude compared to that of electrons interacting with the light field in succession (one after another). These highly correlated electrons are the result of momentum and energy exchange between the participating electrons via the external quantum light field. Our findings pave the way to the creation and control of highly correlated free electrons for applications including quantum information and ultrafast imaging.

Analytical Model of Optical-Field-Driven Subcycle Electron Tunneling Pulses from Two-Dimensional Materials.

We develop analytical models of optical-field-driven electron tunneling from the edge and surface of free-standing two-dimensional (2D) materials. We discover a universal scaling between the tunneling current density (J) and the electric field near the barrier (F): In(J/|F|ß) ∝ 1/|F| with ß values of 3/2 and 1 for edge emission and vertical surface emission, respectively. At ultrahigh values of F, the current density exhibits an unexpected high-field saturation effect due to the reduced dimensionality of the 2D material, which is absent in the traditional bulk material. Our calculation reveals the dc bias as an efficient method for modulating the optical-field tunneling subcycle emission characteristics. Importantly, our model is in excellent agreement with a recent experiment on graphene. Our results offer a useful framework for understanding optical-field tunneling emission from 2D materials, which are helpful for the development of optoelectronics and emerging petahertz vacuum nanoelectronics.

Single Atomic Defect Conductivity for Selective Dilute Impurity Imaging in 2D Semiconductors.

Precisely controlled impurity doping is of fundamental significance in modern semiconductor technologies. Desired physical properties are often achieved at impurity concentrations well below parts per million level. For emergent two-dimensional semiconductors, development of reliable doping strategies is hindered by the inherent difficulty in identifying and quantifying impurities in such a dilute limit where the absolute number of atoms to be detected is insufficient for common analytical techniques. Here we report rapid high-contrast imaging of dilute single atomic impurities by using conductive atomic force microscopy. We show that the local conductivity is enhanced by more than 100-fold by a single impurity atom due to resonance-assisted tunneling. Unlike the closely related scanning tunneling microscopy, the local conductivity sensitively depends on the impurity energy level, allowing minority defects to be selectively imaged. We further demonstrate subsurface impurity detection with single monolayer depth resolution in multilayer materials.

AgCl Addition to Chalcopyrite Compound for Ultra-Low Thermal Conductivity in Realizing High ZT Thermoelectric Materials.

Optimizing the performance of thermoelectric materials by reducing its thermal conductivity is crucial to enhance its thermoelectric efficiency. Novel thermoelectric materials like the CuGaTe2 compound are hindered by high intrinsic thermal conductivity, which negatively impacts its thermoelectric performance. In this paper, we report that the introduction of AgCl by the solid-phase melting method will influence the thermal conductivity of CuGaTe2. The generated multiple scattering mechanisms are expected to reduce the lattice thermal conductivity while maintaining sufficient good electrical properties. The experimental results were supported by first-principles calculations confirming that the doping of the Ag will decrease the elastic constants, bulk modulus, and shear modulus of CuGaTe2, which makes the mean sound velocity and Debye temperature of Ag-doped samples lower than those of CuGaTe2, indicating the lower lattice thermal conductivity. In addition, the Cl elements within the CuGaTe2 matrix escaping during the sintering process will create holes of various sizes within the sample. These combined effects of holes and impurities will induce phonon scattering, which further reduces the lattice thermal conductivity. Based on our findings, we conclude that the introduction of AgCl into CuGaTe2 has shown a lower thermal conductivity without compromising the electrical performance, resulting in an ultra-high ZT value of 1.4 in the (CuGaTe2)0.96(AgCl)0.04 sample at 823 K.

High-Performance Sliding Ferroelectric Transistor Based on Schottky Barrier Tuning.

Sliding ferroelectricity associated with interlayer translation is an excellent candidate for ferroelectric device miniaturization. However, the weak polarization gives rise to the poor performance of sliding ferroelectric transistors with a low on/off ratio and a narrow memory window, which restricts its practical application. To address the issue, we propose a facile strategy by regulating the Schottky barrier in sliding ferroelectric semiconductor transistors based on γ-InSe, in which a high performance with a large on/off ratio (106) and a wide memory window (4.5 V) was ultimately acquired. Additionally, the memory window of the device can be further modulated by electrostatic doping or light excitation. These results open up new ways for designing novel ferroelectric devices based on emerging sliding ferroelectricity.

Quantum Interference between Fundamentally Different Processes Is Enabled by Shaped Input Wavefunctions.

This work presents a general framework for quantum interference between processes that can involve different fundamental particles or quasi-particles. This framework shows that shaping input wavefunctions is a versatile and powerful tool for producing and controlling quantum interference between distinguishable pathways, beyond previously explored quantum interference between indistinguishable pathways. Two examples of quantum interference enabled by shaping in interactions between free electrons, bound electrons, and photons are presented: i) the vanishing of the zero-loss peak by destructive quantum interference when a shaped electron wavepacket couples to light, under conditions where the electron's zero-loss peak otherwise dominates; ii) quantum interference between free electron and atomic (bound electron) spontaneous emission processes, which can be significant even when the free electron and atom are far apart, breaking the common notion that a free electron and an atom must be close by to significantly affect each other's processes. Conclusions show that emerging quantum wave-shaping techniques unlock the door to greater versatility in light-matter interactions and other quantum processes in general.

Approaching the Intrinsic Threshold Breakdown Voltage and Ultrahigh Gain in a Graphite/InSe Schottky Photodetector.

Realizing both ultralow breakdown voltage and ultrahigh gain is one of the major challenges in the development of high-performance avalanche photodetector. Here, it is reported that an ultrahigh avalanche gain of 3 × 105 can be realized in the graphite/InSe Schottky photodetector at a breakdown voltage down to 5.5 V. Remarkably, the threshold breakdown voltage can be further reduced down to 1.8 V by raising the operating temperature, approaching the theoretical limit of 1.5 E g \[{{\cal E}_{\bf g}}\] /e, with E g ${{\cal E}_{\bf g}}$ the bandgap of semiconductor. A 2D impact ionization model is developed and it is uncovered that observation of high gain at low breakdown voltage arises from reduced dimensionality of electron-phonon scattering in the layered InSe flake. These findings open up a promising avenue for developing novel weak-light detectors with low energy consumption and high sensitivity.

Cavity spectral-hole-burning to boost coherence in plasmon-emitter strong coupling systems.

Significant decoherence of the plasmon-emitter (i.e., plexcitonic) strong coupling systems hinders the progress towards their applications in quantum technology due to the unavoidable lossy nature of the plasmons. Inspired by the concept of spectral-hole-burning (SHB) for frequency-selective bleaching of the emitter ensemble, we propose 'cavity SHB' by introducing cavity modes with moderate quality factors to the plexcitonic system to boost its coherence. We show that the detuning of the introduced cavity mode with respect to the original plexcitonic system, which defines the location of the cavity SHB, is the most critical parameter. Simultaneously introducing two cavity modes of opposite detunings, the excited-state population of the emitter can be enhanced by 4.5 orders of magnitude within 300 fs, and the attenuation of the emitter's population can be slowed down by about 56 times. This theoretical proposal provides a new approach of cavity engineering to enhance the plasmon-emitter strong coupling systems' coherence, which is important for realistic hybrid-cavity design for applications in quantum technology.

Concentrated thermionic solar cells using graphene as the collector: theoretical efficiency limit and design rules.

We propose an updated design on concentrated thermionic emission solar cells, which demonstrates a high solar-to-electricity energy conversion efficiency larger than 10% under 600 suns, by harnessing the exceptional electrical, thermal, and radiative properties of the graphene as a collector electrode. By constructing an analytical model that explicitly takes into account the non-Richardson behavior of the thermionic emission current from graphene, space charge effect in vacuum gap, and the various irreversible energy losses within the subcomponents, we perform detailed characterizations on the conversion efficiency limit and parametric optimum design of the proposed system. Under 800 suns, a maximum efficiency of 12.8% has been revealed, where current density is 3.87 A cm-2, output voltage is 1.76 V, emitter temperature is 1707 K, and collector temperature is 352 K. Moreover, we systematically compare the peak efficiencies of various configurations combining diamond or graphene, and show that utilizing diamond films as an emitter and graphene as a collector offers the highest conversion efficiency, thus revealing the important role of graphene in achieving high-performance thermionic emission solar cells. This work thus opens up new avenues to advance the efficiency limit of thermionic solar energy conversion and the development of next-generation novel-nanomaterial-based solar energy harvesting technology.

ZnSe Modified Zinc Metal Anodes: Toward Enhanced Zincophilicity and Ionic Diffusion.

Zinc metal is an ideal candidate for aqueous rechargeable batteries due to its high theoretical capacity and natural abundance. However, its commercialization is inevitably challenged by several critical factors such as dendrite growth and parasitic side-reactions, leading to low coulombic efficiency and a limited lifespan. Herein, a modified Zn foil with a zincophilic ZnSe layer deposited by a simple selenization process is proposed. An order of magnitude stronger adsorption capability toward Zn2+ ions and uniform ion diffusion tunnels of ZnSe enables lower nucleation energy barrier and faster ion-diffusion kinetics. Meanwhile, detrimental Zn corrosion in aqueous system is also effectively mitigated. As a result, ZnSe@Zn anode shows reversible Zn plating/stripping (1700 h at 1 mA cm-2 ) with ultra-low voltage hysteresis (41 mV), contributing to exceptional cycling stability over 500 cycles with negligible capacity fading for the ZnSe@Zn/MnO2 full cell.

Designing high-performance nighttime thermoradiative systems for harvesting energy from outer space.

Energy harvesting using thermoradiative systems has been extensively explored in recent years as a novel strategy for further reducing our energy footprint. However, the nighttime application, thermodynamic limit, and optimal design of such a system remain largely unaddressed so far. Here we propose an improved nighttime thermoradiative system (NTS) for electrical power generation by optically coupling Earth's surface with outer space. Our theoretical model predicts that the NTS operating with Earth (deep space) at 300 K (3 K) yields a maximum power density of 12.3Wm-2 with an efficiency limit of 18.5%, which is potentially more advantageous than previous nighttime energy harvesting systems, such as a nighttime thermoelectric generator. We find that optimizing the thickness of the active layer, enhancing thermal infrared emission, and employing a silver backreflector for photon recycling are crucially important in improving system performance. This Letter provides new insights for the optimal designs of NTSs and paves the way toward practical nighttime power generation.

Imaging nodal knots in momentum space through topolectrical circuits.

Knots are intricate structures that cannot be unambiguously distinguished with any single topological invariant. Momentum space knots, in particular, have been elusive due to their requisite finely tuned long-ranged hoppings. Even if constructed, probing their intricate linkages and topological "drumhead" surface states will be challenging due to the high precision needed. In this work, we overcome these practical and technical challenges with RLC circuits, transcending existing theoretical constructions which necessarily break reciprocity, by pairing nodal knots with their mirror image partners in a fully reciprocal setting. Our nodal knot circuits can be characterized with impedance measurements that resolve their drumhead states and image their 3D nodal structure. Doing so allows for reconstruction of the Seifert surface and hence knot topological invariants like the Alexander polynomial. We illustrate our approach with large-scale simulations of various nodal knots and an experiment which maps out the topological drumhead region of a Hopf-link.

Quantum Transport in Two-Dimensional WS2 with High-Efficiency Carrier Injection through Indium Alloy Contacts.

Two-dimensional transition metal dichalcogenides (TMDCs) have properties attractive for optoelectronic and quantum applications. A crucial element for devices is the metal-semiconductor interface. However, high contact resistances have hindered progress. Quantum transport studies are scant as low-quality contacts are intractable at cryogenic temperatures. Here, temperature-dependent transfer length measurements are performed on chemical vapor deposition grown single-layer and bilayer WS2 devices with indium alloy contacts. The devices exhibit low contact resistances and Schottky barrier heights (∼10 kΩ µm at 3 K and 1.7 meV). Efficient carrier injection enables high carrier mobilities (∼190 cm2 V-1 s-1) and observation of resonant tunnelling. Density functional theory calculations provide insights into quantum transport and properties of the WS2-indium interface. Our results reveal significant advances toward high-performance WS2 devices using indium alloy contacts.

Plasmon-Enhanced Resonant Photoemission Using Atomically Thick Dielectric Coatings.

By proposing an atomically thick dielectric coating on a metal nanoemitter, we theoretically show that the optical field tunneling of ultrafast-laser-induced photoemission can occur at an ultralow incident field strength of 0.03 V/nm. This coating strongly confines plasmonic fields and provides secondary field enhancement beyond the geometrical plasmon field enhancement effect, which can substantially reduce the barrier and enable more efficient photoemission. We numerically demonstrate that a 1 nm thick layer of SiO2 around a Au-nanopyramid will enhance the resonant photoemission current density by 2 orders of magnitude, where the transition from multiphoton absorption to optical field tunneling is accessed at an incident laser intensity at least 10 times lower than that of the bare nanoemitter. The effects of the coating properties such as refractive index, thickness, and geometrical settings are studied, and tunable photoemission is numerically demonstrated by using different ultrafast lasers. Our approach can also directly be extended to nonmetal emitters, to-for example-2D material coatings, and to plasmon-induced hot carrier generation.

Morphological and Electronic Dual Regulation of Cobalt-Nickel Bimetal Phosphide Heterostructures Inducing High Water-Splitting Performance.

Electrocatalytic water splitting (EWS) is a key technology for generating clean and sustainable hydrogen, which can store abundant energy but is impeded by the insufficient efficiency of the anode and cathode catalyst. Designing and constructing non-noble metal composite bifunctional electrocatalysts for promoting both the cathodic hydrogen evolution (HER) and anodic oxygen evolution reactions (OER) is clearly of great importance for EWS. Thus, the chemical composition and morphology of cobalt-nickel bimetal phosphide (Ni, Co)2P nanoparticles (NPs) encapsulated in nitrogen-doped carbon nanotube hollow microspheres (NCNHMs) can regulate the redox-active sites and enhance the electron transfer, resulting in superior splitting efficiency. Contributing to the synergistic effects between highly active Co-Ni bimetal phosphide NPs and NCNHMs, the obtained Co-Ni bimetal phosphide/NCNHMs display remarkable electrochemical performance for water splitting compared with Ni2P/NCNHMs. Therefore, the Ni1.4Co0.6P/NCNHMs catalysts achieved through a nitriding-phosphidation strategy derived from a hollow Ni1.4-Co0.6-based metal organic framework (MOF) exhibit superior HER catalytic activity (87.9 mV at 10 mA cm-2 tested in 0.5 M H2SO4 and 64.4 mV at 10 mA cm-2 tested in 1 M KOH) and OER catalytic activity (320.0 mV at 10 mA cm-2 tested in 1 M KOH). The Ni1.4Co0.6P/NCNHMs deliver excellent water-splitting catalytic activity (1.55 V at 10 mA cm-2 tested in 1 M KOH), which is competitive with that of current non-noble metal electrocatalysts. Density functional theory (DFT) simulations and related experimental results suggest that the electron transfer from Co doping and coating with NCNHMs improves the electronic states, which would enhance the binding strength with H-bonds and then promote the electrocatalytic activity.

Microstructural Engineering of Cathode Materials for Advanced Zinc-Ion Aqueous Batteries.

Zinc-ion batteries (ZIBs) have attracted intensive attention due to the low cost, high safety, and abundant resources. However, up to date, challenges still exist in searching for cathode materials with high working potential, excellent electrochemical activity, and good structural stability. To address these challenges, microstructure engineering has been widely investigated to modulate the physical properties of cathode materials, and thus boosts the electrochemical performances of ZIBs. Here, the recent research efforts on the microstructural engineering of various ZIB cathode materials are mainly focused upon, including composition and crystal structure selection, crystal defect engineering, interlayer engineering, and morphology design. The dependency of cathode performance on aqueous electrolyte for ZIB is further discussed. Finally, future perspectives and challenges on microstructure engineering of cathode materials for ZIBs are provided. It is aimed to provide a deep understanding of the microstructure engineering effect on Zn2+ storage performance.

Design Multifunctional Catalytic Interface: Toward Regulation of Polysulfide and Li2 S Redox Conversion in Li-S Batteries.

The polysulfide shuttle effect and sluggish reaction kinetics hamper the practical applications of lithium-sulfur (Li-S) batteries. Incorporating a functional interlayer to trapping and binding polysulfides has been found effective to block polysulfide migration. Furthermore, surface chemistry at soluble polysulfides/electrolyte interface is a crucial step for Li-S battery in which stable cycling depends on adsorption and reutilization of blocked polysulfides in the electrolyte. A multifunctional catalytic interface composed of niobium nitride/N-doped graphene (NbN/NG) along the soluble polysulfides/electrolyte is designed and constructed to regulate corresponding interface chemical reaction, which can afford long-range electron transfer surfaces, numerous strong chemisorption, and catalytic sites in a working lithium-sulfur battery. Both experimental and theoretical calculation results suggest that a new catalytic interface enabled by metal-like NbN with superb electrocatalysis anchored on NG is highly effective in regulating the blocked polysulfide redox reaction and tailoring the Li2 S nucleation-growth-decomposition process. Therefore, the Li-S batteries with multifunctional NbN/NG barrier exhibit excellent rate performance (621.2 mAh g-1 at 3 C) and high stable cycling life (81.5% capacity retention after 400 cycles). This work provides new insights to promote Li-S batteries via multifunctional catalytic interface engineering.

Graphene-Induced in Situ Growth of Monolayer and Bilayer 2D SiC Crystals Toward High-Temperature Electronics.

A reproducible graphene-induced in situ process is demonstrated for the first time for growing large-scale monolayer and bilayer cubic silicon carbide (SiC) crystals on a liquid Cu surface by chemical vapor deposition (CVD) method. Precise control over the morphology of SiC crystals is further realized by modulating growth conditions, thus leading to the formation of several shaped SiC crystals ranging from triangular, rectangular, pentagonal, and even to hexagonal kind. Simulations based on density functional theory are carried out to elucidate the growth mechanism of SiC flakes with various morphologies, which are in striking consistency with experimental observations. In the liquid Cu-assisted CVD system, growth temperature (∼1100 °C) enables sublimation and deposition of silicon oxide (SiO2) derived from quartz tube, while liquid Cu facilitates preformation of graphene originated from methane. The SiO2 and graphene, grown and reacted in situ in the CVD process, are served as the silicon and carbon source for the cubic SiC crystals, respectively. Moreover, the gradual transformation process from SiO2 particles to SiC flakes is directly observed, with several middle stages clearly displayed. The direct in situ growth of SiC crystals offers a novel method for scaled production of SiC crystals and is beneficial to understand its growth mechanism, and thus push forward the way to develop high-temperature and high-frequency electronic devices.

High-Concentration Niobium-Substituted WS2 Basal Domains with Reconfigured Electronic Band Structure for Hydrogen Evolution Reaction.

Extrinsically controlling the intrinsic activity and stability of two-dimensional (2D) semiconducting materials by substitutional doping is crucial for energy-related applications. However, an in situ transition-metal doping strategy for uniform and large-area chemical vapor deposited 2D semiconductors remains a formidable challenge. Here, we successfully synthesize highly uniform niobium-substituted tungsten disulfide (Nb-WS2) monolayers, with a doping concentration of nearly 7% and sizes reaching 100 µm, through a metal dopant precursor route, using salt-catalyzed chemical vapor deposition (CVD). Our results reveal unusual effects in the structural, optical, electronic, and electrocatalysis characteristics of the Nb-WS2 monolayer. The Nb dopants readily induce a band restructuring effect, providing the most active site with a hydrogen adsorption energy of 0.175 eV and hence greatly improving its hydrogen evolution activity. The combined advantages of the unusual physics and chemistry by in situ CVD doping technique open the possibility in designing 2D-material-based electronics and catalysts of novel functionalities.