Au25 Cluster-Based Atomically Precise Coordination Frameworks and Emission Engineering through Lattice Symmetry.

The atomically precise metal nanoclusters (NCs) have attracted significant attention due to their superatomic behavior originating from the quantum confinement effect. This behavior makes these materials suitable for various photoluminescence-based applications, including chemical sensing, bioimaging, and phototherapy, owing to their intriguing optical properties. Especially, the manipulation of inter- or intracluster interaction through cluster-assembled materials (CAMs) presents significant pathways for modifying the photophysical properties of NCs. Herein, two distinct CAMs, Au25-Zn-Hex and Au25-Zn-Rod, were synthesized via forming a coordination bond between [Au25(p-HMBA)18]- (p-H2MBA = 4-mercaptobenzoic acid) and Zn2+. Au25-Zn-Rod exhibited a 6-fold higher luminescence intensity in the near-infrared region compared to Au25-Zn-Hex, attributed to synergistic inter- and intracluster interactions that induce exciton delocalization and structure rigidification at the atomic scale. This study highlights the potential of diverse lattice symmetries in cluster-based frameworks for tuning the photophysical properties, contributing to a deeper understanding of the structure-property relationship in Au NCs.

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.

An Alternating-Electric-Field-Driven Assembly of DNA Nanoparticles into FCC Crystals.

Using an alternating electric field is a versatile way to control particle assembly. Programming DNA-AuNP assembly via an electric field remains a significant challenge despite the negative charge of DNA. In DNA-AuNP assembly, a critical percolation state is delicately constructed, where the DNA bond is loosely connected and sensitive to electric fields. In this state, an FCC crystal structure can be successfully constructed by applying a high-frequency electric field to assemble DNA-AuNPs without altering the temperature, which is favorable for temperature-sensitive systems. In addition, the regulation of electric fields can be adjusted through parameters such as the frequency and voltage, which offers more precise control than temperature regulation does. The frequency and voltage can be used to precisely tune the phase structure of DNA-AuNPs from dissolved to disordered or FCC. These findings broaden the potential of DNA-based crystal engineering, revealing new opportunities in electronic nanocomposites and devices.

WS2 Moiré Superlattices Supporting Au Nanoclusters and Isolated Ru to Boost Hydrogen Production.

Maximizing the catalytic activity of single-atom and nanocluster catalysts through the modulation of the interaction between these components and the corresponding supports is crucial but challenging. Herein, guided by theoretical calculations, a nanoporous bilayer WS2 Moiré superlattices (MSLs) supported Au nanoclusters (NCs) adjacent to Ru single atoms (SAs) (Ru1/Aun-2LWS2) is developed for alkaline hydrogen evolution reaction (HER) for the first time. Theoretical analysis suggests that the induced robust electronic metal-support interaction effect in Ru1/Aun-2LWS2 is prone to promote the charge redistribution among Ru SAs, Au NCs, and WS2 MSLs support, which is beneficial to reduce the energy barrier for water adsorption and thus promoting the subsequent H2 formation. As feedback, the well-designed Ru1/Aun-2LWS2 electrocatalyst exhibits outstanding HER performance with high activity (η10 = 19 mV), low Tafel slope (35 mV dec-1), and excellent long-term stability. Further, in situ, experimental studies reveal that the reconstruction of Ru SAs/NCs with S vacancies in Ru1/Aun-2LWS2 structure acts as the main catalytically active center, while high-valence Au NCs are responsible for activating and stabilizing Ru sites to prevent the dissolution and deactivation of active sites. This work offers guidelines for the rational design of high-performance atomic-scale electrocatalysts.

Tunable Out-of-Plane Reconstructions in Moiré Superlattices of Transition Metal Dichalcogenide Heterobilayers.

The reconstructed moiré superlattices of the transition metal chalcogenide (TMD), formed by the combined effects of interlayer coupling and intralayer strain, provide a platform for exploring quantum physics. Here, using scanning tunneling microscopy/spectroscopy, we observe that the strained WSe2/WS2 moiré superlattices undergo various out-of-plane atomically buckled configurations, a phenomenon termed out-of-plane reconstruction. This evolution is attributed to the differentiated response of intralayer strain in high-symmetry stacking regions to external strain. Notably, in larger out-of-plane reconstructions, there is a significant alteration in the local density of states (LDOS) near the Γ point in the valence band, exceeding 300%, with the moiré potential in the valence band surpassing 200 meV. Further, we confirm that the variation in interlayer coupling within high-symmetry stacking regions is the main factor affecting the moiré electronic states rather than the intralayer strain. Our study unveils intrinsic regulating mechanisms of out-of-plane reconstructed moiré superlattices and contributes to the study of reconstructed moiré physics.

Twist-Controlled Ferroelectricity and Emergent Multiferroicity in WSe2 Bilayers.

Recently, researchers have been investigating artificial ferroelectricity, which arises when inversion symmetry is broken in certain R-stacked, i.e., zero-degree twisted, van der Waals (vdW) bilayers. Here, the study reports the twist-controlled ferroelectricity in tungsten diselenide (WSe2) bilayers. The findings show noticeable room temperature ferroelectricity that decreases with twist angle within the range 0° < θ < 3°, and disappears completely for θ ≥ 4°. This variation aligns with moiré length scale-controlled ferroelectric dynamics (0° < θ < 3°), while loss beyond 4° may relate to twist-controlled commensurate to non-commensurate transitions. This twist-controlled ferroelectricity serves as a spectroscopic tool for detecting transitions between commensurate and incommensurate moiré patterns. At 5.5 K, 3° twisted WSe2 exhibits ferroelectric and correlation-driven ferromagnetic ordering, indicating twist-controlled multiferroic behavior. The study offers insights into twist-controlled coexisting ferro-ordering and serves as valuable spectroscopic tools.

Mimicking Antiferroelectrics with Ferroelectric Superlattices.

Antiferroelectric oxides are promising materials for applications in high-density energy storage, solid-state cooling, and negative capacitance devices. However, the range of oxide antiferroelectrics available today is rather limited. In this work, it is demonstrated that antiferroelectric properties can be electrostatically engineered in artificially layered ferroelectric superlattices. Using a combination of synchrotron X-ray nanodiffraction, scanning transmission electron microscopy, macroscopic electrical measurements, and lateral and vertical piezoresponse force microscopy in parallel-plate capacitor geometry, a highly reversible field-induced transition is observed from a stable in-plane polarized state to a state with in-plane and out-of-plane polarized nanodomains that mimics, at the domain level, the nonpolar to polar transition of traditional antiferroelectrics, with corresponding polarization-voltage double hysteresis and comparable energy storage capacity. Furthermore, it is found that such superlattices exhibit large out-of-plane dielectric responses without involving flux-closure domain dynamics. These results demonstrate that electrostatic and strain engineering in artificially layered materials offers a promising route for the creation of synthetic antiferroelectrics.

Modulation of Kondo Behavior in a Two-Dimensional Epitaxial Bilayer Bi(111)/Fe3GeTe2 Moiré Heterostructure.

Artificial two-dimensional (2D) moiré superlattices provide a platform for generating exotic quantum matter or phenomena. Here, an epitaxial heterostructure composed of bilayer Bi(111) and an Fe3GeTe2 substrate with a zero-twist angle is acquired by molecular beam epitaxy. Scanning tunneling microscopy and spectroscopy studies reveal the spatially tailored Kondo resonance and interfacial magnetism within this moiré superlattice. Combined with first-principles calculations, it is found that the modulation effect of the moiré superlattice originates from the interfacial orbital hybridization between Bi and Fe atoms. Our work provides a tunable platform for strong electron correlation studies to explore 2D artificial heavy Fermion systems and interface magnetism.

Nonlinear Charge Transport and Excitable Phenomena in Semiconductor Superlattices.

Semiconductor superlattices are periodic nanostructures consisting of epitaxially grown quantum wells and barriers. For thick barriers, the quantum wells are weakly coupled and the main transport mechanism is a sequential resonant tunneling of electrons between wells. We review quantum transport in these materials, and the rate equations for electron densities, currents, and the self-consistent electric potential or field. Depending on superlattice configuration, doping density, temperature, voltage bias, and other parameters, superlattices behave as excitable systems, and can respond to abrupt dc bias changes by large transients involving charge density waves before arriving at a stable stationary state. For other parameters, the superlattices may have self-sustained oscillations of the current through them. These oscillations are due to repeated triggering and recycling of charge density waves, and can be periodic in time, quasiperiodic, and chaotic. Modifying the superlattice configuration, it is possible to attain robust chaos due to wave dynamics. External noise of appropriate strength can generate time-periodic current oscillations when the superlattice is in a stable stationary state without noise, which is called the coherence resonance. In turn, these oscillations can resonate with a periodic signal in the presence of sufficient noise, thereby displaying a stochastic resonance. These properties can be exploited to design and build many devices. Here, we describe detectors of weak signals by using coherence and stochastic resonance and fast generators of true random sequences useful for safe communications and storage.

Assembly of Differently Sized Supercharged Protein Nanocages into Superlattices for Construction of Binary Nanoparticle-Protein Materials.

This study focuses on the design and characterization of binary nanoparticle superlattices: Two differently sized, supercharged protein nanocages are used to create a matrix for nanoparticle arrangement. We have previously established the assembly of protein nanocages of the same size. Here, we present another approach for multicomponent biohybrid material synthesis by successfully assembling two differently sized supercharged protein nanocages with different symmetries. Typically, the ordered assembly of objects with nonmatching symmetry is challenging, but our electrostatic-based approach overcomes the symmetry mismatch by exploiting electrostatic interactions between oppositely charged cages. Moreover, our study showcases the use of nanoparticles as a contrast enhancer in an elegant way to gain insights into the structural details of crystalline biohybrid materials. The assembled materials were characterized with various methods, including transmission electron microscopy (TEM) and single-crystal small-angle X-ray diffraction (SC-SAXD). We employed cryo-plasma-focused ion beam milling (cryo-PFIB) to prepare lamellae for the investigation of nanoparticle sublattices via electron cryo-tomography. Importantly, we refined superlattice structure data obtained from single-crystal SAXD experiments, providing conclusive evidence of the final assembly type. Our findings highlight the versatility of protein nanocages for creating distinctive types of binary superlattices. Because the nanoparticles do not influence the type of assembly, protein cage matrices can combine various nanoparticles in the solid state. This study not only contributes to the expanding repertoire of nanoparticle assembly methods but also demonstrates the power of advanced characterization techniques in elucidating the structural intricacies of these biohybrid materials.

Strain-Dependent Effects on Confinement of Folded Acoustic and Optical Phonons in Short-Period (XC)m/(YC)n with X,Y (≡Si, Ge, Sn) Superlattices.

Carbon-based novel low-dimensional XC/YC (with X, Y ≡ Si, Ge, and Sn) heterostructures have recently gained considerable scientific and technological interest in the design of electronic devices for energy transport use in extreme environments. Despite many efforts made to understand the structural, electronic, and vibrational properties of XC and XxY1-xC alloys, no measurements exist for identifying the phonon characteristics of superlattices (SLs) by employing either an infrared and/or Raman scattering spectroscopy. In this work, we report the results of a systematic study to investigate the lattice dynamics of the ideal (XC)m/(YC)n as well as graded (XC)10-∆/(X0.5Y0.5C)∆/(YC)10-∆/(X0.5Y0.5C)∆ SLs by meticulously including the interfacial layer thickness ∆ (≡1-3 monolayers). While the folded acoustic phonons (FAPs) are calculated using a Rytov model, the confined optical modes (COMs) and FAPs are described by adopting a modified linear-chain model. Although the simulations of low-energy dispersions for the FAPs indicated no significant changes by increasing ∆, the results revealed, however, considerable "downward" shifts of high frequency COMs and "upward" shifts for the low energy optical modes. In the framework of a bond polarizability model, the calculated results of Raman scattering spectra for graded SLs are presented as a function of ∆. Special attention is paid to those modes in the middle of the frequency region, which offer strong contributions for enhancing the Raman intensity profiles. These simulated changes are linked to the localization of atomic displacements constrained either by the XC/YC or YC/XC unabrupt interfaces. We strongly feel that this study will encourage spectroscopists to perform Raman scattering measurements to check our theoretical conjectures.

Theory of Moiré Magnetism and Multidomain Spin Textures in Twisted Mott Insulator-Semimetal Heterobilayers.

Motivated by the recent experimental developments in van der Waals heterostructures, we investigate the emergent magnetism in Mott insulator-semimetal moiré superlattices by deriving effective spin models and exploring their phase diagram by Monte Carlo simulations. Our analysis indicates that the stacking-dependent interlayer Kondo interaction can give rise to different types of magnetic order, forming domains within the moiré unit cell. In particular, we find that the AB (AA) stacking regions tend to order (anti)ferromagnetically for an extended range of parameters. The remaining parts of the moiré unit cell form ferromagnetic chains that are coupled antiferromagnetically. We show that the decay length of the Kondo interaction can control the extent of these phases. Our results highlight the importance of stacking-dependent interlayer exchange and the rich magnetic spin textures that can be obtained in van der Waals heterostructures.

Design and calculation of photoelectric properties of resonance enhanced InAs/GaSb type-II superlattices photodetectors with diffraction rings structure.

The resonance enhanced InAs/GaSb type-II Superlattices (T2SLs) infrared detectors with diffraction rings is designed, and the photoelectric characteristics are calculated and studied in this paper. The diffraction rings are designed on the top surface of T2SLs detector to control the incident light inside the device by forming the resonant cavity. We designed the structure of the conventional PIN InAs/GaSb T2SLs photodetector, calculated the energy band structure of the absorption layer, and analyzed the influence of the thickness of the absorption layer on the photoelectric performance. It is proved that blindly increasing the thickness of the absorption layer cannot effectively improve the device performance. We further studied the quantum efficiency (QE) and electric field distribution of the device after adding the diffraction ring structure. The results show that the structure with diffraction rings can significantly improve the QE of the photodetector without increasing the dark current. The interaction between the diffraction ring and the metal contact layer will enhance the localization of the electric field and further increase the light absorption between the semiconductor layers.

Discovery of Self-Assembled 2D Ru/Si Superlattices Boosting Hydrogen Evolution.

2D heterostructuring is a versatile methodology for designing nanoarchitecture catalytic systems that allow for reconstruction and modulation of interfaces and electronic structures. However, catalysts with such structures are extremely scarce due to limited synthetic strategies. Here, a highly ordered 2D Ru/Si/Ru/Si… nano-heterostructures (RSHS) is reported by acid etching of the LaRuSi electride. RSHS shows a superior electrocatalytic activity for hydrogen evolution with an overpotential of 14 mV at 10 mA cm-2 in alkaline media. Both experimental analysis and first-principles calculations demonstrate that the electronic states of Ru can be tuned by strong interactions of the interfacial Ru-Si, leading to an optimized hydrogen adsorption energy. Moreover, due to the synergistic effect of Ru and Si, the energy barrier of water dissociation is significantly reduced. The well-organized superlattice structure will provide a paradigm for construction of efficient catalysts with tunable electronic states and dual active sites.

On-Wire Design of Axial Periodic Halide Perovskite Superlattices for High-Performance Photodetection.

Precise synthesis of all-inorganic lead halide perovskite nanowire heterostructures and superlattices with designable modulation of chemical compositions is essential for tailoring their optoelectronic properties. Nevertheless, controllable synthesis of perovskite nanostructure heterostructures remains challenging and underexplored to date. Here, we report a rational strategy for wafer-scale synthesis of one-dimensional periodic CsPbCl3/CsPbI3 superlattices. We show that the highly parallel array of halide perovskite nanowires can be prepared roughly as horizontally guided growth on an M-plane sapphire. A periodic patterning of the sapphire substrate enables position-selective ion exchange to obtain highly periodic CsPbCl3/CsPbI3 nanowire superlattices. This patterning is further confirmed by micro-photoluminescence investigations, which show that two separate band-edge emission peaks appear at the interface of a CsPbCl3/CsPbI3 heterojunction. Additionally, compared with the pure CsPbCl3 nanowires, photodetectors fabricated using these periodic heterostructure nanowires exhibit superior photoelectric performance, namely, high ION/IOFF ratio (104), higher responsivity (49 A/W), and higher detectivity (1.51 × 1013 Jones). Moreover, a spatially resolved visible image sensor based on periodic nanowire superlattices is demonstrated with good imaging capability, suggesting promising application prospects in future photoelectronic imaging systems. All these results based on the periodic CsPbCl3/CsPbI3 nanowire superlattices provides an attractive material platform for integrated perovskite devices and circuits.

Tuning Dynamic Structural Evolution of Bi24O31Cl10 for Enhancing Piezo-Photocatalytic Nitrogen Oxidation to Nitrate.

Direct nitrogen oxidation into nitrate under ambient conditions presents a promising strategy for harsh and multistep industrial processes. However, the dynamic structural evolution of active sites in surface reactions constitutes a highly intricate endeavor and remains in its nascent stage. Here, we constructed a Bi24O31Cl10 material with moiré superlattice structure (BCMS) for direct piezo-photocatalytic oxidation of nitrogen into nitrate. Excitingly, BCMS achieved excellent nitric acid production (15.44 mg g-1 h-1) under light and pressure conditions. Detailed experimental results show that the unique structure extracts the local strain tensor from the constricting Bi-Bi bond and Bi-O bond for internal structural reconstruction, which promotes the formation of electron and reactive molecule vortexes to facilitate charge transfer as well as N2 and O2 adsorption. Ultimately, these initiatives strengthen electron exchange between the superoxide radical and nitrogen as well as the binding strength of multiple intermediates, which swayingly adjusts the reaction path and energy barriers.

Moiré Engineering of Spin-Orbit Torque by Twisted WS2 Homobilayers.

Artificial moiré superlattices created by stacking 2D crystals have emerged as a powerful platform with unprecedented material-engineering capabilities. While moiré superlattices are reported to host a number of novel quantum states, their potential for spintronic applications remains largely unexplored. Here, the effective manipulation of spin-orbit torque (SOT) is demonstrated using moiré superlattices in ferromagnetic devices comprised of twisted WS2/WS2 homobilayer (t-WS2) and CoFe/Pt thin films by altering twisting angle (θ) and gate voltage. Notably, a substantial enhancement of up to 44.5% is observed in SOT conductivity at θ ≈ 8.3°. Furthermore, compared to the WS2 monolayer and untwisted WS2/WS2 bilayers, the moiré superlattices in t-WS2 enable a greater gate-voltage tunability of SOT conductivity. These results are related to the generation of the interfacial moiré magnetic field by the real-space Berry phase in moiré superlattices, which modulates the absorption of the spin-Hall current arising from Pt through the magnetic proximity effect. This study highlights the moiré physics as a new building block for designing enhanced spintronic devices.

Extended states in random dimer gated graphene superlattices.

Ordered and disordered semiconductor superlattices represent structures with completely opposed properties. For instance, ordered superlattices exhibit extended Bloch-like states, while disordered superlattices present localized states. These characteristics lead to higher conductance in ordered superlattices compared to disordered ones. Surprisingly, disordered dimer superlattices, which consist of two types of quantum wells with one type always appearing in pairs, exhibit extended states. The percentage of dissimilar wells does not need to be large to have extended states. Furthermore, the conductance is intermediate between ordered and disordered superlattices. In this work, we explore disordered dimer superlattices in graphene. We calculate the transmission and transport properties using the transfer matrix method and the Landauer-Büttiker formalism, respectively. We identify and discuss the main energy regions where the conductance of random dimer superlattices in graphene is intermediate to that of ordered and disordered superlattices. We also analyze the resonant energies of the double quantum well cavity and the electronic structure of the host gated graphene superlattice (GGSL), finding that the coupling between the resonant energies and the superlattice energy minibands gives rise to the extended states in random dimer GGSLs.

Mapping of Long-Wavelength Phonon Contribution in the Thermal Transport of Alloyed Semiconductor Superlattices.

The mapping of long-wavelength phonons is important to understand and manipulate the thermal transport in multilayered structures, but it remains a long-standing challenge due to the collective behaviors of phonons. In this study, an experimental demonstration of mapping the long-wavelength phonons in an alloyed Al0.1Ga0.9As/Al0.9Ga0.1As superlattice system is reported. Multiple strategies to filter out the short- to mid-wavelength phonons are used. The phonon mean-free-path-dependent thermal transport properties directly demonstrate both the suppression effect of the ErAs nanoislands and the contribution of long-wavelength phonons. The contribution from phonons with mean free path longer than 1 µm is clearly demonstrated. A model based on the Boltzmann transport equation is proposed to calculate and describe the thermal transport properties, which depicts a clear physical picture of the transport mechanisms. This method can be extended to map different wavelength phonons and become a universal strategy to explore their thermal transport in various application scenarios.

Engineering van der Waals Contacts by Interlayer Dipoles.

Tuning the interfacial Schottky barrier with van der Waals (vdW) contacts is an important solution for two-dimensional (2D) electronics. Here we report that the interlayer dipoles of 2D vdW superlattices (vdWSLs) can be used to engineer vdW contacts to 2D semiconductors. A bipolar WSe2 with Ba6Ta11S28 (BTS) vdW contact was employed to exhibit this strategy. Strong interlayer dipoles can be formed due to charge transfer between the Ba3TaS5 and TaS2 layers. Mechanical exfoliation breaks the superlattice and produces two distinguished surfaces with TaS2 and Ba3TaS5 terminations. The surfaces thus have opposite surface dipoles and consequently different work functions. Therefore, all the devices fall into two categories in accordance with the rectifying direction, which were verified by electrical measurements and scanning photocurrent microscopy. The growing vdWSL family along with the addition surface dipoles enables prospective vdW contact designs and have practical application in nanoelectronics and nano optoelectronics.