3D Bulk Research Articles

Nonmonotonic Photostability of BA2MAn-1PbnI3n+1 Homologous Layered Perovskites.

Layered lead halide perovskites (2D LHPs) are attracting considerable attention as a promising material for a new generation of solar cell devices. LHPs have been presented as a more stable alternative to the more widespread 3D bulk perovskite materials; however, a critical analysis of their photostability is still lacking. In this work, we perform a comparative study between BA2MAn-1PbnI3n+1 (BA─butylammonium and MA─methylammonium) 2D LHPs with different dimensionalities (n = 1-3) and MAPbI3 3D perovskites. We compare different stability testing protocols including photometrical determination of iodine-containing products in nonpolar solvents, X-ray diffraction, and photoluminescence (PL) spectroscopy. The resulting trends of the photostability in an inert atmosphere based on PL spectroscopy measurements demonstrate a nonmonotonic dependence of the degradation rate on the perovskite layer thickness n with a "stability island" at n ≥ 3, which is caused by a combination of antibate factors of electronic structures and chemical compositions in the family of 2D perovskites. We also identify a critical oxygen concentration in the surrounding environment that affects the mechanism and strongly enhances the rate of layered perovskite photodegradation.

Edge state in AB-stacked bilayer graphene and its correspondence with the Su-Schrieffer-Heeger ladder

We study edge states in AB-stacked bilayer graphene (BLG) ribbon where the Chern number of the corresponding two-dimensional (2D) bulk Hamiltonian is zero. The existence and topological features of edge states when two layers ended with the same or different edge terminations (zigzag, bearded, armchair) are discussed. The edge states (non-dispersive bands near the Fermi level) are states localized at the edge of graphene nanoribbon that only exists in certain range of momentum $k_y$. Their existence near the Fermi level are protected by the chiral symmetry with topology well described by coupled Su-Schrieffer-Heeger (SSH) chains model, i.e., SSH ladder, based on the bulk-edge correspondence of one-dimensional (1D) systems. These zero-energy edge states can exist in the whole $k_y$ region when two layers have zigzag and bearded edges, respectively. Winding number calculation shows a topological phase transition between two distinct non-trivial topological phases when crossing the Dirac points. Interestingly, we find the stacking configuration of BLG ribbon is important since they can lead to unexpected edge states without protection from the chiral symmetry both near the Fermi level in armchair-armchair case and in the gap within bulk bands that are away from Fermi level in the general case. The influence of interlayer next nearest neighbor (NNN) interaction and interlayer bias are also discussed to fit the realistic graphene materials, which suggest the robust topological features of edge states in BLG systems.

Two‐dimensional Metal‐organic Frameworks for Electrochemical CO2Reduction Reaction

AbstractTo lower CO2emissions and address the current energy crisis, one of the most promising approaches that converting the captured CO2into valuable chemicals and fuels via electrocatalysis is proposed recently. Metal‐organic frameworks (MOFs) as an emerging multifunctional material have been extensively designed for electrocatalytic reduction of CO2. In terms of chemical and structural properties, 2D MOFs have obvious superiority over 3D bulk MOFs. Specifically, the large porosity and ultrathin structure of the 2D materials contribute to exotic properties such as enhanced electrical conductivity and rapid mass transport during reactions, which are in favor of electrocatalysis. In this review, the design strategies of 2D MOFs are discussed. Then, the recent advances of MOFs and their derivative catalysts with unique 2D structures for CO2reduction are introduced. These examples are expected to provide clues to rational design strategies and synthesis of high‐performance CO2electroreduction, beyond the bulk MOFs.

Flower-like CuCoMoOx nanosheets decorated with CoCu nanoparticles as bifunctional electrocatalysts for hydrogen evolution reaction and water splitting

The development of high efficiency, low-budget and excellent stability of electrocatalysts is a critical factor to improve water decomposition. Herein, a highly active 3D bulk catalysts of core-shell nanostructures, in which Flower-like CuCoMoOx nanosheets embellished with CoCu alloy nanoparticles are evenly wrapped on Cu NWs cores supported on CF, toward overall water splitting is firstly reported. The coaction of CoCu alloy nanoparticles and CuCoMoOx nanosheets with 3D hierarchical and porous nanoarchitecture can accelerate the ionization of H2O molecules and formation of H2. Meanwhile, the connection nanosheets of Cu NWs with noteworthy conductivity, resulting in rapid electron transport, exhibits outstanding both HER and OER activities and remarkable overall water splitting characteristics in the alkaline medium. As an electrocatalyst for HER and OER, the low overpotential of 75 mV and 315 mV at 100 mA cm−2, and low voltage of 1.66 V for overall water splitting are achieved, even superior to the benchmark of IrO2/CF (+) // /Pt/C/CF(-) in 1 M KOH. This work has a significant reference to hydrogen production for the electrolytic water industry.

Cavity mode dephasing via the optomechanical interaction with an acoustic environment

We consider an optomechanical system comprising a single cavity mode and a dense spectrum of acoustic modes and solve for the quantum dynamics of initial cavity mode Fock (i.e., photon number) superposition states and thermal acoustic states. The optomechanical interaction results in dephasing without damping and bears some analogy to gravitational decoherence. For a cavity mode locally coupled to a one-dimensional (1D) elastic string-like environment or two-dimensional (2D) elastic membrane-like environment, we find that the dephasing dynamics depends respectively on the string length and membrane area--a consequence of an infrared divergence in the limit of an infinite-sized string or membrane. On the other hand, for a cavity mode locally coupled to a three-dimensional (3D) bulk elastic solid, the dephasing dynamics is independent of the solid volume (i.e., is infrared finite), but dependent on the local geometry of the coupled cavity--a consequence of an ultraviolet divergence in the limit of a "pointlike" coupled cavity. We consider as possible respective realizations for the cavity-coupled-1D and 2D acoustic environments, an LC oscillator capacitively coupled to a partially metallized strip and a cavity light mode interacting via light pressure with a membrane.

Three-dimensional topological plasmons in Weyl semimetals

We systematically investigate the properties of bulk, surface and edge plasmons in Weyl semimetals in presence of a magnetic field. It is found that unidirectional plasmons with different properties exist on different surfaces, which is in consistent with the nontrivial topology of the three-dimensional (3D) bulk plasmons. These novel plasmons possess momentum-location lock and may travel between surfaces. The anomalous Hall conductivity brings about abundant anisotropic plasmon dispersions, from linear to parabolic or even hyperbolic bands. With the help of a semiclassical picture for the formation of the Weyl orbits, we point out the Fermi-arc plasmons at opposite surfaces can make up another unique 3D topological plasmon. Furthermore, there is a gapless unidirectional edge plasmon protected by the topology whose direction and position can be controlled by external field. Our work thus uncovers topological features of Weyl plasmons, which may have important applications in photoelectric devices based on chiral/topological plasmonics.

Notes from the bulk: Metric dependence of the edge states of Chern-Simons theory

The Abelian Chern-Simons theory is considered on a cylindrical spacetime $\mathbb{R}\ifmmode\times\else\texttimes\fi{}D$, in a not necessarily flat Lorentzian background. As in the flat bulk case with planar boundary, we find that also on the radial boundary of a curved background a Ka\ifmmode \mbox{\c{c}}\else \c{c}\fi{}-Moody algebra exists, with the same central charge as in the flat case, which henceforth depends neither on the bulk metric nor on the geometry of the boundary. The holographically induced theory on the 2D boundary is topologically protected, in the sense that it describes a Luttinger liquid, no matter which the bulk metric is. The main result of this paper is that a remnant of the 3D bulk theory resides in the chiral velocity of the edge modes, which is not a constant like in the flat bulk case, but it is local, depending on the determinant of the induced metric on the boundary. This result may provide a theoretical framework for the recently observed accelerated chiral bosons on the edge of some Hall systems.

Deep Bayesian local crystallography

The advent of high-resolution electron and scanning probe microscopy imaging has opened the floodgates for acquiring atomically resolved images of bulk materials, 2D materials, and surfaces. This plethora of data contains an immense volume of information on materials structures, structural distortions, and physical functionalities. Harnessing this knowledge regarding local physical phenomena necessitates the development of the mathematical frameworks for extraction of relevant information. However, the analysis of atomically resolved images is often based on the adaptation of concepts from macroscopic physics, notably translational and point group symmetries and symmetry lowering phenomena. Here, we explore the bottom-up definition of structural units and symmetry in atomically resolved data using a Bayesian framework. We demonstrate the need for a Bayesian definition of symmetry using a simple toy model and demonstrate how this definition can be extended to the experimental data using deep learning networks in a Bayesian setting, namely rotationally invariant variational autoencoders.

Lattice Dynamics and Optoelectronic Properties of Vacancy-Ordered Double Perovskite Cs2TeX6 (X = Cl–, Br–, I–) Single Crystals

The soft, dynamic lattice of inorganic lead halide perovskite CsPbX3 (X = Cl-, Br-, I-) leads to the emergence of many interesting photophysical and optoelectronic phenomena. However, probing their lattice dynamics with vibrational spectroscopy remains challenging. The influence of the fundamental octahedral building block in the perovskite lattice can be better resolved in zero-dimensional (0D) vacancy-ordered double perovskites of form A2BX6. Here we study Cs2TeX6 (X = Cl-, Br-, I-) single crystals to yield detailed insight into the fundamental octahedral building block and to explore the effect that its isolation in the crystal structure has on structural and electronic properties. The isolated [TeX6]2- octahedral units serve as the vibrational, absorbing, and emitting centers within the crystal. Serving as the vibrational centers, the isolated octahedra inform the likelihood of a random distribution of 10 octahedral symmetries within the mixed-halide spaces, as well as the presence of strong exciton-phonon coupling and anharmonic lattice dynamics. Serving as the absorbing and emitting centers, the isolated octahedra exhibit compositionally tunable absorption (1.50-3.15 eV) and emission (1.31-2.11 eV) energies. Due to greater molecular orbital overlap between neighboring octahedra with increasing halide anion size, there is a transition from a more molecule-like electronic structure in Cs2TeCl6 and Cs2TeBr6-as expected from the effective 0D nature of these single crystals-to a dispersive electronic structure in Cs2TeI6, typical of three-dimensional (3D) bulk single crystals.

Recent Progress in Perovskite‐Based Reversible Photon–Electricity Conversion Devices

AbstractSolution‐processed metal halide perovskites have the advantages of a tunable bandgap, excellent charge transport properties, and suitable exciton binding energy. They therefore emerge as promising semiconductors for efficient perovskite solar cells (PSCs) and bright perovskite light‐emitting diodes (PLEDs). In addition, these devices possess a similar planar–heterojunction architecture, thus novel dual‐functional perovskite light‐emitting solar cells (PLESCs) that can realize electrical‐to‐optical and optical‐to‐electrical conversion on one device are developed. To date, high‐performance PLESCs with 17.2% electroluminescence external quantum efficiency and 25.2% power conversion efficiency are achieved using a holistic optimization approach. However, a comprehensive review focusing on dual‐functional PLESCs with discussion on their development and limitations, remains lacking. Herein, the rapid progress in PLESCs, including 0D quantum dot, 2D Ruddlesden–Popper, and 3D bulk perovskite devices, is reviewed. First, the fundamental understanding of the device structure and working principle of PLESCs is overviewed. Second, the state‐of‐the‐art developments for simultaneously achieving high‐efficiency PSCs and PLEDs, focusing on defect passivation, interface optimization, energy level alignment, and dimensional control, are comprehensively summarized. Finally, the authors offer some perspectives for future trends in the development of promising PLESCs, which can provide guidelines for this emerging field.

Holographic entanglement entropy and modular Hamiltonian in warped CFT in the framework of GMMG model

We study some aspects of a class of non-AdS holography where the 3D bulk gravity is given by generalized minimal massive gravity (GMMG). We consider the spacelike warped AdS_3 (WAdS_3) black hole solution of this model where the 2d dual boundary theory is the warped conformal field theory (WFCT). The charge algebra of the isometries in the bulk and the charge algebra of the vacuum symmetries at the boundary are compatible and this is an evidence for the duality conjecture. Further evidence for this duality is the equality of entanglement entropy and modular Hamiltonian on both sides of the duality. So we consider the modular Hamiltonian for the single interval at the boundary in associated to the modular flow generators of the vacuum. We calculate the gravitational charge in associated to the asymptotic Killing vectors that preserve the metric boundary conditions. Assuming the first law of the entanglement entropy to be true, we introduce the matching conditions between the variables in two side of the duality and we find equality of the modular Hamiltonian variations and the gravitational charge variations in two sides of the duality. According to the results of the present paper we can say with more sure that the dual theory of the warped AdS3 black hole solution of GMMG is a Warped CFT.

Cyclohexylammonium‐Based 2D/3D Perovskite Heterojunction with Funnel‐Like Energy Band Alignment for Efficient Solar Cells (23.91%) (Adv. Energy Mater. 42/2021)

2D Perovskites In article number 2102236, Jong Hyeok Park, Nam-Gyu Park, Hyunjung Shin and co-workers extract photo-generated holes from 3D bulk perovskites of FAPbI3 through cyclohexylamonium-based 2D perovskites. 2D/3D perovskite heterojunction structures are successfully fabricated and the efficient extraction of holes is expressed in this cover picture.

A comprehensive framework for hard-magnetic beams: Reduced-order theory, 3D simulations, and experiments

Thin beams made of magnetorheological elastomers embedded with hard-magnetic particles (hard-MREs) are capable of large deflections under an applied magnetic field. We propose a comprehensive framework, comprising a beam model and 3D finite element modeling (FEM), to describe the behavior of hard-MRE beams under both uniform and constant gradient magnetic fields. First, based on the Helmholtz free energy of bulk (3D) hard-MREs, we perform dimensional reduction to derive a 1D description and obtain the equilibrium equation of the beam through variational methods. In parallel, we extend the existing 3D continuum theory for hard-MREs to the general case of non-uniform fields by incorporating the magnetic body force induced by the field gradient and implementing it in FEM. Then, we validate the beam model and FEM using experiments on a cantilever beam in either a uniform or a constant gradient field, and identify the dimensionless parameters governing the magneto-elastic coupling. Further, a set of comparative numerical studies for different field configurations and magnetization profiles yields additional insight into the beam response. Our study builds on previous work on hard-MRE beams, while providing a more complete framework, both in terms of the methodologies used and the broader range of configurations considered, serving as a valuable predictive toolbox for the rational design of beam-like hard-magnetic structures.

Solid-State Electrochemical Random Access Memory for in-Memory Computing

Novel approaches to computing are needed to accommodate the rapid increases in data processing needs. Standard von Neumann architectures may not be optimal for operations like machine learning due to the high energy costs of moving data between separate processor and memory elements. In-memory computing with combined logic and memory functionality is a promising energy-efficient solution. Filament-based phase-change memory and resistive random access memory are well-investigated approaches to in-memory computing; however, they suffer from unreliable switching due to the stochastic motion of ions within the filament. This results from the positive feedback process associated with filament-based memories.In this work, we develop electrochemical random-access memory (ECRAM) that uses the 3D concentration of oxygen vacancy ions to store information1,2. We sandwich an ion-conducting, electron-blocking solid electrolyte between the reservoir and switching layers, both mixed ionic and electronic conductors (MIECs). Due to essential zero electronic conductivity, the ion conductor blocks the nonuniform joule heating positive feedback that results in filament formation. Instead, oxygen vacancies are uniformly distributed in the 3D bulk. This electrochemical memory cell is therefore sensitive to the average statistical behavior of all ions, as opposed to the stochastic behavior of a small number of ions in the filament. This average ion concentration yields the analogue information state.We show that small, short electrochemical voltage pulses (+/-1.5V, 2 μs) can be used to shuttle vacancies between the MIECs; this enables reliable, linear, and deterministic switching behavior with hundreds of information states. At the same time, the materials are CMOS compatible with long endurance and information retention. The switching and retention times are dependent upon the ability of the materials to allow reversible changes in the oxygen stoichiometry, analogous to an “oxygen battery,” as well as the ionic resistance of the electrolyte. These ECRAM cells not only solve the longstanding problems within resistive memory, but also present significant opportunities for future research onto different nonstoichiometric transition metal oxides.Reference:1 Y. Li et al. “Filament-free bulk resistive memory enables deterministic analogue switching,” Adv Mater, 32, 202003984 (2020)2. Y. Li et al. "Low-Voltage, CMOS-Free Synaptic Memory Based on LiXTiO2 Redox Transistors," ACS Appl. Mater. Interfaces, 11, 42, 38982–38992 (2019) Figure 1

Large magnetoresistance of a compensated metal Cu2Sb correlated with its Fermi surface topology

We report electrical transport properties and electronic structure of a nonmagnetic metal ${\mathrm{Cu}}_{2}\mathrm{Sb}$ single crystal. ${\mathrm{Cu}}_{2}\mathrm{Sb}$ was found to be a compensated metal with high carrier density $\ensuremath{\sim}{10}^{22}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ and high carrier mobility $\ensuremath{\ge}{10}^{3}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$ for both electron and hole carriers. The current-in-plane magnetoresistance at 2 K and 9 T was 730%, while the current-perpendicular-to-plane magnetoresistance at 2 K and 9 T was 2700% without the saturation. Angle-resolved photoemission spectroscopy throughout the three-dimensional (3D) bulk Brillouin zone signified a quasi-two-dimensional (2D) electron pocket axially centered along the M-A line and a 3D hole pocket at the \ensuremath{\Gamma} point, in accordance with the electron-hole compensated nature. The presence of quasi-2D open Fermi surface, in line with the first-principles band-structure calculations, is likely responsible for the observed nonsaturating current-in-plane magnetoresistance. The present result lays the foundation for realizing large magnetoresistance via Fermiology engineering in compensated metals.

A recent advances of blue perovskite light emitting diodes for next generation displays

The halide perovskite blue light emitting diodes (PeLEDs) attracted many researchers because of its fascinating optoelectrical properties. This review introduces the recent progress of blue PeLEDs which focuses on emissive layers and interlayers. The emissive layer covers three types of perovskite structures: perovskite nanocrystals (PeNCs), 2-dimensional (2D) and quasi-2D perovskites, and bulk (3D) perovskites. We will discuss about the remaining challenges of blue PeLEDs, such as limited performances, device instability issues, which should be solved for blue PeLEDs to realize next generation displays.

3D-1D coupling on non conforming meshes via a three-field optimization based domain decomposition

A new numerical approach is proposed for the simulation of coupled three-dimensional and one-dimensional elliptic equations (3D-1D coupling) arising from dimensionality reduction of 3D-3D problems with thin inclusions. The method is based on a well posed mathematical formulation and results in a numerical scheme with high flexibility in handling geometrical complexities. This is achieved by means of a three-field approach to split the 1D problems from the bulk 3D problem, and then resorting to the minimization of a properly designed functional to impose matching conditions at the interfaces. Thanks to the structure of the functional, the method allows the use of independent meshes for the various subdomains.

Thermal- and Light-Induced Evolution of the 2D/3D Interface in Lead-Halide Perovskite Films.

The instability of halide perovskites toward moisture is one of the main challenges in the field that needs to be overcome to successfully integrate these materials in commercially viable technologies. One of the most popular ways to ensure device stability is to form 2D/3D interfaces by using bulky organic molecules on top of the 3D perovskite thin film. Despite its promise, it is unclear whether this approach is able to avoid 3D bulk degradation under accelerated aging conditions, i.e., thermal stress and light soaking. In this regard, it is crucial to know whether the interface is structurally and electronically stable or not. In this work, we use the bulky phenethylammonium cation (PEA+) to form 2D layers on top of 3D single- and triple-cation halide perovskite films. The dynamical change of the 2D/3D interface is monitored under thermal stress and light soaking by in situ photoluminescence. We find that under pristine conditions the large organic cation diffuses only in 3D perovskite thin films of poor structural stability, i.e., single-cation MAPbI3. The same diffusion and a dynamical change of the crystalline structure of the 2D/3D interface are observed even on high-quality 3D films, i.e., triple-cation MAFACsPbI3, upon thermal stress at 85 °C and light soaking. Importantly, under such conditions, the resistance of the thin film to moisture is lost.

Anomalies in bosonic symmetry-protected topological edge theories: Connection to F symbols and a method of calculation

We describe a systematic procedure for determining the identity of a 2D bosonic symmetry protected topological (SPT) phase from the properties of its edge excitations. Our approach applies to general bosonic SPT phases with either unitary or antiunitary symmetries, and with either continuous or discrete symmetry groups, with the only restriction being that the symmetries must be on-site. Concretely, our procedure takes a bosonic SPT edge theory as input, and produces an element $\omega$ of the cohomology group $H^3(G, U_T(1))$. This element $\omega \in H^3(G, U_T(1))$ can be interpreted as either a label for the bulk 2D SPT phase or a label for the anomaly carried by the SPT edge theory. The basic idea behind our approach is to compute the $F$-symbol associated with domain walls in a symmetry broken edge theory; this domain wall $F$-symbol is precisely the anomaly we wish to compute. We demonstrate our approach with several SPT edge theories including both lattice models and continuum field theories.

Anomalous Dimensionality‐Driven Phase Transition of MoTe2 in Van der Waals Heterostructure

AbstractPhase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface‐to‐volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality‐driven phase transition of molybdenum ditelluride (MoTe2) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single‐crystal 2H‐MoTe2 transformed into polycrystalline Td‐MoTe2 with tilt‐angle grain boundaries of 60°‐glide‐reflection and 120°‐twofold rotation. In contrast to conventional nanomaterials, the hBN‐encapsulated MoTe2 exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe2 has a higher 2H‐to‐Td phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe2 with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless Td contacts for 2H‐MoTe2 transistors can be fabricated by using the dimensionality‐driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi‐phase 2D electronics.