Atomic Symmetry Research Articles

Structural Stability of Vacancy and Substitutional Defects in g-GaN: A First-Principles Study

Herein, the first-principles calculations of defected monolayer graphene-like gallium nitride (g-GaN) were elucidated. The optimized geometric revealed that the nearest-neighbor distances of N-N (between N atoms) and Ga-Ga (between Ga atoms) were narrowing in most configurations. Interestingly, the VGa has a similar atomic symmetry as the pure g-GaN, which is D3h. Most defected configurations are degraded into C2V symmetries, while the Stone-Wales configuration has the lowest symmetry of CS. The formation energies of V­­Na systems are lower, which implies better energetic stability. For the divacancies, the VGa system is more stable than the V­­Na system. Moreover, the Stone-Wales configuration is energetically more stable than the interchange configuration. Furthermore, the reaction coordinates represent the geometric evolution of each defected system. The result revealed that the N-atoms are consistent with moving outward while the Ga-atoms move inward only for monovacancies and divacancies configurations. In contrast, the N-atoms move inward while the Ga-atoms move outward for substitutions, interchanges, and Stone-Wales configurations. We believe that this finding will be beneficial as the groundwork for future 2D g-GaN-based semiconductor devices. HIGHLIGHTS The V­­Na defective system is more stable than the VGa system in monovacancies and substitutions. The VGaGa defective system is more stable than the V­­Na system in divacancies. The Stone-Wales system is a stable configuration with a Cs symmetry. GRAPHICAL ABSTRACT

Quasi-1D Moiré superlattices in self-twisted two-allotropic antimonene heterostructures.

Two-dimensional heterostructures, characterized by a twist angle between individual sublayers, offer unique and tunable properties distinct from standalone layers. These structures typically introduce a realm of exotic quantum phenomena due to the appearance of new, long range periodicities associated with Moiré superlattices. Using molecular beam epitaxy, we demonstrate the growth of bi-allotropic 2D-Sb heterostructures on a W(110) substrate composed of twisted α (α-Sb) and β (β-Sb) phases of antimonene. Due to the relatively weak interaction between sublayers, the twist angle is intrinsically determined for each heterostructure, revealing its inherent self-twisted nature. The different atomic lattice symmetries of both allotropes lead to the formation of distinctive quasi-1D Moiré superlattices, while the random nature of the twist angle allows for a wide modulation of the Moiré potential landscape. The observed Moiré patterns on β-Sb/α-Sb heterostructures were compared with a simple model, revealing satisfactory agreement with the experiments and strongly validating the formation of self-twisted β-Sb/α-Sb heterostructures. The samples were characterized in situ using low energy electron microscopy and diffraction techniques providing a real-time tracking of the growth process and insight into the atomic structure of the synthesized nanostructures.

Symmetry-Shaped Singularities in High-Temperature Superconductor H3S.

The superconducting critical temperature of H3S ranks among the highest measured, at 203 K. This impressive value stems from a singularity in the electronic density-of-states, induced by a flat-band region that consists of saddle points. The peak sits right at the Fermi level, so that it gives rise to a giant electron-phonon coupling constant. In this work, we show how atomic orbital interactions and space group symmetry work in concert to shape the singularity. The body-centered cubic Brillouin Zone offers a unique 2D hypersurface in reciprocal space: fully connecting squares with two different high-symmetry points at the corners, Γ and H, and a third one in the center, N. Orbital mixing leads to the collapse of fully connected 1D saddle point lines around the square centers, due to a symmetry-enforced s-p energy inversion between Γ and H. The saddle-point states are invariably nonbonding, which explains the unconventionally weak response of the superconductor's critical temperature to pressure. Although H3S appears to be a unique case, the theory shows how it is possible to engineer flat bands and singularities in 3D lattices through symmetry considerations.

Superlattice Delineated Fermi Surface Nesting and Electron-Phonon Coupling in CaC6

The superconductivity of CaC6 as a function of pressure and Ca isotopic composition was revisited using DFT calculations on a 2c–double hexagonal superlattice. The introduction of superlattices was motivated by previous synchrotron absorption and Raman spectroscopy results on other superconductors that showed evidence of superlattice vibrations at low (THz) frequencies. For CaC6, superlattices have previously been invoked to explain the ARPES data. A superlattice along the hexagonal c-axis direction is also illustrative of atomic orbital symmetry and periodicity, including bonding and antibonding s-orbital character implied by cosine-modulated electronic bands. Inspection of the cosine band revealed that the cosine function has a small (meV) energy difference between the bonding and antibonding regions, relative to a midpoint non-bonding energy. Fermi surface nesting was apparent in an appropriately folded Fermi surface using a superlattice construct. Nesting relationships identified phonon vectors for the conservation of energy and for phase coherency between coupled electrons at opposite sides of the Fermi surface. A detailed analysis of this Fermi surface nesting provided accurate estimates of the superconducting gaps for CaC6 with the change in applied pressure. The recognition of superlattices within a rhombohedral or hexagonal representation provides consistent mechanistic insight on superconductivity and electron−phonon coupling in CaC6.

A highly-stable and low-energy-barrier photoelectrocatalyst (NiFe-LDH@Ti-MOF) for water splitting at high current densities via breaking the atomic structure symmetry

There is a flurry of interest in enhancing the stability and efficiency of photoelectrocatalysts in water splitting at high current densities. In the present study, nickel–iron layered double hydroxide (NiFe-LDH) was decorated with MIP-177-LT (Ti-metal organic framework (MOF)) to fabricate a heteroatom electrocatalyst (NiFe-LDH@Ti-MOF). The overpotentials of the oxygen evolution reaction (OER) turned out to be very low: 199 and 232 mV at current densities of 200 and 600 mA cm−2, in that order. Then, current densities of 1,000 mA cm−2 were applied to the modified electrode over a long period of 800 h, and this generated a higher stability than that of the commercial IrO2. Further, its mass activity proved ∼2 and 7 times as high as those of IrO2 and NiFe-LDH, in that order. Furthermore, for water splitting, the produced photoelectrocatalyst obtained a current density of 1,000 mA cm−2 using only 1.6 V, and this performance remained changeless for 800 h, which can be considered the most superb performance ever reported. The experimental characterization revealed that during the process of doping Ti-MOF on NiFe-LDH, symmetry breaking takes place in the atomic structure of the LDH surface, which increases the active sites in the photoelectrocatalyst. The presence of oxygen in the Ti-MOF active sites improves the efficiency of OER at high current densities and also contributes to the higher stability of multiple heteroatomic interfaces for water splitting. The propounded strategy enables the generation of oxygen and hydrogen from water splitting at high current densities on an industrial scale by changing the symmetry of the atomic structure of heteroatomic photocatalysts.

First-principles definition of ionicity and covalency in molecules and solids.

The notions of ionicity and covalency of chemical bonds, effective atomic charges, and decomposition of the cohesive energy into ionic and covalent terms are fundamental yet elusive. For example, different approaches give different values of atomic charges. Pursuing the goal of formulating a universal approach based on firm physical grounds (first-principles or non-empirical), we develop a formalism based on Wannier functions with atomic orbital symmetry and capable of defining these notions and giving numerically robust results that are in excellent agreement with traditional chemical thinking. Unexpectedly, in diamond-like boron phosphide (BP), we find charges of +0.68 on phosphorus and -0.68 on boron atoms, and this anomaly is explained by the Zintl-Klemm nature of this compound. We present a simple model that includes energies of the highest occupied cationic and lowest unoccupied anionic atomic orbitals, coordination numbers, and strength of interatomic orbital overlap. This model captures the essential physics of bonding and accurately reproduces all our results, including anomalous BP.

Constructing Symmetry-Mismatched RuxFe3-xO4 Heterointerface-Supported Ru Clusters for Efficient Hydrogen Evolution and Oxidation Reactions.

Ru-related catalysts have shown excellent performance for the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR); however, a deep understanding of Ru-active sites on a nanoscale heterogeneous support for hydrogen catalysis is still lacking. Herein, a click chemistry strategy is proposed to design Ru cluster-decorated nanometer RuxFe3-xO4 heterointerfaces (Ru/RuxFe3-xO4) as highly effective bifunctional hydrogen catalysts. It is found that introducing Ru into nanometric Fe3O4 species breaks the symmetry configuration and optimizes the active site in Ru/RuxFe3-xO4 for HER and HOR. As expected, the catalyst displays prominent alkaline HER and HOR performance with mass activity much higher than that of commercial Pt/C as well as robust stability during catalysis because of the strong interaction between the Ru cluster and the RuxFe3-xO4 support, and the optimized adsorption intermediate (Had and OHad). This work sheds light on a promsing approach to improving the electrocatalysis performance of catalysts by the breaking of atomic dimension symmetry.

Local atomic stacking and symmetry in twisted graphene trilayers.

Moiré superlattices formed by twisting trilayers of graphene are a useful model for studying correlated electron behaviour and offer several advantages over their formative bilayer analogues, including a more diverse collection of correlated phases and more robust superconductivity. Spontaneous structural relaxation alters the behaviour of moiré superlattices considerably and has been suggested to play an important role in the relative stability of superconductivity in trilayers. Here we use an interferometric four-dimensional scanning transmission electron microscopy approach to directly probe the local graphene layer alignment over a wide range of trilayer graphene structures. Our results inform a thorough understanding of how reconstruction modulates the local lattice symmetries crucial for establishing correlated phases in twisted graphene trilayers, evincing a relaxed structure that is markedly different from that proposed previously.

Asymmetric Coordination of Iron Single‐Atom Nanozymes with Efficient Self‐Cascade Catalysis for Ferroptosis Therapy

AbstractSingle‐atom nanozymes (SAzymes) hold great promise in tumor therapy due to their maximized atomic utilization and well‐defined electronic structures. However, they still face challenges of activity, specificity, and targeting that impede therapeutic efficacy. Herein, a practical strategy is reported to construct asymmetric N, S‐coordinated Fe SAzymes (Fe‐S/N‐C). Benefiting from the regulatory influence of S atoms on the disruption of local charge symmetry of center Fe atoms, the Fe‐S/N‐C SAzymes exhibit significantly enhanced peroxidase (POD)‐ and glutathione oxidase (GSHOx)‐like activities, with catalytic efficiencies being 6.33 and 47.88 times higher than their symmetric Fe‐N4 counterparts, respectively. Theoretical calculations demonstrate that the asymmetric atomic interface configuration increases electron localization around center Fe sites, thus facilitating the adsorption and activation of H2O2 and O2. By camouflaging with macrophage membranes, the tumor‐targeting nanocatalytic agents (M@Fe‐S/N‐C) trigger enhanced self‐cascade catalysis in the tumor microenvironment for ferroptosis‐based tumor‐specific therapy. These results open up a promising avenue for addressing the limitations associated with current SAzymes‐based tumor therapies.

Enhanced negative refraction in one- and two-dimensional chiral atomic lattices

We consider a physical scheme for the realization of optical lattices that have negative refracting under certain conditions. The lattice is formed by clusters of five-level closed-loop chiral atoms assumed to have a Gaussian density distribution in dipole traps along one or two dimensions. With a suitable combination of strong standing wave control fields, spatial symmetry of atoms is achieved. Besides, the dispersion properties of an atomic lattice can be modified conveniently by adjusting various parameters related to the atoms or the lattice. We show that negative-like refraction is not specific to the atoms themselves, but also emerges due to the periodicity of the lattice. We further reveal that the refraction is negative over specific regions on the lattice and strongly depends on the number of atoms trapped along a particular direction. Our model is effectively controllable remotely and may have applications in negative refraction devices and optical information processing.

Discovery of Ultrasmall Polar Merons and Rich Topological Phase Transitions: Defects Make 2D Lead Chalcogenides Flexible Topological Materials

AbstractAtomic‐scale polar topological configurations, such as skyrmions and merons, have garnered enormous interest due to their rich emergent physical phenomena and promising applications in next‐generation electronics. Despite recent progress in the exploration of 2D ferroelectrics, isolated polar topological structures in 2D lattices have not yet been explored. Here, an original design principle is proposed to remove the point group limit for polar structures while achieving atomic‐scale polar topological structures in non‐ferroelectric monolayers caused by defects in 2D materials. The first‐principles calculations show that an isolated polar meron with a diameter < 3.0 nm is generated in the deficient lead chalcogenide monolayer, and its formation is attributed to the synergic effects of vacancy‐induced radial atomic displacements and symmetry reduction in 2D materials. The emergent polar meron can transform to rich topological configurations under external stimuli or by manipulation of the defect concentrations. Furthermore, this strategy of atomic‐scale symmetry breaking via point defect engineering can be applied to a wide variety of 2D materials to induce polar topological structures. This work generalizes the polar topology from perovskite oxides to 2D materials, facilitating exciting opportunities to create high‐density topological configurations that enable the exploration of meron/skyrmion‐based functional nanodevices.

Flavor Symmetry of Hydrogen Atoms Potentially Affecting the Proton Radius Deduced from the Electron-Hydrogen Scattering

Precise knowledge of such fundamental quantity as the proton charge radius rp is extremely important both for the quantum chromodynamics (for quark-gluon structure) and for atomic physics (for atomic hydrogen spectroscopy). Yet the ambiguity in measuring rp persists for over a dozen of years by now—from the time when in 2010 the muonic hydrogen spectroscopy experiment yielded rp ≈ 0.84 fm in contrast to the form factor experiment by the Mainz group that produced rp ≈ 0.88 fm. Important was that this difference corresponded to about seven standard deviations and therefore was inexplicable. In the intervening dozen of years, more experiments of various kinds were performed in this regard. Nevertheless, the controversy remains, which is why several different types of new experiments are being prepared for measuring rp. In one of our previous papers, we pointed out the factor that was never taken into account by the corresponding research community: the flavor symmetry of electronic hydrogen atoms, whose existence was confirmed by four kinds of atomic or molecular experiments and also evidenced by two kinds of astrophysical observations. Specifically, in that paper there was discussed the possible presence of the second flavor of muonic hydrogen atoms (in the corresponding experimental gas) and its effect on the shift of the ground state of muonic hydrogen atoms due to the proton finite size. In the present paper we analyze the effect of the flavor symmetry of electronic hydrogen atoms on the corresponding elastic scattering cross-section and on the proton charge radius rp deduced from the cross-section. As an example, we use our analytical results for reconciling two distinct values of rp obtained in different elastic scattering experiments: 0.88 fm and 0.84 fm (which is by about 4.5% smaller than 0.88 fm). We show that if the ratio of the second flavor of hydrogen atoms to the usual hydrogen atoms in the experimental gas would be about 0.3, then the extraction of rp from the corresponding cross-section would yield by about 4.5% smaller value of rp compared to its true value. We also derive the corresponding general formulas that can be used for interpreting the future electronic and muonic experiments.

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia.

The electrochemical conversion of nitrate pollutants into value-added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate-to-ammonia reduction reaction (NO3-RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single-atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe-N/P-C) as a NO3-RR catalyst electrode. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single-Fe-atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO3-RR process. The Fe-N/P-C catalyst exhibits 90.3% ammonia Faradaic efficiency with a yield rate of 17980 μg h-1 mgcat-1, greatly outperforming the reported Fe-based catalysts. Furthermore, operando SR-FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO3-RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the enhanced function of heteroatom doping on single atoms for NO3-RR, and provides an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia

AbstractThe electrochemical conversion of nitrate pollutants into value‐added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate‐to‐ammonia reduction reaction (NO3−RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single‐atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe−N/P−C) as a NO3−RR electrocatalyst. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single‐Fe‐atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO3−RR process. The Fe−N/P−C catalyst exhibits 90.3 % ammonia Faradaic efficiency with a yield rate of 17980 μg h−1 mgcat−1, greatly outperforming the reported Fe‐based catalysts. Furthermore, operando SR‐FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO3−RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the critical role of atomic‐level precision modulation by heteroatom doping for the NO3−RR, providing an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.

Is Pseudohalide CN− a Real Halide? A General Symmetry Consideration

Recently, in light of the significant attention devoted to pseudohalide CN− and cyano radical CN physico-chemical property studies and superhalide behavior exploration in CN−-ligated metal compounds, the photoelectron angular distribution nature of pseudohalide CN− has been directly demonstrated via the photoelectron velocity map imaging technique to be comparable to Cl−. For the halide Cl−, photoelectrons were observed at 266 nm (4.66 eV) to peak, perpendicular to the laser polarization associated with the detachment of p-orbital symmetry. For the analogous pseudohalide CN−, photoelectrons were present at a peak in laser polarization at 266 nm, which can be explained as detachment from mainly atomic s-like orbital symmetry. Although both are often regarded as having the same high electron affinity and similarly strong chemical bonding capabilities to stabilize complexes, their photoelectron angular distributions are distinctly different, which indicates their intrinsically different electronic–structure symmetry (HOMO nature). The approach based on symmetry consideration in this work could be utilized to explain the photoelectron angular distributions of pseudohalide and classic halide ligands via the advanced photoelectron velocity map imaging tool.

Interfacial Reconstructed Layer Controls the Orientation of Monolayer Transition-Metal Dichalcogenides.

Growing continuous monolayer films of transition-metal dichalcogenides (TMDs) without the disruption of grain boundaries is essential to realize the full potential of these materials for future electronics and optoelectronics, but it remains a formidable challenge. It is generally believed that controlling the TMDs orientations on epitaxial substrates stems from matching the atomic registry, symmetry, and penetrable van der Waals forces. Interfacial reconstruction within the exceedingly narrow substrate-epilayer gap has been anticipated. However, its role in the growth mechanism has not been intensively investigated. Here, we report the experimental conformation of an interfacial reconstructed (IR) layer within the substrate-epilayer gap. Such an IR layer profoundly impacts the orientations of nucleating TMDs domains and, thus, affects the materials' properties. These findings provide deeper insights into the buried interface that could have profound implications for the development of TMD-based electronics and optoelectronics.

Influence of monohalogenated flexible tails on the mesomorphism of triphenylene-based discotics

A series of triphenylene-based discotic liquid crystals (DLCs) modified by fluorination or chlorination at the end of peripheral flexible chains were examined in this work. DLC derivatives containing one mono fluorine or chlorine-substituted flexible chain 4a-e and six mono fluorine or chlorine-substituted flexible chains 8a & 8b, 10a & 10b were mainly synthesized and molecular structures confirmed by 1H NMR, 19F NMR and MALDI-TOF MS. The mesogenic properties of the obtained compounds were further studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). Impressively, compounds 4a-e were found exhibiting hexagonal columnar phases and the π-π stacking distances of 4a-e were lower than that of the parent molecule HAT5. Due to the influence of atomic radius and molecular symmetry, the mesophase stability of the fluorinated triphenylene compounds was greater than the chlorinated ones. Through the modification of the side chain, the self-assembly behavior of triphenylene was stronger and the intracolumnar distance was decreased, which may give rise to a higher charge transport mobility. Compound 10b showed self-assembly behavior and formed a spherulite texture that could also remain orderly until room temperature. This is rare in reported DLCs and it might be applied as active components of in electronic devices. These ordered stacked discotic molecules may exhibit improved charge carrier mobility as semiconducting materials.

AisNet: A Universal Interatomic Potential Neural Network with Encoded Local Environment Features.

This paper proposes a new interatomic potential energy neural network, AisNet, which can efficiently predict atomic energies and forces covering different molecular and crystalline materials by encoding universal local environment features, such as elements and atomic positions. Inspired by the framework of SchNet, AisNet consists of an encoding module combining autoencoder with embedding, the triplet loss function and an atomic central symmetry function (ACSF), an interaction module with a periodic boundary condition (PBC), and a prediction module. In molecules, the prediction accuracy of AisNet is comparabel with SchNet on the MD17 dataset, mainly attributed to the effective capture of chemical functional groups through the interaction module. In selected metal and ceramic material datasets, the introduction of ACSF improves the overall accuracy of AisNet by an average of 16.8% for energy and 28.6% for force. Furthermore, a close relationship is found between the feature ratio (i.e., ACSF and embedding) and the force prediction errors, exhibiting similar spoon-shaped curves in the datasets of Cu and HfO2. AisNet produces highly accurate predictions in single-commponent alloys with little data, suggesting the encoding process reduces dependence on the number and richness of datasets. Especially for force prediction, AisNet exceeds SchNet by 19.8% for Al and even 81.2% higher than DeepMD on a ternary FeCrAl alloy. Capable of processing multivariate features, our model is likely to be applied to a wider range of material systems by incorporating more atomic descriptions.

Collective radiance of giant atoms in non-Markovian regime

We investigate the non-Markovian dynamics of two giant artificial atoms interacting with a continuum of bosonic modes in a one-dimensional (1D) waveguide. Based on the diagrammatic method, we present the exact analytical solutions, which predict the rich phenomena of collective radiance. For the certain collective states, the decay rates are found to be far beyond that predicted in the the Dicke model and standard Markovian framework, which indicates the occurrence of super-superradiance. The superadiance-to-subradiance transition could be realized by adjusting the exchange symmetry of giant atoms. Moreover, there exists multiple bound states in continuum (BICs), with photons/phonons bouncing back and forth in the cavity-like geometries formed by the coupling points. The trapped photons/phonons in the BICs can also be re-released conveniently by changing the energy level splitting of giant atoms. The mechanism relies on the joint effects of the coherent time-delayed feedback and the interference between the coupling points of giant atoms. This work fundamentally broadens the fields of giant atom collective radiance by introducing non-Markovianity. It also paves the way for a clean analytical description of nonlinear open quantum system with more complex retardation.

Atomic symmetry alteration in carbon nitride to modulate charge distribution for efficient photocatalysis

The periodical distribution of N and C atoms in the carbon nitride skeleton results in intrinsically insufficient light absorption and serious carrier recombination. Herein, an efficient two-step cystine-mediated strategy was developed to alter the structure symmetry of C3N4 via the introduction of alkyl groups and nitrogen vacancies. The experimental analysis and theoretical calculation confirm that the formation of alkyl groups and nitrogen vacancies can modulate band structure and activate n-π* electron transition. Especially, the charge density in CN-25CYS is redistributed with spatial separation of oxidation and reduction sites, suppressing photogenerated charge recombination effectively. Therefore, the distorted carbon nitride (CN-25CYS) exhibits 9.6-times and 15.6-times higher photoreaction rates in hydrogen production and RhB degradation than the pristine one (CN-0CYS), respectively.