Observing the Role of Electron Delocalization in Electronic Transport by Incorporating Actinides into Ligated Metal-Chalcogenide Superatoms.

Since delocalization of electronic states is a prerequisite for exerting unique electron transport properties, early actinides (An) with highly delocalized 5f/6d orbitals are natural candidates. However, given the experimental difficulties of such radioactive compounds and the complex relativistic effects in theoretical studies, understanding the electronic structure and bonding of actinides is underdeveloped on the periodic table. A further challenge is the very complicated electronic structures encountered in the confinement of actinides, as vividly illustrated by the weakly radioactive Th(Thorium)-encapsulated metal chalcogenide clusters, Th@Co6Te8L6 (L = PH3, PMe3, PEt3). Here we report the electronic structure and the electron transport properties of the Th@Co6Te8L6 clusters and compare them with those of the hollow Co6Te8L6 clusters using the nonequilibrium Green's function combined with relativistic density functional theory (NEGF-DFT). We found that the equilibrium conductance in Th@Co6Te8(PH3)6 (0.76 G0) has been greatly improved over that in Co6Te8(PH3)6 (0.03 G0), which has also been verified under an applied different bias voltage. The covalent bonding character between 6d (Th) and 3d (Co) atomic orbitals resulting from steric confinement is the source of the performance enhancement and a most important factor governing the accessibility of such 5f/6d orbitals. The results are of significance to the rapidly developing field of molecular nanoelectronics.

Broadening the targetable space: engineering and discovery of PAM-flexible Cas proteins.

The application of CRISPR-Cas systems has been hindered by their requirement for long protospacer-adjacent motifs (PAMs). Recent engineering and discovery of PAM-flexible Cas proteins have substantially broadened the targetable DNA sequence space, thereby facilitating genome editing and improving derivative technologies such as gene regulation, seamless cloning, and large-scale genetic screens.

Design and Implementation of Broadband Hybrid 3-dB Couplers with Silicon-Based IPD Technology.

Heterogeneous integration (HI) is a rapidly developing field aimed at achieving high-density integration and miniaturization of devices for complex practical radio frequency (RF) applications. In this study, we present the design and implementation of two 3 dB directional couplers utilizing the broadside-coupling mechanism and silicon-based integrated passive device (IPD) technology. The type A coupler incorporates a defect ground structure (DGS) to enhance coupling, while type B employs wiggly-coupled lines to improve directivity. Measurement results demonstrate that type A achieves <-16.16 dB isolation and <-22.32 dB return loss with a relative bandwidth of 60.96% in the 6.5-12.2 GHz range, while type B achieves <-21.21 dB isolation and <-23.95 dB return loss in the first band at 7-13 GHz, <-22.17 dB isolation and <-19.67 dB return loss in the second band at 28-32.5 GHz, and <-12.79 dB isolation and <-17.02 dB return loss in the third band at 49.5-54.5 GHz. The proposed couplers are well suited for low cost, high performance system-on-package radio frequency front-end circuits in wireless communication systems.

Decoding hexanitrobenzene (HNB) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as two distinctive energetic nitrobenzene compounds by machine learning.

Energetic materials (EMs) are a group of special energy materials, and it is generally full of safety risks and generally costs much to create new EMs. Thus, machine learning (ML)-aided discovery becomes highly desired for EMs, as ML is good at risk and cost reduction. This work decodes hexanitrobenzene (HNB) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as two distinctive energetic nitrobenzene compounds by ML, in combination with theoretical calculations. Based on a series of highly accurate models of density, heat of formation, bond dissociation energy and molecular flatness, the ML predictions show that HNB is the most energetic among ∼370 000 000 single benzene ring-containing compounds, while TATB possesses a moderate energy content and very high safety, as determined experimentally. This work exhibits the significant power of ML and presents an instructive procedure for using it in the field of EMs. The ML-aided design and highly efficient synthesis and fabrication combined strategy is expected to accelerate the discovery of new EMs.

High-Stability Light-Element Magnetic Superatoms Determined by Hund's Rule.

Achieving stable high-magnetism light-element structures at nanoscale is vital to the field of magnetism, which has traditionally been ruled by transition-metal elements with localized d or f electrons. By first-principles calculations, we show that superatoms made of pure earth-abundant light elements (i.e., boron and nitrogen) exhibit desired magnetic properties that rival those of rare-earth elements, and the magnetism is dictated entirely by Hund's maximum spin rule. Importantly, the chemical and structural stabilities of the superatoms are not jeopardized by its high spins and are in fact better than those of transition-metal-element-embedded clusters. Our work thus establishes the basic principles for designing novel light-element, high-stability, and high-moment magnetic superatoms.

Ruthenium Complex of sp2 Carbon-Conjugated Covalent Organic Frameworks as an Efficient Electrocatalyst for Hydrogen Evolution.

It is still a great challenge to explore hydrogen evolution reaction (HER) electrocatalysts with both lower overpotential and higher stability in acidic electrolytes. In this work, an efficient HER catalyst, Ru@COF-1, is prepared by complexation of triazine-cored sp2 carbon-conjugated covalent organic frameworks (COFs) with ruthenium ion. Ru@COF-1 possesses high crystallinity and porosity, which are beneficial for electrocatalysis. The large specific surface area and regular porous channels of Ru@COF-1 facilitate full contact between reactants and catalytic sites. The nitrogen atoms of triazines are protonated in the acidic media, which greatly improve the conductivity of Ru@COF-1. This synergistic effect makes the overpotential of Ru@COF-1 about 200 mV at 10 mA cm-2 , which is lower than other reported COFs-based electrocatalysts. Moreover, Ru@COF-1 exhibits exceptionally electrocatalytic durability in the acidic electrolytes. It is particularly stable and remains highly active after 1000 cyclic voltammetry cycles. Density functional theory calculations demonstrate that tetracoordinated Ru-N2 Cl2 moieties are the major contributors to the outstanding HER performance. This work provides a new idea for developing protonated HER electrocatalysts in acidic media.

High-pressure phases of a Mn-N system.

Highly compressed extended states of light elemental solids have emerged recently as a novel group of energetic materials. The application of these materials is seriously limited by the energy-safety contradiction, because the material with high energy density is highly metastable and can hardly be recovered under ambient conditions. Recently, it has been found that high-energy density transition metal polynitrides could be synthesized at ∼100 GPa and recovered at ∼20 GPa. Inspired by these findings, we have studied a high-pressure Mn-N system from the aspects of structure, stability, phase transition, energy density and electronic structure theoretically for the first time. The results reveal that MnN4_P1̄ consisting of [N4]∞2- is thermodynamically stable at 36.9-100 GPa, dynamically stable at 0 GPa and has a noticeably high volumetric energy density of 15.71 kJ cm-3. Upon decompression, this structure will transform to MnN4_C2/m with the transition barrier declining sharply at 5-10 GPa due to the switching of transition pathways. Hence, we propose MnN4_P1̄ as a potential energetic material that is synthesizable above 40 GPa and recoverable until 10 GPa.

Toward a Comprehensive Understanding of Mode-Specific Dynamics of Polyatomic Reactions: A Full-Dimensional Quantum Dynamics Study of the H + NH3 Reaction.

Mode specificity not only sheds light on reaction dynamics but also opens the door for chemical reaction control. This work reports a state-of-the-art full-dimensional quantum dynamics study on the prototypical hydrogen abstraction reaction of hydrogen with ammonia, which serves as a benchmark for advancing our fundamental understanding of polyatomic reaction dynamics. By taking advantage of the (3 + 1) Radau-Jacobi coordinates, the bond-specific probabilities are resolved with the reactant NH3 initiated from either a non-degenerate or degenerate stretching vibrational state. The observed different atom-specific abstraction probabilities from individual states of the degenerate pair are rationalized in the local mode representation according to the different vibrational energy deposited in each N-H bond. It is verified that the three H atoms in NH3 have the same abstraction probability as that from the degenerate pair and the linear combination of the degenerate pair gives the same reaction probability. In addition, the symmetric and asymmetric stretching modes of the reactant NH3 have comparable efficacies on driving the reaction.

Fast 3D Indoor Scene Synthesis by Learning Spatial Relation Priors of Objects.

We present a framework for fast synthesizing indoor scenes, given a room geometry and a list of objects with learnt priors. Unlike existing data-driven solutions, which often learn priors by co-occurrence analysis and statistical model fitting, our method measures the strengths of spatial relations by tests for complete spatial randomness (CSR), and learns discrete priors based on samples with the ability to accurately represent exact layout patterns. With the learnt priors, our method achieves both acceleration and plausibility by partitioning the input objects into disjoint groups, followed by layout optimization using position-based dynamics (PBD) based on the Hausdorff metric. Experiments show that our framework is capable of measuring more reasonable relations among objects and simultaneously generating varied arrangements in seconds compared with the state-of-the-art works.

Remote Passivation in Two-Dimensional Materials: The Case of the Monolayer-Bilayer Lateral Junction of MoSe2.

Two-dimensional (2D) monolayer-bilayer (ML-BL) lateral junctions (LJs) have recently attracted attention due to their straightforward synthesis and resulting clean interface. Such systems consist of an extended ML with a secondary layer present only over half of the system, leading to an interface that is associated with the terminating edge of the secondary half layer. Our first-principles calculations reveal that the edges of the half layer completely lack reconstruction in the presence of unintentional dopants, in this case, Re. This observation is in startling contrast to the known physics of three-dimensional (3D) semiconductor surfaces where reconstruction has been widely observed. Herein, the electrostatics of the reduced dimensionality allows for greater separation between compensating defects, enabling dopants to remotely passivate edge states without needing to directly participate in the chemistry.

Constructing the bonding interactions between endohedral metallofullerene superatoms by embedded atomic regulation.

We present a possible principle that controls intercluster bonding through embedding different kinds of actinide atoms into the centre of fullerenes, thereby exhibiting different bonding forms. Moreover, these superatoms maintain the robustness of electronic structures.

Energy density of high-pressure nitrogen-rich MNx compounds.

In the past decade, a large number of nitrogen-rich MNx compounds have been discovered under high-pressure conditions. In this work, we have evaluated the energy densities of MNx structures with thermodynamic and dynamical stability through first-principles calculations. The results show that the energy densities of MNx consisting of alkali metals and cyclo-N5- are less than ∼0.5 TNT equivalence, whereas the group-III metal nitrides have high-energy density regardless of the type of nitrogen oligomers in the structures. To clarify the energy density difference for MNx composed of different impurities, bivariate Pearson correlation analysis is performed, which reveals that the high-energy density of MNx is related to the large N density, small M atomic radius, short M-N bond length, small MNx ionicity, long N-N bond length and large M formal oxidation state. According to this correlation, H, Be, B, Al, Si and P elements have been proposed as the candidate impurities to synthesize high-energy density MNx.

Interaction potential energy surface between superatoms.

In this work, we report the potential energy surface between superatoms (SPES) based on first-principles theory. The calculations show that superatoms have SPES crossing behavior between different electronic states similar to an atomic potential energy surface (PES). However, unlike atoms, the relative rotation between superatoms results in new dimensions for the SPES. The rotation is divided into two types, around the direction of the line between two superatoms and perpendicular to it. At the equilibrium distance, the former rotation results in maximum energy and charge changes of 0.03 eV and 10-2 e respectively. However, the latter rotation yields changes that are 17 times and 7 times those of the former. These findings help promote the understanding of intermolecular interactions, and will contribute to the bottom-up superatomic-based assembly of novel materials.

Bonding properties of a superatom system with high-Z elements: insights from energy decomposition analysis.

Superatoms with high-Z elements have novel physicochemical properties, and a comprehensive and thorough view of their bonding properties plays a crucial role in the design of superatoms. Now, energy decomposition analysis shows increasingly prominent performance for understanding inter- and intra-molecular interactions, so the bonding properties of typical superatoms with high-Z elements, Th@Au14, have been investigated here. It is found that under different electron occupation types of the fragments, the electrostatic interaction energy, polarization, and exchange repulsion energy change significantly in their intramolecular interaction energy components, resulting in quantitative or even qualitative differences in their main interaction energy. This indicates that the bonding properties of fragments are related to their electronic structures, and even has extraordinary reference value for the future regulation and control of interactions in superatoms with high-Z elements, which has great significance for superatom development.

Photoinduced Vacancy Ordering and Phase Transition in MoTe2.

We show that non-equilibrium dynamics plays a central role in the photoinduced 2H-to-1T' phase transition of MoTe2. The phase transition is initiated by a local ordering of Te vacancies, followed by a 1T' structural change in the original 2H lattice. The local 1T' region serves as a seed to gather more vacancies into ordering and subsequently induces a further growth of the 1T' phase. Remarkably, this process is controlled by photogenerated excited carriers as they enhance vacancy diffusion, increase the speed of vacancy ordering, and are hence vital to the 1T' phase transition. This mechanism can be contrasted to the current model requiring a collective sliding of a whole Te atomic layer, which is thermodynamically highly unlikely. By uncovering the key roles of photoexcitations, our results may have important implications for finely controlling phase transitions in transition metal dichalcogenides.

An effective method to make polymers degrade readily: spatial isomerization.

The widespread application of hydrocarbon polymers has spurred an increasing interest in the study of their degradation mechanism. In general, the chemical inertness of polymers makes their degradation by low-energy processes a challenging problem. Herein, we report a method of spatial isomerization to make polymers degrade easily. The first-principles calculations show that the energy barrier required for degradation reaction is directly related to the spatial arrangement of the polymer, with the isotactic structure and most atactic structures being easier to degrade than the syndiotactic structure. Therefore, a new way to accelerate the degradation by achieving spatial isomerization of polymers has been proposed. Furthermore, the synthesis rates of these structures have also been calculated to support future experiments.

Binding for endohedral-metallofullerene superatoms induced by magnetic coupling.

To design new materials based on artificial superatoms, clarifying their involved interaction is particularly important. In this study, we discuss first-principle calculations to show that the interaction between endohedral metallofullerenes (EMFs) of U@C28 can lead to different chemical and physical adsorption structures. Especially, these structures are derived from different magnetic coupling resonances, and they can transform by changing the distance between U@C28 superatoms. These findings will promote the future development for bottom-up assembling of new functional materials and even devices.

Actinide embedded nearly planar gold superatoms: structural properties and applications in surface-enhanced Raman scattering (SERS).

Planarity is a special property of superatoms, different from atoms. In this work, we predicted a series of nearly planar structures, An@Au6 (An = Ac-1, Th, Pa+1) clusters, using density functional theory (DFT). Calculations of these actinide embedded clusters reveal a 10-electron (1s21p41d4) closed-shell singlet configuration. It is found that all An@Au6 clusters are nearly or purely planar structures with only in-plane two-dimensional occupied superatomic molecular orbitals (SAMOs). In addition, applying them as surface-enhanced Raman scattering (SERS) substrates, the charge-transfer (CT) states at 677 nm (1dmetal-π1*pyridine) can lead to a SERS signal enhancement of 104 for a pyridine-Th@Au6 complex. Our research indicates that actinide embedded nearly planar superatomic clusters have unique optical properties and potential application value.

Remote epitaxy of copper on sapphire through monolayer graphene buffer.

In this work, we show that remote heteroepitaxy can be achieved when Cu thin film is grown on single crystal, monolayer graphene buffered sapphire(0001) substrate via a thermal evaporation process. X-ray diffraction and electron backscatter diffraction data show that the epitaxy process forms a prevailing Cu crystal domain, which is remotely registered in-plane to the sapphire crystal lattice below the monolayer graphene, with the (111) out-of-plane orientation. As a poor metal with zero density of states at its Fermi level, monolayer graphene cannot totally screen out the stronger charge transfer/metallic interactions between Cu and substrate atoms. The primary Cu domain thus has good crystal quality as manifested by a narrow crystal misorientation distribution. On the other hand, we show that graphene interface imperfections, such as bilayers/multilayers, wrinkles and interface contaminations, can effectively weaken the atomic interactions between Cu and sapphire. This results in a second Cu domain, which directly grows on and follows the graphene hexagonal lattice symmetry and orientation. Because of the weak van der Waals interaction between Cu and graphene, this domain has inferior crystal quality. The results are further confirmed using graphene buffered spinel(111) substrate, which indicates that this remote epitaxial behavior is not unique to the Cu/sapphire system.

Quantum oscillation in carrier transport in two-dimensional junctions.

Two-dimensional (2D) junction devices have recently attracted considerable attention. Here, we show that most 2D junction structures, whether vertical or lateral, act as a lateral monolayer-bilayer-monolayer junction in their operation. In particular, a vertical structure cannot function as a vertical junction as having been widely believed in the literature. Due to a larger electrostatic screening, the bilayer region in the junction always has a smaller bandgap than its monolayer counterpart. As a result, a potential well, aside from the usual potential barrier, will form universally in the bilayer region to affect the hole or electron quantum transport in the form of transmission or reflection. Taking black phosphorus as an example, our calculations using a non-equilibrium Green function combined with density functional theory show a distinct oscillation in the transmission coefficient in a two-electrode prototypical device, and the results can be qualitatively understood using a simple quantum well model.