Dual Stabilization of a Tri-Metallofullerene Radical Er3@C80: Exohedral Derivatization and Endohedral Three-Center Bonding.

The enclosed space within fullerene molecules, capable of trapping metal clusters, offers an opportunity to investigate the behavior of metal atoms in a highly confined sub-nanometer environment. However, the studies on trimetallofullerenes M3@C80 have been very limited due to their difficult obtainability. In this paper, we present a new method for obtaining a tri-metallofullerene Er3@C80 through exohedral modification of the fullerene cage. Our findings reveal that Er3@C80 exhibits a radical character and can react with the dichlorobenzene radical to generate a stable derivative Er3@C80PhCl2. Theoretical calculations demonstrate the presence of a three-center two-electron metal-metal bond in the center of the fullerene cage. This bond serves to counterbalance the Coulomb repulsion between the Er ions. Consequently, both exohedral derivatization and endohedral three-center bonding contribute to the substantial stability of Er3@C80PhCl2. Furthermore, molecular dynamics simulations indicate that the Er3 cluster within the molecule possesses a rigid triangle structure. The availability of M3@C80 derivatives opens avenues for future investigations into interactions among metal atoms, such as magnetic coupling, within fullerene cages.

First principles study of a triazine-based covalent organic framework as a high-capacity anode material for Na/K-ion batteries.

The rational design of high-performance anode materials is crucial for the development of rechargeable Na-ion batteries (NIBs) and K-ion batteries (KIBs). In this study, based on density functional theory (DFT) calculations, we have systematically investigated the possibility of a bilayer triazine-based covalent organic framework (bilayer TCOF) as an anode for NIBs and KIBs. The calculation of the electronic band structure shows that the bilayer TCOF is a direct band gap semiconductor with a band gap of 2.01 eV. After the adsorption of Na/K at the most favorable sites, the bilayer TCOF transitions from a semiconductor to a metal state, guaranteeing good electronic conductivity. The low diffusion barriers of the bilayer TCOF are 0.45 and 0.26 eV, respectively, indicating a fast diffusion rate of Na/K ions. In addition, the bilayer TCOF has a theoretical storage capacity of up to 628 mA h g-1. Finally, it is found that the average voltage of the bilayer TCOF for NIBs and KIBs is 0.53 and 0.48 V, respectively. Based on these results, we can conclude that the bilayer TCOF may be a suitable anode material for NIBs and KIBs.

Stability and Electronic Properties of Mixed Rare-Earth Tri-Metallofullerenes YxDy3-x@C80 (x = 1 or 2).

Tri-metallofullerenes, specifically M3@C80 where M denotes rare-earth metal elements, are molecules that possess intriguing magnetic properties. Typically, only one metal element is involved in a given tri-metallofullerene molecule. However, mixed tri-metallofullerenes, denoted as M1xM23-x@C80 (x = 1 or 2, M1 and M2 denote different metal elements), have not been previously discovered. The investigation of such mixed tri-metallofullerenes is of interest due to the potential introduction of distinct properties resulting from the interaction between different metal atoms. This paper presents the preparation and theoretical analysis of mixed rare-earth tri-metallofullerenes, specifically YxDy3-x@C80 (x = 1 or 2). Through chemical oxidation of the arc-discharge produced soot, the formation of tri-metallofullerene cations, namely Y2Dy@C80+ and YDy2@C80+, has been observed. Density functional theory (DFT) calculations have revealed that the tri-metallofullerenes YxDy3-x@C80 (x = 1 or 2) exhibit a low oxidation potential, significantly lower than other fullerenes such as C60 and C70. This low oxidation potential can be attributed to the relatively high energy level of a singly occupied orbital. Additionally, the oxidized species demonstrate a large HOMO-LUMO gap similar to that of YxDy3-xN@C80, underscoring their high chemical stability. Theoretical investigations have uncovered the presence of a three-center two-electron metal-metal bond at the center of Y2DY@C80+ and YDy2@C80+. This unique multi-center bond assists in alleviating the electrostatic repulsion between the metal ions, thereby contributing to the overall stability of the cations. These mixed rare-earth tri-metallofullerenes hold promise as potential candidates for single-molecule magnets.

Antiaromaticity-aromaticity transition of cyclo[16]carbon upon metal encapsulation.

In contrast to aromatic compounds with particular stability, antiaromatic compounds are usually less stable due to their high reactivity and unfavorable formation energies. Cyclo[16]carbon (C16) is a carbon ring molecule with a dual antiaromatic character. In this study, we demonstrate that C16 can be transformed into highly aromatic molecules upon metal encapsulation. The geometrical characteristics, electronic properties and thermodynamic stability of MC16 compounds (M = Ca, Sc, Ti, V, Ce, U) are fully investigated from a theoretical perspective. Based on natural population analysis, atom-in-molecules theory and localized molecular orbital analysis, the nature of the metal-carbon interaction in the MC16 compounds is investigated. It has been proved that the bonding between Ca and C16 corresponds to a typical ionic interaction, while other metal atoms form polar covalent bonds with C16. By analyzing the frontier molecular orbitals and magnetic response of MC16, we have found that all the encapsulated metal atoms donate two electrons to the in-plane π orbitals via either electron transfer or orbital hybridization, which makes the in-plane π orbitals completely satisfy the 4n + 2 (n = 4) Hückel aromaticity rule. The U atom formally transfers four electrons to the carbon ring in total, two to the in-plane π orbitals and two to the out-of-plane π orbitals, which results in the remarkable dual aromaticity feature of UC16. The transformation of aromaticity can be utilized to develop new strategies for the synthesis of novel carbon ring molecules.

Capturing unconventional metallofullerene M@C60 through activation of the unreactive [5,6] bond toward Diels-Alder reaction.

Full control of the regioselectivity in the functionalization of fullerenes is important for production of fullerene derivatives with desirable properties. Cycloaddition reactions of C60 usually take place at the hexagon-hexagon ring junction, i.e. the [6,6] bond of the fullerene cage, whereas the [5,6] bond is generally unreactive. The activation of the [5,6] bond toward Diels-Alder reactions is difficult because of its longer bond length than the [6,6] bond. In this study, we computationally demonstrate that the [5,6] bond of C60 can be efficiently activated by encapsulation of a divalent metal atom such as Ca or Sm. Electron transfer from the metal atom to the fullerene cage and the interaction between the metal cation and the cage play critical roles in enhancing the reactivity of the [5,6] bond. The physical origin of the reactivity enchancement of the [5,6] bond is investigated quantitatively by using the activation strain model and the energy decomposition method. The change in the orbital interaction energy along the intrinsic reaction coordinate has a major effect on the thermodynamics and kinetics of the reactions between Ca@C60 and cyclopentadiene. Both mono- and bis-addition reactions of cyclopentadiene with Ca@C60 prefer to take place at the [5,6] bonds of the fullerene cage thermodynamically, which is distinct from the case of pristine C60. The HOMO-LUMO energy gap of Ca@C60 is remarkably enlarged upon mono- and bis-functionalization with cyclopentadienes. Therefore, the covalent derivatization strategy can be used to capture the unconventional, missing metallofullerene M@C60.

First-principles studies of a two-dimensional electron gas at the interface of polar/polar LaAlO3/KNbO3 superlattices.

We explored the possibility of producing a two-dimensional electron gas (2DEG) in polar/polar (LaAlO3)m/(KNbO3)n perovskite superlattices that have N type and P type interfaces using the first-principles electronic structure calculations. Two different kinds of LaAlO3/KNbO3 superlattices were constructed, namely stoichiometric NP superlattice (NP-SL) and non-stoichiometric NN superlattice (NN-SL). We discovered that the NP-SL undergoes a transition from an insulating to a metallic state when LaAlO3 has more than 3 unit cells. This reveals the completely spin-polarized two-dimensional hole gas (2DHG), as well as 2DEG with an interfacial charge carrier density of n ∼ 1013 cm-2 and an electron effective mass of 0.240me (for 5 unit cells of LaAlO3). In comparison, the NN-SL is intrinsically metallic, and when LaAlO3 has 4.5 unit cells, the structure shows a 2DEG with a larger density (n ∼ 1014 cm-2) and a smaller electron effective mass (0.185me). In addition, the charge carrier properties are highly sensitive to the number of LaAlO3 unit cells in the NP-SL model, while the size effect of LaAlO3 is negligible for the NN-SL one. Our results demonstrate that electronic reconstruction at the interfaces of the stoichiometric structure can produce both the 2DHG and 2DEG, whereas extra electrons are introduced to form solely the 2DEG at the non-stoichiometric structure interfaces. This research provides fundamental insights into the different interfacial electronic properties and the primary mechanism responsible for the formation of polar/polar heterojunction LaAlO3/KNbO3 superlattices.

First-principles study of Ga-vacancy induced magnetism in ß-Ga2O3.

First principles calculations based on density functional theory were performed to study the electronic structure and magnetic properties of ß-Ga2O3 in the presence of cation vacancies. We investigated two kinds of Ga vacancies at different symmetry sites and the consequent structural distortion and defect states. We found that both the six-fold coordinated octahedral site and the four-fold coordinated tetrahedral site vacancies can lead to a spin polarized ground state. Furthermore, the calculation identified a relationship between the spin polarization and the charge states of the vacancies, which might be explained by a molecular orbital model consisting of uncompensated O2- 2p dangling bonds. The calculations for the two vacancy systems also indicated a potential long-range ferromagnetic order which is beneficial for spintronics application.

Estimating Shape and Micro-Motion Parameter of Rotationally Symmetric Space Objects from the Infrared Signature.

Shape serves as an important additional feature for space target classification, which is complementary to those made available. Since different shapes lead to different projection functions, the projection property can be regarded as one kind of shape feature. In this work, the problem of estimating the projection function from the infrared signature of the object is addressed. We show that the projection function of any rotationally symmetric object can be approximately represented as a linear combination of some base functions. Based on this fact, the signal model of the emissivity-area product sequence is constructed, which is a particular mathematical function of the linear coefficients and micro-motion parameters. Then, the least square estimator is proposed to estimate the projection function and micro-motion parameters jointly. Experiments validate the effectiveness of the proposed method.

Manipulating Hetero-Nanowire Films for Flexible and Multifunctional Thermoelectric Devices.

Flexible thermoelectric devices hold significant promise in wearable electronics owing to their capacity for green energy generation, temperature sensing, and comfortable wear. However, the simultaneous achievement of excellent multifunctional sensing and power generation poses a challenge in these devices. Here, ordered tellurium-based hetero-nanowire films are designed for flexible and multifunctional thermoelectric devices by optimizing the Seebeck coefficient and power factor. The obtained devices can efficiently detect both object and environment temperature, thermal conductivity, heat proximity, and airflow. In addition, combining the thermoelectric units with radiative cooling materials exhibits remarkable thermal management capabilities, preventing device overheating and avoiding degradation in power generation. Impressively, this multifunctional electronics exhibits excellent resistance in extreme low earth orbit environments. The fabrication of such thermoelectric devices provides innovative insights into multimodal sensing and energy harvesting.

Redox Modulation of the Reactivity and Regioselectivity in Diels-Alder Reaction of Metallofullerene La@C82 with Cyclopentadiene.

Modulation of the reactivity of metallofullerenes is critical for production of metallofullerene derivatives with desired properties and functionalities. In this study, we investigate the effects of reduction and oxidation on the reactivity and regioselectivity in Diels-Alder reaction of metallofullerene La@C82 by means of density functional theory calculations. Because of the enhanced electron-deficiency characteristic upon oxidation, the oxidized metallofullerene exhibits higher thermodynamic and kinetic reactivity as compared with neutral La@C82 . The regioselectivity in the reaction of La@C82 with cylcopentadiene is remarkably changed after oxidation of the metallofullerene, which is explained in terms of the changes in the geometrical structure and the electronic structure of the metallofullerene. Quantitative analysis based on the activation-strain model demonstrates that the low activation energy barrier for the reaction of the cation La@C82 + with cyclopentadiene originates from small strain energy and large interaction energy between the reactants. Energy decomposition analysis on the transition states of the reactions reveals that the exchange-repulsion interaction energy is one of the critical factors that determine the kinetic reactivity of the metallofullerene. This study not only provides new theoretical insights on how to modulate the reactivity of metallofullerenes, but also offers guideline for future experimental synthesis of new metallofullerene derivatives.

Activation of the Unreactive Bond in C70 Fullerene toward Diels-Alder Reaction by Encapsulation of a Lithium Atom.

Diels-Alder cycloaddition reaction is useful for generation of covalent derivatives of fullerenes. Diels-Alder reactions of C70 and dienes usually take place at the carbon-carbon bond that has a short bond length in C70 , while the bonds with long lengths are generally unreactive. In this paper, we investigated the reactivities of Li+ @C70 and Li@C70 toward Diels-Alder reactions with cyclohexadiene by means of density functional theory calculations. We found that the thermodynamic and kinetic reactivities of the fullerene cage are changed significantly after the encapsulation of the lithium ion or atom. The encapsulated lithium ion causes a remarkable decrease of the activation barrier for the cycloaddition reaction, which can be ascribed to the enhanced orbital interaction between cyclohexadiene and the fullerene cage. The unreactive bond with a long length in C70 is activated efficiently after the encapsulation of the lithium atom. According to the activation-strain model analysis, the improved reactivity of the long bond is associated with the small deformation energy and large interaction energy of the reactants. Unlike conventional Diels-Alder reactions that proceed through concerted mechanism, the reaction of Li@C70 and cyclohexadiene undergoes an unusual stepwise mechanism because of the open-shell electronic structure of Li@C70 .

Prediction of MXene based 2D tunable band gap semiconductors: GW quasiparticle calculations.

MXenes are a large family of layered transition metal carbide/nitride materials that possess a number of desired properties such as flexible chemical composition, high mechanical strength, and excellent structural stability. Although MXene based semiconductors have attracted considerable recent research attention in the search of novel 2D electronic materials, accurate understanding of their electronic properties has not been established. In this work, we carry out fully converged GW quasiparticle calculations for M2CO2 (M = Hf, Zr, and Ti) MXene based 2D semiconductors and alloys using newly developed accelerated GW methods. The quasiparticle band gaps of single-layer Hf2CO2, Zr2CO2, and Ti2CO2 are predicted to be 2.45, 2.13, and 1.15 eV, respectively. The narrow band gap of Ti2CO2 is attributed to the low energy of Ti 3d as compared with the Hf and Zr d states. Considering their chemical similarity, it is expected that Hf2-2xTi2xCO2 semiconductors can be synthesized without difficulties. We show that the quasiparticle band gap of Hf2-2xTi2xCO2 (0 ≤x≤ 1) semiconductor alloy can be continuously tuned from 2.45 to 1.15 eV, offering a unique 2D semiconductor with a moderate and tunable gap for future electronics applications.

2D selenium allotropes from first principles and swarm intelligence.

Combining the particle-swarm optimization method with first-principles calculations, we explore a new category of two-dimensional (2D) monolayers composed of solely the element selenium. Three stable structures are screened from outputs of crystal search computations, namely T-Se (1T-MoS2-like), C-Se (tiled 1D helical chain), and S-Se (square structure). Phonon calculations, as well as formation energy calculations have been performed to confirm the stability of the three phases. The electronic structure calculations show that both T-Se and C-Se are indirect-band-gap semiconductors, with gap values of 1.11 eV and 2.64 eV respectively when using the hybrid HSE06 functional. In particular, C-Se has a centrosymmetry-breaking structure which provides a spontaneous in-plane ferroelectric polarization of about 2.68 × 10-10 C m-1 per layer. Interestingly, S-Se has a Dirac cone that can open up a band gap of 0.11 eV if spin-orbit coupling is included. The tilted Dirac cone of S-Se shows anisotropic band dispersion as characterized with different Fermi velocities of 1.26 × 106 and 0.24 × 106 m s-1 around the Dirac point. Our works enrich the family of 2D materials of selenium allotropes and show that their versatile properties could give rise to potential application in various fields.