Structure, Stability, and Property Modulations of Stoichiometric Graphene Oxide

The recent success in synthesizing graphene monoxide (GMO) with rigorous stoichiometric ratio C:<i>O</i> = 1:1 has highlighted the need to determine its ground state geometry and to explore its physical properties. Using density functional theory and molecular dynamics simulation, we have found a new ether-type configuration of the GMO that is not only lower in energy than any other structures reported thus far, but is also stable up to 2000 K at which previous reported structures dissociate into CO molecules. The dynamic stability of the structure is further confirmed by calculating its phonon spectra. Furthermore, this ether-type structure exhibits anisotropies in mechanical stiffness and in electronic transport. Band gap, carrier concentration, and effective mass can be sensitively modulated by strain or higher oxidation level with C:<i>O</i> = 1:2. This study provides new theoretical insights into geometry, stability, and properties of the hotly pursued graphene oxide with unprecedented applications

Three-Dimensional Metallic Boron Nitride

Boron nitride (BN) and carbon are chemical analogues of each other and share similar structures such as one-dimensional nanotubes, two-dimensional nanosheets characterized by sp² bonding, and three-dimensional diamond structures characterized by sp³ bonding. However, unlike carbon which can be metallic in one, two, and three dimensions, BN is an insulator, irrespective of its structure and dimensionality. On the basis of state-of-the-art theoretical calculations, we propose a tetragonal phase of BN which is both dynamically <i>stable</i> and <i>metallic</i>. Analysis of its band structure, density of states, and electron localization function confirms the origin of the metallic behavior to be due to the delocalized B 2p electrons. The metallicity exhibited in the studied three-dimensional BN structures can lead to materials beyond conventional ceramics as well as to materials with potential for applications in electronic devices

Photoinduced Rippling of Two-Dimensional Hexagonal Nitride Monolayers

Inducing structural changes and deformation using noninvasive methods, such as ultrafast laser technology, is an attractive route to multiple optomechanical and optoelectronic applications. Here, we show how photon excitation could accumulate in-plane stress and induce long-wavelength ripples in two-dimensional (2D) materials. Numerical results based on first-principles calculations and a continuum model predict that long-range nanoscale rippling could emerge under photon excitation in hexagonal nitride single atomic sheets. The photosoftened transverse acoustic mode dominates the out-of-plane distortion of the sheet, and the resultant rippling pattern strongly depends on the boundary condition. We reveal that the wavelength and height of the ripple scale as I–1/3 and I1/6, respectively, where I is the incident light energy flux. Our findings based on multiscale theory and simulations elucidate the interplay between carrier excitation, phonon dispersion, and long-range mechanical deformations, which could find potential usage in flexible electronics and electromechanical devices

Coexistence of Superconductivity and Nontrivial Band Topology in Monolayered Cobalt Pnictides on SrTiO₃

As an intrinsically layered material, FeSe has been extensively explored for potentially revealing the underlying mechanisms of high transition temperature (high-Tc) superconductivity and realizing topological superconductivity and Majorana zero modes. Here we use first-principles approaches to identify that the cobalt pnictides of CoX (X = As, Sb, Bi), none of which is a layered material in bulk form. Nevertheless, all can be stabilized as monolayered systems either in freestanding form or supported on the SrTiO3(001) substrate. We further show that each of the cobalt pnictides may potentially harbor high-Tc superconductivity beyond the Cu- and Fe-based superconducting families, and the underlying mechanism is inherently tied to their isovalency nature with the FeSe monolayer. Most strikingly, each of the monolayered CoX’s on SrTiO3 is shown to be topologically nontrivial, and our findings provide promising new platforms for realizing topological superconductors in the two-dimensional limit

Tuning the Magnetic Properties of Two-Dimensional Electride Gd₂C via Halogenation

The emergent layered magnetic electride Gd2C with exotic spin-polarized anionic electrons as the hallmark has attracted tremendous interests. However, the Gd lattice masked beneath the anionic electrons with large local moments remains to be further exploited in order to understand the collapse of ferromagnetism upon halogenation. Based on first-principles calculations, we reveal that fully passivating the anionic electrons (AEs) in monolayer Gd2C with halogen suppresses conducting states and blocks ferromagnetic exchange paths between Gd ions, leading to the formation of stable Gd2CX2 (X = Cl, Br, or I) with tunable Heisenberg-type antiferromagnetic exchange couplings. Specifically, Gd2CCl2 and Gd2CBr2 order into the out-of-plane Néel phase, while Gd2CI2 prefers the in-plane Zigzag antiferromagnetic ordering, providing a chemical degree of freedom for engineering magnetic configurations. The single-ion anisotropy rooted in the interplay of onsite Coulomb repulsion and spin–orbit coupling is identified as the primary origin of magnetocrystalline anisotropy. Biaxial strains with a small magnitude can trigger magnetic phase switching between the Néel and zigzag orders and enable delicate manipulation of magnetocrystalline anisotropy in the magnitude or easy-magnetization direction. These findings provide instrumental insights for understanding the versatile functionalized magnetic electrides and broadening their applications

Topological Electride Y₂C

Two-dimensional (2D) electrides are layered ionic crystals in which anionic electrons are confined in the interlayer space. Here, we report a discovery of nontrivial Z2 topology in the electronic structures of 2D electride Y₂C. Based on first-principles calculations, we found a topological Z2 invariant of (1; 111) for the bulk band and topologically protected surface states in the surfaces of Y₂C, signifying its nontrivial electronic topology. We suggest a spin-resolved angle-resolved photoemission spectroscopy (ARPES) measurement to detect the unique helical spin texture of the spin-polarized topological surface state, which will provide characteristic evidence for the nontrivial electronic topology of Y₂C. Furthermore, the coexistence of 2D surface electride states and topological surface state enables us to explain the outstanding discrepancy between the recent ARPES experiments and theoretical calculations. Our findings establish a preliminary link between the electride in chemistry and the band topology in condensed-matter physics, which are expected to inspire further interdisciplinary research between these fields

Beyond Graphitic Carbon Nitride: Nitrogen-Rich Penta-CN₂ Sheet

Using first-principles calculations combined with <i>ab initio</i> molecular dynamics and tight binding model, we predict the existence of a kinetically stable two-dimensional (2D) penta-CN₂ sheet, which is isostructural to the recently discovered penta-graphene. The concentration of N in this new carbon nitride sheet exceeds the maximum N content, namely 21.66%, that has been achieved experimentally in honeycomb geometry. It even exceeds the N content found recently in hole-doped carbon nitride C_0.5N_0.5 as well as in porous graphitic C₃N₄. The penta-CN₂ sheet contains N–N single bonds with an energy density of 4.41 kJ/g, higher than that predicted recently in nitrogen-rich B–N compound. Remarkably, penta-CN₂ has an in-plane axial Young’s modulus of 319 N/m, even stiffer than the <i>h</i>-BN monolayer. The electronic band structure of penta-CN₂ exhibits an interesting double degeneracy at the first Brillouin zone edges which is protected by the nonsymmorphic symmetry and can be found in other isostructural chemical analogues. The band gap of penta-CN₂ calculated using HSE06 functional is 6.53 eV, suggesting its insulating nature. The prediction of a stable penta-CN₂ implies that puckering might be more effective than porosity in holding nitrogen in 2D carbon nitrides. This sheds new light on how to design nitrogen-rich C–N compounds beyond N-doped graphene

Topological Nodal-Point Superconductivity in Two-Dimensional Ferroelectric Hybrid Perovskites

Two-dimensional (2D) hybrid organic–inorganic perovskites (HOIPs) with enhanced stability, high tunability, and strong spin–orbit coupling have shown great potential in vast applications. Here, we extend the already rich functionality of 2D HOIPs to a new territory, realizing topological superconductivity and Majorana modes for fault-tolerant quantum computation. Especially, we predict that room-temperature ferroelectric BA2PbCl4 (BA for benzylammonium) exhibits topological nodal-point superconductivity (NSC) and gapless Majorana modes on selected edges and ferroelectric domain walls when proximity-coupled to an s-wave superconductor and an in-plane Zeeman field, attractive for experimental verification and application. Since NSC is protected by spatial symmetry of 2D HOIPs, we envision more exotic topological superconducting states to be found in this class of materials due to their diverse noncentrosymmetric space groups, which may open a new avenue in the fields of HOIPs and topological superconductivity

Topological Band Engineering of Lieb Lattice in Phthalocyanine-Based Metal–Organic Frameworks

Topological properties of the Lieb lattice, i.e., the edge-centered square lattice, have been extensively studied and are, however, mostly based on theoretical models without identifying real material systems. Here, based on tight-binding and first-principles calculations, we demonstrate the Lieb-lattice features of the experimentally synthesized phthalocyanine-based metal–organic framework (MPc-MOF), which holds various intriguing topological phase transitions through band engineering. First, we show that the MPc-MOFs indeed have a peculiar Lieb band structure with 1/3 filling, which has been overlooked because of its unconventional band structure deviating from the ideal Lieb band. The intrinsic MPc-MOF presents a trivial insulating state, with its gap size determined by the on-site energy difference (ΔE) between the corner and edge-center sites. Through either chemical substitution or physical strain engineering, one can tune ΔE to close the gap and achieve a topological phase transition. Specifically, upon closing the gap, topological semimetallic/insulating states emerge from nonmagnetic MPc-MOFs, while magnetic semimetal/Chern insulator states arise from magnetic MPc-MOFs, respectively. Our discovery greatly enriches our understanding of the Lieb lattice and provides a guideline for experimental observation of the Lieb-lattice-based topological states