Unconventional Band Structure via Combined Molecular Orbital and Lattice Symmetries in a Surface-Confined Metallated Graphdiyne Sheet.

Graphyne (GY) and graphdiyne (GDY)-based monolayers represent the next generation 2D carbon-rich materials with tunable structures and properties surpassing those of graphene. However, the detection of band formation in atomically thin GY/GDY analogues has been challenging, as both long-range order and atomic precision have to be fulfilled in the system. The present work reports direct evidence of band formation in on-surface synthesized metallated Ag-GDY sheets with mesoscopic (≈1 µm) regularity. Employing scanning tunneling and angle-resolved photoemission spectroscopies, energy-dependent transitions of real-space electronic states above the Fermi level and formation of the valence band are respectively observed. Furthermore, density functional theory (DFT) calculations corroborate the observations and reveal that doubly degenerate frontier molecular orbitals on a honeycomb lattice give rise to flat, Dirac and Kagome bands close to the Fermi level. DFT modeling also indicates an intrinsic band gap for the pristine sheet material, which is retained for a bilayer with h-BN, whereas adsorption-induced in-gap electronic states evolve at the synthesis platform with Ag-GDY decorating the (111) facet of silver. These results illustrate the tremendous potential for engineering novel band structures via molecular orbital and lattice symmetries in atomically precise 2D carbon materials.

Ferromagnetic TM2BC (TM = Cr, Mn) monolayers for spintronic devices with high Curie temperature.

Transition metal boro-carbide (TM2BC) structures crystallize in the layered orthorhombic structure in their bulk phases. In this study, however, we find that TM2BC (TM = Cr, Mn) prefer a tetragonal (t) crystal structure in their monolayer phases due to the occurrence of strong sp2 bonds between the metal and B/C atoms instead of sp3 + sp2 bonds which exist in the orthorhombic phase. The calculated energy difference between the orthorhombic and the tetragonal structures based on density functional theory (DFT) is more than 1 eV per unit cell. In addition, t-Cr2BC and t-Mn2BC monolayers are dynamically and thermally stable with their magnetic metal electronic structures. For further investigations, we combine our DFT calculations with the Monte Carlo simulations and find that both t-TM2BC monolayers show ferromagnetic properties. The calculated Curie temperatures are 846 K and 128 K for t-Cr2BC and t-MnBC sheets, respectively. In addition, we examine the magnetic anisotropy energies (MAE) of t-TM2BC monolayers and find that both structures prefer out-of-plane as the easy axis magnetization direction and the applied electric field can easily modulate the MAE of the monolayers. Our theoretical calculations reveal that t-TM2BC (TM = Cr, Mn) sheets have great potential for the future design of controllable spintronic devices with their tunable MAE properties.

Electronic structure, cohesive and magnetic properties of iridium oxide clusters adsorbed on graphene.

In this study, we investigated and revealed the electronic properties, geometric structures and binding behavior of small (IrO)n and [Formula: see text] (n = 1-5) clusters within first principles calculations based on the density functional theory. The electronic and magnetic properties of small nanoclusters displayed significant size dependency due to strong quantum confinement effect. Moreover we considered the binding and structural modification of the clusters on graphene surface as a substrate. The cohesive energy per atom of isolated clusters increased with size of the cluster n. This shows that the increase in coordination number results in a more stable nanocluster with increased number of saturated bonds. Pristine (IrO)n and [Formula: see text] clusters presented different structural motives at equilibrium. The ground states of (IrO)n and [Formula: see text] clusters considered in this study were all magnetic except for (IrO)4, [Formula: see text] , and [Formula: see text] . HOMO-LUMO gap EHLG values displayed large variations due to size of the cluster, hence bond saturation. The structural configurations of free standing nanoclusters are slightly modified, when adsorbed on graphene. The adsorption behavior of a cluster on graphene was improved by an applied electric field yielding larger binding energy and larger charger transfer. We observed that electronic and magnetic ground state of the clusters strongly depend on optimized structural configuration for both bare and adsorbed on graphene monolayer.

A tetragonal phase Mn2B2 sheet: a stable room temperature ferromagnet with sizable magnetic anisotropy.

Exploring the magnetic properties of two-dimensional (2D) metal boride (MBene) sheets for spin-based electronics is gaining importance for developing electronic devices. Through combined first-principles calculations and Monte Carlo simulations, we present a new tetragonal Mn2B2 (tetra-Mn2B2) sheet. The tetra-Mn2B2 sheet shows metallic ferromagnetism (2.65 µB per Mn atom) with excellent stability. Moreover, it is demonstrated that the tetra-Mn2B2 sheet holds promise for experimental synthesis within an acceptable range from the results of stability tests of tetra-Mn2B2. We also find that the magnetic anisotropy (MAE) of the 2D tetra-Mn2B2 sheet is significantly increased under an electric field. The Curie temperature (TC) of the tetra-Mn2B2 sheet is calculated as 406 K. This 2D tetra-Mn2B2 with a high Curie temperature can serve as a promising candidate for future magnetoelectronics applications.

Tuning the electronic structure of RhX3 (X = Cl, Br, I) nonmagnetic monolayers: effects of charge-injection and external strain.

Atomistic exploration and electronic modification of 2D materials have been a central focus of research since the breakthrough of graphene. In the present study, we introduce and reveal the structure, stability and electronic features of novel RhX3 (X: Cl, Br, I) monolayer systems within the framework of density functional theory. Phonon dispersion spectra and equilibrium molecular dynamics calculations confirm the stability of the phases studied at room and elevated temperatures. The structures are slightly modified because of thermal excitations and maintain their stabilities up to 1000 K. We show that the electronic and magnetic nature of pristine monolayers can be tuned by external effects, i.e. both mechanically and electrically. RhCl3, RhBr3 and RhI3 monolayers are nonmagnetic and indirect-gap semiconductors intrinsically, but display indirect-to-direct band gap transitions at particular strain values. The systems gain a net magnetic moment and are transformed into metals by negative charging. The optical properties, such as the absorption coefficient, optical conductivity, energy loss spectrum, refractive index and extinction coefficient, are also presented. This interesting class of nanomaterials are promising candidates for several applications in nanotechnology and optoelectronics with good thermal stability, mechanical flexibility, and tunable electronic properties.

Chemical and substitutional doping, and anti-site and vacancy formation in monolayer AlN and GaN.

We investigated the effects of chemical/substitutional doping, hydrogenation, and anti-site and vacancy defects on the atomic, optoelectronic and magnetic properties of AlN and GaN monolayers. Upon doping of selected atoms, AlN and GaN monolayers can acquire magnetic properties, and their fundamental band gaps are modified by the localized gap states. Spin-polarized gap states broaden into bands at patterned coverage of adatoms, whereby half-metallic or magnetic semiconducting properties can be attained. Specific adatoms adsorbed to Ga atoms break the nearest vertical Ga-N bonds in the GaN bilayer in the heackelite structure and result in changes in the electronic and atomic structure. While adjacent and distant pairs of anion + cation vacancies induce spin polarization with filled and empty gap states, anti-site defects remain nonmagnetic; but both defects induce dramatic changes in the band gap. Fully hydrogenated monolayers are stable only for specific buckled geometries, where one geometry can also lead to an indirect to direct band gap transition. Also, optical activity shifts to the ultra-violet region upon hydrogenation of the monolayers. While H2 and O2 molecules are readily physisorbed on the surfaces of the monolayers with weak van der Waals attraction, they can be dissociated into constituent atoms at the vacancy site of the cation. Our study performed within density functional theory shows that the electronic, magnetic and optical properties of AlN and GaN monolayers can be tuned by doping and point defect formation in order to acquire diverse functionalities.

Diffusion quantum Monte Carlo and density functional calculations of the structural stability of bilayer arsenene.

We have studied the structural stability of monolayer and bilayer arsenene (As) in the buckled (b) and washboard (w) phases with diffusion quantum Monte Carlo (DMC) and density functional theory (DFT) calculations. DMC yields cohesive energies of 2.826(2) eV/atom for monolayer b-As and 2.792(3) eV/atom for w-As. In the case of bilayer As, DMC and DFT predict that AA-stacking is the more stable form of b-As, while AB is the most stable form of w-As. The DMC layer-layer binding energies for b-As-AA and w-As-AB are 30(1) and 53(1) meV/atom, respectively. The interlayer separations were estimated with DMC at 3.521(1) Å for b-As-AA and 3.145(1) Å for w-As-AB. A comparison of DMC and DFT results shows that the van der Waals density functional method yields energetic properties of arsenene close to DMC, while the DFT + D3 method closely reproduced the geometric properties from DMC. The electronic properties of monolayer and bilayer arsenene were explored with various DFT methods. The bandgap values vary significantly with the DFT method, but the results are generally qualitatively consistent. We expect the present work to be useful for future experiments attempting to prepare multilayer arsenene and for further development of DFT methods for weakly bonded systems.