We demonstrate that an antiferromagnetic coupling between paramagnetic Fe-porphyrin molecules and ultrathin Co and Ni magnetic films on Cu(100) substrates can be established by an intermediate layer of atomic oxygen. The coupling energies have been determined from the temperature dependence of x-ray magnetic circular dichroism measurements. By density functional theory+U calculations the coupling mechanism is shown to be superexchange between the Fe center of the molecules and Co surface-atoms, mediated by oxygen.

MXene, a two-dimensional layer of transition metal carbides/nitrides, showed great promise for energy storage, sensing, and electronic applications. MXene are chemically exfoliated from the bulk MAX phase; however, mechanistic understanding of exfoliation and subsequent functionalization of these technologically important materials is still lacking. Here, using density-functional theory we show that exfoliation of Ti3C2 MXene proceeds via HF insertion through edges of Ti3AlC2 MAX phase. Spontaneous dissociation of HF and subsequent termination of edge Ti atoms by H/F weakens Al-MXene bonds. Consequent opening of the interlayer gap allows further insertion of HF that leads to the formation of AlF3 and H2, which eventually come out of the MAX, leaving fluorinated MXene behind. Density of state and electron localization function shows robust binding between F/OH and Ti, which makes it very difficult to obtain controlled functionalized or pristine MXene. Analysis of the calculated Gibbs free energy (ΔG) shows fully fluorinated MXene to be lowest in energy, whereas the formation of pristine MXene is thermodynamically least favorable. In the presence of water, mixed functionalized Ti3C2Fx(OH)1-x (x ranges from 0 to 1) MXene can be obtained. The ΔG values for the mixed functionalized MXenes are very close in energy, indicating the random and nonuniform functionalization of MXene. The microscopic understanding gained here unveils the challenges in exfoliation and controlling the functionalization of MXene, which is essential for its practical application.

We report a density functional theory (DFT) study regarding the effects of atomic defects, such as vacancies and adatom adsorption, on the electronic and magnetic properties of phosphorene (a two-dimensional monolayer of black phosphorus). A monovacancy in the phosphorene creates an in-gap state in the band gap of pristine phosphorene and induces a magnetic moment, even though pristine phosphorene is nonmagnetic. In contrast, both planar and staggered divacancies do not change the magnetic properties of phosphorene, although a staggered divacancy creates states in the gap. Our DFT calculations also show that adsorption of nonmetallic elements (C, N, and O) and transition metal elements (Fe, Co, and Ni) can change the magnetic properties of phosphorene with or without vacancies. For example, the nonmagnetic pristine phosphorene becomes magnetic after the adsorption of N, Fe, or Co adatoms, and the magnetic phosphorene with a monovacancy becomes nonmagnetic after the adsorption of C, N, or Co atoms. We also demonstrate that for O-or Fe-adsorbed monovacancy structure the electronic and magnetic properties are tunable via the control of charge on the phosphorene system. These results provide insight for achieving metal-free magnetism and a tunable band gap for various electronic and spintronic devices based on phosphorene.

Oxygen-functionalized MXene, M2CO2 (M = group III–V metals), are emergent formidable two-dimensional (2D) materials with a tantalizing possibility for device applications. Using first-principles calculations, we perform an intensive study on the stability of fully O-functionalized (M2CO2) MXenes. Depending on the position of O atoms, the M2CO2 can exist in two different structural phases. On one side of MXene, the O atom occupies a site which is exactly on the top of the metal atom from the opposite side. On the other side, the O atom can occupy either the site on the top of the metal atom of the opposite side (BB′ phase) or on the top of the C atom (CB phase). We find that for M = Sc and Y the CB phase is stable, whereas for M = Ti, Zr, Hf, V, Nb, and Ta the stable phase is BB′. The electron localization function, the atom-projected density of states, the charge transfer, and the Bader charge analyses provide a rational explanation for the relative stability of these two phases and justify the ground state structure by giving information about the preferential site of adsorption for the O atoms. We also calculate the phonon dispersion relations for both phases of M2CO2. The BB′-Sc2CO2 and the CB-Ti2CO2 are found to be dynamically unstable. Finally, we find that the instability of BB′-M2CO2 (M = Sc and Y) originates from the weakening of M–C interactions, which manifest as a phonon mode with imaginary frequency corresponding to the motion of C atom in the a–b plane. The insight into the stability of these competing structural phases of M2CO2 presented in this study is an important step in the direction of identifying the stable phases of these 2D materials.