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Organic–inorganic hybrid lead halide perovskites have gained significant attention as light-harvesting materials in thin-film photovoltaics due to their exceptional optoelectronic properties and simple fabrication process. The power conversion efficiency of perovskite solar cells (PSCs) has surged beyond 25% in a short time span. Their transition to commercial market is a "work in progress" due to limited long-term operational stability and the persisting environmental concern due to the presence of lead. Comprehensive investigations on the interplay of material composition and interfacial effects on the device performance of PSCs based on methylammonium lead iodide have shown the crucial role of an A-site cation in incipient deterioration of the material through external stimuli (moisture, light, oxygen, or heat). Consequently, a partial or complete replacement of A-site cations by up to four isoelectronic substituents has resulted in many new perovskite compositions. The correlations between the chemical composition and the optoelectronic properties are, however, not always easy to determine. A-site cation management is governed by stability and charge neutrality of the lattice, and the choices include Cs+-cations and organic cations such as CH3NH3+ or CH(NH2)2+ and combinations thereof. Since the size of the cations is an important structural parameter, an adequate compositional engineering of the A-site could effectively optimize the stability by reducing non-radiative defect sites and enhancing carrier lifetimes. This Perspective reflects on the experimental strategies for A-site cation management and their direct impact on the stability and device performance. It also highlights the opportunities and challenges for further research and industrial commercialization of PSCs. [ABSTRACT FROM AUTHOR]

The understanding of the formation of graphene at the atomic scale on Si-terminated 3C-SiC for obtaining high-quality graphene sheets remains elusive, although epitaxial graphene growth has been shown to be a well-known method for economical mass production of graphene/SiC heterojunctions. In this paper, the atomic behavior of carbon atoms on a Si removed 3C-SiC (111) surface for the formation of graphene buffer layer during the early stage of epitaxial graphene growth was investigated using a molecular dynamics simulation. Observation of the behavior of the remaining carbon atoms on the Si-terminated 3C-SiC (111) surface after removal of the silicon atoms revealed that graphene clusters, which were formed by sp2-bonded carbon atoms, start to appear at annealing temperatures higher than 1300 K. Our simulations indicated that the structural stability of the whole system increased as the number of sp2-bonded carbon atoms on the Si-terminated 3C-SiC (111) surface increased. It was also found that the diffusion energy barrier for the migration of carbon atoms from the on-top site to the bridge site on the Si-terminated 3C-SiC (111) surface mainly determines the critical temperature of graphene cluster formation. [ABSTRACT FROM AUTHOR]

The structural and self-assembling characteristic of Pb heterostructures on the Cu (111) substrate in the early stage of the deposition process were investigated using a molecular dynamics simulation and density functional theory. The Pb islands formed on the Cu (111) surface were observed to diffuse actively in lateral directions following the layer-by-layer growth mode. A heptameric hexagonal island was found to be most stable under highly nonequilibrium conditions. This result can be explained by the tendency of Pb heterostructures, which have minimum surface energy, to have the maximum number of Pb–Pb bondings. In addition, the atomic binding energy, the surface diffusion coefficient prefactor, and the surface diffusion energy barrier for Pb adatoms were quantitatively calculated according to various shapes of Pb islands to determine the stability of the corresponding island. [ABSTRACT FROM AUTHOR]

Using the first-principles calculations, the change in magnetism and electronic structures during Cu nucleation in bcc Fe system was investigated. For modeling the Fe-rich bcc FeCu alloy, FexCu1-x (x≥0.75) were employed in the supercell system. Nonmagnetic Cu atoms were precipitated on the (100) plane of bcc Fe crystal, whereas the magnetized Cu atoms preferred to be precipitated on the (110) plane. Magnetization energies showed a linear decrease as the Cu concentration increased in bcc Fe. The magnetic moments of Fe atoms increased for larger concentration of Cu atoms. Electron density of states showed that the enhanced magnetic moment of Fe atoms resulted from the shift of minority spin states toward higher levels, which was associated with the bond formation between Cu and Fe atoms. [ABSTRACT FROM AUTHOR]

Using ab-initio simulations, we investigated the effects of ferroelectric polarization on the magnetic anisotropy of FePt/BaTiO3 heterojunctions. The changed electronic structure at the interface of FePt and BaTiO3 reduced the magnetic anisotropy energy (MAE) under ferroelectric polarization. Through the electronic density of states analysis, it was found that the MAE change is mainly due to the changed hybridization state between d-orbitals of interface Ti and Fe atoms. [ABSTRACT FROM AUTHOR]

Using the ab initio calculations, the changes of the magnetic properties of Co atoms on graphene for biaxial strains on graphene were investigated. Our calculation results showed that the compression of graphene changed the magnetic moments of Co adatoms more significantly than the elongation of graphene. From the 3d-electron density of states of Co atoms, the changes of magnetic properties of Co atoms on graphene were found to result from the significant hybridization between Co 3dyz and C 2pz orbitals. It was demonstrated that this 3d-electron hybridization of Co/graphene system originated from the strain-induced change of the local atomic structure. [ABSTRACT FROM AUTHOR]

Using ab initio calculations, we investigated the changes of the magnetic moment and electronic structures of Fe adatoms on strained graphene sheets. By the uniaxial tensile strains in armchair and zig-zag directions on graphene sheets, the amounts of charge transfers from graphene 2pz orbital to Fe adatom 3d orbitals were linearly increased. The magnetic moments of Fe, however, show the tendency of linear decrements with the uniaxial tensile strains. The increased Fe magnetic moments by uniaxialy graphene compressions resulted from the shifting of spin-minority states of electrons while the decreased Fe magnetic moments were due to the reduction in the spin-majority states of 3dxy-orbitals of the Fe adatom. [ABSTRACT FROM AUTHOR]

Using ab initio calculations, we obtained the surface phase diagram of a Fe/Pt(001) surface and the magnetic anisotropy energies of the equilibrium Fe/Pt(001) surface structures. From the obtained surface phase diagram, Fe-rich L12 B and perpendicular L10 B were found to be the most stable Fe-Pt surface phases. The calculated magnetic anisotropy energies of the Fe-rich L12 B and perpendicular L10 B Fe/Pt(001) structures revealed that the magnetic easy axes of the surface structures prefer to align in the [001] direction. Through systematic calculations, we showed that the magnetic anisotropy reduction in Fe/Pt(001) originates from the changed electron filling in the 3dz2 orbital of Fe atoms due to the surface formation. [ABSTRACT FROM AUTHOR]

Using first principles calculations, we investigated interface structure effects on the magnetic properties of the Fe/BaTiO3 system. On the BaO-terminated surface, a Fe monolayer is formed as two Fe atoms are adsorbed on the top sites of Ba and O in the (1×1) surface unit and a Fe monolayer (ML) is formed on the TiO2-terminated surface as two Fe atoms are adsorbed on the two O top sites. The magnetic anisotropy energy of Fe was higher on the TiO2-terminated surface (1.5 eV) than on the BaO-terminated surface (0.5 eV). The decomposed electron density of the states showed that the stronger hybridization of Fe with the TiO2 layer than with the BaO layer is the most important reason for the higher magnetic anisotropy energy. [ABSTRACT FROM AUTHOR]