Abstract. -By first-principles calculations, we present a doping-dependent phase diagram of LaOMAs (M=V-Cu) family. It is characterized as antiferromagnetic semiconductor around LaOMnAs side and ferromagnetic metal around LaOCoAs. Both LaOFeAs and LaONiAs, where superconductivity were discovered, are located at the borderline of magnetic phases. Extensive Fermi surface analysis suggests that the observed superconductivity is of electron-type in its origin. We discuss possible pairing mechanisms in the context of competing ferromagnetic phases found in this work and the ferromagnetic spin fluctuations. The quaternary oxypnictides LaOMAs crystallize in layered tetragonal structure with P 4/nmm symmetry [9]. Each transition-metal (oxygen) layer is sandwiched by two nearest-neighbor As (La) atomic layers, which form edgeshared tetrahedrons around the M (oxygen) sites. The (MAs)− and (LaO) + triple-layer-subgroups stack alternatively along the c-axis. The positions of La or As sheets are determined by two internal parameters, z La and z As , which define the inter-layer distances of La-O and MAs, respectively. It is important that this series of compounds are chemically stable such that systematical tuning is available without altering the structure and symmetry significantly. For instance, a variety of compounds can be synthesized by the replacement of transition-metal elements, where both the electron doping and hole doping can be realized by replacing O 2− or La 3+ ions. Except the early report for the structure study [9], the detailed studies on the electronic and magnetic properties for this series of compounds are still in its early stage. It was first reported in 2006 that superconductivity can be realized in LaOFeP below 4K, and the T c was increased to 7K by Fdoping [6]. Later, superconductivity with T c about 2K was reported for LaONiP [7], and T c around 26K was reached very recently in LaOFeAs again after F-doping [8]. We will present in this letter that both M=Fe and Ni compounds locate at special positions of the global phase diagram for the series of M substituted compounds. The competing magnetic and superconducting phases found in our global phase diagram provide important clues on the possible pairing mechanism in this class of materials.The phase diagram is constructed from first-principles calculations based on density functional theory with generalized gradient approximation (GGA) of PBE-type [10] for the exchange-correlation potential. We use the plane-

Formation of topological quantum phase on a conventional semiconductor surface is of both scientific and technological interest. Here, we demonstrate epitaxial growth of 2D topological insulator, i.e., quantum spin Hall state, on Si(111) surface with a large energy gap, based on first-principles calculations. We show that the Si(111) surface functionalized with one-third monolayer of halogen atoms [Si(111)-ffiffiffi 3 p × ffiffiffi ffi 3 p -X (X = Cl, Br, I)] exhibiting a trigonal superstructure provides an ideal template for epitaxial growth of heavy metals, such as Bi, which self-assemble into a hexagonal lattice with high kinetic and thermodynamic stability. Most remarkably, the Bi overlayer is atomically bonded to but electronically decoupled from the underlying Si substrate, exhibiting isolated quantum spin Hall state with an energy gap as large as ∼0.8 eV. This surprising phenomenon originates from an intriguing substrate-orbital-filtering effect, which critically selects the orbital composition around the Fermi level, leading to different topological phases. In particular, the substrate-orbital-filtering effect converts the otherwise topologically trivial freestanding Bi lattice into a nontrivial phase; and the reverse is true for Au lattice. The underlying physical mechanism is generally applicable, opening a new and exciting avenue for exploration of large-gap topological surface/interface states.T opological insulators (TIs) (1-3) are distinguished from conventional insulators by robust metallic surface or edge states residing inside an insulating bulk gap. As these topological states are protected by time-reversal symmetry, they have negligible elastic scattering and Anderson localization (4, 5), rendering significant implications in electronic/spintronic and quantum computing devices. In this regard, 2D TI [i.e., the quantum spin Hall (QSH) insulator] has an advantage over its 3D counterpart as the edge states of the QSH insulator are more robust against nonmagnetic scattering, because the only available backscattering channel is forbidden. Many QSH insulators have been discovered (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), and most of them have a small energy gap. Recently, there has been an intensive search for 2D TIs with a large energy gap (17)(18)(19)(20), which is of both scientific and practical interest, such as for room temperature applications. So far, however, most studied systems rely on freestanding films, and the existence of some 2D freestanding films could be in doubt because of their poor thermal or chemical stability; and even if they do exist, growth and synthesis of freestanding film is usually much harder than growth of thin film on substrate. Furthermore, the functional film often needs to be placed on a substrate in a device setting, but the electronic and topological properties of freestanding films will likely be affected by the substrate (21-23). Therefore, it is highly desirable to search for large-gap QSH states existing on a substrate while maintaining a lar...

Zero-dimensional (0D) halides perovskites, in which anionic metal-halide octahedra (MX6)4− are separated by organic or inorganic countercations, have recently shown promise as excellent luminescent materials.

CH3NH3PbI3-based solar cells have shown remarkable progress in recent years but have also suffered from structural, electrical, and chemical instabilities related to the soft lattices and the chemistry of these halides. One of the instabilities is ion migration, which may cause current–voltage hysteresis in CH3NH3PbI3 solar cells. Significant ion diffusion and ionic conductivity in CH3NH3PbI3 have been reported; their nature, however, remain controversial. In the literature, the use of different experimental techniques leads to the observation of different diffusing ions (either iodine or CH3NH3 ion); the calculated diffusion barriers for native defects scatter in a wide range; the calculated defect formation energies also differ qualitatively. These controversies hinder the understanding and the control of the ion migration in CH3NH3PbI3. In this paper, we show density functional theory calculations of both the diffusion barriers and the formation energies for native defects (V I +, MA i +, V MA –, and I i –) and the Au impurity in CH3NH3PbI3. V I + is found to be the dominant diffusing defect due to its low formation energy and the low diffusion barrier. I i – and MA i + also have low diffusion barriers but their formation energies are relatively high. The hopping rate of V I + is further calculated taking into account the contribution of the vibrational entropy, confirming V I + as a fast diffuser. We discuss approaches for managing defect population and migration and suggest that chemically modifying surfaces, interfaces, and grain boundaries may be effective in controlling the population of the iodine vacancy and the device polarization. We further show that the formation energy and the diffusion barrier of Au interstitial in CH3NH3PbI3 are both low. It is thus possible that Au can diffuse into CH3NH3PbI3 under bias in devices (e.g., solar cell, photodetector) with Au/CH3NH3PbI3 interfaces and modify the electronic properties of CH3NH3PbI3.

CsGeI3 may be used as an efficient hole transport material in solar cells although it may not be an excellent solar absorber material due to the deep electron traps induced by iodine vacancies.

Organic−inorganic metal halide hybrids have emerged as a new class of materials with fascinating optical and electronic properties. The exceptional structure tunability has enabled the development of materials with various dimensionalities at the molecular level, from threedimensional (3D) to 2D, 1D, and 0D. Here, we report a new 1D lead chloride hybrid, C 4 N 2 H 14 PbCl 4 , which exhibits unusual inverse excitation-dependent broadband emission from bluish-green to yellow. Density functional theory calculations were performed to better understand the mechanism of this excitation-dependent broadband emission. This 1D hybrid material is found to have two emission centers, corresponding to the self-trapped excitons (STEs) and vacancy-bound excitons. The excitation-dependent emission is due to different populations of these two types of excitons generated at different excitation wavelengths. This work shows the rich chemistry and physics of organic− inorganic metal halide hybrids and paves the way to achieving novel light emitters with excitation-dependent broadband emissions at room temperature.

Solar cells based on methylammonium lead triiodide (MAPbI3) have shown remarkable progress in recent years and have demonstrated efficiencies greater than 20%. However, the long‐term stability of MAPbI3‐based solar cells has yet to be achieved. Besides the well‐known chemical and thermal instabilities, significant native ion migration in lead halide perovskites leads to current–voltage hysteresis and photoinduced phase segregation. Recently, it is further revealed that, despite having excellent chemical stability, the Au electrode can cause serious solar cell degradation due to Au diffusion into MAPbI3. In addition to Au, many other metals have been used as electrodes in MAPbI3 solar cells. However, how the external metal impurities introduced by electrodes affect the long‐term stability of MAPbI3 solar cells has rarely been studied. A comprehensive study of formation energetics and diffusion dynamics of a number of noble and transition metal impurities (Au, Ag, Cu, Cr, Mo, W, Co, Ni, Pd) in MAPbI3 based on first‐principles calculations is reported herein. The results uncover important general trends of impurity formation and diffusion in MAPbI3 and provide useful guidance for identifying the optimal metal electrodes that do not introduce electrically active impurity defects in MAPbI3 while having low resistivities and suitable work functions for carrier extraction.