Two-dimensional Layered Perovskite Research Articles

Structural diversity in hybrid organic-inorganic lead iodide materials.

The structural chemistry of hybrid organic-inorganic lead iodide materials has become of increasing significance for energy applications since the discovery and development of perovskite solar cells based on methylammonium lead iodide. Seven new hybrid lead iodide compounds have been synthesized and structurally characterized using single-crystal X-ray diffraction. The lead iodide units in materials templated with bipyridyl, 1,2-bis(4-pyridyl)ethane, 1,2-di(4-pyridyl)ethylene and imidazole adopt one-dimensional chain structures, while crystallization from solutions containing piperazinium cations generates a salt containing isolated [PbI6](4-) octahedral anions. Templating with 4-chlorobenzylammonium lead iodide adopts the well known two-dimensional layered perovskite structure with vertex shared sheets of composition [PbI4](2-) separated by double layers of organic cations. The relationships between the various structures determined, their compositions, stability and hydrogen bonding between the protonated amine and the iodide ions of the PbI6 octahedra are described.

New hybrid lead iodides: From one-dimensional chain to two-dimensional layered perovskite structure

Two new hybrid lead halides (H2BDA)[PbI4] (1) (H2BDA=1,4-butanediammonium dication) and (HNPEIM)[PbI3] (2) (HNPEIM=N-​phenyl-ethanimidamidine cation) have been synthesized and structurally characterized. X-ray diffraction analyses reveal that compound 1 features a two-dimensional corner-sharing perovskite layer whereas compound 2 contains one-dimensional edge-sharing double chains. The N-​phenyl-ethanimidamidine cation within compound 2 was generated in-situ under solvothermal conditions. The optical absorption spectra collected at room temperature suggest that both compounds are semiconductors having direct band gaps, with estimated values of 2.64 and 2.73eV for 1 and 2, respectively. Results from the density functional theory (DFT) calculations are consistent with the experimental data. Density of states (DOS) analysis reveals that in both compounds 1 and 2, the energy states in the valence band maximum region are iodine 5p atomic orbitals with a small contribution from lead 6s, while in the region of conduction band minimum, the major contributions are from the inorganic (Pb 6p atomic orbitals) and organic components (C and N 2p atomic orbitals) in compound 1 and 2, respectively.

Semimetal and Topological Insulator in Perovskite Iridates

The two-dimensional layered perovskite Sr2IrO4 was proposed to be a spin-orbit Mott insulator, where the effect of Hubbard interaction is amplified on a narrow J_{eff} = 1/2 band due to strong spin-orbit coupling. On the other hand, the three-dimensional orthorhombic perovskite (Pbnm) SrIrO3 remains metallic. To understand the physical origin of the metallic state and possible transitions to insulating phases, we construct a tight-binding model for SrIrO3. The band structure possesses a line node made of J_{eff} = 1/2 bands below the Fermi level. As a consequence, instability towards magnetic ordering is suppressed and the system remains metallic. This line node, originating from the underlying crystal structure, turns into a pair of three-dimensional nodal points on the introduction of a staggered potential or spin-orbit coupling strength between alternating layers. Increasing this potential beyond a critical strength induces a transition to a strong topological insulator, followed by another transition to a normal band insulator. We propose that materials constructed with alternating Ir- and Rh-oxide layers along the (001) direction, such as Sr2IrRhO6, are candidates for a strong topological insulator.

Hybrid cavity polaritons in a ZnO-perovskite microcavity

We realize a Fabry-Perot hybrid microcavity containing an association of a ZnO thin layer and of a layer of the two-dimensional layered perovskite 5-methyl-2-furanmethanamonium lead bromide (MFMPB). From angle-resolved reflectivity experiments performed at low temperature of 5 K, we show that this hybrid cavity works in the strong-coupling regime and that the lower, middle, and upper polariton branches are observed. We show that the middle polariton branch (MPB) contains a significant component of the cavity photon and both of the two exciton species.

Structural, magnetic, electrical and thermal transport properties in two-dimensional perovskite Sr1.05Ln0.95CoO4 (Ln = La, Ce and Nd) compounds

The structural, magnetic, electrical and thermal transport properties have been investigated in two-dimensional layered perovskite Sr1.05Ln0.95CoO4 (Ln = La, Ce and Nd) compounds. The variations of Jahn–Teller distortion of the CoO6 octahedron and the effective magnetic moment indicate that the induced holes are mostly accommodated in the t2g orbital states keeping the intermediate-spin configuration. The detailed comparison of temperature dependence of magnetization, resistivity, thermoelectric power and thermal conductivity suggests that the tolerance factor t plays a very crucial role in the transport behaviours. With decreasing A-site rare earth ionic radius , the tolerance factor t decreases and the distortion of CoO6 octahedron enhances, which leads to the increased bending of the Co–O–Co bond, the narrowing of the bandwidth and the decrease in the mobility of eg electrons. In addition, from the viewpoint of application, the huge Seebeck coefficients (>600 µV K−1) indicate that the present layered cobaltites can be good candidates for thermoelectric materials.

Strong exciton–photon coupling at room temperature in microcavities containing two-dimensional layered perovskite compounds

We have realized Perot–Fabry microcavities containing a two-dimensional layered perovskite-type semiconductor: (C6H5C2H4–NH3)2PbI4 between a dielectric mirror and a metallic mirror. A strong coupling regime between the perovskite exciton and the confined photon mode has been evidenced at room temperature from angular-resolved reflectivity experiments, anticrossings as large as 190 meV are observed between the excitonic and cavity modes. We have shown that the design of the microcavity can be varied at will, so that the detuning or the Rabi splitting can be precisely chosen. The emission of the polaritonic low energy branch has been observed.

Preparation and characterization of organic–inorganic hybrid perovskite (C 4H 9NH 3) 2CuCl 4

Organic–inorganic hybrid perovskite (C 4H 9NH 3) 2CuCl 4 was prepared via the reaction between copper chloride and butylammonium chloride. Its chemical structure was characterized by FT–IR and elemental analysis. Its thin film was obtained by spin-coating, and X-ray diffraction (XRD) measurements indicated the formation of two-dimensional layered perovskites structure, with the c-axis perpendicular to the substrate surface. The electronic structure, thermal properties and electrical properties of the hybrid perovskite (C 4H 9NH 3) 2CuCl 4 were also studied by UV–vis, photoluminescience (PL), TGA, DSC, and Hall measurement.

Fabrication of two-dimensional layered perovskite [NH3(CH2)12NH3]PbX4 thin films using a self-assembly method.

Ultra-thin films of the two-dimensional layered perovskites [NH3(CH2)12NH3]PbX4 (X = Br, I) with the quantum confinement effect have been fabricated by a convenient self-assembly method.

NMR study of the spin state and magnetic properties of layered perovskite cobalt oxides La 2− xSr xCoO 4

We have made 59 Co NMR measurements to microscopically study magnetic properties of the two-dimensional layered perovskite cobalt oxides, La 2− x Sr x CoO 4 (0.5⩽ x⩽1). The temperature dependence of the 59 Co Knight shift 59K clearly shows that La 2− x Sr x CoO 4 (0.5⩽ x⩽1) are paramagnetic above 4.2 K. From the analysis of the 59K versus magnetic susceptibility plots, the hyperfine coupling constant was found to change at x c∼0.75, indicating that the spin state changes at this critical concentration.

Compositional Modulation of Two-Dimensional Layered Perovskite (RNH3)2Pb(Cl, Br, I)4 and Its Optical Properties

The compositional modulation of two-dimensional layered perovskites, (RNH3)2Pb(X, I)4(R: C6H5C2H4-; X: Cl, Br), and optical properties of these materials were investigated. Thin films of microcrystalline (RNH3)2- Pb(X, I)4 were successfully fabricated by the spin-coating method. X-ray diffraction spectra revealed that these materials could be regarded as inorganic/organic superlattice structures. In the spectra obtained for the (RNH3)2Pb(Br, I)4 films, a strong exciton absorption band was observed even at room temperature, and the peak position shifted continuously towards the blue and violet regions. The exciton absorption bands observed in the spectrum of (RNH3)2Pb(X, I)4 mixed crystal were consistent with the 6s to 6p transition of Pb2+. The peak shift of the exciton absorption bands of these materials could be explained by the valence band structure composed of the Pb(6s) orbital which was hybridized with the I(5p) and X(np) orbitals.