Dynamic Structural Evolution and Dual Emission Behavior in Hybrid Organic Lead Bromide Perovskites.

The optoelectronic properties of organic lead halide perovskites (OLHPs) strongly depend on their underlying crystal symmetry and dynamics. Here, we exploit temperature-dependent synchrotron powder X-ray diffraction and temperature-dependent photoluminescence to investigate how the subtle structural changes happening in the pure and mixed A-site cation MA1-xFAxPbBr3 (x = 0, 0.5, and 1) systems influences their optoelectronic properties. Diffraction investigations reveal a cubic structure at high temperatures and tetragonal and orthorhombic structures with octahedral distortion at low temperatures. Steady state photoluminescence and time correlated single photon counting study reveals that the dual emission behavior of these OLHPs is due to the direct-indirect band formation. In the orthorhombic phase of MAPbBr3, the indirect band is dominated by self-trapped exciton (STE) emission due to the higher-order lattice distortions of PbBr6 octahedra. Our findings provide a comprehensive explanation of the dual emission behavior of OLHPs while also providing a rationale for previous experimental observations.

Synthesis and characterization of layered metal sulfates containing M(µ3-OH/F)2(M = Mg, Co) diamond chains.

Cobalt and magnesium sulfates of the compositions, [C4N2H12]2[Co3F2(SO4)3(H2O)2], (1) and [NH4]2[Mg3(OH)2(SO4)3(H2O)2], (2), respectively, have been synthesized under hydro/solvothermal conditions, and are characterized by IR spectra, elemental analysis, powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), and X-ray single-crystal diffraction. 1 and 2 crystallized in the orthorhombic space groups Pnma and Cmc2 respectively. While 1 is templated by the organic piperazinium cation, 2 is obtained in the presence of ammonium ions. The layered structures are formed by the diamond chains comprising of M3(µ3-OH/F)2 units (M = Co: 1, Mg: 2). Magnetic studies of 1 reveal its ferromagnetic nature with a transition at 10.8 K and show it does not exhibit spin-glass freezing. Isothermal magnetization shows a hysteresis loop at 2.5 K with a coercive field of 1200 Oe and remnant magnetization of 0.1µB. A sharp λ-like anomaly is also seen in the heat capacity curve, favoring long range magnetic ordering below Tc.

Two distinct phases of bilayer graphene films on Ru(0001).

By combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy we reveal the structural and electronic properties of multilayer graphene on Ru(0001). We prove that large ethylene exposure allows the synthesis of two distinct phases of bilayer graphene with different properties. The first phase has Bernal AB stacking with respect to the first graphene layer and displays weak vertical interaction and electron doping. The long-range ordered moiré pattern modulates the crystal potential and induces replicas of the Dirac cone and minigaps. The second phase has an AA stacking sequence with respect to the first layer and displays weak structural and electronic modulation and p-doping. The linearly dispersing Dirac state reveals the nearly freestanding character of this novel second-layer phase.

Varying molecular interactions by coverage in supramolecular surface chemistry.

The possibility of modifying the intermolecular interactions of absorbed benzene-carboxylic acids from coordination to hydrogen bonding by changing their surface coverage is demonstrated through a combination of scanning tunnelling microscopy, X-ray photoemission spectroscopy and density functional theory calculations.

Stabilizing monomeric iron species in a porous silica/Mo(112) film.

The stabilization of single Fe atoms in the nanopores of an ultrathin silica film grown on Mo(112) is demonstrated with scanning tunneling microscopy (STM) and density functional theory (DFT). The Fe atoms are able to penetrate the openings in the oxide surface and adsorb in two different binding configurations at the metal-oxide interface. In the energetically preferred site, the Fe stays monomeric even at temperatures above 300 K. In the second configuration that is adopted in 10% of the cases, surface atoms can be attached to the subsurface species, resulting in the formation of Fe surface clusters. The interfacial Fe atoms preserve their magnetic moment, as shown by a distinct Kondo-like response in STM conductance spectra and DFT calculations.

Direct observation of large electronic domains with memory effect in doped manganites.

We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional sensitivity to chemical compositions to investigate high-quality single crystal samples of La(1/4)Pr(3/8)Ca(3/8)MnO3, establishing the formation of distinct insulating domains embedded in the metallic host at low temperatures. These domains are found to be at least an order of magnitude larger in size compared to previous estimates and exhibit memory effects on temperature cycling in the absence of any perceptible chemical inhomogeneity, suggesting long-range strains as the probable origin.