Crystal structure and electronic properties of low-dimensional hexamethylenediaminium lead halide perovskites.

This work reports on hybrid hexamethylenediaminium lead halide perovskites. The materials were prepared using wet synthesis and the subsequent precipitation from aqueous solution. Structural and morphological charactarization studies show their high degree of crystallinity and phase purity. The determined perovskites' structural parameters agree well with the literature reports. The recorded XPS and DRS data allowed for the first schematic representation of the perovskite band structures. The latter match well the results of DFT modeling. It is shown for the first time that the increase in the perovskite bandgaps is solely due to the increase in the anion electronegativity. Namely, as the anion electronegativity increases, the corresponding valence band energy decreases. In contrast, the electronegativity of the anions has no effect on the perovskite conduction band energies. The presented study deepens our understanding of the relationship between the crystal and electronic structures of low-dimensional hybrid halide perovskites.

The effect of organic cations on the electronic, optical and luminescence properties of 1D piperidinium, pyridinium, and 3-hydroxypyridinium lead trihalides.

We present a structural and optoelectronic study of 1D piperidinium, pyridinium, and 3-hydroxypyridinium lead trihalides. In contrast to the piperidinium and pyridinium species whose single inorganic chains [PbX31-]n are separated by organic cations, the 3-hydroxypyridinium compound is characterized by double inorganic chains. According to DFT the valence and conduction bands of the piperidinium lead trihalides are composed of occupied p-orbitals of the halogen anions and unoccupied p-orbitals of the Pb2+ cations. In contrast, the pyridinium species feature low-lying cationic energy levels formed from the cation's π*-orbitals. Thus, electronic transitions between the cationic energy levels and valence bands require less energy than valence to conduction band transitions in the case of piperidinium lead trihalides. The presence of an OH group in the pyridinium ring leads to a bathochromic shift of the cationic energy levels resulting in a decreased energy of transitions from the cationic energy levels to the valence band. Electronic transitions predicted by DFT are observable in experimental optical absorption and luminescence spectra. This study paves the way for creation of 1D perovskite-like structures with desired optoelectronic properties.

Pyridinium lead tribromide and pyridinium lead triiodide: quasi-one-dimensional perovskites with an optically active aromatic π-system.

Two novel hybrid organic-inorganic perovskites, pyridinium lead tribromide and pyridinium lead triiodide, are prepared. Based on the XRD data, we find that the compounds are quasi-one-dimensional crystals of Pna21 symmetry. DFT calculations demonstrate that the valence band of the new compounds is comprised of the occupied p-orbitals of the halogen anions, while their conduction bands CB and CB + 1 are composed mainly of the unoccupied π-orbitals of the aromatic pyridinium rings. The computed DOS matches well with the recorded XPS spectra. The pyridinium lead tribromide and pyridinium lead triiodide absorption peaks derived from the experimental DRS are located at ∼3.1 eV and ∼2.4 eV, respectively. An agreement between the experimentally and theoretically predicted absorption spectra is found. The synthesized compounds extend the range of quasi-one-dimensional organometallic perovskites suitable for optical and possibly electronic applications.

Photoionisation study of Xe.CF4 and Kr.CF4 van-der-Waals molecules.

We report on photoionization studies of Xe.CF4 and Kr.CF4 van-der-Waals complexes produced in a supersonic expansion and detected using synchrotron radiation and photoelectron-photoion coincidence techniques. The ionization potential of CF4 is larger than those of the Xe and Kr atoms and the ground state of the Rg.CF4 (+) ion correlates with Rg(+) ((2)P3/2) + CF4. The onset of the Rg.CF4 (+) signals was found to be only ∼0.2 eV below the Rg ionization potential. In agreement with experiment, complementary ab initio calculations show that vertical transitions originating from the potential minimum of the ground state of Rg.CF4 terminate at a part of the potential energy surfaces of Rg.CF4 (+), which are approximately 0.05 eV below the Rg(+) ((2)P3/2) + CF4 dissociation limit. In contrast to the neutral complexes, which are most stable in the face geometry, for the Rg.CF4 (+) ions, the calculations show that the minimum of the potential energy surface is in the vertex geometry. Experiments which have been performed only with Xe.CF4 revealed no Xe.CF4 (+) signal above the first ionization threshold of Xe, suggesting that the Rg.CF4 (+) ions are not stable above the first dissociation limit.

Interaction energies in hydrogen-bonded systems: a testing ground for subsystem formulation of density-functional theory.

The formalism based on the total energy bifunctional (E[rhoI,rhoII]) is used to derive interaction energies for several hydrogen-bonded complexes (water dimer, HCN-HF, H2CO-H2O, and MeOH-H2O). Benchmark ab initio data taken from the literature were used as a reference in the assessment of the performance of gradient-free [local density approximation (LDA)] and gradient-dependent [generalized gradient approximation (GGA)] approximations to the exchange-correlation and nonadditive kinetic-energy components of E[rhoI,rhoII]. On average, LDA performs better than GGA. The average absolute error of calculated LDA interaction energies amounts to 1.0 kJ/mol. For H2CO-H2O and H2O-H2O complexes, the potential-energy curves corresponding to the stretching of the intermolecular distance are also calculated. The positions of the minima are in a good agreement (less than 0.2 A) with the reference ab initio data. Both variational and nonvariational calculations are performed to assess the energetic effects associated with complexation-induced deformations of molecular electron densities.