[Show abstract][Hide abstract] ABSTRACT: We report on a combined X-ray and UV photoemission spectroscopy study (XPS and UPS) of organic-inorganic perovskites prepared from a solution of lead chloride (PbCl2) and methylammonium iodide (CH3NH3I). The XPS intensities are consistent with a pure iodide perovskite (CH3NH3PbI3), with no detectable chloride left. However, we found that the elimination of chloride results in residual methylamine molecules (CH3NH2) trapped within the perovskite crystal lattice. Furthermore, we show that vacuum annealing or sputtering induce the formation of a thin PbI2 layer at the crystal surface which acts as a surface barrier blocking electron transfer from the underlying perovskite film.
The Journal of Physical Chemistry C 08/2015; 119(37). DOI:10.1021/acs.jpcc.5b05422 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Free-base H2TPP porphyrin layers have been grown on highly oriented pirolitic graphite by organic molecular beam epitaxy. The growth follows a Stranski-Krastanov (SK) mode, as observed by atomic force microscopy (AFM), showing 3D porphyrin structures above a single flat porphyrin layer, which can be considered as an organic wetting layer. The 3D phase precludes a priori the possibility of studying and exploiting the physical-chemical properties of the pure 2D phase. Here we describe a promising post growth strategy to overcome the limitations imposed by the SK film evolution. We are able to reduce the number of 3D structures on the surface without perturbing the porphyrin wetting layer, as experimentally observed by AFM and optical spectroscopy. The strategy described in this work is fully generalizable and, in principle, could be extended to other free-base porphyrinic compounds.
Crystal Research and Technology 08/2014; 49(8). DOI:10.1002/crat.201300406 · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have studied the controlled adsorption of molecules of volatile compounds on thin Langmuir-Blodgett films of porphyrins deposited onto the surface of a quartz microbalance. The modification of the film optical anisotropy has been measured by reflectance anisotropy spectroscopy (RAS), and simultaneously, the amount of adsorbed molecules has been monitored. Optical spectra measured after adsorption of single analytes are interpreted straightforwardly in terms of anisotropies related to the H-like and J-like aggregation modes of porphyrins. By an appropriate selection of analytes, we have then investigated the effect of the exposure to controlled vapor mixtures. The experimental RAS line shape variation with respect to the signal measured before exposure is the sum of the variations produced by single analytes. This shows that the identification of types and even quantities of gases and vapors in gas sensing experiments is possible, and the responses of the sensing layer to single analytes could be listed in a library.
The Journal of Physical Chemistry C 04/2011; 115(16):8189-8194. DOI:10.1021/jp200303t · 4.77 Impact Factor