Photoluminescence of Er-doped Si-SiO2 and Al–Si-SiO2 sputtered thin films

Er-doped Si-SiO₂ and Al–Si-SiO₂ films have been deposited by rf-sputtering being annealed afterwards. Annealing behavior of the Er³⁺: ⁴I_13/2→⁴I_15/2 emission of Er-doped Si-SiO₂ yields a maximum intensity for annealing at 700–800 °C. ⁴I_13/2→⁴I_15/2 peak emission for Er-doped Al–Si-SiO₂ at 1525 nm is shifted from that for Er-doped Si-SiO₂ at 1530 nm and the bandwidth increases from 29 to 42 nm. ⁴I_13/2→⁴I_15/2 emission decays present a fast decaying component related to Er ions coupled to Si nanoparticles, defects, or other ions, and a slow decaying component related to isolated Er ions. Excitation wavelength dependence and excitation power dependence for the ⁴I_13/2→⁴I_15/2 emission correspond with energy transfer from Si nanoparticles. Populating of the ⁴I_11/2 level in Er-doped Si-SiO₂ involves branching and energy transfer upconversion involving two or more Er ions. Addition of Al reduces the populating of this level to an energy transfer upconversion involving two ions.     

Synchrotron powder diffraction on Aztec blue pigments

Some samples of raw blue pigments coming from an archaeological rescue mission in downtown Mexico City have been characterized using different techniques. The samples, some recovered as a part of a ritual offering, could be assigned to the late Aztec period (XVth century). The striking characteristic of these samples is that they seem to be raw pigments prior to any use in artworks, and it was possible to collect a few μg of pigment after manual grain selection under a microscopy monitoring. All pigments are made of indigo, an organic colorant locally known as añil or xiuhquilitl. The colorant is always found in combination with an inorganic matrix, studied by powder diffraction. In one case the mineral base is palygorskite, a rare clay mineral featuring micro-channels in its structure, well known as the main ingredient of the Maya blue pigment. However, other samples present the minerals sepiolite (a clay mineral of the palygorskite family) and calcite. Another sample contains barite, a mineral never reported in prehispanic paints. We present the results of characterization using high resolution powder diffraction recorded at the European Synchrotron Radiation Facility (BM25A, SpLine beamline) complemented with other techniques. All of them gave consistent results on the composition. A chemical test on resistance to acids was done, showing a high resistance for the palygorskite and eventually sepiolite compounds, in good agreement with the excellent resistance of the Maya blue.     

Cage-size effects on the encapsulation of P2 by fullerenes

The classic pnictogen dichotomy stands for the great contrast between triply bonding very stable N₂ molecules and its heavier congeners, which appear as dimers or oligomers. A banner example involves phosphorus as it occurs in nature as P₄ instead of P₂, given its weak π-bonds or strong σ-bonds. The P₂ synthetic value has brought Lewis bases and metal coordination stabilization strategies. Herein, we discuss the unrealized encapsulation alternative using the well-known fullerenes' capability to form endohedral and stabilize otherwise unstable molecules. We chose the most stable fullerene structures from C_n (n = 50, 60, 70, 80) and experimentally relevant from C_n (n = 90 and 100) to computationally study the thermodynamics and the geometrical consequences of encapsulating P₂ inside the fullerene cages. Given the size differences between P₂ and P₄, we show that the fullerenes C₇₀–C₁₀₀ are suitable cages to side exclude P₄ and host only one molecule of P₂ with an intact triple bond. The thermodynamic analysis indicates that the process is favorable, overcoming the dimerization energy. Additionally, we have evaluated the host-guest interaction to explain the origins of their stability using energy decomposition analysis.