Guillermo Carbajal-Franco

With the recently acquired knowledge of the use of a multiphase mixture of precursors under electron beam irradiation (EBI), new possibilities were opened for this technique. In the present work, we obtained quantum dots, nanocrystals, nanoparticles, and grains of PbSe with a sintered appearance using a biphasic mixture of PbSe and PbSeO₃ under EBI. High-energy milling was used to obtain the biphasic mixture of precursors, which is composed of agglomerates with sizes ranging from ~400 to ~1700 nm, but nanoparticles were also present. The structural details of the biphasic mixture were studied using X-ray diffraction and the Rietveld method. The driving force of the EBI caused instantaneous physical and chemical changes due to the high internal energy of the biphasic mixture of precursors. The abrupt release of high internal energy, due to localized heating effects during EBI, gave way to the formation of multi-scale PbSe structures. Large particles with a sintered appearance formed near the electron beam impact point and in regions between ~800 nm and ~1400 nm, while well-defined faceted nanostructures were predominantly observed beyond ~1400 nm. The latter tended to be surrounded by {200} facets as the main growth direction. Furthermore, coalescence was anticipated to occur during EBI. It occurred simultaneously with the sublimation mechanism when the particle size was below the critical size of 10 nm. Multi-scale PbSe structures, obtained via EBI, are promising for developing thermoelectric devices due to their crystallinity and nanostructured features.

For the first time, a mixture of PbTe and Pb- and Te-oxides coated with carbon, under electron beam irradiation (EBI), was transformed into quantum dots, nanocrystals, nanoparticles and grains of PbTe with a sintered appearance. A small portion of non-stoichiometric phases was also obtained. By selecting conditions that favor the instantaneous transformation, the Gibbs free energy barrier is lowered for obtaining different PbTe structures. The driving force associated with the high-energy milling requires 4 h of processing time to reach a complete transformation, while a high-energy source kinetically affects precursor surfaces to cause an abrupt global chemical transformation instantly. Importantly, the size of the PbTe structures increases as they approach the irradiation point, implying a growth process that is affected by the local temperature reached during the EBI. Imaging after the EBI process revealed morphological variations in PbTe, which can be attractive for use in thermoelectric materials. The results of this study provide the first insights into electron-beam-induced reactions using a multiphase mixture of precursors. Therefore, it is believed that this proposal can also be applied to obtain other binary semiconductor structures, even ternary ones.

The sustainable production of energy is a field of interest to which a new requirement is now imposed: the need to be respectful of the environment. New materials and techniques are being developed, but environmental concerns impose the necessity of keeping research active towards the development of green energy. For this reason, we present the study of short polythiophene (PTh) chains (three and five monomers) and their interaction with nickel oxide, looking for properties related to solar photon harvesting in order to produce electricity. The models of the molecules were developed, and the calculations were performed with an M11-L meta-GGA functional, specially developed for electronic structure calculations. The theoretical explorations demonstrated that the geometry of the PTh molecules suffer little distortion when interacting with the NiO molecule. The calculated value of E_g lies between 2.500 and 0.412 eV for a three-ring PTh chain and between 1.944 and 0.556 eV for a five-ring PTh chain. The chemical parameters indicated that, depending on the geometry of the system, the chemical potential varies from 81.27 to 102.38 kcal/mol and the highest amount of electronic charge varies from −2.94 to 21.56 a.u. for three-monomer systems. For five-monomer systems, the values lie within similar ranges as those of the three-monomer systems. The Partial Density of States (PDOS) showed that the valence and conduction electronic bands were composed of states in the NiO and PTh rings, except for a system where there was a non-bonding interaction.

A process control agent is an organic additive used to regulate the balance between fracturing and mechanical kneading, which control the size of the as-milled particles. Tributyl phosphate (TBP) is evaluated to act as surface modifier of PbTe, and it is compared with the results obtained using formaldehyde (CH₂O). In order to elucidate the nature of the interaction between TBP and the PbTe surface, global and local descriptors were calculated via the density functional theory. First, TBP and CH₂O molecules are structurally optimized. Then, vertical ionization energies as well as vertical electron affinities are calculated to elucidate how both molecules behave energetically against removal and electron gain, respectively. The results were compared with those obtained from the electrostatic potential mapped on the van der Waals isosurface. It is inferred that the theoretical insights are useful to propose adsorption modes of TBP and CH₂O on the PbTe surface, which are usable to rationalize the facets exposed by PbTe after the surface treatment. The optimized structures of the compound systems showed a close correlation between the surface energy shift (Δγ) and the PbTe facets exhibited. Finally, a Wulff construction was built to compare the usage of TBP and CH₂O molecules in PbTe morphology.