Solid state physics: advanced spectroscopy, scanning probe microscopy, nanostructure fabrication

Thin films of hybrid solar cells and metal oxide semiconductors -IGZO in particular– and homogeneous PMMA polymers have been studied at the Positron Laboratory (L-NESS centre, Politecnico di Milano, Polo Territoriale di Como). A slow energy positron beam and a positron lifetime spectrometer have been employed for these studies. The positron spectroscopy information have been correlated with electrical and optical properties of the materials. The chemical composition and the morphology of voids and porosities in hybrid solar cells and thin film metal oxide semiconductors have been studied, and a strong correlation between positronium fraction, S-parameter and the electrical properties of these materials has been found. In PMMA polymers, free volume measurements have shown that the optical properties of the material depend on the presence of monomer residual fraction and even slight changes in the dimensions and concentration of free volumes. Positrons have been also applied to the study of positron to positronium converters in reflection geometry for the AEgIS (Antimatter Experiment: gravity Interferometry, Spectroscopy) experiment at CERN, which is aimed at measuring the gravitational acceleration of antihydrogen. It has been shown that Silica Aerogel and MCM-41 emit a high yield of cold positronium in vacuum and therefore are good candidates as sources for the AEgIS experiment. Moreover the properties of Aerogel 85 have been used to simulate a cold positronium yield in vacuum using micrometric silica thin films in transmission geometry. According to simulation results, the use of transmission geometry is very promising. Simulation results will be used as indicators for the design of thin film silica positron to positronium converters. In conclusion, besides acknowledging that the AEgIS experiment is ready to produce antihydrogen in order to measure antigravity, it has been shown that the positron annihilation techniques are suitable for the analysis of the composition, porosities and voids of contemporary thin film based devices and allow disclosing new sets of information on them. Moreover, in fundamental antimatter physics experiments, the use of micrometric mesoporous silica membranes in transmission geometry will remarkably increase efficiency in the production of cold positronium.