Transition metals and erbium doping of silicate glasses by field-assisted solid-state ion exchange

The doping of various glasses with transition metals and rare-earths for altering the physical or electronic structure of a glass, which subsequently yields to a change in the refractive index and in several optical properties, is exploited for its potential applications in optical materials and functional photonic devices. These doped glasses, possibly containing dopant nanoclusters, are as vital to the future development of photonic systems as integrated circuits are to the electronic systems. Number of techniques have been employed to achieve an in-depth and homogeneous diffusion, but all come with some limitations and constraints. Due to advancements in this field and needs to further the exploration of glass modification, a new technique, field-assisted solid-state ion exchange (FASSIE) is introduced. An experimental setup is developed and the problems faced during the very first experiments lead to the modified version of the setup. The technique is successfully realized for doping silicate glasses not only with monovalent but also with multivalent ions with homogeneous and in-depth concentration profiles. An exhaustive description and a complete understanding of the diffusion are still lacking because of its mechanism and physics behind the diffusion process; however, it is a non-equilibrium technique, where the dopant ions diffuse to either replace alkali ions of a glass or fill the gaps/defects in a matrix. In this thesis, the technique is exploited for the diffusion of transition metals, namely, chromium, silver, and gold, as well as rare-earth erbium in various matrices like soda-lime, borosilicate and pure silica glasses. Besides these hard matrices, diffusion of highly mobile silver is also carried out in a micron thick sol-gel silica layer at very low applied electric field strengths. The dopants diffusion into a given matrix depends not only on the nature of the dopant and experimental conditions but also on the local structure and the chemical phenomena occurring at the metal-glass interface. The diffusion of multivalent gold is found to be much favored in borosilicate matrix than in soda-lime glass and the resulting concentration profiles are comparably homogeneous with the possible formation of nanoparticles that are well distributed in the matrix. In addition to the in-depth concentration characterizations, m-line spectroscopy is used for the investigation of optical properties of the doped layers. Preliminary results are reported for samples prepared using either silver or gold as the dopant, suggesting the presence of guided modes, i.e. two modes in the visible and one in the infrared. In the case of pure silica, the diffusion behavior of silver and gold is completely different from what observed in sodium-containing glasses. This diversity is due to their different mobilities and oxidation states as well as to the fact that penetration of dopants actually takes place through defects/impurities and local rearrangements of the structure. Both of these metals present a threshold time of oxidation prior to the diffusion; however, silver is monitored to be extremely sensitive to the experimental parameters, and the diffusion turns out to be completely suppressed at intense fields. This technique is also utilized for the first time for doping soda-lime glass with chromium. The ionic transport proportionally follows the experimental conditions and replaces the depleted alkali of the glass. An accumulation of the dopant occurs well-beneath the surface at low experimental parameters, which can be smoothed or reduced by increasing the temperature and applied electric field. As FASSIE is a non-equilibrium technique and so the constraints posed by the thermodynamics of chemical formation of compounds and bonds may be overcome. The valence state of chromium can be suitably controlled by post-exchange treatments. Finally, this novel technique is utilized for the first time to directly dope surface layers of soda-lime glass with erbium at high concentration, which explore new ways for erbium-doped glasses towards numerous applications. Rutherford backscattering spectrometry confirms the existence of high concentration of erbium in the silicate matrix, while photoluminescence reveals that the incorporated rare-earth ions are in an optically active configuration. An extended x-ray absorption fine structure spectroscopy experiment is then performed to investigate the local environment of the erbium. The analyses show that two main coordination peaks are due to the Er-O and Er-Si coordinations. The structural, optical, and compositional analyses of the doped erbium assess the effectiveness of the technique, making it a new route for the controlled preparation of erbium-doped materials. In general, the presented experimental observations weigh up the novelty of the technique; indicating that high concentration of dopants diffuse into the glass matrix, which are suitable for a subsequent post-exchange treatments and can be utilized for advanced application in the field of photonics materials and thin film integrated optoelectronic technology. Several characterization techniques are used during the experimental work, namely, secondary ion mass and Rutherford backscattering spectrometries, optical absorption, extended x-ray absorption fine structure, ellipsometry, m-line technique and photoluminescence, each of which is discussed in detail with typical experimental results in an appendix at the end of the thesis.