Investigation of Upconversion Properties from Active Crystals in Oxyfluorophosphate Glasses
Ojha, Nirajan (2020)
Ojha, Nirajan
Tampere University
2020
Teknisten tieteiden tohtoriohjelma - Doctoral Programme in Engineering Sciences
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Väitöspäivä
2020-11-06
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-03-1716-4
https://urn.fi/URN:ISBN:978-952-03-1716-4
Tiivistelmä
Phosphate glasses have been of great interest for the fabrication of new lasing glasses. Indeed, as compared to other glass families, phosphate glasses can be prepared using low processing temperature and can incorporate a large amount of rare-earth (RE) ions, such as Er3+ ions. Er3+ ions have been of great interest as they can be used to convert Near InfraRed (NIR) radiation to shorter wavelengths due to the absorption of two or more photons by a process called upconversion. However, due to the high phonon energy of phosphate glasses, a large amount of Er3+ ions are usually required in phosphate glasses to exhibit upconversion.
Changing the site of the RE ions makes it possible to achieve upconversion while using lower amounts of Er3+ ions. The site of the RE ions can be modified by adjusting the glass composition and also by heat treating the glass to precipitate rare-earth doped crystals to obtain glass-ceramics. To be promising glass-ceramics for photonics applications, the glass-ceramics should be transparent with RE doped crystals dispersed evenly in their volume. However, it is crucial to control the nucleation and growth of the crystals when preparing the transparent glass-ceramics. As not all glasses can be heat treated into transparent glass-ceramics with volume precipitation of RE doped crystals, RE doped crystals with known chemistry and spectroscopic properties can be added in a glass matrix using the direct doping method. However, the crystals tend to decompose during the melting. Therefore, they should be added in the glass at lower temperature than the melting temperature to limit their decomposition during the glass preparation.
The main aim of this thesis was to develop new Er3+ doped phosphate glass-based materials for upconversion application. To improve their spectroscopic properties, the glasses were heat treated into transparent glass-ceramics. Due to the presence of Er3+ doped CaF2 crystals in their volume, the glass-ceramics exhibit enhanced upconversion compared to their parent glasses. The nucleation and growth process of the glasses was investigated, and it was shown that it is possible to tailor the transparency and thermal properties of the glass-ceramic by modifying the glass composition without changing its nucleation and growth process. Additionally, the newly developed glasses were developed with large temperature difference between the onset of crystallization and the maximum nucleation temperature confirming that they are promising materials for the fabrication of upconverter glass-ceramics as it is then possible to control the nucleation and growth of crystals in these glasses. Other glasses were also developed in this work; phosphate glasses were successfully prepared with NaYF4: Er3+, Yb3+ nanocrystals using the direct doping method. To limit the decomposition of the crystals during the glass preparation, the fundamental understanding of the direct doping method was advanced using persistent luminescent microparticles. Using optimized doping parameters, the NaYF4: Er3+, Yb3+ nanocrystals containing glass exhibit intense green upconversion confirming the presence of the nanocrystals in the glass. Finally, it was shown that a heat treatment of NaYF4: Er3+, Yb3+ nanocrystals containing glass to grow Ag nanoparticles, usually added in Er3+ doped glass to enhance the upconversion, led to a decrease in the intensity of the upconversion due to an increase of the inter defects in the NaYF4: Er3+, Yb3+ nanocrystals occurring during the heat treatment indicating that a heat treatment of the glass should be avoided after adding the NaYF4: Er3+, Yb3+ nanocrystals.
Changing the site of the RE ions makes it possible to achieve upconversion while using lower amounts of Er3+ ions. The site of the RE ions can be modified by adjusting the glass composition and also by heat treating the glass to precipitate rare-earth doped crystals to obtain glass-ceramics. To be promising glass-ceramics for photonics applications, the glass-ceramics should be transparent with RE doped crystals dispersed evenly in their volume. However, it is crucial to control the nucleation and growth of the crystals when preparing the transparent glass-ceramics. As not all glasses can be heat treated into transparent glass-ceramics with volume precipitation of RE doped crystals, RE doped crystals with known chemistry and spectroscopic properties can be added in a glass matrix using the direct doping method. However, the crystals tend to decompose during the melting. Therefore, they should be added in the glass at lower temperature than the melting temperature to limit their decomposition during the glass preparation.
The main aim of this thesis was to develop new Er3+ doped phosphate glass-based materials for upconversion application. To improve their spectroscopic properties, the glasses were heat treated into transparent glass-ceramics. Due to the presence of Er3+ doped CaF2 crystals in their volume, the glass-ceramics exhibit enhanced upconversion compared to their parent glasses. The nucleation and growth process of the glasses was investigated, and it was shown that it is possible to tailor the transparency and thermal properties of the glass-ceramic by modifying the glass composition without changing its nucleation and growth process. Additionally, the newly developed glasses were developed with large temperature difference between the onset of crystallization and the maximum nucleation temperature confirming that they are promising materials for the fabrication of upconverter glass-ceramics as it is then possible to control the nucleation and growth of crystals in these glasses. Other glasses were also developed in this work; phosphate glasses were successfully prepared with NaYF4: Er3+, Yb3+ nanocrystals using the direct doping method. To limit the decomposition of the crystals during the glass preparation, the fundamental understanding of the direct doping method was advanced using persistent luminescent microparticles. Using optimized doping parameters, the NaYF4: Er3+, Yb3+ nanocrystals containing glass exhibit intense green upconversion confirming the presence of the nanocrystals in the glass. Finally, it was shown that a heat treatment of NaYF4: Er3+, Yb3+ nanocrystals containing glass to grow Ag nanoparticles, usually added in Er3+ doped glass to enhance the upconversion, led to a decrease in the intensity of the upconversion due to an increase of the inter defects in the NaYF4: Er3+, Yb3+ nanocrystals occurring during the heat treatment indicating that a heat treatment of the glass should be avoided after adding the NaYF4: Er3+, Yb3+ nanocrystals.
Kokoelmat
- Väitöskirjat [4768]