Borohydride Synthesis for Development of Rare Earth Doped and Co-Doped Yttrium Borate Phosphors for Luminescent Applications

Abstract

The research endeavor on the yttrium borate (YBO3) phosphors for an array of applications, including light emitting diodes, flat display technology and optoelectronic devices, is escalating with the availability of these borate hosts, which present incentives such as excellent UV absorbance, high thermal and chemical stability and exceptional optical damage threshold. An insight into their inherent potential necessitates the requirement for a meticulous investigation of their photoluminescence properties, especially with respect to the type of dopant used. However, for the preparation of this aforementioned host, the use of boric acid (H3BO3) as a boron source is widely prevalent. The literature gives a plethora of evidences for the difficulties encountered in formation of a phase pure YBO3 on higher temperatures. Apart from that, the most critical parameter of Y:B ratio has a crucial effect on the phase purity of the material. The key objective of this research work is to synthesize yttrium borate (YBO3) host matrix as well as single rare earth doped and co-doped YBO3 phosphors using novel sodium borohydride based solution precursor route, while probing their photoluminescence behavior by varying different parameters emphasizing color tunability including white light for versatile application domains.

In this thesis we report a novel borohydride synthesis route based on the inclusion of sodium borohydride, which acts both as a boron source and as a precipitating agent, at room temperature without any organic additives, for the successful synthesis of yttrium borate based phosphors. New experimental approach has been demonstrated to form phase pure material, which eliminates the addition of any external precipitating agent, with a detailed emission characteristics analysis for the development of single activator Eu-doped YBO3 and Tb-doped YBO3 phosphors. In addition, color tuning in case of single activator doped YBO3 [Eu-doped and Tb doped] was demonstrated with a focused approach of excitation wavelength induced photoluminescence for samples calcined at 800 °C and 1200 °C. The evaluated chromaticity coordinates (CIE) derived from the spectral energy distribution of emission showed a variation with excitation wavelengths, thereby validating the excitation wavelength dependent tunability of a single activator doped YBO3 phosphors. Besides, incredibly distinguishable colors on irradiating the phosphors by switching the standard excitation wavelengths of 254 nm and 365 nm while generating self-adjustable emissions comprising shades of red and pink for Eu-doped YBO3 and tunable shades of green from single activator Tb-doped YBO3 including near white light enriches the contribution of this research.

Furthermore, single phase white light emitting phosphor by co-doping Eu and Tb ions into YBO3 matrix have been developed by this novel route, which also expounds the versatility of the borohydride method for providing different instances of phosphor materials. Again, photoluminescence studies revealed excitation induced tunable emissions in Eu, Tb co-doped phosphors with tunability spanning across yellow, neutral white, pink and red. Moreover, the properties of single activator doped and co-doped YBO3 phosphors thus developed by borohydride route have been comparatively assessed with bulk phosphor prepared by conventional solid state route. The follow up of the properties unanimously depicts that the use of sodium borohydride is virtuous for synthesizing nanophosphors.

Finally, a step forward has been taken towards successfully demonstrating substantiative end-product prototype applications using the phosphor materials prepared in this research work. These include demonstration of a stable white light using blue LED in combination with an optimized single phase YBO3:Eu,Tb phosphor; developing for the first time a novel polyvinylidene fluoride [PVDF] polymer based flexible film which gives homogeneous luminescence when incorporated by the YBO3 based phosphor particles and irradiated under UV radiation of 254 nm; and a patterned film that can be practically applied and tailored further according to specific needs employed in security domain for fraud prevention. This adds another facet to the useful dimensions of applicability pertaining to YBO3 based phosphors.