Development of Lanthanide-doped Luminescent Nanomaterials for Phosphor Based LED and Sensing Applications

Adusumalli, Venkata Nanda Kishor Babu (2018) Development of Lanthanide-doped Luminescent Nanomaterials for Phosphor Based LED and Sensing Applications. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The research attention towards Ln³⁺ doped luminescent nanomaterials is growing continuously because of their characteristic optical properties, like sharp transitions with long luminescence lifetimes, large Stokes shift and peak position less influenced by the crystals field. In addition, Ln ions have the ability to spectral convert the radiations, Both downconversion and upconversion of photon energy is possible. The Ln³⁺ ion luminescence occurs over a wide spectral region from UV to near infrared (NIR). The forbidden nature of the f-f transitions is resulted in low extinction coefficients, making direct photo-excitation of lanthanide ions weaker. One of the strategies to increase the luminescence efficiency of the lanthanide (activator) ions is via luminescence resonance energy transfer (LRET) from any molecule or ion possessing high absorption coefficient and ability to transfer energy to the Ln³⁺ ions. Ce³⁺ions are found to be one of the suitable sensitizer for some of the Ln³⁺ ions. Due to close ionic radius, they can be easily doped along with the Ln³⁺ ions in the host matrix. Among different host matrices, fluorides are quite interesting and widely reported due to their low phonon energy (350- 400 cm⁻¹), which subsequently minimizes the non-radiative transitions. The unique spectroscopic properties of the luminescent lanthanide ions have been explored for several applications. They are widely used in solid state phosphor based light emitting applications, solar cell applications, anti-counterfeiting process, sensing, catalysis and biological applications like bio imaging, drug delivery, photodynamic therapy. Particularly Ln³⁺-based materials towards the development of phosphor based light emitting diodes (LEDs) with particular reference to blue and white LEDs are interesting due to their applications in general illumination and display applications. There are several reports on solid state blue light and white light emitting materials. Single component white light emitting materials are superior over other white light emitting materials. Single component white light can be produce by doping single Ln³⁺ ions which exhibit their emissions from blue region to red region or can be achieved by adjusting the different color combinations like appropriate mixing of blue, green and red or blue and yellow. This can be achieved by adjusting the individual doping concentrations of Ln³⁺ ions in a single-phase host matrix. Although solid state materials are efficient, there are few disadvantages, difficulties in controlling the size and morphology, poor control of color quality and dispersibility of materials. The alternative is to develop colloidal phosphors which exhibit strong blue and white light emission via single band excitation. Developing colloidal phosphors are advantageous as they can be easily coated on to and can be incorporated into sol-gel or glass matrices for the fabrication of devices with ease. In addition, colloidal phosphors can find potential use for detection or bio-imaging. To our knowledge there are only few reports are available on white light or strong blue light emitting materials with single band excitation, particularly in colloidal phosphors. Moreover, the sharp luminescence signals are not much explored for detection particularly for toxic and explosive materials. Chapter 2 discussed the synthesis of water-dispersible Ce³⁺/Tm³⁺-doped NaYF₄ nanocrystals (NCs) via microwave irradiation route. The NCs dispersion displayed a very strong single band blue emission at 450 nm through energy transfer from Ce³⁺ to Tm³⁺ ions. Moreover, the nanocrystals could be easily incorporated into a polymer matrix. The high transparency of the nanocomposite implies that they can easily be coated on a UV LED for developing bright blue phosphor-based LEDs. Chapter 3 reports the tuning of energy transfer efficiency between Ce³⁺ to Ln³⁺ ions (Ln= Tm, Tb, Sm and Dy) by controlling the crystal phase of Ce³⁺/Ln³⁺ doped NaYF₄ NCs prepared via hydrothermal method. With gradual increase in the reaction time, the phase of the NaYF₄ NCs changes from cubic to hexagonal. Despite the smaller particle size of cubic-phase NCs compared to hexagonal-phase NCs, the observed emission intensity of Ln³⁺ ions is relatively more in the cubic phase compared to hexagonal phase NaYF₄ NCs. This is attributed to the comparatively large overlap between Ce3+ emission band with Ln³⁺ ions absorption in cubic-phase NaYF₄ NCs than that in hexagonal NaYF₄ NCs. This resulted in higher energy transfer efficiency between Ce³⁺ to Ln³⁺ ions in the cubic phase compared to the hexagonal phase NCs. This difference is due to the difference in the splitting of 4f5d band of Ce³⁺ ion in the cubic and hexagonal environment with respect to their barycenter. Chapter 4, discussed the preparation of citric acid capped Ce³⁺/Tm³⁺/Tb³⁺/Sm³⁺-doped CaF₂ nanocrystals adopting a green synthetic route. The colloidal nanocrystals display intense white light upon exciting in the UV region (280 nm). The Ce³⁺ ions act as a sensitizer and transfer the energy to three activator ions, i.e. Tm³⁺, Tb³⁺, and Sm³⁺ ions. This resulted in strong blue, green and red emissions, respectively, from Tm³⁺, Tb³⁺ and Sm³⁺ ions, thus producing intense white light emission from a single component phosphor. The dopant ions are chosen such that the activator ions are excited through single band excitation using a suitable sensitizer (Ce3+ ions). In addition, there is hardly any cross-relaxation between the activator ions thus individual tuning of the emission intensity is possible by simply tuning their concentration. The intensity of the white light is quite efficient as they can be excited using a commercial UV LED. Moreover, the high dispersibility of the colloidal nanocrystals allows easy incorporation into a PVA polymer matrix to make transparent thin films without altering the luminescence properties of NCs. These thin films displayed strong white light when excited with UV light. Furthermore, individual emissions i.e. blue, green and red lights were observed from the nanocrystals using RGB colour filters. This suggests the suitability of the material for potential applications in flat panel displays and LED applications. Chapter 5 reports intense white light emission from colloidal Ce³⁺/Tm³⁺/Mn²⁺ doped-NaYF₄ nanocrystals under UV (280 nm) excitation. The strong intensity is ascribed to the sensitization of the Tm³⁺ and Mn²⁺ ions luminescence via energy transfer from Ce³⁺ ions. The resulting strong blue and greenish yellow emissions combine to produce strong white light emission from a single component phosphor. The observed emission pattern resembles with commercially available YAG:Ce³⁺ phosphor emission on 450 nm blue InGaN LEDs. There is hardly any reabsorption of blue emission of Tm³⁺ ion by Mn²⁺ ions. The calculated CIE color coordinates (0.31, 0.37) fall in the white light region. In addition, analysis of the CCT values indicates that the white light emission from NCs can be tuned from cool white light region to daylight region by simple tuning of the dopant ion concentration. The colloidal nature of the NCs allows development of a transparent thin film by simple incorporation of the NCs into polymer matrix. In fact, such thin film shows intense white light emission after deposition on top of UV LED and excited electrically (5 V current). Chapter 6, reports synthesis of para-mercapto benzoic acid (PMBA) capped CaF₂ nanocrystal. Here the role of PMBA is not only to control the size of the nanocrystals by binding to their surface but as a sensitizer to increase the luminescence efficiency of the Ln³⁺ ions. This is supported by the successful synthesis of small (~ 10 nm) nanocrystals of PMBA capped Tb³⁺(2%)-doped CaF₂ nanocrystals which exhibit strong luminescence from Tb3+ ions through energy transfer from PMBA molecules. In addition, the PMBA capped Tb³⁺ (2%)-doped CaF₂ nanocrystals were quite selective towards detecting some of the nitro aromatics like the para-nitrophenol, 2,4-dinitrophenol, trinitrophenol and para-nitrotoluene in water. A strong reduction in the intensity of the Tb³⁺ luminescence was observed in the presence of the above analysts. In addition, within the above four nitro compounds, a difference is noted in the excitation spectra. This will be useful in differentiating the nature of nitroaromatic molecules. The study also suggests that with other nitro compounds there is hardly any effect on the Tb³⁺ luminescence intensity. We believe this material would be potential for the detection some toxic aromatic compounds.

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