Exploration of New Lanthanide Based Inorganic Phosphors Intended for Light Emitting Diodes

Abstract

The work described in the thesis deals with the study of Exploration of new lanthanide based inorganic phosphors with different crystal structures. The objective of the present study is to design and synthesis of Eu3+ based narrow band red emitting, Eu2+ based broad band tunable phosphors and Europium molecular complex in order to explore the possibilities of new phosphor materials as promising candidates for white light emitting diodes (including solid state lighting (SSL)) applications and to probe their structure - composition - property correlations.

Chapter 1 introduces the solid state luminescence in inorganic materials with different crystal structures and the details of the constituents of phosphor materials are discussed with special highlight on rare earth ions. The main aim and importance of the proposed work of the thesis was summarized in this chapter.

Chapter 2, discusses the Eu3+ luminescence in La1.95Eu0.05W2-xMoxO9 (x = 0 – 2, insteps of 0.1) and La2-yEuyW1.6Mo0.4O9 (y = 0 – 2, insteps of 0.2). Powder XRD analysis of La1.95Eu0.05W2-xMoxO9 (x = 0 – 2, in step of 0.1) reveals a phase transition from triclinic to cubic structure with increasing value Mo6+ concentration (x ≥ 0.2). All the compositions show broad charge transfer (CT) band due to CT from O to W/Mo and red emission (due to Eu3+ ions). In order to obtain the strongest red emission, the concentration variation of Eu3+ was performed in La2-yEuyW1.6Mo0.4O9 (x = 0 – 2, in steps of 0.2). Eu1.6La0.4W1.6Mo0.4O9 composition shows high red emission intensity (~8.2 times) compared to that of commercial red phosphor with very good CIE chromaticity coordinates (x = 0.67, y = 0.33). Selected composition has also been synthesized by simple co-precipitation method. The emission intensity of the Eu1.6La0.4W1.6Mo0.4O9 phosphor, synthesized by co-precipitation technique is ~1.78 times higher than that of the one which is synthesized by solid state reaction. The phonon sideband spectrum analysis has been carried out for Eu3+ activated La2M2O9 (M = W and Mo). The temperature dependent photoluminescence studies reveals that the Eu1.4La0.6W1.6Mo0.4O9 compositions loses 30% of efficiency up to 400 K (compared to ~80% for CaS:Eu2+) and the Judd – Ofelt parameters were also calculated for Eu3+ ions. In addition, the Eu3+ concentration quenching studies also been executed in the La2M2O9 (M = W and Mo) phosphors.

Chapter-3, describes the optical properties of Eu3+-substituted La2Li0.5Al0.5O4 red emitting phosphors. Powder X-Ray diffraction (XRD), diffuse reflectance spectra (DRS) and spectroflurometry were used as vital characterizing tools for the phosphors. The europium (Eu) concentration dependence luminescence properties and Judd–Ofelt intensity parameters have been investigated and calculated, respectively. All the compositions showed orange red emission (due to magnetic and electric dipole transition of Eu3+ ion) with appropriate CIE colour gamut under near UV or blue ray excitation. The calculated critical distance showed that the energy transfer occurs between Eu to Eu via exchange mechanism. Eu1.4La0.6Li0.5Al0.5O4 composition showed highest red emission intensity with CIE color saturation compared to that of commercial Eu activated yttrium oxy-sulfide red phosphor. Eu3+ activated double perovskite phosphor gained much interest due to their potential use in white LEDs. In the second part, a series of novel red emitting phosphor compositions, Ca2MgW1-xMoxO6, have been synthesized and studied their optical properties. Further, Eu3+ concentration variation was executed in Ca2MgW0.85Mo0.15O6 host lattice to gain enhanced excitation and emission properties. All the compositions show broad and intense charge transfer (CT) absorption band ranging from UV to blue region, where the near UV-LED (nUV-LED) emission occurs. The Eu3+ emission intensity of significantly improved in the solid solutions of tungstate and molybdate [Ca2MgW1-xMoxO6; x = 0 – 1 and Ca2-2yEuyNayYyMgW0.85Mo0.15O6. Selected compositions show high emission intensity (red) compared to that of Eu3+ substituted parent phases. Dominant electric dipole(ED) transition was observed for all Eu3+ activated Ca2MgW1-xMoxO6 compositions. In this structure, the Eu3+ occupies a highly distorted non– centrosymmetric site. Ca0.9Eu0.05Li0.05MgW0.85Mo0.15O6 composition shows intense red emission under NUV/blue excitation and the emission intensity is found to be ~6 times higher than that of commercial red phosphor (Y2O2S:Eu3+ Nichia) under blue excitation (465 nm).

Chapter-4, describes the luminescence properties of red emitting Li3BaSrLn3-xEux(MO4)8 (Ln = Gd, La and Y, M = W, Mo) and its solid solution phosphors with stratified scheelite structure. All the compositions crystallize in the monoclinic structure with space group P2/c. Host as well as the phosphors (Eu3+ substituted compositions) show broad absorption band in the UV to near UV region, due to the oxygen to molybdenum (absorption edge ~360 nm, O2- → Mo6+) or tungsten (absorption edge ~320 nm, O2- → W6+) CT transition. The photoluminescence (PL) study of Eu3+ substituted compositions show emission at ~615 nm (due to the forced electric dipole (ED) 5D0 →7F2 transition of Eu3+ ion), which confirms that the Eu3+ ion present in the non-centrosymmetric site. In order to study the emission behaviour systematically the Judd Ofelt (J – O) spectral intensity parameters were calculated for all phosphor compositions. The CIE colour coordinate values are well harmonized with the National Television Standard Committee (NTSC) standards. The presently synthesized tungstate, molybdate and its solid solutions (selected compositions) phosphors have been used as reference for contrast between two. The entire compositions show better absorption in the near UV to blue region and emitting red light ~615 nm indicates that this phosphor may find potential application as a red phosphor for white LED. Selected phosphors show extremely high quantum yield and the same were used to integrate with near UV LED and fabricated red LED with forward bias 10mA.

Chapter-5 discusses the Eu3+ activated Ln fluorides (LnF3:Eu3+ - Ln= La, Gd) nanocrsytals synthesized by hydrothermal method using 1-Butyl-3-methylimidazolium tertrafluoroborate [BMIBF4] and NH4F as fluorine precursors. In addition, LaF3 samples through a facile hydrothermal route with hexagonal structures have been synthesized via doping of trivalent rare earth (RE3+) - RE= Eu, Tb, Sm, Dy and Tm) with rod – like and perforated morphologies using NH4F as fluorine precursor. The synthesized nanocrystals (NCs) have been structurally, morphologically characterized and their optical properties have also been investigated by spectroflurometry. The XRD patterns of Eu3+ or RE3+ substituted and unsubstituted LnF3 are indexed based on hexagonal and orthorhombic crystal structure, respectively. The TEM images reveal that the NCs are well dispersed, nearly ellipsoid, with an average size of about 5 nm. The Eu3+ activated NCs show characteristics excitation and emission spectra. The emission spectra show both magnetic (5D0 - 7F1, MD) and electric (5D0 - 7F2, ED) dipole transition with appropriate CIE color co-ordinates; however, the intensity of the MD transition was found to be high, which is accordance with the local site symmetry. The LaF3:Tm3+,Sm3+ ions show the characteristic emission of Tb3+ (green) and Tm3+ (blue), respectively. In Sm3+-doped LaF3, three prominent emission peaks at 561, 597 and 641 nm were found which are belongs to 4G5/2→6H5/2, 4G5/2→6H7/2 (MD) and 4G5/2→6H9/2 (ED) transitions, respectively. The absolute quantum yield calculated for Eu3+ activated LaF3 was found to be 16.8%. Further, Judd–Ofelt calculation has been used to analyze the experimental phenomena of Eu3+ activated LaF3. Dy3+ activated LaF3 show blue and yellow and the corresponding CIE color coordinate show white light emission (CCT value 10650K). The surface functionalized nanophosphor (DBM –LaF3:Eu3+) show extremely dominant forced electric dipole transition and significant enhancement in PLQY (79.7%) was observed. The DBM-LaF3:Eu3+ integrated with InGaN LED shows red emission (under 20 mA forward biased current) with good color saturation (CIE values x = 0.6545, y = 0.3433).

Chapter-6 describes the Eu2+ luminescence in silicate based host lattice. In the first part, a series of novel yellowish – orange emitting phosphor [Ca3-xMxSi2O7 (M = Eu2+)] have been synthesized by high temperature solid state reaction. The powder x – ray diffraction patterns reveals a single phase formation with excellent crystalline nature of the phases. The photoluminescence excitation and emission studies show that the Eu2+ doped phosphors show broad absorption (300 – 525 nm) and emits yellowish – orange–red spectral region. Further efforts have been made to tune the Eu2+ emission by crystal chemical isovalent substitution of Mg2+ in Ca2+ site. Compositionally induced phase transformation has been observed at y ≥ 0.25 in Ca2.985MgxEu0.015Si2O7. The yellowish – orange-red emission (y = 0) tuned to green color (y = 1) and the corresponding CIE color coordinates (x,y) are changed from (0.5647, 0.4202) to (0.3540, 0.5932). In the second part, a series of Eu2+ activated Barium orthosilicate (BaZnSiO4) were synthesized by high temperature solid state reaction. The PL excitation study of Eu2+ shows a broad absorption band in the range of 270 to 450 nm with multiple absorption peak maxima (310, 350 and 400 nm) due to 4f – 5d electronic transition. The emission spectra of all the compositions show green color emission (in the spectral region 450 – 550 nm with a peak maximum at 502 nm and a shoulder at ~490 nm) with appropriate CIE color coordinates. The two emission peaks are due to the presence of Eu2+ in two different Ba sites in the BaZnSiO4 host lattice. The energy transfer between the Eu2+ ions in BaZnSiO4 host are elucidated from the critical concentration quenching data based on the electronic multipolar interaction. All Eu2+ activated BaZnSiO4 phosphor materials can be efficiently excited in the UV to near UV region (270 - 420 nm), making them attractive candidate as green phosphor for SSL – white LEDs.

Chapter-7 describes the design, synthesis and photophysical study of imidazo-bipyridyl based Eu-complexes. Three ancillary ligands based on imidazo-bipyridyl with phenyl (Ph), naphthyl (Np), triphenylamine (TPA) substitution were synthesized and secondhand to formulate the consistent europium(III) ternary complexes using thenoyltrifluoroacetone (TTA) as an anionic ligand. The complete investigation of spectroscopic, photophysical and electrochemical properties was carried out. The attained results for all the ancillary ligands and their corresponding Eu-complexes were compared with one another. All the Eu-complexes reveal a broad excitation band ranging from near UV to blue region, along with high intense emission and apposite color purity. To further understand the ligand to metal energy transfer (ET) process, the geometry of the ligand was optimized and the energy level location (singlet and triplet) was calculated, by using DFT and TD-DFT calculations. On the basis of the theoretical calculation, the ET mechanism was proposed. From PL emission spectra in solid state, complete ET occurs from Ph, Np based ancillary ligands to Eu3+ ion, which yield a pure red emission, whereas the TPA functionalized based Eu complex show incomplete ET. Fortunately, white emission was observed in the TPA based Eu complex in the solid state. The white LED was fabricated by using a white emitting complex integrated with 395 nm emitted LED (InGaN) chips under 20 mA forward-bias current. The excitation source from LED was fully observed by the complex shown for 3Eu and showed yellowish emission in different concentrations (the similar observation also reflected in solid). However, in the case of 1Eu and 2Eu complexes, they showed close to white emission. The CIE chromaticity coordinates are close to the NTSC standard value for white emission, and in addition, the complex 3Eu coated with the blue LED chip (460 nm) by PMMA (1:10) showed bright white emission with CIE x, y value 0.30, 0.33; respectively.

Chapter- 8 describes the design, synthesis and photophysical study of carbazole functionalized imidazobipyridyl based Eu-complexes. Two new ligands based on the imidazo-bipyridyl moiety were synthesized and used to synthesize EuIII-containing ternary complexes using 4,4,4-trifluoro-1-(2-thienyl)-1,3butanedione as an anionic ligand. The EuIII-containing complexes exhibit broad excitation bands in the near-UV-to-blue region of the electromagnetic spectrum, intense emissions, and color purity. To study the energy-transfer process, efforts were made to optimize ligand geometry and locate the exact position of energy levels using density functional theory and time-dependent density functional theory (confirmed experimentally). Based on the calculations, a mechanism of energy-transfer was proposed. The thermal stability of the complexes was found to be good (>250 °C). Temperature-dependent emission studies indicated that the carbazole-functionalized Eu complex is relevant to temperature sensing. The complexes were combined with an InGaN light-emitting diode (395 nm) chip, and the emission profiles under a 20 mA forward-bias current were analysed, show nearly white emission. The results indicate that the complexes reported here have the potential to be used as red-emitting components in warm-white lightemitting diodes. In addition, selected ligand (L-CBZ) and its Eu-complex (Eu(TTA)3CBZ), have been used to fabricate the OLED device. Overall, the optimized OLED device fabricated with ligand shows a ηp, ηc and external quantum efficiency (EQE) of 0.7 lm/A, 2.1 cd/A and 0.5 % at brightness of 100 cd/m2, with a maximum luminance of 2,118 cd/m2. Furthermore, for the Eu-complex, the OLED device fabricated with a 7.5 wt% emitter doped in the bipolar CBP matrix shows a best electroluminscent performance with an ηp, ηc and EQE of 0.3 lm/A, 0.99 cd/A and 1.8 % at 100 cd/m2, along with maximum luminance of 1,077 cd/m2.

Chapter- 9 deals the summary and conclusion as well as future perspective of the work. The present thesis works deals with rational design and synthesis of new and novel class of Eu3+/2+ based phosphors for white LEDs applications. In addition, Eu molecular complexes also been explored for white LED or OLED applications. The observations and the conclusions derived from the present investigations are summarized in this chapter.