Lanthanide Photoluminescence: A Tool for Analyzing Local Heterogeneity in Ferroelectrics and Scope for High Performance Optical Thermometry
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Rare-earth doped phosphors are fascinating both from scientific and technological standpoints because of the great scope it offers for spectral tuning, enabling a large variety of applications: from simple road markers to lighting, telecommunication, biomedical application, etc. Though the intra-4f electronic transitions are parity forbidden, photoluminescence (PL) emissions from rare-earth ions is possible due to coupling with the vibrational state, mixing with wavefunctions of opposite parity, ligand orbitals, or charge transfer state of the host matrix. Consequently, the host matrix plays an important role in determining the PL emission behavior of a given rare-earth ion. Any change in the structural features of the host, induced by chemical modification, temperature, pressure, electric field, is expected to bring about a change in the PL spectrum, providing the basis of spectral tuning. However, the shielding of the 4f levels by the 5d 6s electrons nearly fixes the energy of the Stark bands of a rare-earth ion, irrespective of the ligand environment around it. In this context, spectral tuning merely involves a change in the relative intensities of the Stark bands appearing in different wavelength regions of the PL emission spectrum. The symmetry occupied by the lanthanide ion determines the extent to which the degeneracies of the 2J+1 levels are lifted for the different 2S+1Lj spectroscopic level. The lowering of the local symmetry not only affects the selection rules of intra band transition but also increases the number of Stark manifolds observed in the PL spectrum. The Eu3+ PL emission possesses a special feature called hypersensitive transition i.e. the 5D0 → 7F2 transition of Eu3+ ion is very sensitive to small changes in the local environment and/or the degree of structural heterogeneity. Eu3+ PL has, therefore, been used as a tool for investigating local symmetry in complex structures and in the quantification of the extent of crystallization during thermal annealing of structural glasses. Ferroelectric materials exhibit an intimate relationship between the state of polarization and crystal structure. The polarization state of a ferroelectric can be influenced by factors like compositional modification, electric field, stress, and temperature, which will also affect the structural state. For example, spontaneous polarization disappears on heating a ferroelectric above the Curie point, accompanied by crystal losing its centre of inversion symmetry. Similarly, hydrostatic pressure suppresses ferroelectricity due to the dominance of the repulsive short-range interaction over the long range attractive coulombic interaction. Electric field also can change the crystal symmetry in ferroelectric solid solutions exhibiting at morphotropic phase boundary compositions. Therefore, introduction of optical functionality to ferroelectrics by doping of rare-earth ions can provide opportunities for investigation of the intriguing relationship between photoluminescence (PL) tuning and external stimuli like electric field, strain, and temperature. The discovery of extraordinary piezoelectric response in BaTiO3 based piezoelectric in recent years has stimulated scientific research on the development of alternative for lead based piezoceramics. Generally, the piezoelectric response of the system is enhanced by tuning the composition towards morphotropic/polymorphic phase boundary (MPB/PPB), which separates two ferroelectric phases in the composition temperature phase diagram. Recently alternate strategy has been proposed to achieve superior piezoelectric performance by introduction of local structural distortions to manipulate interfacial energies. Observing the influence of local structural heterogeneity on the piezoelectric performance, it is important to understand the effect of poling on the overall structural heterogeneity of the PPB composition. Modern day ferroelectric-based miniaturized devices require grain size to be restricted in the submicron regime, which can induce internal stress because of the deficiency of ferroelastic domains. Not only the grain size, but the processing techniques itself are also likely to introduce a great deal of strain inhomogeneity in the system. For a better appreciation of the overall performance of such ferroelectric microelectronic devices, it is important to understand how these constraints influence the structure and microstructure on the global and local length scales. Using the great sensitivity of the 5D0 → 7F2 transition of Eu3+ ion, we have studied the structural heterogeneities in case of poling effect on polymorphic phase boundary composition, inhomogeneous lattice strain, and size restricted submicron ferroelectrics. Chapter 1 of the thesis provides an introduction to the fundamental concepts related to ferroelectrics materials. An overview of lanthanide luminescence properties and advantages of Eu3+ photoluminescence as a structural probe is discussed. This is followed by the concepts associated with non-contact luminescence temperature sensing in lanthanide doped materials. The details of the experimental techniques, characterization tools used, and some theory behind these techniques have been provided in chapter 2. A high-performance piezoceramic is inherently a heterogeneous system comprising of a complex network of ferroelectric domain walls and interphase boundaries coupled within and across randomly oriented grains. Chapter 3 discusses how this heterogeneity is affected by a strong electric field in the lead free piezoelectric Ba(Ti1-xSnx)O3 (BTS) in the vicinity of polymorphic phase boundary, using rare-earth PL as a spectroscopic probe. Eu3+ ions are randomly dispersed (in dilute concentration) in the BTS matrix to make use of PL signal as a structural probe. The very high quantum yield of Eu3+ makes it possible to get very good PL intensity even with a very small concentration of Eu3+ in the dielectric matrix, thereby assuring that the probe itself does not interfere too much with the essential characteristics of the system. Detail analysis of the PL profile patterns in different structural regions show one to one correlation between Eu PL spectra (Stark bands) and global crystal structure. In case of poled specimens of PPB composition, 5D0 → 7F2 hypersensitive transition of Eu3+ shows an irreversible increase in the intensity. In contrast, XRD study in situ with electric field shows reversible P4mm →Amm2 phase transformation. This indicates an overall increase in the local low symmetric phases because of the field induced motion of inter phase boundaries. This study proves that the role of poling is not merely limited to make the ceramic piezo-active, but it also plays an important role in enhancing the performance of the MPB/PPB piezoceramic by increasing the overall local structural heterogeneity. Chapter 4 discusses about the local structural heterogeneity developed in BaTiO3 for two different important situations: (i) lattice is inhomogeneously strained and (ii) grain size restricted in the submicron regime. We make use of the great sensitivity of the hypersensitive 5D0 → 7F2 transition of Eu3+ in local crystal environment to probe the structural heterogeneity developed in BaTiO3 for the two conditions. A systematic comparative investigation was performed on the solid solutions series Ba(Ti1–xSnx)O3 with x = 0.0, 0.03, 0.06, and 0.09, doped with Eu2O3 in dilute concentration. We have deliberately introduced inhomogeneous lattice strain in well sintered grains of these compositions by subjecting them to uniaxial loading for an extended duration. XRD patterns show a significant broadening of the XRD Bragg profiles of the pressed specimens in addition to the stabilization of partial orthorhombic phase in pressed x = 0 and 0.03 compositions. PL measurements show an increase in the 5D0 → 7F2 band emission because of the pressing effect. Microstrain calculations (by Williamson-Hall method) of pressed specimens found a one-to-one correspondence between the degree of the residual microstrain and the intensity of the7F2 hypersensitive band. Similarly, Eu3+ hypersensitive band shows an increase in intensity in case of specimens with submicron size grains as compared to the specimens with above micron size grains. Although the global structure remains unchanged in BaTiO3, suggesting the system exhibits considerably large structural heterogeneity by inducing low symmetry distortions on the local scale when the size is restricted in the submicron regime. The ability to tune the PL spectrum by temperature forms the basis of optical thermometry. Compared to others, this mode of thermometry is advantageous because of the non-contact/remote mode of measurement, fast response time, and being able to operate in corrosive and other kinds of harsh environmental conditions. In conventional approach, optical thermometers are designed by using the thermally driven change in the relative intensity of a pair of Stark lines (commonly known as Fluorescence Intensity Ratio, FIR) in the photoluminescence (PL) spectrum of rare-earth doped solids. Chapter 5 discusses about new strategies to develop a highly sensitive optical thermometer wherein judicious exploitation of the temperature dependence of the Raman signal of the crystalline host, in conjunction with the temperature dependence of the photoluminescence (PL) signal of the doped rare-earth ions, can give very large sensitivity. The phenomenon is demonstrated on a Eu, Er doped BaTiO3. In our model system, Eu3+ band intensities decrease at cryogenic temperature whereas Raman band intensities increase with decreasing temperature, resulting a temperature induced change in emission color from orange to green (on cooling). Anomalous quenching of Eu PL at low temperature can be understood as a consequence of trap state mediated relaxation process, and the increase in Raman intensity correlates well with structural ordering in the rhombohedral ferroelectric phase of host BaTiO3. We introduce a new concept of "Raman-Photoluminescence Intensity Ratio (RPIR)" for the realization of very large temperature sensitivity, not possible before. The mechanism leading to the large sensitivity is explained in detail. Another example shows realization of an optical thermometer by the intensity ratio of Er and Eu PL emission doped in nonferroelectric perovskite CaTiO3. On cooling below room temperature, Er3+ band intensities increase with decreasing temperature whereas Eu3+ band intensities decrease with decreasing temperature, providing temperature dependent intensity ratio. Similarly, different rate of phonon induced PL quenching (in Er and Eu) at high temperature realizes optical temperature sensing at high temperature. The strategy discussed in this chapter offers a competitive alternative to the conventional optical thermometers to design highly sensitive non-contact thermometers with good signal discriminability. Finally, in Chapter 6, we have summarized the essential results of this thesis and also suggested prospects for further research.