Rozprawy doktorskie na temat „Temperature dependent Raman measurements”

Using the Hampton University (HU) Mie and Raman lidar, tropospheric temperature profiles were inferred from lidar measurements of anti-Stokes rotational Raman (RR) backscattered laser light from atmospheric nitrogen and oxygen molecules. The molecules were excited by 354.7 nanometer (nm) laser light emitted by the HU lidar. Averaged over 60-minute intervals, RR backscattered signals were detected in narrow 353.35 nm and 354.20 nm spectral bands with full-widths-at-half-maxima (FWHM) of 0.3 nm. During the special April 19-30, 2012, Ground-Based Remote Atmospheric Sounding Program (GRASP) campaign, the lidar temperature calibration coefficients were empirically derived using linear least squares and second order polynomial analyses of the lidar backscattered RR signals and of reference temperature profiles, obtained from radiosondes. The GRASP radiosondes were launched within 400 meters of the HU lidar site. Lidar derived temperature profiles were obtained at altitudes from the surface to over 18 kilometers (km) at night, and up to 5 km during the day. Using coefficients generated from least squares analyses, nighttime profiles were found to agree with profiles from reference radiosonde measurements within 3 K, at altitudes between 4 km and 9 km. Coefficients generated from the second order analyses yielded profiles which agreed with the reference profiles within 1 K uncertainty level in the 4 km to 10 km altitude region. Using profiles from GRASP radiosondes, the spatial and temporal homogeneities of the atmosphere, over HU, were estimated at the 1.5 K level within a 10 km radius of HU, and for observational periods approaching 3 hours. Theoretical calibration coefficients were derived from the optical and physical properties of the HU RR lidar and from the spectroscopic properties of atmospheric molecular nitrogen and oxygen. The theoretical coefficients along with lidar measurements of sky background radiances were used to evaluate the temporal stability of the empirically derived temperature profiles from the RR lidar measurements. The evaluations revealed systematic drifts in the coefficients. Frequent reference radiosonde temperature profiles should be used to correct for the drifts in the coefficients.

For the first time, the cause of the coefficient drifts has been identified as the differences in the aging of the spectral responses of the HU lidar detector pairs. For the first time, the use of lidar sky background measurements was demonstrated as a useful technique to correct for the coefficient drift. This research should advance the derivations of lidar temperature calibration coefficients which can be used for long observational periods of temperature fields without the need for frequent lidar calibrations using radiosondes.

Thesis (Ph. D.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

The development of microscale and nanoscale devices has outpaced the development of metrology tools necessary for their complete characterization. In the area of thermal MEMS technology, accurate measurements across a broad range of temperatures with high spatial resolution are not trivial. Thermal MEMS are devices in which the control and manipulation of temperature is necessary to perform a desired function, and are used in actuation, chemical sensing, nanolithography, thermal data storage, biological reactions and power generation. In order to properly design for reliability and performance issues amongst these devices and verify modeling accuracy, the temperature distribution under device operating conditions must be experimentally determined. Raman spectroscopy provides absolute temperature measurements with spatial scales below 1 micron, which is sufficient for most MEMS devices. In this work, a detailed study of Raman spectroscopy as an optical thermal metrology tool was performed. It is shown that a calibration of the Stokes shift with temperature yields a linear calibration for measurements up to 1000?n polysilicon. These coefficients were determined for polysilicon processed under various conditions (575-620?B and P doping) to assess the effects of microstructural variations on Raman spectra. The Stokes peak was also shown to shift linearly with an applied pure bending stress. In order to make stress-independent thermometry measurements, the ratio of the Stokes to anti-Stokes signal intensities and the Stokes linewidth were calibrated over the same temperature range. Using the calibration data, Raman spectroscopy was implemented for the evaluation of temperature of thermal MEMS. Heated AFM cantilevers and micro-beam heaters were chosen due to their wide range of applications. Different thermal and mechanical boundary conditions were considered by studying both the beams and cantilevers, resulting in varying levels of thermal stress. By using the three calibrations in a complementary fashion, the validity of Raman thermometry was explored. Device temperatures of up to 650?nd their corresponding uncertainties were found, and used to verify FEA modeling. Effects of thermally induced stresses were taken into account and analyzed. Possible uncertainties such as laser heating, spatial and spectral resolution, light collection efficiency, measurement uncertainty, and instrumental drift were reported and elucidated.

Several different laser remote sensing techniques related to the detection of trace chemical species were studied. In particular, a Differential-Absorption lidar (DIAL), a Laser-Induced-Breakdown Spectroscopy (LIBS) lidar, and a Raman lidar were studied. Several of the laser spectroscopic techniques that were used were common throughout these different studies. More precisely, 10.6 μm CO2 laser related spectroscopy was common for the DIAL and LIBS studies, and 266 nm Nd:YAG laser related spectroscopy was used for the LIBS and Raman studies. In the first system studied a tunable CO2 DIAL system was developed for the first time to our knowledge for the potential detection of the explosive Triacetone Triperoxide (TATP) gas clouds. The system has been used to measure gas samples of SF6, and has shown initial absorption measurements of samples of TATP contained within an enclosed optical absorption cell. DIAL/Lidar returns from a remote retroreflector target array were used for the DIAL measurements after passage through a laboratory cell containing the TATP gas. DIAL measured concentrations agreed well with those obtained using a calibrated Ion Mobility Spectrometer. DIAL detection sensitivity of the TATP gas concentration in the cell was about 0.5 ng/μl for a 0.3 m path-length. However, the concentration of TATP was found to be unstable over long periods of time possibly due to re-absorption and crystallization of the TATP vapors on the absorption cell windows. A heated cell partially mitigated these effects. In the second set of studies, a Deep UV LIBS system was developed and studied for the remote detection of solid targets, and potentially chemical, biological, and explosive substances. A 4th harmonic Q-Switched Nd:YAG laser operating at 266 nm was used for excitation of the LIBS plasma at standoff ranges up to 50 m . The LIBS plasma emission covering the range of 240 – 800 nm was enhanced by use of a nearly simultaneous 10.6 μm CO2 laser that increased the LIBS plasma emission by several orders of magnitude. The emission spectrum was used to detect and identify the species of interest. Plasma temperatures on various solid substrates were measured. An increase in the plasma temperature of about 5000 K was measured and analyzed, for the first to our knowledge, due to the addition of the CO2 laser pulse to the LIBS plasma generated by the Nd:YAG laser. An optimum temporal overlap of the two laser pulses was found to be important for the enhancement. Finally, in a third related lidar system, initial 266 nm Raman lidar studies were conducted at detection ranges of 15 m. However, significant spectroscopic background interferences were observed at these wavelengths and additional optical filtering is required.

Les cycles couplés des aérosols, de la vapeur d’eau et des nuages sont à l’heure actuelle un domaine de recherche dynamique au cœur d’enjeux climatiques et météorologiques. Une meilleure compréhension des interactions entre ces différents cycles atmosphériques doit permettre de mieux appréhender les processus conduisant à des évènements météorologiques extrêmes et également de diminuer les incertitudes des projections climatiques, en grande partie liées aux interactions aérosols – nuages. Contribuant à ces efforts, les travaux présentés dans cette thèse sont fondés sur l'analyse d'observations expérimentales de terrain autour d’un instrument de télédétection émergent. Il s’agit d’un lidar Raman météorologique transportable capable de mesurer simultanément la température thermodynamique, le contenu en vapeur d'eau et les propriétés optiques des aérosols, dans la colonne atmosphérique. Cet instrument, développé au LSCE et nommé WALI, permet des observations continues dans la basse et moyenne troposphère avec une précision et des résolutions verticale et temporelle en adéquation avec les objectifs de ruptures énoncés par l’OMM. En premier lieu, le bilan de liaison de la chaîne d’acquisition de la température basée sur la spectroscopie Raman rotationnelle, nouvellement implémentée sur le lidar, a été obtenu à l’aide d’une modélisation directe – inverse. Les premières mesures de température par lidar, conduites durant une période très contrastée en température marquée par l’occurrence d’une vague de froid, ont permis une comparaison avec les sorties de modèles de prévisions météorologiques à méso-échelle (AROME/Météo-France) et globale (ERA5/ECMWF) et l'instrument IASI embarqué sur les satellites de la série METOP. Lors d’une configuration météorologique hivernale analogue ayant induit des épisodes de pollution majeurs en Île-de-France, un suivi des propriétés optiques des aérosols a été effectué. Enfin, une campagne de mesure multi-instruments incluant un volet aéroporté a été conduite aux abords du lac d’Annecy, avec une stratégie originale couplant la télédétection et l'observation in situ. Elle a permis une analyse préliminaire du cycle de l'eau dans un environnement montagneux complexe, incluant les liens entre la vapeur d'eau atmosphérique, les nuages, les aérosols et le lac. Le lidar Raman météorologique s’avère être un outil idoine pour étudier ces processus
The coupled cycles of aerosols, water vapor and clouds are currently a dynamic field of research at the heart of climate and weather challenges. A better understanding of the interactions between these atmospheric cycles should allow to perceive the processes leading to extreme weather events and to reduce the uncertainties of climate projections, largely related to aerosol-cloud interactions. In line with these efforts, the work presented in this thesis are based on the analysis of experimental field observations, around a new tool for remote sensing. It is a transportable meteorological Raman lidar capable of simultaneous measurements of the thermodynamic temperature, water vapor content and optical properties of aerosols in the atmosphere. This instrument, developed at LSCE and called WALI, allows continuous observations in the lower and middle troposphere with a precision, and vertical and temporal resolutions in line with the breakthrough requirements set by the WMO. Firstly, the link budget of the temperature acquisition channel based on rotational Raman spectroscopy, newly implemented on the lidar, has been obtained using direct - inverse modeling. The first temperature measurements by lidar, carried out during a very contrasted period in terms of temperature marked by the occurrence of a cold spell, allowed a comparison with the outputs of mesoscale (AROME/Météo-France) and global (ERA5/ECMWF) weather prediction models and the IASI instrument onboard the METOP series satellites. During a similar winter meteorological configuration that induced major pollution events in Île-de-France, the optical properties of aerosols were monitored. Finally, a multi-instrument measurement campaign, involving aircrafts, was carried out on the shores of the Annecy lake, with an original strategy coupling remote sensing and in situ observations. They allowed preliminary analyses of the water cycle in a complex mountainous environment, including the links between atmospheric water vapor, clouds, aerosols and the lake. A meteorological Raman Lidar turns out to be a suitable tool to study these processes

The theme of this thesis is Raman spectroscopic study of a variety of exotic states of matter under extreme conditions, such as, hydrostatic pressure as high as 25 GPa and a wide temperature range from 77 K to 390 K. Raman spectroscopy and in some cases, X-ray diffraction studies were performed on systems such as; Topological Crystalline Insulator (TCI) SnTe, normal semiconductor SnSe, natural van der Waals heterostructures from the (SnTe)m(Bi2Te3)n (with m = 1 and n = 1, 2) homologous family, which are also predicted to be Topological Insulators (TI), then Excitonic Insulator (EI) Ta2NiSe5, and its S-counterpart, a normal semiconductor Ta2NiS5. In addition, a Ru2O6-layer honeycomb lattice compound, the silver ruthenium oxide AgRuO3 was also studied. The work presented in this thesis is divided into three parts. (1) Pressure dependent Raman studies were performed to look for signatures of topological phase transitions in SnTe, and a comparative study with a normal semiconductor SnSe. High-pressure Raman studies were also performed on SnBi2Te4 and SnBi4Te7 to look for electronic topological transitions as well as structural phase transitions. For structural characterization as a function of pressure, X-ray diffraction measurements using synchrotron source have been pursued. (2) Pressure and temperature dependent Raman studies were performed to look for signatures of stability of the excitonic insulating phase in Ta2NiSe5 and a comparative temperature dependent study on the S-counterpart, Ta2NiS5, to look for signatures of any phase transition. (3) Lastly, our temperature dependent Raman studies on AgRuO3 reveal a signature of subtle phase transition in addition to an antiferromagnetic transition. For pressure-dependent structural characterization, Raman and X-ray diffraction measurements have been pursued.
Department of Science and Technology (DST) India, Fellowship

碩士
國立中央大學
物理學系
101
The oxygen concentrations of ZnSe1-xOx alloys studied in this thesis are in the range of 1.5%x11.6%. Because of the limited oxygen solubility, Nabetani had proposed that ZnSeO alloy composition up to 6.4%. Our highest concentration up to 11.6%.In our previous study indicate the results of photoluminescence (PL) indicate that the relationship between band gap and oxygen composition can be well described in the framework of band anti-crossing model (BAC model). However, the full width of half maxima (FWHM) of signals becomes broader and the intensities become weaker in the higher O concentration range. These results indicate that the crystal structures may have changed. Thus we investigated the crystal structure via Raman spectrum. In 10K Raman scattering experiments, the phonon frequency is influenced by strain and effective mass. With ZnSe mixes O, the phonon frequency become slower than ZnSe, but when oxygen concentration higher than 9.3%, the frequency is dominated by effective mass. The phonon frequency becomes faster. In temperature-dependent Raman scattering, we can find as the oxygen concentration increases, the anharmonic effect will increase. Besides, the FWHM of LOZnSe becomes broader than ZnSe. In the end, we will discuss optical phonon life time. When increasing the oxygen concentration, the life time will become shorter than ZnSe.

碩士
國立清華大學
工程與系統科學系
94
In this study, resonance Raman spectroscopy (RRS) is employed to probe the structural information of CVD-synthesized single-walled carbon nanotubes (SWNTs) in ambient environment at various temperatures ranged from 300 K to 700 K. SWNTs are fabricated crossing as-formed deep trench on SiO2/Si to avoid the interferences from surrounding media such as the substrates. Furthermore, the enhanced RRS from suspended SWNTs (su-SWNTs) also helps to observe the slight changes of principal peaks as the temperature increased. We specially use the well-established Si Raman thermology to monitor and determine the accurate surface temperature of the measured su-SWNTs. The frequency of G+ peak is found downward shifted with the increase of temperatures due to the softening of C-C bonding strengths, where the changes are reversible during different temperature cycles. Temperature coefficients of the characteristics peaks are estimated respectively. The force constant of C-C bond is used to explain why the temperature coefficients of different chiralities are different. The initial force constant, the chiral angle, and the tube diameter all might be the causes of the different temperature coefficients of the individual suspended-SWNTs. However, there are too many phonon modes in various chiralities, so the temperature-dependent Raman feature is affected by other factors. The Raman feature would not be influenced by one factor only.

碩士
輔仁大學
物理學系
98
Poly(3-hydroxybutyrate) (PHB) is one of the biodegradable and thermoplastic polymers. It has been receiving keen interest as new environmentally friendly material that will find applications in plastic industries. However, the rather high crystallinity (55-80%) of PHB keeps it from being a replaceable material for commodity plastics, mainly due to its stiff and brittle mechanical properties. Blended with poly(3- hydroxyvalerate) (PHV), which is also a biodegradable polymer, the copolymer P(HBx-co-HV1-x) shows substantially a reduced crystallinity and increased flexibility compared to those of the PHB homopolymer. The modified mechanical properties make this copolymer more attractive with regard to material applications. In this context, the composition dependent structure and crystallinity of the copolymer P(HBx-co-HV1-x) thin films with x = 0, 0.05, 0.08 were studied by temperature-dependent micro-Raman spectroscopy. Experimental results revealed that the crystallinity of the copolymers decreases with increasing x at room temperature, whereas it remains nearly unchanged until below their melting temperatures. From the study of the parallel helical structures of the copolymer linked by a chain of C–H…O hydrogen bond pair, it shows that the hydrogen bond becomes weakened with increasing x at room temperature. Moreover, the weakening of the hydrogen bonds starts from just above room temperature and proceeds gradually with increase in temperature. This indicates the occurrence of the deformation of helical structures, leading eventually to the collapse of the parallel helical structures in the crystal parts of the copolymers. While thermal effect plays an important role in the stability of the parallel helical structures of the P(HBx-co-HV1-x) copolymer thin films, it affects hardly the crystallinity of the copolymer well below the melting point for x≦0.08. Contrarily, crystallinity as well as parallel helical structures of the thin films are strongly influenced by the PHV content incorporated into the P(HBx-co-HV1-x) copolymer.

碩士
輔仁大學
物理學系碩士班
103
ABSTRACT This work studies the temperature-dependent Raman scattering in A-site doped (Bi1-xAEx)FeO3-δ (x=0.0, 0.05, 0.10, and 0.15; AE= Ca, Sr, Ba) multiferroic ceramics in the low temperature range of -150 – 25 oC. Raman spectra and XRD reveal a rhombohedrally distorted perovskite structure with R3c space group in BiFeO3. However, the doped samples of 5 %, 10%, and 15% have a structure transition towards a nonrhombohedral symmetry with increasing Ca, Sr, and Ba concentrations. The Raman spectra of all doped BFO samples reveal a sudden change in the peak position and FWHM, especially for A1(1) and A1(2) modes. This anomaly may be attributed to an additional magnetic phase transition of spin reorientation. Fe atoms are mainly involved in vibration modes between 150 and 270 cm−1 and A1(1) and A1(2) modes are approximately at ~170 and ~220 cm-1, respectively. Keywords : alkaline-earth (Ca, Sr, Ba) doped BiFeO3, Raman scattering, X-ray diffraction

碩士
國立交通大學
光電工程研究所
85
Temperature dependent photolummescence and Raman spectroscopy were used to analyze the properties of ZnSe doped glass thin films. From the temperature dependent Raman spectra, the phonon lifetime and spectra width decrease at high temperature because of the decay of optical phonons into low energy phonons. From the temperature dependent photolum-inescence, we observed the peaks shift to the lower energy and decrease in intensity with increasing temperature related to the transition of free electron-hole and free electron-acceptor. The former is due to change of the energy gaps of semiconductors with temperature, the latter is due to the thermal quenching process that characterizes by the activation energy induces the carrier transition from the potential minimum of the free state to the nonradiative potential.

碩士
輔仁大學
物理學系
88
ABSTRACT The dielectric constant and polarization-electric field (P-E) hysteresis loops have been measured as a function of temperature in relaxor ferroelectric single crystals (PbZn1/3Nb2/3O3)1-x(PbTiO3)x (PZN-xPT) for x=0.085 and (PbMg1/3Nb2/3O3)1-x(PbTiO3)x (PMN-xPT) for x=0.24, 0.31, 0.32 and 0.33. We found that the coercive field Ec and polarization of P-E hysteresis loop increases with increasing PT content. A sharp ferroelectric transition with an abrupt step-like discontinuity in polarization was observed near 370 K for PMN-0.24PT, 390 K for PMN-0.31PT, 445 K for PMN-0.32PT and 425 K for PMN-0.33PT, respectively. Both thermal hysteresis and discontinuous behavior of polarization imply a metastable state at the phase transition temperature, which can only exist theoretically for first-order transition but not second-order transitions. In lower temperature range, polarization measurement shows a small change near 370 K for PMN-0.31PT, 360 K for PMN-0.33PT and 350 K for PZN-0.085PT along the [001] direction, but an abrupt step-like change is observed for PMN-0.33PT measured along the [111] direction at 360 K. These are all diffuse first-order transitions. The field-cooled-zero-field-heated (FC-ZFH) dielectric constant also exhibits sharp anomalies correspondingly. In addition, the domain structures of PMN-0.31PT observed by polarized light microscopy confirms the phase coexistence in these type mixed crystals.

This thesis presents experimental and related theoretical studies of pyrochlore titanate oxides, boron nitride nanotubes, and multiferroic bismuth ferrite. We have investigated these systems at high pressures and at low temperatures using Raman spectroscopy. Below, we furnish a synoptic presentation of our work on these three systems. In Chapter 1, we introduce the systems studied in this thesis, viz. pyrochlores, boron nitride nanotubes, and multiferroic BiFeO3, with a review of the literature pertaining to their structural, electronic, vibrational, and mechanical properties. We also bring out our interests in these systems. Chapter 2 includes a brief description of the theory of Raman scattering and infrared absorption. This is followed by a short account of the experimental setups used for Raman and infrared measurements. We also present the technical details of high pressure technique including the alignment of diamond anvil cells, gasket preparation, calibration of the pressure, etc. Chapter 3 furnishes the results of our pressure-and temperature-dependent studies of pyrochlore oxides which has been divided into eight different parts. In recent years, magnetic and thermodynamic properties of pyrochlores have received a lot of attention. However, not much work has been reported to address the quasiparticle excitations, e.g., phonons and crystal-field excitations in these materials. A material that shows exotic magnetic behavior and high degree of degenerate ground states can be expected to have low-lying excitations with possible couplings with phonons, thereby, finger-printing various novel properties of the system. Raman and infrared absorption spectroscopies can, therefore, be used to comprehend the novel role of phonons and their role in various phenomena of frustrated magnetic pyrochlores. Recently, there have been reports on various novel properties of these systems; for example, Raman and absorption studies [Phys. Rev. B 77, 214310 (2008)] have revealed a loss of inversion symmetry in Tb2Ti2O7 at low temperatures which has been suggested as the key reason for this frustrated magnet to remain in spin-liquid state down to 70 mK. Powder neutron-diffraction experiments [Nature 420, 54 (2002)] have shown that an application of isostatic pressure of about 8.6 GPa in spin-liquid Tb2Ti2O7 induces a long-range magnetic order of the Tb3+ spins coexisting with the spin-liquid phase ascribing this transition to the breakdown of the delicate balance among the various fundamental interactions. Moreover, Raman and x-ray studies have shown that Tb2Ti2O7,Sm2Ti2O7,and Gd2Ti2O7 undergo a structural transition followed by an irreversible amorphization at very high pressures (~ 40 GPa or above) [Appl. Phys. Lett. 88, 031903 (2006)]. In this chapter, therefore, we present our temperature-and pressure-dependent Raman studies of A2Ti2O7 pyrochlores, where ‘A’ is a trivalent rare-earth element (A = Sm, Gd,Tb, Dy,Ho, Er,Yb, and Lu; and also Y). Since all the group theoretically predicted Raman modes of this cubic lattice are due to oxygen vibrations only, in Part (A), we revisit the phonon assignments of pyrochlore titanates by performing Raman measurements on the O16 /O18 − isotope based Dy2Ti2O7 and Lu2Ti2O7 and find that the vibrations with frequencies below 250 cm−1 do not involve oxygen atoms. Our results lead to a reassignment of the pyrochlore Raman phonons thus proposing that the mode with frequency ~ 200 cm−1, which has earlier been known as an F2g phonon due to oxygen vibration, is a vibration of Ti4+ ions. Moreover, we have performed lattice dynamical calculations using Shell model that help us to assign the Raman phonons. In Part (B), we have explored the temperature dependence of the Raman phonons of spin-ice Dy2Ti2O7 and compared with the results of two non-magnetic pyrochlores, Lu2Ti2O7 and Y2Ti2O7. Our results reveal anomalous red-shift of some of the phonons in both magnetic and non-magnetic pyrochlores as the temperature is lowered. The phonon anomalies can not be understood in terms of spin-phonon and crystal field transition-phonon couplings, thus attributing them to phonon-phonon anharmonic interactions. We also find that the anomaly of the disorder activated Ti4+ Raman vibration (~ 200 cm−1) is unusually high compared to other phonons due to the large vibrational amplitudes of Ti4+-ions rendered by the vacant Wyckoff sites in their neighborhood. Later, we have quantified the anharmonicity in Dy2Ti2O7. We have extended our studies on spin-ice compound Dy2Ti2O7 by performing simultaneous pressure-and temperature-dependent Raman measurements, presented in Part (C). We show that a new Raman mode appears at low temperatures below TC ~ 110 K, suggesting a structural transition, also supported by our x-ray measurements. There are reports [Phys. Rev. B 77, 214310 (2008), Phys.Rev.B 79, 214437 (2009)] in the literature where the new mode in Dy2Ti2O7 at low temperatures has been assigned to a crystal field transition. Here, we put forward evidences that suggest that the “new” mode is a phonon and not a crystal field transition. Moreover, the TC is found to depend on pressure with a positive coefficient. In Part (D), we have presented our results of temperature-and pressure-dependent Raman and x-ray measurements of spin-frustrated pyrochlores Gd2Ti2O7, Tb2Ti2O7,and Yb2Ti2O7. Here, we have estimated the quasiharmonic and anharmonic contributions to the anomalous change in phonon frequencies with temperature. Moreover, we find that Gd2Ti2O7 and Tb2Ti2O7 undergo a subtle structural transition at a pressure of ~ 9 GPa which is absent in Yb2Ti2O7. The implication of this structural transition in the context of a long-range magnetically ordered state coexisting with the spin-liquid phase in Tb2Ti2O7 at high pressure (8.6 GPa) and low temperature (1.5 K), observed by Mirebeau et al. [Nature 420, 54 (2002)], has been discussed. As we have established in the previous parts that the anomalous behavior of pyrochlore phonons is due to phonon-phonon anharmonic interactions, we have tuned the anharmonicity in the first pyrochlore of the A2Ti2O7 series, i.e., Sm2Ti2O7,by replacing Ti4+-ions with bigger Zr4+-ions, presented in Part (E). Our results suggest that the phonon anomalies have a very strong dependence on the ionic size and mass of the transition element (i.e., the B4+-ion in A2B2O7 pyrochlores). We have also observed signatures of coupling between a phonon and crystal-field transitions in Sm2Ti2O7. In Part (F), we have studied spin-ice compound Ho2Ti2O7 and compared the phonon anomalies with the stuffed spin-ice compounds, Ho2+xTi2−xO7−x/2 by stuffing Ho3+ ions into the sites of Ti4+ with appropriate oxygen stoichiometry. We find that as more and more Ho3+-ions are stuffed, there is an increase in the structural disorder of the pyrochlore lattice and the phonon anomalies gradually disappear with increasing Ho3+-ions. Moreover, a coupling between phonon and crystal field transition has also been observed. In Part (G), we have examined the temperature dependence of phonons of “dynamical spin-ice” compound Pr2Sn2O7 and compared with its non-pyrochlore (monoclinic) counterpart Pr2Ti2O7. Our results conclude that the anomalous behavior of phonons is an intrinsic property of pyrochlore structure having inherent vacant sites. We also find a coupling between phonon and crystal-field transitions in Pr2Sn2O7. In the last part of this chapter, Part (H), we present our Raman studies of Er2Ti2O7. Here, we show that in addition to the anomalous phonons, there are modes that originate from photoluminescence transitions and some of these luminescence lines show anomalous temperature dependence which have been understood using the theory of optical dephasing in crystals, developed by Hsu and Skinner [J. Chem. Phys. 81, 1604 (1984)]. Temperature dependence of a few Raman modes and photoluminescence bands suggest a phase transition at 130 K. In Chapter 4, we furnish our pressure-dependent Raman studies of boron nitride multi-walled nanotubes (BNNT) and hexagonal boron nitride (h-BN) and compare the results with those of their carbon counterparts. Using Raman spectroscopy, we show that BNNT undergo an irreversible transition at ~ 12 GPa while the carbon counterpart, multi-walled carbon nanotubes, show a similar transition at a much higher pressure of ~ 51 GPa. In sharp contrast, the layered form of both the systems (i.e. h-BN and graphite) undergo a hexagonal to wurtzite phase at nearly similar pressure (~ 13 GPa of h-BN and ~ 15 GPa for graphite). A molecular dynamical simulation on boron nitride single-walled nanotubes has also been undertaken that suggests that the polar nature of the B−N bonds may be responsible for the irreversibility of the pressure-induced transformations. It is interesting to see that in hexagonal phase both the systems have almost similar mechanical property, but once they are rolled up to make nanotubes, the property becomes quite different. Chapter 5 presents the temperature dependence of the Raman modes of multiferroic thin films of BiFeO3 and Bi0.7Tb0.2La0.1O3. Though there have been several Raman investigations of BiFeO3 in literature, here we emphasize the observation of unusually intense second order Raman phonons. Our results have motivated Waghmare et al. to suggest a theoretical model to explain the anomalously large second order Raman tensor of BiFeO3 in terms of an incipient metal-insulator transition. In Chapter 6, we summarize our findings on the three different systems, namely, pyrochlores, boron nitride nanotubes, and BiFeO3 and highlight a few possible experiments that may be undertaken in future to have a better understanding of these systems.

This thesis presents experimental and related theoretical studies of pyrochlore titanate oxides, boron nitride nanotubes, and multiferroic bismuth ferrite. We have investigated these systems at high pressures and at low temperatures using Raman spectroscopy. Below, we furnish a synoptic presentation of our work on these three systems. In Chapter 1, we introduce the systems studied in this thesis, viz. pyrochlores, boron nitride nanotubes, and multiferroic BiFeO3, with a review of the literature pertaining to their structural, electronic, vibrational, and mechanical properties. We also bring out our interests in these systems. Chapter 2 includes a brief description of the theory of Raman scattering and infrared absorption. This is followed by a short account of the experimental setups used for Raman and infrared measurements. We also present the technical details of high pressure technique including the alignment of diamond anvil cells, gasket preparation, calibration of the pressure, etc. Chapter 3 furnishes the results of our pressure-and temperature-dependent studies of pyrochlore oxides which has been divided into eight different parts. In recent years, magnetic and thermodynamic properties of pyrochlores have received a lot of attention. However, not much work has been reported to address the quasiparticle excitations, e.g., phonons and crystal-field excitations in these materials. A material that shows exotic magnetic behavior and high degree of degenerate ground states can be expected to have low-lying excitations with possible couplings with phonons, thereby, finger-printing various novel properties of the system. Raman and infrared absorption spectroscopies can, therefore, be used to comprehend the novel role of phonons and their role in various phenomena of frustrated magnetic pyrochlores. Recently, there have been reports on various novel properties of these systems; for example, Raman and absorption studies [Phys. Rev. B 77, 214310 (2008)] have revealed a loss of inversion symmetry in Tb2Ti2O7 at low temperatures which has been suggested as the key reason for this frustrated magnet to remain in spin-liquid state down to 70 mK. Powder neutron-diffraction experiments [Nature 420, 54 (2002)] have shown that an application of isostatic pressure of about 8.6 GPa in spin-liquid Tb2Ti2O7 induces a long-range magnetic order of the Tb3+ spins coexisting with the spin-liquid phase ascribing this transition to the breakdown of the delicate balance among the various fundamental interactions. Moreover, Raman and x-ray studies have shown that Tb2Ti2O7,Sm2Ti2O7,and Gd2Ti2O7 undergo a structural transition followed by an irreversible amorphization at very high pressures (~ 40 GPa or above) [Appl. Phys. Lett. 88, 031903 (2006)]. In this chapter, therefore, we present our temperature-and pressure-dependent Raman studies of A2Ti2O7 pyrochlores, where ‘A’ is a trivalent rare-earth element (A = Sm, Gd,Tb, Dy,Ho, Er,Yb, and Lu; and also Y). Since all the group theoretically predicted Raman modes of this cubic lattice are due to oxygen vibrations only, in Part (A), we revisit the phonon assignments of pyrochlore titanates by performing Raman measurements on the O16 /O18 − isotope based Dy2Ti2O7 and Lu2Ti2O7 and find that the vibrations with frequencies below 250 cm−1 do not involve oxygen atoms. Our results lead to a reassignment of the pyrochlore Raman phonons thus proposing that the mode with frequency ~ 200 cm−1, which has earlier been known as an F2g phonon due to oxygen vibration, is a vibration of Ti4+ ions. Moreover, we have performed lattice dynamical calculations using Shell model that help us to assign the Raman phonons. In Part (B), we have explored the temperature dependence of the Raman phonons of spin-ice Dy2Ti2O7 and compared with the results of two non-magnetic pyrochlores, Lu2Ti2O7 and Y2Ti2O7. Our results reveal anomalous red-shift of some of the phonons in both magnetic and non-magnetic pyrochlores as the temperature is lowered. The phonon anomalies can not be understood in terms of spin-phonon and crystal field transition-phonon couplings, thus attributing them to phonon-phonon anharmonic interactions. We also find that the anomaly of the disorder activated Ti4+ Raman vibration (~ 200 cm−1) is unusually high compared to other phonons due to the large vibrational amplitudes of Ti4+-ions rendered by the vacant Wyckoff sites in their neighborhood. Later, we have quantified the anharmonicity in Dy2Ti2O7. We have extended our studies on spin-ice compound Dy2Ti2O7 by performing simultaneous pressure-and temperature-dependent Raman measurements, presented in Part (C). We show that a new Raman mode appears at low temperatures below TC ~ 110 K, suggesting a structural transition, also supported by our x-ray measurements. There are reports [Phys. Rev. B 77, 214310 (2008), Phys.Rev.B 79, 214437 (2009)] in the literature where the new mode in Dy2Ti2O7 at low temperatures has been assigned to a crystal field transition. Here, we put forward evidences that suggest that the “new” mode is a phonon and not a crystal field transition. Moreover, the TC is found to depend on pressure with a positive coefficient. In Part (D), we have presented our results of temperature-and pressure-dependent Raman and x-ray measurements of spin-frustrated pyrochlores Gd2Ti2O7, Tb2Ti2O7,and Yb2Ti2O7. Here, we have estimated the quasiharmonic and anharmonic contributions to the anomalous change in phonon frequencies with temperature. Moreover, we find that Gd2Ti2O7 and Tb2Ti2O7 undergo a subtle structural transition at a pressure of ~ 9 GPa which is absent in Yb2Ti2O7. The implication of this structural transition in the context of a long-range magnetically ordered state coexisting with the spin-liquid phase in Tb2Ti2O7 at high pressure (8.6 GPa) and low temperature (1.5 K), observed by Mirebeau et al. [Nature 420, 54 (2002)], has been discussed. As we have established in the previous parts that the anomalous behavior of pyrochlore phonons is due to phonon-phonon anharmonic interactions, we have tuned the anharmonicity in the first pyrochlore of the A2Ti2O7 series, i.e., Sm2Ti2O7,by replacing Ti4+-ions with bigger Zr4+-ions, presented in Part (E). Our results suggest that the phonon anomalies have a very strong dependence on the ionic size and mass of the transition element (i.e., the B4+-ion in A2B2O7 pyrochlores). We have also observed signatures of coupling between a phonon and crystal-field transitions in Sm2Ti2O7. In Part (F), we have studied spin-ice compound Ho2Ti2O7 and compared the phonon anomalies with the stuffed spin-ice compounds, Ho2+xTi2−xO7−x/2 by stuffing Ho3+ ions into the sites of Ti4+ with appropriate oxygen stoichiometry. We find that as more and more Ho3+-ions are stuffed, there is an increase in the structural disorder of the pyrochlore lattice and the phonon anomalies gradually disappear with increasing Ho3+-ions. Moreover, a coupling between phonon and crystal field transition has also been observed. In Part (G), we have examined the temperature dependence of phonons of “dynamical spin-ice” compound Pr2Sn2O7 and compared with its non-pyrochlore (monoclinic) counterpart Pr2Ti2O7. Our results conclude that the anomalous behavior of phonons is an intrinsic property of pyrochlore structure having inherent vacant sites. We also find a coupling between phonon and crystal-field transitions in Pr2Sn2O7. In the last part of this chapter, Part (H), we present our Raman studies of Er2Ti2O7. Here, we show that in addition to the anomalous phonons, there are modes that originate from photoluminescence transitions and some of these luminescence lines show anomalous temperature dependence which have been understood using the theory of optical dephasing in crystals, developed by Hsu and Skinner [J. Chem. Phys. 81, 1604 (1984)]. Temperature dependence of a few Raman modes and photoluminescence bands suggest a phase transition at 130 K. In Chapter 4, we furnish our pressure-dependent Raman studies of boron nitride multi-walled nanotubes (BNNT) and hexagonal boron nitride (h-BN) and compare the results with those of their carbon counterparts. Using Raman spectroscopy, we show that BNNT undergo an irreversible transition at ~ 12 GPa while the carbon counterpart, multi-walled carbon nanotubes, show a similar transition at a much higher pressure of ~ 51 GPa. In sharp contrast, the layered form of both the systems (i.e. h-BN and graphite) undergo a hexagonal to wurtzite phase at nearly similar pressure (~ 13 GPa of h-BN and ~ 15 GPa for graphite). A molecular dynamical simulation on boron nitride single-walled nanotubes has also been undertaken that suggests that the polar nature of the B−N bonds may be responsible for the irreversibility of the pressure-induced transformations. It is interesting to see that in hexagonal phase both the systems have almost similar mechanical property, but once they are rolled up to make nanotubes, the property becomes quite different. Chapter 5 presents the temperature dependence of the Raman modes of multiferroic thin films of BiFeO3 and Bi0.7Tb0.2La0.1O3. Though there have been several Raman investigations of BiFeO3 in literature, here we emphasize the observation of unusually intense second order Raman phonons. Our results have motivated Waghmare et al. to suggest a theoretical model to explain the anomalously large second order Raman tensor of BiFeO3 in terms of an incipient metal-insulator transition. In Chapter 6, we summarize our findings on the three different systems, namely, pyrochlores, boron nitride nanotubes, and BiFeO3 and highlight a few possible experiments that may be undertaken in future to have a better understanding of these systems.

This dissertation developed the extended energy loss fine structure (EXELFS) technique. EXELFS experiments using the Al K, Fe L23 and Pd M45 edges in the elemental metals gave nearest-neighbor distances which were accurate to within ± 0.1 A. In addition, vibrational mean-square relative displacements (MSRD) derived from the temperature dependence of the EXELFS compared favorably with predictions from published force constant models derived from inelastic neutron scattering data. Thus, information about "local" atomic environments can be obtained not only from K edges, but from L23 and M45 edges as well. This opens up most of the periodic table to possible EXELFS experiments. The EXELFS technique was used to study the local atomic structure and vibrations in intermetallic alloys and nanocrystalline materials. EXELFS measurements were performed on Fe3Al and Ni3Al alloys which were chemically disordered by piston-anvil quenching and high-vacuum evaporation, respectively. Chemical short-range order was observed to increase as the as-quenched Fe3Al and as-evaporated Ni3Al samples were annealed in-situ at 300 C and 150 C respectively. Temperature-dependent measurements indicated that local Einstein temperatures of ordered samples of Fe3Al and Ni3Al were higher than those of the corresponding disordered samples. Within a "pair" approximation, these increases in local Einstein temperatures for the ordered alloys corresponded to decreases in vibrational entropy per atom of 0.48 ± 0.25 kB for Fe3Al and 0.71 ± 0.38 kB for Ni3Al. In comparison, the decrease in configurational entropy per atom between perfectly disordered and ordered A3B alloys is 0.56 kB in the mean-field approximation. These results suggest that including vibrational entropy in theoretical treatments of phase transformations would lower significantly the critical temperature of ordering for these alloys. EXELFS investigations were also performed on nanocrystalline Pd and TiO2. At 105 K, the MSRD in nanocrystalline Pd and TiO2 were found to be greater than that in the corresponding large-grained materials by 1.8 ± 0.3 x 10(-3) A2 and 1.8 ± 0.4 x 10(-3) A2, respectively. Temperature-dependent measurements were inconclusive in measuring differences in local atomic vibrations between the nanocrystalline and large-grained materials.

The electrical properties of ZnO and contacts to ZnO have been investigated using different techniques. Temperature dependent Hall (TDH) effect measurements have been used to characterize the as-received melt grown ZnO samples in the 20 – 330 K temperature range. The effect of argon annealing on hydrogen peroxide treated ZnO samples has been investigated in the 200 – 800oC temperature range by the TDH effect measurement technique. The experimental data has been analysed by fitting a theoretical model written in Matlab to the data. Donor concentrations and acceptor concentrations together with the associated energy levels have been extracted by fitting the models to the experimentally obtained carrier concentration data by assuming a multi-donor and single charged acceptor in solving the charge balance equation. TDH measurements have revealed the dominance of surface conduction in melt grown ZnO in the 20 – 40 K temperature range. Surface conduction effects have proved to increase with the increase in annealing temperature. Surface donor volume concentrations have been determined in the 200 – 800oC by use of theory developed by D. C. Look. Good rectifying Schottky contacts have been fabricated on ZnO after treating the samples with boiling hydrogen peroxide. Electrical properties of these Schottky contacts have been investigated using current-voltage (IV) and capacitance-voltage (CV) measurements in the 60 – 300 K temperature range. The Schottky contacts have revealed the dominance of predominantly thermionic emission at room temperature and the existence of other current transport mechanisms at temperatures below room temperature. Polarity effects on the Schottky contacts deposited on the O-polar and Zn-polar faces of ZnO have been demonstrated by the IV technique on the Pd and Au Schottky contacts at room temperature. Results obtained indicate a strong dependence of the Schottky contact quality on the polarity of the samples at room temperature. The quality of the Schottky contacts have also indicated their dependence on the type of metal used with the Pd producing contacts with the better quality as compared to the Au. Schottky barrier heights determined using temperature dependent IV measurements have been observed to increase with increasing temperature and this has been explained as an effect of barrier inhomogeneities, while the ones obtained from CV measurements have proved to follow the negative temperature coefficient of the II – VI semiconductor material, i.e. a decrease in barrier height with increasing temperature. However, the values have proved to be larger than the energy gap of ZnO, an effect that has been explained as caused by an inversion layer. Copyright
Dissertation (MSc)--University of Pretoria, 2010.
Physics
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