This work presents a comprehensive study on the application of PAT tools to study, monitor and control polymorphism during batch cooling crystallization processes. For the first time, the same techniques were used to control and adjust polymorphic purity of the solid phase but also to investigate the relation between chemical equilibrium in solution and polymorphic outcome of cooling crystallization. Crystallization is an important unit operation used as separation and purification technique. It is widely employed in the pharmaceutical, chemical, agrochemical, food and cosmetics industries but also in the electronic, metallurgic and material industries. More than 90% of the APIs on the market are produced by crystallization, therefore, monitoring and control this process is fundamental to ensure the quality of the final product. The implementation of process analytical technology (PAT) tools during the development stage of APIs has largely helped in better understanding and optimizing both batch and, more recently, continuous crystallization. Polymorphism is the capacity of a compound to crystallize in more than one different crystalline structure, which can have different properties such as density, melting point, bioavailability and solubility. The choice of solvent, pH, kinetic conditions and presence of impurities has very strong effect on the polymorphic outcome of a cooling crystallization in solution. Understanding this phenomenon as well as being able to monitor and control it during industrial crystallization is one the biggest challenges for pharmaceutical industries.

“Nanomaterials” is a generic term used to describe nano-sized crystals and bulk homogenous materials with a structural disorder at the nanoscale. Ancient (and modern) ceramics and glasses derive some of their properties (eg. pliability and low sintering temperature) from the fact that their raw material namely natural clay is nanosized. Furthermore the pigments used to colour ceramics and glasses need to have particle sizes <500 nm for the object to appear homogenously coloured to the human eye. Raman spectroscopy intrinsically probes chemical bonds and is therefore one of the few techniques that has been proven useful to provide information at the nanoscale. It is an excellent tool to study ceramics and glasses as a Raman spectrum can be used to identify phases, analyse amorphous domains in the silicate network and identify pigments on a nano-scale. The characteristics of a glass, ceramic or ceramic glaze derived through its Raman spectrum can then be linked to the technology used to produce an artefact and in this way provide information about its relative age and provenance. Likewise, the identification of pigments and binders in San rock art might provide information about production techniques and assist in the developement of conservation procedures. In this thesis micro-Raman spectroscopy (with X-ray fluorescence, X-ray powder diffraction, electronmicroscopy and photoluminescence as supportive techniques) was utilised to study archaeological artefacts from the Mapungubwe Collection and San rock art. It was possible to re-date celadon shards excavated on Mapungubwe hill in 1934 to the Yuan or even later Ming dynasty in stead of its original classification as Song. A profile of the glass technology used to produce the Mapungubwe oblates, small trade beads from the “royal burials” on Mapungubwe hill was determined and quite a few unique characteristics of the beads may eventually help to establish their provenance. The possible influence of the presence of rock hyraces at rock art sites on the deterioration of rock art were investigated and during the study very rare polymorphs of CaCO3 (vaterite and monohydrocalcite) were discovered in rock hyrax urine. This study was extended to analyse a San rock art fragment and another first was the identification of animal fat on the fragment, but the exact origin of the fat has to be verified by similar experiments. / Thesis (PhD)--University of Pretoria, 2009. / Physics / unrestricted

Surface enhanced Raman scattering (SERS) has achieved much attention since its conception in 1974. The analytical technique overcomes many difficulties associated with conventional Raman whilst also increasing sensitivity. However, the increased interest and work in the field has also identified flaws, many of which are centred on the irreproducibility of the SERS enhancement effect. The majority of the work described in this thesis focusses on the ‘optimisation’ of solid-state and solution based SERS systems. Optimisation plays a crucial role in maximising both enhancement effects and reproducibility. Here criteria are outlined for the synthesis of high performance solid-state SERS substrates and the synthesis of a range of substrates is assessed, each with associated pros and cons. The most successful substrate was synthesised by exploiting redox potentials which allow for the direct deposition of silver onto copper foil. The deposition times and temperatures were optimised sequentially to generate a high performance substrate capable of detecting Rhodamine 6G at trace levels. Reproducibility comparisons of the silver on copper (SoC) substrate were carried out against commercial substrates: Klarite and QSERS, multiple univariate and multivariate methods were used to assess the substrates performance. The results confirmed that the SoC substrate performed better than both the commercial substrates. The work also highlights the importance of using multiple data analysis methods in order to assess the performance of a solid-state SERS substrate. Deposition of the silver surface was also successful on British 2p coins allowing the for the detection and discrimination of illegal and legal drugs when coupled with multivariate data analysis methods such as PCA and PLS. Solution based SERS analyses were also carried out successfully using different optimisation strategies. The initial investigation involved careful control of the individual components of a SERS system (nanoparticles, aggregating agents and analyte) in order to establish a low limit of detection for the increasingly abused ‘legal high’ MDAI. The use of a reduced factorial design was then successfully employed to explore a greater number of SERS variables and define a low limit of detection for the class B drug mephedrone. The robust experimental design also allowed an insight into the importance of each of the individual components within a solution based SERS system. The final piece of work carried out was the SERS discrimination of antibiotics: ampicillin, ticarcillin and carbenicillin. Optimisation of the solution based experiment allowed the in-situ hydrolysis of the β-lactam moiety present in ampicillin rendering it pharmacologically inactive to be followed under acidic conditions at concentrations of 10 ppm.

No description available.

A home-built Raman system, utilizing a pseudo-backscattering geometry, was built in the Physics Department at the Nelson Mandela Metropolitan University (NMMU). The system was then used to analyse a variety of bulk and thin film semiconducting materials currently being studied in the Physics Department. Silicon wafers were exposed to hydrogen plasma. Raman analysis of hydrogen induced platelets (HIPs), resulting from hydrogen plasma treatment of silicon, is reported. ZnO layers were deposited on glass, GaAs, Si, sapphire and SiC-Si substrates by metalorganic chemical vapour deposition (MOCVD) in the Physics Department at the NMMU. It was found that the ZnO layers grown by MOCVD all exhibited a strong E2 (high) phonon mode that dominated the Raman spectra. Furthermore, the spectra lacked the A1 (LO) phonon mode which is usually associated with the O-vacancy, the Zn-interstitial, or complexes of the two, indicating that the layers were all of good quality. The influence of depositing the ZnO thin film on a 3 mm thick SiC layer was also investigated and compared with the deposition of ZnO on Si substrate, in order to reduce the lattice mismatch between ZnO and the Si substrate. The possible shift of the Raman peaks due to the residual strain in the film, if present, could not be resolved. Characterization of GaN and AlxGa1-xN produced by MOCVD at the CRHEA laboratory of the CNRS in Valbonne, France is reported. A sharp peak at 567 cm-1 corresponding to the E2 (high) mode of GaN broadens and shifts to higher wavenumbers as the aluminium content of the AlxGa1-xN is increased. The shift is accompanied by a decrease in the intensity and a broadening of this peak. The broadening was attributed to a general decrease in the quality of the layers which accompanies increased aluminium content in AlxGa1-xN.

The III-V semiconductor compounds (i.e. In Ga As x 1-x , 1 x x InAs Sb - , In Ga Sb x 1-x and Al Ga As x 1-x ) have been studied using room temperature Raman spectroscopy. X-ray diffraction has been used as a complementary characterization technique. In this study all the III-V semiconductor compounds were grown by metal organic chemical vapour deposition (MOCVD) on GaAs and GaSb substrates. The layers were studied with respect to composition, strain variation and critical thickness. Raman spectroscopy has been employed to assess the composition dependence of optical phonons in the layers. The alloy composition was varied, while the thickness was kept constant in order to investigate compositional effects. A significant frequency shift of the phonon modes were observed as the composition changed. The composition dependence of the phonon frequencies were described by linear and polynomial expressions. The results of this study were compared with previous Raman and infrared work on III-V semiconductor compounds. Strain relaxation in InGaAs and InGaSb has been investigated by Raman and X-ray diffraction. Measurements were performed on several series of layers. For each series, the thickness was varied, while keeping the composition constant. For a given composition, the layer thicknesses were such that some layers should be fully strained, some partially relaxed and some fully relaxed. The Raman peak shifts and XRD confirm that a layer grows up to the critical thickness and then releases the strain as the thickness increases. Critical layer thickness values measured in this study were compared with published data, in which various techniques had been used to estimate the critical thickness.

The reinforcement mechanisms in graphene-based nanocomposites have been studied in this project, which primarily consists of three parts: the size and orientation effects of the graphene-based nano-fillers and their interfacial adhesion with the matrix. Overall Raman spectroscopy has been demonstrated to be a powerful technique to study the graphene-based nanocomposites. The deformation of small size graphene has been followed and a new model has been established to consider both the non-uniformity of strain along the graphene and laser intensity within the laser spot, which interprets the observed unusual Raman band shift well. Additionally, the deformation of monolayer graphene oxide (GO) has been followed for the first time. It appears that continuum mechanics is still valid, and the approximately constant strain distribution along the GO flake suggests a better stress transfer efficiency of GO than that of graphene. The spatial orientation of graphene has been studied based on the Raman scattering obtained from transverse sections of graphene, where the Raman bands intensities show a strong polarization dependence. Based on this, a new model has been established to quantify the spatial orientation of graphene in terms of an orientation distribution function, and the spatial orientation of monolayer graphene has been further confirmed by its surface roughness. This model has been extended to a variety of graphene-based materials and nanocomposites. It is also shown how the spatial orientation of graphene-based fillers affects the mechanical properties of the nanocomposites, through the first determination of the Krenchel orientation factor for nanoplatelets. The findings on both the size and orientation effects have been employed to study the deformation mechanics of bulk GO reinforced nanocomposite films. It has been demonstrated for the first time that the effective modulus of GO can be estimated using the Raman D band shift rate, and this is in agreement with the value measured using conventional mechanical testing. The effective modulus of GO is found to be lower than its Young’s modulus, probably due to the mis-orientation, waviness, wrinkling and agglomeration of the GO fillers.

Contemporary laser research involves the development of spectroscopic techniques to understand the microscopic structural aspects of a simple molecular system in chemical and materials to more complex biological systems such as cells. In particular, Raman spectroscopy, which provides bond specific information, has attracted considerable attention. Further with the advent of femtosecond (fs) laser, the recent trend in the field of fs chemistry is to develop nonlinear Raman techniques that allow one to acquire vibrational structural information with both fs temporal resolution as well as good spectral resolution. Among many advanced nonlinear Raman techniques, the development of fs Stimulated Raman scattering (SRS) has gathered momentum in the recent decade due to its ability to (1) provide vibrational structural information of various system including fluorescent molecules with good signal to noise ratio and (2) circumvent the limitation imposed on the spectral resolution by the necessary pulse durations according to the energy-time uncertainty principle where ‘K’ is a constant that depends on the pulse shape) unlike in the case of fs normal resonance Raman spectroscopy. We have developed a technique named “Ultrafast Raman loss spectroscopy (URLS)” that is analogues to SRS, but is more advantageous as compared to SRS and has the potential to be an alternative if not competitive tool as a vibrational structure elucidating technique. The concept and the design of this novel technique, URLS, form the core of the thesis entitled “Ultrafast Raman Loss Spectroscopy (URLS)”. Chapter 1 lays the theoretical groundwork for ultra-short pulses and nonlinear spectroscopy which forms the heart of URLS. It presents a detailed discussion on the basis behind the elementary experimental problems associated with the ultra-short laser pulses when they travel through a medium, the characterization of these ultrashort pulses as well as various non-linear phenomena induced within a medium due to the propagation of these pulses. Chapter 2 focuses on the concept of SRS which resulted into the foundation of URLS. It illustrates the theoretical as well as the experimental aspects of SRS and demonstrates the sensitivity of SRS over normal Raman spectroscopy. Chapter 3 introduces the conceptual and the technical basis which ensued into the development of URLS while Chapter 4 demonstrates its application and efficiency over its analogue SRS. URLS involves the interaction of two laser sources, viz. a picosecond (ps) pulse and a fs white light (WL), with a sample leading to the generation of loss signal on the higher energy (blue) side with respect to the wavelength of the ps pulse unlike the gain signal observed on the lower energy (red) side in SRS. These loss signals are at least 1.5 times more intense than SRS signals. Also, the very prerequisite of the experimental protocol for signal detection to be on the higher energy side by design eliminates the interference from fluorescence, which appears on the red side. Thus, the rapid data acquisition, 100% natural fluorescence rejection and experimental ease ascertain “Ultrafast Raman Loss Spectroscopy (URLS)” as a unique valuable structure determining technique. Further, the effect of resonance on the line shape of the URLS signal has been studied which forms the subject of discussion in Chapter 5. The objective of the study is to verify whether the variation of resonance Raman line shapes in URLS could provide an understanding of the mode specific response on ultrafast excitation. It is found that the URLS signal’s line shape is mode dependent and can provide information similar to Raman excitation profile (REP) in the normal Raman studies. This information can have impact on the study of various dynamical process involving vibrational modes like structural dynamics and coherent control. Chapter 6 demonstrates the application of URLS as a structure elucidating technique for monitoring ultrafast structural and reaction dynamics in both chemical and biological systems using α-terthiophene (3T) as the model system. The objective is to understand the mechanism of the molecular structure dependent electronic relaxation of the first singlet excited state, S1, of α-terthiophene using fs URLS. The URLS data along with the ab-initio calculations indicate that the electronic transition is associated with a structural rearrangement from a non-planar to a planar configuration in the singlet manifold along the ring deformation co-ordinate. The experimental findings suggest that the singlet state decays exponentially with a decay time constant ( 1/e) of about 145 ps and this decay could be assigned to the intersystem crossing (ISC) pathway from the relaxed S1 state to the vibrationally hot triplet state, T1*. Lastly, Chapter 7 summarizes the entire thesis and presents some possible future prospects for URLS. Considering the advantages of URLS, it is proposed that URLS can be exploited [1] to determine the structure of any fluorescent/non-florescent condensed materials and biological systems with a very good spectral resolution (10- 40 cm-1); [2] to obtain the vibrational signature of weak Raman scattering molecules and vibrational modes with relatively small Raman cross-section owing to its high detection sensitivity with good signal to noise ratio; [3] for performing fs time-resolved study by introducing an additional fs pulse for photo-excitation of the molecule and using URLS to probe the excited state dynamics with good temporal (fs) and spectral (10-40 cm-1) resolution; and lastly, [4] the high chemical selectivity of URLS and the fact that the signal is generated only within the focal volume of the lasers where all the beams overlap can be utilized for developing this method into a microscopy for labeled-free effective vibrational study of biological samples. Consequently, it is hoped that this technique, “Ultrafast Raman Loss Spectroscopy (URLS)”, would be a suitable alternative to other nonlinear Raman methods like coherent anti-Stokes Raman spectroscopy (CARS) that has made major inroads into biology, medicine and materials.

In less than a decade of research, graphene has earned a long list of superlatives to its name and is expected to have applications in various fields such as electronics, photonics, optoelectronics, materials, biology and chemistry. Graphene has also attracted a lot of attention because its properties can be engineered either via intrinsic changes or by modification of its environment. Raman spectroscopy has become an ideal characterization method to obtain qualitative and quantitative information on these changes. This thesis investigates the possibility to change, supplement and monitor the electonic and optical properties as well as the chemical reactivity of graphene. It is achieved by i) substrate effect, ii) introduction of defects in the structure of graphene and iii) the combination of graphene with other two- dimensional crystals such as hexagonal boron nitride (h-BN) and transition metal dichacolgenides. In particular, the experimental work presented here describes: I - The influence of the type of substrate on the Raman intensity of graphene. This work leads to the calculation of the Raman scattering efficiency of graphene after CaF2 is found to be a suitable substrate for this kind of study in contrast to Si/SiOx that strongly modulates the Raman intensities. The G peak scattering efficiency is found to be about 200 x 10-5 m-1 Sr-1 at 2.4 eV while that of the 2D peak is one order of magnitude higher, confirming the resonant nature of the 2D peak Raman scattering process. II - An attractive method to produce large (up to several hundreds of microns across) and high quality graphene by anodic bonding. This cheap, fast and solvent-free method also allows introduction of vacancy like defects in the samples in a relatively controllable way. III - The Raman signatures of several types of defect such as sp3 sites, vacancies and substitutional atoms. For low defect concentration (stage 1) the intensitiy ratio I(D)/I(D') is constant and is 13 for sp3 sites, 9 for substitutional atoms and 7 for vacancies. This signature is explained using the local activation model recently proposed to model the amorphization trajectory of graphene with containing vacancy-like defects. IV - Controlled modification of graphene through mild oxygen plasma. The influence of sp3 sites on monolayer and bilayer graphene's electrical properties are discussed. In the case of bilayer under controlled conditions, it is possible to modify only the top layer. This may lead to decoupling between the two layers, which could explain the good mobility measured for this system. The possiblity to use such system as a sensor is discussed. V - The characteristic Raman signature of aligned graphene/h-BN superlattices. The Raman spectrum shows strong changes in perfectly aligned superlattices, which could be attributed to the reconstruction of the Dirac spectrum. VI - A prototype photovoltaic cell made of a graphene and tungsten disulphide (WS2) heterostructure with an external quantum efficiency of about 30%. The beneficial combination of an excellent absorption in WS2 atomically thin films due to the presence of van Hove singularities and graphene used as a transparent, flexible and conductive electrode is demonstrated.

An improved method to evaluate Young's modulus of bacterial cellulose (BC) nanofibrils is presented. This estimation takes into account polarisation configurations, nanofibril orientation and tensile deformation axis direction. A range of 79 - 88 GPa has been obtained showing their great potential to be used as reinforcement in composite materials. BC bio-composites, constituted of a BC layer embedded in-between two matrix layers, have been prepared by compression moulding. The stress-transfer from the matrix to the reinforcement has been quantified using Raman spectroscopy. This has been carried out by following the shift of the Raman band initially located at a wavenumber position of ~1095 cm-1. Polylactide (PLA) was chosen as matrix material due to its biodegradability and bio-sourced origin. Transparent polylactide films were obtained in specific processing conditions to suppress crystallisation. This allowed the laser to penetrate the matrix and interact with the upper layer of BC networks. Several factors that could affect the interface in these composites have been studied. The influence of the culturing time of BC networks on the composite interfaces has been investigated. Higher Raman band shift rates with respect to strain and stress have been measured for composites manufactured using BC networks having a low culturing time. This led to enhanced coupling between PLA and the upper layer of BC networks. Scanning electron microscopy imaging of the tensile fracture surface of these composites revealed that delamination between the BC layers was occurring rather than failure at the BC/PLA interface. Cross-linking of BC networks using glyoxal was performed to consolidate their layered structure. Raman spectroscopy was used to probe the stress-transfer of unmodified and cross-linked BC networks. These data revealed that cross-linked materials exhibit an enhanced stress-transfer both in the dry and wet states compared to unmodified BC networks. Cross-linked BC networks were used to design composites but no significant stress-transfer improvement was observed. As a result, maleated polylactide (MAPLA) was produced and used as a matrix material in order to consolidate the interface between PLA and both the upper and lower layer of cross-linked BC networks. Composites designed using cross-linked BC networks and MAPLA showed a significant stress-transfer improvement over composites designed using unmodified BC networks and PLA. Also the determination of the bulk tensile mechanical properties of the composites revealed a significant increase of relative Young's modulus. This increase is thought to be due to reduced molecular mobility at both the cross-linked BC/MAPLA interface and between cross-linked BC layers. This is further supported by scanning electron imaging of the tensile fracture surfaces.