Academic literature on the topic 'BULK SEMICONDUCTING'

Band-structure calculations of germanium carbide (GeC) show that it is a new indirect wide band gap semiconducting material, which crystallizes in both cubic and hexagonal phases. Through the density functional and total-energy technique in the generalized gradient approximation, the two polytypes 3C and 2H of GeC were studied. According to our calculations, it is a hard material with a percentage of covalency of about 80–90%. Important energy gaps were determined. The bulk modulus, density of states, and charge density were calculated. For the bulk modulus calculations, Murnaghan’s equation of state was used under elastic deformation to measure hardness. Our calculations showed that this semiconducting material crystallizes in zincblend (Eg = 1.76 eV) and wurtzite (Eg = 2.5 eV) structures.

The II-VI semiconducting compounds are of particular interest due to the ability to compositionally tune them to detect infrared radiation in the 0.5 to 30 μm range. With the demand for advanced imaging systems, there is an immediate need for bulk II-VI materials with improved compositional homogeneity and structural perfection. The performance of optical semiconductors is very sensitive to the presence of defects such as dislocations, precipitates, and boundaries.

The armchair graphene nanoribbons (AGNRs) can be either semiconducting or metallic, depending on their widths. We investigate the electronic properties of AGNRs under uniaxial strain and electric field. We find that the bulk gap decreases gradually with the increase of the electric field for semiconducting case, but it cannot vanish completely in an appropriate range, which is similar to that of a single uniaxial strain. However, a suitable combination of electric field and uniaxial strain can lead to that the energy gap completely vanishes and reopens. For the metallic case, the bulk gap can display the same opening and closing behavior under an electric field and uniaxial strain. Finally, an interesting quantum phenomenon is obtained by applying a perpendicular magnetic field.

In this Ph.D. work, we investigate the optoelectronic properties of low-bandgap semiconducting polymers and project the potential for employing these materials in electronic and photonics devices, with a particular emphasis on use in organic solar cells. The field of organic solar cells is well developed and many of the fundamental aspects of device operation and material requirements have been established. However, there is still more work to be done in order for these devices to ultimately reach their full potential and achieve commercialization. Of immediate concern is the low power conversion efficiency demonstrated in these devices so far. In order to improve upon this efficiency, several routes are being explored. Because the optical bandgaps of semiconducting polymers are larger than in inorganic semiconductors, one of the most promising routes currently under exploration is the development of low-bandgap materials. Using polymers with lower band gaps will allow more of the solar irradiance spectrum to be absorbed and converted into electricity and thus possibly boost the overall efficiency. The bandgap of these semiconducting polymers is determined by the chemical structure, and therefore can be tailored through synthesis if the relevant structure-property relationships are well-understood. The materials studied in this work, a new series of Poly(thienylenevinylene) (PTV) derivatives, posses lower band gaps than conventional polymers through a design that incorporates aromatic-quinoid structural disturbances. This type of chemical structure delocalizes the electronic structure along the polymer backbone and reduces the energy of the lowest excited-state leading to a smaller band-gap. We investigate these materials through a variety of techniques including linear spectroscopy such as absorption and photoluminescence, pump-probe techniques like cw-photoinduced absorption and transient photo-induced absorption, and the non-linear electroasborption technique in order to interrogate the consequences of the delocalized electronic structure and its response to optical stimuli. We additionally consider the effects of environmental factors such as temperature, solvents and chemical doping agents. During the course of these investigations, we consider both of the two primary categorical descriptions of structure-property relationships for polymers within the molecular exciton model, namely the role of inter-molecular interactions on the electronic properties through the variation of supermolecular order and the fundamental determination of electronic structure due to specific intra-molecular interaction along the backbone of the polymer chain. We show that the dilution of aromaticity in semiconducting polymers, while being a viable means of reducing the optical band gap, results in a significant increase in the role of electron-electron interactions in determining the electronic properties. This is observed to be detrimental for device performance as the highly polarizable excited state common to polymers gives way to highly correlated state that extinguishes both the emissive properties and more importantly for solar cells, the charge-generating characteristics. This situation is shown to be predominant regardless of the nature of interchain interactions. We therefore show that the method of obtaining low-bandgap polymers here comes along with costly side-effects that inhibit their efficient application in solar cells. Further, we directly probe the efficacy of these materials in the common bulk-heterojunction architecture with both spectroscopy and device characterization in order to determine the limiting and beneficial factors. We show that, while from the point of view of absorption of solar radiation these low-bandgap polymers are more suited for solar cells, the ability to convert the absorbed photons into electron-hole pairs and generate electricity is lacking, due to the internal conversion into the highly correlated state and thus, the absorbed photon energy is lost. For completeness, we fabricate devices and verify that both the charge-transport properties and alignment of charge extraction levels with those of the contacts can not be responsible for the dramatic decrease in efficiency found from these devices as compared to other higher band gap polymers. We thus conclusively determine that the lack of power converison efficiency is governed by the inefficiency of charge-generation resulting from the intrinsic defective molecular structures rendering a low-lying optically forbidden state below the lowest optical allowed state that consumes the majority of the photogenerated excitons. It is emphasized that our means of investigation allow us to truly access the potential of these materials. In contrast, the direct application of these systems in devices and interpretation of the performance is exceedingly complex and may obscure their true potential. In other words, poor performance from a device may be extrinsic in nature and the optimization process may be very costly with respect to both time and materials. The methods used here however, allow us to determine the intrinsic potential. Not only is this beneficial in terms of preserving the resources that would be used on the trial-and-error method for devices, but it also allows us to learn more on a fundamental level about the structure-property relationships and their implications for device performance. The benefits of this increased understanding are two-fold. First, by learning about the fundamental response of a material, a new application may be realized. For example, the rapidly efficient internal conversion process that renders the materials in this study as poor candidates for solar cells may make them useful for photonics applications, as optical switches, for instance. Secondly, this type of investigation has implications for the whole organic electronics community instead of just being limited to the particular material system and the primary application attempted. In this case, we are essentially able to determine a threshold for aromaticty necessary in a structure that will preserve the stability of the ionic excited state that is useful for charge generation in solar cells.

Thermoelectric device has ability to modify thermal energy into electrical energy or vice versa, and such devices may propose a fabulous possibility in settling of energy problem from an environmental-sustainable aspect. Climate change issue and worldwide shortage of energy is creating exaggerated interests in other new spectacular contrivance of power generation with translucent energy sources. Whenever, the absence of any liquid media or other moving parts, a temperature gradient imposed between the hot and cold junction. A thermoelectric material plays an important role in primary power generation (i.e., combustion, chemical reactions, and nuclear decay), and energy conservation both. Several thermoelectric materials are reported. However, among all of these Tin Telluride (SnTe) displays exclusive features like low-toxicity and eco-friendly behaviour etc. The current prevalence illustrates that at temperature 300 K nano-structuring and band engineering may provide a high thermoelectric performance device of SnTe, which offers a substitute for toxic PbTe in similar operational temperature. The huge area competence and innateness of scaling up with comprehensive constraint of material handling provides possible application of sintering in industrial manufacture of coatings and optoelectronic devices. However, metal chalcogenides of the group IV-VI are gained significant attention due to its fascinating physical properties. These narrow bandgap semiconductors are enormously useful for thermoelectric devices, thermo-voltaic, photovoltaic and as well as various optical applications. Whereas, SnTe semiconductor possesses a cubic rock salt structure with a direct band gap 0.18 eV, which is responsible to make it more attractive. Many more applications such as photo detectors, mid-infrared (3-14 μm) detection and thermoelectric devices of SnTe is a proof of its capacity in material research. A Tin Telluride (SnTe) vacuum evaporated thin films has been synthesized at room temperature (RT) on a glass substrate, which has been proven as a significant enhancement in the figure of merit (ZT) value as p-type material. Moreover, a high-resolution X-ray diffraction (HRXRD) outlines indicated towards a polycrystalline nature in bulk solids as well as in thin films both. Surface morphology of composed grains of variable sizes of these films also investigated using scanning electron microscopy (SEM), which has been further supported by atomic force microscopy (AFM) wherein, the surface parameters (like roughness, skewness, and kurtosis) were measured and analyzed to determine topography of the thin film surface. High-resolution transmission electron microscope (HRTEM) also touches to local microstructural features and crystalline structure, which another level investigation has been confirmed using selected area electron diffraction (SAED) pattern analysis. Four probes method has also been used to determine electrical measurements, which confirm that the thin films behave as a semi-metallic nature. We observed that figure of merit of thin films increases with thickness of the film. The maximum ZT value of ∼ 1.02 for the SnTe thin film with thickness 275 nm and ∼ 1.04 for In-doped SnTe thin film of thickness 450 nm has been observed at room temperature measurement. A detailed analyzes of SnTe is indicated that SnTe is the utmost promising material for thin film photovoltaics, thermoelectric device and IR detector. Whereas, present research work also deals with a comparative study of bulk solids and thin films of SnTe and In-doped SnTe compound semiconducting material. Whereas, it has been observed that optical, electrical and thermoelectric properties of thin films alter with the thickness of thin film. The present thesis has been discussed into seven chapters, which brief discussion has been indicated in the following paragraphs. Chapter 1 introduces to the fundamental aspects of thermoelectric materials. Various kinds of thermoelectric materials such as Metal chalcogenides, Superionic conductors, Metal oxides, Silicon-based materials, PGEC thermoelectric materials categorized as Skutterudites, clathrates, half- Heusler alloys, and SnTe has been discussed in this segment (introduction) of the thesis. Literature review of SnTe compound is discussed thoroughly including the detailed explanation about crystalline structures, phase diagrams and related applications. The effect of doping in pure SnTe has also been discussed in detail with an optimal literature survey. Overall, the semiconducting compound materials discussed in this part. This chapter ends with the motivation for the present work. Finally, the objectives of the thesis based on the review of the literature have been incorporated. Chapter 2 describes a brief description of experimental and characterization techniques used in the present investigation for the synthesis of bare SnTe and Indium doped SnTe bulk and thin films. Whereas, a primarily, vertical directional solidification (VDS) and subsequently thermal evaporation technique have been used for the synthesis of the bulk SnTe, In-doped SnTe and their thin films. This chapter includes the details of sophisticated analytical experimental tools like, XRD/ HRXRD, VP-SEM, EDS, TEM/HRTEM, AFM, FTIR, micro-Raman, PL, UV-NIR, TOF-SIMS, EPR, Four probe method (-T) and Herman method (ZT) measurements for different properties. Bulk SnTe and In-doped SnTe in the form of ingot was prepared by the physical route via VDS technique in a vertical muffle furnace. The thin film deposition has been performed on glass, silicon, and NaCl substrate using the thermal vapor deposition instrument and discussed in detail in this chapter. Chapter 3 provides the detailed investigation of the bulk SnTe ingot. This chapter emphasized that SnTe material was prepared by vertical directional solidification (VDS) technique at high temperature via the physical route. Bulk SnTe pallets were used for the structural characterization (XRD). XRD pattern of bulk SnTe confirms the formation of polycrystalline SnTe as well as its atomic-scale range of structural periodicity. From the XRD analysis it is established that both cubic and orthorhombic phases co-exist in bulk SnTe compound. Rietveld refinement of four-time repeated XRD data indicates all best-fitting parameters for analysis of bulk SnTe. SEM and EDX analyses show the existence of cleavage planes on the morphological surface of the bulk SnTe compound. The results obtained from the EDX revealed that the stoichiometry of Sn and Te is maintained perfectly. The microstructural investigations were achieved by employing HRTEM and SAED, which confirm that inter-planar spacing values correlate with XRD data, and various sizes of grains are present in the material. Some grain boundaries have been occurred, establishing about the formations of imperfections in the material. HRTEM micrographs show the disorder in the grain boundaries of different grains, and hence here in most cases, lattice fringes of different grains were merged with each other. FTIR and Micro - Raman spectra revealed that the SnTe is a suitable compound for IR applications. EPR results revealed that holes are present in an abundant concentration, and voids presence makes the material highly paramagnetic. As SIMS spectra revealed the presence of unreacted Tin, Tellurium, SnTe compound, and impurities in the ingot up to ppm level, therefore this may be an appropriate reason for paramagnetic behavior. The confirmation of the p-type nature of SnTe indicates the presence of holes or vacancies in the material, which is also responsible for paramagnetic resonance. The reaction of very – very few oxygen atoms with the material may also be responsible for free electrons in the material, which seems a strongly correlated with Micro-Raman and FTIR results. Electrical properties confirmed that P-type SnTe is semi-metallic, and resistivity is temperature-dependent. These studies explore the feasibility of employing the material in the industrial production line of infrared detectors. Chapter 4 reports the detailed study of SnTe thin films of different thickness deposited on various substrates. By using thermal evaporation equipment, a series of Tin Telluride (SnTe) thin films of varied thicknesses are deposited onto different substrates at 300 K. The morphology, microstructure, topology, optical, elemental mass isotope spectrum, electrical, and thermoelectric properties of SnTe thin films having thicknesses 33 nm to 275 nm have reported here. High-resolution x-ray diffraction (HRXRD) patterns of SnTe thin films revealed the polycrystalline nature with [200], which orientation possessed a cubic structure. Rietveld refinement of XRD data of these thin films indicates all best-fitting parameters for the analysis of crystalline features. The microstructural and morphological structures of all thin films were examined using HRTEM and SEM-EDS, respectively. The distribution of isotopes of various elements in the thin film along with facet and longitudinal channels was expolred by using depth profile determination through the TOF – SIMS technique. Fourier transform infrared spectroscopy spectra reveal the molecular vibrations, narrow bandgap property of material, and suitability of materials in infrared applications. Longitudinal – optical phonon scattering due to the [222] plane orientation is also observed in the micro-Raman spectra at room temperature, which corresponds to a peak in the range 120–130 Raman shift/cm−1. Hence, the change in optical and microstructural properties at the nano-regime resulted in a shift towards the near-infrared region with an enhancment in the thickness of the thin films. Electrical properties enhance with the decrease of thin-film thickness. Whereas, figure of merit (ZT) equal to 1.02 is the highest value for a thin film of thickness 275 nm among all four thin films. Chapter 5 reports the detailed study of doping of the Indium (In) element in SnTe bulk compound and In-doped SnTe thin films with thickness of 50 nm, 245 nm and 450 nm. Rietveld refinement of XRD data of bulk In0.1Sn0.9Te compound and thin films indicates the formation of polycrystalline bulk and thin films. Rietveld refinement of XRD data indicates all best-fitting parameters for analysis of In-doped SnTe thin films. Morphological, microstructural, topological, optical, and thermoelectric properties have also described in this chapter. SEM, TEM and AFM micrographs revealed that nanoparticles (NPs) of different size from 50 nm to 500 nm have been spread along the whole of surface of thin film. These different size NPs can affect the optical properties of these thin films, because due to absorption of light in visible region the variable size of NPs can emit diverse colors radiations. However, the metallic NPs show dissimilar physical and chemical properties in comparison of bulk metals. It is clear that NPs have large surface area to volume ratio hence in the case of NPs huge interactive interface exist between the adjacent particle and thier local surroundings. As per use of any compound rather in the form of bulk material or in the form of nanomaterials the properties of similar chemical compositional material changed significantly. Thermoelectric properties (figure of merit) revealed that ZT = 1.04 is highest for the film of thickness 450 nm. Chapter 6 describes the detailed study of Ultra-fast spectroscopy for SnTe and In-doped SnTe thin films. This chapter describes relationship about the study of optical properties with dynamics of thin film. Ultrafast laser spectroscopy is a sophisticated technique in which ultrashort pulse lasers generally used to study the dynamics of reaction mechanism up to tremendously small-time scales. Various techniques are practiced for the study of the dynamics of holes and electrons, atoms or molecules. The time domain part of frequency-resolved spectroscopy is the main component of ultrafast molecular spectroscopy. In the case of ultrafast spectroscopy, coherent quantum levels lead to time-dependent dynamics which is belongs to the traditional mechanical movement. In ultrafast spectroscopy, a 70-fs pulse is applied to pump the specimen, which is generated as a result of a mode-locked laser beam, amplifier, and optical parametric amplifier (OPA). The mode-locked laser is of MICRA, which generates a 35-fs pulse of 800 nm with a 320mW average power. The pulsed laser is then amplified using a COHERENT amplifier in which the Ti: Sapphire crystal is used to amplifies the pulse laser to 4 W. This laser pulse is then split into 70:30 using a beam splitter in which 30% of the laser beam is passed through the delay stage to provide a 6 ns long delay. The delayed beam is then passed through the Ti: Sapphire crystal to generate a continuum in the NIR range of 800-1600 nm. Simultaneously, the remaining 70% of the laser beam is passed through TOPAs, which is an OPA. The HELIOS spectrometer is used to detect the differential reflectance in which the InGaAs detector is used. The system is calibrated through the in-house built BND in CSIR-NPL. Chapter 7 provides a summary of the research work done in this thesis. It also suggests and outlines open issues and the future course of research in the area of Chalcogenide materials and compound semiconductors.

Die vorliegende Arbeit ist eine umfassende Studie über das Wachstum mittels Molekularstrahlepitaxie (MBE) und die gemessenen Seebeck Koeffizienten und Lochtransport Eigenschaften von p‑Typ Oxiden, eine Materialklasse welche die optische Transparenz und die einstellbare Leitfähigkeit verbindet. Insbesondere, Nickeloxid (NiO) und Zinnmonoxid (SnO) wurden mittels plasmaunterstützter MBE unter Einsatz von einer Metall‑Effusionszelle und einem Sauerstoffplasma gewachsen. Für das NiO Wachstum wurden vor allem die Wachstumsgrenzen bei hohen Temperaturen festgelegt, welche von der Substratstabilität im Falle von Magnesiumoxid und Galliumnitrid abhängen. Es wird die Möglichkeit der Qualitätsbewertung mittels Ramanspektroskopie für Natriumchlorid-Strukturen gezeigt. Untersuchung der NiO Dotierung durch Oberflächen-Akzeptoren und der damit verbundenen Oberflächen‑Loch‑Anreicherungsschicht offenbart eine neue Dotierungsmöglichkeit für p‑leitende Oxide im Allgemeinen. Die metastabile Phase des SnO wird mittels PAMBE unter Verwendung bekannter Wachstumskinetik von Zinndioxid und verschiedener in‑situ Methoden stabilisiert, die anwendungsrelevante thermische Stabilität wird untersucht. Anschließende ex‑situ Charakterisierungen durch XRD und Ramanspektroskopie identifizieren das kleine Wachstumsfenster für das epitaktische Wachstum von SnO. Elektrische Messungen bestätigen die p‑Typ Ladungsträger mit vielversprechenden Löcherbeweglichkeiten welche auch für Hall Messungen zugänglich sind. Temperaturabhängige Hall Messungen zeigen einen bandähnlichen Transport welcher auf eine hohe Qualität der gewachsenen Schichten hindeutet. Die Funktionalität der gewachsenen Schichten wird durch verschiedene Anwendungen nachgewiesen. Zum Beispiel werden pn‑Heteroübergänge wurden durch das heteroepitaktische Wachstum der SnO Schichten auf einem Galliumoxid-Substrat erlangt. Die ersten bisher berichteten SnO-basierten pn‑Übergänge mit einem Idealitätsfaktor unter zwei wurden erreicht.
This thesis presents a comprehensive study on the growth by molecular beam epitaxy (MBE) and the measured Seebeck coefficients and hole transport properties of p‑type oxides, a material class which combines transparency and tunable conductivity. Specifically, Nickel oxide (NiO) and tin monoxide (SnO) were grown by plasma‑assisted MBE using a metal effusion cell and an oxygen plasma. For NiO growth, the focus lies on high temperature growth limits which were determined by the substrate stability of magnesium oxide and gallium nitride. Quality evaluation by Raman spectroscopy for rock‑salt crystal structures is demonstrated. Investigations of NiO doping by surface acceptors and the related surface hole accumulation layer reveal a new doping possibility for p‑type oxides in general. The meta‑stable SnO is stabilized by PAMBE utilizing known growth kinetics of tin dioxide and various in‑situ methods, its application-relevant thermal stability is investigated. Following ex‑situ characterizations by XRD and Raman spectroscopy identify secondary phases and a small growth window for the epitaxial growth of SnO. Electrical measurements confirm the p‑type carriers with promising hole mobilities accessible to Hall measurements. Temperature dependent Hall measurements show band‑like transport indicating a high quality of the grown layers. The functionality of the grown layers is proven by various applications. For example, pn‑heterojunctions were achieved by heteroepitaxial growth of the SnO layers on gallium oxide substrates. The first reported SnO based pn‑junction with an ideality factor below two is accomplished.

Monitoring the working environment of laboratories and industries for pollutants is of primary concern to ensure the healthiness of working personnel. Semiconducting metal oxides (SMOs) are sensitive to the gas ambience and can be tuned for sensing purpose. As SMOs are not selective, an array of sensors with differential selectivity may resolve to great extent. The objective of the thesis is to understand the physicochemical properties and gas sensing characteristics of Cr1-xFexNbO4. Applying principal component analysis to the sensor response data either for selection of features or for differentiation of analysts is also of concern. Preparation of Cr1-xFexNbO4, phase characterization, lattice parameters estimation, morphological and micro chemical analysis (SEM & EDX), electrical characterization by direct current (DC & AC) in the temperature range of 423 K to 573 K, weighted magnetic moment of iron and chromium deduced from susceptibility measurements, spin nature of iron and surface compositions of different valences of chromium and iron deduced from X-ray photoelectron spectroscopy of are presented. The wide dynamic range hydrogen sensing characteristics of CrNbO4 bulk pellets at different temperatures along with the cross-sensitivity towards NH3, NOx(NO+NO2) and PG (petroleum gas) are investigated. The preparation of Cr1-xFexNbO4 thick and thin films by screen-printing and PLD are also presented. The thick films are tested at different temperatures towards hydrogen. The n-type or p-type nature of thick films towards hydrogen with varying iron concentration in Cr1-xFexNbO4 is reported. The thin films are characterized for phase formation, morphology by XRD, SEM and AFM. XPS performed surface characterization. Electrical resistance measurements at different temperatures and preliminary experiments on hydrogen sensing are presented. The probable hydrogen sensing mechanism of CrNbO4 was revealed by X-ray photoelectron spectroscopy. The experimentally observed reduction in metal ion oxidation states upon interacting with hydrogen is best illustrated by Kröger Vink notation. Principal component analysis was applied for three different types of studies: i) The fit parameters of the transient response of CrNbO4 thick films towards hydrogen are analyzed for finding out the better feature for calibration, ii) Different thick films of CrNbO4, Cr0.5Fe0.5NbO4 and FeNbO4 operated at various temperatures for testing H2 and VOCs are analyzed for redundancy in sensor behaviour and iii) Cr0.8Fe0.2NbO4 thick films are studied for sensing H2, NH3 and their mixtures and usefulness of PCA in resolving them in PC-space. In addition, H2 and VOCs are tested at different temperatures and redundancy in temperature is deduced to construct a sensor array with a minimum number of sensors. Finally, a sensor array consisting of Cr0.8Fe0.2NbO4 thick films, operating at different temperatures is built, and qualitative discrimination of analysts in PC-space is demonstrated. Finally, the major findings of the present investigations and suggestions for future aspects of experimentation are provided

Monitoring the working environment of laboratories and industries for pollutants is of primary concern to ensure the healthiness of working personnel. Semiconducting metal oxides (SMOs) are sensitive to the gas ambience and can be tuned for sensing purpose. As SMOs are not selective, an array of sensors with differential selectivity may resolve to great extent. The objective of the thesis is to understand the physicochemical properties and gas sensing characteristics of Cr1-xFexNbO4. Applying principal component analysis to the sensor response data either for selection of features or for differentiation of analysts is also of concern. Preparation of Cr1-xFexNbO4, phase characterization, lattice parameters estimation, morphological and micro chemical analysis (SEM & EDX), electrical characterization by direct current (DC & AC) in the temperature range of 423 K to 573 K, weighted magnetic moment of iron and chromium deduced from susceptibility measurements, spin nature of iron and surface compositions of different valences of chromium and iron deduced from X-ray photoelectron spectroscopy of are presented. The wide dynamic range hydrogen sensing characteristics of CrNbO4 bulk pellets at different temperatures along with the cross-sensitivity towards NH3, NOx(NO+NO2) and PG (petroleum gas) are investigated. The preparation of Cr1-xFexNbO4 thick and thin films by screen-printing and PLD are also presented. The thick films are tested at different temperatures towards hydrogen. The n-type or p-type nature of thick films towards hydrogen with varying iron concentration in Cr1-xFexNbO4 is reported. The thin films are characterized for phase formation, morphology by XRD, SEM and AFM. XPS performed surface characterization. Electrical resistance measurements at different temperatures and preliminary experiments on hydrogen sensing are presented. The probable hydrogen sensing mechanism of CrNbO4 was revealed by X-ray photoelectron spectroscopy. The experimentally observed reduction in metal ion oxidation states upon interacting with hydrogen is best illustrated by Kröger Vink notation. Principal component analysis was applied for three different types of studies: i) The fit parameters of the transient response of CrNbO4 thick films towards hydrogen are analyzed for finding out the better feature for calibration, ii) Different thick films of CrNbO4, Cr0.5Fe0.5NbO4 and FeNbO4 operated at various temperatures for testing H2 and VOCs are analyzed for redundancy in sensor behaviour and iii) Cr0.8Fe0.2NbO4 thick films are studied for sensing H2, NH3 and their mixtures and usefulness of PCA in resolving them in PC-space. In addition, H2 and VOCs are tested at different temperatures and redundancy in temperature is deduced to construct a sensor array with a minimum number of sensors. Finally, a sensor array consisting of Cr0.8Fe0.2NbO4 thick films, operating at different temperatures is built, and qualitative discrimination of analysts in PC-space is demonstrated. Finally, the major findings of the present investigations and suggestions for future aspects of experimentation are provided

Here CdSxSe1-x nanocrystals were studied and characterized by resonant Raman scattering, low frequency inelastic scattering, pressure tuned Raman scattering and photoluminescence. Ordinary Raman scattering and low frequency inelastic scattering were used for composition and size characterization. In the photoluminescence spectra we found that the deep defects where symbatic for compositions in the range close to CdS. For intermediate compositions down to CdSe no marked change in the defect peak positions were observed. This behavior is similar to the bulk case.1 Also the pressure coefficients in our measurements reflected this peculiarity. Resonant Raman scattering at different pressures was applied to examine the electron phonon interaction in the framework of the one dimensional configuration model. We found that the coupling strength (Huang-Rhys parameter) is larger than expected from theory and might indicate an extrinsic effect such as a point defect.2 The pressure dependence of the coupling was also compared with bulk CdS calculated from phonon replicas of the green defect line 3 and we found the same tendency of weakening with pressure. In the case of nanocrystals we were able to observe this beyond the phase transition point of bulk due to their higher stability or superpressure effects.

Samarium monosulfide is a phase transition material which, in bulk, is a semiconductor at ambient temperature and pressure. An increase of pressure or stress leads to a lattice collapse and first-order transition to a metallic state. Reactively evaporated thin films of SmS have a granular structure, and their optical response differs from that of the bulk. The response is modeled using data on the bulk material and a Bruggemen effective medium theory. When the phase transition is induced by mechanical abrasion of the films, only a portion of the material undergoes the lattice collapse. We model these switched films as a mixture of semiconducting and metallic grains in a dielectric matrix.

The growing interest in semiconducting polymers as device materials 1-3 for applications such as thin films transistors, light-emitting diodes, lasers, and photodectors has also stimulated theoretical and experimental interest in low-dimensional organic semiconductors. 4-8 It is expected that organic quantum wells, quantum wires, quantum boxes, and superlattices may exhibit strong excitonic effects and large exciton binding energies [~0.5 -1.0 eV] in part because of the relatively small dielectric constants of organic molecules and polymers [~3 - 4].4-6 In spite of the many theoretical studies which have predicted quantum confinement effects in heterostructured semiconducting polymers,5,6 clear experimental observation of such effects was not reported until very recently. 9,10 One major experimental difficulty is the rather small exciton Bohr radii (aB) in bulk organic semiconductors (aB~1.0 - 1.5 nm) which places severe limitations on suitable techniques for preparing the nanoscale structures.10,11