The emperical pseudopotential method in the large basis approach was used to calculate the electronic bandstructures of bulk semiconductor materials and layered semiconductor heterostructures. The crucial continuous atomic form factor potentials needed to carry out such calculations were determined by using Levenberg-Marquardt optimization in order to obtain optimal cubic spline interpolations of the potentials. The optimized potentials were not constrained by any particular functional form (such as a linear combination of Gaussians) and had better convergence properties for the optimization. It was demonstrated that the results obtained in this work could potentially lead to better agreement between calculated and empirically determined band gaps via optimization / Physics / M. Sc. (Physics)

Ce manuscrit de thèse est une contribution à l’étude du couplage fort lumière-matière dans les microcavités planaires à base d’oxyde de zinc. Nous avons déterminé les propriétés de l’interaction entre excitons et photons au travers de mesures résolues en angle pour des hétéro-structures réalisées par épitaxie par jets moléculaires (EJM) sur silicium. Il a ainsi été possible de démontrer le régime de couplage fort aussi bien aux températures de l’hélium liquide qu’à température ambiante. Un important travail de conception des cavités et de modélisation de leur réponse optique a été effectué dans le but d’obtenir une émission cohérente de lumière basée sur la condensation des polaritons tel que prédit par les travaux théoriques. Les récentes mesures pour une cavité optimisée conduisent à un facteur de qualité voisin de 500 et à une énergie de Rabi très élevée (120 meV). / This thesis manuscript is a contribution to the study of strong light-matter coupling in zinc oxide-based planar microcavities. We have determined the properties of the exciton-photon interaction through angle resolved measurements on structures grown by molecular beam epitaxy (MBE) on silicon. Thus, we have demonstrated strong coupling at both cryogenic and room temperatures. A substantial work on the conception of cavities and on the calculation of their optical response has been realized to obtain a coherent emission of light based on the condensation of polaritons as predicted by theoretical works. The latest measurements on an optimized microcavity lead to a quality factor close to 500 and a high Rabi splitting of 120 meV.

Orientador: Fernando Iikawa / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-06T18:02:06Z (GMT). No. of bitstreams: 1 Godoy_MarcioPeronFrancode_D.pdf: 4057709 bytes, checksum: 0df1e56082150d4109dcf891f05d4da6 (MD5) Previous issue date: 2006 / Resumo: Neste trabalho estudamos as propriedade estruturais e ópticas de pontos quânticos auto-organizados de InP crescidos sobre o substrato de GaAs. Esta estrutura apresenta o alinhamento de bandas tipo-II na interface, confinando o elétron no ponto quântico, enquanto o buraco mantém-se na barreira, próximo à interface devido à interação coulombiana atrativa. As amostras foram crescidas por epitaxia de feixe químico (CBE) no modo Stranskii-Krastanov. Os pontos quânticos apresentam raio médio de 25 nm e grande dispersão de altura (1-5 nm) e ocorre a relaxação parcial do parâmetro de rede, chegando a 2 %, em pontos quânticos superficiais. Do ponto de vista de propriedades ópticas, a fotoluminescência de pontos quânticos superficiais exibe uma eficiente emissão óptica, devido a baixa velocidade de recombinação dos estados superficiais do InP, e reflete a densidade e distribuição bimodal de tamanhos. Além disso, sua emissão óptica em função da intensidade de excitação exibe comportamento diverso em comparação com pontos quânticos cobertos com uma camada de GaAs. Em pontos quânticos cobertos, determinamos a energia de ativação térmica, que varia de 6 a 8 meV, e é associada à energia de ligação do éxciton ou energia de ionização do buraco. O decaimento temporal da luminescência de pontos quânticos é de 1,2 ns, um tempo relativamente curto para um ponto quântico tipo-II. A análise das propriedades magneto-ópticas em pontos quânticos individuais, inédita em QDs tipo-II, permitiu verificar que o fator-g do éxciton é praticamente constante, independentemente do tamanho dos QDs, devido ao fato dos buracos estarem levemente ligados. Por fim, mostramos a versatilidade do sistema acoplando-o a um poço quântico de InGaAs. Este acoplamento introduz mudanças na superposição das funções de onda do par elétron-buraco que permitem a manipulação do tempo de decaimento da luminescência e da energia de ligação excitônica / Abstract: We have investigated structural and optical properties of InP self-assembled quantum dots grown on GaAs substrate. This system presents a type-II band lineup where only electrons are confined in the InP quantum dots. The InP/GaAs quantum dots were grown by chemical beam epitaxy in the Stranskii-Krastanov mode. Our quantum dots present a mean radius of 25 nm and large height dispersion, 1-5 nm, and a partial relieve of the strain up to 2 % is observed. The photoluminescence spectra of surface quantum dots show an efficient optical emission, which is attributed to the low surface recombination velocity in InP. We observed a bimodal dispersion of the dots size distribution, giving rise to two distinct emission bands. A remarkable result is the relatively large blue shift of the emission band from uncapped samples as compared to those for capped dots. In capped quantum dots, we obtained the thermal activation energy, from 6 to 8 meV, which is associated to the exciton binding energy or hole ionization energy. The observed luminescence decay time is about 1.2 ns, relatively short decay time for type II system. We investigated magneto-optical properties using single-dot spectroscopy. The values of the exciton g factor obtained for a large number of single InP/GaAs dots are mainly constant independent of the emission energy and, therefore, of the quantum dot size. The result is attributed to the weak confinement of the holes in InP/GaAs QDs. We have also investigated structures where InP quantum dots are coupled to a InGaAs quantum well. This system permits the manipulation of the wave function overlap between electron-hole in order to control the optical emission decay time and exciton binding energy / Doutorado / Física / Doutor em Ciências

Esta tese compõe-se de contribuições à física das propriedades eletrônicas das heteroestruturas semicondutoras. São investigadas propriedades eletrônicas das duas hetero­estruturas básicas: o poço quântico e a super-rede. Considera-se o poço quântico dopado com impurezas rasas e estudam-se as suas propriedades eletrônicas nos regimes de poço fraca e altamente dopado. No caso de baixa densidade de impurezas é feita uma simulação Monte Carlo. É utilizado um modelo semi-clássico de band de impureza. A interação elétron-elétron é incluída de forma exata e são calculadas as seguintes propriedades do estado fundamental à temperatura zero: densidade de estados de uma partícula, distribuição de carga, energia de Fermi e distribuição do campo elétrico sobre os doadores neutros, todas em função do grau de compensação, da densidade de impurezas e da largura do poço. É observada uma. grande dependência com a compensação. Os resultados são explicados à luz da competição entre os efeitos de desordem e confinamento. É observada a ocorrência de Coulomb Gap característico de sistemas bidimensionais. Mostra-se que a. distribuição de carga possui largura e constante de decaimento determinados independentemente pela compensação e pela concentração de impurezas, respectivamente. Tais resultados são importantes para a caracterização de poços quânticos puros. No limite altamente dopado parte-se de um modelo light-binding desordenado e calcula­se a densidade de estados de uma partícula formada devido ao overlapping entre os estados localizados; utiliza-se o método de Matsubara e Toyosawa. para a obtenção da média sobre configurações. Discutem-se os efeitos da desordem diagonal introduzida pelo potencial de confinamento os quais são comparados com os da. desordem não-diagonal. São apresentados resultados para a densidade de estados em função do grau de confinamento e concentração de impurezas para poços e fios quânticos. Sâo estudadas as propriedades eletrônicas das super-redes sob campo magnético transversal à direção de crescimento. Mostra-se que esta configuração é ideal para o estudo das características básicas das super-redes: a estrutura de mini bandas e o tunelamento. Calculam-se as sub-bandas de condução utilizando a teoria de massa efetiva de muitas bandas. Introduz-se a idéia de massa efetiva renormalizada para barreiras semicondutoras. Comparam-se os resultados com dados experimentais de ressonância ciclotrônica. A ótima concordância obtida demonstra a grande importância e a utilidade do conceito de massa efetiva renormalizada para barreiras semicondutoras, que é uma maneira nova e simples de lidar com as soluções evanescentes. / This thesis is composed of contributions to the theory of electronic properties of semicon­ ductor heterostructures. Electronic properties of the basic two heterostructures (quantum well and superlattice) are investigated. A quantum well doped with shallow impurities is considered and its electronic properties are studied in both limits: lightly and heavily doped. In the first case a Monte Carlo simula­ tion technique is used. A semiclassical impurity band model is used . The electron-electron interaction is included exactly and properties of the ground state such as the density of single particle states, the charge distribution, the Fermi energy and the electric field di tribution on the neutra/ donors are calculated, all of them as a function of the degree of compensation, the impurity concentration and the width of the well. A great dependency with the compensation is observed. The results are explained by the competition between the effects of disorder and confinement. The existence of a Coulomb Gap is verified . The charge distribution is shown to have a width and decay rate given by the degree of compensation and impurity concentration, in this order. Such results are important to characterize pure quantum wells. On the heavily doped limit, a disordered tight-binding model is used and the density of states that is formed by the overlapping of localized states is calculated by using the method of Matsubara and Toyosawa for the configuration average. The diagonal disord er effect introduced by the confinement potential is considered and compared to that of the non­ diagonal disorder. Results of the density of states as a function of the degree of confinement and impurity concentration for quantum wells and wires are presented. The electronic propertie s of a superlattice under a magnetic field which is transversal to the growth direction are studied. Jt is shown that this configuration is id eal for the study of the basic characteristics of the superlattices: the subband structure and the tunneling. The conduction subbands are calculated by using the theory of many bands effective mass. The idea of renormalized effective mass for barriers is introduced. The obtained level spacings are compared with cyclotron resonance experimental data (infrared absorption). The good agreement obtained demonstrates the importance and usefulness of the renormalized effective mass, which is a new and simple way to handle evanescent waves.

Orientador: Fernando Iikawa / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-21T01:34:49Z (GMT). No. of bitstreams: 1 Girardi_TiagoIllipronti_M.pdf: 2091533 bytes, checksum: 1e5e58f3f5149c97c47c1bf44b7aa186 (MD5) Previous issue date: 2012 / Resumo: Neste trabalho, estudamos o efeito de diferentes condições de interface de InP/GaAs nas propriedades ópticas de pontos quânticos auto-organizados, crescidos por epitaxia de feixe químico, no modo Stranskii-Krastanov. Espera-se que os pontos quânticos de InP/GaAs apresentem alinhamento de bandas do tipo II, e somente os elétrons ficam confinados, enquanto os buracos ficam localizados nas camadas de GaAs em volta do ponto quântico, atraídos pelo elétron. No entanto, devido ao efeito de mistura de átomos nas interfaces o perfil de potencial nas interfaces pode ser alterado significativamente, afetando, com isso, as propriedades ópticas dos pontos quânticos. Foram estudadas amostras com as seguintes condições de interface entre a camada de InP e as camadas de GaAs: inclusão ou não de uma camada de InGaP em uma ou nas duas interfaces. O InGaP gera uma barreira para ambos os tipos de portadores de carga em uma junção tanto com o GaAs como InP e evita a difusa de As das camadas de GaAs para a de InP. Através de medidas de fotoluminescência resolvida no tempo, observamos a variação do tempo de decaimento da emissão óptica associada aos pontos quânticos de acordo com as diferentes condições de interface. Foi observado um tempo curto de decaimento em amostras sem a inclusão de InGaP e com a inclusão apenas na interface superior, enquanto foi observado um tempo longo quando incluímos camadas de InGaP em ambas as interfaces. O tempo de decaimento curto é incompatível com o alinhamento de bandas do tipo II, que deveria separar espacialmente o elétron do buraco. A partir desses resultados e estudos anteriores a esse trabalho, pudemos concluir que o tempo curto se deve à mistura de átomos nas regiões de ambas as interfaces, gerando ligas que localizam os portadores próximos um ao outro. O tempo longo na amostra contendo InGaP nas duas interfaces é atribuído à separação espacial do elétron e do buraco. O efeito de mistura de átomos nas interfaces, neste caso, não forma uma liga na interface que localize os dois tipos de portadores próximos um ao outro. Isso pode ser uma alternativa de preparação de pontos quânticos de InP/GaAs onde se mantém separados espacialmente o elétron e o buraco / Abstract: We studied the effect of different interface conditions on the optical properties of InP/GaAs self-assembled quantum dots grown by chemical beam epitaxy in the Stranskii-Krastanov mode. InP/GaAs quantum dots is expected to present type II band alignment, and only electrons are confined, whereas the holes are localized in the GaAs layers around the quantum dot, attracted by the electron. However, due to the atomic intermixing effect in the interface the potential profile can be strongly changed, affecting the optical properties of the quantum dots. We studied samples with the following conditions at the interfaces between the InP layer and GaAs layers: the inclusion, or the lack of, a InGaP layer at one of or both interfaces. InGaP generates a barrier for both types of carriers in a junction with GaAs and InP, and avoid the diffusion of As from the GaAs layers to the InP one. Using time-resolved photo-luminescence, we observed a change of the optical emission decay times associated to the quantum dots as the interface condition is changed. We observed a short decay lifetime in samples without InGaP layers and with the inclusion in the top interface only, whereas we observed a long decay time when we included InGaP layers in both interfaces. The short decay lifetime is incompatible with the type II band alignment, where the electron and the hole should be spatially separated. Using these and other previous results, we concluded that the short decay lifetime is due to the atomic intermixing in both interfaces regions, forming alloys that localize the carriers near each other. The long lifetime observed for sample containing InGaP in both interfaces is attributed to the large electron-hole spatial separation. In this case intermixing effects at the interfaces do not form a potential well to localize the carries near each other / Mestrado / Física / Mestre em Física

Le suivi de la viabilitié, la croissance et le métabolisme cellulaire des bactéries peut contribuer de manière significative au diagnostic précoce de la maladie, mais peut aussi aider à améliorer le rendement des produits bactériens dans des expériences industrielle ou à petite echelle. Les méthodes conventionnelles utilisées pour l'étude de la sensibilité des bactéries aux antibiotiques sont basées principalement sur la culture, une technique qui prend au moins 12 heures pour rendre un résultat. Ce retard conduit au surtraitement d'un large éventail d'infections par des antibiotiques à large spectre, ce qui est coûteux et peut conduire à l'apparition de résistance à ces antibiotiques précieux, tandis que la détection rapide d'une infection virale ou l'absence de bactéries pourrait prévenir de tels traitements et, dans le cas d'une infection bactérienne, l'identification de la sensibilité aux antibiotiques pourrait permettre l'utilisation d'antibiotiques à spectre étroit. Le projet décrit dans le présent document vise à surveiller les activités biologiques des bactéries vivantes immobilisées sur les surfaces biofonctionnalisées de microstructures composées de semi-conducteurs quantiques (QS). Le procédé dépend de la sensibilité de la photoluminescence (PL) émise par des semi-conducteurs à la perturbation du champ électrique induit par la charge électrique des bactéries immobilisées sur la surface de ces structures. Dans la première phase du projet, nous avons étudié une méthode innovante impliquant la surveillance par PL de l'effet de photocorrosion dans des hétérostructures GaAs/AlGaAs. Le maintien d'un équilibre entre la sensibilité et la stabilité du biocapteur dans l'environnement aqueux nous a permis de détecter Escherichia coli K12 dans des solutions salines tamponnées au phosphate (PBS) avec une limite de détection attrayante de 103 UFC/ml en moins de 2 heures. Suite à cette recherche, nous avons émis l'hypothèse que ces hétérostructures pourraient être utilisés pour développer une méthode à faible coût et quasiment en temps reel de la croissance et de la sensibilité des bactéries aux antibiotiques. L'un des éléments clés dans le développement de cette plate-forme de biocapteurs était de démontrer que le GaAs (001), normalement utilisé pour recouvrir les hétérostructures de GaAs/AlGaAs, ne nuira pas à la croissance des bactéries. Dans la deuxième phase du projet, nous avons exploré la capture et la croissance de E. coli K12 sur des surfaces nues et biofonctionnalisées de GaAs (001). Il a été déterminé que la couverture initiale et les taux de croissance de bactéries dépendent de l'architecture de biofonctionnalisation utilisée pour capturer les bactéries: les surfaces biofonctionnalisées avec d'anticorps présentaient une efficacité de capture significativement plus élevée. En outre, on a trouvé que pour des suspensions contenant des bactéries à moins de 105 UFC/ml, la surface des plaquettes de GaAs ne supportait pas la croissance des bactéries, quel que soit le type d'architecture de biofonctionnalisation. Dans la troisième phase du projet, nous avons suivi la croissance et la sensibilité aux antibiotiques de E. coli K12 et E. coli HB101. Tandis que la présence de bactéries retardaient d’apparition du maximum de PL, la croissance des bactéries retardaient encore plus ce maximum. Par contre, en presence d’antibiotiques efficaces, la croissance des bactéries était arrêtée et le maximum de PL est arrivé plus tôt. Ainsi, nous avons pu distinguer entre des E. coli sensibles ou résistantes à la pénicilline ou à la ciprofloxacine en moins de 3h. En raison de la petite taille, du faible coût et de la réponse rapide du biocapteur, l'approche proposée a le potentiel d'être appliquée dans les laboratoires de diagnostic clinique pour le suivi rapide de la sensibilité des bactéries aux antibiotiques. / Abstract : Monitoring the viability, growth and cellular metabolism of bacteria can contribute significantly to the early diagnosis of disease, but can also help improve yield of bacterial products in industrial- or small-scale experiments. Conventional methods applied for investigation of antibiotic sensitivity of bacteria are mostly culture-based techniques that are time-consuming and take at least 12 h to reveal results. This delay leads to overtreatment of a wide range of infections with broad spectrum antibiotics which is costly and may lead to the development of resistance to these precious antibiotics, whereas rapid detection of a viral infection or absence of bacteria could prevent such treatments and, in the case of bacterial infection, identification of antibiotic susceptibility could allow use of narrow spectrum antibiotics. The project outlined in this document aims at monitoring biological activities of live bacteria immobilized on biofunctionalized surfaces of quantum semiconductor (QS) microstructures. The method takes advantage of the sensitivity of photoluminescence (PL) emitting semiconductors to the perturbation of the electric field induced by the electric charge of bacteria immobilized on the surface of these structures. Our hypothesis was that bacteria growing on the surface of biofunctionalized QS biochips would modify their PL in a different, and measurable way in comparison with inactivated bacteria. In the first phase of the project, we investigated an innovative method involving PL monitoring of the photocorrosion effect in GaAs/AlGaAs heterostructures. Maintaining the balance between device sensitivity and stability in the biosensing (aqueous) environment allowed us to detect Escherichia coli K12 in phosphate buffered saline solutions (PBS) at an attractive limit of detection of 103 CFU/mL in less than 2 hours. Following this research, we hypothesised that these heterostructures could be employed to develop a method for inexpensive and quasi-real time monitoring of the growth and antibiotic susceptibility of bacteria. One of the key elements in the development of this biosensing platform was to demonstrate that GaAs (001), normally used for capping PL emitting GaAs/AlGaAs heterostructures, would not inhibit the growth of bacteria. In the second phase of the project, we explored the capture and growth of E. coli K12 on bare and biofunctionalized surfaces of GaAs (001). It has been determined that the initial coverage, and the subsequent bacterial growth rates are dependent on the biofunctionalization architecture used to capture bacteria, with antibody biofunctionalized surfaces exhibiting significantly higher capture efficiencies. Moreover, for suspensions containing bacteria at less than 105 CFU/mL, it has been found that the surface of GaAs wafers could not support the growth of bacteria, regardless of the type of biofunctionalization architecture. In the third phase of the project, we used PL to monitor the growth and antibiotic susceptibility of E. coli K12 and E. coli HB101 bacteria. While immobilization of bacteria on the surface of GaAs/AlGaAs heterostructures retards the PL monitored photocorrosion, growth of these bacteria further amplifies this effect. By comparing the photocorrosion rate of QS wafers exposed to bacterial solutions with and without antibiotics, the sensitivity of bacteria to the specific antibiotic could be determined in less than 3 hours. Due to the small size, low cost and rapid response of the biosensor, the proposed approach has the potential of being applied in clinical diagnostic laboratories for quick monitoring of antibiotic susceptibility of different bacteria.

There was a fame time for silicon in condensed matter physics, then the graphene era came and now topological insulators are gaining lot of attention, but magnetism in condensed matter physics has remained always fascinating starting from the ancient days up to now and it will remain as one of the core topic in basic or applied physics. The improvement in the modern techniques allows one to explore magnetism in different length scales as well as in different time scales. As an effect of the improvement in experimental techniques, different magnetic anomalies are unearthed. As a result theories are getting refined and the area of magnetism progresses. From the material point of view, oxides carry the most diverse nature in condensed matter starting from high temperature superconductivity (HTS), colossal magnetoresistance, metal insulator transition etc. to ferromagnetism (FM), anti-ferromagnetism (AFM), spin glass (SG) and so on. Among this list, SG and HTS are one of the least understood topics in magnetism till today. A large research community is involved in understanding the underlying physics behind these two, especially in transition metal oxides. It has drawn attention not only due to fundamental aspects but also due to various applications in day to day life. This thesis is an attempt to understand these two phenomena in transition metal oxides. As the title of this thesis suggest, it is all about magnetic and magneto-transport properties of certain transition metal oxide (crystalline) addressing the interplay between two competing order parameters to understand the underlying physics behind it from an experimental point of view. We have studied two different kinds of competing interactions: (i) the FM/AFM interplay either in bulk or at the interface of the two layers in thin films; (ii) the interplay between FM and superconductivity (SC) in superconductor (S)/ferromagnet (F) heterostructures. Basically both of these two kinds lead to non-equilibrium phenomena in these oxides. One of such competition is between FM and AFM leading to slow dynamics (glassy physics). Disorder and frustrations are the key ingredients for such slow dynamics. The spin frustration arises either due to geometry or due to competing interactions. For example, in a triangular antiferromagnet due to the triangular geometry spins gets frustrated. Now, if it prevails spin disorder as well then it satisfies both the criteria for a spin glass and hence it gives birth to glassiness. Another kind of competition is the interplay among SC and FM. It is known that SC and FM are two antagonistic quantum phenomena thus in a single material SC (singlet pairing) and FM does not co-exist. However one can realize this by making F/S heterostructures and observe the battle between these two competing order parameters. The spin polarized quasiparticle injection from F creates non equilibrium spin density inside S and thereby suppressing the order parameter of S. Also by choosing an appropriate ferromagnet the vortex motion inside S can be arrested to certain extent which can enhance the critical current density of S. Thus FM/SC interplay has become an alternative way to look at the high temperature superconductivity. This thesis is categorized into nine chapters. The summary of each chapter is as follows: Chapter: 1 contains certain concepts of magnetism and superconductivity which is useful to understand the topics and experiments described in this thesis. Chapter: 2 gives the underlying principles of the various experimental techniques used in this thesis. Chapter: 3 describes the magnetic properties of successfully synthesized five compositions of LixNi(2-x)O2 (0.67<x<0.99) which has five distinct ground states namely antiferromagnet (AF), spin glass (SG), cluster glass (CG), re-entrant spin glass (RSG) and ferromagnet (FM). The SG and CG ground state has been well described by the frequency dependent peak shift. From the power-law divergence of critical slowing down the estimated value of relaxation time indicates the presence of interacting macro spins (spin cluster) rather than individual spins in certain LixNi(2-x)O2 samples possessing CG ground state which is also supported by the Arrhenius law. The shift in the spin freezing temperature with the application of dc field obeys Almeida-Thouless line. It also exhibits memory effect which is generic to the slow dynamics. The remnant magnetization relaxation follows logarithmic decay. Interestingly, the sample having RSG ground state shows memory effect up-to 50K and behaves like a FM above that temperature. FC-ZFC M(T) curve shows a splitting at the ordering temperature. The critical analysis across the ferromagnetic-paramagnetic phase transition yields a self-consistent γ, β and δ value and the spin-spin interaction in this material follows long range mean field model. The critical exponents obey Widom scaling law: δ = 1 + γ β −1. The universality class of the scaling relations is also verified where the scaled m and scaled h collapses into two branches. Finally the magnetic phase diagram illustrates a vivid picture of the gradual evolution of ferromagnetism in LixNi(2-x)O2 through a glassy state. As a concluding remark, we think, the present study of glassy physics in magnetic insulator/semiconductor sets an example to compare them with the conventional metallic spin glass system. Chapter: 4 exhibits the results of the structural, magnetic and transport measurements to elucidate some of the most striking unusual physical responses of bulk SrRuO3. Two set of polycrystalline SrRuO3 samples with unique ordering temperature have been synthesized. In one case, we have taken the stoichiometric weight ratio of precursors that eventually resulted in Ru-deficient SrRuO3(SROD). In the other case, we have taken extra 2% wt. RuO2 deliberately to form stoichiometric SrRuO3(SRO). Both the samples are found to crystallize in orthorhombic crystal structure with Pnma space group. The low temperature magnetization is found to be well described by the Bloch T3/2 law and the magnetization near Tc is found to follow the scaling law; M~(Tc-T)β with β=0.35 and β=0.30 for SRO and SROD respectively, apparently showing the 3D Ising behaviour. This aspect will be elaborated in the next chapter. The magnetic ac susceptibility study exhibits a broad hump far below the ferromagnetic ordering temperature and the frequency dependence of this hump position exhibits the characteristics of multiple relaxations. Most strikingly, we notice a low temperature glassy magnetic behaviour clearly demonstrated by the time dependent memory effect. This is very surprising and unlikely to happen in systems, which have itinerant ferromagnetic character. However, we conjecture that slow domain growth and spin canting could be the cause for such effect. The transport study evidences a crossover from Fermi liquid (FL) to non-Fermi liquid (NFL) behaviour around 40 K and a slope change in dρ/dT vs. T plot in the vicinity of that temperature. Astonishingly, we observe two distinct dips (one around ferromagnetic ordering temperature and the other far below the ferromagnetic ordering temperature) in the temperature dependent MR response. In addition, we also observe the signature of an unusual dip in the temperature dependent coercive field towards low temperature side. The emergence of such unusual magnetic and transport response is strongly believed to be connected with hidden magnetic interactions. Our effort on neutron diffraction study has been able to trace the cause of such cryptic magnetic interaction. The findings of neutron diffraction study evidence the change in the unit cell lattice parameters around 75 K and that could be the central cause behind such anomalous low temperature magnetic responses. It also demonstrates that the octahedral tilt freezes around the FM transition and has a minimum around the low temperature glass transition temperature. Most remarkably we observe a decline in the total integrated magnetic intensity towards the low temperature side indicating the presence of antiferromagnetic like interaction in SrRuO3. Chapter: 5 resolves the ambiguity in determining the crritical exponents in SrRuO3. Most remarkably, the application of scaling law in the FC magnetization leads a systematic change in the values of critical exponent with the measuring field in SRO. The β value changes from 0 to o.44 to to 0.29 (corresponds to mean field to Ising) with the increase in the measurement field from 10 to 2500 Oe. However, the H→0 extrapolation fields β=0.5. In order to substantiate the actual nature, the critical behavior is studied across the phase transition from the M-H isotherms. The critical analysis yields a self-consistent β, γ and δ values and the spin-spin interaction follows long range mean field δ=γ β model 1+. The critical exponents also obey Widom scaling law: δ = 1 + γ β-1 The universality class of the scaling relations is verified where the scaled m and scaled h collapses into two branches. We have also found that Ru deficiency does not affect the nature of the spin-spin interaction (though ferromagnetism gets reduced). Further the directional dependence of the critical exponent reflects the isotropic nature of the magnetic interaction. In other words the spin-spin interaction found to be: i) three dimensional, ii) long range, iii) mean field type and iv) isotropic in SrRuO3. We have also found magnetocaloric effect (calculated from the M-H isotherms) that across the phase transition. The specific heat measurements find sharp jump at the ferromagnetic transition due to the magnetic contribution of the specific heat. Chapter: 6 describes the magnetism at the SrRuO3 (SRO)/LaAlO3 (LAO) interface where SRO is an itinerant ferromagnet (FM) and LAO is non-magnetic (NM) (rather diamagnetic). Most surprisingly SRO/LAO exhibits pronounced exchange bias (EB) effect realized by observing a shift in the field cooled M-H hysteresis. Further investigation results an increasing trend of the strength of the EB with the decreases in the thickness of ferromagnetic layer. This system also displays the training effect which essentially confirms that this effect is due to EB. EB arises due to the uncompensated spins at the FM/AFM interface hence the EB effect in SRO/LAO system is unconventional. However, the origin of such AFM interaction (responsible for EB effect in FM/NM system) at SRO/LAO interface is realized and explained through the temperature dependence of the EB effect. Further, we have extensively investigated EB effect in other analogous ferromagnets, FM/FM bilayers and FM/FM superlattices. We found that La0.7Sr0.3MnO3 (LSMO) grown on LAO exhibits the signature of EB. In contrast to that La0.5Sr0.5CoO3 (LSCO) does not show any signature of EB. All the bilayers (LSMO/SRO, LSMO/LSCO and LSCO/SRO) exhibit EB and have similar kind of temperature dependence. In order to gain more insight we have grown a (LSMO/SRO)8 superlattice and observed a complex magnetic behaviour. It exhibits partial inverted magnetic hysteresis. But the system shows EB effect characterized by the shift in the FC hysteresis and training effect. All these observations essentially demonstrate that the magnetic nature of various ferromagnetisms at the interfaces can be changed by choosing a proper partner (acts like adding perturbations into one of those system which lies close to the instability region). Chapter: 7 presents the magneto-transport properties of three SRO films grown on LAO (100) of thicknesses of 12, 24 and 48 nm are studied extensively. For a one to one comparison one of the sample is also grown on STO(100). The coercivity vs. temperature in SRO(48 nm)/LAO exhibits a plateau at ~40 K. The dR/dT exhibits the low temperature hump in all the samples which very much replicates with the bulk scenario that we observed in SRO. Most strikingly the 12 nm SRO sample exhibits NFL behaviour throughout the temperature range of measurement (10-150 K). Our careful investigation reveals a cross-over from FL to NFL in all SRO thin films. The cross-over temperature increases with the increase in thickness and eventually shifts towards the bulk cross-over value. It is apt to remind that in bulk SRO we have demonstrated (by employing temperature dependent neutron diffraction) that there is a presence of antiferromagnetic like interaction at low temperature giving birth to glassiness in bulk SRO. Further, an attempt is made to understand the low temperature magneto-transport anomaly by looking into the spin fluctuation through the low frequency 1/f noise measurements. It conveys a message that there are two types of magnetic ordering present in SRO giving rise to two peaks in the temperature dependence of the relative variance. Application of magnetic field suppresses both the peaks in the relative variance. This certainly indicates that the origin of such peak is caused by the spin fluctuations and thereby it is of magnetic origin. Further we have looked into the Hall effect of a structured (Hall patterned) SRO thin film and observed regular Hall effect (RHE) as well as anomalous Hall effect (AHE) in it. Most remarkably the temperature dependence of the RHE coefficient changes its sign close to the ferromagnetic transition temperature of SRO. This implies a change of the type of the carrier as the temperature is varied. Based on these results, the carrier concentration of SRO as a function of temperature is determined. Chapter: 8 is about the magnetic and magnetotransport studies on the successfully grown high quality S/F heterostructures. The oxygen content plays a vital role in superconductivity of oxide materials thus for studying FM/SC interplay in oxides we have discussed how to achieve a high quality sample (oxygen stoichiometric). We have observed a great influence of a FM in suppressing the superconductivity in YBa2Cu3O(7-δ) (YBCO) in FM/SC heterostructures. The analysis of the out of plane M-H hysteresis reveals a significant reduction of the critical fields (HC1 and HC2) of the SC (in SRO/YBCO bilayer) which might have a great significance to understand the superconductivity in a better way (from both the perspectives: theory and experiments). Most remarkably we have found 40% enhancement of the critical current density of YBCO in SRO/YBCO bilayer. We have demonstrated that in order to see the effect of spin polarizes quasiparticle (SPQP) injection into YBCO, one should not apply more than 20mA current since Joule heating contribution wins over pair breaking effect. The SPQP injection from SRO into YBCO exhibits pair breaking effect as the TC (of the SC) shift follows I2/3 law. The resistive transitions under various applied magnetic fields and the field dependence of the activation energy confirms that the vortices are in the 2D regimes (it follows power law, U0~Hα withα=0.5) in SRO/YBCO. To get a better insight into the FM/SC interplay we have looked into two of the FM/YBCO combinations (LSCO/YBCO and LSMO/YBCO). We observe that the degree of the spin polarizations of the FMs scales with the suppression of superconductivity in YBCO which means more the spin polarization more is the suppression. We have also found out that spin polarization is not the sole parameter in suppressing superconductivity in SRO/YBCO bilayers. It also depends upon the state of magnetization of the ferromagnet. Further, we observed a significant reduction (one order) of the activation energy in LSCO/YBCO compared to SRO/YBCO which clearly indicates that the vortex dynamics might depend on other aspects as well (of the FM). It also reveals the formation of decoupled pancake vortices (pure 2D regime) in LSCO/YBCO and LSMO/YBCO bilayers whereas in case of YBCO and SRO/YBCO it is of 2D coupled type. Chapter: 9 summarizes the whole work presented in this thesis. It also discusses about few research problems which one need to look at in future.

In this thesis, the strength of the LBIC system is enhanced in different aspects that includes its feasibility as a non-destructive characterization tool, the signal analysis and development of analytical solution to have better understanding on the defects and inhomogeneities in the quantum dot based hetero-structures for device applications, finally understanding its limits due to the size of the laser beam and interpretation of artefacts in the signal appearance due to the presence of co-devices. Chapter#1 provides the introduction and literature survey of the LBIC system. It covers the importance and area of application of the LBIC. Chapter#2 various tools and instrumentation are discussed briefly for the systems that are developed in the lab as well as standard tools utilised for the material characterization. A LBIC instrumentation a novel colloidal quantum dots (CQD) thin film deposition system is discussed. In the last part along with the standard characterization systems a software tool (semiconductor device simulator) is discussed, which is used to visualize and understand the LBIC profile that is obtained experimentally. Chapter#3 provides the information of colloidal synthesis of PbS and HgxCd1-xTe quantum dots. Device fabrication process is explained step by step for the following devices. p-n junction silicon diodes, PbS-CQD/Si hetero-structures, ITO/PbS-CQD/Al crossbar structures and HgCdTe-CQD/Si hetero-structures. Chapter#4 deals with the major constraints imposed on the LBIC due to the need of Ohmic contacts. To overcome this major limitation, in this work, the origin of the signal is studied with the remote contact geometry for silicon p-n junction devices. It was observed that the signals can be collected with the capacitively coupled remote contacts, where LBIC was ultimately demonstrated as contactless measurement tool without any compromise on the measurements and thus obtained physical parameters. The effect of finite laser beam size is also described, which was found to have effect on the actual dimensions measured with the LBIC images. LBIC utility is further enhanced with the Si/CQD based hetero-structure devices, which are the potential candidates in the evolving device technology to be utilized in various modular systems such as PDs and LED applications. Chapter#5 discusses the origin and possible mechanisms for lateral photo-voltage which is closely monitored in the PbS-CQD/Si hetero-junction device systems. Interestingly, it is observed that there are two different line profiles for n and p type Si substrates. Different mechanisms that give rise to this kind of profiles were found to be distinct and are related to the band alignment of the CQD/Si hetero-structure. It lead to the revelation of an interesting phenomenon and believed to be universally observed irrespective of the materials involved in the formation of hetero-junction. Simulations and experimental results are quite consistent and in agreement with each other, which confirm the underlying physical mechanism that connects the LBIC anomalies with the band alignment. Chapter#6 deals with the spatial variations in the transverse photocurrent in the PbS-CQD film which is studied as a function of applied bias. Analytical equation is setup for the photocurrent in the CQD film under applied bias with the help of available transport mechanism and equations from the literature. The spatial non-uniformity that exists in the photocurrent proved to be the result of spatial inhomoginities in the physical parameters. By correlating the spatial data to the analytical equation, it is shown that the inhomoginities can be predicted. This approach is important for the devices, where monolithic detectors are fabricated by depositing CQD film on Read-Out-Integrated-Circuit (ROIC), where the manifestation of non-uniformity can be understood and probably fixed. Chapter#7 HgCdTe CQD based devices are studied for the purpose of photo-detector applications in MWIR (3  5 μm) region. HgxCd1-xTe Colloidal quantum dots are technologically important due to their wide absorption range that covers different regions of the atmospheric window. HgxCd1-xTe are successfully synthesised, which covers the absorption edge up to ~6.25 m in the IR region. Absorption and photo-response studies are carried out on HgxCd1-xTe/Si hetero-junctions under incident IR radiation. It is observed that the band gap of the quantum dots can be tuned easily by controlling the growth time as a parameter, thus moulded HgxCd1-xTe CQD/Si hetero-structures were found to have good photo-response. Chapter#8 the summary and the future direction and scope of the work is discussed. This includes the interesting observations during this thesis work which are not reported here in details.

This thesis reports the successful rational design of three highly active photocatalytic semiconductor nanocrystal (SNC) systems by exploiting morphology effects and the electronic properties of type II semiconductor heterojunctions. Novel architectures of colloidal SNCs are produced with the aim of suppressing exciton recombination and improving charge extraction for the successful initiation of desirable redox chemistry. Rod-shaped niobium pentoxide Nb₂O₅ nanocrystals (NCs) are shown to exhibit significantly enhanced activity (10-fold increase in rate constant) relative to spherical-shaped NCs of the same material. The increase is attributed to Nb5⁺ Lewis acid site rich (001) surfaces, present in higher proportions in the rod morphology, which bind organic substrates from solution resulting in direct interaction with photogenerated charges on the surface of the NC. Building on the insights into morphology-activity dependence, type II semiconductor heterojunctions are exploited for their ability to increase exciton lifetimes and spatially separate charges. Two novel II-VI heterostructured semiconductor nanocrystals (HSNCs) systems are investigated: a series of CdX/ZnO (X = S, Se, Te) HSNCs and ZnS/ZnO HSNCs capped with two different surface ligands. In the first case, substantial photocatalytic activity improvement is observed for HSNCs (relative to pure ZnO analogues) according to the following trend: CdTe/ZnO > CdS/ZnO > CdSe/ZnO. The observed trend is explained in terms of heterojunction structure and fundamental chalcogenide chemistry. In the second case, both ZnS/ZnO HSNCs exhibit activity enhancement over analogous pure ZnO, but the degree of enhancement is found to be a function of surface ligand chemistry. Photocatalytic activity testing of all the materials investigated in this work is performed via the photodecomposition of methylene blue dye in aerated aqueous conditions under UVA (350 nm) irradiation. The synthetic techniques employed for the synthesis of colloidal SNCs investigated in this thesis range from chemical precipitation and solvothermal techniques to several different organometallic approaches. A wide variety of analytical techniques are employed for the chemical, structural and optical characterisation of SNC photocatalysts including: XRD, XPS, TEM, UV-vis absorption, PL spectroscopy and FTIR. Atom Probe Tomography (APT) is employed for the first time in the structural characterisation of II-VI heterojunctions in colloidal HSNCs. Overall, this thesis provides a useful contribution to the growing body of knowledge pertaining to the enhancement of photocatalytic SNCs for useful applications including: solar energy conversion to chemical fuels, the photodecomposition of pollutants and light-driven synthetic chemistry.

Les hétérostructures de taille nanométrique comprenant des matériaux supraconducteurs (S) en contact avec des matériaux métalliques (N) ou ferromagnétiques (F) présentent des propriétés surprenantes. L'effet de proximité dans les structures F/S/F se manifeste par l'effet vanne de spin. Dans les jonctions S/F/S, on voit l'apparition d'une transition de phase 0-p. Ces propriétés dépendent des paramètres internes du ferromagnétiques. Dans la première partie, nous étudions l'influence d'un décalage énergétique et d'une différence de masse effective pour expliquer l'effet de vanne de spin inverse qui se manifeste dans certaines hétérostructures F/S/F. On étudie la transition de phase 0-p dans le cas de décalage énergétique et d'anisotropie des surfaces de Fermi.La seconde partie est consacré à l'étude de l'effet paramagnétique sur le diagramme (H,T) des bicouches S/N et S/S. On demontre qu'il se forme une phase de supraconductivité induite par champ magnétique à fort champ magnétique et faibles températures. Calculée en présence d'un phase supraconductrice inhomogène de type Fulde-Ferrell-Larkin-Ovchinikov (FFLO), on s'interesse également à l'influence des impuretés sur cette nouvelle phase supraconductrice à fort champ magnétique.La troisième partie est dévolue à l'étude des multicouches supraconducteur/métal normal(N). Le but de cette partie est d'étudier l'influence du nombre de couche et de décalage d'énergie sur la température critique, la densité d'état des multicouches S/N/.../N épaisses et de l'effet Josephson dans les multicouches S/N/.../N/S. / The atomic-scaled heterostructures with superconducting and ferromagnetic materials exhibit astonishing properties. For example, the proximity effect in the F/S/F sandwiches leads to the spin-valve effect. In the S/F/S junctions, one can observe 0-p phase transiton. These effects depend on the ferromagnetic properties.In the first part, we study the influence of energy shift and effective mass difference to explain the inverse spin valve effect. We also study the 0-p phase diagram and its dependence on the energy shifts and anisotropic sprectra in S/F/S junctions.The second part is devoted to the study of paramagnetic effet on the (H,T) phase diagram of the S/N and S/S bilayers. We demonstrate the formation of a superconducting field induced phase for high magnetic fields and low temperature. Calculated in presence of the superconducting inhomogeneous Fulde-Ferrell-Larkin-Ovchinikov (FFLO) state, we study the influence of the impurities on this new superconducting phase.The last part deals with the study of superconducting-normal metal(N) multilayers. We calculate the influence of the number of layers and energy shift on the density of state, the thermodynamical properties of the S/N/.../N thick multilayer and the Josephson current in the S/N/.../N/S thick junctions.