L’objectif de ce travail de thèse est d’étudier certaines propriétés locales de structures contenant des nanocristaux de Ge sur leur surface par utilisation de la technique nano-EBIC(courant induit par bombardement électronique et collecté par un nano-contact). La particularité de cette technique qui utilise le même principe que la technique EBIC classique est l’utilisation d’une pointe conductrice d’un AFM (microscope à force atomique) à la place d’une électrode standard. Nous nous sommes intéressés à la détermination de la longueur de diffusion effective (Leff) et l’étude de sa variation en fonction de paramètres tels que l’énergie primaire et la taille des nanocristaux. Leff augmente pour les faibles énergies primaires, passe par un maximum qui dépend de la taille des nanocristaux, puis diminue pour les énergies élevées. Ce comportement de l’évolution de Leff a été expliqué en chapitre 2. Cependant, ce résultat n’a jamais été observé auparavant. C’est pourquoi, nous avons complété ce travail par une étude basée sur la simulation Monte-Carlo, où l’effet de plusieurs paramètres a été analysé. Parmi les paramètres étudiés, on cite la taille et la forme du nano-contact (ou plus précisément la taille de la nano-zone de déplétion qui se forme sous le contact), la vitesse de recombinaison en surface et l’énergie primaire. La simulation donne le même comportement de variation de Leff que dans le cas expérimental. / The objective of this work is to study certain local properties of structures containing on their surface Ge nanocrystals by using the nano-EBIC (Electron beam induced current collected by a nano-contact). The peculiarity of this technique which uses the same principle as the classical EBIC technique is the use of a conductive AFM (atomic force microscope) tip instead of a standard electrode. We were interested in the determination of the effective diffusion length (Leff) and the study of its variation according to parameters such as the primary energy and the size of nanocrystals. Leff increases for weak energies, reaches a maximum which depends on the nanocrystal size, then decreases for high energies. This behavior of the evolution of Leff was explained in chapter 2. However, this result has never been reported previously. That is why we completed this work by a study based on the Monte-Carlo simulation, where the effect of several parameters was analyzed. Among the parameters studied, we quote the size and the shape of the nano-contact (or more exactly the size of the depletion nano-zone formed under the contact), the surface recombination velocity and the primary energy. The simulation gives the same behavior of Leff variation than the experimental case.

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In recent years, Spin Seebeck effect (SSE) emerges as one of the efficient and easiest ways to generate pure spin current for spintronics devices. In this dissertation, we have systematically studied the SSE and related phenomena like spin Hall magneto-resistance (SMR), anomalous Nernst effect (ANE) in functional magnetic oxides for both fundamental understanding of their origins and practical ways to apply into technological devices. The research has been performed on three different systems of topical interest: (i) Y3Fe5O12 (YIG)/Pt and YIG/C60/Pt, (ii) CoFe2O4 (CFO)/Pt and CFO/C60/Pt, and (iii) Nd0.6Sr0.4MnO3 (NSMO). In case of the YIG/Pt structure, we have achieved a new consensus regarding the temperature dependence of the longitudinal SSE (LSSE). For the first time, we have demonstrated the temperature dependence of LSSE in association with the magnetocrystalline anisotropy (HK) and surface perpendicular magnetic anisotropy field (HKS) of YIG in the same YIG/Pt system. We show that on lowering temperature, the sharp drop in LSSE signal (VLSSE) and the sudden increases in HK and HKS at ~175 K are associated with the spin reorientation due to single ion anisotropy of Fe2+ ions. The VLSSE peak at ~75 K is attributed to the HKS and MS (saturation magnetization) whose peaks also occur at the same temperature. The effects of surface and bulk magnetic anisotropies are corroborated with those of thermally excited magnon number and magnon propagation length to satisfactorily explain the temperature dependence of LSSE in the Pt/YIG system. As a new way to reduce conductivity mismatch, promote spin transport, and tune the spin mixing conductance (G) at the YIG/Pt interface, we have deposited an organic semiconductor (OSC), C60, between ferrimagnetic material (FM) and Pt. Transverse susceptibility study on YIG/C60/Pt has shown that the deposition of C60 has reduced HKS at the surface of YIG significantly, due to the hybridization between the dz2 orbital in Fe and C atoms, leading to the overall increase in spin moments and G and consequently the LSSE. Upon lowering temperature from 300 K, we have observed an exponential increase in LSSE at low temperature (a ~800% increment at 150 K) in this system, which is attributed to the exponential increase in the spin diffusion length of C60 at low temperature. On the other hand, similar experiments on CoFe2O4 (CFO)/C60/Pt show a reduction in the LSSE signal at room temperature, due to the hybridization between the dz2 orbital in Co and C atoms that results in the increased magnetic anisotropy. Upon decreasing the temperature below 150 K, we have interestingly observed that LSSE signal from CFO/C60/Pt exceeds that of CFO/Pt and increases remarkably with temperature. This finding confirms the important role played by the spin diffusion length of C60 in enhancing the LSSE. A systematic study of SMR, SSE, and HKS on the YIG/Pt system using the same YIG single crystal has revealed a low-temperature peak at the same temperature (~75 K) for all the phenomena. Given the distinct origins of the SSE and SMR, our observation points to the difference in spin states between the bulk and surface of YIG as the main reason for such a low-temperature peak, and suggests that the ‘magnon phonon drag’ theory developed to explain the temperature-dependent SSE behavior should be adjusted to include this important effect. SSE and ANE studies on NSMO films have revealed the dominance of ANE over SSE in this class of perovskite-structured materials. The substrate-dependent study of the films shows that compressive strain developed due to the large lattice mismatch from LAO gives rise to the enhanced ANE signal. On the same substrate, ANE signal strength increases as the thickness increases. A sign change in ANE has been observed at a particular temperature, which explains that the Anomalous Hall effect (AHE) and ANE in these systems arise due to intrinsic scattering mechanisms. Overall, we have performed the SSE and related studies on the three important classes of functional magnetic oxide materials. We demonstrate the important role of magnetic anisotropy in manipulating the SSE in these systems. With this knowledge, we have been able to design the novel YIG/C60/Pt and CFO/C60/Pt heterostructures that exhibit the giant SSEs. The organic semiconductor C60 has been explored for the first time as a means of controlling pure spin current in inorganic magnetic oxide/metal heterostructures, paying the way for future spintronic materials and devices.

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Catalytic conversion of methanol to hydrocarbons (MTH) provides an alternative route to the production of fuels and important industrial chemicals that are currently mainly produced from the refinery of petroleum. The ability to control the product distribution of MTH according to the demands of specific applications is of crucial importance, which relies on the thorough understanding of the reaction pathways and mechanisms. Despite the significant research efforts devoted to zeolite-catalyzed MTH, it remains a challenge to establish a firm correlation between the physicochemical properties of zeolites and their catalytic activity and selectivity. In this dissertation, we designed a series of experiments to gain fundamental understanding of how the structural and compositional parameters of zeolites influence their catalytic performances in MTH. We investigated different types of zeolites, covering large-pore Beta, medium-pore ZSM-5, and small-pore DDR zeolites, and tune their crystallite size/diffusion length, hierarchical (mesoporous) structure, and Si/Al ratio (density of acid sites) by controlled synthesis or post-synthesis treatments. The influence of mesoporosity of a zeolite catalyst on its catalytic performance for MTH, with zeolite Beta, was first investigated. The shorter diffusion length associated with the hierarchical structure results in a lower ethylene selectivity but higher selectivity towards C4-C7 aliphatics. Then we investigated the correlation between the Al content and the ethylene selectivity by ZSM-5 zeolites with similar crystal sizes but varied Si/Al ratios. We realized that ethylene selectivity is promoted with the increase of aluminum content in the framework. These two observations can be explained by the same mechanistic reason: the ethylene selectivity is associated with the propagation degree of the aromatics catalytic cycle and essentially determined by the number of the acid sites that methylbenzenes would encounter before they exit the zeolite crystallite. Last we explored how to maximize the propylene selectivity by tuning the physicochemical properties of DDR zeolites. Due to the confined pore space in DDR, the propagation of olefins-based catalytic cycle can be preferentially promoted in a tunable manner, which cannot be realized with zeolites having larger pores. Thus, the propylene selectivity increases with increasing the Si/Al ratio and decreasing the crystallite size.

This research investigated the effect of electron injection in III-Nitrides. The combination of electron beam induced current and cathodoluminescence measurements was used to understand the impact of electron injection on the minority carrier transport and optical properties. In addition, the application of the electron injection effect in optoelectronic devices was investigated. The impact of electron injection on the minority carrier diffusion length was studied at various temperatures in Mg-doped p-GaN, p-Al[subscript x]Ga[subscript 1-x]N, and p-Al[subscript x]Ga[subscript 1-x] N/GaN superlattices. It was found that Lsubscript n] experienced a multi-fold linear increase and that the rate of change of L[subscript n] decreased exponentially with increasing temperature. The effect was attributed to a temperature-activated release of the electrons, which were trapped by the Mg levels. The activation energies, E[subscript a], for the electron injection effect in the Mg-doped (Al)GaN samples were found to range from 178 to 267 meV, which is close to the thermal ionization energy of the Mg acceptor. The E[subscript a] observed for Al[subscript 0.15]Ga[subscript 0.85]N and Al[subscript 0.2]Ga[subscript 0.8]N was consistent with the deepening of the Mg acceptor level due to the incorporation of Al into the GaN lattice. The E[subscript a] in the homogeneously doped Al[subscript 0.2]Ga[subscript 0.8]N/GaN superlattice indicates that the main contribution to the electron injection effect comes from the capture of injected electrons by the wells (GaN). The electron injection effect was successfully applied to GaN doped with an impurity (Mn) other than Mg. Electron injection into Mn-doped GaN resulted in a multi-fold increase of the L[subscript n] and a pronounced decrease in the band-to-band cathodoluminescence intensity. The E[subscript a] due to the electron injection effect was estimated from temperature-dependent cathodoluminescence measurements to be 360 meV. The decrease in the band-to-band cathodoluminescence is consistent with an increase in L[subscript n] and these results are attributed to an increase in the minority carrier lifetime due to the trapping of injected electrons by the Mn levels. A forward bias was applied to inject electrons into commercially built p-i-n and Schottky barrier photodetectors. Up to an order of magnitude increase in the peak (360 nm) responsivity was observed. The enhanced photoresponse lasted for over four weeks and was attributed to an electron injection-induced increase of L[subscript n] and the lifetime.