Graphene is a candidate for next generation spintronics devices exploiting its long spin transport length and high carrier mobility. Besides, when put in interaction with a ferromagnet, it may become an active building block, as suggested by recent surface science studies revealing few tenth of a Bohr magneton magnetic moments held by carbon atoms in graphene on iron, and a Rashba spin-orbit splitting reaching about 10 meV in graphene on a high atomic number element such as gold. The extent to which graphene may influence the properties, e.g. magnetic ones, of the materials contacted to it was barely addressed thus far. High quality hybrid systems composed of graphene in contact with magnetic thin layers or nanoclusters are playgrounds for exploring both aspects, the manipulation of the properties of graphene by interaction with other species, and vice versa. In graphene contacted to ultra-thin ferromagnetic layers for instance, strong graphene/ferromagnet interface effects could be employed in the view of manipulating the magnetization in the ferromagnet. The recently discovered close-to-perfect self-organization of nanoclusters on graphene, provides a way to probe magnetic interaction between clusters, possibly mediated by graphene. Three high quality hybrid systems relying on graphene prepared by chemical vapor deposition on the (111) surface of iridium have been developed under ultra-high vacuum (UHV): cobalt ultra-thin and flat films deposited on top of graphene, and intercalated at moderate temperature between graphene and its substrate, and self-organized cobalt- and iron-rich nanoclusters on the 2.5 nm-periodicity moiré between graphene and Ir(111). Prior to these systems, 10 nm-thick Ir(111) single-crystal thin films on sapphire were developed: they were latter employed as a substrate replacing bulk Ir(111) single-crystals usually employed. This new substrate opens the route to multi-technique characterizations, especially ex situ ones which were little employed thus far for studying graphene/metal systems prepared under UHV. Using a combination of in situ surface science techniques (scanning tunneling microscopy, x-ray magnetic circular dichroism, spin-polarized low-energy electron microscopy, auger electron spectroscopy, reflection high-energy electron diffraction) and ex situ probes (x-ray diffraction, transmission electron microscopy, Raman spectroscopy, MOKE magnetometry) the structural, vibrational, electronic, and magnetic properties of the three new graphene hybrid systems were characterized and confronted to first-principle calculations. Several striking features were unveiled. The interface between graphene and cobalt involves strong C-Co interactions which are responsible for a large interface magnetic anisotropy, capable of driving the magnetization out-of-the plane of the surface of an ultra-thin film in spite of the strong shape anisotropy in such films. The effect is maximized in the system obtained by intercalation between graphene and iridium, which comes naturally air-protected. Nanoclusters, on the contrary, seem to weakly interact with graphene. Small ones, comprising ca. 30 atoms each, remain super paramagnetic at 10 K, have no magnetic anisotropy, and it turns out difficult, even with 5 T fields to saturate their magnetization. Besides, the magnetic domains size seem to exceed the size of a single cluster, possibly pointing to magnetic interactions between clusters.

Graphène est un candidat pour la préparation de dispositifs spintroniques de nouvelle génération tirant partie de sa grande longueur de diffusion de spin et de la grande mobilité de ses porteurs de charge. En interagissant avec matériau ferromagnétique, il pourrait en outre devenir un élément actif, comme le suggèrent des études récentes par physique des surfaces, qui mettent en évidence un moment magnétique de quelques fractions de magnéton de Bohr dans le graphène en contact avec du fer, et une séparation en spin des bandes électroniques du graphène, d'environ 10 meV, par un effet Rashba au contact d'un élément de grand numéro atomique (l'or). La façon dont le graphène peut influencer les propriétés, par exemple magnétiques, des matériaux qui y sont contactés, reste peu étudiée. Les systèmes hybrides de haute qualité, constitués de graphène en contact avec des couches minces magnétiques ou des plots de taille nanométrique, sont des terrains de jeu pour explorer les deux aspects, la manipulation des propriétés du graphène par son interaction avec d'autres espèces, et vice versa. Dans le graphène contacté à des couches magnétiques ultra-minces par exemple, de forts effets d'interface pourraient être exploités pour contrôler l'aimantation du matériau magnétique. L'auto-organisation quasi-parfaite récemment découverte pour des plots nanométriques sur graphène, pourrait permettre d'explorer les interactions magnétiques, potentiellement transmises par le graphène, entre plots. Trois systèmes hybrides de haute qualité, intégrant du graphène préparé par dépôt chimique en phase vapeur sur le surface (111) de l'iridium, ont été développés sous ultra-haut vide (UHV) : des films ultra-minces de cobalt déposés sur graphène, et intercalés à température modérée entre graphène et son substrat, ainsi que des plots nanométriques riches-Co et -Fe, organisés avec une période de 2.5 nm sur le moiré entre graphène et Ir(111). Auparavant, des films de 10 nm d'Ir(111), monocristallins, déposés sur saphir, ont été développés. Ces films ont été par la suite utilisés comme substrats en remplacement de monocristaux massifs d'Ir(111). Ces nouveaux substrats ont ouvert la voie à des caractérisations multi-techniques ex situ, peu utilisées jusqu'alors pour étudier les systèmes graphène/métaux préparés sous UHV. Au moyen d'une combinaison de techniques de surface in situ et de sondes ex situ, les propriétés structurales, vibrationnelles, électroniques et magnétiques des trois nouveaux systèmes hybrides ont été caractérisées et confrontées à des calculs ab initio. Un certain nombre de propriétés remarquables ont été mises en évidence. L'interface entre graphene et cobalt implique de fortes interactions C-Co qui conduisent à une forte anisotropie magnétique d'interface, capable de pousser l'aimantation hors de la surface d'un film ultra-mince en dépit de la forte anisotropie de forme dans ces films. Cet effet est optimum dans les systèmes obtenus par intercalation entre graphène et iridium, qui sont par ailleurs naturellement protégés des pollutions de l'air. Les plots nanométriques, au contraire, semblent peu interagit avec le graphène. Des plots comprenant environ 30 atomes restent superparamagnétiques à 10 K, n'ont pas d'anisotropie magnétique, et leur aimantation est difficile à saturer, même sous 5 T. D'autre part, la taille des domaines magnétiques semble dépasser celle d'un plot unique, ce qui pourrait être le signe d'interactions magnétiques entre plots. / Graphene is a candidate for next generation spintronics devices exploiting its long spin transport length and high carrier mobility. Besides, when put in interaction with a ferromagnet, it may become an active building block, as suggested by recent surface science studies revealing few tenth of a Bohr magneton magnetic moments held by carbon atoms in graphene on iron, and a Rashba spin-orbit splitting reaching about 10 meV in graphene on a high atomic number element such as gold. The extent to which graphene may influence the properties, e.g. magnetic ones, of the materials contacted to it was barely addressed thus far. High quality hybrid systems composed of graphene in contact with magnetic thin layers or nanoclusters are playgrounds for exploring both aspects, the manipulation of the properties of graphene by interaction with other species, and vice versa. In graphene contacted to ultra-thin ferromagnetic layers for instance, strong graphene/ferromagnet interface effects could be employed in the view of manipulating the magnetization in the ferromagnet. The recently discovered close-to-perfect self-organization of nanoclusters on graphene, provides a way to probe magnetic interaction between clusters, possibly mediated by graphene. Three high quality hybrid systems relying on graphene prepared by chemical vapor deposition on the (111) surface of iridium have been developed under ultra-high vacuum (UHV): cobalt ultra-thin and flat films deposited on top of graphene, and intercalated at moderate temperature between graphene and its substrate, and self-organized cobalt- and iron-rich nanoclusters on the 2.5 nm-periodicity moiré between graphene and Ir(111). Prior to these systems, 10 nm-thick Ir(111) single-crystal thin films on sapphire were developed: they were latter employed as a substrate replacing bulk Ir(111) single-crystals usually employed. This new substrate opens the route to multi-technique characterizations, especially ex situ ones which were little employed thus far for studying graphene/metal systems prepared under UHV. Using a combination of in situ surface science techniques (scanning tunneling microscopy, x-ray magnetic circular dichroism, spin-polarized low-energy electron microscopy, auger electron spectroscopy, reflection high-energy electron diffraction) and ex situ probes (x-ray diffraction, transmission electron microscopy, Raman spectroscopy, MOKE magnetometry) the structural, vibrational, electronic, and magnetic properties of the three new graphene hybrid systems were characterized and confronted to first-principle calculations. Several striking features were unveiled. The interface between graphene and cobalt involves strong C-Co interactions which are responsible for a large interface magnetic anisotropy, capable of driving the magnetization out-of-the plane of the surface of an ultra-thin film in spite of the strong shape anisotropy in such films. The effect is maximized in the system obtained by intercalation between graphene and iridium, which comes naturally air-protected. Nanoclusters, on the contrary, seem to weakly interact with graphene. Small ones, comprising ca. 30 atoms each, remain super paramagnetic at 10 K, have no magnetic anisotropy, and it turns out difficult, even with 5 T fields to saturate their magnetization. Besides, the magnetic domains size seem to exceed the size of a single cluster, possibly pointing to magnetic interactions between clusters.

Yttria stabilized Zirconia (8YSZ) is an extensively used solid electrolyte, which finds applications in electrochemical sensors, solid oxide fuel cells and gate oxide in MOSFETs. Recent studies report that YSZ thin films are better performers than their bulk counterparts, in terms of ionic conductivity even at moderate temperatures. YSZ thin films also attract attention with the scope of device miniaturization. However, most of the studies available in the literature on YSZ thin films focus mainly on their electrical characterization. In this work, YSZ thin films were deposited, characterized and possible use of sensors were evaluated. In the present work, 8 mol% yttria stabilized zirconia thin films were deposited using RF magnetron reactive sputtering under different deposition conditions. Films with thicknesses ranging from few tens to few hundreds of nanometres were deposited. The deposited films were subjected to morphological, structural, compositional and electrical characterizations. Deposition and annealing conditions were optimized to obtain dense, stoichiometric and crystalline YSZ thin films. The ionic conductivity of 200 nm nanocrystal line thin film was found to be two orders of magnitude higher than the bulk. The ionic conductivity increased with the decrease in film thickness. Compositional analyses of a set of YSZ thin films revealed free surface yttrium segregation. The free surface segregation of dopants can locally alter the surface chemistry and influence the oxygen transfer kinetics across the electrode-electrolyte interface. Although number of reports are available on the segregation characteristics in YSZ bulk, no reports are available on yttria segregation in YSZ thin film. Hence, this work reports detailed investigations on the free surface yttria segregation in YSZ thin films using angle resolved X-ray photoelectron spectroscopy (XPS). Influence of annealing temperature, film thickness, annealing time, and purity on the segregation concentration was determined. It was found that the most important factor that determines the segregation was found to be the target purity. The segregation depth profile analysis showed that the segregation layer depth was proportional to segregation concentration. Free surface segregation reduced the ionic conductivity of the YSZ thin films roughly about a factor. However, segregation did not affect the film’s morphology, grain size, crystallinity and activation energy. The difference in ionic conductivity observed in the segregated and clean YSZ films suggests that dopant free surface segregation could also be one of the reasons for the variable ionic conductivity reported in the literature. For using YSZ in miniaturized devices, micro-structuring of YSZ is important. It has been reported that the wet etching techniques available for YSZ were not repeatable and do not etch annealed YSZ samples. Reactive ion etching (RIE) is better suited for YSZ patterning due to its capability to offer high resolution, easy control and tenable anisotropic/isotropic pattern transfer for batch processing. Although reports are available on the dry etching of zirconia and yttria thin films, no studies were reported on the dry etching of YSZ thin films. In this work, inductively coupled reactive ion etching (ICP-RIE) using fluorine and chlorine chemistries were employed to etch YSZ thin films. Optimized etching conditions were identified by varying different process parameters like, type of gas, gas flow rate, RF power, ICP power, chamber pressure and carrier wafer in the ICP-RIE process. Optimized conditions were chosen by examining the etch depth, composition analyses before and after etch using XPS, selectivity towards SiO2 (which is the most common buffer layer) and surface roughness. Etch chemistries involved in a particular plasma (SF6, Cl2 and BCl3) were discussed with the help of surface composition and etch thicknesses. The results showed that etching YSZ with BCl3 plasma at optimized conditions yielded best results through oxygen-scavenging mechanism. A maximum etch rate of 53 nm/min was obtained in BCl3 plasma using PECVD Si3N4 carrier wafer at an ICP power of 1500 W, RF power of 100 W, chamber pressure of 5 mTorr with 30 sccm BCl3 flow. Sensing devices were designed by employing YSZ thin film as solid electrolyte and nickel oxide and gold thin film as sensing and reference electrodes, respectively to evaluate the possible use of YSZ thin film in miniaturized NO2 sensor. The electrodes were deposited in inter-digitated pattern. Two types of electrodes were designed with different number of fingers in symmetric and asymmetric configurations. The NO2 sensing was performed in the concentration range of 25 to 2000 ppm at three different temperatures, 673, 773 and 873 K in mixed potential and impedance metric modes. The mixed potential type measurements were carried out only for asymmetric cell in two different electrode configurations. The impedance metric type measurements were carried out for both symmetric and asymmetric cells in two different electrode configurations. Preliminary NO2 sensing experiments in both the types of measurements revealed that in devices with electrodes having more fingers were better in performance. In mixed potential type sensors, sensitivity was measured as the amount of voltage generated when the sensor was exposed to a test gas. The generated voltage was found to be proportional to the logarithm of NO2 concentration in the entire measurement range (50 to 2000 ppm) with the regression fitting parameter, adj.R2 around 0.97 to 0.99 in all the cases. A maximum potential of 271 mV was measured with 2000 ppm NO2 at 873 K. The response and recovery times of the sensors were sensitive to the operating temperature. In impedance metric mode, the sensitivities were measured as the variation in the low frequency phase angle (∆ φ) when the gas concentration is changed. The frequency range of the measurement was from 0.01 Hz to100 kHz. The response time in the impedance metric sensors was comparable to that of mixed potential sensors. But the recovery time in impedance metric sensors was much was slower than the mixed potential type for all the concentrations. The sensors showed linear response only in a narrow range of 50 to 500 ppm with regression fitting value, R2 around 0.98 in all the cases. Above 500 ppm, the sensitivity value was observed to be saturated. From the gas sensing studies performed on the miniaturized sensors, it was found that the mixed potential type sensing mode is better than the impedance metric type in YSZ thin film based devices. However detailed interference gas studies were needed before drawing any conclusion. In summary, the studies presented in the work have contributed to the understanding of free surface yttria segregation behaviour in YSZ thin films. Micromachining conditions were optimized for both pristine and annealed YSZ thin films. Suitability of YSZ thin film based miniaturized NO2 gas sensor was evaluated.