Solution-Processable Graphene Oxide as an Efficient Hole Injection Layer for High Luminance Organic Light-Emitting Diodes

The application of solution-processable graphene oxide (GO) as hole injection layer in organic light-emitting diodes (OLEDs) is demonstrated. High luminance of over 53,000 cd m-2 is obtained at only 10 V. The results will unlock a route of applying GO in flexible OLEDs and other electrode applications.Comment: 14 pages, 3 figures, 1 tabl

Hard X-ray standing-wave photoemission insights into the structure of an epitaxial Fe/MgO multilayer magnetic tunnel junction

The Fe/MgO magnetic tunnel junction is a classic spintronic system, with current importance technologically and interest for future innovation. The key magnetic properties are linked directly to the structure of hard-to-access buried interfaces, and the Fe and MgO components near the surface are unstable when exposed to air, making a deeper probing, nondestructive, in-situ measurement ideal for this system. We have thus applied hard X-ray photoemission spectroscopy (HXPS) and standing-wave (SW) HXPS in the few kilo-electron-volt energy range to probe the structure of an epitaxially grown MgO/Fe superlattice. The superlattice consists of 9 repeats of MgO grown on Fe by magnetron sputtering on an MgO(001) substrate, with a protective Al2O3 capping layer. We determine through SW-HXPS that 8 of the 9 repeats are similar and ordered, with a period of 33 ± 4 Å, with the minor presence of FeO at the interfaces and a significantly distorted top bilayer with ca. 3 times the oxidation of the lower layers at the top MgO/Fe interface. There is evidence of asymmetrical oxidation on the top and bottom of the Fe layers. We find agreement with dark-field scanning transmission electron microscope (STEM) and X-ray reflectivity measurements. Through the STEM measurements, we confirm an overall epitaxial stack with dislocations and warping at the interfaces of ca. 5 Å. We also note a distinct difference in the top bilayer, especially MgO, with possible Fe inclusions. We thus demonstrate that SW-HXPS can be used to probe deep buried interfaces of novel magnetic devices with few-angstrom precision

Morphological and micro-structural interface characterization in multilayer inverted polymer-fullerene bulk heterojunction solar cells

Inverted polymer solar cells based on P3HT/PCBM bulk heterojunction were prepared on flexible polyethylene naphthalate (PEN) substrate. The effect of annealing of the PEN/ITO/ZnO multilayer and ZnO/P3HT:PCBM on the structural, morphological, photophysical and photovoltaic properties was investigated and scrutinized directly on the OPV devices using atom probe tomography (APT), scanning electron microscopy (SEM) and microfocus X-ray techniques. We carried out a 3D reconstruction of the interfaces of the multilayer containing PEN/ITO, ZnO/ITO and P3HT:PCBM/ZnO to address the interface micro-structure and its influence on the morphology of the photoactive film. The analyses show that the morphology of the interfaces is affected by the structure of each layer of the BHJ devices causing orientation of P3HT crystals with PCBM aggregates and ZnO, which in turn leads to a significant change of the charge transport across each layer and therefore photovoltaic performances

Role of domain walls in the abnormal photovoltaic effect in BiFeO3

Recently, the anomalous photovoltaic (PV) effect in BiFeO3 (BFO) thin films, which resulted in open circuit voltages (V-oc) considerably larger than the band gap of the material, has generated a revival of the entire field of photoferroelectrics. Here, via temperature-dependent PV studies, we prove that the bulk photovoltaic (BPV) effect, which has been studied in the past for many non-centrosymmetric materials, is at the origin of the anomalous PV effect in BFO films. Moreover, we show that irrespective of the measurement geometry, V-oc as high as 50V can be achieved by controlling the conductivity of domain walls (DW). We also show that photoconductivity of the DW is markedly higher than in the bulk of BFO

All-in-one synthesis of mesoporous silicon nanosheets from natural clay and their applicability to hydrogen evolution

Silicon nanosheets have attracted much attention owing to their novel electronic and optical properties and compatibility with existing silicon technology. However, a cost-effective and scalable technique for synthesizing these nanosheets remains elusive. Here, we report a novel strategy for producing silicon nanosheets on a large scale through the simultaneous molten-salt-induced exfoliation and chemical reduction of natural clay. The silicon nanosheets thus synthesized have a high surface area, are ultrathin (similar to 5 nm) and contain mesoporous structures derived from the oxygen vacancies in the clay. These advantages make the nanosheets a highly suitable photocatalyst with an exceptionally high activity for the generation of hydrogen from a water-methanol mixture. Further, when the silicon nanosheets are combined with platinum as a cocatalyst, they exhibit high activity in KOH (15.83 mmol H-2 per s per mol Si) and excellent photocatalytic activity with respect to the evolution of hydrogen from a water-methanol mixture (723 mu mol H-2 per h per g Si).clos

Magnetism and structure of Ca3Co206 and Ca3Co409 thin films prepared by pulsed laser deposition

Cette thèse porte sur l'étude des propriétés structurales et magnétiques des couches minces de cobaltite de types Ca3C0206 et Ca3C0409 déposées par ablation laser pulsée sur des substrats d'AI20 3(OOl). Ce travail a d'abord consisté en l'élaboration de couches minces polycristallines de Ca3C0206 et épitaxiées de Ca3C0409. En ce qui concerne la cobaltite Ca3C0206, on a pu déterminer la valeur du gap optique et qui est en accord avec les valeurs obtenues par des études théoriques. Le résultat le plus original est qu'on a démontré que les propriétés magnétiques sont très sensibles à la taille des échantillons. On note que les plateaux d'aimantation observés à 2 K dans les couches minces de Ca3C0206 disparaissent pour les couches fines ayant une épaisseur inférieure à 60 nm. Cette disparition des plateaux d'aimantation est accompagnée de l'apparition d'une transition magnétique additionnelle vers 45 K. Nos résultats sont incompatibles avec l'effet tunnel quantique de l'aimantation comme mécanisme à l'origine des plateaux d'aimantation, et soutiennent les prédictions des modèles Monte-Carlo. Quant aux couches minces de Ca3C0409, on a déterminé la nature et l'origine des défauts structuraux qui sont souvent observés par microscopie électronique en transmission. Ces défauts correspondent à la formation de phase parasite de CaC020 4. La formation de cette phase parasite est due à la haute température de dépôt (750°C), à la grande pression appliquée par le substrat d'AI20 3(OOl) (5.5 GPa) à travers les contraintes et à une possible non stoechiométrie locale de la cible. Ces défauts structuraux influencent les propriétés magnétiques, en réduisant le champ coercitif et l'aimantation rémanente par rapport à ceux du Ca3C0409 massif.This thesis focuses on the study of structural and magne tic properties of Ca3C0206 and Ca3C0409 thin films deposited by pulsed laser deposition on Alz03(001) substrates. This work initially involved the elaboration of polycrystalline Ca3C0206 and epitaxial Ca3C0409 thin films. Regarding the Ca3C0206 cobaltite, we were able to determine the value of the optical band gap, which is in agreement with the values obtained by theoretical studies. More interestingly, we have demonstrated that the magnetic properties are very sensitive to sample size. The magnetization plateaus observed at 2 K in Ca3C0206 thin films disappear for thin films with a thickness below 60 nm. This disappearance of the magnetization steps is accompanied by the appearance of an additional magnetic transition around 45 K. Our results are inconsistent with the quantum tunneling of magnetization as a mechanism for the origin of magnetization plateaus, and support more the predictions of Monte Carlo based models. For the Ca3C0409 films, we have determined the nature and the origin of structural defects that are often observed by transmission electron microscopy. These defects are constituted of the CaC020 4 parasitic phase. The formation of this spurious phase is due to the high deposition temperature (750 oC), the high pressure applied by the AI20 3(OOl) substrate (5.5 GPa) and the possibility of a local non-stoichiometry of the target. These structural defects affect the magne tic properties, by reducing the coercivity and remanent magnetization with respect to those of the Ca3C0409 bulk samples

Magnetism and structure of Ca3Co206 and Ca3Co409 thin films prepared by pulsed laser deposition

Cette thèse porte sur l'étude des propriétés structurales et magnétiques des couches minces de cobaltite de types Ca3C0206 et Ca3C0409 déposées par ablation laser pulsée sur des substrats d'AI20 3(OOl). Ce travail a d'abord consisté en l'élaboration de couches minces polycristallines de Ca3C0206 et épitaxiées de Ca3C0409. En ce qui concerne la cobaltite Ca3C0206, on a pu déterminer la valeur du gap optique et qui est en accord avec les valeurs obtenues par des études théoriques. Le résultat le plus original est qu'on a démontré que les propriétés magnétiques sont très sensibles à la taille des échantillons. On note que les plateaux d'aimantation observés à 2 K dans les couches minces de Ca3C0206 disparaissent pour les couches fines ayant une épaisseur inférieure à 60 nm. Cette disparition des plateaux d'aimantation est accompagnée de l'apparition d'une transition magnétique additionnelle vers 45 K. Nos résultats sont incompatibles avec l'effet tunnel quantique de l'aimantation comme mécanisme à l'origine des plateaux d'aimantation, et soutiennent les prédictions des modèles Monte-Carlo. Quant aux couches minces de Ca3C0409, on a déterminé la nature et l'origine des défauts structuraux qui sont souvent observés par microscopie électronique en transmission. Ces défauts correspondent à la formation de phase parasite de CaC020 4. La formation de cette phase parasite est due à la haute température de dépôt (750°C), à la grande pression appliquée par le substrat d'AI20 3(OOl) (5.5 GPa) à travers les contraintes et à une possible non stoechiométrie locale de la cible. Ces défauts structuraux influencent les propriétés magnétiques, en réduisant le champ coercitif et l'aimantation rémanente par rapport à ceux du Ca3C0409 massif.This thesis focuses on the study of structural and magne tic properties of Ca3C0206 and Ca3C0409 thin films deposited by pulsed laser deposition on Alz03(001) substrates. This work initially involved the elaboration of polycrystalline Ca3C0206 and epitaxial Ca3C0409 thin films. Regarding the Ca3C0206 cobaltite, we were able to determine the value of the optical band gap, which is in agreement with the values obtained by theoretical studies. More interestingly, we have demonstrated that the magnetic properties are very sensitive to sample size. The magnetization plateaus observed at 2 K in Ca3C0206 thin films disappear for thin films with a thickness below 60 nm. This disappearance of the magnetization steps is accompanied by the appearance of an additional magnetic transition around 45 K. Our results are inconsistent with the quantum tunneling of magnetization as a mechanism for the origin of magnetization plateaus, and support more the predictions of Monte Carlo based models. For the Ca3C0409 films, we have determined the nature and the origin of structural defects that are often observed by transmission electron microscopy. These defects are constituted of the CaC020 4 parasitic phase. The formation of this spurious phase is due to the high deposition temperature (750 oC), the high pressure applied by the AI20 3(OOl) substrate (5.5 GPa) and the possibility of a local non-stoichiometry of the target. These structural defects affect the magne tic properties, by reducing the coercivity and remanent magnetization with respect to those of the Ca3C0409 bulk samples