Layer-by-Layer Materials for the Fabrication of Devices with Electrochemical Applications

The construction of nanostructured materials for their application in electrochemical processes, e.g., energy storage and conversion, or sensing, has undergone a spectacular development over the last decades as a consequence of their unique properties in comparison to those of their bulk counterparts, e.g., large surface area and facilitated charge/mass transport pathways. This has driven strong research on the optimization of nanostructured materials for the fabrication of electrochemical devices, which demands techniques allowing the assembly of hybrid materials with well-controlled structures and properties. The Layer-by-Layer (LbL) method is well suited for fulfilling the requirements associated with the fabrication of devices for electrochemical applications, enabling the fabrication of nanomaterials with tunable properties that can be exploited as candidates for their application in fuel cells, batteries, electrochromic devices, solar cells, and sensors. This review provides an updated discussion of some of the most recent advances on the application of the LbL method for the fabrication of nanomaterials that can be exploited in the design of novel electrochemical devices.Depto. de Química FísicaFac. de Ciencias QuímicasTRUEUnión Europea. Horizonte 2020Ministerio de Ciencia e Innovación (MICINN)pu

Nanostructured sonogels

Acoustic cavitation effects in sol-gel liquid processing permits to obtain nanostructured materials, with size-dependent properties. The so-called "hot spots" produce very high temperatures and pressures which act as nanoreactors. Ultrasounds force the dissolution and the reaction stars. The products (alcohol, water and silanol) help to continue the dissolution, being catalyst content, temperature bath and alkyl group length dependent. Popular choices used in the preparation of silica-based gels are tetramethoxysilane (TMOS), Si(OCH3)4 and tetraethoxysilane (TEOS), Si(OC 2H5)4. The resultant "sonogels" are denser gels with finer and homogeneous porosity than those of classic ones. They have a high surface/volume ratio and are built by small particles (1 nm radius) and a high cross-linked network with low -OH surface coverage radicals. In this way a cluster model is presented based on randomly-packed spheres in several hierarchical levels that represent the real sonoaerogel. Organic modified silicates (ORMOSIL) were obtained by supercritical drying in ethanol of the corresponding alcogel producing a hybrid organic/inorganic aerogel. The new material takes the advantages of the organic polymers as flexibility, low density, toughness and formability whereas the inorganic part contributes with surface hardness, modulus strength, transparency and high refractive index. The sonocatalytic method has proven to be adequate to prepare silica matrices for fine and uniform dispersion of CdS and PbS quantum dots (QDs), which show exciton quantum confinement. We present results of characterization of these materials, such as nitrogen physisorption, small angle X-ray/neutrons scattering, electron microscopy, uniaxial compression and nanoindentation. Finally these materials find application as biomaterials for tissue engineering and for CO2 sequestration by means the carbonation reaction.Ministerio de Ciencia y Tecnología MAT2005-158

New Polymer and Composite Structures for Photonic Applications

The focus of this thesis is the development of materials and architectures for all-polymer functional structures for photonic applications. The first part concerns the improvement and optimization of colorimetric and fluorescent sensing structures for the detection of various analytes in the vapor phase. Optical-readout sensors are portable and can provide an easy interpretation that needs no specialized training and can be visible to the naked eye. This makes them promising for applications in environmental control, health monitoring and food safety. The objective of the work was to investigate analyte diffusion processes into multilayered structures of polymer submicrometric films, and then optimizing the structure design and expanding the materials used in the field. First, sensors based on vapor diffusion in multilayered polymer dielectric mirrors with structural coloring were developed. Given their clear color change, this typology of sensors has been shown to be promising in the literature. However, as their response is limited by the diffusion speed of molecular species, they can suffer from slow detection of vapor-phase analytes. Next, I examine the use of fluorescent polymer films sensitive to microviscosity changes caused by exposure to volatile organic compounds and observing the changes in fluorescence during said exposure. The effect on the overall diffusion of capping layers deposited on top of the fluorescent polymer was investigated to quantify the effect of the barrier polymer on the selectivity of the sensor. Finally, I employed the solution processing protocols developed for novel low refractive index polymer suspensions that were initially utilized for the sensors to engineer structures for fluorescence control. When two highly reflecting structures encapsulate a luminescent material in a submicrometric space, this changes the photoluminescence properties in structures called optical microcavities. While the highly reflecting structures can be metallic mirrors, these have limited reflectance intensity, high absorbance losses, as well as a lack of tunability. Instead, the use of dielectric mirrors enables very high reflectance at desired wavelengths. In addition, the use of compliant polymer materials allows the future use of these structures to construct more efficient flexible devices. I was able to develop highly reflecting microcavities for emitters in the visible range as well as in the near infrared. Besides achieving high amplification of fluorescence intensity, I was also able to report for the first time a change in the radiative rate of the fluorescence for polymer structures. As these effects were so far only observed in planar structures of inorganic nature or more complex polymer three-dimensional systems, this presents a breakthrough in the field. In this introduction I will give a wide but deep overview of the optics of multilayered polymer films, their diffusion peculiarities, and use for sensing. Furthermore, I will address the topic of solid-state organic fluorophores and controlling their photoluminescence through engineering the dielectric environment. This will be followed by a chapter-by-chapter exploration of the results obtained during the doctoral training as adapted from already published or drafted work. Finally, the outlook and possible future implications and developments of this research will be examined

“Chimie douce”: A land of opportunities for the designed construction of functional inorganic and hybrid organic-inorganic nanomaterials

Abstract“Chimie douce” based strategies allow, through the deep knowledge of materials chemistry and processing, the birth of the molecular engineering of nanomaterials. This feature article will highlight some of the main research accomplishments we have performed during the last years. We describe successively the design and properties of: sol–gel derived hybrids, Nano Building Blocks (NBBs) based hybrid materials, nanostructured porous materials proceeds as thin films and ultra-thin films, aerosol processed mesoporous powders and finally hierarchically structured materials. The importance of the control of the hybrid interfaces via the use of modern tools as DOSY NMR, SAXS, WAXS, Ellipsometry that are very useful to evaluate in situ the hybrid interfaces and the self-assembly processes is emphasized. Some examples of the optical, photocatalytic, electrochemical and mechanical properties of the resulting inorganic or hybrid nanomaterials are also presented

Coupling nitrogen-vacancy centers in diamond to fiber-based Fabry-Pérot microcavities

This thesis investigates the coupling of the fluorescence of nitrogen-vacancy (NV) centers in diamond to tunable optical microresonators at ambient conditions, in particular in the regime of Purcell enhancement. We use fiber-based, open-access Fabry-Pérot cavities optimized for high finesse and ultra-small mode volume. Different regimes of cavity enhancement are studied that are complementary to each other: A first experiment relies on a high-finesse cavity with dielectric mirrors. The scaling laws of Purcell enhancement are explicitly demonstrated by a large-range variation of both the cavity mode volume (V = 16 − 600 µm^3 ) and the quality factor (Q = 6 · 10^3 − 2 · 10^6). We detect an enhancement of the emission spectral density by up to a factor of 300. The full potential of this resonator can be exploited with emitters having a linewidth which is narrower than the resonance linewidth of the cavity. This concept holds promise for the implementation of wavelength-tunable, narrow-band single-photon sources as well as the generation of indistinguishable single-photons at ambient conditions. However, for broad-band emitters like the NV center at room temperature, the emission lifetime is not affected noticeably in this configuration. In order to directly observe lifetime changes and Purcell-enhanced single-photon emission, we manufacture fiber-based cavities with silver-coated mirrors having ultra-small mode volumes, as small as V = 1.0 λ^3 = 0.34 µm^3. We demonstrate cavity-enhanced fluorescence imaging, which allows to locate and analyze several single NV centers with one cavity. The Purcell effect is evidenced by an enhanced fluorescence collection of up to 1.6 · 10^6 photons per second from single-NV centers and a tunable variation of the emission lifetime corresponding to an effective Purcell factor of up to 2. We furthermore investigate a benefcial regime of optical confinement where the Fabry-Pérot cavity mode is combined with additional mode confinement by the diamond nanocrystal itself, enabling sub-λ^3 mode volumes. We perform simulations that predict effective Purcell factors of up to 11 for NV centers and of up to 63 for silicon-vacancy centers, revealing a great potential for bright single-photon sources and effcient spin readout at ambient conditions.Diese Arbeit erforscht die Kopplung der Fluoreszenz von Stickstoff-Fehlstellen-Zentren (NV-Zentren) in Diamant mit durchstimmbaren optischen Mikroresonatoren bei Umgebungsbedingungen, insbesondere im Regime der Purcell Verstärkung. Hierzu benutzen wir faserbasierte, offen zugängliche Fabry-Pérot Resonatoren, die für hohe Finesse und ultrakleine Modenvolumen optimiert sind. Verschiedene, komplementäre Bereiche der Resonatorverstärkung werden untersucht. Ein erstes Experiment basiert auf einem Resonator mit hoher Finesse und dielektrischen Spiegeln. Das Skalierungsverhalten der Purcell Verstärkung wird ausführlich ausgewertet, indem man sowohl das Modenvolumen des Resonators (V = 16 − 600 µm^3 ) als auch dessen Güte (Q = 6 · 10^3 − 2 · 10^6) über einen weiten Bereich verändert. Die spektrale Leistungsdichte der Emission kann durch den Resonator um einen Faktor von bis zu 300 überhöht werden. Das gesamte Leistugsvermögen dieses Resonators kann mit schmalbandigen Emittern ausgenutzt werden, deren Emissionslinienbreite kleiner als die Linienbreite des Resonators ist. Dies ist ein vielversprechender Ansatz für die Umsetzung von schmalbandigen Einzelphotonenquellen mit durchstimmbarer Wellenlänge und für die Erzeugung ununterscheidbarer Einzelphotonen bei Umgebungsbedingungen. Jedoch bleibt die Lebenszeit der Emission für breitbandige Emitter, wie dem NV-Zentrum bei Raumtemperatur, in dieser Anordnung nahezu unbeeinflusst. Um eine Veränderung der Lebenszeit und durch den Purcell-Effekt verstärkte Einzelphotonenemission direkt zu beobachten, stellen wir Faserresonatoren mit silberbeschichteten Spiegeln und ultrakleinen Modenvolumen, bis hinab zu V = 1.0 λ^3 = 0.34 µm^3, her. Wir demonstrieren resonatorverstärkte Fluoreszenzbildgebung, die das Auffinden und Untersuchen von verschiedenen einzelnen NV-Zentren mit einem Resonator erlaubt. Der Purcell-Effekt wird über eine gesteigerte Aufsammlung der Fluoreszenz nachgewiesen, mit einer Rate von bis zu 1.6 · 10^6 Photonen pro Sekunde von einzelnen NV-Zentren und außerdem durch die abstimmbare Veränderung der Emissionslebenszeit, entsprechend einem effektiven Purcell Faktor von bis zu 2. Des Weiteren untersuchen wir ein vorteilhaftes Regime, in dem der Diamant Nanokristall selbst eine zusätzliche Einschränkung der optischen Mode bewirkt, die sich mit der Mode des Fabry-Pérot Resonators verbindet und Modenvolumen unter 1 λ^3 ermöglicht. Simulationen ergeben effektive Purcell Faktoren von bis zu 11 für NV-Zentren und von bis zu 63 für Silizium-Fehlstellen-Zentren, wodurch das große Potenzial für helle Einzelphotonenquellen und für effzientes Spin-Auslesen bei Umgebungsbedingungen aufgezeigt wird