Development of novel EELS methods to unveil nanoparticle properties

[eng] The aim of this thesis has been two-fold. First, to develop new processing and analysis tools and strategies for extracting information from EELS data, and second, to apply the methods to different nanoparticle systems to shed light to relevant phenomena related to their synthesis and properties. In this regard, chapter 1 presented and overview of the EELS fundamentals and of the state of the art of the technique. Chapter 2 was focused on the advanced computational methods related to EELS data analysis. Moreover, the application of cluster analysis to EELS was introduced, showing its possibilities as an image segmenting and phase identification tool. The following chapters were devoted to the investigation of different material science problems related to NPs that take advantage of the capabilities of quantitative EELS. The results were grouped by increasing complexity of the performed analysis, with chapter 3 devoted to characterizations that were mainly carried out using EELS elemental mapping, chapter 4 being related to ELNES analysis and chapter 5 to EELS tomography. In chapter 2, the adaptation of data clustering algorithms to the analysis of EELS data, developed within the scope of the present thesis, has been undertaken. In chapter 3.1 the organic synthesis of FeOx@SiO2 NPs was assessed. Several findings were obtained through the HRTEM, STEM-HAADF and EELS characterization of the FeOx@SiO2 NPs at different stages of its synthesis. In chapter 3.2, concerning Au-Ag-Se and the Au-Ag-S system cation exchange reactions several findings were made. Chapter 4 was devoted to the characterization of different NPs with an emphasis on the direct observation the oxidation state of its constituents through EELS. In chapter 4.1, the synthesis of MnOx/Fe3O4 core/shell NPs was assessed. In chapter 4.2, the measurement of oxidation state at atomic resolution in spinel crystals was proposed as a method to assess cation inversion in the crystal. The necessary methods were developed and applied to iron oxide/manganese oxide core/shell NPs. Chapter 5 was devoted to the combination of EELS and tomography. In chapter 5.1 the synthesis of cobalt oxide/cobalt ferrite (CoO@CFO) core/shell NPs and cobalt oxide/manganese ferrite (CoO@MFO) NPs was investigated. Chapter 5.2 was focused on the achievement of an oxidation state-sensitive tomographic reconstruction.[cat] L’adveniment de la nanotecnologia està portant amb ell l’aparició d’una gran quantitat de nous materials, compostos i aplicacions. En el seu desenvolupament, sovint té lloc fenomenologia sorprenent, o encara no ben entesa. Per omplir aquest forats en el nostre coneixement i poder desenvolupar noves aplicacions és de vital importància esbrinar la configuració estructural i química a nivell subnanomètric d’aquests components. Per la seva gran resolució espacial, la microscòpia electrònica de transmissió ha esdevingut una eina indispensable en aquest context. A més, en un microscopi electrònic es poden combinar una gran varietat de tècniques que poden donar una quantitat d’informació enorme. Una d’aquestes tècniques és l’espectroscòpia de pèrdua d’energia dels electrons (EELS). Aquesta tècnica ha permès en els últims anys el mapejat d’elements químics i ions columna atòmica per columna atòmica, arribant a uns dels nivells més íntims als que es pot conèixer la matèria en estat sòlid. L’objectiu d’aquesta tesi ha estat fer ús de l’EELS i d’altres tècniques emprades en microscòpia electrònica per entendre els processos químics que tenen lloc en diferents síntesis de nanopartícules. En aquest procés s’han desenvolupat també una sèrie d’eines enfocades al processat de les dades d’EELS ja sigui per a facilitar la seva interpretació, limitar problemes derivats de la seva adquisició (i. e. soroll) o calcular propietats concretes del material estudiat. A més aquesta tècnica s’ha combinat amb mètodes de reconstrucció 3D per obtenir una informació completa dels sistemes estudiats

Clustering analysis strategies for electron energy loss spectroscopy (EELS).

In this work, the use of cluster analysis algorithms, widely applied in the field of big data, is proposed to explore and analyse electron energy loss spectroscopy (EELS) data sets. Three different data clustering approaches have been tested both with simulated and experimental data from Fe3O4/Mn3O4 core/shell nanoparticles. The first method consists on applying data clustering directly to the acquired spectra. A second approach is to analyse spectral variance with principal component analysis (PCA) within a given data cluster. Lastly, data clustering on PCA score maps is discussed. The advantages and requirements of each approach are studied. Results demonstrate how clustering is able to recover compositional and oxidation state information from EELS data with minimal user input, giving great prospects for its usage in EEL spectroscopy

Tuning branching in ceria nanocrystals

Branched nanocrystals (NCs) enable high atomic surface exposure within a crystalline network that provides avenues for charge transport. This combination of properties makes branched NCs particularly suitable for a range of applications where both interaction with the media and charge transport are involved. Herein we report on the colloidal synthesis of branched ceria NCs by means of a ligand-mediated overgrowth mechanism. In particular, the differential coverage of oleic acid as an X-type ligand at ceria facets with different atomic density, atomic coordination deficiency, and oxygen vacancy density resulted in a preferential growth in the [111] direction and thus in the formation of ceria octapods. Alcohols, through an esterification alcoholysis reaction, promoted faster growth rates that translated into nanostructures with higher geometrical complexity, increasing the branch aspect ratio and triggering the formation of side branches. On the other hand, the presence of water resulted in a significant reduction of the growth rate, decreasing the reaction yield and eliminating side branching, which we associate to a blocking of the surface reaction sites or a displacement of the alcoholysis reaction. Overall, adjusting the amounts of each chemical, well-defined branched ceria NCs with tuned number, thickness, and length of branches and with overall size ranging from 5 to 45 nm could be produced. We further demonstrate that such branched ceria NCs are able to provide higher surface areas and related oxygen storage capacities (OSC) than quasi-spherical NCs

Gradual transformation of Ag2S to Au2S nanoparticles by sequential cation exchange reactions: binary, ternary, and hybrid compositions

Cation exchange reactions have been exploited in the last years as an efficient tool for the controlled chemical modi-fication of pre-made nanocrystals. In this work, the gradual transformation of Ag2S nanocrystals into Au2S analogues is performed by sequential cation exchange reactions that allow for a fine control of the chemical composition, delivering also two intermediate ternary sulfides based exclusively on noble metals. The role of two different surfactants in the reaction medium has been studied: while dodecylamine is favoring the heterogeneous nucleation of metallic Au on the surface of the semiconductor domains in detri-ment of the cation exchange reaction, the use of tetraoctylammonium bromide turns out to be crucial for the enhancement of the exchange in order to reach full cation substitution, if desired. The presence of Br- anions in the reaction medium represents an additional tool to modulate the morphology of the final nanocrystals, being either solid or hollow depending on their concentration. The synthetic protocol has been successfully conducted in both spherical and rod-like nanocrystals with identical results, leading to a wide variety of binary, ternary and/or hybrid nanostructures that have been carefully characterized

Synthesis and thermoelectric properties of noble metal ternary chalcogenide systems of Ag-Au-Se in the forms of alloyed nanoparticles and colloidal nanoheterostructures

The optimization of a material functionality requires both the rational design and precise engineering of its structural and chemical parameters. In this work, we show how colloidal chemistry is an excellent synthetic choice for the synthesis of novel ternary nanostructured chalcogenides, containing exclusively noble metals, with tailored morphology and composition and with potential application in the energy conversion field. Specifically, the Ag–Au–Se system has been explored from a synthetic point of view, which leads to a set of Ag2Se-based hybrid and ternary nanoparticles including the room temperature synthesis of the rare ternary Ag3AuSe2 fischesserite phase. An in-depth structural and chemical characterization of all nanomaterials has been performed, which proofed especially useful for unravelling the reaction mechanism behind the formation of the ternary phase in solution. The work is complemented with the thermal and electric characterization of a ternary Ag–Au–Se nanocomposite with promising results: we found that the use of the ternary nanocomposite represents a clear improvement in terms of thermoelectric energy conversion as compared to a binary Ag–Se nanocomposite analogue.Peer ReviewedPostprint (author's final draft

Seeded Growth Synthesis of Au-Fe3O4 Heterostructured Nanocrystals : Rational Design and Mechanistic Insights

Multifunctional hybrid nanoparticles comprising two or more entities with different functional properties are gaining ample significance in industry and research. Due to its combination of properties, a particularly appealing example is Au-FeO composite nanoparticles. Here we present an in-depth study of the synthesis of Au-FeO heterostructured nanocrystals (HNCs) by thermal decomposition of iron precursors in the presence of preformed 10 nm Au seeds. The role of diverse reaction parameters on the HNCs formation was investigated using two different precursors: iron pentacarbonyl (Fe(CO)) and iron acetylacetonate (Fe(acac)). The reaction conditions promoting the heterogeneous nucleation of FeO onto Au seeds were found to significantly differ depending on the precursor chosen, where Fe(acac) is considerably more sensitive to the variation of the parameters than Fe(CO) and more subject to homogeneous nucleation processes with the consequent formation of isolated iron oxide nanocrystals (NCs). The role of the surfactants was also crucial in the formation of well-defined and monodisperse HNCs by regulating the access to the Au surface. Similarly, the variations of the [Fe]/[Au] ratio, temperature, and employed solvent were found to act on the mean size and the morphology of the obtained products. Importantly, while the optical properties are rather sensitive to the final morphology, the magnetic ones are rather similar for the different types of obtained HNCs. The surface functionalization of dimer-like HNCs with silica allows their dispersion in aqueous media, opening the path to their use in biomedical applications

Tuning branching in ceria nanocrystals

Branched nanocrystals (NCs) enable high atomic surface exposure within a crystalline network that provides avenues for charge transport. This combination of properties makes branched NCs particularly suitable for a range of applications where both interaction with the media and charge transport are involved. Herein we report on the colloidal synthesis of branched ceria NCs by means of a ligand-mediated overgrowth mechanism. In particular, the differential coverage of oleic acid as an X-type ligand at ceria facets with different atomic density, atomic coordination deficiency, and oxygen vacancy density resulted in a preferential growth in the [111] direction and thus in the formation of ceria octapods. Alcohols, through an esterification alcoholysis reaction, promoted faster growth rates that translated into nanostructures with higher geometrical complexity, increasing the branch aspect ratio and triggering the formation of side branches. On the other hand, the presence of water resulted in a significant reduction of the growth rate, decreasing the reaction yield and eliminating side branching, which we associate to a blocking of the surface reaction sites or a displacement of the alcoholysis reaction. Overall, adjusting the amounts of each chemical, well-defined branched ceria NCs with tuned number, thickness, and length of branches and with overall size ranging from 5 to 45 nm could be produced. We further demonstrate that such branched ceria NCs are able to provide higher surface areas and related oxygen storage capacities (OSC) than quasi-spherical NCs

Synthesis and thermoelectric properties of noble metal ternary chalcogenide systems of Ag-Au-Se in the forms of alloyed nanoparticles and colloidal nanoheterostructures

The optimization of a material functionality requires both the rational design and precise engineering of its structural and chemical parameters. In this work, we show how colloidal chemistry is an excellent synthetic choice for the synthesis of novel ternary nanostructured chalcogenides, containing exclusively noble metals, with tailored morphology and composition and with potential application in the energy conversion field. Specifically, the Ag-Au-Se system has been explored from a synthetic point of view, leading to a set of Ag2Se-based hybrid and ternary nanoparticles, including the room temperature synthesis of the rare ternary Ag3AuSe2 fischesserite phase. An in-depth structural and chemical charac-terization of all nanomaterials has been performed, which proofed especially useful for unravelling the reaction mecha-nism behind the formation of the ternary phase in solution. The work is complemented with the thermal and electric characterization of a ternary Ag-Au-Se nanocomposite with promising results: we found that the use of the ternary nano-composite represents a clear improvement in terms of thermoelectric energy conversion as compared to a binary Ag-Se nanocomposite analogue

3D Visualization of the Iron Oxidation State in FeO/Fe3O4 Core-Shell Nanocubes from Electron Energy Loss Tomography.

Left panel shows the explained variance ratio of the principal component analysis (PCA) decomposition. The six first components, which are enough to explain the whole data set, are plotted in the right panel. Components 0, 3 and 5 show no remarkable features in the Fe L2,3 ionization energy and seem rather related to the background of the spectra due to their power-law behaviour, while component 1 is almost constant and therefore could be related to the dark noise in the detector

3D Visualization of the Iron Oxidation State in FeO/Fe3O4 Core-Shell Nanocubes from Electron Energy Loss Tomography

The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe and Fe ions in a FeO/FeO core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample