Lluís Yedra

Learning from Nature to Improve the Heat Generation of Iron-Oxide Nanoparticles for Magnetic Hyperthermia Applications

The performance of magnetic nanoparticles is intimately entwined with their structure, mean size and magnetic anisotropy. Besides, ensembles offer a unique way of engineering the magnetic response by modifying the strength of the dipolar interactions between particles. Here we report on an experimental and theoretical analysis of magnetic hyperthermia, a rapidly developing technique in medical research and oncology. Experimentally, we demonstrate that single-domain cubic iron oxide particles resembling bacterial magnetosomes have superior magnetic heating efficiency compared to spherical particles of similar sizes. Monte Carlo simulations at the atomic level corroborate the larger anisotropy of the cubic particles in comparison with the spherical ones, thus evidencing the beneficial role of surface anisotropy in the improved heating power. Moreover we establish a quantitative link between the particle assembling, the interactions and the heating properties. This knowledge opens new perspectives for improved hyperthermia, an alternative to conventional cancer therapies.

EEL spectroscopic tomography: Towards a new dimension in nanomaterials analysis

Electron tomography is a widely spread technique for recovering the three dimensional (3D) shape of nanostructured materials. Using a spectroscopic signal to achieve a reconstruction adds a fourth chemical dimension to the 3D structure. Up to date, energy filtering of the images in the transmission electron microscope (EFTEM) is the usual spectroscopic method even if most of the information in the spectrum is lost. Unlike EFTEM tomography, the use of electron energy-loss spectroscopy (EELS) spectrum images (SI) for tomographic reconstruction retains all chemical information, and the possibilities of this new approach still remain to be fully exploited. In this article we prove the feasibility of EEL spectroscopic tomography at low voltages (80 kV) and short acquisition times from data acquired using an aberration corrected instrument and data treatment by Multivariate Analysis (MVA), applied to Fe(x)Co((3-x))O(4)@Co(3)O(4) mesoporous materials. This approach provides a new scope into materials; the recovery of full EELS signal in 3D.

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(2+) and Fe(3+) ions in a FeO/Fe3O4 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.

High-temperature long-term stable ordered mesoporous Ni–CGO as an anode for solid oxide fuel cells

High temperature stable ordered mesoporous nickel–gadolinium-doped ceria cermets were prepared from a silica hard template (KIT-6), by a multistep impregnation process. The resulting cermet consists of an intimately mixed composite at the nanoscale with highly connected nickel and ceria percolation networks that ensure good electronic conductivity and strong penetration of the active area inside the material. The mesoporous cermets were implemented and evaluated as anodes for intermediate-temperature solid oxide fuel cells using gadolinia-doped ceria as the electrolyte. Targeted values of anode/electrolyte area specific resistance were obtained in the intermediate range of temperatures (ASR = 0.25 Ω cm2 at 675 °C). Virtually no degradation of the microstructure and the electrochemical performance was observed for the cermet after more than 200 h of testing at 800 °C in a water vapor saturated 5% H2 in argon atmosphere. This confirmed the stability of the mesostructure under SOFC operating conditions. Finally, fuel cell tests were carried out using an electrolyte-supported SOFC consisting of a double mesoporous layer (NiO–CGO 50 : 50 wt% and NiO–CGO 65 : 35 wt%) working as the anode and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) as the cathode. A maximum value of the power density of 435 mW cm−2 was achieved at 800 °C in water vapor saturated pure H2. The here-presented mesoporous approach gives rise to a new class of high-temperature stable nanostructured composites, electrochemically analogous to a mixed ionic electronic conductor, for intermediate temperature solid oxide fuel cells.

High-surface-area ordered mesoporous oxides for continuous operation in high temperature energy applications

The collapse of nanostructures at high temperature is one of the main drawbacks for the implementation of nanomaterials in some energy applications. An exciting virtual non-degradation up to 1000 °C is presented here for ordered mesoporous gadolinia doped ceria. By using the nanocasting method based on the KIT-6 template, the long-term stability of the material is achieved when extending the self-limited grain growth regime, recently proved for thin films, to open three-dimensional structures. Contrary to widely employed high temperature stabilization treatments inside the template, this work shows the advantage of a counterintuitive and cost-effective thermal treatment at intermediate temperatures, lower than the operation temperature. The evolution of the mesostructure with time at high temperatures, ranging from 800 °C to 1100 °C, is reported in terms of the microstructure (grain size and specific surface area) and catalytic activity (redox ability and oxygen storage capacity). The possibility of extension of this methodology to almost all metal oxides and the capability of working at temperatures significantly over the state-of-the-art open a new avenue for the use of these high-surface area 3D nanostructures in up-to-now forbidden high temperature energy applications such as solid oxide fuel/electrolysis cells, gas separation membranes or high temperature catalysis.

In-situ Isotopic Analysis at Nanoscale using Parallel Ion Electron Spectrometry: A Powerful New Paradigm for Correlative Microscopy

Isotopic analysis is of paramount importance across the entire gamut of scientific research. To advance the frontiers of knowledge, a technique for nanoscale isotopic analysis is indispensable. Secondary Ion Mass Spectrometry (SIMS) is a well-established technique for analyzing isotopes, but its spatial-resolution is fundamentally limited. Transmission Electron Microscopy (TEM) is a well-known method for high-resolution imaging down to the atomic scale. However, isotopic analysis in TEM is not possible. Here, we introduce a powerful new paradigm for in-situ correlative microscopy called the Parallel Ion Electron Spectrometry by synergizing SIMS with TEM. We demonstrate this technique by distinguishing lithium carbonate nanoparticles according to the isotopic label of lithium, viz. 6Li and 7Li and imaging them at high-resolution by TEM, adding a new dimension to correlative microscopy.

Hantzsch dihydropyridines: Privileged structures for the formation of well-defined gold nanostars.

Anisotropic and branched gold nanoparticles have great potential in optical, chemical and biomedical applications. However their syntheses involve multi-step protocols and the use of cytotoxic agents. Here, we report a novel one-step method for the preparation of gold nanostructures using only Hantzsch 1,4-dihydropyridines as mild reducing agents. The substituent pattern of the dihydropyridine nucleus was closely related to the ease of formation, morphology and stability of the nanoparticles. We observed nanostructures such as spheres, rods, triangles, pentagons, hexagons, flowers, stars and amorphous. We focused mainly on the synthesis and characterization of well-defined gold nanostars, which were produced quickly at room temperature (25°C) in high yield and homogeneity. These nanostars presented an average size of 68 nm with mostly four or six tips. Based on our findings, we propose that the growth of the nanostars occurs in the (111) lattice plane due to a preferential deposition of the gold atoms in the early stages of particle formation. Furthermore, the nanostars were easily modified with peptides remaining stable for more than six months in their colloidal state and showing a better stability than unmodified nanostars in different conditions. We report a new approach using dihydropyridines for the straightforward synthesis of gold nanostructures with controlled shape, feasible for use in future applications.

Precessed electron beam electron energy loss spectroscopy of graphene: Beyond channelling effects

The effects of beam precession on the Electron Energy Loss Spectroscopy (EELS) signal of the carbon K edge in a 2 monolayer graphene sheet are studied. In a previous work, we demonstrated the use of precession to compensate for the channeling-induced reduction of EELS signal when in zone axis. In the case of graphene, no enhancement of EELS signal is found in the usual experimental conditions, as graphene is not thick enough to present channeling effects. Interestingly, though it is found that precession makes it possible to increase the collection angle, and, thus, the overall signal, without a loss of signal-to-background ratio.

HIM-SIMS: Correlative SE/Chemical Imaging at the Limits of Resolution.

Techniques for nano-metrology and nano-analyisis are crucial for the ongoing investigation of nanoscale processes in disciplines from materials to life sciences. The Helium Ion Microscope (HIM) has become an ideal tool for imaging and nano-patterning [1]. Imaging with helium and neon ions leads respectively to resolutions of 0.5 nm and ~2 nm for SE based imaging, while structures with sub 20 nm feature sizes may be patterned using Ne. Despite these advantages, the analysis capability of the instrument is currently limited. At beam energies of 35 keV helium ions do not lead to the emission of characteristic X-rays from a sample. While some compositional information can be obtained from back scattered helium [2], identifying elemental information is more difficult due to the multiple collisions that occur at energies below 100 keV [3].

Secondary Ion Mass Spectrometry in the TEM: Isotope Specific High Resolution Correlative Imaging.

In the context of an ever increasing complexity and size reduction of devices in materials science research, there is a pressing need for accurate characterization at high-spatial resolution and highchemical sensitivity. Transmission Electron Microscopy (TEM) can offer sub-Å spatial information combined with electron spectroscopies yielding chemical and electronic information. However, the analytical TEM retains some limitations that make it unsuitable for applications requiring distinguishing isotopes, detecting trace elements below 0.1 at.% concentration or characterization of very light elements such as hydrogen, lithium or boron [1,2]. By contrast, Secondary Ion Mass Spectrometry (SIMS) has superior sensitivity (ppm range) and is capable of distinguishing isotopes and detecting all elements (including low-Z elements), but suffers from an inherent fundamental limitation in spatial resolution [3]. By correlating TEM with SIMS in-situ, it is possible to overcome their individual limitations and obtain information at high-resolution and high-sensitivity simultaneously.