Publications
-
Direct Evidence of a Graded Magnetic Interface in Bimagnetic Core/Shell Nanoparticles Using Electron Magnetic Circular Dichroism (EMCD)
Del-Pozo-Bueno D., Varela M., Estrader M., López-Ortega A., Roca A.G., Nogués J., Peiró F., Estradé S. Nano Letters; 21 (16): 6923 - 6930. 2021. 10.1021/acs.nanolett.1c02089. IF: 11.189
Interfaces play a crucial role in composite magnetic materials and particularly in bimagnetic core/shell nanoparticles. However, resolving the microscopic magnetic structure of these nanoparticles is rather complex. Here, we investigate the local magnetization of antiferromagnetic/ferrimagnetic FeO/Fe3O4 core/shell nanocubes by electron magnetic circular dichroism (EMCD). The electron energy-loss spectroscopy (EELS) compositional analysis of the samples shows the presence of an oxidation gradient at the interface between the FeO core and the Fe3O4 shell. The EMCD measurements show that the nanoparticles are composed of four different zones with distinct magnetic moment in a concentric, onion-type, structure. These magnetic areas correlate spatially with the oxidation and composition gradient with the magnetic moment being largest at the surface and decreasing toward the core. The results show that the combination of EELS compositional mapping and EMCD can provide very valuable information on the inner magnetic structure and its correlation to the microstructure of magnetic nanoparticles. © 2021 The Authors. Published by American Chemical Society.
-
Mechanochromic Detection for Soft Opto-Magnetic Actuators
Güell-Grau P., Escudero P., Perdikos F.G., López-Barbera J.F., Pascual-Izarra C., Villa R., Nogués J., Sepúlveda B., Alvarez M. ACS Applied Materials and Interfaces; 13 (40): 47871 - 47881. 2021. 10.1021/acsami.1c11710. IF: 9.229
New multi-stimuli responsive materials are required in smart systems applications to overcome current limitations in remote actuation and to achieve versatile operation in inaccessible environments. The incorporation of detection mechanisms to quantify in real time the response to external stimuli is crucial for the development of automated systems. Here, we present the first wireless opto-magnetic actuator with mechanochromic response. The device, based on a nanostructured-iron (Fe) layer transferred onto suspended elastomer structures with a periodically corrugated backside, can be actuated both optically (in a broadband spectral range) and magnetically. The combined opto-magnetic stimulus can accurately modulate the mechanical response (strength and direction) of the device. The structural coloration generated at the corrugated back surface enables to easily map and quantify, in 2D, the mechanical deflections by analyzing in real time the hue changes of images taken using a conventional RGB smartphone camera, with a precision of 0.05°. We demonstrate the independent and synergetic optical and magnetic actuation and detection with a detection limit of 1.8 mW·cm-2 and 0.34 mT, respectively. The simple operation, versatility, and cost-effectiveness of the wireless multiactuated device with highly sensitive mechanochromic mapping paves the way to a new generation of wirelessly controlled smart systems. © 2021 American Chemical Society.
-
Probing the meta-stability of oxide core/shell nanoparticle systems at atomic resolution
Roldan M.A., Mayence A., López-Ortega A., Ishikawa R., Salafranca J., Estrader M., Salazar-Alvarez G., Dolors Baró M., Nogués J., Pennycook S.J., Varela M. Chemical Engineering Journal; 405 (126820) 2021. 10.1016/j.cej.2020.126820. IF: 13.273
Hybrid nanoparticles allow exploiting the interplay of confinement, proximity between different materials and interfacial effects. However, to harness their properties an in-depth understanding of their (meta)stability and interfacial characteristics is crucial. This is especially the case of nanosystems based on functional oxides working under reducing conditions, which may severely impact their properties. In this work, the in-situ electron-induced selective reduction of Mn3O4 to MnO is studied in magnetic Fe3O4/Mn3O4 and Mn3O4/Fe3O4 core/shell nanoparticles by means of high-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Such in-situ transformation allows mimicking the actual processes in operando environments. A multi-stage image analysis using geometric phase analysis combined with particle image velocity enables direct monitoring of the relationship between structure, chemical composition and strain relaxation during the Mn3O4 reduction. In the case of Fe3O4/Mn3O4 core/shell the transformation occurs smoothly without the formation of defects. However, for the inverse Mn3O4/Fe3O4 core/shell configuration the electron beam-induced transformation occurs in different stages that include redox reactions and void formation followed by strain field relaxation via formation of defects. This study highlights the relevance of understanding the local dynamics responsible for changes in the particle composition in order to control stability and, ultimately, macroscopic functionality. © 2020 Elsevier B.V.
-
Ultrabroadband light absorbing Fe/polymer flexible metamaterial for soft opto-mechanical devices
Güell-Grau P., Pi F., Villa R., Nogués J., Alvarez M., Sepúlveda B. Applied Materials Today; 23 (101052) 2021. 10.1016/j.apmt.2021.101052. IF: 10.041
Ultrabroadband light absorbers are attracting increasing interest for applications in energy harvesting, photodetection, self-regulated devices or soft robotics. However, current absorbers show detrimental insufficient absorption spectral range, or light angle and polarization dependence. Here we show that the unexplored optical properties of highly-damped plasmonic materials combined with the infrared absorption of thin polymer films enable developing ultrabroadband light-absorbing soft metamaterials. The developed metamaterial, composed of a nanostructured Fe layer mechanically coupled to a thin polydimethylsiloxane (PDMS) film, shows unprecedented ultrabroadband and angle-independent optical absorption (averaging 84% within 300–18000 nm). The excellent photothermal efficiency and large thermal-expansion mismatch of the metamaterial is efficiently transformed into large mechanical deflections, which we exploit to show an artificial iris that self-regulates the transmitted light power from the ultraviolet to the long-wave infrared, an untethered light-controlled mechanical gripper and a light-triggered electrical switch. © 2021 The Authors
