Aïda Varea

We demonstrate here that electrospray is a technique suitable for deposition carbon-based materials. It is versatile enough to be used with graphene oxide (GO), reduced graphene oxide (rGO) and carbon nanofibers (CNF), obtained by chemical methods. A detailed analysis of the effects of the dispersion properties and the main process parameters (voltage and flow rate) on the deposits quality is presented. Rigid and flexible substrates have been coated with good geometry control under ambient conditions. Thickness of GO films onto silicon substrates, optical transmittance of rGO onto fluoride doped tin oxide (FTO) coated glass substrate and electrical resistance of both, rGO and CNF films onto flexible polyamide have been correlated with deposition time. As an example of application, the manufacture of carbon nanofibers gas sensing devices deposited onto a ceramic platform and flexible polyimide by means of electrospray technique are shown.

One dimensional (1D) nanostructures offer a promising path towards highly efficient heating and temperature control in integrated microsystems. The so called self-heating effect can be used to modulate the response of solid state gas sensor devices. In this work, efficient self-heating was found to occur at random networks of nanostructured systems with similar power requirements to highly ordered systems (e.g. individual nanowires, where their thermal efficiency was attributed to the small dimensions of the objects). Infrared thermography and Raman spectroscopy were used to map the temperature profiles of films based on random arrangements of carbon nanofibers during self-heating. Both the techniques demonstrate consistently that heating concentrates in small regions, the here-called “hot-spots”. On correlating dynamic temperature mapping with electrical measurements, we also observed that these minute hot-spots rule the resistance values observed macroscopically. A physical model of a random network of 1D resistors helped us to explain this observation. The model shows that, for a given random arrangement of 1D nanowires, current spreading through the network ends up defining a set of spots that dominate both the electrical resistance and power dissipation. Such highly localized heating explains the high power savings observed in larger nanostructured systems. This understanding opens a path to design highly efficient self-heating systems, based on random or pseudo-random distributions of 1D nanostructures.

Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. In addition, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir₂₀Mn₈₀/Fe₂₀Ni₈₀ microdots: distorted viscous vortex reversal. In contrast to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet, which leads to unexpected asymmetries in the annihilation and nucleation fields. These results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures.

Propagation of surface acoustic waves in lithium niobate crystals with carbon nanofibers or reduced graphene oxide layers deposited by electrospray has been investigated. We show that the ambient air humidity variations affect the phase of the wave noticeably stronger than its amplitude. The response of both phase and amplitude to fast humidity changes is in the sub-second range. The capability of surface acoustic waves of monitoring the fast changes in nano-layer properties for sensing applications has been demonstrated.

Periodic arrays of compositionally graded Cu–Ni alloy nanopillars (100 or 200 nm in diameter and 450 nm in height) have been fabricated by means of potentiostatic electrodeposition into patterned Au/Ti/Si(111) substrates using a single electrolytic solution. The pillars do not bend after the template removal but remain straight and perfectly attached to the substrate. The average composition of the alloy nanopillars can be tuned between 34 and 70 at% of Ni by varying the applied potential from a [Ni(II)]/[Cu(II)] = 95.2 electrolytic solution. The nanopillars are Ni-rich at the bottom with a compositional gradient about ±20–25% above/below the average composition. The magnetic characterization reveals that all the nanopillars are ferromagnetic with coercivity values around 100–150 Oe (in-plane) and 200–500 Oe (out-of-plane), which are larger than those for continuous films of similar average composition. Atom probe tomography of the Cu-rich Cu–Ni nanopillars indicates the segregation of Ni, which would explain their unexpected magnetic character. The graded-anisotropy ferromagnetic Cu–Ni pillars could be potentially applied in several fields including micro/nano-electromechanical systems (MEMS/NEMS), magnetic recording media or spintronics.