Olga Caballero-Calero

High-aspect-ratio and highly ordered 15-nm porous alumina templates

Ordered anodic aluminum oxide (AAO) templates with pores <15 nm in diameter and an aspect ratio (length-to-diameter ratio) above 3 × 10(3) have been fabricated using a nonlithographic approach; specifically, by anodizing aluminum in an ethylene-glycol-containing sulfuric acid electrolyte. The pores are the smallest in diameter reported for a self-ordered AAO without pore aspect-ratio limitations and good ordering, which opens up the possibility of obtaining nanowire arrays in the quantum confinement regime that is of interest for efficient thermoelectric generators. The effect of the ethylene glycol addition on both the pore diameter and the ordering is evaluated and discussed. Moreover, 15-nm-diameter Bi(2)Te(3) and poly(3-hexyl thiophene) (P3HT) nanowires have been prepared using these AAO templates. As known, Bi(2)Te(3) is currently the most efficient thermoelectric bulk material for room-temperature operations and, according with theory, its Seebeck coefficient should be increased when it is confined to nanowires with diameters close to 10 nm. On the other hand, P3HT is one of the main candidates for integrating organic photovoltaic and thermoelectric devices, and its properties are also proposed to increase when it is confined to nanoscale structures, mainly due to molecular orientation effects.

Ordered three-dimensional interconnected nanoarchitectures in anodic porous alumina

Three-dimensional nanostructures combine properties of nanoscale materials with the advantages of being macro-sized pieces when the time comes to manipulate, measure their properties, or make a device. However, the amount of compounds with the ability to self-organize in ordered three-dimensional nanostructures is limited. Therefore, template-based fabrication strategies become the key approach towards three-dimensional nanostructures. Here we report the simple fabrication of a template based on anodic aluminum oxide, having a well-defined, ordered, tunable, homogeneous 3D nanotubular network in the sub 100 nm range. The three-dimensional templates are then employed to achieve three-dimensional, ordered nanowire-networks in Bi2Te3 and polystyrene. Lastly, we demonstrate the photonic crystal behavior of both the template and the polystyrene three-dimensional nanostructure. Our approach may establish the foundations for future high-throughput, cheap, photonic materials and devices made of simple commodity plastics, metals, and semiconductors.

Synthesis and luminescence properties of electrodeposited ZnO films

Zinc oxide (ZnO) films have been grown on gold (111) by electrodeposition using two different OH− sources, nitrate and peroxide, in order to obtain a comparative study between them. The morphology, structural and optical characterization of the films were investigated depending on the solution used (nitrate and peroxide) and the applied potential. Scanning electron microscopy pictures show different morphologies in each case. X-ray diffraction confirms that the films are pure ZnO oriented along the (0002) direction. ZnO films have been studied by photoluminescence to identify the emission of defects in the visible range. A consistent model that explains the emissions for the different electrodeposited ZnO films is proposed. We have associated the green and yellow emissions to a transition from the donor OH− to the acceptor zinc vacancies (VZn−) and to interstitial oxygen (Oi0), respectively. The orange-red emission is probably due to transitions from the conducting band to Oi− and OZn0 defects and the inf...

Fabrication of Bi2Te3 nanowire arrays and thermal conductivity measurement by 3ω-scanning thermal microscopy

Bi2Te3 is well-known for its utility in thermoelectrical applications and more recently as topological insulator. Its nanostructuration has attracted plenty of attention because of its potential capacity to reduce thermal conductivity. Here, we have grown a composite sample made of a Bi2Te3 nanowires (NWs) array embedded in an alumina matrix. We have then performed scanning thermal microscopy (SThM) in a 3ω configuration to measure its equivalent thermal resistance. Using an effective medium model, we could then estimate the mean composite thermal conductivity as well as the thermal conductivity of the NWs to be, respectively, (λC) = (1.68 ± 0.20) W/mK and (λNW) = (1.37 ± 0.20) W/mK, showing a slight thermal conductivity reduction. Up to now, there have been two main techniques reported in literature to evaluate the thermal conductivity of nanostructures: the use of a thermal microchip to probe a single NW once its matrix has been dissolved or the probing of the whole NWs array embedded in a matrix, obtai...

Electrical contact resistances of thermoelectric thin films measured by Kelvin probe microscopy

This work presents an approach for measuring cross plane electrical contact resistances directly using Kelvin Probe Microscopy. With this technique we were able to measure the electrical contact resistances of a cross section of a thermoelectric thin film made of Bi2Te3 sandwiched between two gold electrodes. On the one hand, the bottom gold electrode, which is located on top of the silicon substrate, was used as a cathode in electro-deposition process to grow the sample. On the other hand, the gold electrode on top was made via physical evaporation. The electrical contact resistances measured at both interfaces were 0.11 ± 0.01Ω and 0.15 ± 0.01Ω, respectively. These differences are related to differences between the top and bottom gold/bismuth-telluride film, obtaining smaller contact resistance where the film was grown by electro-deposition.

Tailoring thermal conductivity via three-dimensional porous alumina

Three-dimensional anodic alumina templates (3D-AAO) are an astonishing framework with open highly ordered three-dimensional skeleton structures. Since these templates are architecturally different from conventional solids or porous templates, they teem with opportunities for engineering thermal properties. By establishing the mechanisms of heat transfer in these frameworks, we aim to create materials with tailored thermal properties. The effective thermal conductivity of an empty 3D-AAO membrane was measured. As the effective medium theory was not valid to extract the skeletal thermal conductivity of 3D-AAO, a simple 3D thermal conduction model was developed, based on a mixed series and parallel thermal resistor circuit, giving a skeletal thermal conductivity value of approximately 1.25 W·m−1·K−1, which matches the value of the ordinary AAO membranes prepared from the same acid solution. The effect of different filler materials as well as the variation of the number of transversal nanochannels and the length of the 3D-AAO membrane in the effective thermal conductivity of the composite was studied. Finally, the thermal conductivity of two 3D-AAO membranes filled with cobalt and bismuth telluride was also measured, which was in good agreement with the thermal model predictions. Therefore, this work proved this structure as a powerful approach to tailor thermal properties.

Engineering a Full Gamut of Structural Colors in All-Dielectric Mesoporous Network Metamaterials

Structural colors are a result of the scattering of certain frequencies of the incident light on micro- or nanoscale features in a material. This is a quite different phenomenon from that of colors produced by absorption of different frequencies of the visible spectrum by pigments or dyes, which is the most common way of coloring used in our daily life. However, structural colors are more robust and can be engineered to span most of the visible spectrum without changing the base material, only its internal structure. They are abundant in nature, with examples as colorful as beetle shells and butterfly wings, but there are few ways of preparing them for large-scale commercial applications for real-world uses. In this work, we present a technique to create a full gamut of structural colors based on a low-cost, robust, and scalable fabrication of periodic network structures in porous alumina as well as the strategy to theoretically predict and engineer different colors on demand. We experimentally demonstrat...

Three-Dimensional Bi2Te3 Networks of Interconnected Nanowires: Synthesis and Optimization

Self-standing Bi2Te3 networks of interconnected nanowires were fabricated in three-dimensional porous anodic alumina templates (3D–AAO) with a porous structure spreading in all three spatial dimensions. Pulsed electrodeposition parameters were optimized to grow highly oriented Bi2Te3 interconnected nanowires with stoichiometric composition inside those 3D–AAO templates. The nanowire networks were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and Raman spectroscopy. The results are compared to those obtained in films and 1D nanowires grown under similar conditions. The crystalline structure and composition of the 3D Bi–Te nanowire network are finely tuned by controlling the applied voltage and the relaxation time off at zero current density during the deposition. With this fabrication method, and controlling the electrodeposition parameters, stoichiometric Bi2Te3 networks of interconnected nanowires have been obtained, with a preferential orientation along [1 1 0], which makes them optimal candidates for out-of-plane thermoelectric applications. Moreover, the templates in which they are grown can be dissolved and the network of interconnected nanowires is self-standing without affecting its composition and orientation properties.

Silicon‐Germanium (SiGe) Nanostructures for Thermoelectric Devices: Recent Advances and New Approaches to High Thermoelectric Efficiency

Silicon and germanium present distinct and interesting transport properties. However, composites made of silicon‐germanium (SiGe) have resulted in a breakthrough in terms of their transport properties. Currently, these alloys are used in different applications, such as microelectronic devices and integrated circuits, photovoltaic cells, and thermo‐ electric applications. With respect to thermoelectricity, in the last decades, Si 0.8 Ge 0.2 has attracted significant attention as an energy harvesting material, for powering space appli‐ cations and other industrial applications. This chapter focuses on the recent advances and new approaches in silicon‐germanium (Si 1−xGex) nanostructures for thermoelectric devices with high thermoelectric efficiency obtained through magnetron sputtering.