• Energy Research

Bi2Te3 nanowires with diameters ranging from 25 to 270 nm, ultra-high aspect ratio, and uniform growth front were fabricated by electrodeposition, pulsing between zero current density during the off time and constant potential during the on time (pulsed-current-voltage method, p-IV). The use of zero current density during the off time is to ensure no electrodeposition is carried out and the system is totally relaxed. By this procedure, stoichiometric nanowires oriented perpendicular to the c-axis is obtained for the different diameters of porous alumina templates. In addition, the samples show a uniform growth front with ultra-high aspect ratio single crystal nanowires. The high degree of crystallinity was verified by transmission electron backscatter diffraction. This characterization revealed that the nanowires present both large single crystalline areas and areas with alternating twin configurations.

The out of plane electrical conductivity of highly anisotropic Bi2Te3 films grown via electro-deposition process was determined using four probe current-voltage measurements performed on 4.6 - 7.2 μm thickness Bi2Te3 mesa structures with 80 - 120 μm diameters sandwiched between metallic film electrodes. A three-dimensional finite element model was used to predict the electric field distribution in the measured structures and take into account the non-uniform distribution of the current in the electrodes in the vicinity of the probes. The finite-element modeling shows that significant errors could arise in the measured film electrical conductivity if simpler one-dimensional models are employed. A high electrical conductivity of (3.2 ± 0.4) ⋅ 105 S/m is reported along the out of plane direction for Bi2Te3 films highly oriented in the [1 1 0] direction.

The growth of self-ordered anodic aluminum oxide (AAO) templates with pore diameters in the 140-400 nm range is achieved by anodization in phosphoric acid at low temperatures (-4 °C). The procedure used in this study is able to completely avoid the >burning> of the oxide, highly frequent in anodizations in phosphoric acid solutions at high voltages. The current density measured during the anodizations is rather low, 0.6-0.7 mA/cm2; therefore, low growth rates have been also measured (<2 μm/h). AAO templates present a relatively low porosity value of 8.4%. However, a considerable pore-enlargement-rate (vΔd = 0.636 ± 0.101 nm/h) has been observed as a consequence of the chemical dissolution of the pore walls during the anodization. Thus, the results reported here constitute an exhaustive study on the preparation of large-diameter-pore self-ordered AAO templates that enables both to access to pore diameters up to now inaccessible and to efficiently overcome the difficulties of their fabrication process ascribed to its aggressive reaction conditions. © 2011 Elsevier Inc. All rights reserved. Authors want to thank the ERC 2008 Starting Grant number 240497 for financial support. Peer Reviewed

Nanodispersion of NiO nanoparticles on the surface of ZnO microparticles has been carried out by means of a low energy dry mixing method. This method keeps the shape and size of the ZnO microparticles that served as support to hold the NiO nanoparticles. The dispersion and anchoring of the NiO nanoparticles takes place by means of an electrochemical reaction with the OH-groups present at the ZnO surface. The process took place at room temperature and cation interdiffusion between ZnO and NiO have not been detected by structural analysis. Nevertheless, changes in the optical properties of semiconductor ZnO particles have been observed due to the deposition of the nanoparticles. These changes correlate with the amount of NiO nanoparticles at the ZnO particle surface. The amount of defects, i.e oxygen vacancies, interstitial oxygen and zinc vacancies, rises with the increasing NiO content. The authors express their thanks to the CICYT project MAT2010-21088-C03-01, MAT2008-06330, PLE2009-0073, and to the European Commission ERC-2008-Stg: 240497 for their financial support. MSMG want to acknowledge B. Alen from IMM for technical support. Peer reviewed

Ordered anodic aluminum oxide (AAO) templates with pores of 12 nm in diameter and aspect-ratios (length-to-diameter) of >1000 are prepared by a simple two-step anodization process under mild-anodization conditions. Such a low diameter pores are consequence of the reduction of the dielectric constant of the electrolyte together with the effect of increasing the electrolyte viscosity. We show that the temperature has a strong impact on reducing the pore diameter, as provokes the increase of viscosity of the medium and thus the hindering of the diffusion of species involved in the widening of pores. Moreover, we manage to stop the AAO growth process at the pore nucleation stage and we show that 8 nm in diameter pores are originally formed under these conditions. The presented ultra-narrow pore templates may allow for the preparation of ordered arrays of high aspect-ratio ultra-small one dimensional (1D) nanostructures that could be useful to explore quantum phenomena and integrate quantum-devices. © 2013 Elsevier Inc. All rights reserved. The authors would like to acknowledge financial support from ERC 2008 Starting Grant “Nano-TEC” number 240497. C.V.M. acknowledges CSIC and European Union Funding for JAE PhD fellowship. Peer Reviewed

Bi2Te3 films have been grown by constant and pulsed electrochemical deposition. The pulsed deposition was carried out by alternating between a constant potential (potentiostatic mode) and an open circuit potential (galvanostatic mode, where current density is fixed at 0 mA/cm 2). The Harris texture analysis was performed to determine the degree of preferred orientation. The results showed that the films were strongly oriented along (1 1 0) direction. The morphology and compositions of the films were then analyzed. Finally, their Seebeck coefficient and electrical resistivity were measured and used to determine the thermoelectric Power Factor of the films for a temperature range between 57 and 107 C. © 2013 Springer-Verlag Berlin Heidelberg. The authors would like to acknowledge partial financial support from ERC StG NanoTEC 240497. D.A.B.T. acknowledges support from NSF IRES-1028071. C.V.M acknowledges a JAE Ph.D. grant, O.C.C. acknowledges JAE post-doctoral positions from CSIC, and the European Social Fund and B.A.M. recognizes the NANOTHERMA project (FCCI), MICINN project MAT2008-06330. Peer Reviewed

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.