Marisol S. Martín-González

Insights into the Electrodeposition of Bi2Te3

In this paper, the processes associated with the electrodeposition of bismuth telluride (Bi 2 Te 3 ), a thermoelectric material, are reported along with an analysis of the composition and crystallinity of the resulting films. The electrodeposition can be described by the general reaction 3HTeO 2 + + 2Bi 3+ + 18e - + 9H + → Bi 2 Te 3 + 6H 2 O. Cyclic voltammetry studies of Bi, Te, and Bi/Te dissolved in I M HNO 3 reveal two different underlying processes depending on the deposition potential. One process involves the reduction of HTeO + 2 to Te 0 and a subsequent interaction between reduced Te 0 and Bi 3+ to form Bi 2 Te 3 . A second process at more negative reduction potentials involves reduction of HTeO + 2 to H 2 Te followed by the chemical interaction with Bi 3+ . Both processes result in the production of crystalline Bi 2 Te 3 films in the potential range -0.1 < E < -0.52 V vs. Ag/AgCl (3 M NaCI) on Pt substrates as determined by powder X-ray diffraction (XRD). Electron probe microanalyses and XRD reveal that the films are bismuth-rich and less oriented for more negative deposition potentials.

Thermoelectric composites of poly(3-hexylthiophene) and carbon nanotubes with a large power factor

Composite films of poly(3-hexylthiophene) and single- as well as multi-walled carbon nanotubes are demonstrated to offer a competitive thermoelectric performance. The power factor significantly exceeds values obtained with either constituent alone provided that the conjugated polymer is sufficiently p-doped. The use of single-walled carbon nanotubes consistently results in a higher electrical conductivity with a maximum value above 103 S cm−1 and thus gives rise to a power factor of 25 ± 6 μW m−1 K−2 for a filler content of only 8 wt% and a maximum 95 ± 12 μW m−1 K−2 for 42–81 wt%. Moreover, a carbon nanotube content of 8–10 wt% does not compromise the low bulk thermal conductivity of the polymer matrix, which promises a high figure of merit of at least ZT > 10−2 at room-temperature. All samples are cast on plastic substrates, emphasising their suitability for large-area, flexible thermoelectric applications.

Skutterudites as thermoelectric materials: revisited

The research on skutterudites in the last few years has contributed to a better understanding of the physical processes which play an important role in enhancing their thermoelectric performance and to the discovery of novel filled compounds, with one of the most promising zT values at intermediate temperatures. Skutterudites are still an ongoing field of research, and an improvement of their efficiencies, stabilities, contacts, industrial scalable fabrication processes and other factors are expected in the near future in order to develop viable modules for intermediate temperature range applications, such as in the automobile industry, factories or incinerators. This paper gives a review on the status of research in the field of skutterudites.

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.

Improved power factor of polyaniline nanocomposites with exfoliated graphene nanoplatelets (GNPs)

In this work, exfoliated graphene nanoplatelets (GNPs)/polyaniline (PANI) nanocomposites have been prepared by sequential processing comprising: (i) a first aniline oxidative polymerization step under acidic conditions and (ii) mechanical blending with GNPs at different percentages. Thermoelectric pellets of the hybrid materials have been obtained at suitable circular geometry by means of cold pressing. Thermoelectric parameters have been determined at room temperature (electrical conductivity, Seebeck coefficient and thermal conductivity). Thermoelectric measurements show a drastic enhancement in both electrical conductivity and Seebeck coefficient with the addition of GNPs. A respectable maximum power factor value of 14 μW m−1 K−2 is reached for hybrid materials charged at 50 wt% GNP content, evidencing a 1000-fold enhancement with respect to the raw PANI polymer. The measured thermal conductivity is in the range of 0.5 W m−1 K−1 for pure PANI to 3.3 W m−1 K−1 for 50 wt% GNP content, which matches the parallel thermal resistor model for this nanocomposite.

Thermal properties of electrodeposited bismuth telluride nanowires embedded in amorphous alumina

Bismuth telluride nanowires are of interest for thermoelectric applications because of the predicted enhancement in the thermoelectric figure-of-merit in nanowire structures. In this letter, we carried out temperature-dependent thermal diffusivity characterization of a 40nm diameter Bi2Te3 nanowires∕alumina nanocomposite. Measured thermal diffusivity of the composite decreases from 9.2×10−7m2s−1 at 150Kto6.9×10−7m2s−1 at 300K and is lower than thermal diffusivity of unfilled alumina templates. Effective medium calculations indicate that the thermal conductivity along nanowires axis is at least an order of magnitude lower than thermal conductivity of the bulk bismuth telluride.

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...

Ferromagnetism in bulk Co-Zn-O

The origin of ferromagnetism in diluted magnetic semiconductors is still an open question, yielding a great deal of research across the world. This work focuses on the Co-Zn-O system. Room-temperature ferromagnetism is observed after a partial reaction of Co3O4 and ZnO, which can be ascribed neither to carrier mediation nor segregated cobalt metallic clusters. Another mechanism is yielding room-temperature ferromagnetism. This mechanism is associated with a partial reaction of ZnO and Co3O4 grains, and always appears when the starting phases (Co3O4 and ZnO) are present in the sample, suggesting that interfaces are involved in the origin of the observed ferromagnetism.The origin of ferromagnetism in diluted magnetic semiconductors is still an open question, yielding a great deal of research across the world. This work focuses on the Co-Zn-O system. Room-temperature ferromagnetism is observed after a partial reaction of Co3O4 and ZnO, which can be ascribed neither to carrier mediation nor segregated cobalt metallic clusters. Another mechanism is yielding room-temperature ferromagnetism. This mechanism is associated with a partial reaction of ZnO and Co3O4 grains, and always appears when the starting phases (Co3O4 and ZnO) are present in the sample, suggesting that interfaces are involved in the origin of the observed ferromagnetism.

Review on measurement techniques of transport properties of nanowires.

Physical properties at the nanoscale are novel and different from those in bulk materials.