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

Localization and Directionality of Surface Transport in Bi2Te3 Ordered 3D Nanonetworks

The resistance of an ordered 3D-Bi2Te3 nanowire nanonetwork was studied at low temperatures. Below 50 K the increase in resistance was found to be compatible with the Anderson model for localization, considering that conduction takes place in individual parallel channels across the whole sample. Angle-dependent magnetoresistance measurements showed a distinctive weak antilocalization characteristic with a double feature that we could associate with transport along two perpendicular directions, dictated by the spatial arrangement of the nanowires. The coherence length obtained from the Hikami-Larkin-Nagaoka model was about 700 nm across transversal nanowires, which corresponded to approximately 10 nanowire junctions. Along the individual nanowires, the coherence length was greatly reduced to about 100 nm. The observed localization effects could be the reason for the enhancement of the Seebeck coefficient observed in the 3D-Bi2Te3 nanowire nanonetwork compared to individual nanowires.

High-Performance n-Type Bi2Te3 Thermoelectric Fibers with Oriented Crystal Nanosheets

High-performance thermoelectric fibers with n-type bismuth telluride (Bi₂Te₃) core were prepared by thermal drawing. The nanosheet microstructures of the Bi₂Te₃ core were tailored by the whole annealing and Bridgman annealing processes, respectively. The influence of the annealing processes on the microstructure and thermoelectric performance was investigated. As a result of the enhanced crystalline orientation of Bi₂Te₃ core caused by the above two kinds of annealing processes, both the electrical conductivity and thermal conductivity could be improved. Hence, the thermoelectric performance was enhanced, that is, the optimized dimensionless figure of merit (ZT) after the Bridgman annealing processes increased from 0.48 to about 1 at room temperature.

Plasmon Resonances in 1D Nanowire Arrays and 3D Nanowire Networks of Topological Insulators and Metals

The 1D nanowire arrays and 3D nanowire networks of topological insulators and metals have been fabricated by template-assisted deposition of Bi₂Te₃ and Ni inside anodic aluminum oxide (AAO) templates, respectively. Despite the different origins of the plasmon capabilities of the two materials, the results indicate that the optical response is determined by plasmon resonances, whose position depends on the nanowire interactions and material properties. Due to the thermoelectric properties of Bi₂Te₃ nanowires, these plasmon resonances could be used to develop new ways of enhancing thermal gradients and their associated thermoelectric power.

The Thermoelectric Properties of Spongy PEDOT Films and 3D-Nanonetworks by Electropolymerization

Recently, polymers have been attracted great attention because of their thermoelectric materials' excellent mechanical properties, specifically their cost-effectiveness and scalability at the industrial level. In this study, the electropolymerization conditions (applied potential and deposition time) of PEDOT films were investigated to improve their thermoelectric properties. The morphology and Raman spectroscopy of the PEDOT films were analyzed according to their applied potential and deposition time. The best thermoelectric properties were found in films grown at 1.3 V for 10 min, with an electrical conductivity of 158 ± 8 S/cm, a Seebeck coefficient of 33 ± 1 µV/K, and a power factor of 17 ± 2 µW/K·m². This power factor value is three times higher than the value reported in the literature for electropolymerized PEDOT films in acetonitrile using lithium perchlorate as a counter-ion. The thermal conductivity was found to be (1.3 ± 0.3) × 10^-1 W/m·K. The highest figure of merit obtained at room temperature was (3.9 ± 1.0) × 10^-2 using lithium perchlorate as a counter-ion. In addition, three-dimensional (3D) PEDOT nanonetworks were electropolymerized inside 3D anodic aluminum oxide (3D AAO), obtaining lower values in their thermoelectric properties.

3D Bi2Te3 Interconnected Nanowire Networks to Increase Thermoelectric Efficiency

3D interconnected nanowire scaffoldings are shown to increase the thermoelectric efficiency in comparison to similar diameter 1D nanowires and films grown under similar electrodeposition conditions. Bi₂Te₃ 3D nanonetworks offer a reduction in thermal conductivity (κ_T) while preserving the high electrical conductivity of the films. The reduction in κ_T is modeled using the hydrodynamic heat transport equation, and it can be understood as a heat viscosity effect due to the 3D nanostructuration. In addition, the Seebeck coefficient is twice that of nanowires and films, and up to 50% higher than in a single crystal. This increase is interpreted as a nonequilibrium effect that the geometry of the structure induces on the distribution function of the phonons, producing an enhanced phonon drag. These thermoelectric metamaterials have higher performance and are fabricated with large areas by a cost-effective method, which makes them suitable for up-scale production.

Revisiting anodic alumina templates: from fabrication to applications

Anodic porous alumina, -AAO- (also known as nanoporous alumina, nanohole alumina arrays, -NAA- or nanoporous anodized alumina platforms, -NAAP-) has opened new opportunities in a wide range of fields, and is used as an advanced photonic structure for applications in structural coloration and advanced optical biosensing based on the ordered nanoporous structure obtained and as a template to grow nanowires or nanotubes of different materials giving rise to metamaterials with tailored properties. Therefore, understanding the structure of nanoporous anodic alumina templates and knowing how they are fabricated provide a tool for the further design of structures based on them, such as 3D nanoporous structures developed recently. In this work, we review the latest developments related to nanoporous alumina, which is currently a very active field, to provide a solid and thorough reference for all interested experts, both in academia and industry, on these nanostructured and highly useful structures. We present an overview of theories on the formation of pores and self-ordering in alumina, paying special attention to those presented in recent years, and different nanostructures that have been developed recently. Therefore, a wide variety of architectures, ranging from ordered nanoporous structures to diameter changing pores, branched pores, and 3D nanostructures will be discussed. Next, some of the most relevant results using different nanostructured morphologies as templates for the growth of different materials with novel properties and reduced dimensionality in magnetism, thermoelectricity, etc. will be summarised, showing how these structures have influenced the state of the art in a wide variety of fields. Finally, a review on how these anodic aluminium membranes are used as platforms for different applications combined with optical techniques, together with principles behind these applications will be presented, in addition to a hint on the future applications of these versatile nanomaterials. In summary, this review is focused on the most recent developments, without neglecting the basis and older studies that have led the way to these findings. Thus, it gives an updated state-of-the-art review that should be useful not only for experts in the field, but also for non-specialists, helping them to gain a broad understanding of the importance of anodic porous alumina, and most probably, endow them with new ideas for its use in fields of interest or even developing the anodization technique.

Novel Nanofluidic Cells Based on Nanowires and Nanotubes for Advanced Chemical and Bio-Sensing Applications

Since the first introduction of one-dimensional nanochannels for single-molecule detection, there has been increasing interest in modern nanofluidic systems, such as chemical and biological sensing applications. Recently developed nanowires (NWs) and nanotubes (NTs) have received tremendous attention due to their unique geometrical, physical and chemical properties, which are very attractive in this field. Here, we review the recent research activities in the field of novel nanofluidic cells based on NWs and NTs. First, we give a brief introduction of this field. Then the common synthesis methods of NWs and NTs are summarized. After that, we discuss the working principle and sensing mechanism of nanofluidic devices, which is fundamental to the interaction between these nanostructures and small molecules. Finally, we present the NW- and NT-based devices for chemical and bio-sensing applications, such as gas sensing, pathogen detection, DNA sequencing, and so forth.

Synergistically enhanced thermoelectric performance by optimizing the composite ratio between hydrothermal Sb2Se3 and self-assembled β-Cu2Se nanowires

Sb₂Se₃ and β-Cu₂Se nanowires were synthesized via hydrothermal reaction and a water-evaporation induced self-assembly method, respectively, and a 70%-Sb₂Se₃ and 30%-β-Cu₂Se disk pellet shows enhanced thermoelectric performance.

Electrodeposition of Bi-Te Thin Films on Silicon Wafer and Micro-Column Arrays on Microporous Glass Template

Electrodeposition is an important method for preparing bismuth telluride (Bi₂Te₃)-based thermoelectric (TE) thin films and micro-column arrays. When the concentrations of Bi:Te in electrolytes were 3 mM:4 mM, the TE films satisfied the Bi₂Te₃ stoichiometry and had no dependence on deposition potential. With increasing over-potential, crystal grains changed from lamellar structures with uniform growth directions to large clusters with staggered dendrites, causing a decrease in the deposition density. Meanwhile, the preferred (110) orientation was diminished. The TE film deposited at −35 mV had an optimum conductivity of 2003.6 S/cm and a power factor of 2015.64 μW/mK² at room temperature due to the (110)-preferred orientation. The electrodeposition of TE micro-columns in the template was recently used to fabricate high-power micro-thermoelectric generators (micro-TEG). Here, microporous glass templates were excellent templates for micro-TEG fabrication because of their low thermal conductivity, high insulation, and easy processing. A three-step pulsed-voltage deposition method was used for the fabrication of micro-columns with large aspect ratios, high filling rates, and high density. The resistance of a single TE micro-column with a 60 μm diameter and a 200 μm height was 6.22 Ω. This work laid the foundation for micro-TEG fabrication and improved performance.

Pulsed current-voltage electrodeposition of stoichiometric Bi2Te3 nanowires and their crystallographic characterization by transmission electron backscatter diffraction

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.

Electrodeposition of V-VI Nanowires and Their Thermoelectric Properties

Nanostructuration is an intensive field of research due to the appearance of interesting properties at the nanoscale. For instance, in thermoelectricity the most outstanding improvements obtained lately are related to phenomena that appear as a result of nano-engineering different materials. The thermoelectric effect is the direct conversion from temperature gradients into electricity and vice versa. When going to low dimensions, for example in the particular case of thermoelectric nanowires, the transport properties of phonons are modified with respect to those found in bulk leading to a higher thermoelectric figure of merit z. In more detail, this review tries to compile some of the landmarks in the electrodeposition of Bi₂Te₃-based nanowires. We will focus on the achievements using different templates, electrolytes and deposition modes. We will also summarize the measurements performed in those nanowires and the main conclusions that can be extracted from the published works. Finally, an update of nanowire-based thermoelectric generators is also included.