High density p-type Bi0.5Sb1.5Te3 nanowires by electrochemical templating through ion-track lithography

Thermoelectric Properties of Bismuth Telluride Thin Films Electrodeposited from a Nonaqueous Solution

We report the thermoelectric properties of Bi₂Te₃ thin films electrodeposited from the weakly coordinating solvent dichloromethane (CH₂Cl₂). It was found that the oxidation of porous films is significant, causing the degradation of its thermoelectric properties. We show that the morphology of the film can be improved drastically by applying a short initial nucleation pulse, which generates a large number of nuclei, and then growing the nuclei by pulsed electrodeposition at a much lower overpotential. This significantly reduces the oxidation of the films as smooth films have a smaller surface-to-volume ratio and are less prone to oxidation. X-ray photoelectron spectroscopy (XPS) shows that those films with Te(O) termination show a complete absence of oxygen below the surface layer. A thin film transfer process was developed using polystyrene as a carrier polymer to transfer the films from the conductive TiN to an insulating layer for thermoelectrical characterization. Temperature-dependent Seebeck measurements revealed a room-temperature coefficient of -51.7 μV/K growing to nearly -100 μV/K at 520 °C. The corresponding power factor reaches a value of 88.2 μW/mK² at that temperature.

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.

Electrodeposition of p-Type Sb₂Te₃ Films and Micro-Pillar Arrays in a Multi-Channel Glass Template

Antimony telluride (Sb₂Te₃)-based two-dimensional films and micro-pillar arrays are fabricated by electrochemical deposition from electrolytes containing SbO⁺ and HTeO₂⁺ on Si wafer-based Pt electrode and multi-channel glass templates, respectively. The results indicate that the addition of tartaric acid increases the solubility of SbO⁺ in acidic solution. The compositions of deposits depend on the electrolyte concentration, and the micro morphologies rely on the reduction potential. Regarding the electrolyte containing 8 mM of SbO⁺ and 12 mM of HTeO₂⁺, the grain size increases and the density of films decreases as the deposition potential shifts from −100 mV to −400 mV. Sb₂Te₃ film with nominal composition and dense morphology can be obtained by using a deposition potential of −300 mV. However, this condition is not suitable for the deposition of Sb₂Te₃ micro-pillar arrays on the multi-channel glass templates because of its drastic concentration polarization. Nevertheless, it is found that the pulsed voltage deposition is an effective way to solve this problem. A deposition potential of −280 mV and a dissolve potential of 500 mV were selected, and the deposition of micro-pillars in a large aspect ratio and at high density can be realized. The deposition technology can be further applied in the fabrication of micro-TEGs with large output voltage and power.

Zero Thermal Expansion and Semiconducting Properties in PbTiO3-Bi(Co, Ti)O3 Ferroelectric Solid Solutions

Zero thermal expansion (ZTE) behavior is rare but important for both fundamental studies and practical applications of functional materials. Until now, most available ZTE materials are either electrical insulating oxides or conductive metallic compounds. Very few ZTE materials exhibit the semiconductor feature. Here we report a ZTE in a semiconducting ferroelectric of 0.6PbTiO₃-0.4Bi(Co_0.55Ti_0.45)O_3-δ. Its unit cell volume exhibits a negligible change over a broad temperature range from room temperature to 500 °C. The ZTE is supposed to be correlated with the spontaneous volume ferroelectronstriction. Intriguingly, the present ZTE material also exhibits the semiconducting characteristic accompanied by negative temperature coefficient of resistance. The mechanism of electric conduction is attributed to the electronic hopping from one ion (Ti³⁺) to another (Ti⁴⁺). The semiconductor nature has also been confirmed by the noticeable visible-light absorption with the relatively lower band gap (E_g) value of 1.5 eV, while the ferroelectric property can be well-maintained with large polarization. The first-principles calculations reveal that the drastically narrowed E_g is related to the Co-Ti substitution. The present multifunctional material containing ZTE, semiconducting, and ferroelectric properties is suggested to enable new applications such as the substrate for solar conversion devices.

Towards high refrigeration capability: the controllable structure of hierarchical Bi0.5Sb1.5Te3 flakes on a metal electrode

A high refrigeration capability is achieved in hierarchical Bi_0.5Sb_1.5Te₃ film composed of tens of cactus like flakes.

Thermal conductivity of a single Bi₀.₅Sb₁.₅Te₃ single-crystalline nanowire

Single-crystalline Bi₀.₅Sb₁.₅Te₃ nanowires were fabricated by a template-assisted pulsed electrodeposition technique; the thermal conductivity of a single Bi₀.₅Sb₁.₅Te₃ nanowire of different diameters was characterized through a self-heating 3 ω method. The temperature-dependent resistance measurements prove the semiconductor behavior of the nanowires. The extremely low thermal conductivity of the nanowires was found compared with the corresponding bulk, and the Umklapp peaks shift to a higher temperature as the decreasing nanowire's diameter decreases, which qualitatively agrees with the theoretical calculations based on the Callaway model. The boundary scattering plays an important role in the reduction of the thermal conductivity and in the shift of the Umklapp peak of the Bi₀.₅Sb₁.₅Te₃ nanowires.

Optimization of the electrodeposition process of high-performance bismuth antimony telluride compounds for thermoelectric applications

High-quality films of bismuth antimony telluride were synthesized by electrodeposition from nitric acid electroplating baths. The influence of a surfactant, sodium ligninsulfonate, on the structure, morphology, stoichiometry, and homogeneity of the deposited films has been investigated. It was found that addition of this particular surfactant significantly improved the microstructural properties as well as homogeneity of the films with a significant improvement in the thermoelectric properties over those deposited in the absence of surfactant. A detailed microprobe analysis of the deposited films yielded a stoichiometric composition of Bi(0.35)Sb(1.33)Te(3) for the films electrodeposited in the absence of surfactant and a stoichiometry of Bi(0.32)Sb(1.33)Te(3) for films deposited in the presence of surfactant.