Thermoelectric materials convert heat into electricity and vice versa, offering potential applications in energy harvesting and solid-state cooling. Among these materials, nanowires have emerged as promising candidates due to their unique properties at the nanoscale. The thermoelectric performance of a material is quantified by the dimensionless figure of merit, ZT, which depends on the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature. Nanowires enhance ZT by reducing thermal conductivity through phonon scattering while maintaining or improving electrical properties.

Diameter-dependent phonon scattering plays a critical role in nanowire thermoelectrics. As the diameter of a nanowire decreases, the increased surface-to-volume ratio enhances boundary scattering of phonons, which carry heat. For silicon nanowires, experiments show that thermal conductivity can be reduced by over two orders of magnitude when the diameter is reduced from approximately 100 nm to around 20 nm. This reduction occurs because phonons with mean free paths longer than the nanowire diameter are effectively scattered. Similarly, bismuth antimony telluride (BiSbTe) nanowires exhibit suppressed thermal conductivity due to enhanced phonon-boundary scattering at diameters below 50 nm. The diameter-dependent suppression of thermal conductivity is a key mechanism for improving ZT in nanowires without degrading electronic transport.

Two primary synthesis methods for thermoelectric nanowires are electrodeposition and vapor-liquid-solid (VLS) growth. Electrodeposition offers a low-cost, scalable approach for producing nanowires with controlled composition and morphology. For example, BiSbTe nanowires can be electrodeposited into porous templates, allowing precise diameter control by adjusting the template pore size. The process involves applying an electric potential to reduce metal ions in solution, forming nanowires within the template pores. The composition can be tuned by adjusting the electrolyte composition and deposition parameters, enabling optimization of thermoelectric properties.

VLS growth, on the other hand, is a high-precision method for producing single-crystalline nanowires with well-defined diameters. In this process, a metal catalyst particle, such as gold, forms a liquid alloy with the nanowire material at elevated temperatures. The nanowire grows as the precursor material precipitates from the supersaturated liquid droplet. Silicon nanowires grown via VLS exhibit excellent crystallinity and diameter uniformity, which are crucial for studying diameter-dependent thermoelectric effects. The VLS method also allows doping control during growth, enabling optimization of electrical conductivity and Seebeck coefficient.

Measuring the thermoelectric properties of single nanowires is essential for understanding their performance without the averaging effects present in bulk composites. Single-nanowire ZT measurements require specialized techniques to independently determine the Seebeck coefficient, electrical conductivity, and thermal conductivity. Microfabricated devices with integrated heaters and thermometers enable precise temperature control and measurement across individual nanowires. For instance, suspended platforms with four-probe configurations allow simultaneous electrical and thermal characterization while minimizing heat loss to the substrate.

Experimental studies on single silicon nanowires have demonstrated ZT values exceeding 0.6 at room temperature for diameters around 20 nm, a significant improvement over bulk silicon. The enhancement arises primarily from reduced thermal conductivity while maintaining reasonable electrical properties. Similarly, single BiSbTe nanowires exhibit ZT values above 1.0 due to their inherently low thermal conductivity and high Seebeck coefficient. These measurements confirm that nanowire geometry and diameter control are effective strategies for enhancing thermoelectric performance.

Challenges remain in optimizing nanowire-based thermoelectric materials for practical applications. Contact resistance between nanowires and electrodes can degrade device performance, requiring careful interface engineering. Long-term stability under thermal cycling is another concern, particularly for materials prone to oxidation or structural degradation. Additionally, scalable assembly methods are needed to integrate nanowires into functional devices without compromising their individual properties.

Future research directions include exploring new nanowire compositions, such as heterostructured or alloyed nanowires, to further decouple thermal and electronic transport. Advanced synthesis techniques, such as selective area growth or strain engineering, may enable additional control over phonon and electron dynamics. Computational modeling can guide the design of nanowires with optimized ZT by predicting the effects of diameter, doping, and interface scattering.

In summary, nanowire-based thermoelectric materials leverage diameter-dependent phonon scattering to achieve high ZT values. Electrodeposition and VLS growth provide versatile synthesis routes, while single-nanowire measurements reveal the intrinsic thermoelectric properties. Continued advancements in synthesis, characterization, and device integration will be crucial for realizing the full potential of these materials in energy conversion applications.