Nanowires have emerged as a promising platform for thermoelectric applications due to their unique ability to decouple electronic and phononic transport properties. By reducing dimensionality, nanowires enable enhanced phonon scattering while maintaining reasonable electrical conductivity, leading to improved thermoelectric performance. Key materials such as silicon-germanium (SiGe) alloys and bismuth telluride (Bi₂Te₃) have demonstrated significant potential in nanowire form, with strategies focusing on engineering phonon scattering mechanisms and optimizing the thermoelectric figure of merit (ZT).

The thermoelectric performance of a material is quantified by ZT, defined as ZT = (S²σT)/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity. In bulk materials, these parameters are interdependent, making optimization challenging. Nanowires circumvent this limitation by introducing boundary scattering and quantum confinement effects that selectively suppress lattice thermal conductivity (κ_l) without severely degrading electronic transport.

Phonon scattering in nanowires is primarily governed by their diameter, surface roughness, and interfacial defects. For SiGe nanowires, studies have shown that reducing the diameter below 100 nm leads to a substantial decrease in κ_l due to increased boundary scattering. Surface roughness further enhances this effect by introducing additional scattering centers for mid- and high-frequency phonons. Experimental measurements on SiGe nanowires with diameters around 50 nm report κ_l values as low as 1.5 W/mK, a reduction of over 80% compared to bulk SiGe alloys. This reduction is attributed to the dominant role of boundary scattering, which restricts phonon mean free paths.

Bi₂Te₃ nanowires exhibit similar benefits, with the added advantage of intrinsically low κ_l due to their layered crystal structure. When synthesized with diameters below 50 nm, Bi₂Te₃ nanowires achieve κ_l values approaching 0.8 W/mK. The anisotropic nature of Bi₂Te₃ also allows for preferential phonon scattering along certain crystallographic directions, further enhancing ZT. For instance, nanowires grown along the [110] direction demonstrate higher ZT values compared to those along [001], owing to the directional suppression of phonon transport.

Synthesis methods play a critical role in determining the thermoelectric properties of nanowires. Vapor-liquid-solid (VLS) growth is widely employed for SiGe and Bi₂Te₃ nanowires due to its ability to control diameter, composition, and crystallographic orientation. In VLS growth, a metal catalyst (e.g., gold for SiGe or nickel for Bi₂Te₃) facilitates nanowire nucleation and elongation. By adjusting growth parameters such as temperature, precursor flux, and catalyst size, researchers can tailor nanowire morphology and defect density. For example, SiGe nanowires grown at 450°C with a germanium content of 20% exhibit optimal carrier mobility and Seebeck coefficients, while maintaining low thermal conductivity.

Another effective approach is the introduction of intentional defects, such as twin boundaries or stacking faults, to scatter phonons. In SiGe nanowires, twin boundaries act as phonon barriers, reducing κ_l without significantly impairing electronic transport. Similarly, Bi₂Te₃ nanowires with controlled defect densities exhibit ZT values exceeding 1.0 at room temperature, a marked improvement over bulk counterparts. The precise placement of defects can be achieved through post-growth annealing or by modulating growth conditions during synthesis.

Alloying is another strategy to enhance phonon scattering in nanowires. For SiGe nanowires, varying the germanium content alters both electronic and thermal transport properties. Increasing germanium concentration reduces κ_l due to mass fluctuation scattering, but excessive alloying can degrade electrical conductivity. Optimal compositions typically range between 10% and 30% germanium, balancing these competing effects. In Bi₂Te₃-based nanowires, alloying with antimony (Sb) or selenium (Se) further suppresses κ_l while maintaining high power factors (S²σ). For instance, Bi₂Te₂.7Se0.3 nanowires demonstrate ZT values of 1.2 at 300 K, outperforming pure Bi₂Te₃ nanowires.

Surface passivation is critical for maintaining high electrical conductivity in nanowires. Unpassivated surfaces often exhibit carrier depletion due to dangling bonds or oxidation, which degrades σ. For SiGe nanowires, hydrogen passivation has been shown to restore near-bulk carrier mobilities. In Bi₂Te₃ nanowires, organic ligands or thin dielectric coatings (e.g., Al₂O₃) preserve surface electronic states while minimizing additional thermal conductance.

The integration of nanowires into functional devices presents additional challenges. Contact resistance between nanowires and electrodes can significantly impact overall device performance. For SiGe nanowires, nickel silicide contacts provide low-resistance pathways, whereas Bi₂Te₃ nanowires benefit from optimized gold or platinum contacts. Thermal interface resistance must also be minimized to ensure accurate ZT measurement and efficient heat transfer in applications.

Recent advances in scalable synthesis techniques, such as electrospinning or template-assisted growth, offer pathways for large-scale production of thermoelectric nanowires. These methods enable the fabrication of aligned nanowire arrays with uniform properties, essential for practical devices. For example, electrospun Bi₂Te₃ nanowire mats have demonstrated ZT values comparable to single nanowires, highlighting their potential for flexible thermoelectric modules.

Future directions include the exploration of core-shell nanowire architectures, where a thermoelectric core is encapsulated in a shell with tailored thermal and electronic properties. Such structures could further decouple phonon and electron transport, pushing ZT beyond current limits. Additionally, the integration of machine learning for growth parameter optimization could accelerate the discovery of novel nanowire compositions and morphologies.

In summary, nanowire synthesis optimized for thermoelectric performance relies on precise control of phonon scattering mechanisms through dimensional confinement, defect engineering, and surface modification. SiGe and Bi₂Te₃ nanowires exemplify the potential of this approach, with demonstrated ZT improvements over bulk materials. Continued advancements in synthesis and characterization will be crucial for realizing the full potential of nanowire-based thermoelectrics in energy harvesting and cooling applications.