Quantum dot thermoelectric materials represent a significant advancement in the field of energy conversion, leveraging unique nanoscale phenomena to enhance thermoelectric performance. These materials exploit quantum confinement effects to manipulate charge carriers and phonons, leading to improved thermoelectric efficiency. Key strategies include carrier filtering, phonon scattering, and nanostructuring, all of which contribute to increasing the dimensionless figure of merit, ZT. Among the most studied systems are PbTe, SiGe, and skutterudite-based quantum dot composites, each offering distinct advantages for thermoelectric applications.

Carrier filtering is a critical mechanism in quantum dot thermoelectric materials. By embedding quantum dots within a bulk matrix, energy barriers are created that selectively scatter low-energy carriers while allowing high-energy carriers to contribute to electrical conductivity. This selective scattering increases the Seebeck coefficient without significantly degrading electrical conductivity, thereby enhancing the power factor. In PbTe-based systems, quantum dots introduce potential fluctuations that filter carriers effectively, leading to ZT values exceeding 2.0 at elevated temperatures. Similarly, SiGe quantum dot superlattices demonstrate improved thermoelectric performance due to energy-dependent carrier scattering, which optimizes the trade-off between electrical and thermal transport properties.

Phonon scattering is another essential factor in improving ZT. Quantum dots act as nanoscale phonon scattering centers, disrupting the propagation of heat-carrying phonons and reducing lattice thermal conductivity. In skutterudite-based materials, the incorporation of quantum dots introduces additional interfaces and defects that scatter phonons across a wide range of frequencies. This effect is particularly pronounced in nanostructured skutterudites, where the combination of intrinsic cage-like structures and extrinsic quantum dot inclusions results in ultralow thermal conductivity. PbTe quantum dot composites also benefit from strong phonon scattering, with lattice thermal conductivity values approaching the theoretical minimum due to the high density of interfaces and strain fields.

Nanostructuring plays a pivotal role in optimizing thermoelectric performance. By engineering quantum dot size, distribution, and composition, it is possible to tailor both electronic and thermal transport properties. In PbTe, precise control over quantum dot spacing and alignment creates a phonon glass-electron crystal (PGEC) behavior, where phonons are strongly scattered while electrons move relatively unimpeded. SiGe quantum dot systems leverage the contrast in acoustic impedance between the dots and the matrix to enhance phonon scattering, further reducing thermal conductivity. Skutterudites, when combined with quantum dots, exhibit a hierarchical scattering mechanism that targets phonons across multiple length scales, from atomic-scale defects to mesoscale grain boundaries.

Energy harvesting applications of quantum dot thermoelectric materials are particularly promising for waste heat recovery and portable power generation. PbTe-based quantum dot materials are well-suited for mid-to-high-temperature applications, such as automotive exhaust systems and industrial processes, where they can convert waste heat into usable electricity with efficiencies surpassing conventional bulk thermoelectrics. SiGe quantum dot systems, with their high thermal stability and compatibility with silicon technology, are ideal for aerospace and satellite power generation, where reliability and weight savings are critical. Skutterudite quantum dot composites, owing to their low thermal conductivity and good mechanical properties, are being explored for wearable thermoelectric generators and self-powered sensors.

The interplay between carrier filtering, phonon scattering, and nanostructuring in quantum dot thermoelectric materials underscores their potential to revolutionize energy conversion technologies. Continued advancements in synthesis techniques, such as colloidal quantum dot assembly and epitaxial growth, will further enhance the performance and scalability of these materials. By addressing challenges related to interfacial defects and long-term stability, quantum dot thermoelectrics could soon become a mainstream solution for sustainable energy harvesting.

In summary, quantum dot thermoelectric materials offer a versatile platform for achieving high ZT values through tailored carrier and phonon engineering. PbTe, SiGe, and skutterudite-based systems exemplify the progress made in this field, demonstrating the transformative potential of nanostructuring for energy applications. As research progresses, these materials are poised to play a pivotal role in advancing thermoelectric technology for a wide range of practical applications.