8.2 Types of nanostructured thermoelectric materials

Nanostructured thermoelectric materials come in various forms, each with unique properties. From nanowires and nanocomposites to quantum dots and 2D materials, these structures offer enhanced control over electron and phonon transport.

By engineering materials at the nanoscale, we can boost thermoelectric performance. Quantum confinement effects, increased phonon scattering, and tailored electronic properties all contribute to improved efficiency in energy conversion devices.

Nanostructured Materials

Nanowires and Nanocomposites

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• Nanowires consist of ultra-thin, elongated structures with diameters typically less than 100 nanometers
• Exhibit unique electrical and thermal properties due to their high surface area to volume ratio
• Fabrication methods include vapor-liquid-solid growth, template-assisted synthesis, and electrospinning
• Nanocomposites combine different materials at the nanoscale to create enhanced thermoelectric properties
• Incorporate nanoparticles or nanowires into a bulk matrix material (silicon-germanium nanocomposites)
• Synergistic effects between components lead to improved thermal and electrical conductivity

Nanoparticle Inclusions and Core-Shell Structures

• Nanoparticle inclusions involve embedding small particles (1-100 nm) into a host material
• Serve as scattering centers for phonons, reducing thermal conductivity without significantly affecting electrical conductivity
• Common nanoparticle materials include metal oxides, silicides, and other semiconductor compounds
• Core-shell nanostructures comprise a central core surrounded by one or more shell layers
• Allow for precise control of electron and phonon transport through interface engineering
• Core and shell materials can be selected to optimize band alignment and phonon scattering (PbTe core with PbS shell)

Quantum-Confined Structures

Superlattices: Engineered Multilayer Systems

• Superlattices consist of alternating layers of different materials with nanoscale thicknesses
• Create periodic potential wells that modify the electronic band structure and phonon dispersion
• Quantum confinement effects arise when layer thicknesses approach the de Broglie wavelength of electrons
• Enhance thermoelectric performance through increased density of states and reduced thermal conductivity
• Fabrication techniques include molecular beam epitaxy and atomic layer deposition
• Common superlattice systems include Si/Ge, Bi2Te3/Sb2Te3, and PbTe/PbSe

Quantum Dot Structures: Discrete Energy Levels

• Quantum dots represent nanoscale semiconductor particles with sizes typically below 10 nm
• Exhibit strong quantum confinement effects in all three spatial dimensions
• Discrete energy levels lead to sharp peaks in the density of states, enhancing the Seebeck coefficient
• Fabrication methods include colloidal synthesis, epitaxial growth, and lithographic patterning
• Integration into thermoelectric devices through embedding in bulk matrices or arranging in ordered arrays
• Tunable electronic properties through size control and material selection (PbTe, InAs, CdSe quantum dots)

Low-Dimensional Materials

2D Materials: Atomically Thin Layers

• 2D materials consist of single or few-layer sheets with atomic-scale thickness
• Exhibit unique electronic and thermal properties due to quantum confinement in one dimension
• Graphene serves as a prototypical 2D material with high electrical conductivity and tunable thermoelectric properties
• Transition metal dichalcogenides (MoS2, WSe2) offer semiconducting behavior and potential for thermoelectric applications
• Fabrication methods include mechanical exfoliation, chemical vapor deposition, and liquid-phase exfoliation
• Integration into thermoelectric devices through stacking, intercalation, or composites with bulk materials

Nanowires and Quantum Dot Structures: 1D and 0D Systems

• Nanowires represent one-dimensional structures with quantum confinement in two dimensions
• Exhibit ballistic transport and reduced phonon scattering compared to bulk materials
• Material systems include silicon, germanium, and III-V semiconductors (InSb, GaAs nanowires)
• Quantum dot structures in low-dimensional configurations offer enhanced control over electronic and thermal properties
• Self-assembled quantum dots can be grown epitaxially on substrates (InAs/GaAs quantum dots)
• Ordered arrays of quantum dots create artificial crystals with tailored band structures
• Combination of nanowires and quantum dots allows for hierarchical nanostructuring and optimized thermoelectric performance