h1

TY  - THES
AU  - Wu, Riga
TI  - The role of nanostructure on carrier transport in thermoelectric materials addressed by focused ion beam assisted device fabrication
PB  - RWTH Aachen University
VL  - Dissertation
CY  - Aachen
M1  - RWTH-2025-03936
SP  - 1 Online-Ressource : Illustrationen
PY  - 2025
N1  - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
N1  - Dissertation, RWTH Aachen University, 2025
AB  - Nanostructures, including grain boundaries (GBs), ion segregation, and precipitates, have a profound impact on carrier transport and material properties. The study of these nanostructures and their influence is an important and burgeoning topic. Nevertheless, the majority of studies on GB-related properties primarily emphasize the average grain size, neglecting the significant role played by the complex structure and composition of GBs in determining transport properties. In addition, doping is a crucial method use to tailor material properties, resulting in ion segregation at boundaries and precipitates. These kinds of nanostructures significantly impact carrier concentration, mobility, and conductivity, ultimately determining the thermoelectric (TE) performance. However, the mechanisms underlying ion segregation and precipitates remain unclear due to the limitations in the current macroscopic experiments. Consequently, characterizing the structure and composition of individual nanostructures becomes a prerequisite for comprehending and engineering these materials. To achieve this goal, this thesis focuses on studying the effect of individual nanostructures on the electrical properties of TE materials using a novel technique based on focus ion beam(FIB)-assisted device fabrication. This novel technique is a flexible and comprehensive approach, combining FIB, electron backscatter diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDX), atom probe tomography (APT), and a physical property measurement system (PPMS), in order to investigate the impact of nanostructures, including GBs, twin boundaries, and precipitates, on charge carrier scattering. This correlative study enables the determination of the local microstructure, composition, and transport properties of individual nanostructures. In Chapter IV, the thesis focuses on studying the impact of GBs in Ag-doped PbTe. This investigation reveals the electronic characteristic and chemical composition of individual GBs, underscoring how compositional heterogeneity influences electrical conductivity. Notably, it is found out that Ag segregation at the GBs enhances electrical conductivity, while the second phase, formed by Ag replacing Pb atoms, acts as a scattering center. Chapter V explores the influence of the misorientation angles of individual GBs on charge carrier scattering in slightly Ag-dope PbTe. The study demonstrates that charge carriers experience significantly stronger scattering at high-angle GBs (HAGBs) than at low-angle GBs (LAGBs). The APT measurements indicate a higher fraction of trapping states at HAGBs, along with a complete breakdown of the meta valent bond, which contributes to the increased scattering of charge carriers. Chapter VI investigates the impact of twin boundaries on the electrical transport properties in Bi2T e2.7Se0.3. The study discovers that the presence of two parallel twin boundaries enhances mobility and conductivity. This enhancement is attributed to the coherent interface and uniform composition distribution at twin boundaries. This finding indicates that the twin boundaries do not scatter carriers in this material, providing insights for designing high-performance TE materials. In the last chapter, this thesis examines telurrium(Te)-based materials doped with antimony (Sb), bismuth (Bi), and arsenic (As). The Energy-Dispersive X-ray Spectroscopy(EDX) and APT measurements provide clear evidence for the poor solubility of dopants in the host material, leading to the formation of telluride precipitates. The local transport measurements of pre-characterized precipitates show that the dramatically increased electrical conductivity is due to the incorporation of precipitates, providing guidance for enhancing the performance of single element thermoelectric. The APT characterization reveals the meta valent bonding nature of the precipitates, further aiding in material design. The novel FIB-assisted device fabrication technique presented in this thesis provides a new approach to study specific nanostructures in bulk materials, breaking previous experimental limitations. The findings on charge transport across the GBs and precipitate have implications for various applications, including TEs, memory materials, and mid-infrared devices. The versatility of this technique allows for its adaptation to study other solid materials and explores extraordinary physical properties in newly synthesized materials. Future directions involve using this technique for solar cell grain boundary research and various electrical measurements. This method enables detailed analysis of how individual defects affect charge carrier transport in semiconductors and expands nanostructure research in material science.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2025-03936
UR  - https://publications.rwth-aachen.de/record/1010217
ER  -