Nanostructure-Assisted Phonon Scattering in Lead-Free Thermoelectric Materials: A TEM Investigation of the SnTe System Fengyuan Shi 1 , Shih-Han Lo 1 , Gangjian Tan 2 , Li-Dong Zhao 2 , Mercouri G. Kanatzidis 2,3 and Vinayak Dravid 1 1. Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States 2. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States 3. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States We have used HRTEM and STEM EDX to determine the important roles of endotaxial nanostructures in a lead-free thermoelectric (TE) material, SnCd 0.03 Te, with 2% CdS and 2% ZnS; where maximum ZTs of ~1.3 and ~1.1 are achieved at 873 K, respectively. TE materials convert wasted heat into electric energy. The conversion efficiency of TE materials is characterized by a dimensionless figure of merit ZT = S 2  T/ κ , where S is the Seeback coefficient, σ is the electrical conductivity, T is the absolute temperature and к = к е + κ ι is the total thermal conductivity, including both electronic (к е ) and lattice (κ ι ) contributions. The record-high ZT values resulting from nanoscale precipitates in p-type SnTe make SnTe-based materials ideal candidate for p-type lead chalcogenides [1-3] for high temperature thermoelectric power generation. Figure 1 (a) and (b) show TEM images of nanostructured SnCd 003 Te with 2% CdS and 2% ZnS, respectively. High-density nanoscale precipitates are found in both samples, presented by dark contrast in the images. These precipitates have two orthogonal symmetry variants, as schematically depicted in the inset in the upper right corner of Figure 1 (a). The SAD pattern along [001] orientation with an aperture that captures both the matrix and the precipitates shows only one set of Bragg reflection spots, indicating no notable difference in lattice spacing between the matrix and the precipitate because of their small lattice mismatch. We have also employed statistical analysis to obtain the size distribution of SnCd 0.03 Te-2%CdS and SnCd 0.03 Te-2%ZnS samples along [001] orientation based on Figure 1(a) and (b), respectively. In Figure 1 (c), the histograms of these two samples show notable difference. We found that for SnCd 0.03 Te-2%CdS, most nanoscale precipitates range from 3 to 4 nm in size, but for SnCd 0.03 Te-2%ZnS, the size of the majority precipitate falls within 4.5 to 7 nm range. Comparing ZnS and CdS cases, the larger size of ZnS nanostructures reduces interface area to the matrix, which in relative terms may result in reduced phonon scattering. Figure 2 shows the HRTEM images and STEM EDX results of SnCd 0.03 Te with 2% CdS and 2% ZnS, respectively. An HRTEM image in Figure 2 (a) depicts a coherent nanoscale CdS precipitate embedded in the SnCd 0.03 Te matrix along [001] orientation. No line defects or discontinuities are observed at the matrix/precipitate interface, confirming coherently strained nanoscale endotaxial precipitates. The STEM-EDS results in Figure 2 (b) indicate that the nanoscale precipitates are rich in Cd and S, but the matrix is devoid of sulfur. Similar analysis were applied to SnCd 0.03 Te with 2% ZnS. Zn and S signals present only in the nanoscale precipitates, but not in the matrix. High ZT values of SnTe system with CdS and ZnS nanoscale precipitates are attributed to the enhanced phonon scattering at the nanoscale precipitates/matrix interfaces, which decreases the lattice conductivity κ ι without degrading σ. The ZT = 1.3 in SnCd 0.03 Te-2%CdS is the record-high value for 438 doi:10.1017/S1431927614003912 Microsc. Microanal. 20 (Suppl 3), 2014 © Microscopy Society of America 2014