Supporting Information Extraordinary Off-stoichiometric Bismuth Telluride for Enhanced n- type Thermoelectric Power Factor Kunsu Park,,,# Kyunghan Ahn,,# Joonil Cha,, Sanghwa Lee,, Sue In Chae,, Sung- Pyo Cho, Siheon Ryee, Jino Im, Jaeki Lee, Su-Dong Park, Myung Joon Han, In Chung, *,, Taeghwan Hyeon *,, Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea. School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, Seoul 08826, Republic of Korea. National Center for Inter-University Research Facilities, Seoul National University, Seoul 08826, Republic of Korea. Department of Physics and KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea. Thin Film Materials Research Center, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea. Creative Research Center, Creative and Fundamental Research Division, Korea Electrotechnology Research Institute (KERI), Changwon 51543, Gyeongsangnam-do, Republic of Korea. # Equally contributed to this work. *Corresponding author. E-mail: thyeon@snu.ac.kr (T.H.H); inchung@snu.ac.kr (I.C.) S1
List of Contents 1. Figures S1-S18, Table S1 2. References S2
Figure S1. A phase diagram of Bi-Te binary system. 1 The blue strip represents the region of phase equilibrium near the molar ratio of Bi:Te = 2:3. This work expanded the solubility limit of Bi to Te from 60.2 to 61.2 atomic percentage, depicted in orange, which cannot be achieved by other synthetic methods. S3
Table S1. Summary of the compositions of BT nanotube, KBT nanotube, bulk BT, and bulk KBT by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Samples Normalized ICP K Bi Te Composition BT nanotube - 0.389 0.611 Bi 2 Te 3.14 KBT nanotube 0.014 0.383 0.603 K 0.0722 Bi 2 Te 3.14 bulk BT - 0.389 0.611 Bi 2 Te 3.14 bulk KBT 0.011 0.382 0.607 K 0.0578 Bi 2 Te 3.18 S4
Figure S2. Typical (a) scanning and (b and c) transmision electron microscopy (SEM and TEM) images of BT nanotubes. S5
Figure S3. Scanning TEM (STEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping on a BT nanotube showing its hollow nature. (a) A STEM image of BT nanotube. (b) STEM-EDS result taken along the yellow line in (a) across the BT nanotube confirming hollow nature of BT nanotube. (c) STEM-EDS shows the molar ratio of Bi to Te to be approximately 2:3. S6
Figure S4. Fourier transform infrared spectroscopy (FT-IR) spectra of BT nanotube in comparison with polyvinylpyrrolidone (PVP) showing the presensce of PVP in BT nanotube sample. S7
Figure S5. Thermogravimetric analysis (TGA) of bulk KBT as well as BT and KBT nanotube samples up to 800 K at a rate of 10 K min 1 under Ar flow. The percent weight loss decreases after reaction with KOH. S8
Figure S6. Powder X-ray diffraction (XRD) patterns of BT and KBT nanotube samples in comparison to Bi 2 Te 3, Te, and K 2 Te references. S9
Figure S7. FT mid-infrared spectrum of bulk KBT shows no absorption peaks, confirming nearly complete removal of organic residues after spark plasma sintering (SPS). S10
Figure S8. The optimized crystal structure of Bi 35 Te 55, a supercell of Bi 2 Te 3.14 with Te Bi defect. The red and yellow spheres represent Te and Bi atoms, respectively. Red dotted circle indicates Te Bi antisite, which is a Te atom occupying a Bi site. S11
Figure S9. The solubility of potassium as a function of chemical potential of potassium (μ K ) in units of ev. The maximum solubility of potassium is achieved at µ K ~ 1.75 ev. S12
Figure S10. A cross-sectional high-angle annular dark-field (HAADF) STEM image of BT down the [-100] axis clearly shows typical lamellar structure of Bi 2 Te 3 that is composed of five atomic layers and van der Waals gaps. S13
Figure S11. Elemental mapping of bulk KBT sample examined by STEM-EDS showing a structural motif of (Te Bi Te Bi Te Te Bi Te) atomic arrangement (a terminal atomic layer of Te at the bottom was omitted in the image). An atomic layer of Te replacing that of Bi to form Te Te bonds is clearly shown (white arrow). Bi and Te atoms are in yellow and red colors, respectively. Scale bar, 1 nm. S14
Figure S12. STEM-electron energy loss spectroscopy (EELS) spectra of bulk KBT. (a) The spectrum obtained by pinpointing the interlayer atom (red arrow in (b)) clearly shows a signal for potassium indicated by black arrows (blue line). In contrast, the spectrum by scanning the non-potassium area only shows a signal for carbon. The reference spectra for carbon K edge (1s) and potassium L 23 (2p) edge (KCl crystal) are obtained from EELS Atlas (Gatan Inc. 1983). (b) The EELS spectrum for potassium was collected at the interlayer atom indicated by the red arrow shown in the HAADF-STEM image. S15
Figure S13. HAADF-STEM images of (a) bulk BT and (b) bulk KBT show the increase in interlayer distance from 0.27 nm to 0.32 nm due to potassium incorporation. Calculated Te Te interatomic distances from octahedral environment are (c) 5.7 Å for bulk BT and (d) 6.1 Å for bulk KBT. We consider covalent radius of Te is 1.36 Å and K + ionic radius is 1.38 Å at the octahedral environment, 2 and their structural model for interlayer octahedral space for bulk BT and bulk KBT are shown in (e) and (f), respectively. The increased Te Te interatomic distance of 6.1 Å is sufficient for accepting a K + ion (2 (1.36 Å + 1.38 Å) < 6.1 Å). Red, yellow, and blue dots indicate Te, Bi, and K, respectively. Scale bar, 1 nm. S16
Figure S14. Electron concentration and mobility of bulk BT and KBT as a function of temperature showing increase in (a) electron concentration (n e ) and (b) mobility (µ e ) upon potassium incorporation. S17
Figure S15. Thermal conductivity of Bi 2 Te 3, bulk BT, and bulk KBT perpendicular ( ) and parallel (//) to the pressing direction of SPS showing significant anisotropy in thermal transport of the samples. S18
Figure S16. The cross-sectional high resolution (a) TEM and (b) SEM images of bulk KBT. The stripe pattern shown in the images is the crystal domains stacked along the pressing direction of the SPS process. S19
Figure S17. Comparison of (a) the power factor (PF) and (b) ZT of the present bulk BT and bulk KBT samples with those of the high performance n-type Bi 2 Te 3 -based materials reported previously. All samples are polycrystalline unless noted otherwise. S20
References (1) Massalski, T. B.; Okamoto, H.; Subramanian, P. R.; Kacprzak, L. Binary Alloy Phase Diagrams; ASM International: Materials Park, OH, 1990. (2) Shannon, R. D. Acta Cryst. A 1976, 32, 751-767. (3) Yan, X. A.; Poudel, B.; Ma, Y.; Liu, W. S.; Joshi, G.; Wang, H.; Lan, Y. C.; Wang, D. Z.; Chen, G.; Ren, Z. F. Nano Lett. 2010, 10, 3373-3378. (4) Liu, W. S.; Zhang, Q. Y.; Lan, Y. C.; Chen, S.; Yan, X.; Zhang, Q.; Wang, H.; Wang, D. Z.; Chen, G.; Ren, Z. F. Adv. Energy Mater. 2011, 1, 577-587. S21