Defect Engineering in Bi2SeO2 by Tuning Synthesis Conditions

My presentation for oral talk on "Defect Engineering in Bi2SeO2 by Tuning Synthesis Conditions" at 第17回日本フラックス成長研究発表会

Transcript

1. 第17回日本フラックス成長研究発表会 合成条件の調整によるBi2 SeO2 の欠陥制御 Defect Engineering in Bi2 SeO2 by Tuning Synthesis Conditions Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp ノビツキー アンドレイ Presenter: Novitskii Andrei 森 孝雄 Supervisor: Mori Takao ナノアーキテクトニクス材料研究センター WPI-MANA 物質・材料研究機構 NIMS
2. Bi2 SeO2 : A rising star for semiconductor devices 2 第17回日本フラックス成長研究発表会 X. Ding, et al., Matter 5 (2022) 4274–4314 Novel quasi-2D material: • n-type semiconductor • bandgap of ~0.85 eV • layered crystal structure • adjustable electronic structure • ultra-high electron mobility • persistent quantum oscillations • unique defects • strong spin-orbital coupling • excellent stability • marvelous optoelectronic performance • promising thermoelectric properties Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp C. Chen, et al., Sci. Adv. 4 (2018)
3. Thermoelectric effect 3 第17回日本フラックス成長研究発表会 Thermoelectric figure-of-merit: α is the Seebeck coefficient (thermopower), σ is the electrical conductivity, κel and κlat are the electronic and lattice thermal conductivities, T is the temperature. At a given carrier concentration n: Strategy to improve the zT: • optimization of n • maximization of μw /κlat ratio Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp P. Gorai, et al., Nat. Rev. Mat. 2 (2017) 17053 𝑧𝑇 = 𝛼2𝜎 𝜅𝑒𝑙 + 𝜅𝑙𝑎𝑡 𝑇 𝑧𝑇𝑚𝑎𝑥 ∝ 𝜇𝑤 𝜅𝑙𝑎𝑡
4. Phase boundary mapping and defect chemistry 4 • the predominant defect - selenium vacancy (VSe ) - is the source of n-type conductivity in Bi2 SeO2 • Bi2 SeO2 cannot be doped p-type due to extremely low formation energy of VSe • VSe can vary between 1-2 orders of magnitude depending on the thermodynamic conditions 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
5. Synthesis process 5 mixing and cold-pressing spark plasma sintering hand grinding 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp 5N powders of Bi, Bi2 O3 , and Se solid-state reaction material characterization: • XRD, SEM, EDS • electrical transport • thermal transport • Hall measurements SPS 2Bi2 O3 + 2Bi + 3Se = 3Bi2 SeO2
6. X-Ray diffraction patterns and SEM 6 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp OS is one step solid-state reaction TS is two-step solid-state reaction
7. Electron concentration and Seebeck coefficient 7 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp The higher the synthesis temperature: • the higher the electron concentration n due to the formation of VSe • the higher the weighted mobility μw due to grain size increase. OS is one step solid-state reaction TS is two-step solid-state reaction
8. Thermoelectric properties 8 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp The higher the synthesis temperature: • the higher the electrical conductivity due to increase in n and μ • the lattice thermal conductivity. OS is one step solid-state reaction TS is two-step solid-state reaction
9. Conclusions 9 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp We combine theory and experiment to demonstrate that the TE performance in n-type Bi2 SeO2 can be improved by systematically controlling defect formation with synthesis conditions: • The electron concentration increases by elevating synthesis temperature (TSSR ), leading to an increase in α2σ power factor. • Furthermore, samples with higher VSe concentrations exhibit suppressed thermal conductivity due to point defect-induced phonon scattering and microstructural changes. • Cumulatively, defect engineering yields a fourfold improvement of the zT in self-doped Bi2 SeO2 , ultimately reaching 0.25 at 773 K. • This study underscores TSSR as a rational, accessible experimental parameter for controlling defect formation in Bi2 SeO2 . OS is one step solid-state reaction TS is two-step solid-state reaction
10. Acknowledgements 10 The study was carried out with financial support from the JST Mirai JPMJMI19A1. 第17回日本フラックス成長研究発表会 先生: 森 孝雄 Supervisor: Mori Takao Prashun Gorai Michael Toriyama G. Jeffrey Snyder Illia Serhiienko Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
11. 第17回日本フラックス成長研究発表会 ノビツキー アンドレイ Presenter: Novitskii Andrei 森 孝雄 Supervisor: Mori Takao ナノアーキテクトニクス材料研究センター WPI-MANA 物質・材料研究機構 NIMS ご清聴ありがとうございました Thank you for your attention Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
12. Supporting information Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
13. Microstructure evolution 13 • with the increase in the temperature of the solid-state reaction, the grain size significantly increases. • along with that, the proportion of platelet-like grains aligned along one axis also increases, which is in good agreement with the results of XRD analysis for bulk samples. 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
14. Microstructure evolution 14 *the displayed data is for the samples obtained by two-step solid-state reaction • with the increase in the temperature of the solid-state reaction, the grain size significantly increases. • along with that, it seems (from SEM images) that the proportion of platelet-like grains aligned along one axis also increases, which is in good agreement with the results of XRD analysis for bulk samples. • changes in microstructure, grain size, and preferential grain orientation are also clearly visible in the SEM images of the sample polished surfaces. 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
15. Thermoelectric properties 15 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
16. Electron concentration and Seebeck coefficient 16 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp The higher the synthesis temperature: • the higher the electron concentration n due to the formation of VSe • the higher the weighted mobility μw due to grain size increase. OS is one step solid-state reaction TS is two-step solid-state reaction
17. Pisarenko plot 17 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp OS is one step solid-state reaction TS is two-step solid-state reaction
18. X-Ray diffraction patterns and SEM 18 第17回日本フラックス成長研究発表会 Thermoelectrics A. Shakouri, Annu. Rev. Mater. Res. 41 (2011) 399–431. Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
19. Jonker plot explained 19 Jonker plot is based on Pisarenko formula: 𝛼 = 𝑘𝐵 𝑒 5 2 + 𝑟 + ln 2 2𝜋𝑚∗𝑘𝐵𝑇 Τ 3 2 ℎ3𝑛 , where r is the scattering factor (r = –1/2 for acoustic phonon scattering). In general, this formula is accurate enough up to chemical potential η of ~1 and can be rewritten to show α ∝ lnσ relationship: 𝛼 = 𝑘𝐵 𝑒 5 2 + 𝑟 + ln 2 2𝜋𝑚∗𝑘𝐵𝑇 Τ 3 2𝑒𝜇 ℎ3 − 𝑘𝐵 𝑒 ln 𝑒𝑛𝜇 . A.F. Ioffe, Physics of Semiconductors, Infosearch Ltd., London, 1960 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
20. Why is there annealing after SPS? 20 Based on the electrical transport properties measurements, it appears that the samples after SPS are not in an equilibrium state. Therefore, all the samples after sintering underwent a relatively short annealing at 803K for 6 hours (under vacuum). After annealing, the samples and their properties can be considered stable and reproducible. 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
21. Effect of solid-state reaction time 21 In an attempt to estimate and/or isolate the influence of microstructure, an additional experiment was conducted where a Bi2 SeO2 sample was synthesized using the same two-step solid-state reaction but for a longer duration (573K/12h + 773K/10h instead of 573K/6h + 773K/12h). Based on the behaviour of material shrinkage during sintering, it seemed that the grain size was larger. However, this was not evident via SEM analysis. Interestingly, the electrical transport properties of this sample are identical to those of the sample obtained at a higher temperature and for a shorter duration (573K/6h + 873K/12h). 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
22. Quartz tubes after solid-state reaction 22 773 K 873 K 973 K 1073 K 1173 K clear sign of Se evaporation 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
23. Actual composition from EDS 23 SSR temperature (K) Nominal composition Actual composition Se:Bi ratio Bi:O ratio 773 Bi2 SeO2 Bi2.04 Se0.98 O1.98 0.48 1.03 873 Bi2.04 Se0.98 O1.98 0.48 1.03 973 Bi2.07 Se0.99 O1.94 0.48 1.07 1073 Bi2.02 Se0.98 O2.00 0.49 1.01 1173 Bi2.03 Se0.99 O1.98 0.49 1.03 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
24. EDS mapping 24 773 K *here different contrast attributed to the different orientations of the grains (electron challenging mode) 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
25. EDS mapping 25 873 K *here different contrast attributed to the different orientations of the grains (electron challenging mode) 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
26. EDS mapping 26 973 K *here different contrast attributed to the different orientations of the grains (electron challenging mode) 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
27. EDS mapping 27 1073 K 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
28. EDS mapping 28 1173 K 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp
29. Se-poor / Se-rich samples 29 Bi2 Se0.95 O2 Bi:Se ratio is around 0.45 for Se-poor sample Bi:Se ratio is around 0.48 for ‘stochiometric’ sample Bi:Se ratio is around 0.49 for Se-rich sample 第17回日本フラックス成長研究発表会 Thermal Energy Materials Group NOVITSKII.Andrei@nims.go.jp