A Facile In-situ Reaction Method for Preparing Flexible Sb2Te3 Thermoelectric Thin Films

22 Inorganic p-type Sb 2 Te 3 flexible thin films with eco-friendly and high thermoelectric 23 performance have attracted wide research interest and potential for commercial 24 applications. This study employs a facile in-situ reaction method to prepare flexible 25 Sb 2 Te 3 thin films by rationally adjusting the synthesized temperature. The prepared 26 thin films show good crystallinity, which enhances the electrical conductivity of 27 ~1440 S cm −1 due to the weakened carrier scattering. Simultaneously, the optimized


Introduction
Thermoelectric (TE) can achieve direct conversion between thermal energy and electrical energy, which have significant applications in power generation and refrigeration [1][2][3][4][5] .With increasing demand for micro-electromechanical systems of chip-sensors, wearable electronics, and implantable electronic devices, the TE flexible thin films (f-TFs) have attracted extensive interesting due to high adaptability to various condition with high TE performance [6][7][8][9][10] .The TE performance of f-TFs can be assessed via power factor (S 2 ) 11 , where  and S represent the electric conductivity and Seebeck coefficient, respectively.Herein,  is defined as σ = nheμ, where nh, e, and μ represent carrier concentration, elementary charge, and carrier mobility, respectively 12,13 .The S can be evaluated by Mott formula 14,15 .And the increase of S can be achieved by the decreased nh and increased effective mass (m * ).And it is a significant challenge to simultaneously increase the S and  due to their coupled relationship.Typically, f-TFs are composed of organic f-TFs and inorganic f-TF 16,17 .
Vieira et al. 29 realized a high  of ~320 S cm −1 and S 2  of ~12.0 μW cm −1 K −2 at 298 K for Sb2Te3 f-TFs prepared by thermal evaporation.Shang et al. 30 successfully prepared (00l)-preferential orientation p-type Sb2Te3 f-TFs by magnetron sputtering methods, and approached the  of ~740 S cm −1 and S 2  of ~12.4 μW cm −1 K −2 at 300 K. Sb2Te3 f-TFs prepared by screen-printing technology can approach  of ~250 S cm −1 and S 2  of ~14.3 μW cm −1 K −2 at room temperature 31 .It is concluded that the  of Sb2Te3 f-TFs prepared by most of the preparation methods is lower than 1000 S cm −1 due to poor crystal growth.Consequently, the carrier transport properties were suppressed and the corresponding TE performance of Sb2Te3 f-TFs is also limited.
In the present work, we employed a thermal diffusion method to prepare p-type

Experimental
Sb2Te3 f-TFs preparation process: The p-type Sb2Te3 f-TFs were prepared on a flexible PI substrate by a thermal diffusion method.First, Sb and Te films were deposited on PI substrates by using thermal evaporation, and the purity Sb (99.99 %) and Te (99.99 %) powders were weighed for 0.7025g and 1.5217 g, respectively.The Sb and Te evaporation parameter were as follow: the evaporation power of 18 W and 20 W, the evaporation time of 13 min and 16 min, and the evaporation pressure of 5×10 −5 Torr, respectively, and the thickness of Sb film was ~220 nm and ~520 nm.

Results and discussion
The  The crystal morphology and chemical composition of the as-prepared Sb2Te3 f-TFs were investigated through SEM and SEM-EDS technology (Figure 3). Figure 3a shows the SEM surface morphology of Sb2Te3 f-TFs.All the films depict large particles morphology, suggesting typical dense polycrystalline characterize.Figure 3b presents EDS spectrum and atomic content of Sb2Te3 f-TFs prepared at Tdiff = 623 K.
As can be seen, the chemical stoichiometry of Sb : Te was ~2.0 :  To further study the changes in microstructure in details, transmission Electron Microscopy was employed for the as-prepared Sb2Te3 f-TFs (Figure 4).The lowresolution (Figure 4a) and a high-resolution image (Figure 4b  μ as a function of nh 33 .The μ increases with increasing Tdiff, while the nh varies from around 1.1 × 10 19 cm −3 .It can be suggested that increasing μ is not mainly caused by the changes of nh.The μ is achieved from 63.4 cm 2 V −1 s −1 at Tdiff = 573 K to high value of 79.7 cm 2 V −1 s −1 at Tdiff = 643 K.In addition, the deformation potential coefficient (Edef) calculated by SPB-model roughly decreases with increasing Tdiff.It is suggested that the enhanced crystallinity leads to the weakened carrier scattering, which is the mainly reason for the increased  with increasing Tdiff.Figure 5c presents the room temperature S as a function of Tdiff.The positive values of S show the typical p-type semiconductor characteristics.The S increases and then decreases in the range of 95 -110 μV K −1 with increasing Tdiff.The maximum S of ~106 μV K −1 is achieved at Tdiff = 623 K. Figure 5d shows the comparison between the measured and calculated S (based on SPB-model) as a function of nh.As can be seen, the measured room temperature nh remains nearly constant around 1.1×10 20 cm −3 with increasing Tdiff.Furthermore, the m * calculated by SPB-model changed in the range from 1.26 m0 to 1.37 m0.Besides, the m * increases and then decreases with increasing Tdiff.
Correspondingly, the trend of S with increasing Tdiff is the same as the trend of m * .
And the changes of S can be attributed to the changes of m * .Figure 5e   The ∆R/R0 changes of Sb2Te3 with bending cycles of 1000 and bending radius of 18 mm.

Conclusion
In this work, we successfully prepared Sb2Te3 f-TFs with high TE performance and bending resistance by the thermal diffusion method.Sb2Te3 f-TFs with standard stoichiometric ratios was achieved, which rationally tuned Tdiff and increased the crystallinity of Sb2Te3 f-TFs.With increasing Tdiff, tuning crystallinity increased σ and thus attenuated carrier scattering, achieving high σ of ~1440 S cm −1 at Tdiff = 643 K.
And the moderate S larger than 95 μV K −1 has been achieved due to standard stoichiometric ratios of Sb2Te3 f-TFs.Correspondingly, an excellent room temperature S 2  of 16.0 μW cm −1 K −2 at Tdiff = 623 K has been achieved.Besides, a ΔR/R0 of < 10 % is achieved after 1000 bending cycles with a bending radius of 18 mm, indicating good bending resistance.

ASSOCIATED CONTENT
Sb2Te3 f-TFs on a flexible polyimide (PI) substrate.The Sb and Te precursor film were deposited by thermal evaporation as show in Figure 1a.The schematic diagram of thermal diffusion process and the optical image of as-prepared Sb2Te3 f-TFs are shown in Figure 1b.The schematic diagram of the reaction process of Sb and Te during thermal diffusion process is shown in Figure 1c.Through tuning the thermal diffusion temperature (Tdiff), the Sb2Te3 f-TFs with standard stoichiometric ratios was obtained.Moreover, the moderate Seebeck coefficient of > 95 μV K −1 was achieved at room temperature.Simultaneously, the μ and  increased with increasing Tdiff due to the weakened carrier scattering.Correspondingly, the highest value of S 2  of 16.0 μW cm −1 K −2 at Tdiff = 623 K has been achieved.Besides, our prepared Sb2Te3 f-TFs approach good bending resistance.

Figure 1
Figure 1 (a) The schematic diagram of the Sb and Te f-TFs prepared by thermal evaporation.(b) The schematic diagram of thermal diffusion process; Inset shows the optical image.(c) The schematic diagram of Sb2Te3 f-TFs preparation through thermal diffusion process.
Secondly, the Te and Bi f-TFs were pressurized in the copper molds placed on a heating equipment.And the Tdiff was set as 573 K, 603 K, 623 K, and 643 K, respectively.The as-prepared p-type Sb2Te3 f-TFs is shown in the Inset of Figure 1b.Characterization of the Sb2Te3 f-TFs: X-ray diffraction (XRD, D/max 2500 Rigaku Corporation, CuKα radiaction) was employed to investigate crystal structure.SEM (Zeiss spra 55) and SEM-EDS (Bruker Quantax 200) were used to analyze the surface morphology, and chemical composition of as-prepared Sb2Te3 f-TFs.TEM (FEI Tecnai G2 F20) and TEM-EDS (Bruker XFlash 5030) were also employed to study the crystal structure and chemical composition.The Geometric phase analysis (GPA) was further employed to analyze the lattice strains by the Strain++ software.Hall measurement system (HL5500PC, Nanometrics) was used to record the Hall performance.And SBA458 (Nezsch) was used to measure the S and σ (error bars of 5% for S, 5% for σ).
Figure 2a, and the (015) peaks increased with increasing the Tdiff.Further, the corresponding calculated crystallinity increased with increasing Tdiff as shown in Figure 2b. Figure 2c depicts the calculated lattice parameter a and c, which clearly indicate the absence of any displacement in crystal structure.The valence states of Sb and Te in the Sb2Te3 films were investigated by XPS (Figure 2d-f).Figure 2d-f present the full XPS spectra and XPS spectra of Sb and Te, respectively.Oxidized Sb2Te3 (peaks at 539.35 and 530.19 eV) and Oxygen characterize (O1s peak) are observed in Figure 2e due to the unencapsulated Sb2Te3 f-TFs used in an atmospheric environment.The binding energies at 528.46 and 537.77eV were related to Sb 3d5/2 and Sb 3d3/2 (as shown in Figure 2e) and the corresponding valances state of Sb was +3.The 3d core-level of Te with two peaks at 586.54 and 576.99 eV were related to the oxidized Sb2Te3.Moreover, the binding energies at 583.03 and 572.82 eV are associated to Te 3d3/2 and Te 3d5/2 (as shown in Figure 2f) and the corresponding valances state of Te is -2.

Figure 2
Figure 2 (a) The XRD spectra of as-prepared Sb2Te3 f-TFs; Inset shows the enlarged (015) peaks and the corresponding calculated crystallinity.(b) The lattice parameters.(c) The full XPS spectra.(d-e) XPS spectra of Sb and Te, respectively.
3.0.The corresponding EDS maps is shown in Figure 3c.Uniformly distributed Sb and Te element were obtained, and no obvious enrichment in Te and Sb is detected.Figure 3c shows the atomic content of Sb2Te3 f-TFs, where the chemical stoichiometry of Sb : Te was closed to 2.0 : 3.0 for Sb2Te3 films prepared at Tdiff from 573 K to 623 K.When the Tdiff reaches at 643 K, the chemical stoichiometry of Sb : Te was 2.0 : 2.8.It is suggested that the Te content slightly decreases due to the evaporation of Te at high temperature.

Figure 3
Figure 3 (a) SEM morphology of Sb2Te3 f-TFs prepared at Tdiff = 603 K, 623 K, and 643 K, respectively.(b) EDS spectrum and atomic content of Sb2Te3 f-TFs prepared at Tdiff = 623 K. (c) The corresponding SEM-BSE images and EDS maps.(d) The measured atomic contents of Sb2Te3 f-TFs.
) for Sb2Te3 prepared at Tdiff = 623 K shows a neat arranged of lattices and the corresponding clear diffraction spots, which indicates good crystallinity of Sb2Te3 f-TFs.Figure 4c presents the enlarged image of the yellow square in Figure 4b.It can be seen that the measured value of the crystal spacing of ~3.2 Å can be indexed as the (015) plane of Sb2Te3, which was consistent with the XRD results.The lattice strains of Figure 4c is shown in Figure 4d, and present no any obvious lattice strain concentration along different directions due to the fewer lattice defects and lattice mismatches.In Figure 4e, the TEM-EDS maps depict that the Sb and Te are uniformly distributed throughout the Sb2Te3 nanoparticles.From the TEM-EDS (as shown in Figure 4f), it can be predicted that the chemical stoichiometry of Sb : Te is also closed to 2.0 : 3.0 (in the nano-scale range).

Figure 4 Figure 5 .
Figure 4 (a) Low-resolution TEM image of Sb2Te3 f-TF prepared at Tdiff = 623 K. (b) High-resolution TEM image taken from the yellow square in Figure 4a.(c) The enlarged TEM images of the yellow square in Figure 4b.(d) The corresponding lattice strains of Figure 4c along different direction.(e) The TEM-EDS maps of Sb and Te.(f) The corresponding TEM-EDS spectrum.
compares the measured S 2  and the calculated S 2  based on SPB model at room temperature.All the S 2  value of Sb2Te3 f-TFs at the room temperature is higher than 11 μW cm −1 K −2 .And the maximum value of S 2  approached as high as 16.0 μW cm −1 K −2 at Tdiff = 623 K due to the high  and moderate S. Herein, the prepared Sb2Te3 f-TFs possesses high bending resistance.The resistance change (∆R/R0) is lower than 10 % under the bending cycles of 1000 and bending radius of 18 mm.

Figure 5
Figure 5 (a) The measured room temperature  as a function of Tdiff.(b) Comparison between the measured and SPB-model calculated μ as a function of nh.(c) The measured room temperature S as a function of Tdiff.(d) Comparison between the