In Situ Synthesis of High Thermoelectric Performance Bi 2 Te 3 Flexible Thin Films through Thermal Diffusion Engineering

: Bi 2 Te 3 -based materials are promising candidates for near-room-temperature applications due to their high thermoelectric performance and low cost. Here, an innovative thermal diffusion strategy combined with magnetron sputtering and thermal evaporation methods was employed to fabricate Bi 2 Te 3 ﬂexible thin ﬁlms (f-TFs) on a ﬂexible polyimide substrate. An in situ synthesis of Bi 2 Te 3 f-TFs with good crystallinity was obtained using a straightforward thermal diffusion method through diffusion of Te into a Bi precursor under low vacuum conditions (1 × 10 5 Pa). This method offers easy preparation, low cost, and a large-area ﬁlm preparation for industrialization. The electrical conductivity increases with increasing thermal diffusion temperatures. A high room temperature carrier mobility of ~28.7 cm − 2 V − 1 S − 1 and an electrical conductivity of ~995.6 S cm − 1 can be achieved. Then, a moderate room temperature Seebeck coefﬁcient >100 µ V K − 1 was obtained due to the chemical stoichiometry being close to the standard by optimizing the thermal diffusion temperature. Consequently, a maximum room temperature PF of ~11.6 µ W cm − 1 K − 1 was observed in Bi 2 Te 3 f-TFs prepared using a thermal diffusion temperature of 653 K. The thermal diffusion strategy applied in the thin ﬁlm preparation represents an effective approach for the preparation of high thermoelectric performance Bi 2 Te 3 f-TFs, offering a promising route for future thermoelectric applications.

A range of thin film deposition techniques, including screen printing [32], solvothermal methods [33], and magnetron sputtering [34,35], have been utilized to fabricate highperformance n-type Bi 2 Te 3 -based flexible thin films (f-TFs).Notably, Varghese et al. [32] synthesized Bi 2 Te 3 nanocrystal inks via a microwave-assisted wet chemical approach for screen printing onto polyimide substrates.Subsequent cold pressing and sintering led to thermoelectric films that achieved a PF of approximately 5.65 at 448 K. Rani et al. [33] produced Bi 2 Te 3 nanostructures through solvothermal methods, attaining a PF of 6.5 µW cm −1 K −2 at 385 K.He et al. [34] fabricated Bi 2 Te 3 f-TFs through direct current magnetron sputtering with in situ annealing, achieving a PF of 8.2 µW cm −1 K −2 .Furthermore, Joo et al. [35] generated Bi 2 Te 3 f-TFs using radiofrequency co-sputtering at 573 K and recorded a PF of 9.7 µW cm −1 K −2 .In our previously published work [36], we successfully synthesized high-quality Te-embedded Bi 2 Te 3 f-TFs by using magnetron sputtering in combination with the thermal diffusion method.To further optimize the chemical content of Bi 2 Te 3 f-TFs and improve its thermoelectric performance, the process can be further optimized to prepare Te through thermal evaporation, with the aim of achieving full sublimation of the Te and Bi-Te reaction.
In this study, a thermal diffusion process combined with thermal evaporation and magnetron sputtering was employed to prepare n-type Bi 2 Te 3 f-TFs on polyimide (PI) substrates, as shown in Figure 1a.The thermal diffusion method can provide sufficient energy for the growth of Bi 2 Te 3 crystals, significantly improving carrier transport performance [36].The Bi film was deposited using magnetron sputtering, and the Te film was deposited using thermal evaporation methods.Evaporating Te film results in a higher preparation efficiency, which is more uniform and makes it easier to produce large-area thin films than magnetron sputtering.Additionally, the copper mold is more conducive to Te sublimation and the Bi-Te reaction.Figure 1b shows a schematic diagram of the Te diffusion into Bi to react to form Bi 2 Te 3 during thermal diffusion.Thermal diffusion is the mutual diffusion reaction between Bi and Te to form Bi 2 Te 3 .Te sublimation and diffusion speed are faster due to the high saturated vapor pressure of Te.Bi 2 Te 3 film is on the Bi precursor film after thermal diffusion, while there is no residue of materials in the Te precursor film due to the complete sublimation of Te induced by a high saturated vapor pressure.The σ increased with increasing thermal diffusion temperatures (T diff ) due to the improved carrier mobility induced by the weak carrier scattering.The maximum σ of ~995.6 S cm −1 was achieved at T diff = 653 K.By optimizing the T diff , a S > 95 µV K −1 can be obtained due to the reasonable chemical content regulation.Consequently, a room-temperature PF of ~11.6 µW cm −1 K −2 can be achieved in Bi 2 Te 3 f-TFs under T diff = 653 K.The PF value of our prepared Bi 2 Te 3 f-TFs is competitive with those of films prepared by other scholars, as shown in Figure 1c.And the PF value of the prepared thin film is only slightly lower than the previously reported PF value of 14.65 µW cm −1 K −2 [36].The novelty thermal diffusion method has the advantages of easy preparation, low cost, and large-area film preparation.In this work, the ultra-high performance of the as-prepared intrinsic Bi 2 Te 3 thin film was not achieved due to the lack of regulation.It is expected to achieve significant performance improvement through doping or structural regulation in the future.[35], screen printing (2016) [32], in situ annealing during magnetron sputtering (2020) [34], thermal diffusion method (2022) [36], post-electric current treatment (2023) [37], solvothermal method (2023) [33].Inset is the image of the Bi2Te3 f-TFs.

Thin Film Preparation
A thermal diffusion method combined with thermal evaporation methods was employed to synthesize n-type Bi2Te3 thin films on a flexible PI substrate.High purity tellurium power (99.99%,Macklin, Shanghai, China) weighing 0.6 g was used to deposit the Te film on the PI substrate using thermal evaporation methods, as shown in section I of Figure 1a.The thermal evaporation parameters were as follows: a thermal evaporation power of 20 W, an evaporation time of 13 min, and a thermal evaporation pressure of 5 × 10 −5 Torr.The thickness of the as-deposited Te film was ~500 nm.A high purity Bismuth target (99.99%)was used to deposit the Bi precursor film on the PI substrate using magnetron sputtering methods, as shown in section II of Figure 1a.The magnetron sputtering parameters were as follows: a radio frequency power of 25 W, a duration of 30 min, and a working pressure of 1 Pa.The thickness of the Bi film was ~150 nm.Subsequently, both the Te and Bi films were pressurized in a copper mold placed on the heating equipment in a glove box (1 × 10 5 Pa), as shown in section III of Figure 1a.Compared to the high vacuum experimental conditions used in our previous work [36], this experimental method is conducive to large-scale production.The Tdiff was set to 608 K, 623 K, 638 K, and 653 K, respectively.The as-deposited n-type Bi2Te3 f-TFs is shown in the inset of Figure 1c.

Thin Film Preparation
A thermal diffusion method combined with thermal evaporation methods was employed to synthesize n-type Bi 2 Te 3 thin films on a flexible PI substrate.High purity tellurium power (99.99%,Macklin, Shanghai, China) weighing 0.6 g was used to deposit the Te film on the PI substrate using thermal evaporation methods, as shown in section I of Figure 1a.The thermal evaporation parameters were as follows: a thermal evaporation power of 20 W, an evaporation time of 13 min, and a thermal evaporation pressure of 5 × 10 −5 Torr.The thickness of the as-deposited Te film was ~500 nm.A high purity Bismuth target (99.99%)was used to deposit the Bi precursor film on the PI substrate using magnetron sputtering methods, as shown in section II of Figure 1a.The magnetron sputtering parameters were as follows: a radio frequency power of 25 W, a duration of 30 min, and a working pressure of 1 Pa.The thickness of the Bi film was ~150 nm.Subsequently, both the Te and Bi films were pressurized in a copper mold placed on the heating equipment in a glove box (1 × 10 5 Pa), as shown in section III of Figure 1a.Compared to the high vacuum experimental conditions used in our previous work [36], this experimental method is conducive to large-scale production.The T diff was set to 608 K, 623 K, 638 K, and 653 K, respectively.The as-deposited n-type Bi 2 Te 3 f-TFs is shown in the inset of Figure 1c.

Characterization of the Thin Film
The crystalline structure of the sample was investigated using X-ray diffraction (XRD, D/max 2500, Rigaku Corporation, Tokyo, Japan, utilizing CuKα radiation).Scanning electron microscopy (SEM, Zeiss-spra 55, Oberkochen, Baden-Württemberg, Germany) and SEM coupled with energy dispersive spectroscopy (SEM-EDS, Bruker Quantax 200, Billerica, MA, USA) were used to analyze the surface morphology and chemical composition.The Hall properties were measured using a Hall measurement system (HL5500PC, Nanometrics, Ottawa, ON, Canada), while the S and σ values were measured using a SBA458 system (Nezsch, Selb, Bavaria, Germany).

Results and Discussion
To elucidate the crystalline structure of the Bi 2 Te 3 f-TFs prepared through thermal diffusion, an XRD analysis was employed, and XRD spectra are shown in Figure 2a.All the XRD peaks can be indexed to Bi 2 Te 3 (PDF #15-0863).It can be seen that the three main diffraction peaks correspond to the (006), (015), and (0010) planes of Bi 2 Te 3 .The strongest (015) peaks suggest a preferred orientation, as shown in Figure 2a and Section I of Figure 2a.As well, the intensity of the (015) diffraction peaks increased with increasing T diff , indicating the increase in crystallinity.Te peaks (PDF #36-1452) can be observed in Figure 2a.The enlarged (012) and (110) Te peaks in Section II of Figure 2a  diffraction peaks correspond to the (006), (015), and (0010) planes of Bi2Te3.The strongest (015) peaks suggest a preferred orientation, as shown in Figure 2a and Section I of Figure 2a.As well, the intensity of the (015) diffraction peaks increased with increasing Tdiff, indicating the increase in crystallinity.Te peaks (PDF #36-1452) can be observed in Figure 2a.The enlarged (012) and (110) Te peaks in Section II of Figure 2a further confirm the existence of Te.The obvious Te peak appears in the Bi2Te3 f-TFs prepared at Tdiff > 623 K.More Te atoms can be sublimated and diffused into the Bi precursor thin film at a high Tdiff.The over-sublimated Te remained in the Bi precursor during cooling, leading to the formation of Te.
To further understand the Te content evolution, a detailed semi-quantitative investigation of Te can be measured in Figure 2b.As can be seen, the Te content increases and Bi content decreases with increasing Tdiff.At Tdiff = 608 K, the low Te content of Bi2Te3 is attributed to the poor diffusion reaction.When the Tdiff increased to 623 K, the chemical content of Bi and Te was close to the standard chemical content of Bi2Te3 (40% and 60%) due to the fully thermal diffusion process.When Tdiff increased to over 638 K, the Te content was above 60% due to the Te over-sublimation.This Te-rich content is consistent with the XRD results.To further characterize the valence states of Sb and Te in the Bi2Te3 f-TFs, XPS studies were employed, as shown in Figure 2c-e  To further understand the Te content evolution, a detailed semi-quantitative investigation of Te can be measured in Figure 2b.As can be seen, the Te content increases and Bi content decreases with increasing T diff .At T diff = 608 K, the low Te content of Bi 2 Te 3 is attributed to the poor diffusion reaction.When the T diff increased to 623 K, the chemical content of Bi and Te was close to the standard chemical content of Bi 2 Te 3 (40% and 60%) due to the fully thermal diffusion process.When T diff increased to over 638 K, the Te content was above 60% due to the Te over-sublimation.This Te-rich content is consistent with the XRD results.To further characterize the valence states of Sb and Te in the Bi 2 Te 3 f-TFs, XPS studies were employed, as shown in Figure 2c-e The surface and cross-section morphology of the Bi 2 Te 3 f-TFs was analyzed using SEM technology, as shown in Figure 3.The uniformly distributed nanoparticles of the Bi 2 Te 3 f-TFs prepared at T diff = 608 K, 623 K, 638 K, and 653 K can be observed in Figure 3a-d, respectively.This well-crystallized structure is consistent with the XRD results.The size of the Bi 2 Te 3 particles increased with increasing T diff .Finally, the large flake-like particles could be obtained at T diff > 638 K, as shown in Figure 3d, indicating a high crystallinity.The cross-section SEM images of Bi 2 Te 3 f-TFs are shown in Figure 3e.Highly dense crystals could be observed in all the Bi 2 Te 3 f-TFs.And the thicknesses of the Bi 2 Te 3 f-TFs prepared at T diff = 608 K, 623 K, 638 K, and 653 K were ~293 nm, ~386 nm, ~300 nm, and ~280 nm, respectively.Figure 3f   The surface and cross-section morphology of the Bi2Te3 f-TFs was analyzed using SEM technology, as shown in Figure 3.The uniformly distributed nanoparticles of the Bi2Te3 f-TFs prepared at Tdiff = 608 K, 623 K, 638 K, and 653 K can be observed in Figure 3a-d, respectively.This well-crystallized structure is consistent with the XRD results.The size of the Bi2Te3 particles increased with increasing Tdiff.Finally, the large flake-like particles could be obtained at Tdiff > 638 K, as shown in Figure 3d, indicating a high crystallinity.The cross-section SEM images of Bi2Te3 f-TFs are shown in Figure 3e.Highly dense crystals could be observed in all the Bi2Te3 f-TFs.And the thicknesses of the Bi2Te3 f-TFs prepared at Tdiff = 608 K, 623 K, 638 K, and 653 K were ~293 nm, ~386 nm, ~300 nm, and ~280 nm, respectively.Figure 3f   Figure 4 shows the electrical performances of the as-prepared Bi2Te3 f-TFs synthesized at different Tdiff.The temperature-dependent  of Bi2Te3 f-TFs is shown in Figure 4a.As can be seen,  decreased with increasing T, and the maximum  was achieved at room temperature.When Tdiff increased from 608 K to 653 K, the room-temperature  significantly increased from ~359.19 S cm −1 to ~995.6 S cm −1 .To further understand the electric performance evolution, the Hall performance of Bi2Te3 f-TFs was investigated.The single parabolic band (SPB) model was employed to analyze the electrical performance.The ne-dependent µ curves calculated using the SPB model are shown in Figure 4b.The µ increased with increasing Tdiff, leading to an increase in .Correspondingly, the deformation potential coefficient (Edef) decreased with increasing Tdiff, as shown in Figure 4c.This is consistent with the increase in µ, potentially resulting from reduced carrier scattering in the Te-embedded Bi2Te3 heterostructure.Compared to the highest σ of Figure 4 shows the electrical performances of the as-prepared Bi 2 Te 3 f-TFs synthesized at different T diff .The temperature-dependent σ of Bi 2 Te 3 f-TFs is shown in Figure 4a.As can be seen, σ decreased with increasing T, and the maximum σ was achieved at room temperature.When T diff increased from 608 K to 653 K, the room-temperature σ significantly increased from ~359.19 S cm −1 to ~995.6 S cm −1 .To further understand the electric performance evolution, the Hall performance of Bi 2 Te 3 f-TFs was investigated.The single parabolic band (SPB) model was employed to analyze the electrical performance.The n e -dependent µ curves calculated using the SPB model are shown in Figure 4b.The µ increased with increasing T diff , leading to an increase in σ.Correspondingly, the deformation potential coefficient (E def ) decreased with increasing T diff , as shown in Figure 4c.This is consistent with the increase in µ, potentially resulting from reduced carrier scattering in the Te-embedded Bi 2 Te 3 heterostructure.Compared to the highest σ of 567.69 S cm −1 in our previous work [36], the high σ in this work is due to the decreased energy filtration effect caused by the composition approaching the standard stoichiometric ratio.Figure 4d plots the T-dependent S, and the negative S values confirm the traditional n-type semiconductor behavior.Additionally, S exhibited a decreasing trend with increasing T diff values from 608 K to 653 K.All the room-temperature S values were above 100 µV K −1 due to the near standard stoichiometric ratio, and the maximum S of −151.95 µV K −1 in Bi 2 Te 3 f-TFs prepared at T diff = 608 K was obtained.n e decreased with increasing T from 608 K to 638 K, and then n e slightly increased.It can be seen that the change in S is attributed to the changes in n e , regardless of the effective mass (m * ) evolution.Figure 4f shows the T-dependent PF, and the largest PF values were obtained at room temperature.The PF value of Bi 2 Te 3 f-TFs prepared at T diff = 653 K was obviously higher than that of T diff < 638 K.The largest PF of ~11.6 µW cm −1 K −2 was achieved at T diff = 653 K due to the corresponding high σ and S. As T diff further increased, the thermoelectric performance decreased due to severe chemical content segregation, as shown in the Figure S1 and Table S1.
567.69 S cm −1 in our previous work [36], the high σ in this work is due to the decreased energy filtration effect caused by the composition approaching the standard stoichiometric ratio.Figure 4d plots the T-dependent S, and the negative S values confirm the traditional n-type semiconductor behavior.Additionally, S exhibited a decreasing trend with increasing Tdiff values from 608 K to 653 K.All the room-temperature S values were above 100 µV K −1 due to the near standard stoichiometric ratio, and the maximum S of −151.95 µV K −1 in Bi2Te3 f-TFs prepared at Tdiff = 608 K was obtained.ne decreased with increasing T from 608 K to 638 K, and then ne slightly increased.It can be seen that the change in S is attributed to the changes in ne, regardless of the effective mass (m * ) evolution.Figure 4f shows the T-dependent PF, and the largest PF values were obtained at room temperature.The PF value of Bi2Te3 f-TFs prepared at Tdiff = 653 K was obviously higher than that of Tdiff < 638 K.The largest PF of ~11.6 µW cm −1 K −2 was achieved at Tdiff = 653 K due to the corresponding high  and S. As Tdiff further increased, the thermoelectric performance decreased due to severe chemical content segregation, as shown in the Figure S1 and Table S1.

Conclusions
In this research, we successfully synthesized Bi2Te3 f-TFs with good crystallinity using a specialized thermal diffusion strategy combined with magnetron sputtering and thermal evaporation methods.The σ increased with increasing T due to the increase in µ induced by the decrease in Edef.The maximum room-temperature σ of ~995.6 S cm −1 was obtained at Tdiff = 653 K.By optimizing Tdiff, a near-standard stoichiometric ratio can be achieved, leading to a moderate S > 100 µV K −1 .Finally, a high room-temperature PF reached ~11.6 µW cm −1 K −2 due to the high σ and S. The successful preparation of thin films at low vacuum levels is particularly beneficial for large-scale production of thin films.

Conclusions
In this research, we successfully synthesized Bi 2 Te 3 f-TFs with good crystallinity using a specialized thermal diffusion strategy combined with magnetron sputtering and thermal evaporation methods.The σ increased with increasing T due to the increase in µ induced by the decrease in E def .The maximum room-temperature σ of ~995.6 S cm −1 was obtained at T diff = 653 K.By optimizing T diff , a near-standard stoichiometric ratio can be achieved, leading to a moderate S > 100 µV K −1 .Finally, a high room-temperature PF reached ~11.6 µW cm −1 K −2 due to the high σ and S. The successful preparation of thin films at low vacuum levels is particularly beneficial for large-scale production of thin films.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/coatings13122018/s1, Figure S1: The thermoelectric performance of Bi 2 Te 3 prepared at 668 K; Table S1: The atomic percent of Bi 2 Te 3 prepared at 668 K.
Author Contributions: N.C. and D.A made substantial contributions to the conceptualization, design of methodology, and writing-original draft.D.A. and Z.Z.provided administration.J.G., Y.C. and W.B. performed data acquisition, provided administrative, technical, and supervision, support, and reviewed and edited the manuscript.All authors have read and agreed to the published version of the manuscript.
further confirm the existence of Te.The obvious Te peak appears in the Bi 2 Te 3 f-TFs prepared at T diff > 623 K.More Te atoms can be sublimated and diffused into the Bi precursor thin film at a high T diff .The over-sublimated Te remained in the Bi precursor during cooling, leading to the formation of Te.
. The typical binding energy of Bi and Te can be observed in the XPS full spectra, as shown in Figure2c.The binding energies at 157.0 eV and 162.3 eV correspond to Bi 4f7/2 and Bi 4f5/2, respectively, as shown in Figure2d, indicating a Bi valence state of +3.The binding energies at 572.9 eV and 583.3 eV correspond to Te3d5/2 and Te3d7/2, respectively, as shown in Figure2d, suggesting a Te valence state of −2.The XPS spectra further prove the formation of Bi2Te3.

Figure 2 .
Figure 2. (a) XRD spectra of the as-prepared Bi 2 Te 3 f-TFs.Inset (I) highlights the enlarged (015) peaks of Bi 2 Te 3 ; inset (II) presents the enlarged (012) and (110) peaks of Te.(b) The measured chemical contents of Bi and Te.(c) The XPS full spectra.(d,e) The XPS spectra of Bi and Te, respectively.
. The typical binding energy of Bi and Te can be observed in the XPS full spectra, as shown in Figure 2c.The binding energies at 157.0 eV and 162.3 eV correspond to Bi 4f 7/2 and Bi 4f 5/2 , respectively, as shown in Figure 2d, indicating a Bi valence state of +3.The binding energies at 572.9 eV and 583.3 eV correspond to Te3d 5/2 and Te3d 7/2 , respectively, as shown in Figure 2d, suggesting a Te valence state of −2.The XPS spectra further prove the formation of Bi 2 Te 3 .Coatings 2023, 13, 2018 5 of 8 exhibits the SEM-BSE of the Bi 2 Te 3 f-TFs prepared at T diff = 653 K and the corresponding EDS mapping.As can be seen, most of the Bi and Te elements were uniformly distributed.Minor Te-enriched regions can be observed, as shown the marked white circle, resulting from over-sublimation of Te in Bi 2 Te 3 .The EDS maps further validated the Te-embedded Bi 2 Te 3 composite film.

Figure 2 .
Figure 2. (a) XRD spectra of the as-prepared Bi2Te3 f-TFs.Inset (I) highlights the enlarged (015) peaks of Bi2Te3; inset (II) presents the enlarged (012) and (110) peaks of Te.(b) The measured chemical contents of Bi and Te.(c) The XPS full spectra.(d,e) The XPS spectra of Bi and Te, respectively.
exhibits the SEM-BSE of the Bi2Te3 f-TFs prepared at Tdiff = 653 K and the corresponding EDS mapping.As can be seen, most of the Bi and Te elements were uniformly distributed.Minor Te-enriched regions can be observed, as shown the marked white circle, resulting from over-sublimation of Te in Bi2Te3.The EDS maps further validated the Te-embedded Bi2Te3 composite film.

Figure 3 .
Figure 3. (a-d) SEM images of Bi 2 Te 3 f-TFs prepared at T diff = 608 K, 623 K, 638 K, and 653 K, respectively.(e) The corresponding cross-sectional SEM images of Bi 2 Te 3 f-TFs.(f-i) The SEM-BSE of Bi 2 Te 3 f-TFs prepared at T diff = 653 K and the corresponding EDS maps.BSE: Backscattered electron.