Boosting Thermoelectric Performance of Bi2Te3 Material by Microstructure Engineering

Abstract Due to the intrinsic contradiction of electrical conductivity and Seebeck coefficient in thermoelectric materials, the enhancement for the power factor (PF) is limited. Since the PF decides the output power, strategies to the enhancement of PF are of paramount importance. In this work, Bi2Te3/Sb and Bi2Te3/W multilayer films are proposed to enhance the thermoelectric properties. Both systems possess extremely high conductivity of ≈5.6 × 105 S m−1. Moreover, the electrical conductivity and Seebeck coefficient simultaneously increase as temperature rising, showing the overcome of the intrinsic contradiction. This results in ultrahigh PFs of 1785 µWm−1 K−2 for Bi2Te3/W and of 1566 µWm−1 K−2 for Bi2Te3/Sb at 600 K. Thermal heating of the Bi2Te3/Sb multilayer system shows compositional changes with subsequent formation of Bi‐Te‐Sb phases, Sb‐rich Bi‐Te precipitates, and cavities. Contrary, the multilayer structure of the Bi2Te3/W films is maintained, while Bi2Te3 grains of high‐crystalline quality are confined between the W layers. In addition, bilayer defects in Bi2Te3 and smaller cavities at the interface to W layers are also observed. Thus, compositional and confinement effects as well as structural defects result in the ultrahigh PF. Overall, this work demonstrates the strategies on how to obtain ultrahigh PFs of commercial Bi2Te3 material by microstructure engineering using multilayer structures.


Introduction
The invention of wearable devices and micro-electronics has raised the demand for sustainable power supply that can be integrated within the electronic systems to generate power. [1]Self-powered energy supplies can largely satisfy the requirement. [2]Conventional methods for power manufacturing have difficulties in their miniaturization. [3]In the last decades, thermoelectric (TE) generators have attracted considerable attention due to their unique capability of converting heat into electrical power directly. [4]Moreover, TE thin films can overcome current issues of bulk TE powders as they are easy to be integrated into micro-scale systems. [5]The energy conversion property of TE devices is valued by the dimensionless TE figure of merit ZT = S 2 T −1 , where S, ,  and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively.The properties of TE devices greatly rely on the inherent properties of TE materials. [6]Particularly, power factor (PF) (S 2 ) of TE materials directly decides the output power of TE devices. [7]However, TE materials with high ZT and PF are very rare. [7,8]While ZT is limited by thermal conductivity of TE material being intrinsic properties of the material, the PF depends on S and , which can be tuned.Therefore, growing investigations have focused on exploring potential TE materials and developing strategies to improve their PF and ZT.
Bulk Bi 2 Te 3 exhibits supreme intrinsic TE properties at room temperature. [9]Bi 2 Te 3 received great attention since commercially available power generating TE modules rely on that material. [10]TE films based on Bi 2 Te 3 are also a hot topic currently. [11]Manzi et al. demonstrated the use of plasma-jet printed and electromagnetic field-assisted printed colloidal TE Bi 2 Te 3 nanoflakes. [12]The material exhibit PF of 70 μWm −1 K −2 at room temperature.Chiba et al. prepared flexible films based on Bi 2 Te 3 nanoplates and single-walled carbon nanotubes via drop casting.The PF of 630 μWm −1 K −2 was achieved, indicating the high performance of flexible nanocomposite films. [13]Ashfaq et al. demonstrated the that effectiveness of thermal evaporation as an approach for synthesizing the Sr doped Bi 2 Te 3 thin films with PF of 1561 μWm −1 K −2 at room temperature. [14]Moreover, confinement effects enhance PF significantly.In the case of Bi 38 Te 55 Se 7 and Bi 15 Sb 29 Te 56 nanowires, PFs of 2820 and 1750 μWm −1 K −2 at room temperature, respectively, were reported. [15]owever, this showed the impact of composition on the PFs in addition.
Besides, multilayer TE thin films have been developing in the past.Guo et al. fabricated Ni/Bi 0.5 Sb 1.5 Te 3 TE material with multilayer structure, which exhibits the PF greater than 4 mWm −1 K −2 at 300 K. [16] Liao et al. measured TE properties of spurted Sb/Bi-Sb-Te multilayer thin films after electrical stress.The PF of 1.36 mWm −1 K −2 was achieved at 603 K. [17] Kim et al. deposited Sb 2 Te 3 /Bi 2 Te 3 multilayer films via magnetron sputtering system, whose PF reached 493 μWm −1 K −2 at room temperature. [18]arlier, Mahan et al. proposed a special method to enhance the PF by combining materials exhibiting high electron concentration (e.g., metals) with semiconductors to introduce the distribution of carriers corresponding to the asymmetrical Fermi level. [19]his resulted in a significant improvement of the PF.At the same time, the high interface density can also reduce thermal conductivity effectively, thus improving the overall TE properties. [20]hus, this article proposes the integration of W and Sb layers with Bi 2 Te 3 layers in the form of Bi 2 Te 3 /Sb and Bi 2 Te 3 /W multilayer structures for combining their own advantages.Multilayer TE thin films commonly exhibit better TE properties than conventional TE materials due to the existence of interface optimization and band engineering within the films to refine the carrier transportation. [21]Moreover, multilayer TE thin films can utilize the advantages of various properties from different materials, and by stacking these materials on top of each other, they can take full utilization of their respective advantages to improve the overall TE properties. [21] performance is also related to the cycle of the layers.As reported, [22] the layer number impact TE properties of TE materials.It was shown that the carrier concentration and mobility decreased, but the resistivity and Seebeck coefficient increased slightly and significantly, respectively, hereby increasing the power factor with increasing layer number.Consequently, keeping approximate same total thickness, the Bi 2 Te 3 /Sb and Bi 2 Te 3 /W multilayer thin films can result in a strong scattering effect compared with pure single-layer Bi 2 Te 3 .The interlayers can reduce the mean free path of phonons, thus hindering phonon transport and decreasing the thermal conductivity.At the same time, the energy filtering effect of the semiconductor/metal interface can increase the Seebeck coefficient.Therefore, the mechanism of excellent TE properties of multilayer TE thin films is explored by measuring the TE properties and characterizing the microstructure of the multilayers.Overall, this work opens new frontiers in the development of TE materials with advanced properties by microstructure engineering.

Microstructure of Bi 2 Te 3 /Sb and Bi 2 Te 3 /W Multilayers
Figure 1 shows XRD patterns and Raman spectra of as-deposited and thermally annealed Bi 2 Te 3 /Sb and Bi 2 Te 3 /W thin films.In Figure 1a, the as-deposited Bi 2 Te 3 /Sb thin film show diffraction peaks corresponding to rhombohedral Bi 2 Te 3 (JCPDS NO. 8-27).This indicates the formation of nano crystallites of Bi 2 Te 3 in the thin film at room temperature.After annealing to 500 Figure 1c,d shows Raman spectra of Bi 2 Te 3 /Sb and Bi 2 Te 3 /W multilayer thin films, respectively.The vibration modes A 1 1g , E 2 g , and A 2 1g are related to Bi 2 Te 3 and exhibit a small evolution with increasing of heating temperature. [23]However, the E 2 g of the Bi 2 Te 3 /Sb reveals larger shoulder at 400 K, while it splits into two peaks at 500 K.In addition, the peaks are broader in Bi 2 Te 3 /Sb multilayers than in Bi 2 Te 3 /W multilayers and single-layer Bi 2 Te 3 thin film (Figure S2, Supporting Information).This is a signature of intermixing with subsequent formation of amorphous Bi-Te-Sb phases.Taking the XRD and Raman patterns together, the peaks related to Bi 2 Te 3 are the strongest peaks in all the samples.This shows that the crystallization of Bi 2 Te 3 is dominant in the thin films.However, Sb tend to react with Bi 2 Te 3 during heating, while W remained amorphous in the film (confirmed by TEM below).
The effect of interdiffusion between the layers in Bi 2 Te 3 /Sb multilayer thin films was further revealed by nanoscale investigations using cross-sectional TEM. Figure 2a shows the HAADF-STEM image of Bi 2 Te 3 /Sb thin film after thermal heating at 600 K.The corresponding EDX maps of Bi, Te, and Sb are shown in Figure 2b,c,d, respectively.HAADF images and EDX maps revealed vanishing of the multilayer structure, although some Sbrich layers are remained (Figure 2d).In addition, interdiffusion led to the precipitation of Sb-rich Bi-Te phases and the formation of large cavities in the Bi 2 Te 3 /Sb multilayers.Thus, the thin film after thermal heating represents an alloy consisting of Bi-Te-Sb phases and Sb-rich Bi-Te precipitates.The average composition of the Bi 2 Te 3 /Sb multilayers was measured to be 32.35 at.% of Bi, 47.65 at.% of Te and 20 at.% of Sb, while the average composition of Bi-Te-Sb phase being measured in specimen areas without Sb precipitates was identified to be 34.8 at.% (STD 0.75) of Bi, 50.7 at.% (STD 0.42) of Te and 14.5 at.% (STD 1.1) of Sb.The homogeneous area close to the substrate revealed the following composition 34.5 at.% of Bi, 50.8 at.% of Te and 14.7 at.% of Sb.Interestingly, the ratio of Bi to Te in the all cases is close to 1.5, showing the presence of Bi 2 Te 3 as matrix.The composition of Sbrich Bi-Te precipitates was identified to be 52.5 at.% (STD 8.5) of Sb, 20.8 at.% (STD 2.8) of Bi and 26.7 at.% of Te (STD 5.9).
Contrary to the Bi 2 Te 3 /Sb multilayers, the multilayer structure of Bi 2 Te 3 /W thin films remained unaffected after thermal heating at 600 K. EDX maps of Figure 2f,g depict well-separated Bi 2 Te 3 and W layers.Moreover, as in the previous case, cavities were also formed in the multilayers at the W/ Bi 2 Te 3 interface (also Figure S3, Supporting Information).However, their distribution is more homogenous since they were formed at the interfaces between W and Bi 2 Te 3 , which are rough.Moreover, the size of the cavities is smaller, while their density is large compared to the Bi 2 Te 3 /Sb multilayers.
The grain sizes of Bi 2 Te 3 in the Bi 2 Te 3 /W multilayers differ from the grain sizes of Bi 2 Te 3 in the Bi 2 Te 3 /Sb multilayers.Figure 3 depicts high-resolution TEM images of the multilayers.The grain boundaries in the Bi 2 Te 3 /W thin film are sharper and more defined compared to the Bi 2 Te 3 /Sb thin film.Although, the grain sizes of Bi 2 Te 3 in the Bi 2 Te 3 /W multilayers are varied in lateral direction (from ≈25 to 100 nm), the sizes are restricted in the growth direction by the W layers.So, the thickness of the grains is limited to ≈35 nm, giving much narrow distribution of Bi 2 Te 3 grain sizes.Contrary, the grain sizes of Bi 2 Te 3 can be varied in all growing direction in the case of the Bi 2 Te 3 /Sb multilayers.Moreover, Bi 2 Te 3 grains in the Bi 2 Te 3 /W multilayer showed high crystallinity.Figure 3c depicts atomic-resolution HAADF-STEM image of a part of c-oriented Bi 2 Te 3 grain (The full grain is shown in Figure S3, Supporting Information).The image reveals that Bi 2 Te 3 crystal consists of building units with quintuple layers and the layers contain five atomic planes in the order of -Te-Bi-Te-Bi-Te.The building units are bonded together by weak van der Waals forces.In accordance with XRD measurements, the Bi 2 Te 3 grains were formed with strong (001) texture (Figure S3, Supporting Information).Furthermore, stacking defects were also observed within of Bi 2 Te 3 grains.The defects are confined between two atomic layers of Bi and Te and represent localized stacking faults (Figure 3c,d).The defects are typical for layered chalcogenidebased compounds. [24]Moreover, it was shown that such bilayer defects can enhance charge carrier mobility and thus the electrical conductivity of Bi 2 Te 3 (also TE material) as well as they can reduce lattice thermal conductivity by inhibiting the phonon transport. [25]

TE Properties of Bi 2 Te 3 /Sb and Bi 2 Te 3 /W Multilayers
Figure 4 shows the carrier concentration and mobility of Bi 2 Te 3 /Sb and Bi 2 Te 3 /W multilayers as a function of annealing temperature.The negative values of carrier concentration demonstrate n-type conduction, indicating that electrons are the major carriers.The carrier concentration in Bi 2 Te 3 /Sb (Figure 4a) exhibits fluctuations to some degree during annealing before 450 K.This can be due to compositional changes occurring on annealing as revealed by TEM measurements.After annealing at temperatures of 450 and 500 K, the carrier concentration of Bi 2 Te 3 /Sb thin films decreased significantly from −2.455 × 10 22 to −2.091 × 10 21 cm −3 .Then, the carrier concentration tends to stabilize from  500-650 K.This implies that the reaction induced by interdiffusion between Bi 2 Te 3 and Sb has been almost completed.However, the carrier concentration in Bi 2 Te 3 /W multilayers remains constant during annealing, which is due to high quality Bi 2 Te 3 grains.Unlike carrier concentration, the mobility (Figure 4b) of the thin films shows a continuous increasing trend with temperature.It can be ascribed to that the gradual crystallization of the multilayers leads to the high mobility.This is beneficial for the enhancement of electrical conductivity and the achievement of excellent TE properties.
Figure 5 displays the evolution of electrical conductivity, Seebeck coefficient and PF of Bi 2 Te 3 /Sb and Bi 2 Te 3 /W multilayers as a function of heating temperature.Figure 5a shows that all sam-ples exhibit extremely high electrical conductivity.According to Figure 5a, Bi 2 Te 3 /Sb possesses the lowest electrical conductivity of 2.5 × 10 5 S m −1 at room temperature.With the heating temperature increasing, the electrical conductivity increased slightly due to interdiffusion.While the electrical conductivity of Bi 2 Te 3 /W exhibits a significantly increasing trend with the highest value of 5.6 × 10 5 S m −1 at 650 K.This can be ascribed to the controllable multilayer structure of Bi 2 Te 3 /W with the confined Bi 2 Te 3 grains in good crystallinity.The overall electrical conductivity increases with temperature, indicating semiconductor behavior of the samples.TE materials with lower internal resistance could dissipate less energy while operating, which is critical in promoting the energy conversion efficiency. [26]igure 5b depicts the temperature-dependent variation of Seebeck coefficient.The coefficients are negative for all samples, which means that the thin films are n-type semiconductors.The Seebeck coefficient increases significantly as temperature rising, where Bi 2 Te 3 /Sb exhibits the highest Seebeck coefficient of −64 μVK −1 at 600 K.While the crystallization of Bi 2 Te 3 /Sb multilayers can lead to the increase in the carrier concentration, the crystal interfaces and cavities can scatter carriers and reduce mobility, which leads to the low electrical conductivity and high Seebeck coefficient.Above 600 K, the Seebeck coefficient decreases due to the bipolar effect. [27]Figure 5c represents the calculated PFs.The overall PF increases with annealing temperature.Bi 2 Te 3 /W exhibits the highest PF of 1785 μWm −1 K −2 at 600 K. Beyond the expectation, the results show that the electrical conductivity and Seebeck coefficient could increase at the same time with temperature rise.Though the Seebeck coefficient is low, the extremely high electrical conductivity contributes to much higher PF of Bi 2 Te 3 /W multilayers than Bi 2 Te 3 /Sb multilayers and single-layer Bi 2 Te 3 thin film (Figure S4, Supporting Information), indicating the effect of carrier transport modulation within the multilayer nanocomposite films.This indicates that the samples can conquer the intrinsic contradictory relationship between the electrical conductivity and Seebeck coefficient, which contributes considerably to such high PF.
Moreover, in Figure 5b, there is an upward trend observed in S-T plot, coinciding with an increase in electrical conductivity (Figure 5a).Two cases are warrant for discussion: Bi 2 Te 3 /Sb multilayers and Bi 2 Te 3 /W multilayers.In the first case, Figure 5b reveals an upward trend in Seebeck coefficient at temperatures above 600 K, concurrently with a decrease in electrical conductivity (Figure 5a).This trend may be attributed to additional microstructural and compositional changes occurring at higher temperatures.In the second case, Figure 5b shows a slight decrease in Seebeck coefficient at temperatures higher than 550 K, while electrical conductivity continues to rise (Figure 5a).Given that only the Bi 2 Te 3 phase is formed in Bi 2 Te 3 /W multilayers, the formation of more defects at higher temperatures, including larger voids, might explain this behavior.Although, in general, nanoscale voids reduce the thermal conductivity of TE materials, positively influencing the ZT value, larger voids can have the opposite effect.Similar trends in Seebeck coefficient and electrical conductivity were observed in Ag-Mn-Sb-Te and Mg-Zn-Sb alloy. [28,29]However, the decrease in Seebeck coefficient did not significantly impact the overall ZT value in these cases, as the formation of nanoscale voids contributed to the reduction of thermal conductivity. [29]3.Discussion on the Enhancement of PFs in Bi 2 Te 3 /Sb and Bi 2 Te 3 /W Multilayers High electrical conductivity at room temperature can be attributed to the presence of high intrinsic conductive layers of Sb and W. With temperature rise, however, interdiffusion between the layer in the Bi 2 Te 3 /Sb multilayers resulted in compositional changes with subsequent formation of Bi-Te-Sb and Sbrich Bi-Te grains as well as cavities.The formation of new grains and grain boundaries lead to a reduction in carrier concentration (Figure 4a), which results in the enhancement of Seebeck coefficient.[30] However, the Bi 2 Te 3 /W multilayers are more stable, and the multilayer structure is maintained after thermal heating.Moreover, confinement of Bi 2 Te 3 grains between W layers and their high crystalline quality led to higher electrical conductivity with temperature rise, while the cavities and bilayer defects will result in low thermal conductivity, which is beneficial for ZT value.The confined Bi 2 Te 3 crystals cause insufficient grain boundaries for effectively blocking carriers, which contribute to lower Seebeck coefficient compared to the Bi 2 Te 3 /Sb multilayers.It should be noted that the successive W layers caused the short circuit in the test, which also caused the measured Seebeck coefficient to be lower than the actual value.The carrier transportation in the sample can be divided into two parts.In the direction parallel to the layers, the carrier transportation is little hampered due to the strong conductive W interlayers, while confined Bi 2 Te 3 grains of high-quality lead to the increase of grain boundaries, which are also beneficial for filtering carriers.In the direction vertical to the layers, the multilayer structure would provide more interfaces and opportunities for blocking and filtering.The contact interfaces between semiconductor and metal would induce Schottky barrier, [31] which is critical for filtering low-energy carriers and regulating carrier concentration in a reasonable scale.The combination of semiconductor and metal can modify electronic properties because of the coupling and synergistic effects.The interfaces of different energy band structures can optimize the carrier transportation, even refine the overall TE properties.In conclusion, the interlayers of Sb and W behave differently in boosting the overall TE properties.The phase boundary modulation of Bi 2 Te 3 /Sb and the multilayer interface engineering of Bi 2 Te 3 /W allow the samples to conquer the intrinsic contradiction between the electrical conductivity and the Seebeck coefficient.Thus, the synchronously elevated electrical conductivity and Seebeck coefficient together facilitate excellent PF.This provides a feasible way for investigation and fabrication of TE materials with high properties.

Conclusion
Bi 2 Te 3 /Sb and Bi 2 Te 3 /W multilayer nanocomposite TE thin films were designed by using magnetron sputtering.The films are modified in terms of microstructure to optimize their TE properties.Because of the multilayer structure, the intrinsic contradictory relationship between electrical conductivity and Seebeck coefficient can be overcome.As a result, the multilayer samples exhibit very high electrical conductivity, which contributes to reduce energy dissipation while generating electricity.The experimental outcomes showed that the microstructure of Bi 2 Te 3 /Sb and Bi 2 Te 3 /W thin films can rationally regulate the carrier concentration and filter low-energy carriers to enhance the electrical conductivity and Seebeck coefficient at the same time.With the simultaneous elevation of electrical conductivity and Seebeck coefficient, Bi 2 Te 3 /W multilayers exhibits the highest PF of 1785 μWm −1 K −2 at 600 K. Overall, this work offers a novel approach to overcome the intrinsic contradictions of TE materials and provides the strategies for designing advanced novel multilayer/nanocomposite TE thin films with enhanced properties.

Figure 1 .
Figure 1.XRD patterns of as-deposited and annealed a) Bi 2 Te 3 /Sb and b) Bi 2 Te 3 /W thin films.Raman spectra of as-deposited and annealed c) Bi 2 Te 3 /Sb and d) Bi 2 Te 3 /W thin films.

Figure 2 .
Figure 2. Cross-sectional HAADF-STEM images of a) Bi 2 Te 3 /Sb and e) Bi 2 Te 3 /W multilayers thermally heated at 600 K. Corresponding EDX elemental maps of Bi, Te and Sb are shown in (b), (c), (d), respectively, for the Bi 2 Te 3 /Sb and in (f), (g), (h), respectively, for the Bi 2 Te 3 /W.Dark areas surrounded by light/grey background in (a) and (e) correspond to cavities.

Figure 3 .
Figure 3. a,b) High-resolution TEM images of Bi 2 Te 3 /Sb and Bi 2 Te 3 /W multilayers after thermally heating at 600 K. Cavities appear with bright contrast in the images.W marks W layers. c) Atomic-resolution HAADF-STEM micrograph of Bi 2 Te 3 grain formed in the Bi 2 Te 3 /W.Insert depict line profiles extracted along the line shown in (c).T marks Te layers, B marks Bi layers and I marks Bi/Te bilayers.d) Magnified HAADF image of area in (c) marked by rectangle.The bilayer defect is marked by red arrows.vdW depicts van der Waals gaps in (c) and (d).

Figure 4 .
Figure 4. a) Carrier concentration and b) mobility of the as-deposited Bi 2 Te 3 /Sb and Bi 2 Te 3 /W films as a function of annealing temperature.