A exible micro-thermoelectric device from carbon nanotube-epitaxially grown (Bi,Sb)2Te3 nanocrystal

: 22 Flexible thermoelectric (TE) materials have attracted increasing interest due to their potential 23 applications in energy harvesting and high-spatial-resolution thermal management. However, a high- 24 performance flexible micro-TE device (TED) compatible with the modern electronics fabrication 25 process has not yet been developed. Here we report a general van der Waals epitaxial growth approach 26 to fabricating a freestanding and flexible hybrid comprised of single-wall carbon nanotubes and 27 highly ordered (Bi,Sb) 2 Te 3 nanocrystals. High power factors ranging from ~1,680 to ~1,020 µW m − 1 28 K − 2 in the temperature range of 300-480 K, combined with a strongly depressed thermal conductivity 29 yield an average figure of merit of ~0.81. A prototype flexible micro-TED module consisting of two 30 p-n hybrids was then fabricated, which demonstrated an unprecedented open circuit voltage of ~22.7 31 mV and a power density of ~0.36 W cm − 2 under a ~30 K temperature difference, and a net cooling 32 temperature of ~22.4 K and a heat absorption density of ~92.5 W cm − 2 . 33 Our show the low-angle tilt grain boundary (GB) is important in determining the carrier and phonon transport properties as well as mechanical flexibility. We designed and fabricated a prototype micro-TEC/TEG module consisting of two p-n couples of the freestanding and flexible SWCNT-(Bi,Sb) 2 Te 3 hybrid, which has a record-high power generation and cooling ability and excellent flexibility. This work demonstrates a promising strategy flexible micro-TEDs with high

With the meteoric growth of modern electronics, there has been a significantly increased requirement 37 for energy harvesting and thermal management technologies for use in portable electronics 1,2 and on-38 chip integrations 3-5 . Recycling environmental waste heat into electrical energy using TE technology 39 based on the Seebeck effect is considered one of the most promising solutions 6,7 . Such TE generators 40 (TEGs) complement chemical or solar batteries by extending their life or even replacing them 8 . 41 Thermal management based on the Peltier effect of TE materials is a positive cooling technology 9 , 42 and a TE cooler (TEC) enables site-specific, green, and on-demand cooling, handling a large heat TEDs 13-15,19,24-31 is usually far worse than that of conventional ones. Second, the micro-TEDs are 51 greatly needed in nW to mW self-powered nano-micro electronic devices without recharging 4 , and in 52 the thermal management of electronics requiring high precision, high-spatial-resolution and fast 53 temperature control 5,10 . However, because of the natural brittleness of conventional inorganic TE 54 materials, it is very difficult to reduce their dimensions using traditional top-down processing to 55 fabricate on-chip micro-TEDs. Bottom-up approaches using thin-film technology have therefore been 56 developed 4,5,10 , but the main drawback is their poor TE performance compared with the conventional 57 bulk TEDs. Third, in micro-TEDs where the heat flow is perpendicular to the device plane 4,5,10 , 58 difficulties in the fabrication and maintenance of the temperature gradient are very challenging. In 59 contrast, micro-TEDs with a lateral structure where the heat flow is parallel to the device plane, can 60 be more easily fabricated by integrated circuit (IC) compatible technologies, which allows the TEDs 61 to fit various complex packaged electronic devices. But such a micro-TED has poor performance 62 because of the heat loss/short circuit through the substrate 25 . Consequently, freestanding thin-film TE   Here we show a general epitaxial growth approach to preparing freestanding and flexible hybrids 73 of single-wall carbon nanotube (SWCNT)-nanocrystal materials, which does not require a small 74 lattice mismatch and allows fine dopant tuning for property optimization. P-type SWCNT-75 (Bi,Sb)2Te3 hybrid TE films with well-aligned crystallographic orientations were fabricated using a 76 layer-by-layer growth process, as illustrated in Fig. 1. This hybrid TE material has an average figure 77 of merit (ZT) of ~0.81 in the temperature range of 300-370 K, because of its highly ordered 78 microstructure and optimized composition. Our experimental and computational evidence show that 79 the low-angle tilt grain boundary (GB) is important in determining the carrier and phonon transport 80 properties as well as mechanical flexibility. We designed and fabricated a prototype micro-TEC/TEG 81 module consisting of two p-n couples of the freestanding and flexible SWCNT-(Bi,Sb)2Te3 hybrid, 82 which has a record-high power generation and cooling ability and excellent flexibility. This work 83 demonstrates a promising strategy to fabricate flexible micro-TEDs with high performance.

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Epitaxial growth of a SWCNT-nanocrystal hybrid 85 Since different crystallographic orientations mean different surface/interface atomic and electronic 86 structures, microstructure control is crucial for designing novel structural and functional materials 87 with desirable physical and chemical properties. This is especially true for materials that have 88 anisotropic TE properties, where obtaining a high anisotropy in the polycrystalline material similar 89 to that in the single-crystal is an effective approach to achieving a high TE performance 8 . The   is achieved when the Bi:Sb ratio is ~0.5:1.5. A dense Bi0.5Sb1.5Te3 film deposited on a SiO2/Si wafer 154 with the (000l)-textured and a non-(000l)-textured hybrid were also prepared for comparison. The the (000l)-textured hybrid is ~42% higher than that of the non-(000l)-textured one and ~35% lower 169 than that of the (000l)-textured dense film ( Supplementary Fig. 21a). with an increase of Th (Fig. 4b), which could be caused by the increased Th and TE performance 38 .  The ability of the micro-TEG module to convert thermal energy to electric power was also tested 231 with the cold-side temperature maintained at RT (Fig. 4f,g and Supplementary Note 8). The output TE voltage increased linearly with the temperature gradient (ΔT g ) and the calculated total || value 233 of a p-n couple is close to the sum of measured || values of p-and n-type hybrids ( Fig. 3b and 234 Supplementary Fig. 23), indicating that the contact thermal resistance in the micro-TED module is 235 negligible. When the load resistance matches the micro-TEG module internal resistance, the 236 maximum output power (P g max ) is obtained, which increases with ΔT g 2 . When the ΔT g reaches ~30 Sb 2 Te 3 nanocrystals with 5º, 15º and 30º tilt-angle GBs after 5 bending cycles.

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The flexibility of the hybrid film was estimated by measuring its relative change of resistance (R/R0) 273 at different bending radii (Fig. 5a,b). The bending strain ( b ) can be approximately estimated from

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The flexibility of the micro-TED module was also evaluated. To realize bending at the microscale, 282 an AFM was used to push the suspended SiNx membrane connected to a p-n couple. From the 283 displacement of moving end of the p-n couple, the bending radius was estimated ( Supplementary Fig.   284 25). We can see that due to the freestanding structure and thickness effect, the p-n couple can be 285 freely deformed with a bending radius down to ~0.2 mm without any obvious increase in resistance, 286 revealing its excellent flexibility (Fig. 5a). Moreover, the p-n couple also shows good electrical 287 stability after ~1,000 bending cycles (Fig. 5b). MD simulations show the (Bi,Sb)2Te3 nanocrystals in 288 the (000l) orientation with a low-angle tilt GB along the <12 ̅ 10> direction have a much better bending 289 flexibility than others (Fig. 5c-f, Supplementary Note 11 and Supplementary Fig. 26), which is in  this work were performed to first compute the static energy. The kinetic evolution of the SWCNT- The data supporting the findings of this study are provided in the Supplementary Information.

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Additional relevant data are available from the corresponding authors upon reasonable request.

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Code availability 376 The code or subroutines that support the findings of this study are available from the corresponding  Competing interests 396 The authors declare no competing interests.

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Supplementary information is available for this paper at https://XXX.