High-performance freestanding thermoelectric membrane by intact exfoliation of van der Waals epitaxial bismuth antimony telluride lm

Separation of epitaxial thin lms on growth substrate and transferring onto other materials for functional heterostructures have boosted the transformative impact on science and technology. However, this scheme has proved challenging in thin lm thermoelectrics, but promised a vast range of applications beyond the limited device congurations of bulk thermoelectrics. Here, we demonstrate that the Bi 0.5 Sb 1.5 Te 3 (BST) epitaxial thin lm on sapphire substrate grown by spontaneous van der Waals epitaxy (vdWE) is exfoliated and transferred onto versatile materials, creating the high-quality freestanding thermoelectric membranes. Unprecedented millimeter-size vdWE BST membranes are produced by etching pseudomorphic Te monolayer on the surface of sapphire substrate in dilute HF solution. The intact exfoliation and direct transfer for vdWE BST membranes ensures a high thermoelectric performance, maintaining the high-quality crystallinity and subsequently showing the remarkable zT value (~0.9 at 300 K) in thin lm thermoelectrics. These results represent the realization of long pursued but yet to be demonstrated high-performance thin lm thermoelectrics, paving the way for design and fabrication of arbitrary shaped thermoelectric devices.


Introduction
With the increasing demand of sustainable and environmentally friendly energy supply, thermoelectrics have attracted a great interest in diverse elds ranging from electronics to energy storage and conversion because they can directly convert thermal energy to electricity 1 . Compared with bulk thermoelectrics, which have been widespread by using a at and rigid module device 2-4 , thin lm thermoelectrics have shown many attractive advantages such as micro-cooling ability for solving heat dissipation issues in electronics and energy scavenging from human body heat [5][6][7] . Indeed, thin lm thermoelectrics have a potential for curved and exible module devices, which promise a versatile energy harvesting from the close contact to various arbitrary geometries of heat sources 6,[8][9][10][11] . While organic thin lm thermoelectrics have a relevance for such devices, a low thermoelectric performance has been a major obstacle 12,13 . This provokes the inevitable use of inorganic Bi 2 Te 3 -based state-of-the-art materials for the curved and exible thermoelectrics 8,14-16 , but remains challenging due to the anisotropic brittle crystal and mechanical incompatibility.
Thermoelectric Bi 2 Te 3 -based thin lms have been achieved by the conventional heterogeneous epitaxial growth on single crystalline substrates with a small lattice mismatch and atomically at surface 17,18 , which suppress the formation of structural and atomic defects. However, this heterogenous epitaxial thin lm growth limits the applications to the curved and exible thermoelectrics due to the clamping effect, inherited from strong bond between overlayer lm and substrate at interface, of epitaxial thin lms on the at substrate 19,20 . Furthermore, it is extremely di cult to grow high quality epitaxial thin lms on the materials with large lattice mismatch or on the exible matters 21 . These obstacles have been solved by using the exible scaffold materials such as carbon nanotube anchored with highly oriented Bi 2 Te 3 nanocrystals, providing a promising way for practical exible thermoelectric devices 22 . Out of heterogeneous epitaxy scheme, the van der Waals epitaxy (vdWE) has proved an e cient growth of highperformance Bi 2 Te 3 -based thin lms on the various substrates [23][24][25] . Indeed, the layer structured Bi 0.5 Sb 1.5 Te 3 (BST) alloy, which are widespread p-type material for room temperature refrigeration and power generation, has been successfully grown via the vdWE on the graphene transferred amorphous silica substrate and on the pseudomorphic Te monolayer at the surface of sapphire substrate, showing comparable electrical properties to single crystal BST alloy. Considering the previous lift-off techniques for freestanding complex-oxide membranes 20 , this vdWE BST thin lm, which has a weak bonding at the vdW hetero-interface between Te-terminated quintuple layer and pseudomorphic Te monolayer on sapphire substrate, has a potential for intact exfoliation and direct transfer on curved or exible supports for freestanding thermoelectric membranes. The naturally-formed pseudomorphic Te monolayer on the sapphire substrate during in-situ spontaneous vdWE growth is of bene t in practical lift-off process in chemical solution, working as a sacri cial layer. Additionally, the sacri cial Te monolayer is expected to have a high etch selectivity, allowing to overcome the well-known shortcoming, a slow release rate 21,26 , of chemical lift-off for larger substrates.

Results
Here, we demonstrate a high-performance freestanding BST thermoelectric membrane on curved glass and exible polyethylene terephthalate (PETE) ( Supplementary Fig. 1), in which the vdWE BST thin lms grown by pulsed laser deposition (PLD) method are intactly separated from sapphire substrate and transferred onto various supporters using chemical lift-off technique (Fig. 1). First, we prepared vdWE BST thin lm on the sapphire substrate by spontaneous vdW epitaxy 25 . As highlighted, the in-situ formation of pseudomorphic Te monolayer on the surface of sapphire substrate is the key in the growth of VdWE BST thin lms, allowing the weak vdW bonding between the Te monolayer and the outermost Te atomic layer of the Te-Bi/Sb-Te-Bi/Sb-Te quintuple layer (Fig. 1a). Importantly, this diminishes a strong clamping effect between lm and substrate in the growth of heterogeneous epitaxial lms and promises an easy exfoliation by chemical etching of the Te monolayer or mechanical peeling-off technique using scotch tape. While the latter mechanical method requires a special process such as thermal heating and additional metal layer deposition, the former can allow the selective etching of Te monolayer in a dilute HF solvent. For preserving the epitaxial BST thin lm during the chemical etching of the sacri cial Te monolayer, approximately 100 nm thick polymethyl methacrylate (PMMA) was spin-coated on the top of the BST thin lm (Fig. 1b, c). When the etching is completed, the thin PMMA-capped BST lm is exfoliated from sapphire ( Supplementary Fig. 2) and becomes oating in the solvent. Once the BST lm with PMMA overlayer is transferred onto other substrates, the PMMA overlayer is completely removed using acetone, IPA, and DI water. To verify the suggested concept and methodology for transferring vdWE BST lms, SiO 2 /Si wafer, at and curved glasses, and exible polyethylene terephthalate (PETE) lms were chosen for the destination substrates (Fig. 1e). A strong adhesion to the substrates was achieved by baking at 373-393 K, ending the whole procedures for creating the rst freestanding thermoelectric membranes.
Bene ting from the sacri cial pseudomorphic Te monolayer, the crystallinity of epitaxial BST thin lm can be maintained during the transfer as shown by the scanning electron microscopy (SEM) observations and X-ray diffraction (XRD) measurements (Fig. 2). SEM cross-sectional images clearly show that the nano-grained structure of epitaxial BST thin lm is hardly damaged by the chemical etching ( Fig. 2d-f).
We conducted XRD f and w scans for the as-grown vdWE BST thin lm on sapphire substrate and the transferred BST membranes on at glass and exible PETE (Fig. 2g-i and Supplementary Fig. 3). The f scans of (105) re ections for the all samples ( Fig. 2g-i) exhibit a six-fold symmetry, indicating that the inplane crystallinity of epitaxial BST thin lm is well maintained in the transferred membranes on both at glass and exible PETE supports without any collapse of expitaxy. Except the diffraction peaks of substrates, all samples show only intense (00l) peaks of BST structure ( Supplementary Fig. 2). Additionally, the w rocking curves of the (0015) peak ( Supplementary Fig. 4), which show the decreased peak intensity and broadened FWHM of the transferred membranes on the curved glass and exible PETE, implying that the out-of-plane crystallinity of the transferred membranes slightly degrades due to the inherent brittleness of inorganic BST alloy.
The crystallinity and composition of the transferred BST membranes are also examined by the highresolution scanning transmission electron microscopy (HR-STEM). Figure 3a shows low magni cation top-view high-angle annular dark eld (HAADF) image of exfoliated BST membrane. The nano-domain structure is consistent with those observed in cross-sectional SEM images. Low magni cation energy dispersive spectroscopy (EDS) analysis shows the Bi, Sb and Te elements are homogeneously distributed in the membrane, indicating that no impurity phase occurs during the chemical etching and transfer ( Fig. 3b-e). Furthermore, cross-sectional image of transferred BST membrane on SiO 2 /Si wafer clearly evidences the intact transfer of epitaxial BST thin lm, maintaining the layered structure with quintuple BST layers in the membrane (Fig. 3f, g). A high crystallinity of BST membrane is con rmed by the perfect (1)  These structural features, as represented by the nano-grained structure with a high crystallinity (Fig. 3a, b), give an expectation for a high power factor (PF) comparable to that of single crystal and a low thermal conductivity similar to the nanostructured BST bulk, enabling a high performance thermoelectric freestanding BST membrane.
Thermoelectric transport properties are measured along the in-plane direction for as-grown epitaxial BST lms and transferred BST membranes at room temperature (Fig. 4). The room-temperature electrical conductivity (s) and Seebeck coe cient (S) values of as-grown epitaxial BST lms are ranged in 1300-1700 S cm -1 and 140-170 mV K -1 , respectively, resulting the PF of 3.0-4.0 mW m -1 K -2 at 300 K. The s values of the transferred membranes increased to ~1900 S cm -1 on at glass and decreased to ~1100 S cm -1 on curved glass and ~1250 S cm -1 on exible PETE (Fig. 4a). Accordingly, average roomtemperature S values showed the trade-off relation (Fig. 4a). The average PF values were ranged in 2.5-3.4 mW m -1 K -2 for the transferred membranes (Fig. 4b). We have measured the out-of-plane thermal conductivity (k out ) by using a conventional 2w method as shown in the inset of Fig. 4c. To estimate the in-plane thermal conductivity (k in ), we used the reported anisotropy ratio of k in /k out in bulk single crystalline BST is ~2.0 27 for the as-grown lms and transferred membranes owing to the high quality crystallinity. The k out and k in values of transferred membranes show the similar values with as-grown vdWE lms. The estimated k in values are lower than that of bulk single crystalline BST due to the nanogained structure. In-plane lattice thermal conductivity (k L,in ) for as-grown lms and transferred membranes on at and curved glass substrates were calculated by subtracting the electronic part from k in (see Supplementary Note 2 and Supplementary Fig. 7 for details). The k L,in values for all the lms and membranes are ~0.4 W m -1 K -1 , which is slightly higher than that (~0.3 W m -1 K -1 ) of nanostructured bulk BST with the high-density dislocation array at grain-boundary 28 . This indicates that the in-plane thermal conduction suffers from the intensive phonon scattering at boundaries between nano-sized mosaic domains as shown in Fig. 3a, b. The resultant thermoelectric gure of merit zT (= sS 2 T/k, where T is absolute temperature and k is the total thermal conductivity) values are shown in Fig. 4d with the reported zT values of Bi 2 Te 3 -based thin lms for comparison. The in-plane zT values for as-grown epitaxial BST lms are ranged in 1.0~1.6, which is comparable to that of state-of-the-art nanostructure bulk BST 29 and is the highest value in the reported BST thin lm 6,22,30,31 . Moreover, the estimated roomtemperature zT values for the transferred BST membranes are comparable to those of zone melted BST ingots used in the commercialized modules 32,33 . These values are also comparable to those of highperformance BST thin lms on the rigid substrates, proving the merit of present lift-off and transferring processes for the vdWE thin lms. By considering the projected e ciency of 4.6% of the transferred membranes (see Supplementary Note 3 for details) when the cold-and hot-side temperature are 300 K and 350 K, respectively, the present work is promising for the diverse arbitrary thermoelectric devices.
Among the transferred membranes, we conducted the bending test of the membrane on exible PETE (Fig. 5). From the original s, S and PF values of vdWE thin lm as a reference, all transport parameters of the membrane on exible PETE, its bended state with a radius of 15 mm and attened state showed no obvious changes but slight decreases (Fig. 5b-d), which are mainly attributed to the decrease of s coming from the generation of structural microcracks (Supplementary Fig. 8). This unfavorable structural defect in the membrane is accelerated in the cyclic exible bending tests. As shown in Fig. 5e-g, the S values upon bending cycles are almost unchanged while s values are largely degraded, leading to the loss of 40% of original PF at 100 bending cycles in a bending radius of ~10 mm. As observed microcracks in the bended membranes, a bending deformation creating a high density microcrack is responsible for the decreased performance in the membranes on exible PETE. However, it is apparent that there is sustainable performance, which was found from the membranes on curved glass, strongly suggesting that the present millimeter-sized membranes are more appropriate for the application of curved surface of heat source rather than the exible thermoelectric devices.

Discussion
In summary, we developed a strategy to fabricate high-performance freestanding thermoelectric membranes from the chemical exfoliation of high-quality vdWE BST thin lms on sapphire and intact transfer onto versatile substrates. Although the millimeter-sized BST membranes are di cult to apply for the exible thermoelectric devices, our demonstration is promising for the practical devices on the curved surface of versatile heat source such as human body. The zT value of ~0.9 at 300 K, which is comparable or much higher than those of previous BST-based lms, will stimulate the fabrication of highperformance n-type Bi 2 Se 2.7 Te 0.3 thermoelectric membranes using this method. Our demonstrations advance thermoelectric thin lm research by allowing unrestricted transfer onto diverse substrates for freestanding membranes.

Methods
Material preparation. Bi 0.5 Sb 1.5 Te 3 lms were deposited on a-Al 2 O 3 (001) single crystal substrate (10 ´ 10 mm 2 ) by PLD with the uence of 1.2-1.50 J cm -2 of KrF excimer laser (l = 248 nm) using a 2-inch diameter Bi 0.5 Sb 1.5 Te 3 target, while the target were prepared in the laboratory by using conventional solid state reaction followed by consolidation by spark plasma sintering. The substrate temperature was set at 200-230 ºC, while the argon partial pressure was at 10 mTorr. Films were deposited with ~130 nm in thicknesses. XRD analyses of 2q, w and f scans for the lms using X-ray diffractometer (Rigaku Smartlab, Rigaku Co.) with Cu Ka1 radiation (l = 1.540592 Å).
Thermoelectric properties measurement. The in-plane s and S were measured at room temperature by ZEM-3 system (Advanced Riko, Japan). The out-of-plane k out of the lms were measured with the 2w method (TCN-2w, Advanced Riko, Japan). The Hall measurement was performed by using the van der Pauw method (8400 HMS, Lake Shore Cryotronics, Inc., USA).
Microstructure characterization. The top and cross section morphology of as grown BST lm and BST lm transferred to at glass substrate were analyzed by SEM equipped with an EDS detector (FESEM, JSM-7000F, JEOL, Japan). Exfoliated lm for STEM was prepared using either a dual-beam focused ion beam (AURIGA CrossBeam Workstation, Carl Zeiss, Germany) or direct pulverizing by super sonication. The HAADF-STEM images were obtained from a probe-corrected STEM (JEM-ARM200F, JEOL, Japan) equipped with a cold eld emission source, operating at 200 kV. To improve the image resolution, successive images were acquired with short time intervals and averaged for the same area.

Data availability
The data that supports the ndings of this study are available from the corresponding author upon reasonable request.   Morphology and XRD analysis of as-grown vdWE BST lm and transferred BST membranes on at glass and exible PETE. a-c Photos and top-view SEM images of as-grown BST lm (a) and transferred BST membranes on at glass (b) and PETE (c) substrates. d-f Cross-section SEM images of the vdWE BST lms (d) and transferred BST membranes on glass (e) and PETE (f) substates. g-i scan measured by XRD for as-grown vdWE BST lm (g) and transferred BST membranes on at glass (h) and exible PETE (i), respectively.

Supplementary Files
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