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High-performance, semiconducting membrane composed of ultrathin, single- organic

Tatsuyuki Makitaa,b,c, Shohei Kumagaia,b, Akihito Kumamotod, Masato Mitania,b, Junto Tsurumie, Ryohei Hakamatania,b, Mari Sasakia,b, Toshihiro Okamotoa,b,c,f, Yuichi Ikuharad, Shun Watanabea,b,c,f,1 , and Jun Takeyaa,b,c,e,1

aMaterial Innovation Research Center (MIRC), Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan; bDepartment of Advanced , Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan; cAIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory (OPERANDO-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Kashiwa, Chiba 277-8561, Japan; dInstitute of Engineering Innovation, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan; eInternational Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan; and fPrecursory Research For Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved November 18, 2019 (received for review June 10, 2019)

Thin film (TFTs) are indispensable building blocks in 11). With recent developments in synthetic and device any electronic device and play vital roles in switching, processing, manufacturing, the performance of OTFTs has improved sig- and transmitting electronic information. TFT fabrication processes nificantly, with low-temperature solution processability, reason- inherently require the sequential deposition of metal, semicon- ably high field-effect mobility >10 cm2·V−1·s−1, and excellent ductor, and dielectric layers and so on, which makes it difficult environmental durability (12–15). to achieve reliable production of highly integrated devices. The Small-molecule OSCs can spontaneously form a highly integration issues are more apparent in organic TFTs (OTFTs), par- ordered assembly with weak van der Waals interactions. Under ticularly for solution-processed organic semiconductors due to optimum crystallization growth conditions, large single-crystal limits on which underlayers are compatible with the printing tech- thin films of OSCs of areas up to 100 cm2 can be produced (12, nologies. We demonstrate a ground-breaking methodology to 16–20). This raises the possibility of fabricating stable functional integrate an active, semiconducting of OTFTs. In this method, single-crystal thin film OSCs even in an ultrathin membrane a solution-processed, semiconducting membrane composed of form, similar to biological membranes such as cell membranes few-molecular-layer–thick single-crystal organic semiconductors is (21). For solution-processed OSCs, however, the formation of exfoliated by water as a self-standing ultrathin membrane on the high-quality single is limited by surface , wetta- water surface and then transferred directly to any given under- bility, roughness, etc., of the substrates/underlayers (11), and layer. The ultrathin, semiconducting membrane preserves its orig- inal single crystallinity, resulting in excellent electronic properties with a high mobility up to 12 cm2·V−1·s−1. The ability to achieve Significance transfer of -scale single crystals with almost no deteriora- tion of electrical properties means the present method is scalable. Organic thin film transistors (OTFTs) are promising building The demonstrations in this study show that the present transfer blocks in next-generation electronic devices due to the com- method can revolutionize and constitute a key patibility with the solution process of organic semiconductors step forward in TFT fabrication processes. (OSCs). Generally, OTFT processes inevitably face the seri- ous issue that the available substrates are limited to those organic semiconductors | thin film | organic single crystal that are solution-process compatible. In a striking contrast to conventional OTFT fabrication processes, we successfully he development in the early 1950s of various tech- demonstrate simple water exfoliation together with a transfer Tniques for inorganic material has made possible the mass method for solution-processed organic single-crystal semicon- production of single-crystal semiconductors of high quality (1, 2). ductors. An ideal single-crystalline form can be maintained Modern Si technology is based on single-crystal wafers, which is during the process, which results in excellent electronic perfor- an ideal platform for both efficient carrier transport and reliable mance. The methodology presented in this study allows the production of electronic devices (3). Freestanding single-crystal ideal production of OTFTs on a wide range of destination sub- wafers have found applications in optoelectronic technologies strates with sufficient scalability and expands the possibility because they can be formed as thin membranes which can be for OSCs to be employed in printed electronic devices. employed directly as a substrate. Because both the front and back Author contributions: T.M. and A.K. conceived the proof-of-concept of the water exfoli- surfaces of a wafer are available to integrate multiple layers by ation method; T.M., M.S., Y.I., S.W. and J. Takeya designed research; T.M., S.K., A.K., and homoepitaxial deposition, semiconductive membranes are use- R.H. performed research; M.M. and T.O. contributed new reagents/analytic tools; T.M., ful for manufacturing extremely high-density integrated circuits J. Tsurumi, and S.W. analyzed data; A.K. and T.M. performed TEM measurements and and sophisticated microdevices such as microelectromechanical analyzed data with significant input from Y.I.; S.K. and R.H. assisted in performing and analyzing X-ray measurements; M.M. and T.O. synthesized and purified the DNBDT com- systems (MEMS) (4, 5). Unlike devices fabricated directly on pound; S.W. and J. Takeya supervised research; and T.M., S.W., and J. Takeya wrote the wafers, thin film transistors (TFTs) require the sequential depo- paper.y sition of thin films of an active layer [for example, The authors declare no competing interest.y amorphous (6), compound semiconductors (7), and amor- This article is a PNAS Direct Submission.y phous metal oxide semiconductors (8, 9)], a dielectric layer, This open access article is distributed under Creative Commons Attribution-NonCommercial- and metallic contacts on top of the supporting substrate. This NoDerivatives License 4.0 (CC BY-NC-ND).y complicates the device fabrication process because TFTs inher- 1 To whom correspondence may be addressed. Email: [email protected] or ently have multiple heterointerfaces that need to be controlled. [email protected] Similar to most inorganic TFTs, organic TFTs (OTFTs) com- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ posed of organic semiconductors (OSCs) have a stacked-device doi:10.1073/pnas.1909932116/-/DCSupplemental.y structure, regardless of crystallinity and deposition method (10, First published December 19, 2019.

80–85 | PNAS | January 7, 2020 | vol. 117 | no. 1 www.pnas.org/cgi/doi/10.1073/pnas.1909932116 Downloaded by guest on September 28, 2021 Downloaded by guest on September 28, 2021 faC a crystal of of direction the denotes arrow (C white growth. The substrate. mica a on casting C C of of structure ular 1. Fig. bench- called process our solution a of method, edge-casting continuous film a via thin single-crystal crys- 3,11-dinonyldinaphtho[2,3-d:2 b:4,5- a single material, its this, and mark maintaining single-crystal substrate For while organic superhydrophilic it tallinity. ideal a exfoliate/laminate on an to defects then deposit without to film thin is Films. Semiconducting method Thin present of Exfoliation Water Results OTFTs. of of production durability ideal the and has allows scalability 90% method transfer than excellent this greater The of demonstrated. factor reliability 12 been to high up reasonably mobility a a and with performance device measurements, a diffraction X-ray and and diffraction ver- electron comprehensively 10 by as ified to process, entire up the of during coverage maintained areal single-crystal be large A remarkably substrate. a then given with can any form and this onto water super- directly by transferred a simply In be on exfoliated predeposited is OSC substrate manufacturing. hydrophilic single-crystal perfect OTFT a method, conventional OTFT revolutionizes scalable a toward method guarantees transfer which (see the fabrication displays, of OLED potential flexible the large-area of the in production adopted been mass already invented has been exam- method has This TFTs for (28–31). active TFTs, of transfer metal-oxide temperature by room ple, driven displays light-emitting (OLED) organic flexible diode In (22–27). investigated been have electronics. organic in developments further processes allow fabrication will TFT on limitations thin these OSC Eliminating single-crystal films. sur- homogeneous optimum fabricating for and to energy uniformity, face durability stability, the thermal for solvent, requirements do polymeric organic strong inks are fluorinated there substrate, OSC Hence, hydrophobic the Inc.). many highly been example, as a have extreme on such homogeneously an OSCs spread As of not date. underlayers to for limited candidates appropriate aiae al. et Makita at kV 80 of voltage accelerating an temperature. with room performed were measurements C a of tern C B A ee edmntaeagon-raigmtoooythat methodology ground-breaking a demonstrate we Here, films OSC of techniques transfer issues, these Regarding H 19 9 500 C DBTN igecytlti l arctdvacniuu edge- continuous via fabricated film thin single-crystal –DNBDT–NW 9 9 DBTN hnfimflaigo h ae ufc.(E surface. water the on floating film thin –DNBDT–NW 0 Molec- (A) film. thin OSC solution-processed a of exfoliation Water dtipee( ]dithiophene  m Photograph (D) exfoliation. water the of illustration Schematic ) 9 DBTN hnfimtaserdot E rd h TEM The grid. TEM a onto transferred film thin –DNBDT–NW Water C S 9 –DNBDT–NW film Mica substrate IAppendix SI Immersion 9 rs-oaie irsoyimage microscopy Cross-polarized (B) –DNBDT–NW. S C 9 –DNBDTNW C 9 H o ealddiscussion). detailed for 19 E D [100] zoneaxis Floating C Mica substrateleftbehind ,wsgrown was 1A), (Fig. ) 9 –DNBDT–NW film Dish filledwithwater 002 0 CYTOP ,3 h e othe to key The 011 0 -d cm 020 0 ]benzo[1,2- 2 ADpat- SAED ) · V cm −1 (AGC 2 ·s can −1 ugsstesuccessful the the Then, suggests of domains 1B) large (Fig. of substrate formation the mica the a of on cross-polarized with obtained A image nm). film microscope (∼10 were thin layers optical shearing molecular single-crystal few substrate uniform a of sub- and a thickness the achieve supply, of to solution temperature controlled of the speed as such strate, conditions deposition a the be as to employed contact measured was water was the mica angle which natural for (33). thin substrate, template of group superhydrophilic piece our cleaved by freshly developed A (32), ” “meniscus-guided epaesbtaeadte netdadpae aedown face placed and inverted on then technique and edge-casting substrate continuous A the template substrate. by a mica fabricated raw is a a film on there on that wrinkles than empirically and strate found cracks is It fewer substrate. are template substrate a glass as or pared mica cleaved freshly with A A 3A. treated Fig. structure. crys- in single elec- shown stacked OSC is transfer in a to tals component procedure into a a of as implemented illustration schematic used when be devices potentially tronic can film thin Technique. OSC Transfer of Development structure. formation pre- assembled the surface the broken, stabilizes of water effect hardly hydrophobic the and the stable because on sumably structurally floating are films exfoliation thin after comprehensively. OSC phenomena the exfoliation needed been Importantly, water are the studies has describe quantitative method further to TEM However, carbon-coated exfoliation 39). ultrathin (38, of similar grids preparation the in sur- established “self-cleaning” and well superhydrophilic the light-induced (34–37), for as low faces made known been relatively UV/O have observations, a effect, a Similar having 2C). example, substrate a (Fig. for on angle, performed contact be a of can exfoliation film Young–Dupr and the well-established Indeed, OSC the Appendix). using the analyzed of quantitatively between analogy be infiltration can an which water 2D), (Fig. a to layers mica with superhydrophilic face 3 a of angle has a contact substrate high 108 creates mica a of This of cleaved water observation of plane. the angle substrate with contact energy. consistent the surface be surface, to the hydrophobic can in perpendicular method difference stands present the the of C in terms substrate in understood hydrophilic a from Exfoliation. of DNBDT Mechanism of crystals single of bulk 16). a (14, analogs for lattice expected with packing bone assembly molecular of the constants original demon- where its spots preserves crystallinity, membrane diffraction single ultrathin (TEM) present observed the microscope that The strate pattern. electron (SAED) transmission 1E transferred tion a Fig. were sin- of measured. samples whether and grid film assess thin To the cracks substrate. obtained, onto or the were boundaries on crystals grain films gle as OSC can such the exfoliation in defects perfect exist if exfo- A even perfect film. obtained thin achieve be millimeter-scale to a required seconds few of are a liation process only that immersion Note the 1D. Fig. micro- of in cross-polarized by photograph the the verified under in water, film scope the bluish of a of surface observation the the on floats and the substrate step, mica simple this With 1C. C Fig. in illustrated schematically as 9 9 –DNBDTNW –DNBDTNW C 9 –DNBDTNW b UV 7 = /O ◦ .97 hnfimi xoitdimdaeyfo the from immediately exfoliated is film thin a ihyhdohbcaklcanthat chain alkyl hydrophobic highly a has 3 .Teciia ifrnei h sur- the in difference critical The 2B). (Fig. PNAS ogv uehdohlcsraei pre- is surface superhydrophilic a give to and A ˚ hw eetdae lcrndiffrac- electron area selected a shows h xoito fa S hnfilm thin OSC an of exfoliation The ∼3 | mc a etyimre nwater, in immersed gently was /mica aur ,2020 7, January ◦ c ◦ C uigcniuu edge-casting, continuous During . 6 = 9 .I otat freshly a contrast, In 2A). (Fig. –DNBDTNW . C 19 neflae single-crystal exfoliated An 9 UV –DNBDTNW geswt herring- a with agrees A 3 ˚ tetdgassubstrate glass -treated C C /O 9 9 | –DNBDTNW –DNBDTNW 3 o.117 vol. tetdgassub- glass -treated pto up ´ qain(SI equation e | o 1 no. ∼1 hnfilm thin mm thin thin | 81 2 .

APPLIED PHYSICAL SCIENCES ABC108° 3° 3° derived from transfer characteristics show no “kink-down” behavior (SI Appendix, Fig. S7), which supports the validity of the estimated values (42–44).

C –DNBDT–NW Mica UV/O treated Glass OTFT Fabrication on Commercially Available Food Wrap. We provide 9 3 another example of the effectiveness of the present transfer tech- D nique: OTFT fabrication on food wrap (Fig. 5A). Commercially OSC thin film available food wrap is normally composed of polyvinylidene chlo- Infiltration ride (PVDC), polyvinyl chloride (PVC), or polyethylene (PE). Mica substrate Highly hydrophobic surface Although they are cheap, easily mass produced, and show good Water Superhydrophilic surface barrier properties, they are not compatible as a substrate for ∼ ◦C Fig. 2. Mechanism of water exfoliation. (A–C) Contact angle profiles for OSCs because of their poor thermal stability ( 120 ) and durability against organic solvents. Approximately 10 µm of food surfaces of (A) C9–DNBDT–NW-, (B) mica-, and (C) UV/O3-treated glass substrates. (D) Schematic illustration of the water infiltration. wrap was fixed on a glass plate by adhesive tape. A gold gate electrode was deposited through a shadow mask. An insulating polymer, parylene was deposited with a thickness of 200 nm via on the destination substrate. A few droplets of water are then chemical vapor deposition. Then, a C9–DNBDT–NW thin film applied near the edge of the template substrate, leading to was transferred from the template substrate. Finally, a 40-nm– immediate water infiltration and resulting in the exfoliation of thick gold source and drain electrodes were deposited through a the C9–DNBDT–NW thin film by the mechanism discussed shadow mask followed by electrical isolation of the OSCs via a above. Because the solution-processed OSC film is sufficiently YAG . Note that during the whole procedure, the substrate thin [composed of a few molecular layers (16)], the exfoliated is not heated or exposed to any organic solvents, except for the C9–DNBDT–NW thin film is physisorped onto the destination radiation heat during the deposition of the parylene and gold substrate by the electrostatic force. This process does not require electrodes. The formation of single-crystalline thin films was con- the use of organic solvents or substrate heating, which means firmed by cross-polarized optical microscopy images of a device it can be used for a wide range of destination substrates. As fabricated on food wrap (Fig. 5B); an almost completely black an ultimate example, Fig. 3B shows a photograph of a leaf on image is obtained when the crystal growth direction is parallel or 2 which a C9–DNBDT–NW thin film of ∼10 × 10 mm was trans- perpendicular to the angle, which indicates that the ferred. Note that the surfaces of leaves are considered the most crystal axes are highly oriented. The transistor characteristics of inappropriate surfaces for solution growth of OSCs because the the fabricated bottom-gate top-contact OTFT are shown in Fig. 5 surface is weak to heating, uneven, and hydrophobic. The square- C–E. All 25 transistors operate with an average mobility of 11.4 ± shaped area surrounded by dashed lines repels water more than 1.7 cm2·V−1·s−1 and reached as high as 13 cm2·V−1·s−1. Again, the other areas, which suggests the C9–DNBDT–NW film was this result suggests that exfoliated OSC single-crystal thin films successfully deposited. To confirm the crystallinity of the trans- can show excellent electronic properties without fatal deterio- ferred C9–DNBDT–NW thin film, an X-ray diffraction (XRD) ration during the fabrication process. The excellent mechanical measurement was performed. Fig. 3C shows the diffraction pat- flexibility and adhesiveness of food wrap may be useful for many tern of the C9–DNBDT–NW film transferred on a 30-µm–thick applications; Fig. 5F shows an OTFT array fabricated on food glass substrate. The observed spots can be assigned to a her- wrap attached to an apple. OTFT fabrication on a polyethylene ringbone packing structure, which is consistent with the results terephthalate (PET) film was also successfully demonstrated (SI of the TEM measurement and agrees with the bulk structure of Appendix, Fig. S10). DNBDT analogs.

OTFT Fabrication with a Highly Hydrophobic Gate Dielectric Layer. The transfer technique described above enables the deposition A of single-crystal OSC thin films on any given hydrophobic Glass/C9–DNBDT–NW Remove glass substrate. Fig. 4A shows the device configuration of fabricated Drop water bottom-gate top-contact OTFTs using the present transfer technique. As a gate dielectric layer, SiO2 coated with a fluorinated polymer, CYTOP, was employed. Generally, CYTOP gives an ideal surface with extremely low trap density (40), while its highly hydrophobic nature hinders direct solution Destination substrate Transferred C9–DNBDT–NW film growth of single-crystal OSCs. Fig. 4B shows an observation BC of a transferred thin film using a laser confocal microscope, confirming the successful transfer of millimeter-sized domains of C9–DNBDT–NW thin film. Subsequently, 2,3,5,6-tetrafluoro- 7,7,8,8-tetracyanoquionodimethane (F4-TCNQ) (41) and 40-nm–thick gold were deposited through a shadow mask Covered with highly hydrophobic OSC film followed by patterning of OSC thin film using an yttrium– b* aluminum–garnet (YAG) laser. Fig. 4 C–E shows the transistor c* characteristics of the fabricated OTFT. The transfer charac- teristics in both the saturation and linear regimes exhibit a Fig. 3. Transfer technique for OSC thin film. (A) Schematic illustration of negligibly small hysteresis and high on–off ratio of larger than the transfer technique. A few droplets of water are applied near the edge 8 of a template substrate, represented here by UV/O -treated glass, with a 10 . The mobility was derived from the transconductance to 3 ∼ cm2·V−1·s−1 ± cm2·V−1·s−1 C9–DNBDT–NW thin film placed on the destination substrate, after which be 12 (9.5 1.8 on average for the template substrate is carefully removed. (B) Photograph of a leaf on 24 measured transistors). The mobility of the present OTFT which a single-crystal thin film of C9–DNBDT–NW was transferred. The area reaches as high a value as those previously reported from surrounded by the dashed square is covered with the C9–DNBDT–NW thin our group with DNBDT analogs (14, 16). It should be also film. (C) X-ray oscillation photograph of the transferred C9–DNBDT–NW film emphasized that the gate voltage dependences of the mobility measured on a 30-µm–thick glass substrate.

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APPLIED PHYSICAL SCIENCES |

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Fig. 6. OTFT array made of a transferred inch-size C9–DNBDT–NW thin film. (A) Photograph of an inch-size C9–DNBDT–NW thin film deposited on a SiO2/Si wafer coated with parylene. (B) Schematic illustration of the present device configuration. (C) Transfer characteristics of the 100 fabricated OTFTs. The transfer characteristics of the present device are shown in D for the saturation regime and in E for the linear regime. (F) Output curves of the present device. L and W are 100 µm and 500 µm, respectively. (G–I) Histograms of (G) µsat,(H) Vth, and (I) reliability factor for the 100 fabricated OTFTs.

identical to the direction where the degree of crystal packing ularly, solution-processed OTFTs exhibit excellent electronic is the strongest. Fortunately, the most preferable charge car- properties without sacrificing their bulk properties. The uni- rier conduction always goes along the strongest packing direction versality and scalability of this technique were also confirmed. (intuitively, the transfer integral is likely the largest along the The technique demonstrated here is compatible with roll-to-roll strong packing direction). Therefore, these microcracks have a production processes together with low-cost printing technolo- negligible effect on the field-effect mobility. In fact, we success- gies for OSC thin films, which presents a great opportunity fully verified that the original field-effect mobility is almost pre- for OSCs to be employed in a wide range of electronic device served even after transfer and still within the benchmarked value applications. of larger than 10 cm2·V−1·s−1, which shows that the present transfer method does not fatally hamper the charge carrier con- Materials and Methods duction. From the microscopic viewpoint of charge transport Single-Crystal OSC Film Transfer. A natural mica (purchased from Nilaco Corporation) was cut into the desired shape, and a cleaved facet was nature, the mean-free path of the present C9–DNBDT–NW was estimated to be on the order of 10 nm (45). The obser- formed to give a superhydrophilic surface. Also, samples of EAGLE XG glass (Corning Inc.) with a thickness of 0.7 mm were treated with UV/O vation of ideal mobility preserved even after transfer manifests 3 for 15 min. Single-crystal thin films of C9–DNBDT–NW were grown from itself that these microcracks are never generated within a 0.02 wt% 3-chlorothiophene solution using continuous edge-casting, as 10-nm scale. described in our previous work (33). During the thin film growth, the Any substrate can be used as the template substrate, as long substrate was heated to 90 ◦C and sheared with a shearing speed of as its surface is superhydrophilic, specifically for which the water 20 µm·s−1. After the OSC film growth, the substrate was cut into pieces contact angle is less than 10◦. A wide variety of substrates can and inverted on the destination substrate to be transferred. A few droplets be used as the destination substrate, which is one of the most of ultrapure water were applied near the point of contact between the / important results in this study. Note, however, that the desti- two substrates. Finally, the mica substrate or UV O3-treated glass substrate nation substrate needs to be water resistant because it is likely was carefully taken away to complete the transfer. The template substrate to be exposed to water, although the time frame for the con- is recyclable. Details of the TEM measurement, XRD measurement, device fabrication, tact with water is very short. Most importantly, the substrates on and electrical measurements are described in SI Appendix. which the OSCs are deposited are not limited by the require- ments inevitably associated with solution processes, such as Data Availability. The chemicals, device preparation procedures, characteri- temperature resistance, hydrophobicity, roughness, and solvent zation methods, and supplementary data are detailed in SI Appendix. tolerance. Clearly, this overturns the common understanding for ACKNOWLEDGMENTS. T.M. was supported by a grant-in-aid through a conventional OTFT fabrication processes and revolutionizes the Japan Society for the Promotion of Science (JSPS) Research Fellowship. fundamental technologies for vertically stacked electronic device T.O. acknowledges support from PRESTO, JST through the project “Scien- manufacturing. tific Innovation for Energy Harvesting Technology” (Grant JPMJPR17R2). In conclusion, we have successfully developed a method S.W. acknowledges support from PRESTO, JST through the project “Hyper- nanospace Design Toward Innovative Functionality” (Grant JPMJPR151E). to obtain high-performance, semiconducting membranes com- A.K. and S.W. acknowledge support from the Leading Initiative for Excel- posed of few-molecular-thickness OSCs. The simple water exfo- lent Young Researchers of JSPS. This work was also supported in part by liation step allows the formation of self-standing OSC thin JSPS Grants-in-Aid for Scientific Research (KAKENHI) grants (JP17H06123, films floating on the water surface. The preservation of single JP17H06200, and JP17H03104). TEM measurement was conducted at the Advanced Characterization Nanotechnology Platform of the University of crystallinity, confirmed by electron and X-ray diffraction mea- Tokyo, supported by the “Nanotechnology Platform” of the Ministry of surements, indicates the utility of these electronic membranes Education, Culture, Sports, Science and Technology, Japan. We thank Ms. as components of next-generation electronic devices. Partic- Kayoko Sato for assistance with experiments.

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