Oxide Heteroepitaxy for Flexible Optoelectronics - ACS Applied

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Oxide Heteroepitaxy for Flexible Optoelectronics Yugandhar Bitla, Ching Chen, Hsien-Chang Lee, Thi Hien Do, Chun-Hao Ma, Le Van Qui, Chun-Wei Huang, Wen-Wei Wu, Li Chang, Po-Wen Chiu, and Ying-Hao Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10631 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 8, 2016

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ACS Applied Materials & Interfaces

Oxide Heteroepitaxy for Flexible Optoelectronics

Yugandhar Bitla,† Ching Chen,† Hsien-Chang Lee,† Thi Hien Do,‡ Chun-Hao Ma,§ Le Van Qui,† Chun-Wei Huang,† Wen-Wei Wu,† Li Chang,† Po-Wen Chiu,§ and Ying-Hao Chu†,‡,∆,* †

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

‡

Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

§

Institute of Electronics Engineering, National Tsing Hua University, 30013 Hsinchu, Taiwan

∆

Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan

KEYWORDS: Transparent conducting oxides • Flexible electronics • Optoelectronics • Muscovite mica • Heteroepitaxy

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Abstract The emerging technological demands for flexible and transparent electronic devices have compelled researchers to look beyond the current silicon-based electronics. However, fabrication of devices on conventional flexible substrates with superior performance are constrained by the trade-off between processing temperature and device performance. Here, we propose an alternative strategy to circumvent this issue via the heteroepitaxial growth of transparent conducting oxides (TCO) on the flexible mica substrate with comparable performance to their rigid counterparts. Taking the examples of ITO and AZO for the case study, a strong emphasis is laid upon the growth of flexible yet epitaxial TCO relying muscovite’s superior properties compared to conventional flexible substrates and its compatibility with the present fabrication methods. Besides excellent optoelectro-mechanical properties, an additional functionality of high-temperature stability, normally lacking in the current state-of-art transparent flexitronics, is provided by these heterostructures. These epitaxial TCO electrodes with good chemical and thermal stabilities as well as mechanical durability can significantly contribute to the field of flexible, lightweight and portable smart electronics.


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Introduction Over the last decade, there have been dramatic technological advances in portable electronics, flexible electronics, multifunctional windows, and numerous other devices that feature transparent electrodes.1-3 Transparent conducting oxides (TCO) have served as fundamental components in advanced technological devices spanning solar cells, light emitting diodes, photodiodes, thin film transistors, photocatalysis, flat panel displays, gas sensors and energy efficient windows. As transparent, flexible, and portable electronics continue to proliferate, the development of fabrication routes to TCO with superior performance has become a subject of study. An ideal TCO would be fully transparent in a wide range of wavelengths as well as show metal-like conduction. However, the realization of intended excellent properties of highly crystalline epitaxial TCO thin films with good mechanical flexibility are severely impeded by the amorphous nature of most commercial flexible substrates. The routinely used transparent yet flexible substrate like ultra-thin glass is fragile and costly while polymer substrates are not thermally stable and hence, hinder the growth of high-quality films. In view of the above considerations, non-magnetic and insulating muscovite mica serves as an ideal substrate for flexible optoelectronic applications due to its high transparency, atomically smooth surface, thermal and chemical stabilities, flexibility, mechanical durability, and compatibility with present fabrication methods. The monoclinic mica KAl2(AlSi3O10)(OH)2 possesses a two-dimensional structure with an AlO6 octahedral layer sandwiched by two (Si,Al)O4 tetrahedral layers. The sandwich structures are stacked further by a layer of potassium cations between the negative silicon oxide layers. Moreover, unlike the conventional substrates, the crystalline structure of mica allows the van der Waals epitaxial growth of oxide thin films resulting in optimal performance.4-7 These outstanding properties make muscovite mica a favorite substrate for flexible and transparent electronics that are central to all future smart functional devices. The research on TCO materials has been mainly focused around the minor variants of 3 Environment ACS Paragon Plus


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n-type In2O3, SnO2, and ZnO films.2,3 Among the known TCOs, ITO is the most favorite one owing to its unique combination of optical transparency, electrical conductivity, and ease of fabrication. However, indium is rare, expensive, chemically unstable in reducing ambient, toxic to humans and the environment.8 After years of development, Al-doped ZnO (AZO) is perhaps the best indium-free TCO which comes closer to ITO's performance and yet addresses the aforementioned issues of In. In the last decade, an amorphous transparent semiconducting oxide2, InGaZnO4 (IGZO) with high mobility ~10 cm2/Vs (30-50 times higher than a-Si), low leakage current, high transparency and moderate conductivity has been extensively studied and commercialized. However, it is expected that these oxides will still underperform against their single crystalline counterpart. Therefore, it is imperative that the response of flexible heteroepitaxial oxides will be much faster than IGZO due to their higher mobilities. In this work, we demonstrate the realization of epitaxial TCO/mica electrodes taking ITO and AZO as examples for flexible optoelectronic applications by investigating their optical, electrical, chemical and thermal properties as well as mechanical flexibility and durability. The ITO/mica and AZO/mica heterostructures not only retain the superior properties of epitaxial films but also exhibit good flexibility and durability. Moreover, these epitaxial TCO films can further facilitate the growth of high-performance planar epitaxial devices that are stable even at elevated temperatures. If functional materials can be designed and developed on them, such efforts will have a significant impact on flexible optoelectronic device applications.

Results and Discussion The phase identification and crystal structure of the heterostructures were examined by highresolution x-ray diffraction. Figure 1(a) shows typical out-of-plane 2θ-θ scans of the 500 nm thick ITO/mica and AZO/mica heterostructures along with the reference mica substrate. The observation of only ITO(lll) and AZO(001) diffraction peaks on muscovite (00l) suggests the 4 Environment ACS Paragon Plus

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epitaxial nature of TCO thin films without other secondary phases. The out-of-plane lattice constants of ITO and AZO films extracted from the XRD peak positions are 4.87 Å and 5.212 Å with slight compressive strains of 0.18% and 0.11%, respectively. In this heteroepitaxy, the interaction between TCOs and muscovite is very weak due to the nature of van der Waals epitaxy.4,5 Thus, the strain state of TCOs is independent of the thickness (Fig. S1). The observed strain is more related to the growth condition as different defects induced during the process dictate the strain state of TCO films. Furthermore, the Φ-scans of the ITO{440}, AZO{101}, and muscovite {202} reflections were employed to analyze the in-plane structural relationship as shown in figure 1b. The observation of three muscovite {202} peaks at 120° intervals indicates different stacking sequence between sheet units along the c axis9 while cubic bixbyite ITO{440} and hexagonal AZO {101} exhibit six peaks at 60° intervals indicate the growth of single crystalline AZO and multi-domain ITO films on Mica substrate. Different domains of ITO are separated by 60o rotation around [111] ITO. Based on the XRD results, the epitaxial relationship between TCO and Mica can be determined as (222)ITO//(001)Mica and [

0]ITO//[010]Mica for the ITO/mica heterostructure, and (001)AZO//(001)Mica and

[010]AZO//[010]Mica for the AZO/mica heterostructure. The Rocking curves measured to obtain the critical information about the crystalline quality resulted in the full width at half maximum (FWHM) of about ~2.3° and ~1° for ITO(222) and AZO (002) peaks, respectively, as shown in figure 1(c). The larger value of FWHM for ITO film is attributed to the multi-domain feature. Based on the XRD results, the schematics of epitaxial ITO and AZO thin films grown on (001) native muscovite are displayed in figures 1(d)-(e). In order to characterize the detailed microstructure on the TCO/mica heterostructure and further confirm the heteroepitaxy, the film-substrate interface was examined by transmission electron microscopy (TEM). Figure 2 shows high-resolution cross-sectional TEM images taken along [100]Mica and [010]Mica zone axes revealing clear, defect-free and semi-coherent (a) ITO/YSZ, YSZ/mica, and (b) AZO/Mica interfaces. The corresponding Fast Fourier 5 Environment ACS Paragon Plus


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Transform patterns of selected area of ITO (red), YSZ (orange), AZO (blue) and muscovite (yellow) marked in 2(a)-(b) are shown in the bottom panels, respectively. The reciprocal lattices of ITO, YSZ, AZO and muscovite are clearly indexed and consistency of the epitaxial relationship with the XRD results is confirmed. It has been demonstrated that ITO exhibits best epitaxy on insulating YSZ10,11 due to their excellent lattice matching ( 1000 nm), TCO beyond λp(Plasma wavelength) become optically opaque due to the free-electron plasma resonance. The optical energy gap as a function of thickness is plotted for ITO and AZO in the inset of figure 3(b). The optical band gap of 4.1(2) eV for AZO and 3.7(2) eV for ITO are obtained and exhibit weak decrement with thickness. The Hall measurements on the TCO films confirmed their n-type semiconducting nature. The thickness dependent electrical 6 Environment ACS Paragon Plus

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conductivities of TCO/mica (Filled circles), epitaxial TCO10-16 (Asterisk symbol), polycrystalline TCO12,17-32 (Open circles) and TCO on flexible substrates17-19,24,25,33-47 (Open circles with cross marks) are compared in figure 3(c). It can be seen that the conductivity of TCO/mica increases with thickness and has values comparable to epitaxial TCO obtained on rigid substrates. The mobility of TCO/mica as a function carrier concentration displayed in figure 3(d) are on par with epitaxial TCO as they overcome the grain boundary scattering inherent to those on flexible substrates. The extracted mobility and carrier concentration for various thicknesses show that the electrical properties of TCO films improve with the thickness and are comparable to the reported ones; the best parameters are achieved for ~ 150(250) nm ITO(AZO) thick films with a resistivity as low as ρ ~ 5(30) 10-5 Ωcm and a mobility as high as µ ~ 50(20) cm2 /Vs. Therefore, these films can be used as transparent conducting films in the visible light range. The optical transmittance of TCO/mica as a function of sheet resistance presented in figure 4(a) displays higher transmittance and lower sheet resistance comparable to epitaxial TCO. Thus, it is evident that the improved structural quality of TCO/mica is responsible for superior optoelectronic performance than that observed on conventional soft flexible substrates. This gets further reflected in the figure of merit (ΦTC), an important parameter for judging the performance of TCO, defined by48

, where T is the average

optical transmittance and Rs is the sheet resistance. It is known that the higher the ΦTC the better the quality of the transparent conducting films.49 ΦTC measured as a function of TCO thickness is shown in figure 4(b) for TCO/mica, epitaxial TCO, polycrystalline TCO and TCO on flexible substrates. Higher figure of merit values for epi TCO and TCO/mica can be identified due to their improved structural quality. Furthermore, an attempt is made to extend the TCO/mica electrodes practical application by performing a range of cyclability tests, which they might actually undergo during or after the possible manufacturing processes and operation conditions of smart devices. In view of 7 Environment ACS Paragon Plus


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TCO-based flexible optoelectronic applications, the operational stability and mechanical durability of TCO/mica was tested under two different bending conditions (flex-in mode: TCO is under compressive strain and flex-out mode: TCO is under tensile strain as depicted in the inset of figure 5). Figures 5(a)-(b) show that the values of measured sheet resistance remain approximately constant under bending radius of 4 mm (7 mm) and 6 mm (9 mm) during both the compressive and tensile deformations, respectively, for ITO(AZO) indicating the robustness of epitaxial TCO/mica under bending for flexible device application. In spite of inherent fragility and lack of flexibility of TCO, flexible mica substrate enables better flexing of TCO/mica via the heteroepitaxy maintaining high crystalline quality. These critical bending radius values are better than those reported for ITO on mica,36,51 PEN,52,53 PET54-56 and AZO on PES33. It is evident from figures 5(a)-(b) that the critical bending radius of curvature decreases with TCO thickness. It is noteworthy that the critical bending radius or the critical strain for TCO (Figure S2) under compressive strain ( AZO) is higher than the tensile strain ( values are comparable to

= 0.75% for ITO and 0.80% for

= 0.41% for ITO and 0.46% for AZO). These

= 1.1% for ITO50, 1.03% for AZO33 and

= 1.7% for ITO50,

1.74% for AZO33. However, the critical strains of TCOs are strongly dependent on the film and substrate thickness. Furthermore, the conduction stability of TCO/mica as functions of bending cycles and time in flex-in mode was examined. The AZO/mica exhibited very stable sheet resistance even after 2000 bending cycles under a radius of curvature of 8 mm as shown in Fig. 5(c) while ITO/mica exhibited a 10% increase in its resistance beyond 100 cycles. The inherent fragile nature of ITO may impose this limitation. However, the current TCO electrodes still exhibit better durability than those on conventional soft substrates as a result of their high crystalline quality. Note that the properties of TCO were stable under compressive bending for longer durations of time in open atmosphere at room temperature as shown in figure 5(d). This shows long-term optoelectronic stability of TCO under mechanical flexing.


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Furthermore, the thermal and chemical stabilities of TCO/mica are of paramount importance for their practical applications. The commonly used polymer substrates exhibit curling57 when the temperature rises above 150oC and get easily damaged thereafter. This often leads to such device failures which need operation at high temperature. This issue is effectively tackled by TCO/mica which maintains its inherent metallic character under bending even at high temperatures as shown in Figure S3. Unlike, the most flexible TCOs, the TCO/mica electrode displays thermal stability at elevated temperatures (as high as 300 oC) as shown in Figure S3. This is due to the underlying mica’s higher melting point than the conventional polymer substrates. Additionally, the TCO/mica electrodes dipped in various chemicals including acetone, ethanol, xylene, toluene and chlorobenzene for at least 10 min duration show almost no change in their resistivity values ascertaining their chemical inertness (Figure S4). No visible damage was noticed on their surface.

Conclusions In conclusion, high quality transparent conducting oxide heteroepitaxy on flexible and transparent muscovite has been successfully demonstrated. Furthermore, these electrodes retain the superior performance of the epitaxial TCO yet display good flexibility at the same time robust against chemical and thermal constraints. The TCO/mica heterostructures also provide a platform to integrate functional devices epitaxially on them and hence, their strong commercialization in various walks of life is anticipated. Due to the high melting point of mica, the devices designed on top of these templates can be stable at elevated temperatures. Moreover, our process can be transferred to the sputtering process for large scale production. Therefore, the current study paves the way for an exciting new avenue to the next generation flexible smart electronics or simply flexitronics.



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Experimental Details Sample preparation. 1. Pulsed Laser Deposition (PLD): The epitaxial TCO thin film was deposited on freshly cleaved muscovite mica via pulsed laser deposition using commercial ITO (90% In2O3 and 10% SnO2) and AZO (98% ZnO and 2% Al2O3) targets. No surface treatment and precleaning of mica substrate was required in the process. The deposition chamber was initially evacuated to a base pressure of 10-6 Torr. The deposition process carried out at a substrate temperature of 420 °C in 0.1 mTorr oxygen pressure with a KrF excimer laser (λ = 248nm, Coherent) operated at 10 Hz laser repetition rate and laser fluence of 1J/cm2. However, prior to ITO deposition, a Yttria-stabilized zirconia (YSZ) buffer layer less than 10 nm in thickness was deposited at a substrate temperature of 395 °C in 0.1 mTorr oxygen pressure. 2. Sputtering: The scaled-up epitaxial AZO thin film was deposited on a (001) native muscovite mica by RF magnetron sputtering using AZO target (98% ZnO and 2% Al2O3). No surface treatment and pre-cleaning of mica substrate was required in the process. The deposition chamber was initially evacuated to a base pressure of 10-6 Torr and maintained at 1 mTorr with Ar flow of 20 sccm. During the deposition, the mica substrate was heated to 400 ˚C and rf power was controlled at 50 W. After deposition, the AZO thin films were annealed in the sputtering chamber at 500 ºC and 10-6 Torr to improve electrical conductivity. Structural Characterization. Crystal structure of TCO/Mica was investigated by a Bruker D8 high-resolution x-ray diffractometer using monochromatic Cu Kα1 radiation (

1.54056Å)

Cross-sectional TEM specimens were prepared by focused ion beam (FIB) technique (FEI Nova 200). TEM specimens were then examined in a JEOL JEM ARM 200F microscope to reveal interface information. Optoelectronic measurements. The optical spectra were collected in the transmission mode using a Perkin-Elmer Lambda-900 spectrometer (200－2600 nm). The Hall and resistivity measurements were performed in the physical property measurement system (Quantum 10 Environment ACS Paragon Plus

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Design). Bending tests. Computer aided home-built bending set-up combined with optical microscope was used for bending tests. The measurements were performed with the TCO/mica electrodes with initial length L, subject to compression or tension under an external force applied through the bending stage to change it to L-dL. The electrode was bent from one end while the other end was fixed and the vertical distance was measured using the microscope. The setup allows the length of the sample to be bent by providing the displacement as small as 1 µm on the bending stage.

Associated Content Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. The critical strain, thermal and chemical stabilities of TCO/mica are provided.

Author Information Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgments This work is supported by the Ministry of Science and Technology under Grant Nos. MOST 103-2119-M-009-003-MY3 and MOST 104-2628-E-009-005-MY2.



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[23]. Agashe, C.; Kluth, O.; Hu¨pkes, J.; Zastrow, U.; Rech, B.; Wuttig, M. Efforts to Improve Carrier Mobility in Radio Frequency Sputtered Aluminum Doped Zinc Oxide Flms. J. Appl. Phys. 2004, 95, 1911-1917. [24]. Yang, T. L.; Zhang, D. H.; Ma, J.; Ma, H. L.; Chen, Y. Transparent Conducting ZnO:Al Films Deposited on Organic Substrates Deposited by R.F. MagnetronSputtering. Thin Solid Films 1998, 326, 60 – 62. [25]. Shen, H. L.; Zhang, H.; Lu, L. F.; Jiang, F.; Yang, C. Preparation and Properties of AZO Thin Films on Different Substrates. Prog. Nat. Sci.: Mater. Int. 2010, 20, 44-48. [26]. Agura, H.; Suzuki, A.; Matsushita, T.; Aoki, T.; Okuda, M. Low Resistivity Transparent Conducting Al-doped ZnO Films Prepared by Pulsed Laser Deposition. Thin Solid Films 2003, 445, 263 – 267. [27]. Mendelsberg, R. J.; Lim, S. H. N.; Zhu, Y. K.; Wallig, J.; Milliron, D. J.; Anders, A. Achieving High Mobility ZnO:Al at Very High Growth Rates by DC Filtered Cathodic Arc Deposition. J. Phys. D: Appl. Phys. 2011, 44, 232003. [28]. Lee, D.-J.; Kim, H.-M.; Kwon, J.-Y.; Choi, H.; Kim, S.-H.; Kim, K.-B. Structural and Electrical Properties of Atomic Layer Deposited Al-Doped ZnO Films. Adv. Funct. Mater. 2011, 21, 448-455. [29]. Kim, Y.; Lee, W.; Jung, D.-R.; Kim, J.; Nam, S.; Kim, H.; Park, B. Optical and Electronic Properties of Post-Annealed ZnO:Al Thin Films. Appl. Phys. Lett. 2010, 96, 171902. [30]. Cornelius, S.; Vinnichenko, M.; Shevchenko, N.; Rogozin, A.; Kolitsch, A.; Möller, W. Achieving High Free Electron Mobility in ZnO:Al Thin Films Grown by Reactive Pulsed Magnetron Sputtering. Appl. Phys. Lett. 2009, 94, 042103. [31]. Sakata, H.; Noguchi, S. Electrical Properties of Sn-doped In2O3 Prepared by Reactive Evaporation. J. Phys. D: Appl. Phys. 1981, 14, 1523-1529. [32]. Kim, J. H.; Jeon, K. A.; Kim, G. H.; Lee, S. Y. Electrical, Structural and Optical Properties of ITO Thin Films Prepared at Room Temperature by Pulsed Laser Deposition. Appl. Surf. Sci. 2006, 252, 4834-4837. [33]. Choi, H. R.; Eswaran, S. K.; Lee, S. M.; Cho, Y. S. Enhanced Fracture Resistance of Flexible ZnO:Al Thin Films in-Situ Sputtered on Bent Polymer Substrates. ACS Appl. Mater. Interfaces 2015, 7, 17569-17572. [34]. Yun, J.; Park, Y. H.; Bae, T. S.; Lee, S.; Lee, G. H. Fabrication of a Completely Transparent and Highly Flexible ITO Nanoparticle Electrode at Room Temperature. ACS Appl. Mater. Interfaces 2013, 5, 164−172. 14 Environment ACS Paragon Plus

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[35]. Wu, H.; Hu, L.; Carney, T.; Ruan, Z.; Kong, D.; Yu, Z.; Yao, Y.; Cha, J. J.; Zhu, J.; Fan, S.; Cui, Y. Low Reflectivity and High Flexibility of Tin-Doped Indium Oxide Nanofiber Transparent Electrodes. J. Am. Chem. Soc. 2011, 133, 27–29. [36]. Usami, H.; Nakasa, A.; Adachi, M.; Suzuki, E.; Fujimatsu, H.; Ohashi, T.; Yamada, S.; Tsugita, K.; Taniguchi, Y. Preparation of Flexible and Heat-Resisting Conductive Transparent Film by The Pyrosol Process. Thin Solid Films 2006, 515, 2310 – 2315. [37]. Kim, N.; Kang, H.; Lee, J. H.; Kee, S.; Lee, S. H.; Lee, K. Highly Conductive AllPlastic Electrodes Fabricated Using a Novel Chemically Controlled Transfer-Printing Method. Adv. Mater. 2015, 27, 2317-2323. [38]. Wang, L. M.; Chen, Y. J.; Liao, J. W. Characteristics of Indium–Tin Oxide Thin Films Grown on Flexible Plastic Substrates at Room Temperature. J. Phys. Chem. Sol. 2008, 69, 527-530. [39]. Park, S. K.; Han, J. I.; Kim, W. K.; Kwak, M. G. Deposition of Indium–Tin-Oxide Films on Polymer Substrates for Application in Plastic-Based Flat Panel Displays. Thin Solid Films 2001, 397, 49-55. [40]. Sandoval-Paz, M. G.; Ramírez-Bon, R. Indium Tin Oxide Films Deposited on Polyethylene Naphthalate Substrates by Radio Frequency Magnetron Sputtering. Thin Solid Films 2009, 517, 2596 – 2601. [41]. Han, H.; Mayer, J. W.; Alford, T. L. Effect of Various Annealing Environments on Electrical and Optical Properties of Indium Tin Oxide on Polyethylene Napthalate. J. Appl. Phys. 2006, 99, 123711. [42]. Yang, C.; Lee, S.; Lin, T.; Chen, S. Electrical and Optical Properties of Indium Tin Oxide Films Prepared on Plastic Substrates by Radio Frequency Magnetron Sputtering. Thin Solid Films 2008, 516, 1984 –1991. [43]. Guillén, C.; Herrero, J. Structural, Optical and Electrical Characteristics of ITO Thin Films Deposited by Sputtering on Different Polyester Substrates. Mater. Chem. Phys. 2008, 112, 641–644. [44]. Kim, D.-H.; Park, M. R.; Lee, G. H. Preparation of High Quality ITO Films on a Plastic Substrate Using RF Magnetron Sputtering. Surf. Coat. Technol. 2006, 201, 927 – 931. [45]. Guillén, C.; Herrero, J. Influence of The Film Thickness on The Structure, Optical and Electrical Properties of ITO Coatings Deposited by Sputtering at Room Temperature on Glass and Plastic Substrates. Semicond. Sci. Technol. 2008, 23, 075002. 15 Environment ACS Paragon Plus


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[46]. Hao, L.; Diao, X.; Xu, H.; Gu, B.; Wang, T. Thickness Dependence of Structural, Electrical and Optical Properties of Indium Tin Oxide (ITO) Films Deposited on PET Substrates. Appl. Surf. Sci. 2008, 254, 3504–3508. [47]. Lee, J.; Jung, H.; Lee, J.; Lim, D.; Yang, K.; Yi, J.; Song, W. C. Growth and Characterization of Indium Tin Oxide Thin Films Deposited on PET Substrates, Thin Solid Films 2008, 516, 1634–1639. [48]. Haacke, G. New Figure of Merit for Transparent Conductors. J. Appl. Phys. 1976, 47, 4086–4089. [49]. Ray, S.; Banerjee, R.; Basu, N.; Batabyal, A.; Barua, A. Properties of Tin Doped Indium Oxide Thin Films Prepared by Magnetron Sputtering. J. Appl. Phys. 1983, 54, 3497-3501. [50]. Chen, Z.; Cotterell, B.; Wang, W.; Guenther, E. Soo-Jin Chua, A Mechanical Assessment of Flexible Optoelectronic Devices. Thin Solid Films 2001, 394, 201-205. [51]. Peng, H.; Dang, W.; Cao, J.; Chen, Y.; Wu, D.; Zheng, W.; Li, H.; Shen, Z. X.; Liu, Z. Topological Insulator Nanostructures for Near-Infrared Transparent Flexible Electrodes. Nature Chem. 2012, 4, 281–286. [52]. Paeng, D.; Yoo, J. H.; Yeo, J.; Lee, D.; Kim, E.; Ko, S. H.; Grigoropoulos, C. P. Low-Cost Facile Fabrication of Flexible Transparent Copper Electrodes by Nanosecond Laser Ablation. Adv. Mater. 2015, 27, 2762-2767. [53]. Kim, N.; Kang, H.; Lee, J. H.; Kee, S.; Lee, S. H.; Lee, K. Highly Conductive AllPlastic Electrodes Fabricated Using a Novel Chemically Controlled Transfer-Printing Method. Adv. Mater. 2015, 27, 2317-2323. [54]. Im, H. G.; An, B. W.; Jin, J.; Jang, J.; Park, Y. G.; Park, J. U.; Bae, B. S., A HighPerformance, Flexible and Robust Metal Nanotrough-Embedded Transparent Conducting Film for Wearable Touch Screen Panels. Nanoscale 2016, 8, 3916-3922. [55]. Feng, C.; Liu, K.; Wu, J. S.; Liu, L.; Cheng, J. S.; Zhang, Y.; Sun, Y.; Li, Q.; Fan, S.; Jiang, K. Flexible, Stretchable, Transparent Conducting Films Made from Superaligned Carbon Nanotubes. Adv. Funct. Mater. 2010, 20, 885-891. [56]. Deng, B.; Hsu, P. C.; Chen, G.; Chandrashekar, B. N.; Liao, L.; Ayitimuda, Z.; Wu, J.; Guo, Y.;, Lin, L.; Zhou, Y.; Aisijiang, M.; Xie, Q.; Cui, Y.; Liu, Z.; Peng, H. Roll-toRoll Encapsulation of Metal Nanowires between Graphene and Plastic Substrate for High-Performance Flexible Transparent Electrodes. Nano Lett. 2015, 15, 4206-4213. [57]. Zardetto, V.; Brown, T. M.; Reale, A.; Di Carlo, A. Substrates for Flexible Electronics: A Practical Investigation on the Electrical, Film Flexibility, Optical, 16 Environment ACS Paragon Plus

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Temperature, and Solvent Resistance Properties. J. Polym. Sci. B Polym. Phys., 2011, 49, 638–648.



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FIGURE CAPTIONS Figure 1. (a). The typical 2θ-θ scan of the ITO/mica, AZO/mica films along with reference muscovite. (b) ϕ-scans at ITO{440}, mica{202} and AZO{101} diffraction peaks. (c) The Rocking curves at ITO(222) and AZO(002) diffraction peaks. Schematic of (d) ITO/mica and (e) AZO/mica. Figure 2. The cross-sectional TEM image of (a) ITO/YSZ/mica and (b) AZO/mica interfaces along with the selected area diffraction patterns of ITO, AZO, YSZ and mica in bottom panels. Figure 3. The optical transmittance spectra of (a) ITO and (b) AZO with varying thickness on mica. The inset (a) shows the image of native mica, ITO/mica and AZO/mica and inset (b) displays the optical band gaps. (c) Thickness-dependent conductivity and (d) Hall mobility as a function of carrier concentration of TCO/mica (Filled circles), epitaxial TCO10-16 (Asterisk symbol), polycrystalline TCO12,17-32 (Open circles) and TCO on flexible substrates17-19,24,25,3347 (Open circles with cross marks). Figure 4 (a) Average optical transmittance Tavg in the visible range as a function of the sheet resistance Rs and (b) Figure of merit as a function of film thickness for TCO/mica (Filled circles), epitaxial TCO10-16 (Asterisk symbol), polycrystalline TCO12,17-32 (Open circles) and TCO on flexible substrates17-19,24,25,33-47 (Open circles with cross marks). Figure 5. The sheet resistance as a function of bending radius of curvature under (a) flex-in (compression strain) and (b) flex-out (tension strain) modes for various ITO(filled circles) and AZO (open circles) thicknesses. The insets show the schematic of these modes. The stability and durability of thin and thick ITO(filled circles) and AZO (open circles) electrodes as a function of (c) bending cycles and (d) time.


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Oxide Heteroepitaxy for Flexible Optoelectronics - ACS Applied

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