Flexible Visible-Blind Ultraviolet Photodetectors Based on ZnAl

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Functional Inorganic Materials and Devices

Flexible Visible-Blind Ultraviolet Photodetectors Based on ZnAl-Layered Double Hydroxide Nanosheet Scroll Chan-Woo Jeon, Sang-Seok Lee, and Il-Kyu Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12082 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Flexible Visible-Blind Ultraviolet Photodetectors Based on ZnAlLayered Double Hydroxide Nanosheet Scroll Chan-Woo Jeon, Sang-Seok Lee, and Il-Kyu Park* Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea *Corresponding author. Tel.: +82029706349, fax: +82029736657 e-mail address: [email protected] (I. K. Park)

Visible-blind ultraviolet (UV) photodetectors have achieved great attentions for realizing internet of things technologies as well as for monitoring the level of UV exposure to humans. To realize next-generation flexible and visible-blind UV photodetectors, new functional material systems with easy fabrication, selectively strong UV light absorption, environment friendliness, and high stability regardless of ambient conditions need to be developed. Herein, flexible visible-blind UV photodetectors are successfully fabricated based on two-dimensional ZnAl-layered double hydroxide (LDH) nanosheets with scroll structures grown on flexible substrates. The ZnAl-LDH nanosheet scrolls exhibit highly resistive semiconducting properties with a band gap of 3.2 eV and work function of 3.64 eV. The photodetector based on the ZnAl-LDH shows photoresponse in the UV spectral range below 420 nm, indicating visible-blind spectral response. In addition, the UV photodetector shows a maximum responsivity of 17 mA/W under illumination with 365 nm light. Moreover, the flexible photodetector shows reproducible photoresponse even after 1,000 bending cycles, which indicates the acceptable stability of the ZnAl-LDH nanosheet scrolls.

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Keywords: Layered double hydroxide; Scroll structure; Visible-blind UV photodetector; Flexible device; Nanosheet


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1. INTRODUCTION Recently, with increasing demand for the realization of wireless sensor networks, photodetectors converting optical signals into electrical signals have received much attention.1–4 Particularly, ultraviolet (UV) photodetectors are useful for a wide variety of applications such as air purification, optical receivers for optical integrated circuits, ozone hole monitoring, leak detection, and missile plume detection.1–4 In addition, the UV radiation from solar radiation is regarded to have profound effects, both harmful and advantageous, on humankind.2–4 Even though moderate skin exposure to UV light is beneficial for health as it activates the synthesis of vitamin D or removes various germs, excessive UV radiation exposure can be harmful as it can cause skin cancer or accelerate aging. Therefore, monitoring the level of UV exposure to humans has become important, for which attachable and portable photodetectors are required. High-performance UV photodetectors should exhibit high UV sensitivity, fast response speed, low power consumption, significant visible-light rejection, and a linear photocurrent-optical power correlation.2–4 Particularly, to avoid the background signals of visible and infrared radiations, “visible-blind” UV photodetectors are highly desirable.4 In addition, modern smart sensor technologies demand novel and flexible UV photodetectors for more compatible applications. Therefore, various wide-band gap semiconductors such as ZnO, SnO2, ZnS, (Al)GaN, SiC, and diamond either in bulk or nanostructure form have been fabricated for UV photodetectors.4–7 Among them, ZnO has been regarded as an excellent building block for visible-blind photodetectors because of its wide band gap (3.2 eV), environmental friendliness, and ease of fabrication.4–7 Despite the intensive efforts devoted to the development of ZnO-based UV photodetectors, the fabrication of highly



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reliable and flexible UV photodetectors with high performances have not yet been realized.4–7 To realize next-generation flexible and visible-blind UV photodetectors, new material systems with ease of fabrication, environmental friendless, selectively strong UV light absorption, and high stability in harsh operating conditions need to be developed. Layered-double

hydroxides

(LDHs),

one

of

the

promising

two-dimensional

nanostructures, have received much attention due to their novel optical, electrical, magnetic, and electrochemical properties.8–13 The general formula of a LDH is [M(II)1−xM(III)x (OH)2]x+[An−x/n·yH2O]x−, where M(II) and M(III) are divalent and trivalent metal cations, respectively, and An− is an exchangeable n-valent anion, such as CO32−, Cl−, NO3−, and CH3COO−. The properties of LDHs can be modulated by combining with a wide variety of metal cations and anions. Among them, ZnAl-LDH is an excellent visible-blind UV-absorbing material because of its wide band gap, easy fabrication by low-temperature hydrothermal synthesis, low cost, and environmental friendliness.12–17 Particularly, because of the advantages of multiple sheet structure, i.e., a scroll structure, it exhibits excellent flexibility or bendability. In this paper, we demonstrate for the first time the feasibility of using an LDH nanosheet with a scroll structure as an active layer in a flexible and visible-blind UV photodetector (Figure 1(a)). We fabricated the photodetector in the lateral structure and analyzed it.18

2. EXPERIMENTAL DETAILS 2.1. Fabrication of Samples ZnAl-LDH nanosheets having a scroll structure was grown on glass and acetate substrates by two-step low-temperature hydrothermal synthesis, which includes the


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deposition of an Al thin film as the seed layer and hydrothermal synthesis for the main LDH growth, as shown in Figure 1(a). The growth of LDHs was not significantly affected by the substrates because the Al thin film acted as a seed layer as well as trivalent metal ion source.15,16 After cleaning the substrates, a 50-nm-thick Al thin film was deposited on the substrates using a thermal evaporation system. The ZnAl-LDH layer was grown by immersing the Al-deposited substrates into an aqueous solution of 120 mM zinc (II) nitrate hexahydrate [Zn(NO3)2∙6H2O] and hexamethylenetetramine (HMTA; C6H12N4). The ZnAl-LDH layer was grown at 90 ºC for 1 h. After the reaction, the samples were cleaned using deionized water in an ultrasonic bath for 5 min to completely remove the homogeneously formed particles in the solutions during synthesis. To fabricate the photodetector based on the ZnAl-LDH nanostructures, a symmetrical metal-semiconductor-metal structure was formed, as shown in the schematic of the full device structure in Figure 1(a). For the electrode, a 300-nm-thick Cu metal layer was deposited using the thermal evaporation system through a shadow mask in an interdigit electrode (IDE) pattern. The channel width and length (opening) of the IDE were 4 mm and 200 μm, respectively. By using an acetate plate, a fully flexible ZnAlLDH-based photodetector was fabricated, as shown in Fig. 1(a).

2.2. Characterization and Measurement of Samples. The structural properties of the ZnAl-LDHs were investigated by field-emission scanning electron microscopy (Hitachi, SU-8010), optical microscopy (Olympus BX40), and grazing incidence X-ray diffraction (GIXRD, Rigaku D-max-2500-pc) using a Cu Kα radiation source. The optical properties of the ZnAl-LDHs were characterized by UV-vis absorption spectroscopy (Agilent Cary 100) and photoluminescence (PL) spectroscopy



using a 20 mW He-Cd laser (325 nm) at room temperature. The work function of the ZnAl-LDH was measured by ultraviolet photoelectron spectroscopy (UPS, AXIS SUPRA, Kratos). The I-V measurements were performed using a Keithley 2450 source meter. The photoresponse of the device was measured using a light source of light-emitting diode (LED) arrays operating at the wavelengths of 365, 375, 385, 395, 420, and 450 nm. The optical power of the UV LED was controlled by an input current and measured using an ultraviolet intensity meter (UV Karl-Suss UV Intensity Meter Model 1000).

3. RESULTS AND DISCUSSION Figure 1(b) shows the surface morphology of the ZnAl-LDH on a flexible acetate substrate. ZnAl-LDH grows in the direction perpendicular to the substrate and shows high-density scrolled multiple sheet structures. The morphology of the ZnAl-LDH was affected by the Zn source concentration in the solution (see Figure S1 in supplementary information for more details). At a high Zn source concentration, the morphology of the ZnAl-LDH changed from porous flower-like structure to a more closely packed scroll structure due to a high nucleation rate.17 The closely packed nanosheet scroll structure can be beneficial for facilitating charge carrier transport as well as for enhancing the bending reliability. The cross-sectional image of the full device structure in Figure 1(c) reveals that a uniform Cu metal thin film fully covers the LDH surface. Because the Al thin film deposited on the acetate substrate was used as a seed layer, the ZnAl-LDH nanostructures adhered on the acetate surface strongly. Even after multiple folding, as shown in Figure 1(a), the photodetector was not delaminated from the substrate. This is because of the high-quality interface between the LDH and the acetate surface and the closely packed LDH scroll structures.


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As shown in Fig. 2(a), the out-of-plane GIXRD profile of the ZnAl-LDH shows only two peaks at 9.6° and 19.4°, which correspond to the (003) and (006) reflections of the ZnAl-LDH phase with R-3m rhombohedral symmetry. This confirms that the ZnAl-LDH nanostructures formed a highly crystalline layered structure without any other phases or impurities. It should be noted that these two diffraction peaks appeared at lower angles compared with those reported in literature,17,18 which indicates an increase in the interplanar distance of the ZnAl-LDH in this case. This is because of the difference in the Zn2+/Al3+ ratio depending on the synthesis condition. Because the ionic radii of Al3+ (0.675 nm) is smaller than that of Zn2+ (0.88 nm), the interplanar distance can change with a variation in the Zn2+/Al3+ ratio.14,17 The Al3+ was supplied only by the Al thin film, while the Zn2+ was sufficiently supplied by the solution even after the dissolved Al depleted. Therefore, Zn2+ occupies a larger proportion in the lattice than Al3+ does. Figure 2(b) shows the UV-visible absorption and photoluminescence (PL) spectra of the ZnAl-LDH. The bandgap of the ZnAl-LDH was estimated by fitting the UV-visible absorption spectrum to the optical absorption coefficient (α) versus photon energy curve by the Tauc/Davis-Mott expression as follows:19 (𝛼ℎ𝜈)1/𝑛 = 𝐴(ℎ𝜈 ― 𝐸𝑔),

(1)

where hν is the photon energy, A is the proportionality constant, and Eg is the band gap energy. In Equation (1), n is determined by the electronic transient characteristics in the semiconductor (n = 0.5 and 2 for direct and indirect transitions, respectively). As shown in Figure 2(b), the (αhν)1/n curve is plotted against the photon wavelength with n = 0.5, which indicates that the ZnAl-LDH is a direct-bandgap semiconductor. A band gap of 3.28 eV was obtained for ZnAl-LDH from the straight line of the plot. The PL spectrum shows a peak at 397 nm (3.12 eV), and the difference between the absorption edge and



PL peak energy (Stokes shift) was about 0.16 eV. This small Stokes shift indicates that the ZnAl-LDH is highly crystalline without band gap states. The position of the Fermi level (EF) of the ZnAl-LDH was determined as 2.92 eV above the maximum of the valence band edge by UPS performed using He (I) radiation (hν = 21.2 eV). Based on the optical absorption and UPS results, the work function (Φ) and electron affinity (χ) were estimated as 3.64 and 3.28 eV, respectively. As shown in the energy band structure (Figure 2(d)), the ZnAl-LDH is a wide-band gap semiconductor that can serve as an intrinsically visible-blind UV photodetector. Based on the ZnAl-LDH nanosheet scroll, a flexible photodetector device was fabricated using Cu as symmetric IDEs. Because the work function of Cu metal (4.7 eV) is larger by 1.06 eV than that of ZnAl-LDH, the contact between Cu and ZnAl-LDH is a Schottky contact, as shown in the inset of Figure 3(a). The back-to-back Schottky contact is beneficial to minimize the dark current in the photodetector.20 Figure 3(a) shows the current-voltage (I-V) curve measured in dark condition. The device shows symmetric I-V characteristics with a very low dark current of less than 500 nA under an applied bias of less than 5 V. However, a drastic increase in current was observed with an increase in voltage over 8 V. The drastic increase in current at a large applied bias is due to tunneling through the one of the forward-biased Schottky contact. The symmetric electrical properties and low dark current are attributed to the large band gap of ZnAl-LDH and back-to-back Schottky contact of the device. The photocurrent of the photodetector was evaluated by subtracting the I-V results measured in dark from that measured under light of various wavelengths and intensities. Figure 3(b) shows the photocurrent under illumination of light of different wavelengths. As the light wavelength decreases from 420 to 365 nm, the photocurrent increases


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consistently. Figure 3(c) shows the variation in photocurrent under on and off switching of light of various wavelengths. By controlling the optical power of the UV LED (Popt = number of incident photon × photon energy/time), the number of incident photon was adjusted to be the same for all the wavelengths. When the light was turned on, the current increased rapidly and saturated, while the current dropped rapidly when the light was turned off. Repeatability is also a key parameter for a photodetector. As shown in Figure 3(c), the on/off ratio of current remains constant even during repeated operations. The reproducibility of the photocurrent indicates the high stability of the photodetector. With an increase in light wavelength from 365 to 420 nm, the photocurrent decreased drastically. At the longer wavelength than 450 nm, the photodetector did not show response as shown in Fig. 3(c). Figure 3(d) shows the spectral response of the photodetector. The responsivity (R) was calculated by Equation (2):21

R

I ph PS ,

(2)

where Iph is the photocurrent, S is the effective area of the device, i.e., exposed LDH area between the IDE pattern, and P is the areal optical power density. It is evident that the responsivity increased significantly above the threshold excitation energy of 3.22 eV (385 nm), which is very close to the band gap energy of the ZnAl-LDH (3.28 eV). This indicates that the photocurrent mechanism in the ZnAl-LDH is induced by the generation of electron-hole pairs, as shown in the inset of Figure 3(b). A photon energy less than the band gap energy of ZnAl-LDH cannot generate electron-hole pairs, and thus, photocurrent is not observed. Therefore, the ZnAl-LDH-based photodetector is an intrinsically visible-blind UV photodetector.



Figure 4(a) and (b) show the I-V curves and on-off switching properties with variation in the intensity of light of 365 nm wavelength. As can be seen, the photocurrent increases with increasing light intensity, and the photodetector shows consistent on-off switching behaviors even with variation in light intensity. This is because the number of photogenerated charge carriers is proportional to the absorbed photon flux. To check the long-term stability of the device, we measured the photocurrent for the photodetectors stored in the air condition for three months (see Figure S2 in supplementary information for more details). The photocurrent level of the photodetector showed no degradation even after three months. This indicates the ZnAl-LDH nanostructures can be a stable light absorbing media. We further investigated the dynamic performance of the photodetector by measuring its response time. Even though the response time of the photodetector should be calculated by considering exponential decay function, for convenience the rise and decay times have been estimated by the time taken for the initial current to increase or decrease to 90% and 10% of the maximum output current, respectively.22,23 As shown in Figure 4(c), the rise and decay times of the photodetector are 0.91 and 1.68 s, respectively. These are much faster responses compared with those of the much investigated ZnO-based UV photodetectors, whose typical response time ranges from few tens to hundreds of seconds.24,25 In this study, the decay time is longer than the rise time, which indicates that charge traps and defect states are involved in this process.26 The recovery time is mainly affected by the lifetime of the electronic trap states at the semiconductor-metal interface or defects in the semiconductors. If these trap states remain filled for a long time after illumination, response speed will be limited by a slower decay time as in this case. In addition, the photodetector showed negligible polarizationdependent photoresponse which would be due to would be due to random orientation of


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the ZnAl-LDH scroll structure (see Figure S3 in supplementary information for more details). The dynamics of the photocarrier transport and polarization-dependent photoresponse require further investigations. The dependences of photocurrent and responsivity on light intensity are shown in Figure 4(d). The photocurrent increases linearly with increasing light intensity. The linear light intensity (P)-dependent photocurrent (Iph) indicates negligible loss of photogenerated charges by the complex process of photon absorption, recombination, and charge trapping within the semiconductor.7,21 The linear dynamic range (LDR) is also an important parameter for photodetectors, because they need to operate in a broad light intensity range.27 In the measured illumination intensity range, the photodetector showed linear behavior. Figure 4(d) shows the responsivity as a function of light intensity for a light wavelength of 365 nm. The responsivity decreases with increasing light intensity, which is primarily attributed to electron-trap saturation and recombination of electronhole pairs during transit time.21,26 With an increase in light intensity, the traps are filled with electrons and the quasi-Fermi level of the ZnAl-LDHs increases. This leads to an increase in free electrons, resulting in an increase in the electron-hole recombination rate. The maximum responsivity is 17 mA/W, which is smaller than that reported previously for UV photodetectors based on other material systems.4-7 The specific detectivity (D*) was 2.07x1010 which was calculated based on the measured dark current and responsivity.28 Table 1 shows the comparison of the critical parameters for the present ZnAl-LDH-based and other characteristic two-dimensional nanostructure-based photodetectors. Considering the not well-optimized device structure, the performance of the ZnAl-LDH-based photodetector showed comparable performances.



To further investigate the bending reliability of the flexible photodetector, the photocurrent was measured when the photodetector was in a bent state. With a decrease in curvature radius, the photocurrent decreased slightly, as shown in Figure 5(a). The photocurrent reduced by 42% as the curvature radius of the photodetector decreased from flat to 5 mm, as shown in Figure 5(b). This can be attributed to the formation of cracks between the ZnAl-LDH nanosheet scrolls. These cracks can increase the electrical resistance by cutting the current paths and scattering the charge carrier transport. The bending time-dependent on-off switching properties of the photodetector were measured at a curvature radius of 6 mm. The bending and release of the photodetector was repeated up to 1,000 cycles. As shown in Figure 5(c), the photocurrent values decreased consistently with increasing bending times, while the on/off response behavior remained constant. As the bending cycle increased up to 1,000 cycles, the photoresponse decreased to 36% of the original photoresponse (Figure 5(d)). The photocurrent was maintained even after 1,000 bending cycles, indicating the acceptable stability of the ZnAl-LDH nanosheet scroll. To investigate the origin of performance degradation during bending of the device, the microstructures of the ZnAl-LDH-based photodetector before and after 1,000 bending cycles were compared. As shown in Figure 6(a) and (b), the LDH area exhibit uniform morphologies without cracks before bending. However, after 1,000 bending cycles, cracks appeared on the LDH area and most of the Cu electrode was detached from the LDH surface, as shown in Figure 6(c) and (d). It should be noted that the delamination of the Cu thin film from the LDH was more severe than the formation of cracks within the LDH. When the photodetector is in the bent state, cracks can form on the LDH surface because the LDH nanosheet scroll is under shear stress, as shown in Figure 6(f). After


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releasing the bending stress, the nanosheets returned to their original state by recovering the cracks. The schematic diagram of the shape evolution of the ZnAl-LDH surface during bending and relief operations is shown in Figure 6(g). On the other hand, the stress can concentrate at the interface between Cu and the LDH layers because of the difference in the stiffness of these two materials. This results in delamination of the Cu thin film from the LDH surface. Therefore, crack formation and electrode detachment resulted in performance degradation after repeated bending and stress relief. Considering relatively thick (~1 μm) ZnAl-LDH layers, the bending performance shown here is acceptable as compared with that of other flexible photodetectors comprising two-dimensional nanostructures having few monolayer sheets. The scrolled nanosheet structure is beneficial for not only facilitating charge carrier transport, but also enhancing the bending reliabilities of flexible devices.

4. CONCLUSIONS In conclusion, in this work we present the flexible and visible-blind UV photodetector based on ZnAl-LDH nanosheet scrolls integrated on a substrate. The ZnAlLDH showed semiconducting properties with a bandgap energy of 3.2 eV and work function of 3.64 eV. The I-V curve of the photodetector showed a typical symmetric Schottky contact with a very low dark current of less than 500 nA under an applied bias of less than 5 V. The photodetector based on the ZnAl-LDH showed a visible-blind spectral response and photoresponse at wavelengths below 420 nm. The UV photodetector showed a maximum responsivity of 17 mA/W and a much faster response speed than those of the previously reported ZnO-based UV photodetectors. Because of the facile recovery of the nanosheet scroll structures under mechanical stress, the ZnAl-



LDH-based UV photodetector showed reproducible photoresponse even after 1,000 bending cycles.

Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2018R1A2B6006968).


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References [1] Sang, L.; Liao, M.; Sumiya, M. A Comprehensive Review of Semiconductor

Ultraviolet Photodetectors: From Thin Film to One-Dimensional Nanostructures, Sensors 2013, 13, 10482–10518. [2] Chen, H.; Liu, K.; Hu, L.; Al-Ghamdi, A. A.; Fang, X. New Concept Ultraviolet

Photodetectors, Materials Today 2015, 18, 493–502. [3] Peng, L.; Hu, L.; Fang, X. Low‐Dimensional Nanostructure Ultraviolet

Photodetectors, Adv. Mater. 2013, 25, 5321–5328. [4] Xie, C.; Lu, X. T.; Tong, X. W.; Zhang, Z. X.; Liang, F. X.; Liang, L.; Luo, L. B.;

Wu, Y. C. Recent Progress in Solar‐Blind Deep‐Ultraviolet Photodetectors Based on Inorganic Ultrawide Bandgap Semiconductors, Adv. Func. Mater. 2019, 29, 1806006. [5] Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.;

Lo, Y. H.; Wang, D. ZnO Nanowire UV Photodetectors with High Internal Gain, Nano Lett. 2007, 7, 1003–1009. [6] Fang, X.; Bando, Y.; Liao, M.; Zhai, T.; Gautam, U. K.; Li, L.; Koide, Y.; Golberg,

D. An Efficient Way to Assemble ZnS Nanobelts as Ultraviolet-light Sensors with Enhanced Photocurrent and Stability, Adv. Func. Mater. 2010, 20, 500–508. [7] Kind, H.; Yan, H. Q.; Messer, B.; Law, M.; Yang, P. D. Nanowire Ultraviolet

Photodetectors and Optical Switches, Adv. Mater. 2002, 14, 158–160. [8] Wang, Q.; O’Hare, D. Recent Advances in the Synthesis and Application of Layered

Double Hydroxide (LDH) Nanosheets, Chem. Rev. 2012, 112, 4124–4155. [9] Guo, X.; Zhang, F.; Evans, D. G.; Duan, X. Layered Double Hydroxide Films:

Synthesis, Properties and Applications, Chem. Comm. 2010, 46, 5197–5210.



[10] Xu, M.; Wei, M. Layered Double Hydroxide‐based Catalysts: Recent Advances in

Preparation, Structure, and Applications, Adv. Func. Mater. 2018, 28, 1802943. [11] Mohapatra, L.; Parida, K. A Review on the Recent Progress, Challenges and

Perspective of Layered Double Hydroxides as Promising Photocatalysts, J. Mater. Chem. A 2016, 4, 10744–10766. [12] Liu, X.; Ge, L.; Li, W.; Wang, X.; Li, F. Layered Double Hydroxide Functionalized

Textile for Effective Oil/Water Separation and Selective Oil Adsorption, ACS Appl. Mater. Interfaces 2015, 7, 791–800. [13] Xu, S.; Pan, T.; Dou, Y.; Yan, H.; Zhang, S.; Ning, F.; Shi, W.; Wei, M. Theoretical

and Experimental Study on MIIMIII-layered Double Hydroxides as Efficient Photocatalysts Toward Oxygen Evolution from Water, J. Phys. Chem. C 2015, 119, 18823–18834. [14] Baek, S. H.; Nam, G. H.; Park, I. K. Morphology Controlled Growth of ZnAl-Layered

Double Hydroxide and ZnO Nanorod Hybrid Nanostructures by Solution Method, RSC Advances 2015, 5, 59823–59829. [15] Cho, D. K.; Jeon, C. W.; Park, I. K. Growth and Optical Band Gap of CdAl-Layered

Double Hydroxide Thin Structures on Rigid Substrate, J. All. Comp. 2018, 737, 725– 730. [16] Cho, D. K.; Park, I. K. Evolution of Structural and Chemical Properties of CoAl-

Based Layered Double Hydroxides Grown on Silicon Substrate, Ceram. Int. 2018, 44, 8556–8561. [17] Baek, S. H.; Nam, G. H.; Park, I. K. Morphological Evolution of ZnAl-Layered

Double Hydroxide Nanostructures Grown on Al2O3/Si Substrate, Sci. Adv. Mater. 2016, 8, 2142–2146.


Page 16 of 29

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[18] Miao, J.; Zhang, F.; Recent Progress on Highly Sensitive Perovskite Photodetectors,

J. Mater. Chem. C 2019, 7, 1741–1791. [19] Tauc, J. Amorphous and Liquid Semiconductors, Springer Science & Business Media,

2012. [20] Soares, S. F. Photoconductive Gain in a Schottky Barrier Photodiode, Jpn. J. Appl.

Phys. 1992, 31, 210–216. [21] Rose, A. Concepts in Photoconductivity and Allied Problems, Krieger Publishing

Company, New York 1978. [22] Jin, Z.; Wang, J. Flexible High-Performance Ultraviolet Photoconductor with Zinc

Oxide Nanorods and 8-hydroxyquinoline, J. Mater. Chem. C 2014, 2, 1966–1970. [23] Miao, J.; Zhang, F. Recent Progress on Photomultiplication Type Organic

Photodetectors, Laser Photonics Rev. 2019, 13, 1800204. [24] Jin, Y.; Wang, J.; Sun, B.; Blakesley, J. C.; Greenham, N. C. Solution-Processed

Ultraviolet Photodetectors Based on Colloidal ZnO Nanoparticles, Nano Lett. 2008, 8, 1649–1653. [25] Tian, W.; Zhai, T.; Zhang, C.; Li, S. L.; Wang, X.; Liu, F.; Liu, D.; Cai, X.;

Tsukagoshi, K.; Golberg, D.; Bando, Y. Low‐Cost Fully Transparent Ultraviolet Photodetectors Based on Electrospun ZnO‐SnO2 Heterojunction Nanofibers, Adv. Mater. 2013, 25, 4625–4630. [26] Huang, K.; Zhang, Q.; Yang, F.; He, D. Ultraviolet Photoconductance of a Single

Hexagonal WO3 Nanowire, Nano Res. 2010, 3, 281–287.



[27] Wang, W.; Zhang, F.; Du, M.; Li, L.; Zhang, M.; Wang, K.; Wang, Y.; Hu, B.; Fang,

Y.; Huang, J. Highly Narrowband Photomultiplication Type Organic Photodetectors, Nano Lett. 2017, 17, 1995–2002.

[28] Wang, W.; Zhang, F.; Li, L.; Gao, M.; Hu, B.; Improved Performance of

Photomultiplication Polymer Photodetectors by Adjustment of P3HT Molecular Arrangement, ACS Appl. Mater. Interfaces 2015, 7, 22660–22668. [29] Ou, Q.; Zhang, Y.; Wang, Z.; Yuwono, J. A.; Wang, R.; Dai, Z.; Li, W; Zheng, C;

Xu, Z. Q.; Qi, X; Duhm, S; Medhekar, N. V.; Zhang, H.; Bao, Q.; Strong Depletion in Hybrid Perovskite p–n Junctions Induced by Local Electronic Doping, Adv. Mater. 2018, 30, 1705792. [30] Li, Z.; Qiao, H.; Guo, Z.; Ren, X.; Huang, Z.; Qi, X.; Dhanabalan, S. C.; Ponraj, J.

S.; Zhang, D.; Li, J.; Zhao, J.; Zhong, J.; Zhang, H.; High-Performance PhotoElectrochemical Photodetector Based on Liquid-Exfoliated Few-Layered InSe Nanosheets with Enhanced Stability, Adv. Funct. Mater. 2018, 28, 1705237. [31] Xing, C.; Huang, W.; Xie, Z.; Zhao, J.; Ma, D.; Fan, T., Liang, W.; Ge, Y.; Dong, B.;

Li, J.; Zhang, H.; Ultrasmall bismuth quantum dots: facile liquid-phase exfoliation, characterization, and application in high-performance UV–Vis photodetector, ACS Photonics 2018, 5, 621–629. [32] Xie, Z.; Xing, C.; Huang, W.; Fan, T.; Li, Z.; Zhao, J.; Xiang, Y.; Guo, Z.; Li, J.;

Yang, Z.; Dong, B., Qu, J.; Fan, D.; Zhang, H.; Ultrathin 2D Nonlayered Tellurium Nanosheets: Facile Liquid‐Phase Exfoliation, Characterization, and Photoresponse


Page 18 of 29

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with High Performance and Enhanced Stability, Adv. Funct. Mater. 2018, 28, 1705833. [33] Zhang, Q.; Jie, J.; Diao, S.; Shao, Z., Zhang, Q.; Wang, L.; Deng, W.; Hu, W.; Xia,

H.; Yuan, X.; Lee,S. T.; Solution-Processed Graphene Quantum Dot Deep-UV Photodetectors, ACS Nano 2015, 9, 1561–1570.



Figure captions Figure 1. (a) Schematic of fabrication procedure of ZnAl-LDH scroll-based UV photodetector and photograph of a UV photodetector fabricated on a flexible substrate. (b) Surface image of ZnAl-LDH grown on an acetate substrate. (c) Cross-sectional view of the full device structure of the photodetector.

Figure 2. (a) GIXRD profile of ZnAl-LDH. (b) UV-visible absorption and photoluminescence spectra of ZnAl-LDH. (c) UPS spectrum of ZnAl-LDH measured using He I radiation (21.2 eV). Energy band diagram of ZnAl-LDH.

Figure 3. (a) I-V curve of the device measured in dark condition. The inset shows the equilibrium band diagram of the device. (b) Variation in photocurrent with light wavelength. The inset is the energy band diagram showing the operation principle of the photodetector. (c) On-off switching properties measured under illumination with different light wavelengths at a bias of 2 V. (d) Spectral response of the device measured at an applied bias of 5 V.

Figure 4. (a) Variation in photocurrent with light intensity at a light wavelength of 365 nm. (b) Light intensity-dependent on-off switching properties with a light wavelength of 365 nm at a bias of 2 V. (c) Rise time and recovery (fall) time of the photocurrent under illumination of 2.6 mW/cm2 with 365 nm light wavelength at Vapplied = 2 V. (d) Photocurrent and responsivity as functions of incident light power.


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Figure 5. (a) Variation in photocurrent with curvature radius under illumination of 3.6 mW/cm2 with 365 nm light wavelength at V applied = 2 V. (b) Curvature radius-dependent normalized photocurrent. (c) On-off switching properties of the photodetector with variation in bending-relief cycling time under illumination with 365 nm wavelength light at a bias of 2 V. (d) Bending-relief cycling time-dependent photocurrent.

Figure 6. Optical microscope images of LDH area and Cu electrode edge: (a), (b) before bending test and (c), (d) after 1,000 bending-relief cycles. After 1,000 bending cycles, cracks are found near the electrode and the Cu electrodes are detached from the LDHs. The inset shows the SEM image of cracks on LDH after 1,000 bending cycles. (e) Optical microscope image of the photodetector with IDE. The area marked by dotted red square is closely observed using an optical microscope before and after the bending test. (f) Surface SEM image of ZnAl-LDH during bending with a curvature radius of 7.5 mm. (g) Schematic diagram of shape evolution of ZnAl-LDH surface during bending and relief operations.



Figures

Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



Table 1. Comparison of the critical parameters for the present ZnAl-LDH-based and other characteristic two-dimensional nanostructure-based photodetectors.


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Table of contents A flexible visible-blind UV photodetector is demonstrated based on two-dimensional ZnAllayered double hydroxide nanosheet scroll structures, which exhibit wide band gap semiconducting properties. The photodetector shows visible-blind spectral response with fast response time less than 1 s and reproducible photo-response even after 1,000 bending cycles owing to highly stable scroll structure.


Flexible Visible-Blind Ultraviolet Photodetectors Based on ZnAl

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