gas dual sensor using Ag

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Functional Inorganic Materials and Devices

High-performance flexible ZnO nanorod UV/ gas dual sensor using Ag nanoparticle template Do Kyun Kwon, Yoann Porte, Kyung Yong Ko, Hyungjun Kim, and Jae-Min Myoung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13046 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Applied Materials & Interfaces

High-Performance Flexible ZnO Nanorod UV/Gas Dual Sensor Using Ag Nanoparticle Template Do-Kyun Kwon,† Yoann Porte,† Kyung Yong Ko,‡ Hyungjun Kim‡ and Jae-Min Myoung*,†

†Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Soedaemun-gu, Seoul 03722, Republic of Korea ‡School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Soedaemun-gu, Seoul 03722, Republic of Korea

*E-mail) [email protected]

KEYWORDS: Ag nanoparticle, ZnO nanorod, UV/gas dual sensor, Solution process, Networkstructured flexible device 1

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ABSTRACT Flexible zinc oxide (ZnO) nanorod (NR) ultraviolet (UV)/gas dual sensors using silver (Ag) nanoparticle (NP) templates were successfully fabricated on polyimide (PI) substrate with nickel (Ni) electrodes. Arrays of Ag NP were used as a template for the growth of ZnO NRs, which could enhance the flexibility and the sensing properties of the devices through the localized surface plasmon resonance (LSPR) effect. The Ag NPs were fabricated by rapid thermal annealing (RTA) process of Ag thin films and ZnO NRs were grown on Ag NPs to maximize the surface area and form networks with rod-to-rod contacts. Due to the LSPR effect by Ag NPs, the UV photoresponse of the ZnO NRs was amplified and the depletion region of ZnO NRs was formed quickly due to the Schottky contact with the Ag NPs. As a consequence, ZnO NR UV/gas dual sensor grown on Ag NP template with a diameter of 28 nm exhibited the outstanding UV sensing characteristics with an UV on-off ratio of 3628, and rising time (tr) and decay time (td) of 3.52 and 0.33 s upon UV exposure, along with excellent NO2 sensing characteristics with a stable gas on-off ratio of 288.5, and tr and td of 38 and 62 s upon NO2 exposure. Furthermore, the sensors grown on Ag NP template exhibited good mechanical flexibility and stable sensing properties without significant degradation even after the bending test up to 10,000 cycles at the bending radius of 5 mm.

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INTRODUCTION Over the last few years, the implementation of electronic devices on flexible substrates has been possible due to the progress in fabrication processes. Among the variety of existing flexible electronic devices, flexible sensors have gained considerable attention in wearable electronics owing to their ability to detect environmental hazards such as ultraviolet (UV) radiation and the emission of contaminants and toxic gases.1-5 In particular, the detection of such pollutants is essential for the health of living organisms as well as environmental and industrial requirements, and thus the development of various sensors for UV and toxic gas detection is required.6-9 In this regard, zinc oxide (ZnO), which is a metal oxide semiconductor, has attracted considerable attention as an UV ray and toxic gas sensing material owing to its wide direct optical bandgap of 3.37 eV, biological inertness, and simple sensing mechanism based on the change in resistivity.10-14 Moreover, ZnO can be easily fabricated with nanorod (NR) which possesses a large surface area relative to the volume of the active layer, thus improving the UV ray and gas sensing characteristics.15-19 However, despite these advantages, the performance of the existing flexible ZnO UV/gas dual sensors remains limited in terms of flexibility and sensing properties. Furthermore, the growth of ZnO NRs generally requires to use ZnO seed layer because it is difficult to grow ZnO NRs on different materials.20-22 In order to overcome these limitations and obtain excellent sensing properties and flexibility, various studies such as auxiliary functionalization using polymer or metal, application of porous structure material and tetrapod structure have been carried out.4,23-27 However, the polymer absorbs not only UV radiation but also visible light, which can alter the UV sensing efficiency, and is not stable under the high temperatures at which a gas sensor generally operates.28 In addition, porous structure materials have poor mechanical stability owing to their fragile 3



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structure, and tetrapod structures are manufactured through a high-temperature vacuum process which is problematic when using flexible substrates.4,27 In this perspective, using a metallic material such as silver (Ag) nanoparticle (NP) for auxiliary functionalization, the sensing performance can be improved while increasing the stability and flexibility of the device. The incident light interacts with the Ag NPs through localized surface plasmon resonance (LSPR) effect, resulting in a strong scattering of the incident light and an intensified absorption by the ZnO NRs, thus improving the sensing performance.29-30 Furthermore, Schottky junctions are formed at the ZnO NRs/Ag NPs heterojunctions which quickly generate charge depletion regions at the vicinity of the junction, resulting in fast sensing response.29 More importantly, since Ag NPs possess a lattice spacing similar to that of wurtzite ZnO, they can be used as a template for the growth of ZnO NRs without using an additional seed layer.21 In this study, we suggest a new approach to improve the performance and flexibility of the ZnO NR UV/gas dual sensor by using Ag NP template through a simple hydrothermal process. Ag NPs were used to improve the sensing properties through LSPR effect and to serve as a seed layer for the growth of ZnO NRs. In addition, the ZnO NRs were grown on Ag NPs in a threedimensional structure to maximize the surface-to-volume ratio of the nanostructure. ZnO NR UV/gas dual sensor grown on Ag NP template demonstrated excellent properties of UV sensing and NO2 gas detection. Furthermore, after 10,000 cycles of dynamic bending test at the bending radius of 5 mm, the device was still operating well without significant deterioration.

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EXPERIMENTAL Fabrication of Ag NPs on the substrate. Firstly, Ag layers with different thicknesses were deposited on 50 µm-thick polyimide (PI)-covered glass substrates by e-beam evaporator. After the deposition of Ag layers, they were loaded immediately into a rapid thermal annealing (RTA) system in order to prevent the oxidation of Ag by O2 and H2O in the atmosphere. Then, after 10 min-H2 gas purging, the samples were annealed at 500 °C for 1 min in order to thermally agglomerate the Ag layers into Ag NPs. After annealing treatment, the Ag films with thicknesses of 7, 9, and 12 nm were changed into the Ag NP arrays with diameters of 28, 38, and 95 nm, respectively. For comparison with Ag NPs, ZnO NPs with a diameter of 30 nm were synthesized by hydrothermal method13 and their arrays were prepared on PI-covered glass substrates by alcohol-assisted single-layer assembly technique developed by our group.15 Device fabrication. The ZnO NRs were grown on the Ag NPs and ZnO NPs by a simple hydrothermal process. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma-Aldrich, 0.3 g) and sodium hydroxide (NaOH, Sigma-Aldrich, 1.6 g) were dissolved in 50 mL DI water to make 40mM concentration solution and then maintained in an oven for 30 min at 85 °C. After the reaction, the samples were washed and dried at 100 °C for 10 min.9 Finally, a 200 nm-thick top Ni electrode was deposited on the ZnO NRs by RF magnetron sputtering at room-temperature for 30 min with an RF power of 150 W.9 Characteristization. The morphologies of the Ag NPs and the ZnO NRs were observed by field-emission scanning electron microscopy (FESEM, JSM-6701F, JEOL). The crystalline structures of the Ag NPs and the ZnO NRs were analyzed by high-resolution transmission electron microscopy (HRTEM, JEM-ARM 200F, JEOL) and X-ray diffraction measurement (XRD, Ultima IV, Rigaku). The optical properties of the ZnO NRs grown on the Ag NPs were 5



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analyzed by room-temperature photoluminescence (PL, Dongwoo Optron) measurements by using a IK3252R-E He‒Cd laser (λ = 325 nm) as an excitation source. The UV sensing performance was investigated by semiconductor parameter analyzer (Agilent B1500A, Agilent Technologies) in the dark and under UV illumination at room-temperature with a relative humidity of 34%. For an accurate measurement, the UV intensity at λ = 365 nm was quantified by an UV detector (HD 2102-1, Delta Ohm). The gas sensing performance was measured in a sealed-gas sensing chamber connected to an electrical feed-through and gas inlet and outlet at 270 °C with a relative humidity of 0%. The NO2 gas was diluted with N2 gas and dry air was used as the purging gas. Finally, the bending properties of the devices were evaluated by using a bending machine (Flexible Materials Tester, Hansung Systems Inc.) with different bending radii from infinity to 1 mm.

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RESULTS AND DISCUSSION

Figure 1. Schematics of the fabrication procedure for the flexible ZnO NR UV/gas dual sensor grown on Ag NP template. The fabrication process of the flexible ZnO NR UV/gas dual sensors using Ag NP template is schematically illustrated in Figure 1. Ag thin film deposited on the PI substrate was changed into Ag NPs by RTA process. Then, ZnO NRs were grown on the exposed Ag NPs using a hydrothermal method. Here, the Ag NPs are used as a template for the growth of ZnO NRs because the Ag (111) plane (2.89 Å) has a lattice spacing similar to the a-axis lattice constant (3.25 Å) of the ZnO wurtzite structure.31 This allows the ZnO NRs to grow on the Ag NPs along the c-axis. The as-grown ZnO NRs have a large surface-to-volume ratio, which plays a significant role in achieving superior sensing properties from the device. Moreover, the island configuration of the Ag NPs used as a template for the growth of the NRs is advantageous for applications in flexible devices as it greatly improves the mechanical resistance of the device.32 7



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Finally, the top Ni electrode with a width of 250 µm and a length of 2000 µm was deposited on the ZnO NRs by using a shadow mask. The Ni electrode with a high work function value (5.1 eV) forms a Schottky barrier with ZnO, minimizing the leakage current by increasing the barrier height with ZnO NRs, which leads to a better sensitivity of the sensor.9,33 Also, the Ni electrode is suitable for sensors operating at high temperatures owing to its high melting point and chemical stability.

Figure 2. Top-view SEM images of the Ag NPs after RTA process and the 40º-tilted SEM image of the ZnO NRs grown on Ag NP template (insets). The Ag NPs with average diameters of 28, 38, and 95 nm were prepared from Ag thin films with different thicknesses of (a) 7, (b) 9, and (c) 12 nm. (d) XRD pattern and (e) PL spectra of the ZnO NRs grown on Ag NP templates with different diameters and ZnO NP template. Figure 2 shows the morphology and the structural and optical properties of the ZnO NRs grown on Ag NP templates with different diameters. Figure 2a-c shows the top SEM images of 8


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the Ag NP templates with various diameters. The Ag NPs were prepared using Ag films with thicknesses of 7, 9, and 12 nm, and after RTA process the Ag NPs were formed with a diameter of 28, 38, and 95 nm, respectively. (Details about the average Ag NPs size distribution are shown in Figure S1). The Ag NPs formed from Ag film with a thickness of 7 nm exhibited an uniform and spherical shape. However, as the Ag film thickness increased, the number of Ag NPs decreased significantly and the uniformity in size decreased. The insets of the Figure 2a-c show the tilted-view SEM images of the ZnO NRs grown on Ag NP templates with different diameters. As a diameter of the Ag NP was changed from 28, through 38, to 95 nm, the diameter of ZnO NR grown on the Ag NP template was changed from 40, through 60, to 80 nm, respectively. The ZnO NRs grown on the Ag NP template showed an urchin structure and formed the rod-to-rod contacts between ZnO NRs. The length of the ZnO NRs was optimized to be 300 nm in order to form the network structure between the ZnO NRs. Also, to compare the effect of the Ag NPs on the sensor characteristics, a sensor was fabricated using ZnO NPs shown in Figure S2a as a template for the growth of the ZnO NRs instead of Ag NPs. As shown in Figure S2b, the ZnO NRs with a diameter of 40 nm were grown on the ZnO NP templates with a diameter of 30 nm, that was similar to the ZnO NRs grown on Ag NPs with a diameter of 28 nm. To examine the crystallinity of the ZnO NRs grown on Ag NP templates with different diameters and ZnO NP template, XRD analyses were performed as shown in Figure 2d. All samples exhibited an XRD pattern corresponding to the hexagonal wurtzite structure with the (1010) and (0002) planes. The strong (0002) peak indicates that the ZnO NRs were grown along the c-axis. Unlike the ZnO NRs grown on ZnO NP template, the XRD spectra of the ZnO NR grown on Ag NP template exhibited the diffraction peaks corresponding to the (111) and (200) 9



planes of the Ag NPs (JCPDS #04-0783). Figure S3 shows the enlarged XRD spectra of the ZnO NRs grown on Ag NP templates with different diameters. As the diameter of the Ag NPs increased, the intensity of the Ag diffraction peaks at 38.5 and 44 ° also increased. Although, the intensity of the Ag diffraction peaks was weak, their presence confirms that Ag remains in the form of NP without disappearing during the growth of ZnO NRs. In addition, the XRD spectra show a very weak peak at 32.5 °, which corresponds to the (111) plane of Ag2O, confirming the partial oxidation of the Ag NPs occurred during the synthesis of ZnO NRs. To analyze more details about oxidation of Ag NPs, TEM-energy dispersive X-ray spectroscopy (EDS) analyses were performed, and the mapping images of ZnO NRs grown on Ag NP template are shown in Figure S4. These images confirm that the partial oxidation of Ag NP is not serious even after hydrothermal process because almost no O element was detected in Ag NP. Figure 2e presents the PL spectra of the ZnO NRs grown on the Ag NP templates with different diameters and the ZnO NP template. The PL spectra of the ZnO NRs exhibit similar characteristics consisting of two major emission bands, a near band-edge emission (NBE) centered at 390 nm and wide deep-level emission (DLE) centered at 550 nm. It can be observed that the ZnO NRs grown on Ag NP template exhibit a strong NBE intensity than those grown on the ZnO NP template. In addition, the NBE intensity of the ZnO NRs increased as the diameter of the Ag NP decreased. The dramatic increase of PL intensity in the NBE region can be attributed to the LSPR effect of the Ag NPs used as a template. The LSPR effect has been extensively studied for optoelectronic device applications, consisting of a collective vibration phenomenon of free electrons generated by the interaction between an incident light and a noble metal nanostructure with a large curvature.34,35 An incident light with the appropriate wavelength can interact more efficiently with the ZnO NRs grown on the Ag NP template rather than with a 10


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single ZnO nanostructure due to the scattering and absorption of the incident light through the LSPR effect of the metal nanostructures.35 This is because the excited electrons of the Ag NP by absorbing the incident light are transferred to the ZnO NR due to the energy bandgap difference between the Ag NP and the ZnO NR.34 Therefore, the light absorption is enhanced in the Ag NPs due to the LSPR effect, and electrons excited by the energy of the absorbed wavelength are transmitted to the ZnO NRs, thereby amplifying the PL intensity. Especially, the characteristic wavelength of the LSPR varies with the diameter of the Ag NPs.36 When the diameter of the Ag NP increases, the characteristic wavelength range at which the LSPR occurs increases and broadens gradually, as it is demonstrated by the red shift observed in the NBE.36 This phenomenon, in which the LSPR wavelength is red shifted as the diameter of the Ag NP increases, is in agreement with a decrease in PL intensity observed in the NBE region as the diameter of Ag NP increases. On the other hand, unlike in the NBE region, the PL intensity increased in the DLE region as the diameter of the Ag NP increased. In the case of ZnO NRs grown on ZnO NP template, a broad DLE region is observed around 530 nm due to the presence of intrinsic defects in the ZnO structure. In the case of ZnO NRs grown on Ag NP template with a diameter of 28 nm, the PL intensity in the DLE region is similar to the ZnO NRs grown on ZnO NP template. However, as the diameter of the Ag NP increases, the PL intensity in the DLE region of the ZnO NRs grown on Ag NP template gradually increases, and the specific wavelengths of the LSPR shift to longer wavelengths. As a consequence, the LSPR occurs over a wider range of wavelengths, thus increasing the FWHM of the DLE peak.36,37

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Figure 3. Photoresponse of ZnO NR UV sensors grown on Ag NP templates with different diameters and ZnO NP template; (a) normalized UV response as a function of time and (b) Ion/Ioff, tr, and td. To investigate the effect of the Ag NPs on the sensor, the ZnO NR UV sensors grown on Ag NP templates with different diameters and ZnO NP template were fabricated. Figure 3a shows the normalized UV response of the sensors as a function of time. The normalized UV response of the devices was measured at an UV intensity of 2.74 mW · cm-2 under a reverse bias of 4.0 V. UV on-off ratio of the sensor is defined by the ratio of the current changes and is expressed as follows,38 - /

(1) 12


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where Ion and Ioff are the currents of the sensor after and before exposure to the UV light, respectively. The ZnO NR UV sensors grown on Ag NP templates with different diameters exhibited outstanding UV response compared to the sensor grown on ZnO NP template. This is because the UV detection is amplified by the LSPR effect from the Ag NP template. When decreasing the diameter of the Ag NP from 95 nm to 28 nm, the normalized UV response of the ZnO NR UV sensor grown on Ag NP template increased dramatically by 78% because of the improved NBE intensity as shown in Figure 2e. The actual current value corresponding to the normalized UV response of Figure 3a is shown in Figure S5. As the size of Ag NP increases, the rod-to-rod contacts of the ZnO NRs grown on Ag NP template also decrease because the density of aligned Ag NPs decreases. Therefore, as the particle size increases, the sensor current decreases. Figure 3b exhibits Ion/Ioff, rising time (tr), and decay time (td) of the sensors with different templates. The Ion/Ioff of the sensor grown on Ag NP template was decreased from 3628 to 2068 with increasing diameter of Ag NP from 28 to 95 nm, while it reached only 354 for the sensor grown on ZnO NP template. In the case of the sensors grown on Ag NP template, tr and td did not show significant variations and remained within the range of 3.52 to 4.97 s and 0.33 to 0.39 s, respectively, whereas the sensor grown on ZnO NP template exhibited significantly higher times with 25 and 4.36 s, respectively. Moreover, the ZnO NR UV sensor grown on Ag NP template with a diameter of 28 nm exhibited the best characteristics for UV detection with Ion/Ioff of 3628, tr of 3.52 s, and td of 0.33 s.

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Figure 4. Energy band diagrams of the surface of (a) the ZnO NRs without Ag NP template and (b) the ZnO NRs grown on Ag NP template. The LSPR effect is not the only phenomenon induced by the Ag NPs improving the UV sensing characteristics of the devices. The UV sensing mechanism of the ZnO-based sensor reacting through changes in electrical resistance is greatly affected by the surface depletion region. So, it is noteworthy that the Ag NP template plays an important role in achieving the better sensitivity of the device by affecting the surface depletion region of ZnO NR. The mechanism of improving the sensor performance through the surface depletion region is represented in Figure 4 as a schematic energy band diagram of the theoretical model. Figure 4a shows the energy band diagrams of a ZnO NR-based sensor without Ag NP template in dark and under illumination. In the dark, oxygen molecules adsorbed on the surface of the ZnO NRs capture electrons in the conduction band of ZnO NRs and become oxygen ions. Under UV illumination, electron-hole pairs are generated in ZnO NRs, and oxygen ions adsorbed on the surface are instantaneously combined with the generated holes leaving free electrons in the 14


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conduction band minimum (CBM) of the ZnO NRs. As a consequence, these generated free electrons increase the conductivity of the ZnO NRs. Then, when the UV light is turned off again, the electron-hole recombines and the conductivity decreases. At this moment, the oxygen molecules reabsorbed on the surface of the ZnO NRs become oxygen ions, forming a depletion region with a low conductivity. This process is very slow and takes a long time to recover before the UV irradiation.28,39 On the other hand, the band structure of the ZnO NRs grown on Ag NP template exhibits a different pattern, as shown in Figure 4b. Since the work function of Ag (Φm ~ 4.5 eV) is larger than the electron affinity of ZnO (χ ~ 4.35 eV), electrons flow from ZnO NR to Ag NP, which causes Ag NP and ZnO NR to form a Schottky barrier after alignment of their Fermi energy level.9,40-41 In order to demonstrate the Schottky junction between Ag and ZnO, an Ag/ZnO NRs/Ag structure was constructed as shown in the Figure S6. This I-V curve indicates that Schottky contact is obtained from the Ag/ZnO NRs/Ag structure. When the UV light is irradiated on this structure, the oxygen ions adsorbed on the surface are immediately combined with the generated holes, leaving free electrons in the ZnO NRs and increasing the conductivity of the ZnO NRs. However, unlike the ZnO NRs without Ag NP template, when the UV light is turned off again, surface depletion region at the vicinity of the junction between ZnO NR and Ag NP is formed more rapidly due to the electron flow from ZnO NR to Ag NP for alignment of Fermi energy levels.28 Because of the rapidly formed surface depletion region, the ZnO NR UV sensor grown on the Ag NP template exhibits a significantly faster decay time for UV detection than the ZnO NR UV sensor without Ag NP template, as shown in Figure 3. In addition, considering electron transport path of the optimized ZnO NR UV sensor grown on Ag NP template (Figure S7), although the Schottky barrier is formed only at the bottom of the ZnO NRs, Ag NPs still 15



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greatly influence the characteristics of the sensor due to the rod-rod contact with ZnO NRs and the horizontal structure of the electrodes.

Figure 5. UV and gas sensing properties of the optimized ZnO NR UV/gas dual sensor grown on Ag NP template with a diameter of 28 nm; (a) UV response with different UV intensities (0.26, 1.05, 1.98, and 2.74 mW · cm−2) as a function of time, (b) Ion/Ioff, tr, and td with different UV intensities, (c) UV response during 4 UV on–off sensing cycles at an UV intensity of 2.74 mW · cm−2, (d) gas response with different gas concentrations (50, 100, 200, 300, 400, and 500 ppm) as a function of time at 270 ℃, (e) Ron/Roff, tr, and td with different gas concentrations, and (f) gas response during 4 gas on–off sensing cycles at a NO2 concentration of 500 ppm. Figure 5 shows the UV and NO2 gas sensing characteristics of the optimized ZnO NR UV/gas dual sensor grown on Ag NP template with a diameter of 28 nm. Figure 5a-c show the UV 16


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sensing properties of the sensor at 4 V. This sensor responded strongly to UV regardless of the direction of applied voltage and showed the largest current change at 4 V, as shown in Figure S8a. Therefore, all of the UV sensing characteristics of the sensor were measured at a voltage of 4 V. When increasing the UV intensity from 0.26 to 2.74 mW · cm−2, the UV response of the sensor increased proportionally as shown in Figure 5a. Figure 5b shows Ion/Ioff, tr, and td with different UV intensities. As the UV intensity increased from 0.26 to 2.74 mW · cm−2, Ion/Ioff increased from 389 to 3628, while tr and td decreased from 3.89 to 3.52 s and from 0.76 to 0.33 s, respectively. Figure 5c shows the UV response of the sensor during 4 UV on-off sensing cycles under an UV intensity of 2.74 mW · cm-2. This optimized sensor maintained stable UV sensing characteristics without showing significant degradation in performance during 4 consecutive cycles. Moreover, the UV responsivity of the sensor with different UV intensities is shown in Figure S9. The UV responsivity of the sensor is expressed as follows,42-44 /

(2)

where Iph is the photocurrent, P is the incident power density, and S is the effective area of the photosensitive region, respectively. As shown in Figure S9, the UV responsivity of the sensor is maintained without significant change from 85.38 to 95.44 A/W although the UV intensity increases from 0.26 to 2.74 mW · cm−2. To confirm the gas sensing characteristics of the optimized sensor, the device performances were measured under NO2 gas exposure at a constant voltage of 1 V and 270 ℃ as shown in Figure 5d-f. Since the gas response of the sensor is nearly constant at all voltages as shown in Figure. S8b, the measurement is performed at 1 V. Moreover, the operating temperature of the NO2 gas sensor was optimized at 270 ℃ as shown in Figure S10. When the operating temperature of the NO2 gas sensor was below 200 ℃, the gas response was very low. However, as the 17



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temperature increased above 200 ℃, the gas response increased sharply, until it reached 270 ℃, where the gas response stabilized. Therefore, all subsequent analyses were performed at 270 ℃. Compare with the sensing mechanism of the sensor for UV light, in which the current is increased when the UV is irradiated, the sensing mechanism of the sensor for NO2 gas is different. When the sensor is exposed to NO2 gas, the current of the sensor is decreased. This is due to the formation of a depletion region when NO2 gas is adsorbed on the surface of the ZnO NRs, which decreases the concentration of free electrons.14 So, the gas on-off ratio of the sensor is defined by the ratio of the resistance changes and is expressed as follows,45 Gas on-off ratio = /

(3)

where Ron and Roff are the resistances of the sensor after and before exposure to the NO2 gas, respectively. As the gas concentration increased, the gas response of the sensor increased gradually as shown in Figure 5d. Figure 5e shows Ron/Roff, tr, and td with different NO2 gas concentrations. As the NO2 gas concentration increased from 50 to 500 ppm, Ron/Roff increased from 86.6 to 288.5 and tr and td increased from 29 to 38 s and from 22 to 62 s, respectively. The high Ron/Roff of 288.5 and the fast tr and td of 38 and 62 s under a NO2 concentration of 500 ppm are due to the large surface area of the three-dimensional nanostructures of the ZnO NR UV/gas dual sensor grown on Ag NP template. The maximized surface area within these threedimensional nanostructures promotes a smooth diffusion of the NO2 gas and an effective surface reaction with the ZnO NRs. In addition, the network structure created by the rod-to-rod contacts of the ZnO NRs grown on Ag NP template plays an important role in improving the performances of the sensor. The rod-to-rod contacts between the ZnO NRs can be interpreted in a similar way to grain boundaries, leading to the formation of potential barriers at the contact points of the ZnO NRs.14 As the potential barrier of the ZnO NRs increases, the amount of 18


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electron transfers decreases, which results in an amplified change in current when NO2 is adsorbed at the junction of ZnO NRs. Therefore, the network structure of the ZnO NR UV/gas dual sensor grown on Ag NP template is an essential element for the improvement of gas sensing properties. Furthermore, at 500 ppm, the sensor exhibited stable gas sensing characteristics without any significant deterioration in performances during 4 gas on-off sensing cycles, as shown in Figure 5f. To distinguish the measurement capabilities of the dual sensor under UV illumination and NO2 gas exposure, the response of the sensor was measured when exposed to UV and NO2 simultaneously, as shown in Figure S11. Under UV illumination, the resistance of the ZnO NRs decreases. If NO2 gas is simultaneously injected under UV illumination, the NO2 gas combines with the photoelectrons generated in the ZnO NRs due to its high electron affinity, which leads to an increase in the resistance of the ZnO NRs. So, as shown in Figure S11a, the sensor exposed only to NO2 or exposed to UV and NO2 simultaneously shows different Roff due to UV illumination, i.e. Roff of sensor is significantly lower when exposed to UV and NO2 simultaneously than when exposed only to NO2. Therefore, by using the difference in Roff, it is possible to distinguish the effect of UV illumination and NO2 gas exposure on the dual sensor. Figure S11b shows the linear gas response of the sensor when exposed to UV and NO2 simultaneously from Figure S11a. Compared to the sensor exposed only to NO2 in Figure 5f, the sensor exposed to UV and NO2 simultaneously showed excellent gas response with exponential resistance changes. For an accurate comparison, the quantitative characteristics of gas response when exposed to UV and NO2 simultaneously and when exposed only to NO2 are compared in Figure S11c. Ron/Roff of the sensor when exposed to UV and NO2 simultaneously was 409, which was higher than the ratio of 288.5 when exposed only to NO2. Also, when exposed to UV and 19



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NO2 simultaneously, tr and td were 30 and 26 s, respectively, significantly faster than 38 and 62 s when exposed only to NO2. The high Ron/Roff and the fast tr and td of the sensor when exposed to UV and NO2 simultaneously are due to the LSPR effect of Ag NP. As the UV absorption of Ag NP is increased by the LSPR effect, more electrons oscillate in Ag NP. At this time, when the NO2 gas is injected, the NO2 molecules are adsorbed by the oscillating electrons of the Ag NP due to high electron affinity.34,46 As a result, the oscillating electrons of Ag NP are reduced, and it consequently improves the NO2 sensing properties of the ZnO NR sensor grown on Ag NP template.

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Figure 6. UV and NO2 gas sensing properties of the flexible ZnO NR UV/gas dual sensor grown on Ag NP template with a diameter of 28 nm; (a) camera image of the bent device with the bending radius of 5 mm, (b) Ion/Ioff, tr, and td with different bending radii at an UV intensity of 2.74 mW · cm−2, (c) UV response as a function of time during the dynamic bending test up to 10,000 cycles at the bending radius of 5 mm under an UV intensity of 2.74 mW · cm−2, (d) Ion/Ioff, tr, and td during the dynamic bending test, (e) gas response as a function of time during the dynamic bending test up to 10,000 cycles at the same radius under a fixed NO2 concentration of 500 ppm and at 270 ℃, and (f) Ron/Roff, tr, and td during the dynamic bending test. 21



Finally, a mechanical bending test was performed to evaluate the flexibility of the optimized ZnO NR UV/gas dual sensor grown on Ag NP template with a diameter of 28 nm. Figure 6a shows a camera image of the bent sensor on PI substrate with the bending radius of 5 mm. Figure 6b shows Ion/Ioff, tr, and td of the dual sensor during a static bending test with different bending radii from ∞ (flat state) to 1 mm at an UV intensity of 2.74 mW · cm-2 under a reverse bias of 4.0 V. As the sensor was bent from flat state to the bending radius of 5 mm, Ion/Ioff, tr, and td were slightly changed from 3628, 3.52 s, and 0.33 s to 3715, 4.85 s, and, 0.45 s, respectively. However, with further decrease of the bending radius from 5 to 1mm, Ion/Ioff decreased significantly from 3715 to 1893 and tr and td increased from 4.85 to 8.55 s and from 0.45 to 0.87 s, respectively. As a consequence, the dynamic bending tests were performed at the bending radius of 5 mm. Figure 6c-d show the changes in the UV sensing characteristics of the dual sensor during the dynamic bending test up to 10,000 cycles at the same bending radius. As shown in Figure 6c, the UV response of the dual sensor decreased by about 25% of the initial UV response before the bending test with increasing bending cycle from 0 to 10,000. Especially, when the number of bending cycles increased from 0 to 4,000, the UV response showed a sharp decrease of 20%. However, in the subsequent bending test from 4,000 to 10,000 cycles, the UV response decreased only by 5%, demonstrating stable operation. Figure 6d shows the corresponding quantitative UV sensing characteristics of the sensor as a function of the bending cycle at the bending radius of 5 mm. As the bending cycle increased from 0 to 10,000, the sensor showed stable characteristics with the gradually decreased Ion/Ioff 3628 to 2753, and the slightly increased tr and td from 3.52 to 5.33 s and from 0.33 to 0.41 s, respectively. Moreover, Figure 6e-f show the changes in the NO2 gas sensing characteristics of the dual sensor under the same dynamic bending test conditions as the UV light sensing and at a fixed 22


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NO2 gas concentration of 500 ppm. With increasing bending cycle from 0 to 10,000, the gas response of the sensor gradually increased by about 26% as shown in Figure 6e. When the number of bending cycles increased from 0 to 4,000, Ron/Roff increased by 16%, while it increased by 10% with increasing bending cycle from 4,000 to 10,000 as shown in Figure 6f. Consequently, as the bending cycle increased from 0 to 10,000, Ron/Roff and tr increased from 288.5 to 363.8 and from 38 to 68 s, respectively, while the td remained almost unchanged from 62 to 60 s. After 10,000 bending cycles, the gas response of the sensor appeared to be improved under NO2 gas exposure, while the UV response decreased under UV light exposure. This improved gas response is due to the increase in sensor resistance, which was caused by the degradation of the sensor during the dynamic bending test. The degradation of the ZnO NR UV/gas dual sensors grown on Ag NP template after dynamic bending cycle was confirmed by SEM images shown in Figure S12. After 10,000 cycles of repetitive bending tests, fine cracks appeared on the top Ni electrode deposited on ZnO NRs. Despite the degradations, the ZnO NR UV/gas dual sensor grown on Ag NP template remained operational even after 10,000 bending cycles. This mechanical stability of the dual sensor is due to the structural advantage of the ZnO NRs grown on Ag NP template, which has an urchin structure with the rod-to-rod contacts between ZnO NRs. Moreover, to compare the performance of the current flexible ZnO NR UV/gas dual sensor with the previous ZnO nanomaterial-based UV and NO2 gas sensors, the device parameters are summarized in Table 1 and 2. Due to the uncommon nature of studies involving flexible ZnO NR UV/gas dual sensor, the characteristics of UV and NO2 sensors have been compared individually. The ZnO NR UV/gas dual sensor grown on the Ag NP template presented in this study showed superior performances in both the UV and the NO2 sensing compared to the other 23



Page 24 of 43

reported ZnO-based sensors. In addition, unlike previous studies, the devices demonstrated stable mechanical properties by maintaining stable UV sensing and NO2 sensing characteristics even after 10,000 dynamic bending test with the bending radius of 5 mm.

Table 1. Comparison of UV sensing properties of the ZnO-based UV sensor.

Synthesis method

Wavelength of UV light (nm)

Ion/Ioff

tr (s)

td (s)

Bending radius (mm)

Number of bending cycle

Ref.

Low temp, solution

350

-

10.3

14.2

-

-

32

Low temp, solution

365

2.3

1

1

6.5

-

47

Low temp, solution

365

22

3.2

66.3

12

10000

48

Low temp, solution

320

6.2

0.77

0.73

-

200

49

Low temp, solution

365

3628

3.52

0.33

5

10000

This work

24


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Table 2. Comparison of NO2 gas sensing properties of the ZnO-based gas sensor.

Synthesis method

Operating Temp. (°C)

Ron/Roff

tr (s)

td (s)

Bending radius (mm)

Number of bending cycles

Ref.

Low temp. Solution

250

3.3

25

21

-

-

50

Low temp. Solution

300

14.9

52

101

-

-

51

Low temp. Solution

290

264

-

-

-

-

52

Low temp. Solution

270

218.1

25.0

14.1

5

10000

14

Low temp, solution

270

288.5

38

62

5

10000

This work

25



Page 26 of 43

CONCLUSIONS In summary, we have fabricated flexible ZnO NR UV/gas dual sensors using Ag NP template on PI substrate. The Ag NPs were used as a template to grow the ZnO NRs and acted as an amplifying material for the detection of UV light through the LSPR effect. As a diameter of the Ag NP decreased from 95 to 28 nm, the Ion/Ioff of the dual sensor grown on Ag NP template increased significantly from 2068 to 3628. The Ag NPs not only enhanced the Ion/Ioff of the ZnO NRs due to the LSPR effect, but also induce fast decay times for UV sensing due to rapidly formed surface depletion region on the ZnO NRs. The optimized ZnO NR UV/gas dual sensor grown on Ag NP template with a diameter of 28 nm exhibited the outstanding UV sensing characteristics with an Ion/Ioff of 3628, a tr of 3.52 s, and a td of 0.33 s at an UV intensity of 2.74 mW · cm−2. Moreover, the same sensor showed excellent NO2 gas sensing characteristics with a Ron/Roff of 288.5, and fast tr and td of 38 and 62 s at 500 ppm. Such characteristics have been attributed to the large surface area and the rod-to-rod contacts of ZnO NRs, which greatly improve the gas sensing properties. Furthermore, the ZnO NR UV/gas dual sensor grown with the Ag NP template exhibited stable sensing characteristics with a 25% decrease in UV response and a 26% increase in gas response even after 10,000 bending cycles at the bending radius of 5 mm. Therefore, it is believed that using Ag NP templates for ZnO NR UV/gas sensors is a very effective way to not only improve sensing performance, but also ensure mechanical stability.

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ASSOCIATED CONTENT Supporting Information Size distribution of the Ag NPs on PI substrates prepared by RTA process as a function of Ag film thickness. Top-view SEM image of the ZnO NP layer and magnified SEM image of the ZnO NPs. The 40º-tilted SEM image of the ZnO NRs grown on ZnO NP template. Magnified XRD spectra of the ZnO NRs grown on Ag NP templates with a diameter of 28, 38, and 95 nm. TEM-EDS mapping images of the ZnO NR grown on Ag NP template. UV response of the ZnO NR UV sensors grown on different templates as a function of time. I-V characteristics of Ag/ZnO NRs/Ag and scheme of the MSM structure under dc bias. Schematic illustration of Schottky barrier and electron path of ZnO NRs grown on Ag NP template. I-V characteristics of ZnO NR UV/gas dual sensor grown on Ag NP template at UV on-off and gas on-off conditions. UV responsivity of the optimized ZnO NR UV sensor grown on Ag NP template with different UV intensities. NO2 gas response of the sensor with different operating temperatures at a NO2 concentration of 500 ppm. Response of the ZnO NR gas/UV dual sensor grown on Ag NP template when exposed to UV and NO2 simultaneously. Top-view SEM images of the UV/gas dual sensor before and after bending test of 10,000 cycles at the bending radius of 5 mm.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes 27



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The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1301-07.

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Table of Contents

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Figure 1. Schematics of the fabrication procedure for the flexible ZnO NR UV/gas dual sensor grown on Ag NP template. 46x25mm (300 x 300 DPI)



Figure 2. Top-view SEM images of the Ag NPs after RTA process and the 40º-tilted SEM image of the ZnO NRs grown on Ag NP template (insets). The Ag NPs with average diameters of 28, 38, and 95 nm were prepared from Ag thin films with different thicknesses of (a) 7, (b) 9, and (c) 12 nm. (d) XRD pattern and (e) PL spectra of the ZnO NRs grown on Ag NP templates with different diameters and ZnO NP template. 102x58mm (300 x 300 DPI)


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Figure 3. Photoresponse of ZnO NR UV sensors grown on Ag NP templates with different diameters and ZnO NP template; (a) normalized UV response as a function of time and (b) Ion/Ioff, tr, and td. 111x149mm (300 x 300 DPI)



Figure 4. Energy band diagrams of the surface of (a) the ZnO NRs without Ag NP template and (b) the ZnO NRs grown on Ag NP template. 114x73mm (300 x 300 DPI)


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Figure 5. UV and gas sensing properties of the optimized ZnO NR UV/gas dual sensor grown on Ag NP template with a diameter of 28 nm; (a) UV response with different UV intensities (0.26, 1.05, 1.98, and 2.74 mW · cm−2) as a function of time, (b) Ion/Ioff, tr, and td with different UV intensities, (c) UV response during 4 UV on–off sensing cycles at an UV intensity of 2.74 mW · cm−2, (d) gas response with different gas concentrations (50, 100, 200, 300, 400, and 500 ppm) as a function of time at 270 ℃, (e) Ron/Roff, tr, and td with different gas concentrations, and (f) gas response during 4 gas on–off sensing cycles at a NO2 concentration of 500 ppm. 105x62mm (300 x 300 DPI)



Figure 6. UV and NO2 gas sensing properties of the flexible ZnO NR UV/gas dual sensor grown on Ag NP template with a diameter of 28 nm; (a) camera image of the bent device with the bending radius of 5 mm, (b) Ion/Ioff, tr, and td with different bending radii at an UV intensity of 2.74 mW · cm−2, (c) UV response as a function of time during the dynamic bending test up to 10,000 cycles at the bending radius of 5 mm under an UV intensity of 2.74 mW · cm−2, (d) Ion/Ioff, tr, and td during the dynamic bending test, (e) gas response as a function of time during the dynamic bending test up to 10,000 cycles at the same radius under a fixed NO2 concentration of 500 ppm and at 270 ℃, and (f) Ron/Roff, tr, and td during the dynamic bending test. 159x143mm (300 x 300 DPI)


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Table of Contents 54x36mm (150 x 150 DPI)


gas dual sensor using Ag

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