Diffusion-Driven Al-Doping of ZnO Nanorods and Stretchable Gas

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Diffusion-Driven Al-Doping of ZnO Nanorods and Stretchable Gas Sensors Made of Doped ZnO Nanorods/Ag Nanowires Bilayers Gitae Namgung, Qui T.H. Ta, Woochul Yang, and Jin-Seo Noh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17336 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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ACS Applied Materials & Interfaces

Diffusion-Driven Al-Doping of ZnO Nanorods and Stretchable Gas Sensors Made of Doped ZnO Nanorods/Ag Nanowires Bilayers Gitae Namgung,† Qui Thanh Hoai Ta,† Woochul Yang,‡ and Jin-Seo Noh*,† †

Department of Nano-Physics, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam-si, Gyeonggi-do 13120, Korea ‡

Department of Physics, Dongguk University, 30 Phildong-ro 1gil, Jung-gu, Seoul 04620, Korea

*Author to whom correspondence should be addressed: Jin-Seo Noh: e-mail: [email protected], phone: +82 317505611

Keywords: Stretchable gas sensors, Al-doped ZnO nanorods, drive-in diffusion, silver nanowires, bilayers

A crystal-damage-free nano-doping method, which utilized the vacuum drive-in diffusion of Al into ZnO nanorods, was developed. In this method, vertical ZnO nanorod arrays that were grown by chemical bath deposition beforehand were deposited with Al thin film and subsequently heat-treated under a high vacuum. At an optimum condition, the surface Al atoms were completely diffused into ZnO nanorods, resulting in Al-doped ZnO nanorods. Stretchable gas sensors were fabricated by sequentially drop-casting Al-doped ZnO nanorods and silver nanowires on PDMS substrate. The resistance and response of the sensor could be optimized through the elaborate control of relative densities of Al-doped ZnO nanorods and silver

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nanowires. The sensor showed a high response of 32.3% to 10 ppm of NO2 gas at room temperature, even under a large strain of 30%. The NO2-sensing mechanism of Al-doped ZnO nanorods/silver nanowires bilayer sensors is discussed on the basis of synergistic interplay of Aldoped ZnO nanorods and silver nanowires.


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1. INTRODUCTION The gradually increasing emission of toxic gases from diversifying sources has pushed our daily lives towards risk. For instance, nitrogen dioxide (NO2), which is a main component of automobile exhaust gas, threatens the environment and human being’s health than ever before. The sources of toxic gases are unevenly localized, and real-time gas detection synchronized with our daily motion is on demand to relieve the danger of toxic gas exposure. A patch-type sensor may be ideal since it can be easily attached and detached on multiple places such as skin, clothes, and shoes. To patch a sensor on those objects that experience dynamic deformations, some stretchability is essential. Furthermore, the sensor should run at room temperature both to remove the heater patterns and to avoid potential burns. Zinc oxide (ZnO) is an n-type semiconductor with a large band gap (3.37 eV) and wurtzite crystal structure. Due to its high response to various gases like NO2, acetone, and formaldehyde, it has been intensively investigated as a gas-sensing material. To improve its gas-sensitivity, a variety of low-dimensional nanostructures have been explored, including nanoparticles,1-3 nanowires,4-12 nanorods,13-19 and nanofibers.20-23 Despite the big progress in the sensing performance, all the nanostructured ZnO sensors were rigid and operated at elevated temperatures. These features may prohibit ZnO nanostructures from being applied for stretchable gas sensors. A tactics to reduce the operation temperature of ZnO-based sensors is to dope it with metallic elements such as aluminum (Al),21,24-28 manganese (Mn),2,22 palladium (Pd),17,20 copper (Cu),29 and gallium (Ga).30 Al-doped ZnO (AZO) has been most actively studied owing to the low-cost and high-performance. For the synthesis of AZO nanostructures, in-situ chemical doping21,24,25 and ion implantation31,32 have been conventionally employed. However, these methods may cause misoriented crystal growth33 and crystal damage though efficient.31,32,34,35



Although AZO has a reduced resistivity compared to pure ZnO,24,25,34,35 the networks of its lowdimensional nanostructures are expected to still have high resistance. Moreover, its roomtemperature gas-sensing capability may not be good enough. For these reasons, a secondary structural component is necessary, which can further reduce the sensor resistance and facilitate gas-responding process. Silver nanowires (AgNWs) can play these roles effectively.16,36,37 Studies on the stretchable gas sensors have been rarely reported. Narrowing the scope down to ZnO-based stretchable sensors, there has been only one report. Gutruf et al. fabricated microtectonic ZnO on polydimethylsiloxane (PDMS) substrate and demonstrated that it could respond to both NO2 and hydrogen (H2) gases under a strain at room temperature. However, the largest strain applied to their sensor was just 5%.38 In this work, we realized stretchable gas sensors, employing AZO nanorods (AZO NRs)/AgNWs bilayers on PDMS. The AZO NRs were synthesized through a distinctive series of processes, i.e., wet ZnO NR growth, Al sputtering, and thermal drive-in diffusion. It was found that the sensors could respond to NO2 gas even under large strains up to 30% at room temperature.

2. EXPERIMENTAL SECTION 2.1. Synthesis of AZO NRs Schematic procedures for the synthesis of AZO NRs are presented in Fig. 1. At first, ZnO NR arrays were vertically grown on the silicon (Si) substrate, using a seed-assisted growth method previously reported.39 For that, a ZnO seed layer 120 nm in thickness was deposited by radio frequency (RF) magnetron sputtering. ZnO NRs were grown on this seed layer, employing a chemical bath deposition (CBD) method. For the CBD growth, the Si substrate with a ZnO seed layer was immersed in a mixture solution of 50 mM zinc nitrate hexahydrate


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(Zn(NO3)2·6H2O, CAS number 10196-18-6, purity = 98%) and 50 mM hexamethylenetetramine (HMTA, C6H12N4, CAS number 100-97-0, purity ≥ 99.0%) at 90 °C for an hour. In the next step, Al thin films were deposited on the vertically grown ZnO NR arrays by RF magnetron sputtering. The chamber was first pumped down to the high vacuum level of ~10-6 Torr, and then argon (Ar) gas was fed into the chamber with a flow rate of 16 sccm to control the working pressure at 14 mTorr. The sputtering proceeded for 30 – 120 s under a RF power of 100 W, which resulted in Al films with a thickness range of 2 – 8 nm. To collect the Al-coated ZnO NRs, two sheets of nanorod arrays were gently rubbed face to face and sonicated in ethanol for 20 min. Those nanorods dispersed in ethanol were drop-cast on a quartz slab and dried in a convection oven at 60 °C. The last step is thermal drive-in diffusion of Al into ZnO NRs by heat treatment.34,35,40 To minimize the potential oxidation of Al, this heat treatment was performed in a vacuum furnace. Al-coated ZnO NRs on the quartz slab were put in a tube furnace with a diameter of 2 in, and heated at 500 – 600 °C for 4 – 6 h under vacuum of ~10-5 Torr. Thus-synthesized AZO NRs were separated from the quartz slab by being immersed in deionized (DI) water and sonicated for 20 min.

2.2. Synthesis of AgNWs AgNWs were synthesized by a polyol method.37,41 Three reactant solutions of copper (II) chloride

dihydrate

(CuCl2·2H2O,

CAS

number

10125-13-0,

purity

≥

99.0%),

polyvinylpyrrolidone (PVP, Mw ~40,000, CAS number 9003-39-8), and silver nitrate (AgNO3, CAS number 7761-88-8, purity ≥ 99.0%) were prepared, using ethylene glycol (EG, C2H6O2, CAS number 107-21-1, purity = 99.8%) as solvent. At first, 50 ml of EG was preheated at 151.5°C for 1 h with magnetic stirring, then 0.4 ml of 4 mM CuCl2 solution was added to it. In



15 min, 15 ml of 0.176 M PVP solution was injected into the solution. Finally, 15 ml of 0.95 M AgNO3was added over the injection time of 30 min, leading to color change to bright gray. Then, the mixture solution was cooled down to room temperature for 24 h. The AgNWs were collected through centrifugation and three times of washing.

2.3. Fabrication of Stretchable Gas Sensors Stretchable gas sensors basically had bilayer structures consisting of an AZO NRs underlayer and an AgNWs overlayer on PDMS substrate. For their fabrication, AZO NRs were dispersed and sonicated for 20 min in DI water. The colloid concentration was set at 0.35 mg/ml. The AZO NRs solution was drop-cast on a PDMS substrate (20.0 × 10.0 × 0.4 mm in size) and dried at 60°C for 1 h (Fig. 1g). Meanwhile, three AgNWs solutions were prepared with different concentrations of 0.5, 0.75, and 1.0 mg/ml. These AgNWs solutions were individually coated over the preformed AZO NRs films and dried again at 60°C for 1 h. Sonication was performed for 2 min before drop-casting the AgNWs solutions, and the effect of repetitive AgNWs dropcasting on the sensor performance was also examined. Furthermore, stretchable sensors made of only AZO NRs or ZnO NRs/AgNWs bilayer were independently prepared for comparison.

2.4. Characterization and Gas-Sensing Tests The shape, size, and distribution of ZnO NRs, Al-coated ZnO NRs, and AZO NRs were analyzed using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7500F). Those analyses were made both on nanorods stuck to the Si substrate and on nanorods taken off the substrate. High-resolution lattice fringe, overall composition, and element distribution of individual nanorods were examined by a field emission transmission electron microscope (FE-


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TEM, JEOL JEM-2100F) mounted with an energy-dispersive X-ray spectrometer (EDX). Electron energy loss spectroscopy (EELS) analysis was also performed to trace doping-induced energy change, using another TEM (FEI, Tecnai G2 F30ST). The crystal structures and any inclusion of secondary phases of those nanorods were determined by X-ray diffraction (XRD, PANalytical X’Pert Pro MPD) with Cu K radiation. In addition, X-ray photoelectron spectroscopy (XPS, K-Alpha-Thermo electron) was employed to examine any changes in binding states of major elements after Al-doping. The NO2-sensing performance of stretchable gas sensors was evaluated using a home-made sensing system with a specially devised chamber. A sample stage equipped with a stretching machine could apply tensile strains up to ~150%, simultaneously with the real-time measurement of sample resistance. The volume capacity of the chamber was 682 cm3. A sample was loaded onto the stage and two points on the sample surface were electrically connected to two counterpins using a 0.1 mm-thick gold (Au) wire. NO2 gas was diluted to desired concentrations in the range of 1 – 100 ppm by carefully adjusting the gas flow rates of NO2 and carrier gas (N2). The change of electrical resistance of the sensor upon exposure to NO2 was measured by a multimeter (Keithley 2001), and all the measured resistances were recorded into a computer by a Lab View program. All the gas-sensing tests were performed at room temperature. Besides the stretchable sensors, the gas-sensing properties of stationary sensors on Si substrate were also studied.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of AZO NRs



Fig. 2 shows stepwise SEM images of as-synthesized ZnO NRs, Al-coated ZnO NRs, and AZO NRs. The images in the left column were obtained from nanorods sticking to Si substrate, while those in the right column from nanorods cast on quartz substrate. It is found from Fig. 2a that high-density ZnO NRs are almost vertically aligned. Their average diameter and length are estimated at 66 and 567 nm from Fig. 2b (also see the cumulative size distributions in Fig. S1 for more information). Fig. 2c and d are SEM images of Al-coated ZnO NRs. Here, Al thin film was deposited by RF sputtering for 1 min, the thickness of which was expected to be ~4 nm. The overall distribution and size of the Al-coated ZnO NRs look very similar to those of assynthesized ZnO NRs. A small difference is that the upper parts of the Al-coated ZnO NRs are slightly thicker than the rest, as can be seen in Fig. 2d. This phenomenon becomes more evident when the Al film thickness increases (see an 8 nm-thick Al film case in Fig. S2 c and d for comparison). These suggest that Al film is deposited preferably on the upper parts of ZnO NRs. The shape and size of Al-coated ZnO NRs remain consistent after heat treatment, as confirmed for AZO NRs in Fig. 2e and f. Fig. 3 displays TEM images of Al-coated ZnO NRs (before heat treatment) and AZO NRs (after heat treatment). Al sputtering was performed for 30 s in this case. Both low-resolution and high-resolution images are presented together to see the overall shape and atomic arrangements. The slightly tapered nanorod shapes in Fig. 3a and c are the same as those observed from SEM. Clear lattice fringes of ZnO and Al are found for Al-coated ZnO NRs (Fig. 3b). Al film thickness is on average 1.3 nm. The interface between ZnO NR core and Al shell is sharp without a noticeable transition layer. These prove that the Al sputtering at the given conditions does not damage the crystal qualities of both ZnO and Al. Most notably, the Al shell layer disappears after vacuum-annealing at 600 oC for 4 h. This indicates that Al atoms were thermally driven into the


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ZnO core during the heat treatment. There are no isolated Al or AlxOy phases found inside ZnO. The similar disappearance of surface Al layer also occurs after heat treatment at a lower temperature (500 oC), as shown in Fig. S3. The lattice fringe of AZO looks apparent in the highresolution image. In order to further analyze elemental distribution after the heat treatment, TEM-EDX element mapping was performed. Fig. 4 shows the element maps and EDX profile of AZO NRs, which were synthesized at the same conditions as for those in Fig. 3c and d: 30 s-long sputtering followed by vacuum-annealing at 600 oC for 4 h. Other than dense Zn and O dots (red and green dots), relatively low-density of Al dots (blue dots) are distributed throughout the entire nanorods, signifying the uniform distribution of Al atoms inside AZO NRs. No agglomerates of Al are noticed. The Al concentration of AZO NRs is estimated at 5 at% from an EDX profile (Fig. 4f). The sparse but uniform distribution of Al atoms is also observed from another sample that was heat-treated at different conditions (500 oC, 4 h), as shown in Fig. S4. Additionally, EDX lineprofiling was conducted for three samples. Al intensity, which is detected mostly from surface Al atoms, becomes weaker after heat treatment, particularly at 600 oC (Fig. S5). These results support that the thermal drive-in diffusion under vacuum is an effective way to dope ZnO NRs with a metallic element, resulting in uniform dopant distribution but no crystal damage.

3.2. XRD, XPS, and EELS Analyses To elucidate any heat-treatment-induced crystallographic changes, XRD measurements were carried out. Fig. 5 exhibits XRD patterns of an Al-coated ZnO NRs sample and two AZO NRs samples that were vacuum-annealed at different temperatures (500, 600 oC) for 4 h. The main peaks of the Al-coated ZnO NRs come from ZnO body, which are assigned to the wurtzite



crystal structure of ZnO crystal (JCPDS No. 36-1451). A weak characteristic peak at 2 = 39.3o, which is indexed to (111) plane of Al, is attributed to the thin Al overlayer. Interestingly, the characteristic peak of Al is greatly shrunk for an AZO NRs sample (500 oC, 4 h), and it finally disappears for another AZO NRs sample (600 oC, 4 h), presumably due to the completion of thermal drive-in diffusion of surface Al atoms. AZO NRs retain the sharp characteristic peaks of ZnO even after the heat treatment, indicating that the thermally driven Al-doping does not cause any crystallographic deterioration. To further confirm the thermally driven Al-doping, XPS and TEM-EELS analyses were performed. In Fig. 6, Al-coated ZnO sample represents ZnO NRs with an Al surface layer while Al-coated ZnO sample is their counterparts after heat treatment at 600 oC for 4 h. It is found from overall XPS spectra (Fig. 6a) that Al, Zn, and O all exist in both samples. Several differences can be witnessed from element-specific XPS spectra (Fig. 6b – e). Al 2p peaks appear at 72.8 and 73.2 eV for Al-coated ZnO and Al-doped ZnO, respectively (Fig. 6b). The Al peak position of Al-coated ZnO is very close to that (72.7 eV) of pure Al, whereas Al-doped ZnO shows a clear peak shift towards a higher binding energy. Reflecting that the Al 2p peak position of Al2O3 is 74.3 eV, it is inferred that Al atoms form partial chemical bonds with O atoms by replacing Zn atoms in ZnO NRs. On the other hand, three O 1s peaks are possible in ZnO, supposedly appearing at 529.9, 531, and 532.2 eV. Those peaks have origins in O atoms of ZnO crystal, O2- ions in oxygen-deficient regions, and O atoms bound on the surface of the ZnO, respectively.42 Comparing Fig. 6c and Fig. 6d, a lower energy peak (529.4 eV) of Al-doped ZnO is weakened while a higher energy peak (530.8 eV) strengthened, with respect to the corresponding peak intensities of Al-coated ZnO. This implies that O vacancies are generated by the Al-doping, presumably due to the valence mismatch between Zn and Al. TEM-EELS spectra


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support the propriety of the above reasoning. In Fig. 6f, O K-edge position (538.3 eV) of AZO NRs is shifted to a lower energy loss, compared to the position (539.2 eV) of a control ZnO film. This may be attributed to the slight weakening of O inner electron bonding strength by Al-O bond formation and O vacancy generation.

3.3. NO2-Sensing Performance of AZO NR Arrays on Si Substrate To examine the gas-sensing capability of AZO NRs, we first conducted gas-sensing tests on AZO NR arrays standing on the Si substrate with a ZnO seed layer. Temperature-dependent NO2-sening results are provided in Fig. 7. Here, AZO NRs were obtained by vacuum-annealing Al-coated ZnO NR arrays (Al layer sputtered for 30 s) at 600 oC for 4 h. At elevated temperatures, the AZO NR arrays show sharp, large, and NO2 concentration-dependent response signals. For instance, the response to 100 ppm of NO2 reaches 120.4% at 350 oC. Surprisingly, the relatively sharp response signals still sustain at room temperature, although recovery speeds are somewhat slowed down. At room temperature, responses to 50 and 100 ppm of NO2 are calculated to be 4.2 and 10.5%, respectively. This result offers the possibility to implement stretchable NO2 sensors utilizing AZO NRs. The effect of heat treatment conditions on the gassensing performance was also studied and presented in Fig. S6 – S8.

3.4. NO2-Sensing Performance and Strain-Dependence of Stretchable Gas Sensors Stretchable NO2 sensors were fabricated, following the process flow depicted in Fig. 1. AZO NRs/AgNWs bilayer was employed as a gas-detecting layer, instead of AZO NRs single layer. By the introduction of AgNWs overlayer, the resistance of a stretchable sensor could be significantly reduced and its NO2 sensitivity was improved a lot. Fig. 8 shows response curves



of stretchable sensors with a fixed amount of AZO NRs but varying amounts of AgNWs. The bilayers were formed by drop-casting AZO NRs colloid and AgNWs colloid sequentially, and the colloid concentrations were adjusted to control the relative densities of AZO NRs and AgNWs in the stretchable sensor. For all the sensors in Fig. 8, the concentration of AZO NRs colloid was fixed at 0.35 mg/ml, while AgNWs concentration changed from 0.5 to 1 mg/ml. As can be seen from Fig. 8a – c, the unstrained resistance of the sensor decreases as the AgNWs colloid concentration increases (the density of AgNWs increases), whereas the responses are lower at higher concentrations. This manifests that the AgNWs overlayer effectively reduces the sensor resistance, but may deteriorate the NO2 sensitivity of the sensor in case of overdose, which is attributed to the NO2-blocking effect of dense AgNWs film. For the best case (Fig. 8a), the bilayer sensor detects 10 ppm of NO2 with a high response of 87.9% at a zero strain. However, this sensor did not normally work at strained states due to the extremely high resistance, confirming the importance of moderate sensor resistance. Instead, lower-resistance sensors turned out to show NO2 responses even under high strains. Fig. 8d and e show response curves of a sensor with higher density of AgNWs (in Fig. 8c) at tensile strains of 15 and 30%, respectively. The sensor normally responds to NO2 gas at both strains, even though the sensor resistance is very high and response signals contain much noise. The responses to 10 ppm NO2 are calculated to be 29.4 and 32.3% for the respective strains of 15 and 30%, which are larger than an unstrained value (9.6%). These results demonstrate that the sensor of this work could operate in the much wider strain range than a stretchable sensor previously reported,38 although the response became smaller. Another sensor with an intermediate density of AgNWs also exhibits NO2 sensitivity under strains, but its response and noise level become worse (Fig. 8f). For the comparative study, a stretchable sensor was fabricated with a ZnO NRs/AgNWs bilayer,


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the colloid concentrations of which were same as the former AZO NRs/AgNWs bilayer. As presented in Fig. S9, its resistance is substantially higher and response signals are more irregular at both unstrained and 15%-strained states. Moreover, it did not work under a 30% strain. These are indicative that AZO NRs play a key role in the enhanced NO2 sensing. Gas-sensing performance of the stretchable sensor with an AZO NRs/AgNWs bilayer was also examined on carbon monoxide (CO) and ammonia (NH3) gases. The sensor was the same as in Fig. 8c. Its response curves at zero strain are shown in Fig. S10. The response signals look very noisy for both gases, and the sensor resistance barely changes at the presence of the gases. Furthermore, there were no gas-sensing behaviors found at strained states. In view of the aforementioned NO2-sensing performance of the sensor, these indicate that the stretchable sensor has the fairly good gas-selectivity. To further improve the performance of the stretchable sensor, we slightly modified the bilayer structure. Instead of pure AZO NRs, a mixture of AZO NRs and AgNWs was used as an underlayer. Fig. 9 shows the response signals of the modified sensor at unstrained and 30% strained states. Here, the mixture layer was formed by drop-casting a mixture colloid of AZO NRs and AgNWs with a weight ratio of 3:2, while AgNWs overlayer formed using the same colloid concentration as the former (1 mg/ml). It is found that both response and noise level are improved. The responses to 50 ppm of NO2 are 29.6 and 93.6%, respectively for zero strain and a 30% strain. Fig. 10 displays SEM images of bilayer of a stretchable sensor at unstrained and 15%strained states. It is observed from Fig. 10a that low density of AgNWs are randomly distributed over a very dense AZO NRs film. The dense AZO NRs film is locally torn under an applied strain, as seen in Fig. 10b. The width of thus-generated cracks is estimated to be 200 – 500 nm



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from Fig. 10c. At the same time, AgNWs are spatially rearranged to be more parallel to the straining direction. For this event, the junctions between neighboring nanowires may play as pivots. As a consequence of this rearrangement, spatial junction density and the quality of contact decrease. For these reasons, the sensor resistance rapidly increases with an increment in external strain.

3.5. Gas-Sensing Mechanism of the AZO NRs/AgNWs Bilayer-Based Stretchable Sensors To explain the NO2-sensing performance of a stretchable sensor under a strain, a potential mechanism is proposed in Fig. 11. In ambient condition, oxygen molecules (O2) are easily adsorbed on the surface of AZO NRs, and the adsorbed O2 extracts an electron from the AZO surface, consequently becoming an oxygen ion (O2–).3,14,15,19,22,27,28,43,44 In the meantime, AgNWs play as a catalyst to dissociate O2 into oxygen atoms (O) due to the superb O2-dissociation capability of Ag.36,45 The dissociated O atoms move to AZO NRs, where they again merge into O2, then the O2 may be ionized to be O2– by taking an electron away from the surface of AZO NRs.46 The electron transfer from AZO NRs to O2 induces a depletion layer around the AZO surface, which functions as a barrier to electron transport (Fig. 11a). Once the bilayer is exposed to NO2 gas, the O2– ions strongly interact with NO2 molecules, obeying the Eq. (3) below.3,44 Consequently, electrons are easily donated to NO2, which leads to the expansion of depletion layer and the decrease of electron concentration in AZO NRs (Fig. 11b). This is why the sensor resistance increases at the presence of NO2. The above processes can be expressed as follows: O2 + e – → O2–

(AZO NRs)

(1)

O2  2O  O2

(AgNWs)

(2)

NO2 + O2– + 2e – → NO2– + 2O–

(AZO NRs)


(3)

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When a tensile strain is applied to the stretchable sensor, AgNWs are locally rearranged to be better aligned to the straining direction, resulting in the decrease in both the junction density and junction quality (Fig. 11c). At the same time, nanocracks are generated in AZO NRs film. Owing to these phenomena, the sensor resistance quickly increases. Upon removing the external strain, the nanocracks are recovered and the AgNWs are back to their original arrangement due to the excellent elasticity of PDMS substrate. The larger responses under a high strain compared to unstrained responses (see Fig. 8 and Fig. 9) may be attributed to nanocracks, through which more NO2 molecules permeate deeper into the AZO NRs film.

4. CONCLUSIONS AZO NRs were prepared by the vacuum drive-in diffusion of Al into ZnO NRs. For this, ZnO NR arrays were first vertically grown on a Si substrate with a pre-sputtered ZnO seed layer, using a CBD method. Al thin film with a thickness of 2 – 4 nm was deposited on the ZnO NR arrays, then the Al-coated ZnO NRs were heat-treated under high vacuum. After the heat treatment at 600 oC for 4 h, the Al surface layer disappeared while Al atoms were distributed throughout the entire ZnO NRs. Neither crystal damage nor phase separation resulted from this thermal drive-in diffusion. Stretchable gas sensors were fabricated on a PDMS substrate, employing bilayers of AZO NRs and AgNWs. Here, AgNWs were introduced to reduce the sensor resistance and facilitate the gas-reactivity of AZO NRs. The stretchable sensors were able to detect 10 ppm of NO2 gas at room temperature, even under large strains up to 30%. It was revealed that the response under a high strain was rather larger than the response at zero strain. A plausible mechanism was proposed to explain the observed NO2-sensing performance at both room temperature and strained state.



Supporting Information Cumulative size distributions, stepwise morphological evolution, HR-TEM image and EDX element maps, annealing time-dependent EDX line profiles, gas-sensing performance of AZO NRs on Si, gas-sensing performance of ZnO NRs/AgNWs bilayers on PDMS, gas-sensing performance on other gases

ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03932515).


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References (1) Fan, F.; Feng, Y.; Bai, S.; Feng, J.; Chen, A.; Li, D. Synthesis and Gas Sensing Properties to NO2 of ZnO Nanoparticles. Sens. Actuator B-Chem. 2013, 185, 377-382. (2) Han, N.; Liu, H.; Wu, X.; Li ,D.; Chai, L.; Chen, Y. Pure and Sn-, Ga-and Mn-Doped ZnO Gas Sensors Working at Different Temperatures for Formaldehyde, Humidity, NH3, Toluene and CO. Appl. Phys. A 2011, 104, 627-633. (3) Rai, P.; Yu, Y. T. Citrate-Assisted Hydrothermal Synthesis of Single Crystalline ZnO Nanoparticles for Gas Sensor Application. Sens. Actuator B-Chem. 2012, 173, 58-65. (4) Ahn, M. W.; Park, K. S.; Heo, J. H.; Kim D. W.; Choi, K. J.; Park, J. G. On-Chip Fabrication of ZnO-Nanowire Gas Sensor with High Gas Sensitivity. Sens. Actuator BChem. 2009, 138, 168-173. (5) Ramgir, N. S.; Sharma, P. K.; Datta, N.; Kaur, M.; Debnath, A. K. Room Temperature H2S Sensor Based on Au Modified ZnO Nanowires. Sens. Actuator B-Chem. 2013, 186, 718-726. (6) Wan, Q.; Li, Q. H.; Chen Y.J.; Wang, T. H.; He, X. L.; LI J. P.; Lin, C. L. Fabrication and Ethanol Sensing Characteristics of ZnO Nanowire Gas Sensors. Appl. Phys. Lett. 2004, 84, 3654-3656. (7) Ahn, M. W.; Park, K. S.; Heo, J. H.; Park, J. G.; Kim, D. W.; Choi, K. J.; Lee, J. H.; Hong, S. H. Gas Sensing Properties of Defect-Controlled ZnO-Nanowire Gas Sensor. Appl. Phys. Lett. 2008, 93, 263103.



(8) Kim, K.; Song, Y. W.; Chang, S.; Kim, I. H.; Kim, S.; Lee, S. Y. Fabrication and Characterization of Ga-Doped ZnO Nanowire Gas Sensor for the Detection of CO. Thin Solid Films 2009, 518, 1190-1193. (9) Hsueh, T. J.; Chang, S. J.; Hsu, C. L.; Lin, Y. R.; Chen, I. C. Highly Sensitive ZnO Nanowire Ethanol Sensor with Pd Adsorption. Appl. Phys. Lett. 2007, 91, 053111. (10) Drobek, M.; Kim, J. H.; Bechelany, M.; Vallicari, C.; Julbe, A.; Kim, S. S. MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity. ACS Appl. Mater. Interfaces 2016, 8, 8323-8328. (11) Drobek, M.; Kim, J. H.; Bechelany, M.; Vallicari, C.; Leroy, E.; Julbe, A.; Kim, S. S. Design and Fabrication of Highly Selective H2 Sensors Based on SIM-1 NanomembraneCoated ZnO Nanowires. Sens. Actuator B-Chem. 2018, 264, 410-418. (12) Weber, M.; Kim, J. H.; Lee, J. H.; Kim, J. Y.; Iatsunskyi, I.; Coy, E.; Drobek, M.; Julbe, A.; Bechelany, M.; Kim, S. S. High Performance Nanowires Hydrogen Sensors by Exploiting the Synergistic Effect of Pd Nanoparticles and MOF Membranes. ACS Appl. Mater. Interfaces. 2018, 10, 34765-34773. (13) Oh, E.; Choi, H. Y.; Jung, S. H.; Cho, S.; Kim, J. C.; Lee, K. H.; Kang, S. W.; Kim, J.; Yun, J. Y.; Jeong, S. H. High-Performance NO2 Gas Sensor Based on ZnO Nanorod Grown by Ultrasonic Irradiation. Sens. Actuator B-Chem. 2009, 141, 239-243. (14) Şahin, Y.; Öztürk, S.; Kılınç, N.; Kösemen, A.; Erkovan, M.; Öztürk, Z. Z. Electrical Conduction and NO2 Gas Sensing Properties of ZnO Nanorods. Appl. Surf. Sci. 2014, 303, 90-96.


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(15) Liao, J.; Li, Z.; Wang, G.; Chen, C.; Lv, S.; Li, M. ZnO Nanorod/Porous Silicon Nanowire Hybrid Structures as Highly-Sensitive NO2 Gas Sensors at Room Temperature. Phys. Chem. Chem. Phys. 2016, 18, 4835-4841. (16) Xiang, Q.; Meng, G.; Zhang, Y.; Xu, J.; Xu, P.; Pan, Q.; Yu, W. Ag Nanoparticle Embedded-ZnO Nanorods Synthesized via a Photochemical Method and Its Gas-Sensing Properties. Sens. Actuator B-Chem. 2010, 143, 635-640. (17) Kashif, M.; Ali, M. E.; Ali, S. M. U.; Hashim, U. Sol–Gel Synthesis of Pd Doped ZnO Nanorods for Room Temperature Hydrogen Sensing Applications. Ceram. Int. 2013, 39, 6461-6466. (18) Zeng, Y.; Zhang, T.; Yuan, M.; Kang, M.; Lu, G.; Wang, R.; Fan, H.; He, Y.; Yang, H. Growth and Selective Acetone Detection Based on ZnO Nanorod Arrays. Sens. Actuator B-Chem. 2009, 143, 93-98. (19) Lupan, O.; Chai, G.; Chow, L. Novel Hydrogen Gas Sensor Based on Single ZnO Nanorod. Microelectron. Eng. 2008, 85, 2220-2225. (20) Wei, S.; Yu, Y.; Zhou, M. CO Gas Sensing of Pd-Doped ZnO Nanofibers Synthesized by Electrospinning Method. Mater. Lett. 2010, 64, 2284-2286. (21) Zhao, M.; Wang, X.; Cheng, J.; Zhang, L.; Jia, J.; Li, X. Synthesis and Ethanol Sensing Properties of Al-Doped ZnO Nanofibers. Curr. Appl. Phys. 2013, 13, 403-407. (22) Mao, Y.; Ma, S.; Li, X.; Wang, C.; Li, F.; Yang, X.; Zhu, J.; Ma, L. Effect of Mn Doping on the Microstructures and Sensing Properties of ZnO Nanofibers. Appl. Surf. Sci. 2014, 298, 109-115.



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(23) Wang, W.; Huang, H.; Li, Z.; Zhang, H.; Wang, Y.; Zheng, W.; Wang, C. Zinc Oxide Nanofiber Gas Sensors via Electrospinning. J. Am. Ceram. Soc. 2008, 91, 3817-3819. (24) Navale, S. C.; Ravi, V.; Mulla, I. S.; Gosavi, S. W.; Kulkarni, S. K. Low Temperature Synthesis and NOx Sensing Properties of Nanostructured Al-Doped ZnO. Sens. Actuator BChem. 2007, 126, 382-386. (25) Navale, S. C.; Ravi, V.; Srinivas, D.; Mulla, I. S.; Gosavi, S. W.; Kulkarni, S. K. EPR and DRS Evidence for NO2 Sensing in Al-Doped ZnO. Sens. Actuator B-Chem. 2008, 130, 668-673. (26) Sahay, P. P.; Nath, R. K. Al-Doped Zinc Oxide Thin Films for Liquid Petroleum Gas (LPG) Sensors. Sens. Actuator B-Chem. 2008, 133, 222-227. (27) Sahay, P. P.; Nath, R. K. Al-Doped ZnO Thin Films as Methanol Sensors. Sens. Actuator B-Chem. 2008, 134, 654-659. (28) Hou, Y.; Soleimanpour, A. M.; Jayatissa, A. H. Low Resistive Aluminum Doped Nanocrystalline Zinc Oxide for Reducing Gas Sensor Application via Sol–Gel Process. Sens. Actuator B-Chem. 2013, 177, 761-769. (29) Ong, W. L.; Huang, H.; Xiao, J.; Zeng, K.; Ho, G. W. Tuning of Multifunctional CuDoped

ZnO

Films

and

Nanowires

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Enhanced

Piezo/Ferroelectric-like

and

Gas/Photoresponse Properties. Nanoscale 2014, 6, 1680-1690. (30) Kevin, M.; Tho, W. H.; Ho, G. W. Transferability of Solution Processed Epitaxial Ga:ZnO Films; Tailored for Gas Sensor and Transparent Conducting Oxide Applications. J. Mater. Chem. 2012, 22, 16442-16447.


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(31) Chen, Z. Q.; Maekawa, M.; Yamamoto, S.; Kawasuso, A.; Yuan, X. L.; Sekiguchi, T.; Suzuki, R.; Ohdaira, T. Evolution of Voids in Al+-Implanted ZnO Probed by a Slow Positron Beam. Phys. Rev. B 2004, 69, 035210. (32) Chen, Z. Q.; Maekawa, M.; Kawasuso, A.; Sakai, S.; Naramoto, H. Annealing Process of Ion-Implantation-Induced Defects in ZnO: Chemical Effect of the Ion Species. J. Appl. Phys. 2006, 99, 093507. (33) Bhagavannarayana, G.; Kushwaha, S. K.; Parthiban, S.; Meenakshisundaram, S. The Influence of Mn-Doping on the Nonlinear Optical Properties and Crystalline Perfection of Tris (Thiourea) Zinc (II) Sulphate Crystals: Concentration Effects. J. Cryst. Growth 2009, 311, 960-965. (34) Kim, Y.; Kang, S. Aluminum-Doped ZnO Nanorod Array by Thermal Diffusion Process. Mater. Lett. 2009, 63, 1065-1067. (35) Tong, C.; Yun, J.; Chen, Y. J.; Ji, D.; Gan, Q.; Anderson, W. A. Thermally Diffused Al:ZnO Thin Films for Broadband Transparent Conductor. ACS Appl. Mater. Interfaces 2016, 8, 3985-3991. (36) Tarwal, N. L.; Rajgure, A. V.; Patil, J. Y.; Khandekar, M. S.; Suryavanshi, S. S.; Patil, P. S.; Gang, M. G.; Kim, J. H.; Jang, J. H. A Selective Ethanol Gas Sensor Based on SprayDerived Ag–ZnO Thin Films. J. Mater. Sci. 2013, 48, 7274-7282. (37) Luan, Y.; Zhang, S.; Nguyen, T. H.; Yang, W.; Noh, J. S. Polyurethane Sponges Decorated with Reduced Graphene Oxide and Silver Nanowires for Highly Stretchable Gas Sensors. Sens. Actuator B-Chem. 2018, 265, 609-616.



(38) Gutruf, P.; Zeller, E.; Walia, S.; Nili, H.; Sriram, S.; Bhaskaran, M. Stretchable and Tunable Microtectonic ZnO‐Based Sensors and Photonics. Small 2015, 11, 4532-4539. (39) Son, N. T.; Noh, J. S.; Park, S. Role of ZnO Thin Film in the Vertically Aligned Growth of ZnO Nanorods by Chemical Bath Deposition. Appl. Surf. Sci. 2016, 379, 440-445. (40) Ruske, F.; Roczen, M.; Lee, K.; Wimmer, M.; Gall, S.; Hüpkes, J.; Hrunski, D.; Rech, B. Improved Electrical Transport in Al-Doped Zinc Oxide by Thermal Treatment. J. Appl. Phys. 2010, 107, 013708. (41) Lee, J. H.; Lee, P.; Lee, D.; Lee, S. S.; Ko, S. H. Large-Scale Synthesis and Characterization of Very Long Silver Nanowires via Successive Multistep Growth. Cryst. Growth Des. 2012, 12, 5598-5605. (42) Dwivedi, V. K.; Srivastava, P.; Prakash, G. V. Photoconductivity and Surface Chemical Analysis of ZnO Thin Films Deposited by Solution-Processing Techniques for Nano and Microstructure Fabrication. J. Semicond. 2013, 34, 033001. (43) Sadek, A. Z.; Choopun, S.; Wlodarski, W.; Ippolito, S. J.; Kalantar-zadeh, K. Characterization of ZnO Nanobelt-Based Gas Sensor for H2, NO2, and Hydrocarbon Sensing. IEEE Sens. J. 2007, 7, 919-924. (44) Yan, D.; Hu, M.; Li, S.; Liang, J.; Wu, Y.; Ma, S. Electrochemical Deposition of ZnO Nanostructures onto Porous Silicon and Their Enhanced Gas Sensing to NO2 at Room Temperature. Electrochim. Acta 2014, 115, 297-305.


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Figure captions Fig. 1. Schematic procedures for preparing AZO NRs and a stretchable gas sensor. (a) ZnO seed layer growth on Si substrate. (b) Growth of ZnO NR arrays on the ZnO seed layer by CBD. (c) Al thin film deposition on the ZnO NR arrays. (d) Removal of the Al-coated ZnO NRs from the substrate by sonication. (e) Drop-casting and drying of the Al-coated ZnO NRs on a quartz slab followed by vacuum-annealing. (f) Taking the AZO NRs away from the quartz by sonication. (g) Drop-casting of the AZO NRs colloid on PDMS and drying in oven. Then, the drop-casting of AgNWs colloid on the AZO NRs film is performed.

Fig. 2. SEM images of (a, b) ZnO NRs, (c, d) Al-coated ZnO NRs, and (e, f) AZO NRs after vacuum-annealing. (e) and (f) were obtained from annealing conditions of 550 °C, 6h and 600 °C, 4h, respectively. The images in the left column are for NRs sticking to the Si substrate, while images in the right column for NRs cast on the quartz.

Fig. 3. TEM images of (a, b) Al-coated ZnO NRs and (c, d) AZO NRs. (a, b) Low-resolution images. (c, d) High-resolution images near the surface. AZO NRs were obtained by vacuum-annealing at 600 °C for 4h.

Fig. 4. (a) TEM image and (b-g) TEM-EDX element maps of AZO NRs. (b) represents overall map superposed with NRs image, and (c) through (e) show maps of Zn, O, and Al. Scale bar 200 nm. (f) TEM-EDX spectrum of the AZO NRs.


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Fig. 5. XRD patterns of Al-coated ZnO NRs and AZO nanorods vacuum-annealed at different conditions of 500 °C, 4h and 600 °C, 4h.

Fig. 6. (a) Overall XPS spectra and (b-e) element-specific XPS spectra of Al-coated ZnO and Aldoped ZnO samples. (f) Oxygen K-edge EELS spectra of an AZO NRs sample and a control ZnO film.

Fig. 7. NO2-sensing responses of AZO NR arrays on the Si substrate, which were obtained by vacuum-annealing at 600 oC for 4 h. Test temperature: (a) 350 °C, (b) 200 °C, (c) room temperature.

Fig. 8. NO2-sensing responses of AZO NRs/AgNWs bilayers on PDMS. The concentration of AgNWs was adjusted with fixing AZO NRs concentration at 0.35 mg/ml: (a) 0.5 mg/ml, (b, f) 0.75 mg/ml, (c-e) 1 mg/ml. (d) through (f) show responses under 15 and 30% strains.

Fig. 9. NO2-sensing responses of a mixture layer/AgNWs bilayer on PDMS. Here, the mixture layer was composed of AZO NRs and AgNWs film with a weight ratio of 3:2. (a) and (b) presents responses under zero strain and a 30% strain, respectively.

Fig. 10. SEM images of the AZO NRs/AgNWs bilayer of a stretchable sensor under (a) zero strain and (b) 15% strain. (c) Magnified image of the selected region.

Fig. 11. Schematic picture illustrating the operation mechanism of a stretchable NO2 sensor made of AZO NRs/AgNWs bilayers. Potential status of the sensor (a) in atmosphere, (b) under NO2 gas, and (c) under a strain.



Page 26 of 37

Figure 1

(b) (c) Al

(a) ZnO

ZnO seed layer Si substrate

(g) (d)

(f)

(e) Quartz


Page 27 of 37 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


Figure 2

(a)

(b)

200 nm

300 nm

(d)

(c)

300 nm

300 nm

(e)

(f)

300 nm


200 nm


Page 28 of 37

Figure 3

(a)

(b) ZnO 0.25 nm ZnO (101)

Al 5 nm

200 nm

(c)

(d)

AZO 200 nm

5 nm


Page 29 of 37

Figure 4

(a)

(c)

(b)

(d)

Zn

(e)

O

(f)

Element Zn O Al

Intensity (a.u.)

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


0

2

4

6

8

10

Beam Energy (keV)


Al

At% 45.8 49.2 5.0

12

14


Figure 5

Intensity (a.u.)

AZO NRs (600 oC, 4 h)

Al (111)

AZO NRs (500 oC, 4 h)

Al-coated ZnO NRs

37

38

39

40

41

42

43

44

45

30

40

50

60

2 (Degree)


ZnO (112) ZnO (201)

ZnO (103)

ZnO (110)

ZnO (102)

Al (111)

ZnO (002) ZnO (101)

ZnO (100)

2 (Degree)

Intensity (a.u.)

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

Page 30 of 37

70

Page 31 of 37

Figure 6

(a)

(b)

Al-doped ZnO Al-coated ZnO

Zn2p1/2

Intensity (a.u.)

Al2p

0

400

800

73.2eV 72.8eV

68

1200

70

530

532

Intensity (a.u.)

Intensity (a.u)

530.8eV

528

526

534


Zn 2p Zn 2p2/3 1020.4 eV

1030

Zn 2p1/2 1043.6 eV

1040

528

530

532

534

Binding Energy (eV)

1050

(f)

AZO ZnO

O K-edge

538.3eV

Intensity (a.u.)

(e)

1020

530.8eV

529.4eV

Binding Energy (eV)

1010

76

Al-doped ZnO

Al-coated ZnO

526

74

(d) O 1s

530.2eV

O 1s

72

Binding Energy (eV)

Binding Energy (eV)

(c)


Al 2p

Intensity (a.u.)

Zn2p3/2

O1s

Intensity (a.u.)

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


539.2eV

530

540

Binding Energy (eV)


550

560

570

580

Energy Loss (eV)

590

600


Figure 7

Resistance (k)

(a)

50

10 ppm

50 ppm

100 ppm

350 oC

40 30 20 10

N2

N2

0

2000

N2 4000

N2

6000

8000

10000

Time (s)

Resistance (k)

(b)

24

10 ppm

50 ppm

100 ppm

200 oC

22 20 18 16 14

N2

N2

0

2000

N2 4000

N2

6000

8000

10000 12000

Time (s)

(c)

46

100 ppm

50 ppm

RT

45

Resistance (k)

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

Page 32 of 37

44 43 42 41 40

N2

N2 0

2000

N2

4000

6000

8000 10000 12000

Time (s)


Page 33 of 37

Figure 8

(b)

(a) Resistance ()

220

10 ppm 50 ppm

0.5 mg/ml ( = 0)

200 180 160 140 120 100 80 60

N2 0

N2

N2 2000

80

10 ppm

100 ppm

4000

N2

6000

50 ppm

70 65 60

50 45

8000 10000 12000

0.75 mg/ml ( = 0)

55

0

N2

N2

N2 1000

2000

Time (s)

N2 4000

5000

6000

(d) 50 ppm

100 ppm 1 mg/ml ( = 0)

1100 1050 1000 950 900 850

N2 0

N2

N2 2000

4000

8000

50 ppm

1 mg/ml ( = 15%)

65 60 55 50 45 40

N2

10000

0

N2

N2 2000

Time (s) 240

(f) 10 ppm

50 ppm

100 ppm 1 mg/ml ( = 30%)

220 200 180 160 140 120 100

N2

N2

0

2000

N2 4000

4000

N2 6000

8000

Time (s)

N2 6000

650

50 ppm

100 ppm 0.75 mg/ml ( = 30%)

550 500 450 400 350

8000

10 ppm

600

Resistance (M)

260

100 ppm

70

N2

6000

10 ppm

75

Resistance (M)

10 ppm

1150

Resistance ()

3000

Time (s)

(c)

(e)

100 ppm

75

Resistance (M)

240

Resistance ()

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


N2

N2

N2

0

2000

4000

Time (s)

Time (s)


N2 6000


Figure 9

(a)

(b)

1300

50 ppm

100 ppm

50 ppm

300

100 ppm

Resistance ()

1200

Resistance ()

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

Page 34 of 37

1100 1000 900 800 700

N2

600

N2

500 0

N2

250 200 150

N2 100

1000 2000 3000 4000 5000 6000 7000

Time (s)

N2 0

2000

N2 4000

Time (s)


6000

8000

Page 35 of 37 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


Figure 10

=0

(a) AZO NRs

AgNWs

2 µm

 = 15%

(b)

2 µm

(c) AZO NRs

300 nm



Page 36 of 37

Figure 11

O2

(a)

O2

O2– O2–

O Depletion Layer

O O

AZO nanorod O2–

O2–

O2–

O2–

O2– NO2

(b)

O2– O2–

NO2 NO2

NO2– O– AZO nanorod NO2– O– NO2– O– O2– NO2– O– O2– NO2–

O– NO2– O–

(c)

PDMS

Nanocracks

Apply strain

AZO nanorods film


AgNWs

Page 37 of 37 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


TOC Graphic

Ambient O2

NO2 O2

No strain NO2

NO2

Large strain NO2

NO2 AgNWs AZO NRs

NO2

NO2

NO2

Nanocracks


Diffusion-Driven Al-Doping of ZnO Nanorods and Stretchable Gas

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