Hierarchical Metal Oxide Wrinkles as Responsive Chemical Sensors

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Hierarchical Metal Oxide Wrinkles as Responsive Chemical Sensors Woo-Bin Jung, Soo-Yeon Cho, Geun-Tae Yun, Junghoon Choi, Yesol Kim, Minki Kim, Hohyung Kang, and Hee-Tae Jung ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01098 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

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ACS Applied Nano Materials

Hierarchical Metal Oxide Wrinkles as Responsive Chemical Sensors Woo-Bin Jung,†,‡,⊥ Soo-Yeon Cho,†,‡,§,⊥ Geun-Tae Yun, †,‡ Junghoon Choi, †,‡ Yesol Kim, †,‡ Minki Kim, †,‡ Hohyung Kang†,‡

and Hee-Tae Jung*,†,‡

† Department of Chemical and Biomolecular Engineering (BK-21 Plus), Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 34141, Republic of Korea ‡ KAIST Institute for NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 34141, Republic of Korea § Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA E-mail: [email protected] ⊥

These authors contributed equally to this work

Keywords: metal oxide, wrinkle, hierarchical wrinkle, ridge, acetone, gas sensor

Abstract Easily observed wrinkle structures comprise various materials. Because evolutionary wrinkle structures exhibit a broad surface area, light scattering, and hydrophobicity, the structural advantages of wrinkles have been exploited in several research fields. Wrinkle structures have been applied to various materials, as well as used in metals, graphene, and polymers; however, it is extremely difficult to prepare metal oxide1 ACS Paragon Plus Environment

ACS Applied Nano Materials 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

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based wrinkle structures because of the brittle nature of metal oxide. In this study, a universal method for synthesizing hierarchical metal-oxide wrinkle (hereafter referred to as the H-wrinkle) structures with precise morphology control was developed. Metal oxide H-wrinkles were prepared using a polymer sacrificial layer and transferred for consecutive thermal oxidation on the target substrate. To investigate the morphological properties of the H-wrinkle, the CuO H-wrinkle was applied to gassensing devices; it showed a sensitivity 6.07 times greater than that of the pristine CuO film sensors. This significant improvement corresponding to the unique morphological properties of H-wrinkles, rendering considerable adsorption sites and high surface permeability and subsequently permitting the facile penetration of gas into the sensing layers. In this study, we demonstrated a first example of the use of the metal oxide wrinkle structure in electronic devices and thus confirmed its use in various applications.

INTRODUCTION

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A wrinkle structure is obtained as a result of physical instability between two layers. The wrinkle structure prepared in this manner, which can be easily observed, exists in various scales.1-5 This structure has been utilized in several studies because of its various advantages. For example, a wrinkle structure on a hydrophobic surface exhibits a superhydrophobic surface that permits the facile roll-off of water, and a solar cell or a surface-enhanced Raman spectroscopy substrate with a wrinkled structure maximizes light scattering and hence increases efficiency.6-11 In addition, wrinkle structures have been widely used in catalysis and bio-fields as surfaces of large wrinkled structures because these structures can render an extremely large surface area as well as affect the cell orientation and differentiation.12-16 Because of these advantages, wrinkle structures using various materials such as graphene, metal, and polymer have been intensively examined.17-20 Wrinkle structures using these materials have been widely used in solar cells, electronic devices, transparent electrodes, bio-fields, and other applications.9, 21, 22 In particular, metal- or graphene-based wrinkle structures have been utilized in research and pressure sensors for preparing flexible conductive substrates.23, 24 In particular, unlike graphene



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and metals, metal oxides exhibit a suitable band gap and excellent catalytic properties. They can be utilized in various applications such as solar cells, catalysts, sensors, and transistors. However, it is difficult to maximize their performance because of their extremely simple morphology, i.e., typically as thin films. Hence, a wrinkle structure applied to metal oxide materials is good for improving performance. Thus far, only one H-wrinkle structure based on metal oxide has been reported; however, this wrinkle structure does not exhibit the precise control of the hierarchical morphology.25

In this study, we developed a new method for producing a hierarchical metal oxide wrinkle structure with precise control of the hierarchical morphology. The wrinkle structure on a polystyrene (PS) substrate was transferred onto a wafer and oxidized by thermal annealing. Versatile metal oxide semiconductors (viz., CuO, NiO, and TiO2) were successfully fabricated by a universal process. By using a sacrificial layer and by tuning its concentration on the primary wrinkle, the wavelength of H-wrinkle could be independently controlled. To investigate the unique morphological properties of the Hwrinkle, gas-sensing devices were fabricated using the CuO H-wrinkle. The sensitivity was up to 6.07 times greater than that of pristine film sensors. This high sensitivity 4 ACS Paragon Plus Environment

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corresponded to the unique H-wrinkle structure with a surface area greater than that of the general wrinkle. A suspended structure was realized to further increase the pathway of target gas molecules. This study presented a new formation method for a wrinkle structure based on metal oxide, which can be used in various applications in the future, such as electronic devices.

EXPERIMENTAL SECTION Metal film preparation. To prevent the formation of the wrinkle structure during direct deposition of metal on the polystyrene film (2 cm × 2 cm, thickness = 100 µm), metal film on the PS film (7 cm × 7cm) is prepared by transfer method. Poly(methyl methacrylate) (PMMA; Aldrich, MW = 10,000 g/mol) is dissolved in chlorobenzene to 10 wt% concentration. The PMMA solution is spin-coated onto the deposited metal film on the SiO2/Si wafer. The PMMA-coated metal film is then floated on 2 M KOH solution (90 °C), and PMMA–metal film and SiO2/Si wafer are separated. The floated PMMA–metal film is transferred on the PS film and annealed at 80 °C. Finally, the metal film on the PS film is obtained by removing the PMMA layer.



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Preparation of polypyrrolidone skin layer. To fabricate the hierarchical wrinkle, a polypyrrolidone (PVP; Aldrich, MW = 40000 g/mol) skin layer is prepared on the existing G1 wrinkle. A solution of PVP in ethanol (0.2 ml) is added dropwise on the G1 wrinkle and spin-coated.

Metal oxidation. The metal wrinkle transferred on the SiO2 wafer was oxidized in a furnace. Cu, Ti, and Ni were oxidized under Ar, at 450, 600, and 650 °C, respectively, for 2 h.

Device fabrication and measurements. The Au electrode (70 nm in thickness) was deposited on the sample with a predeposited Ti adhesion layer (5 nm in thickness) by e-beam evaporation to measure the resistance using a customized stainless-steel mask. The fabricated sensing devices were mounted in a sensing chamber designed to measure resistance signals using a data acquisition module (Agilent 34970A). The heating module, which utilized external power sources, was set up exactly below the sensing chamber plate (operating temperature of 200–350 °C). Air and volatile organic compound (VOC) gases (viz., toluene, ammonia, ethanol, propanal, and acetone) were used as the reference gas and target analytes, respectively. Gas flow into the 6 ACS Paragon Plus Environment

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sensing chamber was controlled using a gas delivery system designed in-house to measure the response of the sensor to the analytes. The total flow rate was maintained at 400 sccm. Air and VOC gases were injected for 15 min and 5 min, respectively. A serial gas dilution system with a mass flow controller (MFC, Brooks 5850E) was used to regulate the acetone gas concentration from 0.125 ppm to 1000 ppm.26 The size of the sealed gas-sensing chamber was 10 cm (width)  5 cm (length)  8 mm (height).

Characterization. Scanning electron microscopy (SEM, S-4800, Hitachi) images were recorded to observe the morphology of the wrinkled structure. Cross-sectional images of the wrinkle structure were observed on a focused ion-beam SEM instrument (Helios Nanolab 450 F1, FEI Company). For the quantitative estimation of the wavelength, SEM images were processed by the ImageJ program. First, the line profile across the several waves of first-generation (G1) and second-generation (G2) wrinkles in the SEM image was created by using the ImageJ program, and the wavelength (valley-tovalley) was measured in the graph. Finally, several valley-to-valley distances were measured, and the average wavelength of G1 and G2 was calculated.



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RESULTS AND DISCUSSION Figure 1a shows a schematic of the fabrication of the metal oxide wrinkles by oxidation after the wrinkle was transferred onto the SiO2/Si substrate. After the metal film was prepared on the PS film, strain relief was carried out with subsequent heating above the glass transition temperature (Tg) of the PS film (~120 C) in the oven. The areal strain (ε, ε = (A0  Af)/A0), where A0 and Af respectively denote the areas before and after strain relief, was controlled by simply changing the heating time in the oven. After shrinking of the PS film, the metal film changed to a wrinkle morphology because of instability between the metal and PS films. The pre-strained PS film shrunk above the Tg, and the metal film with higher modulus showed an undulated form on the shrunk PS film. For the oxidation of the metal wrinkle by thermal annealing, the metal wrinkle was transferred on a thermally stable substrate such as a SiO2/Si wafer. Afterward, the metal wrinkle was allowed to float on dichloromethane (DCM) solvent and was transferred onto the SiO2/Si wafer. Finally, the metal oxide wrinkle was obtained by thermal oxidation at an appropriate temperature for each metal. This final wrinkle structure was a delaminated buckle morphology because there was no


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polymer layer under the wrinkle structure and a void space formed between the wrinkle and the SiO2/Si wafer. Figure 1b shows the scanning electron microscopy (SEM) images of the wrinkles of the representative metal oxide, CuO, with different areal strain values. Notably, the wrinkle structure even after thermal annealing was perfectly maintained. With the increase in strain (ε) from 0.25 to 0.75, the amplitude of the CuO wrinkle increased, and the wavelength remained at ~1.6 μm because of the linear model of the wrinkle structure27. By this method, various metal oxide wrinkles could be fabricated. Figure 1c and 1d show wrinkles of three metal oxides (viz. CuO, NiO, and TiO2, respectively) and their X-ray diffraction (XRD) patterns. Although all of the metal films exhibited the same thickness (~20 nm), the wavelength and shape of the metal oxide wrinkle were different because of the different Young’s moduli. The CuO, TiO2, and NiO wrinkles have moduli of 2.68 ± 0.19 µm, 3.10 ± 0.34 µm, and 7.00 ± 1.64 µm, respectively. The result of the wavelength has similar trend to the modulus: 116 GPa for Ti =, 110–128 GPa for Cu, and 200 GPa for Ni. The metal oxide film and wrinkle exhibited the same XRD pattern, indicative of the complete oxidation of the metal oxide film and wrinkle by thermal annealing.



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For a large surface area of the metal oxide wrinkle, an H-wrinkle was fabricated using a sacrificial skin layer such as polyvinylpyrrolidone (PVP). The H-wrinkle structure comprised multiple wrinkles at the same time. Primary wrinkles were formed, and the PVP sacrificial layer was coated (Figure 2a). The further shrinking of the PVPcoated wrinkles led to wrinkles of large wavelengths, and the sacrificial layer was wiped off with ethanol after wrinkle formation. Hence, an H-wrinkle structure in the form of small wrinkles in larger wrinkles could be formed. Similar to the above method, the H-wrinkle structure prepared by this method was allowed to float on DCM and transferred onto the SiO2 substrate. Thereafter, a metal oxide H-wrinkle structure was obtained by oxidation. In the case of hierarchical wrinkles, the shape changed during transfer. The hierarchical wrinkle changed from continuous to discontinuous, as shown in the SEM images before and after transfer (Figure 2b). Continuous G2 wrinkles were sinusoidal wrinkles, but discontinuous wrinkles were similar to the ridge forms. In the ridge system, flat parts between the ridges were observed.

Figures 2c and 2d respectively show the change in the wrinkle wavelength before and after transfer onto the SiO2 substrate. Typically, when the wrinkle structure floated 10 ACS Paragon Plus Environment

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on a liquid, the wavelength changed because it could spread (Figure S1). As can be observed in the SEM images (Figure 2c), the H-wrinkle structure prepared with a low concentration of the sacrificial layer did not exhibit visible traces after migration. The H-wrinkle structure prepared with a sacrificial layer concentration of greater than or equal to 8 wt% revealed that the second wrinkle structure is maintained even after the transfer. However, not only the change in the primary wrinkle structure but also the shape of the secondary wrinkle changed after the transfer. Prior to the transfer, the secondary wrinkles that formed were extremely similar to the primary wrinkles; but after the transfer, secondary wrinkles were farther apart, with a ridge structure instead of a wrinkle structure. The corrugated structure exhibited a sinusoid, but the ridge structure exhibited a feature with a delaminated part and a flat part in the middle. According to the results obtained by measurements of the size change before and after transfer, the wavelength of the first wrinkle increased from 1.2 μm to 1.6 μm. In the case of the secondary wrinkles, the size and shape changed simultaneously; hence, the comparison of the ridge width revealed that the size of the secondary



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wrinkles decreases under all conditions. The trend of the ridge width is proportional to the concentration of PVP solution, similar to the trend for the results of G2 wavelength.

To demonstrate the morphological novelty and potential applications of metal oxide wrinkle systems, we fabricated chemiresistor-type gas-sensing devices with the CuO film, CuO wrinkle, and CuO H-wrinkle. Metal oxide semiconductor is the most widely used channel material for gas sensors because of its high sensitivity, rapid response/recovery times, highly flexible production, and large number of gases detected.28,

29

Typically, simple film structures of metal oxide have been used for

commercial sensing channels materials; however, these structures can render low sensitivity because of their morphological limitations, including low surface-to-volume ratio and low penetration pathway.30,

31

On the contrary, the H-wrinkle structures

exhibited considerably higher sensitivity with significantly more adsorption sites and high surface permeability, permitting the facile penetration of gas into sensing layers. To demonstrate this point, we transferred as a test case, the synthesized CuO film, CuO wrinkle, and CuO H-wrinkle on Si/SiO2 substrates and integrated them with a 70 nm thick Au microelectrode with a Ti layer via electron-beam evaporation using a 12 ACS Paragon Plus Environment

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predesigned shadow metal mask. Figure 3a–c shows the top-view SEM images of the sensing devices integrated with the CuO film, CuO wrinkle, and CuO H-wrinkle, respectively. As can be clearly observed, the 100 μm electrodes were uniformly deposited on the CuO channel even with the complex H-wrinkle structures. In addition, magnified SEM images (Figure S2) revealed that the high surface area of the wrinkle surfaces with hierarchical ridges is well maintained, indicating that large amounts of gas molecules can be adsorbed on sensing channels.32 To investigate the effects of morphology on the electrical conductivities of sensing devices, the baseline channel resistances of the CuO film, CuO wrinkle, and CuO H-wrinkle sensors were measured at an operating temperature of 300 °C (Figure 3d). The channel resistance of the Hwrinkle was two to three times greater than those of the film and wrinkle sensors, corresponding to the electrical hindrance along dense hierarchical ridges and contact resistances from the wrinkled channel and flat electrodes.33 Such a device was loaded on a gas-sensing measurement setup built in-house (see Figure S3 in the Supporting Information for details of the entire gas-delivery system). A constant drain bias was applied to the two-probe sensor, and the change in the electrical resistance of the



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sensor upon exposure to CH3COCH3 (acetone) was monitored and recorded as the sensing signal at the operating temperature of 300 °C (ΔR/Ra; Ra and ΔR respectively denote the baseline resistance of the sensor exposed to dry air and the change in resistance after exposure to analytes).34 Figure 3e shows the real-time sensing of acetone (0.125 to 5 parts per million (ppm)) of the CuO film, CuO wrinkle, and CuO Hwrinkle sensors. Since CuO was a p-type semiconducting channel, all sensors exhibited positive responses to acetone with resistance increase due to the sudden elimination of the charge-transfer region (hole accumulation layer) near the CuO surfaces.31,

35

Notably, the CuO wrinkle and CuO H-wrinkle sensors exhibited

significantly higher response amplitudes for all acetone concentrations. We observed a slight baseline drift for the CuO H-wrinkle sensor, which is attributed to the nonrecovered acetone on the hierarchical wrinkle surfaces. Compared with the CuO film and CuO wrinkle sensors, it could be harder to recover all the attached analyte on the hierarchical surfaces at the same time since a substantial amount of analytes adsorbed on the hierarchical surfaces. Particularly, the calculated response amplitudes from real-time data revealed that the CuO wrinkle and CuO H-wrinkle


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sensors exhibited the maximum sensitivity, i.e., respectively 2.02- and 6.07-times higher sensitivity, to 0.125 ppm acetone compared with that of the CuO film sensor (Figure 3f). In addition to response amplitudes, the CuO wrinkle and CuO H-wrinkle sensors exhibited the significantly faster response time (τ 90%, time taken to reach 90% of the minimum resistance), i.e., 90 s less than that of the CuO film sensor (Figure S4). Figure 4 clearly shows the improved sensing mechanism of the CuO wrinkle and CuO H-wrinkle sensors. Cross-sectional SEM images revealed the larger surface area of the CuO wrinkle with the shrinkage of the CuO thin films (top left, Figure 4a). However, overlapping dead spaces formed along with the enhanced surface area because of the wrinkling process (top right, Figure 4a). For the CuO H-wrinkle, high ridge structures clearly formed with completely open spaces without any overlapping dead space, as can be observed for the CuO wrinkle systems. With completely open ridge structures, not only the top surface but also the bottom surface of the CuO Hwrinkle was exposed to the ambient.36, 37



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The precise morphological characterization of each structure in the schematic clearly reveals the adsorption behavior of gas molecules on the CuO film, CuO wrinkle, and CuO H-wrinkle (Figure 4b–d, respectively). On a typical film structure of the CuO channel, limited amounts of gas molecules adsorbed, leading to low response amplitudes.30 As a result of the wrinkling of the CuO film, large amounts of analytes were adsorbed on the channel with considerable surface adsorption sites. However, only part of the enhanced surface area of wrinkled CuO was exploited for surface detection because of the overlapping dead space formed by wrinkling (Figure 4a). Accordingly, the enhancement factor for the sensitivity of the CuO wrinkle was only ~2.02, and not ~4, corresponding to the actual enhancement factor for the surface area of the wrinkle systems by 75% strain. Finally, two factors contributed toward significant sensitivity enhancement for the CuO H-wrinkle. First, only completely open wrinkle structures without any overlapping dead spaces were present, rendering the maximum sites for the adsorption of acetone analytes. Second, the top and bottom surfaces of the CuO films were completely exposed to ambient. Accordingly, the CuO H-wrinkle not only provided considerably higher adsorption sites but also induced high


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surface permeability, thereby permitting facile penetration into sensing layers. Thus, the significantly large enhancement of the response amplitudes (6.06-fold) and response time can be achieved by the simple hierarchical wrinkling of the CuO film. Overall, the importance of the H-wrinkle systems and effects on sensing performances were clearly demonstrated.

CONCLUSIONS In conclusion, a new method for creating a hierarchical wrinkle structure based on metal oxide was developed. Because the developed method utilizes a polymer sacrificial layer, the size of hierarchical wrinkles could be independently controlled. Through this method, a hierarchical metal oxide structure was used for the gas sensor, and a higher sensitivity to acetone was achieved because of its structural feature. This developed method can be applied to catalysts, biosensors, solar cells, nanoelectronic devices, and others because it can be extended to various metal oxide materials and can be used to facilitate fabrication on rigid substrates.

ASSOCIATED CONTENT



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The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Detailed SEM images of hierarchical wrinkle. PVP thickness variation with various concentration of PVP solution. Response time analysis of the CuO film, CuO wrinkle, and CuO H-wrinkle sensors. AFM images of transferred wrinkle structure with different strain and hierarchy.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Tel: 82-42-350-3971

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT W.-B.J. and S.-Y.C. contributed equally to this work. This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and future Planning, Korea (MSIP, NRF-2018R1A2B3008658) and the 18 ACS Paragon Plus Environment

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KAIST GCORE(Global Center for Open Research with Enterprise) grant funded by the Ministry of Science and ICT (N11190229).

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Figure 1. Fabrication of the metal oxide wrinkle. (a) Schematic of the fabrication of the metal oxide wrinkle. (b) CuO wrinkle formed by the variation of the areal strain from 0.25 to 0.75. (c) CuO, NiO, and TiO2 wrinkles. (d) XRD patterns of the metal oxide thin film and wrinkle.


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Figure 2. Hierarchical metal oxide wrinkle. (a) Schematic of the fabrication of a hierarchical metal oxide wrinkle. (b) Hierarchical wrinkle before and after transfer. After transfer, the morphology of the hierarchical wrinkle changed to the ridge form, which exhibited flat regions between ridges. (c) SEM images of the Cu wrinkles before transfer and CuO wrinkles after transfer. (d) Change in the wavelength and ridge width after the transfer of the wrinkle and h-wrinkle.



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Figure 3. Sensing characterization of the CuO film, CuO wrinkle, and CuO H-wrinkle. Top-view SEM images of the (a) CuO film, (b) CuO wrinkle, and (c) CuO H-wrinkle sensors with electrode integration. (d) Channel resistances of the CuO film, CuO wrinkle, and CuO H-wrinkle sensors. (e) Real-time CH3COCH3 (acetone) sensing response of the CuO film, CuO wrinkle, and CuO H-wrinkle sensors. (f) Acetone response amplitudes (ΔR/Ra) of the CuO film, CuO wrinkle, and CuO H-wrinkle sensors.


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Figure 4. Sensing mechanism of the CuO wrinkle and CuO H-wrinkle systems. (a) Cross-sectional SEM images of the CuO wrinkle (top) and CuO H-wrinkle (bottom) sensing channels (left: low magnification, right: high magnification). Schematic of the adsorption behavior of gas molecules on the (b) CuO film, (c) CuO wrinkle, and (d) CuO H-wrinkle.



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Keyword: metal oxide, wrinkle, hierarchical wrinkle, ridge, acetone, gas sensor Woo-Bin Jung,†,‡,⊥ Soo-Yeon Cho,†,‡, §,⊥ Geun-Tae Yun, †,‡ Junghoon Choi, †,‡ Yesol Kim, †,‡ Minki Kim, †,‡ Hohyung Kang†,‡ and Hee-Tae Jung*,†,‡ Hierarchical Metal Oxide Wrinkles as Responsive Chemical Sensors ToC figure


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Figure 1. Fabrication of the metal oxide wrinkle. (a) Schematic of the fabrication of the metal oxide wrinkle. (b) CuO wrinkle formed by the variation of the areal strain from 0.25 to 0.75. (c) CuO, NiO, and TiO2 wrinkles. (d) XRD patterns of the metal oxide thin film and wrinkle.

ACS Paragon Plus Environment


Figure 2. Hierarchical metal oxide wrinkle. (a) Schematic of the fabrication of a hierarchical metal oxide wrinkle. (b) Hierarchical wrinkle before and after transfer. After transfer, the morphology of the hierarchical wrinkle changed to the ridge form, which exhibited flat regions between ridges. (c) SEM images of the Cu wrinkles before transfer and CuO wrinkles after transfer. (d) Change in the wavelength and ridge width after the transfer of the wrinkle and h-wrinkle.


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Figure 3. Sensing characterization of the CuO film, CuO wrinkle, and CuO H-wrinkle. Top-view SEM images of the (a) CuO film, (b) CuO wrinkle, and (c) CuO H-wrinkle sensors with electrode integration. (d) Channel resistances of the CuO film, CuO wrinkle, and CuO H-wrinkle sensors. (e) Real-time CH3COCH3 (acetone) sensing response of the CuO film, CuO wrinkle, and CuO H-wrinkle sensors. (f) Acetone response amplitudes (ΔR/Ra) of the CuO film, CuO wrinkle, and CuO H-wrinkle sensors.



Figure 4. Sensing mechanism of the CuO wrinkle and CuO H-wrinkle systems. (a) Cross-sectional SEM images of the CuO wrinkle (top) and CuO H-wrinkle (bottom) sensing channels (left: low magnification, right: high magnification). Schematic of the adsorption behavior of gas molecules on the (b) CuO film, (c) CuO wrinkle, and (d) CuO H-wrinkle.


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ToC graphics


Hierarchical Metal Oxide Wrinkles as Responsive Chemical Sensors

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