ZnO Nanowall Networks for Sensor Devices: From Hydrothermal Synthesis to Device Demonstration

This paper reports the synthesis of ZnO nanowalls (NWLs) on various substrates by low cost and scalable hydrothermal approach targeting for flexible sensor device applications. Prototype flexible piezoelectric sensor devices using the synthesized ZnO NWLs were demonstrated through a transfer process. The roles of precursor chemical concentration and aluminum seed layer thickness in determining the morphology and the growth rate of the as grown ZnO NWLs were investigated. Effects of thermal annealing on the morphology and the crystalline quality of the NWLs were studied. To fabricate the sensor devices, ZnO NWL layers were transferred on to different substrates using thin Polymethyl methacrylate (PMMA) layer. Wafer scale transfer of thermally annealed ZnO NWLs to flexible substrates has been achieved. The demonstrated prototype devices can generate up to 300 mV of output voltage with external applied forces. Simulation model using COMSOL Multiphysics was developed to study the piezoelectric properties of the ZnO NWLs based devices. The simulation results agree well with the experimental data. The demonstrated ZnO NWLs based flexible devices are applicable for wearable nanodevices and motion sensors that will benefit from their large surface areas. © The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0221807jss]

Recently, wearable electronics have become one of the trending technologies for their potential applications in personal fitness, medical care and communication. A compact, flexible and sustainable piezoelectric device can play important roles in wearable electronics such as: (i) energy harvesting, which converts mechanical energy into electricity and powers up devices; (ii) pressure sensors, which detect forces and transform them into electrical signals for further analysis.
Semiconductor material ZnO possesses a large bandgap of ∼3.37 eV, a large exciton binding energy of 60 meV 1 and decent piezoelectric properties. Because of such unique properties, ZnO is a promising candidate for various device applications such as sensors/biosensors, photodetectors/solar cells, resistive randomaccess memory and surface acoustic wave devices. [2][3][4][5][6][7] As a result, considerable efforts are going on to synthesize ZnO based nanostructures with one-dimensional (1D) and two-dimensional (2D) morphologies such as nanowires, [8][9][10] nanorods, 1,11,12 nanowalls 13-15 and nanobelts [16][17][18] by various growth techniques. ZnO nanostructures have become the focus of intensive research due to their applicability in nanoscale devices. ZnO nanowalls (NWLs) are featured with interconnected honeycomb-like structure with large surface-to-volume ratio, and good mechanical durability. Due to its planar structure, the traditional top-down fabrication process is still applicable. Material synthesis processes such as chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE) require expensive apparatus and processes which are time consuming and typically conducted at elevated temperatures. On the other hand, solution based synthesis process is low cost and scalable. It yields high throughput at much lower operating temperatures. Consequently, numerous studies have been published in the literature on the synthesis of ZnO thin films, 19 nanorods, 20-22 nanowires [23][24][25] and nanowalls [26][27][28] using hydrothermal synthesis process.
Mechanical transfer process for thin films has been developed with graphene, and other 2D materials, proving flexibility for multiple application purposes. [29][30][31] The polymethyl methacrylate (PMMA)mediated transfer method is one of the common transfer tech-niques. With this transfer method, there are fewer restrictions on substrate and contact selection which allows a wide range of device designs. 32 Devices based on transferred ZnO nanostructures have been reported previously for field effect transistors, 33 strain sensors 34 and nanogenerators. 35 In this paper, we demonstrated flexible ZnO nanowalls based piezoelectric device fabrication using the combination of hydrothermal synthesis and mechanical transfer of thermally annealed ZnO NWLs. Here, we have developed a process that allows to achieve wafer scale transfer of high crystalline quality ZnO NWLs to flexible substrates through (i) hydrothermal synthesis of large area and uniform ZnO NWLs; (ii) thermal annealing of ZnO NWLs; and (iii) wafer scale mechanical transfer of ZnO NWLs by using PMMA. The process was optimized by considering the trade-off between the thermal annealing temperature and feasibility of mechanical transfer. The maximum output voltage from the fabricated sensor devices is around 300 mV, which is comparable with the similar devices reported in the literature. [35][36][37] Experimental Synthesis of ZnO NWLs.- Figure 1 shows the schematic of the ZnO NWLs growth procedure. SiO 2 -on-Si, Si, polyimide (PI) and polyethylene terephthalate (PET) were used as the growth substrates. Prior to the growth, the substrates were cleaned with acetone and isopropanol, rinsed by deionized water and dried with nitrogen flow. Aluminum (Al) seed layer with different thicknesses (100 nm, 200 nm) were deposited on the substrates via thermal evaporation. The ZnO  NWLs were grown in an aqueous solution of zinc nitrate hexahydrate (ZNH, Zn(NO 3 ) 2 · 6H 2 O) and hexamethylenetetramine (HMT, (CH 2 ) · 6N 4 ) at 80 • C for 30 minutes. To study the effects of the chemical concentration on the morphology of the synthesized ZnO NWLs, several experiments were conducted using different molar ratios of ZNH and HMT (2:1, 1:1 and 1:2). For these experiments, a constant ZNH concentration of 25 mM was used with the total solution volume of 40 mL. After finishing the growth, the samples were removed from the solution, rinsed with DI water and dried with nitrogen. The effects of thermal annealing at elevated temperatures (T annealing = 350 • C, 600 • C, 800 • C) for 3 hrs on the ZnO NWLs surface morphology and crystalline quality were also investigated.
Mechanical transfer.-The mechanical transfer process of the ZnO NWLs is illustrated in Fig. 2. PMMA solution (anisole as solvent, 7% concentration, MicroChem) was drop-coated on top of the grown sample and deposited uniformly by film applicator. Then the sample was dried on hot plate for 1 hr at 60 • C. Using a blade, the PMMA film with ZnO NWLs attached was exfoliated from the growth substrate. The film was then embedded between two glass slides and put on hot plate for 2 hrs at 90 • C. Heat curing of the film helped to release the strain and maintained the flatness of the PMMA film. After curing, the film was released from the glass slides and placed on the designated area of the target substrate. The film was hold in place on the substrate by anchoring its corners, protecting itself from disturbance in air. Furnace was then used to heat up and evaporate PMMA (350 • C, 2 hrs) and left ZnO NWLs on the target substrate. The post-transferred ZnO NWLs were characterized by optical imaging and FESEM to demonstrate the integrity and effectiveness of the transfer process. Figure 3 shows the schematic of the prototype device. Polyimide (PI, HPP-ST 2mil, DuPont) served as both top and bottom substrates and Ti/Au was deposited as contact. The size of the central contact area is 1 cm × 1 cm. Au stripe of 3 mm wide is extended to one side of the substrate to serve as the contact pad for wirings. The ZnO NWL films were transferred to the bottom PI substrate and then packed with top substrate. The areas of the transferred films were larger (1.5 cm × 1.5 cm) as compared to those of the Au contact pads, avoiding crosstalk between top and bottom contacts after transfer. Epoxy adhesive (Poxy Pak, Henkel) was applied on the edges of PI substrates for device packaging. Copper wires were connected to the outer contact pads to form extended connection. To confirm the sources of the voltage pulses, we also fabricated one reference device with no ZnO NWLs embedded in between the PI substrates. Numerical simulation was also conducted using COMSOL Multiphysics to study the piezoelectric properties of the ZnO NWLs.

Piezoelectric device design and fabrication.-
Characterization.-The morphology, crystalline quality and optical properties of the ZnO NWLs were characterized by using field emission scanning electron microscopy (FESEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. FESEM images were taken with Helios 650. The SEM images were taken with Tescan Vega-3. XRD spectra were collected on a Bruker Discover D8 X-Ray Diffractometer with Cu Kα radiation (1.54 Å). PL spectra were measured at room temperature using a Jobin Yvon-spex-Fluorog-3-Spectrofluorimeter with a 450 W Xenon lamp as the light source. Piezoelectric voltage measurement was logged by Tektronic TPS 2024B Oscilloscope and retrieved through RS-232 port.

Results and Discussion
To study the effects of HMT molar concentration on the morphology of the synthesized ZnO NWLs, syntheses were conducted at 80 • C for 30 mins. For all the experiments, the ZNH concentration was fixed at 25 mM while varying the HMT concentration in the range between 12.5 mM and 50 mM. Figure 4 shows the top view SEM images of the as-grown ZnO NWLs on SiO 2 -on-Si substrate covered with 100 nm Al film using different HMT concentrations. For HMT concentrations of 25 mM and 12.5 mM ( Fig. 4a and Fig. 4b), the morphologies of the ZnO NWLs are similar. As the concentration of the HMT increases to 50 mM, the synthesis process resulted in the formation of flower-like ZnO nanoparticles on top of the NWLs networks. Tang. et al. observed the similar phenomenon previously. 38 The higher concentration of HMT provides more Zn(OH) 4 2− ions (Eqs. 1-3) in To investigate the effects of Al seed layer thickness on the growth rate, cross sectional SEM was conducted. It is known that the use of Al seed layer is critical to promote the formation of NWL structure instead of nanorod structure. [41][42][43] Al oxidizes in the solution and forms AlO 2 − (Eq. 4). The binding of AlO 2 − to the Zn 2+ terminated surface suppresses the growth of ZnO along the [0001] polar orientation and thus promotes its growth in the lateral orientation. 41,42 Al Figure 5 shows the cross-sectional SEM images of the ZnO NWLs grown on SiO 2 -on-Si substrate with different Al thicknesses using the same concentration of ZNH and HMT (molar ratio = 1:1). The growths were conducted for 30 mins. The ZnO NWLs grown on the 100 nm thick Al layer had a thickness of ∼1.6 μm, which corresponds to a growth rate of ∼3.2 μm/hr. On the other hand, the ZnO NWLs grown on 200 nm thick Al layer had a thickness of ∼3.3 μm which corresponds to a growth rate of ∼6.4 μm/hr. Our studies indicate that the ZnO NWLs growth rate increases with the increase of the seed layer thickness, due to the increase of the AlO 2 − concentration in the solution. 43 However, for both cases, the formation of the nanoparticles on top of the NWLs occurs with longer growth time. This indicates the shift of growth kinetic from the formation of nanowalls to nanoflowers due to the decrease of AlO 2 − concentration while the precipitation of ZnO is still fast. This is due to the screening of Al film by NWLs as it grows. Due to the large defect concentration in hydrothermal synthesized ZnO materials, thermal annealing process is typically utilized to improve the crystalline quality of the material, thus to improve the piezoelectric properties of the material for sensor device application. 36 To study the effects of thermal annealing on the surface morphology, crystalline quality and optical properties of the synthesized ZnO NWLs, several annealing experiments were conducted at different temperatures (350 • C, 600 • C and 800 • C). All the experiments were conducted under the flow of oxygen and argon for 3 hrs. Figure 6 shows the top view SEM images of the as-grown and annealed ZnO NWLs with different annealing temperatures. As can be seen from the images, ZnO NWL structure regresses with increased annealing temperature. This indicates the ZnO NWLs tends to agglomerate during high temperature annealing process.  annealing, ZnO (103) peak appeared, and the NWLs exhibited sharp (201) peak with higher intensity. The intensities of the (110), (103), and (200) peaks also increased with increasing annealing temperature. This indicates that high-temperature thermal annealing can improve the crystalline quality of the ZnO NWLs. Figure 8 shows the PL spectra of the as-grown and annealed ZnO NWLs. The NWLs exhibited near band edge emission at ∼380 nm and broad deep level emission around 540-580 nm. As the annealing temperature increases, the overall intensity of the spectrum decreases, which is partially due to the reduced NWL thickness as NWLs agglomerate. Note that the intensity of the green luminescence reduces significantly as the annealing temperature increases. It has been reported previously that the green emission is related to the oxygen vacancies (V O ) in ZnO. 44,45 The relatively high intensity of the green emission peak in the as-grown ZnO NWLs is due to the strong bonding  From our systematic studies, we identified the optimal synthesis condition of ZnO NWLs (ZNH: HMT = 1:1) and growth substrate (SiO 2 /Si with 200 nm Al seed layer) for the studies of mechanical transfer process. Since thermal annealing process enhances the crystalline quality of the material, we tried different annealing temperatures (350 • C, 600 • C, 800 • C for 3 hr) for ZnO NWLs prior to the transfer process. It was observed that high annealing temperature tends to build strong bonding between ZnO NWLs and its growth substrate, making the exfoliation process challenging. Mechanical transfer of ZnO NWL with Tannealing >600 • C causes significant damage to the film and consequently deteriorates the performance of the device. Thus, we identified that T annealing = 600 • C is the optimum temperature of thermal annealing for transfer process. Figure 9a shows the photo image of the ZnO NWLs transferred to the Si substrate. The size of the transferred NWL film was 1.5 cm × 1.5 cm. Note that there were cracks on the transferred NWL film. These defects were caused by the scratching from the blade during the exfoliation process. As shown in the SEM image of Fig. 9b, the surface morphology of the ZnO NWLs was maintained after the transfer process.
To demonstrate the piezoelectricity of the transferred ZnO NWLs, we fabricated the prototype devices. Figure 10 shows the prototype device fabricated on flexible PI substrate. The voltage output characteristic for a typical device is shown in Fig. 11a. As shown in the figure, pulse signals were generated with periodically applied finger force on top of the device. Fig. 11b plots the output signal as a function of time and with applied external force and delayed release. The positive output signal with applied force is from the compressive deformation of the ZnO NWLs, and the negative signal with the release of the external force is caused by material restoration. The maximum output voltage recorded was ∼ 300 mV from the finger applied force. These devices showed similar output voltage characteristics as compared to the previously reported nanogenerators fabricated using ZnO nanorods [35][36][37] and ZnO nanowalls. 26,41 For the reference device with no NWLs embedded between the electrodes, no output voltage was observed.
To study the piezoelectric properties of the ZnO NWLs, COMSOL Multiphysics was used to develop a model with a honeycomb-like structure as shown in Fig. 12a. The simulated device is composed of ZnO hollow hexagon structure with electrodes at the top and bottom. Periodic boundary conditions were set on the sidewalls of the single hexagonal unit, sharing the boundary condition of deformation, elastic strain and electric potential with 6 neighboring hexagon units (see Supporting Information). Three geometric parameters were specified: nanowall height (h), nanowall lateral length (a), and nanowall thickness (t). In the simulation, the vertical pressure was applied on top of the device. The bottom electrode was fixed in position and electrically grounded. The potential difference between the top and bottom electrodes was calculated.
Based on the synthesized ZnO NWL structure, the following parameters were used in the simulation: a = 250 nm, t = 50 nm, h = 2.5 μm. These parameters were extracted from experimental characterization of the grown ZnO NWLs. From the simulation results, we observed that the voltage output is linearly proportional to both applied pressure and nanowall height. This is due to the deformation of ZnO NWLs in the vertical direction under applied force. A larger deformation builds up higher piezoelectric potential between the top and bottom electrodes. Fig. 12b plots the contour image of the electric potential of the hexagonal unit cell with the periodic boundary condition (see Supporting Information for details of the simulation). With an approximate 10 N of applied force and 0.5 cm × 0.5 cm contact  area during our device test, 400 kPa was estimated as the typical pressure applied to the devices under test. In the simulation, with the set pressure of 400 kPa, the corresponding output voltage is ∼210 mV. The simulation results are consistent with our experimental data.

Conclusions
In summary, we demonstrated the flexible prototype piezoelectric sensor device using the ZnO NWLs synthesized by hydrothermal process with aluminum as the seed layer. Comprehensive studies have been conducted to understand the effects of growth condition, seed layer thickness and thermal annealing process on material properties of ZnO NWLs. PMMA-medicated mechanical transfer process was used to transfer the as-grown ZnO NWLs to target substrates. Prototype piezoelectric ZnO NWLs sensor devices with a typical output voltage of 100 mV and maximum output voltage of 300 mV were fabricated. The experimental results match well with simulation. The developed transfer process is also applicable for various substrates with different active materials. Solution synthesis of high quality ZnO NWLs and the mechanical transfer process pave the way to achieve low cost and scalable flexible electronic and biomedical sensor devices.