State Key Laboratory of Material-oriented Chemical Engineering and School of Chemical Engineering, Nanjing Tech University, PR China
Received Date: January 08, 2016; Accepted Date: January 27, 2016; Published Date: January 30, 2016
Citation: Li Y, Gao Z, Qin WW, Wen QJ, Jun MX, et al. (2016) Nano Size Related Piezoelectric Efficiency in a Large ZnO Thin Film, Potential for Self-Powered Medical Device Application. Biochem Anal Biochem 5:243. doi:10.4172/2161-1009.1000243
Copyright: © 2016 Li Y, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Large-area piezoelectric ZnO films with different gran size has been synthesized by sol-gel technique using different annealing temperatures from 550 to 700°C. The piezoelectric efficiency (PE) of those deposited films is characterized by Piezoelectric Force Microscopy (PFM). All synthesized films exhibit a crystal structure. The width of the rock curve of  characterized by x-ray diffraction decreases with the annealing temperature, suggesting a better c-axis orientated ZnO film formed at higher annealing temperature. The grain size of the grown films are found to continuously increase from 20 to 60 nm when the annealing temperatures increase from 550 to 700°C. The piezoelectric efficiency (PE, d33) of the films exhibit strong grain size dependence, i.e., the PE initially increase with the annealing temperature and then decreasing with a further annealing temperature increased. The maximum PE value appears in the film annealed at 650°C. The peculiar piezoelectric properties (d33) can be explained by the competing between the crystalline, which favors a larger d33 due to the enhanced dipole polarization, and the grain size, which results in a piezoforce release at large grain size due to domain wall size and motion.
Grain Size; Sol-gel; Nanogenerator; ZnO thin film; Piezoelectric efficiency; Biosensor
In vivo sensors have a wide application for real-time biomedical monitoring . Single-use or non-rechargeable batteries to support implanted device limits the application of those promising implanted sensors, because of requiring to be surgically replaced at the end. The human body, in fact, is a natural power source, owning potential energy such as chemical energy (glucose), hydraulic energy (body fluid and blood flow), and blood vessel contraction). The challenges remaining are how to efficiently convert them into electrical energy . The nanogenerator based on ZnO nanowire has demonstrated to enable the collection of the energy from the environmental mechanical energy and to drive electrical device in the microwatt power range [3-6]. The fundamental mechanism of a nanogenerator (NG) is related to a piezoelectric potential generated in nano wire (NW) under an external force, and a current flows on a loading resistor [7,8]. Although such prototyping work has demonstrated the basic principle of the nanogenerator, the performance must drastically improve to make nanogenerators for practical applications. NG process must be low cost and easy for mass production. Thus, it is necessary to develop strategies towards achieving cost-effective NG in order to consistently scavenge the mechanical energy from the environmental sources.
Piezoelectric thin film power generator has emerged as a promising candidate for vibrating energy collector [9-12]. Recently, high power output from lead zirconate titanate (PZT) piezoelectric thin films onto flexible substrates has suggested a direct application of piezoelectric films as piezoelectric generators without the requirement of NW fabrication . In comparison with NW-type nanogenerators, thin film nanogenerators possess numerous advantages, simple and mature process, well-controlled film properties, and, more important, compatibility with current semiconductor IC technology for mass production [13-15].
A number of techniques, chemical vapor deposition , spraypyrolysis , molecular beam epitaxy , vacuum arc deposition , magnetron sputtering [20-22] and pulsed laser deposition (PLD) , have been involved in the growth of ZnO thin films. Sol-gel method has emerged as an important growth technique because it enables the surface modification of zinc oxide with selected organic compounds. It can offer simplicity and low cost mild conditions of synthesis required by large-scale nanogenerator application . In this paper, we prepare ZnO film on Si substrate by Sol-gel technique. Different film structure and grain sizes of ZnO thin films were generated by post-annealing at different temperatures from 550 to 700°C. Structurally, emphasis is placed on XRD characterization of the films. The grain size of the films was characterized using Atomic Force Microcopy (AFM) and Scanning Electron Microscopy (SEM). The local piezoelectric properties of these films are obtained using PFM. The observed relationship between the piezoelectric (PE) property and the film structure is theoretically explained.
To performance PFM characterization, Pt bottom electrodes with 5 nm in thickness were prepared on 6-inches Si (100) substrates using vacuum thermal evaporation. The 0.35M ZnO precursor solution was gained by dissolving high-purity zinc acetate dehydrate (Zn (CH3COO) 2·2H2O, 99.0%) in 2-methoxyethanol (C3H8O2, 99.0%). When the solution stirred at 60°C for 2h, the monoethanolamine (MEA, C2H7NO, 99.0-100.3%) was slowly added into the solution as a stabilizer which makes the solution clear. The molar of ratio MEA to zinc acetate was kept at 1.0. The solution is then aged for more than 24h at room temperature before it is used. For ZnO film growth, the Pt/Si substrates are in-turn cleaned in an ultrasonic bath with acetone, ethanol, and distilled water. Then, Pt/Si substrates were dried in nitrogen and ready for coating. The precursor solution is spin-coated on the Pd/Si (100) substrates at 3000 rpm for 30s. After each coating, the as-coated film is preheated from room temperature to 350°C for 10 minutes. The spincoating and preheating process was repeated six times to make a dense film with a 70 nm in thickness. Finally, the films are annealed at 550°C, 600°C, 650°C and 700°C in air for 1hour in a quartz-tube furnace.
The structure of the deposited film is determined using XRD analyses with a Philips PW3710 system (Cu-Kα radiation, λ=0.15406 nm). AFM (Oxford A350) is using to characterize the morphology of the films. The grain size of the films is characterized using XRD and Scanning Electron Microscopy (Hitachi 7800, 20 keV). PFM (Oxford A350) is a similar to typical contact mode imaging tool, except an AC voltage is applied to the surface of a piezoelectric sample through the tip of a conductive AFM cantilever. When a voltage is applied, the piezoelectric thin film undergoes a change in shape, leading to a change in tip-to-surface height. This height change can be tracked and measured with a piece of equipment called a lock-in amplifier, which tracks the contact resonance frequency of the tip-to-surface contact.
Figure 1 shows a XRD pattern for the film annealing at various temperatures and the details are summarized in Table 1. The XRD spectra of all four samples exhibit a strong (002) peak, which indicates that they have a preferential growth orientation along the c-axis perpendicular to the substrate surface. As the annealing temperature is increased, the relative intensity of the (002) diffraction peak increases rapidly and its full width at half maximum (FWHM) decreases gradually. Regarding the ZnO film grown at 700°C, the intensity of the (002) diffraction peak is maximum and its full width at half maximum (FWHM) is found to be 0.26°C. These results indicate that the crystallinity of the ZnO thin films is enhanced as the annealing temperature is increased. Using the Scherrer formula, DXRD=0.94λ/(βcos θβ) (where λ, θβ, and β are the x-ray wavelength, the Bragg diffraction angle, and the line width at half maximum of the diffraction peak, respectively), the mean crystallite sizes (DXRD), as presented in the Table 1, are calculated to be 27.0, 28.7, 30.9 and 34.7 nm for the films annealed at 550, 600. 650, 700°C, respectively.
|Substrate||Annealing temperature(°C)||2θ (deg)||FWHM (deg)||Grain size (nm)|
Table 1: Data summary of the peak position, full width at half maximum (FWHM)
and calculated grain size at various annealing temperatures.
Figure 2 shows a typical cross-sectional dark-field TEM micrographs of a ZnO film on a Si substrate annealed at 650°C. The thickness of the film obtained from the TEM image is approximately 70 nm. Columnar growth can be clearly observed for the film deposited on the Si substrate. The shape of the grains in the upper region of the structure is convex, because the upper region is more stable and closer to equilibrium morphology due to atomic mobility and stress relaxation, in compared to the lower region . The inset of Figure 2 shows the selected area electron diffraction data obtained with the electron beam parallel to the [21,30] zone axis of ZnO. The indexed diffraction pattern confirms the hexagonal structure of the ZnO thin film.
The average grain size at film surface with a data more accuracy of ±4 nm is also characterized by SEM. Figure 3 shows that the grain size of the film increases with the annealing temperature. A comparison of Figures 4a and 4d clearly indicates that the larger grains have sharper interfaces, while the smaller grains are separated by distinct inter- granular regions, in particular for the samples with smaller average grain sizes. Because these images show only grains with a particular crystallographic orientation, so individual grains can be identified. The grain size characterized by SEM is typically in close agreement with the calculated values of dXRD in grain size trend. We can identify a couple of grains in which the crystalline order persists for ~ 8-9 nm. The width of the inter-granular region is ∼ 5 nm in this film. This type of grain boundary is structurally similar to that recently observed in ZnO films .
The Piezoelectric Effect (PE) of thin ZnO films is characterized by PFM for standard piezoelectric phenomena . In all the piezoelectric measurements, the interaction between the tip and electric field was ignored . The displacement as a function of applied voltage for the films is shown in Figure 4a. The amount of the displacement under an AC voltage, which does not follow the trend of grain size and c-axis orientation, initially increases with grain size and then decreases when the annealing temperature is further increased. The maximum displacement point appears for the film annealed at 650°C. The piezoelectric coefficient d33, as showed in Figure 4b, can be determined from the slope of the resulting plot of the amplitude of displacement vs. that of applied voltage, which represents peak-to-peak values of both the displacement and the applied voltage. To reference the piezoelectric response to the piezoelectric material, the PE response on a bare single crystalline Si (100) substrate is also calculated. The piezoelectric constant d33 is approximately 0.26 pm/V for the bare Si substrate, which is at a similar level (<0.5 pm/V) as the value in the reference literature [28,29]. As a result, the impact of initial substrate roughness on the PE result can be ignored. The piezoelectric constant for the ZnO thin film initially increases from 6.5 pm/V to 32.5 pm/V when the annealing temperature increases from 550 to 650°C. The piezoelectric constant then decreases to 17 pm/V when the annealing temperature further increases to 700°C. A maximum PE value of 32.5 pm/V appears for the film annealed at 650°C.
The peculiar behavior of the PE constant at different grain sizes may be primarily attributed to competing effects between the grain size and the c-axis orientation of the ZnO films. ZnO’s wurtzite structure has a hexagonal unit cell of space group C6mc and lattice parameters a = 0.3296 and c = 0.52065 nm. The oxygen anions and Zn cations form a tetrahedral unit. The entire structure lacks central symmetry. The structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O2- and Zn2+ ions stacked alternatively along the c-axis. Although the entire unit cell of ZnO is neutral, the distribution of the cations and anions could adopt a specific configuration determined by crystallography such that some surfaces can be terminated entirely with cations or anions, resulting in positively or negatively charged surfaces or polar surfaces. The most common polar surface is the basal plane. The oppositely charged ions produce positively charged Zn- (0001) and negatively charged O-(0002) polar surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis as well as a divergence in the surface energy[30-32]. More c-axis oriented film growth at a high annealing temperature diminishes the d spacing along the c-axis and enhances ZnO polarity, leading to a high PE constant.
Grain size is an important factor in affecting on the piezoelectric properties [33-36]. Obviously, the film stress plays a role in piezoelectric property, but cannot explain the dependence of d33 on the grain size observed in our experiment because of the maximum d33 appearing in the ZnO film annealed at 650°C. The reduction of the ratio c/a with the reduction of the grain size cannot also explain the strong grainsize dependence of d33 because it is related directly to the decrease of spontaneous polarization. Accordingly, an intrinsic mechanism related to ion shift, which preserve the piezoelectric crystal structure, used to explain for the observed behavior is not likely. In contrast, the coupling between the grains and the motion of the domain walls are considered to very likely provide a dominating contribution to the observed behavior. At a case of small grain, the direction of spontaneous polarization is aligned opposing among neighbor grain, which is caused by strong coupling due to narrow grain boundary. Therefore the collective piezoelectric response is small, because the spontaneous polarization is cancelled each other. When the grain grow, the coupling interaction among grains is weakened, but external force can be released at wide grain boundary, leading a decrease in piezoelectric response. Therefore an optimum grain size can result in a maximum PE. Some similar studies are previously carried out [37-40]. The domain wall density has been considered as the origin of the strong dielectric grain-size dependence of ε' observed in unpoled ferroelectric BaTiO3 ceramics  and of the large enhancement in d33 observed in poled fine-grained BaTiO3 ceramics [38-40]. In addition, the area dimension of the 90°-domain wall should also be an important factor that greatly influences d33 [41-43]. This behavior is well-consistent with the PE value of the ZnO films observed at different grain sizes exhibit size-dependent PE behavior and d33 values .
In this paper, high-quality ZnO thin films with preferred c-axis orientation are prepared on single-crystal Si (100) via sol-gel growth method and then annealed in the temperature range of 550 to 700°C. XRD, SEM and PFM are utilized for the characterization of structural and piezoelectric properties. While all films annealed in the temperature range of 550 to 700°C exhibit c-axis orientation, the films with higher annealing temperatures are grown with larger grain sizes simultaneously. The piezoelectric efficiency of the films initially increases with the grain size and then decreases with further increase in grain size. The peculiar piezoelectric properties (d33) can be explained by the competition between c-axis orientation, which favors a larger d33 due to enhanced static asymmetry, and grain size, which is likely related to motion of domain wall. ZnO films with high PE values are potential candidates for building nanogenerator for energy harvesting applications.
This work was supported in part by Jiangsu Province Key Project for Society Development under contract BE2012759, National High Technology Development Program under contract 2014AA020604 and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).