Piezoelectric characterization of a single zinc oxide nanowire using a nanoelectromechanical oscillator

In this paper, we present a nanoelectromechanical oscillator with a single semiconducting zinc oxide nanowire (ZnO) doubly clamped and suspended on two metal electrodes by which the piezoelectric property on the growth of the ZnO nanowire along the c-axis, [0001], is characterized by the resonant frequency shift of the oscillator. We report that the resonance of the nanowire oscillator can be detected in ambient air and the effective piezoelectric coefficient on the growth of a ZnO nanowire along the c-axis, [0001], is significantly larger than that of bulk (0001) ZnO.     

A study on rapid growth and piezoelectric effect of ZnO nanowires array

This work presents a rapid and simple synthesis procedure for ZnO nanowires (NWs) array by using the vapor–solid (VS) method. Experimental results indicate that the length and diameter of the grown ZnO NWs are associated with the temperature effect, while the growth density of NWs is strongly related to gas flux during the VS process. Additionally, the synthesized ZnO NWs possess specific crystalline qualities, making them highly promising for piezoelectric device applications. Therefore a piezoelectric type nanogenerator based on the ZnO NWs is also designed in this work, with a high output of piezoelectric current of 0.6 μA cm⁻² obtained as well. Our results further demonstrate the feasibility of applying piezoelectric energy via the rapidly grown ZnO NWs array.     

Controlled growth of ZnO nanorods on common paper substrate and their application for flexible piezoelectric nanogenerators

A simple and effective method of synthesizing nanorods (NRs) and the ability to control the size and aspect ratio of them are crucial for fabricating nanodevices. In this paper, we present a systematic study of the growth of ZnO NRs on common paper substrates using a hydrothermal approach by adjusting the growth conditions. By a slight variation of the solution concentration and the growth time, significant changes in morphology and size (aspect ratio) of the obtained ZnO NRs have been controlled. Moreover, the piezoelectric power generation from ZnO-paper nanogenerators grown with different precursor concentration and growth time are also investigated. It is found that the morphology and aspect ratio of NRs have significant influence on the piezoelectric behavior. This type of flexible piezoelectric nanogenerator will have potential applications in implantable biosensors and wearable self-powered electronic devices.     

内嵌倾斜压电柱复合材料板的压电振动特性分析

针对1-3型内嵌倾斜压电柱复合板结构,建立压电复合板结构的有限元模型,并推导出变形关系和形函数;应用拉格朗日方程,建立了单元结构的运动方程。针对不同倾斜角度压电柱复合板进行有限元仿真分析,研究了复合压电板的正压电特性,并对复合压电板的能量损耗因子进行分析。研究结果表明：在相同压力下,压电柱的倾斜角度对复合板的弯曲模态频率影响较大。随着压电柱体倾斜角度的增大,模态频率降低,弯曲变形增大;同时,倾斜柱体棱长变长,产生电压增大。悬臂板在压力作用下,沿宽度方向产生反对称电势。在压电陶瓷柱的倾角达到57时,损耗因子达到最大。采用内嵌式倾斜压电柱复合板结构,降低了压电板的脆性,保证大尺寸压电板的结构均匀性和应用。     

Vertical ZnO nanowire growth on metal substrates

Vertical growth of ZnO nanowires is usually achieved on lattice-matched substrates such as ZnO or sapphire using various vapor transport techniques. Accomplishing this on silicon substrates requires thick ZnO buffer layers. Here we demonstrate growth of vertical ZnO nanowires on FeCrAl substrates. The pre-annealing prior to growth appears to preferentially segregate Al and O to the surface, thus leading to a self-forming, thin pseudo-buffer layer, which then results in vertical nanowire growth as on sapphire substrates. Metal substrates are more suitable and cheaper than others for applications in piezoelectric devices, and thin self-forming layers can also reduce interfacial resistance to electrical and thermal conduction.     

Mg/sub x/Zn/sub 1-x/O: a new piezoelectric material

Piezoelectric ZnO thin films have been successfully used for multilayer surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) is a new piezoelectric material, which is formed by alloying ZnO and MgO. Mg/sub x/Zn/sub 1-x/O allows for flexibility in thin film SAW device design, as its piezoelectric properties can be tailored by controlling the Mg composition, as well as by using Mg/sub x/Zn/sub 1-x/O/ZnO multilayer structures. We report the metal-organic chemical vapor deposition (MOCVD) growth, structural characterization and SAW evaluation of piezoelectric Mg/sub x/Zn/sub 1-x/O (x<0.35) thin films grown on (011~2) r-plane sapphire substrates. The primary axis of symmetry, the c-axis, lies on the Mg/sub x/Zn/sub 1-x/O growth plane, resulting in the in-plane anisotropy of piezoelectric properties. SAW test devices for Rayleigh and Love wave modes, propagating parallel and perpendicular to the c-axis, were designed and fabricated. Their SAW properties, including velocity dispersion and piezoelectric coupling, were characterized. It has been found that the acoustic velocity increases, whereas the piezoelectric coupling decreases with increasing Mg composition in piezoelectric Mg/sub x/Zn/sub 1-x/O films.     

Improved seedless hydrothermal synthesis of dense and ultralong ZnO nanowires

Seedless hydrothermal synthesis has been improved by introducing an adequate content of ammonia into the nutrient solution, allowing the fabrication of dense and ultralong ZnO nanowire arrays over large areas on a substrate. The presence of ammonia in the nutrient solution facilitates the high density nucleation of ZnO on the substrate which is critical for the nanowire growth. In order to achieve an optimal growth, the growth conditions have been studied systematically as a function of ammonia content, growth temperature and incubation time. The effect of polyethyleneimine (PEI) has also been studied but shown to be of no benefit to the nucleation of ZnO. Ultradense and ultralong ZnO nanowires could be obtained under optimal growth conditions, showing no fused structure at the foot of the nanowire arrays. Due to different reaction kinetics, four growth regimes could be attributed, including the first fast growth, equilibrium phase, second fast growth and final erosion. Combining this simple method with optical lithography, ZnO nanowires could be grown selectively on patterned areas. In addition, the as-grown ZnO nanowires could be used for the fabrication of a piezoelectric nanogenerator. Compared to the device of ZnO nanowires made by other methods, a more than twice voltage output has been obtained, thereby proving an improved performance of our growth method.     

Simple and cost-effective fabrication of size-tunable zinc oxide architectures by multiple size reduction technique

Abstract

We present a simple size reduction technique for fabricating 400 nm zinc oxide (ZnO) architectures using a silicon master containing only microscale architectures. In this approach, the overall fabrication, from the master to the molds and the final ZnO architectures, features cost-effective UV photolithography, instead of electron beam lithography or deep-UV photolithography. A photosensitive Zn-containing sol–gel precursor was used to imprint architectures by direct UV-assisted nanoimprint lithography (UV-NIL). The resulting Zn-containing architectures were then converted to ZnO architectures with reduced feature sizes by thermal annealing at 400 °C for 1 h. The imprinted and annealed ZnO architectures were also used as new masters for the size reduction technique. ZnO pillars of 400 nm diameter were obtained from a silicon master with pillars of 1000 nm diameter by simply repeating the size reduction technique. The photosensitivity and contrast of the Zn-containing precursor were measured as 6.5 J cm^?2 and 16.5, respectively. Interesting complex ZnO patterns, with both microscale pillars and nanoscale holes, were demonstrated by the combination of dose-controlled UV exposure and a two-step UV-NIL.     

Piezoelectric potential output from ZnO nanowire functionalized with p-type oligomer

We have studied the piezoelectric potential output of a ZnO wire/belt functionalized with p-type oligomer (2,5-Bis(octanoxy)-1,4-bis(4-formyl phenylene vinylene) benzene) (OPV2) when it was deflected by an atomic force microscope (AFM) tip in contact mode. In comparison to the ZnO wire/belt without oligomer coating, an extra positive voltage peak was observed prior to the appearance of a negative potential peak. The paired positive and negative voltage peaks are the results of tip contact to the stretched and the compressed side of the wire/belt, corresponding to the positive and negative local piezoelectric potential, respectively. The p-n junction between OPV2 and ZnO serves as a "diode" that controls the flow of current. When the nanowire/nanobelt is first bent by the AFM tip, the diode is reversely biased and the piezoelectric charges are stored in the ZnO wire/belt. As the AFM tip further bends the wire/belt, the local piezoelectric potential is continuously accumulated to a value that is large enough to break through the diode. Then the free charges from the external circuit can flow in and neutralize/screen part of the piezoelectric charges, resulting in a positive pulse in the output signal. When the AFM tip continues to scan to reach the compressed side of the ZnO wire/belt, the p-n junction is forwardly biased. Neutralizing/screening the residual and the newly created piezoelectric charges leads to the flow of current from the tip to the ZnO wire/belt, resulting in a negative voltage pulse. This study supports the charging and discharging model proposed for the piezoelectric nanogenerator.     

Electromechanical coupling coefficient k15 of polycrystalline ZnO films with the c-axes lie in the substrate plane

The (1120) textured polycrystalline ZnO films with a high shear mode electromechanical coupling coefficient k15 are obtained by sputter deposition. An over-moded resonator, a layered structure of metal electrode film/(1120) textured ZnO piezoelectric film/metal electrode film/silica glass substrate was used to characterize k15 by a resonant spectrum method. The (1120) textured ZnO piezoelectric films with excellent crystallite c-axis alignment showed an electromechanical coupling coefficient k15 of 0.24. This value was 92% of k15 value in single-crystal (k15 = 0.26).     

Selective MOCVD growth of ZnO nanotips

ZnO is a wide bandgap semiconductor with a direct bandgap of 3.32eV at room temperature. It is a candidate material for ultraviolet LED and laser. ZnO has an exciton binding energy of 60 meV, much higher than that of GaN. It is found to be significantly more radiation hard than Si, GaAs, and GaN, which is critical against wearing out during field emission. Furthermore, ZnO can also be made as transparent and highly conductive, or piezoelectric. ZnO nanotips can be grown at relatively low temperatures, giving ZnO a unique advantage over the other nanostructures of wide bandgap semiconductors, such as GaN and SiC. In the present work, we report the selective growth of ZnO nanotips on various substrates using metalorganic chemical vapor deposition. ZnO nanotips grown on various substrates are single crystalline, n-type conductive and show good optical properties. The average size of the base of the nanotips is 40 nm. The room temperature photoluminescence peak is very intense and sharp with a full-width-half-maximum of 120 meV. These nanotips have potential applications in field emission devices, near-field microscopy, and UV photonics.     

Piezoelectric constants for ZnO calculated using classical polarizable core-shell potentials

We demonstrate the feasibility of using classical atomistic simulations, i.e.?molecular dynamics and molecular statics, to study the piezoelectric properties of ZnO using core-shell interatomic potentials. We accomplish this by reporting the piezoelectric constants for ZnO as calculated using two different classical interatomic core-shell potentials: that originally proposed by Binks and Grimes (1994 Solid State Commun. 89 921-4), and that proposed by Nyberg et al (1996 J. Phys. Chem. 100 9054-63). We demonstrate that the classical core-shell potentials are able to qualitatively reproduce the piezoelectric constants as compared to benchmark ab initio calculations. We further demonstrate that while the presence of the shell is required to capture the electron polarization effects that control the clamped ion part of the piezoelectric constant, the major shortcoming of the classical potentials is a significant underprediction of the clamped ion term as compared to previous ab initio results. However, the present results suggest that overall, these classical core-shell potentials are sufficiently accurate to be utilized for large scale atomistic simulations of the piezoelectric response of ZnO nanostructures.     

The effect of metal layers on the morphology and optical properties of hydrothermally grown zinc oxide nanowires

If the silicon industry is to successfully integrate ZnO nanowires (NWs) into existing devices to fully utilise the piezoelectric or optical properties of ZnO NWs, then a detailed understanding of the effect of metal interconnects on the morphology of the NWs during growth needs to be obtained. In this study, ZnO NWs were hydrothermally grown at 90 °C on Au, Ni and a Si substrate control to mimic the typical surfaces of a MetalMUMPs MEMS chip. The growth rate was significantly affected by the metal film below the ZnO seed layer, which was mainly attributed to changes in the roughness and grain size of the seed layer deposited, with the growth rate decreasing with increasing roughness. The growth rate on Si and Au surfaces also increased when isolated from the Ni samples, suggesting that Ni cations released in the solution could also inhibit growth by electrostatically attaching to the NWs surface and acting as a barrier to the incorporation of zinc ions. Furthermore, photoluminescence studies show the addition of metal layers to the substrate reduces the optical quality of the produced ZnO NWs.     

Pyramidal nanostructures of zinc oxide

Zinc oxide (ZnO) nanostructures have been prepared by pulsed laser deposition of the oxide onto Si(100) substrate at 600 degrees C. An examination of the morphology using atomic force microscopy and scanning electron microscopy reveals well formed pyramidal structures consistent with the growth habit of ZnO. A domain matched epitaxy across the interface makes the ZnO pyramids orient along the axes of Si(100) surface. The pyramidal nanostructures signify an intermediate state in the growth of hexagonal nanorods of ZnO. The hardness of the nanostructures as well as their response to oxygen gas have been investigated using nanoindentation and conducting probe methods respectively. ZnO nanostructures are much harder than their bulk. The hardness of ZnO pyramids obtained by nanoindentation is 70 +/- 10 GPa which is about one order more that of bulk ZnO. Besides, the nanostructures exhibit high sensitivity towards oxygen. A 70% increase in the resistance of ZnO nanostructures is observed when exposed to oxygen atmosphere.     

Piezoelectric and semiconducting coupled power generating process of a single ZnO belt/wire. A technology for harvesting electricity from the environment

This paper presents the experimental observation of piezoelectric generation from a single ZnO wire/belt for illustrating a fundamental process of converting mechanical energy into electricity at nanoscale. By deflecting a wire/belt using a conductive atomic force microscope tip in contact mode, the energy is first created by the deflection force and stored by piezoelectric potential, and later converts into piezoelectric energy. The mechanism of the generator is a result of coupled semiconducting and piezoelectric properties of ZnO. A piezoelectric effect is required to create electric potential of ionic charges from elastic deformation; semiconducting property is necessary to separate and maintain the charges and then release the potential via the rectifying behavior of the Schottky barrier at the metal-ZnO interface, which serves as a switch in the entire process. The good conductivity of ZnO is rather unique because it makes the current flow possible. This paper demonstrates a principle for harvesting energy from the environment. The technology has the potential of converting mechanical movement energy (such as body movement, muscle stretching, blood pressure), vibration energy (such as acoustic/ultrasonic wave), and hydraulic energy (such as flow of body fluid, blood flow, contraction of blood vessels) into electric energy that may be sufficient for self-powering nanodevices and nanosystems in applications such as in situ, real-time, and implantable biosensing, biomedical monitoring, and biodetection.     

Mass sensitivity of thickness-twist modes in a rectangular piezoelectric plate of hexagonal crystals

Mass sensitivity of thickness-twist vibration modes in a rectangular plate piezoelectric resonator of 6 mm crystals are obtained from the three-dimensional equations of linear piezoelectricity. The boundary conditions at the finite-plate boundaries all have been taken into consideration. The solutions obtained are exact, which is relatively few for piezoelectric problems over finite domains. The results are fundamental and useful to the understanding and design of piezoelectric resonators and acoustic wave sensors made of polarized ceramics, ZnO and AlN.     

Piezoelectric properties of zinc oxide films on glass substratesdeposited by RF-magnetron-mode electron cyclotron resonance sputteringsystem

There are various types of electron cyclotron resonance (ECR) sputtering systems, DC-mode, RF-mode, etc. We reported that zinc oxide (ZnO) films on glass substrates deposited by DC-mode ECR and RF-mode ECR sputtering systems had shown excellent piezoelectric properties and c-axis orientations. The RF-mode ECR sputtering system was capable of depositing ZnO films on glass substrates without evidence of column and fiber grains in cross section and driving a 1.1 GHz fundamental Rayleigh surface acoustic wave (SAW). In this paper, the properties of ZnO film deposited by an RF-magnetron-mode ECR sputtering system, which has added magnets to the outside of a cylindrical zinc metal (Zn) target of the RF-mode ECR sputtering system, are investigated. It is confirmed that the SAW filters using ZnO films on an interdigital transducer (IDT)/glass substrate deposited by the RF-magnetron-mode ECR sputtering exhibit almost the same effective electromechanical coupling factors (keff) as the theoretical keff values calculated by finite element method (FEM) using the constants of ZnO single crystal (measured keff values are 97% of the theoretical values) and 0.6~3.7 dB lower insertion loss in comparison with the films deposited by the DC-mode ECR and the RF-mode ECR sputtering system     

Recent advances in ZnO materials and devices

Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.     

Fabrication and characterization of ZnO nanoneedle array using metal organic chemical vapor deposition

High density and vertically well-aligned ZnO nanoneedle arrays were fabricated on the ZnO thin film deposited on silicon substrates. The ZnO buffer layer and nanoneedles were synthesized by metal organic chemical vapor deposition using diethylzinc and oxygen gas. The ZnO buffer film was grown at 250 degrees C and the growth temperature of nanoneedles was in the range of 480-500 degrees C. As-grown ZnO nanoneedles showed single crystalline structure of ZnO (002). The crystalline properties of three samples (A: as-deposited ZnO buffer layer, B: annealed buffer film, C: ZnO nanoneedles) were compared using XRD and Raman spectroscopy. The synthesized ZnO nanoneedles (sample C) showed highest crystalline quality among three samples. The field emission properties of ZnO nanoneedles were investigated, which showed low turn on field of 4.8 Vmicrom(-1) and high field enhancement factor of 3.2 x 103.     

Design and fabrication of a piezoelectric transducer for wind-power generator

This study investigates the feasibility of a high-performance ZnO piezoelectric transducer for wind-power generation applications. The piezoelectric transducer is constructed of a Cu/ZnO/ITO/PET structure. Closely examining the ITO/PET substrate by a nano indenter reveals a low Young's modulus of 6.62 Gpa for specific deflections. The ZnO piezoelectric film of 965 nm is deposited on ITO/PET substrate using a RF magnetron sputtering system at room temperature. A copper layer is attached to the ZnO/ITO/PET structure to construct piezoelectric transducers. Both scanning electron microscopy and X-ray diffraction indicate that, among the favorable characteristic of the ZnO piezoelectric film include a rigid surface structure and a high c-axis preferred orientation. According to cantilever vibration theory, a transducer with a cantilever length of 9.9 mm and vibration area of 1.5 cm² is designed for natural wind. An appropriate mass loading of 0.57 g on the cantilever is critical for increasing the vibration amplitude and promoting the generated power of a piezoelectric transducer. Finally, an open circuit voltage of 1.87 V for the ZnO piezoelectric transducer at a vibration frequency of 100 Hz is obtained by an oscilloscope. After rectifying and filtering, the output power of the generator exhibits an available benefit of 0.07 μW/cm² with the load resistance of 5 MΩ.