Appl Phys A (2011) 102: 705–711 DOI 10.1007/s00339-010-5962-z
Flexible piezoelectric harvesting based on epitaxial growth of ZnO
Wen-Yang Chang· Te-Hua Fang · Cheng-I Weng ·Shin-Shing Yang
Received: 11 January 2010 / Accepted: 30 June 2010 / Published online: 28 July 2010 © Springer-Verlag 2010
Abstract A flexible piezoelectric harvester based on the
epitaxial growth of an array ZnO nanorods with zigzag lay-ers to enhance bending and compression deformation is in-vestigated. The effects of the growth temperature, growth time, and growth concentration for ZnO epitaxial growth are determined on the flexible substrate. Scanning elec-tron microscopy (SEM) and X-ray diffraction (XRD) were used to analyze the nanostructures and crystalline charac-teristics of the nanorods, respectively. Nanorod piezoelec-tric harvesting with screen printing technology was inte-grated on the polyimide substrate. The results show that epitaxial ZnO nanorods at a concentration ratio of (1:4), a growth time of 4 hours, and a growth temperature of 90°C have perfect crystal morphology for piezoelectric harvest-ing. The current-voltage characteristics exhibit Schottky-like behavior. During the harvesting process, the current out-put was highly reproducible and repeatable when the ul-trasonic wave equipment was turned on and off. The cur-rent output after bending increases with increasing
curva-W.-Y. Chang
Department of Mechanical and Computer-Aided Engineering, National Formosa University, Yunlin 632, Taiwan
T.-H. Fang (
)Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan e-mail:[email protected]
C.-I. Weng
Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan
S.-S. Yang
Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan
ture radius due to a compression force during the bending process.
1 Introduction
Flexible electronics devices can bend, expand, and be placed on irregular surfaces, and they have been gradually devel-oped for large-area applications and low-cost manufactur-ing [1,2]. In most flexible electronics, studies have focused on printable resistance, transistors, and sensor devices, etc. on flexible substrate [3–5]. Low-power generators based on flexible electronics can be applied to wireless devices, such as radio-frequency identification (RFID), doormats with wireless transmitters, and micro-actuators for self-energy harvesting. Solar cells, thermal energy, and mechanical vi-brations can be harvested for energy. The solar cells have limited use in low-light conditions despite their excellent power density. Thermal energy is a low transduction into electrical energy due to power consumption. Mechanical vi-bration by a piezoelectric material is used in numerous ap-plications. Piezoelectric energy harvesting mainly includes zirconium titanate (PZT), aluminum nitride (AlN), barium titanate (BaTiO3), polyvinylidene fluoride (PVDF), and zinc
oxide (ZnO). Generally speaking, crystal and ceramic piezo-electric materials in bulk and film forms are not suitable for flexible substrates due to higher temperature sinter.
However, ZnO is one of the most important functional and piezoelectric materials, with a direct wide bandgap (3.2 eV at 300 K) and large exciton binding energy (60 meV) [6,7]; it has many applications [8]. For example, ZnO film has been extensively used in acoustic devices because of its strong piezoelectric effect [9] and stable mechanical be-havior [10]. In addition, one-dimensional nanorods provide much larger length-to-diameter and surface-to-volume ratios than do piezoelectric materials in bulk and film forms. ZnO
706 W.-Y. Chang
nanorod arrays have attracted attention due to their physical and photoelectrical properties; they have potential applica-tions for lasers [11], transistors [12], and solar cells [13,14]. ZnO nanorods can be synthesized using various techniques, such as sputtering deposition [15], molecular beam epitaxy (MBE) [16], the hydrothermal method [17], and the vapor-liquid-solid (VLS) method [18]. For applications in flexible harvesting, piezoelectric materials require low temperature fabrication and enough flexibility to be implemented on a substrate. ZnO can be synthesized using the chemical solu-tion method at low temperatures (<100◦C).
Recently, piezoelectric ZnO nanorods have been demon-strated. Aligned nanorods were deflected with a conductive atomic force microscopy (AFM) tip in contact mode for ef-ficiently converting nanoscale mechanical energy into elec-tric energy [19]. The harvesting mechanism from an array of aligned piezoelectric ZnO nanorods was covered by a zigzag silicon electrode [20].
In the present study, flexible piezoelectric harvesting based on array epitaxially grown ZnO nanorods with a zigzag seed layer to enhance bending and compression for large-area applications and low-cost manufacturing are in-vestigated. The effects of the growth temperature, growth time, and growth concentration on ZnO epitaxial growth were determined. The surface morphology, crystalline struc-ture, and electronic characteristics of the ZnO nanorods were analyzed. The feasibility of a flexible piezoelectric harvester for applications in flexible electronics devices is demonstrated.
2 Experiments
2.1 Fabrication
The flexible piezoelectric harvesting is based on polyimide (PI) films with double sided copper foils [21]. The PI is
DuPont Kapton, which has a density of 1.42× 103 kg/m3 at 23◦C, a glass transition temperature Tg of 360◦C, and a
volume resistivity of 1.5× 1017ohm-m. The flexible energy harvester includes two PI films, one cover layer, and bump protrusions. Two PI films were used as the top and bottom films of the flexible energy harvester. The top film contains the row electrodes, zigzag seed layer, and the bump struc-tures. The bottom film includes the column electrodes, ZnO nanorods, and a cover film, which was laminated into the PI film using hot pressing to form the post layer for support-ing the membrane of the top film. The energy harvestsupport-ing mechanism is based on zigzag seed layer and ZnO nanorods to contact together after a force is applied. The zigzag seed layer and ZnO nanorods are initially closed; the piezoelec-tric characteristics are then generated by different external forces.
The fabrication procedure is shown in Fig.1. First, the electrode patterns on the top PI film (i), with a 12-µm-thick copper were defined for row electrodes using the photolitho-graphy method and the via-holes for pass-through holes were drilled using punching method [22]. The top film was then put into a solution of CuSO4 for the electroless
plat-ing of Au, to a thickness of about 5 µm, to avoid the ox-idation of the copper foil and to enhance the zigzag seed layer growth. After the top electrode fabrication, the film was turned over. A thixotropy material was printed on the top film using a screen stencil mask to form bump struc-tures (ii). The printing header was a rubber squeegee with a durometer of 70 and a printing angle of 45◦. The bump structures were formed after the film was cured at a temper-ature of 150◦C for 45 minutes. The zigzag seed layer, with a thickness of about 80 nm, was then deposited on the elec-trodes by reactive sputtering at a pressure of 6×10−6Torr for 10 minutes (iii). During the deposition of the zigzag seed layer, the RF power, rotation rate of the holder, cooling time,
Fig. 1 Fabrication procedure of epitaxial ZnO harvesting based on flexible substrates
Flexible piezoelectric harvesting based on epitaxial growth of ZnO 711
Fig. 10 Current responses using a ultrasonic wave vibration, (a) with-out bending, and (b) under bending
with ZnO nanorods and a zigzag layer was fabricated using two PI films, one cover layer, and bump protrusions. The flexible harvester generated output current and exhibited Schottky-like current–voltage characteristic. The results of this study can be applied to many fields of flexible electron-ics. They provide useful information for designing and fab-ricating large-area flexible electronics harvesting devices.
Acknowledgements This work was supported by the National Sci-ence Council of Taiwan under Grants Nos. NSC 96-2628-E150-005-MY3 and NSC-97 2221-E150-069 96-2628-E150-005-MY3.
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