Evaluation of the piezoelectric properties and voltage generation of flexible zinc oxide thin films

Abstract

Local piezoresponse and piezoelectric output voltage were evaluated on ZnO thin films deposited by radio-frequency magnetron sputtering on hard Si/Ti/Au and flexible Cu-coated polyimide substrates. Three different thicknesses of ZnO films were studied (285 nm, 710 nm, and 1380 nm), focusing on characteristics like crystallinity, grain size, surface roughness, and morphology. Independent of the nature of the metal layer and the substrate, our results show that thicker films presented a higher level of crystallinity and a preferential orientation along the c-axis direction, as well as a lower density of grain boundaries and larger crystal sizes. The improvement of the crystalline structure of the material directly enhances its piezoelectric properties, as confirmed by the local characterizations performed by piezoresponse force microscopy and by the evaluation of the output voltage generation under the application of a periodical mechanical deformation on the whole film. In particular, the highest value of the d33 coefficient obtained (8 pm V(-1)) and the highest generated output voltage (0.746 V) belong to the thickest films on hard and flexible substrates, respectively. These results envision the use of ZnO thin films--particularly on flexible substrates--as conformable, reliable, and efficient active materials for use in nanosensing, actuation, and piezoelectric nanogenerators.

Development of a Flexible Lead-Free Piezoelectric Transducer for Health Monitoring in the Space Environment

“…From the slope of the linear characteristic an estimation of the d 33 PE coefficient was also possible, with an average value of 12.3 pm/V that well agrees with the 12.84 pm/V theoretically determined by direct first-principles density functional calculations [64]. The direct PE effect of flexible ZnO-based structures very similar to the ones reported in this work was already investigated and reported in a previously published paper [39]. In that case a PE output voltage generation of 0.361 V was obtained for a 700 nm-thick ZnO film under the application of a periodical mechanical stimulus of 30 N. The sensitiveness of our sputtered ZnO thin films towards input mechanical forces was thus already pointed out, and further supported the investigation of such materials as PE transducers for HMS.…”

“…ZnO is of great interest thanks to the co-presence of both semiconducting and PE properties, together with the possibility of being easily synthesized in a lot of different micro/nano structures, like porous [38] and compact [39] thin films, micro/nanowires [40,41], nanorods [42], or nanobelts [43]. Among all of the aforementioned structures, the thin film one is of particular interest since many deposition techniques, like magnetron sputtering [44], pulsed-laser deposition [45], sol-gel [46], chemical vapor deposition [47], and atomic layer deposition [48] can be exploited for growing high-quality ZnO layers.…”

“…Moreover, a strict control on the deposition rate, chemical composition, stoichiometry, and morphology of the final sputtered thin films can be simply achieved by properly changing one or more deposition parameters, such as gas flows and partial pressure, substrate temperature, and/or the supplied direct current (DC)/RF signal power. Different works reported about the successful use of ZnO thin films and nanostructures as flexible PE devices [39], nanogenerators [49], anodes for Li-ion batteries [50], photoanodes for dye-sensitized solar cells [51], and for wetting controllable systems [52]. The possibility of growing ZnO micro/nanostructures also promoted the study and development of ZnO-based composite materials, which were successfully tested as PE elements for HMS applications or for other applications [40].…”

Piezoelectricity and charge trapping in ZnO and Co-doped ZnO thin films

“…This value is in good agreement with previous d 33 measurements on the same material. [32][33][34][35] On the other hand, the d 33 distribution of Zn 0.95 Co 0.05 O, in Fig. 2(i) is much wider and may be fitted by a double-peak Gaussian function centered around 13pmV -1 , with a standard deviation of 10pmV -1 , and 24pmV -1 with a standard deviation of 21pmV -1 .…”

“…[28][29][30][31] Indeed, recently several PFM investigations on ZnO nanostructures and thin films have been carried out. [32][33][34][35] Here, we present PFM experiments on both undoped and Co-doped ZnO thin films, aiming at investigating the spatial distribution of the out-of-plane piezoelectric matrix element d 33 , which measures the material displacement in the same direction as the applied out-of-plane electric field. Moreover, we present a qualitative understanding of the surface polarization by studying the phase of the piezoelectric response with respect to an AC external bias.…”