Xiao Juanxiu, Ong Wei Li, Guo Ziming, Ho Ghim Wei, Zeng Kaiyang
†Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore, 117576.
‡Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore, 117576.
ACS Appl Mater Interfaces. 2015 Jun 3;7(21):11412-22. doi: 10.1021/acsami.5b01988. Epub 2015 May 18.
This paper reports the localized electrical, polarization reversal, and piezoelectric properties of the individual hexagonal ZnO nanorods, which are grown via the hydrothermal method and textured with [0001] orientation. The studies are conducted with conductive atomic force microscopy (c-AFM) and piezoresponse force microscopy (PFM) techniques. The correlation between the resistance switching and polarization reversal is discussed. The c-AFM results show that there is less variation on the set or reset voltage in nanorod samples, compared to that of the ZnO thin film. With increasing aspect ratio of the nanorods, both set and reset voltages are decreased. The nanorods with low aspect ratio show unipolar resistance switching, whereas both unipolar and bipolar resistance switching are observed when the aspect ratio is larger than 0.26. The PFM results further show the ferroelectric-like property in the nanorods. Comparing with that of the ZnO thin film, the enhanced piezoresponse in the nanorods can be attributed to the size effect. In addition, the piezoresponse force spectroscopy (PFS) experiments are conducted in ambient air, synthetic air, and argon gas. It shows that the depolarization field in the nanorod may be due to the moisture in the environment; moreover, the increased piezoresponse may relate to the absence of oxygen in the environment. It is also shown that the piezoelectric responses increase nonlinearly with the aspect ratio of the nanorods. By comparing the piezoresponse hysteresis loops obtained from the nanorod samples of as-grown, air-annealed and vacuum-annealed, it is found that the oxygen vacancies are the origin of the polarization reversal in ZnO nanorods. Finally, the tradeoff between the electrical and ferroelectric-like properties is also observed.
本文报道了通过水热法生长并具有[0001]取向织构的单个六方ZnO纳米棒的局部电学、极化反转和压电特性。研究采用导电原子力显微镜（c-AFM）和压电力显微镜（PFM）技术进行。讨论了电阻开关与极化反转之间的相关性。c-AFM结果表明，与ZnO薄膜相比，纳米棒样品的设置或重置电压变化较小。随着纳米棒长径比的增加，设置和重置电压均降低。低长径比的纳米棒表现出单极电阻开关，而当长径比大于0.26时，则观察到单极和双极电阻开关。PFM结果进一步表明纳米棒具有类铁电特性。与ZnO薄膜相比，纳米棒中增强的压电响应可归因于尺寸效应。此外，在环境空气、合成空气和氩气中进行了压电力谱（PFS）实验。结果表明，纳米棒中的去极化场可能是由于环境中的水分；此外，增强的压电响应可能与环境中缺氧有关。还表明，压电响应随纳米棒的长径比非线性增加。通过比较生长态、空气退火和真空退火的纳米棒样品获得的压电响应滞后回线，发现氧空位是ZnO纳米棒中极化反转的起源。最后，还观察到了电学和类铁电特性之间的权衡。
