Abstract :
[en] The polarity in ZnO nanowires is an important issue since it strongly affects surface configuration and reactivity, nucleation and growth, electro-optical properties, and nanoscaleengineering device performances. However, measuring statistically the polarity of ZnO nanowire arrays grown by chemical bath deposition and elucidating its correlation with the polarity of the underneath polycrystalline ZnO seed layer grown by the sol–gel process represents a major difficulty. To address that issue, we combine resonant x-ray diffraction (XRD) at Zn K-edge using synchrotron radiation with piezoelectric force microscopy and polarity-sensitive chemical etching to statistically investigate the polarity of more than 107 nano-objects both on the macroscopic and local microscopic scales, respectively. By using high temperature annealing under an argon atmosphere, it is shown that the compact, highly c-axis oriented ZnO seed layer is more than 92% Zn-polar and that only a few small O-polar ZnO grains with an amount less than 8% are formed. Correlatively, the resulting ZnO nanowires are also found to be Zn-polar, indicating that their polarity is transferred from the c-axis oriented ZnO grains acting as nucleation sites in the seed layer. These findings pave the way for the development of new strategies to form unipolar ZnO nanowire arrays as a requirement for a number of nanoscaleengineering devices like piezoelectric nanogenerators. They also highlight the great advantage of resonant XRD as a macroscopic, non-destructive method to simultaneously and statistically measure the polarity of ZnO nanowire arrays and of the underneath ZnO seed layer.
Commentary :
The authors would like to thank SOLEIL and CRG-F committee for beamtime allocation (20140419), the BM2-D2AM staff at ESRF for his assistance during the experiment and Hervé Roussel (LMGP, Grenoble, France) for his assistance regarding the XRR measurements. This work was partly supported by the Carnot Institute Energies du Futur through the project CLAPE and by Grenoble INP via a Bonus Qualité Recherche grant through the project CELESTE. Funding from la Région Rhône-Alpes via the Research Cluster Micro– Nano and from the Nanosciences Foundation of Grenoble is also acknowledged. SG held a doctoral fellowship from la Région Rhône-Alpes. The authors would also like to thank the facilities, and the scientific and technical assistance of the CMTC characterization platform of Grenoble INP supported by the Centre of Excellence of Multifunctional Architectured Materials ‘CEMAM’ n°ANR-10-LABX-44-01 funded by the ‘Investments for the Future’ Program. RP and VC held doctoral and post-doctoral fellowships from Labex CEMAM and Nanosciences Foundation of Grenoble, respectively. D D F was supported by Nanosciences Foundation of Grenoble and the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.
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