Highly Uniform Epitaxial ZnO Nanorod Arrays for Nanopiezotronics
© to the authors 2009
Received: 14 January 2009
Accepted: 24 March 2009
Published: 7 April 2009
Highly uniform and c-axis-aligned ZnO nanorod arrays were fabricated in predefined patterns by a low temperature homoepitaxial aqueous chemical method. The nucleation seed patterns were realized in polymer and in metal thin films, resulting in, all-ZnO and bottom-contacted structures, respectively. Both of them show excellent geometrical uniformity: the cross-sectional uniformity according to the scanning electron micrographs across the array is lower than 2%. The diameter of the hexagonal prism-shaped nanorods can be set in the range of 90–170 nm while their typical length achievable is 0.5–2.3 μm. The effect of the surface polarity was also examined, however, no significant difference was found between the arrays grown on Zn-terminated and on O-terminated face of the ZnO single crystal. The transmission electron microscopy observation revealed the single crystalline nature of the nanorods. The current–voltage characteristics taken on an individual nanorod contacted by a Au-coated atomic force microscope tip reflected Schottky-type behavior. The geometrical uniformity, the designable pattern, and the electrical properties make the presented nanorod arrays ideal candidates to be used in ZnO-based DC nanogenerator and in next-generation integrated piezoelectric nano-electromechanical systems (NEMS).
Vertically aligned ZnO nanorods (NRs) and nanowires (NWs) are attracting much interest for several applications such as nanophotonics [1, 2], dye-sensitized solar cells [3, 4], electron field emitters [5, 6], surround-gate field effect transistors , and nanopiezotronics . A number of preparation methods by high temperature vapor transport  and low temperature chemical synthesis [10, 11] were developed. For comparison, the NR arrays can be classified from several aspects: physical and geometrical properties of the individual building blocks and their uniformity in length, in diameter, and in axis-to-substrate angle. The NRs/NWs can be distributed either randomly or in a well-defined way. The above applications require different kinds of nanostructures concerning their geometrical parameters. For instance, photonic crystals with well-defined defects are of importance in nanophotonics [12, 13]. Another demanding application is the construction of ZnO NW-based DC current generator, where the NWs convert the mechanical energy of a vibrating Pt-coated, zig-zag-shaped electrode to electric energy by exploiting the piezoelectric nature of ZnO . Even for nanosensors, however, the generated power density (~80 nW/cm2) should be significantly increased. As Liu et al.  have pointed out the output voltage of the system, being now typically in the order of ~10 mV, can be drastically improved by increasing the number of the active NW-s, i.e., the ones which are in continuously contact with the zigzag top electrode. Therefore, two approaches were proposed: improving the uniformity of the NWs on one hand and patterning the array according to the dimension and shape of the top electrode. Vertical ZnO nanoarrays arranged in a designed pattern were recently produced by a few groups using different techniques [16, 17], however, either the geometrical non-uniformity of the NWs or the low density of the vertical microcrystals (~1 NR/μm2) makes their use in nanogenerator application difficult. Moreover, if the nanostructure is produced by vapor–liquid–solid (VLS) method the metal catalyst droplet on the top of the NW can hinder the formation of the required Schottky contact at the top electrode/NW interface.
Here, we demonstrate alternative fabrication routes which fulfill all the above crucial requirements by providing highly uniform, crystallographically oriented NRs in the 100-nm diameter range, in predefined, dense patterns. Our method benefits of the catalyst free, low temperature epitaxial growth, and the direct writing nanolithography. We have tried two options for the formation of NR arrays. In the first, the desired nucleation pattern was drawn in a polymethyl-methacrylate (PMMA) layer, which was subsequently removed resulting in an all-ZnO structure. In the second route, the nucleation pattern was realized in a hard metal coating; therefore, the fabricated NRs were electrically contacted at the anchoring surface.
All ZnO NR Array
Anchored NR Array
Summary of the growth parameters and the obtained nanorod dimensions
Surface polarity (Zn/O)
Hole diameter (nm)
Inter-rod distance (nm)
Nanorod density (NR/μm2)
Growth concentration (mM)
Growth time (min)
Feret’s diameter (nm)
The obtained nanostructures were visualized by a Hitachi S4800 field emission scanning electron microscope (FESEM). Transmission electron microscope (TEM) images were obtained by a 200 kV JEOL JEM-2010 instrument. The electrical characterization of the individual NWs was carried out in air by conductive AFM technique by means of a SII NanoTechnology Inc., SPA-400 instrument equipped with Keithley 4200-SCS semiconductor parametric analyzer. The spring constant and resonant frequency of the used Au-coated cantilever is 1.4 N/m and 26 kHz, respectively.
Results and Discussion
Similar phenomenon was described by other groups, as well, albeit they used high temperature vapor transport methods. Wang et al.  explained the self-attraction by the accumulated Coulomb charges at the NR/Au catalyst droplet interface when charged by the primary electrons during SEM observation. Han et al.  have also observed self-attracted NWs prepared by catalyst-free vapor–solid (VS) preparation method. Therefore, the charging cannot be ascribed to the presence of catalysts.
In our case, the NR tip attachment can be attributed to surface tension of water during the drying process, as it was described by Segawa et al.  for hybrid organic–inorganic NR. We believe that further down-scaling is limited mainly by the resolution of our e-beam lithography facility rather than by growth kinetics.
In order to correctly describe the electrical behavior by an equivalent circuit and to separate the contributions of contact resistance, internal resistance of the NR, surface conductance, and piezoelectricity induced Schottky barrier height change, a refinement of the measurement technique and further systematic investigation is required. Still, in our work the successful formation of a rectifying Schottky contact between ZnO NR and the measuring tip could reproducibly be obtained. This was pointed out by Liu et al.  to be a necessary requirement for the operation of the DC nanogenerator with vibrating top contact.
We have demonstrated that by using homoepitaxial chemical growth method highly uniform, single crystalline NR arrays arranged in a predefined pattern can be prepared. By changing the growth parameters, diameter and length of the NRs can be tuned in the range of 90–170 nm and 500 nm–2.3 μm, respectively. The monodispersity of the diameter of single crystalline NRs can be <2% by maintaining an excellent uniformity in the longitudinal dimension. We exploited two alternative synthesis routes using soft and hard under-layer to obtain all-ZnO and metal contacted, anchored NR arrays, respectively. The former one can be a promising candidate for nanopillar-based photonic crystals, especially if a refractive index contrast between the NR and the ZnO substrate is realized. On the other hand, anchored NR arrays contacted on the bottom are promising structures for nanopiezotronics. The arrays show excellent uniformity in length and the dense pattern (~30 NR/μm2) can be adjusted to the top vibrating electrode of the nanogenerator. Thereby a significant improvement in the output voltage, hence a more efficient energy harvesting can be predicted.
This work was supported by the “Nanotechnology Network Project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan, and by the Hungarian Fundamental Research Found (OTKA) under contract PD 77578. The authors are grateful to Prof. Y. Bando for the valuable suggestions and to Mr. Y. Misawa, Mr. S. Hara, Mr. K. Tamura, and Mr. A. Ohi for professional help with sample preparation.
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