Polarity-Dependent Growth Rates of Selective Area Grown ZnO Nanorods by Chemical Bath Deposition

Abstract

Polarity is known to affect the growth and properties of ZnO single crystals and epitaxial films, but its effects are mostly unknown in ZnO nanorods. To leave polarity as the only varying parameter, ZnO nanorods are grown by chemical bath deposition under identical conditions and during the same run on O- and Zn-polar ZnO single crystals patterned by electron beam lithography with the same pattern consisting of 15 different domains. The resulting well-ordered O- and Zn-polar ZnO nanorod arrays with high structural uniformity are formed on all the domains. The comparison of their typical dimensions unambiguously reveals that Zn-polar ZnO nanorods have much higher growth rates than O-polar ZnO nanorods for all the hole diameter and period combinations. The distinct growth rates are explained in the framework of the surface reaction-/diffusive transport-limited elongation regime analysis, which yields a much larger surface reaction rate constant for Zn-polar ZnO nanorods. The origin of the difference is attributed to polarity-dependent dangling bond configurations at the top polar c-faces of ZnO nanorods, which may further be affected by polarity-dependent interactions with the ionic species in aqueous solution. These findings show the relevance of considering polarity as an important quantity in ZnO nanorods.

Formation mechanisms of ZnO nanowires on polycrystalline Au seed layers for piezoelectric applications

“…The transition between these two growth regimes is mainly dependent upon the c-plane top surface area ratio and upon the bulk solution concentration of Zn 2+ ions [55]. Cossuet et al recently determined that for values of c-plane top surface area ratios below 0.26 and 0.42 for well-ordered Zn-and O-polar ZnO NWs, respectively, and for the same bulk concentration of Zn 2+ ions of 30 mM as in the present investigation, the growth of ZnO NWs is limited by the surface reaction [45]. To determine the growth regime at work, ZnO NWs grown by CBD for a reduced duration of 50 min were formed on the 5, 20, and 100 nm thick Au seed layers.…”

“…The c-plane growth rates were calculated by performing the ratio between the mean length of ZnO NWs and their elongation time, which was estimated to be of 10 min as it takes around 40 min to reach the growth temperature of 90°C. The surface reaction-/ diffusive transport limited model shows that there exists a linear relationship between the reciprocal c-plane growth rate and c-plane top surface area ratio, which was experimentally confirmed by Cossuet et al in the case of a 30 mM bulk concentration of Zn 2+ ions [45]. The reciprocal c-plane growth rate versus c-plane top surface area ratio is plotted in figure 8 and directly compared to the model predictions reported for Zn-and O-polar ZnO NWs grown on prepatterned Zn-and O-polar ZnO single crystals, respectively.…”

“…Although the polarity of the NWs cannot be deduced in the present case due to the experimental uncertainties, our experimental data are in good agreement with the theoretical model and state that the growth of ZnO NWs is limited by the surface reaction at the c-plane top facet. Nonetheless, when considering a longer NW growth time around 3 h, the c-plane growth rate gets much lower than the model predictions, the discrepancy being attributed to a very slow nucleation process (leading to a non-constant density over the elongation time) and to dynamical processes in aqueous solution that should be taken into account when the spontaneous growth is employed (in contrast to the SAG in [45]).…”

“…Reciprocal c-plane growth rate versus c-plane top surface area ratio of ZnO NWs grown by CBD for 50 min on 5, 20, and 100 nm thick Au seed layers. The dark solid lines are the expected fit for Zn and O-polar ZnO NWs, respectively, according to [45]. The bulk solution concentration of Zn 2+ ions is 30 mM.…”

Effects of Polyethylenimine and Its Molecular Weight on the Chemical Bath Deposition of ZnO Nanowires

“…9 In the present configuration with a large c -plane surface over the total surface ratio, the elongation of ZnO NWs is limited by the mass transport of the reactants and not by the surface reaction rate. 43,44 The diffusive transport of Zn(II) ions acting as the limiting reactant in aqueous solution may somehow be hampered by the protonated PEI through repulsive electrostatic interactions, and its magnitude may depend on the PEI concentration and M w .…”