Homoepitaxial regrowth habits of ZnO nanowire arrays
© Liu et al; licensee Springer. 2011
Received: 29 September 2011
Accepted: 7 December 2011
Published: 7 December 2011
Synthetic regrowth of ZnO nanowires [NWs] under a similar chemical vapor transport and condensation [CVTC] process can produce abundant ZnO nanostructures which are not possible by a single CVTC step. In this work, we report three different regrowth modes of ZnO NWs: axial growth, radial growth, and both directions. The different growth modes seem to be determined by the properties of initial ZnO NW templates. By varying the growth parameters in the first-step CVTC process, ZnO nanostructures (e.g., nanoantenna) with drastically different morphologies can be obtained with distinct photoluminescence properties. The results have implications in guiding the rational synthesis of various ZnO NW heterostructures.
One-dimensional ZnO nanowires [NWs] have attracted increasing interests for their potential applications in various functional devices, including solar cells [1, 2], nanosensors , nanogenerators , and nano-optoelectronics . Compared with other semiconductor NWs, ZnO has a large band gap (3.37 eV) and high exciton binding energies (60 meV) and thus can be used as ultraviolet laser or light-emitting diodes. In addition, ZnO nanostructures also exhibit the richest morphologies reported so far (e.g., nanocomb, nanoring, nanobridge, nanonail, nanobelt) [2, 6–9], making it coveted as building blocks for functional devices. Nonetheless, the growth of ZnO NWs is sensitive to the fabrication conditions and is often difficult to control, i.e., slight changes of the growing parameters may result in drastic differences in morphologies and subsequent semiconducting or optical properties . Although tremendous efforts have been made to understand various growth mechanisms, new morphologies such as NW forests , microtowers , or micropyramids  keep on emerging through regrowth processes, which continue to challenge growth theories and require systematic investigations. Dissimilar to the conventional catalyst-mediated vapor-liquor-solid process, the homoepitaxial growth of ZnO nanostructures on pre-existing ZnO NWs may follow a vapor-solid mechanism since no metal catalysts are involved. It is expected that ZnO NWs with more abundant morphologies can be fabricated using regrowth techniques.
In this work, we investigate the regrowth habits of ZnO NWs using a chemical vapor transport and condensation process [CVTC] . We find that the epitaxial regrowth habits are not only determined by the growth conditions in the second-step CVTC process, but also greatly influenced by the initial morphologies and properties of the ZnO NW templates. When the growth conditions in the first-step CVTC process are changed, the epitaxial regrowth of NWs can either follow axial, radial, or both directions. We demonstrate the successful fabrication of ZnO 'nanoantenna' structures through homoepitaxial regrowth control mechanisms . Our findings offer the new possibility of growing novel nanostructures and have implications in guiding the fabrication of various ZnO NW heterostructures, such as axial p-n junctions and/or core-shell (multi-shell) structures.
ZnO nanostructures were grown using a two-step CVTC method in a tube furnace (Lindberg blue, Thermo Scientific, Waltham, MA, USA). During the first-step CVTC process, a-plane sapphire substrate was coated with a 1.5-nm-thick Au catalyst. Equal amounts of ZnO powders (99.99%; Alfa Aesar, Ward Hill, MA, USA) and graphite powders (99.9%; Alfa Aesar, Ward Hill, MA, USA) were loaded into an alumina boat to provide Zn vapors. Argon flow with a rate of 12 sccm was used as the carrier gas. The growth temperature was set at 910°C with a ramping rate of 50°C/min. The initial growth times of samples A, B, and C were 10 min, 10 min, and 30 min, respectively. Before growth, the Au catalyst film of sample B was exposed to a focused ion beam [FIB] with a Ga+ ion source (30 KeV, 0.19 nA, 900 ms). After the first CVTC process, ZnO NWs were taken out and used as the templates for the second-time CVTC growth. The second-time growth parameters are nearly identical to those in the first step, except that the growth time of all the samples was set to 10 min. The Third- and fourth-time growths of sample A follow the same growth conditions as those used in the second-time growth.
The morphologies of all samples were characterized by scanning electron microscopy [SEM] (FEI Quanta FEG, FEI Co., Hillsboro, OR, USA). Room temperature photoluminescence [PL] spectra were collected using Flurolog-3-p type spectrometer in the air. The crystal structures and orientations were analyzed using X-ray diffractometry [XRD] (D8 ADVANCE, Bruker AXS GmbH, Karlsruhe, Germany) and transmission electron microscopy [TEM] (FEI Tecnai G2, FEI Co., Hillsboro, OR, USA).
Results and discussions
Bending, bundling, or coiling of ZnO NW arrays are commonly reported in the literature. Electrostatic interactions due to the polar <0001> surface with different termination atoms were considered as the main cause of bending and coiling . This mechanism, however, may have difficulties in understanding why most NW bundles observed here are touched from side surfaces (i.e., ±(1-210) surface). We speculate, in Figure 2e, that while the NWs grow longer, some of them tend to incline due to the geometric instability. Since ZnO NWs have piezoelectric properties, this causes the side surfaces to be either positively or negatively charged. The opposite charges exist depending on the stretching or compression of the NWs, which could lead to the attraction of NW tops because of the large electrostatic force.
The detailed mechanism why ZnO nanowires have preferential regrowth along the radial direction remains unclear. Ga+ ion implantation in the Au film during the FIB processing might be one of the important factors. Recent experiments in the hydrothermal growth of ZnO NWs suggest that positively charged Ga generally tends to segregate along the side surface of the ZnO NWs , which may catalyze the radial growth of the ZnO NWs since Ga-Zn metal has a very low eutectic point.
We notice that the base lengths of nanoantennas are shorter than those of the original NW templates, suggesting that in addition to simultaneously axial and radial growth, diffusion-assisted NW self-assembling has occurred. Our static annealing experiments of NWs grown at 30 min have indeed revealed the shortening and widening of the NWs. This offers a plausible explanation of the shorter bases we see in nanoantennas. During the forming process of the bases, the tops of relatively longer NWs remain unmelted and protrude out of the bases, which act as the nucleation centers and promote the growth of nanoantenna tips. The asymmetric positioning of the tips relevant to their bases suggests that the mechanism of the nanoantenna is likely to be distinguished from the formation mechanism of microtowers reported in the literature .
We demonstrate that the regrowth of ZnO NWs by a two-step CVTC process can lead to NWs with different morphologies and properties. By varying the growth parameters in the first-step CVTC process, ZnO NWs can preferentially grow along the axial, radial or both directions. Unlike Si or Ge NWs where axial or core-shell heterostructures can be fabricated by varying different gaseous precursors, most ZnO NWs are grown by evaporating solid ZnO powder which makes it difficult to switch the precursor. Our two-step CVTC processes suggest a simple yet effective method to fabricate axial or core-shell ZnO NW heterostructrues, such as p/n ZnO junctions. Moreover, ZnO nanoantennas first reported here can be readily obtained by our two-step CVTC process and are good candidates for field emitters and/or nanosensors.
We acknowledge the financial support of the National Natural Science Foundation of China (51072119), Shanghai Nano (1052nm03000), Shanghai Rising Star Program (09QA1404100), Innovation Program of Shanghai Municipal Education Commission (12ZZ139), and STCSM (10231201103).
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