Among semiconductor nanomaterials, ZnO has been one of the most interesting systems due to their extraordinary properties and remarkable multifunction capability. Zinc oxide (ZnO) is a II-VI compound semiconductor with wide band gap (Eg = 3.37 eV) and large exciton binding energy (Eb = 60 meV) at room temperature. The development of ZnO can be traced since 1912. The present renaissance of ZnO research started in the mid 1990s. Due to the unique properties, ZnO nanostructures, particularly NWs, are potential candidates for applications in solar cells, gas sensors, light emitting diodes, ultraviolet lasers and field effect transistors (FETs), etc..
Various vapor route approaches such as pulse laser deposition, metal-organic chemical vapour deposition and atomic layer deposition, have been used for synthesizing ZnO NWs. Amongst them, CVD is the most popular approach uses by researchers to produce ZnO NWs. Two requirements are needed to produce vertically aligned ZnO NWs. Firstly, substrates/epilayers with small lattice mismatch to ZnO (heteroepitaxial growth) or highly c-oriented ZnO seed layers (homoepitaxial growth/catalyst-free growth) are necessary for facilitating the growth of aligned NWs. Secondly, a moderate synthesis condition is required to maintain the epitaxial relationship between the substrate and the ZnO NWs during the CVD process so as to achieve aligned growth of NWs.
Figure 1 shows the catalyst-free growth of ZnO NWs on highly c-oriented ZnO seed layer using CVD.
Various vapor route approaches such as pulse laser deposition, metal-organic chemical vapour deposition and atomic layer deposition, have been used for synthesizing ZnO NWs. Amongst them, CVD is the most popular approach uses by researchers to produce ZnO NWs. Two requirements are needed to produce vertically aligned ZnO NWs. Firstly, substrates/epilayers with small lattice mismatch to ZnO (heteroepitaxial growth) or highly c-oriented ZnO seed layers (homoepitaxial growth/catalyst-free growth) are necessary for facilitating the growth of aligned NWs. Secondly, a moderate synthesis condition is required to maintain the epitaxial relationship between the substrate and the ZnO NWs during the CVD process so as to achieve aligned growth of NWs.
Figure 1 shows the catalyst-free growth of ZnO NWs on highly c-oriented ZnO seed layer using CVD.
Fig. 1. (a) Vertically aligned ZnO NWs grown on highly c-oriented ZnO seed layer, (b) Top view of ZnO NWs shows that the tip of NWs are hexagonal shape.
[Reference: Pung, S. Y.;Choy, K. L.;Hou, X. et al.; Nanotechnology 2008, 19, 435609.]
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