Facile, wafer-scale compatible growth of ZnO nanowires chemical bath deposition: assessment of zinc ion contribution and other limiting factors.

ZnO is a highly promising, multifunctional nanomaterial having various versatile applications in the fields of sensors, optoelectronics, photovoltaics, photocatalysts and water purification. However, the real challenge lies in producing large scale, well-aligned, highly reproducible ZnO nanowires (NWs) using low cost techniques. This large-scale production of ZnO NWs has stunted the development and practical usage of these NWs in fast rising fields such as photocatalysis or in photovoltaic applications. The present article shows an effective, simple approach for the uniform, aligned growth of ZnO NWs on entire silicon wafers (sizes 3 or 4 inches), using a low-temperature Chemical Bath Deposition (CBD) technique. In addition to this, a systematic study of the substrate size dependent growth of NWs has been conducted to better understand the effect of the limitation in the deposition rate of Zn ions on the growth of NWs. The growth rate of ZnO NWs is seen to have a strong relationship with the substrate size. Also, the loading efficiency of the Zn ions is higher in ZnO NWs grown on a 3-inch silicon wafer in comparison to those grown on a small piece. An in-depth time dependent growth study conducted on entire 3-inch wafers to track the morphological evolution (length, diameter and number of the NWs) reveals that the growth rate of the length of the NWs reaches a saturation state in a short time span of 20 min. Assessment of the overall homogeneity of the NWs grown on the 3-inch wafer and simultaneous growth on two entire 4-inch silicon wafers has also been demonstrated in this article. This demonstration of large-scale, well-aligned controllable, aligned growth of ZnO NWs on entire silicon wafers is a first step towards NW based devices especially for applications such as photovoltaic, water purification, photocatalysis or biomedical applications.