Low Temperature Growth of Horizontal ZnO Nanorods
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Low Temperature Growth of Horizontal ZnO Nanorods Abdiel Rivera, Anas Mazady and Mehdi Anwar Electrical and Computer Engineering, University of Connecticut, Storrs CT, 06269. E-mail: [email protected] ABSTRACT We report the growth of ZnO horizontal (NRs) on p-Si substrate at low temperature without any assisting mechanism. The NRs were grown at 90°C on a ZnO film previously deposited using metal-organic chemical vapor deposition. The horizontal nanowires have diameters in the range of 200 – 500nm and lengths between 1 – 7 µm, depending upon the duration of the growth and the ratio of the precursors. The density of the NRs was controlled by varying the concentration of zinc nitrate (Zn(NO3)2) while keeping hexamethylenetetramine (HMTA) constant. Density of horizontal NRs increased with lower zinc nitrate concentration (from 11.35 to 3.29 mMol) for a growth duration of 18hrs. Increased zinc nitrate concentration of 3.29mMol resulted in an asymmetric growth along the vertical axis due to oxygen termination giving rise to slower growth rates. INTRODUCTION ZnO can be grown via hydrothermal synthesis in a variety of nanostructures by controlling the stirring time [1], pH of the solution [2], type of reactant [3], and concentration of the reactants [4]. In the particular case of NWs/NRs, a temperature range of 70-95°C is usually reported, varying the time, the seed layer thickness [5] and Zn+2 /OH- ratio to control orientation, density and dimensions of the NWs/NRs. Y. Sun. et al [6] noticed that the ratio of Zn+2 /OH- ions in the reaction and growth duration determine whether the NWs/NRs would be grown at an angle with respect to the substrate or in parallel. Within the first two hours of the growth, Zn2+ and OHions decreased substantially. After two hours, the concentration of OH- ions decreased slower than Zn2+ ions favoring the continuous growth of parallel NWs/NRs along (0002). Most reported horizontal NWs/NRs are grown by confining the growth along the surface using patterns, dislocations and catalyst. Commonly, the side walls of patterned ZnO films are used as nucleation sites for the growth of NWs/NRs. For example, Y. Qin et al. reported the growth of ZnO films by patterning strip lines with a Cr layer on top to inhibit the nucleation of the nanowires [7]. The seed layer was grown using magnetron sputtering, patterned using optical lithography and nanowires were grown using zinc nitrate and hexamethylenetetramine (HMTA). Similarly, Kim et al. [8] reported growth of NWs parallel to glass substrates that were seeded from the side walls of a patterned ZnO layer. The seed layer was grown at different angles (0 60°, 80°) using RF sputtering, resulting in different NWs diameter possibly due to the nucleation size. The low temperature growth of horizontal ZnO NRs on p-Si substrate was achieved by tailoring the process chemistry, i.e. the zinc nitrate/HMTA ratio. A ratio of 11.35mMol:25mMol is at the transition, and any increase in the ratio leads to dominantly vertical NRs.
EXPERIMENT ZnO horizontal nanorods were gr
