Low-temperature Hydrothermally Grown 100 μm Vertically Well-aligned Ultralong and Ultradense ZnO Nanorod Arrays with Improved PL Property

The hydrothermal synthesis of ZnO nanorods (NRs) has been investigated using ammonium hydroxide and polyethyleneimine as additives to the conventional nitrate based synthesis route, to obtain thin-films of well-aligned, ultradense and ultralong nanostructures. ZnO NRs longer than 60 μm were obtained in a one-cycle growth run and rod lengths ~ 100 μm by a two-cycle growth. The lengths of the rods were distributed uniformly across the substrate in all samples and highly dense NR arrays were observed. These conditions were obtained by a careful review of the nucleation and growth kinetics for this material system, such that the supersaturation of the solution was only relieved by precipitation on and in the presence of crystalline ZnO, and by the exploitation of a second growth phase due to the chelating behave of PEI and the products of HMTA. Also, the growth behavior was correlated to the solution pH values. The structural and optical data were found to be supportive of the growth conditions. The photoluminescence (PL) spectra from as-grown ultralong ZnO NRs exhibited a strong broad (580-625 nm) visible emission peak. However, annealing in a forming gas atmosphere at 623K (350°C) revealed a PL spectrum with a significantly decreased visible emission and an increased near band gap UV emission at 379 nm. Thus, the mechanisms associated with ammonium hydroxide and PEI addition provide a simple route for synthesizing ultralong and dense arrays of ZnO NRs at low temperature i.e. 368K (95°C).