Characteristics of ZnO thin films prepared by acidic sol method

Zinc oxide (ZnO) thin films are grown using an acidic sol by dip coating. The acidic sol is prepared from a methanolic solution of zinc chloride using formic acid (HCOOH) as the stabilizer and poly(vinyl acetate) (PVAc) as the tackifier. Crystalline ZnO thin films are obtained following annealing at 480 degrees C for 1 h. The effects of dip-coating conditions on the microstructure and optical properties of the ZnO thin films are investigated using a variety of analytical tools. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) show that line ZnO crystallites comprise the bottom layer of the films, and large crystallites emerge on the surface of the fine crystallite layer. Selected area diffraction (SAD) patterns indicate that both the fine and large crystallites exhibit a hexagonal wurtzite crystal structure. The particle size distribution of the bottom fine crystallites is below 20 nm, while that of the large crystallites varies from 500 to 800 nm, depending on the number of dip-coating times. The ZnO films have an absorption edge that starts at approximately 370 nm and cuts off at 290 nm. As calculated from the absorption edge, the ZnO films have optical gaps of about 4.02-4.11 eV.