Strain effects in ZnO thin films and nanoparticles

We grew Stranski-Krastanow-type ZnO thin film and Volmer-Weber-type self-assembled ZnO nanocrystals using magnetron sputtering methods. The evolution of surface roughness and strain effects in thin ZnO films on Al2O3(0001) substrate and ZnO nanocrystals on Pt(111) surface studied by synchrotron x-ray scattering. The well-aligned two-dimensional (2D) planar layer dominated in layer-by-layer growth at the highly strained initial growth stage in the thin films. As the film thickness increased, the discrete nucleations on the 2D planar layer continuously grew until the ZnO film reached the strain relaxed steady-state regime. The accumulated strain energy in the thin film grown at low temperature slowly relaxed while the strain energy in the high temperature system rapidly relaxed. When the three-dimensional islands on the 2D surface of thin ZnO film grown at the low and high temperatures were quickly developed by strain relaxation, the critical exponent beta were roughly 0.693 and 1.579, respectively. The thickness-dependent strain of Volmer-Weber-type ZnO nanocrystals relaxed slowly, compared to that of Stranski-Krastanow-type thin film.