Microwave-Assisted Nonaqueous Sol-Gel Synthesis: From Al:ZnO Nanoparticles to Transparent Conducting Films

Al:ZnO nanopartides with different doping levels were synthesized by a microwave-assisted nonaqueous sol gel route in benzyl alcohol and subsequently processed into transparent conducting films. The crystal structure, crystal size and shape, and doping level of the nanoparticles were analyzed in detail by Rietveld refinement from powder X-ray diffraction (PXRD) data and transmission electron microscopy (TEM). Study of the thermal stability gave evidence that the material did not phase-separate up to 600 degrees C. Films were prepared on fused silica substrates by dip-coating from Al:ZnO nanoparticle dispersions, followed by a microwave-assisted densification step. According to scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements, the films were homogeneous over large areas with a root-mean-square (Rms) roughness of about 10 nm. A minimum resistivity of 2.35 x 10(-2) Omega.cm was achieved for a 357-nm-thick Al:ZnO film with an initial Al-to-Zn mol ratio of 1:9 after postannealing under N-2. The changes of the electrical properties of the films could be well-explained on the basis of varying initial doping levels and crystal sizes of the nanoparticle building blocks. The average transmittance of the films in the visible light range was higher than 90%, and especially for green light it reached up to 95%.