Nanostructured surface

Surface morphology and optical properties of ZnMeO ( Me: Co, Ni ) thin films prepared by RF - sputtering
Stolyarchuk I.D., Dan’kiv O.O., Hadzaman I.V., Stolyarchuk A.I., Krypak A.O., Kuzyk O.V.,
Department of Physics, Drohobych Ivan Franko State Pedagogical University, 24 I.Franko str., 82100 Drohobych, Ukraine. 
E-mail: i.stolyarchuk@dspu.edu.ua

Zinc oxide (ZnO) based materials demonstrate attractive properties for different modern technological applications. ZnO has a wide band gap and a high exciton binding energy (Eg = 3.37 eV; Eb = 60 meV). For this reason ZnO is good candidate for optoelectronic applications and, also, for detection of harmful and toxic gases. Many recent publications have been dedicated to transition metal (Mn, Co, Ni) doped ZnO from point of view searching of room temperature ferromagnetic ordering due to magnetic ion doping. There is also suggestion that doping ZnO with transition metals could be promise way to enhance the gas sensor sensitivity.
In this work, we report on the fabrication of undoped ZnO and ZnMeO (Me: Mn, Co, Ni) thin films by RF-plasma sputtering technique. Content of transition metals in the deposited films has varied in range of 0 – 10 %. X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM) were main techniques for structural and morphological analysis of the fabricated samples. According to HR-TEM and AFM analysis the ZnCoO thin films are composed of closely packed nanocrystallites with nanorod shape, whereas the ZnNiO thin films show uniform columnar microstructure. The shift of the absorption edge due to decrease the energy band gap Eg with increasing cobalt content and complex dependence of the energy band gap on content of nickel was observed in optical absorption spectra of the studied films. 
The revealed morphological features and related optical properties are very important for applications in gas sensors since surface roughness is directly proportional to the gas sensitivity due to the increased contact area with gaseous specious. As for example, the carbon monoxide and ethanol gas sensing properties (response and recovery times) of the ZnMeO (Me: Co, Ni) thin films were investigated and the obtained results and sensing mechanisms are discussed.