3.3. Atomic force microscopy
Fig. 4 depicts the tapping mode AFM image of a CeO2 thin film of thickness 240 nm, annealed at temperature 500 ◦C, and from the image of Fig. 4, it is evident that CeO2 thin films exhibit large nicely separated conical columnar microstructure with the measured roughness of (area Ra = 2.38 nm) and (area RMS= 3.11 nm). When the mean free path of free charge carriers becomes comparable with the dimensions of the grains, the surface influence on the charge mobility becomes dominant [16]. Thus, the gassensing properties are based on surface reactions between the
sensing material and the gases [17]. Surface roughness is directly proportional to the gas sensitivity of the film since larger roughness results in larger contact area with the gaseous species. This is a manifestation of the importance of surface-to-volume ratio in gas-sensing applications [16]. Another feature associated with columnar microstructure, which is also of crucial importance in gas-sensing applications, is porosity. Porosity arises due to the presence of voids in the columnar microstructure. These voids provide direct access for the gas molecules to flowin fromthe environment [18]. XPS analysis (Section 3.2) provides further evidence on the porosity of the films. This observation of high porosity confirms the similar findings of XPS and optical results discussed in Sections 3.1 and 3.2.