Experimental and theoretical investigation of the effect of alkali (Li, Na and K) doping on the properties of nickel oxide thin films: Comparative study

Pure and 6% alkali-doped NiO thin films (alkali A = Li, Na, K) were prepared by a sol-gel spin coating method and deposited on glass substrates. XRD analysis showed that the prepared films belonged to a cubic structure with (111) plane as preferential growth orientation for undoped and K-doped samples and (200) for Li and Na doping. An optical study based on (UV-Visible) showed that the band gap tends to decrease with alkali doping and achieves a minimal value with Na doping. The Urbach energy increases systematically with the decrease of the optical band gap. The resistivity measurements showed that alkali doping led to a significant decrease in the resistivity value. The lowest value was achieved for the 6% Na-doped sample. Structural, optical and elastic properties of pure and 6% A-doped NiO were performed using the first principal method based on density functional theory. The optimization of the geometry of the studied samples revealed that the lattice parameters changed after doping. The band structure and density of states calculations showed that undoped and alkali doped samples exhibited an indirect band gap and the doped samples had comparatively narrower band gaps. The elastic constants Cij, Bulk modulus B, Shear modulus G, Young modulus, and Poison ratio of doped and alkali-doped NiO were further investigated.