# Facile Synthesis of Highly Conductive Vanadium-Doped NiO Film for Transparent Conductive Oxide
## 3 Results
### Study of Film Properties  
The morphology of the surface of thin films of pristine NiO and 3 at.%V:NiO was studied using atomic force microscopy. Figure 6a-d shows AFM topography and three-dimensional (3D) images of NiO and 3 at.%V:NiO on FTO substrates. A clear observation of particle size reduction was also confirmed by the AFM topographic images of 3 at.%V:NiO film. Both the pristine and 3 at.%V:NiO films showed a closely packed uniform morphology and exhibited root mean square (RMS) surface roughness values of 2.94 nm and 2.49 nm, respectively. This reduction in surface roughness in the film after doping indicates that a small amount of vanadium in the NiO lattice will be beneficial for the various transparent conductive oxide applications [1; 17; 20; 22].  
Figure 5: (**a–c**) XPS spectra of NiO: (**a**) survey spectra (inset: high-resolution spectra of V 2p), high-resolution (**b**) Ni 2p\({}_{3/2}\) and (**c**) O 1s; (**d–f**) XPS spectra of 3 at.%V:NiO: (**a**) survey spectra (inset: high-resolution spectra of V 2p), high-resolution (**b**) Ni 2p\({}_{3/2}\) and (**c**) O 1s.  
We also investigated the influence of V doping on the electrical conductivity of the NiO thin films by a photovoltaic measurement system. For that, 20 mg/mL NiO or 3 at.%V:NiO dispersed in DI water was spin-coated onto a precleaned FTO substrate (16 \(\Omega\)). Then, the substrates were annealed at 60 \({}^{\circ}\)C for 10 min to remove residual water. The electrodes were then transferred into a thermal evaporator (JVAC, South Korea) for the deposition of silver. The silver electrode (active area 0.2 cm\({}^{2}\)) was thermally evaporated with a thickness of 100 nm at a deposition rate of 0.4 nm per second through a shadow mask. The current-voltage (I-V) curves of the fabricated FTO/NiO and 3 at.%V:NiO/Ag films were measured by a source meter. The following equation used to calculate the conductivity of the films:  
\[\sigma=\frac{d}{A\times R}\]  
where \(\sigma\) is the conductivity, d is the thickness of the films, A is the active area, and R is the resistance obtained from the I-V curve. As shown in Figure 7, replacing the pristine NiO film with a 3 at.%V:NiO film significantly increased the vertical current, indicating the enhancement of electrical conductivity upon V doping. The conductivity values of the pristine NiO and 3 at.%V:NiO films were estimated to be \(2.9\times 10^{-6}\) Scm\({}^{-1}\) and \(3.9\times 10^{-6}\) Scm\({}^{-1}\), respectively. The conductivity of 3 at.%V:NiO film, compared to that of pristine NiO film, was enhanced by about 34.4% due to the enrichment of Ni\({}^{3+}\) ions in the 3 at.%V:NiO sample [25]. This point was extensively discussed in the XPS analysis. The previously reported values are within the range for NiO films depending on the film composition and deposition condition [32]. A recent work by [PERSON] et al. observed the same trend of an increase in conductivity for nitrogen-doped NiO film (\(2.8\times 10^{-7}\) Scm\({}^{-1}\)) compared to pristine NiO film (\(1.39\times 10^{-7}\) Scm\({}^{-1}\)) [26]. Our results demonstrate that V doping can be a powerful way to control the electrical conductivity of NiO.  
Figure 6: AFM topography and 3D AFM images of (**a**,**b**) NiO NPs and (**c**,**d**) 3 at.%V:NiO.