Influence of Co concentration on properties of NiO film by sparking under uniform magnetic field

Nickel oxide (NiO) films cover numerous electronic applications, including transparent conducting oxides and hole transport layer, because of its high transparency and wide band gap. A sparking discharge is a new and unique method for the deposition of NiO films due to non-complex operation and non-requirement of a vacuum atmosphere. Unfortunately, NiO films by the sparking method display a porous surface with inferior crystallinity. By assisting a uniform magnetic field in the sparking method, the porous and the crystallinity of NiO are improved. However, electrical properties of the NiO films deposited by this strategy are poor. In order to improve the electrical properties of NiO, a substitutional of Ni ions by Co ions is considered. In this study, we report an influence of Co concentration on properties of NiO films by sparking under a uniform magnetic field. Our results indicate that an increase in Co concentration to 0.1 M improves the crystallinity and increases a carrier concentration of NiO, resulting in a reduction of the resistivity. This consequence is in agreement with the increase in a number of higher-valence Ni3+ because of the Co2+ substituted Ni2+. Based on our research, Co-NiO film is promising materials for a transparent conductor.

To investigate the electrical properties of Co-NiO films, Hall effect along with a van der Pauw method was used.

Results and discussion
Morphology and crystallinity of Co-NiO films. Figure 1a-e indicate the SEM images of Co-NiO films at various concentrations. The formations of an irregular shape and a non-uniform size of submicron particles at the surface of NiO films are observed in Fig. 1b-e. Especially in Fig. 1b, submicro-rod structures are seen. However, the higher concentration of Co demonstrates that the submicron particles become smaller and more uniform. In Fig. 1f according to JCPDS No. 47-1049, XRD patterns of samples display a sharp peak at 43.32°, which is attributed to the (200) plane with the d-spacing of 0.209 nm and indexed to cubic symmetry with a space group of Fm-3m. By increasing the concentration of Co higher than 0.1 M, the intensity of peaks is sig- www.nature.com/scientificreports/ nificantly decreased. This result can be interpreted that the bigger Co ions substitute Ni ions in the NiO lattice, causing the formation of defects in the crystal structure. In order to further study the crystallinity and defect of films, we use the following equation 19 : where D is the crystallite size, is the wavelength of X-ray (1.5418 Å), β is the full width at half maximum (FWHM) of the diffraction peak, θ is Bragg angle of the diffraction peak and δ is the dislocation density. By deriving from (1) As a result, the reduction of the T avg is related to the increase in the thickness of films and the reflection of an incident light caused by submicron particles. Based on this result, films prepared by our method have a better T avg than films prepared by a spray pyrolysis method due to an effect of crystallinity on transmittance 21 . The sparking under the uniform magnetic field can produce films with better crystallinity for the (200) plane, resulting in a higher T avg compared with the spray pyrolysis method. However, the increase in the concentration of Co indicates that the T avg of our films is lower than the films (T avg = 85%) prepared by the spray pyrolysis method because of the effect of the submicron particles as mentioned above.
The absorption spectra are shown in Fig. 3b. It has been observed that the increase in Co concentration not only increases the absorption intensity but also indicates the redshift of the absorption edges. This shift can be interpreted that the Ni 2+ in the NiO lattice is substituted by Co 2+ , which reduces the band gap of the NiO film and increases the concentration of holes due to the Co 2+ having a lower number of electrons in 3d level (3d 7 ) than Ni 2+ (3d 8 ). To further verify the optical band gaps of the samples, we use Tauc's relation expressed as 19 : where h is Planck's constant, ν is the photon's frequency, A is a proportionality constant, E g is the band gap, α is the absorption coefficient and n is equal to 2 or 1/2 for indirect and direct transitions. By deriving from Tauc's relation as shown in Fig Chemical state of Co-NiO film. The chemical composition and the chemical bond of Co-NiO film were investigated by XPS. Note that the results of the investigation are displayed in Tables 1 and 2. Figure 4a illustrates the survey spectra of Co-NiO films, which reveals that the composition of the films is Ni, Co, O and C without other elementals. The appearance of carbon is caused by the contamination, which is commonly found in the airexposed sample. As shown in Fig. 4b, the Co 2p core level spectrum contains the deconvoluted peaks of Co 1, Co 2, Co 3 and Co 4, which was assigned to Co 2+ and Co 3+22-25 . The deconvolutions of the O 1s core level spectrum are shown in Fig. 4c. It was found that the peaks of O 1 and O 2 corresponded to O 2−26,27 . While the O 3 and O 4 peaks were in accordance with C(O)OH and O=C, respectively 28 . The C 1s core level spectrum displayed in Fig. 4d indicates five deconvoluted peaks of C 1, C 2, C 3, C 4 and C 5. These peaks were related to C-C, C-O, C=O, C=O and O=C-O, respectively [29][30][31][32] . To understand the influence of Co concentration on the quantity of Ni 3+ and Ni 2+ , the Ni 2p core level spectrum of Co-NiO films was examined. As shown in Fig. 5a-e, it was found that the Ni 2p core level spectra of 0 M, 0.05 M, 0.1 M, 0.15 M and 0.2 M Co-NiO films consist of satellite and sublevel peaks of Ni 2p 3/2 and Ni 2p 1/2 . These sublevel peaks were resolved into six peaks (Ni 1 to Ni 6), which Ni 1, Ni 2, Ni 4 and Ni 5 peaks were assigned to Ni 2+ and the remaining peaks (Ni 3 and Ni 6) were registered to Ni 3+33,34 . The Ni 1 to Ni 6 peaks of 0 M, 0.05 M, 0.1 M, 0.15 M and 0.2 M Co-NiO films have a similar location but their intensity is different because the number of Ni 3+ and Ni 2+ ions in each condition is not equal, depending on Co concentration. With the increase in Co concentration, the Ni 3+ ions in the NiO lattice increased as shown in Fig. 5f. The following decrease of the Ni 3+ ions at the concentration of Co higher than 0.1 M was originated by the formation of CoO at the surface and grain boundaries of NiO, which act as carrier traps and prevent the substitution of Ni 2+ ions by Co 2+ ions. A similar explanation was found by other reports of doped-metal oxides 35,36 . Table 2 illustrates the chemical composition of Co-NiO films. It was found that the decrease in an atomic concentration (at%) of Ni 2p is affected by the increase in the content of Co, which is generally found in the report related to dopant 37 . To support the results of the chemical composition in Co-NiO films evaluated by XPS, the (1) D = 0.9 /β cos θ  Table 2. It has been observed that the values of Ni and Co measured by EDS are lower than the values from XPS. This is caused by receiving different information. For XPS, information on chemical composition comes from the surface of the film while EDS provides the information not only from the film but also from the glass substrate. Therefore, the quantity of Ni and Co elements are diminished by the chemical compositions of the glass substrate. Nevertheless, the percentage of change in at % of Ni and Co evaluated by EDS is similar to the result of XPS. Figure 6 presents the EDS mapping of Co-NiO films.

Peak Component Position BE (eV) FWHM (eV) Assignment References
Co 2p  where R is the resistance, A is the area and L is the length between electrodes. As shown in Table 3, the decrease in resistivity from 9.9 × 10 -3 to 7.89 × 10 -3 Ω cm was caused by increasing the number of higher-valence Ni 3+ due to the Co 2+ substituted Ni 2+38 . According to this result, Co-NiO films fabricated by our method indicate the lower resistivity than Co-NiO films prepared by the solution methods 22,39 . It is well known that the resistivity is inversely related to the crystallite size 40 . Our method provides the film with two orders of magnitude bigger crystallite size than the films deposited by the solution method due to the influence of the uniform magnetic field on the sparking method as mentioned in our previous report 19,39 . At the higher concentration than 0.1 M, the resistivity of the films increased. This result is attributed to the increase of defects and the decrease of Ni 3+ , which is in agreement with the dislocation density and the carrier traps obtained by XRD and XPS. The electrical conductivity of Co-NiO film is shown in Fig. 7b. Obviously, the electrical conductivity of the films depends on the Co concentration, which is correlated with the resistivity in Table 3. To further investigate the electrical properties of Co-NiO film, Hall effect measured by the van der Pauw method was used. Note that the schematic diagram of the van der Pauw method is shown in Fig. 7c. The hall coefficient (R H ) and the carrier concentration (n) of samples are given by the following equation 41 :

Conclusions
In this work, the Co-NiO film was successfully deposited by the two steps, which the first step is the deposition of NiO film by spark discharge under the uniform magnetic field and the second step is the spin coating of Co solution on NiO film. The increase in Co concentration from 0.05 to 0.1 M helps to improve the crystallinity and increases the carrier concentration of NiO film, resulting in a decrease of resistivity. This consequence is in agreement with the increase in the number of higher-valence Ni 3+ because of the Co 2+ substituted Ni 2+ , which confirms our assumption. By considering with optical and electrical properties, 0.05 M Co-NiO film is promising applications in TCO.  To separate the small particles from the big particles, the Lorentz force and the net force were used, which shows in Fig. 8. The spark discharge has four spark head. Each head consists of anode and cathode made of nickel wires (99.98%, Advent Research Materials Ltd) with a diameter of 0.5 mm. The distance between anode and cathode was 1 mm. The distance between the spark head and substrate was 5 mm. The operating voltage was approximately 3 kV. The pulse frequency of the sparking was 13 Hz. The spark discharge was operated under ambient air.

Methods
Fabrication of Co-NiO film. The flow chart for the fabrication of Co-NiO film is shown in Fig. 9. The 10 × 10 mm 2 glass samples were cleaned by sonication in distilled water, acetone and ethanol, respectively. After that, the glass samples were dried by flowing the N 2 gas.