Nickel oxide (NiO) is used in many specialized applications credit to the fact that it is a transparent and conductive p-type semiconductor material. Essentially, the mode of nickel oxide deposition has its advantages that tend to be suitable for specific applications. The effect of dopants on high bandgap semiconductors where improved electrical conductivity has been reported can be anticipated for the p-type NiO. Other factors also play role in the materials’ conductivity; for instance, defects within the material can either increase or decrease conductivity. Interestingly, it has been reported that for such metal oxides, the change in material conductivity can be achieved by introducing oxygen vacancies. To this end, numerous attempts to synthesize metal oxides, such as NiO, from salt containing the appropriate metal have been published. So far, basic and simple approaches have been presented; however, no report has exploited the possibility of utilizing a simple wire wound coating for the deposition of the thin films at nanoscale. Nearly a century ago, the Wire – bar coating was used in making the wax papers. Later this technique was used to spread a material in liquid state, such as paint, or ink, over a substrate such as paper, card, film and plastic sheet. Now, this research work is exploiting the same process for the deposition of thin film of functional nanomaterials (nickel oxide). Needless to say, this process is compatible with industrial large-scale roll-to-roll and scalable manufacturing.
In a recent publication, researchers from the Emerging Technologies Research Center at De Montfort University: Krishna Nama Manjunatha (PhD student then) and Professor Shashi Paul proposed a novel approach, where their focus was to investigate the possibility of using a wire wound coating technique for deposition of thin films at nanoscale. In addition, they investigated the doping of NiO using three different metal salts that have different valences (Cu1+, Zn2+, and Ga3+) as dopants for NiO thin films, to assess the effect of monovalent, bivalent and trivalent ions towards doping. Their work is published in the research journal, Applied Surface Science.
Essentially, they aimed at assessing the synthesis and doping (different valences) of NiO thin films from metal organic compounds at low temperatures for wire-wound coated films. In this view, the two researchers commenced their empirical investigation with the preparation of precursor solution where a series of M-doped NiO; where M = Cu, Zn and Ga, were prepared by dissolving metal organic compounds in a methanol solvent. Next, thin films of NiO were deposited on a k-bar using a facile yet economical deposition process. Eventually, the deposited NiO films were characterized by XRD and FTIR spectroscopy.
The authors reported that resistivity of NiO was not just altered by doping, but involved much complex mechanisms including defect density, Oxygen and Nickle vacancy, grain boundaries with uncompensated Ni2+ ions, localization of oxygen orbitals in the valence band maximum and many more. Overall, the significance of aliovalent cations, especially monovalent compared to trivalent was elucidated.
In summary, the study by Professor Shashi Paul and Krishna Nama Manjunatha provided an in-depth assessment of the changes in the structural, optical and electrical properties of NiO and a comparison amongst different metals (dopants) with different valences. The study also provided a proof of concept towards a simple and yet economical technique to prepare doped and un-doped NiO thin films with transmittance similar to standard TCOs and resistivity values similar to films deposited by vacuum based processes, such as sputtering and ALD. In a statement with Advances in Engineering, Professor Shashi Paul, the author further highlighted that their study clarified that aliovalent cations, especially monovalent compared to trivalent, provided controlled doping and increased the conductivity of thin films.
Krishna Nama Manjunatha, Shashi Paul. Wire-bar coating of doped Nickle oxide thin films from metal organic compounds. Applied Surface Science volume 488 (2019) page 903–910