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Abstract
This thesis investigates the synthesis and resistive switching properties of nickel oxide (NiO) nanostructures grown on strontium titanate, SrTiO3 and niobium doped SrTiO3 substrates of (001) orientation a phase separation approach. Pulsed laser deposition of a bismuth nickel oxide (BiNiO3) perovskite precursor is carried out under conditions which induce perovskite decomposition into bismuth oxide and NiO. Post-deposition process conditions are then finetuned to exploit the volatile nature of bismuth oxide which then leaves behind an array of self-assembled NiO nanostructures.
Varying synthesis parameters allows us to find the optimum conditions for controlling the morphology and chemical properties. Firstly deposition partial O2 pressures were examined to show the influence on the morphology and phase of the samples. Results show the optimum pressure to be 50 mTorr. Next the temperature was modified. A substantial effect was observed with both crystal shape and inter-particle distance. The 900 °C sample was shown to be the optimum temperature. The influence of laser pulses on volume and nanocrystal shape was then investigated. 5 000 pulses showed the formation of square pyramidal hut shaped structures, 10 000 showed some hut shapes as well as truncated pyramids and 20 000 showed a film-like growth indicating a metastable state of joined hexagonal structures. Finally the effects of annealing conditions were considered. Samples annealed in a vacuum showed round structures composed of Ni2O3. 100 mTorr O2 partial pressure showed NiO phases however the structures were undefined and 1 Torr was found to promote formation of defined nanostructures and NiO(200) phase constituents.
In understanding the resistive switching properties of these individual nanostructures conductive– atomic force microscopy was used. The role of the interface as well as the interplay between the nanostructure morphology and resistive switching properties and the governing mechanisms are required in order to create efficient switching devices based on nickel oxide. We find that the nanostructures display prominent extrinsic bipolar switching characteristics in a specific height range (~20-15 nm). Heights lower than this show conductive behaviour and taller heights display a strong rectifying behaviour. The maximum ON/OFF ratio is at ~103 at a read voltage of ~+0.4 V. This ratio is found to decrease with increasing height of the nanostructure. Linear fittings of I-V loops reveal that low and high resistance states follow Ohmic-conduction and Schottky-emission mechanism, respectively. This switching behaviour (dependence on height) is attributed to the modulation of the carrier density at the nanostructure-substrate interface due to the applied electric field.