Epitaxial NiO nanocrystals: a dimensional analysis
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esearch Letters
Epitaxial NiO nanocrystals: a dimensional analysis Jeffrey Cheung, Mahmut Baris Okatan, Jivika Sullaphen, Xuan Cheng, and Valanoor Nagarajan, School of Materials Science and Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia Yong-Lun Chen and Ying-Hao Chu, Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Address all correspondence to V. Nagarajan at [email protected] (Received 29 January 2013; accepted 2 April 2013)
Abstract We present the study of the synthesis of (001) nickel oxide (NiO) epitaxial nanocrystals grown on (001) strontium titanate (SrTiO3) single crystal substrates. Pulsed laser deposition of the bismuth nickel oxide (BiNiO3, BNO) perovskite precursor followed by post-deposition processing is carried out to form the NiO nanocrystals. A detailed analysis of the dimensions of nanocrystals reveals that the morphology attained differs from the thermodynamically expected equilibrium shape. The deviations from the equilibrium shape are found to follow a systematic trend where the in-plane basal dimensions, that is, the length and width of the nanocrystals grown differ in discretized dimensions. This discretization suggests that for a given interfacial area of nanocrystals there are multiple stable basal rectangular geometries attainable.
Nickel oxide (NiO) is a basic metal oxide used in diverse applications ranging from solar devices to electronic materials.[1–3] It is a p-type semiconductor with wide intrinsic band gap of ∼4 eV,[4] which adopts the rock-salt crystal structure at room temperature. The demonstration of conduction modulation (resistive switching)[5–9] as well as novel electrochemical properties[10–12] has spawned a resurgent scientific interest in NiO, particularly in its nanostructured form. Recently, we demonstrated a size-dependent resistive switching in epitaxial NiO nanocrystals where the resistive switching mechanism was found to be interface-mediated.[13] It was shown that the applied electric field controls the minority carrier distribution and hence alters conductivity. In addition to electrical properties, NiO also shows intriguing surface-driven polar anisotropies,[14–17] a burgeoning area attracting tremendous interest currently for photocatalysis. It is clear that in order to effectively exploit nanocrystal NiO for electronic and electrochemical applications, detailed insight into surface and shape control and an understanding of underlying drivers of morphology is desired. This communication discusses the dimensional and morphological aspects of (001) oriented NiO nanocrystals fabricated via pulsed laser deposition on (001) SrTiO3 oxide substrates. The role of typical epitaxy process parameters (such as temperature, pressure, etc.) and laser pulses for controlled nanocrystal synthesis is investigated. It is found that the nanocrystals do not follow the thermodynamically expected equilibrium shapes and dimensions, of square-based pyramids (the shape with lowest total surface
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