Investigation of Ni@CoO core-shell nanoparticle films synthesized by sequential layer deposition

Abstract

Films of Ni@CoO core-shell nanoparticles (NP Ni core size d ≈ 11 nm) have been grown on Si/SiOx and lacey carbon supports, by a sequential layer deposition method: a first layer of CoO was evaporated on the substrate, followed by the deposition of a layer of pre-formed, mass-selected Ni NPs, and finally an overlayer of CoO was added. The Ni NPs were formed by a magnetron gas aggregation source, and mass selected with a quadrupole mass filter. The morphology of the films was investigated with Scanning Electron Microscopy and Scanning Transmission Electron Microscopy. The Ni NP cores have a shape compatible with McKay icosahedron, caused by multitwinning occurring during their growth in the source, and the Ni NP layer shows the typical random paving growth mode. After the deposition of the CoO overlayer, CoO islands are observed, gradually extending and tending to merge with each other, with the formation of shells that enclose the Ni NP cores. In situ X-ray Photoelectron Spectroscopy showed that a few Ni atomic layers localized at the core-shell interface are oxidized, hinting at the possibility of creating an intermediate NiO shell between Ni and CoO, depending on the deposition conditions. Finally, X-ray Magnetic Circular Dichroism at the Ni L2,3 absorption edge showed the presence of magnetization at room temperature even at remanence, revealing the possibility of magnetic stabilization of the NP film

Steering the magnetic properties of Ni/NiO/CoO core-shell nanoparticle films: The role of core-shell interface versus interparticle interactions

“…The data also showed that NiO was formed during the deposition of the CoO overlayer. Oxidation of the external layers of the Ni cores, as previously reported [20,21], can be ascribed to a slightly higher Table I together with the corresponding nominal thickness of the CoO overlayer, t 3 CoO . It must be stressed that t 3 CoO is a quantity expressing the amount of CoO deposited during step 3 of the NP growth method summarized in the previous section (growth of the third layer) and estimated by the evaporation rate measured by the quartz microbalance, while t CoO is an estimate of the CoO shell average thickness in Ni@NiO@CoO NPs as obtained by analysis of the XPS Co 2p components [38].…”

“…3) of the sequential deposition, with the morphology of a percolated film. SEM images of Ni@NiO@CoO NPs grown on Si/SiO x have been previously reported [20]; at increasing coverage of the CoO overlayer, the islands cover completely the Ni NP cores and extend over neighboring NPs, forming nanostructures of cubic shape with size between 15 and 50 nm at the highest overlayer thickness studied (t 3 CoO = 6 nm). XPS analysis confirmed that the oxide shell consisted mainly of CoO (at variance with previous experiments [21], where the NPs prepared with a similar technique had an oxide shell composed of Co 3 O 4 and Co suboxides) and showed also that an interfacial NiO shell was formed during the deposition of the CoO overlayer.…”

“…As stated in previous papers [18][19][20] with H cool = +50 kOe. DC demagnetization (DCD) curves were recorded as follows.…”

“…Therefore, optimization of the different parameters governing the EB can be pursued, with the ultimate goal of reaching magnetic stability at RT for NPs with smaller diameter values. In a recent preliminary work, this method was also applied to Ni@CoO NP assemblies [20], with a Ni core diameter of approximately 11 nm. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) images revealed the core-shell structure on the obtained NPs, and from the analysis of X-ray photoelectron spectroscopy (XPS) data it was found that a thin oxidized Ni shell, with thickness of few atomic layers was formed, presumably at the core-shell interface.…”

Low pressure bottom-up synthesis of metal@oxide and oxide nanoparticles: control of structure and functional properties

“…Since the electronegativity values of Ni and Co (1.88 Pauling) are very close, it is reasonable to deduce that formation of NiO can take place, as well as oxidized Co. XMCD data taken at remanence from Ni@CoO NP films synthesized with sequential layer deposition revealed the existence of a dichroic signal at room temperature, a possible indication of stabilization of the Ni NP cores in a FM phase [91].…”