Chemical Solution Deposition of Ordered 2D Arrays of Room-Temperature Ferrimagnetic Cobalt Ferrite Nanodots

Abstract

Over the past decades, the development of nano-scale electronic devices and high-density memory storage media has raised the demand for low-cost fabrication methods of two-dimensional (2D) arrays of magnetic nanostructures. Here, we present a chemical solution deposition methodology to produce 2D arrays of cobalt ferrite (CFO) nanodots on Si substrates. Using thin films of four different self-assembled block copolymers as templates, ordered arrays of nanodots with four different characteristic dimensions were fabricated. The dot sizes and their long-range arrangement were studied with scanning electron microscopy (SEM) and grazing incident small-angle X-ray scattering (GISAXS). The structural evolution during UV/ozone treatment and the following thermal annealing was investigated through monitoring the atomic arrangement with X-ray absorption fine structure spectroscopy (EXAFS) and checking the morphology at each preparation step. The preparation method presented here obtains array types that exhibit thicknesses less than 10 nm and blocking temperatures above room temperature (e.g., 312 K for 20 nm diameter dots). Control over the average dot size allows observing an increase of the blocking temperature with increasing dot diameter. The nanodots present promising properties for room temperature data storage, especially if a better control over their size distribution will be achieved in the future.

Self-Assembly of Block Copolymers

“…A final calcination step removes all organic components and transformed the iron ions of the inner shell into iron oxide, leading to inorganic hollow double-shell nanospheres [23]. A similar approach is demonstrated on vapor-annealed thin films of a PS-b-PEO diblock copolymer by Jin Xu et al [24], where the cylindrical PEO domains are loaded with iron and cobalt ions, followed by an oxidative and thermal treatment to yield regularly arranged, ferrimagnetic cobalt ferrite nanodots with suitable transition temperature to superparamagnetic nanodots, making them interesting candidates for memory storage applications. A systematic study of vapor annealing to equilibrate polystyrene-block-poly(2-vinylpyridine) diblock copolymer films and corresponding homopolymers using in situ spectroscopic ellipsometry and different microscopies is presented by Xiao Cheng et al, who found the temperatures of the substrate and the solvent vapor to be main controlling parameters influencing the dynamics of equilibration [25].…”

Progress and perspective on polymer templating of multifunctional oxide nanostructures

“…10), 136 thereby leading to self-assembly into different morphologies. [137][138][139][140][141][142] Self-assembly in BCP thin films is more complicated. Parameters like film thickness, substrate interaction with each block, and free surface selectivity can influence the morphology significantly.…”

“…This is one of the few reports of the fabrication of complex oxides using BCP templating. 142,152 Horizontal morphologies (with no or limited long range order) such as lamella or horizontal cylinders are not often used for templating of metal oxides using CSD. Previously, metallic nanostructures have been formed using lamellar BCP and CSD to…”

“…III A and III B generally only include crystallinity, with one report of electrical properties as a function of the number of nanostructured layers in a 3D nanomesh 149 and one report of magnetic properties. 142 Publications on materials fabricated by non-BCP polymeric templating (Sec. III C) do report material properties more often; these…”

“…However, the resulting nanostructures generally have a non-unified crystal orientation, resulting in an overall polycrystalline structure. 140,142,196,197 Epitaxial quality through CSD growth was so far only reported for metal oxides grown in non-BCP polymeric templates. 118,190,194 Limited by the phase transition temperature or decomposition temperature of the polymer templates, relatively low deposition temperatures are typically used during polymer-templated ALD.…”