Inorganic nanocomposites have characteristic structures that feature expanded interfaces, quantum effects, and resistance to crack propagation. These structures are promising for the improvement of many materials including thermoelectric materials, photocatalysts, and structural materials. Precise control of the inorganic nanocomposites’ morphology, size, and chemical composition is very important for these applications. Here, we present a novel fabrication method to control the structures of inorganic nanocomposites by means of a self-assembled block copolymer template. Different metal complexes were selectively introduced into specific polymer blocks of the block copolymer, and subsequent removal of the block copolymer template by oxygen plasma treatment produced hexagonally packed porous structures. In contrast, calcination removal of the block copolymer template yielded nanocomposites consisting of metallic spheres in a matrix of a metal oxide. These results demonstrate that different nanostructures can be created by selective use of processes to remove the block copolymer templates. The simple process of first mixing block copolymers and magnetic nanomaterial precursors and then subsequently removing the block copolymer template enables structural control of magnetic nanomaterials, which will facilitate their applicability in patterned media, including next-generation perpendicular magnetic recording media.
Inorganic nanocomposites have characteristic structures that feature expanded interfaces, quantum effects, and restrained crack propagation. These structures are promising for the improvement of many materials including thermoelectric materials [
Templating processes that use organic materials as molds are powerful methods for controlling inorganic nanocomposites’ structures [
Many studies have focused on the synthesis of inorganic nanostructures of porous materials [
We recently demonstrated a synthetic route to inorganic nanocomposites by means of self-assembled block copolymer templates [
Our goal is to develop a nanofabrication process that yields controlled structures of magnetic nanomaterials and that is simpler than the processes used conventionally in the semiconductor industry. In this report, we present a novel fabrication method to control the structures of inorganic nanocomposites by means of a self-assembled block copolymer template. Metal complexes were selectively introduced into a specific polymer block of the block copolymer, and subsequent removal of the block copolymer template by oxygen plasma treatment produced hexagonally packed porous structures. In contrast, calcination removal of the block copolymer template yielded nanocomposites of metallic spheres in a matrix of a metal oxide. These results demonstrate that different nanostructures can be created by selective use of processes to remove the block copolymer templates.
Ferrocene (bis(cyclopentadienyl) iron, >98%, Sigma-Aldrich), acetylacetonate platinum(II) (Pt(acac)2, 99%, Sigma-Aldrich), and acetylacetonate iron(III) (Fe(acac)3, 99%, Wako) were dissolved in a 0.5 wt% solution of polystyrene-
The block copolymer templates were removed by oxygen plasma treatment or by calcination in air. The sample films on the copper TEM grids were treated in oxygen plasma operating at 100 W for 2 or 3 min using a low-temperature plasma asher (PR300, Yamato Scientific Co., Ltd.). The sample films in petri dishes were calcined at 550°C for 6 h in flowing air.
Scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) of the resulting samples were conducted with a high-resolution transmission electron microscope (JEM-2010FEF, JEOL) operating at an accelerating voltage of 200 keV. X-ray diffraction patterns were collected with Cu K
A STEM image of the spin-coated sample that was prepared with PS-P4VP (25.5 k/24 k) and three metal complexes (ferrocene, Pt(acac)2, and Fe(acac)3) showed spherical structures (Figure
Dark-field scanning transmission electron microscopy image of a spin-coated sample prepared from PS-P4VP (25.5 k/24 k) and three metal complexes (ferrocene, Pt(acac)2, and Fe(acac)3).
The EDS results in Table
Energy-dispersive spectroscopy results of spin-coated samples prepared from PS-P4VP (25.5 k/24 k) and metal complexes.
Fe (%) | Pt (%) | |
---|---|---|
Sphere | 59.9 | 40.1 |
Matrix | 93.8 | 6.2 |
(a) Dark-field scanning transmission electron microscopy image of spin-coated samples prepared from PS-P4VP (25.5 k/24 k) and Fe(acac)3; (b) energy-dispersive spectroscopy mapping image for Fe (green).
These results indicate that the self-assembled spherical structure of P4VP enabled selective introduction of Fe(acac)3 and Pt(acac)2, whereas ferrocene was selectively introduced into the PS matrix, which was formed from selective dissolution of PS in toluene.
As seen in Figure
Dark-field scanning transmission electron microscopy image of a sample after treatment with oxygen plasma for 2 min.
(a) Dark-field scanning transmission electron microscopy images of a sample after treatment with oxygen plasma for 3 min; energy-dispersive spectroscopy mapping images for (b) Fe and (c) Pt.
The EDS mappings of Fe and Pt show that the matrix of the porous structure is composed of Fe and Pt (Figures
In the STEM image of the sample calcined at 550°C, bright spheres (10 nm in diameter) surrounded by a dark matrix are observed (Figure
Energy-dispersive spectroscopy results for a calcined sample.
Fe (%) | Pt (%) | |
---|---|---|
Sphere | 38.3 | 61.7 |
Matrix | 92.4 | 7.6 |
Dark-field scanning transmission electron microscopy image of a calcined sample.
In contrast to the results observed upon removal of the block copolymer template by calcination, in which the spherical structures remained, the spheres became pores after treatment in oxygen plasma. This difference in morphology was attributed to the difference between the PS and P4VP blocks’ susceptibility to oxidation. In the calcination process, the PS and P4VP blocks decomposed within the same temperature range of 370–460°C [
The X-ray diffraction profile of the calcined sample confirms the existence of two crystalline phases (Figure
X-ray scattering profile of a calcined sample, indicating the presence of FePt (closed circles) and Fe2O3 (open squares).
A novel fabrication method to control the structures of inorganic nanocomposites by templating a self-assembling block copolymer is demonstrated. Metal complexes were selectively introduced into a specific polymer block, and subsequent removal of the block copolymer by oxygen plasma treatment produced hexagonally packed porous structures. In contrast, calcination removal of the block copolymer template yielded a nanocomposite of metallic spheres in a matrix of a metal oxide. We have therefore demonstrated that different nanostructures can be created by selective use of processes to remove the block copolymer template. The process demonstrated herein, consisting of simple mixing of the block copolymer and magnetic nanomaterial precursors and subsequent removal of the block copolymer template, enables the structural control of magnetic nanomaterials, which will facilitate their application in patterned media, including next-generation perpendicular magnetic recording media. This novel method to control nanostructures of inorganic nanocomposites can be applied to other chemical species, including other metals, oxides, carbides, nitrates, and sulfides. This method also is promising for the improvement of nanocomposites’ properties in many applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.