The Tunable Assembly of Magnetite Mesocrystals, a Natural Compass
The direction of this unit crystal alignment of the mesocrystal nanoparticles is controlled by using the ligands.
The results of the research were published in Nature Communications.

 

 

 

▲ (From left) Prof. Kim Young-keun (corresponding author), Dr. Park Bum-chul (first author); and Dr. Ko Min-jun (first author).

 

 

Professor Kim Young-keun’s group in the Department of Materials Science and Engineering at the College of Engineering conducted a joint study with Dr. Kim Young-kwang’s group at Virtual Lab Inc. and investigated the change of a mesocrystalline microstructure in the process of generating iron oxide mesocrystals in the presence of the surface ligand oxalate, proposing a new approach to controlling the magnetic properties of nanocrystalline materials.
* Mesocrystals : Mesocrystals refer to aggregates of very small unit crystals. They are drawing attention for potential industrial applications because they show collective properties that are not found in single unit crystals.
* Ligands : Ligands refer to the molecules or ions bound to a material, and they play critical roles in determining the properties of those materials.
 
The results of the research were published online in Nature Communications, a multidisciplinary journal dedicated to publishing high-quality research in all areas of the biological, health, physical, chemical and Earth sciences, on March 3.

 

Nano-sized iron oxide particles are highly valuable because their magnetic properties can be adjusted according to specific applications in biomedicine and electronic engineering. Because the properties of iron oxide mesocrystals are dependent upon not only the properties of the unit crystals but also on their methods of binding, it is necessary to accurately understand the mesocrystal formation process. Several theoretical pathways have been suggested, but none has been verified experimentally because an oriented attachment is difficult to control during the chemically performed mesocrystal synthesis.
* Oriented attachment : One of the crystal growth pathways in which unit crystals are attached onto the same crystal face, where they will grow.

 

The research team was able to control the alignment of unit crystals in the mesocrystal formation process by controlling the type of ligand that is attached onto the surface of the iron oxide unit crystals. The researchers found that the misalignment of the unit crystals makes it difficult to mutually combine their magnetic properties for producing mesocrystals with the desired magnetic properties in comparison with well-aligned unit crystals. In addition, they analyzed the impacts of ligands on the alignment of the unit crystals by using the density functional theory.
* Surface energy: Surface energy refers to the energy generated by the exposure of the outermost electrons of a material. Nanomaterials, which have a larger surface area than bulk materials, show much higher surface energy. Since the higher the energy level a material possesses the less stable it is, nanomaterials tend to bind to each other.
 
The research team explained the significance of their research: “In the process of assembling the iron oxide unit crystals into mesocrystals, we controlled the degree of oriented attachment by using the ligands. In addition, we showed the impacts of the crystallographic alignment of iron oxide mesocrystals on their magnetic properties through the experiment. We are now able to design the materials according to their fields of application.”
 
The research was supported by the Mid-Career Researcher Program of the Ministry of Science and ICT, and the Basic Creative and Adventurous Research Program of the Ministry of Education.

 

 

 

[ Terms ]

 

1. Mesocrystals
  ○ Mesocrystals refer to aggregates of very small unit crystals. They are drawing attention for potential industrial applications because they show collective properties that are not found in single unit crystals.

2. Ligands
  ○ Ligands refer to the molecules or ions bound to a material, and they are known to affect the physical and chemical properties of materials.

3. Surface energy
  ○ Surface energy refers to the energy generated by the exposure of the outermost electrons of a material. Nanomaterials, which have a larger surface area than bulk materials, show much higher surface energy. Since the higher the energy level a material possesses the less stable it is, nanomaterials tend to bind to each other.

4. Oriented attachment
  ○ One of the crystal growth pathways in which unit crystals are attached onto the same crystal face, where they will grow.

 

 

[Description of Figures]

 

(Figure 1) Control of an iron oxide mesocrystal crystallization pathway via oriented attachment.
(a) Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns of iron oxide synthesis intermediates and iron oxide mesocrystals at different reaction times. At 1.5 hours from the beginning of the reaction, the iron oxide unit crystals began to be generated from the intermediates and gradually bind to each other to grow. (c) Crystallization pathway of the phase transformation (dotted line) from the intermediates to the iron oxide and the oriented attachment (solid line). (d) High resolution TEM (HR-TEM) and fast Fourier transform (FFT) images of Regions 1 and 2 at 1.5 hours from the beginning of the synthesis. The crystalline face is the same, as the unit crystals undergo oriented attachment in Region 1, while the crystalline face is not the same, as the intermediates formed as unit crystals are included in Region 2.

 

 

(Figure 2) Quantitative analyses of crystallographic alignment.
(a) The orientation factor obtained at the {311} plane from the azimuthal profile of the SAED pattern of a single mesocrystal of ac-MC including acrylate as a ligand, pa-MC including polyacrylate as a ligand, and mg-MC including magnesium-adsorbed polyacrylate as a ligand. The orientation factor represents how consistent the alignment direction of the unit crystals in the mesocrystals is. The orientation factor was highest in the ac-MC and lowest in the mg-MC. (b) Crystallite sizes measured from the HR-TEM images (blue) and calculated by using the Debye–Scherrer equation (gray). (c) The individual mesocrystals showed uniform and similar size distributions.