Mixing and matching different materials to allow new functionalities to arise is one of the major focal points in materials science. Recently, the elegant combination of mesoscopically structured crystalline materials (mesocrystals) and oxide matrices forming “mesocrystal-embedded functional oxide systems” has enabled materials scientists to tune the properties of new nanocomposites and to derive versatile properties not found in the individual components.

Mesocrystals (MCs) represent a new class of nanostructured solid materials, consisting of crystallographically oriented nanoparticles. They are considered an intermediate state between a polycrystal with randomly aggregated subunits and a single crystal. MCs combine the favorable characteristics of nanoparticles together with a structure in the micro- or macroscale, allowing for much easier handling. An additional advantage of MCs is that they give rise to novel orientation-dependent material characteristics. They are thus expected to enhance the performance of materials in various applications.

In a recent prospective article for MRS Communications, Jan-Chi Yang of the National Cheng Kung University, Ying-Hao Chu of the National Chiao Tung University in Taiwan, and their team focus on MC-embedded functional oxide systems. “In this work we highlight breakthroughs and achievements of MCs research in systems where the link between the nanoparticles is a physical solid architectural environment,” Yang says.

There are different ways in which the nanocrystals can link in a mesocrystalline system. When the method of preparation is based on colloidal suspensions, nanocrystals naturally assemble assisted by organic/polymer matrices, physical fields, or mineral bridges. When the assembly of nanocrystals is based on the use of an oxide matrix, then the matrix plays a decisive role in tailoring the structure and functionalities of the MC.

Apart from developing methods for successfully producing MC-superstructures of metal oxides, research in MC-embedded oxide systems is focusing on three main themes, according to Yang. “One is how to use the environment to improve the properties of nanocrystals. The second is how to use the nanocrystals to assist the matrix functionality. And the last very important aspect is to use different functional materials to trigger new functionalities in a MC,” he says.

For example, Liao et al. have suggested that in a system of a CoFe₂O₄ (CFO) MC embedded in a BiFeO₃ (BFO) matrix, the magnetic anisotropy of the system can be changed by altering the orientation of the CFO MC. In another study, Liu et al. used various perovskite matrices to modulate the strain state and corresponding magnetic properties of a CFO mesocrystal.

Tetsuro Majima of Osaka University studies the alignment of nanoparticles into ordered mesocrystal superstructures. “We have newly found a facile and general approach for synthesizing metal oxide MCs with superstructures by a topotactic structural transformation and developing MCs containing two different metals.” In a topotactic reaction the orientations of the produced crystals are determined by the orientation of the initial crystal. Through this method, TiOF₂ was incorporated as an n-type F-dopant source in TiO₂ mesocrystals with visible-light absorption.

According to Majima, nanoplate anatase TiO₂ mesocrystals largely enhanced photoinduced separation of electronic charges. These dissociated charges also exhibited a long lifetime, resulting in a system with greatly increased photoconductivity and photocatalytic activity. “The efficient charge transfer occurred between n-type and p-type semiconductor nanoparticles in the composite mesocrystals,” Majima adds. “This behavior,” which is difficult to achieve in traditional disordered systems consisting of crystalline nanoparticles, “is desirable for their applications ranging from catalysis, optoelectronics and sensing, to energy storage and conversion,” says Majima.

Caroline Ross, professor at the Department of Materials Science and Engineering at the Massachusetts Institute of Technology, has worked on oxide nanocomposite systems, mainly focusing on magnetic and multiferroic properties. “We think these systems offer an ideal opportunity to couple two properties via the strain at the interface, in a ferromagnetic phase and a ferroelectric phase,” she says.

In recently published work in ACS Nano, methods of fabricating highly ordered CoxNi_1−xFe₂O₄ MCs embedded in a BFO matrix were demonstrated. Ross and her colleagues showed how changing the magnetic material composition leads to a magnetically interacting system of pillars (embedded) in a ferroelectric matrix.

“Varying the composition of the pillars provides independent control of anisotropy and magnetostatic interactions, which will be useful in investigations of magnetically interacting arrays, artificial spin ice structures, and magnetic frustration, as well as for magnetic quantum cellular automata devices,” she says.

Yang believes that although the combination of materials could be limitless, scientists face two big challenges. “The first is the material selection itself, because when it comes to material-growth and the self-assembly of the mesocrystal system, the material compatibility is a big issue,” he says. “Furthermore, producing MCs for large-scale applications is the greatest challenge ahead.”

Read the abstract in MRS Communications