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Copper, nickel play important role in development of next-gen 2D materials

Researchers at Tokyo University of Science, Kyoto Institute of Technology and the University of Cambridge found that by mixing two metal ion solutions together, they were able to enhance the structural properties of coordination nanosheets.

In a paper published in the journal Advanced Materials, the team behind the discovery explains that coordination nanosheets are a new and emerging class of two-dimensional materials that have potential applications in electronic devices, batteries, and catalytic systems. They consist of metal ions and organic ligand molecules, linked to each other to form one framework, via coordination bonds. These nanosheets act as building blocks, which can be mixed and matched to produce a large variety of planar structures.

According to the scientists, in 2013, benzenehexathiolato (BHT) was discovered as a powerful organic ligand in coordination nanosheets. It was observed that upon changing the element used in the metal centers, it is possible to create BHT-based nanosheets with vastly different structural properties.

However, the synthesis of BHT-based coordination nanosheets via solution-based processes has proven challenging because the resultant nanosheets lack crystallinity, indicating the formation of small crystalline domains with poor orientation control. These structural shortcomings hinder the nanosheets’ performance and limit the possibilities of studying their structure-property relationships.

This is where the new development comes in. Led by Hiroshi Nishihara, the Japan and UK-based group investigated whether BHT-based coordination nanosheets developed by the introduction of two metal ions could overcome the aforementioned challenges.

To do so, the researchers prepared heterometallic nanosheet films at a liquid-liquid interface by changing the mixing ratio of two metal ions, copper and nickel, in an aqueous solution. Simply put, they poured an aqueous solution containing these two metal ions onto an organic solution containing a BHT precursor.

To their surprise, they found that a new structural phase had formed at the interface between the two phases, with intermediate ratios of nickel and copper. Moreover, they found that this NiCu2BHT film possessed much higher crystallinity than pure copper and nickel films.

“Our results indicate that the nanosheets grow in a specific direction and with a fixed composition, NiCu2BHT, at the liquid–liquid interface when the two metal ions are mixed at an appropriate ratio,” Nishihara said in a media statement. “It is extraordinary that such simple mixing of different metal ions resulted in a unique structure with 2D periodicity and enhanced crystallinity, even in relatively thick films.”  

With an increase in crystallinity, notable improvements were also observed in the performance of the heterometallic nanosheets. Electrical conductivity measurements together with the analysis of film morphology revealed that these films have lower activation energies and higher conductivities than copper films. In fact, the researchers observed conductivities of up to 1300 S/cm with a dependency on temperature similar to that of good metal conductors. 

Finally, the team analyzed the underlying mechanisms that led to this improvement in crystalline order and suggested that NiCu2BHT films may naturally arrange themselves into a bilayer structure that releases the structural strain of the material. 

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