Novel 2-D Material Offers a Band Gap and Self Assembly

Image: MIT and Harvard University

The competitive field of two-dimensional materials has added another rival to graphene to its ranks. A collaboration between MIT and Harvard University researchers has yielded what observers are heralding as a major advance in the synthetic design of novel semiconducting materials. The Boston-area researchers have developed a new 2-D material that not only has an inherent band gap—which graphene lacks—but self-assembles, promising easier avenues to mass production.

The material is a combination of nickel and an organic compound called 2,3,6,7,10,11-hexaiminotriphenylene (HITP). The resulting material belongs to a class of materials known as metal-organic frameworks (MOFs) that are compounds in which metal ions are coordinated to rigid organic molecules to form a porous material that can be one-, two-, or three-dimensional.

The research, which was published in the Journal of the American Chemical Society ("High Electrical Conductivity in Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2, a Semiconducting Metal–Organic Graphene Analogue"),  demonstrated that the new compound, Ni3(HITP)2, has the same hexagonal honeycomb structure as graphene.

One of the attractive characteristics the researchers demonstrated with this particular MOF is that its properties can be tuned to a desired capability simply by adding more or less of the two constituent parts. This could lead to the development of photovoltaics in which the solar cell could be manipulated to capture different wavelengths of light that match the solar spectrum.

The MIT-Harvard team performed their studies of the material in its bulk form rather than as flat sheets, making the record-breaking measurements for the MOF all the more impressive. By using two-probe and van der Pauw electrical measurements, the researchers revealed that the bulk (pellet) and surface (film) specific conductivity values of the materials were 2 Siemens/centimeter-1 (S/cm-1) and 40 S/cm-1, respectively—both records for MOFs, and among the best for any coordination polymer.

“There’s every reason to believe that the properties of the particles are worse than those of a sheet,” said MIT assistant professor of chemistry Mircea Dincă in a press release. “But they’re still impressive.”

In addition to the material’s potential applications to photovoltaics, the researchers envision that it could be used in the creation of exotic materials such as magnetic topological insulators, or materials that exhibit quantum Hall effects.

“They’re in the same class of materials that have been predicted to have exotic new electronic states,” said Dincă in the release. “These would be the first examples of these effects in materials made out of organic molecules. People are excited about that.”

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