Tailoring two-dimensional materials

A new class of materials has created a buzz in the scientific world. Two-dimensional planes of atoms or molecules can be stronger, more flexible, and better conductors of heat and electricity than their bulk counterparts. Thanks to those advantages, the materials are promising candidates for previously unimagined high-tech applications in computing, energy generation, nanodevices, and more.

What’s more, 2D materials are “all surface,” meaning their entire chemical structure is exposed and can be potentially modified to further optimize the materials’ properties, said Joshua Goldberger , a chemist at the Ohio State University. Goldberger, his colleagues, and his students are researching new methods to synthesize novel 2D materials and tailor their electronic properties by tinkering with the surface bonds.

Goldberger presented some of his latest work on 24 July at the annual meeting of the American Crystallographic Association in Philadelphia.

Synthesizing a 2D foundation

Graphene continues to generate excitement, but the atom-thick sheets of pure carbon are relatively inert when it comes to chemical reactions. Goldberger’s group is more interested in graphane, a 2D sheet of carbon and hydrogen, and in similar materials. The presence of hydrogen forces the main bonds in the materials into a tetragonal sp³ configuration, which makes it easier for chains of atoms, called ligands, to attach to the surface.

Researchers in Goldberger’s lab were the first—in 2013—to produce germanane (GeH), an analogue of graphane made from the element germanium. Like silicon, which occupies the same column of the periodic table, germanium is a semiconductor. Companies have invested much money and time figuring out how to process germanium into electronic chips. According to Goldberger, the industrial know-how would give novel 2D materials made from germanium a head start over carbon-based materials on the road to commercialization.

Germanium doesn’t naturally form single-atom-thick flakes. To make germanane the researchers took crystals of calcium germanide and then dissolved away the calcium atoms with hydrochloric acid. The hydrogen ions from the acid filled the gaps left by the calcium.

After eight days of slowly stirring a CaGe₂ and HCl mixture, the researchers were able to tease apart large crystalline flakes of germanane. The flakes proved highly stable in air and water and have a direct band gap of approximately 1.56 eV, meaning they efficiently emit and absorb light in the near-IR.

Following the initial success, Goldberger and his colleagues developed a new technique to make germanane by growing CaGe₂ films on a substrate of crystalline germanium and immersing the sheets in HCl. The new technique holds promise as a way to grow large-area germanane films, provided the underlying germanium substrate is smooth.

Tuning with covalent chemistry

Creating hydrogen-terminated bonds in 2D sheets is just the beginning of the researchers’ journey toward their main goal: customizing the electronic properties of the new materials.

As a next step, Goldberger and his colleagues synthesized a carbonized version of germanane, GeCH₃, by using carbon-containing solutions instead of HCl. Replacing the H termination in GeH with the longer CH₃ chain strained the lattice structure of the material, making the bond larger. In turn, the slight geometric shift caused the band gap to increase by approximately 0.1 eV, enough to reach the red end of the visible spectrum. The shift also boosted thermal stability.

Longer ligands should strain the lattice even more. Goldberger’s group is currently testing how well they can fine-tune the band gap of germanium-based 2D materials by attaching larger and larger chemical chains. They have promising initial results that they hope to publish soon.

Catherine Meyers is a writer in the media services division of the American Institute of Physics.