The physical properties of materials often differ significantly between 3D crystals and 2D structures. These properties can be useful for applications, e.g, in electronics. Tuning, for example, the bandgap could allow researchers to tailor the both the electrical and optical properties of 2D materials for specific applications.

Andras Kis and colleagues, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, have studied atomically thin PtSe₂ and found that the material changes from metallic to semiconducting behavior when its thickness is reduced. The team used mechanical exfoliation to produce ultrathin PtSe₂ crystals and atomic force microscopy (AFM) imaging to measure the thickness of the material. Then the conduction characteristics were measured in vacuum.

The team found that PtSe₂ crystals with a thickness of ca. 13 nm have a metallic behavior and low resistance at room temperature. Thinner crystals (ca. 2.5 nm) have semiconducting properties. This effect is due to a shrinking density of states with decreasing thickness. According to the researchers, the tuning range for the PtSe₂ bandgap is much bigger than for other 2D materials. In addition, the material is air-stable. This could allow the creation of electronic circuits on an atomic scale and other applications in nano-and optoelectronics.

• Thickness-modulated metal-to-semiconductor transformation in a transition metal dichalcogenide,
  Alberto Ciarrocchi, Ahmet Avsar, Dmitry Ovchinnikov, Andras Kis,
  Nat. Commun. 2018.
  https://doi.org/10.1038/s41467-018-03436-0