Friday, June 2, 2017

2-D material terms deliberately rhyming with graphene: Xenes and MXenes

How thin can a material get? Answer: As thin as a single atomic layer is thick. That is a thickness (thinness) in the ångström or low-nanometer range. Ultrathin!

There is a lot of excitement about materials with their third dimension falling into this range, since by tailoring materials layer by layer and stacking layers of different elemental composition, chemists can engineer functional interfaces and the nanodevices for the future—in principle, and with proof of concept for certain chemical elements and element combinations.

The successful epitaxial growth of layers of the carbon allotrope graphene—an atomic-scale,  hexagonal lattice of carbon atoms—in the 1970s triggered studies of other ultrathin material layers.

Chemists like to express chemical similarity by using a class-specific suffix and make chemical class names rhyme. In the terms “Xene” and “MXene” the suffix is “-ene,” rhyming with graphene. The letter X symbolizes elements of the boron, carbon and nitrogen group from the p-block of the periodic table. The letter M stands for transition metal elements.

The term “Xene” refers to a monoatomic sheet, buckled or with the center of all layer atoms in one plane. With X referring to carbon, boron, silicon, phosphorous, germanium or tin (Latin: stannum), the corresponding 2-D materials are named graphene, borophene, silicene, phosphorene, germanene and stanene, respectively. With the exception of graphene, which has a flat-sheet structure, the Xenes have a buckled or corrugated shape [1].

A typical MXene (pronounced “maxene”) is a 2-D transition metal carbide, transition metal nitride or transition metal carbonitride. The graphenelike sheet of Ti3C2 is an example. Most MXenes have been synthesized by selecting titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium and/or molybdenum as the transition metal [2-4].

Keywords: chemistry, chemical nomenclature, materials science, nanoscience, ultrathin films, interfaces.

Selected literature and more to explore

[1] Mitch Jacoby: 2-D materials go beyond graphene. Chem & Eng News 2017, 95 (22), 36-40 [].

[2] Babak Anasori et al.: Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes). ACS Nano 2015, 9 (10), pp. 9507-9516 [].

[3] Joseph Halim et al.: X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes). Appl. Surf. Sci. 2016, 362, pp. 406-417 [].

[4] Patrick Urbankowski et al:. Synthesis of two-dimensional titanium nitride Ti4N3 (MXene). Nanoscale 2016, 8 (22), pp. 11385-11391 [!divAbstract].