(Invited) Xenes: A New Emerging Two-Dimensional Materials Platform for Nanoelectronics
Authors: Molle Alessandro 
Published: February 2020

The recent discoveries of silicene, germanene, and stanene, commonly denoted as Xenes (with X being Si, Ge, Sn), and their ligand-functionalized “Xane” derivatives, brings to the forefront an important class of two-dimensional (2D) elementary crystals. Similar to graphene, these materials are comprised of group IV atoms arranged in a honeycomb lattice, but with varying degrees of buckling that inherently stem from the interplay of sp2 and sp3 hybrid bonds.[1,2] An example of a generic buckled Xene lattice is sketched in the Figure below. So far, Xenes were synthesized as epitaxially grown monolayers on substrates. Substrates suitable for the Xene epitaxy are commensurate with the free-standing Xene lattice. Once grown on a substrate, the Xene self-arranges in a number of surface phases depending on the allowed commensurability relationships. The silicene-on-silver will be taken as a representative example of a polymorphic Xene lattice where different phases can be induced as a function of varying growth conditions. The toll to pay in order to have a Xene lattice is a strong interaction with its hosting substrate, usually metallic in character. This fact is currently limiting an easy exploitation of Xene for functional applications and claims for specific handling process enabling the integration of Xene into a device platform. An overview of the state-of-the-art on the Xenes-on-substrates will be here given. To address the device integration issues, the delamination of an encapsulated silicene monolayer will be presented as paradigmatic process for the fabrication of a silicene transistor [3] that can be universally extended to other epitaxial Xenes. The option of a van der Waals epitaxy of Xene on non-interacting substrates will be also reviewed by specifying model systems where to perform experiments.[4] The silicene epitaxy on a MoS2substrate will be brought as an experimental case in point in this respect.[5]
In principle, the electronic structure of Xene can range from trivial insulators, to semiconductors with tunable gaps, to semimetallic, depending on the substrate, chemical functionalization, and strain. Furthermore, over 16 different topological insulator states are predicted to emerge, herein including the quantum spin Hall (QSH) state at room temperature, which, if realized, would enable new classes of nanoelectronic and spintronic devices. The physical foundations of a QSH state in Xenes will be briefly introduced. The access to a QSH state in Xenes would classify them as 2D topological insulator thus paving the way to a disruptive concept of a topological field effect transistor. [1] The pre-requisites of topological FET will be here outlined and other candidates in the 2D materials family suitable for this purpose will be taken into account.
The extreme versatility in electronic structure that can be accessed solely by changing the group IV element, the degree of spin-orbit coupling, the functionalization chemistry, and the substrate for example, make the Xenes a multifunctional 2D platform for nanoelectronics. For instance, silicene is predicted to band gap engineering as a function of its buckled phase, its band structure ranging from a graphene-like semimetal to a semiconductor with parabolic bands. Starting from these theoretical predictions, the details of the electrical transport measured on a silicene transistor will be discussed and critical points will be singled out. Finally, perspectives and challenges in the synthesis and exploitation of Xenes in nanotechnology will be discussed.
[1] M. Houssa, A. Dimoulas, and A. Molle, J. Phys.: Condens. Matter 27, 253002 (2015) (Topical Review).
[2] C. Grazianetti, E. Cinquanta, and A. Molle, 2D Materials 3, 012001 (2016) (Topical Review).
[3] L. Tao, et al., Nature Nanotech. 10, 227 (2015).
[4] E. Scalise et al., 2D Materials 1, 011010 (2014).
[5] D. Chiappe, et al., Adv. Mater. 26, 2096 (2014).

Figure 1