The Xenes Generations: A Taxonomy of Epitaxial Single-Element 2D Materials Carlo Grazianetti, Christian Martella, and Alessandro Molle* Isolation of graphene is today a milestone in condensed matter physics that paves the way to a new entire class of 2D synthetic materials referred to as Xenes with no analogous bulk layered allotropes. The booming rush to discover first novel and unprecedented materials flows into two generations of Xenes: the first one strictly relates to carbon being made of elements of the IV/14 column/group of the periodic table, and the second one includes elements of the adjacent columns. From borophene to tellurene, with rare exceptions, the physics and chemistry of the elements are rewritten and herein reviewed in terms of their fundamental and peculiar properties. Particular attention is paid to the epitaxial methodologies and configurational details, aiming at determining key points for nanotechnology applications of the Xenes afforded by scalability, quality, and stability aspects. Finally, the ongoing efforts to devise and realize applications based on the Xenes are summarized. 1. Introduction In view of incipient challenges on how to move the semiconduc- tor technology forward to a post-silicon era of electronic devices, in 2015, Guy Le Lay stated that “the material to replace silicon in the electronics industry could, in fact, be silicon itself in the form of silicene.” [1] The statement was originally grounded on the pio- neering discovery of a 2D silicon-made framework epitaxially grown on a (111)-terminated silver surface with a quasiplanar structure, [2] and then captured by the isolation of this atomically thin layer into an operation field-effect transistor (FET) with Dirac-like transport. [3] That was the key to discriminate the so-termed silicene, namely, a single atomic layer reproducing a 2D hexagonal lattice, from a silicon-induced decoration of the Ag(111) surface. As such, silicene can be integrally lifted out from its pristine substrate and independently behaves as an active layer in a device or as a stabilized atomically thin mem- brane. Another quality of silicene that came up since the original concept of the free-standing lattice is the buckled character of its bonding connections that necessarily stems from an enhanced orbital overlap otherwise too short in a perfectly planar arrange- ment to guarantee thermodynamic stability. [4,5] After silicene, a generation of Xenes, silicene-like crystals made of X atoms, has followed up, gather- ing an increasing number of elements X from group 13 (the boron group) up to group 16 (the chalcogen group) and pass- ing through group 15 (the pnictognes group) in the periodic table of elements, thus constituting an emerging platform of novel materials for nanotechnology. [6] Historically, this evolution can be articu- lated in two different stages: a first genera- tion limited to group 14 (the carbon/silicon group) with mutually similar characteris- tics, issues, and a common inspiration toward the quest for a 2D topological insu- lating state, [7] and an ongoing second gen- eration, out of group 14, where the epitaxial methods were extended to confine down to the 2D level those X elements that seem- ingly do not possess a stable graphene-like allotrope, not being in a straightforward kinship with carbon as silicon and germa- nium. A general taxonomy of the Xenes as so far reported is shown in Figure 1 where Xenes are grouped according to the periodic table and key structural properties. The two Xene gen- erations will be briefly described in the following sections. Attention will be then paid to classify the methodologies for the synthesis of the Xenes, spotlight characteristic aspects of the Xenes, and state application perspectives. 2. Xenes: First Generation Xenes were initially inspired by the conceptual option to reduce elements belonging to the group 14 column of the periodic table (out of carbon) in the 2D form of graphene. A historical view on this first generation of Xenes is shown in Figure 2 including representative evidences of each single member on its debut. Mastering silicon at the 2D level for hyperscaled silicon-based elec- tronics is a technology driver for the Xene development that moti- vated the initial focus on silicene as a target channel for ultrathin body transistors owing to silicon ubiquity in microelectronics. [11] In a later stage, spanning elements in the group 14 column was basi- cally inspired by the ambition to make a 2D elementary crystal with nontrivial topology, namely, a 2D topological insulator at room tem- perature, eventually aiming at topological quantum electronics. [7] 2.1. Silicene Silicene was first synthesized as an epitaxial layer on Ag(111) substrates, [2,15] therein displaying a multiphase character, [16] Dr. C. Grazianetti, Dr. C. Martella, Dr. A. Molle CNR-IMM, Agrate Brianza Unit via C. Olivetti 2, Agrate Brianza 20864, Italy E-mail: alessandro.molle@mdm.imm.cnr.it The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/pssr.201900439. DOI: 10.1002/pssr.201900439 REVIEW@RRL www.pss-rapid.com Phys. Status Solidi RRL 2020, 14, 1900439 1900439 (1 of 11) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim