Two‐Dimensional Semiconductors: Present and Future Challenges

Abstract

There is a booming interest in two-dimensional (2D) materials, which are nowadays dominating the landscape of the nanotechnology field. Cutting-edge research is moving towards new mono-elemental 2D materials beyond graphene. [1] To reflect the strong interest of the scientific community towards this hot topic, a Symposium specifically addressing the applications of 2D semiconductor materials has been organized within the frames of the E-MRS Spring Meeting 2019 which took place in Nice (France). This Special Issue showcases the results and outlines the ongoing challenges presented by the participants of the Symposium.The quest for 2D elementary crystals behaving as topological insulators pushed the discovery of new members of the 2D world. As reviewed [2] by A. Molle and co-workers, this regards the family of M-Xenes, which are mono-elemental 2D materials originating from the boron (B, Ga), carbon (C, Si, Ge, Sn, Pb), pnictogen (P, As, Sb, Bi), and chalcogen (Se, Te) groups of the periodic table. Historically, one can distinguish the first generation of Xenes, strictly related to the carbon group and inspired by graphene, [3] whose most famous representative is silicene. [4] The second generation of Xenes includes elements of the adjacent columns, most of which do not possess a stable layered allotrope and thus are synthesized by epitaxial growth. A notable example is phosphorene, [1] synthesized mainly by mechanical or liquid exfoliation or by plasma treatment, [5] that has gained great interest for its peculiar chemical and physical properties. [6] Thus it is a highly promising candidate for applications in nanoelectronics, energy, and the biomedical field. Following this growing interest, current research is addressing heavier elements of the same group, [2] such as arsenene, antimonene, and bismuthene that have the advantage of larger spin-orbit coupling and higher ambient stability than phosphorene.Next to mono-elemental 2D materials, transition metal dichalcogenides (TMD) take on great importance for their complementary properties. For instance, VSe 2 is characterized by piezoelectricity and room-temperature ferromagnetism, coupled with a good ambient stability. [7] Among TMD, the fundamental properties of MoS 2 are studied, [8] and it is also explored as a semiconductor in novel transistors based on vertical van der Waals heterostructures with graphene. [9] WS 2 is a good candidate for optoelectronic devices, because it has the highest band gap (2.0 eV) in the visible region and a very strong photoluminescence emission. [10] Hence, there is the need to synthesize TMD nanosheets on a large scale, for example by moving towards a chemical wet procedure such as solvent-assisted exfoliation. [11] Heterostructures of interest are also built from graphene with aluminum nitride or oxide. In this way, the Fermi level can be finely tuned, achieving an improvement over graphene-based devices. [12] Alternatively, fabrication of one-dimensional graphene nanoribbons [13] by chemical vapor deposition on Ge(111) has been s...