2D Semiconductors

The field of two-dimensional (2D) materials is currently one of the most actively-researched areas in condensed matter physics. From an initial interest in graphene, a semimetal, and semiconductors such as BN and MoS2 [1], interest has shifted back to other elemental 2D materials such as silicene [2], phosphorene [3], etc. Additionally, roadmaps towards commercialization have been developed [4]. The goal of this special issue of Journal of Physics: Condensed Matter is to assemble articles on various aspects of 2D materials, from different material systems, so as to provide a diverse picture of current research. All too often, focus is either on one material or one characterization or one application. Fittingly, the issue starts with a contribution on graphene. While perfect graphene does not have a band gap, one can occur with the introduction of effects such as adatoms, finite size and defects. Hollen et al [5] use scanning tunneling microscopy and tunneling spectroscopy to study native defects in graphene islands grown by ultrahigh vacuum decomposition of ethylene on Cu(1 1 1). The defects were identified to be single vacancies with an electronic signature that is primarily visible in the Cu surface state. The contribution of Li et al [6] studies the interaction of silicene with silver. Silicene was probably the first 2D material post-graphene demonstrated (theoretically) to have a Dirac cone [2], the linear dispersion which leads to exciting new properties compared to conventional 3D semiconductors which tend to have quadratic dispersions. The interaction of silicene with silver is important since silicene has primarily been made on Ag(1 1 1) and the nature of the interaction is still under question [7]. Here, using scanning tunneling microscopy and Raman spectroscopy, it is shown that Ag atoms deposited on (3 × 3) and ( 3 3 × ) silicene, with all the growth happening in ultrahigh vacuum molecular beam epitaxy, do not interact with the silicene. After graphene and silicene comes, naturally, germanene. Young et al [8] present an investigation of the electrical properties of multilayer hydrogenated germanene (also known as germanane). This was made by wet chemistry. They reveal that doping can be achieved in a water environment. Resistivity measurements were performed on bulk crystals, showing large anisotropy and environmental effect, with a decrease of as much as three orders of magnitude in water. Excitons are the elementary excitations of a semiconductor when it is excited optically. From a model calculation, Tseng et al [9] show that exciton lifetimes can be extended in monolayer transition-metal dichalcogenides, using dark exciton states that are addressed using pulsed infrared light. This effect could potentially be applied for energy and quantum information storage. Most research on 2D materials has focused on inorganic materials, yet there are also organic systems. Peng et al [10] provide a theoretical study of one such system which they term covalent organic frameworks (COF). They describe bottom-up strategies for constructing these 2D-COF and carry out first-principles calculations to study their photocatalytic activity under visible light. The proposal is to use them for visible-light driven hydrogen production from water. We thank all contributors to this special issue.