Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Abstract

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Exploring 2D structures of indium oxide of different stoichiometry

“…This monolayer can further form metallic bilayers and even periodic 3D-layered-like structures and considered as an important prediction towards the realization of 3D metallic Si crystal [10]. We note that the most common polymorph of 2D silicon is honeycomb buckled and that a comprehensive survey into polymorphism in group IV 2D materials and their experimental verification, and in general post-dichalcogenide 2D materials, can be found in a most recent review [11].…”

“…The structural and electronic properties of 2D InO monolayer is presented in Figure 1 (panels a, c, e, and g). We find that 2D InO forms a monolayer structure of InSetype [11]. An evocative reference for it is a planar hexagonal monolayer of h-Sc 2 O 3 , h-V 2 O 3 , and h-Mn 2 O 3 [13], h-Y 2 O 3 [14], and h-Al 2 O 3 [15].…”

“…Particularly, 2D InO is perceived for being able to promote cutting-edge applications as to high-speed electronics by considering predictions for its outstanding electron mobility [8]. A more detailed study into the atomic structure of 2D InO is motivated by its recently reported experimental realization in spatial confinement by MOCVD [12] which has shown consistency with the structure of monolayer of InSe-type group III metal chalcogenides [11]. We further explore 2D structures of indium oxide with composition In 2 O 3 by involving a type of stable structure predicted previously for atomically thin 2D metal oxides of d-block elements such as hexagonal h-Sc 2 O 3 , h-V 2 O 3 , and h-Mn 2 O 3 [13], and h-Y 2 O 3 [14], and p-block elements such as h-Al 2 O 3 [15].…”

Novel Polymorph of GaSe

“…[ 15 ] In other words, a specific composition can form different crystal structures of entirely different symmetry and periodicity. [ 16 ] A special form of polymorphism is polytypism, which applies to closed‐packed or layered materials, where polytypes are characterized by constituent layers with identical structures but different periodicities perpendicular to the layer plane (i.e., different stacking sequences). [ 16 ] The well‐known polytypes of GaSe are named as ε‐(2R), β‐(2H), γ‐(3R), and δ‐(4H), based on different stacking sequences between the adjacent layers ( Figure a–d).…”

“…[ 16 ] A special form of polymorphism is polytypism, which applies to closed‐packed or layered materials, where polytypes are characterized by constituent layers with identical structures but different periodicities perpendicular to the layer plane (i.e., different stacking sequences). [ 16 ] The well‐known polytypes of GaSe are named as ε‐(2R), β‐(2H), γ‐(3R), and δ‐(4H), based on different stacking sequences between the adjacent layers ( Figure a–d). Despite the similarities in band structure, [ 17 ] the electronic and optical properties show particularities due to the presence or lack of symmetry.…”