Abstract
Two-dimensional (2D) transition metal oxides (TMOs) are an emerging class of nanomaterials. Using density functional theory and ab initio molecular dynamics (AIMD) simulations, we carried out a systematic study of atomically thin metal oxide phases with compositions MO, M2O3, and MO2, for transition metal elements Sc, Ti, V, Cr, and Mn. We identified nine thermally stable structures that may be realized as free-standing nanosheets: hexagonal h-Sc2O3, h-V2O3, and h-Mn2O3; hexagonal t-VO, t-CrO, and t-MnO; and square sq-TiO, sq-VO, and sq-MnO. The t-MO phases are novel hexagonal structures which emerged naturally from phase transformations observed during AIMD simulations. The 2D TMOs were found to exhibit a wide range of remarkable electronic and magnetic properties, indicating that they are bright candidates for electronic and spintronic applications. Most exceptional in this regard is h-V2O3, that is the only phase that has been experimentally realized so far, and was found to be a ferromagnetic half-metal with Dirac-cone-like bands.
Highlights
The discovery of graphene and its fascinating properties[1] has incited a whole new field of research dedicated to the exploration of two-dimensional (2D) nanomaterials
Using top–down approaches, where monolayers are isolated from layered bulk structures, and bottom-up approaches, where thin films are obtained through deposition of molecules on substrates, added to graphene is a wide range of 2D materials consisting of elements other than carbon, such as h-BN, the mono-elemental 2D semiconductors silicene, phosphorene, and germanene, Mxenes, i.e., transition metal carbides, nitrides, and carbonitrides, and transition metal dichalcogenides (TMDs), such as MoS2.2,3 The exploration of 2D nanomaterials has only started little over a decade ago, but is rapidly progressing
density functional theory (DFT) calculations by Vittadini et al on 1–6 monolayer (ML) thick films derived from the main low-index surfaces of h-V2O3,15,16,23 we examined the stability of buckled 2D transition metal oxides (TMOs) phases, rerunning all simulations with buckled instead of planar anatase TiO2 revealed two anomalously stable films that are a restructuring of anatase TiO2 into a lepidocrocite-TiO2 nanosheet (derived from the 2 ML (001) film) and a novel TiO2 phase (derived from the 4 ML (101) film).[24]
Summary
The discovery of graphene and its fascinating properties[1] has incited a whole new field of research dedicated to the exploration of two-dimensional (2D) nanomaterials. Bulk phase transition metal oxides (TMOs) are known to have unique properties originating from their itinerant and strongly correlated transition metal (TM) d electrons. This strong electron correlation, where the TM d electrons experience a strong Coulomb repulsion from one another, leads to physical phenomena such as the metal–insulator transition[4,5] (MIT), where, upon tuning parameters such as temperature, pressure, and chemical composition, and often along with a structural and magnetic phase transition, the electronic structure of a system will undergo a MIT A TMO can exist as various stable polymorphs, stoichiometrically identical but crystallographically different phases, that for MnO2, for example, add up to a total number of six stable phases found so far[11] and may transform into one another with energy barriers as low as
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