Although single- or multi-layer transition metal oxides (TMOs) have a longer history than other atomically thin materials and comprise a range of earth-abundant minerals that have been used for millenia as construction materials, pigments, lubricants and for heat management, they have received scant attention compared to other types of atomically thin materials – such as the more popular graphene and transition metal chalcogenides. This is despite the fact that TMOs are used routinely and are continuing to become a focus in many developing areas of research and industry. However, a team from RMIT and Monash universities in Australia and the National Institute for Materials Science in Japan have now helped to filled this knowledge gap, presenting a wide-ranging overview of atomically thin and layered TMOs to help demonstrate their interesting functionalities.

The physical and chemical properties of TMOs are determined typically by strongly correlated d electrons, and they are highly tunable due to the diversity of their chemical composition and crystal structure, as well as the comparative ease in inducing oxygen defects. In a review article in Applied Materials Today, Kourosh Kalantar-Zadeh and colleagues therefore argue that 2D and layered metal oxides have much to offer and should be explored further. They provide a comprehensive overview regarding 2D and layered TMOs, as well as the fundamentals and applications of planar TMOs and a look ahead to the prospects and pathways to new developments being offered by such TMOs.

"The number of oxygen atoms can be tuned to obtain specific crystal phases with various physical and chemical properties"Kourosh Kalantar-Zadeh

As in TMOs the transition metal s electrons are strongly pulled by oxygen, which plays an important role in the formation of specific electronic orbitals, and consequently the structural, physical and chemical properties are determined mostly by the strongly correlated d electrons, 2D TMOs tend to present different physical and chemical properties compared to their bulk counterparts. This produces a variety of unusual electronic properties, such as high temperature superconductivity and multiferroicity, and unique optical, mechanical and thermal phenomena. In addition, by reducing the thickness of TMOs, their catalytic and chemical characteristics can be changed.

As Karantar-Zadeh points out, “the number of oxygen atoms can be tuned to obtain specific crystal phases with various physical and chemical properties”. This makes these the basis of many different electronic components – they already play a major role in applications ranging from optics, electronics, catalysis and commercial energy storage/harvesting systems, to uses in sensors and biosystems – with research on their superconductivity also showing some fascinating outcomes.