Most of the large variety of 2D materials are derived from their parent van der Waals (vdWs) crystals, thus their atomic struc tures are identical to the bulk counterparts with some minor lattice constant differ ence due to the reduced dimensionality and vanished interlayer interaction. Simi larly, some 2D monolayers, of which a typical example would be silicene, [10] stem from bulk materials with covalent bonds. Although their bonding may significantly change (silicene: sp 2 ; bulk silicon: sp 3 ) when they go from the bulk material to the 2D monolayer, their atomic structures are analogous. However, a slight atomic relaxation may occur in order to balance the reduced bonding coordination in the 2D forms. Therefore, in these cases, the determination of the 2D atomic structures is quite straightforward.In contrast, most metal oxides feature strong interlayer ionic bonds. The lack of a strong interlayer interaction in their 2D forms usually introduces dangling bonds, leading to strong surface polarization which induces surface instability of 2D metal oxides. Pronounced lattice relaxation, prominent struc tural reconstruction and substrate effects have been identified as the main mechanisms for compensating such strong sur face polarization in 2D metal oxides, as have been observed for a Pd 5 O 4 overlayer on Pd(111), [22] a strained PdO(101) layer on Pd (100), [23] a Ag 1.83 O trilayer on Ag(111), [24] a RhO 2 trilayer on Rh(111), [25] multiple phases of 2D Mn oxides on Pd(100), [26] and TiO 2 on rutile TiO 2 (011). [27] All of these significant changes increase the difficulty of synthesizing 2D metal oxides, as well as pose a challenge to computational structure prediction methods. Notwithstanding, more recently, spectacular progress has been made in prediction, design, preparation, and charac terization of oxide monolayers owing to the advancement of growth technologies and novel synthesis routes, as well as the development of computational and theoretical methods. [28][29][30][31][32][33] The structural reconstructions in combination with the elec tron confinement in 2D and the large surfacetovolume ratio endow 2D transition metal oxides (TMOs) with stunning physical/chemical properties. Moreover, the 2D TMOs showThe discovery of graphene has stimulated dramatic research interest on other 2D materials including transition metal oxide (TMO) monolayers in order to realize novel functionalization and applications. Due to reduced bonding coordination and strong surface polarization, the structures of most TMOs in the monolayer limit are very different from their bulk counterparts, as well as their physical and chemical properties. In this brief review, the authors sum marize recent research progress on atomically thin TMO layers. The focus is on the structural properties of the TMOs and their interaction with the sub strates from the computational point of view. The authors also introduce the potential applications of the TMO 2D materials on supercapacitors, photo catalysts, batteries, and sensors.

The nitrogen-doping induced changes in optical properties and electronic structures of SrTiO3 films have been investigated by using spectroscopic ellipsometry and x-ray photoemission spectroscopy. Combined with the first-principles calculations, it is found that the localized N 2p states above O 2p states are attributed to the new absorption edge at 500nm and the photoactivity in the visible light region. Our results are consistent with both recent experimental and theoretical studies on nitrogen-doped TiO2, where the visible light responses arise from the localized N 2p states slightly above the valence-band edge rather than the band gap narrowing.

Ge 3 N 4 dielectrics were prepared on Ge surface by in situ direct atomic source nitridation. The thermal stability and band alignments for Ge3N4∕Ge interfaces have been studied by using high-resolution x-ray photoemission spectroscopy. The in situ thermal treatment shows that Ge3N4 film has higher temperature thermal stability up to 550°C in vacuum. The conduction- and valence-band offsets at Ge3N4∕Ge interface are quite asymmetrical with the values of 2.22 and 1.11eV, respectively.

The kinetics of the interfacial layer (IL) growth between Hf aluminates and the Si substrate during high-temperature rapid thermal annealing (RTA) in either N2 (∼10 Torr) or high vacuum (∼2×10−5 Torr) is studied by high-resolution x-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy. The significant difference of the IL growth observed between high vacuum and relatively oxygen-rich N2 annealing (both at 1000 °C) is shown to be caused by the oxygen species from the annealing ambient. Our results also show that Hf aluminates exhibit much stronger resistance to oxygen diffusion than pure HfO2 during RTA in N2 ambient, and the resistance becomes stronger with more Al incorporated into HfO2. This observation is explained by the combined effects of (i) smaller oxygen diffusion coefficient of Al2O3 than HfO2, and (ii) higher crystallization temperature of the Hf aluminates.

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