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Hwang, Jae-Yeol,Kim, Young-Min,Lee, Kyu Hyoung,Ohta, Hiromichi,Kim, Sung Wng American Chemical Society 2017 NANO LETTERS Vol.17 No.10
<P>Demands on high-quality layer structured two-dimen sional (2D) thin films such as pnictogen chalcogenides and transition metal dichalcogenides are growing due to the findings of exotic physical properties and potentials for device applications. However, the difficulties in controlling epitaxial growth and the unclear understanding of van der Waals epitaxy (vdWE) for a 2D chalcogenide film on a three-dimensional (3D) substrate have been major obstacles for the further advances of 2D materials. Here, we exploit the spontaneous vdWE of a high-quality 2D chalcogenide (Bi0.5Sb1.5Te3) film by the chalcogen-driven surface reconstruction of a conventional 3D sapphire substrate. It is verified that the in situ formation of a pseudomorphic Te atomic monolayer on the surface of sapphire, which results in a dangling bond-free surface, allows the spontaneous vdWE of 2D chalcogenide film. Since this route uses the natural surface reconstruction of sapphire with chalcogen under vacuum condition, it can be scalable and easily utilized for the developments of various 2D chalcogenide vdWE films through conventional thin-film fabrication technologies.</P>
Directing Oxygen Vacancy Channels in SrFeO<sub>2.5</sub> Epitaxial Thin Films
Khare, Amit,Lee, Jaekwang,Park, Jaeseoung,Kim, Gi-Yeop,Choi, Si-Young,Katase, Takayoshi,Roh, Seulki,Yoo, Tae Sup,Hwang, Jungseek,Ohta, Hiromichi,Son, Junwoo,Choi, Woo Seok American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.5
<P>Transition-metal oxides (TMOs) with brown-millerite (BM) structures possess one-dimensional oxygen. vacancy channels (OVCs), which play a key role in realizing high ionic conduction at low temperatures. The controllability of the vacancy channel orientation, thus, possesses a great potential for practical applications and would provide a better visualization of the diffusion pathways of ions in TMOs. In this study, the orientations of the OVCs in BM-SrFeO2.5 are stabilized along two crystallographic directions of the epitaxial thin films. The distinctively orientated phases are found to be highly stable and exhibit a. considerable difference in their electronic structures and optical properties, which could be understood in terms of orbital anisotropy. The control of the OVC orientation further leads to modifications in the hydrogenation of the BM-SrFeO2.5 thin films. The results demonstrate a strong correlation between crystallographic orientations, electronic structures, and ionic motion in the BM structure.</P>