2D MoB MBene and various dimensional defects in MoAlB MAB phase thin films
Two-dimensional (2D) inorganic transition metal boride nanosheets are emerging materials for energy application due to their unique properties. Typical processing routes involve chemical etching of bulk material synthesized via solid-state reaction at temperatures above 1000 °C. In our work, we investigate the formation of MoB MBene domains in MoAlB thin films grown at 700 °C along with various other defects such as 90-degree twist boundaries and compositional defects.
2D MBenes are derivatives of ternary MAB phases and hold promising potential towards various energy harvesting applications. Their synthesis via etching of bulk material or within thin films is complex and often involves local structural transformation and defect formation. Accordingly, the analysis of the local atomic arrangement of the MBene and other defects in MoAlB is important for developing new synthesis strategies and to understand the role of defects in general [1,2].
Our studies are dedicated to unravel MBene and other defect formation in thin MoAlB MAB films deposited on sapphire substrates which are grown at moderated temperatures. The thin films are deposited by direct current magnetron sputtering at Aachen University using either two targets (MoB and Al)  or a MoAlB compound target [1,2]. The films are investigated down to the atomic scale using advanced aberration corrected (scanning) transmission electron microscopy ((S)TEM) techniques. For imaging high angle annular dark field (HAADF) or annular bright field (ABF) detectors are employed, while the local composition is studied by energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS).
Cross sectional HAADF STEM indicates a straightforward processing pathway for the direct formation of MoAlB-MoB MBene-AlOx heterostructures without using chemical etching (figure 1a) . The deintercalated Al reacts with residual oxygen and forms amorphous AlOx phase adjacent to the MoB MBene. In the vicinity of the MoB MBene and within the MoAlB grains other defects such as twist boundaries and compositional defects are found (figure 1b) [1,2]