Defects in an orthorhombic MoAlB MAB phase thin film grown at moderate synthesis temperature.

Rajib Sahu,Christina Scheu,Jan-Ole Achenbach,Dimitri Bogdanovski,Jochen M Schneider

doi:10.1039/d1nr07792a

Rajib Sahu, Christina Scheu + Show 3 more

Open Access

https://doi.org/10.1039/d1nr07792a

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Abstract

Here, we report on atomic scale characterization of various defects in a MoAlB (MAB) phase thin film grown by DC sputtering at a synthesis temperature of 700 °C. Aberration-corrected scanning transmission electron microscopy reveals the formation of an intergrown metastable Mo3Al2B4 phase accompanied by thermally stable 90° twist boundaries, coexisting within the pristine MoAlB matrix. The concurrent formation of these structural defects in the MoAlB matrix can be rationalized based on minute differences in formation enthalpies as shown via density functional theory calculations. The specific structural nature of both the twist boundary and compositional defect (Mo3Al2B4) in a MoAlB matrix is hitherto unreported in literature. Apart from these defects, faceted grain boundaries are observed. In the vicinity of amorphous AlOx regions, Al is deintercalated and a 2D MoB MBene phase is formed as reported before. Besides these amorphous AlOx regions, a few nanometer-sized 3D MoB clusters are found. The advancement of aberration-corrected scanning transmission electron microscopy significantly improves characterization from 1D to 3D defects which is important for thin film materials design for the moderate synthesis temperature range. The reported defects might play an important role in the formation of 2D MoB MBenes.

Highlights

We study various defects with different dimensionalities in an orthorhombic MoAlB MAB phase thin film grown at 700 °C by direct current (DC) magnetron sputtering, using a combined experimental and computational approach
A range of 10–16, 18–34, and 78–200 mrad was set as a collection angle for annular bright field (ABF), annular dark field (ADF) and high-angle annular dark field (HAADF) imaging, respectively
Projector-augmented waves (PAW)[28] with a cut-off energy of 500 eV were used for basis set representation, with electron exchange and correlation treated via the generalized gradient approximation-based functional parametrized by Perdew, Burke and Ernzerhof (PBE).[29]

Summary

Introduction

M4AlB6, and Mo7Al6B7.2,3 Unlike two-dimensional (2D) materials, where weak van der Waals interactions dominate the interlayer bonding, metallic M–Al interactions and strong covalent/ionic M–B bonds are present in MAB phases.[4,5] Both the orthorhombic and hexagonal MoAlB MAB phases with an equistoichiometric 1 : 1 : 1 composition are potential candidates for various applications such as protective coatings[6,7] and battery electrodes.[4,8,9,10,11] This is due to their good corrosion resistance,[12] high temperature stability,[12,13] and excellent electrical and thermal conductivities[14] etc. Achenbach et al reduced the growth temperature by 42% compared to the reported values and successfully synthesized a crystalline orthorhombic phase by direct current (DC) magnetron sputtering.[7] Recently, Evertz et al. Established that by adopting growth conditions that enable both surface and bulk diffusion, fully dense MoAlB coatings are grown.[17] It is well known that plasma-assisted thin film deposition methods often lead to the formation of metastable phases,[18,19] including a large number of defects of different dimensionalities.[18,19,20,21,22,23]. We study various defects with different dimensionalities in an orthorhombic MoAlB MAB phase thin film grown at 700 °C by DC magnetron sputtering, using a combined experimental and computational approach. The formation of MBene was observed in close vicinity of these defects, as reported earlier.[24]

Experimental

Density functional theory

Results and discussion

Conclusions