Oxygen vacancy induced structural variations of exfoliated monolayerMnO2sheets

Abstract

We report findings on the structural stability of exfoliated monolayer MnO 2 sheets. Our study reveals that monolayer MnO 2 sheets display two specific kinds of structural modification under electron irradiation. An atomic reconstruction ͑2 ϫ 1͒ and a phase of MnO, induced by ordered oxygen vacancies, were identified by transmission electron microscopy techniques and further characterized by comparison with density-functional theory calculations. These findings are expected to significantly broaden current knowledge of the structural stability of ultrathin layered sheets. Single atomic-layered crystalline sheets, particularly graphene, have attracted significant attention recently due to their unique two-dimensional ͑2D͒ structures and new physicochemical properties.1-6 Strictly speaking, no perfect 2D crystalline sheet structure would exist in free space unless it became an inherent part of a bulk crystal. [7][8][9][10] It has been demonstrated that suspended graphene sheets are not perfectly flat but exhibit intrinsic microscopic corrugations or ripples out of their 2D planes. 8,10 In addition to graphene, metal oxide monolayer sheets derived from delamination of layered transition metal oxide compounds represent a class of 2D nanostructures which are scientifically interesting as well as having potentially very important technological applications.11,12 However, until now, little has been known about the degree of stability of these suspended monolayer ͑atomic or molecular͒ sheets possessing extremely large surface area and, if structural variations do occur, what types of structures could result. Undoubtedly, knowledge about the stability of these monolayer sheets will play a vital role in interpreting their properties [1][2][3][4] and in exploring their potential applications. 5,13 Atomic structures and reconstructions on surfaces of single crystals have been studied for more than 20 years since the noted 7 ϫ 7 Si ͑111͒ surfaces were discovered [14][15][16] and atomic structures of single-walled carbon nanotubes ͓one-dimensional ͑1D͔͒ have also been investigated extensively. [17][18][19] More recently, Ishigami et al. 7 reported the atomic structure of graphene formed on a SiO 2 substrate. They observed both triangular and hexagonal lattices in the scanning tunneling microscopy images and suggested that the formation of these structures could be attributed to the film curvature and/or charge traps on the SiO 2 surface.On the other hand, Meyer et al. 8 and Stankovich et al. 9 revealed significant local curvature within graphene and suggested that this curvature was caused by the lattice deformations. Nevertheless, no direct evidence of atomic reconstructions was provided with respect to these monolayer ͑molecular or atomic͒ sheets, limiting the pace of progress toward understanding their stability or the mechanism for their corrugations or curvatures. In fact, for 2D sheets with large numbers of surface atoms ͑i.e., on both faces͒ and likewise adatoms, 20 the atomic reconstructions or ordered rearrangements,...

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