Cr silicate as a prototype for engineering magnetic phases in air-stable two-dimensional transition-metal silicates
Nassar DoudinKayahan SaritasJin‐Cheng ZhengJ. Anibal BoscoboinikJerzy T. SadowskiPadraic ShaferAlpha T. N’DiayeMin LiSohrab Ismail‐BeigiEric I. Altman
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Abstract Identifying environmentally inert, ferromagnetic two-dimensional (2D) materials with high Curie temperatures ( T c ) down to the single layer limit has been an obstacle to fundamental studies of 2D magnetism and application of 2D heterostructures to spin-polarized devices. To address this challenge, the growth, structure and magnetic properties of a 2D Cr-silicate single layer on Pt(111) was investigated experimentally and theoretically. The layer was grown by sequentially depositing SiO and Cr followed by annealing in O 2 . Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and low energy electron microscopy all indicated a well-ordered layer that uniformly covered the surface, with STM and LEED indicating that the silicate relaxed to its favored lattice constant. Further experimental characterizations demonstrated that the Cr was nominally 3+ but with a lower electron density than typical trivalent Cr compounds. Comparison with theory identified a Cr 2 Si 2 O 9 structure that resembles a single layer of a dehydrogenated dioctahedral silicate. Magnetic circular dichroism in x-ray absorption spectroscopy revealed a ferromagnetically ordered state up to at least 80 K. Theoretical analysis revealed that the Cr in a dehydrogenated Cr-silicate/Pt(111) is more oxidized than Cr in freestanding Cr 2 Si 2 O 9 H 4 layers. This greater oxidation was found to enhance ferromagnetic coupling and suggests that the magnetism may be tuned by doping. The 2D Cr-silicate is the first member of a broad series of possible layered first-row transition metal silicates with magnetic order; thus, this paper introduces a new platform for investigating 2D ferromagnetism and the development of magnetoelectronic and spintronic devices by stacking 2D atomic layers.Keywords:
Magnetism
Low-energy electron diffraction
Bragg’s 1913 publication of the principles of X-ray crystallography came only a year after von Laue’s discovery of X-ray diffraction from crystals. Structure determination (of small molecules) with high-energy electron diffraction followed by just three years the 1927 discovery of electron diffraction by Davisson and Germer. By contrast, low-energy electron diffraction (LEED) would require four more decades before yielding its first structure determinations (of surfaces) around 1970. The delay was primarily due to the need for ultra-high vacuum and to a lesser extent to the need for a suitable theory to model multiple scattering. This review will sketch the development of surface crystallography by LEED and describe its principles and present capabilities.
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We determine the atomic structure of the (111) surface of an epitaxial ceria film using low-energy electron diffraction (LEED). The 3-fold-symmetric LEED patterns are consistent with a bulk-like termination of the (111) surface. By comparing the experimental dependence of diffraction intensity on electron energy (LEED-I(V) data) with simulations of dynamic scattering from different surface structures, we find that the CeO2(111) surface is terminated by a plane of oxygen atoms. We also find that the bond lengths in the top few surface layers of CeO2(111) are mostly undistorted from their bulk values, in general agreement with theoretical predictions. However, the topmost oxygen layer is further from the underlying cerium layer than the true bulk termination, an expansion that differs from theoretical predictions.
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Low-Energy Electron-Diffraction Studies of the Interaction of Oxygen with a Molybdenum (100) Surface
Oxygen adsorption on the (100) surface of molybdenum single crystals was examined by low-energy electron-diffraction (LEED) in a display-type apparatus. Continuous diffraction-pattern observations from an initially clean surface were made at reaction temperatures between room temperature and 850°C. Below 650°C, no new ordered structures were observed. At 750°C, four ordered structures were formed on the surface with increasing oxygen coverage. The thermal characteristics and kinetics of formation of these oxides are discussed. In addition, models of surface structure with less than a monolayer oxygen coverage are presented.
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Abstract Low-energy electron diffraction (LEED) is a technique for investigating the crystallography of surfaces and overlayers adsorbed on surfaces. This article describes the principles of diffraction from surfaces, and elucidates the method of sample preparation to achieve diffraction patterns. The article describes the limitations of surface sensitive electron diffraction and discusses the applications of LEED with examples.
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This discussion of a few preliminary experiments with nickel points out some of the potential uses of low energy electron diffraction in improving our understanding of many types of surface phenomena. The first, and probably the most basic use, is in the study of clean surfaces. As illustrated in this article, the physical properties of the surface layer of atoms may be totally unlike those in the bulk of the crystal. It is necessary to understand such phenomena before a thorough understanding of chemical effects on surfaces can be achieved. The adsorption of gases, oxidation and corrosion, and the formation of epitaxial layers can all be studied in great detail by low energy electron diffraction.
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Low-energy electron diffraction (LEED) is a technique for investigating the crystallography of surfaces and overlayers adsorbed on surfaces. This article provides a brief account of LEED, covering the principles and measurements of diffraction from surfaces. Some of the processes involved in sample preparation are described. In addition, the article discusses the limitations of surface-sensitive electron diffraction and the applications of LEED with examples.
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