Scanning Tunneling Microscopy of the Ytterbium Nanofilm Surface and Layers of Oxygen Molecules Adsorbed on It
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ICAL ELECTRONICS
Scanning Tunneling Microscopy of the Ytterbium Nanofilm Surface and Layers of Oxygen Molecules Adsorbed on It M. V. Kuz’mina,* and M. A. Mittseva aIoffe
Institutes, St. Petersburg, 194021 Russia *e-mail: [email protected]
Received March 11, 2020; revised March 11, 2020; accepted March 12, 2020
Abstract—The surfaces of Yb–Si(111) and O–Yb–Si(111) structures (with a thickness of ytterbium nanofilms of 16 monolayers (6.08 nm)) have been investigated for the first time using scanning tunnel microscopy, which has provided data on the morphology and phase composition of these surfaces. It is found that, prior to oxygen adsorption, the nanofilms exhibit a high degree of homogeneity over their thickness, grow in accordance with a mechanism very close to the layer-by-layer growth, and have a homogeneous crystalline structure. After oxygen adsorption, an island layer of oxygen molecules is formed with a thickness of 0.112 nm. It is shown that the nanofilms morphology in the regions of the film surface coated by a monomolecular oxygen film changes significantly, while the morphology of the surface regions uncoated with the adsorbed layer remains unchanged. DOI: 10.1134/S1063784220080125
INTRODUCTION Considerable attention is paid at present to analysis of nanometer-size structures [1, 2]. In particular, a large number of publications are devoted to investigation of adsorption properties of such structures. In a considerable number of such structures, oxygen and carbon monoxide molecules are used as an adsorbate [3–8]. A disadvantage of these publications is that the type of the adsorption bond of these molecules with the surface of nanoobjects, size dependences of their adsorption properties, and a possible influence of the adsorbate on their bulk properties have not been investigated systematically. In contrast to these publications, our investigations are mainly devoted to analysis of the type of the adsorption bond of molecules with the surface of nanoobjects (films with a thickness of a few nanometers) and the effect of the adsorbate on their bulk properties. For example, it was shown in [9–12] that CO molecules adsorbed on ytterbium nanofilms form a donor–acceptor bond with the substrate surface by two unshared electrons. It was also established that, for an ytterbium film thickness from 1 to 16 monolayers (ML) (0.38–6.08 nm), the adsorption of O2 and CO molecules is accompanied with its transition from the bivalent (electron configuration [Xe]4f 14 6s2, henceforth referred to as Yb2+) to the trivalent metal state (configuration [Xe]4f 13 5d1 6s2, Yb3+). According to our results, this transition is due to the Coulomb interaction of electrons localized on the surface, which form the bond of molecules with the surface, and con-
duction electrons of ytterbium. The result of this interaction is the passage of a part of conduction electrons from the metal to the adsorbed molecules. This passage is accompanied with the lowering of the 5d energy level of ytterbium to the position lower than the Fermi l
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