STM observation of the initial Ag overlayer formation on the Ni(110) surface

抄録

Alloys between different kind of metals on surfaces often exhibit novel structures comparing to those in the bulk. For example, Ni and Au do not form alloys in the bulk form, but Au adsorbed on the topmost surface layer of Ni is known to induce surface two-dimensional alloying by replacing some of the Ni atoms on the surface with Au [1]. Since ultrathin films on metal surfaces will a unique structure, it will have high expectations for the discovery of a novel growth modes from the conventional thin film growth modes (FM, VW, SK) and for the development of new functionalities.

It is known that Ni-Ag, like Ni-Au, does not mix atoms in bulk form, and that Ag thin films on a Ni(111) surface grow in layer-by-layer and form a moire pattern due to the lattice mismatch [2], and that Ag(111) is formed on a Ni(100) surface has been confirmed in our laboratory. Although there have been many studies on metal heteroepitaxial growth by STM since the 1990s, there have been few observations of thin films of fcc metals at the atomic level, and the growth pattern of thin films, including interdiffusion and alloy formation, has not been clearly established at present. In this study, we studied the Ag-induced surface structures on the Ni(110) surface (Ni(110)-Ag) that has not been observed by ultra-high vacuum scanning tunneling microscopy (UHV-STM) and low-energy electron diffraction (LEED), depending on the growth and annealing conditions.

UHV-STM observation of the Ag-deposited surface on Ni(110) substrates at room temperature showed that Ag did not alloy with the substrate Ni atoms, but formed a reticulate structure. When deposited at 200°C, the reticular structure was aligned to form a stripe structure along the [001] direction. A 10×10 nm² STM image (Fig. 1) of the RT deposition shows that three regions coexist. Region A is a bright region with a periodic protrusion in the [001] direction, which corresponds to the substrate atomic rows. Therefore, Ag atoms are considered to grow epitaxially in the [11-0] direction to form bright bands in the [001] direction. They occasionally form wavy structures with a 3× periodicity in the [11-0] direction. Region B is noisy, where Ag atoms do not match the periodicity of the substrate in the [11-0] row and they are considered to fluctuate in the row. Region C is a trough structure with a depression of about 120 pm and is considered to be a region where no Ag atoms exist.

LEED observation showed one-tenth of the satellites spots near integer-order spots of the Ni(110) 1×1 structure in the [11-0] direction, indicating that epitaxially grown Ag atoms have a super structure along the [11-0] row.

References

[1] L. Pleth Nielsen et al., Phys. Rev. Lett. 74, 1159 (1995).

[2] S. Nakanishi et al., Phys. Rev. B 62, 13136 (2000).