Determination of Na submonolayer adsorption site on Cu(111) by low-energy ion blocking

Abstract

The structure of a submonolayer coverage of sodium adsorbed on a Cu͑111͒ surface at room temperature has been investigated using time-of-flight scattering and recoiling spectrometry. The effect of the adsorbed Na atoms on the angular distribution of scattered 2 keV H + ions is analyzed by molecular dynamics and scattering and recoiling imaging code simulations. It is shown that at a coverage = 0.25 monolayer, Na atoms preferentially populate the fcc threefold surface sites with a height of 2.7± 0.1 Å above the first-layer Cu atoms. At a lower coverage of = 0.10 ML, there is no adsorption site preference for the Na atoms on the Cu͑111͒ surface.The adsorption and coadsorption of alkali metals on metal surfaces are of considerable interest in surface science due to both their fundamental importance and technological applications. 1,2 Details of the adsorption of Na on Cu͑111͒ have been revealed through experimental 3-8 and theoretical studies. 9-11 In low-energy electron diffraction ͑LEED͒ studies, 4-6 either a p͑2 ϫ 2͒ pattern 4,5 or a ring pattern ͑less ordered phase͒ 6 have been observed at a coverage of = 0.25. The coverage is defined as the ratio between the number of adsorbed Na atoms and the number of Cu atoms in the outermost substrate layer. In low-temperature scanning tunneling microscopy ͑STM͒ studies, 7,8 a structure transition that included some intermediate phases, such as a mixing of p͑3 ϫ 3͒ and p͑2 ϫ 2͒ structures up to a coverage of = 0.25 with one Na atom per unit cell was suggested. However, the adsorption site preference of Na on Cu͑111͒ has not yet been conclusively revealed experimentally. Contradicting results about the adsorption site have also been obtained from various theoretical calculations 9-12 because the binding energy/atom between the hollow-site-centered site and the top-bridge site adsorption structures may be as small as 5 meV/atom at = 0.25. 10 A technique that can resolve such a question must have ultrahigh surface sensitivity. Advances in dechanneling and blocking effects in low-energy ion scattering ͑LEIS͒ have the unique ability to analyze the structure of the outermost atomic layers of materials. In this work, the LEIS technique of time-of-flight scattering and recoiling spectrometry ͑TOF-SARS͒ ͑Ref. 13͒ has been used to determine the Na adsorption site and adsorption height at low coverage on a "smooth" Cu͑111͒ surface.Low keV hydrogen ions ͑H + ͒ were used as the probing beam, with a typical accuracy of Ͻ0.1 Å for determination of interatomic spacings. 13 Since the energy of the ions is in the keV regime, the scattering cross sections are substantially enhanced as compared with the meV scattering counterpart. Consequently, the ions can only penetrate into the shallow subsurface layers where dechanneling readily occurs. Scattering and recoiling imaging code ͑SARIC͒ simulations [14][15][16] show that only the outermost four Cu layers of the Cu͑111͒ surface can be "seen" by most of the incoming H ions. These Cu atoms serve as point scatters and give rise to a flux of scatter...

Mechanisms Leading to Alkali Oxidation on Metal Surfaces

“…At room temperature, Na on Cu(111) forms a disordered phase up to the formation of the (3/2 × 3/2) structure (0.44 ML) with Na atoms occupying 3-fold hollow sites . On the contrary, on Ni(111) a (√3 × √3)R30°-Na structure has also been recently revealed for intermediate coverages (0.33 ML).…”

H− survival probability during collisions with Na/Cu(111)

“…We choose this particular coverage because, as shown in our previous studies, the Na atoms are adsorbed at the three-fold fcc site with an adsorption height of 5.2 a.u. [20,27]. We perform the calculations under the following assumptions: (1) In order to maximize the RCT effects, the H − ions collide directly with Na atoms.…”

“…As shown in our previous studies, the Na atoms are adsorbed at the three-fold fcc site with an adsorption height of 5.2 a.u. [20,27]. In both theoretical calculations and experiment, the projectile is approaching the Na/Cu(111) surface with an energy of 2 keV, and at a fixed incidence angle of 30°measured from surface.…”

“…In the past, we have successfully used H − ions in our calculations as incident projectile, to reproduce experimental data obtained with H + as incident projectile [16,17]. The Time-of-Flight Analysis of Scattered and Recoiled Particles (TOF-SARS) technique [16,19,20] is used in the experiment.…”

Charge transfer during H/H− collisions with Cu(100) and Cu(111) surfaces

“…Such studies are crucial for progress in various applied fields, such as plasma wall interactions in fusion devices, catalysis, space science, and film deposition . In addition, charge transfer is essential in adsorption and desorption processes, chemical reactions, quenching of metastable states, and in the ion formation during scattering and sputtering experiments …”