Scanning tunnelling microscopy of MgO ultrathin films on Ag(001) (original) (raw)
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Growth-mode and interface structure of epitaxial ultrathin MgO/Ag(001) films
Journal of Physics: Condensed Matter, 2021
MgO ultrathin films are of great technological importance as electron tunneling barrier in electronics and spintronics, and as template for metallic clusters in catalysis and for molecular networks for 2D electronics. The wide band-gap of MgO allows for a very effective decoupling from the substrate. The films morphology and the detailed structure of the interface are crucial for applications, controlling the electronic transfer. Using surface x-ray diffraction, we studied the growth-mode and the structure of MgO/Ag(001) ultrathin films elaborated by reactive molecular beam epitaxy as function of the substrate temperature. We observed that deposition of about 1 monolayer results in an MgO(001) film in coherent epitaxy, with the oxygen atoms on top of silver as predicted by DFT calculations, and an interlayer distance at the interface of about 270 pm. Under well-defined conditions, a sharp MgO bilayer is formed covering a fraction of the substrate surface.
Interface Oxygen Induced Internal Structures of Ultrathin MgO Islands Grown on Ag(100)
The Journal of Physical Chemistry C, 2020
When imaged by scanning tunneling microscopy (STM) at constant current under nontopographic conditions, nanosized MgO islands grown on Ag(100) may show a fractal dark internal structure and/or appear embedded in the surface. On the contrary, when imaged under topographic conditions, the same islands appear as nearly flat plateaus. Islands showing different contrast and a different contrast dependence on sample bias have been often reported in the literature, but a thorough explanation is still missing. Here we demonstrate, on the basis of of experimental data and computational results, that such structure is due to the accumulation of oxygen at the MgO islands− substrate interface. Interestingly, interfacial oxygen concentrates at the center of the islands.
Structure and morphology of the Ag/MgO(001) interface during in situ growth at room temperature
Physical Review B, 1999
The structure and morphology of Ag deposits grown at room temperature on high-quality MgO͑001͒ surfaces have been investigated in situ, from 0.2 to 300 equivalent monolayers ͑ML͒ of Ag deposited. Surface x-ray diffraction and grazing incidence small angle x-ray scattering parallel and perpendicular to the surface were combined. Nucleation, growth, and coalescence of islands are found from 0.2 ML. The average in-plane width, height, and in-plane separation of growing islands are deduced and are found to reproduce well the Mg 1s x-ray photoemission spectroscopy spectrum previously reported by other authors. The height over width ratio of the islands is ϳ0.37Ϯ0.05 at all stages of the deposit. Ag grows in cube-on-cube epitaxy with respect to the MgO͑001͒ substrate. A very unusual evolution of the state of strain in Ag with increasing amount of Ag deposited is observed. Below 4-6 ML ͑island width smaller than 90 Å͒, the small Ag islands are coherent with the MgO. Below 1 ML ͑island width smaller than 35 Å͒, they have their bulk lattice parameter, and between 1 and 4 ML they become more and more strained by the MgO substrate, with an average lattice parameter intermediate between those of Ag and of MgO. Around 4-6 ML, the islands reach a critical size and misfit dislocations are introduced at the edges. Above 30 ML, the film is almost continuous, and the interfacial misfit dislocations reorder to form a square network, oriented along ͗110͘ directions. Stacking faults appear in Ag at this stage. A small amount of twinned Ag also starts to grow around 4 ML. This unusual evolution of the strain in the Ag islands and the following introduction of misfit dislocations are interpreted on the basis of a one-dimensional Frenkel-Kontorova model involving a very weak Ag-MgO interaction and a weak corrugation of the interatomic potential. Quantitative measurements and analysis of the MgO crystal truncation rods ͑CTR's͒ during growth were shown to provide different structural parameters of the interface that are important for theoretical calculation, especially the epitaxial site, above oxygen atoms of the substrate, and the interfacial distance (2.52Ϯ0.1 Å). The origin of the interference along the CTR's is discussed according to the strain state of the epitaxial Ag. ͓S0163-1829͑99͒14131-6͔ PHYSICAL REVIEW B 15 AUGUST 1999-II VOLUME 60, NUMBER 8 PRB 60 0163-1829/99/60͑8͒/5858͑14͒/$15.00 5858
Thickness-dependent strain in epitaxial MgO layers on Ag()
Surface Science, 2002
Thin MgO films (1-20 ML) were synthesised by evaporating the metallic component in oxygen atmosphere on Ag(0 0 1). The reduced lattice misfit (3.1%) between the (0 0 1) surface unit meshes of the Ag fcc and MgO rock salt structure leads to the formation of epitaxial MgO layers with the (0 0 1)MgOk(0 0 1)Ag and [1 0 0]MgOk[1 0 0]Ag orientation relationships. The oxide layers were structurally characterized by LEED and modulated electron emission, while their surface morphology was studied by STM. In spite of the weak interaction between the oxide overlayer and the substrate, the occurrence of a significant tetragonal distortion of the MgO structure has been detected. We investigated in detail the strain in the oxide layer as a function of the thickness. Thin (<6 ML) films exhibit an expansion along the growth axis with respect to the equilibrium phase. This vertical strain progressively reduces as the thickness increases and disappears at about 9 ML. A Poisson ratio larger than the bulk MgO value has been obtained for the 3 ML film, while the bulk value is appropriate films thicker than 6 ML. STM data indicate that an almost complete substrate coverage is obtained at 3 ML deposition. Ó
Scanning Tunneling Microscopy of the Ordered Water Monolayer on MgO(001)/Ag(001) Ultrathin Films
The Journal of Physical Chemistry C, 2019
Two-dimensionally ordered monolayers of water on MgO(001) have been extensively studied in the past using diffraction and spectroscopic and computational methods, but direct microscopic imaging has not been reported so far. Here, we present a scanning tunneling microscopy (STM) study, supported by infrared and X-ray photoelectron spectroscopy, of the c(4 × 2)-10H 2 O and p(3 × 2)-6H 2 O structures prepared on ultrathin MgO(001)/Ag(001) films. For the applied tunneling conditions, the contrast in the STM images originates from the hydroxyl groups, which result from water dissociation within the monolayer. The observed periodicities match the structures for the energetically most favorable c(4 × 2) and p(3 × 2) monolayer phases obtained from density functional calculations. Although the molecular water species within the monolayers, which are essential for the stabilization of the hydroxyl groups, could not be resolved, the STM results presented in this study provide further confirmation of the predicted structural models of the c(4 × 2)-10H 2 O and p(3 × 2)-6H 2 O monolayers.
Topography and work function measurements of thin MgO(001) films on Ag(001) by nc-AFM and KPFM
Physical Chemistry Chemical Physics, 2010
The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are À1.1 and À1.4 eV, respectively, in good agreement with recent calculations and experiments.
Stoichiometry and morphology of MgO films grown reactively on Ag(100)
Applied Surface Science, 1999
The stoichiometry and the morphology of MgO adlayers epitaxially grown on Ag 100 are studied by means of X-ray Ž. Ž. photoelectron spectroscopy XPS and high-resolution spot profile analysis low-energy electron diffraction SPA-LEED. For comparison, Mg deposited at 200 K is oxidized after and during growth, respectively. The post-oxidized film is dominated by stoichiometric MgO but shows also non-stoichiometric species. Annealing the film to 700 K reduces the latter components. Strong differences have been found for the epitaxy of both kinds of MgO films, depending on the preparation Ž. condition. The post-oxidized film shows a clear diffraction pattern with 12-fold symmetry due to MgO 100 grains in three rotational orientations only after annealing to 700 K. Depositing and oxidizing simultaneously the Mg the oxide film shows a Ž. better film quality no metallic Mg, a small MgO fraction and epitaxy with 1 = 1 already at 200 K.
To simulate the properties of ultrathin layers of magnesium oxide epitaxially grown on silver (001), we have adopted a periodic slab model, consisting of six layers of Ag covered on both sides with an MgO monolayer. All calculations have been performed with the CRYSTAL98 program. Several DFT functionals were tried and a rich basis set was adopted. The electronic and structural properties of the two bulk materials (Ag and MgO) were quite accurately reproduced. The presence of the metallic substrate was found to have an appreciable influence on the structural and electronic features of the oxide surface. In the most stable configuration (O ions directly above Ag atoms, Mg ions in the hollow sites), the surface is corrugated, and there is a net transfer of electrons from the overlayer to the metal, leading to a substantial reduction of the work function of the metal and to a decrease of the electrostatic field at the surface. The reactivity properties of the supported oxide surface have been investigated by studying the interaction of the composite material with water molecules.
Physical Review B, 2005
Submonolayer structures of MgO epitaxially deposited on Ag͑001͒ have been simulated theoretically in order to gain information on the mode of growth. The model adopted consists in a thin silver slab covered on both sides by MgO "polymers" ͑narrow ribbons monoatomic in height͒ sufficiently distant to prevent interactions among them. An ab initio DFT periodic technique has been adopted, whose adequacy has previously been checked for the case of perfect complete MgO/ Ag overlayers. Two orientations of the polymers have been considered, corresponding to two modes of growth experimentally observed: a nonpolar ͑ NP͒ and a polar ͑P͒ one, the latter being limited at the two borders by rows of bicoordinated O and Mg ions. We have considered both the case where the polymers sit ͑are "supported"͒ on the flat metal surface, and where they are "embedded" in grooves at the surface. The results of the simulations ͑equilibrium geometries, electronic structure and energy data͒ can be summarized as follows: ͑i͒ Interaction with the metal substrate enormously contributes to reducing the instability of the polar border; nevertheless, both for islands supported or embedded at the silver surface, the role of the metal is not sufficient to stabilize the P-with respect to the NP orientation. ͑ii͒ Steps or vacancy depressions at Ag͑001͒ with edges oriented along the silver ͗110͘ symmetry direction are considerably favored ͑by almost 25% per unit length͒ over the ͗100͘ oriented edges in agreement with observations; the preexistence of the former type of steps may favor kinetically the nucleation of P-oriented islands leaning against them. ͑iii͒ The formation of islands embedded in the metal is in all cases thermodynamically favored with respect to supported ones; this means that, in favorable conditions, the growing oxide film might displace silver atoms and penetrate to some extent in the metal. ͑iv͒ The electronic and electrostatic features at polar borders suggest that they could be characterized by very high chemical activity.