Laboratory For Surface Modification

Structure and dynamics of SF₆ on Ru(0001) by N.S.Faradzhev, D.O.Kusmierek, B.V.Yakshinskiy, and T.E.Madey

1. The aim of the present study is to understand the adsorption properties and surface geometry of sulfur hexafluoride (SF₆) on ruthenium single crystal.
2. The study has been carried out using temperature-programmed desorption (TPD), low-energy electron diffraction (LEED) and electron-stimulated desorption ion angular distributions (ESDIAD) of both positive and negative ions.

3. In SF₆, the sulfur atoms have octahedral coordination, making it a highly symmetrical molecule. This molecule is known to be chemically inert and thus has some importance in high voltage applications.

4. The substrate is ruthenium single crystal, which has a hexagonal close-packed crystal lattice. The distance between neighboring fluorine atoms in the free SF₆ molecule is about 80% of the distance between adjacent ruthenium atoms. So the substrate and adsorbate geometries are alike but not ideal.

5. Temperature-programmed desorption studies measured for various doses of SF₆ on ruthenium reveal two features at about 90 to 100 K which exist in a wide range of doses. Their saturation is attributed to formation of first adsorbed layer. As the dose increases, the new peaks at lower temperatures are gradually formed indicating the growth of next adsorbed layers. The low temperatures and, thus, low values of desorption activation energies indicate that sulphur hexafluoride on ruthenium is primarily physisorbed.
6. Analysis of uptake curve, which demonstrates adsorption kinetics, indicates that at this adsorption temperature for coverages from fractional monolayer to multilayer the sticking coefficient of SF₆ on the substrate is nearly equal to unity.

7. F⁺ and F^- ions are the main fragments escaping from the surface in the submonolayer regime under electron bombardment. At the coverages, when SF₆ on the surface does not exceed 1 molecular layer, we observe strong off-normal emission of both positive and negative fluorine ions. These halo-like patterns indicate that there is no preferential azimuthal orientation of S-F bonds whereas their polar angles are similar.

8. Heating the sample to ~90K leads to dramatic changes of ESDIAD patterns. As you can see, for both ions, the halo splits into six off-normal beams indicating rearrangement of molecules. The similarity of ESDIAD patterns for both F+ and F- fragment proves that the ions escape from molecules in the same chemical states.

9. These observations are consistent with the following model of adsorption. SF₆ molecules adsorb on Ru by 3 F atoms with the other 3 pointing away from the surface. At lower temperatures the molecules are randomly oriented on the surface. Heating the sample induces rearrangement of the adsorbed layer. 6-fold symmetry of the patterns indicates the existence of 2 preferential azimuthal orientations of the molecules or 2 domains. The intensities of the signals from both domains are nearly identical. This means that the surface areas occupied by both domains are equal.

10. The origin of different domains can be explained by the presence of steps on the surface. The substrate is (0001) single crystal with hexagonal close-packed crystal structure. Each second layer is laterally shifted relative to the previous one. So there exists an ABAB stacking order, and near surface region of the crystal is characterized by 3-fold symmetry. Hence, the heating may lead to opposite orientation of molecules adsorbed on A- and B-terminated areas.

11. LEED studies do not reveal a long range order in the formed layer.

References:

1.
Faradzhev N.S., Kusmierek D.O., Yakshinskiy B.V., and Madey T.E.
Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) Low.Temp.Phys. 2003 / Fizika Nizkih Temperatur, 2003, V.29, N3 , pp. 286-295

2. Faradzhev N.S., Kusmierek D.O., Yakshinskiy B.V., Solovev S., and Madey T.E
Electron-stimulated desorption of F+ and F- from SF₆ on Ru(0001): Structure and Bonding Surf.Sci., 2003, V.528, N 1-3, pp.20-26

	3. In SF₆, the sulfur atoms have octahedral coordination, making it a highly symmetrical molecule. This molecule is known to be chemically inert and thus has some importance in high voltage applications.
	4. The substrate is ruthenium single crystal, which has a hexagonal close-packed crystal lattice. The distance between neighboring fluorine atoms in the free SF₆ molecule is about 80% of the distance between adjacent ruthenium atoms. So the substrate and adsorbate geometries are alike but not ideal.

	7. F⁺ and F^- ions are the main fragments escaping from the surface in the submonolayer regime under electron bombardment. At the coverages, when SF₆ on the surface does not exceed 1 molecular layer, we observe strong off-normal emission of both positive and negative fluorine ions. These halo-like patterns indicate that there is no preferential azimuthal orientation of S-F bonds whereas their polar angles are similar.
	8. Heating the sample to ~90K leads to dramatic changes of ESDIAD patterns. As you can see, for both ions, the halo splits into six off-normal beams indicating rearrangement of molecules. The similarity of ESDIAD patterns for both F+ and F- fragment proves that the ions escape from molecules in the same chemical states.
	9. These observations are consistent with the following model of adsorption. SF₆ molecules adsorb on Ru by 3 F atoms with the other 3 pointing away from the surface. At lower temperatures the molecules are randomly oriented on the surface. Heating the sample induces rearrangement of the adsorbed layer. 6-fold symmetry of the patterns indicates the existence of 2 preferential azimuthal orientations of the molecules or 2 domains. The intensities of the signals from both domains are nearly identical. This means that the surface areas occupied by both domains are equal.
	10. The origin of different domains can be explained by the presence of steps on the surface. The substrate is (0001) single crystal with hexagonal close-packed crystal structure. Each second layer is laterally shifted relative to the previous one. So there exists an ABAB stacking order, and near surface region of the crystal is characterized by 3-fold symmetry. Hence, the heating may lead to opposite orientation of molecules adsorbed on A- and B-terminated areas.

1.	Faradzhev N.S., Kusmierek D.O., Yakshinskiy B.V., and Madey T.E.	Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001)	Low.Temp.Phys. 2003 / Fizika Nizkih Temperatur, 2003, V.29, N3 , pp. 286-295
2.	Faradzhev N.S., Kusmierek D.O., Yakshinskiy B.V., Solovev S., and Madey T.E	Electron-stimulated desorption of F+ and F- from SF₆ on Ru(0001): Structure and Bonding	Surf.Sci., 2003, V.528, N 1-3, pp.20-26