Narrow Results

In this work we study the adsorption of Cs on S-covered Si(100)-(2 × 1) and Si(100)-(1 × 1) surfaces, as well as the adsorption of S on Cs-covered Si(100)-(2 × 1). The experiment was performed in an ultrahigh vacuum (UHV) chamber with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and work function (WF) measurements. Predeposited S increases the binding energy and the maximum amount of Cs that can be deposited on the surface. The presence of S inhibits the pattern of the characteristic WF curve of Cs on clean Si(100)-(2 × 1), i.e. an initial decrease to a minimum value, Φ_min, followed by an increase toward the value Φ_max of metallic Cs. The WF, instead, decreases to a value close to that of saturated Cs on clean Si(100)-(2 × 1), where it forms a plateau. This is characteristic of the covalent bonding of Cs with the semiconductor substrate. Independently of the sequence of Cs and S deposition, (a) the transition Si(100)-(2 × 1) → Si(100)-(1 × 1) occurs when Θ_S > 0.5 ML, and (b) the sites of Cs and S remain the same, with the Cs atoms residing between the S atoms. Heating of the S/Cs/Si composite surfaces to ~ 650 K causes a reorganization of the Cs and S adatoms in a tendency to form a Cs–S complex. The issue of site preference for Cs and S adatoms has been discussed in detail in the structural models provided.

We have studied the adsorption of sulfur at regular and defect sites of the MgO (001) surface using cluster models embedded in a large array of point charges by the density functional method. The calculated results indicate that it is a chemical adsorption regarding sulfur at both the regular site and the defect site of the MgO (001) surface. Especially for sulfur adsorbed at different oxygen vacancy sites (F, F⁺ and F²⁺ centers) and different magnesium vacancy sites (V, V^- and V^2- centers), it has very large adsorption energies, which reflects the fact that the MgO (001) surface with the vacancies is an excellent adsorbent for sulfur adsorption. Besides, we find that the adsorbed sulfur is almost inserted into the lattice for sulfur adsorbed at the magnesium vacancy site of the MgO (001) surface. The adsorption energy of sulfur on the MgO (001) surface with magnesium vacancies is much larger when compared to that on the MgO (001) surface with oxygen vacancies. At the same time, it is also found that the S behaves as an electron acceptor except that it is adsorbed at the magnesium vacancy site behaving as an electron donor.

The electronic structure of S adsorption on goethite (110) surface has been studied by ASED-MO cluster calculations. For S location, the most exposed surface atoms of goethite surface were selected. The calculations show that the surface offers several places for S adsorption. The most energetically stable system corresponds to S location above H atom.

We studied in detail the configurations that correspond to the higher OP values. For these configurations, the H-S and Fe-S computed distances are 2.1 and 3.7 Å, respectively. The H-S and Fe-S are mainly bonding interaction with OP values of 0.156 and 0.034, respectively. The Fe-S interaction mainly involves Fe 3d_x2-y2 atomic orbitals with lesser participation of Fe 4p_y and Fe 3d_yz atomic orbitals. The O-S interaction shows the same bonding and antibonding contributions giving a small OP value. The O-S interaction involves O 2p orbitals. There is an electron transfer to the Fe atom from the S atom. On the other hand, there is an electron transfer to S atom from the H and O atoms, respectively.

The electronic structure of H₂S adsorbed on the goethite (110) surface has been studied by ASED-MO cluster calculations. We have studied both the perpendicular and the parallel H₂S molecular adsorption on the FeOOH(110) surface. We have analyzed the adsorption configuration energies including rotation. The parallel species does not rotate during adsorption and corresponds to the most stable configuration. We have also studied the bonding contributions for the minimum energy configuration and the density of states plots.