Michel Loudet

Studies of 1T TiS2 by STM, AFM and XPS: The mechanism of hydrolysis in air

Scanning tunnelling microscopy, atomic force microscopy and X-ray photoelectron spectroscopy were used to study the surface of TiS2, a layered material. STM and AFM images in air and at room temperature revealed the trigonal symmetry of the lattice. After a few minutes exposure to air, we have shown by AFM and XPS the growth of a thin layer of TiO2 on the surface.

Electronic structure of intercalated metal disulfides (Ag13TiS2 and Fe13TiS2) studied by XPS and theoretical calculations

Abstract The effect of metal intercalation (silver and iron) into lT-Cdl 2 -type TiS 2 layered crystals, expressed as M x TiS 2 , has been studied by X-ray photoelectron spectroscopy (XPS) and self-consistent electronic calculations (augmented sphere wave method). The spectra are found to depend strongly on the guest metals, and we have shown how the XPS valence bands are modified by the ‘host-guest’ interactions by using calculated densities of states. In the first approximation, the chemical shift of the core peaks are correlated with Mulliken population analysis.

Interpretation of scanning tunneling microscopy and atomic force microscopy images of 1T-TiS2

Abstract The surface of 1T-TiS2 was examined by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The STM and AFM images of this compound were interpreted on the basis of the partial electron density ρ(r,EF) and total electron density ρ(r) of a slab which consists of six (001) 1T-TiS2 layers. Electronic structure calculations were performed using the ab-initio Hartree–Fock program crystal . It was found that the bright spots in experimental STM images correspond to sulfur atoms at both positive and negative bias voltages. The AFM image showed a periodicity which can be explained by the atomic corrugation at the surface. Structural defects on the surface were also investigated, and their interpretation constitutes experimental proof that only sulfur atoms were detected by scanning probe microscopies.

Experimental (XPS/STM) and theoretical (FLAPW) studies of model systems M1/4TiS2 (M=Fe, Co, Ni): influence of the inserted metal

Abstract The effects of metal insertion (iron, cobalt and nickel) into 1T-CdI2-type TiS2 layered crystals, expressed as MxTiS2, have been studied by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and band structure calculations (FLAPW method). The stoichiometry x=1/4 was chosen because of specific crystallographic features of the compounds studied. We focused our interest on the role played by chalcogen atoms. S 2p core spectra are found to depend strongly on their chemical surroundings (Ti or Ti and M) and on the guest metal. We imaged the top sulfur plane (001) for Fe1/4TiS2, Co1/4TiS2 and Ni1/4TiS2 and note that the results also depend on the compound considered. Theoretical calculations have been carried out in order to improve our knowledge of the electronic structure of M1/4TiS2 compounds and attempts are made to rationalize the experimental data.

Electronic structure (XPS and ab-initio band structure calculation) and scanning probe microscopy images of α-tin sulfide

Abstract The bulk and surface electronic structure of α-SnS is investigated by X-ray photoelectron spectroscopy and band structure calculations, using ab-initio Hartree-Fock program CRYSTAL. Calculated band structure and density of states are consistent with recent calculations using other methods. Scanning tunnelling microscopy (STM) and Atomic Force microscopy (AFM) images were examined by calculating the partial electron density ϱ ( r , E f ) and total electron density ϱ ( r ) of a slab which consists of height (100) α-SnS layers. It was found that bright spots in experimental STM images correspond to sulphur and tin atoms in positive bias voltage and to the outermost layer (tin atoms only) in negative bias voltage. However, the simulated image related to filled states cannot explain the new periodicity of the experimental pattern in low negative polarisation, which suggests the appearance of a surface charge density wave. The performed AFM image show a periodicity which can be simply explained by the atomic corrugation at the surface.

Surface analysis of two misfit layer compounds - (PbS)1.18(TiS2) and (PbS)1.18(TiS2)2 - By scanning probe microscopies (AFM and STM) and X-ray photoelectron spectroscopy (XPS)

Abstract Scanning tunneling microscopy, atomic force microscopy and X-ray photoelectron spectroscopy were used to study the surface of two misfit layer compounds, (PbS) 1.18 (TiS 2 ) and (PbS) 1.18 (TiS 2 ) 2 . For the cleaved compounds, STM and AFM images in air and at room temperature revealed the trigonal symmetry of the transition metal dichalcogenide TiS 2 -1T. After a few hours of exposure to air, AFM and XPS show that the surfaces are strongly modified, with different features according to the mono or the bilayer misfit compounds considered. Depending on the time exposure to air, these results were interpreted in terms of hydrolysis and oxidation reactions.

Further theoretical analyses (2D and 3D) of Ni1/4TiS2 probed by XPS/STM studies

The surface of Ni1/4TiS2 was investigated by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Electronic calculations were performed using the ab initio Hartree-Fock program crystal. The XPS results (chemical shift of core peaks) are satisfactorily accounted for through a Mulliken population analysis. Concerning the STM results, we imaged the top sulfur plane (0 0 1) and interpret the data on the basis of the partial electron density of a slab which consist of seven (0 0 1) Ni1/4TiS2 layers. It was found that the bright spots in experimental STM images correspond to sulfur atoms in a single metallic environment (Ti atoms). Beside higher electronic density, these atoms are highlighted because of their proximity to the tip compared with sulfur atoms in a double environment (Ti and Ni).

Electronic and structural properties of Ti vacancies on the (001) surface of TiS2: Theoretical scanning tunneling microscopy images

Various defects--either bright or dark triangular defects--are observed on the (001) titanium disulfide surface by ultrahigh vacuum scanning tunneling microscopy. The experimental interpretations of the images available in the literature suggest that a fraction of Ti atoms could be displaced from the octahedral site they occupied to vacant sites of the crystal structure, leading to more or less correlated defects. In this paper, the authors have performed ab initio periodic linear combination of atomic orbitals-generalized gradient approximation (LCAO-GGA) calculations on (5x5) and (4x4) biperiodic supercells to model the electronic and geometrical involvements of Ti vacancy. The relaxed atomic structures of each system and the wave-function character of the defect states are carefully analyzed before the theoretical scanning tunneling microscopy images are generated within the Tersoff-Hamann approximation. The relaxed structure of the Ti vacancy shows an inward movement of the neighboring sulfur atoms at the surface. However, the occupied electronic states of the vacancy at the Fermi level are mainly developed on the atomic orbitals of the first sulfur neighbors at the surface, leading to bright triangular zones on the simulated image.

Differentiated contrasts for M1/4TiS2 (M = Fe, Ni) UHV-STM images

Abstract The specific crystallographic features of M1/4TiS2 (M=Ni, Fe) lead us to carry out UHV-Scanning Tunneling Microscopy studies, in order to bring the different atoms chemical environment of the structure and the host–guest interactions to light. We imaged the top sulfur planes (001) for both compounds and proposed an explanation to the two well differentiated contrast observed and a surface structural arrangement. S2p XPS core spectra were recorded to highlight the different chemical surrounding for chalcogen atoms and their differentiation depending of the compound considered (Ni or Fe1/4TiS2).