Ti vacancies on the (001) surface of TiS 2 detected by scanning tunneling microscopy: A combined experimental and theoretical study Emilie Amzallag, Herve ´ Martinez * , Isabelle Baraille, Michel Re ´rat, Michel Loudet, Danielle Gonbeau Universite ´ de Pau et des Pays de l’Adour, IPREM e ECP/CNRS UMR5254, He ´lioparc Pau Pyre ´ne ´es, 2 av. Pierre Angot, 64053 Pau cedex 9, France Received 18 January 2007; received in revised form 6 April 2007; accepted 12 April 2007 Available online 21 April 2007 Abstract Various defects e either bright or dark triangular defects e are observed on the titanium disulfide surface (001) by ultra high vacuum scan- ning 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, we have performed ab initio periodic LCAO-B3LYP calculations on a (5  5) biperiodic supercell to model the electronic and geometrical in- volvements of Ti vacancies and to generate the theoretical STM images within the TersoffeHamann approximation. The relaxed structure of the Ti vacancy shows an inward movement of the neighboring sulfur atoms at the surface. However, the occupied vacancy electronic states at the Fermi level are mainly developed on the atomic orbitals of the first S neighbors at the surface, leading to bright triangular zones on the simulated image. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Density functional calculations; Ab initio; Scanning tunneling microscopy (STM); Disulfur titanium; Vacancy; Surface defect 1. Introduction Surface defects created by cleavage are poorly understood and there is an obvious need for more work in this field. In par- ticular, point defects alter the local electronic density that can change the local chemical behavior of the surface. Thus, the fundamental electronic and structural properties of defects may be important information for understanding the chemical reactivity of a surface. In this area, scanning tunneling micros- copy (STM) has proven to be a useful tool for the description of surface properties at the atomic level; however, theoretical calculations are usually needed in order to properly interpret acquired images. In this paper, we have detected different kinds of defects characteristic of TiS 2 by UHV STM. This compound is gener- ally considered as a model system for intercalation chemistry which is a major subject of our group. Some of the detected defects have never been identified. Those defects already re- ported in the literature [1] have been explained on the basis of Ti vacancies in octahedral sites or Ti migration toward tet- rahedral sites. These features may cause local relaxation of the TiS 2 layers and involve the displacements of some sulfur atoms inside the surface. These differences of the sulfur atoms’ height can explain the modulating intensity of the im- age, because of the exponential link of the tunneling current with the surface to tip distance. However, these qualitative ex- planations do not consider the electronic implications of the defect creation, nor the geometrical consequences. This paper thus takes into account these phenomena and proposes a ratio- nal explanation of some defect identified in STM by applying the periodic ab initio DFT (density functional theory) method * Corresponding author. Tel.: þ33 5 59 40 75 99; fax: þ33 5 59 40 76 22. E-mail address: herve.martinez@univ-pau.fr (H. Martinez). 1293-2558/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2007.04.010 Solid State Sciences 9 (2007) 594e599 www.elsevier.com/locate/ssscie