Probing defect species on real surfaces from the analysis of the spectral profile of admolecules Hanen Zorgati a , Moncef Said a , Christophe Ramseyer b , Claude Girardet b, * a UR Physique des Solides, Faculté des Sciences de Monastir, 5019 Monastir, Tunisia b Laboratoire de Physique Moléculaire, UMR 6624, Université de Franche-Comté, 25030 Besançon, France article info Article history: Received 3 November 2008 Accepted for publication 29 January 2009 Available online 11 February 2009 Keywords: Surface control Infrared spectroscopy Physisorption Defects abstract The analysis of changes in the vibrational spectrum of infrared active molecules adsorbed on a ionic sur- face containing point or extended defects can be an efficient method to determine the nature and density of surface defects. We study the infrared response of ammonia molecules deposited on a ionic surface of MgO containing charge vacancies and dipolar defects in various concentrations and distributions and show significant changes assigned to the defects signature. A Monte Carlo approach is used to randomly deposit the probe molecules on the surface displaying random or regularly arranged defects at low temperature. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Probing the quality of surfaces at the nanoscopic scale has be- come a challenge to stimulate progress in nanotechnology. Indeed real surfaces generally display point and extended defects originat- ing either from thermodynamic principles or from local instabili- ties. Identification and characterization of defect sites on oxide surfaces is important in manufacture of electronic devices where defects cause significant degradations and in heterogenous cataly- sis as nucleation and reaction sites. Atomic vacancies and divacan- cies, and dangling chemical bonds, are responsible for the occurrence of local charge defects and of dipolar electric moments on ionic surfaces [1]. For instance, freshly cleaved MgO(1 0 0) sur- face shows many defects [2] which cannot be easily controlled (edges, corners, steps, kinks, ...). While MgO substrate cleaved in vacuum contains monoatomic steps and oxygen vacancies [3], water adsorption produces hydroxyl groups OH on this surface [4]. Another example is given by the formation of dipolar chains on vicinal semiconducting surfaces, as shown [5] by adsorption of hydrogen on the steps of Si(1 1 1). Aside from the microscopic probes (STM, AFM, ...) [6,7] which can however have an influence on the defects, direct spectroscopic approaches for evaluating the surface quality generally require high vacuum conditions and/or resolution in terms of signal over noise ratio, which is often redhibitory when the defect density on the surface is low. Therefore, indirect detection of defects using probe molecules has been currently performed. Various techniques have been used to probe vacancies on metals and metal oxides using CO, N 2 and CO 2 molecules [8,9]. Reflection–adsorption infra- red spectra of CO adsorbed on thin MgO films have been reported [10] in the temperature range 30–90 K. The comparison of the spectra recorded on single crystals and powder samples shows an increased concentration of low-coordinated adsorption sites on powders, indicating chemisorption processes. CO spectra col- lected at 60 K on powders with different surface areas were used to distinguish among various chemisorption sites through the fre- quency shift of the vibrational signals [11]. However, most of these studies concern chemisorption of the probe molecules directly above the defects, and only the strong spectroscopic signals corresponding to chemisorbed sites are usu- ally analyzed. The situation is fully different when the molecules do not chemically bind to the substrate, even at the defect sites. Studies of the adsorption, at low temperature, of small infrared ac- tive molecules on ionic surfaces even carefully prepared to mini- mize the defect occurrence, have shown that the corresponding spectra generally exhibit weak additional signals [12], interpreted as the influence of surface defects, in the vicinity of the intense vibrational peak due to molecule physisorption on regular sites. The spectral response of the probe molecules on this ideal surface (no defect) can then be used as the reference state to analyze the defect signature on a real surface containing defects. Molecules such as CO, CO 2 , NH 3 , etc. physisorbed on ionic and semiconduct- ing surfaces are well-known [13–17] to display strong intensity spectral signals and they can be used as efficient probes of surface morphology. Moreover, favoring the admolecule migration towards the defects on the surface by controlled temperature 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2009.01.042 * Corresponding author. Tel.: +33 3 81666483; fax: +33 3 81666475. E-mail address: claude.girardet@univ-fcomte.fr (C. Girardet). Surface Science 603 (2009) 887–894 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc