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Halogen Chemistry on Solid Surfaces
 

Introduction

The study of surface halogen chemistry in ultrahigh vacuum was started at the beginning of the seventies. The first papers were published in 1974-1975. The main purpose was to obtain the information about chemical state and structure of halogen layers on the surface in reaction process. Unfortunately in that time the experimental technique for study so complicated processes as chemical surface reactions was rather limited and did not permit to get the information on atomic level. Since the appearance of local probes for surface study such as Scanning Tunneling Microscope and Atomic Force Microscope the scientists have obtained the possibility to study the surface processes with atomic resolution.

Mechanism of Metal Chlorination

In the first our study of chlorination of Cu(111) in 1988-89 we obtained that chemical state of surface is dependent on the coverage of chlorine. It was established that at chlorine coverage less than one atomic layer only chemisorbed chlorine state can exist on the surface, but at two- or multiatomic layers of chlorine the surface chemical state is copper chloride. Another words the surface demonstrates bistability of chemical state dependent only on the chlorine coverage. We made two suppositions: first, such phenomenon can be general for surface chlorination process and secondly, it does not depend on the way of creation of chlorine coverage. Now we have confirmed these two points on the base of research of Cu(100) and Ag(111) chlorination and AgCl deposition on Ag(111).

Atomic Structures of Chlorine Monolayer Coverage

In situ use of STM for chlorination process permits us to understand the real space atomic structure of monolayer coverage and to obtain the lattice parameters. Application of Fourier Analysis to STM images enhances the accuracy of interatomic distance determination up to 0.01 nm. The complicated structures formed on the closed parked (111) planes of Ag and Cu were decoded as Ag(111)-(17x17)Cl and a set of one axis compressed chlorine structures on copper surface.

Local Structure of Chloride Phase

One of the unclear points of the reaction is the mechanism of nucleation and growth of chloride phase. We use STM, EELFS and XPD methods for local structure measurements. The direct evidence of AgCl islands formation on the surface upon chlorine adsorption at room temperature on Ag(111) has been obtained. Specific feature of modified silver surface is AgCl islands surrounded by ditches with 5-15 nm depth and untreated atomic plateaus. Atomic structure and chemical state of surface area empty of AgCl islands corresponds to saturated chlorine monolayer with (17x17)-Cl structure. It was also established that primary electron in AES can cause the transformation of silver chloride film. A strong influence of electrons on silver chloride film could be an origin of different interpretation of electron spectroscopy data. Atomic structure of 2D AgCl islands formed at low temperature corresponds to bulk structure of silver chloride. On Ag(100) and Cu(100) chloride phase grows in (111) direction as four domain structure. Interatomic distances measured by EELFS for 2D silver and copper chloride films formed on (111) planes at low temperature are close to the bulk values.

Current State of Affairs

Now there are several research centers in the world which are intensively studying the mechanism of surface halogenation of metals and semiconductors. Among them we'd like to mark Pittsburgh Surface Science Center (Prof. J.T. Yates), University of Washington in Seattle (Profs. A. Szabo and T. Engel), IBM Research Division (Prof. J. Boland) in USA, Cambridge University (Prof. D. King, and Dr. R. Lambert) and Nottingham University (Prof. R.G. Jones) in Great Britain, Florence University in Italy (Profs. J. Rovida and U. Bardi) and Tohoku University in Japan (Prof. T. Sakurai) as the places of the most interesting works in the field of surface chemistry.

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