STM study of Bi-on-Cu(1 0 0) Paul Wynblatt a , Dominique Chatain b, * , Alain Ranguis b a Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA b Centre de Recherche en Matie `re Condense ´e et Nanosciences 1 – CNRS, Campus de Luminy – case 913, 13288 Marseille Cedex 9, France Received 21 November 2006; accepted for publication 19 January 2007 Available online 4 February 2007 Abstract Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 · 2), c(9 p 2 · p 2)R45°, and p(10 · 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 · 2) phase transforms to the c(9 p 2 · p 2)R45° phase by a first order transition, whereas the transition from c(9 p 2 · p 2)R45° to p(10 · 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step–step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step–step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu–Bi alloys is temperature dependent. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Surface structure, morphology; Surface phase transition; Bismuth adsorption; Copper (1 0 0); Surface diffusion 1. Introduction Bi is known to form a variety of structures on Cu(1 0 0), depending on coverage. These structures have been studied by LEED in the 1970s [1,2] and more recently by X-ray dif- fraction in the late 1990s [3,4]. Our interest in these struc- tures stems from recent work performed on the diffusion of Bi over Cu(100) [5], in which the interpretation of Bi- coverage profiles depends on a knowledge of the 2-d phases which exist at different coverages, as well as the coverage ranges over which the different phases can coexist. No pre- vious studies of this system by scanning tunneling micros- copy (STM) have been reported, although STM has proved to be very useful to determine the surface structures present in other metal-on-metal systems [6–8]. Our primary purpose in this study was to confirm the presence in the dif- fusion profiles of previously identified phases, and to gath- er information on the nature of phase coexistence at the surface. The question of phase coexistence is particularly important in the interpretation of diffusion profiles, as dis- cussed below. In addition, we have made observations of the effects of Bi adsorption on the stability of the step struc- tures of Cu(1 0 0). These observations are relevant to previ- ous work on the equilibrium crystal shapes of pure Cu and of Bi-saturated Cu [9,10], and are therefore also included in the present paper. The structures of Bi on Cu(1 0 0) identified by Meyer- heim et al. [3,4] are summarized in Table 1. In this table, we define the coverage of a particular structure by the ratio of the Bi 2-d atom density to the maximum Bi-coverage in equilibrium with 3-d Bi islands, h MAX . Since some previous authors [1–4] have defined coverage with respect to Cu(1 0 0) sites, we also give those values in parentheses. The third column of the table indicates whether the 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.01.050 * Corresponding author. Tel.: +33 (0)6 60 30 28 90; fax: +33 (0)4 91 41 89 16. E-mail address: chatain@crmcn.univ-mrs.fr (D. Chatain). 1 Laboratoire propre du CNRS, associe ´ aux Universite ´s d’Aix-Marseille 2 et 3. www.elsevier.com/locate/susc Surface Science 601 (2007) 1623–1629