Applied Surface Science 311 (2014) 643–647 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc 11-Hydroxyundecyl octadecyl disulfide self-assembled monolayers on Au(1 1 1) Erol Albayrak a , Semistan Karabuga b , Gianangelo Bracco c , M. Fatih Danıs ¸ man d,∗ a Department of Materials and Metallurgical Engineering, Ahi Evran University, Kırs ¸ ehir 40000, Turkey b Department of Chemistry, Kahramanmaras ¸ Sütc ¸ ü ˙ Imam University, Kahramanmaras ¸ 46030, Turkey c CNR-IMEM and Department of Physics, University of Genoa, via Dodecaneso 33, Genoa 16146, Italy d Department of Chemistry, Middle East Technical University, Ankara 06800, Turkey a r t i c l e i n f o Article history: Received 2 May 2014 Received in revised form 15 May 2014 Accepted 16 May 2014 Available online 28 May 2014 Keywords: Unsymmetric disulfides Self-assembled monolayers Au(1 1 1) Helium diffraction Low energy atom scattering Supersonic molecular beam deposition a b s t r a c t Here, we report a helium atom diffraction study of 11-hydroxyundecyl octadecyl disulfide (CH 3 (CH 2 ) 17 S S (CH 2 ) 11 OH, HOD) self-assembled monolayers (SAMs) produced by supersonic molecular beam deposition (SMBD). Two different lying down monolayer phases were observed depend- ing on the substrate temperature. At low temperatures a poorly ordered phase was observed, while the diffraction patterns of the film grown at high temperatures were similar to that of mercaptoundecanol (MUD) SAMs reported previously in the literature. The transition from the low temperature phase to the high temperature phase is due to S S bond cleavage at the surface. Desorption from several different chemisorbed and physisorbed states were observed with energies in the same range as observed for MUD and octadecanelthiol (ODT) SAMs. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Thiol self-assembled monolayers (SAMs) on Au(1 1 1) surfaces continue to attract attention due to their uses in many differ- ent applications and their role as a model system in fundamental surface science [1–4]. Their fundamental properties, like crystal structures, adsorption dynamics and mechanism are very impor- tant to understand to improve the performance of the applications that utilize these SAMs [1–5]. Alkanethiol SAMs on Au(1 1 1) are well studied systems and at low coverage they adopt lying down striped phases with (p × √ 3) rectangular unit cells where p is the stripe periodicity, as depicted in Fig. 1, which depends on the alkyl chain length.[1,2,6] At full coverage, however, thiol molecules stand up and form the well-known ( √ 3 × √ 3)R30 ◦ structure [1,2]. Unsymmetric disulfide (R S S R ′ , will be referred to UDS) SAMs, on the other hand, are particularly important for preparing mixed monolayers and for understanding thiol film formation mechanism and kinetics. To this end, disulfides where the unsymmetry was provided by using alkyl groups with different chain lengths or with different ∗ Corresponding author. Tel.: +90 0312 2107618; fax: +90 0312 2103200. E-mail address: danisman@metu.edu.tr (M.F. Danıs ¸ man). functionalities (like hydroxylated and fluorinated ones) were intensively studied [7–16]. One of the main motivations in these studies was to elucidate whether a phase separation takes place or not, which has implications in preparation of patterned surfaces and which is a way of determining if S S bond cleaves during adsorption. In all of these studies disulfides was grown from solution and the film coverage was controlled by adjusting the concentration of the solution and/or the immersion time. Resulting films were mostly studied by scanning probe microscopy and spec- troscopy techniques. While in many cases no phase separation was observed for the as grown full coverage (high density) UDS SAMs [11–13], phase separation after annealing have also been reported [14,15]. In addition for 11-hydroxyundecyl octadecyl disulfide (CH 3 (CH 2 ) 17 S S (CH 2 ) 11 OH, HOD) SAM phase separation was reported at low coverage evidenced by the formation of two different corrugation periodicities consistent with the lengths of separated thiolates [10]. Here we present a helium atom diffraction study of the HOD SAMs grown in vacuum by means of supersonic molecular beam deposition (SMBD) which allows a very precise control of film cov- erage. Using helium diffraction enabled us to monitor film growth in real time and follow the evolution of film structure as a function coverage and substrate temperature. We identified two different lying down monolayer phases one of which was observed only when the film was grown at low substrate temperatures (T ≈ 200 K). http://dx.doi.org/10.1016/j.apsusc.2014.05.128 0169-4332/© 2014 Elsevier B.V. All rights reserved.