Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Adsorption of hexacontane on atomically-flat surfaces of graphite and Au (111) Ya.Yu. Lopatina ⁎ , A.A. Marchenko Institute of Physics, National Academy of Science of Ukraine, 46, Nauky Ave, Kyiv 03028, Ukraine ARTICLE INFO Keywords: n-alkanes n-hexacontane Self-assembled organic monolayers Scanning tunneling microscopy Liquid-solid interface ABSTRACT We report the results of comparative study of monolayer films of n-hexacontane (C 60 H 122 ) adsorbed on atom- ically flat graphite and reconstructed Au(111) surfaces. Scanning tunneling microscopy (STM) revealed poly- morphism in monolayer on Au(111) surface. It was shown that under the same conditions of deposition hex- acontane molecules may form two packing structures. The effect of collective reorientation of molecules was revealed on graphite surface. Obtained results are explained in terms of commensurability between alkyl chain and substrate. 1. Introduction The interest in the adsorption of normal alkanes (n-С n Н 2n+2 ) is due to the fact that they serve as building blocks of numerous organic compounds and their derivatives. These studies give insight in the mechanisms of self-organization of more complex organic molecules, in particular polymers and biological molecules [1–4], making n-alkanes an attractive model objects. Moreover, the addition of aliphatic radicals to a wide class of organic molecules increases their solubility in tradi- tional solvents and opens possibilities of their use in modern liquid technologies [5–7]. Since the alkyl environment of molecules can sig- nificantly affect the self-organization processes, the investigations of the adsorption behavior of n-alkanes are important for obtaining monolayers with predetermined parameters and structures [8–10]. The adsorption of n-alkanes on atomically flat surfaces has been intensively studied by scanning tunneling microscopy (STM) in ambient conditions. Systematic research of homologous series from decane (n = 10) to pentacontane (n = 50) have shown that these molecules form highly ordered lamella-like monolayers. It was found that type of lamellae is determined by the substrate. Thus, on graphite the main axes of molecules are perpendicular to the furrows between lamellae (so-called “rectangular” packing) regardless of alkyl chain length [11–13]. On the Au(111) surface rectangular packing is observed only for alkanes with n multiple to 16 (n = 16, 32, 48). For other n molecules form an angle of 60° with the direction of the lamella (“oblique” packing) [14–17]. Nonmonotonic character of the adsorption on Au (111) is also evident in the stability of monolayers and different me- chanisms of melting [14]. Difference in adsorption behavior on Au(111) and graphite surfaces was explained by the difference in the period of commensurability of the alkyl chain with the substrate (1.9 nm and 12.3 nm respectively). The aim of our work is to determine whether the nature of the ad- sorption of long-chain alkanes (n > 50) is consistent with the estab- lished regularities. This question remains open because with the alkyl chain length approaching to the period of commensurability with the graphite substrate different adsorption behavior of n-alkanes at n > 50 can be expected. To continue the research of the homologous series of alkanes we focused on STM-investigation of n-hexacontane (C 60 H 122 ) monolayers. In order to elucidate substrate effect we used highly or- iented pyrolytic graphite (HOPG) and Au(111) surfaces. 2. Experimental details Basal plane of HOPG was obtained by shearing of a graphite monocrystal. As an Au(111) surface thin gold films evaporated onto mica were used (Phasis, Switzerland). In order to get atomically flat terraces gold substrates were annealed in a propane–butane flame. Chemical purity of Au(111) surface was confirmed by the presence of herringbone reconstruction pattern [18]. Solid granules of n-hex- acontane (C 60 H 122 ) were dissolved in n-tetradecane (С 14 Н 30 ). All che- micals were purchased from Sigma-Aldrich and used as received. To obtain monolayers a drop of the solution was deposited on the fresh substrate in air at room temperature. The structure of monolayers was observed using scanning tunneling microscope (STM) adapted to liquid/solid interface. STM tips were mechanically cut from a 0.25 mm Pt/Ir (80:20) wire. Calibration of https://doi.org/10.1016/j.tsf.2018.09.033 Received 22 March 2018; Received in revised form 16 July 2018; Accepted 13 September 2018 ⁎ Corresponding author. E-mail address: yaroslava.lopatina@gmail.com (Y.Y. Lopatina). Thin Solid Films 665 (2018) 164–167 Available online 14 September 2018 0040-6090/ © 2018 Elsevier B.V. All rights reserved. T