Self-assembled monolayers of radical molecules physisorbed on HOPG(0 0 0 1) substrate studied by scanning tunnelling microscopy and electron paramagnetic resonance techniques P. Krukowski a, *, Z. Klusek a , W. Olejniczak a , R. Klepaczko a , M. Puchalski a , P. Dabrowski a , P.J. Kowalczyk a , K. Gwozdzinski b a Division of Physics and Technology of Nanometer Structures, Solid State Physics Department, University of Lodz, Pomorska 149/153, 90-236 Lodz, Poland b Department of Molecular Biophysics, University of Lodz, Banacha 12/16, 90-237 Lodz, Poland 1. Introduction It is commonly believed that using the innovative technique called electron spin noise-scanning tunnelling microscopy (ESN- STM) the electron spin signal originating from a single paramagnetic molecule deposited on a substrate can be observed [1–6]. The efficacy of this technique in detecting an unpaired spin of a single electron is enormously interesting bearing in mind the wide field of potential applications: in spintronics [7–9], quantum computations [10–12], and memory devices [13]. Recently, it has been also shown that the electron spin signal originating from an inorganic sample, i.e. a submonolayer oxide film on an Si(1 1 1)-(77) surface terminated by oxygen dangling bonds can be observed in an ESN- STM configuration [14]. However, much more work must be done before ESN-STM can be considered as a standard spectroscopic technique. The main problems in the ESN-STM configuration are largely concerned with the instrumentation (improvement of impedance matching, decreasing the parasitic capacitances, using low noise radio frequency (rf) amplifiers and short cables, operation at low temperatures, the need for a permanent magnet with high field homogeneity) [15,16] as well as with preparing well-defined test samples having paramagnetic properties which fulfil the experimental requirements [17]. Hence, there are only a limited number of studies presenting paramagnetic molecular systems suitable for ESN-STM studies. In particular, (a) Pong et al. [18] showed that BDPA (a,g-bisdiphenylene-b- phenylallyl) radicals molecules can be deposited by thermal evaporation, microcontact printing, and solvent deposition on HOPG(0 0 0 1) and Au(1 1 1) substrates. (b) Mas-Torrent et al. [19] utilized different interactions (van der Waals, electrostatic, coordination bond, covalent bond) during deposition of PTM (polychlorotriphenylmethyl) radicals on selected substrates. (c) Niermann et al. [20] showed that galvinoxyl (2,6-di-tert-butyl- 4-3,5-di-tert-butyl-4-oxycyclohexa-2,5-dienyliden-emethyl- phenoxy) forms a self-assembled monolayer on Au(1 1 1) with two different molecular configurations. Applied Surface Science 255 (2009) 8769–8773 ARTICLE INFO Article history: Received 27 March 2009 Received in revised form 6 May 2009 Accepted 2 June 2009 Available online 17 June 2009 PACS: 76.30.v 75.75.+a 07.79.Cz Keywords: STM HOPG Radicals Self-assembled monolayer ABSTRACT In this paper, we present a combined STM and EPR study on the adsorption and self-organization of monolayers formed from 2-(14-Carboxytetradecyl)-2-ethyl-4,4-dimethyl-3-oxazolidinyloxy (16DS) and 4 0 ,4 0 -Dimethylspiro(5a-cholestane-3,2 0 -oxazolidin)-3 0 -yloxy (CSL) adsorbed on a highly oriented pyrolitic graphite HOPG(0 0 0 1) substrate. Both 16DS and CSL molecules are persistent free radicals containing a paramagnetic doxyl group. The STM measurements of 16DS on HOPG(0 0 0 1) were performed at the liquid–solid interface while the studies of CSL on HOPG(0 0 0 1) were carried out under ultrahigh vacuum conditions. It was found that the 16DS molecules on the HOPG(0 0 0 1) surface form a highly-ordered monolayer with a domain structure. The high-resolution STM images show structural details of 16DS molecules on HOPG(0 0 0 1) revealing the paramagnetic doxyl group. In contrast, CSL molecules on HOPG(0 0 0 1) form a well-ordered monolayer without domain structure. The EPR results indicate that both compounds deposited on HOPG(0 0 0 1) substrate are not reduced and retain their paramagnetic character. We believe that the molecular systems described can be used in single spin detection experiments using an electron spin noise-scanning tunnelling microscopy (ESN-STM) technique. In particular, the possibility of obtaining contrast spin signals from the paramagnetic and diamagnetic parts of molecules increases the significance of our results. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +48 42 635 4150; fax: +48 42 665 5130. E-mail address: pkrukowski@std2.phys.uni.lodz.pl (P. Krukowski). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.06.035