Published: March 15, 2011 r2011 American Chemical Society 4713 dx.doi.org/10.1021/la105063u | Langmuir 2011, 27, 4713–4720 ARTICLE pubs.acs.org/Langmuir UPS, XPS, and NEXAFS Study of Self-Assembly of Standing 1,4-Benzenedimethanethiol SAMs on Gold Luca Pasquali, †,§ Fabio Terzi, ‡ Renato Seeber, ‡ Stefano Nannarone, †,§ Debasish Datta, ||,^,# C  eline Dablemont, ^,# Hicham Hamoudi, ^,# Maurizio Canepa, 3 and Vladimir A. Esaulov* ,^,# † Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universit  a di Modena e Reggio Emilia, Via Vignolese 905, 41100 Modena, Italy ‡ Dipartimento di Chimica, Universit  a di Modena e Reggio Emilia, Via Campi 183, 41100 Modena, Italy § CNR-IOM, s.s.14, km 163.5 in Area Science Park, 34012 Trieste, Italy ) Seth Anandram Jaipuria College, 10, Raja Naba Krishna Street, Kolkata 700 005, India ^ Universit  e-Paris Sud, Institut des Sciences Mol  eculaires d’Orsay, Orsay, France # CNRS, UMR 8214 Institut des Sciences Mol  eculaires d’Orsay, ISMO, B^ atiment 351, UPS-11, 91405 Orsay, France 3 CNISM and Dipartimento di Fisica, Universit  a di Genova, Via Dodecaneso 33, 16146 Genova, Italy ABSTRACT: We report a study of the self-assembly of 1,4-benzenedimethanethiol monolayers on gold formed in n-hexane solution held at 60 °C for 30 min and in dark conditions. The valence band characteristics, the thickness of the layer, and the orientation of the molecules were analyzed at a synchrotron using high resolution photoelectron spectroscopy and near edge X-ray adsorption spectroscopy. These measurements unambiguously attest the formation of a single layer with molecules arranged in the upright position and presenting a free ÀSH group at the outer interface. Near edge X-ray absorption fine structure (NEXAFS) measurements suggest that the molecular axis is oriented at 24° with respect to the surface normal. In addition, valence band features could be successfully associated to specific molecular orbital contributions thanks to the comparison with theoretically calculated density of states projected on the different molecular units. ’ INTRODUCTION Dithiol molecules, in particular, molecules containing aro- matic rings, have been the object of much investigation because of potential use in, for example, molecular electronics thanks to their conducting properties and the possibility to graft the thiol ends to two metal electrodes. 1À7 Furthermore, dithiol molecules self-assembled on surfaces can, under suitable conditions, bond to the surface with one ÀSH group, while the second ÀSH group is left dangling at the surface of the organic film and available to act as a binding site, for example, to a metal nanoparticle. This has been proposed as an interesting method to study the properties of isolated nanoparticles. 8 Regarding these and other applica- tions, a major hurdle, as opposed to alkanethiols, 9À17 is the possibility that both thiol ends bind to the metal surface, instead of leaving one end free. 18À23 Frequently, mixed phases may result depending upon assembly conditions and there is an ongoing debate in the literature 18 about the possibility of obtaining ordered monolayer films of standing-up dithiol molecules. Despite many attempts, the conditions for the reproducible formation of highly ordered compact self-assembled monolayers (SAMs), with free ÀSH end groups at the outer interface, depending subtly on factors such as the quality of the substrate and reagents, solvent strength, photochemical action, and so forth were until recently not clearly defined. Recently, some of us have shown that under specific assembly conditions, 19,20 that is, when dithiols were assembled in well Received: December 22, 2010 Revised: February 24, 2011