Langmuir 2005, 21, 4779-4781
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Scanning Tunneling Microscope Imaging of (CH3S)2 on Cu(111) Michiaki Ohara,* Yousoo Kim, and Maki Kawai* Surface Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan Received January 17, 2005. In Final Form: April 8, 2005 Scanning tunneling microscope (STM) images of isolated molecules of dimethyl disulfide, (CH3S)2, adsorbed on the Cu(111) surface were successfully obtained at a sample temperature of 4.7 K. A (CH3S)2 molecule appears as an elliptic protrusion in the STM images. From density functional theory calculation, it was suggested that the bright part in the protrusion corresponds to the molecular orbital which is widely spread around H atoms in each CH3 group in the (CH3S)2 molecule. The STM images revealed that the molecules have a total of six equivalent adsorption orientations on Cu(111), which are given by the combination of three equivalent adsorption sites and two conformational isomers for each adsorption site.
Introduction
Experiment
Self-assembled monolayers (SAMs) have attracted much attention because of their unique highly ordered structures and promising applications for sensors, catalysts, and molecular devices.1 Among the many varieties of SAMs, monolayers of alkanethiols [CH3(CH2)nSH] or dialkyl disulfides [CH3(CH2)nS-S(CH2)nCH3] on noble metal surfaces, especially gold (Au) and copper (Cu) (111), have been most widely studied owing to their chemical stability and easy preparation.2-6 However, despite the extensive practical utilization of these SAMs, fundamental aspects of the SAM formation from individual organosulfur molecules on the noble metal (111) surfaces still remain unclear. To understand them precisely, it is essential to investigate the initial adsorption state, such as adsorption sites, structures, and orientations, of isolated organosulfur molecules on the surface. The scanning tunneling microscope (STM) study of single molecules at cryogenic temperature provides us a chance to observe the molecules under extremely stable conditions because the dissociation of the S-S bond of the disulfide has been known to easily occur even at temperatures as low as 140 K.7-9 In the present study, (CH3S)2 was adsorbed at a substrate temperature of 50 K and nondissociatively adsorbed (CH3S)2 molecules were successfully observed on the Cu(111) surface with a lowtemperature scanning tunneling microscope (LT-STM) under ultrahigh vacuum (UHV) conditions. From the observed STM images, the adsorption site and orientation of (CH3S)2 on the surface were proposed. These observations would be helpful to understand the initial stage of the SAM formation.
All experiments were performed in an UHV chamber (
