Image Contrast Analysis of STM Images of Self-Assembled Dioctadecyl Chalcogenides on Graphite at the Liquid-Solid Interface

• Published in: Chemphyschem 2005-11, Vol.6 (11), p.2383-2388
• Main Authors: Fukumura, Hiroshi, I-i, Daizo, Uji-i, Hiroshi, Nishio, Satoru, Sakai, Hitoshi, Ohuchi, Akihiko
• Format: Article
• Language: English
• Subjects: chalcogenides; Chemistry; Exact sciences and technology; General and physical chemistry; interfaces; scanning probe microscopy; self-assembly; Solid-liquid interface; Surface physical chemistry; tunneling conductivity
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200500215

The structures of self‐assembled monolayers of dioctadecyl selenide (CH3(CH2)17)2Se and dioctadecyl telluride (CH3(CH2)17)2Te, as well as dioctadecyl ether (CH3(CH2)17)2O and dioctadecyl sulfide (CH3(CH2)17)2S, on graphite at the liquid–solid interface were systematically investigated by scanning tunneling microscopy (STM). Both dioctadecyl selenide and telluride formed monolayer structures in which the tilt angle between the molecular axis of the alkyl chain and the lamellae axis was 90°, while dioctadecyl ether assembled with a tilt angle of 60°. Dioctadecyl sulfide was found to make two different self‐assembled structures having tilt angles of 60 and 90°. When selenide was embedded in ether compounds in mixed self‐assembled monolayers, the alkyl chains of the selenide became blurred, implying that the alkyl chains in the monolayers were unstable. This is in contrast with the structure of co‐adsorbed monolayers of the ether and sulfide compounds, where the images of all alkyl chains had high spatial resolution. For the co‐adsorbed monolayers, the image contrast of chalcogen atoms was normalized compared with that of alkyl chains of the ether compound in the same image frame. The normalized image contrast was found to be independent of the measurement conditions involving tip shapes, having the following trend, Te>Se>S>C>O. The difference in the normalized image contrast among chalcogen atoms are discussed based on fundamental parameters like polarizability and atomic radii. A new analytical method is proposed to compare scanning tunneling microscopic images obtained with different probes. The method is based on the assumption that the height distribution of alkyl chains at solution/graphite interfaces should follow the same Gaussian curve irrespective of the curvature radii of the tunneling probes. The figure shows that selenium and tellurium embedded in self‐assembled monolayers are clearly identified.