Self-Assembled Monolayers of Cobalt(II)− (4-tert-Butylphenyl)-Porphyrins:  The Influence of the Electronic Dipole on Scanning Tunneling Microscopy Images

• Published in: Journal of the American Chemical Society 2004-12, Vol.126 (51), p.16951-16958
• Main Authors: Arima, Valentina, Fabiano, Eduardo, Blyth, Robert I. R, Della Sala, Fabio, Matino, Francesca, Thompson, Julie, Cingolani, Roberto, Rinaldi, Ross
• Format: Article
• Language: English
• Subjects: Adsorption and desorption kinetics; evaporation and condensation; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Physics; Solid-fluid interfaces; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0470970

Self-assembled monolayers (SAMs) of cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin, a promising material for optical, photoelectrochemical, and chemical sensor applications, were prepared on Au(111) via axial ligation to 4-aminothiophenol, and studied by several surface science techniques. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements showed the apparent topology of the Au(111) herringbone structure reconstruction, but with bias-dependent contrast images and asymmetric I/V characteristics. Photoelectron spectroscopy confirmed the presence of metalloporphyrins on the surface, whereas near-edge X-ray absorption (NEXAFS) measurements revealed that the porphyrin ring was tilted by about 70° with respect to the surface plane. The above effects are ascribed to the presence of oriented molecular dipole layers between the metal and the organic material as confirmed by a comparison with first-principles density-functional theory calculations. The measured bias-dependent STM profiles have been reproduced by a simple monodimensional tunneling model.