Adsorption of Organic Molecules

The adsorption of organic molecules on electrode surfaces in electrolyte solutions has also long been an important subject in electrochemistry for elucidating the role of molecular properties and the atomic structure of electrode surfaces [39]. In spite of a large number of efforts made to obtain STM images of organic molecules in air and UHV [18], there has not been much success in resolving molecular structures at electrode surfaces with in-situ STM in electrolyte solutions. Although a few recent reports described in-situ STM images of molecular adlayers in electrolyte solutions [40], the resolution achieved by STM imaging in solution seems to be still limited, compared with that in UHV [18],

In the previous work performed mainly in electrochemical UHV technique and in electrolyte solutions, well-defined single-crystal surfaces of metals such as Pt, Rh, Au, and Ag were most frequently used as the substrate for the study of adsorption of organic molecules because of relatively strong tendencies of those surfaces to immobilize organic molecules [1, 2, 39]. However, it was found in our preliminary studies with in-situ STM that the relatively large molecules investigated did not form ordered adlayers on a well-defined Pt(lll) nor on Au(lll) in solution, probably because of the slow surface mobility of adsorbed molecules. On the other hand, highly ordered pyrolytic graphite (HOPG) and similar layered crystals such as M0S2 are popular choices as the substrate for the study of adsorbed molecules because of the ease of preparing a clean and atomically flat surface [18, 40]. It is well known that liquid crystals such as alkylcyano-biphenyls form ordered adlayers on HOPG and M0S2, and they were successfully visualized by STM [18]. These successes can be explained, in our opinion, by relatively weak van der Waals-type interactions of the substrates with adsorbates, allowing self-ordering processes to proceed. However, HOPG is known to exhibit fairly large corrugation amplitudes for the individual carbon atoms in STM images, which sometimes prohibit resolution of small corrugations from adsorbed organic molecules. In fact, in our preliminary experiments, only the atomic structure of HOPG was observed in solutions containing organic molecules.

We have long been interested in finding a more appropriate substrate to investigate the adsorption of various organic molecules in solution, and we recently reported, for the first time, a successful in-situ STM imaging of a highly ordered adlayer of 5,10,15,20-tetrakis(N-methylpyridiniiun-4-yl)-21 H,23H-porphine (TMPyP) on the I-Au(l 11) in HCIO4 [16]. We also obtained recently further evidence that the I-Au(l 11) electrode can be employed as an ideal substrate for in-situ STM imaging of various adsorbed organic molecules in solution [17], Organic substances investigated were purposely selected based on their characteristic shapes: square, triangular, and linear. TMPyP hexamethyl-p-rosaniline (Crystal Violet; CV), and 4,4- bis (N-methylpyridinium)-p-phenylenedivinylene (PPV) were all found to form highly-ordered molecular arrays on top of the iodine monolayer adsorbed on Au(l 11). In-situ STM images shown in Fig. 4 with near-atomic resolution revealed their orientation, packing arrangement, and even the internal structure of each molecule. It is particularly

Fig. 4. In-situ STM images of TMPyP (a), CV (b), and PPV (c) on I/Au(l 11) [16, 17].

emphasized that the iodine layer on Au(l 11) played a crucial role in the formation of highly ordered TMPYP arrays [16, 17]. Relatively weak van der Waals-type interaction on the iodine adlayers seems to be an important key factor in the formation of the ordered molecular arrays.

The adsorption of these molecules has also been investigated on various iodine-modified electrodes such as I/Ag(lll), I/Pt(lll) and I/Rh(lll). Highly ordered adlayers of TMPyP were found to form on I/Au(lll) and I/Ag(lll), whereas less-ordered adlayers formed on I/Pt(lll) and I/Rh(lll) [41]. The surface mobility of iodine on Pt(l 11) is very slow, as shown in Fig. 1(b). TMPyP is adsorbed on I/Pt(l 11) as isolated molecules with a disordered arrangement. These results suggest that the surface mobility of the iodine adatoms also plays an important role in the ordering process of the organic adlayer.

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