Molecular Tracer Dynamics in Crystalline Organic Films at the Solid−Liquid Interface

Abstract

A molecular tracer method for scanning tunneling microscopy allowed the observation of individual molecular events at a crystal surface in solution. Long-chain alkanes or ethers were coadsorbed with similar length thioethers on a graphite surface. Due to the distinctive contrast of the sulfur atoms, it was possible to track individual thioether tracer molecules. The exchange of tracer molecules between the adsorbed monolayer and the overlying solution was measured at equilibrium. Rates depended on chain length, temperature, and neighboring molecules. Tracers at boundaries between crystal domains revealed specific molecular processes driving boundary fluctuations and domain growth. In addition to adsorption-desorption driven rearrangements, collective motion within the monolayer has been seen. Longer chain ethers exhibit two phases having different molecular configurations at grain boundaries and different microstructural habits. Transition between phases was promoted by the scanning probe.

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

“…This tendency is generally observed in almost all self-assembled monolayers of alkane derivatives with long alkyl chains, which suggests that all of the methylene groups of a long alkyl chain lie in the hollows of the graphite lattice. [9,18] However, the case of the square defect in dioctadecyl selenide monolayers is an exception where the angle between the axis of the alkyl chain and that of the graphite lattice is 908, which is different from the common directions (60 and À608). This unusual configuration can lead to the reduction of the adsorption energy between the molecules in the square defect and the graphite substrate, making the defect metastable.…”

“…The structures observed with these molecules are essentially similar to those reported by Padowitz et al, although the lengths of alkyl chains were a little shorter in their experiments. [17,18] 15 CH 3 ) make the type I structure. In our experiments, dioctadecyl sulfide showed both structures.…”

“…[5][6][7][8][9] De Schryver and co-workers have extensively applied this methodology to study chemical reactions involving interfaces in addition to adsorption-desorption dynamics. [10][11][12][13][14][15][16] Padowitz et al also observed exchange dynamics on graphite at the solid-liquid interface using dihexadecyl ether (CH 3 -(CH 2 ) 15 ) 2 O and dihexadecyl sulfide (CH 3 (CH 2 ) 15 ) 2 S. [17,18] They found that these molecules co-crystallized and were distinguishable in STM images due to the strong image contrast of sulfur atoms with methylene or oxygen groups. Although they have shown that atomic species can be identified with STM imaging, further investigations using molecules containing chalcogen atoms in higher periodic lines such as selenium and tellurium have not been made yet.…”

Scanning Tunneling Microscopy and Spectroscopy of Donor–Acceptor–Donor Triads at the Liquid/Solid Interface

“…This means that the formation of a mixed monolayer goes along with a pronounced exchange activity until the equilibrium composition of the monolayer is reached after which almost no dynamic events are observed. Compared to an earlier investigation of molecular exchange dynamics of simple long-alkyl-chain derivatives, [17] the compounds under investigation show an extremely slow desorption rate constant upon reaching the equilibruim composition. A similar result was obtained by us for a mixture of alkylated isophthalic acid derivatives where individual molecules were observed not to exchange for more than 20 min.…”

Use of Specific Functionalised Tips with STM: A New Identification Method of Ester Groups and Their Molecular Structure in Self‐Assembled Overlayers

“…For example, alkanes and alkanols adsorbed on graphite tend to adopt in most cases an all-trans conformation with their molecular axes parallel to each other in order to optimise the intermolecular and molecule-substrate interactions. [15][16][17] Self-assembly was also studied with a number of substituted alkanes, [18][19] long-chain ethers, [20] fatty acids, [21] or benzene derivatives. [22][23][24] Moreover, the STM has a wider application field rather than just providing images of surface structures.…”