Real Space Observation of Nanoscale Molecular Domains

Scanning tunneling microscopy with very high gap impedance reveals several stable molecular conformations of organic monolayers self-assembled from alkanethiols on gold. Thermal annealing of the preassembled films favored well-packed large molecular domains that were studied in real space with unprecedented detail and accuracy. At room temperature the system has several stable molecular conformations, which implies that chemisorption energies balance the energy of interaction between molecular chains. A similar probability of different conformations indicates almost equal total energies, which makes it an interesting - and challenging - object for molecular dynamics simulations. Our results contribute to a long standing scientific debate on the structure of self-assembled monolayers and provide the basis for an improved understanding of organic interfaces.

Figure 1: STM image of dodecanethiol on gold(111) prepared by adsorption from a millimolar solution of dodecanethiol in ethanol for 2h at room temperature an 48 h at 50 degrees Celsius in the same solution. Monoatomic gold steps, depressions and a network of lines linking depressions delimit ordered domains in the SAM. Domains differ by their phase, their chain tilt or the origin of sulfur adsoption. Molecular arrangements are hexagonal with an addtional rectangular superlattice. The phases originate from the particular distribution in the unit cell of molecules with different twist angles that move end groups at different height levels in and out of the average plane of the monolayer by 0.03 nm.

Bruno Michel <bmi@zurich.ibm.com>

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Last modified: Wed, 15 May 1996 11:44