Molecules of benzene-1,4-dithiol were self-assembled onto the two facing gold electrodes of a mechanically controllable break junction to form a statically stable goldsulfur-aryl-sulfur-gold system, allowing for direct observation of charge transport through the molecules. Current-voltage measurements at room temperature demonstrated a highly reproducible apparent gap at about 0.7 volt, and the conductancevoltage curve showed two steps in both bias directions. This study provides a quantative measure of the conductance of a junction containing a single molecule, which is a fundamental step in the emerging area of molecular-scale electronics.

respectively (20). I11 the case of two-photon absorption, the line-shape f~~n c t i o r l(21 ), where n is the transition frequency, v is the laser frequency, and r is the molec-SCIENCE VOL. 271 22 MARCH 1996 22. C. SchGnenberger, J. Jonitsma, J. A M. Sondag-Huethorst. L G. J. Fokkink, J. Phys. CChem. 99.3259(1 995). 23. We thank W. Emst for the loan of a microwave source.

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Alkanethiolate self-assembled monolayers on Au{111} are used for the two-dimensional matrix isolation of conjugated organic thiolates, studied for their potential to act as molecular wires. The conjugated organic molecules are inserted from dilute solution into boundaries between structural domains and at substrate step edges of preassembled n-alkanethiolate monolayers so as to preserve the order of the initial alkanethiolate lattice. In contrast, when both molecules are codeposited from a single solution, the structure of the assembled monolayer shows no ordered molecular lattice of the alkanethiol component. Scanning tunneling microscopy measurements on the isolated conjugated component molecules show them to have enhanced conductivity compared to neighboring alkanethiolates.