Electrical conduction through adsorbed monolayers

Electrical conduction was studied in Al/adsorbed monolayer/Al junctions. The adsorbed monolayers were long chain saturated fatty acids (23 to 14 carbon atom chain), short chain perfluorinated fatty acids (10 to 7 carbon atom chain), and n-octadecyl-trichlorosilane. The current observed decreased rapidly with decreasing temperature from 295 K to approximately 77 K at which point the decrease in current with decreasing temperature became very small. At room temperature (295 K) there was no definite relation between the dc conductivity and the length of the fatty acids. At 77 K and lower temperatures, however, the dc conductivity was exponentially dependent on the fatty acid chain length. These results suggest that at 77 K and lower temperatures the conduction mechanism is tunneling through the monolayer. The tunneling barrier height was found to be of the order of 2.8 and 5.2 eV for fatty acids and perfluorinated fatty acids, respectively. By artificially producing molecular holes in monolayers it was shown that in the presence of such holes the current increases by at least one order of magnitude while the effective barrier height is lowered by 10%. Differences between the present results and those previously obtained with monolayers deposited from an air–water interface (Langmuir–Blodgett monolayers) are discussed.