Measurement of negative differential resistance properties of dipyridinium self-assembled monolayers by ultrahigh-vacuum scanning tunneling microscopy

We fabricated organic monolayers onto a Au(111) substrate by the self-assembly method using 4-{4[4-(4-{1-[4-(4-acetylsulfanyl-phenylethynyl)-phenyl]-2,6-diphenyl-pyridinium-4-yl}-phenyl)-2,6-diphenyl-pyidinium-1-yl]-phenylethynyl}-phenylthioacetate (dipyridinium). Also, we selected an organic single molecule in the organic monolayers and measured their current-voltage (I-V) properties by ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). Self-assembly monolayers (SAMs) were prepared on a Au(111) substrate that had been thermally deposited onto freshly cleaved, heated mica. The Au substrate was exposed to a 1 mM solution of 1-dodecanethiol in ethanol for 24 h to form a monolayer. After thoroughly rinsing the sample, it was exposed to a 0.3 mM solution of dipyridinium in tetrahydrofuran (THF) for 30 min. Finally, the electrical property of the organic monolayers was investigated by UHV-STM. The applied voltages were from -2.0 to +2.0 V at 299 K. The vacuum condition is 6 x 10(-8) Torr. As a result, we confirmed the property of a negative differential resistance (NDR) in the positive-voltage region in between 1.5 and 2.0 V.