Studies of the electrical properties of large molecular adsorbates

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Abstract
The scanning tunneling microscope (STM) was used to investigate the conductivity of organic aggregates on gold electrodes. DNA fragments can form very stable aggregates in the presence of tris (hydroxymethyl)aminomethane buffer salt. With a submonolayer coverage, the tip can be moved back and forth between "clean" gold (in contact with the electrolyte) and an organic adsorbate patch. In these conditions, a systematic variation of the current voltage (I-V) characteristics indicates that the electrical characteristics are not dominated by a contaminant particle on the STM tip. The dependence of image contrast on tip bias was also studied over the range + 0.3 to - 0.3 V. It is found that: (a) the image contrast does not depend strongly on tip bias; (b) the I-V curves over what appears to be "clean" gold under the electrolyte are similar to those observed over clean gold maintained in an inert atmosphere; (c) the I-V curves over an adsorbate patch are diode-like; (d) the I-V curves show no sign of a large voltage offset of the sort that is characteristic of an electrochemical interface; (e) the conductance over the organic adsorbate can be much larger than that over "clean" gold. It is concluded from (a) and (d) that current is most likely carried by electrons, not ions. Here (c) is inconsistent with (a) unless the closing of the STM servoloop causes significant modification of the electronic properties of the adsorbate (by adjusting "pressure" in the tunnel gap). The I-V curves over the organic adsorbate have an unusual dependence on initial set-point voltage, consistent with such an effect.