| A variable temperature ultra-high vacuum (UHV) scanning tunneling microscope (STM) is utilized for measuring the electrical properties of isolated organic molecules adsorbed to the degenerately doped n- and p-type silicon surfaces at room temperature and 80 K. Current-voltage curves taken under these conditions show negative differential resistance (NDR) at positive sample bias for p-type doping and negative bias for n-type doping. These results qualitatively agree with a theoretical model proposed by Supriyo Datta (Purdue University). Due to the enhanced stability of the STM at cryogenic temperatures, repeated measurements can be routinely taken over the same molecule. Taking advantage of this improved stability, current-voltage curves on isolated cyclopentene molecules are demonstrated to be repeatable and possess negligible hysteresis for a given tip-molecule distance. On the other hand, subsequent measurements with variable tip position show that the NDR voltage increases with increasing tip-molecule distance. Using a one-dimensional capacitive equivalent circuit and a resonant tunneling model, this behavior can be quantitatively explained, thus providing insight into the electrostatic potential distribution across a semiconductor-molecule-vacuum-metal tunnel junction. This model also provides a quantitative estimate for the alignment of the highest occupied molecular orbital (HOMO) of cyclopentene with respect to the Fermi level of the silicon substrate, thus suggesting that this experimental approach can be used for performing chemical spectroscopy at the single molecule level on semiconductor surfaces. Overall, these results serve as the basis for a series of design rules that can be applied to silicon-based molecular electronic devices. |
