Conduction, storage, and leakage in particle-on-SAM nanocapacitors

Individual gold nanoparticles exhibit discrete capacitances of the order of 1 aF, and they can be tethered to a conductive substrate using a bi-functional monolayer of a suitable organic molecule. However the conduction, retention and leakage of charge by such an attached "nanocapacitor" will be an important issue in any practical application of this concept. Here we investigate the electrical properties of the particles using a combination of scanning tunneling spectroscopy and numerical modeling based on equalizing Wentzel-Kramers-Brillouin style tunneling currents. Application of the model provides the voltage division across the structure, and, together, with an estimate of the capacitance of the particle, provides an indication of likely stored charge and energy and its decay. The methodology was tested with I-V data measured for an Au{111}-α,α\´-p-xylyldithiol-Au nanoparticle system in air. About 25 eV can be stored on the nanoparticles using a charging voltage of 3 V, corresponding to up to twenty electrons. However, leakage of the charge will occur by tunneling in approximately 6×10^-9 s. Therefore, these nanocapacitors would discharge completely in any electric circuit slower than about 1.5 GHz.