Molecular Electronic Junctions Achieved High Thermal Switch Ratios in Atomistic Simulations

The development of devices that improve thermal energy management requires thermal regulation with efficiency comparable to the ratios <i>R</i> ∼ 10⁵ in electric regulation. Unfortunately, current materials and devices in thermal regulators have only been reported to achieve <i>R</i> ∼ 10. We use atomistic simulations to demonstrate that Ferrocenyl (Fc) molecules under applied external electric fields can alter charge states and achieve high thermal switch ratios <i>R</i> = <i>G</i>_<i>q</i>/<i>G</i>₀, where <i>G</i>_<i>q</i> and <i>G</i>₀ are the high and low limiting conductances. When an electric field is applied, Fc molecules are positively charged, and the SAM-Au interfacial interaction is strong, leading to high heat conductance <i>G</i>_<i>q</i>. On the other hand, with no electric field, the Fc molecules are charge neutral and the SAM-Au interfacial interaction is weak, leading to low heat conductance <i>G</i>₀. We optimized various design parameters for the device performance, including the Au-to-Au gap distance <i>L</i>, the system operation temperature <i>T</i>, the net charge on Fc molecules <i>q</i>, the Au surface charge number <i>Z</i>, and the SAM number <i>N</i>. We find that <i>G</i>_<i>q</i> can be very large and increases with increasing <i>q</i>, <i>Z</i>, or <i>N</i>, while <i>G</i>₀ is near 0 at <i>L</i> > 3.0 nm. As a result, <i>R</i> > 100 was achieved for selected parameter ranges reported here.