Low-temperature cotunneling electron transport in photo-switchable molecule-nanoparticle networks

We report the temperature-dependent (4.2–300 K) electron transport properties (current–voltage) of photo-switchable two-dimensional arrays of gold nanoparticles (10 nm in diameter) functionalized by azobenzene derivatives. Under UV-light irradiation at 4.2 K, the azobenzene moieties are switched from the trans-to-cis isomers, leading to an increase of the current. In both conformations, at low temperature (<77 K) and low voltage (<1 V), the voltage- and temperature-dependent current behaviors show that electron cotunneling is the dominant transport mechanism. The number of cotunneling events Ncot slightly increases from ≈1.4 to 1.7 upon trans-to-cis isomerization of the azobenzenes. The nanoparticle Coulomb charging energy is not significantly modified (≈15 meV) by the azobenzene isomerization. This weak increase of Ncot is explained by the modest cis/trans current ratio (≤10) and the limited numbers of nanoparticle–molecule–nanoparticle junctions inserted between the two nanoscale electrodes (<50 nm apart) connecting the network.