Electrostatic Redox Reactions and Charge Storage in Molecular Electronic Junctions

Abstract

The electronic properties of a molecular junction (MJ) consisting of a redox-active molecular layer and a 15 nm thick layer of aluminum oxide (AlOx) between conducting carbon contacts were compared to the same device lacking AlOx. For the electron acceptor naphthalene diimide (NDI), a negative bias applied to an NDI/AlOx MJ injects electrons into the NDI, which are blocked from further transport by the oxide. The electrons stored in the NDI more than double the charge storage over that of an equivalent dielectric parallel plate capacitor, and the dynamics of charging and discharging are completely distinct from those of a parallel plate. Replacement of NDI with an electron donor tetraphenylporphyrin (TPP) layer reverses the polarity of the charge/discharge process with electrons leaving the TPP layer under positive bias. The charge/discharge kinetics are temperature and bias dependent, indicating the electrons injected into the NDI layer result in nuclear reorganization to the configuration of an NDI– anion. The devices exhibit electronic properties resembling a “dynamic chemical capacitor”, in which carriers are stored in the molecular layer and the charging kinetics are controlled by reorganization energy, temperature, and applied bias. The molecule/oxide MJs are analogous to an electrochemical cell lacking ions, double layers, and solvent and involve only a single half reaction. In addition to providing kinetic information about basic electron transfer reactions underlying electrochemistry, electron donor–acceptor reactions, and electrostatic effects in organic electronics, the molecule/oxide MJs may have valuable applications in charge storage, memory, and as capacitors with a range of response times tunable by variations in structure and reorganization energy.