Negative Differential Resistance in Spin-Crossover Molecular Devices

We demonstrate, based on low-temperature scan-ning tunneling microscopy (STM) and spectroscopy, a pro-nounced nega t i v e differential resistance (NDR) in spin-crossover (SCO) molecular devices, where a FeI I SCO molecule is deposited on surfaces. The STM measurements reveal that the NDR is robust with respect to substrate materials, temperature, and the number of SCO layers. This indicates that the NDR is intrinsically related to the electronic structure of the SCO molecule. Experimental results are supported by density functional theory (DFT) with non-equilibr i u m Green's function (NEGF) calculations and a generic theoretical model . While the DFT+NEGF calculations reproduce NDR for a special atomically sharp STM tip, the effect is attributed to the energy-dependent tip density of states rather t h a n the molecule itself . We, therefore, propose a Coulomb blockade model invol v i n g three molecular orbitals with very different spatial localization as suggested by the molecular electronic structure.