Theory Meets Experiment in Low-Dimensional Structures with Correlated Electrons

The choice of the transition metal atom in an organometallic complex decisively affects its physiochemical properties such as its color or magnetic moment. Towards rationalization of the impact of the choice of transition metal atom on the electronic and spintronic properties of organometallic wires, we study the electric transport through Fe, Co and Ni containing 1D coordination polymers contacted by the tip of a scanning probe microscope (SPM). The coordination polymers are synthesized in-situ by co-deposition of the metal atoms and the quinonediimine (2,5-diamino-1,4-benzoquinone-diimine) ligand onto Au(111). Scanning tunneling microscopy combined with atomic force microscopy is used for characterization as well as lifting and transport experiments. All three coordination polymers are found to be structurally equivalent, with a coordination geometry consistent with the expected formal M(II) oxidation states. This implies dehydrogenation of the quinonediimine ligand. Wires with lengths over 100 nm can be obtained. Transport measurements are performed by contacting one end of the surface-adsorbed polymer with the SPM tip. The combination of STM and AFM allows for simultaneous measurement of the current, conductance and force gradient as a function of bias voltage and lifting height. The transition metal element contained in the organometallic wire is found to have a profound impact on its electronic properties: lifting of Fe and Ni wires leads to the opening of a band gap at heights of a few nanometers. In contrast, Co containing polymers do not exhibit no gap. In addition, conductance measurements reveal pronounced steps at bias voltages of a few millivolts for Fe and Co wires but not for Ni ones. These conductance steps are tentatively attributed to spin excitations.