Tuning interfacial spin filters from metallic to resistive within a single organic semiconductor family

A metallic spin filter is observed at the interface between Alq(3) adsorbates and a Cr(001) surface. It can be changed to a resistive (i.e., gapped) filter by substituting Cr ions to make Crq(3) adsorbates. Spin-polarized scanning tunneling microscopy and spectroscopy show these spin-dependent electronic structure changes with single molecule resolution. Density functional theory calculations highlight the structural and electronic differences at the interfaces. For Alq(3), a charge-transfer interaction with the substrate leads to a metallic spin filter. For Crq(3), direct covalent interactions mix molecular orbitals with the substrate surface state to make two well-separated interfacial hybrid orbitals.