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

A metallic spin filter is observed at the interface between $\mathrm{Al}{\mathrm{q}}_{3}$ adsorbates and a Cr(001) surface. It can be changed to a resistive (i.e., gapped) filter by substituting Cr ions to make $\mathrm{Cr}{\mathrm{q}}_{3}$ 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 $\mathrm{Al}{\mathrm{q}}_{3}$, a charge-transfer interaction with the substrate leads to a metallic spin filter. For $\mathrm{Cr}{\mathrm{q}}_{3}$, direct covalent interactions mix molecular orbitals with the substrate surface state to make two well-separated interfacial hybrid orbitals.