Tuning Molecule-Mediated Spin Coupling in Bottom-Up-Fabricated Vanadium-Tetracyanoethylene Nanostructures

We have fabricated hybrid magnetic complexes from V atoms and tetracyanoethylene ligands via atomic manipulation with a cryogenic scanning tunneling microscope. Using tunneling spectroscopy we observe spin-polarized molecular orbitals as well as Kondo behavior. For complexes having two V atoms, the Kondo behavior can be quenched for different molecular arrangements, even as the spin-polarized orbitals remain unchanged. This is explained by variable spin-spin (i.e., V-V) ferromagnetic coupling through a single tetracyanoethylene (TCNE) molecule, as supported by density functional calculations.

Figure 1

(a) STM image of TCNE molecules and V atoms adsorbed on Ag(100). (b) The same area after moving TCNE molecules toward V atoms using STM manipulation following arrows in (a), thus forming V-TCNE complexes. The arrow indicates how a V-TCNE complex will be moved toward a second V atom to form a ${\mathrm{V}}_2\mathrm{TCNE}$ complex [see Figs. 2d, 2e]. Both STM images are plotted with the same scale bar for apparent height. Imaging parameters: 1 V and 5 pA.

Figure 2

Highly resolved STM images and structural models of (a) TCNE, (b) V-TCNE, (c) $\mathrm{V}(\mathrm{TCNE}{)}_2$, (d) ${\mathrm{V}}_2\mathrm{TCNE}@27°$, and (e) ${\mathrm{V}}_2\mathrm{TCNE}@11°$ on Ag(100). The models are derived from the STM images. Note that ${\mathrm{V}}_2\mathrm{TCNE}@11°$ has a shorter V-V distance compared to ${\mathrm{V}}_2\mathrm{TCNE}@27°$. All STM images are plotted with the same height scale bar shown in Fig. 1. Imaging parameters: 1 V and 5–50 pA.

Figure 3

(a) SP-DFT-calculated spin-resolved LDOS of a free anionic V-TCNE complex. Large-bias STS spectrum is shown for (b) V-TCNE and (c) $\mathrm{V}(\mathrm{TCNE}{)}_2$. Highly resolved low-bias STS is shown for (d) TCNE, (e) V-TCNE, and (f) $\mathrm{V}(\mathrm{TCNE}{)}_2$. For clarity, the Ag-background signal [green curve in (d)] was subtracted from spectra in (b), (c), (e), and (f).

Figure 4

Large-bias (a),(b) and highly resolved low-bias (c),(d) STS spectra of ${\mathrm{V}}_2\mathrm{TCNE}@27°$ and ${\mathrm{V}}_2\mathrm{TCNE}@11°$, respectively (Ag-background spectrum is subtracted). The inset in (b) shows the calculated spin-resolved LDOS of a free anionic ${\mathrm{V}}_2\mathrm{TCNE}$ complex.