Intramolecularly resolved Kondo resonance of high-spin $\mathrm{Fe}\left(\mathrm{II}\right)$-porphyrin adsorbed on $\mathrm{Au}(111)$

Using cryogenic scanning tunneling microscopy, we measured the electronic states and Kondo resonance of single $\mathrm{Fe}\left(\mathrm{II}\right)$-porphyrin molecules adsorbed on a $\mathrm{Au}(111)$ surface with intramolecular resolution. We found that the $\mathrm{Fe}\left(\mathrm{II}\right)$ ion introduces a spin-polarized molecular state near the Fermi level. Tunneling spectroscopy revealed that this state gives rise to Kondo resonance exhibiting characteristics different from those of the $\mathrm{Fe}\left(\mathrm{II}\right)$ spin state. Spin-polarized density functional theory calculations revealed that the molecule was weakly adsorbed on the surface, yet still switches its spin configuration from $S=1\text{to}2$. The spin switching was found to be driven by three effects: a structural distortion of the macrocyclic ring from planar to saddle shaped, a weak chemical bonding between the Fe and the Au surface atom underneath, and weakened Fe-N bonds due to $\mathrm{Au}(111)$-molecule charge transfer.

Figure 1

(a) STM topograph ($-1.0\mathrm{V}$, $0.3\mathrm{nA}$) of on-surface-synthesized $\mathrm{FeTPP}$ molecules in a close-packed island. The dashed line indicates the main axis. A structural model is overlaid on one of the molecules. (b) Site-specific $dI/dV$ spectra measured at the points marked in (a) following the same color coding, and at the center of a ${\mathrm{H}}_2\mathrm{TPP}$ in (f). The spectra are shifted vertically for clarity. (c)–(e) STS images of a single $\mathrm{FeTPP}$ molecule at the indicated bias voltages. (f)–(h) STM topograph ($-1.0\mathrm{V}$, $0.3\mathrm{nA}$) and STS images at the indicated bias voltages of a single ${\mathrm{H}}_2\mathrm{TPP}$ molecule. The scale bars are 1 nm and (c)–(h) are in the same scale.

Figure 2

(a)–(c) Spatial evolution of normalized Kondo resonance in a $\mathrm{FeTPP}$ molecule. Spectra in (b) and (c) are measured along the red (main axis) and blue dashed lines in (a), respectively. The Fano fitting is shown by red and blue curves. (d) Spatially resolved normalized magnitude $A/y_0$, line-shape factor $q$, and Kondo temperature $T_K$ derived from the Fano fitting of the spectra in (b) and (c) following the same color coding. (e), (f) $2\mathrm{D}q\mathrm{and}{T}_K\mathrm{maps}(2.0\times 2.0\mathrm{n}{\mathrm{m}}^2)$.

Figure 3

DFT optimized structure of $\mathrm{FeTPP}$ (a) and ${\mathrm{H}}_2\mathrm{TPP}$ (h) adsorbed on a three-layer Au substrate (only the top layer is shown). (b), (c) Magnified side views of an adsorbed $\mathrm{FeTPP}$ and a free $\mathrm{FeTPP}$ showing the structural deformation of the macrocyclic ring upon adsorption. (d) PDOS of free $\mathrm{FeTPP}$, adsorbed $\mathrm{FeTPP}$ and adsorbed ${\mathrm{H}}_2\mathrm{TPP}$ (vertically shifted for clarity). (e)–(g) Constant PDOS contour maps of $\mathrm{FeTPP}$ at the indicated energy. (i), ( j) Constant PDOS contour maps of ${\mathrm{H}}_2\mathrm{TPP}$ at the indicated energy.

Figure 4

(a) Spin-polarized PDOS of Fe and porphyrin backbone of $\mathrm{FeTPP}$ adsorbed on an Au substrate. (b) Net spin density of free and adsorbed $\mathrm{FeTPP}$ (up spin and down spin are in red and blue, respectively). (c) Spin-polarized $3d$ orbitals of Fe in free, free saddle-shaped $(S=1\mathrm{and}2)$ and adsorbed saddle-shaped molecules (from top to bottom).