Electronic and magnetic properties of molecule-metal interfaces: Transition-metal phthalocyanines adsorbed on Ag(100)

We present a systematic investigation of molecule-metal interactions for transition-metal phthalocyanines (TMPc, with TM = Fe, Co, Ni, Cu) adsorbed on Ag(100). Scanning tunneling spectroscopy and density functional theory provide insight into the charge transfer and hybridization mechanisms of TMPc as a function of increasing occupancy of the 3$d$ metal states. We show that all four TMPc receive approximately one electron from the substrate. Charge transfer occurs from the substrate to the molecules, inducing a charge reorganization in FePc and CoPc, while adding one electron to ligand $\pi$ orbitals in NiPc and CuPc. This has opposite consequences on the molecular magnetic moment: In FePc and CoPc the interaction with the substrate tends to reduce the TM spin, whereas, in NiPc and CuPc, an additional spin is induced on the aromatic Pc ligand, leaving the TM spin unperturbed. In CuPc, the presence of both TM and ligand spins leads to a triplet ground state arising from intramolecular exchange coupling between $d$ and $\pi$ electrons. In FePc and CoPc the magnetic moment of C and N atoms is antiparallel to that of the TM. The different character and symmetry of the frontier orbitals in the TMPc series leads to varying degrees of hybridization and correlation effects, ranging from the mixed-valence (FePc, CoPc) to the Kondo regime (NiPc, CuPc). Coherent coupling between Kondo and inelastic excitations induces finite-bias Kondo resonances involving vibrational transitions in both NiPc and CuPc and triplet-singlet transitions in CuPc.

Figure 1

Spin-polarized electronic structure of gas-phase TMPc. (Un-) occupied states are represented by (dashed) full lines. The contribution of different $d$ states is indicated in percentage.

Figure 2

(a) Topographic image of CoPc and NiPc molecules codeposited at room temperature on the Ag(100) surface ($I=0.49$ nA, $V=-1.0$ V), image size $13.4\times 13.4$ nm${}^2$. The green/red arrow indicates the $+/-{30}^{\circ }$ rotation of the molecular axis with respect to the surface lattice vectors (white arrows). (b) Adsorption geometry of NiPc on Ag(100) calculated by DFT. The different elements are labeled. N${}_a$ and N${}_p$ refers to aza and pyrrole N, and C${}_A$ and C${}_B$ to the two C atoms of the benzene ring used to measure chiral distortions (Table ).

Figure 3

Bias-dependent topographic images of all TMPc. For CuPc and NiPc negative voltage images with maximum chiral contrast are displayed. The contrast of CoPc and FePc does not vary in the range of negative voltages investigated. For positive voltage, all TMPc appear achiral in the bias range $V<+$1 V.

Figure 4

(a) $dI/dV$ spectra, acquired on the TM ions (red, bottom) and benzene rings (blue, top). The latter spectra have been offset for clarity. MOs, interface (IR), and Kondo (K) resonances are labeled. The weak Kondo peak at the ion site is smeared out by the larger $V_{\mathrm{mod}}$ used as compared to Fig. 7. Tunneling conditions ($I$, $V$): 1 nA, $-$1.0 V (FePc, CoPc), 3 nA, $-$2.2 V (NiPc), 3 nA, $-$2.0 V (CuPc). (b) Computed spin-polarized DOS of TMPc (black), and its projection onto the TM $d$ states. (c) Computed spin-polarized PDOS projected onto MOs.

Figure 5

Constant current $dI/dV$ maps of (a) CuPc and (b) CoPc. A topographic image of each molecule recorded simultaneously during the acquisition of the maps is displayed on the left side. The color scale is adjusted for maximum contrast in each map. NiPc and FePc present similar MO distributions as CuPc and CoPc, respectively.

Figure 6

Constant current $dI/dV$ maps of interfacial resonances in FePc arising from the hybridization between molecular and substrate electrons, marked as IR in Fig. 4b. The topography of FePc recorded during the acquisition of the maps is displayed on the left side.

Figure 7

[(a)–(d)] Conductance spectra acquired around $E_F$ on the TM ions (red) and benzene rings (blue). The zero-bias (elastic), vibrational, and triplet-singlet Kondo resonances observed in CuPc and NiPc are labeled as $K_e$, $K_v$, and $K_{\mathrm{ts}}$, respectively. Tunneling conditions ($I$, $V_b$): 1 nA, $-$100 mV (FePc, CoPc), 1.1 nA, $-$100 mV (NiPc), 2 nA, $-$100 mV (CuPc). [(e) and (f)] $d^{2}I/d{V}^2$ maps acquired at $-$3 and $-$5 mV, respectively, showing the intensity distribution of the elastic Kondo resonance ($K_e$) of CuPc and NiPc and its close resemblance to the $2e_g$ resonances.

Figure 8

Temperature dependence of (a) ${\Gamma }_K$ and (b) $G_K$ normalized to the intensity at 5 K ($G_0$) of the elastic Kondo resonance of CuPc. The solid line in (a) is a fit of ${\Gamma }_K$ using Eq. (1) giving $T_K=27\pm 2$ K. The solid (dashed) line in (b) is a fit of $G_K/G_0$ for the $S=1/2$ (underscreened $S=1$) case of Eq. (2).

Figure 9

Fit of the differential conductance ($dI/dV$) spectra of NiPc and CuPc measured at (a) the position of the TM ion and (b) the benzene ring, using Fano and Lorentzian functions for the zero and finite bias peaks, respectively, and step functions for the inelastic conductance steps. The bottom graph in (a) shows the second derivative ($d^{2}I/d{V}^2$) of the spectra measured on Ni and Cu. The energies obtained from the fits are indicated by dashed lines in the $d^{2}I/d{V}^2$ spectra. The bars correspond to the intensity of calculated Raman (yellow) and infrared (gray) modes of gas phase CuPc.[85] The bottom graph in (b) shows the theoretical PDOS obtained for the $2e_g$ orbital of CuPc for $I=25$ meV (solid line) and 0 meV (dashed line) calculated within the noncrossing approximation (see text for details).

Figure 10

Spin density of neutral (top) and anionic (bottom) gas-phase TMPc in $D_{4h}$ symmetry. Red (yellow) indicate spin up (down). The isovalue for the spin density is 0.005 $e^{-}$/Å${}^3$.

Figure 11

Spin-density distribution of FePc (a) adsorbed on Ag(100) and (b) in the gas-phase anion. Red/yellow denote spin up/down. The isovalue for the spin density is 0.005 $e^{-}$/Å${}^3$ in both cases.