Magnetic configurations of open-shell molecules on metals: The case of CuPc and CoPc on silver

For nanostructured interfaces between open-shell molecules and metal surfaces that involve charge transfer upon adsorption, the investigation of molecular magnetic properties is an interesting yet difficult task, because in principle different magnetic configurations with distinct properties can be found. Here, we study the magnetic properties of CuPc-Ag and CoPc-Ag interfaces, which constitute interesting test cases because charge is transferred to the initially open-shell Pc molecules upon adsorption. Using hybrid density functional theory, we examine the stability of the various magnetic configurations occurring at these nanoscale interfaces, as well as for the corresponding gas-phase anions, and compare our findings to those of previous experimental studies. For CuPc-Ag, we identify a high-spin triplet configuration as the most likely configuration at the interface, whereas for CoPc-Ag a quenching of the total magnetic moment is found. Interestingly, such quenching is consistent with two distinctly different interfacial electronic configurations. These important differences in the magnetic properties of CuPc and CoPc on Ag are rationalized by variations in the interaction of their central metal atoms with the substrate. Our work facilitates a deeper understanding of the magnetic configuration and interlinked electronic-structure properties of molecule-metal interfaces. Furthermore, it highlights the necessity of an appropriate choice of methodology in tandem with a detailed evaluation of the different emerging magnetic properties.

Figure 1

Density of states projected onto the molecule (PDOS) of the different magnetic configurations of CuPc-Ag(111): the singlet (a), triplet (b), and doublet (c) configuration. For relevant features in the PDOS, an isosurface representation of the molecular contributions to the local density of states (LDOS) is shown below. Energy regions (integration window 0.5 eV) corresponding to the schematic representations of the LDOS are marked by numbers in the respective PDOSs.

Figure 2

Isosurface representation of the spin density for the triplet (a) and doublet (b) configuration of CuPc-Ag(111), see text for details. Regions of positive sign are plotted in red, and regions of negative sign are plotted in blue.

Figure 3

PDOS of the different magnetic configurations of CoPc-Ag(111): (a) closed-shell and (b) open-shell singlet configurations. The molecular contributions of the LDOS corresponding to relevant features in the PDOS of the two configurations are shown below. Energy regions (integration window 0.5 eV) corresponding to the schematic representations of the LDOS are marked by numbers in the respective PDOSs.

Figure 4

Isosurface representation of the spin density for the open-shell singlet configuration of CoPc-Ag(111), see text for details. Regions of positive sign are plotted in red and regions of negative sign are plotted in blue.

Figure 5

Spin polarization, P(E), for the open-shell singlet configuration of CoPc-Ag(111), calculated using Eq. (1).

Figure 6

Broadened eigenvalue spectrum (using a Gaussian with $\sigma =0.1$ eV) of the singlet and triplet configuration of the CuPc anion in the gas phase, calculated with HSE06 (panels a and c) and OT-RSH (panels b and d). Relevant HSE06-calculated orbitals are also shown. The regions corresponding to the schematic representations of the orbitals are marked by numbers in the respective spectra. We note that, as expected, the OT-RSH energy gap is larger than the one calculated with HSE06, owing to the different description of the long-range exchange interaction. Therefore, the energies of the occupied and unoccupied orbitals are aligned separately to the HOMO (left panels) and LUMO (right panels).

Figure 7

Broadened eigenvalue spectrum (using a Gaussian with $\sigma =0.1$ eV) of the closed-shell singlet and open-shell singlet configuration of the CoPc anion in the gas phase, calculated with HSE06 (panels a and c) and OT-RSH (panels b and d). Relevant HSE06-calculated orbitals are also shown. The regions corresponding to the schematic representations of the orbitals are marked by numbers in the respective spectra. We note that, as expected, the OT-RSH energy gap is larger than the one calculated with HSE06, owing to the different description of the long-range exchange interaction. Therefore, the energies of the occupied and unoccupied orbitals are aligned separately to the HOMO (left panels) and LUMO (right panels).

Figure 8

Three-dimensional representation of the charge-density rearrangements occurring upon adsorption of CuPc and CoPc on Ag(111) for their various magnetic configurations. Regions of electron accumulation are plotted in red, and regions of electron depletion are plotted in blue. For each configuration, a top and a side view are shown.