Supporting information for: Coverage-driven. Electronic Decoupling of Fe-Phthalocyanine from a. Ag(111) Substrate

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1 Supporting information for: Coverage-driven Electronic Decoupling of Fe-Phthalocyanine from a Ag() Substrate T. G. Gopakumar,, T. Brumme, J. Kröger, C. Toher, G. Cuniberti, and R. Berndt Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, D-2498 Kiel, Germany, Institute for Materials Science and Max Bergmann Center of Biomaterials, Dresden University of Technology, D-69 Dresden, Germany, and Institut für Physik, Technische Universität Ilmenau, D Ilmenau, Germany gopa@physik.uni-kiel.de Phone: Fax: To whom correspondence should be addressed Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, D-2498 Kiel, Germany Institute for Materials Science and Max Bergmann Center of Biomaterials, Dresden University of Technology, D-69 Dresden, Germany Institut für Physik, Technische Universität Ilmenau, D Ilmenau, Germany S

2 S: Projected density of states of Fe s and Ag s states Figure shows the calculated PDOS of s states of the central Fe atom (shaded area) of a single FePc on Ag() [Figure a] and of a FePc in the (6 3) structure on Ag() [Figure b]. The black lines depict s states of Ag atoms, which form the bridge adsorption site of FePc. The other full line (red) shows s states of Ag surface atoms, which are located between FePc molecules of adjacent unit cells. While the positions of the Fe s states are correlated with those of the Fe d z 2 states (dashed line), the positions of bridge Ag s states are determined by the location of the Fe d xz /d yz states. Furthermore, the Ag s states of bridge atoms become narrower upon forming the superstructure, which explains the narrowing of peaks in the experimentally obtained di/dv spectra. Moreover, in the (6 3) structure positions and lineshapes of s states of Ag atoms between FePc molecules are similar to those of Ag bridge atoms between and ev. No such correlation is observed for the single FePc. This finding is further indication of a modified hybridization between molecule and surface. Fe s Ag s (surface) Fe d 2 z Ag s (bridge) a PDOS (ev - ).5.5 single FePc b FePc - (6 3) E - E F (ev) Figure : Calculated projected density of states (PDOS) for (a) a single fully relaxed FePc molecule and (b) a fully relaxed FePc in the (6 3) structure on Ag() [shaded area: Fe s states, lines: s states of a Ag atom forming the bridge site (black), s states of a Ag atom between FePc molecules in the periodically repeated cells (red), dashed line: Fe d z 2 orbitals scaled by /4]. S2

3 Table : Calculated total energies (in Ry, Ry=3.6eV) used for determining the adsorption energy of single FePc on Ag() and the simulated(6 3) superstructure. single FePc (6 3) superstructure Ag()-FePc Ag() FePc S2: Adsorption energies The hybridization of FePc with Ag() enables an estimate of the total binding energy using density functional calculations. The total energies of the components used are listed in Table. The total binding energy of a single FePc on the Ag() surface can be obtained directly by comparing the total energy of the combined Ag()-FePc system with the separate total energies of the Ag() surface and of the free-standing FePc. The sum of the total energies of the two separate surface and FePc systems yields a value of Ry, the difference with Ag()-FePc being.553ry. This is the total energy gain for a single FePc molecule upon adsorption. For the(6 3) superstructure we have Ry and thus an adsorption energy of.4896 Ry. The difference of.57 Ry compared to the single-fepc adsorption energy directly evidences a weakening of the surface-molecule interaction, through the formation of the(6 3) superstructure. The adsorption energies calculated for FePc are comparable with the literature.,2 S3: Density of states of single FePc and FePc embedded in a (6 3) structure in vacuum The formation of the (6 3) superstructure raises questions regarding the nature of the interaction between the molecules leading to the observed effects, and how this interaction changes the electronic structure compared to a single FePc molecule. In order to directly investigate the interactions between the molecules, calculations were performed for a free-standing FePc layer with the same distance between the molecules of the repeated unit cell as had been measured in the S3

4 experiments. Including the Ag() surface would be computationally very expensive since for the simulation of the full superstructure, the 8 FePc molecules alone would already contain 26 atoms. More importantly, the density of states (DOS) of the silver substrate would tend to obscure small interactions between the molecules, making these effects difficult to visualize. Due to the plane-wave basis set and the periodic boundary conditions used by the PWscf code it is, in general, difficult to use this method to calculate the electronic structure for a single isolated molecule. Thus, we calculated the electronic structure for both the single isolated FePc and the free-standing (6 3) superstructure with the SIESTA package which uses a localized basis set. 3 Due to the local nature of this basis set, atoms of different repeated unit cells do not interact if the distance is larger than the cutoff radii used for the basis orbitals. We used the generalized gradient approximation and a local basis set with one s and two p orbitals for C, two s orbitals for H, two polarized s and p orbitals for N and one polarized s orbital and two polarized p and d orbitals for Fe. The cut-off radii for the pseudo-atomic orbital basis set within SIESTA was set using an energy shift of.ry. The DOS for the isolated FePc and the free-standing(6 3) superstructure is shown in Figure 2. Panel (a) shows the DOS using an artificial broadening of.ev. The main difference close to the Fermi energy can be seen for the states near.25ev. The isolated FePc has two states near this energy, whereas the DOS for an FePc in the(6 3) superstructure seems to be more complex close to.25ev. In order to understand the changes the DOS was replotted with a smaller broadening of.ev as shown in Figure 2(b). Due to the reduced broadening one can now clearly see that one state splits into two upon forming the(6 3) superstructure. This splitting can be explained by the lifting of the degeneracy between the two in-plane directions in the superstructure, referred to as the short and long axes of the unit cell in the main text. Figure 3 shows the calculated integrated local density of states (ILDOS) isosurface for the states within the energy window indicated by blue dashed lines in Figure 2. While the states are localized at the molecule for the isolated FePc as shown in Figure 3a, the ILDOS for the free-standing (6 3) layer shown in Figure 3b clearly shows a connection between the individual molecules as S4

5 broadening:. ev DOS (arb. units) - single FePc FePc - (6 3) broadening:. ev a E - E F (ev) Figure 2: Calculated spin-resolved density of states (DOS) for a single fully relaxed FePc molecule in vacuum (red) and for a fully relaxed FePc in a free-standing (6 3) structure (black). Spin-up and spin-down DOS appear with, respectively, positive and negative sign. In panel (a) a broadening of.ev was used to plot the DOS, whereas in panel (b) the broadening was reduced to.ev. Blue dashed lines indicate the region which was used to plot the integrated local density of states as shown in Figure 3. b Figure 3: Spatial contours of integrated local density of states for (a) a single fully relaxed FePc molecule in vacuum and (b) a fully relaxed FePc in a free-standing (6 3) structure (cyan: H, yellow: C, blue: N, red: Fe, grey isosurface: integrated local density of states). The upper and lower energy boundaries for integration are shown in Figure 2 as blue dashed lines. S5

6 indicated by black dashed lines. Even if the rotation of the different FePc molecules towards each other might be hindered on the Ag() substrate this hybridization between the FePc is still possible. The calculations described here are additional evidence for an increased molecule-molecule interaction in the(6 3) superstructure. di / dv (ns) 2 2 a b Center spectrum: 2nd layer A Center spectrum: 3rd layer B C B' C' A'. Sample Voltage (V).5 Figure 4: Constant-height di/dv spectra of FePc ligands adsorbed atop the (a) first and (b) second molecular layer (feedback loop parameters:.75v, na). Peaks labelled A, B, C in (a) and A, B, C in (b) indicate pairs of peaks, which shift by similar magnitudes, i. e., from.39 (A) to.89v (A ), from.67 (B) to.2v (B ) and from.88 (C) to.44v (C ). S4: Spectra of di/dv of FePc centers in second and third molecular layers Spectra of di/dv have been acquired atop the center of FePc molecules in the second [Figure 4a] and third [Figure 4b] molecular layer. Peak shifts from A to A, B to B and C to C are similar to the values observed from spectra of FePc ligands (Figure 5, manuscript). In the second-layer spectra [Figure 4a] negative differential resistance (NDR, di/dv < ) is observed for voltages.4v < V <.8V. We do not analyze this phenomenon systematically here. However, it is not likely that energetically localized tip states convoluted with the molecular density of states 4 are at the origin of the observation. If this scenario were correct then NDR would likewise be expected S6

7 for the ligand spectra (Figure 5, manuscript), which do not exhibit NDR. Rather, voltage-dependent changes in the tunneling barrier height 5 are more likely the explanation of NDR in the present case. References () Heinrich, B. W.; Iacovita, C.; Brumme, T.; Choi, D.-J.; Limot, L.; Rastei, M. V.; Hofer, W. A.; Kortus, J.; Bucher, J.-P. J. Phys. Chem. Lett. 2,, 57. (2) Casarin, M.; Marino, M. D.; Forrer, D.; Sambi, M.; Sedona, F.; Tondello, E.; Vittadini, A.; Barone, V.; Pavone, M. J. Phys. Chem. C 2, 4, 244. (3) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. J. Phys.: Condens. Matter 22, 4, (4) Lyo, I.-W.; Avouris, Ph. Science 989, 245, 369. (5) Grobis, M.; Wachowiak, A.; Yamachika, R.; Crommie, M. F. Appl. Phys. Lett. 25, 86, 242. S7