Site- and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyanines

Transcription

1 Site- and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyanines Cornelius Krull 1, Roberto Robles 2, Aitor Mugarza 1, Pietro Gambardella 1,3 1 Catalan Institute of Nanotechnology (ICN), UAB Campus, E Barcelona, Spain 2 Centre d Investigacions en Nanociència i Nanotecnologia (CIN2), UAB Campus, E Barcelona, Spain 3 Institució Catalana de Recerca i Estudis Avançats (ICREA) and Departament de Física, Universitat Autonoma de Barcelona, E Barcelona, Spain Contents I. Rare LiMPc configurations II. III. IV. DFT calculations Vibronic excitations Atom-by-atom manipulation sequence V. Supplementary References Supplementary Figures S1 S5 Supplementary Table SI NATURE MATERIALS 1

2 I. Rare LiMPc configurations More than 75% of the observed LiMPc species correspond to the configurations L A and M described in the main text. In some instances, however, we observed different LiMPc configurations, which we report in Fig. S1. The first case (Fig. S1a) corresponds to a configuration obtained after manipulating a LiCuPc-L A species. After trying to push the Li atom away from the molecule, this remained bonded to the aza-n site, as can be observed in the topographic image. Such configuration corresponds to the LiCuPc-L B complex obtained by DFT (see Fig. 2a). Further attempts to separate this complex resulted in the diffusion of the whole molecule without changing the position of the Li atom relative to the Pc ring. This complex appears therefore to be stable and its electronic structure is very similar to LiCuPc-L A, as revealed by the di/dv spectra shown in Fig. S1a. A second case is that of a LiCuPc complex with the ligand axes rotated by 45º with respect to the high symmetry directions of the substrate rather than the usual ±30º common to both undoped and doped molecules (Fig. S1b). A bright benzene ring is observed, indicating the presence of a Li ion similar to the LiCuPc-L A case. Here, however, the di/dv spectrum is similar to that of LiNiPc-L A, corresponding to a charge transfer of Q = 1, rather than LiCuPc-L A with Q = 2. After lateral manipulation of the complex, the configuration changes to that of the stable LiCuPc-L A configuration with Q = 2 (not shown). A third configuration of doped CuPc complexes exhibits a bright protrusion which is laterally shifted from the center (Fig. S1c). The shift is of approximately 1 Å, as measured from the topographic profile of the image. The di/dv spectrum is very similar to that of the stable LiCuPc-M species, indicating that charge transfer occurs through the d-states. Finally, Fig. S1d shows an image of a fuzzy LiNiPc complex. The bright center and four-fold symmetry of the complex suggest that this is an M type configuration. The unstability of this 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION configuration, evidenced by the fuzziness of the image, is in full agreement with its large formation energy of about1 ev calculated by DFT for LiNiPc-M (see Table SI). Figure S1. Rare LiMPc configurations. Topography (left), schematics (right), and di/dv spectra (bottom) obtained for: a, LiCuPc-L B, with the Li bonded to the aza-n. Both di/dv spectra measured on the Cu and benzene sites are very similar to those obtained for the L A species (dashed lines). b, LiCuPc-L A azimuthally rotated by +45º with respect to the high symmetry directions of the substrate. The spectrum is very similar to that of LiNiPc-L A, corresponding to a charge state for the 2e g orbital of Q = 1. c, Asymmetric LiCuPc-M, indicating that the Li ion is laterally shifted from the center of the molecule. The spectra is very similar to the symmetric case (dashed line) discussed in the text. A background spectrum acquired on Ag(100) has been subtracted to the data in this case in order to enhance molecular features. d, Unstable LiNiPc-M. The fuzzy appearance is attributed to the unstability of the Li ion at the center. Spectroscopy could not be performed on this type of molecules. NATURE MATERIALS 3

4 II. DFT calculations Charge density maps In the ligand-doped molecules (L) there is a clear correspondence between the position of the Li atom and the 2e g acceptor level that shifts below the Fermi level. This is also evidenced by the charge density maps calculated by DFT (Fig. S2, left) of the formerly degenerate 2e g orbital, which splits into a filled state localized along the axis containing the Li ion, and an empty state at higher energy along the orthogonal axis, as observed by STM. Note that empty states present an increased charge density at the Cu ion due to the energy overlap with the b 1g state (see Fig. 2e). In contrast, the M configuration (right) retains the four-fold symmetry of the doubly degenerate 2e g state. Figure S2. Charge density distribution of the 2e g resonance of the L and M configurations of LiCuPc computed by DFT. The left panel shows the two orthogonal isocharge contours originating from the splitting of the 2e g state for ligand-doped molecules. The right panel shows the four-fold symmetric contour of the 2e g state for metal-doped species. 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION STM topographic images Topographic images of LiCuPc complexes have been simulated using the Tersoff and Hamman theory 1,2 using the method of Ref. 3. A set of images obtained at -1.5 V for each configuration is displayed in Fig. S3. Comparison between the calculated and experimental images allows discriminating between different configurations. For example, in the case of the alkali adatom u interacting with a benzene ring, only the L A configuration reproduces the double-lobe structure observed in the experimental images of the charged ring. This is also in agreement with the lower energy calculated for this configuration relative to L A0. Finally, the image calculated for the M configuration, with brighter intensity at the center, can be univocally associated to the doping of the metal site, consistently with the experimental results presented in the main text. Figure S3. Topographic images of the different LiCuPc configurations computed by DFT. The images are obtained at V bias = -1.5 V. The different features, such as the double-lobe structure and the bright center of the L A u and the M configurations Charge transfer, magnetic moment and total energy The values for the total energy, magnetic moment and charge transfer displayed in Fig. 2 are listed in Table SI for the undoped and singly doped molecules. It can be noted that the total charge is higher than that derived from the experimental di/dv spectra. This overestimation is related to the intrinsic difficulty of DFT in treating electron correlation, especially in highly delocalized orbitals such as the NATURE MATERIALS 5

6 2e g π-orbital. 4-6 For the same reason, the spin related to the presence of unpaired electrons in this orbital is missing in the calculations. As a consequence, we consider only quantitative values obtained for the transition-metal site (TM), related to the localized d-orbitals. The charge evolution in the 2e g orbital is studied by following relative trends. For instance, the different charge states of u u the LiCuPc-L A and LiNiPc-L A complexes derived from the experimental data is reflected in the lower charge computed for the latter. N (MPc) N (TM) N (Li) N (Ag) m (TM) E 0 Li CuPc NiPc Li u L A o L A L B L C M Table SI. Charge transfer, magnetic moment, and formation energies of doped LiMPc complexes calculated by DFT. Differential charge N (in electron units) of the adsorbed molecules, transition metal ion (TM), Li ion, and Ag substrate relative to gas-phase MPc and unperturbed Ag surface. Magnetic moment m (in units of B ) of the metal ions. Relative formation energy E (in ev). Rows corresponding to CuPc (NiPc) are grey (white). Note that, due to the hybridized 3d x2-y2 /2p character of the molecular b 1g orbital, the magnetic moment projected onto the metal ion captures only part of the S = ½ spin present in this orbital. 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION III. Vibronic excitations Electrons tunneling to a molecular orbital having relatively large lifetime can inelastically excite vibrational states of the same orbital. This is often the case for molecules deposited on insulating layers, as the molecular orbitals hybridize only weakly with the substrate, which leads to an extended lifetime for the transient state. Such vibronic excitations will appear in di/dv spectra as equidistant peaks overlaying the actual molecular orbital resonance. 7-9 The energy spacing is equal to the energy of the excited vibration. The detailed analysis of the peak structures found below the Fermi level for all LiMPc-L A complexes shows a series of equidistant peaks. The energy of the vibrational excitation is found to be between 173 and 193 mev. Literature values for the C-C and C-N stretching modes lie exactly within this energy range ( mev). 10 Furthermore, PES measurements for thin films of CuPc on HOGP 10,11 and in the gas-phase 12 show a vibronic coupling of the HOMO of CuPc at the energy of 150 mev. We therefore interpret the multi-peak structure below E F as a vibronic progression inside the molecules. Its presence indicates an increase of the electron lifetime within the molecules, and hence a Li-induced decoupling from the substrate. 7 Figure S4. Vibronic coupling in L A species. Fitting of the vibronic satellite peaks with Lorentzian functions: a, Example of the fit obtained for the singly occupied 2e g orbital in LiNiPc-L A. b, The energy separation obtained for the multiple peaks is around 170 mev in both LiCuPc and Li 2 CuPc species, and around 190 mev in LiNiPc. NATURE MATERIALS 7

8 IV. Atom-by-atom manipulation sequence Although the LiMPc species form spontaneously by depositing Li atoms at low temperature, doping levels higher than one were rarely achieved due to the low coverage of Li required for the detailed study of individual molecules. Doping levels higher than two were studied for CuPc by using the STM tip to manipulate Li adsorbates. By laterally moving individual Li atoms one-by-one, up to six dopants could be introduced in the same molecule (see Methods for technical details). Figure S5 shows such a manipulation sequence including successful as well as failed doping attempts. We observed that dragging Li atoms towards the benzene rings yields L A configurations and that the molecules can rotate upon incorporation of dopants. We explored also the possibility of forming configurations with dopants occupying sites along the ligand axis orthogonal to that already occupied by other dopants. In such cases the molecule would rotate in order to incorporate the dopant in the same ligand axis (see doping with the 3 rd Li atom in Fig. S5). di/dv spectroscopy measurements carried out in between manipulation events were found to induce diffusion or rotation of metastable configurations. This resulted in an internal rearrangement of dopants, as shown in Fig. S4 for the case of Li 3 CuPc. In this case the dopants could even be accommodated in orthogonal axes. However, this configuration proved particularly unstable, preventing us from performing spectroscopy measurements. The topography of the stable Li 4 CuPc to Li 6 CuPc configurations suggests accommodation of the Li atoms along the same ligand axis, consistently with the widening of the 2eg - 2eg gap observed in Fig NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION Figure S5. Atom-by-atom doping of CuPc induced by lateral manipulation of Li atoms. Arrows indicate the direction of the lateral motion of the tip between successive images. NATURE MATERIALS 9

10 V. Supplementary References 1. Tersoff, J., Hamann, D. R. Theory and Application for the Scanning Tunneling Microscope. Phys. Rev. Lett. 50, (1983). 2. Tersoff, J., Hamann, D. R. Theory of the scanning tunneling microscope. Phys. Rev. B 31, (1985). 3. Bocquet, M.-L., Lesnard, H., Monturet, S., Lorente, N. In Computational Methods in Catalysis and Materials Science; van Santen, R. A., Sautet, P., Eds.; Wiley-VCH: Weinheim, Germany. 4. Mugarza, A. et al. Electronic and magnetic properties of molecule-metal interfaces: Transition-metal phthalocyanines adsorbed on Ag(100). Phys. Rev. B 85, (2012). 5. Mugarza, A. et al. Spin coupling and relaxation inside molecule metal contacts. Nat Commun 2, 490 (2011). 6. Cohen, A. J., Mori-Sánchez, P. & Yang, W. Insights into Current Limitations of Density Functional Theory. Science 321, (2008). 7. Nazin, G. V., Wu, S. W., Ho, W. Tunneling rates in electron transport through double-barrier molecular junctions in a scanning tunneling microscope. Proc. Natl. Acad. Sci. U.S.A. 102, (2005). 8. Wu, S. W., Nazin, G. V., Chen, X., Qiu, X. H., Ho, W. Control of Relative Tunneling Rates in Single Molecule Bipolar Electron Transport. Phys. Rev. Lett. 93, (2004). 9. Qiu, X. H., Nazin, G. V., Ho, W. Vibronic States in Single Molecule Electron Transport. Phys. Rev. Lett. 92, (2004). 10. Kera, S., Yamane, H., Ueno, N. First-principles measurements of charge mobility in organic semiconductors: Valence hole vibration coupling in organic ultrathin films. Prog. Surf. Sci. 84, (2009). 11. Kera, S., Yamane, H., Sakuragi, I., Okudaira, K. K., Ueno, N. Very narrow photoemission bandwidth of the highest occupied state in a copper-phthalocyanine monolayer. Chem. Phys. Lett. 364, (2002). 12. Evangelista, F. et al. Electronic structure of copper phthalocyanine: An experimental and theoretical study of occupied and unoccupied levels. J. Chem. Phys. 126, (2007). 10 NATURE MATERIALS