Molecules at surfaces

We study metal-organic structures on solid surfaces, which can be ensembles of adsorbed molecules or self-assembled systems of organic molecules and metal atoms, by synchrotron-radiation-based x-ray spectroscopy. Near-edge absorption fine structure in x-ray absorption spectroscopy (NEXAFS) yields the adsorption geometry and, together with x-ray photoelectron spectroscopy (XPS), gives information about the electronic properties. Magnetic properties are determined using x-ray magnetic circular dichroism in x-ray absorption (XMCD). Our goal is to control in a defined way the magnetism of such structures. To that end, an improved understanding of the magnetic properties and the relevant mechanisms responsible for magnetic coupling between metal atoms or between adsorbate and substrate is imperative.

Example 1: Coupling between adsorbed magnetic molecules and a ferromagnetic substrate through graphene

We demonstrate that an antiferromagnetic coupling between paramagnetic Co-porphyrin molecules and an ultrathin Ni magnetic layer is present even if a layer of graphene separates the molecules from the Ni substrate. Because of the unique properties of graphene, this is important for future applications in a molecule-based organic magnetoelectronics. The coupling energy has been determined from the temperature dependence of x-ray magnetic circular dichroism measurements. By density functional theory+U calculations the coupling mechanism is shown to be mediated via the π electronic system of graphene, while no covalent bonds between the molecule and the substrate are established, and the molecules sit at a distance of ≈ 3.3 Å above the graphene plane.

These experiments have been conducted in collaboration with K. Tarafder and P. M. Oppeneer of Uppsala University (Sweden).

Example 2: Ring-opening reaction of nitrospiropyran on Au(111)

We have found that 1,3,3-trimethylindolino-6-nitrobenzopyrylospiran (nitrospiropyran) adsorbs in the closed-ring form on Au(111), but reverts to its merocyanine isomer form with open ring if the sample temperature exceeds 300 K. Such a thermally activated ring-opening reaction shows that on the surface the merocyanine form is thermodynamically more stable, thus reversing the stability with respect to the free molecule. This is attributed to the larger interaction of the adsorbed merocyanine with the metal and the formation of strongly hydrogen-bonded molecular dimers.

These experiments have been conducted in collaboration with G. Schulze, M. Koch, K. J. Franke, F. Leyssner, M. Wolf, P. Tegeder, and J. I. Pascual, also Freie Universität Berlin.

Example 3: Coupling of rare-earth magnetic moments in endohedral fullerenes

Endohedral fullerenes are nanometer-sized objects that contain atoms inside a carbon cage. The carbon cage helps to isolate encapsulated magnetic species from aggressive environments and can confine three rare-earth ions in a relatively small space. We have investigated the magnetic coupling in C80 fullerenes, filled with Gd3N (see figure). We have discovered a ferromagnetic coupling between the gadolinium moments inside the same molecule. If the molecules are adsorbed on a ferromagnetic Ni film, they also interact with the substrate through the carbon cage. There are at least two different species of Gd3N@C80 on Ni: A more abundant one that couples weakly antiferromagnetic to the substrate, and a more strongly ferromagnetically coupled, but less abundant one. This leads to a peculiar change in the sign of the Gd magnetic moment as a function of temperature.

These experiments have been conducted in collaboration with C. Schmidt, S. Waßerroth, G. Ahmadi, B. W. Heinrich, M. Schneider, P. W. Brouwer, and K. J. Franke, also Freie Universität Berlin, and E. Weschke, Helmholtz-Zentrum Berlin für Materialien und Energie.

Example 4: Spin-crossover molecules on surfaces

Spin-crossover molecules are a particular calls of molecules that change their magnetic moment as a function of temperature. If they could be adsorbed on a metal surface while maintaining their spin-crossover properties, they would comprise interesting building blocks for a molecular magneto-electronic logic devices. We have achieved a thermally induced, fully reversible, gradual spin crossover in a submonolayer of [FeII(NCS)2L] (L: 1-{6-[1,1-di(pyridin-2-yl)ethyl]- pyridin-2-yl}-N,N-dimethylmethanamine) deposited on a highly oriented pyrolytic graphite (HOPG) substrate in ultrahigh vacuum, as proven by x-ray absorption spectroscopy measurements. These results show that by using a carbon-based substrate the spin-crossover behavior can be preserved even for molecules that are in direct contact with a solid surface.

These experiments have been conducted in collaboration with D. Wiedemann and A. Grohmann (Technische Universität Berlin) and W. Kroener and P. Müller (Universität Erlangen-Nürnberg).

Publications about this research:

C. F. Hermanns, K. Tarafder, M. Bernien, A. Krüger, Y.-M. Chang, P. M. Oppeneer, and W. Kuch
Magnetic Coupling of Porphyrin Molecules Through Graphene
Adv. Mater. 25, 3473 (2013).
DOI: 10.1002/adma.201205275

C. F. Hermanns, M. Bernien, A. Krüger, C. Schmidt, S. T. Waßerroth, G. Ahmadi, B. W. Heinrich, M. Schneider, P. W. Brouwer, K. J. Franke, E. Weschke, and W. Kuch
Magnetic Coupling of Gd3N@C80 Endohedral Fullerenes to a Substrate
Phys. Rev. Lett. 111, 167203 (2013).
DOI: 10.1103/PhysRevLett.111.167203

T. R. Umbach, M. Bernien, C. F. Hermanns, A. Krüger, I. Fernández-Torrente, P. Stoll, J. I. Pascual, K. J. Franke, and W. Kuch
Ferromagnetic Coupling of Mononuclear Fe Centers in a Self-Assembled Metal Organic Network on Au(111)
Phys. Rev. Lett. 109, 267207 (2012).
DOI: 10.1103/PhysRevLett.109.267207

M. Bernien, D. Wiedemann, C. F. Hermanns, A. Krüger, D. Rolf, W. Kroener, P. Müller, A. Grohmann, and W. Kuch
Spin Crossover in a Vacuum-Deposited Submonolayer of a Molecular Iron(II) Complex
J. Phys. Chem. Lett. 3, 3431 (2012).
DOI: 10.1021/jz3011805

M. Bernien, J. Miguel, C. Weis, Md. E. Ali, J. Kurde, B. Krumme, P. M. Panchmatia, B. Sanyal, M. Piantek, P. Srivastava, K. Baberschke, P. M. Oppeneer, O. Eriksson, W. Kuch, and H. Wende
Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates
Phys. Rev. Lett. 102, 047202 (2009).
DOI: 10.1103/PhysRevLett.102.047202

M. Piantek, G. Schulze, M. Koch, K. J. Franke, F. Leyssner, A. Krüger, C. Navío, J. Miguel, M. Bernien, M. Wolf, W. Kuch, P. Tegeder, and J.I. Pascual
Reversing the Thermal Stability of a Molecular Switch on a Gold Surface: Ring-Opening Reaction of Nitrospiropyran
J. Am. Chem. Soc. 131, 12729 (2009).
DOI: 10.1021/ja901238p