Interaction of Cobalt Atoms, Dimers, and Co4 Clusters with

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Interaction of Cobalt Atoms, Dimers and Co Clusters with Circumcoronene: a Theoretical Study Tomás Alonso-Lanza, Angel Mañanes, and Andrés Ayuela J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04369 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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The Journal of Physical Chemistry

Interaction of Cobalt Atoms, Dimers and Co4 Clusters with Circumcoronene: a Theoretical Study Tomás Alonso-Lanza,∗,† Ángel Mañanes,‡ and Andrés Ayuela† †Centro de Física de Materiales CFM-MPC CSIC-UPV/EHU, Donostia International Physics Center (DIPC), Departamento de Física de Materiales, Fac. de Químicas, UPV-EHU, 20018 San Sebastián, Spain ‡Departamento de Física Moderna, Universidad de Cantabria, Santander, Spain E-mail: [email protected]

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Abstract We study the interaction of circumcoronene molecule C54 H18 with cobalt in the form of atoms, dimers and clusters of four atoms. We find that the cobalt atom prefers to be on a hollow site in the edge zone. The cobalt dimer is bonded perpendicularly to another different hollow site in the edge zone. The Co4 cluster adopts a tetrahedral shape with a face parallel to circumcoronene, placing each of the three atoms over contiguous hollow sites, starting from the edge. Cobalt remains bonded to circumcoronene as Co2 molecules or Co4 clusters, rather than spread as isolated atoms, because the cobalt-cobalt interaction is stronger than the cobalt-carbon bond. The interaction with cobalt clusters induces small magnetic moment on circumcoronene. The magnetic moment of cobalt is reduced when bonding to circumcoronene. There is charge transfer between those systems and its direction depends on the relative position of the cluster with respect to circumcoronene. We then propose that the presence of cobalt over circumcoronene or similar polycyclic-aromatic hydrocarbons can be detected in experiments, looking at the carbon isomer shift which arises due to the contact charge densities at the nuclei.

Introduction Polycyclic-aromatic hydrocarbons (PAHs) such as benzene and coronene (C24 H12 ) are found in the interstellar medium 1,2 and obtained as by-products in combustion processes. 3 PAHs show high stability because of the high delocalization of electrons, which allows these molecules to keep its aromaticity. 4 Larger PAH molecules such as circumcoronene C54 H18 have also been reported. 5–8 Circumcoronene shows π delocalized bonds in the form of seven local sextets and six double bonds, which can be represented in a Clar structure. 9 Key aspects of circumcoronene have been experimentally investigated e.g., the interaction with ion beams, 10 fragment losses, 11 the NMR spectra, 12 and induced electron currents when applying perpendicular magnetic fields. 13 Circumcoronene, as other PAHs, is found in the interstellar 2


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medium, 2 accounting for the IR spectra that is experimentally obtained. Several studies are considering the IR spectra related to neutral and charged circumcoronene. 14–16 These charge states can be obtained after adsorbing atoms which can be bonded to circumcoronene in different sites (Fig. 1). Small PAHs, such as benzene and coronene, show cobalt adsorption. Note that PAHs complexes with late transition metals like cobalt have been proposed to be used as ultimate memories in spintronics. 17 A cobalt atom is bonded to benzene on the ring centre. 18 Transition metal atoms are reported to interact with benzene molecules 19 leading to two main different structures: multiple-decker for the early elements and bowl-like for the late elements such as cobalt. 20 The hollow site predicted for cobalt over benzene allows benzene to keep its aromaticity, 4 which means that its properties are highly retained. The magnetism of the cobalt-benzene system is mainly determined by the cobalt atom, which displays 1.1 µB with a small negative magnetic moment induced in benzene. 21 Larger coronene can be evaporated using laser ablation 22 and its properties has been studied using photoelectron spectroscopy 22 and collisions with charged ions. 23 The interaction between iron and coronene has been studied both theoretically by density functional theory calculations, 24 and experimentally by photoelectron spectroscopy 24 and mass selected laser photodissociation. 25 Cobalt and coronene interaction was studied both theoretically by density functional theory and experimentally using photoelectron spectroscopy by Kandalan et al. 26 Large circumcoronene molecule comes next and it has been studied interacting with iron atoms, that preferred to bond on hollow sites by the edges. 27,28 However, depositing cobalt in the form of a single Co atom and small Co clusters on circumcoronene remains yet to be investigated. Here we study the graphene flake immediately larger than coronene molecule that keeps an hexagonal shape, which is called circumcoronene. We deposit small cobalt clusters of one, two and four atoms over it. This system can display interesting electronic and magnetic properties due to the interaction of the 3d orbitals of the cobalt atoms with the 2p orbitals of the C54 H18 molecule. The possible induced magnetic moment in the graphene flake due to the

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electron-correlation, 31 highly accurate Quantum Monte Carlo calculations supported the reliability of this minimum for graphene. 32 Furthermore, the large size of circumcoronene and the large number of studied geometries would imply a prohibitive computational cost when accurate methods beyond DFT-GGA for the electron correlation were to be used. We carry out them mainly with the siesta method. 33 We choose the generalized gradient approximation for exchange-correlation with the PBE flavor 34 in order to solve these equations self-consistently. The wavefunctions of independent particles are expanded in basis sets of DZP atomic orbitals. We use norm-conserving relativistic Troullier-Martins 35 pseudopotentials for the core electrons of carbon, hydrogen and cobalt atoms. The reference electronic configuration for cobalt has 1.50 electrons in the 4s level and 7.50 electrons in the 3d level. We used fractional charges for the occupations of the pseudopotential attending to the electron configurations for the ground state of the free cobalt atom that we obtained in test adf calculations. It has been shown that for small transition metal atoms the ground state electronic structure lies in between 3d n−1 4s 1 and 3d n−2 4s 2 . 29,36 The electron configurations and the value of the radii were thoroughly tested by computing energy differences between all-electron and pseudopotential calculations for several excited states„ such as 3d 7 4s 2 , 3d 7 4s 1 4p1 , 3d 7 4s 1 and 3d 8 4s 1 . The pseudopotentials were also tested in real calculations, for example, in cobalt bulks. We obtained an hcp structure, with a first neighbors distance of 2.52 Å and a magnetic moment per atom of 1.65 µB , which are in agreement with experimental results and similar all-electron calculations. 37 We tested the pseudopotentials in graphene and benzene molecules obtaining reasonable results for their main properties. For our isolated systems we choose a large enough unit cell box of 40x40x40 Å, and atoms are fully relaxed. We have tried different cell sizes for the unit cell to ensure that self-interactions among molecules are negligible. We use the same computational parameters in all the calculations: an electronic temperature of 25 meV and a mesh cutoff of 400 Ry. Additionally, we used another density functional theory codes to validate our results (Amsterdam Density Functional adf). 38 Then, to deal with electron correlation beyond standard DFT codes to

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several levels, we include the Hubbard term (vasp) for cobalt d electrons 39–41 using values of Uef f = U − J = 2 and 3 eV, representing a range of reasonable values as found in the literature 42–46 for transition metal systems. We also study the van der Waals interactions for some particular cases by using the implementation of a vdW-DF 47 functional within the vasp method. 48,49 Finally, we use the elk 50 method, an all-electron full-potential linearised augmented-plane wave (FP-LAPW) method, to compute the hyperfine parameters for some particular configurations. Further computational details of elk calculations are given in the corresponding section.

Results and discussion Stability Co atom on circumcoronene We deposit a cobalt atom onto circumcoronene and relax the coordinates starting from several sites. We include top, bridge and hollow sites for the cobalt atom in the three main regions of circumcoronene: central, intermediate and edge. We describe in Fig. 1 the nomenclature to specify the different configurations computed for each system with Co atoms, Co dimers, and Co4 clusters. Taking into account that C54 H18 has D6h symmetry, the possible sites reduce substantially. The computed configurations are schematically shown in Fig. 2. The binding energies per atom are also included. They are calculated as Eb = (EC54 H18 + n ∗ ECo − EC54 H18 +Con )/n in eV, where n is the number of cobalt atoms n=1,2,4. Note that several of the input geometries suffer large geometry transformations: some cases relax and fall into other proposed geometries, and few relaxed to even new structures. It seems clear from the values of the bonding energy that the hollow sites are in general preferred over the top and bridge sites. For some configurations the cobalt atoms move from the top and bridge sites to the near hollow sites during relaxation, as in the case of CB, IB1, IB3 or

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IT2 among others. Among the hollow sites in the different zones, we find that the farther from the center of circumcoronene the more stable the hollow site becomes, as given by the sequence CH

Interaction of Cobalt Atoms, Dimers, and Co4 Clusters with

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