Limits...
Femtomagnetism in graphene induced by core level excitation of organic adsorbates.

Ravikumar A, Baby A, Lin H, Brivio GP, Fratesi G - Sci Rep (2016)

Bottom Line: The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale.On the other hand, when graphene is covalently functionalized, the system is magnetic in the ground state showing two spin dependent mid gap states localized around the adsorption site.At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via Cozzi 55 - 20125 Milano, Italia.

ABSTRACT
We predict the induction or suppression of magnetism in the valence shell of physisorbed and chemisorbed organic molecules on graphene occurring on the femtosecond time scale as a result of core level excitations. For physisorbed molecules, where the interaction with graphene is dominated by van der Waals forces and the system is non-magnetic in the ground state, numerical simulations based on density functional theory show that the valence electrons relax towards a spin polarized configuration upon excitation of a core-level electron. The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale. On the other hand, when graphene is covalently functionalized, the system is magnetic in the ground state showing two spin dependent mid gap states localized around the adsorption site. At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore.

No MeSH data available.


Related in: MedlinePlus

The minimum energy configurations of organic molecules from the top and perspective view for (a,b) pyridine (c,d) 4-picoline radical and (e,f) pyridine radical adsorbed on graphene. The yellow and red spheres represent C atoms of graphene and of the molecule, respectively. The blue spheres stand for N and smaller black ones for H atoms. The perspective view shown in (f) highlights the localized distortion in the graphene lattice when it covalently bonds with pyridine radical. There, acc represents the molecule-graphene bond length and aZ is the displacement of the carbon atom of graphene covalently bonded to the radical, as measured with respect to its nearest neighbours.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4835731&req=5

f1: The minimum energy configurations of organic molecules from the top and perspective view for (a,b) pyridine (c,d) 4-picoline radical and (e,f) pyridine radical adsorbed on graphene. The yellow and red spheres represent C atoms of graphene and of the molecule, respectively. The blue spheres stand for N and smaller black ones for H atoms. The perspective view shown in (f) highlights the localized distortion in the graphene lattice when it covalently bonds with pyridine radical. There, acc represents the molecule-graphene bond length and aZ is the displacement of the carbon atom of graphene covalently bonded to the radical, as measured with respect to its nearest neighbours.

Mentions: We start by investigating the most stable configurations of pyridine, 4-picoline radical and pyridine radical adsorbed on graphene taking into account the translational as well as rotational degrees of freedom of the molecule. To check the stability of the system and quantitatively understand the energies involved during the adsorption process, we define the adsorption energy as . Here is the total energy of the optimized molecule-graphene system, Eg and Em are the total energies of the isolated graphene substrate and of the gas phase molecule/radical, respectively. The adsorption energies are summarized in Table 1, along with most relevant structural parameters. We find that pyridine placed parallel to graphene, with its N atom at the center of a graphene ring, is the most stable configuration at low molecular concentration (Fig. 1(a,b)) and its interaction with the substrate is dominated by van der Waals forces. These results are supported by various experimental and theoretical studies of closed shell organic molecules adsorbed on graphene585960. Our most stable configuration displays the pyridine ring symmetrically located with its nitrogen atom at the center of the graphene ring and oriented similar to AB stacking found in graphite. Another arrangement on graphene in which the nitrogen atom is above one of the carbon atoms of graphene has similar adsorption energy, just 6 meV less stable. A small energy difference between these configurations60 points to high molecular diffusivity. If we neglect the van der Waals interactions, the adsorption strength is underestimated by one order of magnitude while the adsorption bond length is seriously overestimated. We next consider the 4-picoline (4-Methlypyridine) radical (where we have removed one of the hydrogen atoms of the methyl group) which forms a covalent bond with the free pz orbitals of graphene as shown in Fig. 1(c,d). For this system, van der Waals forces also play an important role to access the minimum energy configuration since the pyridine π cloud remains almost parallel to graphene, at an angle of ~14° and at an average distance of 3.16 Å with respect to the substrate plane. Finally the pyridine radical, when adsorbed on graphene, forms a mostly covalent bond and orients itself perpendicular to the graphene plane as seen in Fig. 1(e). Figure 1(f) shows the perspective view of this configuration. A localized deformation of the graphene lattice due to covalent interaction with the molecule lifts the carbon atom of graphene61 by a quantity aZ, defined as the height difference of the bonding carbon atom of graphene with respect to its nearest neighbours. This is visualized clearly in the Fig. 1(f) along with the graphene-molecule bond length (acc). The results for the adsorption energies are better understood by taking pyridine radical as a reference. Picoline radical, in comparison, shows a lower value of acc and aZ, consistently with its lower reactivity. The more negative computed value of Eads is due to a stronger vdW interaction of its almost planar π cloud. In comparing pyridine molecule vs radical, we remark that in the case of covalently bonded systems on graphene, the deformation of the substrate requires a large energy cost that is lowering significantly the adsorption energy: almost 1 eV in similar systems62. Our adsorption energy value for pyridine radical is comparable to that of other radicals published therein. On the contrary, vdW-bonded pyridine does not suffer this deformation cost and is stabilized by a larger dispersion interaction, hence the overall larger value of Eads. Our effort will be to understand the effect of chemisorption and physisorption on the system magnetism in the ground and core excited states.


Femtomagnetism in graphene induced by core level excitation of organic adsorbates.

Ravikumar A, Baby A, Lin H, Brivio GP, Fratesi G - Sci Rep (2016)

The minimum energy configurations of organic molecules from the top and perspective view for (a,b) pyridine (c,d) 4-picoline radical and (e,f) pyridine radical adsorbed on graphene. The yellow and red spheres represent C atoms of graphene and of the molecule, respectively. The blue spheres stand for N and smaller black ones for H atoms. The perspective view shown in (f) highlights the localized distortion in the graphene lattice when it covalently bonds with pyridine radical. There, acc represents the molecule-graphene bond length and aZ is the displacement of the carbon atom of graphene covalently bonded to the radical, as measured with respect to its nearest neighbours.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4835731&req=5

f1: The minimum energy configurations of organic molecules from the top and perspective view for (a,b) pyridine (c,d) 4-picoline radical and (e,f) pyridine radical adsorbed on graphene. The yellow and red spheres represent C atoms of graphene and of the molecule, respectively. The blue spheres stand for N and smaller black ones for H atoms. The perspective view shown in (f) highlights the localized distortion in the graphene lattice when it covalently bonds with pyridine radical. There, acc represents the molecule-graphene bond length and aZ is the displacement of the carbon atom of graphene covalently bonded to the radical, as measured with respect to its nearest neighbours.
Mentions: We start by investigating the most stable configurations of pyridine, 4-picoline radical and pyridine radical adsorbed on graphene taking into account the translational as well as rotational degrees of freedom of the molecule. To check the stability of the system and quantitatively understand the energies involved during the adsorption process, we define the adsorption energy as . Here is the total energy of the optimized molecule-graphene system, Eg and Em are the total energies of the isolated graphene substrate and of the gas phase molecule/radical, respectively. The adsorption energies are summarized in Table 1, along with most relevant structural parameters. We find that pyridine placed parallel to graphene, with its N atom at the center of a graphene ring, is the most stable configuration at low molecular concentration (Fig. 1(a,b)) and its interaction with the substrate is dominated by van der Waals forces. These results are supported by various experimental and theoretical studies of closed shell organic molecules adsorbed on graphene585960. Our most stable configuration displays the pyridine ring symmetrically located with its nitrogen atom at the center of the graphene ring and oriented similar to AB stacking found in graphite. Another arrangement on graphene in which the nitrogen atom is above one of the carbon atoms of graphene has similar adsorption energy, just 6 meV less stable. A small energy difference between these configurations60 points to high molecular diffusivity. If we neglect the van der Waals interactions, the adsorption strength is underestimated by one order of magnitude while the adsorption bond length is seriously overestimated. We next consider the 4-picoline (4-Methlypyridine) radical (where we have removed one of the hydrogen atoms of the methyl group) which forms a covalent bond with the free pz orbitals of graphene as shown in Fig. 1(c,d). For this system, van der Waals forces also play an important role to access the minimum energy configuration since the pyridine π cloud remains almost parallel to graphene, at an angle of ~14° and at an average distance of 3.16 Å with respect to the substrate plane. Finally the pyridine radical, when adsorbed on graphene, forms a mostly covalent bond and orients itself perpendicular to the graphene plane as seen in Fig. 1(e). Figure 1(f) shows the perspective view of this configuration. A localized deformation of the graphene lattice due to covalent interaction with the molecule lifts the carbon atom of graphene61 by a quantity aZ, defined as the height difference of the bonding carbon atom of graphene with respect to its nearest neighbours. This is visualized clearly in the Fig. 1(f) along with the graphene-molecule bond length (acc). The results for the adsorption energies are better understood by taking pyridine radical as a reference. Picoline radical, in comparison, shows a lower value of acc and aZ, consistently with its lower reactivity. The more negative computed value of Eads is due to a stronger vdW interaction of its almost planar π cloud. In comparing pyridine molecule vs radical, we remark that in the case of covalently bonded systems on graphene, the deformation of the substrate requires a large energy cost that is lowering significantly the adsorption energy: almost 1 eV in similar systems62. Our adsorption energy value for pyridine radical is comparable to that of other radicals published therein. On the contrary, vdW-bonded pyridine does not suffer this deformation cost and is stabilized by a larger dispersion interaction, hence the overall larger value of Eads. Our effort will be to understand the effect of chemisorption and physisorption on the system magnetism in the ground and core excited states.

Bottom Line: The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale.On the other hand, when graphene is covalently functionalized, the system is magnetic in the ground state showing two spin dependent mid gap states localized around the adsorption site.At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via Cozzi 55 - 20125 Milano, Italia.

ABSTRACT
We predict the induction or suppression of magnetism in the valence shell of physisorbed and chemisorbed organic molecules on graphene occurring on the femtosecond time scale as a result of core level excitations. For physisorbed molecules, where the interaction with graphene is dominated by van der Waals forces and the system is non-magnetic in the ground state, numerical simulations based on density functional theory show that the valence electrons relax towards a spin polarized configuration upon excitation of a core-level electron. The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale. On the other hand, when graphene is covalently functionalized, the system is magnetic in the ground state showing two spin dependent mid gap states localized around the adsorption site. At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is not magnetic anymore.

No MeSH data available.


Related in: MedlinePlus