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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

Screening charge for (a) pyridine, (b) 4-picoline radical and (c) pyridine radical. The isovalue is 0.0034 e/Å3 and regions of electron accumulation/depletion are depicted in red and blue regions, respectively.
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f5: Screening charge for (a) pyridine, (b) 4-picoline radical and (c) pyridine radical. The isovalue is 0.0034 e/Å3 and regions of electron accumulation/depletion are depicted in red and blue regions, respectively.

Mentions: A further distinction between the different adsorption systems can be inferred by calculating the screening charge which is defined as (here ρFCH and ρGS are electron density of the system in the core ionized and the ground state, respectively), whose integral amounts to the additional valence electron. Let us consider first the results for physisorbed pyridine shown in Fig. 5(a). Here, the screening is mostly concentrated in the molecular region where a polarization of the molecular orbitals towards the core hole as well as a filling of the LUMO, can be appreciated. By partitioning Δρ in terms of projection onto atomic orbitals according to Löwdin population analysis6566, we found that 70% of the added charge can be attributed to the atoms of the molecule and the remaining 30% to graphene, in agreement with the occupation of the states seen in Fig. 3(a). In the case of the chemisorbed molecules, we have a more pronounced participation by the graphene substrate to the screening: one switches from the spin-polarized mid gap state occupied by one electron, to the spin-degenerate hybrid one. MIDGAP-l depicted in Fig. 4(a,c), is now doubly occupied. The participation of graphene to the screening charge is moderately larger for pyridine radical than for 4-picoline radical. This can be seen visually by comparing Δρ and noticing that larger charge lobes on graphene are depicted in Fig. 5(c) than in Fig. 5(b). Correspondingly, the projection of Δρ on the atoms of the molecule amounts to 63% for the 4-picoline radical and 59% for the pyridine radical cases.


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

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

Screening charge for (a) pyridine, (b) 4-picoline radical and (c) pyridine radical. The isovalue is 0.0034 e/Å3 and regions of electron accumulation/depletion are depicted in red and blue regions, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: Screening charge for (a) pyridine, (b) 4-picoline radical and (c) pyridine radical. The isovalue is 0.0034 e/Å3 and regions of electron accumulation/depletion are depicted in red and blue regions, respectively.
Mentions: A further distinction between the different adsorption systems can be inferred by calculating the screening charge which is defined as (here ρFCH and ρGS are electron density of the system in the core ionized and the ground state, respectively), whose integral amounts to the additional valence electron. Let us consider first the results for physisorbed pyridine shown in Fig. 5(a). Here, the screening is mostly concentrated in the molecular region where a polarization of the molecular orbitals towards the core hole as well as a filling of the LUMO, can be appreciated. By partitioning Δρ in terms of projection onto atomic orbitals according to Löwdin population analysis6566, we found that 70% of the added charge can be attributed to the atoms of the molecule and the remaining 30% to graphene, in agreement with the occupation of the states seen in Fig. 3(a). In the case of the chemisorbed molecules, we have a more pronounced participation by the graphene substrate to the screening: one switches from the spin-polarized mid gap state occupied by one electron, to the spin-degenerate hybrid one. MIDGAP-l depicted in Fig. 4(a,c), is now doubly occupied. The participation of graphene to the screening charge is moderately larger for pyridine radical than for 4-picoline radical. This can be seen visually by comparing Δρ and noticing that larger charge lobes on graphene are depicted in Fig. 5(c) than in Fig. 5(b). Correspondingly, the projection of Δρ on the atoms of the molecule amounts to 63% for the 4-picoline radical and 59% for the pyridine radical cases.

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