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Spin-spin and spin-orbit interactions in nanographene fragments: A quantum chemistry approach
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0002-1763-9383
2012 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 10, 104702- p.Article in journal (Refereed) Published
Abstract [en]

The relativistic behavior of graphene structures, starting from the fundamental building blocks - the poly-aromatic hydrocarbons (PAHs) along with other PAH nanographenes - is studied to quantify any associated intrinsic magnetism in the triplet (T) state and subsequently in the ground singlet (S) state with account of possible S-T mixture induced by spin-orbit coupling (SOC). We employ a first principle quantum chemical-based approach and density functional theory (DFT) for a systematic treatment of the spin-Hamiltonian by considering both the spin-orbit and spin-spin interactions as dependent on different numbers of benzene rings. We assess these relativistic spin-coupling phenomena in terms of splitting parameters which cause magnetic anisotropy in absence of external perturbations. Possible routes for changes in the couplings in terms of doping and defects are also simulated and discussed. Accounting for the artificial character of the broken-symmetry solutions for strong spin polarization of the so-called "singlet open-shell" ground state in zigzag graphene nanoribbons predicted by spin-unrestricted DFT approaches, we interpolate results from more sophisticated methods for the S-T gaps and spin-orbit coupling (SOC) integrals and find that these spin interactions become weak as function of size and increasing decoupling of electrons at the edges. This leads to reduced electron spin-spin interaction and hence almost negligible intrinsic magnetism in the carbon-based PAHs and carbon nanographene fragments. Our results are in agreement with the fact that direct experimental evidence of edge magnetism in pristine graphene has been reported so far. We support the notion that magnetism in graphene only can be ascribed to structural defects or impurities.

Place, publisher, year, edition, pages
2012. Vol. 136, no 10, 104702- p.
National Category
Physical Sciences Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-93387DOI: 10.1063/1.3687002ISI: 000301664600033Scopus ID: 2-s2.0-84858969231OAI: oai:DiVA.org:kth-93387DiVA: diva2:516000
Funder
Swedish e‐Science Research Center
Note

QC 20120417

Available from: 2012-04-17 Created: 2012-04-16 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Spin-Orbit and Spin-Spin Coupling in the Triplet State
Open this publication in new window or tab >>Spin-Orbit and Spin-Spin Coupling in the Triplet State
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The underlying theory of “Spin” of an electron and its associated inter-actions causing internal fields and spectral shift to bulk-magnetism iswell established now. Our understanding of spin properties is significant andmore useful than ever before. In recent years there seems to be an enormousinterest towards application oriented materials that harness those spin prop-erties. Theoretical simulations remain in a position to “assist or pilot” theexperimental discovery of new materials.In this work, we have outlined available methodologies for spin coupling inmulti-reference and DFT techniques. We have benchmarked multi-referencespin-Hamiltonian computation in isoelectronic diradicals - Trimethylenemethane(TMM) and Oxyallyl. Also with DFT, parameters are predicted with anewly discovered TMM-like stable diradicals, reported to have large positiveexchange interactions. Excellent agreement were obtained and our findingsemphasize that the dipole-dipole interactions alone can predict the splittingof triplet states and that DFT spin procedures hold well in organic species.We have extended our spin-studies to a highly application oriented ma-terial - nanographene. Using our novel spin-parameter arguments we haveexplained the magnetism of graphene. Our studies highlight a few signifi-cant aspects - first there seems to be a size dependency with respect to thespin-Hamiltonian; second, there is a negligible contribution of spin-orbit cou-pling in these systems; third, we give a theoretical account of spin restrictedand unrestricted schemes for the DFT method and their consequences forthe spin and spatial symmetry of the molecules; and, finally, we highlightthe importance of impurities and defects for magnetism in graphene. Wepredict triplet-singlet transitions through linear response TDDFT for thetris(8-hydroxyquinoline) aluminium complex, an organic molecule shown tohave spintronics applications in recent experiments. Our spin studies werein line with those observations and could help to understand the role of thespin-coupling phenomena.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. 67 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2012:17
Keyword
Spin-spin, Spin-Orbit, D and E parameters, ZFS, graphene, TMM, OXA, diradicals, tris(8-hydroxyquinonline) aluminium, magnetic anisotropy, magnetism, triplet
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-95761 (URN)978-91-7501-366-4 (ISBN)
Public defence
2012-06-07, FB42, AlbaNova Universitetscentrum, Stockholm, 14:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note
QC 20120531Available from: 2012-05-31 Created: 2012-05-29 Last updated: 2012-05-31Bibliographically approved

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