Spin Doublet Point Defects in Graphenes: Predictions for ESR and NMR Spectral Parameters

Abstract

An adatom on a graphene surface may carry a magnetic moment causing spin-half paramagnetism. This theoretically predicted phenomenon has recently also been experimentally verified. The measurements of defect-induced magnetism are mainly based on magnetometric techniques where artifacts such as environmental magnetic impurities are hard to rule out. Spectroscopic methods such as electron spin resonance (ESR) and paramagnetic nuclear magnetic resonance (pNMR) are conventionally used in the development of magnetic materials, e.g., to study paramagnetic centers. The present density functional theory study demonstrates with calculations of the ESR g-tensor and the hyperfine coupling tensors, as well as the pNMR shielding tensor, that these spectroscopies can be used to identify the paramagnetic centers in graphenes. The studied defects are hydrogen and fluorine adatoms on sp(2)-hybridized graphene, as well as hydrogen and fluorine vacancies in the sp(3)-hybridized graphane and fluorographene, respectively. The directly measurable ESR and pNMR parameters give insight into the electronic and atomic structures of these defects and may contribute to understanding carbon-based magnetism via the characterization of the defect centers. We show that missing hydrogen and fluorine atoms in the functionalized graphane and fluorographene, respectively, constitute sp(2)-defect centers, in which the magnetic resonance parameters are greatly enhanced. Slowly decaying adatom-induced magnetic resonance parameters with the distance from the sp(3)-defect, are found in pure graphene.

Influence of Polymorphs and Local Defect Structures on NMR Parameters of Graphite Fluorides

“…Defects from more localized F vacancies were also explored including the two adjacent fluorine defects to produce an “ene” (ene­[2F]) or a four-fluorine vacancy to produce two adjacent “ene” defects (ene­[4F]). In this study no defects containing unpaired electrons (missing F or C) were considered, but paramagnetic point defects have previously been addressed by DFT methods . The calculated 19 F NMR spectra for the ene­[2F] and ene­[4F] defects are shown in Figure d,e.…”

Magnetic impurities in single-walled carbon nanotubes and graphene: a review

“…167 With this motivation, the characteristic spectroscopic parameters of the spin doublet point defects in graphene (ESR g-tensor and pNMR shielding tensor) were calculated by Vahakangas et al The authors considered the interactions of hydrogen and fluorine adatoms with graphene, graphane and fluorographene. 168…”

Nuclear Spin Induced Optical Rotation of Functional Groups in Hydrocarbons

“…10,11 The basis set co2 is a completenessoptimized basis set 40 developed and tested for NSOR. 16 Additional benchmark tests against larger completeness-optimized basis sets 16,41 have shown that it provides a sufficient quality with modest computational effort (see Fig. S1 in ESI †).…”

“…Recently, a branch of spectroscopy related to NMR, known as nuclear magneto-optic spectroscopy (NMOS), has been explored both experimentally [4][5][6][7][8][9][10][11] and theoretically. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] In contrast to classical NMR, which detects the signal through electromagnetic induction from the precessing nuclear magnetic moments, NMOS measures how these magnetic moments change the polarization state of the light passing through the sample. The interactions between the nuclei and the light, which are mediated by the electron cloud of the molecule, have shown promise for opening new types of spectroscopic measurements, such as observation of dipolar couplings in isotropic liquids 26 or mapping the properties of excited states.…”

“…The interactions between the nuclei and the light, which are mediated by the electron cloud of the molecule, have shown promise for opening new types of spectroscopic measurements, such as observation of dipolar couplings in isotropic liquids 26 or mapping the properties of excited states. 23 So far, five NMOS effects have been described: nuclear spininduced optical rotation (NSOR), [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] nuclear spin-induced circular dichroism (NSCD), [19][20][21][22][23] nuclear spin-induced Cotton-Mouton effect (NSCM), 24,25 nuclear spin-induced Cotton-Mouton effect in an external magnetic field (NSCM-B), 26,27 and nuclear quadrupole-induced Cotton-Mouton effect (NQCM). 28,29 Out of these, only NSOR, a circular birefringence caused by nuclear magnetic moments oriented along the light beam, has been experimentally measured so far.…”