Preferential antiferromagnetic coupling of vacancies in graphene onSiO2: Electron spin resonance and scanning tunneling spectroscopy

Abstract

Monolayer graphene grown by chemical vapor deposition and transferred to SiO2 is used to introduce vacancies by Ar + ion bombardment at a kinetic energy of 50 eV. The density of defects visible in scanning tunneling microscopy (STM) is considerably lower than the ion fluence implying that most of the defects are single vacancies as expected from the low ion energy. The vacancies are characterized by scanning tunneling spectroscopy (STS) on graphene and HOPG. A peak close to the Dirac point is found within the local density of states of the vacancies similar the the peak found previously for vacancies on HOPG. The peak persists after air exposure up to 180 min, such that electron spin resonance (ESR) at 9.6 GHz can probe the vacancies exhibiting such a peak. After an ion flux of 10/nm 2 , we find an ESR signal corresponding to a g-factor of 2.001-2.003 and a spin density of 1-2 spins/nm 2 . The peak width is as small as 0.17 mT indicating exchange narrowing. Consistently, the temperature dependent measurements reveal antiferromagnetic correlations with a Curie-Weiss temperature of -10 K. Thus, the vacancies preferentially couple antiferromagnetically ruling out a ferromagnetic graphene monolayer at ion induced spin densities of 1 − 2/nm 2 .

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

“…The results (Figure 3) for the isotropic g-factor of the H@G and F@G systems, albeit not entirely converged due to the above reasons, are in good agreement with the experimental magnitude in the range g = 2.001 − 2.003 obtained for carbon vacancy, 21 another type of point defect in graphene. In the present study, relative anisotropies of the g-tensor of around 0.05% (Fig.…”

“…This enhancement arises from the interaction of the nuclear spins with the large magnetic moment of 21 the unpaired electron(s). 25 In pNMR, an ensemble of 2S+1 thermally populated states need to be considered.…”

“…12 One of the most powerful spectroscopy techniques for obtaining such local information of paramagnetic defects in solids is electron spin resonance (ESR), 19,20 which has not been yet fully exploited in graphene research. 21 The characterization capability of the method has been shown, e.g., in ref., 22 where low-temperature superparamagnetic-like states were observed via ESR measurements of graphene flakes containing extended defect structures, originating from heavily sonicated nanographite samples. In ref., 22 density-functional theory (DFT) modeling of the defects was also carried out, with the finding that hydrogen adatoms on graphene are associated with a significant magnetic moment.…”

Probing Electron Spin Resonance in Monolayer Graphene

“…The magnitude of magnetic moments due to vacancy defects varies depending on their type and other properties of the system [39][40][41][42][43]. Irradiation of graphene with Ar + ions [44] can result in vacancy defects with g = 2.001 − 2.003 while N + ion bombardment [45] can implant ions that produce a g-factor of 2. However, we observe no change in g-factor at all as a function of charge carrier density, as could be expected from charge carrier density dependent Kondo screening.…”

Spin relaxation in disordered graphene: Interplay between puddles and defect-induced magnetism

“…We choose to use a smaller concentration because we expect the defects concentrations to be considerably lower in samples where defects are not deliberately created. We further justify our choice by recalling that there is a degree of uncertainty in the estimation of the defects concentrations 63,64 , and that in real experiments the defects tend to distribute over the two sublattices and to form other structures, such as divacancies, that induce defect states around ε = 0 (see, for instance, Ref. 32).…”