Magnetic properties of sulfur-doped graphene
Introduction
Carbon nanomaterials are regarded as one of the best-suited platforms for spintronics due to their low density, inherently low spin-orbit coupling, and large spin-flip scattering lengths [1], [2]. Ideally, any sp2 carbon system is expected to exhibit diamagnetic behavior due to the existence of π-electron orbital magnetism [3]. However, the origin of anomalous ferromagnetic ordering and paramagnetic response in sp2 carbon systems has puzzled researchers for many decades [4], [5], [6]. This unexpected presence of magnetic ordering in nanocarbons is a major impediment for realizing long spin-flip scattering lengths required for spintronic applications. Although there have been many efforts to understand the presence of magnetism in pure carbon-based nanomaterials [7], [8], [9], [10], [11], many of them have been either controversial or irreproducible. This is likely due to the presence of unintentional magnetic impurities (e.g., residual Fe catalyst particles in carbon nanotubes), poorly characterized defects, and intrinsic topology (e.g., curvature in C60 and nanotubes). Graphene, a two-dimensional atom-thick layer of sp2 carbon, is well suited for elucidating the origin of magnetism due to its fairly simple honeycomb lattice with unique electronic and optical properties [12], [13]. Furthermore, the properties of many carbon nanomaterials (e.g., fullerenes, carbon nanotubes, graphite, and some polycyclic aromatic molecules) are often theoretically derived from their underlying graphene lattice.
Many theoretical studies have predicted that point defects in graphene exhibit a non-zero magnetic moment, which can possibly interact with each other resulting in a long-range ferromagnetic ordering [7], [8], [14], [15], [16], [17], [18]. Nair et al. reported a purely paramagnetic behavior in highly defective fluorinated and ion-irradiated graphene, implying the absence of any defect-defect interactions leading to ferromagnetic (FM) ordering [19], [20]. On the contrary, others have observed signatures of FM in defected graphene indicating possible interactions between defect-induced magnetic moments [21], [22], [23]. Collectively, both intrinsic defects (e.g., vacancies and edges) and extrinsic dopants (e.g., fluorine dopants, ion-irradiation induced pores, and unintentional magnetic impurities) have been proposed to increase paramagnetic response of graphene, and in some cases even cause FM ordering through defect-defect interactions. As we and others have previously shown, the nature of defects plays a critical role in an unexpected magnetic ordering in many nanostructured materials (particularly, nanograined oxides) derived from non-magnetic bulk, for example ZnO [24], [25], [26], [27]. Accordingly in this study, we controllably doped graphene nanoplatelets (GnPs) with sulfur (an extrinsic defect) to tune different magnetic interactions between intrinsic (e.g., between vacancies) and extrinsic defects (e.g., vacancy and S-dopant). Our X-ray photoelectron spectroscopy (XPS) studies and density functional theory (DFT) clearly evince the formation of covalent bonds between S dopants and intrinsic defects. While we observed that pristine GnPs prepared using the chemical exfoliation method exhibited a weak FM ordering due to the presence of intrinsic defects, we found that the FM ordering systematically decreased with increasing S dopants suggesting that the interactions between S-dopants and intrinsic defects demagnetize GnPs.
Section snippets
Experimental procedure
Grade M GnPs (xGnP-M-5, 99.95 at% carbon and 0.05 at% sulfur) were purchased from XG Sciences, Inc. (Michigan, USA). Pristine GnPs consist of short stacks of graphene sheets with an average thickness of approximately 6–8 nm and average size of 5 µm (see Fig. S1). In the chemical exfoliation process, as-received GnPs (5 g) were exfoliated in 100 ml of N-Methylpyrrolidone (NMP) for 2 h using a 1/8″ tip sonicator at 120 W, and then vacuum filtered using a 0.45 µm nylon membrane. Subsequently, the collected
Results and discussion
As shown in Fig. 1, finite areas of hysteresis loops provide a clear evidence for FM in both pristine and doped GnP samples at 300 and 5 K, with saturation values (Ms) ~0.06 (pristine), 0.017 (1.5 wt% S GnP), and 0.043 emu/g (3 wt% S GnP). The FM is embedded in a large diamagnetic (DM) background (see insets in Fig. 1a and b), which arises from the underlying graphene lattice. Although graphite/graphene is diamagnetic, the presence of defects (as it will be discussed later) induce weak FM, similar
Conclusions
In summary, our experiments showed that the magnetism in graphene is sensitive to the nature of the defects. While pristine graphene with naturally occurring edges and vacancies (i.e., intrinsic defects) exhibits a non-zero magnetic moment, the addition of S-dopants was found to quench this magnetic ordering. In fact, we found that sulfur doping drastically changes the magnetic behavior of the as-prepared samples. The zero-field-cooling (ZFC) and field-cooling (FC) in M vs. T measurements
Author contribution
J.Z., R.P. and A.M.R. designed the experiments. J.Z., and L.O. synthesized pristine and doped samples. J.Z., J.H., A.H. and A.Z. have done the magnetic characterization. P.A. performed XPS measurements. H.P., A.W. and J.W. performed the DFT calculations. J.Z., R.P., J.H. and A.M.R. analyzed the data. J.Z., R.P., A.M.R., H.P. and A.W. drafted the results.
Acknowledgments
The computational work was supported by DOE-BES-DMS (DEFG02-99ER45795). We used computational resources of the NERSC, supported by the U.S. DOE (DE-AC02- 05CH11231), and the Ohio Supercomputing Center. J. H would like to acknowledge the support of NSF DMR 1307740. A. M. R and R. P acknowledge the support from US National Science Foundation grant CMMI-1246800 award. The authors acknowledge Drs. E. M. Wylie and Brian Powell for their help with ICP-MS.
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