Abstract
A comparative study has been made for the non-catalyst based few layer graphene (FLG) and Fe-catalyst based multiwall carbon nanotubes (MWCNTs). Magnetic and electronic properties of FLG and MWCNTs were studied using magnetic M-H hysteresis loops and synchrotron radiation based X-ray absorption fine structure spectroscopy measurements. Structural defects and electronic and bonding properties of FLG/MWCNTs have been studied using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). The work functions of FLG and MWCNTs are 4.01 eV and 3.79 eV, respectively, obtained from UPS (He-I) spectra. UPS (He-II) results suggest that the density of states (DOS) of MWCNTs is higher than FLG and is consistent with Raman spectroscopy result that shows the defect of MWCNTs is higher than FLG. The magnetic coercivity () of the MWCNTs (~750 Oe) is higher than FLG (~85 Oe) which could be used for various technological magnetic applications.
1. Introduction
Graphene, a one-atom thick sheet of carbon atoms arranged in a hexagonal lattice, has many desirable properties. It has become vey useful for the electronic and magnetic-storage devices [1–7]. Graphene materials have long spin relaxation time and length due to the small spin-orbit coupling, so graphene is most likely used for the spintronics applications. Spintronics is a field of research, where both the fundamental research and innovative device applications coexist with each other. Spintronics devices are pervasive, and most notably they can be found in computers’ hard disks. They function due to coupling of magnetism and electric current [6, 7]. Similar to the modulation of physical and chemical properties of carbon nanotubes, graphene has been proposed for versatile applications [8–10]. In the other way, the ferromagnetic materials such as “Fe-”catalyst based carbon nanotubes (CNTs) have potential application in various areas such as “magnetic-storage media” that include the traditional tapes and videocassettes, hard disks for mainframe computers, floppy disks for personal computers (PC), and portable ZIP and MO disks (magnetooptic) with high storage capacity [11, 12]. So it is important to study the magnetic properties of carbon nanostructured materials like graphene/few layer graphene (FLG) and carbon nanotubes (CNTs)/multiwall-CNTs for these applications.
In this work, we have studied the few layer graphene (FLG) and multiwall carbon nanotubes (MWCNTs) to elucidate their electronic structure, bonding properties, and magnetic behaviours and compared them with a view on the magnetic device application like low-dimensional high-density magnetic recording media.
2. Experimental Details
Microwave plasma enhanced chemical vapor deposition (PECVD) process was used to grow the non-catalyst based vertically aligned few layers graphene (FLG) thin films on bare Si (100) substrates using CH4 as carbon precursor, whereas the MWCNTs were grown by catalytical chemical vapor deposition (CVD) using camphor (as carbon source) and the ferrocene (Fe-catalyst) in the ratio of 20 : 1. Details of growing process of FLG and MWCNTs could be available elsewhere [13, 14]. The surface morphology was measured using scanning electron microscopy. The electronic properties were measured using X-ray absorption near edge structure spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). The microstructure and defects are studied using Raman spectroscopy and electron field emission, whereas the field-dependent magnetization of FLG and MWCNTs was studied using a SQUID-type magnetometer. The core-level C 1s X-ray photoelectron microscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements of the FLG and MWCNT were performed on the monochromatic Al = 1486.6 eV excitation energy and a He lamp (He-II) with 40.8 eV excitation energy, respectively, using the KRATOS-AXIS SUPRA X-ray photoelectron spectroscopy (XPS) equipment at UNISA (Florida Science Campus), South Africa. Work functions were determined from the UPS He-I spectra using excitation energy 21.22 eV. Raman spectra measurements were carried out using 647 nm excitation wavelengths with the LASER beam spot size ~1 μm and the incident power was ~1 mW.
3. Results and Discussion
The scanning electron microscopy (SEM) images of FLG and MWCNTs are shown in Figures 1(a) and 1(b). It is evident that the synthesized graphene is vertically aligned to the underlying substrate and is randomly intercalated forming a porous mesh-like network. In case of MWCNTs, most of the nanotubes are not completely vertically aligned and some of them are entangled. Figure 1(b) reveals that the Fe nanoparticles are embedded in the end of the MWCNTs, offering excellent oxidation protection, which can be effectively exploited in magnetic nanoprobes and spintronics application. Figures 1(c) and 1(d) display the electron field emission (EFE) measurements of emitted current density () and applied electric field () and inset shows the corresponding Fowler-Nordheim (F-N) plots of FLG and MWCNTs, respectively. The figure shows that there exists a threshold of electric field, beyond which increases roughly exponentially. Fowler-Nordheim (F-N) plots (see inset figure) clearly illustrate the threshold electric field or turn-on electric field (). The values of were obtained with linear curve fitting in the high electric field region and were found to be 26.5 V/μm for FLG and 4.7 V/μm for MWCNTs indicating that MWCNTs have better field emission properties than FLG. It is noted that is defined as the field at which exceeds 0.1 μA/cm2 and is deduced from a linear fitting of the F-N plots in the high applied electric field. The notable feature of MWCNTs shows that the current density () is 3.82 mA/cm2, when the electric field is 10 V/μm, whereas no current is observed in case of FLG at electric field 10 V/μm. X-ray photoemission images of FLG and MWCNT are shown in Figures 1(e) and 1(f), respectively, where red particles are carbon atoms and blue particles are oxygen atoms which are obtained at the kinetic energy of ~1202.00 eV (C 1s) and 954.14 eV (O 1s), respectively. These images show the uniform distribution of carbon and oxygen atoms on the FLG and MWCNTs surface. The images further imply that the oxygen atoms are less in MWCNTs compared to FLG.
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Figure 2(a) shows the Raman spectra of pristine FLG graphene and MWCNTs. The Raman spectra of MWCNTs, the most prominent bands of interest, are (i) the D band at ~1360 cm−1, which is disorder-activated with symmetry, (ii) the G band at ~1580 cm−1, assigned to the Raman-allowed phonon mode with symmetry, and (iii) the first overtone of the D band, that is, 2D band at ~2700 cm−1 [15]. In case of the FLG, the characteristic peaks are G band at ~1583 cm−1 and 2D band at ~2664 cm−1 [15]. The other defects induced peaks at ~1335 and 1617 cm−1 are also observed, which have been assigned to D band and D′ band activated by defects via double-resonance process [15]. The D peak is defect activated via an intervalley double-resonance process and its intensity provides a convenient measure for the amount of disorder [16–20]. The peaks at ~2460 cm−1 arise in both FLG and MWCNTs via a combination of (D + D′) bands and are defect activated [21–23]. Commonly, the intensity ratio of D band over G band / can serve as a convenient measurement of the amount of defects in graphitic materials [17]. The / ratio of MWCNTs and FLG are 0.70 and 0.44, respectively, indicating MWCNTs have higher defects than FLG.
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Figure 2(b) displays the C K-edge XANES spectra of FLG and MWCNTs and the reference graphite. The C K-edge XANES spectra of reference graphite are vertically shifted for clarity. For graphitic materials in general, X-ray absorption near edge structure (XANES) spectra can be divided into three regions characterized by specific resonance energies [21]. The first region of resonance appears around eV and the C- resonance appears around eV, and a broad region between 290 eV and 315 eV corresponds to resonance. The presence of the and C- resonances serves as a fingerprint for the existence of sp2 hybridized C-C bonds and C-H bonds, respectively. These spectra reflect a transition from the C 1s core state to the p-like final states above the Fermi level. The features associated with sp2 C of the graphite; MWCNTs are at ~285.5, 286.3, and 286.1 eV, respectively, whereas the feature observed at ~293 eV is assigned to the sp3 C states [21, 22]. The C K-edge XANES spectra of the FLG show features at ~286.1 eV, ~292.6 eV, and ~291.6 eV which can be attributed to the unoccupied , , and excitonic states transitions, respectively [22, 23]. The peak at ~288.1 is defined as C-H and the peak at ~288.9 eV is the interlayer graphite structure and these two peaks are not prominent in the MWCNTs.
Figure 3 shows the C 1s core-level spectra of pristine FLG and MWCNTs. The C 1s spectra of FLG and MWCNTs are fitted with three Gaussian components. In case of FLG, the peak at ~284.4 eV assigned to sp2 hybridized C atoms in graphene, the peak at ~285.2 eV assigned to sp3 that hybridized C atoms arises from C-H and C-C bonds, and the peak at ~286.3 eV was assigned as C-O bonds. All these peaks are shifted ~0.19 eV for MWCNTs. The work functions determined from He-I spectra (~40.8 eV) as shown in Figure 4. The work functions are estimated from the linear fitting of the edge of spectra as shown in inset Figure 4. The estimated work functions of FLG and MWCNTs are 4.01 eV and 3.79 eV, respectively, which are comparable with the values reported by Ago et al. [24]. As shown in Figure 5, the UPS spectra corresponding to valence-band density of states (DOS) of FLG and MWCNTs obtained at He-II radiation (~40.8 eV). These spectra exhibit significant changes in electron states below the Fermi level after being normalized to possess the same integrated intensity with respect to 0–19 eV binding energies. The spectrum of pristine FLG shows five band features, which are assigned to (i) C 2p π at ~4.8 eV, (ii) crossing of C 2p π and C 2p σ bands around 6.7 eV, (iii) C 2p σ at 9.2 eV, (iv) C 2s-2p hybridized state at 11.9 eV, and (v) C 2s σ band at ~13.5 eV [25]. In case of MWCNTs, all these peaks are shifted ~0.15 eV as indicated that the Fermi edge is shifted ~0.15 eV as shown in inset Figure 3 that further implies reducing the work function. It is also observed that the relative intensity of the pπ- and pσ-derived DOS of MWCNTs is higher than FLG. This could be accounted for by the presence of defects sites, as identified in the Raman spectra.
PPMS were used at room temperature (300 K), to investigate the hysteresis loop of the magnetic behaviour. Figures 6(a) and 6(b) plot the resulting magnetic moment (M) and μ0H of as-grown MWCNTs and FLG, respectively. The magnetic moment of the FLG is nearly 104 times lower than those of MWCNTs. However, we strongly believe the higher ferromagnetic behaviour in MWCNTs is attributed due to Fe nanoparticles. The coercivity () of the MWCNTs is ~750 Oe that is counterpart of bulk Fe (0.9 Oe) [26] and Ni and Co nanowire arrays [23, 27] and it is comparable with values obtained elsewhere [28, 29]. On the other hand, the coercivity () of the FLG is ~85 Oe and is nine times lower than MWCNT. The difference of these magnetic properties is due to the differences of defects, DOS, and work functions. These findings are encouraging for various technological applications and suggest higher magnetic stability of these graphene-based FLG and MWCNTs materials.
(a) MWCNT
(b) Graphene
4. Summary and Conclusion
In summary, we have studied the electronic and magnetic properties of MWCNTs and FLG. We found that the defects and DOS are higher in MWCNTs than FLG in this study. The work functions of FLG and MWCNTs are 4.01 eV and 3.79 eV, respectively. The room temperature magnetization of MWCNTs is found to be ~104 times higher than FLG. The coercivity (Hc) of the MWCNTs is also higher than FLG. These ferromagnetic properties could open up the possibility of producing highly tunable graphene-based applications devices, including spintronic nanodevices, magnetoresistance, and magnetic memory devices. The results of MWCNTs also demonstrate the potential applications in magnetic recording media of the isolated and well-distributed magnetic particles in the magnetic metal-encapsulated carbon nanostructures.
Competing Interests
The authors declare no competing financial interests and no conflict of interests regarding the publication of this paper.
Acknowledgments
The author Sekhar Chandra Ray acknowledges National Research Foundation, South Africa, for financial support.