Article pubs.acs.org/JPCC
Tuning the Electronic and Magnetic Properties of NitrogenFunctionalized Few-Layered Graphene Nanoflakes Navneet Soin,*,†,‡ Sekhar C. Ray,*,‡,§ Sweety Sarma,§ Debarati Mazumder,§ Surbhi Sharma,∥ Yu-Fu Wang,⊥ Way-Faung Pong,⊥ Susanta Sinha Roy,# and André M. Strydom∇ †
Institute for Materials Research and Innovation (IMRI), School of Engineering, University of Bolton, Deane Road, Bolton BL3 5AB, United Kingdom § Department of Physics, College of Science, Engineering and Technology, University of South Africa, Private Bag X6, Florida, 1710, South Africa ∥ School of Biosciences, University of Birmigham, Edgbaston B15 2TT, United Kingdom ⊥ Department of Physics, Tamkang University, Tamsui 251, Taipei Taiwan # Department of Physics, School of Natural Sciences, Shiv Nadar University, Gautam Budh Nagar 201314, Uttar Pradesh India ∇ Highly Correlated Matter Research Group, Physics Department, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa ABSTRACT: In this article, we report the modification of the electronic and magnetic properties of few-layered graphene (FLG) nanoflakes by nitrogen functionalization carried out using radiofrequency plasma-enhanced chemical vapor deposition (rf-PECVD) and electron cyclotron resonance (ECR) plasma processes. Even though the rf-PECVD N2 treatment led to higher N-doping levels in the FLG (4.06 atomic %) as compared to the ECR process (2.18 atomic %), the ferromagnetic behavior of the ECR FLG (118.62 × 10−4 emu/g) was significantly higher than that of the rf-PECVD FLG (0.39 × 10−4 emu/ g) and pristine graphene (3.47 × 10−4 emu/g). Although both plasma processes introduce electron-donating N atoms into the graphene structure, distinct dominant nitrogen bonding configurations (pyridinic, pyrrolic) were observed for the two FLG types. Whereas the ECR plasma introduced more sp2-type nitrogen moieties, the rf-PECVD process led to the formation of sp3-coordinated nitrogen functionalities, as confirmed through Raman measurements. The samples were further characterized using X-ray absorption near-edge spectroscopy (XANES), and X-ray and ultraviolet photoelectron spectroscopies revealed an increased electronic density of states and a significantly higher concentration of pyrrolic groups in the rf-PECVD samples. Because of the formation of reactive edge structures and pyridinic nitrogen moieties, the ECRfunctionalized FLG samples exhibited highest saturation magnetization behavior with the lowest field hysteretic features. In comparison, the rf-PECVD samples displayed the lowest saturation magnetization owing to the disappearance of magnetic edge states and formation of stable nonradical-type defects in the pyrrole type structures. Our experimental results thus provide new evidence regarding the control of the magnetic and electronic properties of few-layered graphene nanoflakes through control of the plasma-processing route. leading to magnetic ordering in graphene.3−10 However, experimental validation can be carried out only for “bulk” graphene film samples, as the magnetic signal from monolayer graphene would be too weak for detection. Because of the delocalized π-bonding and the lack of any localized magnetic moments, pristine graphene is nonmagnetic and, as such, cannot be utilized for spintronics.14 Therefore, the introduction of a magnetic moment and the subsequent synthesis of ferromagnetic graphene or its derivatives with high magnet-
1. INTRODUCTION Intrinsic magnetism in materials without d or f electrons has attracted much interest, especially for carbon-based materials and, in particular, graphene. There has been a long-standing interest in the development of ferromagnetic graphene to realize its applications in spintronic devices through the combination of spin and charge.1−4 The introduction of a magnetic response in graphene through the introduction of edges, vacancy defects, or adsorbed atoms has been investigated using both theoretical and experimental means.1−13 In the literature, density-functional-theory- (DFT-) based simulations have predicted that defects, zigzag edges, and vacancies will lead to unpaired spins, thus inducing a magnetic moment and © 2017 American Chemical Society
Received: February 21, 2017 Revised: June 7, 2017 Published: June 7, 2017 14073
DOI: 10.1021/acs.jpcc.7b01645 J. Phys. Chem. C 2017, 121, 14073−14082
Article
The Journal of Physical Chemistry C
extinguished, and the heaters were switched off to allow the samples to return to ambient temperature under a steady N2 flow. The synthesis conditions used in the course of this work were similar to those employed in our previous works, where further details can be found.28,33 2.2. Nitrogen Plasma Doping of FLG Samples. Nitrogen doping/functionalization of FLG samples was carried out using two separate procedures: rf-PECVD and ECR. As compared to the rf plasmas, high uniformity, enhanced plasma densities (1011−1013 cm−3), and low ion energies (as low as 10−20 eV) are the main features of ECR plasmas.28,33 The ECR plasma itself was generated through the interactions between the magnetic field produced by the electromagnets and the electric field at microwave frequency. For a 2.45 GHz microwave frequency, a magnetic field of 875 G is required for ECR resonance conditions. The low working pressures required for stable ECR operation necessitated that the chamber be pumped to pressures of less than 9 × 10−2 Pa using a combination of vendor-provided turbomolecular and rotary pumps. For N-doping of the FLG samples, N2 was leaked into the chamber to achieve a working pressure of ∼0.025 Pa, after which a 150 W ECR plasma was ignited and maintained for 5 min; the resulting samples are denoted as FLG:N(ECR).28 Similarly, for an another set of samples, the postdeposition Ndoping process was carried out in an rf-PECVD chamber [resulting samples denoted as FLG:N(PECVD)] using N2 at a low pressure of ∼2 × 10−6 Torr and a power of 200 W.34 2.3. Characterization. Raman spectroscopic measurements were carried out using a 632.8-nm helium−neon laser (ISA LabRam) focused using a 50× lens to yield a spot size of 2−3 μm, thus providing a spectral resolution of
