Graphene, an interesting 2D system has a rare electronic structure of two inverted Dirac cones touching at a single point, with great electron mobility and promising microelectronics applications. In the present article, a theoretical investigation has been performed on the structural, electronic, and magnetic properties of pristine graphene nanosheet and also the effect of 3d transition metal (TM) co-doped in graphene nanosheet within the density functional theory framework. 3d TM is categorized into two groups: Cr- group (Cr-Cr, Cr-Mn, and Cr-Fe) and Ni-group (Ni-Cr, Ni-Ti, Ni-Mn). After co-doping TM atoms on graphene, it still holds its planar shape which refers to the stability of these co-doped graphene nanosheets. This is also confirmed by the increasing bond length of carbon and TM atoms on graphene nanosheets. Highest zero-point energies have been found of -12049.24eV and -10936.87eV respectively for Cr-Cr and Ni-Cr co-doped graphene nanosheet. According to Mullikens charge and electron density differences, all the TM atoms can act as electron donors while the graphene nanosheet is electron acceptor. All the TMs co-doped graphene nanosheet show metallic behavior in terms of band structures and DOS plots except Ti-Ni which has shown a little band gap. In terms of electronic properties, Cr-Cr and Ni-Cr co-doped graphene nanosheets are found most stable among the other studied systems and they can exhibit magnetic behavior as there is a variation in their up and down spin as shown in spin polarized DOS. Thats why they are beneficial to the application of various magnetic devices as well as sectors. Besides Cr-group co-doped graphene nanosheet can exhibit better magnetic properties than Ni-group.
For the past decades, graphene which is a one-atom-thick planar sheet of sp2 hybridized carbon atoms closely packed in a honeycomb lattice structure, has caught significant consideration to be utilized as a cut-ting edge electronic material. This is possible because of its remarkable properties including great current density, chemical inertness, optical transmission, huge thermal conductivity, ballistic transport and marvelous hydrophobicity at nano scale range (Geim et al., 2008), (Chen et al., 2008). Micro-mechanical cleavage was the method used for first graphene extraction from graphite and this method permitted simple fabrication of first-rate graphene crystallites and further prompted tremendous experimental activities (Clintock, 2012).
Fundamentally graphene is a huge aromatic macro-molecule with no band gap and it can absorbs electro-magnetic energy homogeneously over the electro-magnetic spectrum, from infrared through visible to ultraviolet (Avouris, 2010). As a whole, great surface area renders to improve its reactivity. Again, since graphene has plentiful surface area for reactions to occur, it is likewise of attention in aspects of medicine, biology and bioengineering for the reason it is suitable for a biologically well-suited electrode and drug deli-very material (Vanesa et al., 2012).
Both theoretical (Chen et al., 1997; Menon et al., 2000) and experimental (Binns et al., 1996; Nagao et al., 1998) investigations have been done to understand the behavior of TM atoms when it interacted with graphite or a graphene. These investigations demon-strate that carbon spz orbitals hybridize intensely with the ‘d orbitals of the TM atoms. Half-metallic systems have been found by the interaction process of magnetic atoms with nanotubes that are of interest for nano-magnets and spintronics devices (Alphenaar et al., 2001). The great moments of Fe and Co doped struc-tures and the half metallic behavior for specific single-atom Co doped structures could be suitable for mag-netic device and spintronics functions (Yagi et al., 2004). It has also investigated that doping of the edges of the similar graphene nanoribbon with various atoms, from s-type to d-type transition metals; we can achieve numerous electronic and magnetic properties. In fact, different dopants doped the similar ribbon which can be insulator, semiconductor, metal, ferro-magnetic and anti ferromagnetic. Doping in 3d transi-tion metals can be supplied systems with FM or AF circumstances at the edges. Doping of the edge with little densities of Fe or Mn can cause in half-metallic or half-semiconductor ribbon. Consequently, graphene nanoribbons give an extensive range of promising electronic and magnetic phenomenon like similar ribbon structure but various dopant TM atoms (Pathi-ranage, 2021; Gorjizadeh et al., 2008).
In our investigation, the structural, electronic and magnetic properties of pristine graphene and 3d transition metal co-doped graphene nanosheet have been studied. In this article, focus has been made on the co-doping of 3d TM atoms onto graphene nanosheet. We have investigated how these co-doped transition metals (Cr-Cr, Cr-Mn, Cr-Fe, Ni-Ti, Ni-Cr, Ni-Fe) impact different physical properties of 2D graphene nanosheet.
Computational Details
In this study, we perform spin-polarized ab initio cal-culations using the Cambridge Serial Total Energy (CASTEP) code (Segall et al., 2002) within the frame-work of density functional theory (Rajagopal et al., 1973; Kohn & Sham, 1965). In this code, the Kohn–Sham equations are solved and the wave functions of valence electrons are expanded in a basis set of plane waves with kinetic energy smaller than specified cut-off energy, Ecut. A nonlocal ultra-soft pseudo-potential of the Vanderbilt-type represents the presence of tightly-bound core electrons and also described the electron-ion interaction. The exchange correlation potential is treated within the Perdew-Burke-Ernzerhof, (1996) version of the generalized gradient approxi-mation (PBE-GGA) (Perdew et al., 1996). A super cell contained 56 carbon (C) atoms is considered as the basic model for calculation which is a plane graphene nanosheet. Here 1×3× 2 Monkhorst-Pack grid (Monk-horst & Pack, 1976) in the Brillouin zone of the unit cell was employed for the geometry optimization so that it can generate an even grid of k-points along the three axes in reciprocal space. We use PBE gradient corrected functional and ultra soft pseudo potentials with plane-wave kinetic energy cutoff energies of 310 eV. Graphene nanosheet is built of perfect geometry with an initial C-C distance of 1.42 Å. Geometry optimization has been performed for all investigating nanosheets with the Broyden-Fletcher Goldfarb-Shan-no (BFGS) minimization technique, with the flowing ambit for converged structures: energy change per atom less than 1×10−5eV, residual force less than 0.03 eV/ Å, stress below 0.05 GPa and the displacement of atoms during the geometry optimization less than 0.001 Å.
In the TM co-doped graphene nanosheets, for efficient comparison, every time we have replaced the C20 and C30 atom of the pristine nanosheet by the targeted TMs. Thus, different structural, electronic and magne-tic properties of the 3d TM co-doped graphene nano-sheets were compared and investigated by the pro-cesses discussed above.
When a substance is doped by any hetero element its structural properties are changed radically which leaves impact to the geometric and structural pro-perties of the mother substance. Thus structural sta-bility analysis is an important observation when a material is doped by other hetero elements. In this investigation, it is noticed that optimized pristine grap-hene and Cr-Cr, Cr-Mn, Cr-Fe co-doped graphene nanosheets are perfect planer. Besides, in co-doped structures, bond lengths are distorted around the TM atoms. In the pristine graphene nanosheet, every time we have replaced C20 by Cr atom and C30 is by Cr, Mn and Fe atoms respectively. Here, C20 can be considered as TM1 position and the position of C30 is TM2. The charge transfer is the transference of fractional electronic charge between the complex sys-tem which creates an electrostatic attraction in the complex system or molecule. This electrostatic attar-ction is the root of forming stabilizing force in a complex system and the portion which transfer charge is recognized as electron donor and the receiving entities is known as electron acceptor. The charge transfer occurs due to the difference in the electron density of the constituents of a system i.e. some have excess of electrons and some have lack of electrons.
For better understanding, we have divided our study of 3d TM co-doped graphene nanosheet into two groups in this section. They are Cr co-doped (Cr-Cr, Cr-Mn, Cr-Fe) group and Ni co-doped (Ni-Ti, Ni-Cr, Ni -Fe) group. Here 3d TMs are Ti, Cr, Mn, Fe, Ni.
Effect of Cr-3d TM co-doping in Graphene Nano-sheet
Structural Properties
In this article, we have theoretically investigated the structural, electronic and magnetic properties of pris-tine graphene and 3d transition metal co-doped grap-hene nanosheet by using DFT framework. We have also analyzed the effect of TM co-doping in graphene nanosheet. From structural property, we have analyzed the Mulliken charge transfer, bond lengths, electron density difference and zero point energy for pristine graphene and 3d transition metal co-doped graphene nanosheet. From the zero point energy, we can say that Cr-Cr and Ni-Cr doped graphene are more stable than other Cr- and Ni- co-doped structures. The zero point energies for Cr-Cr and Ni-Cr co-doped graphene are respectively - 12049.24eV and -10936.87eV. Band structure of TMs co-doped graphene nanosheet reveals metallic behaviors for all the co-doped structure except Ti-Ni which has shown a little band gap. Spin polarized Density of states (DOS) of Cr and Ni group indicate that Cr group co-doped graphene nanosheet show better magnetic properties.
Fig 6: Band structures (a-d) and total DOS spectra (e) for pristine and Ni-Ti, Ni-Cr, Ni -Fe co doped graphene nanosheet.
We are very thankful to PUST Research Cell for their fund allocation to setup the Computational Lab at the Department of Physics in Pabna University of Science and Technology for giving me opportunity to complete this work. We also thankfully acknowledge the Higher Education Quality Enhancement Program (HEQEP) subproject CP-3415, University Grant Commission (UGC) of Bangladesh, and the World Bank for the financial assistance to set up the Computational Physics (CP) Research Lab in the Dept. of Physics at Jahangirnagar University.
The authors declare that they have no known com-peting financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Academic Editor
Dr. Wiyanti Fransisca Simanullang Assistant Professor Department of Chemical Engineering Universitas Katolik Widya Mandala Surabaya East Java, Indonesia.
Department of Physics, Pabna University of Science and Technology, Pabna-6600, Bangladesh.
Nishat M, Islam R, and Rahman MA. (2021). Investigation of the effect of 3d TM-TM atom co-doped in graphene nanosheet: DFT based calculations, Int. J. Mat. Math. Sci., 3(6), 122-132. https://doi.org/10.34104/ijmms.021.01220132