Article pubs.acs.org/JPCC
Transition Metal Embedded Two-Dimensional C3N4−Graphene Nanocomposite: A Multifunctional Material Dibyajyoti Ghosh,† Ganga Periyasamy,†,∥ and Swapan K. Pati*,‡,§ †
Chemistry and Physics of Materials Unit, ‡Theoretical Sciences Unit, and §New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, 560064, India S Supporting Information *
ABSTRACT: The lack of intrinsic spin polarization in graphene as well as in its several composites limits their usage as suitable spintronic material. Using long-range dispersion corrected density functional theory, we explore the structural, electronic, magnetic, and optical properties of recently synthesized [Liu, Q.; Zhang, J. Langmuir 2013, 29, 3821−3828] two-dimensional graphitic carbon nitride (g-C3N4) stacked graphene (C3N4@graphene) where 3d transition metals (TMs) are embedded in the cavity of g-C3N4 (TM-C3N4@ graphene). The incorporation of TMs modifies the structure of C3N4@graphene negligibly and keeps graphene almost as in its pristine form. TM inclusion makes the narrow-gap semiconducting C3N4@graphene as metallic. Charge-transfer analysis shows that the TMC3N4 transfers electrons from the 3d-orbital of TM to the conduction band of graphene, making it n-doped in nature. Importantly, Cr, Fe, Co, and Ni embedded C3N4@graphene shows long-range ferromagnetic coupling among TMs in their ground state. The magnetic ordering appears due to suitable ferromagnetic d−p exchange interaction, which is absent in paramagnetic V- and Mn-C3N4@graphene sheets. Furthermore, calculated high charge carrier densities of the n-doped graphene layer in these nanocomposites are quite promising for its usage in ultrafast electronics. Performing Heisenberg model based Monte Carlo simulations, we predict the Curie temperatures for Crand Fe-C3N4@graphene as 381 and 428 K, respectively. Moreover, these sheets also demonstrate prominent visible light response, which gives us a clue about their probable photocatalytic activity. Thus, the present study exhibits the true multifunctional behavior of TM-C3N4@graphene by demonstrating its usage in various fields, such as memory devices, spintronics, ultrafast electronics, photocatalysis, etc.
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INTRODUCTION The successful synthesis of graphene in 2004 has brought a revolution in the field of material science research.1 Because of the presence of several extraordinary chemical, electrical, optical, thermal, and mechanical properties, graphene and other analogous 2-dimensional (2D) layered materials, such as boron nitride (BN), molybdenum disulfide (MoS2), silicene, and tungsten disulfide (WS2), have attracted a huge research interest.2−7 Several research groups have already demonstrated practical applications of these materials in electronic integrated circuits,8 field effect transistors,9 supercapacitors,10 optics,11,12 catalysis,13,14 bioimaging,15,16 toxic gas-trapping,17 etc. Moving further, recently, layered nanocomposites, made of different intrinsic 2D sheets, have been synthesized and found to modify the performance of parent’s sheets in terms of several applications.18 For example, Dean et al. were able to improve the performance of graphene-based electronic devices by using BN layers as a substrate.19 This composite also shows very high electron mobility.20 Recently, Lee et al. have successfully fabricated a transparent and flexible MoS2 field effect transistor on top of a graphene/BN layer, where BN and graphene layers act as dielectric and gate, respectively.21 Because of their low-dimensionality, these single-layered as well as composite sheets could also be explored as potential materials for high-performance spintronics devices.22−24 More© XXXX American Chemical Society
over, the capability to control charge carrier density, which is very essential for electronic as well as photonic applications, has been achieved in these materials.25,26 However, the absence of an intrinsic spin center, which is the prime requirement for spintronics, restricts their application in this field. In general, 3d transition metals (TMs) are deposited on top of the singlelayered sheets to be used as a source of spin moment.27,28 However, these TM-incorporated sheets have a major limitation as TMs on top of the layer are quite mobile in nature and form clusters due to strong d−d interaction.29,30 Although recent studies demonstrate that mobile TMs can be frozen by creating regular functionalized point defects, formation of regular defects are still experimentally difficult and need extreme caution.27,31 In this regard, newly synthesized and well-characterized TM-deposited graphene−graphitic carbon nitride (TM-C3N4@graphene) nanocomposites can be a promising solution.32 In these sheet materials, TM atoms create localized spin polarization and n-doped graphene shows high electron carrier density. The combination of these two makes these sheets perfect candidates for spintronic applications. Received: April 5, 2014 Revised: May 16, 2014
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zone of the unit cell and the (2 × 2) supercell of TM-C3N4@ graphene, respectively. We have carried out geometry optimizations without imposing any kind of symmetry constraints, and interatomic forces are relaxed up to 0.01 eV/ Å. We have computed Heisenberg model based Monte Carlo (MC) simulation by taking a supercell of 100 × 100 sites and involving 4 × 106 MC steps to study magnetic ordering at finite temperature. For optical property calculation, we have computed the frequency-dependent dielectric matrix using the GGA functional and a k-mesh of 13 × 13 × 1 to sample the Brillouin zone finely. Previous study has proved that the GGA functional can reproduce the experimental spectra of these kinds of materials efficiently.33 The imaginary part of the dielectric matrix is used further to calculate absorption spectra.42 To know about the structural stability of these structures, we have formulated the binding energy as follows27
The polymer of heptazine units forms a g-C3N4 layer of this nanocomposite. It is energetically the most stable among all graphitic carbon nitrides (see Figure S1, Supporting Information).33 Getting stacked with graphene in third dimension via relatively weak van der Waals forces, it forms a stable nanocomposite (C3N4@graphene).33 This nanocomposite is a narrow-gap semiconductor and already has been used for several applications, such as optoelectronics, electrocatalysts, photocatalysts, etc.34 Very recently, Liu et al. have synthesized Co embedded C3N4@graphene, which shows better electrocatalytic performance in an oxygen reduction reaction.32 From their experiment, it is evident that Co atoms remain embedded in the g-C3N4 cavity uniformly without forming any cluster or oxide. We believe that, following the same procedure, one can easily deposit other TMs uniformly in the C3N4@graphene layer. As these materials possess regular spin ordering, the most essential requirement for memory and spintronics applications, one should investigate their electronic and magnetic properties thoroughly. To the best of our knowledge, usage of this TMC3N4@graphene as magnetic 2D sheets still remained entirely unexplored. In the present study, using density functional theoretical calculations, we have systematically demonstrated structural, electronic, and magnetic properties of TM-C3N4@ graphene where TMs are 3d TMs, except Sc and Ti (i.e., V, Cr, Mn, Fe, Co, Ni, Cu, and Zn). Our studies evidently show the metallic nature of all of these sheets. Cr, Fe, Co, and Ni atoms couple ferromagnetically, whereas V- and Mn-containing sheets are paramagnetic in nature. Furthermore, performing Monte Carlo (MC) simulations, we find that the Curie temperature (TC) of Cr and Fe embedded ferromagnetic sheets is well above the room temperature. Apart from exciting magnetic properties, the graphene layer of these composites shows considerable (∼10 times) enhancement in charge carrier density due to its n-doping. We also computed optical absorption spectra, which show several low-energy peaks, clearly demonstrating their improved visible light response.
E bind = Esheet‐TM − (Eg‐C3N4 + Egraphene + E TM)
where Eg‑C3N4, Egraphene, and ETM are the energies of bare g-C3N4, graphene, and isolated TM atom, respectively. Esheet‑TM denotes the energy of C3N4@graphene with TM atoms embedded within it.
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RESULTS AND DISCUSSION To compute the geometric and electronic properties of metalfree C3N4@graphene nanocomposites, we keep the 1 × 1 gC3N4 unit on top of the 3 × 3 supercell of graphene, from which we find the lattice mismatch between two layers to be very small (i.e., 1.5%).43 Importantly, as can be seen from Figure S1 (Supporting Information), the adhesion of g-C3N4 on top of graphene removes its intrinsic structural buckling and makes it planar to have better π−π stacking between two layers. The interlayer distance between these two planar sheets is ∼3.04 Å, which compares well with previous reports.43 The presence of a g-C3N4 sheet over graphene breaks the potential periodicity of graphene, leading to its band-gap opening at the Γ-point (graphene fold K-point) as can be seen in Figure S2 (Supporting Information). As can be seen from Figure 1, the deposition of TM atoms at the cavity made by Nedge atoms of C3N4@graphene generates a
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COMPUTATIONAL DETAILS We have used spin-polarized density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) for the present study.35 We have taken care of exchange-correlation using Perdew−Burke−Ernzerhof (PBE) functional within the Generalized Gradient Approximation (GGA).36 As it is well-known that only the GGA functional cannot accurately describe partially filled d-orbital containing systems, we have applied the GGA+U method.37 In this method, localized d-orbitals are taken care with Coulomb and exchange corrections, whereas s- and p-orbitals are described by only the GGA functional. We have considered correlation energy (U) as 4 eV and exchange energy (J) as 1 eV, which are well testified by several other studies focused on 3d transitionmetal-based organometallic complexes.28,38,39 We have used the projected augmented wave (PAW) method40,41 with a planewave basis set with a cutoff energy of 400 eV for all systems. We also have included Vosko−Wilk−Nusair modification for spinpolarized calculations to interpolate the correlation energy.42 As all the systems are two-dimensional sheets, we have avoided spurious interaction in nonperiodic directions (i.e., z direction here) by creating a vacuum of 20 Å in the z direction. To find the magnetic ground state of these systems, we have considered a (2 × 2) supercell. For periodic calculations, Monkhorst−Pack 7 × 7 × 1 and 5 × 5 × 1 k-point grids (total number of k-points is 25 and 13, respectively) are used to sample the 2D Brillouin
Figure 1. Atomic configurations of Cr-C3N4@graphene: (a) unit cell, (b) zoomed top view of electron-rich cavity where TM atoms get trapped, and (c) side view of Cr-C3N4@graphene sheet. Blue, gray, and light blue balls are nitrogen, carbon, and Cr atoms, respectively. Graphene sheet has been shown in wireframe mode for clarity.
consistent distribution of these TMs (Cr atoms for example) over this nanocomposite sheet. Depending upon the nature of spin−spin interaction among the 3d-orbitals of TMs and ligand 2p-orbitals (i.e., d−p exchange), we can have suitable 2D magnetic sheets. B
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show that these metal atoms stay nearer to two Nedge atoms and relatively far from the other four Nedge. Like the situation in Jahn−Teller distortion, the doubly occupied d-orbitals always circumvent direct interaction with p-orbitals of Nedge and gain stabilization energy by destroying the above-mentioned symmetry. However, one should note that the overall structures of TM-C3N4@graphene remain almost planar with nearly the same interlayer distance as it is for g-C3N4@graphene. The negative binding energy as tabulated as in Table 1 proves the structural stability of these nanocomposites. Moreover, calculated interface adhesion energies (see the Supporting Information for details) as tabulated in Table S1 show that the TM-C3N4 layer gets stacked on top of graphene quite strongly. Plotting the spin density distribution for all TM embedded nanocomposite sheets (except Cu and Zn), we evidently find that most of the magnetic moments are localized at the metal atom and Nedge atoms, as can also be seen in the spin density plot for Fe-C3N4@graphene in Figure 3a,b. Notably, adhesion
Interestingly, as can be seen in Figure 1, the structural reformation of the nanocomposite due to the incorporation of TM in the Nedge cavity is negligible. The graphene sheet remains almost unaltered, whereas the Ninner atoms of g-C3N4 go out of plane a little (by ∼0.2−0.5 Å depending upon the TM) for all sheets. Furthermore, careful investigation shows that V, Mn, Cu, and Zn atoms stay in exactly the middle of the cavity, whereas others remain in a slightly distorted hexagonal field (see Table S1 in the Supporting Information). Under a hexagonal crystal field environment, 2p-orbitals of Nedge atoms of g-C3N4 act as interacting ligand orbitals, which breaks the degeneracy of 3d-orbitals of the TM atom and splits them into four energy states, as can be seen from Figure 2a. Quite
Figure 2. (a) The crystal field splitting of TMs in a hexagonal environment (energy is not scaled) and (b) pDOS of d-orbitals on the Cr atom of Cr-C3N4@graphene (symbols: solid black, dashed red, dashed dotted maroon, dashed double dotted deep green, and double dashed dotted cyan represent dxy, dyz, dz2, dxz, and dx2−y2 orbitals, respectively). In the inset of (b), pDOS of p-orbitals on the Nedge of the same structure has been plotted. Note that positive and negative pDOS values correspond to majority and minority spins, respectively.
Figure 3. (a) Top and (b) side views of spin density of Fe-C3N4@ graphene with an isosurface of 0.025 e/Å3 where up and down spin densities are represented as orange and yellow (not visible here) colored surfaces, respectively. (c) Calculated spin-resolved band structure and corresponding DOS of Fe-C3N4@graphene (Γ(0,0,0), M(1/2,1/2,0), K(2/3,1/3,0) are high-symmetry K-points. Symbols: black and red solid lines denote majority and minority spin bands, and blue dotted line shows Fermi level).
obviously, as can also be seen from pDOS of 3d-orbitals of Cr of Cr-C3N4@graphene in Figure 2b, three d-bands, which are out of the xy plane (considering xy plane as composite’s plane), become much stabilized (∼1 eV) than the other two d-bands, i.e., dx2−y2 and dxy, which are in the xy plane and face the 2px/ 2py-orbitals of Nedge directly. Now, for V2+, three 3d-electrons singly fill up these lowest three d-bands, leaving dx2−y2 and dxy empty. However, for Mn2+ and Cu1+/Zn+, all the d-orbitals are occupied with single and a pair of electrons, respectively. Thus, both directly interacting d-orbitals (i.e., dx2−y2 and dxy) of these TM atoms face the hexagonal crystal field with symmetrical occupancy of zero electrons (for V), one electron (for Mn), or two electrons (for Cu, Zn), resulting in incorporation of them at almost the exact center of the cavity. Other TMs, i.e., Cr, Fe, Co, and Ni, have unsymmetrical fillings of the d-orbitals, causing the breakage of the perfect hexagonal symmetry around them. Tabulated optimized TM−Nedge distances in Table S1
of TM-C3N4 on a graphene sheet does not break the spin symmetry of the latter. We can exclude Cu- and Zn-C3N4@ graphene for further discussions about magnetism as they are d10 systems and possess a nonmagnetic ground state. Now, we determine the nature of magnetic coupling between two neighboring TM atoms through d−p exchange where electrons of d-orbitals of TM and p-orbitals of Nedge from the g-C3N4 layer get involved in magnetic interaction. A 2 × 2 supercell has been considered for all 2D sheets to determine the properties (see Figure S3, Supporting Information). The energies of the
Table 1. Binding Energies per Unit Cell of TM-C3N4@graphene, Magnetic Moment per TM, Exchange Energies for 2 × 2 Supercell, Electric Field Strength inside the Nanocomposite, and Carrier Concentration in Graphene Are Tabulated metal binding energy (eV) magnetic moment per unit cell (μB) ΔEex (meV) Eint (V/nm) electron carrier density (×1013 cm−2)
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
−3.66 3 2 0.57 3.9
−1.99 4 −179 0.50 3.2
−2.65 5 4 0.64 5.0
−2.20 4 −201 0.63 3.5
−1.92 3 −11 0.44 3.3
−1.40 1 −14 0.42 2.6
−1.49 0 0 0.43 2.1
−1.2 0 0 0.58 3.0
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antiferromagnetic. By analyzing the pDOS of the d-orbitals of Cr and p-orbitals on Nedge in Figure 2b, it is apparent that interacting d-orbitals are a hybrid consisting of majority spin bands of dx2−y2 and dxy, which overlap with pz/py of the same spin of the ligand, but this results in a total zero effective overlap due to symmetry of orbitals. As a result, Cr-C3N4@ graphene gives rise to a ferromagnetic ground state as also described in Heitler−London theory.45 For Fe, the situation is a bit different, as can be seen in Figure S5 (Supporting Information). Here, the interacting d-orbitals (i.e., near to EF) are dxz/dyz, which also results in ferromagnetic d−p exchanges after interacting with ligand py orbitals, where the effective overlap is zero. For details, see the Supporting Information. Ferromagnetism in Co and Ni embedded sheets can also be explained considering the same approach. Note that, as the interacting d-orbitals, i.e., minority spin-band of dyz/ dxz, just touches the Fermi level, the d−p exchange coupling is weak for these two systems (i.e., Co and Ni), which results in a smaller exchange energy, as tabulated in Table 1. Quite excitingly, our study predicts that sheets embedded with Cr and Fe atoms have quite high ferromagnetic exchange energies, i.e., −179 and −201 meV per supercell, respectively. These values are well comparable with other already reported 2D magnetic sheets.28,46,47 The large ferromagnetic exchange energy makes this material a highly favorable candidate for realizing a high-temperature 2D ferromagnetic system. We have explored this point further in of our study. Importantly, as these TM deposited g-C3N4 sheets get physisorbed on top of graphene, the characteristic Dirac cone of the latter still remains at the Γ-point (i.e., graphene fold Kpoint) but with a finite gap. This kind of phenomenon is quite common for other physisorption-based graphene nanocomposites.48−50 The plotted band structures of TM-C3N4@graphene in Figure 3b (for Fe-C3N4@graphene) and in Figure S6 (Supporting Information) (for others) show the upward shift of the Fermi level. This indicates a net electron transfer from TMC3N4 to graphene, which is also evident from Bader charge analyses as well as the electron density difference plot (Figure 5), as discussed in next paragraph. Depending upon the charge transfer, the shift of the Fermi level varies in a range of 0.27− 0.51 eV (see Table S1, Supporting Information). These transferred electrons partly populate the composite’s conduction band, which has a major contribution from pz-orbitals of carbon atoms of graphene. Partially occupied graphene-based bands as well as bands arising from spin-polarized 3d-orbitals show a dispersive nature and pass through the Fermi level, giving rise to the metallic nature of this organometallic nanocomposite (see Figure 3b and Figure S6). Furthermore, pDOSs of d-orbitals of TM atoms also show the proper origin of local magnetic moments of these sheets. For V and Mn, as shown in Figure 4a,b, the majority spin bands arising from delectrons remain as highly localized with nondispersive singly occupied states, staying far below the Fermi level. This results in a total magnetic moment of 3 and 5 μB/unit cell for V- and Mn-functionalized sheets, respectively. For other TM embedded sheets (i.e., Cr, Fe, Co, and Ni), although the metal atoms remain in a slightly distorted hexagonal crystal field environment, d-bands of these TM atoms split in the same manner as for V/Mn, i.e., 2 (dxz, dyz) + 1 (dz2) + 1 (dxy/dx2−y2) + 1 (dx2−y2/ dxy) in the increasing energy order. For Cr atoms, as can be found in Figure 2b, majority spin bands from higher energetic dxy and dx2−y2 orbitals also get populated and give rise to a magnetic moment of 4 μB/unit cell. Because of the increase of
systems, where TM atoms get coupled ferromagnetically (FM) and antiferromagnetically (AFM), are computed separately to calculate the exchange energy (Eex = EFM − EAFM). The corresponding spin densities of these two magnetic states of FeC3N4@graphene can be seen in Figure S4 (Supporting Information). Negative and positive values of Eex indicate ferromagnetic and antiferromagnetic ground states, respectively. The Eex and magnetic moment per unit cell are tabulated in Table 1. Calculated Eex energies show that, among all of these TMs, Cr, Fe, Co, and Ni atoms interact magnetically with each other when they get embedded in a C3N4@graphene sheet. It can be understood by looking at the pDOS of dorbitals on the TM atoms of the nanocomposite, as shown in Figure 4. It is well-known that the electrons that occupy spin-
Figure 4. Projected DOS of d-orbitals on the TM atoms of TMC3N4@graphene where TM is (a) V, (b) Mn, (c) Fe, (d) Co, (e) Ni, and (f) Cu.
polarized bands near the Fermi level only can get involved in magnetic coupling with other ligand bands of the same energy. The magnetic coupling is very weak for V- and Mn-based systems as their singly occupied d-orbitals remain well below the Fermi level (see Figure 4). Consequently, these sheets show paramagnetism in their ground state. However, the Fermi levels of other nanocomposites, i.e., Cr, Fe, Co, and Ni embedded C3N4@graphene, are populated by spin-polarized d-orbitals, which results in an effective magnetic interaction among TMs. Calculated exchange energies evidently show a ferromagnetically coupled ground state for all of them. This ferromagnetic interaction between TMs can be understood with the help of Goodenough-Kanamori-Anderson rules.44 For systems under study, TM−ligand−TM forms a 180° angle, and depending upon the interacting 3d-orbitals of TM and 2p-orbitals of Nedge, the magnetic coupling nature can be either ferromagnetic or D
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Calculated Eint values, tabulated in Table 1, are well comparable with other similar charge separated systems.48 The electron doping also results in an enhancement in the carrier concentration in the graphene layer. As tabulated in Table 1, considering a unit cell of the nanocomposites, we find the carrier concentration of graphene in the range of 2.1 × 1013 to 5.0 × 1013 cm−2, which is quite comparable with other ndoped graphene systems.26,48 As Samuels et al. demonstrated for bilayer graphene, the creation of such high carrier density by applying conventional gate voltage is quite difficult.48 However, for TM-C3N4@graphene, it can be achieved very easily, and these materials can be ideal for ultrafast electronics. High exchange energies for Cr- and Fe-C3N4@graphene due to d−p interactions have encouraged us to find their magnetic behavior under finite temperature. In this regard, several groups have performed Monte Carlo (MC) simulations using the Ising model to estimate the Curie temperature (TC).28,46,47 For our study, we have implemented MC simulations considering the complete Heisenberg Hamiltonian. The Hamiltonian can be formulated as53 Figure 5. (a) Top and (b) side views of three-dimensional transferred charge density plot at the interface between graphene and Cr-C3N4. Yellow and light blue isosurfaces show charge accumulation and depletion with respect to isolated graphene and Cr-C3N4 layer. The red arrow shows the direction of built-in electric field. The isovalue used here is 0.002 e Å−3.
H=
∑ Jij (SixSjx + SiySjy + SizSjz) ij
where i, j and Jij are the nearest-neighbor magnetic sites and exchange coupling constant between them, respectively. Six, Siy, and Siz are x, y, and z components of total spin (Stot) at the ith magnetic center of the 2D sheet. To use this Hamiltonian for MC simulations, we need to know the J values (both its magnitude and sign). Detail calculations to estimate J can be found in the Supporting Information. From our calculations, the estimated J values for Cr and Fe embedded sheets come out as −4.97 and −5.58 meV, respectively. By plotting the average of Siz per unit cell vs temperature in Figure 6, it is evident that
d-electrons in Fe, Co, and Ni atoms (i.e., 6, 7, and 8 electrons), all majority spin bands as well as minority spin bands start getting occupied, which eventually reduce the magnetic moment of corresponding units cells (Figure 4c−e). The local magnetic moments of these three metal atoms in unit cells appear as 4, ∼3, and 1 μB, respectively. At this point, one should note that all TM atoms remain in their corresponding high-spin states; i.e., the spin pairing at low-lying d-bands results in higher energy than the single occupancy at higher energy d-orbitals. As mentioned, Cu- and Zn-C3N4@graphene show nonmagnetic behavior in their electronic ground state. Unlike other TM atoms, Cu remains in a +1 oxidation state in Cu-C3N4 due to back-donation of electrons from ligand to metal. Complete filling of d-orbitals, i.e., d10, also gives extra stabilization to the +1 oxidation state of Cu. Band structure calculations show the metallic character of these systems, as revealed in Figure S6. Performing Bader charge analysis51 as tabulated in Table S1, we find that graphene accepts electrons from the C3N4-TM layer and becomes n-doped. Plotted three-dimensional charge density difference of Cr-C3N4@graphene in Figure 5 also clearly demonstrates the charge depletion from Cr-C3N4 and accumulation of that in graphene. Because of the difference in electrostatic potentials of these two layers, charge transfer occurs. This induced electron couples strongly in between two layers and remains localized to the inner layer (i.e., closer to CrC3N4) of graphene. Interestingly, it is in contrast to the C3N4@ graphene nanocomposite, where electron transfer occurs in the opposite direction. Thus, the incorporation of TM in the gC3N4 cavity changes the doping nature of graphene from p-type to n-type. This charge separation between two layers eventually creates a built-in electric field inside the nanocomposite. Knowing the energy gap at the Γ-point (ΔE) and the distance between two layers (d) and considering the tight-binding model, we can approximately formulate the internal electric field (Eint) as48,52
Figure 6. Values of the average of Siz per unit cell of Fe-C3N4@ graphene with respect to the temperature. The transition from ferromagnetic to paramagnetic state occurs (i.e., Curie temperature) at 428 K.
these sheets retain their ferromagnetic spin ordering perfectly even at room temperature. We find the TC values of Cr- and Fe-C3N4@graphene as 381.4 and 428.2 K, respectively (see Figure 6). The values of TC for Cr-C3N4@graphene and FeC3N4@graphene are much higher than the reported TC of dilute magnetic semiconductors as well as of several other 2D sheets.28,46,5455 Importantly, strong ferromagnetic ordering in these sheets is completely intrinsic and it does not need any application of external stimuli like strain.47,56 Because of the considerably high Curie temperatures, we strongly believe that these sheets will attract huge attention for advanced magnetic device industries.
E int ≈ ΔE /d E
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OPTICAL PROPERTIES Pure g-C3N4 is a wide-band semiconductor and has poor photoabsorption efficiency due to its inability to absorb visible light. 57 However, studies have shown that, when it stacks with graphene, a new optical transition occurs due to strong coupling between these two layers and, eventually, the visible photoresponse increases a lot.43 For present systems, the inclusion of transition metals closes the band gap of C3N4@ graphene completely and induces more levels near the Fermi energy, indicating a much greater enhancement of performance in terms of visible light response. To have a quantitative picture, we have calculated the absorption spectra of these nanocomposite sheets by evaluating the imaginary part of the dielectric function as shown in Figure 7.
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CONCLUSIONS In conclusion, performing DFT-based computations, we have discussed the structural, electronic, magnetic, and optical properties of recently experimentally synthesized TM embedded C3N4@graphene sheets.32 The stacking of TM-C3N4 on top of graphene preserves the pristine properties of the latter. Nevertheless, the conduction band of graphene gets populated due to charge transfer from the organometallic layer to it. The Fermi level of the whole nanocomposite shifts upward due to additional charge and its charge rearrangement, giving rise to metallicity in these sheets. Interestingly, the range and nature of magnetic ordering in these organometallic sheets are determined by the relative arrangement of d-orbitals of TM and p-orbitals of the ligand (i.e., Nedge atoms) near the Fermi level. The TMs with spin-polarized d-orbitals near to the Fermi level, which couple effectively with p-orbitals of the same energy, exhibit long-range magnetic ordering. For the sheets embedded with Cr, Fe, Co, and Ni, the interacting 3d-orbitals produce effective zero overlap with the ligand 2p-orbitals, giving rise to a ferromagnetic d−p exchange. In addition to this, the n-doped graphene shows an almost 10 times enhancement of carrier density, demonstrating its probable usage in advanced electronics. The charge separation between two layers eventually produces a high built-in electric field inside them. Monte Carlo simulations based on the Heisenberg model confirm the ferromagnetic ordering of spin in Cr- and FeC3N4@graphene at room temperature. The calculated TC’s of these sheets (i.e., 381 and 428 K) are much higher than those of recently reported phthalocyanine-based organometallic and C3N4-based organic sheets.55,49,53Moreover, as the new states appear near the Fermi level of the nanocomposite, the visible light response of these systems also gets modified to a large extent. The prominent absorption peaks at the low-energy range (i.e.,
