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Functionalization of two-dimensional C4N by atoms adsorption: A first-principles investigation
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113649
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Physica E: Low-dimensional Systems and Nanostructures journal homepage: www.elsevier.com/locate/physe
Functionalization of two-dimensional C4N by atoms adsorption: A firstprinciples investigation
T
Min Xu, Hongyan Wang*, Songsong Sun, Hengtao Li, Xiumei Li, Yuanzheng Chen, Yuxiang Ni School of Physical Science and Technology, Key Laboratory of Advanced Technology of Materials, Ministry of Education of China, Southwest Jiaotong University, Chengdu, 610031, China
A B S T R A C T
To design and improve the electronic structure of 2D materials, many methods can be used. Among them, surface adsorption is an effective way. Here, the adsorption characteristics of 20 different atoms, including period-2 (B, C, N, O, F), period-3 (Al, Si, P, S, Cl), and 3d transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed Dumbbell C4N (DB C4N) are explored by first-principles calculations. In addition, the adsorption energy, the density of states (DOS), the energy band, the charge transfer, and total magnetic moments of each adatom-C4N system are also investigated. The result shows that DB C4N exhibits good adsorption capability to foreign atoms, and its adsorption energy is stronger than that of g-C2N, C3N, and SiC. Moreover, adsorption of N-, O-, F-, and Cl-atom can induce the p-type doping feature in the C4N monolayer. Except for C, N, O, S and Zn atom, all the other adatoms studied in this paper can induce magnetism in the C4N monolayer. Therefore the electronic structure and magnetic properties of the C4N monolayer can be efficiently modified by adsorbing corresponding atom, thus expanding the applications of the C4N monolayer in the field of catalyst, solar cells, sensors, and electronic devices.
1. Introduction Since the discovery of monolayer graphene in 2004 [1], great progress has been made in many applications based on graphene due to its exceptional properties, such as massless relativistic Dirac fermion behavior [1,2], high-speed mobility of carriers [3], high thermal conductivity, and half-integer Hall conductance [4,5]. After the synthesis of two dimensional (2D) single-layer graphene, a large number of other two-dimensional materials have been discovered. Among them, Carbon nitride materials are the important type of 2D materials. Varieties of new 2D carbon nitride materials (e.g. g-C3N4, C2N, C3N, g-C14N12, gC10N9, g-C6N6, etc.) have been proposed in experiment and theory [6,7]. Recently, a novel 2D carbon nitride material, called Dumbbell C4N (DB C4N), was predicted by Li et al. [8] According to the C3N monolayer structure and the DB structure of Silicene/Germanene/Stanene, the new rich carbon C4N monolayer with Dumbbell structure is proposed by adsorbing C atom on the N atom positions of the C3N monolayer. Two configurations, DB C4N-I and DB C4N-II, can be obtained based on the positions of the raised C/N atoms, whose Fermi velocity is 2.6 × 105 m/s and 2.4 × 105 m/s, respectively. The Fermi velocity of the C4N monolayer is so high that it is possible to become ideal materials for building high-speed electronic devices. Moreover, the two new structures, DB C4N-I and DB C4N-II, are the first predicted Dirac carbon nitride materials with no spin-polarization. As is known, *
graphene present zero-band-gap semiconducting electronic character, which limits its further applications [9,10]. Therefore, a lot of research has been done on the modification of graphene and extension of its application. In order to manipulate and improve the electronic structures of 2D materials, a great deal of efforts, such as adsorption [11–13], doping [14–16], and strain [17,18], can be made to modify 2D materials to further broaden its applications. Among those methods, the surface adsorption is an effective access, because the 2D materials have a large surface area exposed to the exterior environment. By introducing impurity atom on surface of 2D materials, the electronic structure of the system can be tuned, leading to the appealed characters. For example, adatom-silicene systems can exhibit semiconducting, half-metal, metal behaviors by adsorbing various atoms on the silicene monolayer [19]. The studies of Pang et al. showed that alkali atoms adsorption on germanene yielded a band-gap opening [20]. The work of Zheng et al. indicated that 3d transition metals (TM) adsorbed on g-C2N monolayer can also effectively change the electronic structures of g-C2N monolayer [21]. In addition, 3d TM atoms adsorbed on the C2N monolayer could serve as promising single-atom catalysts [22].The non-metallic and semi-metallic atoms adsorption can induce the semiconductor C3N to become a metal [23]. These findings indicate that surface absorption can be used to modify and improve the electronic structures of the twodimensional materials and expand its applications. Functionalization of the C4N monolayer through the adsorption of atom seems to be a
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.physe.2019.113649 Received 14 January 2019; Received in revised form 18 July 2019; Accepted 22 July 2019 Available online 23 July 2019 1386-9477/ © 2019 Elsevier B.V. All rights reserved.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113649
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Fig. 1. Illustration of top and side views of the C4N, and C4N 2 × 2 × 1 supercell (dashed black line) (a). The raised C atoms are labeled as CR and the planar C atoms are labeled as CP. The four adsorption sites H1, H2, TC, and BCC-1 of A side (b) and the four adsorption sites H3, H4, TN, and BCC-2 of B side (c). The yellow (gray) symbols are the carbon (nitrogen) atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. The electronic band structure (a) and partial density of states (b) of the unit cell of a pristine C4N monolayer.
spin polarization was included in all calculations. In addition, a dipole correction [27] was used in our calculations. To avoid the interactions between periodically repeated images, a vacuum layer of at least 15 Å was used. A kinetic energy cutoff of 520 eV was adopted and the Brillouin zone was sampled with special points of a 15 × 15 × 1 grid, as proposed by Monkhorst and Pack [28]. Furthermore, atomic relaxation was performed until the change of total energy was less than 10−5 eV and the Hellmann-Feynman force on each atom was less than 0.02 eV/ Å. As it is known, to obtain an accurate description of atoms containing d electrons, a finite onsite Coulomb interaction is necessary to be considered. Thus, we used the GGA + U method [29,30] for the 3d transition metal atom adsorbed on the C4N systems, we adopt an onsite Coulomb interaction of U = 4.0 eV in our calculations. While this is a constant correction, it can be considered as an acceptable average for all 3d-ions as a trend. To simulate the atom adsorption on the C4N monolayer, the DB C4NI configuration was chosen, which is the same as that predicted by Li et al. [8] One atom was assumed to absorb on the 2 × 2 × 1 C4N
promising way to extend the application of the C4N. In the present work, DB C4N-I configuration in which C atoms bonded all N atoms on the same side is studied. The adsorption of 20 different adatoms including period-2 (B, C, N, O, F), period-3 (Al, Si, P, S, Cl), and TM (Sc ~ Zn) atoms on the C4N monolayer is studied using first-principles density functional theory (DFT). The electromagnetic characteristics of the C4N monolayer including the adsorption energy, geometry, the density of states (DOS), the total magnetic moment, the Bader charge transfer, and work function are investigated. 2. Computational details In the present work, first-principles calculations were performed using Vienna Ab-initio Simulation Package (VASP) [24]. To describe the exchange and correlation, the Perdew-Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA) [25] was employed. The DFT-D2 method of Grimme [26] was used to consider the van der Waals interaction between the adatom and the C4N monolayer. The 2
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113649
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Fig. 3. Average planar potential in the z direction for pristine C4N (a) and functionalized by adatoms: (b) C, (c) S, and (d) Mn.
Fig. 4. The band structure of the pristine C4N (a) and functionalized by Period-2 adatoms: (b) N, (c) O.
supercell with 40 atoms containing 32 C and 8 N atoms. As shown in Fig. 1(b) and (c), the eight possible adsorption sites for the single adatom on the C4N monolayer can be considered: (i) above the center of hexagonal carbon rings of A side (H1), (ii) on top of the upper carbon atoms of A side (TC), (iii) above the center of hexagonal carbon nitrogen rings of A side (H2), (iv) above the bridge of hexagonal carbon rings of A side (BCC-1), (v) above the center of hexagonal carbon rings of B side (H3), (vi) on top of the nitrogen atoms of B side (TN), (vii) above the center of hexagonal carbon nitrogen rings of B side (H4), (viii) above the bridge of hexagonal carbon rings of B side (BCC-2). The adatom is expected to occupy one of the sites mentioned above. The adsorption energy for the adatom-C4N system can be defined as Eq. (1)
Table 1 The favorable adsorption site, the binding energies (Eads ), the bond length between the adatom and the closest surface atom (Dads ), the energy band gap (Eg ), the total magnetic moments ( μtot ), and the Bader charge transfers from the adatom to C4N for the hybrid system. Metallic and half-metallic structures are denoted as M and HM, respectively. Atom
Site
Eads (eV)
Dads (Å)
Eg (eV)
μ tot (μβ)
Δρ (e)
B C N O F Al Si P S Cl
H2 TC TC TC TC H2 H2 H2 H2 TC
−4.68 −4.13 −3.96 −6.67 −4.99 −3.46 −4.39 −3.98 −4.47 −3.74
1.60 1.40 1.28 1.23 1.36 2.13 2.01 1.91 1.87 1.75
0 HM 0.43 0.48 0.89 M HM HM 0.27 0.76
0.99 0.00 1.00 0.00 1.00 0.25 0.18 0.87 0.00 1.00
−1.17 +0.02 +0.83 +1.04 +0.66 −1.08 −1.12 −0.58 −0.05 +1.11
Eads = EC4 N+adatom − EC4 N − Eadatom
(1)
where EC4 N+adatom , EC4 N , and Eadatom are the total energies of the adatom-C4N system, the pure C4N monolayer, and the isolated adatom, respectively. To analyze the charge distribution, the grid-based Bader scheme is used [31]. The charge difference (Δρ ) is explained as Eq. (2) 3
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113649
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Fig. 5. The TDOS and PDOS of the C4N monolayer decorated with B, C, N, O, and F adatoms. The black solid line represents TDOS and red line represents the PDOS of functionalizing adatoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Differential charge density of the C4N monolayer functionalized by Period-2 adatoms: (a) B and (b) N. Color coding consists of pink for charge gain and purple for charge loss. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Δρads = ρC4 N+ adatom − ρC4 N − ρadatom
3. Results and discussion
(2)
here ρC4 N+ adatom , ρC4 N and ρadatom are the total charges of the adatomC4N system, the pure C4N monolayer, and the isolated atom, respectively.
3.1. Optimized structure and electronic properties of C4N The 2 × 2 × 1 supercell containing 32 C atoms and 8 N atoms is constructed to understand the electronic properties of the pristine C4N monolayer. The optimized structure of the C4N monolayer is shown in 4
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113649
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Fig. 7. The band structure of the C4N monolayer functionalized by Period-3 adatoms: (a) Al, (b) P, and (c) S.
Fig. 8. The TDOS and PDOS of the C4N monolayer decorated with Al, Si, P, S, and Cl adatoms. The black solid line represents TDOS and red line represents the PDOS of functionalizing adatoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.2. Electronic structures of the adsorbed the C4N monolayer
Fig. 1(a). In the C4N monolayer, carbon and nitrogen atoms are bonded together by sp3 hybridization, which is similar to the C atoms in diamond. The optimized lattice constant of the C4N model is 4.775 Å, which is in good agreement with previously reported results [8]. The bond lengths of CP-CP, CP-CR and N-CP are 1.491 Å, 1.567 Å and 1.558 Å, respectively. The band structure and corresponding projected density of states (PDOS) of pure C4N unit cell are shown in Fig. 2(a) and (b), respectively. Analogous to the band structure of graphene (silicene/ germanene/stanene) [32,33], zero band gap at the K point is found in Fig. 2(a), suggesting a Dirac cone at the K point. It can be seen that the conduction band edge and valence band edge are dominated by the p orbitals of C and N atoms.
20 different atoms including period-2 (B, C, N, O, F), period-3 (Al, Si, P, S, Cl) and transition metal (Sc ~ Zn) atoms are adsorbed on the eight possible adsorption sites of Dumbbell C4N (DB C4N), shown in Fig. 1(b) and (c). In order to understand the effect of surface atom adsorption on the electronic structures of the C4N monolayer, the adsorption energy, the density of states (DOS), the energy band gap, the charge transfer, and total magnetic moments of each adatom-C4N system are calculated. Adatom adsorption on the C4N monolayer is expected to alter the Fermi level of the system, therefore a large change in work function relative to pure C4N can be found from Fig. 3. The 5
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113649
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Fig. 9. Differential charge density of the C4N monolayer functionalized by Period-3 adatoms: (a) Si and (b) Cl. Color coding consists of pink for charge gain and purple for charge loss. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
predicted to be 5.77, 4.90 and 4.53 eV, respectively (see Fig. 4).
Table 2 The favorable adsorption site, the binding energies (Eads ), the bond length between the adatom and the closest surface atom (Dads ), the energy band gap (Eg ), the total magnetic moments ( μtot ), and the Bader charge transfers from the adatom to C4N for the hybrid system. Metallic structures is denoted as M. Atom
Site
Eads (eV)
Dads (Å)
Eg (eV)
μ tot (μβ)
Δρ (e)
Sc Ti V Cr Mn Fe Co Ni Cu Zn
BCC-1 BCC-1 H2 H2 H2 H2 H2 H2 H2 H2
−3.50 −3.44 −3.04 −2.64 −2.54 −1.23 −3.00 −2.91 −1.48 −0.63
2.24 2.22 2.18 2.16 2.15 2.04 2.03 1.95 1.99 2.05
0.05 0.15 0.09 0.17 0.31 M 0.09 0.11 M 0.07
0.94 2.07 3.17 4.13 5.00 2.00 1.12 0.08 0.05 0.00
−1.48 −1.12 −1.01 −0.80 −0.95 −0.60 −0.52 −0.39 −0.42 −0.67
3.2.1. Absorption of the Period-2 (B, C, N, O, F) nonmetallic atoms We studied the adsorption behavior of Period-2 (B, C, N, O, F) nonmetallic atoms adsorbed on the C4N monolayer. The period-2 nonmetallic atoms B, C, N, O and F are trend to be adsorbed at the different favorable site of the C4N monolayer. The atom B prefers to adsorb at the H2 site, while the others period-2 nonmetallic atoms C, N, O and F prefer to adsorb at the TC site. The adsorption energies are 4.68 eV for B, 4.13 eV for C, 3.96 eV for N, 6.67 eV for O, and 4.99 eV for F. In the cases of O-C4N system, it has the greatest adsorption energy. According to the band gaps presented in Table 1, the B-C4N system remains zero band gaps, while the N-, O- and F-C4N systems are 0.43 eV, 0.48 eV and 0.89 eV, respectively. Interestingly, in the case of C-C4N induces the half-metallic properties composed of a conducting spin-up channel and an insulating spin-down channel with an electronic band gap of 0.43 eV. In addition, the Fermi levels shift toward the valence band edge, indicating that N, O and F atom adsorptions can produce the p-type carrier in C4N the monolayer (as shown in Fig. 5). The adsorption of B, N and F produces magnetism for the Period-2 atoms adsorbed the C4N monolayer, while the C- and O-C4N system is nonmagnetic (see Table 1). During the period-2 atoms functionalization process, the C4N monolayer may act as electron donator and acceptor.
work function (Φ ) is defined as:Eq. (3)
Φ = E vac − EF
(3)
where E vac and EF are the potentials of the vacuum level and Fermi level energy, respectively. Our calculated work function of pure C4N is equal to 5.15 eV and functionalized C4N monolayer with C, S, and Mn are
Fig. 10. The band structure of the C4N monolayer functionalized by 3d-transition metal adatoms: (a) Mn, (b) Fe, and (c) Zn. 6
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113649
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Fig. 11. The TDOS and PDOS of the C4N monolayer decorated with Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn adatoms. The black solid line represents TDOS and red line represents the PDOS of functionalizing adatoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
In B-C4N system, the maximal charge gain −1.17 e is found, while the charge loss happened in the case of C-, N-, O- and F-C4N system. The charge density difference of B-, N-adsorbed C4N is shown in Fig. 6. The pink and purple zone represents the gain and loss of electron, respectively. The adsorption of B, N, and F atoms can induce the magnetism in the C4N monolayer because of the presence of unpaired electrons.
sites prefer to H2 site for Al, Si, P, S atoms, while Cl atom prefers to Tc site (see Table 1). Among the different systems considered here, Al-C4N system has the weakest adsorption energy (3.46 eV), whereas S-C4N system has the strongest adsorption energy (4.47 eV). The bond lengths between the adatoms and the closest surface atoms (Dads ) of the period3 adatoms are longer than those of the period-2 adatoms except N adatom, which may be associated with the atomic number and atomic radius. As shown in Table 1, Al-C4N system shows the metallic properties,
3.2.2. Absorption of the Period-3(Al, Si, P, S, Cl) atoms Compared to the period-2 adatoms (B, C, N, O, F), the adsorption 7
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113649
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Fig. 12. Differential charge density of the C4N monolayer functionalized by 3d-transition metal adatoms: (a) Fe and (b) Ni. Color coding of pink and purple illustrates the charge gain and charge loss, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
system yields a gap opening of 0.05, 0.15, 0.09, 0.17, 0.31, 0.09, 0.11 and 0.07 eV, respectively (as shown in Table 2). In addition, Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu atoms adsorption can effectively induce magnetism in the C4N monolayer (as shown in Table 2). The maximum magnetic moment 5.00 μβ is found for Mn-C4N system. The magnetic moments of TM-adsorbed C4N monolayer are mainly from TM atoms, and the monolayer only contributes 0.0012μβ. The band structures of Mn-, Fe-, and Zn-C4N systems are shown in Fig. 10. It is clearly seen that the spin-up and spin-down band structure of Mn- and Fe-C4N system show asymmetry, indicating that these systems show magnetic. It is found that Zn-adsorbed C4N system is nonmagnetic, which is in agreement with Zn-adsorbed C3N/C2N/germanene [20,22,23]. According to Bader charge analysis in Table 2, 0.39–1.48 electrons are transferred from Sc ~ Zn to C4N, respectively. The charge density difference of Fe-, Ni-adsorbed C4N is shown in Fig. 12. The pink and purple zone represents the gain and loss of electron, respectively.
while S- and Cl-C4N system yield a gap opening of 0.27 and 0.76 eV, respectively. For Si-C4N system, it exhibits half-metallic properties because its Fermi energy level only crosses through spin-up energy band. In addition, in the case of P-C4N system, its Fermi energy only crosses through spin-down energy band, and spin-down channel with an electronic band gap of 0.28 eV. The band structures of Al-, P-, and S-adsorbed C4N are shown in Fig. 7. For Al- and P-C4N systems, the spin-up and spin-down band structure are asymmetry, that is to say, these two systems are magnetic, while S-C4N system shows nonmagnetic. Interestingly, we can also see from Fig. 8 that the Fermi level shifts toward the valence band edge, which indicates that Cl atom adsorption can produce a p-type carrier in the C4N monolayer. According to Bader charge analysis, the Al-, Si-, P- and S-C4N systems transfer 1.08, 1.12, 0.58 and 0.05 electrons to C4N, respectively. It should be noted that only in the case of Cl-C4N system, 1.11 electrons are transferred from C4N to Cl atom because of the strong electronegativity of Cl atom (as shown in Table 1). The charge density difference of Si-, Cl-adsorbed C4N is shown in Fig. 9. The pink and purple zone represents the gain and loss of electron, respectively.
4. Conclusion In summary, on the basis of the first-principles calculations, we have systematically investigated the various atoms absorbed on the C4N monolayer, which include period-2 (B, C, N, O, F), period-3 (Al, Si, P, S, Cl), and 3d-transition metals (Sc ~ Zn) atoms. Based on the adsorption energy, we found that foreign adatoms prefer to be adsorbed on the site near carbon atoms, and most atoms favor bonding on the H2 site of the C4N. Adatom-C4N system presents various electromagnetic properties. N, O, F, and Cl atom adsorption can effectively induce p-type carriers in the C4N monolayer. Interestingly, pure C4N behaves as a Dirac material, while semimetallic C4N can show semiconducting, half-metal, or metal behavior depending on the adatoms type. It should be noted that the half-metal nature of C-, Si-, P-decorated C4N has great potential for spintronic device applications. Except for C-, O-, S- and Zn-C4N system, the other adsorption system induce the magnetism in the C4N monolayer. The tunable of the electronic characterization and the magnetic properties of the C4N monolayer by adsorbing atoms can extend the 2D material applications in catalyst, solar cells, sensors, electronic and spintronic devices.
3.2.3. Absorption of the transition metal (Sc–Zn) adatoms We next studied the adsorption characteristics of ten elements of the 3d transition metal adatoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed on the C4N monolayer. The corresponding results are summarized in Table 2. Among the 3d transition metal adatoms considered here, only Sc and Ti are adsorbed on the BCC-1 site. However, the H2 site is the most favorable adsorption site for V-, Cr-, Mn-, Fe-, Co-, Ni-, Cu-, and Zn-C4N systems. The trend is similar with the TM-adsorbed two dimensional (2D) C3N monolayer [23].The adsorption energy of TMadsorbed C4N monolayer from Sc to Zn are −3.50, −3.44, −3.04, −2.64, −2.54, −1.23, −3.00, −2.91, −1.48 and −0.63 eV, respectively. The adsorption energy sustained increase from Sc to Mn, and then continuous decrease from Co to Zn, which is similar to 3d TM atoms adsorption on C2N monolayer [22]. The Sc-C4N system has the largest adsorption energy, whereas the Zn-C4N system has the weakest adsorption energy due to its large atomic number and atomic radius. Interestingly, these values are much larger than those for the adsorption of 3d TM single atoms on C3N [23]. To gain deeper insight into the interaction between the 3d TM atoms and the C4N monolayer, the TDOS and PDOS have been performed for the 3d TM-C4N systems and the calculated results were listed in Fig. 11. From TDOS and PDOS profiles of C4N interacting with TM foreign atoms (see Fig. 11), Fe- and Cu-C4N system is found to turn into metallic properties, while in the case of Sc-, Ti-, V-, Cr-, Mn-, Co-, Ni- and Zn-C4N
Conflicts of interest There are no conflicts to declare.
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Acknowledgements [15]
This work is supported by the National Science Foundation of China (Grant No. 11774294), Sichuan Science and Technology program (Grant No. 2017JY0056 and 2018HH0088).
[16]
[17]
Appendix A. Supplementary data
[18]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.physe.2019.113649.
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Physica E: Low-dimensional Systems and Nanostructures journal homepage: www.elsevier.com/locate/physe
Functionalization of two-dimensional C4N by atoms adsorption: A firstprinciples investigation
T
Min Xu, Hongyan Wang*, Songsong Sun, Hengtao Li, Xiumei Li, Yuanzheng Chen, Yuxiang Ni School of Physical Science and Technology, Key Laboratory of Advanced Technology of Materials, Ministry of Education of China, Southwest Jiaotong University, Chengdu, 610031, China
A B S T R A C T
To design and improve the electronic structure of 2D materials, many methods can be used. Among them, surface adsorption is an effective way. Here, the adsorption characteristics of 20 different atoms, including period-2 (B, C, N, O, F), period-3 (Al, Si, P, S, Cl), and 3d transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed Dumbbell C4N (DB C4N) are explored by first-principles calculations. In addition, the adsorption energy, the density of states (DOS), the energy band, the charge transfer, and total magnetic moments of each adatom-C4N system are also investigated. The result shows that DB C4N exhibits good adsorption capability to foreign atoms, and its adsorption energy is stronger than that of g-C2N, C3N, and SiC. Moreover, adsorption of N-, O-, F-, and Cl-atom can induce the p-type doping feature in the C4N monolayer. Except for C, N, O, S and Zn atom, all the other adatoms studied in this paper can induce magnetism in the C4N monolayer. Therefore the electronic structure and magnetic properties of the C4N monolayer can be efficiently modified by adsorbing corresponding atom, thus expanding the applications of the C4N monolayer in the field of catalyst, solar cells, sensors, and electronic devices.
1. Introduction Since the discovery of monolayer graphene in 2004 [1], great progress has been made in many applications based on graphene due to its exceptional properties, such as massless relativistic Dirac fermion behavior [1,2], high-speed mobility of carriers [3], high thermal conductivity, and half-integer Hall conductance [4,5]. After the synthesis of two dimensional (2D) single-layer graphene, a large number of other two-dimensional materials have been discovered. Among them, Carbon nitride materials are the important type of 2D materials. Varieties of new 2D carbon nitride materials (e.g. g-C3N4, C2N, C3N, g-C14N12, gC10N9, g-C6N6, etc.) have been proposed in experiment and theory [6,7]. Recently, a novel 2D carbon nitride material, called Dumbbell C4N (DB C4N), was predicted by Li et al. [8] According to the C3N monolayer structure and the DB structure of Silicene/Germanene/Stanene, the new rich carbon C4N monolayer with Dumbbell structure is proposed by adsorbing C atom on the N atom positions of the C3N monolayer. Two configurations, DB C4N-I and DB C4N-II, can be obtained based on the positions of the raised C/N atoms, whose Fermi velocity is 2.6 × 105 m/s and 2.4 × 105 m/s, respectively. The Fermi velocity of the C4N monolayer is so high that it is possible to become ideal materials for building high-speed electronic devices. Moreover, the two new structures, DB C4N-I and DB C4N-II, are the first predicted Dirac carbon nitride materials with no spin-polarization. As is known, *
graphene present zero-band-gap semiconducting electronic character, which limits its further applications [9,10]. Therefore, a lot of research has been done on the modification of graphene and extension of its application. In order to manipulate and improve the electronic structures of 2D materials, a great deal of efforts, such as adsorption [11–13], doping [14–16], and strain [17,18], can be made to modify 2D materials to further broaden its applications. Among those methods, the surface adsorption is an effective access, because the 2D materials have a large surface area exposed to the exterior environment. By introducing impurity atom on surface of 2D materials, the electronic structure of the system can be tuned, leading to the appealed characters. For example, adatom-silicene systems can exhibit semiconducting, half-metal, metal behaviors by adsorbing various atoms on the silicene monolayer [19]. The studies of Pang et al. showed that alkali atoms adsorption on germanene yielded a band-gap opening [20]. The work of Zheng et al. indicated that 3d transition metals (TM) adsorbed on g-C2N monolayer can also effectively change the electronic structures of g-C2N monolayer [21]. In addition, 3d TM atoms adsorbed on the C2N monolayer could serve as promising single-atom catalysts [22].The non-metallic and semi-metallic atoms adsorption can induce the semiconductor C3N to become a metal [23]. These findings indicate that surface absorption can be used to modify and improve the electronic structures of the twodimensional materials and expand its applications. Functionalization of the C4N monolayer through the adsorption of atom seems to be a
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.physe.2019.113649 Received 14 January 2019; Received in revised form 18 July 2019; Accepted 22 July 2019 Available online 23 July 2019 1386-9477/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Illustration of top and side views of the C4N, and C4N 2 × 2 × 1 supercell (dashed black line) (a). The raised C atoms are labeled as CR and the planar C atoms are labeled as CP. The four adsorption sites H1, H2, TC, and BCC-1 of A side (b) and the four adsorption sites H3, H4, TN, and BCC-2 of B side (c). The yellow (gray) symbols are the carbon (nitrogen) atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. The electronic band structure (a) and partial density of states (b) of the unit cell of a pristine C4N monolayer.
spin polarization was included in all calculations. In addition, a dipole correction [27] was used in our calculations. To avoid the interactions between periodically repeated images, a vacuum layer of at least 15 Å was used. A kinetic energy cutoff of 520 eV was adopted and the Brillouin zone was sampled with special points of a 15 × 15 × 1 grid, as proposed by Monkhorst and Pack [28]. Furthermore, atomic relaxation was performed until the change of total energy was less than 10−5 eV and the Hellmann-Feynman force on each atom was less than 0.02 eV/ Å. As it is known, to obtain an accurate description of atoms containing d electrons, a finite onsite Coulomb interaction is necessary to be considered. Thus, we used the GGA + U method [29,30] for the 3d transition metal atom adsorbed on the C4N systems, we adopt an onsite Coulomb interaction of U = 4.0 eV in our calculations. While this is a constant correction, it can be considered as an acceptable average for all 3d-ions as a trend. To simulate the atom adsorption on the C4N monolayer, the DB C4NI configuration was chosen, which is the same as that predicted by Li et al. [8] One atom was assumed to absorb on the 2 × 2 × 1 C4N
promising way to extend the application of the C4N. In the present work, DB C4N-I configuration in which C atoms bonded all N atoms on the same side is studied. The adsorption of 20 different adatoms including period-2 (B, C, N, O, F), period-3 (Al, Si, P, S, Cl), and TM (Sc ~ Zn) atoms on the C4N monolayer is studied using first-principles density functional theory (DFT). The electromagnetic characteristics of the C4N monolayer including the adsorption energy, geometry, the density of states (DOS), the total magnetic moment, the Bader charge transfer, and work function are investigated. 2. Computational details In the present work, first-principles calculations were performed using Vienna Ab-initio Simulation Package (VASP) [24]. To describe the exchange and correlation, the Perdew-Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA) [25] was employed. The DFT-D2 method of Grimme [26] was used to consider the van der Waals interaction between the adatom and the C4N monolayer. The 2
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Fig. 3. Average planar potential in the z direction for pristine C4N (a) and functionalized by adatoms: (b) C, (c) S, and (d) Mn.
Fig. 4. The band structure of the pristine C4N (a) and functionalized by Period-2 adatoms: (b) N, (c) O.
supercell with 40 atoms containing 32 C and 8 N atoms. As shown in Fig. 1(b) and (c), the eight possible adsorption sites for the single adatom on the C4N monolayer can be considered: (i) above the center of hexagonal carbon rings of A side (H1), (ii) on top of the upper carbon atoms of A side (TC), (iii) above the center of hexagonal carbon nitrogen rings of A side (H2), (iv) above the bridge of hexagonal carbon rings of A side (BCC-1), (v) above the center of hexagonal carbon rings of B side (H3), (vi) on top of the nitrogen atoms of B side (TN), (vii) above the center of hexagonal carbon nitrogen rings of B side (H4), (viii) above the bridge of hexagonal carbon rings of B side (BCC-2). The adatom is expected to occupy one of the sites mentioned above. The adsorption energy for the adatom-C4N system can be defined as Eq. (1)
Table 1 The favorable adsorption site, the binding energies (Eads ), the bond length between the adatom and the closest surface atom (Dads ), the energy band gap (Eg ), the total magnetic moments ( μtot ), and the Bader charge transfers from the adatom to C4N for the hybrid system. Metallic and half-metallic structures are denoted as M and HM, respectively. Atom
Site
Eads (eV)
Dads (Å)
Eg (eV)
μ tot (μβ)
Δρ (e)
B C N O F Al Si P S Cl
H2 TC TC TC TC H2 H2 H2 H2 TC
−4.68 −4.13 −3.96 −6.67 −4.99 −3.46 −4.39 −3.98 −4.47 −3.74
1.60 1.40 1.28 1.23 1.36 2.13 2.01 1.91 1.87 1.75
0 HM 0.43 0.48 0.89 M HM HM 0.27 0.76
0.99 0.00 1.00 0.00 1.00 0.25 0.18 0.87 0.00 1.00
−1.17 +0.02 +0.83 +1.04 +0.66 −1.08 −1.12 −0.58 −0.05 +1.11
Eads = EC4 N+adatom − EC4 N − Eadatom
(1)
where EC4 N+adatom , EC4 N , and Eadatom are the total energies of the adatom-C4N system, the pure C4N monolayer, and the isolated adatom, respectively. To analyze the charge distribution, the grid-based Bader scheme is used [31]. The charge difference (Δρ ) is explained as Eq. (2) 3
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Fig. 5. The TDOS and PDOS of the C4N monolayer decorated with B, C, N, O, and F adatoms. The black solid line represents TDOS and red line represents the PDOS of functionalizing adatoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Differential charge density of the C4N monolayer functionalized by Period-2 adatoms: (a) B and (b) N. Color coding consists of pink for charge gain and purple for charge loss. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Δρads = ρC4 N+ adatom − ρC4 N − ρadatom
3. Results and discussion
(2)
here ρC4 N+ adatom , ρC4 N and ρadatom are the total charges of the adatomC4N system, the pure C4N monolayer, and the isolated atom, respectively.
3.1. Optimized structure and electronic properties of C4N The 2 × 2 × 1 supercell containing 32 C atoms and 8 N atoms is constructed to understand the electronic properties of the pristine C4N monolayer. The optimized structure of the C4N monolayer is shown in 4
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Fig. 7. The band structure of the C4N monolayer functionalized by Period-3 adatoms: (a) Al, (b) P, and (c) S.
Fig. 8. The TDOS and PDOS of the C4N monolayer decorated with Al, Si, P, S, and Cl adatoms. The black solid line represents TDOS and red line represents the PDOS of functionalizing adatoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.2. Electronic structures of the adsorbed the C4N monolayer
Fig. 1(a). In the C4N monolayer, carbon and nitrogen atoms are bonded together by sp3 hybridization, which is similar to the C atoms in diamond. The optimized lattice constant of the C4N model is 4.775 Å, which is in good agreement with previously reported results [8]. The bond lengths of CP-CP, CP-CR and N-CP are 1.491 Å, 1.567 Å and 1.558 Å, respectively. The band structure and corresponding projected density of states (PDOS) of pure C4N unit cell are shown in Fig. 2(a) and (b), respectively. Analogous to the band structure of graphene (silicene/ germanene/stanene) [32,33], zero band gap at the K point is found in Fig. 2(a), suggesting a Dirac cone at the K point. It can be seen that the conduction band edge and valence band edge are dominated by the p orbitals of C and N atoms.
20 different atoms including period-2 (B, C, N, O, F), period-3 (Al, Si, P, S, Cl) and transition metal (Sc ~ Zn) atoms are adsorbed on the eight possible adsorption sites of Dumbbell C4N (DB C4N), shown in Fig. 1(b) and (c). In order to understand the effect of surface atom adsorption on the electronic structures of the C4N monolayer, the adsorption energy, the density of states (DOS), the energy band gap, the charge transfer, and total magnetic moments of each adatom-C4N system are calculated. Adatom adsorption on the C4N monolayer is expected to alter the Fermi level of the system, therefore a large change in work function relative to pure C4N can be found from Fig. 3. The 5
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Fig. 9. Differential charge density of the C4N monolayer functionalized by Period-3 adatoms: (a) Si and (b) Cl. Color coding consists of pink for charge gain and purple for charge loss. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
predicted to be 5.77, 4.90 and 4.53 eV, respectively (see Fig. 4).
Table 2 The favorable adsorption site, the binding energies (Eads ), the bond length between the adatom and the closest surface atom (Dads ), the energy band gap (Eg ), the total magnetic moments ( μtot ), and the Bader charge transfers from the adatom to C4N for the hybrid system. Metallic structures is denoted as M. Atom
Site
Eads (eV)
Dads (Å)
Eg (eV)
μ tot (μβ)
Δρ (e)
Sc Ti V Cr Mn Fe Co Ni Cu Zn
BCC-1 BCC-1 H2 H2 H2 H2 H2 H2 H2 H2
−3.50 −3.44 −3.04 −2.64 −2.54 −1.23 −3.00 −2.91 −1.48 −0.63
2.24 2.22 2.18 2.16 2.15 2.04 2.03 1.95 1.99 2.05
0.05 0.15 0.09 0.17 0.31 M 0.09 0.11 M 0.07
0.94 2.07 3.17 4.13 5.00 2.00 1.12 0.08 0.05 0.00
−1.48 −1.12 −1.01 −0.80 −0.95 −0.60 −0.52 −0.39 −0.42 −0.67
3.2.1. Absorption of the Period-2 (B, C, N, O, F) nonmetallic atoms We studied the adsorption behavior of Period-2 (B, C, N, O, F) nonmetallic atoms adsorbed on the C4N monolayer. The period-2 nonmetallic atoms B, C, N, O and F are trend to be adsorbed at the different favorable site of the C4N monolayer. The atom B prefers to adsorb at the H2 site, while the others period-2 nonmetallic atoms C, N, O and F prefer to adsorb at the TC site. The adsorption energies are 4.68 eV for B, 4.13 eV for C, 3.96 eV for N, 6.67 eV for O, and 4.99 eV for F. In the cases of O-C4N system, it has the greatest adsorption energy. According to the band gaps presented in Table 1, the B-C4N system remains zero band gaps, while the N-, O- and F-C4N systems are 0.43 eV, 0.48 eV and 0.89 eV, respectively. Interestingly, in the case of C-C4N induces the half-metallic properties composed of a conducting spin-up channel and an insulating spin-down channel with an electronic band gap of 0.43 eV. In addition, the Fermi levels shift toward the valence band edge, indicating that N, O and F atom adsorptions can produce the p-type carrier in C4N the monolayer (as shown in Fig. 5). The adsorption of B, N and F produces magnetism for the Period-2 atoms adsorbed the C4N monolayer, while the C- and O-C4N system is nonmagnetic (see Table 1). During the period-2 atoms functionalization process, the C4N monolayer may act as electron donator and acceptor.
work function (Φ ) is defined as:Eq. (3)
Φ = E vac − EF
(3)
where E vac and EF are the potentials of the vacuum level and Fermi level energy, respectively. Our calculated work function of pure C4N is equal to 5.15 eV and functionalized C4N monolayer with C, S, and Mn are
Fig. 10. The band structure of the C4N monolayer functionalized by 3d-transition metal adatoms: (a) Mn, (b) Fe, and (c) Zn. 6
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Fig. 11. The TDOS and PDOS of the C4N monolayer decorated with Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn adatoms. The black solid line represents TDOS and red line represents the PDOS of functionalizing adatoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
In B-C4N system, the maximal charge gain −1.17 e is found, while the charge loss happened in the case of C-, N-, O- and F-C4N system. The charge density difference of B-, N-adsorbed C4N is shown in Fig. 6. The pink and purple zone represents the gain and loss of electron, respectively. The adsorption of B, N, and F atoms can induce the magnetism in the C4N monolayer because of the presence of unpaired electrons.
sites prefer to H2 site for Al, Si, P, S atoms, while Cl atom prefers to Tc site (see Table 1). Among the different systems considered here, Al-C4N system has the weakest adsorption energy (3.46 eV), whereas S-C4N system has the strongest adsorption energy (4.47 eV). The bond lengths between the adatoms and the closest surface atoms (Dads ) of the period3 adatoms are longer than those of the period-2 adatoms except N adatom, which may be associated with the atomic number and atomic radius. As shown in Table 1, Al-C4N system shows the metallic properties,
3.2.2. Absorption of the Period-3(Al, Si, P, S, Cl) atoms Compared to the period-2 adatoms (B, C, N, O, F), the adsorption 7
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Fig. 12. Differential charge density of the C4N monolayer functionalized by 3d-transition metal adatoms: (a) Fe and (b) Ni. Color coding of pink and purple illustrates the charge gain and charge loss, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
system yields a gap opening of 0.05, 0.15, 0.09, 0.17, 0.31, 0.09, 0.11 and 0.07 eV, respectively (as shown in Table 2). In addition, Sc, Ti, V, Cr, Mn, Fe, Co, Ni and Cu atoms adsorption can effectively induce magnetism in the C4N monolayer (as shown in Table 2). The maximum magnetic moment 5.00 μβ is found for Mn-C4N system. The magnetic moments of TM-adsorbed C4N monolayer are mainly from TM atoms, and the monolayer only contributes 0.0012μβ. The band structures of Mn-, Fe-, and Zn-C4N systems are shown in Fig. 10. It is clearly seen that the spin-up and spin-down band structure of Mn- and Fe-C4N system show asymmetry, indicating that these systems show magnetic. It is found that Zn-adsorbed C4N system is nonmagnetic, which is in agreement with Zn-adsorbed C3N/C2N/germanene [20,22,23]. According to Bader charge analysis in Table 2, 0.39–1.48 electrons are transferred from Sc ~ Zn to C4N, respectively. The charge density difference of Fe-, Ni-adsorbed C4N is shown in Fig. 12. The pink and purple zone represents the gain and loss of electron, respectively.
while S- and Cl-C4N system yield a gap opening of 0.27 and 0.76 eV, respectively. For Si-C4N system, it exhibits half-metallic properties because its Fermi energy level only crosses through spin-up energy band. In addition, in the case of P-C4N system, its Fermi energy only crosses through spin-down energy band, and spin-down channel with an electronic band gap of 0.28 eV. The band structures of Al-, P-, and S-adsorbed C4N are shown in Fig. 7. For Al- and P-C4N systems, the spin-up and spin-down band structure are asymmetry, that is to say, these two systems are magnetic, while S-C4N system shows nonmagnetic. Interestingly, we can also see from Fig. 8 that the Fermi level shifts toward the valence band edge, which indicates that Cl atom adsorption can produce a p-type carrier in the C4N monolayer. According to Bader charge analysis, the Al-, Si-, P- and S-C4N systems transfer 1.08, 1.12, 0.58 and 0.05 electrons to C4N, respectively. It should be noted that only in the case of Cl-C4N system, 1.11 electrons are transferred from C4N to Cl atom because of the strong electronegativity of Cl atom (as shown in Table 1). The charge density difference of Si-, Cl-adsorbed C4N is shown in Fig. 9. The pink and purple zone represents the gain and loss of electron, respectively.
4. Conclusion In summary, on the basis of the first-principles calculations, we have systematically investigated the various atoms absorbed on the C4N monolayer, which include period-2 (B, C, N, O, F), period-3 (Al, Si, P, S, Cl), and 3d-transition metals (Sc ~ Zn) atoms. Based on the adsorption energy, we found that foreign adatoms prefer to be adsorbed on the site near carbon atoms, and most atoms favor bonding on the H2 site of the C4N. Adatom-C4N system presents various electromagnetic properties. N, O, F, and Cl atom adsorption can effectively induce p-type carriers in the C4N monolayer. Interestingly, pure C4N behaves as a Dirac material, while semimetallic C4N can show semiconducting, half-metal, or metal behavior depending on the adatoms type. It should be noted that the half-metal nature of C-, Si-, P-decorated C4N has great potential for spintronic device applications. Except for C-, O-, S- and Zn-C4N system, the other adsorption system induce the magnetism in the C4N monolayer. The tunable of the electronic characterization and the magnetic properties of the C4N monolayer by adsorbing atoms can extend the 2D material applications in catalyst, solar cells, sensors, electronic and spintronic devices.
3.2.3. Absorption of the transition metal (Sc–Zn) adatoms We next studied the adsorption characteristics of ten elements of the 3d transition metal adatoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed on the C4N monolayer. The corresponding results are summarized in Table 2. Among the 3d transition metal adatoms considered here, only Sc and Ti are adsorbed on the BCC-1 site. However, the H2 site is the most favorable adsorption site for V-, Cr-, Mn-, Fe-, Co-, Ni-, Cu-, and Zn-C4N systems. The trend is similar with the TM-adsorbed two dimensional (2D) C3N monolayer [23].The adsorption energy of TMadsorbed C4N monolayer from Sc to Zn are −3.50, −3.44, −3.04, −2.64, −2.54, −1.23, −3.00, −2.91, −1.48 and −0.63 eV, respectively. The adsorption energy sustained increase from Sc to Mn, and then continuous decrease from Co to Zn, which is similar to 3d TM atoms adsorption on C2N monolayer [22]. The Sc-C4N system has the largest adsorption energy, whereas the Zn-C4N system has the weakest adsorption energy due to its large atomic number and atomic radius. Interestingly, these values are much larger than those for the adsorption of 3d TM single atoms on C3N [23]. To gain deeper insight into the interaction between the 3d TM atoms and the C4N monolayer, the TDOS and PDOS have been performed for the 3d TM-C4N systems and the calculated results were listed in Fig. 11. From TDOS and PDOS profiles of C4N interacting with TM foreign atoms (see Fig. 11), Fe- and Cu-C4N system is found to turn into metallic properties, while in the case of Sc-, Ti-, V-, Cr-, Mn-, Co-, Ni- and Zn-C4N
Conflicts of interest There are no conflicts to declare.
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Acknowledgements [15]
This work is supported by the National Science Foundation of China (Grant No. 11774294), Sichuan Science and Technology program (Grant No. 2017JY0056 and 2018HH0088).
[16]
[17]
Appendix A. Supplementary data
[18]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.physe.2019.113649.
[19]
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