Functionalized Single-Atom-Embedded Bilayer Graphene and

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Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Functionalized Single-Atom-Embedded Bilayer Graphene and Hexagonal Boron Nitride Keisuke Takahashi* and Lauren Takahashi Center for Materials research by Information Integration (CMI2), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

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S Supporting Information *

ABSTRACT: Single-atom-embedded bilayer graphene and two-dimensional hexagonal boron nitride are proposed in terms of first-principles calculations. In particular, a series of 68 different single atoms are embedded within bilayer graphene and boron nitride. It is revealed that the magnetic moment and bandgap behave differently depending on the atomic element used for doping where it becomes possible to form a magnet, conductor, semiconductor, or insulator. The electronic and geometrical properties of bilayer graphene and boron nitride are, in principle, able to be tailored and tuned, thereby expanding on how twodimensional materials are functionalized and designed. KEYWORDS: graphene, boron nitride, single atom, functionalized graphene, bandgap, magnetic moment 8 × 1 special k points of the Brillouin zone sampling is applied where periodic boundary conditions are applied in the x- and y-axes with 10 Å of vacuum conditions in the z-axis.16 The bandgaps of the designed two-dimensional materials are calculated using the exchange correlation of GLLB-sc for accurate estimations.17 Note that the positive and negative energies for bandgap represent metals and semimetals, respectively. The formation energy (Ef) of a single atom (SA) with layered two-dimensional materials (2D) is calculated using eq 1:

T

he design of two-dimensional materials has experienced active interest from the research community upon the initial discovery of graphene due to the unique properties that are induced by an ultrathin layer of atom alignment.1,2 The challenge faced by two-dimensional materials research lies in how to tailor and functionalize the two-dimensional material in order to achieve target materials properties.3,4 Several approaches have been taken in order to functionalize the two-dimensional materials. Doping is a commonly taken approach where the atom is substituted by other elements, resulting in the change of electronic properties and reactivities.5,6 Another extraordinary approach is to design the material by combining different two-dimensional materials to form a heterostructure two-dimensional material.7,8 Thus, it has been demonstrated that functionalizing the two-dimensional material is achievable in principle. Single-atom and two-dimensional materials are reported to be well coupled to each other.9,10 In particular, electronic structures as well as reactivity of two-dimensional materials can be functionalized by the introduction of a single atom.11−13 One can consider that the single atom can be a key component for tuning the properties of two-dimensional materials. Here, designing functionalized two-dimensional materials by placing a single atom between graphene and two-dimensional boron nitride is proposed. In particular, a series of single atoms are embedded between layers and the change of electronic structures are investigated in terms of first-principles calculations. The grid-based projector augmented wave (GPAW) method within density functional theory calculations is implemented with spin polarization calculations.14 Exchange correlation of vdW-DF with spin polarization calculations is used in order to consider the effect of van der Waals forces induced by graphene and two-dimensional hexagonal boron nitride.15 8 × © XXXX American Chemical Society

Ef = E[2D + SA] − E[2D] − E[SA]

(1)

Note that a negative binding energy indicates an exothermic reaction. The structural models of single-atom-embedded bilayer graphene and two-dimensional hexagonal boron nitride are designed. Bilayer graphene and boron nitride are first modeled where AA stacking is chosen. While both AB and AA stacking are observed experimentally and found to be controlled via exposure of ethylene gas, AA stacking is chosen as it has been reported that single atoms prefer the hollow sites of graphene.18−21 The binding energies of graphene and hexagonal boron nitride layers are calculated to be −0.30 and −0.21 eV, respectively. The distances between the graphene and hexagonal boron nitride layers are calculated to be 3.69 and 3.55 Å, respectively, which has good agreement with previously calculated distances via vdW-DF.22 A single atom is then placed at the hollow site between the graphene and two-dimensional boron nitride layers as shown Figure 1a,b Received: October 30, 2018 Accepted: December 26, 2018 Published: December 26, 2018 A

DOI: 10.1021/acsaelm.8b00036 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Electronic Materials where atomic numbers 1−42, 44−57, and 72−83 are considered.

energy, distance between layers, magnetic moment of the embedded atom, and bandgap are visualized in Figure 2. Details of the numerical values are listed in the Supporting Information. The formation energy is first evaluated when embedding single atoms between graphene layers. Figure 2a plots the formation energies (eV) against the atomic number of the embedded atom, where many instances form exothermically. In particular, the following 45 elements result in exothermic energy when embedded between layers of graphene: H, Li, Be, B, C, N, O, F, Na, S, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, As, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, In, Xe, Cs, Ba, La, Hf, Ta, W, Re, Os, Ir, Pt, and Pb where Na, K, and Cs doped graphene are also observed experimentally.23−25 Alternatively, the following 23 elements result in endothermic energy when embedded between layers of graphene: He, Ne, Mg, Al, Si, P, Ar, Zn, Ga, Ge, Se, Br, Kr, Ag, Cd, Sn, Sb, Te, I, Au, Hg, Tl, and Bi. These results reveal a trend where exothermic reactions tend to occur when the single atom is a transition metal and endothermic reactions tend to occur when the single atom is an alkali metal, alkaline earth metal, a poor metal, or a nonmetal (which fall in groups 1, 2, and 12−18). The distance between the layers of graphene is investigated next. Figure 2b plots the distance between layers against the atomic number of the embedded atom. Details of the distances can be seen in the Supporting Information. The distance between the layers in single Mn atom embedded bilayer graphene is reported to be 3.39 Å, which shows good agreement with the reported calculated distance of 3.39 Å.26 In general, the distance either slightly decreases from the graphene-only distance of 3.69 Å or increases the distances up to ∼4.0 Å. However, a large increase of distance happens when

Figure 1. Atomic models of single-atom-embedded bilayer graphene and boron nitride. The (a) side view and (b) top view of bilayer graphene. The (c) side view and (d) top view of two-dimensional hexagonal boron nitride. Atomic color code: C, gray; B, pink; N, blue; single atom, gold.

The effects of embedding a single atom between graphene layers as seen in Figure 1 are first investigated. Formation

Figure 2. Bilayer graphene doped with a single atom where the atomic number of the element used for doping is plotted against (a) formation energy (eV), (b) distance between layers (Å), (c) the magnetic moment of the single atom (μB), and (d) the bandgap of single-atom-embedded graphene layers (eV). Plot points where the graphene doped with an element is found to have an exothermic (negative) formation energy are colored in red. B


Letter

ACS Applied Electronic Materials

Figure 3. Bilayer boron nitride doped with a single atom where the atomic number of the element used for doping is plotted against (a) formation energy (eV), (b) distance between layers (Å), (c) the magnetic moment of the single atom (μB), and (d) the bandgap of single-atom-embedded graphene layers (eV). Plot points where the graphene doped with an element is found to have an exothermic (negative) formation energy are colored in red.

them candidates for applications requiring semiconductor properties. On the other hand, a bandgap of ∼0 is achieved with Mn, Re, and Rh, which have exothermic formation energy, thereby making them conductor candidates. Note that there are no cases where a bandgap is >3 eV, which is an insulator. Hence, it can be understood from Figure 2d that tuning the band structure of bilayer graphene is achievable in principle by changing the element of the inserted atom. Single-atom-embedded two-dimensional hexagonal boron nitride as shown in Figure 1b is also investigated. The corresponding formation energy, distance between the twodimensional boron nitride layers, magnetic moment of the embedded single atom, and bandgap are shown in Figure 3. Details of the numerical values are listed in the Supporting Information. Formation energies of single-atom-embedded layers of twodimensional hexagonal boron nitride are first investigated. Figure 3a shows the formation energy of the embedded-atom two-dimensional boron nitride layers against the atomic number of the embedded atom. As can be seen, roughly a little less than half of the elements have an exothermic formation energy. More specifically, the following 30 elements result in an exothermic formation energy when embedded between two-dimensional boron nitride layers: H, Li, C, N, F, Sc, Mn, Fe, Co, Ni, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ba, La, Hf, Re, Os, Ir, Pt, Au, Tl, Pb, and Bi. Alternatively, the following elements are found to result in endothermic formation energies: He, Be, B, O, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, As, Se, Br, Kr, Ag, Cd, In, Sn, Sb, Te, I, Xe, Cs, Ta, W, and Hg. There are 15 fewer atoms that induce an exothermic energy when embedded between layers of two-dimensional boron nitride than when compared to the cases seen with layers of graphene.

elements in groups 1, 17, and 18 are embedded. This suggests that elements that fall into groups 1, 17, and 18 would induce large increase in distance between bilayer graphene. The magnetic moment of the single atom between graphene layers is also evaluated. As can be seen in Figure 2c, many elements are found to have a magnetic moment of 0 μB. A closer look shows that the following 13 elements have a magnetic moment >0.1 μB when embedded between layers of graphene: V, Cr, Mn, Co, As, Br, Zr, Nb, Rh, Sb, I, Hf, and Ta. However, when compared to the formation energy shown in Figure 2a, Br, Sb, and I are found to induce formation energies that are endothermic. In particular, high magnetic moments of 2.25 μB and 1.94 μB are observed when V and Cr are embedded between graphene layers, respectively, where both V and Cr induce exothermic formation energy. In summary, it can be understood that there are a few high magnetic moment cases in bilayer graphene while most of the magnetic moments of atom-embedded bilayer graphene are ∼0 μB. The bandgap of single-atom-embedded graphene layers is similarly investigated. Figure 2d plots the bandgaps of the atom-embedded graphene layers against the atomic number of the embedded atom. As can be seen, atom-embedded graphene layers can exhibit either metal or semimetal states depending on the embedded element. The following eight single atoms induce a bandgap that falls within a semiconductor range between approximately 1 and 3 eV: Ne, S, Ar, K, Kr, Rb, Xe, and Cs. Na-doped graphene layers have an exothermic formation energy of −1.02 eV with a bandgap of 0.93 eV, which shows good agreement with experimental reports of experimentally observable Na-adsorbed graphene where the Na is found to increase the bandgap of the graphene.25 Additionally, exothermic formation energy is calculated for the cases of S, K, Rb, Xe, and Cs, making C


Letter

ACS Applied Electronic Materials

suggesting that the bandgap of the atom-embedded bilayer two-dimensional boron nitride is tunable in principle. In conclusion, single-atom-embedded bilayer graphene and two-dimensional hexagonal boron nitride are investigated in terms of first-principles calculations. In particular, 68 different single atoms are embedded between bilayers of graphene and two-dimensional boron nitride. Calculations reveal that single atoms of non-transition metals tend to have exothermic formation energies for both cases of bilayer graphene and twodimensional boron nitride. Additionally, the atomic element affects the degree of change of distance between layers with different trends that appear for the cases of graphene and the cases of two-dimensional boron nitride. Higher magnetic moments are also observed in bilayer two-dimensional boron nitride when compared to the graphene case. More importantly, bandgaps of both bilayer graphene and twodimensional boron nitride can be greatly modified by the introduction of a single atom where bilayer graphene can become a conductor or semiconductor while bilayer twodimensional boron nitride can behave as a conductor, semiconductor, or insulator depending on the chosen single atom. Thus, a series of calculations suggest that tuning the distance between layers, magnetic moments, and bandgaps of bilayer graphene and two-dimensional boron nitride can be achieved, in principle, by embedding the appropriate single atom.

Additionaly, similar to what was witnessed with the case of graphene layers, a trend appears where non-transition metals tend to form endothermic formation energies when embedded between two-dimensional boron nitride layers. Figure 3b shows the distances between layers of twodimensional hexagonal boron nitride against the atomic numbers of the embedded atoms. An initial glance at Figure 3b reveals that the distance between layers appear to gradually increase as the atomic number increases. As it can be also seen in graphene case, there are some spikes of peaks in layer distance in Figure 3b. In particular, an element from groups 1, 16, 17, and 18 tends to induce large layer distances, similar to the graphene case as shown in Figure 2b, although there is an exception with Mn. Hence, one can consider that groups around noble gases in periodic table can affect the layer distance. Magnetic moments of single-atom-embedded layers of twodimensional hexagonal boron nitride are then investigated. It is reported that doping 3d and 5d transition metal atoms into a vacancy of a single layer of two-dimensional hexagonal boron nitride induces a magnetic moment and that a magnetic moment is generally induced when single 3d and 5d transition metal atoms are placed between layers of bilayer boron nitride.27 As shown in Figure 3c, strong magnetic moments appear when elements are embedded between layers of twodimensional boron nitride. The following 31 elements are shown to exhibit magnetic moments greater than 0.1 μB: H, B, C, N, O, F, Al, P, Cl, Sc, V, Mn, Fe, Co, As, Br, Y, Zr, Nb, Mo, Ru, Rh, Sb, I, Ba, La, Hf, Ta, W, Re, and Os. Out of these elements, the following nine have endothermic formation energies: B, O, Al, P, V, As, Br, Sb, and I. Figure 3c also illustrates that embedding atoms between two-dimensional boron nitride layers is more likely to result in high magnetic moments compared to the graphene case. In particular, Mn, Y, Mo, Ta, W, and Re have high magnetic moments of 5.35 μB, 5.05 μB, 5.06 μB, 4.75 μB, 5.15 μB, and 5.16 μB, respectively, where Mn, Y, Mo, and, Re exhibit exothermic formation energy. These results show that embedding single atoms between layers of two-dimensional boron nitride can induce higher magnetic moments, leading toward good candidates for supporting magnetic atoms. There is an immediate distinction between the cases of graphene and two-dimensional boron nitride when observing the effect of the embedded atom on the bandgap. Here, as seen in Figure 3d, there are fewer instances of semimetals when compared to the cases involving graphene shown in Figure 2d. However, there are more insulators observed in the case of two-dimensional boron nitride than in the case of graphene. In particular, the following 18 single atoms result in a bandgap above 3 eV which is an insulator: He, Li, Ne, Na, S, Ar, K, Ni, Cu, Se, Kr, Rb, Pd, Ag, Xe, Cs, Hg, and Tl. Of these atoms, Li, Ni, Rb, Pd, and Tl have exothermic formation energies. This shows good agreement with previous studies that show that two-dimensional boron nitride forms insulators.28 In addition, an approximately zero bandgap can be induced when Mn, Fe, Co, Ru, Rh, W, and Os are introduced to bilayer twodimensional boron nitride. Lastly, bandgaps for semiconductors, which range between approximately 1 and 3 eV, are observed in cases of H, Cr, Zn, Ga, Br, Cd, In, Te, Pt, and Au where H, Pt, and Au have exothermic formation energies. These results show that single-atom-embedded bilayer boron nitride can take the form of a conductor, semiconductor, or insulator depending on the single atom chosen for embedding,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.8b00036.

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Details of the formation energy, distance between layers, the magnetic moment of the embedded atom, and the bandgap calculated for single-atom-embedded bilayer graphene and boron nitride; energy convergence test of bilayer graphene and boron nitride by the number of k points (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Keisuke Takahashi: 0000-0002-9328-1694 Lauren Takahashi: 0000-0001-9922-8889 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by Japan Science and Technology Agency (JST) CREST Grant JPMJCR17P2, JSPS KAKENHI Grant-inAid for Young Scientists (B) Grant JP17K14803, and Materials research by Information Integration (MI2I) Initiative project of the Support Program for Starting Up Innovation Hub from JST. Computational work is supported in part by the Hokkaido University Academic Cloud Information Initiative Center, Hokkaido University, Sapporo, Japan.

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Functionalized Single-Atom-Embedded Bilayer Graphene and

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