Novel Electronic and Magnetic Properties of Graphene Nanoflakes in

Article pubs.acs.org/JPCC

Novel Electronic and Magnetic Properties of Graphene Nanoflakes in a Boron Nitride Layer Yungang Zhou,†,‡ Zhiguo Wang,*,†,‡ Ping Yang,‡ and Fei Gao*,‡ †

Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, Peopleʼs Republic of China ‡ Pacific Northwest National Laboratory, Post Office Box 999, Richland, Washington 99352, United States ABSTRACT: Novel electronic and magnetic properties of various-sized graphene nanoflakes (GNFs) embedded in a boron nitride (BN) layer were studied by ab initio methods. The feasibility of synthesizing hybrid GNF-BN structure, a desirable quantum dot structure, was explored. In this structure, photoexcited electrons and holes occupy the same spatial regionthe GNF regionwhich offers an effective way to generate a GNF-based light-emitting device and adjust its emitted optical properties by controlling the size and array of GNF in the BN layer. On the basis of the important magnetism properties of embedded GNF, we propose a specific configuration to obtain a large spin. Together with the high stability of spin alignment, the proposed configuration can be exploited for spintronic devices.

1. INTRODUCTION Simple graphene-based devices have become feasible, which demonstrates that the initial concept proposed for graphene can be manufactured.1 Graphene nanoflake (GNF), a zerodimensional dot, can be synthesized by cutting the graphene sheets.2,3 Understanding the GNF properties is scientifically important because the basic functional components of future electronic or spintronic devices will be fabricated at the nanometer scale to increase their performance with miniaturization.4 The quantum confinement in GNF enables its rich electronic properties.5−7 Some theoretical investigations have demonstrated that the magnetism of GNF, mainly from the localized states of edged C atoms, can be governed by its size and geometry.8 However, one disadvantage is that the isolated, zigzag-edged GNF (as reported in zigzag-edged graphene9) without chemical termination is relatively metastable. The edged reconstruction may take place at room temperature and suffer from irregular edges and mechanical delicacy, making it hard to fabricate and assemble. To overcome these difficulties, we have investigated the formation of GNF confined by fields of boron nitride (BN) sheet as a host material. In principle, GNF remains planar and can be connected with the same lattice sheet without compromising its mechanical integrity and having better electronic and magnetic properties, as shown in the present study. Over the past few years, research gains have been made in fabricating hybrid carbon−boron nitride (C-BN) nanostructures to explore new nanomaterials with novel properties.10−14 Hybrid C-BN nanotube, considered as a roller from hybrid CBN sheet, has been proposed theoretically15−17 and fabricated experimentally.18−20 The prime advantage of this configuration is that its band structure can be controlled by changing atomic © 2012 American Chemical Society

composition and ratio. For example, it can be converted from a nonmagnetic semiconductor to a spin-polarized metal and then to a nonmagnetic semiconductor by increasing the BN component ratio, creating a pathway for possible nanoelectronic devices.15 Sharing the same lattice structure with graphene and the BN layer, the hybrid graphene-BN layer is expected to have intriguing electronic structure. Recently, a uniform, continuous C-BN film with a large area was efficiently synthesized.21 The experimental work mainly concentrated on the synthesis of this film and investigated its stability. Although the bandgaps of several cases with different C concentrations, forming random C domains, have been measured, there is no systematic investigation, particularly associated with different configurations. Theoretical investigations will provide significant insights into electronic and magnetic properties of these new functional materials. GNF-based quanta have been predicted through selective hydrogenation or by applying gate voltages locally.22−25 We expect that different hybrid GNF-BN configurations will enable tailoring of physical properties on BN-based structures and magnify the possible applications of GNF-based quantum dots in BN layer. In this paper, we have systematically studied the electronic structures of triangular GNFs embedded in BN layers with different size. These triangular GNFs are very interesting because of three issues: (a) they are unstable as isolated GNFs but become stable when embedded in BN layers, (b) only triangular nanoflakes in BN layer can induce magnetic moment, Received: January 17, 2012 Revised: February 29, 2012 Published: March 7, 2012 7581

dx.doi.org/10.1021/jp300593q | J. Phys. Chem. C 2012, 116, 7581−7586

The Journal of Physical Chemistry C

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of C, B, N, and total atoms in the hybrid GNF-BN configuration, respectively, satisfying the relationship nC + nB + nN = n. Moreover, the binding energy per GNF embedded in BN layer is determined from Eb = (EGNF + EBN − EGNF‑BN), where EGNF, EBN, and EGNF‑BN denote the energies of isolated GNF, BN layer with corresponding nanopore, and hybrid GNF-BN configuration, respectively.

and (c) it is of great interest to note that photoexcited electrons and holes occupy the same spatial regionthe GNF region that offers an effective way to generate a GNF-based lightemitting device and to adjust its emitted optical properties by controlling the size and array of the GNFs in BN layer.

2. COMPUTATIONAL METHODS All calculations were performed via generalized gradient approximation (GGA) with the Vienna ab initio simulation package (VASP).26−28 The pseudopotentials are constructed from 3, 4, and 5 valence electrons for B, C, and N atoms, respectively. Electronic wave functions were expanded by use of a plane-wave basis set with a cutoff energy of 520 eV. A hybrid GNF-BN configuration consisting of 216 atoms (12 × 9 supercell) was used with a vacuum space of 15 Å to avoid interactions between the supercell and its images. Two hybrid styles based on the edged interaction between GNF and host BN are consideredone containing 3 C−B bonds and 3n C− N bonds (CON1) and the other consisting of 3 C−N bonds and 3n C−B bonds (CON2) at the heterointerface (here n is the number of carbon atoms at each nanoflake edge; refer to Figure 1). For each style, five configurations corresponding to

3. RESULTS AND DISCUSSION In all hybrid configurations (Figure 1), C, B, and N atoms are positioned in the same plane after optimization, where two configurations are considered, as noted by CON1 and CON2. Calculated C−N and C−B heterobinding lengths are about 1.42 and 1.54 Å, respectively, suggesting the interaction between C and N is stronger than that between C and B atoms.36 The inner C−C binding lengths in GNF are calculated to expand slightly, ranging from 0.01 to 0.02 Å (i.e., 0.7%− 1.4%), as compared to the perfect lattice structure of graphene. Hence, the embedded GNF remains planar and can be positioned in the BN host without compromising its mechanical integrity due to a small lattice mismatch. The relative stability of these hybrid nanostructures is important for practical applications. Thus, we investigated the formation energies (Ef) of the hybrid GNF-BN configurations with various GNF sizes (Figure 2a). Clearly, the formation energies of CON1 and CON2 are larger than the value of the pristine BN sheet and increase monotonically with the GNF size. A direct result from this comparison is that the GNF-BN configurations become less stable than a clean BN layer. However, the difference is quite small, which suggests the hybrid GNF-BN configuration is relatively stable, especially for CON1 configurations with small GNF size. Experimental realization of hybrid C-BN film has been demonstrated by the synthesis of uniform, continuous hybrid C-BN film,21 and theoretical investigation has proven C atoms doped in BN sheet have stable hexagonal configurations and can form onedimensional nanoribbons under suitable chemical potential conditions.13 To further explore the stability of the hybrid GNF-BN configuration, the binding energies (Eb) of CON1 and CON2 embedded in the BN layer have been determined (Figure 2b). Obviously, the binding energies, increasing from 20 to 100 eV in both styles, are large enough to prevent the GNF’s disassociation from the BN layer at room temperature. Of note, the binding energies in CON1 are somewhat larger than those of CON2 as determined by the different number of C−N and C−B bonds in these configurations. Notably, the lattice defects, such as B and N vacancies, in the BN layer have been recently characterized and compared to those observed in graphene.37 Specifically, a freestanding, single BN layer with various triangle-shaped holes was successfully created,38 which may also provide a simple way of synthesizing hybrid GNF-BN structure by adsorption or doping processes. To explore the feasibility of these processes, the mobility of C adatoms on the surface BN sheet needs to be determined. The possible migration paths of an adatom C on a BN layer from the initial B site (the most stable site for a C atom on a BN layer with the adsorption energy39 of 0.84 eV) to the nearestneighbor B site, B−(T2)−B, B−(T1)−B, B−T1−B, B−H−B, and B−T2−B, are investigated. The corresponding diffusion energy barriers (Er) were 0.7, 0.08, 0.24, 0.35, and 0.8 eV, respectively (Figure 2c), indicating the diffusion of adatom C occurs easily at typical growth temperatures.36 Hence, the good stability of the previously mentioned hybrid GNF-BN

Figure 1. Schematic view of two types (CON1 and CON2) of GNF embedded in BN layer with different GNF sizes. The solid lines show the 12 × 9 supercell for structural optimization. Green, gray, and brown spheres in these configurations represent B, N, and C atoms, respectively.

the C concentrations of 3% (C6), 6% (C13), 10% (C22), 15% (C33), and 21% (C46) are considered. For structural optimizations, Brillouin zone integration was performed with a 4 × 4 × 1 k-point grid. For electronic structural optimization, a 6 × 6 × 1 k-point grid was used. All calculations were carried out with spin polarization, and the atomic positions of the structure were relaxed until all the force components were smaller than 0.01 eV/Å. The accuracy of our calculations was carefully checked with a pristine graphene (BN) layer, and the calculated binding lengths of 1.42 (1.45) Å, bond angles of 120° (120°), and 0 eV gap (4.6 eV gap) electronic structure in graphene (BN layer) are in good agreement with previous results.29−35 To determine the relative stabilities of these hybrid GNF-BN configurations, the formation energy per atom is estimated as Ef = (EGNF‑BN − nCuC − nBuB − nNuN)/n, where E GNF‑BN is the total energy of the hybrid GNF-BN configuration; uC, uB, and uN are the chemical potentials of C, B, and N referenced to graphene, a-rhombohedral B, and gaseous N, respectively; and nC, nB, nN, and n are the numbers 7582



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Figure 2. (a) Formation energy, Ef, per atom in a 12 × 9 supercell for CON1 and CON2 with different GNF sizes, relative to the value of zero in a pristine BN layer. (b) Binding energy, Eb, of the GNF embedded in BN layer for CON1 and CON2 with different GNF sizes. (c) Energy barriers, Er, for adatom C to diffuse on BN layer along different paths, where the energy barriers are calculated relative to that of the bridge site.

Figure 3. (a) Spin-polarized DOS and charge densities of the CBM and VBM for hybrid C13-BN structure, where the Fermi level in DOS is indicated by a blue dashed line and the yellow isosurface in the local geometric structures corresponds to the value of +0.002 e/Å3. (b) Band structure of a quantum dot device for GNF-BN structure in CON1, where GapGNF+N and GapBN−N denote the band gaps of GNF plus C-nearby N atoms (GNF + N) and BN host minus C-nearby N atoms (BN − N), respectively. ΔEC and ΔEV represent the conduction-band and valence-band offsets, respectively. (c) Band gaps of GNF + N (GapGNF+N in CON1) and GNF + B (GapGNF+B in CON2) with different GNF sizes.

the configurations of C13 + N and BN − N can be viewed as semiconducting materials with band gaps of 3.7 and 4.6 eV, respectively. In contrast, in CON2, the band gaps of C13 +B (configuration of C13 plus C-nearby B atom) and BN − B (configuration of BN host minus C-nearby B atoms) are about 4.2 and 4.5 eV, respectively. The difference of the band gaps between CON1 and CON2 occurs from the different heterointerfaces, indicating a significant role of C, B, and N atoms at the interface on the electronic properties of hybrid C13-BN configurations. This is different from graphane ribbon, where edged H atoms have no contribution to either valence band maximum (VBM) or conduction band minimum (CBM),41 but in agreement with edged graphene nanoribbons (GNR) and BN nanoribbons (BNNR).42 Corresponding spatial charge distributions of the VBM and CBM are presented

configurations and the adatom C’s high mobility on the BN layer can be used for engineering GNF quantum dots in a BN layer with atomic precision. By use of a focused electron beam, as in the report on graphene,40 one is able to precisely displace B and N atoms and create a triangle-shaped hole in the BN. When the temperature is high enough, the adsorbed C atoms will migrate on the BN surface until they are trapped to the holes of the BN layer, which possibly may provide the opportunity for precise fabrication of GNF in a BN layer with various sizes. The spin-polarized density of states (DOS) of the GNF (e.g., C13 in CON1) and BN host are shown on the left in Figure 3a. Here, we denote the configuration of C13 plus C-nearby N atoms as C13 + N, and the configuration of BN host minus Cnearby N atoms as BN − N. It is of interest to note that both 7583



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(Figure 3a, right), which are consistent with the DOS analysis that both the VBM and CBM are contributed by C13 + N. It clearly indicates a construction of type I heterojunction43 in which quantum confinement can be achieved, offering an effective way to attain GNF-based quantum dot devices (Figure 3b). For C13 in CON1, the conduction-band offset (ΔEC) and valence-band offset (ΔEV) are about 0.3 and 0.6 eV, respectively. Photoexcited electrons and holes will occupy the same spatial region (GNF + N region). Hence, hybrid GNFBN configuration can be used as a desirable light-emitting device, where a maximized wave function overlap of electrons and holes yields a high radiative recombination rate. Figure 3c displays the dependence of band gap on GNF size (n) for GNF + N (in CON1) and GNF +B (in CON2). From n = 13 to 46, band gap in both configurations changes (from 3.7 to 3.0 eV for GNF + N and from 4.2 to 2.1 eV for GNF + B). The decrease of band gap with increasing size is akin to a typical band-gap behavior of quantum dot,8 consistent with observations of hybrid C-BN nanotubes.11 These results demonstrate that the emitted optical properties can be adjusted via controlling the size and array of GNF in the BN layer. For both fundamental and applied interests, magnetism at the nanoscale is an exciting research field. An isolated triangular GNF is found to display a linear-scaling net spin due to topological frustration of the π-bonds.4 The BN layer with triangular nanohole exhibits strong spin transport anisotropy around the Fermi level and can be used as electronic circuits.44 Here, we further explored the magnetic properties of triangular GNF-BN structures and found that the embedded GNF (for n ≥ 13) with zigzag edge can create intrinsic spin, agreeing with the finding in isolated GNF.4 The magnetic moments calculations in CON1 and CON2 show similar results for all of the cases considered. Thus, our main focus is on the analysis of the electronic structures of CON1 (as follows). For C13-BN configuration (using C13 in CON1 as an example), the energy difference between spin-polarized and spin-unpolarized states is about 0.3 eV. As such, the polarized state is more favorable. Due to the degradation of s states to magnetism, we present only the spin-polarized p states of C13 and host BN (Figure 4a, top). In C13, some spin-polarized states emerge at the regions of −2.0 to −3.0 eV and −14.0 to −14.5 eV. At lower energy regions (from −14.0 to −14.5 eV), the states caused by the subtle balance of electron transfers among π orbitals at edge atoms45 exhibit the peculiar sharp peak nature. With little dispersion of these sharp peaks, it is expected that the corresponding polarized electrons will localize at the edges. In the energy regions from −2.0 to −3.0 eV, the spin states are split, and the up- and down-spin states are attributed by two triangular interpenetrating sublattices in C13, respectively. The finding of bipartite lattices supports Lieb’s theorem prediction in terms of the spin of the exact ground state of the Hubbard model.46 Unpaired electrons in these regions are polarized, which induces the polarization of electrons in host BN due to the hybrid. These results are in agreement with a previous theoretical report that stated the flat polarized bands can be generated when the borders with zigzag shape are induced in a hexagonally bonded honeycomb sheet.45 Hence, the total magnetic moment of the C13-BN structure becomes about 1.0μB, larger than that of 0.5μB in an isolated triangular GNF.4 The energy-resolved densities associated with these spin states (energy intervals from −14.0 to −14.5 eV and −2.0 to −3.0 eV in C13 andfrom −14.0 to −14.5 eV and −2.0 to −3.0 eV in the BN host) are presented at the bottom in

Figure 4. Spin-polarized DOS and energy-resolved spin-polarized charge density (pspin‑up − pspin‑down) for (a) C13-BN configuration and (b) C46-BN configuration. The Fermi level in DOS is indicated by a blue dashed line, and the yellow and blue isosurfaces in the local geometric structures correspond to the values of +0.007 and −0.006 e/ Å3, respectively.

Figure 4a. The corresponding magnetic moments of these energy ranges are about 0.40, 0.10, 0.35, and 0.15 times μB, respectively. It can be concluded that the polarized electrons in the C13-BN structure mainly locate at the C and N atoms at the borders of GNF, whereas the contribution of the inner C atoms in C13 is relatively small due to antiferromagnetic alignment of the spin moments. However, almost no polarized electrons are detected in the B atoms at the heterointerface. On the basis of these results, it is anticipated that hybrid GNF-BN configurations with a larger GNF size (containing a large number of C and N atoms at the heterointerface) should display substantial magnetism. For n = 46, the total magnetic moment of hybrid GNF-BN configuration becomes about 4.0μB, as shown by the spin-polarized DOS and magnetic densities in Figure 4b. This configuration still is semiconducting with a band gap of 3.0 eV, indicating the high stability of spin alignment at room temperature. Finally, we propose a modified structure to obtain a large spin-based building block consisting of an array of spinpolarized triangular GNFs. Here, two principles need to be considered: (1) avoiding the coupled interaction between nearby GNFs, which leads to the reduction of net magnetic 7584



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Figure 5. (a) Isosurface of spin-polarized charge density (pspin‑up − pspin‑down) and (b) spin-polarized DOS in GNF-BN with a C coverage of 36%. The Fermi level in DOS is indicated by the blue dashed line, and the yellow and blue isosurfaces in the local geometric structures correspond to the values of +0.007 and −0.006 e/Å3, respectively. (c) Changes of total energy (Etotal), C−C binding length of embedded GNF (LC−C), and magnetic moment (M) with time from ab initio molecular dynamics simulation of this hybrid configuration.

GNF in the BN layer suggests a possibility of tunable light emission at certain wavelengths. In contrast to previous studies of an isolated GNF, we found that electrons of the bordered N atoms in CON1 (B atoms in CON2) are remarkably polarized due to hybridization, leading to a total magnetic moment nearly twice as large as that of the isolated GNF. The surprisingly large magnetic moment in GNF-BN, with 36% coverage and high stability of spin alignment, renders the GNF-BN structure an excellent candidate for spintronic devices.

moment, and (2) structural stability with increasing GNF size. Corresponding to the substituted concentration of 36% C in the BN layer, a desirable configuration in which the separated distance between the nearby GNFs is about 7.5 Å was consideredlarge enough to avoid the coupled interaction (Figure 5a). The formation energy of such an array (0.02 eV for each atom) is similar to that of the coverage in 21% C. Polarized electrons mainly located at the heterointerface are merely simple combinations of a single C46 embedded in BN structures. These features imply that the magnetic moment of GNF-BN configuration can increase to a coverage of 36%. Hence, the hybrid GNF-BN layer with a similar array of GNFs can result in a large net magnetic moment (16μB), which is technologically important for magnetic nanoscale material such as magnetic recording and permanent magnetism. As expected, this configuration still is a large gap semiconductor with a 2.0 eV gap (Figure 5b), which suggests the high stability of spin alignment and is consistent with the results on hightemperature magnetism of sp electrons.47 To further explore the stability of this configuration, ab initio molecular dynamics simulations are carried out for 1 ps at room temperature (T = 300 K). The MD simulations clearly indicate that the geometry and magnetic moment are almost constants, with small fluctuations in the total energy, C−C binding length in inner GNF, and the magnetic moment with time (Figure 5c). In fact, the C−C, C−B, C−N, and N−B binding energies in this configuration are much larger than the thermal energy. Hence, the remarkably large magnetic moments, along with its stability, render GNF-BN structure an excellent candidate for spintronic devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (F.G.) or [email protected] (Z.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the U.S. Department of Energy’s (DOE) Office of Science, Office of Basic Energy Sciences Materials Sciences and Engineering (MSE) Division and Pacific Northwest National Laboratory (PNNL). The authors also wish to thank the Molecular Science Computing Facility in the Environmental Molecular Sciences Laboratory at the Pacific Northwest National Laboratory for a grant of computer time.

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REFERENCES

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4. CONCLUSIONS In conclusion, hybrid GNF-BN structures with different sizes and arrays of GNFs in the BN layer have been investigated by first-principles calculations. These structures are energetically stable and can be used to achieve a pronounced quantum confinement effecta possible pathway for nanophotonic applications. The gradual decrease of band gap with the size of 7585



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Novel Electronic and Magnetic Properties of Graphene Nanoflakes in

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