Hydrogenated Graphene Nanoflakes: Semiconductor to Half-Metal

Feb 11, 2012 - Energetic stability of graphene nanoflakes and nanocones. Natalie Wohner , Pui Lam , Klaus Sattler. Carbon 2014 67, 721-735 ...
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Hydrogenated Graphene Nanoflakes: Semiconductor to Half-Metal Transition and Remarkable Large Magnetism Yungang Zhou,†,‡ Zhiguo Wang,† Ping Yang,‡ Xin Sun,‡ Xiaotao Zu,† and Fei Gao*,‡ †

Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States

‡

ABSTRACT: The electronic and magnetic properties of graphene nanoflakes (GNFs) can be tuned by patterned adsorption of hydrogen. Controlling the H coverage from bare GNFs to half hydrogenated and then to fully hydrogenated GNFs, the transformation of small-gap semiconductor → half-metal → wide-gap semiconductor occurs, accompanied by a magnetic → magnetic → nonmagnetic transfer and a nonmagnetic → magnetic → nonmagnetic transfer for triangular and hexagonal nanoflakes, respectively. The half hydrogenated GNFs, associated with strong spin polarization around the Fermi level, exhibit the unexpected large spin moment that is scaled squarely with the size of flakes. The induced spin magnetizations of these nanoflakes align parallel and lead to a substantially collective character, enabling the half-hydrogenated GNFs to be spin-filtering flakes. These hydrogenation-dependent behaviors are then used to realize an attractive approach to engineer the transport properties, which provides a new route to facilitate the design of tunable spin devices.

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have been demonstrated on the top of Ru(0001) surface.17 Recently, a simple and effective method for producing GNFs directly from graphite using sonication and cationic cetyltrimethylammonium bromide (CTAB) as a stabilizer has been proposed from which the flakes can be dispersed in common organic solvents such as dimethyl formamide (DMF).18 Theoretical calculations show that these nanoflakes are able to retain their plane structures up to 1500 K.19 All the shaped GNFs are predicted to have zero intrinsic spin except the zigzag-edged triangular GNF in which a linear-scaling net spin arises due to the topological frustration of π-bonds.15 Local density approximation calculations with spin-polarization show that the degenerated nonbonding states in triangular GNF split near the Fermi level and create a small energy gap.15 Hence, the electronic and magnetic properties of GNFs are totally different from those of graphene and GNRs, triggering intense studies on these novel GNF-based nanostructures. A topic of considerable interest enables us to tune the electronic or magnetic properties at nanoscale. Charge transfer and field-effect doping can be applied to manipulate charge carrier concentrations, but achieving nanoscale control remains a challenge. One of the alternative approaches is to induce hydrogen coverage on these novel nanostructures. By plasma treatment, two-dimensional (2D) fully hydrogenated graphene (graphane) has been experimentally fabricated.20 One-dimensional fully hydrogenated GNR (fH-GNR) realized by cutting the graphane, similar to the case of GNRs, or by hydrogenating

INTRODUCTION The progress in the fabrication of graphene-based lower dimensional structures has been realized by carving onedimensional (1D) ribbons.1 Interestingly, the electronic properties of graphene change in a nontrivial manner when it approaches lower dimensions that are completely different from graphene.2−8 Zigzag-edged ribbons inducing the localized edge states introduce two flat bands at the Fermi energy, which shows a metallic character, while armchair edged ribbons can be either metallic or semiconducting depending on their widths.9−12 Theoretical simulations of Z-shaped graphene nanoribbon (GNR) revealed that a quantum dot can be trapped at the junction, with complete confinement of electronic states.13 On the basis of the Z-shaped GNR, a quantum computation scheme was proposed, with qubits encoded in electron or hole spin states localized at zigzag edges.14 Further confinement toward graphene quantum dot turns out to be a challenge that triggers the intense studies of graphene nanoflake (GNF). In fact, understanding the properties of GNF is significant because the basic functional components of future electronics or spintronic devices will be at nanometer scale to uphold the trend of increased performance with miniaturization.15 Substantial efforts have been made to produce the novel nanoflakes experimentally. Using a microwave plasma enhanced chemical vapor deposition strategy, Shang et al. successfully synthesized the multilayer GNF films, which was proposed as a mean to create a new class of nanostructured electrodes for biosensing and energy storage/conversion applications.16 With scanning tunneling spectroscopy, hexagonal GNFs with well-defined zigzag edges © 2012 American Chemical Society

Received: January 5, 2012 Revised: February 10, 2012 Published: February 11, 2012 5531

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Figure 1. (a) Optimized geometric structures of H-GNF(tri=4) and H-GNF(hex=3). The small (red) and large (brown) balls represent H and C atoms, respectively. (b) The corresponding spatial spin density distribution (pspin‑up−pspin‑down) for H-GNF(tri=4) and H-GNF(hex=3). The yellow and blue isosurfaces correspond to the values of +0.006 e/Å3 and −0.006 e/Å3, respectively. (c) Schematic illustration of π resonating model for the local structure with and without H adsorption.

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COMPUTATIONAL METHODS The first principle periodic calculations based on density functional theory are performed using the Vienna ab initio simulation package (VASP). Within generalized gradient approximation (GGA), we considered Perdew−Burke−Ernzerhof (PBE) exchange and correlation functionals. The electronic wave functions were expanded using a plane-wave basis set with a cutoff energy of 520 eV. In our calculations, the triangular and hexagonal GNFs with hydrogen passivated zigzag edges were established in a supercell of 35 × 35 × 20 Å, which is sufficiently large to avoid the interactions between the adjacent flakes. For general convention, the size of GNFs was classified by the number (n) of zigzag-edged atoms along the same edge of flakes. C atoms in sublattice A and B are denoted as CA and CB, and the adsorbed H atom is defined as Had. The structures of bare triangular GNF, bare hexagonal GNF, single-H adsorbed triangular GNF, single-H adsorbed hexagonal GNF, half-hydrogenated triangular GNF, and half-hydrogenated hexagonal GNF are represented by GNF(tri), GNF(hex), HGNF(tri), H-GNF(hex), hH-GNF(tri), and hH-GNF(hex), respectively. For the optimizations of these structures, brillouin zone integration was performed using a 4 × 4 × 1 k-point grid, and for electronic structural optimization, a 6 × 6 × 1 k-point grid was used. All the calculations were carried out with spinpolarization, 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 using a pristine graphene layer, and the calculated binding length of 1.42 Å, binding angle of 120°, and 0 eV gap electronic structure in graphene are in good agreement with previous results.27−29

GNRs, similar to the formation process from graphene to graphane, also has been explored.21 Previous theoretical studies suggested that graphane and fH-GNR, with favorable formation energies and wide-band gaps, are nonmagnetic, obviously different from the cases of bare graphene and GNR, respectively.22,23 In particular, owing to the reversibility of hydrogenation, a flexibility to manipulate hydrogen coverage on graphene becomes a possibility.15 The magnetism or half metallicity can be achieved via selective hydrogenation in graphene and GNR-based nanostructures.24−26 However, the detailed mechanisms for these observed phenomena in hydrogenated GNFs are largely unknown. In this work, we performed ab initio density functional theory calculations to systematically explore the patterned adsorption of hydrogen on GNFs. The stability of these structures was evaluated by calculating the formation energies and binding energies. In contrast to a bare GNF, the hydrogenated GNFs have unique and very interesting properties, especially for the magnetic properties in half hydrogenated GNFs (hH-GNFs): (a) the induced total spin of hH-GNFs squarely scales with the size of flakes, remarkably larger than that of bare GNFs, triangle GNF (a linear-scaling net spin), and hexagonal GNF (no spin); (b) compared with the bare GNFs, the hH-GNFs with different shapes can be magnetized. Thus, the present method provides a greatly convenient technique to fabricate the C-based nanomagnet, which is independent of GNFs shape; (c) in hH-GNFs, the electronic structure exhibits a half-metal character, associated with the strong spin polarization around the Fermi level; (d) the induced spin magnetizations in hH-GNFs align parallel, i.e., ferromagnetic spin ordering, and exhibit a substantial collective character through long-range magnetic coupling. This is a crucial issue for the spin-circuit, which has often been overlooked in the previous studies of doping-induced local spin states in nanostructures. Hence, on the basis of these intriguing findings, we proposed a simple and effective approach to engineer the transport properties of electrons by controlling the coverage of hydrogenation on GNRs, which may provide a new path to explore spintronics at nanoscale.

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RESULTS AND DISCUSSION The honeycomb latticed GNFs with different shapes have been theoretically studied in detail.30−34 Total spin of these structures is formed by two interpenetrating sublattices, A and B, agreeing with the prediction of Lieb’s theorem.30 For self-consistency of the present study, we have considered a 5532

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series of calculations of a single-H on GNFs and found that the top site is the preferred site (Figure 1a) for both GNF(tri) with n = 4 (GNF(tri=4)) and GNF(hex) with n = 3 (GNF(hex=3)) structures, same as the adsorption of Had on graphene in earlier work.27 In GNF(tri=4), the adsorption energy difference between the two sublattice-sites is about 0.8 eV, in favor of the adsorption on sublattice B site, while in GNF(hex=3), the energy difference can be neglected. We hereafter mainly consider the adsorption of H on the sublattice B site in following calculations. For both cases, each Had atom is chemically bonded to the substrate CB atom in flakes with a binding length of 1.13 Å, leading to the out-of-plane corrugation of 0.3 Å. The bond angle of three underneath C−C−C is about 115°, between the values of 120° in pristine graphene and 109° in diamond, suggesting a change of some sp2-like orbital characters to the more covalent sp3-like characters. However, the induced distortion only occurs in the neighboring C atoms of the Had atom, understood from the optimal C−C bond lengths. The spin moments of GNF(tri=4) and GNF(hex=3) are found to be about 1.5 and 0 μB, respectively, agreeing with the previous work.15 After Had adsorption above the CB atom, it is of great interest to note that Had induces the magnetic transition on GNFs that the spin-down charges of substrate CB dispel and certain spin-up charges are detected around the Had atom, leading the spin moment of GNF(tri=4) and GNF(hex=3) to be about 2.0 and 0.5 μB, respectively (Figure 1b). As a comparison, if the H atom was positioned on a CA atom, the spin moments of GNF(tri=4) and GNF(hex=3) become about 1.0 and −0.5 μB, respectively. In order to qualitatively explain the transition of spin pattern and the resulting magnetic properties of these structures, a typical π resonating model was used (Figure 1c). In this model, the π electron system can be described as a combination of conventional alternated double bond structures (Figure 1c, top). The H atom is covalently bonded with the C atom, which breaks one of the CC double bonds and leads to the superfluity of unpaired electrons (Figure 1c, bottom). Owing to the bond switching, the unpaired electron of nearby C is not stable, further moving around Had and substrate CA atoms. Consequently, the spin-up charges of these atoms are reinforced. Thus, for the adsorption of each H atom on GNF, a spin moment of 0.5 μB can be induced due to the spin coupling of the unpaired electrons. To efficiently utilize such chemical doping with H atoms, the larger coverage of H on GNFs is of considerable interest due to its potential for enhanced magnetism. Here, we explore this by covering all the CB atoms of a GNF by Had atoms only, and the corresponding configurations of hH-GNF(tri) with n = 4 (hHGNF(tri=4)) and hH-GNF(hex) with n = 3 (hH-GNF(hex=3)) are shown in Figure 2. The energy difference between spin-polarized and spin-unpolarized states of hHGNF(tri=4) and hH-GNF(hex=3) is about 0.42 and 0.86 eV, respectively, in favor of the polarized state, evidencing the relatively large magnetic interaction energy. As reported in graphene and BN layers, 2D planar honeycomb structure due to the strong π binding between the nearby atoms can remain,8,35 whereas a puckering due to the weak π bonding is needed to maintain the stability of Si layer.36 Graphane presents an extreme where all CB and CA atoms are saturated by H atoms at the opposite sides, and thus no C−C π bonding exists, resulting in a relatively large C−C binding length.20 Consequently, each C atom forms a typical sp3 hybridization, leading to the out-of-plane corrugation of 0.45 Å.37 Thus, the

Figure 2. (a) Optimized geometric structures of hH-GNF(tri=4) and hH-GNF(hex=3). (b) The corresponding spatial spin density distribution for hH-GNF(tri=4) and hH-GNF(hex=3).

half saturation of C atoms in hH-GNF(tri=4) and hHGNF(hex=3) breaking the π-bonding is expected to distort the planar geometry, i.e., the surfaces are buckled about 0.31 Å for both cases (Figure 2a). The relaxed C−C bond lengths, ranging from 1.48 to 1.51 Å, are larger than those in graphene (1.42 Å)8 and smaller than those in the Si layer (2.26 Å)38 and graphane (1.52 Å).37 Numerous efforts have been devoted to investigate the electronic and magnetic properties of GNFs, which represents a key issue for their applications in spin memory, transistors, and solid-state qubits. Previous studies showed that the spin magnetizations of A and B sublattices in bare GNFs are aligned antiparallel, i.e., antiferromagnetic spin ordering, indicating a negative exchange interaction between the nearest neighboring C atoms.15,30 A GNF(tri) has the different number of C atoms in each sublattice, namely, NA ≠ NB, and thus satisfies the total spin moment M = (NA − NB)/2 = (n − 1)/2. However, other structures, such as a GNF(hex), lead to M = 0 due to NA = NB. As studied for a single-H adsorbed GNF above, each Had can break one of the CC double bonds, which induces 0.5 μB spin moment on GNF, and thus, the total spin for triangular and hexagonal hH-GNFs can be determined to be M = (5n + n2)/4 and M = 3n2/2, respectively. These results demonstrate a remarkable increase in spin polarization, and the total spin almost scales squarely with the size of flakes, significantly larger than that in bare flakes (Figure 3a). According to the rule developed here, it is expected that all shaped hH-GNFs can introduce a large spin. However, for the bare flakes with a triangular shape, only a very small spin can be induced. This finding offers a great convenience for carving out a portion of half-hydrogenated graphene (graphone) experimentally. In applications, such a C-based nanomagnet may be a component of the break-through technologies for making lightweight permanent magnets, serving as an excellent probe tip in magnetic atomic force microscopy, and utilizing in pharmaceutical systems in the living body, such as magnetic attraction drug delivery. For a given n, hH-GNF(tri=4) and hH5533

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sublattices, while the band gap in the latter is from the ionic potential difference between N and B atoms. In Figure 4, a split

Figure 4. Spin-polarized PDOS of GNF(tri=4), H-GNF(tri=4), hHGNF(tri=4), GNF(hex=3), H-GNF(hex=3), and hH-GNF(hex=3) structures. The red, black, magenta, and blue curves represent spin-up s-electrons, the spin-down s-electrons, the spin-up p-electrons, and the spin-down p-electrons, respectively.

of spin states in GNF(tri=4) which gives a gap of 0.4 eV, agreeing with previous calculated results.15 Far away from the Fermi level, the located energy of spin-up states is slightly lower than that of spin-down states, while they share a similar distribution in reciprocal space. Hence, the magnetic property of GNF(tri=4) is mainly contributed by the p-electrons near the Fermi level. It is well-known that the electronic property differences between the triangular and hexagonal structures are closely associated with their geometry structures. GNF(hex=3) with a gap of 1.1 eV is nonmagnetic, completely different from GNF(tri=4). Once a Had positions on the CB atom of GNF(tri=4), the spin-up charges of nearby CA atoms are reinforced, which causes the valence states just below the Fermi level to board and cross the Fermi level, showing a typical pdopant. In the H-GNF(hex=3) structure, a new spin-up state occurs at the Fermi level. Hence, the total spin moments for both the cases are increased by 0.5 μB. A typical half-metallic behavior is observed, in which one level in the spin-up channel crosses the Fermi level, and the corresponding level in the spindown channel does not. This special distribution of electronic states indicates an interesting transition of electronic character, i.e., semiconductor → half-metal. After half-hydrogen coverage, the charge alignment of antiferromagnetic spin ordering is completely transferred to that of ferromagnetic spin ordering, and the hH-GNFs are drastically converted to half-metals from the inspection of their spin-polarized partial density of states (PDOS). The relative stability of these hydrogenated nanoflakes is important for practical applications. Thus, we have investigated the formation energies of hH-GNF(tri) and hH-GNF(hex) configurations with various sizes. Here, the formation energy (Ef) is defined as Ef = ECH − χHμH − χCμC, where ECH is the cohesive energy per atom of an hydrogenated GNF, and χi (i = H or C) is the mole fraction of the i atom in the GNF. μH denotes the binding energy per atom of an H2 molecule, and μC is taken as the cohesive energy per atom of a single graphene layer. This approach has been widely employed to investigate

Figure 3. (a) Variation of spin moments, M, of GNF(tri), GNF(hex), hH-GNF(tri), and hH-GNF(hex) structures as a function of size (n). (b) The variation of formation energies, Ef, of hH-GNF(tri) and hHGNF(hex) structures as a function of size (n). (c) The variation of the total energies, Etotal, and the spin moment, M, with time from molecular dynamics simulations for hH-GNF(tri=4) and hH-GNF(hex=3) structures.

GNF(hex=3) configurations present a total spin of 9.0 and 13.5 μB, significantly larger than the values of 1.5 and 0 μB in GNF(tri=4) and GNF(hex=3) configurations, respectively. The corresponding spin charges are shown in Figure 2b. Clearly, the spin moment is mainly dominated by the electrons of CA atoms (90%), while the electron contribution of the Had above CB atoms is relatively small (10%). The spin magnetizations of these electrons align parallel, i.e., ferromagnetic spin ordering, and exhibit a substantial collective character by long-range magnetic coupling. For a doped system, one of the crucial issues for its applications in spintronic devices is whether the local spin moments induced by defect states can lead to a collective magnetism, which is an essential requirement for any spintronic application. However, this critical issue, as well as its implications, was often overlooked in the previous studies of doping-induced magnetism of nanostructures. Furthermore, the carbon-based materials such as nanotube or graphene have been reported to hold the promise of extremely long spin relaxation and decoherence time due to the very small spin− orbit and hyperfine coupling in carbon.39 Hence, this unusual property, together with the strong spin polarization of the collective character, renders the hH-GNF an excellent candidate for spintronic devices. Previous theoretical studies have reported that the energy gaps of GNRs originate from the staggered sublattice potentials because of the antiferromagnetic spin ordering.40 Hence, the clean GNFs studied here can be considered as a magnetic analogue of BN layers because the former has a small gap originating from the exchange potential difference in the two 5534

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the relative stability of some nanostructures based on GNRs and BN nanoribbons (BNNRs).41,42 Clearly, the formation energies of both hH-GNF(tri) and hH-GNF(hex) configurations increase monotonically with the GNF size, as shown in Figure 3b, implying that small GNFs are more likely to form than larger ones. The negative values of all the flakes indicate a stronger stability than that of the experimentally available graphene, suggesting a great possibility of fabricating these interesting nanostructures. However, it should be noted that it may be difficult to experimentally fabricate the hydrogenated GNFs by directly exposing the flakes to H2 since the sufficient dissociation is limited without having a transition metal (TM) or using a plasma implantation device to directly introduce H atoms. Alternatively, it is possible to incorporate TM (like Pt) to dissociate H2 and to potentially control the H concentration by gas pressure (H2 gas) or cut the available graphone layer.43 To further explore the stability of the hydrogenated flakes, the binding energies (Eb) of H atoms on GNFs have been defined as Eb = (EGNF + nEH − EhH‑GNF)/n, where EGNF, EhH‑GNF, and EH denote the energies of an isolated GNF, a hydrogenated GNF, and an H atom, respectively, and n represents the number of H atoms. The calculated binding energies were found to be about 1.7 and 2.4 eV for hHGNF(tri=4) and hH-GNF(hex=3) configurations, respectively. Obviously, the binding energies are large enough to prevent the dissociation of Had from the GNFs at the generally operational conditions. In order to confirm this result, ab initio molecular dynamics (MD) simulations are carried out with a time step of 1 fs at room temperature (T = 300 K). After 2000 steps, the geometries of the both configurations still remain, and the total energy and spin moment are almost constants (Figure 3c). In fact, the H−C binding energies in these configurations are much larger than the thermal energy at room temperature. Hence, the stable spin alignment renders hH-GNF structure an excellent candidate for the spintronic device. In view of the potential applications of these hydrogenated flakes, we have proposed simple models for spintronic circuit devices, as shown in Figure 5a, in which the hH-GNF structures with all the CB atoms covered by H atoms are sandwiched between two conducting Z-GNRs. Our spin density calculations show that the spin-up states with the same alignment, without including the antiparallel coupled spin of GNR edge states, are localized on the hydrogenated flakes, offering a excellent transmission channel for the spin-up electrons. Oppositely, if all the CA atoms of the hH-GNF structures are covered by H atoms, the spin-down states with the same alignment can be seen at the flakes, displaying a significantly different character that opens a transport channel only for spindown electrons and blocks the channel for spin-up electrons (Figure 5b). A flexible manipulation of hydrogen coverage on GNFs becomes possible due to the reversibility of hydrogenation.20 Similar to graphane,37 by fully hydrogenating GNFs (fHGNFs), their electronic and magnetic properties are transformed from magnetic half-metal to nonmagnetic semiconductor, and the corresponding energy gaps are found to be about 4.4 and 4.3 eV for fH-GNF(tri=4) and fHGNF(tri=3), respectively. A reason for this is that all the π bonds of C atoms (containing CA and CB atoms) are saturated by H atoms without unpaired electrons, which quenches the magnetism. Thus, in these cases, the transport of both spin electrons will be blocked (Figure 5c). This is in contrast to a bare conducting Z-GNR without hydrogenation where both the

Figure 5. Schematic illustration of (a) the hH-GNF structures with all CB atoms covered by H atoms, (b) the hH-GNF structures with all CA atoms covered by H atoms, and (c) the fH-GNF structures with all CA and CB atoms covered by H atoms, sandwiched between two conducting Z-GNRs, forming possible spin switch devices. The spatial spin density distributions of sandwiched GNFs are plotted.

spin-up and spin-down electrons can rapidly transport along the ribbon. On the basis of these findings, the transport properties can be engineered by controlling the hydrogenation coverage of GNRs, which may provide a new path to explore spin devices such as spin switch at the nanometer scale. Similar spin-filter and spin-valve have also been observed in C atomic chains and B-doped GNRs.44,45 Although the experimental evidence to support these models so far is limited, the essential conditions have been fulfilled: selective hydrogenation of graphene has been employed to adjust graphene’s electronic properties;20,46 using chemical,47,48 sonochemical,49 and lithographic2 methods, as well as through the unzipping of carbon nanotubes,50−53 the reliable production of 1D GNRs has been reported, and particularly a bottom-up approach was proposed recently for the atomically precise fabrication of GNRs in small width.4 Hence, further experimental studies are expected to fascinate these attractive and important findings.

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CONCLUSIONS The electronic and magnetic properties of hydrogenated GNFs have been investigated by ab initio density functional theory calculations. Half-hydrogenated GNFs exhibit a remarkably large spin moment that is scaled squarely with the size of the flakes, as well as strong spin polarization around the Fermi 5535

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level. The induced spin magnetizations in these nanoflakes align parallel and exhibit a substantial collective character by longrange magnetic coupling, which is a crucial issue for the spincircuit that is often overlooked in previous studies on dopinginduced magnetism of nanostructures. By further hydrogenation, fully hydrogenated GNFs, in which all the π bonds of C atoms in GNFs are saturated by H atoms and no unpaired electrons are left, become nonmagnetic semiconductors. Thus, the intriguing electronic and magnetic properties of GNFs can be effectively engineered by tuning hydrogenation on nanoflakes. On the basis of these important findings, a simple and effective approach is proposed to engineer the transport properties by controlling the coverage of hydrogenation on GNRs, which may lead to a new path of exploring spin devices, such as spin switch at nanometer scale.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported from the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under Contract DEAC05-76RL01830. Z.G.W. was financially supported by the Young Scientist Foundation of Sichuan (09ZQ026-029). A portion of this research was performed using the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research.

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