Tuning electronic and magnetic properties of

APPLIED PHYSICS LETTERS 95, 103108 共2009兲

Tuning electronic and magnetic properties of graphene by surface modification Jian Zhou,1 Miao Miao Wu,1 Xiao Zhou,1 and Qiang Sun1,2,a兲 1

Department of Advanced Materials and Nanotechnology, Peking University, Beijing 100871, People’s Republic of China 2 Center for Applied Physics and Technology, Peking University, Beijing 100871, People’s Republic of China

共Received 23 June 2009; accepted 16 August 2009; published online 9 September 2009兲 We have demonstrated that the electronic and magnetic properties of graphene sheet can be delicately tuned by surface modification. Applying an external electric field to a fully hydrogenated graphene sheet can unload hydrogen atoms on one side, while keeping the hydrogen atoms on the other side, thus forming a half-hydrogenated graphene sheet, where the unpaired electrons in the unsaturated C sites give rise to magnetic moments, coupled through extended p-p interactions. Furthermore, the electronic structure of the resulting half-hydrogenated graphene sheet can be further tuned by introducing F atoms on the other side, making a nonmagnetic semiconductor with a direct band gap. © 2009 American Institute of Physics. 关doi:10.1063/1.3225154兴 Graphene 共G兲 has been attracting tremendous interest and attentions in the past years since its synthesis in 2004.1 The carbon atoms in the sheet are sp2 hybridized and each has an unsaturated dangling bond, making it flexible to perform various surface modifications. Sofo et al.2 predicted theoretically a hydrogenated graphene called graphane with each carbon atom adsorbing one hydrogen atom, forming an sp3 hybridized carbon frame 共labeled as H-G-H兲. They showed that although a graphene sheet is metallic, graphane is a semiconductor with energy gap of 3.5 eV. The theoretical prediction has been confirmed in a recent experiment conducted by Elias et al.3 who synthesized the graphane sheet by exposing graphene in hydrogen plasma. Here a question arises: between the metallic graphene and the semiconducting graphane, can we further tune the properties? In this study, using first-principles calculation, we show that due to the flexible bonding features of carbon atoms there are many options for us to manipulate the surface modification for novel properties. We have found that the system can be changed from a nonmagnetic metallic sheet to a magnetic semiconductor with a small indirect band gap, and to a nonmagnetic semiconductor with a large direct band gap. Our first-principles calculations are based on spin polarized density functional theory with generalized gradient approximation 共GGA兲共Ref. 4兲 for exchange-correlation potential in the form of Becke–Lee–Yang–Parr.5 All the calculations are performed using DMOL3 共Ref. 6兲 package, and the periodic boundary condition is used to simulate infinite graphane sheet. The vacuum space of 15 Å is used in the direction normal to the graphane sheet in order to avoid interactions between two layers. For geometric relaxation, we use the unit cell consisting of two C atoms, as shown in Fig. 1共a兲, and for electronic structural calculations, fourfold unit cell consisting of eight C atoms is also used. In these two cases, the reciprocal space is presented by Monkhorst–Pack special k-point scheme7 with 11⫻ 11⫻ 2 and 6 ⫻ 6 ⫻ 2 grid meshes, respectively. We have used effective core potential with double-numerical plus polarization 共DNP兲 basis set, a兲

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which includes a polarization p-function on all hydrogen atoms. The atoms are relaxed without any symmetry constraints, the criteria of convergences of energy, force, and displacement are set as 1 ⫻ 10−5 Ha, 0.001 Ha/Å, and 0.005 Å, respectively. In DMOL3 package the static potentials arising from externally applied electric field can be included by adding one potential term −e⌽ to the Hamiltonian of system. The validity of this method has been verified by Delley8 who has studied the dissociation of molecules in strong electric fields and found that the bond length and vibrational frequency as a function of the field can be fitted very well to the Morse potential model which includes an additional external electric field term.

FIG. 1. 共Color online兲 Top view 共a兲 and side view 共b兲 of relaxed geometric structure of graphane. The dashed rhombus is for the unit cell. 共c兲 is for the band structure and PDOS. G = 共0 , 0 , 0兲, K = 共−1 / 3 , 2 / 3 , 0兲, and F = 共0 , 1 / 2 , 0兲 in the Brillouin zone. G represents ⌫ point and K is for Dirac point of graphene.

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FIG. 2. 共Color online兲 Schematics of charge polarization 共a兲, the variations of Mulliken charges 共b兲 and RC–H distance 共c兲 with e-field.

For the accuracy test, we first examine the geometric and electronic structures of graphane with no external e-field applied 共Fig. 1兲. C–C bond length and C–H bond length in our optimized structure are 1.532 and 1.106 Å, in good agreement with the results of Sofo et al.,2 namely, 1.52 and 1.11 Å, respectively. The band structure and the partial density of state 共PDOS兲 are given in Fig. 1共c兲, which shows a semiconducting feature with a band gap of ⬃4.0 eV, qualitatively in agreement with the previous report.2 Next we tune the geometry of graphane sheet using an external e-field. Before presenting the calculated results, we discuss the physics picture. In graphane, all the H sites are equivalent, interacting covalently with C sites via ␴ bonding. However, when an electric field is applied to this graphane sheet, due to the polarization of the electric field, the H atoms on the two sides 共labeled as H1 and H2兲 will not be equivalent again 关see Fig. 2共a兲兴. Because electrons are negatively charged, they move along the direction opposite to the applied electrical field. Therefore, more electrons will be ac-

Appl. Phys. Lett. 95, 103108 共2009兲

cumulated near to H1, making it negatively charged with the charge of QH1; while for H2, electrons will move away from it, making it positively charged with the charge of QH2. Because of the big difference in atomic size and in electronic configurations between C atom and H atom, the electrons in C atom are easier to get polarized as compared to H atom. Therefore we can expect that the absolute value of QH1 is larger than that of QH2. This nonequivalence in charges states between H1 and H2 will produce different forces at H1 and H2 in the applied electric field. Accordingly H1 will move faster than H2 does. We can expect that with the increase in applied electric field, H1 and H2 atoms will move away from the C frame, but H1 will be desorbed first, leaving H2 there and forming a half-hydrogenated graphene sheet. The above analyses based on basic physics are confirmed in our calculations. In Fig. 2共b兲 we show the charges of QH1, QH2, QC1, and QC2 changing with the applied electrical field, we can clearly see the differences of their behaviors in e-field. In Fig. 2共c兲 the dependences of the distances C1–H1 and C2–H2 on the applied electric field are also plotted. In e-field, the bond lengths between C and H atoms are elongated, especially the distance between C1–H1 increases much faster than that of C2–H2 under relatively strong e-field, in agreement with the physics picture discussed above. We found that when the magnitude of e-field increases to 0.189 a.u. 共1 a.u. = 5.14⫻ 1011 V / m兲, the optimized C1–H1 bond length became 1.54 Å, with Mulliken charges Q1 of ⫺0.678 a.u. However, when we increased e-field further, to 0.190 a.u., H1 is desorbed with the equilibrium distance more than 5 Å, while the bond length between H2 and C2 elongated to 1.6 Å. When the e-field is turned off, geometry reoptimization suggests that atom H2 is rebounded to C2 with the bond length of 1.1 Å. The above results show that applying an electric field to graphane sheet can produce a half-hydrogenated graphene 共labeled as G-H兲. Now the following questions arise: since a graphene sheet is metallic, while the hydrogenated structure is a semiconductor. Is the half-hydrogenated graphene metallic or semiconducting? Is it magnetic or nonmagnetic? To answer these questions, we calculated the band structure and PDOS of G-H system 关Fig. 3共a兲兴, which shows that the structure is an indirect band gap semiconductor with very small band gap. Detailed analysis revealed that ␣ electrons are at a relatively lower energy level than the ␤ electrons. The valence band is contributed by ␣ electrons. The valence band maximum is at K point, while the conductance band minimum is at G 共⌫兲 point. The band gap is found to be 0.43 eV. Furthermore, due to the unpaired electron in the unsaturated C site, magnetic moment appears, each unit carries 1␮B, mainly from C-2p orbital and ferromagnetically coupled. Detailed analysis indicates that the unsaturated C site carries the magnetic moment of 0.922␮B, which polarizes its neighboring C and H giving the moment of ⫺0.141 and +0.219␮B, respectively. Therefore the ferromagnetic coupling between the unsaturated C sites is mediated by the hydrogenated C atom between them via p-p interactions forming the configuration of 共¯ ↑ ↓ ↑ ¯兲. The magnetic state is further confirmed by comparison with the nonmagnetic state, the former is 0.15 eV lower in energy than the latter. As we know that the magnetic properties of graphene-based structures have currently attracted tremendous interest. It has been found that zero-dimensional graphene nanodots, one-dimensional nan-


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FIG. 3. 共Color online兲 Relaxed geometric structure and electronic structure of half-hydrogenated graphene 共a兲, half-fluorinated and half-hydrogenated graphene 共b兲, fully fluorinated graphene 共c兲. The arrows and numbers in 共a兲 are for magnetic moments.

oribbons, two-dimensional 共2D兲 nanoholes that consist of zigzag edges can all exhibit magnetism. Here we show that half-hydrogenation provides us a novel way to get a magnetic sheet while keeping the structural integrity. From above we have seen that a metallic graphene sheet can be fully hydrogenated when exposed to H plasma, forming a wide gap semiconducting graphane sheet, which can also become half-hydrogenated by using an electric field, making it magnetic semiconducting. To further tuning the properties, we consider using F atom for the surface modification in the half-hydrogenated graphene sheet. In fact, attentions have been already paid to fluorinated carbon nanostructures such as fullerenes 关C60F18,9,10 C60F36,11,12 C60F48,12–14 C60F60,15 and C58F18 共Ref. 16兲兴 and carbon nanotubes 共CNTs兲.17–19 For example, Kudin et al.18 showed that different structures of fluorine absorptions on CNT can modulate the conducting properties from metallic to semiconducting. In the following we discuss the properties of the partially and fully fluorinated graphene sheets. We calculated the structures of surface decoration of fluorine atoms on the half-hydrogenated graphene sheet 关Fig. 3共b兲兴. We have found that binding energy per F atom is 4.69 eV much stronger than C–H bond, and C–F bond length is 1.393 Å, longer than C–H bond 共1.106 Å兲. For this halfhydrogenated and half-fluorinated graphene sheet 共labeled as F-G-H兲, the band structure and PDOS suggest that it is a direct band gap semiconductor with no magnetic properties. The band gap is increased to 3.70 eV, but less than that of fully hydrogenated graphene sheet. Finally, when fully fluorinated 关labeled as F-G-F, see Fig. 3共c兲兴, the system is still a direct band gap semiconductor, with band gap of 3.48 eV, which is even less than that of F-G-H system. In conclusion, we have demonstrated that the graphene sheet provides us a flexible platform to manipulate the surface modification for tuning the electronic and magnetic properties. From G to H-G-H to G-H to F-G-H and to F-G-F, the properties can be tuned from metallic to semiconducting, from nonmagnetic to magnetic, or from direct gap to indirect

gap depending on the species and coverage of atoms used for the surface modifications. This work is partially supported by grants from the National Natural Science Foundation of China 共Contract Nos. NSFC-10744006 and NSFC-10874007兲. 1

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