Semiconductor to Metal to Half-Metal Transition in Pt-Embedded

Subscriber access provided by George Mason University Libraries & VIVA (Virtual Library of Virginia)

Article

Semiconductor to Metal to Half-Metal Transition in Pt Embedded Zigzag Graphene Nanoribbons Xiaohui Hu, Neng Wan, Li-tao Sun, and Arkady V. Krasheninnikov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp502365m • Publication Date (Web): 01 Jul 2014 Downloaded from http://pubs.acs.org on July 10, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Semiconductor to Metal to Half-Metal Transition in Pt Embedded Zigzag Graphene Nanoribbons Xiaohui Hu1, Neng Wan 1, Litao Sun1* and Arkady V. Krasheninnikov2 1

SEU-FEI Nano-Pico Center, Key Lab of MEMS of Ministry of Education, Southeast

University, 210096 Nanjing, China 2

Department of Applied Physics, Aalto University, FI-00076 Aalto, Finland

*E-mail: [email protected]

ABSTRACT: The electronic and magnetic properties of Pt embedded zigzag graphene nanoribbons (Pt-ZGNRs) are investigated using density-functional theory calculations. It is found that Pt-ZGNRs exhibit a semiconductor–metal–half-metal transition as the position of Pt substitutional impurities in the ribbon changes from the center to edge sites. This behavior can be attributed to the interaction between Pt impurities and edge states of ZGNRs, which governs the electron occupation of the edge states. The transition always occurs independent of ribbon width. However, Pt impurity concentration is important for obtaining this transition. Our results demonstrate that Pt-ZGNRs can be used as versatile electronic devices.

Keywords: graphene, doping, transition-metal impurities, electronic structure calculations, half-metallicity

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. INTRODUCTION In spite of many fascinating properties of graphene,1,2 such as relativistic massless dispersion and high mobility, the gapless band structure of graphene creates many challenges in some applications of this material, e.g., in logical nanodevices. Thus, it is highly desirable to develop a technique to tune the electronic structure of graphene and make it semiconducting. Investigations have indicated that electronic gap in graphene systems can be opened by carving ribbons of a finite width from graphene sheets.3–5 The electronic structure of graphene nanoribbons (GNRs) and band gap value can be controlled by edge shape,6–8 as edge reconstructions lower the energy of the system,9,10 so that in principle one can obtain metals or semiconductors with the desired gap. More interestingly, zigzag GNRs (ZGNRs) are known to have localized edge states, which are antiferromagnetically coupled between the two edges.11–13 It is predicted that there are two most stable spin configurations, one with the spins at both edges being ferromagnetically (FM) coupled; another is that the spins are coupled antiferromagnetically (AFM). The AFM configuration is more stable than the FM one. It has been shown that the ferromagnetic ZGNRs are metallic, but the antiferromagnetic ZGNRs are semiconducting.6,14,15 The electronic properties of GNRs are sensitive to many factors, such as doping, edge modification, defect and deformation.16–19 Among all the possibilities, impurity doping is one of the most frequently adopted ways for tuning the properties of graphene. For instance, the effects of transition metal atoms embedded in single

2


Page 2 of 23

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and double vacancies on the electronic and magnetic properties of graphene have been theoretically studied. 20 Chan et al. have analyzed the structural and electronic properties of 12 different metal adatoms on graphene.21 Rigo et al. have studied the structural, magnetic and transport properties of a single Ni atom adsorbed on ZGNRs.22 Longo et al. have investigated the properties of Nin and Fen nanostructures on ZGNRs.23 A model was developed to describe how the energetics of the GNRs doped with a single impurity depends on the impurity position.24 Experimentally, metal atoms (Pt, Co, and In) have been successfully incorporated into graphene in substitutional positions using ion25 or electron26 irradiation-mediated techniques. The effects of transition metals on the electronic doping and scattering in graphene have been investigated using molecular-beam epitaxy combined with in situ transport measurements.27 Recently, Gan et al. observed Pt atom diffusion on graphene surface and found that Pt atom tend to move to the edge sites of graphene flakes.28 One can infer that the presence of Pt atom should also have a considerable effect on the properties of GNRs. Thus it is important to understand the atomic structure, energetics and electronic properties of Pt atoms on GNRs. Earlier theoretical studies29 provided many insights into the effects of the edges on the migration of gold atom on GNRs, and helped to interpret the experimental results. In this paper, we also use first-principles calculations to study how Pt atom impurities influence the electronic properties of the ZGNR. We demonstrate that upon introduction of Pt impurities, the electronic properties of the ZGNR are remarkably modified, and the ZGNR

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

exhibits a semiconductor–metal–half-metal transition as the position of Pt impurities changes from the center to edge sites. We further explain the transition through the interaction of Pt impurity and ribbon edge states. 2. METHOD AND MODEL Calculations were performed using the Vienna Ab Initio Simulation Package (VASP) code

30,31

based on density-functional theory (DFT). The exchange and

correlations were described by the Perdew–Burke–Ernzerhof (PBE) functional32 under the generalized gradient approximation (GGA). The ion-electron interactions were treated with the projector augmented wave (PAW) approximation.33,34 The plane wave cutoff energy was set to 500 eV, and a higher value had little effect on the results. The structures were relaxed until the energy and the force on each atom were less than 10−5 eV and 0.01 eV/Å, respectively. A 15 × 1 × 1 k-point mesh with Monkhorst–Pack scheme was used to sample the 1D Brillouin zone. The intervals between the ribbons were maintained at 15 Å for both layer–layer and edge–edge distances to simulate isolated ribbons. Following an established convention,17 ZGNRs were characterized by the number of zigzag chains (N) across the ribbon width. In our study, we considered the case of N = 8 ZGNR as the typical system. All edge C atoms were saturated by H atoms to avoid dangling bonds. Doping was modeled in the supercell made of the 4 unit cells by substituting a Pt atom for a C atom in the ZGNRs. Eight different substitution sites were considered from the center to the edge of the ribbon, as indicated by the numbers (1–8) in Figure 1. For each of the considered configurations, full relaxation

4


Page 4 of 23

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was performed and the optimized configurations were presented in Figure S1 in the Supporting Information. In configurations 1–7, the Pt impurity form three covalent bonds with the nearest C atoms, and C–Pt bond length is from 1.93 to 1.95 Å, in accordance with previous calculations.20 In configurations 7 and 8, Pt atom replaced the edge C atom, and C–Pt bond length is from 1.91 to 1.98 Å. As Pt atomic radius is larger than that of the C atom, the Pt atom is displaced outwards from the ZGNR plane by a distance 1.40 – 1.94 Å.

Figure 1. Atomic structure of ZGNR with substitutional Pt impurities used in the simulations. The numbers (1–8) denote different substitution sites. The white and gray balls represent H and C atoms, respectively. 3. RESULTS AND DISCUSSION A. Electronic and Magnetic Properties of Pt-ZGNRs with Pt Impurity at Various Sites. To get insight into the relative stability of Pt-ZGNRs, the binding energies of Pt atom at different sites were calculated. The binding energies Eb, defined as the difference between the energy of the Pt atom in the substitutional position of ZGNRs and the energy of the reconstructed single vacancy plus the 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

energy of the isolated Pt atom, are presented in Figure 2. The typical values of Eb vary from -8.3 to -7.2 eV, which suggests the high stability of the substitutional Pt atom in ZGNR. It means that Pt atom binds strongly to single vacancy, as reported previously for graphene.20 It is evident that the edge configuration (site 8) is energetically more favorable than the other sites. This indicates that the edge (site 8) of ZGNR is the most favorable site for Pt impurity.

Figure 2. Binding energies of ZGNRs with the Pt atom at different substitution sites. In order to explore the modifications of the electronic and magnetic properties induced upon introduction of Pt impurities, we performed spin-polarized total energy calculations for the Pt-ZGNRs with the following three magnetic phases: (i) nonmagnetic state, denoted by NM; (ii) the antiferromagnetic state, with the ferromagnetically ordered spins at each edge, but with opposite directions of the spin localized on different edges, denoted by AFM; and (iii) the ferromagnetic state, ferromagnetically ordered spins at each ribbon edge and the same spin directions between two edges, denoted by FM. The FM and AFM states were obtained by the 6


Page 6 of 23

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electronic structure optimization starting with high initial values of the spins on the edge atoms. The relative energies of NM, AFM, and FM states for the Pt-ZGNRs are shown in Table 1. It is found that when the Pt atom is embedded at the ZGNR inner (sites 1–5), we find that the AFM state is more stable compared to their corresponding FM state. On the contrary, for the edge configurations (sites 6–8), the coupling between the edge and the impurity leads to the same final state. Table 1. The total energies of NM, AFM, and FM states of Pt-ZGNRs, along with the total magnetic moment of the lowest energy state for each configuration. The NM state energy of the Pt atom at site 1 is set to zero. Site

1

2

3

4

5

6

7

8

ENM (eV)

0

-0.04

0.01

-0.15

-0.01

-0.64

-0.67

-2.69

EAFM (eV)

-0.25

-0.27

-0.24

-0.35

-0.23

-0.77

-0.81

-2.85

EFM (eV)

-0.23

-0.26

-0.22

-0.34

-0.21

-0.77

-0.81

-2.85

µtot (µB)

0

0

0

0

0

1.28

0.35

0.35

As evident from table 1, when Pt atom is embedded at the inner sites (1–5), the total magnetic moment of the system is the same as for the pristine ZGNR (0 µB). However, for the edge configurations (6–8), the system acquires a finite magnetic moment upon introduction of Pt impurity, which can be associated with the coupling between the edge and the impurity states. The effect of the Pt-edge coupling is evident from the spin densities in Figure 3. In fact, the spin polarization of the right 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

edge is locally suppressed by the presence of the Pt atom for the edge configurations (Figure 3(b) –3(d)). It is well known that the spin polarization in ZGNR originates from the edge states at the Fermi energy. The interaction between the Pt atom and the right edge C atoms destroys the edge states and suppresses the spin polarization, which destroys the symmetry of spin distributions, consequently inducing the magnetic moment. The phenomenon is analogous to the effect of edge defects and impurities on the magnetic structure of ZGNRs.35 The edge magnetism suppression was also observed in the Au and Ti atoms embedded ZGNRs.36,37

Figure 3. Spin density distribution of Pt-ZGNRs. (a) configuration 1, (b) configuration 6, (c) configuration 7, and (d) configuration 8. The isovalues for the blue and red isosurfaces are 0.005 and -0.005 e/Å3, respectively.

8


Page 8 of 23

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To probe the effect of Pt impurity position on the electronic properties of the ZGNR, we calculated the band structures for different configurations. The pristine 8-ZGNR has an AFM ground state and shows a band gap of 0.45 eV. Figure 4(a) shows the band structure of configuration 1. The system maintains its semiconducting character and has a band gap of 0.17 eV. It is well known that for the pristine ZGNR, the energy bands near the Fermi level are mainly associated with the edge states.6 The charge density analysis indicates that the lowest unoccupied spin-down state results from the edge C atoms, whereas the highest occupied spin-up state is mostly localized at the Pt and neighboring C atoms. Therefore, the occupied orbitals of the Pt atom and the neighboring C atoms have much higher energy than that of the edge C atoms, which contribute to the decrease of the band gap. The band structures of the system with impurities at other inner sites (2–5) are similar to that of configuration 1, which are presented in Figure S2 in the Supporting Information. All of them have semiconducting character, their band gaps are 0.13, 0.26, 0.16 and 0.26 eV for configurations 2–5, respectively. The configuration 6 exhibits a metallic character, and the charge densities are localized along the right edge C atoms. This corresponds to the occupation of the spin-up edge state, which is empty in the clean ZGNR and is promoted to across the Fermi level as a consequence of the interaction with the Pt atom. Conversely, the corresponding occupied spin-down edge state tends to become half occupied. In configurations 7 and 8, it can be seen from Figures 4(c) and (d) that the spin-up channel is semiconducting, while the spin-down channel is metallic with a band crossing the Fermi level. Thus, the system is half-metallic and able to provide

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

100% spin-polarized current. The charge densities originate from the Pt atom and the right edge C atoms. Different from configuration 6, the spin-up edge state is preserved at the conduction band, as well as the spin-down edge state across the Fermi level. Comparing the band structures for the different Pt sites, we can notice that the energy bands are much more affected when the separation between Pt impurity and the edge decreases.

Figure 4. The band structures of Pt-ZGNRs and the corresponding charge densities of the states near Fermi level at the X point for (a) configuration 1, (b) configuration 6, (c) configuration 7, and (d) configuration 8. The blue solid line and red dashed line denote spin-up and spin-down channels, respectively. The Fermi level is indicated by a horizontal olive dashed line. The isovalue is set to be 0.006 e/Å3.

10


Page 10 of 23

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Usually, GGA exchange and correlation functionals underestimate the band gap of semiconductors. To assess their effects on the electronic structure of the doped ribbons, we performed test calculations with the hybrid HSE06 functional,38–40 which could give more accurate results. Taking the configuration 8 as an example, the band structure was calculated using the HSE06 functional and presented in Figure S3 in the Supporting Information. The results show that its spin-up gap is increased from 0.13 eV to 0.17 eV, while the spin-down band preserved crossing the Fermi level. So the configuration 8 remains to be half-metallic, confirming our results. Since the HSE06 calculations are computationally expensive while the GGA correctly describes the trends of the band gap in Pt-ZGNRs, we employ the GGA approach in bulk of the calculations. As mentioned above, the near-gap states of the pristine ZGNR mainly originate from the edge states. Thus, we can understand the band structure transition from the edge effect. The projected density of states (PDOS) of the Pt atom and the right edge C atom is shown in Figure 5. For configuration 1, we did not observe interactions between the electronic states of Pt atom and the right edge C atom around the Fermi level, as indicated in Figure 5(a). The impurity Pt atom at the inner site hardly affects the edge states of the ZGNR, and the system preserves its semiconducting characteristic. The configuration 6 shows continuous PDOS around the Fermi level for both spin channels in Figure 5(b), responsible for the metallic character. For configuration 7, the environment of the Pt atom varies significantly, directly bonding to the right edge C atoms. Figure 5(c) shows that the hybridization between Pt atom and

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the right edge C atom happened within an energy interval between 0 and 0.2 eV above the Fermi level for spin-up channel. Whereas the PDOS is continuous for spin-down channel at the Fermi level, it is almost zero. This dominates the half-metallicity. In configuration 8, where Pt atom directly replaced a right edge C atom, the PDOS is similar to that of configuration 7, and it also shows half-metallicity. Thus the results indicate that the system exhibit semiconductor, metal, and half-metal transitions with respect to the position of Pt atom in the ZGNR.

Figure 5. PDOS of Pt atom embedded at different sites of the ZGNR for (a) configuration 1, (b) configuration 6, (c) configuration 7, and (d) configuration 8. The red and blue lines indicate the PDOS of Pt atom and the right edge C atom. The positive and negative values represent spin-up and spin-down channels, respectively. 12


Page 12 of 23

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B. Dependence of the Electronic Properties on Ribbon Width. We calculated the electronic structure of a series of Pt-ZGNRs with different widths and Pt-embedded sites, which include center sites (site 1), subedge sites (site 6), and edge sites (site 8), to test the dependence of the electronic properties on ribbon width, and the results are presented in Table 2. It is found that the system always exhibits the semiconducting character for the center site-embedded Pt-ZGNRs. This can be attributed to the interaction between Pt atom and the edge C atoms. When Pt impurity is embedded at the center site, the interaction between Pt atom and edge C atoms is weak, which does not influence the edge states around the Fermi level. So ZGNR preserves its semiconducting character. For the subedge site-embedded Pt-ZGNRs, all the systems display the metallic character independent of ribbon width. This indicates that the interaction between Pt atom and the edge C atoms does not change with ribbon width. With the increase in the ribbon width, all edge site-embedded Pt-ZGNRs display half-metallicity. Similar to the above cases, though ribbon width changes, the environment of Pt atom replacing the edge C atom does not vary much. This transition trend is found to be independent of the ribbon width, pointing out that the ribbon showing the site-induced transition from semiconductor to metal then to half-metal can be used as an electric switch in nanodevices.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

Table 2. The electronic character (EC) of the Pt-ZGNRs with different widths N. N(c), N(s), and N(e) denote the ribbon width of center site, subedge site and edge site embedded ZGNR. ∆ (eV) denote the band gap of Pt-ZGNRs. ∆α (eV) and

∆ β (eV) represent for the spin up gap and the spin down gap, respectively. (S, semiconductor; M, metal; HM, half-metal). N(c)

∆

EC

N(s)

∆

EC

N(e)

∆α

∆β

EC

6

0.18

S

6

0

M

6

0.13

0

HM

7

0.10

S

7

0

M

7

0.13

0

HM

8

0.17

S

8

0

M

8

0.14

0

HM

9

0.13

S

9

0

M

9

0.15

0

HM

10

0.21

S

10

0

M

10

0.16

0

HM

11

0.20

S

11

0

M

11

0.15

0

HM

12

0.22

S

12

0

M

12

0.16

0

HM

C. Dependence of the Electronic Properties on Pt Impurity Concentration. Furthermore, the effect of doping concentration has also been investigated by choosing different supercell sizes characterized by the number N of unit cells in the supercell. Here, we consider the case of N = 3, 4 and 5, denoted as Pt-3ZGNR, Pt-4ZGNR and Pt-5ZGNR, respectively. Taking the edge site-embedded Pt-ZGNRs as an example, we calculated their band structures, and the results are shown in Figure 6. It is found that doping concentration is important for the electronic properties

14


Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of Pt-ZGNRs. For a higher concentration of Pt impurity, Pt-3ZGNR presents a semiconductor character. For a lower concentration of Pt impurity, Pt-5ZGNR demonstrates a metal character. Only Pt-4ZGNR shows the half-metallicity. So the semiconductor to metal to half-metal transition in Pt-ZGNRs is sensitive to the Pt impurity concentration, an appropriate doping concentration would be the key factor to realize this transition in Pt-ZGNRs.

Figure 6. Band structures of Pt-ZGNRs with different doping concentrations. (a) Pt-3ZGNR, (b) Pt-4ZGNR and (c) Pt-5ZGNR. The blue solid line and red dashed line denote the spin-up and spin-down channels, respectively. The Fermi level is marked by a horizontal olive dashed line. D. The random distribution of Pt impurities in the edge of ZGNRs. As the incorporation of Pt impurities into the edges may be random in the experiments, we also studied the effects of random distribution of Pt impurities at the edge of ZGNRs on the electronic structure. Three different configurations of Pt impurities in the edge of ZGNRs are considered and presented in Figure 7. It is found that its spin-up gap is

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.11, 0.10, and 0.07 eV, respectively, while the spin-down band preserved crossing the Fermi level. So the system remains to be half-metallic, the random distribution of Pt impurities in the edge of ZGNRs does not change the half-metallicity.

Figure 7. Three different configurations of Pt impurities in the edge of ZGNRs and the corresponding band structures. The blue solid line and red dashed line denote the spin-up and spin-down channels, respectively. The Fermi level is marked by a horizontal olive dashed line. The blue/gray/white balls correspond to Pt/C/H atoms respectively.

16


Page 16 of 23

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. CONCLUSIONS Using density-functional theory calculations, we have investigated the electronic and magnetic properties of Pt-ZGNRs. It is found that Pt impurities will considerably modify

the

band

structure

of

the

ZGNR,

and

induce

a

semiconductor–metal–half-metal transition depending on the position of Pt impurity in the ribbon. This behavior can be attributed to the interaction between Pt impurity and the edge states of ZGNRs. This transition can always be present independent of the ribbon width. However, an appropriate doping concentration would be the key factor to realize this transition in Pt-ZGNRs. Our results demonstrate that Pt-ZGNRs can be used as versatile electronic devices. ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China 2011CB707601, the National Natural Science Foundation of China 61274114, 61370042, 61006011 and 113279028, and Natural Science Foundation of Jiangsu Province BK2012024. AVK acknowledges support from the Academy of Finland through project 263416. Computational resources were supported by National Supercomputing Center in Tianjin and CSC in Finland. Supporting Information Available Optimized configuration of ZGNRs with Pt atom at different substitution sites, the band structures of Pt-ZGNRs for configurations 2–5, band structures of the configuration 8 calculated by the GGA with PBE and HSE06 functional. This information is available free of charge via the Internet at http://pubs.acs.org.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

REFERENCES (1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183–191. (2) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109–162. (3) Son, Y. W.; Cohen, M. L.; Louie, S. G. Half-Metallic Graphene Nanoribbons. Nature 2006, 444, 347–349. (4) Jia, X. T.; Hofmann, M.; Meunier, V.; et al. Controlled Formation of Sharp Zigzag and Armchair Edges in Graphitic Nanoribbons. Science 2009, 323, 1701–1705. (5) Han, M. Y.; Özyilmaz, B.; Zhang, Y. B.; Kim, P. Energy Band-Gap Engineering of Graphene Nanoribbons. Phys. Rev. Lett. 2007, 98, 206805. (6) Son, Y. W.; Cohen, M. L.; Louie, S. G. Energy Gaps in Graphene Nanoribbons. Phys. Rev. Lett. 2006, 97, 216803. (7) Barone, V.; Hod, O.; Scuseria, G. E. Electronic Structure and Stability of Semiconducting Graphene Nanoribbons. Nano Lett. 2006, 6, 2748–2754. (8) Reich, S.; Maultzsch, J.; Thomsen, C.; Ordejon, P. Tight-Binding Description of Graphene. Phys. Rev. B 2002, 66, 035412. (9) Koskinen, P.; Malola, S.; Häkkinen, H. Self-Passivating Edge Reconstructions of Graphene. Phys. Rev. Lett. 2008, 101, 115502.

18


Page 18 of 23

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Wassmann, T.; Seitsonen, A. P.; Saitta, A. M.; Lazzeri, M.; Mauri, F. Structure, Stability, Edge States, and Aromaticity of Graphene Ribbons. Phys. Rev. Lett. 2008, 101, 096402 (11) Nakada, K.; Fujita, M. Edge state in graphene ribbons: Nanometer Size Effect and Edge Shape Dependence. Phys. Rev. B 1996, 54, 17954. (12) Fujita, M.; Wakabayashi, K.; Nakada, K.; Kusakabe, K. Peculiar Localized State at Zigzag Graphite Edge. J. Phys. Soc. Jpn. 1996, 65, 1920–1923. (13) Lee, H.; Son, Y. M.; Park, N.; Han, S.; Yu, J. Magnetic Ordering at the Edges of Graphitic Fragments: Magnetic Tail Interactions Between the Edge-Localized States. Phys. Rev. B 2005, 72, 174431. (14) Yang, L.; Park, C. H.; Son, Y. W.; Cohen, M. L.; Louie, S. G. Quasiparticle Energies and Band Gaps in Graphene Nanoribbons. Phys. Rev. Lett. 2007, 99, 186801. (15) Kim, W. Y.; Kim, K. S. Prediction of Very Large Values of Magnetoresistance in a Graphene Nanoribbon Device. Nat. Nanotechnol. 2008, 3, 408–412. (16) Martins, T. B.; Miwa, R. H.; da Silva, A. J. R.; Fazzio, A. Electronic and Transport Properties of Boron-Doped Graphene Nanoribbons. Phys. Rev. Lett. 2007, 98 196803. (17) Cervantes-Sodi, F.; Csányi, G.; Piscanec, S.; Ferrari, A. C. Edge-Functionalized and Substitutionally Doped Graphene Nanoribbons: Electronic and Spin Properties. Phys. Rev. B 2008, 77, 165427. (18)

Li,

Y.

F.;

Zhou,

Z.;

Shen,

P.

W.;

Chen,

19


Z.

F.

Spin

Gapless


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Semiconductor-Metal-Half-Metal Properties in Nitrogen-Doped Zigzag Graphene Nanoribbons. ACS Nano 2009, 3, 1952–1958. (19) Hu, X. H.; Sun, L. T.; Krasheninnikov, A. V. Tuning Electronic and Magnetic Properties of Zigzag Graphene Nanoribbons. Appl. Phys. Lett. 2012, 100, 263115. (20) Krasheninnikov, A. V.; Lehtinen, P. O.; Foster, A. S.; Pyykkö, P.; Nieminen, R. M. Embedding Transition-Metal Atoms in Graphene: Structure, Bonding, and Magnetism. Phys. Rev. Lett. 2009, 102, 126807. (21) Chan, K. T.; Neaton, J. B.; Cohen, M. L. First-Principles Study of Metal Adatom Adsorption on Graphene. Phys. Rev. B 2008, 77, 235430. (22) Rigo, V. A.; Martins, T. B.; da Silva A. J. R.; Fazzio, A.; Miwa, R. H. Electronic, Structural, and Transport Properties of Ni-Doped Graphene Nanoribbons. Phys. Rev. B 2009, 79, 075435. (23) Longo, R. C.; Carrete, J.; Ferrer, J.; Gallego, L. J. Structural, Magnetic, and Electronic Properties of Nin and Fen Nanostructures (n =1–4) Adsorbed on Zigzag Graphene Nanoribbons. Phys. Rev. B 2010, 81, 115418. (24) Power, S. R.; de Menezes, V. M.; Fagan, S. B.; Ferreira, M. S. Model of Impurity Segregation in Graphene Nanoribbons. Phys. Rev. B 2009, 80, 235424. (25) Wang, H.; Wang, Q.; Cheng, Y.; Li, K.; Yao, Y.; Zhang, Q.; Dong, C.; Wang, P.; Schwingenschlögl, U.; Yang, W.; et al. Doping Monolayer Graphene with Single Atom Substitutions. Nano Lett. 2012, 12, 141–144. (26) Rodríguez-Manzo, J. A.; Cretu, O.; Banhart, F. Trapping of Metal Atoms in

20


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vacancies of Carbon Nanotubes and Graphene. ACS Nano 2010, 4, 3422–3428. (27) Pi, K.; McCreary, K. M.; Bao, W.; Han, W.; Chiang, Y. F.; Li, Y.; Tsai, S.-W.; Lau, C. N.; Kawakami, R. K. Electronic Doping and Scattering by Transition Metals on Graphene. Phys. Rev. B 2009, 80, 075406. (28) Gan, Y.; Sun, L.; Banhart, F. One- and Two-Dimensional Diffusion of Metal Atoms in Graphene. Small 2008, 4, 587–591. (29) Zhang, W.; Sun, L. T.; Xu, Z. J.; Krasheninnikov, A. V.; Huai, P.; Zhu, Z. Y.; Banhart, F. Migration of Gold Atoms in Graphene Ribbons: Role of the Edges. Phys. Rev. B 2010, 81, 125425. (30) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15–50. (31) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (33) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979. (34) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758–1775. (35) Huang, B.; Liu, F.; Wu, J.; Gu, B. L.; Duan, W. H. Suppression of Spin Polarization in Graphene Nanoribbons by Edge Defects and Impurities. Phys. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Rev. B 2008, 77, 153411. (36) Hu, X. H.; Zhang, W.; Sun, L. T.; Krasheninnikov, A. V. Gold-Embedded Zigzag Graphene Nanoribbons as Spin Gapless Semiconductors. Phys. Rev. B 2012, 86 195418. (37) Yu, G. D.; Lü, X. L.; Zheng, Y. S.; Tian, W. J. Suppression of Edge Magnetism in a Titanium-Embedded Zigzag Graphene Nanoribbon. J. Appl. Phys. 2012, 111, 033707. (38) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207–8215. (39) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Erratum: “Hybrid Functionals Based on a Screened Coulomb Potential” [J. Chem. Phys.118, 8207 (2003)]. J. Chem. Phys. 2006, 124, 219906. (40) Krukau, A. V.; Vydrov, O. A.; Izmaylov, A. F.; Scuseria, G. E. Influence of the Exchange Screening Parameter on the Performance of Screened Hybrid Functional. J. Chem. Phys. 2006, 125, 224106.

22


Page 22 of 23

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents (TOC)

23


Semiconductor to Metal to Half-Metal Transition in Pt-Embedded

Recommend Documents