Tunable electron and hole doping in FeCl3 intercalated grapheneJames Nathaniel and Xiao-Qian Wang Citation: Applied Physics Letters 100, 213112 (2012); doi: 10.1063/1.4722817 View online: http://dx.doi.org/10.1063/1.4722817 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Hydrogen intercalation of single and multiple layer graphene synthesized on Si-terminated SiC(0001) surface J. Appl. Phys. 116, 083502 (2014); 10.1063/1.4893750 Theory and synthesis of bilayer graphene intercalated with ICl and IBr for low power device applications J. Appl. Phys. 114, 063702 (2013); 10.1063/1.4817498 Electronic structures of an epitaxial graphene monolayer on SiC(0001) after metal intercalation (metal=Al, Ag,Au, Pt, and Pd): A first-principles study Appl. Phys. Lett. 100, 063115 (2012); 10.1063/1.3682303 Doping of graphene adsorbed on the a-SiO2 surface Appl. Phys. Lett. 99, 163108 (2011); 10.1063/1.3653261 Ultrafast carrier dynamics in pristine and FeCl 3 -intercalated bilayer graphene Appl. Phys. Lett. 97, 141910 (2010); 10.1063/1.3497644

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

155.97.178.73 On: Sun, 23 Nov 2014 13:30:32

Tunable electron and hole doping in FeCl3 intercalated graphene

James Nathaniel and Xiao-Qian Wanga)

Department of Physics and Center for Functional Nanoscale Materials, Clark Atlanta University, Atlanta,Georgia 30314, USA

(Received 21 March 2012; accepted 12 May 2012; published online 24 May 2012)

We have studied the electronic characteristics of FeCl3 intercalated bilayer graphene under a

perpendicularly applied electric bias. Evolution of the electronic structure of FeCl3 intercalated

bilayer graphene as a function of the applied electric bias is performed using first-principles

density-functional theory including interlayer van der Waals interactions. The calculation results

demonstrate that the hole-doped graphene layers associated with the high electronegativity of

FeCl3 transform into electron-doped layers tuned by the applied bias. The implications of

controllable electronic structure of intercalated graphene for future device applications are

discussed. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4722817]

Graphene, a monolayer of sp2-hybridized carbon net-

work, has attracted considerable attention in the fields of

optoelectronics, sensors, and hydrogen storage.1–3 Among a

variety of graphene-based materials, graphene intercalation

compounds (GICs) are formed by insertion of molecular

layers with various chemical species between graphite

layers.4,5 A host of potential applications using different

intercalants has been proposed. Weak van der Waals (vdW)

binding between graphene and intercalant layers makes

GICs versatile and allows for tunable electronic properties.

As the interlayer distance of graphite is dramatically

increased due to the presence of intercalants, the modified

layer distance strongly affects the electronic coupling

between graphene sheets.6 As a result, the electrical, thermal,

and magnetic properties can be tailored by simply choosing

from various intercalantes.4,7,8 Consequently, intercalation

of graphene-based compounds represents an efficient

approach to modify the properties of graphene.

In virtually all potential applications of graphene, an

in-depth understanding of the electronic characteristics is

pivotal to the integration of graphene into future nanoelec-

tronic devices.7–12 In this regard, interlayer interactions in

GIC play an crucial role in determining the electronic struc-

ture properties. Recent experimental advances have demon-

strated the preparation of ferric chloride (FeCl3) intercalated

few-layer graphene by two-zone vapor transport method.5,13

While the FeCl3 intercalation effectively decouples the adja-

cent graphene layers, the intrinsic hole-doping may lead to

graphene oxidization and constrain the electrical properties.

To explore the feasibility of a controlled doping, herein we

employ a dispersion-corrected density-functional theory

(DFT) that incorporates interlayer vdW interactions to inves-

tigate the corresponding electronic structure.19 The calcu-

lated dependence of the electronic structure on the applied

electric bias reveals that there is a notable change of the na-

ture of doping from hole doping to electron doping. As such,

the intercalation can be utilized for graphene-based devices.

Since DFT is deficient in treating long-range interac-

tions,14,15 it is necessary to take into account the dispersion

corrections associated with vdW forces. This is particularly

the case for FeCl3 intercalated graphene under the influence

of an electric field. Our first-principles calculations are based

on dispersion-corrected DFT with general gradient approxi-

mation (GGA) for exchange-correlation potential for which

Perdew, Burke, and Ernzerhof (PBE) exchange-correlation

functional16 was used as implemented in DMol3 package.17

We employed the dispersion correction with GGA using the

Tkatchenko-Scheffler (TS) scheme,14 which exploits the

relationship between polarizability and volume. The TS dis-

persion correction accounts for the relative variation in dis-

persion coefficients of differently bonded atoms by

weighting values taken from the high-quality first-principles

database with atomic volumes derived from partitioning of

the self-consistent electronic density.

To facilitate the calculation of an electric field, the Ham-

iltonian has been extended to include not only the potential

arising from the nuclear charges as external potential but

also the static potentials arising from an externally applied

electric field. The static potentials were constructed by inves-

tigating the dissociation of molecules in strong electric fields

and subsequently fitting the bond length and vibrational fre-

quency as a function of the field all the way up to the dissoci-

ation limit by analytical formula. In our calculations, a static

electric field was applied perpendicular to the graphene layer

plane,17 and electric bias was varied from 0 to 2.06 V/A.

FIG. 1. Crystal structure of two layers of graphene intercalated with FeCl3.

The graphene layers sandwich FeCl3. Cl and Fe atoms are represented with

light and dark grey, respectively.a)Electronic mail: xwang@cau.edu.

0003-6951/2012/100(21)/213112/3/$30.00 VC 2012 American Institute of Physics100, 213112-1

APPLIED PHYSICS LETTERS 100, 213112 (2012)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

155.97.178.73 On: Sun, 23 Nov 2014 13:30:32

Shown in Fig. 1 is the optimized structure of double layer

of graphene intercalated by FeCl3. A supercell with lattice

constants a ¼ b ¼ 12:12 A periodic in the xy plane was con-

structed, which amounts to combining 2� 2 unit cells of

FeCl3 and 5� 5 cells of graphene as suggested by previous

works.4,13 The stacking sequence of the bilayer graphene is

the AA type with the graphene atoms in different layers sit-

ting on top of each other.4 It is worth clarifying that the char-

acteristic AA stacked adjacent layers is promoted by FeCl3intercalation, in contrast to the Bernal (AB) stacking in graph-

ite.4 A vacuum of 10 A was used to avoid the interactions

between replicas. The unit cell has P�31m (D3d) symmetry.4,13

The double numerical plus polarization basis set

was employed, along with a kinetic energy change of

3� 10�4eV in the orbital basis. Appropriate Monkhorst-

Pack k-point grids of 4� 4� 1 for FeCl3 intercalated bilayer

graphene was sufficient to converge with the integration of

the charge density. The optimization of the atomic positions

proceeds until the change in energy is less than 5� 10�5eV

per cell.14,18 The optimized geometry yields an interlayer

distance of 9.405 A between graphene layers and 4.689 A

between FeCl3 and graphene layers. The calculated results

are in excellent agreement with experimental observations

based on an x-ray diffraction study of an interlayer distance

of 9.37 A between graphene layers and a distance of 4.69 A

from the FeCl3 and graphene layers.11,13 It is worth mention-

ing that although the local density approximation (LDA)

approach provides qualitatively correct pictures and remains

the popular choice for investigations of electric-field effects,

our calculations reveal that dispersion corrected GGA leads

to substantial improvements over the LDA results regarding

the layer distances. The dispersion-correction method,

coupled to suitable density functional, has been shown to

account for the long-range dispersion forces with remarkable

accuracy,18 particularly in rectifying the considerable weak

bonding in the GGA approach.19

We illustrate in Fig. 2 the evolution of band structures

of FeCl3 intercalated graphene with perpendicularly applied

electric fields. In the absence of an electric bias, the FeCl3layer serves as an acceptor.9,10,12 The FeCl3 layer sand-

wiched between two graphene layers accepts electrons from

both the layers, leading to hole-doped graphene. The upward

shift of the Dirac point of �1 eV is reminiscent of the heavy

hole doping. The extracted upshift of the Dirac point for pris-

tine FeCl3-intercalated graphene is in very good agreement

with previous theoretical calculation result,13 and is in good

conformity with experimental observations.5 As seen in Fig.

2, the FeCl3 intercalated layer displays flat bands in the elec-

tronic band structure. These dispersionless bands are con-

nected to the d-orbital of Fe, which can facilitate

photoexcited transition between energy levels with the assis-

tance of optical phonons.20

Upon the application of an electric bias, the band struc-

ture undergoes profound modifications. With the increase

of the electric field magnitude, the corresponding electronic

band structures display monotonic downward shifting of

the the Dirac point (the crossing at K). As readily observ-

able in Fig. 2, the Dirac point shifts to the Fermi level (at

0 eV) at an electric field of 1.03 V/A. Downward shifts of

the Dirac point to ��1 eV and ��2 eV are observed with

increasing the electric field to 1.54 and 2.06 V/A, respec-

tively. However, when the electric bias is further increased,

FIG. 2. Calculated band structures of two layers of graphene intercalated

with FeCl3: (a) no electric bias, (b) 1.03 V/A electric bias, (c) 1.54 V/A elec-

tric bias, and (d) 2.06 V/A electric bias, respectively.

FIG. 3. Extracted charge density distribution for near-gap

states at the band center of the conduction band maximum

(CBM) and valence band minimum (VBM) in the absence of

electric bias.

213112-2 J. Nathaniel and X.-Q. Wang Appl. Phys. Lett. 100, 213112 (2012)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

155.97.178.73 On: Sun, 23 Nov 2014 13:30:32

the Dirac point shifts back towards the Fermi level, along

with changes of the number of flat bands above the Fermi

level.

Closer scrutiny of the band structure characteristics

reveals that the shift of the Dirac point is attributed to the

charge transfer associated with flat bands near the Fermi

level. We show in Fig. 3 the extracted charge density of

near-gaps states, which correspond to flat bands in the band

structure. As seen in Fig. 3, the electronic charges are con-

fined at Fe or Cl sites. Specifically, the hybridization of

graphene-based dispersed bands and FeCl3-based flat bands

leads to charge transfer between graphene layers and FeCl3,

and between Fe and Cl.

We summarize in Table I the changes in the layer dis-

tances, along with the extracted Mulliken charges of various

components. As shown in Table I, the FeCl3 intercalants

accept electrons from graphene layers. As there is a symme-

try for the bilayer intercalated graphene system, each gra-

phene layer donates half of the charge as FeCl3 receives. It is

worth noting that with the application of an electric bias, the

charge transfer takes place not only between FeCl3 and gra-

phene but also between Fe and Cl. As a consequence, the flat

bands originated from the FeCl3 can shift and contract as the

electric bias increases in magnitude. This is in conformity

with the corresponding changes of the band structure in that

the ferric chloride flat bands upshifts higher as the graphe-

ne’s Dirac point shifts downwards. Overall, the electric bias

can be used to control the hole doping of the FeCl3 intercala-

tion. With the application of the electric bias, the hole-doped

graphene layers can turn into n-type doping ones. It is worth

noting that the grouping of flat bands with the increase of the

applied electric bias. The FeCl3 stretches to move closer to

the graphene layers, resulting an effective repelling of the

graphene sheets from each other. In the case of electric-field-

induced electron doping, the shifted Dirac point opens a

small gap ED ’ 0:1eV.11

In summary, we have investigated the evolution of band

structure of FeCl3-intercalated graphene layers with the

application of an electric field. Our results demonstrate that

the hole-doped pristine GIC can transform into electron-

doped with the electric bias. The tunable doping is of partic-

ular importance to the optical responses as the band filling

and electron-hole interactions lead to enhanced excitonic

effect on the optical absorption.21As the tailoring of doping

plays an important role in graphene-based electronics devi-

ces and optoelectronics, we hope the advocated tunable dop-

ing with electric bias can promote future experimental

studies in this direction.

This work was supported by the National Science Foun-

dation under Grant No. DMR-0934142 and the Air Force

Office of Scientific Research under Grant No. FA9550-10-1-

0254.

1K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.

Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 (2004).2C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J.

Hass, A. N. Marchenkov, E. H. Conrad, P. N. First, and W. A. de Heer,

Science 312, 1191 (2006).3Y. B. Zhang, Y. W. Tan, H. L. Stormer, and P. Kim, Nature (London) 438,

201 (2005).4M. S. Dresselhaus and G. Dresselhaus, Adv. Phys. 51, 1 (2002).5W. J. Zhao, P. H. Tan, J. Liu, and A. C. Ferrari, J. Am. Chem. Soc. 133,

5941 (2011).6T. Ohta, A. Bostwick, T. Sevller, K. Horn, and E. Rotenberg, Science 313,

951, (2006).7T. E. Weller, M. Ellerby, S. S. Saxena, R. P. Smith, and N. T. Skipper,

Nat. Phys. 1, 39 (2005).8N. Emery, C. Herold, M. d’Astuto, V. Garcia, C. Bellin, J. F. Mareche, P.

Lagrange, and G. Loupias, Phys. Rev. Lett. 95, 087003 (2005).9N. Jung, N. Kim, S. Jockusch, N. J. Turro, P. Kim, and L. Brus, Nano Lett.

9, 4133 (2009).10T. Abe, M. Inaba, Z. Ogumi, Y. Yokota, and Y. Mizutani, Phys. Rev. B

61, 11344 (2000).11J. W. Yang, G. Lee, J. S. Kim, and K. S. Kim, J. Phys. Chem. Lett. 2,

2577, (2011).12J. T. Sun, Y. H. Lu, W. Chen, Y. P. Feng, and A. T. S. Wee, Phys. Rev. B

81, 155403, (2010).13D. Zhan, L. Sun, Z. H. Ni, L. Liu, X. F. Fan, Y. Wang, T. Yu, Y. M. Lam,

W. Huang, and Z. X. Shen, Adv. Funct. Mater. 20, 3504 (2010).14A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. 102, 073005 (2009).15X. J. Liu, C. Z. Wang, Y. X. Yao, W. C. Lu, M. Hupalo, M. C. Tringides,

and K. M. Ho, Phys. Rev. B 83, 235411 (2011).16J. P. Perdew, K. Burke, and Y. Wang, Phys. Rev. B 54, 16533 (1996).17DMol3, Accelrys Software Inc., San Diego, CA, 2011.18D. K. Samarakoon and X.-Q. Wang, Appl. Phys. Lett. 100, 103107 (2012).19M. D. Williams, D. K. Samarakoon, D. W. Hess, and X.-Q. Wang, Nano-

scale 4, 2962 (2012).20X. Q. Zou, D. Zhan, X. F. Fan, D. Lee, S. K. Nair, L. Sun, Z. Ni, Z. Q.

Luo, L. Liu, T. Yu, Z. X. Shen, and E. E. M. Chia, Appl. Phys. Lett. 97,

141910 (2010).21L. Yang, Nano Lett. 11, 3844 (2011).

TABLE I. Calculated electric bias (�) induced change in distance between

graphene layer and FeCl3 (d0), along with that between graphene layers (d1),

respectively. The corresponding Mulliken charges in FeCl3 and Fe are also

listed.

� (V/A) d0 (A) d1 (A) FeCl3 (e) Fe (e)

0 4.689 9.405 �1.49 1.17

1.028 4.683 9.416 �1.81 1.19

1.543 4.682 9.423 �1.60 1.56

2.057 4.677 9.426 �2.61 0.30

213112-3 J. Nathaniel and X.-Q. Wang Appl. Phys. Lett. 100, 213112 (2012)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

155.97.178.73 On: Sun, 23 Nov 2014 13:30:32