japplphys_110_086101

8/12/2019 JApplPhys_110_086101

1/4

Identification of graphene crystallographic orientation by atomic forcemicroscopyC. M. Almeida, V. Carozo, R. Prioli, and C. A. AcheteCitation: J. Appl. Phys. 110, 086101 (2011); doi: 10.1063/1.3642991View online: http://dx.doi.org/10.1063/1.3642991View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v110/i8Published by theAmerican Institute of Physics.Additional information on J Appl PhysJournal Homepage: http://jap.aip.org/Journal Information: http://jap.aip.org/about/about_the_journalTop downloads: http://jap.aip.org/features/most_downloaded

Information for Authors: http://jap.aip.org/authors

Downloaded 05 Jun 2013 to 160.36.192.221. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
http://jap.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=C.%20M.%20Almeida&possible1zone=author&alias=&displayid=AIP&ver=pdfcovhttp://jap.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=V.%20Carozo&possible1zone=author&alias=&displayid=AIP&ver=pdfcovhttp://jap.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=R.%20Prioli&possible1zone=author&alias=&displayid=AIP&ver=pdfcovhttp://jap.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=C.%20A.%20Achete&possible1zone=author&alias=&displayid=AIP&ver=pdfcovhttp://jap.aip.org/?ver=pdfcovhttp://link.aip.org/link/doi/10.1063/1.3642991?ver=pdfcovhttp://jap.aip.org/resource/1/JAPIAU/v110/i8?ver=pdfcovhttp://www.aip.org/?ver=pdfcovhttp://jap.aip.org/?ver=pdfcovhttp://jap.aip.org/about/about_the_journal?ver=pdfcovhttp://jap.aip.org/features/most_downloaded?ver=pdfcovhttp://jap.aip.org/authors?ver=pdfcovhttp://jap.aip.org/authors?ver=pdfcovhttp://jap.aip.org/features/most_downloaded?ver=pdfcovhttp://jap.aip.org/about/about_the_journal?ver=pdfcovhttp://jap.aip.org/?ver=pdfcovhttp://www.aip.org/?ver=pdfcovhttp://jap.aip.org/resource/1/JAPIAU/v110/i8?ver=pdfcovhttp://link.aip.org/link/doi/10.1063/1.3642991?ver=pdfcovhttp://jap.aip.org/?ver=pdfcovhttp://jap.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=C.%20A.%20Achete&possible1zone=author&alias=&displayid=AIP&ver=pdfcovhttp://jap.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=R.%20Prioli&possible1zone=author&alias=&displayid=AIP&ver=pdfcovhttp://jap.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=V.%20Carozo&possible1zone=author&alias=&displayid=AIP&ver=pdfcovhttp://jap.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=C.%20M.%20Almeida&possible1zone=author&alias=&displayid=AIP&ver=pdfcovhttp://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/274207555/x01/AIP-PT/Janis_Test_APL/sliceCV2pt_rsi_spm.jpg/6c527a6a7131454a5049734141754f37?xhttp://jap.aip.org/?ver=pdfcov

8/12/2019 JApplPhys_110_086101

2/4

Identification of graphene crystallographic orientation by atomicforce microscopy

C. M. Almeida,1,a) V. Carozo,1,2 R. Prioli,3 and C. A. Achete1,21Divisao de Metrologia de Materiais, Instituto Nacional de Metrologia, Normalizacao e Qualidade Industrial(INMETRO), Duque de Caxias, RJ, 25250-020, Brasil2Departamento de Engenharia Metalurgica e de Materiais, Universidade Federal do Rio de janeiro,Rio de Janeiro RJ, 21941-972, Brasil3Departamento de Fsica, Pontifcia Universidade Catolica do Rio de Janeiro, Marques de Sao Vicente 225,Rio de Janeiro, 22453-900, Rio de Janeiro, Brasil

(Received 10 August 2011; accepted 19 August 2011; published online 18 October 2011)

The direct determination of the crystallographic orientation of graphene sheets was performed using

lattice resolution atomic force microscopy images. A graphene sample, micromechanically exfoliated

onto a SiO2 substrate showing well defined crystal edges, was imaged in lateral force mode. The

lateral force images reveal the periodicity of the graphene hexagonal structure allowing the

visualization of the lattice symmetries and determination of the crystal orientation. Crystal edges

predominantly formed by zigzag or armchair directions were identified. The nature of the edges was

confirmed by Raman spectroscopy. VC 2011 American Institute of Physics. [doi:10.1063/1.3642991]

Since graphene, a bi-dimensional carbon lattice, wasfirst isolated using adhesive tape,1 its outstanding mechani-

cal2 and electronic3 properties have attracted a lot of atten-

tion. Applications have been foreseen but, in order to be

used in technological applications, development of graphene

growing techniques and methods for crystal orientation

needs to be achieved. The knowledge of graphene crystallo-

graphic orientation is particularly important due to the fact

that the energy states at the graphene edges aredependent on

the crystallographic orientation of its border.4 Either quasi-

metallic or semiconducting behavior, depending on the

atomic structure of the edges, may be obtained in graphene.5

Therefore, to explore the graphene properties in a device, it

is necessary to know the graphene crystalline orientation.

The identification of the crystallographic orientation of

graphene edges has been done by Raman spectroscopy. The

intensity of disorder-induced Raman feature (D band at

1350 cm1) in graphite6 and graphene7,8 was found to be

correlated to the atomic nature of the crystal edge. The ob-

servation of graphene edges has also been performed by

transmissionelectron microscopy9,10 and scanning tunneling

microscopy.1113

One of the capabilities of atomic force microscopy

(AFM) is the possibility to achieve high resolution images of

a crystal lattice in real-space. Atomic resolution images gen-

erated by AFM are produced in non-contact mode AFM, andare often carried out in ultra high vacuum conditions.14 How-

ever, high resolution images produced in contact mode are

also achieved in vaccum,15 ambient,16 or liquid conditions.17

The observation of the atomic-scale features of a surface was

first achieved by AFM operated in the lateral force mode.18

The lateral force images presented the stick and slip motion

of a tip sliding on the basal plane of a graphite surface at am-

bient conditions and low loads. Lateral force images with the

periodicity of the crystal atomic corrugation were also per-formed.19 Lattice-resolved frictional patterns of graphite

were obtained using carbon nanotube tips,20 resulting in high

resolution images of lateral forces acting between the tip and

the surface. Despite the fact that novel tips were used to

achieve high resolution AFM images, lattice resolution

images can also be achieved using conventional tips under

ambient conditions.

In this letter, the crystallographic orientation of gra-

phene sheets deposited onto an insulated substrate was iden-

tified. The sheet orientation was determined using lateral

force images in contact-mode atomic force microscopy. The

advantage of using AFM is the possibility to directly visual-

ize the graphene without any electrical contact or conductive

substrate and with minimal sample preparation. Moreover,

the identification of the crystallographic orientation of the

graphene edges was confirmed by Raman spectroscopy.

Graphene flakes were micromechanically exfoliated

onto an oxidized Si substrate with the use of an adhesive

tape.1 The substrate was thermally oxidized in a dry oxygen

atmosphere, creating a SiO2 film with thickness of 300 nm.

The number of graphene sheets was determined using optical

microscopy and Raman spectroscopy.

AFM measurements were performed in contact mode

under ambient conditions. All lateral force images were

acquired using a Si3N4V-shape cantilever with normal and tor-sional spring constants of 0.0756 0.001 N/m, and 68.562.6

N/m, respectively.21 A scanner with 1 lm2 maximum scan

range was used. The microscope gains were set close to zero

and the scanning was performed at 40 Hz using the constant

height mode. The sample was positioned in the microscope

stage so that the angle between the crystal edge and the scan-

ning direction was known.

Raman experiments were performed with a confocal

system equipped with a 532 nm wavelength laser, using the

backscattering configuration with a 600 lines/mm grating.

The lateral resolution was limited by a beam diameter

a)Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0021-8979/2011/110(8)/086101/3/$30.00 VC 2011 American Institute of Physics110, 086101-1

JOURNAL OF APPLIED PHYSICS 110, 086101 (2011)

http://dx.doi.org/10.1063/1.3642991http://dx.doi.org/10.1063/1.3642991http://dx.doi.org/10.1063/1.3642991http://dx.doi.org/10.1063/1.3642991http://dx.doi.org/10.1063/1.3642991http://dx.doi.org/10.1063/1.3642991

8/12/2019 JApplPhys_110_086101

3/4

of 360 nm using a 100x objective lens. The measurements

performed at graphene edges were done with a linear laser

polarization oriented parallel to the crystal edges.6,7

In Fig. 1, an optical image of a graphene sheet is pre-

sented. It shows that the graphene obtained by micromechan-

ical cleavage presents straight edges with tens of

micrometers in length. The angles observed between these

edges are multiples of 30

. Irregular edges with a fewmicrons in length are also observed as well as a dark region

with several graphene layers.

A lateral force image obtained at the graphene is shown

in Fig. 2(a). The periodic structure of the surface with its

threefold symmetry is observed. In the inset of the figure, the

Fast Fourier Transform (FFT) of Fig.2(a)is shown. The FFT

presents the symmetries observed in the image. Figure2(b)

presents the line profiles marked in Fig. 2(a). Intrapeak dis-

tances dI and dII of 0.246 0.02 nm and 0.386 0.02 nm are

measured, respectively, on the upper and lower profiles. The

parameters dI and dII corresponds to the distances between

graphene hexagons along the specific crystallographic direc-

tions of the sheet highlighted by the solid and dashed lines inFig.2(a).

Figure 3 shows a sketch of the monolayer of carbon

atoms 1.42 A apart in the hexagonal crystal lattice forming a

graphene. The solid and dashed lines represent specific crys-

tallographic directions known as zigzag and armchair. The

distances between the center of consecutives hexagons are

dZZ 0.246 nm and dAC 0.426 nm for the zigzag and arm-

chair directions, respectively.22

The optical image of the graphene presented in Fig. 1

can be seen in Fig. 4. In the inset of the Fig. 4, an AFM

image of the sheet oriented as performed in respect to the

graphene edge is shown. From the AFM image it is possible

to identify the crystalline orientations of the sheet. The

marked border I is preferentially oriented along the zigzag

direction, while border II is preferentially oriented along the

armchair direction.

In Fig. 5, Raman spectra taken from edges I and II are

shown. A difference between the relative intensity of the G-

peak (IG) and D-peak (ID) in both edges was observed. While

edge I presents an ID/IG ratio of 0.15, edge II presents ID/IGequal to 0.25.

The exfoliation process leads to cleavage and fracture of

the graphite crystal. Figure1indicates that during the cleavageprocess, fracture of the graphene occurs preferentially along

specific crystallographic directions. The number of atomic

bonds per unit length is about 0.41 along the zigzag and 0.47

along the armchair directions. The cleavage along any other

direction would need a higher number of breaking bonds per

unit length and is less likely to occur.

The determination of the crystallographic orientation of

the graphene was performed with lateral force microscopy

images. Figure 2 shows that the AFM tip moves on top of

the graphene following a stick-slip friction pattern. The pro-

files taken along specific directions [Fig. 2(b)] reveal the pe-

riodicity of the crystal lattice. The periodicity of the lattice is

confirmed by the FFT. The distances dI 0.24 nm and

FIG. 1. (Color online) Optical image of a graphene flake deposited via micro-

mechanical exfoliation onto a SiO2 substrate. The edges of the sheet follow

straight lines that present an angular spread of multiples of 30. The inset

shows a topography atomic force microscopy (AFM) image with the height

profile of the edge highlighted with a square (z range from 0 to 1.96 nm). The

edge presents a profile of approximately 0.660.1 nm.

FIG. 2. (Color online) (a) Lattice resolution lateral force image of the hex-

agonal arrangement of the graphene surface acquired in contact mode. (b)

The FFT spectrum showing the lattice symmetries is shown in the inset. Theline profiles highlighted in Fig. 2(a)are shown. The profiles I and II present

intrapeak distancesdI 0.2426 0.02 nm anddII 0.3766 0.02 nm.

086101-2 Almeidaet al. J. Appl. Phys.110, 086101 (2011)


8/12/2019 JApplPhys_110_086101

4/4

dII 0.38 nm correspond to the distances between graphene

hexagons, composed by thesp2 carbon atoms, along the zig-

zagdZZand armchairdACdirections, as predicted by the gra-

phene sketch shown in Fig.3.

The lateral force image shown in Fig. 2(a)can be usedto identify the crystallographic orientation of the graphene

sheet if the angle between the graphene layer is known in

respect to the scanning direction. In Fig. 4, the superposi-

tion of the AFM image with the optical image of the gra-

phene is presented. Hexagons were drawn to show the

crystallographic orientation of the graphene determined by

AFM. Positioning the hexagons onto the optical image, the

crystallographic orientation of edges I and II can be deter-

mined. Edge I was identified as preferentially oriented

along the zigzag direction, while edge II was identified as

being preferentially oriented in the armchair direction.

To confirm the orientation of the edge Raman measure-

ments were carried out. In the Raman spectra, the G-bandpeak appears as asp

2 carbon signature. It is an in-plane dou-

ble degenerated phonon mode originated at the center of the

Brillouin zone. The D-band is due to a double resonance

Raman scattering process andis related to the break of crys-

tal translational symmetry.23 In the armchair edge, the D-

peak was reported to be stronger than the one in zigzag edge

orientation, due to the momentum conservation.7,8 As shown

in Fig.4, the ratio between the intensities of the D-band and

the G-band is bigger for the edge II, confirming that this

edge is mainly formed by carbon atoms following the arm-chair orientation, while edge I is formed by atoms following

the zigzag orientation. These results strengthen the measure-

ments preformed with the lateral force images.

In summary, the crystallographic orientation of a gra-

phene sheet was determined using lateral force microscopy

images. Crystal edges predominantly formed by carbon

atoms following the zigzag or armchair directions were iden-

tified. Our results were confirmed by Raman spectroscopy. It

was demonstrated that the crystallographic orientation of

graphene sheets can be done without minimal sample prepa-

ration, using only lattice resolution images taken by atomic

force microscopy.

This work was supported by: Conselho Nacional de

Desenvolvimento Cientfico e Tecnologico CNPq and Fun-

dacao Carlos Chagas Filho de Amparo a Pesquisa do Estado

do Rio de Janeiro FAPERJ.

1K. S. Novoselovet al.,Science 306, 666 (2004).2C. Leeet al.,Science 321, 385 (2008).3K. I. Bolotinet al.,Solid State Commun.146, 351 (2008).4K. Nakadaet al.,Phys. Rev. B 54, 17954 (1996).5

X. Jia, M. Hofmann, et al.,Science323, 1701 (2009).6L. G. Cancadoet al.,Phys. Rev. Lett. 93, 247401 (2004).7Y. Youet al.,Appl. Phys. Lett. 93, 163112 (2008).8C. Casiraghiet al.,Nano Lett. 9, 1433 (2009).9C . O. Giritet al.,Science323, 1705 (2009).

10

P. Koskinenet al.,Phys. Rev. B 80, 073401 (2009).11Y. Niimiet al.,Appl. Surf. Sci.241, 43 (2005).

12K. A. Ritter and J. W. Lyding, Nat. Mater. 8, 235 (2009).13

S. Neubecket al.,Appl. Phys. Lett. 97, 053110 (2010).14S. Scharet al.,Appl. Surf. Sci. 210, 43 (2003).15R. Bennewitzet al.,Phys. Rev. B 60, R11 301 (1999).16Y. Ganet al.,Surf. Sci.601, 1064 (2007).17Y. Kuwahara,Phys. Chem. Miner. 26, 198 (1999).18

C. M. Mateet al.,Phys. Rev. Lett. 59, 1942 (1987).19D. R. Baselt and J. D. Baldeschwieler, J. Vac. Sci. Technol. B 10, 2316

(1992).20W.-C. Laiet al., Nanotechnology21, 055702 (2010).21

S. Timoshenko and J. N. Goodier, Theory of Elasticity (McGraw-Hill,

New York, 1951).22P. Delhaes, Graphite and Precursors (CRC Press, Boca Raton, FL,

2001).23

C. Thomsen and S. Reich, Phys. Rev. Lett. 85, 5214 (2000).

FIG. 3. (Color online) Sketch showing the honeycomb crystal lattice of a

graphene sheet. The two lines represent possible crystallographic orienta-

tions of the lattice: zigzag (solid) and armchair (dashed). The distances

between two consecutive hexagon centers in both directions are shown in

the figure:dZZ 0.246 nm anddAC 0.426 nm.

FIG. 4. (Color online) Optical image of the graphene sheet with the high-

lighted edges I and II. The inset shows a lattice resolution AFM image of the

sheet oriented as performed, allowing the identification of the graphene crys-

talline orientation. Superimposing the hexagons onto the optical image, the

crystallographic orientation of the edges I (zigzag) and II (armchair) are show.

FIG. 5. (Color online) Raman spectra taken in booth edges I (solid) and II

(dashed) highlighted in Fig. 4. The spectrum from edge I presents an ID/IGratio of 0.15, while an ID/IG ratio of 0.25 is found at edge II. The spectra

show the predominance of carbon bounds following the zigzag direction at

edge I and armchair direction at edge II.

086101-3 Almeidaet al. J. Appl. Phys.110, 086101 (2011)
http://dx.doi.org/10.1126/science.1102896http://dx.doi.org/10.1126/science.1157996http://dx.doi.org/10.1016/j.ssc.2008.02.024http://dx.doi.org/10.1103/PhysRevB.54.17954http://dx.doi.org/10.1126/science.1166862http://dx.doi.org/10.1103/PhysRevLett.93.247401http://dx.doi.org/10.1063/1.3005599http://dx.doi.org/10.1021/nl8032697http://dx.doi.org/10.1126/science.1166999http://dx.doi.org/10.1103/PhysRevB.80.073401http://dx.doi.org/10.1016/j.apsusc.2004.09.091http://dx.doi.org/10.1038/nmat2378http://dx.doi.org/10.1063/1.3467468http://dx.doi.org/10.1016/S0169-4332(02)01477-0http://dx.doi.org/10.1103/PhysRevB.60.R11301http://dx.doi.org/10.1016/j.susc.2006.11.057http://dx.doi.org/10.1007/s002690050177http://dx.doi.org/10.1103/PhysRevLett.59.1942http://dx.doi.org/10.1116/1.586061http://dx.doi.org/10.1088/0957-4484/21/5/055702http://dx.doi.org/10.1103/PhysRevLett.85.5214http://dx.doi.org/10.1103/PhysRevLett.85.5214http://dx.doi.org/10.1088/0957-4484/21/5/055702http://dx.doi.org/10.1116/1.586061http://dx.doi.org/10.1103/PhysRevLett.59.1942http://dx.doi.org/10.1007/s002690050177http://dx.doi.org/10.1016/j.susc.2006.11.057http://dx.doi.org/10.1103/PhysRevB.60.R11301http://dx.doi.org/10.1016/S0169-4332(02)01477-0http://dx.doi.org/10.1063/1.3467468http://dx.doi.org/10.1038/nmat2378http://dx.doi.org/10.1016/j.apsusc.2004.09.091http://dx.doi.org/10.1103/PhysRevB.80.073401http://dx.doi.org/10.1126/science.1166999http://dx.doi.org/10.1021/nl8032697http://dx.doi.org/10.1063/1.3005599http://dx.doi.org/10.1103/PhysRevLett.93.247401http://dx.doi.org/10.1126/science.1166862http://dx.doi.org/10.1103/PhysRevB.54.17954http://dx.doi.org/10.1016/j.ssc.2008.02.024http://dx.doi.org/10.1126/science.1157996http://dx.doi.org/10.1126/science.1102896

japplphys_110_086101

Documents