The structural and electrical evolution of chemical vapor deposition grown graphene by electron beam irradiation induced disorder

The structural and electrical evolution of chemical vapor deposition grown graphene by electron beam irradiation induced disorder

CARBON 5 9 ( 2 0 1 3 ) 3 6 6 –3 7 1 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon The structural and electri...

576KB Sizes 0 Downloads 17 Views

CARBON

5 9 ( 2 0 1 3 ) 3 6 6 –3 7 1

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

The structural and electrical evolution of chemical vapor deposition grown graphene by electron beam irradiation induced disorder M.Z. Iqbal, O. Kelekci, M.W. Iqbal, Jonghwa Eom

*

Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

The defect formation mechanism in chemical vapor deposition grown single layer graph-

Received 28 January 2013

ene devices has been investigated by increasing electron beam (e-beam) irradiation doses

Accepted 15 March 2013

gradually up to 750 e/nm2. The evolution of D peaks in Raman spectra provides an evi-

Available online 24 March 2013

dence of strong lattice disorder due to e-beam irradiation. Particularly, the trajectory of D and G peak intensities ratio (ID/IG) suggests that the transformation of graphene from crystalline to the nanocrystalline and then towards amorphous form with increasing irradiation dose. The defect parameters were calculated by phenomenological model of amorphization trajectory for graphitic materials. The mobility decreasing gradually from 1200 to 80 cm2/V s with gradual increase of irradiation dose, which implies the formation of localized states in e-beam irradiated graphene. The Dirac point is shifted towards negative gate voltage which indicates the n-doping in graphene with increasing e-beam irradiation dose.  2013 Elsevier Ltd. All rights reserved.

1.

Introduction

In the recent time, graphene has been attracting much attention due to its fascinating properties, such as extremely high mobility, quantum electronic transport and high elasticity [1,2]. These properties demonstrate the probable application of graphene in future solid-state devices and potential of being an alternative to traditional semiconductors. The growth on metal substrate by chemical vapour deposition (CVD) is the most promising and the cheapest technique for the production of large area graphene, among various methods [3,4]. It is also compatible with the current large scale integrated circuit fabrication processes [4–7]. However, the absence of a band-gap in the pristine graphene makes it unsuitable for transistors with a high on–off ratio [8]. Hence, tailoring the electronic properties by means of defects and geometrical confinement is important in order to realize

graphene as a competitive material for future electronics [9,10]. Recently, graphene based nanodevices are being studied extensively. Fabrication and characterizations of graphene devices often require an extensive use of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which are sources of e-beam. The irradiation of ebeam may damage the lattice of graphene and can create some defects. Recently, polymethyl methacrylate (PMMA) has also been used as insulating layer to the top gate electrode in field effect transistor of graphene devices [11]. Creating the PMMA insulating layer on top of the graphene requires significant amount of e-beam dose. Therefore, the intentional or unintentional use of source of e-beam (SEM and TEM) strongly affects the intrinsic properties of graphene [12,13]. Some research groups have been reported the effect of electron and ion beam radiation on exfoliated graphene [12–19]. Balandin and co-workers have extensively investigated the

* Corresponding author. E-mail address: [email protected] (J. Eom). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.03.030

CARBON

5 9 (2 0 1 3) 3 6 6–37 1

effects of e-beam irradiation on the structural, electronic and thermal properties of mechanical exfoliated graphene sheets [12,20–24]. Recently, some effects of e-beam irradiation on CVD grown graphene have been reported [25]. The lattice defects due to irradiation can be considered as a potential source of intervalley scattering, which could in principle induce insulating behavior in the e-beam irradiated graphene [26]. However, a detailed investigation is still required to study the effects of e-beam irradiation on CVD grown graphene for practical use and basic science interest. Here we present the systematic study of effect of e-beam irradiation on CVD grown graphene. The effects of electron beam of various doses (125–750 e/nm2) are investigated by Raman spectroscopy and transport measurements. The resistivity versus back gate voltage measurement shows the shift of Dirac point towards negative gate voltage with increasing the dose of e-beam irradiation. The shift of Dirac point position towards negative gate voltage is an indication of electron doping and it is also confirmed by red shift of G and 2D peak in Raman spectra. The growth of D and G peak intensities ratio with increasing irradiation dose leads to amorphization trajectory, which advocates that the structure of graphene film transforms from crystalline to nanocrystalline and then to amorphous form. The crystalline size is estimated by Tuinstra–Koening and Ferrari–Robertson relation and the defect parameters are theoretically calculated, which is in excellent agreement with experimental results.

2.

Experimental

The CVD graphene samples were synthesized by same method as previously discussed in our paper [25,27,28]. The CVD grown graphene film on Cu foil was transferred after spincoating a thin layer of PMMA on the SiO2 (300 nm)/Si (pdoped) substrate [25,27,28]. The CVD grown graphene on the SiO2/Si substrate was kept in acetone for 1 day to completely remove the PMMA layer from the graphene surface and then was rinsed in methanol and dry with nitrogen gas. The transferred CVD graphene on SiO2 substrate was pre-patterned with big electrodes and alignment marks (Cr/Au of 5/30 nm) fabricated by photolithography. The unwanted CVD graphene was removed by combination of photolithography and oxygen plasma etching techniques. The inner electrode were made by e-beam lithography and evaporation of Cr/Au (5/ 55 nm) for transport measurement. The exact location of exposure area is identified by using alignment marks. The e-beam irradiation of different dose was conducted by using Raith GmbH lithography system, which permits an accurate control of the irradiation dose and the location of exposed area. The experiment was performed with accelerating voltage of 20 keV of e-beam, working distance of 3.55 mm. Faraday cup of the sample stage was used to measure the beam current, which was 14.9 pA for this experiment. The dose was applied in such a way that the electrodes circumvented the exposure to the e-beam irradiation of graphene channel. Raman spectra were measured with a Renishaw micro-spectrometer over a wave number from 1100 to 3200 cm1 with the laser power 1 mW and wavelength of 514 nm at room temperature. The back-gate dependent electrical measure-

367

ments were performed to examine the modification in resistivity by e-beam irradiation. Fig. 1(a) is the scanning electron micrograph image of the fabricated device and Fig. 1(b) shows the schematic of graphene channel by e-beam exposure.

3.

Results and discussion

3.1. Evaluation of Raman spectra of e-beam irradiated graphene Fig. 2 shows the Raman spectra of pristine and e-beam irradiated graphene samples at room temperature for various doses from 125 to 750 e/nm2. The D and G peaks appear around 1347 and 1587 cm1, respectively as shown in Fig. 2(a) and these values are similar to previously reported values obtained in pristine CVD grown graphene [29]. However, the very small D peak in pristine CVD graphene indicates a high quality graphene. The D peak is attributed to A1g symmetry phonons near the K-zone boundary. These phonons are not Raman active due to the momentum conservation in the scattering, and require a defect for their activation [30]. The G peak corresponds to the in-plane bond stretching motion of the pairs of carbon atoms with E2g optical phonon at the Brillouin zone centre. The increase of D and D 0 peaks with increasing e-beam irradiation suggests that the disorder has been induced. Initially intensity of both D and D 0 peaks increases with increasing the dose after each irradiation step. However, after certain e-beam dose this trend becomes reverse and it was observed for both D and D 0 peak. The 2D peak is the second order of the D peak and appears around 2690 cm1, which

Fig. 1 – (a) Scanning electron micrograph (SEM) image of the device fabricated by simultaneous process of photo- and ebeam lithography. The graphene appears as dark color in the middle of micrograph. (b) The schematic of e-beam exposure on graphene channel.

368

CARBON

(a)

5 9 ( 2 0 1 3 ) 3 6 6 –3 7 1

(b)

2D

D'

-

2

-

2

-

2

-

2

-

2

750 e /nm

80

80 G D

-

D'

625 e /nm

2

750 e /nm

500 e /nm -

2

-

2

-

2

60

Intensity (a.u.)

625 e /nm

60

Intensity (a.u.)

G

D

500 e /nm 375 e /nm

40

-

2

250 e /nm

375 e /nm 40

250 e /nm -

-

2

20

-

-

2400

Pristine

0

Pristine

1500

2

125 e /nm

2

125 e /nm

0 1200

2

187.5 e /nm

20

187.5 e /nm

1300 1350 1560

2700 -1

1620 -1

Raman Shift (cm )

Raman Shift (cm )

Fig. 2 – (a) Raman shift for various e-beam irradiation doses. The peaks D, G and 2D appear around 1347, 1587 and 2690 cm1, respectively. The disorder induced D and D 0 peaks is raised after e-beam irradiation. (b) Evaluation of Raman spectra by the multiple Lorentzian curve fittings of D, G and D 0 peaks, respectively with measured data.

Position of D peak FWHM of D peak

-1

FWHM of D peak (cm )

30

(a)

-1

Position of D peak (cm )

1360

1350 25

1340

20 1330

0

200

400

600 -

800

2

e-beam irradiation dose (e /nm ) 1590 2695 -1

1585

20

2690

15

La (nm)

-1

Position of G peak Position of 2D peak

1580

10

2685 5 0

1575

Position of 2D peak (cm )

(b) Position of G peak (cm )

originates from a process where momentum conservation is satisfied by two phonons with opposite wave vectors and no defects are required for their activation, and are thus always present. The change of D, G and D 0 peaks affected by e-beam irradiation are evaluated by the multiple Lorentzian curve fittings as shown Fig. 2(b). Fig. 3(a) shows the shift in D peak position and full width half maximum (FWHM) of D peak as a function of e-beam irradiation dose. The FWHM of D peaks and shift in peak positions for the pristine and e-beam irradiated graphene were obtained by using Lorentzian fit in Raman spectra from Fig. 2(b). FWHM of D peak decreases up to certain value of irradiation doses and for higher doses it restores towards the original magnitude. The D peak becomes sharper from 125 to 375 e/nm2 irradiation doses, and the magnitude of FWHM has smaller value in this regime. The positions of G and 2D peak as functions of irradiation dose are shown in Fig. 3(b) and these peaks shifted towards lower wave number with gradual increase in irradiation dose similar as D peak. The inset of Fig. 3(b) shows the crystalline size (La) of graphene as a function of e-beam irradiation dose. The value of La decreases rapidly as e-beam irradiation dose increase, which indicates that graphene transforms from crystalline to amorphous state. Fig. 4(a) shows the intensity ratio of D and G peak (ID/IG) as a function of crystalline size. This plot is divided into two regions. Initially, the ID/IG ratio increases up to certain value of irradiation dose and in the second region when it attains a certain limit, it drops down gradually by further increasing the e-beam irradiation dose. This trend can be explained by the model of amorphization trajectory proposed by Ferrari and Robustson [31]. The first trend, i.e. increase of ID/IG, specifies that the crystalline graphene transformed into nanocrystalline form and the second trend, i.e. the gradual decrease in ID/IG ratio, suggests that nanocrystalline graphene transforms

0

200

400

600

2680

800 2

e-beam irradiation dose (e-/nm )

2675

1570 0

200

400

600 -

800

2

e-beam irradiation dose (e /nm )

Fig. 3 – (a) Position and full width half maximum (FWHM) of D peak as a function of e-beam irradiation dose. (b) Position of G and 2D peak as a function of e-beam irradiation dose. Inset: Crystalline size (La) as a function of e-beam irradiation dose.

CARBON

value of ID/IG ratio is consistent with previous observation for CVD graphene [25]:

2.5

(a)

ID/IG

2.0

ID CðkÞ ¼ La IG ID ¼ C0 ðkÞL2a IG

Experimental data Fitting data

1.5

ID ¼ IG

1.0 0.5 0.0 0

3

6

9

12

15

18

La (nm)

(b)

log (ID/IG)

1

Experimental data 0.1 Fitting data 1

369

5 9 (2 0 1 3) 3 6 6–37 1

10

log (La) Fig. 4 – Implementation of Eq. (3) on ID/IG as a function of average distance La between defects, induced by e-beam irradiation. Instead of integrated area ratio, we use intensity ratio because below La  2 nm, the G and D 0 peaks overlap. The red solid line is the theoretical modelling data of Eq. (3) and black points are the experimental data. (b) Theoretical fit obtained by using Eq. (3) and experimental data of ID/IG vs. La are plotted on a log–log scale for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

into amorphous carbon film. It is due to the large number of defects introduced at higher dose and most parts of nanocrystalline graphene are converted into sp2 amorphous carbon film. The region below 375 e/nm2 e-beam irradiation doses corresponds to the nanocrystalline phase and crystalline size (La) of this phase can be estimated by Tuinstra–Koening relation ðID =IG / 1=La Þ as shown in Eq. (1), where C(k)  4.4 nm (at exciting laser light k = 514 nm) [32]. However, the second region having above 375 e/nm2 e-beam irradiation doses follows the Ferrari and Robertson relation ðID =IG / L2a Þ as shown in Eq. (2) [31]. Here C 0 (k)  0.58/nm2 was estimated by consistency of ID/IG at the irradiation dose of 375 e/nm2. The resulting plot of ID/IG ratio as a function of calculated La for both regions is shown in Fig. 4(a). The ID/IG ratio decreases with increasing La in the region where La > 2 nm, i.e. nanocrystalline state, and the ratio increases in the region where La < 2 nm, i.e. amorphous state. This region is represented as amorphous sp2 carbons film having lower value of La. The

r2  r2S CA 2A rA  2r2S

ð1Þ ð2Þ " exp

pr2S L2a

!  exp

 !# p r2A  r2S L2a

" þ CS 1  exp

pr2S L2a

!# ð3Þ

Here, we have fitted Eq. (3) to calculate the structural disorder in graphene as previously reported model used to calculate structural disorder by Ar+ ion bombardment [33]. The two regions with different length scales are defined by structurally disordered and active regions. The rS and rA are the radii of structurally disordered and activated region respectively and, here rA is always greater than rS. As the defect density grows up, the D band intensity increases and then reaches a maximum. Meanwhile the activated regions start to overlap and these regions eventually saturate. By further increasing the defect density in graphene, the D band intensity decreases because structurally disordered areas start to dominate. The CA parameter in Eq. (3) is a measure of the maximum possible value of the ID/IG ratio in graphene. Theoretically, the parameter CA could be possible only when mixing of K–K 0 wave vector are allowed. However, in this situation there would be no structural change in the hexagonal network of carbon atoms. Therefore, CA can be defined in terms of electron–phonon coupling elements between the C and K points [34]. The CS parameter is the value of the ID/IG ratio in the highly disordered limit. The red line curve is in excellent agreement with the experimental results in Fig. 4(a), by taking the parameters CA = 5.20048, CS = 0.34096, rA = 1.56084 and rS = 1.08089 nm. The experimental data and fitting curve are plotted in log–log scale for clarity in Fig. 4(b). The electron energy loss is an important parameter to describe the influence of irradiation effect. We can estimate the electron energy loss, DK, in the graphene by using the relation DK = (dE/dx) · t. Here, t is the thickness of graphene layer and dE/dx is electron stopping power which is governed by inelastic interactions with the target material. It contributes to various physical phenomena such as ionization of the target atoms and electronic excitations, and leads to local bond breaking and amorphization. It has value 2.89 eV/nm for graphitic materials [35]. Using t = 0.34 nm [36], the estimated energy loss is found to be 0.893 eV for graphene [35,37].

3.2.

Transport measurements

The effect of e-beam irradiation dose in graphene lattice modification is studied by measuring the resistivity as a function of back-gate voltage (Vg) for various e-beam irradiation doses shown in Fig. 5(a). Resistivity was calculated by using q = RW/ L, where R is the measured resistance, W is width and L is the length of graphene channel. The Dirac point is shifted from 18 to 9.5 V with controlled e-beam exposure and resistance persistently increased with higher dose exposure. This implies that e-beam irradiation tunes the transport properties and it transforms graphene doping from hole type to electron type doping. In order to analyze the difference between pristine and irradiated samples quantitatively, we employ the semiclassical Drude model to calculate the mobility, l = (neq)1,

370

CARBON

5 9 ( 2 0 1 3 ) 3 6 6 –3 7 1

50

- 2 750 e /nm - 2 625 e /nm - 2 450 e /nm - 2 375 e /nm - 2 250 e /nm - 2 187.5 e /nm - 2 125 e /nm

(a) Resistivity (kΩ )

40 30 20 10

Pristine

0

-40

-20

0

20

40

Vg (V)

2

Mobility (cm /V sec)

(b) 1200 900

Resistivity at Dirac point (kΩ )

1500

Vg= 40V Vg= 30V Vg= -20V Vg= -30V Vg= -40V

50 40 30 20 10 0

600

200

400

600 -

800 2

e-beam irradiation dose (e /nm )

300 0 0

200

400

600 -

800

2

Irradiation dose (e /nm ) Fig. 5 – Resistivity of graphene as a function of back gate voltage (Vg) for various doses. (b) Mobility of graphene as a function of e-beam irradiation dose at different gate voltage. Inset shows the resistivity at Dirac point as a function e-beam irradiation dose.

where q is the resistivity, n ¼ Cg j Vg  VDirac j =e and Cg is gate capacitance taken to be 115 aF/lm2 for 300 nm SiO2 substrate [38], which was same as obtained from our Hall measurements. Fig. 5(b) shows the trend of mobility of graphene device as a function of irradiation dose and the mobility is found to be decreasing gradually from 1200 to 80 cm2/V s as e-beam irradiation dose increases. The inset of Fig. 5(b) shows the resistivity at Dirac point as a function of irradiation dose and it is observed that resistivity rapidly increases with increasing e-beam irradiation dose. The increase of device resistance supports the amorphization trajectory, which suggests graphene’s transformation to the nanocrystalline and then to amorphous form with increasing irradiation of e-beam. Therefore, we can conclude that graphene becomes amorphous rather than recrystalline at higher e-beam irradiation.

follows the amorphization trajectory with increasing irradiation dose, which implies that graphene transforms from crystalline to nanocrystalline form and then after a certain limit it transforms into amorphous form. The crystalline size was calculated by Tuinstra–Koening and Ferrari–Robertson relations and the quantification of defect parameters was performed by fitting the phenomenological model of amorphization trajectory for graphitic materials, which is consistent with experimental results. In transport measurements, the Dirac point shifted towards negative gate voltage indicating the n-doping in graphene. The resistance increased persistently and mobility decreased gradually from 1200 to 80 cm2/V s with higher dose exposure. This can be attributed to the formation of disordered states in e-beam irradiated graphene.

Acknowledgements 4.

Conclusions

We have introduced the defects in inductively coupled plasma enhanced chemical vapor deposition grown graphene by ebeam irradiation. Raman spectra and transport measurements elucidated the mechanisms of disorder formation in graphene. The appearance of large D peak is attributed to lattice damage of graphene layer upon applying e-beam irradiation. The evolution of D and G peak intensities ratio (ID/IG)

This work was supported by Nano-Material Technology Development Program (2012M3A7B4049888), Priority Research Centers Program (2012-0005859), and Mid-career Researcher Program (2010-0010861) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. This work was also supported by Converging Research Center Program through the Ministry of Education, Science and Technology (2012K001310).

CARBON

5 9 (2 0 1 3) 3 6 6–37 1

R E F E R E N C E S

[1] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6(3):183–91. [2] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306(5696):666–9. [3] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Largescale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009;457(7230):706–10. [4] Bae S, Kim H, Lee Y, Xu XF, Park JS, Zheng Y, et al. Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nat Nanotechnol 2010;5(8):574–8. [5] Avouris P. Graphene: electronic and photonic properties and devices. Nano Lett 2010;10(11):4285–94. [6] Wu YQ, Lin YM, Bol AA, Jenkins KA, Xia FN, Farmer DB, et al. High-frequency, scaled graphene transistors on diamond-like carbon. Nature 2011;472(7341):74–8. [7] Yang H, Heo J, Park S, Song HJ, Seo DH, Byun KE, et al. Graphene barristor, a triode device with a gate-controlled Schottky barrier. Science 2012;336(6085):1140–3. [8] Xia FN, Farmer DB, Lin YM, Avouris P. Graphene field-effect transistors with high on/off current ratio and large transport band gap at room temperature. Nano Lett 2010;10(2):715–8. [9] Jafri SHM, Carva K, Widenkvist E, Blom T, Sanyal B, Fransson J, et al. Conductivity engineering of graphene by defect formation. J Phys D Appl Phys 2010;43(4):045404–11. [10] Banhart F, Kotakoski J, Krasheninnikov AV. Structural defects in graphene. ACS Nano 2011;5(1):26–41. [11] Huard B, Sulpizio JA, Stander N, Todd K, Yang B, GoldhaberGordon D. Transport measurements across a tunable potential barrier in graphene. Phys Rev Lett 2007;98(23): 236803–6. [12] Teweldebrhan D, Balandin AA. Modification of graphene properties due to electron-beam irradiation. Appl Phys Lett 2009;94(1):013101–3. [13] Xu ZW, Chen L, Li JL, Wang R, Qian XM, Song XY, et al. Oxidation and disorder in few-layered graphene induced by the electron-beam irradiation. Appl Phys Lett 2011;98(18):183112–3. [14] Teweldebrhan D, Balandin AA. Response to ‘‘Comment on Modification of graphene properties due to electron-beam irradiation’’. Appl Phys Lett 2009;95(24):246102–3. [15] Childres I, Jauregui LA, Foxe M, Tian JF, Jalilian R, Jovanovic I, et al. Effect of electron-beam irradiation on graphene field effect devices. Appl Phys Lett 2010;97(17):173109–11. [16] Kim KJ, Choi JH, Lee H, Lee HK, Kang TH, Han YH, et al. Effects of 1 MeV electron beam irradiation on multilayer graphene grown on 6H-SiC(0 0 0 1). J Phys Chem C 2008;112(34):13062–4. [17] Compagnini G, Giannazzo F, Sonde S, Raineri V, Rimini E. Ion irradiation and defect formation in single layer graphene. Carbon 2009;47(14):3201–7. [18] Mathew S, Chan TK, Zhan D, Gopinadhan K, Barman AR, Breese MBH, et al. The effect of layer number and substrate on the stability of graphene under MeV proton beam irradiation. Carbon 2011;49(5):1720–6. [19] Krasheninnikov AV, Nordlund K. Ion and electron irradiationinduced effects in nanostructured materials. J Appl Phys 2010;107(7):071301–70.

371

[20] Balandin AA. Thermal properties of graphene and nanostructured carbon materials. Nat Mater 2011;10(8):569–81. [21] Nika DL, Ghosh S, Pokatilov EP, Balandin AA. Lattice thermal conductivity of graphene flakes: comparison with bulk graphite. Appl Phys Lett 2009;94(20):203103–5. [22] Calizo I, Balandin AA, Bao W, Miao F, Lau CN. Temperature dependence of the Raman spectra of graphene and graphene multilayers. Nano Lett 2007;7(9):2645–9. [23] Liu G, Teweldebrhan D, Balandin AA. Tuning of graphene properties via controlled exposure to electron beams, Ieee T Nanotechnol 2011;10(4):865–70. [24] Calizo I, Bejenari I, Rahman M, Liu G, Balandin AA. Ultraviolet Raman microscopy of single and multilayer graphene. J Appl Phys 2009;106(4):043509–13. [25] Iqbal MZ, Singh AK, Iqbal MW, Seo S, Eom J. Effect of e-beam irradiation on graphene layer grown by chemical vapor deposition. J Appl Phys 2012;111(8):084307–11. [26] Chen JH, Cullen WG, Jang C, Fuhrer MS, Williams ED. Defect scattering in graphene. Phys Rev Lett 2009;102(23):236805–8. [27] Singh AK, Iqbal MW, Singh VK, Iqbal MZ, Lee JH, Chun SH, et al. Molecular n-doping of chemical vapor deposition grown graphene. J Mater Chem 2012;22(30):15168–74. [28] Iqbal MW, Singh AK, Iqbal MZ, Eom J. Raman fingerprint of doping due to metal adsorbates on graphene. J Phys Condens Matter 2012;24(33):335301–7. [29] Suk JW, Kitt A, Magnuson CW, Hao YF, Ahmed S, An JH, et al. Transfer of CVD-grown monolayer graphene onto arbitrary substrates. ACS Nano 2011;5(9):6916–24. [30] Cancado LG, Jorio A, Ferreira EHM, Stavale F, Achete CA, Capaz RB, et al. Quantifying defects in graphene via Raman spectroscopy at different excitation energies. Nano Lett 2011;11(8):3190–6. [31] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000;61(20):14095–107. [32] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53(3):1126–30. [33] Lucchese MM, Stavale F, Ferreira EHM, Vilani C, Moutinho MVO, Capaz RB, et al. Quantifying ion-induced defects and Raman relaxation length in graphene. Carbon 2010;48(5):1592–7. [34] Lazzeri M, Attaccalite C, Wirtz L, Mauri F. Impact of the electron–electron correlation on phonon dispersion: failure of LDA and GGA DFT functionals in graphene and graphite. Phys Rev B 2008;78(8):081406R–9R. [35] Tanuma S, Powell CJ, Penn DR. Calculations of stopping powers of 100–30 keV electrons in 31 elemental solids. J Appl Phys 2008;103(6):063707–20. [36] Morozov SV, Novoselov KS, Schedin F, Jiang D, Firsov AA, Geim AK. Two-dimensional electron and hole gases at the surface of graphite. Phys Rev B 2005;72(20):201401R–4R. [37] Shinn E, Hu¨bler A, Lyon D. Perdekamp MG, Bezryadin A, Belkin A. Nuclear energy conversion with stacks of graphene nanocapacitors. Complexity 2013;18(3):24–7. [38] Tan YW, Zhang Y, Bolotin K, Zhao Y, Adam S, Hwang EH, et al. Measurement of scattering rate and minimum conductivity in graphene. Phys Rev Lett 2007;99(24):246803.