Optical investigation of reduced graphene oxide by

APPLIED PHYSICS LETTERS 99, 141911 (2011)

Optical investigation of reduced graphene oxide by spectroscopic ellipsometry and the band-gap tuning Y. Shen,1,2 P. Zhou,1,a) Q. Q. Sun,1,b) L. Wan,3 J. Li,3 L. Y. Chen,2 D. W. Zhang,1 and X. B. Wang3,c) 1

ASIC & System State Key Lab, School of Microelectronics, Fudan University, Shanghai 200433, China Department of Optical Science & Engineering, Fudan University, Shanghai 200433, China 3 School of Materials Science and Engineering, Hubei University, Wuhan 430062, China 2

(Received 25 July 2011; accepted 16 September 2011; published online 5 October 2011) Spectroscopic ellipsometry was used to characterize the optical response of few layer reduced graphene oxide and graphene oxide in visible range. Lorentz oscillator model is added to analyze the ellipsometric parameters. The experiment shows the optical response of few layer reduced graphene oxide and monolayer exfoliated graphene in visible range is quite similar with slight difference due to the structure defects. The Lorentz oscillator model gives experimental support to C 2011 American Institute investigate the band-gap tuning through the reduction process in details. V of Physics. [doi:10.1063/1.3646908] Graphene is a flat monolayer of carbon atom densely packed in a honeycomb lattice which has stimulated extensive researches nowadays due to its unique electronic and optical properties.1,2 The monolayer mechanical exfoliated graphene (MEG) has received significant attention because of its extraordinary outstanding properties,3–5 however zero band-gap limits mass production to device applications. Chemical reduction of graphene oxide (GO) have yielded promising results on acquisition of tunable band-gap.6–8 The optical properties of few layer reduced graphene oxide (FRGO) fabricated by chemical reduction method9 remain still ambiguous. The optical response of MEG through spectroscopic ellipsometry (SE) method has been performed.9,10 The works focus on modeling the monolayer graphene film based on the optical properties of bulk graphite. The ellipsometry method is very sensitive to characterize film in the range of several nanometers. Besides, the ellipsometry can obtain more parameters to determine the optical constants of films more precisely in a nondestructive way. Some models11,12 were applied to explore the dispersion of optical constants of MEG. However, decorated with several kinds of functional groups such as epoxide, hydroxyl, and carboxyl groups and with variable coverage of oxygen and hydroxyl groups,13 the band structure of GO and FRGO thus remains unclear. To study the layer number and band structure of few-layer GO and FRGO by nondestructive, SE method is useful and important since it is hard to be derived directly. In this letter, Lorentz oscillator model is added to analyze the ellipsometric parameters of GO and FRGO. We describe the optical response and band-gap tuning of GO and FRGO films transferred on SiO2/Si substrate in visible range. Meanwhile, the SE method can also be used to evaluate the layer number of FRGO and opens a way to analyze the coverage proportion and band-gap distribution of GO and FRGO. GO was synthesized from natural graphite as our previous report.14 As-synthesized GO was suspended in distilled

water and sonicated for 1 h, yielding a 0.025 mg mL1 GO aqueous dispersion. The resulting homogeneous GO dispersion was reduced using hydrazine solution (35 wt. % in water) and ammonia solution (28 wt. % in water) to obtain the FRGO dispersion. Fabrication of GO and FRGO films was carried out by vacuum filtration of the dispersion using a mixed cellulose ester membrane filter, press against the surface of the SiO2/Si wafer, and dry.15 Raman spectra are measured on GO and FRGO on SiO2/ Si substrate. The measurements are taken at room temperature with the excitation wavelength of 514 nm. Fig. 1(a) shows Raman spectra of GO and FRGO. The most intense peak (G band) at 1580 cm1 is assigned to the E2g mode of the relative motion of sp2 carbon atoms which is also the prominent peak in Raman spectra of mechanical cleaved graphene.8,16,17 The G-peak has shifted from 1600 cm1 to

a)

FIG. 1. (Color online) (a) Comparison of the Raman spectra of GO and FRGO. (b) The TEM image of our sample with FRGO marked by the red frame. (c) The zoom in of the FRGO depicted in (a).

Electronic mail: [email protected]. Electronic mail: [email protected]. c) Electronic mail: [email protected]. b)

0003-6951/2011/99(14)/141911/3/$30.00

99, 141911-1

C 2011 American Institute of Physics V

Downloaded 06 Oct 2011 to 202.120.224.53. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

141911-2

Shen et al.

Appl. Phys. Lett. 99, 141911 (2011)

FIG. 2. (Color online) (a) The experimental and calculated elllipsometric parameters tan(W) and cos(D) of GO with the incidence angle of 75 . (b) The calculated refractive index (n) and extinction coefficient (k) of GO. (c) The experimental and calculated elllipsometric parameters tan(W) and cos(D) of FRGO with the incidence angle of 75 . (d) The calculated refractive index (n) and extinction coefficient (k) of FRGO.

1588 cm1 due to the transition from sp3 to sp2 after the reduction of graphene oxide. The peak at 1350 cm1 (D band) (absent in MEG) caused by the disorder band in graphite edge is seen in the Raman spectra of both GO and FRGO,18 which indicates the structural imperfection induced by hydroxyl and epoxide groups on the carbon basal plane.4 The value of ID/IG in Raman spectra of FRGO depicted in Fig. 1(a) indicates less sp3 in the sample than in GO8 which reveals a high degree of GO reduction with sp2 cluster created through removal of oxygen has been achieved during the process. Additionally, the ratio of G/2D peak (2700 cm1) intensity identifies the layer number of graphene sheets.16,19,20 After reduction, the FRGO film is imaged by transmission electron microscopy (TEM). According to the result depicted in Fig. 1(b), the FRGO film transferred on SiO2/Si substrate is marked by the red frame. Fig. 1(c) is zoom in from the selected red frame of Fig. 1(b). Between the two interfaces, the FRGO is approximately 4.2 nm. The thickness of MEG is reported to be 0.33 0.4 nm,21–23 thus the sample should be 10 layers reduced GO. In order to investigate the optical constant and band-gap structure of GO and FRGO, spectroscopic ellipsometry in visible range 275-826 nm with different angle of incidence at 65 , 70 , and 75 is applied. The ellipsometric parameters W and D are defined by q ¼ rs =rp ¼ tan w expðiDÞ;

(1)

where rp and rs represents the complex reflection coefficients of polarized light parallel and perpendicular to the incidence plane, respectively. A three-phase model of substrate SiO2/GO(FRGO)/ambient is designed to analyze the SE spectra. In this model, unknown parameters of film thickness (d) and dielectric constant (e) for GO and FRGO are defined as fitting variables. In order to study the distribution of band-gap energy, Lorentz oscillation model is adopted to characterize the dielectric function of GO described as follows:

e ¼ e1 þ ie2 ¼ e1 1 þ

X i

! A2i ; C2i E2 jvi E

(2)

where e1 is the light-frequency dielectric constant, Ai, Ci, and vi are the amplitude, center energy, and damping coefficient of each oscillator in eV, respectively. The Ai value also represents the percentage contribution of oscillator i in the whole system. The complex refractive index can be calculated from the dielectric function in qffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1=2 1 e21 þ e22 þ e1 n¼ ; 2 k¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1=2 1 e21 þ e22 e1 : 2

(3)

Theoretical studies have demonstrated that the energy bandgap of GO varies among 2.8, 2.1, and 1.8 eV at the decrease of coverage of oxygen and hydroxyl groups from 75% to 50%, and a further reduction can lead GO to be conducting.13 The thickness of monolayer GO should be 1 nm (Ref. 24) which is larger than graphene (0.33 0.4 nm) (Ref. 25) due to the presence of hydroxyl and epoxide groups, structure defects, and absorbed water molecules.24–26 The low-energy plasma excitations p* of the electrons in GO occurs at around 5 eV.27 Consequently, we use oscillators with different energy to characterize the different energy densities of states for GO due to different coverage. Fig. 2(a) shows the experimental and calculated tan(W) and cos(D) of GO fit well in visible range and Fig. 2(b) shows the calculated refractive index (n) and extinction coefficient (k) of GO. The calculated four oscillators were designed in data analyze with key parameters listed in Table I. The calculated thickness of GO is 9.08 nm that indicates a 10 layer GO, which in good agreement with the layer number we verified from Raman spectra. One (5.0 eV) of the four oscillators represents the lowenergy plasma excitations p* of the electrons in GO and the


141911-3

Shen et al.

Appl. Phys. Lett. 99, 141911 (2011)

TABLE I. Main parameters of the fitting results for GO and FRGO; Amplitude Ai and center energy Ci have units of eV; d is the thickness of film; and the frequency dielectric constant e1 is dimensionless.

D (nm) e1 A1 C1 (eV) A2 C2 (eV) A3 C3 (eV) A4 C4 (eV)

GO

FRGO

9.08 6 0.049 3.2217 6 0.0017 2.0215 2.8032 6 0.133 1.4385 6 0.0051 5.0031 6 0.0762 0.4366 6 0.0037 2.1493 6 0.064 6.8171 6 0.2475 1.8325 6 0.052

4.10 6 0.027 2.903 6 0.069 2.5334 6 0.094 4.6002 6 0.032 2.697 6 0.061 0.02 6 0.005 0.0955 6 0.004 4.7 6 0.047 — —

other three (2.8 eV, 2.1 eV, and 1.8 eV) represent different coverage of mixed hydroxyl groups and oxygen atoms.13 From the value of amplitude of the oscillators listed in Table I which represents the percentage contribution of in the whole system, it can be seen that the most dominant gap is 1.8 eV which resorted to 50% coverage, followed is 2.8 eV which resorted to 75% coverage, and the energy gap of 2.1 eV does tiny contribution to the whole system. The experimental and calculated tan(W) and cos(D) of FRGO with incidence angle of 75 is shown in Fig. 2(c). The energy of the three oscillators used in fitting process is approximately 4.6 eV, 4.7 eV, and 20 meV. 4.6 eV is due to the effects of resonance excitons on the inter-band transition which has a red-shift by 0.5 eV because of a van Hove singularity in graphene’s density of states.10 4.7 eV represents the p* peak of plasma excitation. 20 meV is the calculated bandgap energy of FRGO since chemical reduction decreases the band-gap energy. The calculated n and k of FRGO is shown in Fig. 2(d). The slight difference of n and k between our work and others’ might be caused by the structure defects such as impurities and air cushion generated during the reduction process while others’ sample are all MEG. The experimental and calculated curves fit so well which indicated the optical response of FRGO and MEG is quite similar. The schematic diagram of transition of band-gap from GO to FRGO is depicted in Fig. 3. During the reduction process with the functional groups removed gradually, the band-gap of GO

FIG. 3. (Color online) Schematic diagram of the transitions of band-gap through the reducing process from GO to FRGO.

decreases from 2.8 eV, 1.8 eV to 0.02 eV, increase the conductivity of FRGO also with the transition from sp3 to sp2 hybridization of carbon atoms which can be verified by Raman spectra. In summary, we use Lorentz oscillator model to analyze the ellipsometric parameters of GO and FRGO. It has been observed that the optical response of FRGO and MEG in visible range is quite similar which provides a possible way to make FRGO a feasible substitution for MEG in optical aspect. Additionally, SE method with Lorentz oscillator model is proved to be suitable to analyze the transition of band-gap through the process of reduction of GO and provide a nondestructive and reliable solution for measuring the layer number of graphene-based material. This work was supported by the NSFC (61076114), Shanghai Educational Develop Foundation (10CG04), SRFDP (20100071120027), and National Science and Technology Major Project (2011ZX02707). 1

P. Matyba, H. Yamaguchi, G. Eda, M. Chhowalla, L. Edman, and N. D. Robinson, ACS Nano 4, 638 (2010). 2 K. S. Subrahmanyam, P. Kumar, A. Nag, and C. N. R. Rao, Solid State Commun. 150, 1774 (2010). 3 K. 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). 4 G. Eda, G. Fanchini, and M. Chhowalla, Nat. Nanotechnol. 3, 270 (2008). 5 V. C. Tung, L. M. Chen, M. J. Allen, J. K. Wasse, K. Nelson, R. B. Kaner, and Y. Yang, Nano Lett. 9, 1949 (2009). 6 I. K. Moon, J. Lee, R. S. Ruoff, and H. Lee, Nat. Commun. 73, 1 (2010). 7 P. Kumar, K. S. Subrahmanyam, and C. N. R. Rao, e-print arXiv:1009.1028 8 A. C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000). 9 U. Wurstbauer, C. Ro¨ling, U. Wurstbauer, W. Wegscheider, M. Vaupel, P. H. Thiesen, and D. Weiss, Appl. Phys. Lett. 97, 231901 (2010). 10 V. G. Kravets, A. N. Grigorenko, R. R. Nair, P. Blake, S. Anissimova, K. S. Novoselov, and A. K. Geim, Phys. Rev. B 81, 155413 (2010). 11 A. Gray, M. Balooch, S. Allegret, S. D. Gendt, and W.-E. Wang, J. Appl. Phys. 104, 053109 (2008). 12 J. W. Weber, V. E. Calado, and M. C. M. V. D. Sanden, Appl. Phys. Lett. 97, 091904 (2010). 13 D. W. Boukhvalov and M. I. Katsnelson, J. Am. Chem. Soc. 130, 10698 (2008). 14 J. Wang, X. Wang, L. Wan, Y. Yang, and S. Wang, Chin. J. Chem. 28, 1935 (2010). 15 L. Wan, S. Wang, X. Wang, B. Dong, Z. Xu, X. Zhang, B. Yang, S. Peng, J. Wang, and C. Xu, Solid State Sci. 13, 468 (2011). 16 A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S.Novoselov, S. Roth, and A. K. Geim, Phys. Rev. Lett. 97, 187401 (2006). 17 F. Tuinstra and J. L. Koenig, J. Chem. Phys. 53, 1126 (1970). 18 K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prud’homme, I. A. Aksay, and R. Car, Nano Lett. 8, 36 (2008). 19 X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, and R. S. Ruoff, Science 324, 1312 (2009). 20 J. Lee, K. S. Novoselov, and H. S. Shin, ACS Nano 5, 608 (2010). 21 Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts, and R. S. Ruoff, Adv. Mater. 22, 3906 (2010). 22 K. F. Mak, M. Y. Sfeirb, J. A. Misewichb, and T. F. Heinz, Proc. Natl. Acad. Sci. U.S.A. 107, 14999 (2010). 23 F. J. Nelson, V. K. Kamineni, T. Zhang, E. S. Comfort, J. U. Lee, and A. C. Diebold, Appl. Phys. Lett. 97, 253110 (2010). 24 H. Chang, Z. Sun, Q. Yuan, F. Ding, X. Tao, F. Yan, and Z. Zheng, Adv. Mater. 22, 4872 (2010). 25 K. A. Mkhoyan, A. W. Contryman, J. Silcox, D. A. Stewart, G. Eda, C. Mattevi, S. Miller, and M. Chhowalla, Nano Lett. 9, 1058 (2009). 26 I. Jung, M. Vaupel, M. Pelton, R. Piner, D. A. Dikin, S. Stankovich, J. An, and R. S. Ruoff, J. Phys. Chem. C 112, 8499 (2008). 27 T. Eberlein, U. Bangert, R. R. Nair, R. Jones, M. Gass, A. L. Bleloch, K. S. Novoselov, A. Geim, and P. R. Briddon, Phys. Rev. B 77, 233406 (2008).
