- Email: [email protected]
Effects of rapid thermal annealing on properties of HfAlO films directly deposited by ALD on graphene
Materials Letters 137 (2014) 200–202
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of rapid thermal annealing on properties of HfAlO films directly deposited by ALD on graphene Li Zheng a,b, Xinhong Cheng a,n, Duo Cao a,b, Zhongjian Wang a, Dawei Xu a, Chao Xia a,b, Lingyan Shen a,b, Yuehui Yu a a b
State Key Laboratory of Functional Materials for Informatics, SIMIT, Chinese Academy of Sciences, Shanghai 200050, China University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2014 Accepted 27 August 2014 Available online 6 September 2014
In this work, we investigated the effects of post rapid thermal annealing (RTA) at 800 1C on structure and optical properties of HfAlO films directly grown on graphene by atomic layer deposition (ALD). Raman spectra indicated that no defects were introduced into graphene through film growth and RTA process. X-ray photoelectric (XPS) spectra showed that RTA contributed to thermal decomposition of hydroxides. Transmission electron microscopy (TEM) images and grazing incidence X-ray diffraction (GIXRD) patterns indicated that the amorphous morphology stability of HfAlO during the RTA process, and spectroscopic ellipsometry (SE) showed that RTA had few effects on optical properties of HfAlO on graphene. & 2014 Elsevier B.V. All rights reserved.
Keywords: Dielectrics Microstructure Annealing Graphene HfAlO films
1. Introduction Graphene, a monolayer of carbon atoms arranged in a honeycomb lattice, has been the subject of considerable interests since its discovery [1]. Its high carrier mobility, combined with the mechanical and thermodynamic stability, makes it a promising material for a wide range of applications, especially functioning as a semiconducting channel of nanoelectronic devices [2–4]. The fabrication of graphene-based field effect transistors (GFETs) requires a uniform gate dielectric on graphene. High-κ dielectrics, such as Hf-based oxides, are essential components in aggressively scaled GFETs due to their ability of increasing the gate switching voltage and suppressing the gate leakage current [5,6]. ALD may be an optimal technique for high-κ dielectrics growth, owing to its precise control of the film thickness and uniformity [7]. However, the graphene surface is chemical insert and ALD-dielectrics can only be deposited on graphene edges or defect sites [8,9]. Surface functionalization was used to allow ALD-dielectrics growth on graphene [10–12]. Nevertheless, the carrier mobility of graphene is degraded significantly after functionalization. Our previous work tried to deposit high-κ dielectrics directly onto graphene by ALD with assistance of pre-H2O treatment [13,14]. The physically absorbed H2O on graphene could act as deposition sites and allow the subsequent dielectrics growth.
n
Corresponding author. E-mail address: [email protected] (X. Cheng).
http://dx.doi.org/10.1016/j.matlet.2014.08.146 0167-577X/& 2014 Elsevier B.V. All rights reserved.
Up till now, it is not found the related study on the effects of postRTA on the high-κ dielectrics properties directly grown on graphene by ALD. The annealing process is a necessary and indispensable step in the standard complementary-metal–oxide–semiconductor (CMOS) integration process. This study is significant since it shows the compatibility between high-κ dielectrics, graphene and CMOS devices integration. In this work, the effects of post-RTA at 800 1C for 30 s on elemental constituents, crystal structure and optical properties of Aldoped HfO2 (HfAlO) films on graphene were explored. HfAlO was atomic-layer-deposited directly on non-functional grapheme, and no additional defects were introduced into graphene through growth and post-RTA process confirmed by Raman spectra.
2. Experimental Graphene was grown on Cu foils by a chemical vapor deposition method at 1050 1C [15]. After growth, graphene was transferred onto Si and SiO2/Si, respectively. Acetone was utilized to remove the photoresist residue on graphene. HfAlO was deposited from tetrakis (ethylmethylamino)hafnium and trimethylaluminum (purchased from J&K) in a commercial ALD reactor. First, four cycles of H2O were introduced into ALD chamber to act as deposition sites. Second, 3 nm thickness of HfAlO was deposited at 100 1C on graphene. Third, the chamber temperature was elevated to 200 1C to deposit another 6 nm thickness of HfAlO. Details about the optimization of preparation process can be found in our previous work [13,14]. After dielectrics growth, post-RTA at 800 1C for 30 s was implemented on HfAlO films.
L. Zheng et al. / Materials Letters 137 (2014) 200–202
Raman spectra were performed to indicate whether post-RTA introduced any defects into graphene. In addition, XPS, TEM and SE were applied for the investigation of the post-RTA effects on the elemental constituents, microstructure and optical properties of HfAlO on graphene, respectively.
3. Results and discussion Raman analysis: Raman spectra were utilized to determine the quality of graphene under different conditions. As shown in Fig. 1a,
Fig. 1. Raman spectra of graphene: (a) pristine graphene; (b) graphene with HfAlO films; (c) graphene with HfAlO films after post-RTA.
201
pristine graphene had a weak D-band peak at 1350 cm 1, a G-band peak at 1558 cm 1, and a sharp 2D-band peak at 2659 cm 1 with a full width at half maximum of 40 cm 1. In addition, the I2D/IG ratio was greater than 1.25. All the Raman characteristic peaks indicated that graphene was monolayer with few defects. The Raman spectrum of graphene with HfAlO films was almost the same as the pristine one, besides a lower I2D/IG radio, shown in Fig. 1b. The decrease of I2D/IG radio could be caused by the non-adiabatic removal of the Kohn anomaly from the Γ point [16,17]. After post-RTA, no raise of defectrelated D-band was detected, shown in Fig. 1c, implying no damage was introduced into graphene during post-RTA process. It was worth mentioning that the I2D/IG ratio was further decreased, indicating stronger adhesion of HfAlO films on graphene after post-RTA. XPS analysis: To determine whether post-RTA was benefited to the properties of HfAlO on graphene grown directly by ALD, samples were measured by XPS. All the XPS peaks were calibrated with the C 1s peak located at 284.8 eV. As shown in Fig. 2a and b, Al 2p peak could be fitted as a symmetric single peak at 74.8 eV; the peak positions of Hf 4f5/2 and Hf 4f7/2 were at 18.8 eV and 17.2 eV, respectively, and the binding energy difference was 1.6 eV. These results clearly showed the existence of Al3 þ and Hf4 þ . The peak position of Hf 4f in HfAlO was shifted to a higher binding energy after post-RTA as shown in Fig. 3e, which was caused by the stronger incorporation of Al-oxide into Hf-oxide during the post-RTA process. As shown in Fig. 2c, deconvolution of O 1s peak revealed three distinct components. The stronger peak located at 531.4 eV originated from hydroxyl groups. The other two peaks located at 533 eV and 530.5 eV originated from Al–O bonds and Hf–O bonds, respectively. After post-RTA, the Al–O bond upshifted while the hydroxide bond downshifted, shown in Fig. 2f, indicating the property improvement of HfAlO. TEM analysis: TEM was carried out to investigate the effects of post-RTA on microstructure of HfAlO on graphene. The TEM images of HfAlO on graphene before and after post-RTA were illustrated in Fig. 3a and b, respectively. HfAlO films deposited at two temperatures were both amorphous. The initial 3 nm thickness of HfAlO was loose due to low-temperature deposition. But fortunately, the initial loose film was similar to a seed layer, and benefited for the nucleation and compactness of the subsequent 6 nm HfAlO growth at 200 1C. After post-RTA, HfAlO films maintained amorphous due
Fig. 2. XPS peak spectra of HfAlO on graphene. Al 2p (a), Hf 4f (b) and O1s (c) of HfAlO on graphene; Al 2p (d), Hf 4f (e) and O1s (f) of HfAlO on graphene after post-RTA.
202
L. Zheng et al. / Materials Letters 137 (2014) 200–202
Fig. 3. TEM images HfAlO on graphene before (a) and after (b) post-RTA. (c) GIXRD patterns of HfAlO on graphene before and after post-RTA.
Fig. 4. Optical analysis of HfAlO on graphene with and without post-RTA: (a) refractive index; (b) absorption coefficients; (c) complex permitivity.
to Al2O3 permeation, as shown in Fig. 3b. This result was consistent with GIXRD analysis. As shown in Fig. 3c, no diffraction peaks were detected for HfAlO films annealed at 800 1C, indicating Al-oxide permeation could act as a network modifier and stabilize the amorphous phase of HfAlO films. Optical properties analysis: The optical constants such as refractive index, absorption coefficients and complex permittivity of HfAlO on graphene were analyzed by SE measurements. As shown in Fig. 4a, the refractive index of HfAlO on graphene increased with wavelength in the ultra-violet region and reached a maximum at 400 nm. Then, it began a slow decline in the visible light region. As shown in Fig. 4b, HfAlO on graphene had an absorption edge in the ultra-violet region while it was transparent in the visible light region. Both refractive index and absorption coefficients were related to the microstructure of HfAlO films [18–20]. After postRTA, no obvious changes of refractive index or absorption coefficients were detected, indicating HfAlO maintained amorphous during the RTA process. Fig. 4c illustrated the complex permittivity of HfAlO on graphene with and without post-RTA. Both the real part εr and imaginary part εi of the complex permittivity were almost unaltered after post-RTA. Therefore, post-RTA had few effects on optical properties of HfAlO on graphene. 4. Conclusion In summary, we have investigated the effects of post-RTA on HfAlO directly deposited on graphene by ALD. After post-RTA, the hydroxyl groups are significantly reduced and HfAlO maintain amorphous due to Al-oxide permeation. The refractive index, extinction coefficients and complex permittivity of HfAlO on graphene are almost unaltered after post-RTA, indicating high quality of the dielectric films. No additional defects are introduced into graphene, and no observable change of HfAlO morphology is detected after post-RTA even at 800 1C. Therefore, the growing process of HfAlO on graphene by H2O-based ALD is compatible with the fabrication of GFETs.
Acknowledgements This work is funded by the National Natural Science Foundation of China (Grant no. 11175229). We would like to thank Prof. Zengfeng Di, Dr. Gang Wang, Dr. Xiaohu Zheng and Dr. Haoran Zhang for technical assistance and discussions.
References [1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dobonos SV, et al. Science 2004;306:666–9. [2] Nolotin KI, Sikes KJ, Jiang Z, Klima M, Fudenberg G, Hone J, et al. Solid State Commun 2008;146:351–5. [3] Lee C, Wei X, Kysar JW, Hone J. Science 2008;321:385–8. [4] Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Nano Lett 2008;8:3498–502. [5] Chae SH, Yu WJ, Bae JJ, Duong DL, Perello D, Jeong HY, et al. Nat Mater 2013;12:403–9. [6] Zou K, Hong X, Keefer D, Zhu J. Phys Rev Lett 2010;105:126601. [7] Wang X, Tabakman SM, Dai H. J Am Chem Soc 2008;130:8152–3. [8] Lee BK, Park SY, Kim HC, Cho K, Vogel EM, Kim MJ, et al. Appl Phys Lett 2008;92:203102. [9] Xuan Y, Wu YQ, Shen Y, Qi M, Capano MA, Cooper JA, et al. Appl Phys Lett 2008;92:013101. [10] Farmer DB, Lin Y, Afzali-Ardakani A, Avouris P. Appl Phys Lett 2009;94:213106. [11] Kim S, Nah J, Jo I, Shahrjerdi D, Colombo L, Yao Z, et al. Appl Phys Lett 2009;94:062107. [12] Hollander MJ, LaBella M, Hughes ZR, Zhu M, Trumbull KA, Cavalero R. Nano Lett 2011;11:3601–7. [13] Zheng L, Cheng X, Cao D, Wang Z, Xu D, Xia C, et al. J Vac Sci, A 2014;32:01A103. [14] Zheng L, Cheng X, Cao D, Wang G, Wang Z, Xu D, et al. ACS Appl Mater Interfaces 2014;6:7014–9. [15] Celebi K, Cole MT, Choi JW, Wyczisk F, Legagneux P, Rupesinghe N, et al. Nano Lett 2013;13:967–74. [16] Pisana S, Lazzeri M, Casiraghi C, Novoselov KS, Geim AK, Ferrar1 AC, et al. Nat Mater 2007;6:198–201. [17] Das A, Pisana S, Chakraborty B, Piscanec S, Saha SK, Waghmare UV, et al. Nat Nanotechnol 2008;3:210–5. [18] Chaaya AA, Viter R, Baleviciute I, Bechelany M, Ramanavicius A, Gertnere Z, et al. J Phys Chem C 2014;118:3811–9. [19] Hong YE, Kim YS, Do K, Lee D, Ko DH, Ku JH, et al. J Vac Sci, A 2005;23:1413. [20] Aarik J, Mandar H, Kirm M, Pung L. Thin Solid Films 2004;466:41–7.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of rapid thermal annealing on properties of HfAlO films directly deposited by ALD on graphene Li Zheng a,b, Xinhong Cheng a,n, Duo Cao a,b, Zhongjian Wang a, Dawei Xu a, Chao Xia a,b, Lingyan Shen a,b, Yuehui Yu a a b
State Key Laboratory of Functional Materials for Informatics, SIMIT, Chinese Academy of Sciences, Shanghai 200050, China University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2014 Accepted 27 August 2014 Available online 6 September 2014
In this work, we investigated the effects of post rapid thermal annealing (RTA) at 800 1C on structure and optical properties of HfAlO films directly grown on graphene by atomic layer deposition (ALD). Raman spectra indicated that no defects were introduced into graphene through film growth and RTA process. X-ray photoelectric (XPS) spectra showed that RTA contributed to thermal decomposition of hydroxides. Transmission electron microscopy (TEM) images and grazing incidence X-ray diffraction (GIXRD) patterns indicated that the amorphous morphology stability of HfAlO during the RTA process, and spectroscopic ellipsometry (SE) showed that RTA had few effects on optical properties of HfAlO on graphene. & 2014 Elsevier B.V. All rights reserved.
Keywords: Dielectrics Microstructure Annealing Graphene HfAlO films
1. Introduction Graphene, a monolayer of carbon atoms arranged in a honeycomb lattice, has been the subject of considerable interests since its discovery [1]. Its high carrier mobility, combined with the mechanical and thermodynamic stability, makes it a promising material for a wide range of applications, especially functioning as a semiconducting channel of nanoelectronic devices [2–4]. The fabrication of graphene-based field effect transistors (GFETs) requires a uniform gate dielectric on graphene. High-κ dielectrics, such as Hf-based oxides, are essential components in aggressively scaled GFETs due to their ability of increasing the gate switching voltage and suppressing the gate leakage current [5,6]. ALD may be an optimal technique for high-κ dielectrics growth, owing to its precise control of the film thickness and uniformity [7]. However, the graphene surface is chemical insert and ALD-dielectrics can only be deposited on graphene edges or defect sites [8,9]. Surface functionalization was used to allow ALD-dielectrics growth on graphene [10–12]. Nevertheless, the carrier mobility of graphene is degraded significantly after functionalization. Our previous work tried to deposit high-κ dielectrics directly onto graphene by ALD with assistance of pre-H2O treatment [13,14]. The physically absorbed H2O on graphene could act as deposition sites and allow the subsequent dielectrics growth.
n
Corresponding author. E-mail address: [email protected] (X. Cheng).
http://dx.doi.org/10.1016/j.matlet.2014.08.146 0167-577X/& 2014 Elsevier B.V. All rights reserved.
Up till now, it is not found the related study on the effects of postRTA on the high-κ dielectrics properties directly grown on graphene by ALD. The annealing process is a necessary and indispensable step in the standard complementary-metal–oxide–semiconductor (CMOS) integration process. This study is significant since it shows the compatibility between high-κ dielectrics, graphene and CMOS devices integration. In this work, the effects of post-RTA at 800 1C for 30 s on elemental constituents, crystal structure and optical properties of Aldoped HfO2 (HfAlO) films on graphene were explored. HfAlO was atomic-layer-deposited directly on non-functional grapheme, and no additional defects were introduced into graphene through growth and post-RTA process confirmed by Raman spectra.
2. Experimental Graphene was grown on Cu foils by a chemical vapor deposition method at 1050 1C [15]. After growth, graphene was transferred onto Si and SiO2/Si, respectively. Acetone was utilized to remove the photoresist residue on graphene. HfAlO was deposited from tetrakis (ethylmethylamino)hafnium and trimethylaluminum (purchased from J&K) in a commercial ALD reactor. First, four cycles of H2O were introduced into ALD chamber to act as deposition sites. Second, 3 nm thickness of HfAlO was deposited at 100 1C on graphene. Third, the chamber temperature was elevated to 200 1C to deposit another 6 nm thickness of HfAlO. Details about the optimization of preparation process can be found in our previous work [13,14]. After dielectrics growth, post-RTA at 800 1C for 30 s was implemented on HfAlO films.
L. Zheng et al. / Materials Letters 137 (2014) 200–202
Raman spectra were performed to indicate whether post-RTA introduced any defects into graphene. In addition, XPS, TEM and SE were applied for the investigation of the post-RTA effects on the elemental constituents, microstructure and optical properties of HfAlO on graphene, respectively.
3. Results and discussion Raman analysis: Raman spectra were utilized to determine the quality of graphene under different conditions. As shown in Fig. 1a,
Fig. 1. Raman spectra of graphene: (a) pristine graphene; (b) graphene with HfAlO films; (c) graphene with HfAlO films after post-RTA.
201
pristine graphene had a weak D-band peak at 1350 cm 1, a G-band peak at 1558 cm 1, and a sharp 2D-band peak at 2659 cm 1 with a full width at half maximum of 40 cm 1. In addition, the I2D/IG ratio was greater than 1.25. All the Raman characteristic peaks indicated that graphene was monolayer with few defects. The Raman spectrum of graphene with HfAlO films was almost the same as the pristine one, besides a lower I2D/IG radio, shown in Fig. 1b. The decrease of I2D/IG radio could be caused by the non-adiabatic removal of the Kohn anomaly from the Γ point [16,17]. After post-RTA, no raise of defectrelated D-band was detected, shown in Fig. 1c, implying no damage was introduced into graphene during post-RTA process. It was worth mentioning that the I2D/IG ratio was further decreased, indicating stronger adhesion of HfAlO films on graphene after post-RTA. XPS analysis: To determine whether post-RTA was benefited to the properties of HfAlO on graphene grown directly by ALD, samples were measured by XPS. All the XPS peaks were calibrated with the C 1s peak located at 284.8 eV. As shown in Fig. 2a and b, Al 2p peak could be fitted as a symmetric single peak at 74.8 eV; the peak positions of Hf 4f5/2 and Hf 4f7/2 were at 18.8 eV and 17.2 eV, respectively, and the binding energy difference was 1.6 eV. These results clearly showed the existence of Al3 þ and Hf4 þ . The peak position of Hf 4f in HfAlO was shifted to a higher binding energy after post-RTA as shown in Fig. 3e, which was caused by the stronger incorporation of Al-oxide into Hf-oxide during the post-RTA process. As shown in Fig. 2c, deconvolution of O 1s peak revealed three distinct components. The stronger peak located at 531.4 eV originated from hydroxyl groups. The other two peaks located at 533 eV and 530.5 eV originated from Al–O bonds and Hf–O bonds, respectively. After post-RTA, the Al–O bond upshifted while the hydroxide bond downshifted, shown in Fig. 2f, indicating the property improvement of HfAlO. TEM analysis: TEM was carried out to investigate the effects of post-RTA on microstructure of HfAlO on graphene. The TEM images of HfAlO on graphene before and after post-RTA were illustrated in Fig. 3a and b, respectively. HfAlO films deposited at two temperatures were both amorphous. The initial 3 nm thickness of HfAlO was loose due to low-temperature deposition. But fortunately, the initial loose film was similar to a seed layer, and benefited for the nucleation and compactness of the subsequent 6 nm HfAlO growth at 200 1C. After post-RTA, HfAlO films maintained amorphous due
Fig. 2. XPS peak spectra of HfAlO on graphene. Al 2p (a), Hf 4f (b) and O1s (c) of HfAlO on graphene; Al 2p (d), Hf 4f (e) and O1s (f) of HfAlO on graphene after post-RTA.
202
L. Zheng et al. / Materials Letters 137 (2014) 200–202
Fig. 3. TEM images HfAlO on graphene before (a) and after (b) post-RTA. (c) GIXRD patterns of HfAlO on graphene before and after post-RTA.
Fig. 4. Optical analysis of HfAlO on graphene with and without post-RTA: (a) refractive index; (b) absorption coefficients; (c) complex permitivity.
to Al2O3 permeation, as shown in Fig. 3b. This result was consistent with GIXRD analysis. As shown in Fig. 3c, no diffraction peaks were detected for HfAlO films annealed at 800 1C, indicating Al-oxide permeation could act as a network modifier and stabilize the amorphous phase of HfAlO films. Optical properties analysis: The optical constants such as refractive index, absorption coefficients and complex permittivity of HfAlO on graphene were analyzed by SE measurements. As shown in Fig. 4a, the refractive index of HfAlO on graphene increased with wavelength in the ultra-violet region and reached a maximum at 400 nm. Then, it began a slow decline in the visible light region. As shown in Fig. 4b, HfAlO on graphene had an absorption edge in the ultra-violet region while it was transparent in the visible light region. Both refractive index and absorption coefficients were related to the microstructure of HfAlO films [18–20]. After postRTA, no obvious changes of refractive index or absorption coefficients were detected, indicating HfAlO maintained amorphous during the RTA process. Fig. 4c illustrated the complex permittivity of HfAlO on graphene with and without post-RTA. Both the real part εr and imaginary part εi of the complex permittivity were almost unaltered after post-RTA. Therefore, post-RTA had few effects on optical properties of HfAlO on graphene. 4. Conclusion In summary, we have investigated the effects of post-RTA on HfAlO directly deposited on graphene by ALD. After post-RTA, the hydroxyl groups are significantly reduced and HfAlO maintain amorphous due to Al-oxide permeation. The refractive index, extinction coefficients and complex permittivity of HfAlO on graphene are almost unaltered after post-RTA, indicating high quality of the dielectric films. No additional defects are introduced into graphene, and no observable change of HfAlO morphology is detected after post-RTA even at 800 1C. Therefore, the growing process of HfAlO on graphene by H2O-based ALD is compatible with the fabrication of GFETs.
Acknowledgements This work is funded by the National Natural Science Foundation of China (Grant no. 11175229). We would like to thank Prof. Zengfeng Di, Dr. Gang Wang, Dr. Xiaohu Zheng and Dr. Haoran Zhang for technical assistance and discussions.
References [1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dobonos SV, et al. Science 2004;306:666–9. [2] Nolotin KI, Sikes KJ, Jiang Z, Klima M, Fudenberg G, Hone J, et al. Solid State Commun 2008;146:351–5. [3] Lee C, Wei X, Kysar JW, Hone J. Science 2008;321:385–8. [4] Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Nano Lett 2008;8:3498–502. [5] Chae SH, Yu WJ, Bae JJ, Duong DL, Perello D, Jeong HY, et al. Nat Mater 2013;12:403–9. [6] Zou K, Hong X, Keefer D, Zhu J. Phys Rev Lett 2010;105:126601. [7] Wang X, Tabakman SM, Dai H. J Am Chem Soc 2008;130:8152–3. [8] Lee BK, Park SY, Kim HC, Cho K, Vogel EM, Kim MJ, et al. Appl Phys Lett 2008;92:203102. [9] Xuan Y, Wu YQ, Shen Y, Qi M, Capano MA, Cooper JA, et al. Appl Phys Lett 2008;92:013101. [10] Farmer DB, Lin Y, Afzali-Ardakani A, Avouris P. Appl Phys Lett 2009;94:213106. [11] Kim S, Nah J, Jo I, Shahrjerdi D, Colombo L, Yao Z, et al. Appl Phys Lett 2009;94:062107. [12] Hollander MJ, LaBella M, Hughes ZR, Zhu M, Trumbull KA, Cavalero R. Nano Lett 2011;11:3601–7. [13] Zheng L, Cheng X, Cao D, Wang Z, Xu D, Xia C, et al. J Vac Sci, A 2014;32:01A103. [14] Zheng L, Cheng X, Cao D, Wang G, Wang Z, Xu D, et al. ACS Appl Mater Interfaces 2014;6:7014–9. [15] Celebi K, Cole MT, Choi JW, Wyczisk F, Legagneux P, Rupesinghe N, et al. Nano Lett 2013;13:967–74. [16] Pisana S, Lazzeri M, Casiraghi C, Novoselov KS, Geim AK, Ferrar1 AC, et al. Nat Mater 2007;6:198–201. [17] Das A, Pisana S, Chakraborty B, Piscanec S, Saha SK, Waghmare UV, et al. Nat Nanotechnol 2008;3:210–5. [18] Chaaya AA, Viter R, Baleviciute I, Bechelany M, Ramanavicius A, Gertnere Z, et al. J Phys Chem C 2014;118:3811–9. [19] Hong YE, Kim YS, Do K, Lee D, Ko DH, Ku JH, et al. J Vac Sci, A 2005;23:1413. [20] Aarik J, Mandar H, Kirm M, Pung L. Thin Solid Films 2004;466:41–7.