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Article

Water Incorporation in Graphene Transferred onto SiO/Si Investigated by Isotopic Labeling 2

Nicolau Molina Bom, Gabriel Vieira Soares, Myriano Henriques de Oliveira, Joao Marcelo J. Lopes, Henning Riechert, and Claudio Radtke J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06780 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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The Journal of Physical Chemistry

Water Incorporation in Graphene Transferred onto SiO2/Si Investigated by Isotopic Labeling §

Nicolau Molina Bom, †,*Gabriel Vieira Soares, ‡Myriano Henriques de Oliveira Junior, #João Marcelo J. Lopes, #Henning Riechert and ║Cláudio Radtke

§

Programa de Pós Graduação em Microeletrônica, UFRGS, Porto Alegre-RS, Brazil †

‡

Instituto de Física, UFRGS, Porto Alegre-RS, Brazil

Departamento de Física, ICEx, UFMG, Belo Horizonte-MG, Brazil #

Paul-Drude-Institut für Festkörperelektronik, Berlin, Germany ║

*Corresponding

Instituto de Química, UFRGS, Porto Alegre-RS, Brazil

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ABSTRACT

H and O incorporation in single-layer graphene (SLG) transferred onto SiO2/Si substrates following annealing in water vapor (H2O) was investigated. The use of isotopically enriched water in conjunction with nuclear reaction analysis enabled to specifically investigate the effect of water annealing, among other modification agents of graphene like ambient exposure. Results revealed that incorporation of these species occur by two distinct mechanisms, depending on the temperature range considered. From 100 to 300 °C, physisorption of H2O molecules in the SLG/SiO2/Si structure is the dominant process. For 400°C and above, chemisorption is favored due to the creation of defects in the graphene lattice. Transport measurements demonstrated that the observed physico-chemical and structural modifications have a huge impact on the electrical characteristics of these structures.

Keywords: graphene, isotopic tracing, ion beam analysis, water


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INTRODUCTION

Graphene is the two-dimensional building block for carbon allotropes, exhibiting fascinating physical properties1-3 such as very high carrier mobility4 and near-ballistic transport at room temperature5. Moreover, the possibility to make very thin channels will allow continuing the downscaling in field-effect transistors (FETs) channel lengths and achieve higher device processing speeds6. Owing to these characteristics, graphene is considered one of the most promising contenders for future nanoelectronic devices. Considering its synthesis, diverse methods have been intensively studied. Although graphene obtained by micromechanical exfoliation is still popular in laboratory and offer the most promising properties, it is not suitable to electronics industry. The implementation of graphene in semiconductor technology will only be possible through a large-scale fabrication approach. Chemical vapor deposition (CVD) of single-layer graphene (SLG) on catalytic metal7,8 surfaces is a well-established method to fulfill this purpose. However, SLG prepared by CVD needs to be transferred onto a non-metallic substrate, aiming at device fabrication. Poly(methyl methacrylate) (PMMA) is the most common supporting film used in the transfer, being removed after the process. Nevertheless, it is known that PMMA residues can still remain on graphene even after removal procedures,9-11 degrading its charge transport characteristics due to the high chemical sensitivity of the graphene surface12. Furthermore, it is expected that a non-negligible number of H2O molecules is inserted at the SLG/substrate interface during its transfer, in consequence of the substrate’s hydrophilicity. This has been associated to the hysteresis observed in the gate characteristics of graphene devices13,14. Processing and modification of graphene properties for technological purposes are also challenging. In particular, the adsorption of different species in SLG/SiO2/Si structures is of


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central importance. Understanding these processes is necessary in order to meet the physical requirements for industrial applications of graphene. Previous work shows that hydrogenation of SLG/SiO2/Si resulted in the appearance of Raman disorder-induced D band, which is assigned to a change in carbon hybridization from sp2 to sp3 due to the formation of C-H bonds, causing a significant decrease in carrier mobilities15. Charge carrier concentration is also increased by adsorbed NH3, NO2, CO, H2O, and O2 gas molecules16-18. In the latter case, the hole-doping due to superoxide anions (O2-) is greatly promoted by water vapor. This effect is attributed to water solvation and electrostatic binding of O2- to the SiO2 surface, where the electrons are provided by graphene19. In addition, it should be mentioned that an increase in hole-doping was observed even following vacuum annealings12,18,20. The reasons for this effect are twofold: (i) heating brings graphene closer to the undulated oxide surface, and this coupling increases the charge transfer that enables doping20; (ii) after thermal treatment, the number of adsorbates is reduced, providing adsorption sites for H2O and O2 molecules when the sample is exposed to air13,18. Suzuki et al.13 propose that heating prior to air exposure leads to defect formation in graphene lattice, which would also act as adsorption sites. Supporting this assumption, DFT calculations of Cabrera-Sanfelix and Darling21 demonstrated that water interaction with graphite is much stronger at defective sites. Besides O and H incorporation in the basal plane of graphene, diffusion of water at the graphene/SiO2 interface was also observed22: Raman spectroscopy visualized intercalation of water from the edge to the center underneath graphene. Considering the discussion above, we systematically investigated the incorporation of H and O in SLG transferred onto SiO2/Si substrates, upon annealing in water vapor. In order to quantify the incorporation of these elements specifically related to the annealing step, we used water enriched in 2H (Deuterium, D) and

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O rare isotopes (natural abundances of 0.0115 and


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0.205 %, respectively). Thus, eventual modifications (due to air exposure, for example) do not interfere in the quantification procedure. The main objective is to circumvent experimental hurdles of commonly used ex-situ characterization techniques that picture the effect of H and O incorporation following the whole processing history of the sample, including ambient exposure. An example is the investigation of Suzuki and coworkers13, who used Raman spectroscopy to characterize graphene transferred onto SiO2/Si samples submitted to heating in vacuum followed by air exposure. The observed hole doping was attributed to air adsorbates at defect sites created by heating. The incorporation of the rare isotopes in the SLG/SiO2/Si structure takes place both at the adjacency of graphene and in the SiO2 layer. In order to better estimate the amount of D and

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O exclusively incorporated due to the graphene layer, we prepared SiO2/Si samples

(without transferred graphene, sample B in Fig. 1a) annealed concomitantly to their graphenecovered counterparts (sample A in Fig. 1a). The difference in the amounts of D and

18

O

incorporated in these samples (as depicted in Fig. 1a) corresponds to the concentration of these isotopes related to the graphene’s contribution. As discussed below, these concentrations are underestimated. Anyway, we could investigate the reactivity of graphene towards water exposure as a function of temperature. Moreover, comparing these results with structural and transport properties of the resulting graphene layer we could infer about physico-chemical modifications induced by the annealing. The present findings suggest different H2O incorporation mechanisms depending on the temperature range, associated to modifications in the structure (and consequently in the reactivity) of the graphene layers.


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Figure 1. (a) Preparation procedure of SLG/SiO2/Si and SiO2/Si samples. The first step consisted of outgassing in vacuum (200°C, 30 min) followed by annealing (100 to 600ºC) for 1h, in 10 mbar of D218O. The difference in the amounts of D and 18O between samples A and B represent the amount of these isotopes present in the sample as a result of the graphene transfer. (b) Sketch of the experimental setup used in nuclear reaction analysis (NRA) of induce the

18

18

O. Protons of 730 keV

O(p,α)15N nuclear reaction producing α particles. These particles reach a surface

barrier detector covered by a Mylar film which absorbs backscattered protons. NRA quantifies specifically

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O as depicted in the sketch. A similar experimental setup is used for D

quantification.


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EXPERIMENTAL METHODS

Samples consisted of commercially available CVD grown graphene (grown on polycrystalline Cu foils) transferred onto SiO2 films (285 nm thick) on Si (purchased from Graphene Supermarket™). The SLG covers the whole substrate surface. Counterpart SiO2/Si samples (without graphene) were prepared for comparison. A static pressure, resistively heated quartz tube furnace was used for the experiments, which was pumped down to 2 x 10-7 mbar prior to water vapor loading. As illustrated in Fig. 1a, the samples first underwent an outgassing step in vacuum (200 °C, 30 min). Following this step, no PMMA residuals were detected as shown by photoemission and atomic force microscopy (AFM) experiments (see the Supporting Information (SI)). Afterwards, annealings were performed at temperatures ranging from 100 to 600 ºC for 1 h, in 10 mbar of water vapor simultaneously enriched in the

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O and D rare isotopes (hereafter

called D218O). This vapor pressure corresponds approximately to the water partial pressure in air of 30 % relative humidity at 25 ºC. Following annealing in water, samples were cooled to room temperature and then exposed to air prior to subsequent analysis. 18O and D quantifications were accomplished by NRA, using the

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O(p,α)15N nuclear reaction23 at 730 keV (1013

18

O/cm2

sensitivity and 5 % accuracy) and the D(3He,p)4He nuclear reaction24 at 400 keV (1012 D/cm2 sensitivity and 5% accuracy), respectively (see Fig. 1b). NRA enables quantification of the 18O and D rare isotopes without the interference of the most abundant ones. Raman spectroscopy was carried out with the 482.5 nm-line of a Kr+ ion laser with spatial resolution of 1 µm. Several measurements were performed at different locations of the samples in order to obtain information concerning the homogeneity of the layers. Magneto transport measurements in a


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large area (> 5 mm) Van-der-Pauw geometry were employed to evaluate the carrier density and mobility values, using low magnetic fields (0.6 T) at room temperature. Measurements were performed after the samples were removed from the furnace.

RESULTS AND DISCUSSION

Fig. 2a shows the D areal density as a function of D218O annealing temperature for SLG/SiO2/Si (solid circles) and for bare SiO2/Si (open circles). The D areal densities in samples prior to annealing were below the detection limit of the technique. At 100 ºC, we observe a slight D incorporation, which is similar for both samples. When the annealing is performed in the 200400 ºC temperature range, a higher D uptake in SLG/SiO2/Si is observed in comparison to bare SiO2/Si. Areal concentrations are between 0.7-1.2 ×1015 D/cm2, corresponding to 18-30 % of the C concentration in a graphene monolayer (~3.9 ×1015 C/cm2). One may relate this relatively high D concentration to its pronounced attachment to graphene, especially at defective regions, such as grain boundaries (graphene grown on Cu foils by CVD is typically polycrystalline25) or even in the basal plane of graphene, changing its hybridization from sp2 to sp3. Nevertheless, water intercalation at the SLG/SiO2 interface and physisorption cannot be ruled out. Besides NRA quantification, a complementary analytical tool like Raman spectroscopy is necessary to infer about structural modifications and defect creation in SLG as a result of annealing. By this way, a clearer picture of how water interacts with SLG is possible. These issues will be discussed below. At 600 ºC, the SLG sample shows a significant decrease in D uptake, while for the bare SiO2/Si the D areal density shows an opposite behavior. Such observation is consistent with


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reported results evidencing that hydrogen bonded to graphene is unstable for temperatures above 450 ºC15. The increase verified in SiO2 films can be attributed to the formation of silanol groups (SiOH) in the SiO2 surface26.


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Figure 2. (a) D and (b) 18O areal densities as functions of annealing temperature for SLG/SiO2/Si (solid symbols) and SiO2/Si (open symbols). (c)

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O (solid circles) and D (open circles) areal

densities incorporated in graphene as function of annealing temperature, obtained from the difference between SLG/SiO2/Si and SiO2/Si curves in parts (a) and (b). Lines are only to guide the eyes. Figure 2b shows the

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O areal density as a function of annealing temperature. Since in both

samples the SiO2 film is 285 nm-thick, one can use the relation 1 × 1015 O/cm2 = 0.226 nm to determine the total amount of oxygen in this film23. The amount of the 18O isotope, considering its natural abundance of 0.2 % is about 2.5 × 1015 18O/cm2. This value is obtained by NRA for non-annealed samples. At 100 ºC, no significant 18O incorporation is observed for both samples. Once again, in the 200-400 ºC temperature range a higher uptake is observed for SLG/SiO2/Si samples. At 600 ºC, the

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O areal density keeps increasing for both samples, reaching

approximately the same value (~ 1.3 × 1016 O/cm2). Moreover, it’s noteworthy that at temperatures ≥ 400 °C the incorporation raise is more sloped for bare SiO2/Si samples, in comparison with those covered with SLG. This effect can be assigned to the isotopic exchange between SiO2 and the gas phase. This trend is not followed by the SLG/SiO2/Si structure, probably due to the fact that the graphene can act as an additional diffusion path for water27, hindering the isotopic exchange. Fig. 2c shows

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O and D amounts obtained by the difference

between samples with and without graphene (as depicted in Fig. 1), enabling to estimate the uptake taking place strictly related to the graphene layer. The ratio between

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O and D

concentrations of Fig. 2c in the 200-300 ºC range is approximately constant, which point to a common mechanism operating at these temperatures. However, one should keep in mind that these quantities are underestimated, as the SLG can act as a diffusion barrier. Thus, the supply of


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reactive species at the SiO2 surface in the case of SLG/SiO2/Si may be lower than for bare SiO2/Si. In order to verify structural and physico-chemical modifications of graphene associated to annealing in water vapor, Raman measurements were performed in SLG/SiO2/Si samples before and after annealings. Fig. 3a shows the collection of Raman spectra representative of samples after each annealing condition. Very similar spectra were obtained from different points of the surface (1 cm2), evidencing the homogeneity of the SLG. Spectra show two main features: the G (~1591 cm-1) peak, associated to E2g phonon at the Brillouin zone center28; and 2D peak (~2701 cm-1), corresponding to a double-resonance process29,30. The spectrum for pristine SLG evidences a very weak defect-induced D peak (~1353 cm-1)30,31. For the sample annealed at 100 ºC, no significant changes in peak shapes are observed, consistent with the absence of D and 18O uptake as checked by NRA (Fig. 2). As temperature is increased, one observes that the integrated intensity ratio between G and 2D peaks (AG/A2D) stays approximately constant up to 300 °C, strikingly increasing at 600 °C (Fig. 3b, circles). AG/A2D has been shown to be related to oxidative doping of graphene32-35, indicating that O incorporation in graphene is favored for temperatures ≥ 400 °C. In addition, the steep increase of AG/A2D at 600 oC may also be a consequence of the induced structural disorder in the SLG lattice. It’s noteworthy that, at this temperature, the structural disorder observed by Raman is not reversible, as evidenced by the spectrum of the same sample following an additional annealing in vacuum (see the Supporting Information (SI)). In order to probe this disorder, the evolution of the defect-induced D peak as function of temperature was analyzed. Fig. 3b (squares) shows the maximum intensity ratios between the D and G peaks (ID/IG), which is inversely related to the mean distance between defects36,37. We


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considered ID/IG instead of the areal ratio due to changes in the shape of the D peak at higher temperatures. The ID/IG curve exhibits a significant increase in the higher temperature regime (400-600 °C), evidencing the structural disorder induced by annealings. Moreover, it is possible to observe a significant broadening of the D and of G peaks at these conditions (Fig. 3a). This observation strongly supports the hypothesis that high temperature annealings (≥ 400 °C) cause large-range modifications in the graphene lattice. It’s noteworthy that in this high temperature regime one observes the highest 18O incorporation, as verified by NRA. It should be mentioned that these Raman spectral changes are very similar to those obtained for SLG/SiO2/Si annealed in vacuum and subsequently exposed to air (not shown).


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Figure 3. (a) Raman spectra of SLG/SiO2/Si samples before and after annealings. The G, 2D, and D bands are indicated. The spectra are normalized to the G peak intensity. a. u. stands for arbitrary units. (b) Dependence of the ID/IG (squares) and AG/A2D (circles) ratios on annealing temperature, corresponding to the Raman spectra in part (a).

Hints of the mechanisms underlying this behavior were provided by DFT calculations of Cabrera-Sanfelix21, which demonstrate that the dissociative chemical adsorption of H2O in graphite lattice preferentially occurs at defective zones, where C vacancies in the structure act as incorporation sites. In the case of perfect graphite, the high energetic barrier for chemisorption favors physical adsorption of H2O molecules on the surface. Correlating this model with the evolution of the Raman spectra observed here, it is possible to state that high temperature annealing (≥ 400 °C) increases the concentration of defects in the SLG network as evidenced by the relative intensity of the D peak (Fig. 3b). As a consequence, the reactivity of graphene layer is enhanced, probably leading to the dissociation of H2O molecules and subsequent chemisorption of O. It’s noteworthy that the 18O incorporation curve (Fig. 2b) of SLG/SiO2/Si is more sloped for T ≥ 400 °C, evidencing higher O uptake rates at this temperature range. For lower temperatures, the physisorption of H2O is presumably the dominant incorporation mechanism, since there is no sufficient energy to overcome the energy barrier for chemisorption. This assumption is valid for defect-free regions of SLG, since defective regions (like SLG’s edges, for example) are more reactive. The creation of defects in the graphene lattice, observed at higher temperatures, may be assigned to temperature-induced amorphization of the SLG38,39. Another possibility for explaining this effect is related to graphene-substrate interaction: as the SLG comes closer to SiO2 surface due to annealings, the corrugation of the layer may increase


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the stress, weakening the C-C bonds and enhancing its reactivity. X-ray photoelectron spectroscopy analysis (see the Supporting Information (SI)) with synchrotron radiation evidences that new carbon-oxygen functionalities are present in the SLG/SiO2/Si sample following annealing at 600 oC. A similar mechanism was verified for epitaxial graphene on Ru(0001) substrates40,41. Moreover, the SiO2 substrate can also influence H2O incorporation as evidenced in organic functionalization experiments of graphene with diazonium salts42. In view of this scenario, NRA and Raman data evidence that physisorption of H2O molecules is favored below 400 °C, and defect-induced chemical adsorption is the dominant mechanism for higher temperatures. In order to establish a connection between the observed structural and physico-chemical changes and the charge carrier characteristics, transport measurements were carried out. The results are shown in Fig. 4. The pristine SLG sample exhibits p-doping, with a value around 4 × 1012 cm-2 (in agreement with previous reports7,16), and carrier mobility about 940 cm2/V.s. For the sample annealed at 200 ºC, the mobility suffers a sharp decrease in comparison to the value measured for the pristine sample. This is followed by an increase in doping. Both changes may be associated to the D and

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O incorporation in the graphene layer (Figs. 2a and 2b). The

formation of oxygen-graphene complexes at SLG/SiO2 interface is responsible for the electron transfer19, and this effect is intensified with the presence of H2O molecules. As temperature rises, the doping exhibits a linear dependence in the 200-400 °C range, while charge mobility keeps approximately constant (~ 340 cm2/V.s). At 600 °C, the charge mobility decreases once again, and the charge density goes up to ~1.4 × 1013 cm-2. The behavior observed here corroborates the model previously proposed, in which defect-induced chemisorption takes place at higher temperatures, severely degrading the transport characteristics of the structure. The influence of


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the substrate in the p-doping cannot be ruled out, since the SLG is brought closer to substrate as an effect of heating, intensifying the electrostatic interaction between C atoms from graphene and O atoms from the SiO2 layer16.

Figure 4. (Upper part) Mobilities and (lower part) carrier densities of SLG/SiO2/Si samples as function of annealing temperature, obtained by magneto transport measurements. CONCLUSIONS In summary, we have investigated the effects of H2O absorption on the structural and transport properties of graphene layers upon annealing in a wide range of temperature. The use of isotopically enriched water in conjunction with NRA enabled to specifically quantify H and O incorporation as a result of annealing in water, evidencing that the incorporation behavior is strongly dependent on the annealing temperature for both species. The reactivity of graphene towards water exposure presents a threshold around 400 oC: (i) below this threshold, the increase in p-doping is associated to the physical adsorption of H2O in the SLG/SiO2/Si structure and to


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the closer contact between SLG and the SiO2 surface as an effect of heating; (ii) for 400°C and above, the concentration of defects sharply increases, which favor the chemical adsorption of O atoms in the graphene network as evidenced by NRA. Thus, the structural disorder verified in the SLG could be associated to the adsorption of species from the gas phase (identified by isotopic labeling), hypothesis previously suggested in literature. Finally, the connection between physicochemical modifications induced by annealings and the electrical properties of the SLG/SiO2/Si structures implies critical consequences for future applications in electronic devices based on graphene.

ACKNOWLEDGMENTS We would like to thank the financial support of INCT Namitec, INCT INES, MCT/CNPq, CAPES, FAPEMIG, FAPERGS, and LNLS-National Synchrotron Light Laboratory, Brazil. The authors would also like to deeply acknowledge the technical assistance of the Accelerator Group, especially the VUV and Soft X-ray Spectroscopy Group . We would also like to thank A.H. Englert and D.E. Weibel for AFM and photoemission analyses, respectively.


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Table of contents (TOC) image


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(a) Preparation procedure of SLG/SiO2/Si and SiO2/Si samples. The first step consisted of outgassing in vacuum (200 °C, 30 min) followed by annealing (100 to 600 ºC) for 1h, in 10 mbar of D218O. The difference in the amounts of D and 18O between samples A and B represent the amount of these isotopes present in the sample as a result of the graphene transfer. (b) Sketch of the experimental setup used in nuclear reaction analysis (NRA) of 18O. Protons of 730 keV induce the 18O(p,α)15N nuclear reaction producing α particles. These particles reach a surface barrier detector covered by a Mylar film which absorbs backscattered protons. NRA quantifies specifically 18O as depicted in the sketch. A similar experimental setup is used for D quantification. 312x195mm (96 x 96 DPI)


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(a) D and (b) 18O areal densities as functions of annealing temperature for SLG/SiO2/Si (solid symbols) and SiO2/Si (open symbols). (c) 18O (solid circles) and D (open circles) areal densities incorporated in graphene as function of annealing temperature, obtained from the difference between SLG/SiO2/Si and SiO2/Si curves in parts (a) and (b). Lines are only to guide the eyes. 82x180mm (300 x 300 DPI)



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(a) Raman spectra of SLG/SiO2/Si samples before and after annealings. The G, 2D, and D bands are indicated. The spectra are normalized to the G peak intensity. a. u. stands for arbitrary units. (b) Dependence of the ID/IG (squares) and AG/A2D (circles) ratios on annealing temperature, corresponding to the Raman spectra in part (a). 82x127mm (300 x 300 DPI)


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(Upper part) Mobilities and (lower part) carrier densities of SLG/SiO2/Si samples as function of annealing temperature, obtained by magneto transport measurements. 82x93mm (300 x 300 DPI)


Water Incorporation in Graphene Transferred onto ... - ACS Publications

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