Near room temperature reduction of graphene oxide Langmuir-Blodgett monolayers by hydrogen plasma.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 11708

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Near room temperature reduction of graphene oxide Langmuir–Blodgett monolayers by hydrogen plasma Gulbagh Singh,a V. Divakar Botcha,a D. S. Sutar,b Pavan K. Narayanam,a S. S. Talwar,a R. S. Srinivasac and S. S. Major*a Langmuir–Blodgett monolayer sheets of graphene oxide (GO) were transferred onto Si and SiO2/Si, and subjected to hydrogen plasma treatment near room temperature. GO monolayers were morphologically stable at low power (15 W) plasma treatment, for durations up to 2 min and temperatures up to 120 1C. GO monolayers reduced under optimized plasma treatment conditions (30 s duration at 50 1C) exhibit a sheet thickness of (0.5–0.6) nm, high sp2-C content (75%), a low O/C ratio (0.16) and a significant redshift of Raman G-mode to 1588 cm1, indicating efficient de-oxygenation and a substantial decrease of defects. A study of the valence band electronic structure of hydrogen plasma reduced GO monolayers shows an increase of DOS in the vicinity of the Fermi level, due to the increase of C 2p-p states, and a substantial decrease of work function. These results, along with conductivity measurements and transfer characteristics, reveal the p-type nature of hydrogen plasma reduced GO monolayers, displaying a con-

Received 28th February 2014, Accepted 16th April 2014

ductivity of (0.2–31) S cm1 and a field effect mobility of (0.1–6) cm2 V1 s1. Plasma treatment at higher temperatures results in a substantial increase in sp3-C/damaged alternant hydrocarbon content and

DOI: 10.1039/c4cp00875h

incorporation of defects related to the hydrogenation of the graphitic network, as evidenced by multiple Raman features, including a large red-shift of D-mode to 1331 cm1 and a high I(D)/I(G) ratio, and supported

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by the appearance of mid-gap states in the vicinity of the Fermi level.

1. Introduction The unique electrical, optical and mechanical properties of graphene have led to enormous possibilities of innovative applications. The oxidative exfoliation of graphite into a single layer of graphene oxide (GO), which can be converted to reduced GO, is a potential route to achieve the goal of large scale production of a low cost alternative to graphene for a variety of applications in nano-electronics, opto-electronics, sensors and photovoltaic devices.1–3 GO sheets are insulators, containing various oxygen functional groups, such as C–O, CQO, COOH, and –OH, on the basal plane and the periphery, which are responsible for the disruption of p-conjugation of the graphitic network. A variety of reduction approaches have been employed for the removal of oxygen functional groups and restoration of the graphitic network and to convert GO sheets into reduced GO sheets.4,5 a

Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India. E-mail: [email protected]; Fax: +91-22-25767552; Tel: +91-22-25767567 b Central Surface Analytical Facility, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India c Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India

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For device applications, it is highly desirable to carry out solid state reduction of the precursor GO sheets on a substrate. Several approaches, such as drop casting,6 spin casting,7 dip coating,8 spray coating9 and electrophoretic deposition,10 have been used for the deposition of GO sheets on solid substrates, most of which, often yield agglomerates of GO and overlapping sheets. In contrast, the Langmuir–Blodgett (LB) technique can facilitate a controlled transfer of GO monolayer sheets of desired morphology onto a variety of substrates.11,12 This is because the presence of hydroxyl and ionizable carboxylic groups, together with the hydrophobic graphene domains, imparts an overall amphiphilic character to the GO sheets, which can be spread at the air–water interface and transferred as LB monolayers of GO. There are mainly two approaches that have been employed for the reduction of GO. In the first approach, GO dispersions have been reduced in solution and then transferred onto solid substrates. For the reduction of GO in solution, various chemical reagents, such as hydrazine (N2H4) and its derivatives,13 sodium borohydride (NaBH4),14 alkali (NaOH, KOH),15 glucose (C6H12O6),16 hydrohalic acids,17 vitamin C (ascorbic acid),18 sulfur containing compounds,19 TiO2 and silver nanoparticles,20,21 and metal powders, have been used.22 Hydrothermal steaming of GO23 and hydrothermal treatment in ammonia24 and urea25 have

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also been used for the reduction of GO. In the other approach, GO dispersions have been used to deposit GO films by solution based methods, followed by solid state reduction, which is usually carried out by heat treatment in an inert/vacuum/H2 atmosphere.26,27 GO monolayers have also been reduced by heat treatment at relatively high temperatures (B1000 1C) in alcohol vapor28 and by using a copper cap layer in a H2 atmosphere.29 It has recently been reported that the graphitic network of free standing hydrazine reduced GO films can be restored by heat treatment at high temperatures in the range of 1500–2700 1C.30 The reduction of GO sheets has also been reported by subjecting the sheets to hydrogen plasma,31 methane plasma32 and methane–argon mixture plasma treatments.33 Simultaneous reduction and doping of GO sheets has been carried out by heating in ammonia34 and by ammonia plasma treatment.35 In this work, GO monolayer sheets transferred onto Si and SiO2/Si substrates have been subjected to hydrogen plasma treatment, under different conditions of power, temperature and the duration of plasma exposure. A comprehensive investigation of the morphological stability, chemical and electronic structure of hydrogen plasma treated GO monolayers has been carried out to identify the window of process parameters, in which, optimal reduction of GO monolayers takes place. The extent of removal of oxygen functionalities, the sp2-C content of reduced GO, and changes in sp3-C/damaged alternant hydrocarbon structure have been investigated by X-ray photoelectron spectroscopy (XPS). Raman spectroscopy has been used to assess the extent of reduction and the presence of defects in plasma reduced GO, including those related to hydrogenation. These studies have also been extended to explore the effects due to hydrogenation of the graphitic network that dominate after prolonged hydrogen plasma treatment, particularly at higher temperatures. The changes in valence band electronic structure of GO monolayers have been investigated by ultraviolet photoelectron spectroscopy (UPS) and corroborated with the XPS and Raman studies under all the conditions of hydrogen plasma treatment, explored in this work. Electrical conductivity and field effect mobility measurements of plasma reduced GO monolayers have been carried out in bottom-gated field effect geometry to demonstrate that p-type, reduced GO monolayers, with substantially improved electrical characteristics, can be obtained by hydrogen plasma treatment near room temperature.

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out at 15 W and 30 W for various durations between 10 s and 5 min. The surface morphology of as-transferred and hydrogen plasma treated GO sheets was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A Raith 150-Two instrument operated at 10 kV was used for SEM measurements and a Digital Instrument’s Veeco-Nanoscope IV Multimode scanning probe microscope was used in tapping mode for AFM measurements. Micro-Raman spectroscopy was performed using a Horiba Jobin Yvon HR800 confocal Raman microprobe equipped with a 514 nm Ar+ laser and the peak fitting analysis was carried out using Gaussian functions. XPS and UPS data have been recorded using a Thermo-VG Scientific photoelectron spectrometer (Multilab) equipped with a concentric hemispherical analyzer. The XPS measurements were performed in ultrahigh vacuum at a base pressure o107 Pa, using a monochromatic 100 W Al Ka (1486.6 eV) X-ray source. The Si-2p signal from the substrate was used as internal reference. Avantage V3.9 software was used for XPS data analysis. For XPS peak fitting, the background was considered to be of Shirley type and each component peak was considered to be a mixture of Lorentzian and Gaussian (L/G B 30%). For UPS measurements, a He discharge lamp operated to emit He-I (21.2 eV) and He-II (40.8 eV) flux was used. For UPS data analysis, Avantage V3.9 software was used with Shirley type background fitting and by considering each component peak as Gaussian. The overall energy resolution was 0.7 eV for XPS and 0.1 eV for UPS measurements. For work function measurements by He-I UPS, the sample was biased to 3 V to accelerate low energy secondary electrons. To remove adsorbed moisture, all the samples were annealed at 120 1C for 1 hour at B107 Pa, prior to XPS and UPS measurements. The electrical conductivity of GO and plasma reduced GO monolayers was measured using a two-probe arrangement and bottom-gated field effect transistor (FET) geometry was employed to measure the field effect mobility. These measurements were carried out using a Keithley 4200-SCS semiconductor characterization system. The device structures were fabricated by transferring isolated GO monolayer sheets onto the SiO2/Si substrate by the LB technique, over which, Cr/Au (5 nm/ 100 nm) source and drain electrodes were patterned by photolithography and deposited by sputtering. A 150 nm thick aluminium back gate contact was deposited by thermal evaporation. The channel length and width were in the range of 10–20 mm and 10–40 mm, respectively.

2. Experimental details GO was synthesized by oxidative exfoliation of graphite powder (Bay carbon, SP-1) by Hummers–Offeman’s method.36 The LB technique was employed for the transfer of GO monolayers onto Si and SiO2/Si substrates under optimized conditions, as reported earlier.12 Reduction of as-transferred GO monolayers was carried out by hydrogen plasma treatment at different temperatures in a DC plasma system. Before plasma treatment, the chamber was evacuated to a base pressure of 104 Pa. Subsequently, hydrogen was introduced to maintain a working pressure of B50 Pa. Hydrogen plasma treatment was carried


3. Results and discussion GO sheets were subjected to hydrogen plasma treatment at 15 W and 30 W at room temperature (RT) as well as at slightly elevated temperatures (50 1C, 70 1C, 90 1C and 120 1C) for various durations (10 s, 30 s, 1 min, 2 min and 5 min). Fig. 1 shows the SEM images of hydrogen plasma treated GO sheets for typical cases. The SEM image of GO sheets after RT plasma treatment at 15 W for 2 min shows a uniform distribution of closely spaced sheets of 20–60 mm, lying flat on the substrate surface.

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These features are similar to those reported earlier32 for the as-transferred GO sheets (not shown here), and hence reveal that plasma treatment up to 2 min under the above conditions does not cause any noticeable change in the overall morphology and density of the sheets. In sharp contrast, plasma treatment for 5 min under the same conditions results in substantial damage to GO sheets, as shown in the inset of Fig. 1(b). It is also found that at higher power, the morphological damage of GO sheets begins at progressively shorter durations of plasma treatment. This is shown typically for 30 W power in the inset of Fig. 1(c), in which, severe damage of GO sheets is seen even after short duration plasma treatment for 30 s. Hence, in the rest of this work, hydrogen plasma treatment has been carried out at 15 W. GO sheets were also subjected to hydrogen plasma treatment at elevated temperatures. Fig. 1(d) shows a typical SEM image of GO sheets after plasma treatment at 70 1C for 30 s at 15 W, in which, practically no change in the overall morphology and density of the sheets is seen. A similar behavior was seen for plasma treatment up to 120 1C under these conditions. Fig. 2 shows the AFM images of as-transferred and typical plasma treated GO sheets. In all the cases, sheet thickness was measured at several locations by height profiling at sheet-substrate edges as well as sheet-sheet edges (for partly overlapping sheets).37 Typical height profiles at single sheet-substrate edges are shown as insets. The rms roughness of the Si substrate was found to be B0.1 nm. In the case of as-transferred GO sheets, the sheet thickness was found to be in the range of (1.0–1.3) nm, which is in good agreement with the reported results of GO monolayers.12,38 Similar measurements were performed on GO sheets after RT plasma treatment for 10 s, 30 s, 1 min and 2 min, and a typical case is shown in Fig. 2(b). In all these cases, practically no changes were observed in the morphology and thickness of the

Fig. 1 SEM images of GO monolayer sheets after hydrogen plasma treatment at (a) 15 W for 2 min at RT, (b) 15 W for 5 min at RT, (c) 30 W for 30 s at RT, and (d) 15 W for 30 s at 70 1C. The insets show higher magnification images.


Fig. 2 AFM images of (a) as-transferred GO monolayers and hydrogen plasma treated (at 15 W power) GO monolayers for 30 s at (b) RT, (c) 70 1C and (d) 120 1C. The corresponding height profiles are shown as insets.


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sheets after plasma treatment at RT. The AFM images were also recorded after plasma treatment of GO sheets for 30 s at 50 1C, 70 1C, 90 1C and 120 1C, and typical cases are shown in Fig. 2(c) and (d). In all these cases, although no changes are observed in the overall surface morphology of the sheets, the sheet thickness was found to decrease to (0.5–0.6) nm. The decrease in sheet thickness of GO monolayers after plasma treatment at elevated temperatures to values normally associated with pristine graphene39 is attributed to substantial removal of oxygen functional groups from the GO network, as will be discussed below. The presence of oxygen functional groups in GO monolayers, before and after hydrogen plasma treatment, has been assessed by high resolution XPS study of the C-1s core level. The results, corresponding to variation of the duration and the temperature of plasma treatment, are summarized in Tables 1 and 2, respectively, and the C-1s spectra of typical cases are shown in Fig. 3. The de-convoluted C-1s spectrum of as-transferred GO monolayers is shown in Fig. 3(a) and the corresponding peak positions along with the relative integrated intensities of the de-convoluted components are listed in Table 1. The peaks at 284.5 eV and 285.4 eV are, respectively, assigned to sp2-C (graphitic) and the damaged alternant hydrocarbon structure/ sp3-C, while the component at 289.7 eV is attributed to the p–p* shake up satellite of the 284.5 eV peak.40,41 The peaks at 286.3 eV, 287.2 eV, and 288.4 eV are, respectively, attributed to oxygen functionalities, namely, C–O, CQO, and COOH.40 The O/C ratio estimated from the C-1s peak for GO monolayers is 0.50 (Table 1). The contributions from the sp3-C/damaged alternant hydrocarbon structure and other oxygen functionalities to C-1s peak are referred to as ‘non-graphitic’ carbon. The ratio of the total intensity of nongraphitic carbon peaks to that of the graphitic carbon (sp2-C) peak is denoted as ‘X’ and is 1.44 for GO monolayers.

Fig. 3 C-1s core level spectra of (a) as-transferred GO monolayers and hydrogen plasma treated (at 15 W power) GO monolayers for (b) 30 s at RT, (c) 2 min at RT, (d) 30 s at 50 1C and (e) 30 s at 120 1C.

Table 1 Peak positions and integrated intensity values (%) of the de-convoluted components of the C-1s core level of as-transferred GO and RT hydrogen plasma treated GO monolayers. The plasma treatment durations (at 15 W power) are as indicated. The values of O/C and X are also listed

De-convoluted peak positions (eV) Plasma treatment duration

sp2-C (284.5)

sp3-C (285.4 0.1)

C–O (286.3 0.1)

CQO (287.2 0.1)

COOH (288.3 0.2)

p–p* (289.5 0.2)

O/C ratio

X

GO 10 s 30 s 1 min 2 min

40 60 68 66 62

16 11 13 11 15

18 14 9 15 12

18 8 6 4 6

7 5 2 2 3

1 2 2 2 2

0.50 0.32 0.19 0.23 0.24

1.44 0.61 0.43 0.47 0.56

Table 2 Peak positions and integrated intensity values (%) of the de-convoluted components of the C-1s core level of 30 s hydrogen plasma treated GO monolayers. The plasma treatment temperatures (at 15 W power) are as indicated. The values of O/C and X are also listed

De-convoluted peak positions (eV) Plasma treatment temperature

sp2-C (284.5)

sp3-C (285.4 0.1)

C–O (286.2 0.1)

CQO (287.2 0.1)

COOH (288.3 0.2)

p–p* (289.4 0.2)

O/C ratio

X

RT 50 1C 70 1C 90 1C 120 1C

68 75 70 64 54

13 10 14 19 27

9 9 9 10 12

6 3 3 3 3

2 2 2 2 2

2 1 2 2 2

0.19 0.16 0.16 0.17 0.19

0.43 0.31 0.39 0.52 0.79



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The C-1s spectra recorded after plasma treatment of GO monolayers at room temperature for different durations are also included in Fig. 3, which show the dominance of the sp2-C peak, along with small contributions from the other components. The corresponding peak positions and the relative integrated intensities of the de-convoluted components are listed in Table 1. It is noticed that the contributions from sp3-C/damaged alternant hydrocarbon structure, C–O, CQO and COOH groups decrease substantially after the plasma treatment. The contribution of sp2-C content to the C-1s peak intensity is 40% in GO monolayers, which increases initially with an increase in the duration of plasma treatment up to 30 s, to reach a value of 68% and shows a marginal decrease for longer durations of plasma treatment. The contribution of sp3-C/damaged alternant hydrocarbon structure (285.4 eV) decreases substantially for plasma treatment duration up to 30 s, but increases marginally for longer duration plasma treatments. The O/C ratio decreases initially with an increase in the duration of plasma treatment and reaches a value of 0.19 after 30 s. It is however found to marginally increase for longer duration plasma treatments, which may be attributed to possible loss of carbon. In accordance with the above, ‘X’ decreases substantially with an initial increase in plasma treatment duration up to 30 s, to reach a value of B0.43, but shows a small increase for longer duration plasma treatments. Fig. 3 also includes the C-1s spectra recorded after plasma treatment of GO monolayers for 30 s at different temperatures. The de-convoluted components of the C-1s peak and their relative integrated intensities are listed in Table 2. It is seen that plasma treatment at 50 1C results in a significantly higher value of sp2-C content (75%), compared to plasma treatment at room temperature under similar conditions. It may be mentioned that such high values of sp2-C content (B80%) have only been reported after reduction of GO sheets at much higher temperatures (B800 1C or above).42,43 However upon further increase of plasma treatment temperature up to 120 1C, a progressive decrease of sp2-C content is seen. The sp3-C/damaged alternant hydrocarbon content has also been found to attain a minimum value after plasma treatment at 50 1C, but increases substantially with further increase of plasma treatment temperature. Table 2 also lists the O/C ratio, which decreases to 0.16 after plasma treatment at 50 1C, but increases marginally for higher temperature plasma treatments. In accordance with the above, ‘X’ decreases from 1.44 for GO to its lowest value of 0.31 after plasma treatment of 30 s at 50 1C. The decrease of ‘X’ is indicative of the de-oxygenation of GO monolayers and reduction in sp3-C domains, which accompanies a substantial increase in graphitic carbon (sp2-C) content, and thus the extent of the aromatic network within the plasma treated GO monolayers. The significant increase in ‘X’, seen after plasma treatment at higher temperatures, is primarily due to the increase in the sp3-C/damaged alternant hydrocarbon content. It is thus inferred that hydrogen plasma treatment for durations longer than 30 s and temperatures above 50 1C does not result in further removal of oxygen but possibly results in the creation of defects in the graphitic network. Raman spectra were recorded for GO monolayers after RT plasma treatment for different durations, as well as, after


Paper Table 3 Raman peak positions for as-transferred GO monolayers and RT hydrogen plasma treated GO monolayers. The plasma treatment durations (at 15 W power) are as indicated

Plasma treatment duration

D-band (cm1)

GO 10 s 30 s 1 min 2 min

1339 1338 1338 1338 1338

G 0 -band (cm1)

G-band (cm1) 1 1 1 1 1

1602 1591 1590 1592 1592

2 1 1 1 1

2687 2667 2666 2666 2671

6 3 5 5 3

S3-band (cm1) 2926 2920 2921 2923 2925

3 3 3 3 1

Table 4 Raman peak positions for 30 s hydrogen plasma treated GO monolayers. The plasma treatment temperatures (at 15 W power) are as indicated

Plasma treatment temperature

D-band (cm1)

RT 50 1C 70 1C 90 1C 120 1C

1338 1338 1338 1336 1331

G 0 -band (cm1)

G-band (cm1) 1 1 1 1 1

1590 1588 1591 1597 1607

1 1 1 2 1

2666 2669 2671 2673 2674

5 2 2 2 4

S3-band (cm1) 2921 2924 2924 2927 2905

3 2 1 2 2

plasma treatment for 30 s at different temperatures. The corresponding peak positions for the Raman modes are listed in Tables 3 and 4, and typical Raman spectra are shown in Fig. 4. Two prominent peaks associated with the G- and D-modes and two weak peaks associated with G0 - and S3-modes of graphite44–46 are seen in all the spectra. The G-mode is known to appear at B1582 cm1 for bulk graphite44 and in the range of 1584– 1588 cm1 for graphene.3,47,48 The D-mode is a disorder activated Raman mode due to bonding defects in the graphitic plane, and is reported to appear at B1350 cm1.44–46 The G0 -mode is a second order double resonant Raman scattering mode from zone boundary phonons,44–46 and the S3 peak is a defect induced second order D + G combination mode.46 For GO monolayers, the G-band peaks at (1602 2) cm1, as shown in Fig. 4(a). The G-band is found to red-shift up to 30 s of RT plasma treatment (Table 3) and attains a value of (1590 1) cm1, as also seen in Fig. 4(a). RT plasma treatment for longer durations however results in comparatively smaller redshifts of the G-band, as is evident from Table 3. Fig. 4(a) also includes the typical Raman spectra of GO monolayers after plasma treatment for 30 s at 50 1C and 120 1C. It is seen from Table 4 that the G-band red-shifts to its lowest value of (1588 1) cm1 after plasma treatment for 30 s at 50 1C. However, after plasma treatment for 30 s at 70 1C and 90 1C, the red-shifts of the G-band are substantially smaller. It is noteworthy here that the large red-shift of the G-band to values close to that associated with graphene after plasma treatment for 30 s at 50 1C is consistent with XPS results, which have shown that under these conditions, ‘X’ attains its lowest value (Tables 1 and 2). Interestingly, the G-band is found to blue-shift to (1607 1) cm1 after plasma treatment at 120 1C, as shown in Fig. 4(a). The G-bands of the as-transferred and plasma treated GO monolayers, shown in Fig. 4(a), have also been de-convoluted into component peaks to analyze the observed shifts. The de-convoluted G-band spectra (Fig. 4(b)) reveal the presence of additional


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Fig. 4 (a) Raman spectra of as-transferred GO monolayers and hydrogen plasma treated GO monolayers (at 15 W power) for 30 s duration at different temperatures (as indicated) and (b) the corresponding de-convoluted G-mode peaks.

smaller components, namely, D00 -mode (B1550 cm1) and D0 -mode (B1615 cm1). The D00 -mode is a disorder induced first order scattering mode49,50 and the D0 -mode peak is also a disorder induced feature originating from the double resonance Raman process.44 The appearance of G-mode at 1598 cm1 along with broad D0 - and D00 -modes of considerable intensities is indicative of the substantial presence of defects in GO monolayers.51 Fig. 4(b) shows that plasma treatment up to 50 1C results in red-shift of G-mode and a decrease in the intensities of D0 - and D00 -modes relative to the G-mode. In particular, the red-shift of G-mode to 1588 cm1 seen after plasma treatment for 30 s at 50 1C indicates that the reduction due to hydrogen plasma treatment is most effective under these conditions. The substantial decrease in the intensities of D0 - and D00 -modes seen in this case is also indicative of the decrease in defects and disorder, which is consistent with the increase in sp2-C content and decrease of the O/C ratio, as inferred from XPS results. It is also found that plasma treatment at 120 1C results in a substantially smaller red-shift of G-mode to 1594 cm1 and the D0 -mode exhibits a substantial increase in intensity relative to the G-mode. The large intensity of D0 -mode in this case results in the observed blue-shift of the G-band to B1607 cm1, after plasma treatment at higher temperatures. Tables 3 and 4 also show the positions of the D-band, which appears at (1339 1) cm1 for the GO monolayers and does not exhibit any significant shift after plasma treatment up to 70 1C. With further increase in temperature, the D-band red-shifts progressively, attaining a value of (1331 1) cm1 after plasma treatment at 120 1C. The position of the D-mode at 1330–1332 cm1 is characteristic of sp3 hybridized C–C bonds in diamond52 and has been observed in hydrogenated amorphous carbon53 and


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more recently in hydrogen functionalized graphene.54 The redshift of the D-band observed after the higher temperature plasma treatment is thus attributed to hydrogenation of the graphitic network. This inference is supported by the drastic increase in sp3-C/damaged alternant hydrocarbon structure and consequently the value of ‘X’ (as seen from XPS results in Table 2), as well as the substantial increase in the intensity of D 0 -mode, after plasma treatment under these conditions. Fig. 4(a) shows that after plasma treatment at RT and 50 1C, the D-band becomes significantly narrower and relatively intense. The integrated intensity ratio of the D-band to the G-band (I(D)/I(G) ratio) for GO monolayers is (2.7 0.1), which decreases marginally to (2.3 0.1) after plasma treatment for 30 s up to 50 1C. With further increase in temperature, the D-band exhibits progressive broadening, shown typically in Fig. 4(a) after plasma treatment for 30 s at 120 1C. The corresponding I(D)/I(G) ratio is (4.2 0.2). The disordered carbon content, bonding defect and sp3/sp2 fraction in carbon based materials have usually been characterized in terms of the I(D)/I(G) ratio.46,55 It has been shown that the I(D)/I(G) ratio increases with the decrease in sp2-C content and the size of sp2-C bonded domains in the case of crystalline graphitic forms56 and is usually found to decrease during the reduction of GO monolayers in the solid state.12,32 It may be mentioned that the I(D)/I(G) ratios for GO and reduced GO have been usually estimated without de-convoluting the G-band and are found to be in the range of 1–2, as reported earlier.12,35 The larger values of the I(D)/I(G) ratio observed in the present work are essentially due to the removal of contributions from D 0 - and D00 -modes to the G-band. It may be pointed out that the decrease of the I(D)/I(G) ratio after plasma treatment of GO monolayers for 30 s up to 50 1C is relatively moderate (compared to, for example, in ref. 5) in spite of the large sp2-C content and a small O/C ratio, observed under these conditions. This may be attributed to the possible influence of defects/disorder associated with hydrogen plasma treatment, as has been recently reported for hydrogen functionalized graphene.54 The substantial increase in the I(D)/I(G) ratio after plasma treatment at 120 1C also supports this inference. Fig. 4(a) also shows the G 0 - and S3-bands for typical cases. The G 0 -band peaks at (2687 6) cm1 in GO monolayers and red-shifts to B2670 cm1 for all the plasma treated monolayers. In contrast to the behavior of the G 0 -band, the S3-band does not show any noticeable shift after plasma treatment. The G 0 -band also becomes sharper and relatively intense after plasma treatment at RT and 50 1C (as compared to GO monolayers). The integrated intensity ratio of the G 0 - to S3-band (I(G 0 )/I(S3)) is 0.43 for GO monolayers and increases to 0.72 after 30 s plasma treatment at 50 1C. The increase in the I(G 0 )/ I(S3) ratio is consistent with the recovery of sp2-C bonds in the graphitic structure due to the plasma treatment.46,56 However, Fig. 4(a) also shows that after plasma treatment at 120 1C, the intensity of the G 0 band becomes relatively small (I(G 0 )/I(S3) ratio is 0.38), and the S3-band in this case appears as a sharp peak at B2905 cm1 along with a shoulder at B2925 cm1. The changes in the vicinity of B2930 cm1 have been attributed to the


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formation of C–H bonds in hydrogen functionalized graphene.54 These features are thus attributed to hydrogenation related effects at higher temperature plasma treatment, as discussed above. UPS has been extensively used to study the valence band (VB) electronic structure of carbon-based materials and has recently been employed to study the VB features of chemically reduced GO monolayers and nitrogen doped reduced GO monolayers.35,57,58 In the present work, He-II photoelectron spectra have been recorded for as-transferred and plasma treated GO monolayers to examine the changes in VB electronic structure and density of states (DOS) at the Fermi level. The Fermi level position is referenced to the zero binding energy position. Fig. 5(a) shows the He-II VB spectra of GO and RT plasma treated (for different durations) GO monolayers after the subtraction of Shirley type background and de-convolution into Gaussian component peaks. As a reference, the He-II UPS VB spectrum of crystalline graphite (not shown) was also recorded, which showed five component peaks corresponding to C 2p-p (B3.5 eV), C 2p-(p–s) overlap (B5.5 eV), C 2p-s (6.0–8.0 eV), C 2s–2p mixed state (8.8–10.5 eV) and C 2s state (B13.5 eV), as reported earlier.58,59 It was possible to de-convolute the VB spectra of GO monolayers by considering only four components, C 2p-p (B3.7 eV), C 2p-s (B6.2 eV), C 2s–2p mixed state (B9.3 eV) and C 2s state (B13.5 eV), without taking into account the C 2p-(p–s) overlap peak. This is consistent with the presence of a very weak C 2p-p peak, which is essentially due to the relatively small sp2-C content of GO monolayers, as revealed by XPS results. Fig. 5(a) also shows that the intensity of the C 2p-p band increases considerably during the initial stages (up to 30 s) of RT plasma treatment. However, with further increase in the duration of plasma treatment, the intensity of the C 2p-p band decreases marginally. Concomitant with the increase of the C 2p-p band, a strong C 2p-(p–s) overlap band is seen in all the plasma treated monolayers. It is also noticed that the C 2p-s, C 2s–2p mixed and C 2s bands shift towards higher binding energies in all the RT plasma treated monolayers. Fig. 5(b) shows the zooming of the region near the Fermi level for all the VB spectra. The valence band edge (VBE) was determined by locating the onset of rapid increase of DOS and is shown by arrow marks in all the cases. The GO monolayers show negligible DOS near the Fermi level, with the VBE located at B1.5 eV below the Fermi level. For all the RT plasma treated monolayers, a steeper increase of DOS is seen in the vicinity of the Fermi level, as compared to the GO monolayers. This indicates that the VBE is closer to the Fermi level in all the RT plasma treated monolayers. Particularly, after RT plasma treatment of 30 s, the VBE is located near 0 eV. However, the VBE shifts away from the Fermi level and is located at B0.4 eV and B0.7 eV after RT plasma treatment for 1 min and 2 min, respectively. Fig. 6(a) shows the He-II VB spectra after plasma treatment for 30 s at elevated temperatures. The de-convoluted VB spectrum after plasma treatment at 50 1C shows a substantial increase in the intensity of the C 2p-p peak, which corroborates well with the high sp2-C content (75%), in this case. Plasma treatment at higher temperatures results in a progressive decrease of the intensity of the C 2p-p peak, which becomes practically negligible after


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Fig. 5 (a) The de-convoluted He-II valence band spectra of astransferred GO monolayers and hydrogen plasma treated GO monolayers (at 15 W power) at room temperature for different durations (as indicated) and (b) the corresponding magnified regions near the Fermi level.

plasma treatment at 120 1C. Fig. 6(a) also shows the appearance of new features in the VB spectra after plasma treatment at 90 1C and 120 1C. It is found that in both these cases, the intensity of the C 2p-s band increases with plasma treatment temperature and an additional peak is observed in the range 11.8–12.0 eV, which is attributed to the C–H 2p-s states.60 These features are attributed to the larger sp3/damaged alternant hydrocarbon content (Table 2), which may be responsible for the disruption of the sp2 network. Fig. 6(b) shows the zooming of the region near the Fermi level in the VB spectra recorded after plasma treatment at elevated temperatures. The VB spectrum after plasma treatment at 50 1C shows features similar to those after RT plasma treatment for the same duration (30 s), displaying VBE near 0 eV and a marginal increase in the DOS near the Fermi level. However, plasma treatment at higher temperatures results in the shift of VBE away from the Fermi level, which is located at B0.4 eV, B1.2 eV and B2.8 eV for 70 1C, 90 1C and 120 1C plasma treated monolayers, respectively. In the cases where the Fermi level is located sufficiently away from the VBE


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nature of hydrogen plasma reduced GO monolayers. Plasma treatment for longer durations or at higher temperatures, however, results in the decrease of sp2-C content and hence, the C 2p-p states, which manifest in the shift of the VBE away from the Fermi level. The secondary electron thresholds of the He-I UPS spectra are shown in Fig. 7 for as-transferred and plasma treated GO monolayers. The work function in all the cases has been estimated from the threshold energy. Fig. 7(a) shows that the work function is 4.4 eV for the GO monolayers, and decreases initially with the increase in the duration of plasma treatment, and saturates to 3.6 eV for 30 s and higher durations. Fig. 7(b) shows that the work function decreases to 3.4 eV after plasma treatment at 50 1C and remains practically unchanged at higher temperatures. The dependence of the work function on the duration and temperature of plasma treatment is, respectively, shown in Fig. 7(c) and (d). The work function of epitaxially grown graphene is reported to be 4.0 eV63 and the higher work

Fig. 6 (a) The de-convoluted He-II valence band spectra of hydrogen plasma treated GO monolayers (at 15 W power) for 30 s duration at different temperatures (as indicated) and (b) the corresponding magnified regions near the Fermi level.

(e.g., after plasma treatment at 90 1C and 120 1C), an additional feature, showing an increase in DOS at the Fermi level has been noticed. It is interesting to note that the DOS at the Fermi level becomes unusually large after plasma treatment at 120 1C. It has been reported61,62 that hydrogenation of graphene results in the opening of the band gap with mid-gap states, which can be observed, depending on their location with respect to the Fermi level. The large DOS in the vicinity of the Fermi level observed after plasma treatment of GO monolayers at 120 1C is thus attributed to considerable hydrogenation of the graphitic network, as evidenced from multiple features seen in Raman studies in this case, which include, a large red-shift of the D-band and a high I(D)/I(G) ratio. The shift of the VBE towards the Fermi level, seen in Fig. 5(b) and 6(b) after plasma treatment up to 30 s and 50 1C is attributed to the increase in the intensity of C 2p-p states, which is in tune with the increase in sp2-C content of the reduced graphene oxide monolayers obtained by hydrogen plasma treatment. The location of the Fermi level near the VBE is also indicative of the p-type


Fig. 7 Secondary electron threshold region of the He-I spectra of (a) as-transferred GO monolayers and hydrogen plasma treated GO monolayers (at 15 W power) at room temperature for different durations (as indicated), (b) hydrogen plasma treated GO monolayers (at 15 W power) for 30 s at different temperatures (as indicated). The corresponding work functions are plotted against (c) the duration of plasma treatment at RT and (d) the temperature of plasma treatment for a duration of 30 s.


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function of GO is attributed to the presence of surface dipole moments of oxygen functional groups, which are responsible for the disruption of p-conjugation in the graphitic network.64 The decrease of the work function of GO monolayers due to plasma treatment is thus attributed to the removal of oxygen functional groups and an increase in sp2-C content. The slightly smaller values of the work function (relative to graphene) observed in this work are possibly due to the presence of surface hydrogen, as proposed recently.65 The effect of de-oxygenation and restoration of the graphitic network on the electrical properties of hydrogen plasma treated

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GO monolayers has been investigated. Fig. 8(a) shows the schematic of a bottom gated FET and the SEM image of a GO monolayer in two-probe contact geometry. Fig. 8(b) shows the typical IDS–VDS curves of the as-transferred and plasma reduced GO monolayers. The GO monolayers show insulating behavior with conductivity in the range of (106–105) S cm1, which increases to (103–102) S cm1, after plasma treatment for 30 s at RT. The reduced GO monolayer obtained by plasma treatment for 30 s at 50 1C displays the highest conductivity values in the range of (0.2–31) S cm1, which is attributed to the efficient de-oxygenation of GO monolayers and substantial reduction in defects, under these conditions. These values are comparable to the conductivity of reduced GO monolayers (corresponding to the reported66,67 sheet resistance of (200– 1000) kO sq1), obtained by heat treatment in the range of (800– 1000) 1C. Fig. 8(c) shows the typical transfer characteristics (IDS–VG curve at VDS = 1 V) of the plasma reduced GO monolayer (for 30 s at 50 1C) and the inset displays the output characteristics (IDS–VDS at different VG) of the device. The FET exhibits ambipolar behavior and the charge neutrality point is observed at B25 V for this device. This is indicative of the p-type nature of the plasma reduced monolayers, which is consistent with UPS results. The field-effect mobility has been extracted from the slope (DIDS/DVG) in the linear region of the IDS–VG curve, using the equation m = (L/W CGVDS) (DIDS/DVG), where L and W are the channel length and width, respectively, and CG is the gate capacitance. The field effect mobility of holes has been estimated for 30 devices and its values were found to be in the range of (0.1–6) cm2 V1 s1. The values of field effect mobility observed in the present work are comparable to those usually reported (0.1–12 cm2 V1 s1) for chemically reduced GO monolayers, for example, after hydrazine treatment, followed by heat treatment at temperatures in the range of 400 1C to 1000 1C.67,68

4. Conclusions

Fig. 8 (a) Schematic of a bottom-gated FET and SEM image of a typical GO monolayer in two probe contact geometry, (b) I–V plots of the astransferred GO monolayer and the hydrogen plasma treated GO monolayers (at 15 W power) for 30 s duration at RT and 50 1C and (c) transfer characteristics of the bottom-gated FET employing a hydrogen plasma treated (at 15 W for 30 s at 50 1C) GO monolayer as a channel. The inset of (c) shows the output characteristics of the device.


The effect of hydrogen plasma treatment on LB monolayers of GO has been studied under different conditions near room temperature. GO monolayers remain morphologically stable after plasma treatment up to 2 min and temperatures up to 120 1C at relatively low power (15 W). At higher power, a substantial damage of GO sheets begins to take place for much shorter durations of plasma treatment, even at room temperature. Plasma treatment of GO monolayers at 15 W for 30 s at a temperature of 50 1C results in a substantial decrease of the O/C ratio to 0.16 and a corresponding increase in their sp2-C content to 75%. Plasma treatment at higher temperatures and longer durations does not cause further removal of oxygen but results in the creation of defects in the graphitic network, leading to an increase in the sp3-C/damaged alternant hydrocarbon content. A sheet thickness of (0.5–0.6) nm and a large red-shift of G-mode to 1588 cm1 are seen after plasma treatment of GO monolayers for 30 s at 50 1C. These values are close to the corresponding values reported for graphene, and corroborate well with the high sp2-C content and a low O/C ratio of


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the plasma reduced GO monolayers, demonstrating that the reduction process is most effective under these conditions. Raman studies of GO monolayers, plasma treated under different conditions, have shown multiple features, such as changes in D-, D 0 -, D00 -, G- and G 0 -bands and variations in I(D)/I(G) and I(G0 )/I(S3) ratios, which corroborate well with the corresponding changes in sp2-C, and sp3-C/alternant damaged hydrocarbon contents and the O/C ratio, as measured by XPS. These features have elucidated the influence of defects/disorder associated with hydrogenation, which may accompany the de-oxygenation of GO monolayers during hydrogen plasma treatment under different conditions. Dominant effects due to hydrogenation of the graphitic network are seen after plasma treatment at higher temperatures (particularly at 120 1C), which appear in the form of a drastic increase in sp3-C/damaged alternant hydrocarbon content and several Raman features, such as red-shift of the D-band to B1331 cm1, a high I(D)/I(G) ratio, a substantial increase in the intensity of D 0 -mode and changes in the S3-band. The study of valence band electronic structure by UPS has shown an increase in the intensity of C 2p-p states, leading to a steep increase of DOS in the vicinity of the Fermi level, and a substantial decrease in the work function after plasma treatment of GO monolayers for 30 s at RT and 50 1C. These features are consistent with the increase in graphitic carbon content and the substantial removal of oxygen functional groups that take place under these conditions. Both UPS and FET characteristics confirm the p-type nature of the plasma reduced GO monolayers, displaying a conductivity of (0.2–31) S cm1 and a field effect mobility of (0.1–6) cm2 V1 s1. Longer duration and higher temperature plasma treatments result in the shift of VBE away from the Fermi level, which corroborates well with the decrease in C 2p-p states due to the increase in sp3-C content. This is attributed to considerable effects arising from the hydrogenation of the graphitic network, under these conditions, which is also reflected by the observation that the saturation value of the work function (3.4 eV) is marginally smaller than that of pristine graphene. A substantial shift of the VBE away from the Fermi level and the appearance of non-zero DOS due to mid-gap states are seen after plasma treatment at higher temperatures, consistent with the observed Raman features, indicating hydrogenation of the graphitic network. This work also demonstrates that a controlled hydrogen plasma treatment of LB monolayers of GO sheets near-room temperature results in the efficient removal of oxygen and an increase in sp2-C content, leading to the formation of p-type, reduced GO monolayers with high electrical conductivity and field effect mobility, which are comparable to the values usually obtained at much higher temperatures by solid state reduction methods.

Acknowledgements The authors would like to thank FIST (Physics)-IRCC Central SPM facility, IIT Bombay for AFM measurements, Central Surface Analytical Facility, IIT Bombay for XPS and UPS measurements, Centre for Research in Nanotechnology and Science


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(CRNTS), IIT Bombay for Raman spectroscopy measurements and Centre for Excellence in Nanoelectronics (CEN), IIT Bombay for SEM and electrical measurements.

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