Improved electrical and optical characteristics of transparent graphene thin films produced by acid and doping treatments

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Improved electrical and optical characteristics of transparent graphene thin films produced by acid and doping treatments Qing Bin Zheng, Mohsen Moazzami Gudarzi, Shu Jun Wang, Yan Geng, Zhigang Li, Jang-Kyo Kim * Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

A R T I C L E I N F O

A B S T R A C T

Article history:

New thermal and chemical schemes, which include (i) a modified thermal treatment, (ii)

Received 18 November 2010

acid treatment in a HNO3 bath and (iii) doping by immersing in a SOBr2 solution, are devel-

Accepted 25 February 2011

oped to treat graphene films to improve the electrical conductivity and transparency. It is

Available online 4 March 2011

shown that a longer thermal treatment at 1100 C as well as additional acid and doping treatments reduce the sheet resistance by about 20–50% with improved transmittance. The final product has a sheet resistance of 1600 X/sq and a transparency of 82%, which is quite sufficient to replace the transparent conducting films made from indium tin oxide for many existing applications in photovoltaic cells and optoelectronics. The transmittance and sheet resistance measured after 3 months of exposure to air confirms the stability of the improved characteristics after the additional treatments.  2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Transparent conductors (TCs) have been used in a wide variety of photoelectronic devices, such as liquid crystal displays (LCD), organic light emitting diodes (OLED), photovoltaic cells, sensors, lasers, etc. [1,2]. Indium tin oxide (ITO) and fluorine tin oxide (FTO) are the dominant materials to produce TCs owing to their excellent electrical conductivity and optical transparency. Nevertheless, there are important disadvantages of these metal oxides, such as increasing material cost of indium, degraded performance when applied on flexible substrates due to brittleness, the limitation of the element indium on earth, the instability of these materials in the presence of acid or base, the susceptibility to ion diffusion into polymer layers, the limited transparency in the near infrared region, and the current leakage of FTO devices caused by FTO structural defects [3]. In response to these limitations, there have been significant efforts in search of alternative materials that possess comparable or even better characteristics, including high

electrical conductivity, excellent transparency and good stability. Graphene, a two-dimensional mono-layer of sp2-bonded carbon atoms, has attracted significant interests recently because of the unique transport properties [4]. Due to the high optical transmittance and electrical conductivity, graphene is being considered as a transparent conductive electrode. Compared with traditional electrodes made from ITO or FTO, graphene films have higher mechanical strength, flexibility, chemical stability and are considered much cheaper to produce. Several techniques, including chemical vapor deposition (CVD) [5], spin- or spray coating [6,7], transfer printing [8,9], Langmuir–Blodgett (L–B) deposition [10,11] and electrophoretic deposition [12], have been devised to fabricate graphene-based TCs. However, methods for enhancing the electrical conductivity of graphene films without impairing the film transparency – a prerequisite for many applications – need to be further developed. The main strategies for improving the electrical conductivity of graphene films have been based on various doping treatments [13–15].

* Corresponding author: Fax: +852 2358 1543. E-mail address: [email protected] (J.-K. Kim). 0008-6223/$ - see front matter  2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.02.064

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Considerable research has been conducted to enhance the electrical conductivities of thin films made from carbon nanotubes (CNTs), especially single-walled CNTs, while maintaining their high transmittance. Various techniques have been developed, including simple immersion of CNT films in HNO3 [16], SOCl2 [17], HNO3 followed by SOCl2 [18] or SOBr2 [19]. CNT films were fabricated after dispersion in sodium dodecyl sulfate (SDS), followed by immersion in HNO3 [15], which gave rise to an excellent conductivity with a negligible change in transmittance in the visible range. The enhancement was attributed to the removal of remaining SDS and the subsequent densification of the film with improved cross-junction resistance between the CNT networks as well as improved metallicity of CNTs. A simple room-temperature post-deposition process was developed [18], where the HNO3 and SOCl2 treatments reduced the sheet resistivity of CNT thin films. The enhancement in transport properties due to the SOCl2 treatment was related to the formation of acyl chloride functional groups. The CNT films modified using SOBr2 outperformed the counterparts modified using SOCl2 [19], due to the formation of new conducting paths between CNTs via S atoms when SOBr2 was used. In particular, the p-type doping associated with Br2 intercalation of CNT bundles was effective in significantly reducing the electrical resistivity [20]. However, very few studies have hitherto been reported on similar acid and doping treatments of graphene sheets. It is unknown whether these treatments are effective in enhancing the electrical conductivity of graphene thin films. Our recent study [21] demonstrated that the Br2 vapor treatment enhanced the electrical conductivity of graphite nanoplatelets (GNPs) by increasing the ionic component of Br functionalities. Based on the experimental evidence, the present study aims at creating chemical functionalities on graphene thin films using the acids and SOBr2 doping treatment to achieve desired effects of enhanced electrical conductivity and transparency. The mechanisms behind the improvements in these characteristics are identified by establishing the correlation with the surface morphology, roughness, chemistry and functional groups of the thin films.

2.

Experimental

2.1.

Preparation of graphene thin films

The materials and the process employed to produce graphene thin films were essentially similar to our previous studies [9,22]. Graphene oxide (GO) was synthesized through oxidation and exfoliation of natural graphite flakes (Asbury Graphite Mills). The graphene colloid was produced after chemical reduction of GO using hydrazine [9,22]. Quartz slides were used as substrate for transfer printing of graphene films. 25 mm · 25 mm square slides were washed in an acetone bath under ultrasonication to remove any organic contamination, which were immersed in a piranha solution (VolH2SO4: VolH2O2 = 7:3) for 1 h, followed by rinsing with distilled water and drying in a vacuum oven for 30 min. The quartz substrates were stored in an (amino propyl) trithoxysilane solution (3% in toluene) for 1 h to allow the graphene film to be easily transferred.

Graphene films were prepared on the cellulose acetate (CA) filter membranes with an average pore size of 200 nm through vacuum filtration of graphene suspension. At a given location on the porous membrane, the rate of filtration decreased when the thickness of graphene layer increased and the permeation rate of solvent was controlled by the accumulation of the graphene sheets on the pores. Therefore, the filtration process was self-regulating and the film thickness could be controlled by simply varying either the concentration of graphene suspension or the filtration volume. The CA membranes with graphene films on were cut into appropriate sizes when they were still wet and subsequently pasted onto the quartz substrates with the film surface facing the substrate surface. After the film was fully dried on the substrate, the CA membrane was dissolved in an acetone bath, leaving the transparent graphene film on the substrate.

2.2.

Reduction of graphene thin films

Significant research efforts have been directed toward proper reduction of GO into a more pure form of graphene to restore the sp2 carbon structure and thus increase the electrical conductivity. A recently study identified that the electrical resistance of GO sheets depended on the duration of thermal treatment: the longer was the thermal treatment duration, the higher was the degree of graphitization and reduction of GO sheets [23]. The interlayer distance between GO sheets was reduced significantly depending on the annealing time, which was attributed to the removal of water molecules, hydrogen atoms from hydroxyl groups in the annealed GO [24]. Based on these findings and the preliminary experiment, the thermal treatment conditions were modified in this study by adopting a longer treatment time of 1 h at 1100 C. The dried graphene films on quartz slides were loaded inside a ceramic container with open ends. The container was introduced into a furnace (Thermcraft/Eurotherm) with controlled vacuum and gas flows. A vacuum of 10 5 Torr was established before heating. Films were heated with a continuous flow of ultra-pure argon, at a rate of 10 C/min, held at 400 C for 3 h at 10 3 Torr, and were allowed to cool to room temperature for about 20 min. Subsequently, the films were heated to 1100 C at a rate of 10 C/min and the temperature was held constant at 1100 C for 1 h, after which the system was allowed to cool down. At room temperature, ambient air was admitted to the furnace and the films were recovered. Longer periods of thermal treatment at 1100 C, including 1.5 h, were also tested with the same procedure, which resulted in severe damage to the quartz substrate. The choice of 1100 C as the graphitization temperature was confirmed previously [9,25] and a flow of argon gas would eliminate the problem of film loss by reaction with residual oxygen. To further improve the optical transparency and electrical characteristics of graphene films, a series of additional treatments were employed, as schematically illustrated in the flow chart in Fig. 1. The graphene films after the thermal treatment were subject to two additional processes: (i) dipping in a HNO3 bath (70%) for 3 h and drying with gentle nitrogen flow; and (ii) dipping in a SOBr2 bath (97% reagent grade, Aldrich Inc.) for 24 h and drying with gentle nitrogen flow.

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Natral graphite Modified Hummer’s method

Graphite oxide Standard method

Ultrasonication

Graphene oxide Chemical reduction by hydrazine

Graphene colloid Pretreated substrates

Final graphene films

Fabrication of graphene films through transfer printing

Thermal treatment 400 ºC 3 h

SOBr2 treatment for 24 h

Thermal treatment 1100 ºC 1 h

HNO3 (70%) treatment for 3 h

Additional treatments

Fig. 1 – Flow chart for preparation of graphene thin films.

2.3.

Characterization

Scanning electron microscope (SEM, JSM-6700F, JEOL) and field emission transmission electron microscopy (TEM, 2010F, JEOL) were used to characterize the structure and morphology of GO. The tapping-mode atomic force microscope (AFM, Scanning Probe Microscope-NanoScope, Digital Instruments) was employed to evaluate the surface morphology and the thickness of graphene films. The transparency was characterized using the UV/VIS spectroscopy (Perkin Elmer Lambda 20). The sheet resistance of the films was measured using the four-point probe method (Scientific Equipment & Services). In order to reduce the contact resistance between the probes and the film surface, the four contact points were coated with silver paste. The influence of additional treatments on the conductive properties of graphene films, including carrier type and concentration, were evaluated by employing a four-probe resistivity/Hall measurement system (HL5500PC, Bio-Rad). The bonding types between the atoms were analyzed using the time-of-flight secondary ion mass spectrometry (ToF-SIMS, PHI 7200). The primary Cs+ ion source was operated at 8 kV and the scanning area was 200 lm square. Secondary ions in the mass range from 0 to 600 amu were collected. The elemental compositions and the assignments of the carbon peaks were characterized using the X-ray photoelectron spectroscopy (XPS, PHI5600 Physical Electronics), which was equipped with a monochromatic Al Ka X-ray source operated in a residual vacuum of 5 · 10 9 Torr. The high-resolution spectra were deconvoluted using a multitak software (provided by Physical Electronics).

3.

Results and discussion

3.1.

Characterization of GO

The L–B method was used to prepare the specimen for morphological characterization. Typically, as prepared GO was redispersed in a mixed solution of water and methanol (1:5) [10], then the GO solution (10 mL) was dropped at a rate of 100 lL/min onto the water surface in the L–B trough (KSV Instruments Ltd., MiniMicro LB System). After compression of the GO film on the water surface, a freshly piranha-solution-cleaned Si/SiO2 substrate was vertically dipped into the

solution and slowly pulled out at a constant speed of 0.1 mm/min, allowing for the deposition of GO sheets on the substrate surface. The initial characterization of GO was carried out using a SEM microscope. As shown in Fig. 2a, the individual GO sheets had sizes up to a few tens of square micrometers. Since the SEM images are representative of electronic structure, not topography, AFM is needed to establish the thickness and surface roughness of graphene sheets. The representative AFM image (Fig. 2b) indicates that the flat, mono-layer GO sheets had a thickness of 0.8 nm. The high resolution TEM image of GO sheet (Fig. 2c) and the corresponding selected area electron diffraction (SAED) pattern (shown in the inset of Fig. 2c) further confirmed that the single layer GO partially collapsed into an amorphous structure.

3.2.

Surface morphology and roughness of graphene films

The typical surface morphologies of graphene films (with initial thickness of 38.7 nm and 78.0 nm) before and after the additional treatments are shown in Table 1. The mean surface roughness values of the films before the thermal treatment were 12.9 and 16.2 nm, which were reduced to 5.3 and 6.4 nm, after the thermal treatment; and to 7.7 and 8.6 nm, respectively, after the additional treatments. The removal of oxygenated functional groups and graphitization of the films at the high temperature were mainly responsible for the reduction of surface roughness. Fig. 3 summarizes the changes in roughness of graphene films plotted as a function of film thickness at different stages of production. The surface roughness in general increased with increasing thickness for all films studied regardless of thermal or chemical treatments. This observation was expected because the graphene films were prepared through vacuum filtration, and thicker films tended to be influenced more by the roughness of filter membranes than thinner films. It follows then that the inherent surface roughness of graphene films can be further reduced by choosing a filter membrane with a smaller pore size. It is interesting to note that the surface roughness of the films after additional treatments was marginally higher than those obtained only with thermal treatment, which may be associated with the etching effect of nitric acid applied in this study.

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Fig. 2 – (a) Typical SEM image of GO sheets; (b) AFM image (4 lm · 4 lm scan) of GO sheets, showing mono-layer of thickness 0.738 nm; (c) high resolution TEM image of a GO sheet with corresponding selected area electron diffraction pattern (SAED) in the inset.

3.3.

Transparency

A thicker film resulted in a higher degree of absorption of light and the corresponding transparency of the film decreased with increasing film thickness at all treatment stages studied. A comparison of the transmittance of the films obtained at different stages is shown in Fig. 4, confirming the reduction of transparency after the thermal treatment. The darkening of graphene films after the thermal treatment is mainly due to the reduction of graphene and the adsorption of small particles on the other side of the quartz substrates, which is a kind of artifact. After the acid and doping treatments, the transparency was restored beyond that of the thermal treatment. As shown in Fig. 5, the films obtained in this study in general had higher transparency than the previous results for similar film thicknesses, indicating high efficiency of the chemical treatments in improving the transparency. The removal of impurities that may present on both sides of the substrates by the acid treatment could also contribute to the improvement.

3.4.

Surface chemistry

XPS analysis was performed to provide information on elemental compositions of the GO suspension and graphene films after thermal treatment and the additional treatments, and thus to better understand their surface chemistries that are directly correlated to their optical and electrical properties. Fig. 6 compares the general spectra as well as the C1s XPS spectra of GO and the graphene films obtained after thermal treatment and the additional treatments. The C 1s signal of graphene consists of four different chemically shifted components that can be deconvoluted into: C@C/C–C in aromatic rings at 284.6 eV; C–O at 286.1 eV; C@O at 287.5 eV; and C(=O)– (OH) at 289.2 eV [26]. The components at 285.8, 287.6 and 289.5 eV were attributed to the C–O, C@O and C(=O)–(OH) groups, representing 38.5%, 9.0% and 1.1% of the carbon signals for GO (Fig. 6b), whereas the concentrations of the corresponding functional groups after additional treatments were changed to 31.9%, 9.0% and 2.4%, respectively (Fig. 6f). The peak at 286.4 eV arose from C–S, with about 2.3% signal

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Table 1 – AFM images of graphene films at different stages. 38.7 nm thick graphene film

78.0 nm thick graphene film

As prepared

After thermal treatment

After additional treatments

composition, in agreement with the S 2p spectrum to be explained later. Table 2 summarizes the elemental compositions of GO and the graphene films obtained after thermal treatment and the additional treatments, which are also com-

pared with the results obtained previously [9] corresponding to the graphene films prepared after thermal treatment or ‘standard process’. The much lower oxygen concentration in the final graphene films than the other two samples indicates

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Before thermal treatment

40

After additional treatments Standard process [9]

Ra (nm)

30

20

10

0 0

50

100

150

200

250

300

Film thickness (nm) Fig. 3 – Comparison of surface roughness of graphene films.

120

Before thermal treatment After thermal treatment After additional treatments t=39nm

Transmittance (%)

100

80 t=89nm

60 40 t=286nm

20

0 400

500

600

700

800

900

1000

1100

Wavelength (nm) Fig. 4 – UV–vis spectra of graphene films obtained at different treatment stages.

Before thermal treatment After thermal treatment After additional treatments Standard process [9]

Transmittance (%)

80

60

40

20

0 0

50

100

150

200

250

300

Film thickness (nm) Fig. 5 – Comparison of transmittance at different treatment stages.

that the additional chemical treatments effectively removed the oxygen-containing functional groups on the graphene surface.

It is also worth noting that the final graphene films exhibited high Br, S and N contents apparently arising from the chemicals employed in the acid and doping treatments, which may also be responsible for the lower sheet resistance of the graphene films prepared in this study. The analysis of the Br 3d spectra shown in Fig. 7a indicates that there are two types of C–Br bonds, namely the charge-transfer complexes and the covalent bonds. The intense Br 3d component at a core-level binding energy of 70.1 eV is typical of C–Br covalent bonds, whereas the less intense component with a lower binding energy of 69.3 eV is assigned to ionic bromine [27]. The two types of C–Br bonds in the post-treated graphene films were further confirmed by ToF-SIMS as shown in Fig 8. Atomic bromine exhibits two isotopic peaks at molar masses 79 and 81, respectively; molecular bromine, Br2, exhibits a triplet at 158, 160 and 162. It is noted that this kind of p-type doping associated with –Br can lead to the formation of charge-transfer complexes within the graphene sheets, which results in a charge redistribution in the system, significantly increasing the electrical conductivity of graphene films [21]. Fig. 7b shows the analysis of the S 2p spectra. The peak at around 169 eV can be attributed to S with an oxidation state VI and SO covalent bonds. The presence of signal corresponding to –SO3 (m/z = 80) and –SO4H (m/z = 80) as shown in typical ToF-SIMS spectra in Fig. 8, confirms the successful attachment of these functional groups onto the graphene films. An additional peak occurred at 163.6 eV, identical to that of an organic C–S bond [28], indicating that–C–S–C– bonds existed in the final graphene films. The possibility of covalent cross-linking between the graphene sheets through –C–S–C– bonds may have also contributed to the enhanced electrical conductivity. The components near 399–402 eV in the N 1s spectrum (Fig. 7c) are attributed to the NO, N–C groups, which originate mainly from the HNO3 treatment. Judging from the ToF-SIMS results shown in Fig 8, three different types of nitrogen compounds, namely –CN (m/z = 26), –CNO (m/z = 42), and –NO2 (m/ z = 46), were identified. These results are consistent with N 1s XPS results, confirming the attachment of nitrogen atoms on graphene surface via covalent bonding due to the HNO3 acid treatment. Although the treatment in HNO3 likely introduced some defects on the graphene surface and edges, it does not appear to have any detrimental effect on the electrical properties of graphene thin films. This phenomenon can be explained by the fact that some dangling bonds or defects formed by the HNO3 treatment are immediately passivated by –OH or –COOH groups. A nucleophilic substitution by bromide takes place once –OH or –COOH groups are in contact with SOBr2. Then, the graphene sheets form ionic bonds with –Br or –SOBr groups, imparting a beneficial effect on the conductivity of graphene films.

3.5.

Enhanced electrical conductivity

The sheet resistance of graphene films measured at different treatment stages is shown in Fig. 9. The results indicate that the sheet resistance of the films before the thermal treatment was relatively high (104–106 O/sq). After the thermal treatment, the graphitized films had substantially reduced resistance of 100–2000 O/sq, about 3 orders of magnitude lower

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4

-O1S

× 10

(a)

2.0

(b)

-C-C -C-O

Intensity (a.u)

0.5

0.0 1400

1200

-O KLL -O KLL

1.0

-C KLL

C/S

-C1S

1.5

1000

-C=O

-CO-O

800

600

400

200

0

300

295

Binding energy (eV)

(d)

280

1.5

1200

-O1S

0.0 1400

-C-O -C=O

-O KLL -O KLL

-C KLL

1.0

0.5

1000

-CO-O

800

600

400

200

0

298

294

1200

1000

800

600

Intensity (a. u) 400

278

200

Binding energy (eV)

-C-O -C=O -CO-O

-C-S

-Br3d

-Br3P3 -S2P

-Br3S -S2S

-N1S

-O KLL -O KLL

-C KLL

C/S 0.0 1400

282

-C-C

(f)

-C1S

(e)

1.0

0.5

286

4

-O1S

1.5

290

Binding Energy (eV)

Binding energy (eV) × 10

-C-C

Intensity (a.u.)

(c)

2.0

C/S

285

4

-C1S

×10

290

Binding Energy (eV)

0

300

295

290

285

280

Binding Energy (eV)

Fig. 6 – XPS general spectra and curve fitting of C1s spectra of (a and b) GO, (c and d) after thermal treatments, (e and f) after additional treatments.

than that measured before the thermal treatment. The thermal treatment removed part of the oxygenated functional groups and helped graphitization of graphene films, which in turn restored the p-electron system in graphene. After the acid and doping treatments, the sheet resistance was fur-

ther reduced by about 20–50% to 50–1600 O/sq depending on the film thickness. It was reported that the increase in the electrical conductivity of HNO3-treated CNT films arose from the removal of SDS [16]. In our case, the etching effect of the acid treatment cleaned the graphene films with a beneficial

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Table 2 – Elemental compositions of GO, graphene film after thermal treatments, after additional chemical treatments and previous results based on a standard process [9]. Element (At.%)

C

N

O

S

Br

GO After thermal treatments Final product Standard process [9]

71.91 80.91 71.40 80.1

0.49 0.52 2.93 –

27.48 18.43 16.79 19.9

0.11 – 4.80 –

– – 4.08 –

(a)

SO42-

(b)

Intensity (a.u)

Intensity (a.u)

C-Br

Ionic Bromine

S-O C-S

0 82

80

78

76

74

72

70

68

66

64

62

178 176

Binding Energy (eV)

NO

Intensity (a.u)

(c)

172 170 168 166 164 162 160 158

Binding Energy (eV)

C-N

412 410 408 406 404 402 400 398 396 394 392

Binding Energy (eV) Fig. 7 – XPS detail spectra of the final graphene thin films after additional treatments: (a) Br 3d, (b) S 2p, and (c) N 1s.

effect on electrical conductivity. Fig 10 presents schematic illustrations of the doping effect: SOBr2 molecules are attached to graphene sheets, and –Br or –SOBr functional groups are bonded after doping with SOBr2. These molecules and functional groups with strong electronegativity acted as electron acceptors, allowing the Fermi level to move toward the valence band and thus increasing the hole density in

graphene, giving rise to a significantly reduced electrical resistivity. In summary, the combined effect of both HNO3 cleaning and SOBr2 doping resulted in enhanced electrical and optical properties. The effect of HNO3 treatment is two fold, namely the removal of the impurities originated from thermal annealing and the etching of graphene film beneficial to elec-

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Fig. 8 – ToF-SIMS spectra of graphene film after thermal treatments (a and b), and after additional chemical treatments (c and d).

trical conductivity. SOBr2 doping treatment introduced –Br or –SOBr functional groups with a strong electronegativity onto the graphene surface and acted as electron acceptors improving the electrical conductivity.

Sheet resistance (Ω/sq)

1000000

100000

Before thermal treatment After thermal treatment After additional treatments

2000

1000

0 0

50

100

150

200

250

Film thickness (nm) Fig. 9 – Comparison of sheet resistance at different treatment stages.

300

In order to further explain the enhanced electrical conductivity, the carrier type and sheet carrier concentration at different stages were determined from the Hall coefficient as measured by a Hall measurement system (HL5500PC, BIORAD) using the Van der Pauw method and the results are summarized in Table 3. The graphene films remained a positive carrier, i.e. hole carrier, throughout the whole processes. It is worth noting that the sheet carrier concentration increased by about 3 orders of magnitude after thermal treatment, which arose mainly from the removal of the oxygenated functional groups and the graphitization. After additional treatments, the sheet carrier concentration further increased by nearly 200% due to the doping effect. It is seen that the –Br or –SOBr functional groups with a strong electronegativity acted as electron acceptors, thus increasing the hole density in graphene. The degree of increase in sheet carrier concentration was functionally very similar to the reduction in resistivity due to the two additional processes (Fig. 9). In summary, the current results of sheet resistance vs transparency are compared with the values collected from the literature, as shown in Fig. 11. When compared to our previous results [9], the sheet resistance obtained in this study was significantly reduced, the reduction being most pro-

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Br

12000

O

S

Sheet resistance (Ω/sq)

(a)

4 9 ( 2 0 1 1 ) 2 9 0 5 –2 9 1 6

C

(b)

Br S O

Br S O

Br

HO

HO

Present study Standard process [9] Blake et al [7] Zhu et al [29] Wang et al [2] wang et al [3] Wu et al [30] Reina et al [31] Kim et al [32] Becerril et al [33]

9000

6000

Spraycoating

3000

Br S O

O

CVD

0

CVD

OH Br HO O

80

100

O

HO

O

OH

O

Br S

substrate materials, such as copper and nickel, which have to be removed chemically after the growth. The high cost of the single crystal substrates and the ultrahigh vacuum conditions necessary to maintain for the CVD growth significantly limit the use of the method for large scale applications [31]. As described above, films derived from aqueous suspensions of graphene sheets through the transfer printing method can potentially overcome these limitations.

Br

Br O

O S Br

Br Br O

60

Fig. 11 – Comparison with previous results of our group and the results from representative literatures [2,3,7,9,29–33].

Br

OH

Br

40

Transparency (%)

O S

O

O

20

Br OH

Br S

OH OH O OH

Br

O

S

Br

Br

O OH

Fig. 10 – Schematic illustrations of doping effect: (a) molecular structure of a SOBr2 molecule absorbed onto graphene surface; (b) final chemical structure of graphene sheet after additional treatments.

3.6.

Stability of graphene films

The degradation of the electrical conductivity and transmittance enhanced by various chemical processes in service environment is a critical issue for practical applications [14]. To evaluate whether these properties remain stable after the additional treatments, the transmittance and the sheet resistance were measured after 1.5 and 3 months of exposure to ambient air and the results are presented in Table 4. The exposure to air for an extensive period of

nounced at high transparencies above 75%. The current results clearly outperformed the data taken from the literature for all levels of resistivity and transparency, with the exception of the data reported for graphene films prepared by the CVD method. The CVD method usually requires using specific

Table 3 – Carrier type and sheet carrier concentration of graphene films at different stages. Sample 38.7 nm thick 78.0 nm thick

Before thermal treatment (cm 2)

After thermal treatment (cm 2)

After additional treatment (cm 2)

+2.30 · 1011 +7.11 · 1011

+2.66 · 1014 +3.63 · 1014

+5.64 · 1014 +7.09 · 1014

Table 4 – Comparison of sheet resistance (Rs) and transmittance (T) of graphene films at different stages of exposure to air. Thickness (nm)

After thermal treatment Rs (O/sq)

38.7 78.0 89.0 172.8 221.2 250.8 285.9

2031.3 1190.8 815.7 747.6 545.0 282.9 81.8

T (%) 73.4 54.1 50.2 38.6 28.5 20.9 3.6

After additional treatment Rs (O/sq) 1598.0 911.0 589.0 335.8 261.7 190.2 55.7

T (%) 81.7 74.4 55.7 41.6 38.1 35.1 16.4

After exposure for 1.5 months Rs (O/sq) 1773.8 1132.6 706.8 394.6 366.8 216.4 70.1

T (%) 80.2 70.4 55.3 40.1 36.9 31.2 15.5

After exposure for 3.0 months Rs (O/sq) 1791.5 1223.2 784.5 410.3 378.8 230.9 81.1

T (%) 78.1 65.8 53.5 38.6 35.7 30.7 15.4

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3 months caused only marginal degradation of the overall absorbance with the transmittance values remaining largely unchanged for the whole film thicknesses. Meanwhile, the sheet resistance increased by about 5–20% after the initial 1.5 months of exposure, especially for those in the range of film thickness 100–240 nm, but remained almost unchanged after an extensive exposure for 3 months. A possible reason for the initial degradation of the conductivity is the loss of bromide functional groups that have an ameliorating effect of enhancing the electrical conductivity. The acyl bromide groups are reactive with water, and thus it is likely that the doped functional groups on the graphene film surface may have reacted with moisture present in air during ageing. To reduce the possibility of decomposition and thus to retain the improved electrical conductivity of p-doped transparent CNT films [34], it was proposed to apply a thin layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT/PSS).

4.

Conclusions

The transparency and electrical conductivity of graphene thin films were improved by a series of chemical treatments after the thermal reduction. They include a treatment by HNO3 and functionalization in a SOBr2 solution. The acid treatment helped remove the impurities on the graphene film as well as substrate surface, while –Br or –SOBr functional groups with a strong electronegativity acted as electron acceptors thus increasing the hole density in graphene. The electrical conductivity of the resulting graphene films increased by about 20–50% along with enhancement of the transmittance. The comparison of resistivity vs transparency with the literature values clearly indicates outperformance of the graphene films obtained in this study.

Acknowledgements This project was financially supported by Henkel International (ICIPLC001.07/08) and the Hong Kong Research Grant Council (614010). Q.B.Z. is partly supported by Postgraduate Scholarship through NanoTechnology Program of the School of Engineering at HKUST. Technical assistance from the Materials Characterization and Preparation Facilities (MCPF) of HKUST is appreciated.

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