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NANO: Brief Reports and Reviews Vol. 6, No. 5 (2011) 409–418 c World Scientific Publishing Company DOI: 10.1142/S1793292011002780
UV-LIGHT-ASSISTED OXIDATIVE sp3 HYBRIDIZATION OF GRAPHENE ¨ FETHULLAH GUNES ¸ , GANG HEE HAN, HYEON-JIN SHIN, SI YOUNG LEE, MEIHUA JIN, DINH LOC DUONG, SEUNG JIN CHAE, EUN SUNG KIM, FEI YAO, ANASS BENAYAD, JAE-YOUNG CHOI and YOUNG HEE LEE∗ BK21 Physics Division, Department of Energy Science Sungkyunkwan Advanced Institute of Nanotechnology Center for Nanotubes and Nanostructured Composites Graphene Center at Sungkyunkwan University, Suwon 440-746, Republic of Korea Graphene Center at Samsung Advanced Institute of Technology (SAIT ) P. O. Box 111, Suwon 440-600, Republic of Korea ∗
[email protected] Received 17 May 2011 Accepted 24 May 2011 We report the transformation of electronic structures of sp2 graphene to sp3 graphene by UVlight-assisted oxidation. Two distinctive oxidation mechanisms were observed during this metal– insulator transition: (i) At low-oxidation regime, p-doping behavior by oxygen species extracting electrons from graphene and (ii) at high-oxidation regime, n-doping behavior by electron-hopping via strongly localized oxide states. We also found that the dominant oxygen-related functional group by UV-light-assisted oxidation was an epoxide group rather than hydroxyl group, which differed from conventional graphite oxide. Keywords: Graphene oxide; UV-light; transition from metallic to insulator; transparent conducting films.
1. Introduction
However, due to excessive amount of defects generated during those processes, reduced graphene oxides cannot exhibit the properties of intrinsic graphene.12 Recently, hydrogenated graphene, called graphane, was synthesized, showing a good example of hybridization and metal–insulator transition.13 Similarly, several methods to tune the conduction of graphene from the semimetallic to the insulating state have been studied both theoretically and experimentally, including artificial defect creation method such as ozone exposure,14 high-temperature
Graphene has been intensively investigated due to its one-atom-thick structure with unique properties.1–5 Having all atoms on the surface makes graphene easily convertible by environmental effects or various chemical species. Electronic and atomic structures of graphene can be easily altered by modifying its surface with dopants, molecules, or any other chemicals.6–11 For instance, oxidation or chemical modification of graphite is one common way of separating graphitic layers into monolayer graphene oxide and reducing them into graphene. 409
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oxidation,15 and energetic irradiation by positive ions, protons and electrons.16–18 Theoretical calculations for ozone-treated graphene revealed that ozone molecules adsorbed on the graphene basal plane can form epoxide groups and oxygen molecules, resulting in a continuous transition from conductive to insulating state.19 UV-light treatment was used to generate ozone and oxidize graphene to locally introduce sp3 -type defects in order to observe the magnetoresistance transport phenomena,14 the physical and chemical properties related to sp3 hybridization under different environmental gases remain undiscovered. Here, we investigated a nondestructive method of forming stable oxidation of graphene by UVlight irradiation at room temperature with various ambient gases such as O2 and H2 gas. The transition of electronic structures from sp2 hybridization (metallic) to sp3 hybridization (insulating) occurred within less than 10 min of UV-light irradiation without deteriorating graphene basal plane. The dominant oxygen-related functional group by UV-light oxidation was an epoxide group rather than hydroxyl group, opposite trend of conventional graphite oxide (GO). At longer UV-light irradiation, a type conversion of majority carrier from hole to electron was also observed independent of ambient gas. These were confirmed consistently by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electron diffraction (ED) patterns and Hall measurements.
2. Sample Preparation and Experimental Details 2.1. Preparation of monolayer graphene film and GO powder Large-area monolayer graphene films were synthesized on Cu foil by chemical vapor deposition (CVD) method and transferred onto other substrates, as explained in Ref. 10. GO powder was produced by a modified Brodie method with 24-h oxidation time using precursor graphite as described in Ref. 20.
2.2. UV-light treatment Graphene sheets transferred on SiO2 /Si, sapphire and h-BN substrate were placed into a sealed box (30 × 20 × 10 cm3 ) with UV-light bulb (254 nm– 20 mW/cm2 , low-pressure Hg lamp) attached to the top-inside and with connected gas sources. Before
turning the UV-light on, samples were kept in O2 or H2 gas flow for more than 5 min in order to remove the air inside the box and to obtain effective surface contact with target gas. Then, samples were kept under the illumination of UV-light for different time lengths.
2.3. Characterizations Raman spectroscopy (Renishaw, RM-1000 Invia) with excitation energy of 2.41 eV (514 nm, Ar+ ion laser) was used to characterize the optical properties of the graphene films on sapphire and SiO2 /Si substrates. The angle-resolved XPS spectrometry (QUANTUM 2000, Physical Electronics, USA) was performed with an angle of 7◦ to the surface using focused monochromatized Al Kα radiation (1486.6 eV) in order to determine the atomic concentrations of bare carbon and oxygen terminated carbon atoms. Current–Voltage (I − V ) characteristics of graphene transistors were measured by a source-measure unit (Keithley 236.237) using a probe station at room conditions. Sheet resistance measurements were performed by a fourpoint method (Keithley 2000 multimeter) at room temperature. High-resolution transmission electron microscopy (HR-TEM) (JEM 2100F, JEOL) was used to investigate the surface morphology and ED patterns of monolayer graphene before and after UV-light treatment. Graphene sheets for HR-TEM were transferred on SiO2 /Si substrates as described before and then directly transferred onto Cu grids by etching SiO2 layer in diluted HF acid solution and further dilution. UV-light-treated TEM samples exposed to illumination after transferring them on TEM grids in order to obtain top and bottom graphene surface contact with oxygen species. The sheet carrier density, mobility, and sheet resistance of graphene were measured by the van der Pauw method using Hall measurement (ACCENT semiconductor UK/HL 5500PC) at room temperature.
3. Results and Discussions 3.1. Mechanism of UV-light hybridization Figure 1 shows a schematic of UV-light-assisted oxygenation of conducting graphene film converting into insulating graphene oxide. Ozone (O3 ) generation by UV-light has been known for decades.21 Low-pressure mercury lamp with 254-nm wavelength has been used for efficient surface cleaning.
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Fig. 1. The schematic of graphene under ambient conditions (a) without and (b) with UV-light irradiation. Carbon, oxygen and hydrogen atoms are indicated in different colors. Functional groups such as epoxide and hydroxyl groups are indicated in the right side. Oxygen, water, and ozone molecules are indicated at the bottom (color online).
Although 254-nm wavelength itself is not sufficient enough to produce ozone directly, partial 185-nm wavelength (5%) UV-light generation from lowpressure Hg lamp makes it possible to have simultaneous production of ozone at 185 nm and ozone decomposition into oxygen ions (O− ) at 254 nm (95%).22 When O2 molecules are exposed to UVlight at 185-nm wavelength, they transform into ozone (O3 ) molecules. Resulting ozone molecules then decompose into highly oxidizing oxygen ions (O− ). The resulting oxygen atoms (O− ) either attach to other oxygen molecules (O2 ) in the medium to be stabilized to generate more ozone (O3 ) molecules or attack the basal graphene to form epoxides, hydroxyl, and carboxyls with carbon atoms. In our experiment, graphene sample was exposed to UV-light at room temperature. C–O–C epoxide and C–OH hydroxyl groups are likely to be formed during UV-light exposure, rather than etching, since the process is conducted in rather mild conditions. It is also worth mentioning that the graphene surface becomes hydrophilic after UVlight exposure. This is in good contrast with the previous report that graphene layers can be etched by high-temperature oxygen treatment.15 The CVD-grown monolayer graphene was transferred onto sapphire substrate (see Sec. 2 for details). The pristine monolayer graphene in Fig. 2(a) shows a small D-band near 1350 cm−1 , which corresponds to transverse optical phonon near the K point and indicates sp3 hybridization of carbon network, G-band near 1590 cm−1 , which is related to optical E2g phonon at the Brillouin zone center and indicates sp2 hybridization of carbon network, and G -band around 2694 cm−1 ,
which is also known as 2D-band, an overtone of D-band.23 Small D/G and large G /G intensity ratios indicate high quality of the CVD-grown monolayer graphene. At 5-min UV-light exposure under oxygen atmosphere, D-band intensity was enhanced and broadened, while G -band intensity was reduced considerably. It is also noted that a shoulder near 1620 cm−1 was developed, which is known as D -band. This phonon mode is usually Raman-inactive but becomes active due to phonon confinement caused by the defects.24 The presence of defects involving oxygen and/or hydrogen atoms could be a source of phonon confinement in our case. No peak splitting of G-band into G+ and G− peaks indicates that our process does not involve strain-induced doping effect.25 With further UVlight exposure to 10 min, D-band intensity was not altered much, while D -band disappeared and G -band intensity increased. The disappearance of the shoulder (D -band) at 1620 cm−1 is attributed to the kicking-out of oxygen species by an overexposure of UV-light, in good agreement with the previous report in removing oxygen species by UVlight treatment.15,26 After thermal annealing of the sample at 1000◦ C under high vacuum, the D-band intensity was substantially reduced back to that of the pristine sample, implying that the increase of sp3 hybridization is caused not by the deterioration of carbon atoms in the graphene plane but by the chemisorption of oxygen atoms onto the carbon network. Widths of the observed D-band and G-band were much narrower than those of the GO samples synthesized by the modified Brodie method.20 Furthermore, G -band peak was still retained during irradiation, implying rather simple single modal
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(a)
(b)
(c)
(d) Fig. 2. Raman spectra of the pristine monolayer graphene (red), 3-min UV-light-irradiated one (magenta, on Si/SiO2 ), 5-min UV-light-irradiated one (green), 10-min UV-light-irradiated one (blue), annealed one (orange) after irradiation of 10 min, and GO powder (black) on (a) sapphire and (b) Si/SiO2 substrate under O2 gas ambient, and (c) under H2 gas ambient on Si/SiO2 substrate. Ar laser with an excitation wavelength of 514 nm was used. Magnified G-band from the same data set in (a) was shown in the right panel. D/G and G’/G intensity ratios and peak positions are indicated in the figure. The Raman spectra taken on sapphire substrate is the averaged data obtained from five different points from the graphene for each condition. All the peaks were normalized by the G-band intensity. (d) C1s peaks from X-ray diffraction spectra of pristine graphene, 10-min UV-light-irradiated graphenes under H2 and O2 gas ambient, and GO powder. All spectra were deconvoluted into the four peaks of C–C/C=C (green), COOH (blue), C–OH (magenta), and C–O–C (purple). All peaks were normalized by C–C/C=C peak intensity (color online).
oxidation to occur. With increasing UV-light irradiation time, the G-band and G -band peak positions were upshifted, revealing a p-type doping behavior by oxidation, and were recovered to those with annealing at 1000◦ C. It is also interesting to observe (D + D )-band near 2950 cm−1 , known as a defect-related peak, which occurs from two phonons with different momentum.13 This peak was not observed in the pristine graphene but in GO, again strongly indicating that the sp3 hybridization by oxidation observed in our case is oxygen-related. Our observation of creating sp3 hybridization by
oxidation should be contrasted with the previous destructive high-temperature oxygen etching.15 Similar sp3 hybridization was also observed even with different substrate (SiO2 /Si), as shown in Fig. 2(b). Larger D/G and D /G intensity ratios were observed compared to that with sapphire substrate. A similar experiment to that with O2 ambient on SiO2 /Si substrate shown in Fig. 2(b) was done with hydrogen gas environment. Higher D/G intensity ratio similar to Fig. 2(b) but larger G /G intensity ratio was observed in this case, as shown in Fig. 2(c). The full width at half maximum was
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narrower compared to the case of O2 ambient on SiO2 /Si substrate. The shoulder peak still remained at 10-min UV-light exposure. These suggest that hydrogen gas was also effective to create sp3 disorder, while maintaining better hexagonal symmetry in the graphene plane.
23.8% of the GO powder showing that more effective conversion of graphene to graphene oxide was achieved by UV-light compared to conventional GO. This can be understood by the larger surface area exposed to oxygen species in the case of monolayer graphene unlike the multilayered GO which has limited accessibility to the oxidizing agents. Reference data related to C/O ratios were also shown in Table 1.24,29 Highly ordered pyrolized graphene and functionalized single graphene sheet samples show similar C/O ratio compared to our CVD-grown graphene, owing to high quality of our graphene samples. The C1s peak obtained from graphene on BN substrate after annealing at 650◦ C was similar to that of the graphene transferred to Au substrate without any further heat treatment, proving that our results were not affected from any residual contamination that might be introduced during transfer process (see Supplementary Information S1). C/O ratio of the UV-light-treated graphene both under H2 and O2 environment was similar to the conventional GO sample. This large oxygen content observed here is also in agreement with large D/G ratio obtained from Raman spectra in Fig. 2, demonstrating the effectiveness of UVlight oxidation of graphene. Figure 2(d) shows XPS C1s spectra, which are deconvoluted into several peaks: C–C and C=C related peak near 284.4 eV, epoxide (C–O–C) associated peak near 285.5 eV, hydroxyl (C–OH) group near 286.3 eV, and carboxyl (COOH) group near 288.5 eV.30 As shown in Table 2, the amount of C–O–C bonds under
3.2. Chemical and electronic structure More detailed chemical composition and binding nature of sp3 hybridization under different ambient gases were obtained by analyzing XPS data. Graphene samples for the XPS were transferred to hexagonal boron nitride (h-BN) substrate and annealed at high vacuum (10−7 Torr) and at 650◦ C for 5 h before measurement. h-BN substrate was chosen due to its high thermal stability and chemical inertness to minimize interaction with graphene layer.27 By using BN substrate, we eliminated the possible coupling with the substrate at hightemperature.28 As shown in Table 1, which was extracted from XPS spectra (see Supplementary Fig. S1), the amount of oxygen species was incremented to 33.7% under UV-light irradiation with hydrogen gas from 9.0% of the pristine CVDgrown sample. The ozone gas generated by UVlight irradiation is still the dominant reactant even under hydrogen environment. The oxygen content was further incremented to 28.4% with oxygen gas. The oxygen content was much higher than
Table 1. Carbon, oxygen and their relative ratios obtained by the areas under C1s and O1s peaks from XPS from 10-min UV-light-treated sample under O2 and H2 environment on h-BN substrate. GO powder and some reference data are also shown for comparison. 10-min UV-light treatment
(C1s) C%: (O1s) O%: C/O:
Pristine
in H2
in O2
GOapowder
91.0 9.0 10.1
66.3 33.7 2.0
71.6 28.4 2.5
76.2 23.8 3.2
Reference data from the literature
C/O: a
HOPGb
FGSc
TEGOb
GOb
10.8–14.9
10–20
5.6–7.9
1.7–2.5
GO powder is prepared as described in Ref. 20. HOPG = Highly ordered pyrolized graphene29 ; TEGO = Thermally expanded GO29 ; GO = Graphite oxide.29 c FGS = Functionalized single graphene sheets.24 b
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Pristine Gr UV in H2 UV in O2 GOpowder
C–C/C=C (%)
(C–O–C) (%)
C–OH (%)
COOH (%)
284.4 eV
285.5 eV
286.3 eV
288.5 eV
74 27.1 36.4 39.2
8.6 20.3 21.1 12.6
4.5 11.5 7.4 21.1
3.9 7.4 6.7 3.3
UV-light oxidation is about 21%, independent of environmental gases, and is much larger than 12.6% of the GO sample. On the other hand, the amount of hydroxyl group is much smaller than 21.1% of the GO sample, whereas the amount of carboxyl groups is almost two times larger. The sample UVtreated in H2 shows much higher hydroxyl and carboxyl groups and a bit lesser C–O–C amount compared to that of the sample UV-treated in O2 . As a consequence, the amount of sp2 bonds of the UV-light-treated samples is smaller than that of the GO sample, indicating the effective oxidation of graphene plane. The ozone generated by UV-light irradiation may couple with environmental gas. For instance, in the case of H2 ambient, the generated ozone may couple with H2 gas to form –OH or –OOH such that more hydroxyl and carboxyl groups are to be formed. In our case, more C–OH groups are formed in the case of H2 ambient compared to O2 ambient. This also explains why more C–O–C groups are formed in our case compared to GO where oxides are formed in aqueous solution that contains –OH groups.31 Even under O2 ambient, C–OH and COOH groups are formed, strongly suggesting the involvement of moisture in the reaction, which was unintentionally introduced in the reaction chamber. The different chemical composition of our UV-lighttreated graphene from that of the conventional GO will give rise to different chemical stability and electrical properties, which is important in the use of practical applications in the future. The effect of sp2 hybridization can be visualized in I − V characteristics, as shown in Fig. 3(a). A dramatic increase of the resistance was observed after 3-min UV-light irradiation (inset). At 10-min UV-light irradiation, the resistance was incremented by a factor of 109 compared to the pristine graphene sample. Shown in
(a)
(b) Fig. 3. (a) Room temperature I − V curves of the pristine (red), 3-min (black), 5-min (green), 10-min (blue) UV-lighttreated devices on SiO2 /Si substrate. The channel length and width were 4 mm and 2 mm, respectively, with Au source and drain electrode. (b) Sheet resistance changed by UV-light exposure time. Inset shows graphene monolayer on a Cu TEM grid after being exposed to UV-light for 10 min, maintaining the stable planar structure (color online).
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Fig. 3(b) is the sheet resistance increase with UV exposure time. The sheet resistance of the pristine graphene (∼300 Ω/sq) was increased to 1 MΩ/sq at 9-min irradiation (the sheet resistance with 10-min irradiation was not measurable). This implies that UV-light irradiation simply converts the electronic structures of graphene from metallic to insulating state. HR-TEM image obtained from graphene transferred onto TEM Cu-grid after
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UV-light treatment for 10 min was shown in the inset of Fig. 3(b). The oxidized graphene still remained intact in spite of oxidation on both sides of graphene. To confirm the stability of graphene under UV-light irradiation, the graphene film was transferred to carbon-coated Cu TEM grid and UVlight-treated for 10 min under oxygen ambient. In this case, both sides of the suspended graphene
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4. HR-TEM images of (a) the pristine and (b) 10-min UV-light-irradiated monolayer graphene. ED patterns obtained from the same position are shown in the HR-TEM images from (c) the pristine and (d) 10-min UV-light-irradiated samples and ((e) and (f)) the corresponding ED patterns. The extracted bond lengths are in excellent agreement with theoretical calculations.
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Table 3. Electrical properties of UV-light-treated graphene films with treatment time dependence: Sheet resistance, mobility, Hall coefficient, and sheet carrier density. Graphene thickness is estimated as 1 nm for the calculation of sheet carrier density. Condition: UV treatment time: Rs (ohm/sq): Mobility (cm2 /V-s): Hall coefficient (m2 /C): Sheet carrier density: (cm−2 ):
Pristine
UV treatment in O2
UV treatment in H2
—
3 min
5 min
10 min
3 min
5 min
10 min
464.2 384 +17.8 +3.50E+13
5354 19.9 +10.7 +5.85E+13
8.74E+06 15.9 −1.39E+04 −4.50E+10
> Over limit — — —
1126 195 +21.9 +2.85E+13
2.53E+04 7.11 +18 +3.48E+13
6.75+06 53.6 −3.62E+04 −1.73E+10
are expected to be oxidized. Nevertheless, the graphene layer remained stable, as shown in Figs. 4(a) and 4(b). ED patterns in momentum space were obtained from these samples (Figs. 4(c) and 4(d)). A clear hexagonal spot (guided by green long-dashed line) was obtained from the pristine graphene. On the other hand, the hexagonal ring was slightly shrunken, indicated by red dashed line. This shrinkage originates from C–OH and C–O–C which extends the C–C bond length to 148 pm and 150 pm, and in-plane periodicity to 253 pm from 142 pm and 246 pm of the pristine graphene, respectively, as shown in Figs. 4(e) and 4(f).32 In order to understand the doping effect of UVlight irradiation, Hall measurement was carried out with graphene films transferred on Si/SiO2 substrates. The results were summarized in Table 3. The mobility of the pristine graphene film decreased rapidly with UV-light exposure time for O2 case, which is attributed to enhance sp3 hybridization by oxidation, and was not measurable at 10-min UVlight exposure time. It is noted in the Hall coefficient that the carrier type was converted from hole to electron at 5-min UV-light exposure time under O2 ambient. In the case of H2 environment, the mobility decreased at 3-min UV-light exposure time but at 10-min UV-light exposure, the mobility increased compared to that of 3-min exposure time. The carrier type was again converted from hole to electron at 10-min UV-light exposure time. The electron carrier density is smaller by three orders of magnitude than hole carrier density. This type conversion to n-type is also in congruent with the downshift of the G-band peak position, shown in Figs. 2(b) and 2(c). The G-band peak position was upshifted first, showing p-type behavior at the beginning, and then shifted back, indicating n-type conversion by molecular doping in both environments after 5 min
for O2 and after 10 min for H2 . With mild oxidation, energy dispersion is no longer Dirac-like linear but becomes rather quadratic, with momentum similar to typical semiconductors or insulators. At heavy oxidation, the Fermi level might be shifted into the valence band, as observed in AuCl3 doping.10,33,34 This case is similar to highly degenerate semiconductors. Hopping might occur through strongly localized oxide states, as observed in GOs.14,35
4. Conclusions An effective way of transforming electronic structures of graphene from sp2 hybridization to sp3 hybridization by UV-light-induced oxidation was investigated. Hybridized graphene films show metal–insulator transition by UV-light illumination under O2 and H2 environment. The p-type carriers were generated due to the increase of the oxygen content at initial stage of UV-light irradiation followed by the generation of n-type carriers at later stage by electron-hopping via strongly localized oxide states, independent of the ambient gases. This hybridization strategy by UV-light illumination could be implemented in graphene nanoelectronics by direct patterning of large-area graphene films by the local UV-light exposure technique.
Acknowledgments This work was financially supported by WCU (World Class University) program through the KRF funded by MEST (R31-2008-000-100290), the IRDP of NRF (2010-00429) through a grant provided by MEST in Korea, NRF (20100029653) through a grant provided by MEST in Korea.
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Fig. S1. Survey spectra of X-ray photoelectron spectroscopy from pristine graphene, UV-light-treated graphene for 10 min in O2 and H2 environment on BN substrate and graphite oxide powder on Au-coated Si/SiO2 substrates (Left). C1s peaks from pristine graphene right after transfer on Au-coated Si/SiO2 substrate and from pristine graphene after transferred and annealed at 650◦ C for 5 h on BN substrate showing similar quality (Right).
Supplementary Information Survey spectra of XPS from pristine graphene, UVlight-treated graphene for 10 min in O2 and H2 environment and GO powder and C1s peaks from pristine graphene before and after heat treatment (Fig. S1).
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