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Doping Graphene with an Atomically Thin Two Dimensional Molecular Layer Haena Kim, Hyun Ho Kim, Jeong In Jang, Seong Kyu Lee, Geon-Woong Lee, Joong Tark Han,* and Kilwon Cho* Graphene has attracted much attention since its experimental discovery[1] owing to its excellent properties.[2–8] Among various properties of graphene, high flexibility, high transmittance (97.7%)[9] and high conductivity (106 S/cm)[10] enable graphene to be applied to the transparent conducting electrode of soft electronic devices.[5,10–16] To employ graphene in such electrodes, improved conductivity and work function control are required, which can both be realized through doping of graphene. Recently, a variety of doping methods have been reported,[17,18] including substitutional doping, electrochemical doping, and molecular contact doping. Molecular contact doping[10,14–16,19–23] is the most suitable method for device fabrication because it does not generate atomic defects in graphene. Various dopants have been used in molecular contact methods, including aromatic molecules,[19] metal nanoparticles,[20] self-assembled monolayers (SAMs),[14,22] solvent molecules,[21] acids,[10,15] and vapor-phase molecules.[16] However, these studies concentrated on the doping strength, and research into the other properties of doped graphene that are necessary for its applications in high-performance graphene-based devices remains insufficient. In particular, preventing charge mobility degradation, modulating the work function, and maintaining the high stability, transmittance, and roughness of doped graphene are important for the realization of high-performance devices. Nevertheless, the previous studies rarely referred to these characteristics in detail. Therefore, the development and analysis of methods for doping graphene that satisfy these requirements and provide strong doping effects are important challenges. Here, we report a novel method for doping graphene with atomically thin and chemically versatile graphene oxide (GO) sheets. GO contains functional groups such as hydroxyl, epoxy, carbonyl, and carboxyl groups[24] that withdraw electrons from graphene, so it can be used as a p-type dopant material. As we will demonstrate, graphene doped with atomically thin GO has H. Kim,[+] H. H. Kim,[+] S. K. Lee, K. Cho Department of Chemical Engineering Polymer Research Institute Pohang University of Science and Technology Pohang 790-784, Republic of Korea E-mail: [email protected] J. I. Jang, G.-W. Lee, J. T. Han Nano Carbon Materials Research Group Korea Electrotechnology Research Institute Changwon 641-120, Republic of Korea E-mail: [email protected] [+]These
authors contributed equally to this work.
DOI: 10.1002/adma.201403196
Adv. Mater. 2014, DOI: 10.1002/adma.201403196
an intrinsic transmittance and a roughness that are similar to those of graphene, and exhibits high stability in ambient air at room temperature. In addition, two-dimensional micro-sized GO sheets have the morphological advantage that they can be used to dope graphene homogeneously. As a result, the generation of electron-hole (e-h) puddles is minimized, and mobility degradation after doping is prevented. Controlled doping of graphene can be achieved by varying either the coverage or the degree of functionality of the GO sheets. As the coverage of GO increases from 0 to 100%, the work function of the GO-doped graphene gradually increases from 4.56 eV up to 4.99 eV. We also demonstrate that the degree of oxidation and reduction of the GO dopant can be controlled, and thereby the work function of the GO-doped graphene can be finely tuned. Furthermore, organic thin film transistors (OFETs) were fabricated with the work function tuned graphene/GO electrodes and it was found that GO doping improved the electrical performances of these devices. To assess the feasibility of doping graphene with GO, we firstly examined the p-type doping phenomena obtained with a single micro-sized GO sheet. Highly oxidized GO sheets[25] were spin-coated onto hexamethyldisilazane (HMDS)/SiO2/ Si substrates to exclude substrate doping effects and investigate the GO doping effect accurately[26] and then graphene obtained with chemical vapor deposition (CVD) on Cu foil was transferred by using the conventional wet transfer method[27] (Figure 1a). During the transfer of the graphene onto GOcoated HMDS/SiO2/Si wafers, low molecular weight PMMA (120k) was used as a supporting layer to minimize the residue effect.[28] By using field emission scanning electron microscopy (FESEM), we confirmed that the size distributions of the synthesized GO sheets ranged from several tens of nanometers to several micrometers (see the Supporting Information, Figure S1) and that the graphene was successfully transferred onto the GO-coated substrate (Figure 1b). Highly oxidized GO can be utilized as a strong p-type dopant of graphene because of the charge transfer interactions between graphene and the electron-withdrawing groups in GO. To investigate the doping effect of atomically thin GO sheet, Kelvin probe force microscopy (KPFM) and Raman spectroscopy were performed. KPFM was used here to measure the difference between the surface potentials of doped and pristine graphene.[10,29,30] KPFM image corresponding with an AFM image (Figure 1c) shows that the surface potential of the graphene/single GO sheet is negatively shifted by 120 mV with respect to that of the graphene on the HMDS-treated SiO2 surface (Figure 1d). This result indicates that graphene is p-doped by the atomically thin GO sheet.[29]
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Figure 1. (a) Structure of graphene/GO on an HMDS-treated SiO2/Si substrate. (b) SEM image of monolayer graphene on a GO sheet. (c) AFM image and height profile, (d) Kelvin probe force microscopy, and (e) Raman map of the 2D-band shift of monolayer graphene on a GO single sheet. (f) Variations in the conductance of GO-doped and HNO3-doped graphene with time at atmospheric pressure and room temperature. (g) Gate-dependent I-V characteristics of graphene and GO-doped graphene. (h) AFM images of graphene with and without GO.
Raman spectroscopy measurements were also performed to investigate the doping effects of the GO sheets. When graphene is doped, the positions and the full width at half maximum (FWHM) values of the G and 2D peaks in the Raman spectrum are known to be changed.[31,32] The 2D peak of graphene on an atomically thin single GO sheet is blue-shifted with respect to that of graphene transferred onto HMDS-treated SiO2, as shown in Figure S2. In addition, we obtained the Raman 2D band map of graphene transferred onto GO/HMDS-treated SiO2, as shown in Figure 1e. The Raman map of the 2D peak shift contains a bright region in the graphene/GO area. This change in the 2D peak confirms that the graphene is p-doped by only a single GO sheet. Compared to other doping methods,[10,14–16,19–21] GO doping has advantages: stable and strong p-doping that maintains the intrinsic properties of graphene such as charge carrier mobility, transmittance, and roughness. Further, the doping effects of GO on graphene are also stable for more than 40 days at room temperature and atmospheric pressure. As shown in Figure 1f, the conductance of nitric acid doped graphene decreases by approximately half over 40 days, whereas the conductance of
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GO-doped graphene is almost constant. The sheet resistance of graphene was also found to decrease from 600 to 292 ohm/sq as a result of GO doping. The gate-dependent I-V characteristics of graphene and GOdoped graphene show that the hole mobility of graphene is almost unaffected by doping. The hole mobility of GO-coated graphene was found to be 3,330 cm2/Vs, which is only slightly lower than that of pristine graphene (equivalent device position before GO coating), 3,500 cm2/Vs (Figure 1g). However, this value is higher than those obtained with other doping methods: self-assembled monolayer doping, ∼1,800 cm2/Vs[14]; AuCl3 doping, 1,735 cm2/Vs[33]; CYTOP doping, 810 cm2/Vs[34]. In general, molecular contact doping methods[14,33,34] decrease the charge mobility because the inhomogeneous adsorption of dopants generates e-h puddles. The two-dimensional GO sheets homogeneously withdraw electrons from graphene, so the number of e-h puddles generated by doping is minimized and the initial mobility before doping can be almost preserved. Maintaining the charge mobility of graphene has been an important challenge for research into doping because of the linear dependence of the electrical conductivity on the
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87%, and ∼100% at the GO solution concentrations 0.1, 0.2, 0.4, 0.6, and 0.8 g/L, respectively (see the Supporting Information, Figure S5). Graphene was transferred onto each GOcoated SiO2/Si substrate, the graphene/GO film on a SiO2/Si substrate was transferred onto an HMDS-treated substrate, and then ultraviolet photoemission spectroscopy (UPS) measurements were performed, as shown in inset of Figure 2b. The work functions were calculated by using the following equation,
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mobility (σ = e(nμe + pμh ), n: electron concentration, p: hole concentration).[35] Graphene can be effectively doped with atomically thin GO sheets, which enables the preservation of its intrinsic roughness and transparency. The root-mean-square surface roughness (RRMS) of the graphene/GO film is 0.55 nm, which is similar to that of pristine graphene (0.35 nm) due to the single layer of GO sheet beneath the graphene, as shown in Figure 1h and Figure S3. The transmittance of graphene was found to decrease by less than 1% from 97.5% to 96.7% as a result of GO doping, which means that the GO sheet affects the transmittance of graphene only slightly (see the Supporting Information, Figure S4). These advantages improve the contact at the interface between deposited materials and graphene, and prevent performance degradation due to optical losses in vertical devices such as organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). The doping of graphene can be finely tuned in this method by varying the coverage of GO and the number of its functional groups. Firstly, the effects of varying the coverage of GO on the p-doping of graphene were investigated by measuring the variation in the graphene work function. GO solutions with various concentrations were used to provide coverage variation in the range 0.1 to 0.8 g/L; the coverage of spincoated GO on the SiO2/Si substrate was verified with FE-SEM (Figure 2a). From the FE-SEM images, the coverages of GO on the SiO2/Si substrates were calculated to be 19%, 35%, 65%,
φ = ω − Esec − EFE where Esec is the onset of the secondary emission, and EFE is the Fermi edge (-128.1 eV) under a sample bias of -20 V. In the case of pristine graphene, the work function was calculated to be 4.56 eV. As the coverage of GO increases from 0 to ∼100%, the work function of GO-doped graphene gradually increases from 4.56 to 4.99 eV, as shown in Figure 2b. This trend implies that more holes are incorporated into graphene as the coverage of GO increases, which results in down-shifts in the Fermi level. The effects of varying the degree of functionality of the GO sheets on the p-doping of graphene were also investigated. The hydroxyl (C-OH), epoxide (C-O-C), carbonyl (>C = O), and carboxyl (-COOH) functional groups in GO withdraw electrons from graphene, so the doping level of graphene can be modulated by controlling the degree of functionality.[29] There
Figure 2. (a) FE-SEM images of GO on Si substrates for various GO coverages. (b) The variation of the work function of the graphene/GO films with the GO coverage. (inset: ultraviolet photoemission spectra (in the secondary electron emission region) of graphene with various GO coverages on HMDS/SiO2/Si substrates.) (c) UPS spectra of graphene on GO samples with different degrees of oxidation. UPS spectra of graphene on GO (d) for two different reduction temperatures and (e) various reduction times.
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are two approaches to the control of the number of functional groups in GO: i) control of the oxidation process during the synthesis of GO from graphite and ii) control of the reduction process after coating GO onto the target substrate. Firstly, we investigated the effects of varying the degree of oxidation of GO on the p-doping of graphene. Two types of GO were synthesized from graphite through the different number of oxidation process and the C/O ratios of these GO samples were determined to be 1.14 and 0.69 with element analysis. The X-ray photoelectron spectroscopy (XPS) results in Figure S6a indicate that the relatively highly oxidized GO sheets (C/O ratio 0.69) contain more C-OH (285.3 eV), C-O-C (286.8 eV), >C = O (287.8 eV), and -COOH (288.7 eV) than the less oxidized GO sheets (C/O ratio 1.14). This functionality of two GO samples was also confirmed by FT-IR spectra, as shown in Figure S6b. In Figure 2c, it can be seen that the work functions of GOdoped graphene with these two samples are shifted by 0.29 eV and 0.43 eV, respectively. This result shows that the more oxidized GO sheets withdraw more electrons from graphene, which increases the level of p-doping of the graphene. In addition, we examined the effects of varying the degree of reduction of GO on the p-doping of graphene. GO sheets coated onto substrates were thermally reduced and their degree of reduction was regulated by varying the annealing temperature or the reduction time. To investigate the effects of varying the reduction temperature, we performed UPS measurements for graphene transferred onto GO/substrates after the annealing of GO at two different temperatures (150 °C and 200 °C) in vacuum for 1 h. It has been reported that epoxide, carboxyl, and hydroxyl groups are partially reduced by annealing at 150 °C and that additional reduction of these groups and the
carbonyl groups is obtained by annealing at 200 °C.[36] These changes in the functionality of the GO sheets affect the doping of graphene: the work function of GO-doped graphene was measured with UPS and found to decrease from 4.99 to 4.57 eV, as shown in Figure 2d. In addition, we investigated the effects of varying the GO annealing time. The annealing time was varied from 0 min to 60 min at 150 °C, and the variation in the degree of functionality of GO with the annealing time was determined with XPS, as shown in Figure S7. These results show that the number of functional groups of GO decreases as the reduction time increases, so the work function of GO-doped graphene decreases with increases in the reduction time, as shown in Figure 2e. We have thus verified experimentally the efficient control of the electronic states of graphene by varying the degree of functionality of GO through oxidation or reduction processes. We fabricated OFETs with GO-doped graphene (graphene/ GO) electrodes to assess the viability of graphene/GO as a transparent flexible electrode for organic electronics. Bottom contact organic thin film transistors (OFETs) were fabricated by using the amorphous p-type semiconductor poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) and graphene films with or without GO as the source (S) / drain (D) electrodes. Figure 3a shows a schematic diagram of the device fabrication process. Aluminum was thermally evaporated onto graphene/ GO-transferred substrates as a sacrificial layer by using shadow masks,[22] and then uncovered graphene/GO regions were completely etched with O2 plasma, followed by HMDS treatment of the etched region. The Al layer was then wet-etched, and PTAA was spin-coated onto the pre-patterned graphene/GO electrodes. The field-effect mobility of the fabricated OFET devices
Figure 3. (a) Schematic diagram of the fabrication process of the OFETs with graphene/GO electrodes. (b) Transfer characteristics (VD = −60 V) of p-type PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) FETs with graphene/GO as S/D electrodes. (c) Output characteristics of PTAA FETs with graphene/GO as S/D electrodes. (d) Schematic energy band diagram for PTAA, pristine graphene, and GO-doped graphene.
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Experimental Section Synthesis of Graphene Oxide: GO sheets were prepared via the exfoliation of graphite oxide powder, which was produced from natural graphite (Alfa Aesar, 99.999% purity, –200 mesh) by using a modified Hummers method.[25,37] The graphite powder was dispersed in water by stirring, and the highly functionalized graphite oxide with carboxylic acid groups was removed by the centrifugation of a 2 g/L aqueous GO solution. The sediment graphite oxide solution was diluted in water to a concentration of 400 mg/L, and then sonicated for 1 h to exfoliate the graphite oxide into GO sheets. Characterization: The graphene films were characterized with atomic force microscopy and Kelvin probe force microscopy (AFM and KPFM, Veeco NanoScope 8) operating in tapping mode, field emission scanning electron microscopy (FESEM, Hitachi S-4800), confocal Raman spectroscopy (WITec) with an excitation wavelength of 532 nm, UV-vis spectroscopy (Varian, Cary-5000), FT-IR spectroscopy (Nicolet 6700, Thermo Scientific), and ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (at the 8A2 and 4D beamlines at the Pohang Accelerator Laboratory in Korea), and the electrical characteristics of the OFETs were measured by using a Keithley 2400 under vacuum conditions.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by a grant (Code No. 2011–0031628, 2013M3A6A5073177) from the Center for Advanced Soft Electronics
Adv. Mater. 2014, DOI: 10.1002/adma.201403196
under the Global Frontier Research of the Ministry of Science, ICT and Future Planning, Korea. The authors thank the Pohang Accelerator Laboratory for providing the synchrotron radiation sources at 8A2 and 4D beamlines used in this study.
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with GO doping was found to increase from 3.23 × 10−4 to 2.26 × 10−3 cm2/Vs and their on/off current ratios were significantly enhanced (Figure 3b). These enhanced electrical properties are attributed to the low sheet resistances of the graphene/ GO electrodes and the optimization of the alignment between the work function of the graphene/GO electrodes and the HOMO level of PTAA. Figure 3c shows the output characteristics of the PTAA devices using graphene S/D electrodes with or without GO. In addition to the increase in the saturation current due to the introduction of the GO doping layer, the S-shaped non-ohmic behavior at low VD is remarkably reduced. Thus the work function of the graphene/GO S/D electrodes is well matched with the HOMO level of PTAA, which results in a reduction in the injection barrier from 0.55 eV to 0.11 eV (Figure 3d). We have demonstrated the use of atomically thin and chemically versatile GO sheets as p-type dopants of CVD-graphene. Our doping method solves previously challenging issues in graphene doping; this method enables the strong, stable, largescale, low-temperature, and controllable p-doping of graphene with preserved charge mobility, intrinsic roughness, and transmittance. The Fermi level of the GO-doped graphene can be controlled by varying the coverage and degree of functionality of GO (oxidation or reduction). Furthermore, high-performance OFETs were fabricated with work function tuned graphene/GO electrodes. This study has demonstrated the potential of GO as a p-dopant and the usefulness of this doping method for optoelectronics such as organic photovoltaic cells (OPVs), organic light-emitting diodes (OLEDs), and OFETs.
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Doping Graphene with an Atomically Thin Two Dimensional Molecular Layer Haena Kim, Hyun Ho Kim, Jeong In Jang, Seong Kyu Lee, Geon-Woong Lee, Joong Tark Han,* and Kilwon Cho* Graphene has attracted much attention since its experimental discovery[1] owing to its excellent properties.[2–8] Among various properties of graphene, high flexibility, high transmittance (97.7%)[9] and high conductivity (106 S/cm)[10] enable graphene to be applied to the transparent conducting electrode of soft electronic devices.[5,10–16] To employ graphene in such electrodes, improved conductivity and work function control are required, which can both be realized through doping of graphene. Recently, a variety of doping methods have been reported,[17,18] including substitutional doping, electrochemical doping, and molecular contact doping. Molecular contact doping[10,14–16,19–23] is the most suitable method for device fabrication because it does not generate atomic defects in graphene. Various dopants have been used in molecular contact methods, including aromatic molecules,[19] metal nanoparticles,[20] self-assembled monolayers (SAMs),[14,22] solvent molecules,[21] acids,[10,15] and vapor-phase molecules.[16] However, these studies concentrated on the doping strength, and research into the other properties of doped graphene that are necessary for its applications in high-performance graphene-based devices remains insufficient. In particular, preventing charge mobility degradation, modulating the work function, and maintaining the high stability, transmittance, and roughness of doped graphene are important for the realization of high-performance devices. Nevertheless, the previous studies rarely referred to these characteristics in detail. Therefore, the development and analysis of methods for doping graphene that satisfy these requirements and provide strong doping effects are important challenges. Here, we report a novel method for doping graphene with atomically thin and chemically versatile graphene oxide (GO) sheets. GO contains functional groups such as hydroxyl, epoxy, carbonyl, and carboxyl groups[24] that withdraw electrons from graphene, so it can be used as a p-type dopant material. As we will demonstrate, graphene doped with atomically thin GO has H. Kim,[+] H. H. Kim,[+] S. K. Lee, K. Cho Department of Chemical Engineering Polymer Research Institute Pohang University of Science and Technology Pohang 790-784, Republic of Korea E-mail: [email protected] J. I. Jang, G.-W. Lee, J. T. Han Nano Carbon Materials Research Group Korea Electrotechnology Research Institute Changwon 641-120, Republic of Korea E-mail: [email protected] [+]These
authors contributed equally to this work.
DOI: 10.1002/adma.201403196
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an intrinsic transmittance and a roughness that are similar to those of graphene, and exhibits high stability in ambient air at room temperature. In addition, two-dimensional micro-sized GO sheets have the morphological advantage that they can be used to dope graphene homogeneously. As a result, the generation of electron-hole (e-h) puddles is minimized, and mobility degradation after doping is prevented. Controlled doping of graphene can be achieved by varying either the coverage or the degree of functionality of the GO sheets. As the coverage of GO increases from 0 to 100%, the work function of the GO-doped graphene gradually increases from 4.56 eV up to 4.99 eV. We also demonstrate that the degree of oxidation and reduction of the GO dopant can be controlled, and thereby the work function of the GO-doped graphene can be finely tuned. Furthermore, organic thin film transistors (OFETs) were fabricated with the work function tuned graphene/GO electrodes and it was found that GO doping improved the electrical performances of these devices. To assess the feasibility of doping graphene with GO, we firstly examined the p-type doping phenomena obtained with a single micro-sized GO sheet. Highly oxidized GO sheets[25] were spin-coated onto hexamethyldisilazane (HMDS)/SiO2/ Si substrates to exclude substrate doping effects and investigate the GO doping effect accurately[26] and then graphene obtained with chemical vapor deposition (CVD) on Cu foil was transferred by using the conventional wet transfer method[27] (Figure 1a). During the transfer of the graphene onto GOcoated HMDS/SiO2/Si wafers, low molecular weight PMMA (120k) was used as a supporting layer to minimize the residue effect.[28] By using field emission scanning electron microscopy (FESEM), we confirmed that the size distributions of the synthesized GO sheets ranged from several tens of nanometers to several micrometers (see the Supporting Information, Figure S1) and that the graphene was successfully transferred onto the GO-coated substrate (Figure 1b). Highly oxidized GO can be utilized as a strong p-type dopant of graphene because of the charge transfer interactions between graphene and the electron-withdrawing groups in GO. To investigate the doping effect of atomically thin GO sheet, Kelvin probe force microscopy (KPFM) and Raman spectroscopy were performed. KPFM was used here to measure the difference between the surface potentials of doped and pristine graphene.[10,29,30] KPFM image corresponding with an AFM image (Figure 1c) shows that the surface potential of the graphene/single GO sheet is negatively shifted by 120 mV with respect to that of the graphene on the HMDS-treated SiO2 surface (Figure 1d). This result indicates that graphene is p-doped by the atomically thin GO sheet.[29]
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Figure 1. (a) Structure of graphene/GO on an HMDS-treated SiO2/Si substrate. (b) SEM image of monolayer graphene on a GO sheet. (c) AFM image and height profile, (d) Kelvin probe force microscopy, and (e) Raman map of the 2D-band shift of monolayer graphene on a GO single sheet. (f) Variations in the conductance of GO-doped and HNO3-doped graphene with time at atmospheric pressure and room temperature. (g) Gate-dependent I-V characteristics of graphene and GO-doped graphene. (h) AFM images of graphene with and without GO.
Raman spectroscopy measurements were also performed to investigate the doping effects of the GO sheets. When graphene is doped, the positions and the full width at half maximum (FWHM) values of the G and 2D peaks in the Raman spectrum are known to be changed.[31,32] The 2D peak of graphene on an atomically thin single GO sheet is blue-shifted with respect to that of graphene transferred onto HMDS-treated SiO2, as shown in Figure S2. In addition, we obtained the Raman 2D band map of graphene transferred onto GO/HMDS-treated SiO2, as shown in Figure 1e. The Raman map of the 2D peak shift contains a bright region in the graphene/GO area. This change in the 2D peak confirms that the graphene is p-doped by only a single GO sheet. Compared to other doping methods,[10,14–16,19–21] GO doping has advantages: stable and strong p-doping that maintains the intrinsic properties of graphene such as charge carrier mobility, transmittance, and roughness. Further, the doping effects of GO on graphene are also stable for more than 40 days at room temperature and atmospheric pressure. As shown in Figure 1f, the conductance of nitric acid doped graphene decreases by approximately half over 40 days, whereas the conductance of
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GO-doped graphene is almost constant. The sheet resistance of graphene was also found to decrease from 600 to 292 ohm/sq as a result of GO doping. The gate-dependent I-V characteristics of graphene and GOdoped graphene show that the hole mobility of graphene is almost unaffected by doping. The hole mobility of GO-coated graphene was found to be 3,330 cm2/Vs, which is only slightly lower than that of pristine graphene (equivalent device position before GO coating), 3,500 cm2/Vs (Figure 1g). However, this value is higher than those obtained with other doping methods: self-assembled monolayer doping, ∼1,800 cm2/Vs[14]; AuCl3 doping, 1,735 cm2/Vs[33]; CYTOP doping, 810 cm2/Vs[34]. In general, molecular contact doping methods[14,33,34] decrease the charge mobility because the inhomogeneous adsorption of dopants generates e-h puddles. The two-dimensional GO sheets homogeneously withdraw electrons from graphene, so the number of e-h puddles generated by doping is minimized and the initial mobility before doping can be almost preserved. Maintaining the charge mobility of graphene has been an important challenge for research into doping because of the linear dependence of the electrical conductivity on the
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87%, and ∼100% at the GO solution concentrations 0.1, 0.2, 0.4, 0.6, and 0.8 g/L, respectively (see the Supporting Information, Figure S5). Graphene was transferred onto each GOcoated SiO2/Si substrate, the graphene/GO film on a SiO2/Si substrate was transferred onto an HMDS-treated substrate, and then ultraviolet photoemission spectroscopy (UPS) measurements were performed, as shown in inset of Figure 2b. The work functions were calculated by using the following equation,
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mobility (σ = e(nμe + pμh ), n: electron concentration, p: hole concentration).[35] Graphene can be effectively doped with atomically thin GO sheets, which enables the preservation of its intrinsic roughness and transparency. The root-mean-square surface roughness (RRMS) of the graphene/GO film is 0.55 nm, which is similar to that of pristine graphene (0.35 nm) due to the single layer of GO sheet beneath the graphene, as shown in Figure 1h and Figure S3. The transmittance of graphene was found to decrease by less than 1% from 97.5% to 96.7% as a result of GO doping, which means that the GO sheet affects the transmittance of graphene only slightly (see the Supporting Information, Figure S4). These advantages improve the contact at the interface between deposited materials and graphene, and prevent performance degradation due to optical losses in vertical devices such as organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). The doping of graphene can be finely tuned in this method by varying the coverage of GO and the number of its functional groups. Firstly, the effects of varying the coverage of GO on the p-doping of graphene were investigated by measuring the variation in the graphene work function. GO solutions with various concentrations were used to provide coverage variation in the range 0.1 to 0.8 g/L; the coverage of spincoated GO on the SiO2/Si substrate was verified with FE-SEM (Figure 2a). From the FE-SEM images, the coverages of GO on the SiO2/Si substrates were calculated to be 19%, 35%, 65%,
φ = ω − Esec − EFE where Esec is the onset of the secondary emission, and EFE is the Fermi edge (-128.1 eV) under a sample bias of -20 V. In the case of pristine graphene, the work function was calculated to be 4.56 eV. As the coverage of GO increases from 0 to ∼100%, the work function of GO-doped graphene gradually increases from 4.56 to 4.99 eV, as shown in Figure 2b. This trend implies that more holes are incorporated into graphene as the coverage of GO increases, which results in down-shifts in the Fermi level. The effects of varying the degree of functionality of the GO sheets on the p-doping of graphene were also investigated. The hydroxyl (C-OH), epoxide (C-O-C), carbonyl (>C = O), and carboxyl (-COOH) functional groups in GO withdraw electrons from graphene, so the doping level of graphene can be modulated by controlling the degree of functionality.[29] There
Figure 2. (a) FE-SEM images of GO on Si substrates for various GO coverages. (b) The variation of the work function of the graphene/GO films with the GO coverage. (inset: ultraviolet photoemission spectra (in the secondary electron emission region) of graphene with various GO coverages on HMDS/SiO2/Si substrates.) (c) UPS spectra of graphene on GO samples with different degrees of oxidation. UPS spectra of graphene on GO (d) for two different reduction temperatures and (e) various reduction times.
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are two approaches to the control of the number of functional groups in GO: i) control of the oxidation process during the synthesis of GO from graphite and ii) control of the reduction process after coating GO onto the target substrate. Firstly, we investigated the effects of varying the degree of oxidation of GO on the p-doping of graphene. Two types of GO were synthesized from graphite through the different number of oxidation process and the C/O ratios of these GO samples were determined to be 1.14 and 0.69 with element analysis. The X-ray photoelectron spectroscopy (XPS) results in Figure S6a indicate that the relatively highly oxidized GO sheets (C/O ratio 0.69) contain more C-OH (285.3 eV), C-O-C (286.8 eV), >C = O (287.8 eV), and -COOH (288.7 eV) than the less oxidized GO sheets (C/O ratio 1.14). This functionality of two GO samples was also confirmed by FT-IR spectra, as shown in Figure S6b. In Figure 2c, it can be seen that the work functions of GOdoped graphene with these two samples are shifted by 0.29 eV and 0.43 eV, respectively. This result shows that the more oxidized GO sheets withdraw more electrons from graphene, which increases the level of p-doping of the graphene. In addition, we examined the effects of varying the degree of reduction of GO on the p-doping of graphene. GO sheets coated onto substrates were thermally reduced and their degree of reduction was regulated by varying the annealing temperature or the reduction time. To investigate the effects of varying the reduction temperature, we performed UPS measurements for graphene transferred onto GO/substrates after the annealing of GO at two different temperatures (150 °C and 200 °C) in vacuum for 1 h. It has been reported that epoxide, carboxyl, and hydroxyl groups are partially reduced by annealing at 150 °C and that additional reduction of these groups and the
carbonyl groups is obtained by annealing at 200 °C.[36] These changes in the functionality of the GO sheets affect the doping of graphene: the work function of GO-doped graphene was measured with UPS and found to decrease from 4.99 to 4.57 eV, as shown in Figure 2d. In addition, we investigated the effects of varying the GO annealing time. The annealing time was varied from 0 min to 60 min at 150 °C, and the variation in the degree of functionality of GO with the annealing time was determined with XPS, as shown in Figure S7. These results show that the number of functional groups of GO decreases as the reduction time increases, so the work function of GO-doped graphene decreases with increases in the reduction time, as shown in Figure 2e. We have thus verified experimentally the efficient control of the electronic states of graphene by varying the degree of functionality of GO through oxidation or reduction processes. We fabricated OFETs with GO-doped graphene (graphene/ GO) electrodes to assess the viability of graphene/GO as a transparent flexible electrode for organic electronics. Bottom contact organic thin film transistors (OFETs) were fabricated by using the amorphous p-type semiconductor poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) and graphene films with or without GO as the source (S) / drain (D) electrodes. Figure 3a shows a schematic diagram of the device fabrication process. Aluminum was thermally evaporated onto graphene/ GO-transferred substrates as a sacrificial layer by using shadow masks,[22] and then uncovered graphene/GO regions were completely etched with O2 plasma, followed by HMDS treatment of the etched region. The Al layer was then wet-etched, and PTAA was spin-coated onto the pre-patterned graphene/GO electrodes. The field-effect mobility of the fabricated OFET devices
Figure 3. (a) Schematic diagram of the fabrication process of the OFETs with graphene/GO electrodes. (b) Transfer characteristics (VD = −60 V) of p-type PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) FETs with graphene/GO as S/D electrodes. (c) Output characteristics of PTAA FETs with graphene/GO as S/D electrodes. (d) Schematic energy band diagram for PTAA, pristine graphene, and GO-doped graphene.
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Experimental Section Synthesis of Graphene Oxide: GO sheets were prepared via the exfoliation of graphite oxide powder, which was produced from natural graphite (Alfa Aesar, 99.999% purity, –200 mesh) by using a modified Hummers method.[25,37] The graphite powder was dispersed in water by stirring, and the highly functionalized graphite oxide with carboxylic acid groups was removed by the centrifugation of a 2 g/L aqueous GO solution. The sediment graphite oxide solution was diluted in water to a concentration of 400 mg/L, and then sonicated for 1 h to exfoliate the graphite oxide into GO sheets. Characterization: The graphene films were characterized with atomic force microscopy and Kelvin probe force microscopy (AFM and KPFM, Veeco NanoScope 8) operating in tapping mode, field emission scanning electron microscopy (FESEM, Hitachi S-4800), confocal Raman spectroscopy (WITec) with an excitation wavelength of 532 nm, UV-vis spectroscopy (Varian, Cary-5000), FT-IR spectroscopy (Nicolet 6700, Thermo Scientific), and ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (at the 8A2 and 4D beamlines at the Pohang Accelerator Laboratory in Korea), and the electrical characteristics of the OFETs were measured by using a Keithley 2400 under vacuum conditions.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by a grant (Code No. 2011–0031628, 2013M3A6A5073177) from the Center for Advanced Soft Electronics
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under the Global Frontier Research of the Ministry of Science, ICT and Future Planning, Korea. The authors thank the Pohang Accelerator Laboratory for providing the synchrotron radiation sources at 8A2 and 4D beamlines used in this study.
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with GO doping was found to increase from 3.23 × 10−4 to 2.26 × 10−3 cm2/Vs and their on/off current ratios were significantly enhanced (Figure 3b). These enhanced electrical properties are attributed to the low sheet resistances of the graphene/ GO electrodes and the optimization of the alignment between the work function of the graphene/GO electrodes and the HOMO level of PTAA. Figure 3c shows the output characteristics of the PTAA devices using graphene S/D electrodes with or without GO. In addition to the increase in the saturation current due to the introduction of the GO doping layer, the S-shaped non-ohmic behavior at low VD is remarkably reduced. Thus the work function of the graphene/GO S/D electrodes is well matched with the HOMO level of PTAA, which results in a reduction in the injection barrier from 0.55 eV to 0.11 eV (Figure 3d). We have demonstrated the use of atomically thin and chemically versatile GO sheets as p-type dopants of CVD-graphene. Our doping method solves previously challenging issues in graphene doping; this method enables the strong, stable, largescale, low-temperature, and controllable p-doping of graphene with preserved charge mobility, intrinsic roughness, and transmittance. The Fermi level of the GO-doped graphene can be controlled by varying the coverage and degree of functionality of GO (oxidation or reduction). Furthermore, high-performance OFETs were fabricated with work function tuned graphene/GO electrodes. This study has demonstrated the potential of GO as a p-dopant and the usefulness of this doping method for optoelectronics such as organic photovoltaic cells (OPVs), organic light-emitting diodes (OLEDs), and OFETs.
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