Charge Transport in Thick Reduced Graphene Oxide Film

Article pubs.acs.org/JPCC

Charge Transport in Thick Reduced Graphene Oxide Film Ho-Jong Kim,†,‡ Daehee Kim,†,§ Suyong Jung,*,† Sam Nyung Yi,∥ Yong Ju Yun,⊥ Soo Kyung Chang,‡ and Dong Han Ha*,† †

Center for Quantum Measurement Science, Korea Research Institute of Standards and Science, Daejeon 34113, Republic of Korea Department of Physics, Yonsei University, Seoul 03722, Republic of Korea § Department of Chemistry, Sungkyunkwan University, Suwon 16419, Republic of Korea ∥ Department of Electronic Material Engineering, Korea Maritime and Ocean University, Busan 49112, Republic of Korea ⊥ Department of Materials Chemistry and Engineering, Konkuk University, Seoul 05029, Republic of Korea ‡

S Supporting Information *

ABSTRACT: We have investigated temperature-dependent charge transport behavior in thick reduced graphene oxide (RGO) film. Our results show that charges transport through two parallel percolating conducting pathways. One contains large disordered regions as one of its constituents, so its conductance is determined dominantly by variable range hopping (VRH). The other is composed of small and medium disordered regions and crystalline sp2 domains, so its conductance is determined by a serial connection of quantum tunneling and thermal activation. The more oxygen functional groups are removed from GO film upon progressive reduction, the lower the potential barriers between the crystalline sp2 domains and disordered regions become. The contribution of thermal activation to total conductance does not appear evidently for highly reduced GO film having low potential barriers, but thermal activation causes the conductance of moderately reduced film to change continuously, even at low temperatures where the VRH is almost frozen out.

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quantum tunneling added VRH”.9,14−17 In addition, Baek et al. found that introducing potential barriers between crystalline sp2 domains and disordered regions explains the RGOresistance behavior better than the VRH model does, especially at low temperatures.18 A recent electrostatic force microscopy study on charge propagation in a GO/RGO/GO ribbon showed that the potential energy inside the GO domain is higher than that of the RGO region.19 In this work, we investigate the charge transport behavior in thick RGO films by monitoring the film height, electrical conductance, and Raman spectra upon progressive chemical/thermal reduction. Our analysis shows that there exist two types of parallel percolating conducting pathways in the RGO film; one contains large disordered regions as one of the constituents, and the other is composed of small and medium disordered regions and crystalline sp2 domains. To fully understand the temperaturedependent charge transport behavior, three different charge transport modes for disordered systems, phonon-assisted VRH, fluctuation-induced quantum tunneling, and thermal activation, are taken into account together.

INTRODUCTION Graphene oxide (GO), a highly disordered derivative of graphene, is a novel material being considered for diverse practical applications such as a gas/molecule sensor, transparent conducting layer, and energy storage material.1−3 Moreover, GO can be used in the form of not only thin films but also flexible and stretchable textiles, for which GO is expected to further expand its applications at a low cost and on a large scale. GO is an electrically insulating material with abundant oxygen functional groups attached on its surface. Previous studies have shown that the structural and electrical properties of GO can be tuned through controlled chemical and thermal postreduction processes.4−9 Chemically/thermally reduced graphene oxide (RGO) flakes are composed of nanometer-sized conducting crystalline sp2 domains randomly interspersed with insulating highly disordered regions where oxygen functional groups are attached.9−12 The charge transport behavior of RGO is quite different from that of pristine graphene, where ballistic charge transport behavior over several microns and charge carrier mobility as high as >105 cm2 V−1 s−1 are observed.13 A detailed understanding of the charge transport mechanism of RGO is greatly needed to optimize RGO-based device applications. Previous studies have discussed the charge transport behavior of RGO on the basis of “phonon-assisted variable range hopping (VRH)” and “fluctuation-induced © 2015 American Chemical Society

Received: November 3, 2015 Revised: December 2, 2015 Published: December 4, 2015 28685

DOI: 10.1021/acs.jpcc.5b10734 J. Phys. Chem. C 2015, 119, 28685−28690

Article

The Journal of Physical Chemistry C

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EXPERIMENTAL METHODS

GO films are prepared by attaching GO flakes, obtained from natural graphite by using a modified Hummers and Offenman method, on top of bovine serum albumin (BSA)-coated SiO2(300 nm)/Si substrates.6,20 For BSA functionalization, we incubated SiO2/Si substrates in the solution of 0.5 wt % BSA for 30 min, rinsed with distilled water, and finally dried with nitrogen gas. We dropped approximately 200 μL of GO aqueous solution at a concentration of 1 mg/mL with a pH value from 3 to 4 on a BSA-coated SiO2/Si substrate (8 mm × 12 mm). After 30 min treatment, we rinsed the substrate with distilled water and dried with nitrogen gas. GO flakes and BSA molecules are charged in aqueous solution, which depends on the pH value. Thus, the attractive force between GO flakes and BSA allows GO film to grow thick in aqueous solution with a pH value from 3 to 4. The average thickness of GO flakes is approximately 1 nm, and the lateral size varies between 0.2 and 2.0 μm (Figure S1 of the Supporting Information). GO film is chemically reduced at 40 °C by immersing the film in a solution of 2.0 mL of hydroiodic acid (57 wt % in H2O) and 5.0 mL of acetic acid (>99.7%). Subsequently, we rinsed RGO film with a saturated sodium bicarbonate (NaHCO3) solution and then with distilled water and finally dried it with nitrogen gas at room temperature. We pattern the RGO film into rectangular shapes for electric transport measurements using photolithography and oxygen plasma treatment. Oxygen plasma etching of RGO film is carried out in the conditions of oxygen flow rate of 10 standard cubic centimeters per minute (sccm), a pressure of 20 Pa, a plasma power of 20 W, and a frequency of 2.45 GHz. Metal electrodes are defined by e-beam evaporation of Cr(5 nm)/ Au(40 nm) layers and a lift-off process. Our previous measurements showed that RGO film chemically reduced in hydroiodic acid solution becomes saturated at the regime of high defect density even after a lengthy reduction process.7 Thus, we additionally reduced the film thermally at 350 °C for 4 h in Ar−H2 mixed atmosphere to get highly reduced GO film. For convenience, we labeled the chemically reduced GO film with and without further thermal reduction treatments as “TRGO film” and “C-RGO film”, respectively. The thickness of the RGO film was measured using an atomic force microscope (AFM), and electrical conductance variations were monitored with a two-probe configuration in a vacuum chamber, varying the temperature between 10 and 300 K. Raman experiments were performed in a backscattering geometry under ambient conditions using a laser line of 514.5 nm as the incident light.

Figure 1. (a) Optical microscope image of RGO film on BSA/SiO2/Si substrate, electrically contacted by Cr(5 nm)/Au(40 nm) electrodes. (b) AFM height images of C-RGO (white-blue solid line) and T-RGO (red dotted line) films. C-RGO film is thermally annealed to form TRGO film. The thickness of the as-prepared BSA adhesive layer is 3−4 nm. (c) Raman spectra of C-RGO (red and blue solid lines) and TRGO (red and blue dotted lines) films at different chemical reduction times.

From the measurement, we can speculate the RGO film shown in Figure 2b to be composed of 80−90 layers; the thickness of the as-prepared BSA adhesive layer between the RGO film and substrate was measured to be 3−4 nm.

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RESULTS AND DISCUSSION Figure 1a shows RGO film patterned into a rectangle 100 μm wide and 500 μm long. As depicted in Figure 1b, the thickness of C-RGO film/BSA was measured to be approximately 40 nm, which was later shrunk a bit after additional thermal reduction (T-RGO film). Previous experiments showed that the thickness of a GO monolayer is approximately 1 nm, but it decreases gradually as the oxygen functional groups are removed from the GO surface upon progressive reduction.21,22 AFM height images clearly indicate that the thermal reduction process shrinks the film’s thickness. XRD (X-ray diffraction) patterns and FT-IR (Fourier transform infrared spectroscopy) spectra also confirmed that some of remaining oxygen functional groups were removed during the thermal reduction process, further decreasing film thickness (Figures S2 and S3).23−30

Figure 2. Temperature dependence of the normalized conductance of RGO film after chemical (filled marks) and thermal (open marks) reduction. The measurements of T-RGO film were carried out after the thermal reduction of the corresponding C-RGO film. The time denotes the chemical reduction time. Conductance was measured with dc bias voltage of Vsd = 1 mV within the linear I−V relation region. The inset shows the I−V curves of RGO film chemically reduced for 3 min.

Figure 1c shows Raman spectra of GO film with and without chemical/thermal reduction. Compared with the spectrum of as-prepared GO film (black solid line), the width of each Raman peak of RGO gets narrower so that the D′ peak at approximately 1620 cm−1 is clearly visible as a shoulder of the G peak. The G peak is a stretching mode of the C−C bond of 28686


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Figure 3. Natural logarithm of conductance versus T−1/4 for (a) chemically and (b) thermally reduced GO films. (a′) and (b′) are from the selected area of (a) and (b) marked with dotted boxes, respectively. Measurements of T-RGO film (open marks) were taken after thermal reduction of the respective C-RGO film (filled marks). The time denotes the chemical reduction time. Circular and triangular marks indicate the experimental results, and solid and dotted lines indicate the fittings using eqs 1 and 2, respectively.

the sp2 structure of graphene, and both the D and D′ peaks are related to defects in graphene structure.31,32 For the C-RGO films reduced for 15 s and 3 min, note that there is little difference in the Raman parameters, indicating the degree of reduction of GO, for example the position and width of each Raman peak and the intensity ratio of the D peak to G peak (I(D)/I(G)). Thus, we can argue that the structure of the CRGO film reduced for 15 s is substantially the same as that of the C-RGO film reduced for 3 min. In other words, the chemical reduction of GO film proceeds much faster than that of GO-coated fiber bundle, where it takes time for the reducing agents to go down the narrow gaps between the densely arranged nanofibers.7 After thermal reduction, we found that the position of the G peak was red-shifted from ∼1589 to ∼1585.5 cm−1, and I(D)/I(G) decreased from 1.78 to 1.61, indicating an increase of the fraction of crystalline sp2 domains. Our Raman spectra clearly show that the structures of the RGO film transformed from the topologically disordered state “Stage 2” to the nanocrystallized state “Stage 1” in the “three-stage model of increasing disorder”.33,34 Although some of the remaining oxygen functional groups were further removed during the thermal reduction process, the intense and broad D peak indicates that a sizable fractions of the T-RGO film still remains as disordered regions. RGO flakes are reported to be composed of nanometer-size conducting crystalline sp2 domains randomly interspersed with oxidized disordered regions.9−12 GO film is initially insulating because of highly disordered regions covering a large fraction of the GO flakes, but parallel percolating conducting pathways develop between the electrodes as the oxygen functional groups are removed during reduction. Figure 2 shows the normalized conductance variation of RGO film as a function of temperature. Note that the extrapolated conductance at the zero temperature limit for C-RGO film is close to zero,

suggesting that the majority of conducting pathways contain large disordered regions, where VRH is frozen out at very low temperatures. By contrast, the T-RGO film shows different temperature-dependent behavior, that is, a large fraction of the room-temperature conductance exists at the zero temperature limit. During the additional thermal reduction process, each disordered region is shrunk (moreover, some of them might vanish) so that the number of conducting pathways containing only small disordered regions, which the charges can quantum tunnel through or pass over, is greater. Our conductance measurements confirm that oxygen functional groups are further removed from the chemically reduced film by additional thermal annealing, which is also supported by the previously discussed AFM and Raman results. In addition, we have found that hole mobility extracted from the transfer characteristic (I−Vg curve) of RGO film is remarkably improved by the additional thermal reduction process, and the extracted hole mobilities are similar to previously reported results35,36 (Figure S4). Figure 3 shows the conductance variation of RGO film as a function of T−1/4 between 10 and 300 K. As the temperature decreases, the conductance of C-RGO film keeps decreasing, while that of T-RGO film is approximately flattened at low temperatures. After thermal reduction, the room temperature conductance is increased by a few times for both films. The increment in the conductance of RGO film by thermal reduction is enhanced at low temperature as shown in Figures 3a and 3b; conductance at 10 K is improved by an order of magnitude. Previous studies showed that the resistance of RGO film has approximately linear dependence on the distance between electrodes, meaning that the charge transport behavior in RGO film is dominantly determined by the RGO film and not by the contact resistance.16 28687


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The Journal of Physical Chemistry C We numerically fit our data to eq 1, which is a threedimensional (3D) VRH model supplemented with parallel fluctuation-induced quantum tunneling (solid lines in Figure 3). ⎛ H ⎞ G(T ) = G h exp⎜ − 1/ n ⎟ + Gt , ⎝ T ⎠

n=4

(1)

where H is a hopping parameter dependent inversely on both the density of states (DOS) near the Fermi level and the localization length. n − 1 is the dimensionality of the conduction, and Gh and Gt are the parameters for the relative contributions of VRH and the quantum tunneling mechanism. The temperature-independent fluctuation-induced tunneling term Gt is added to the VRH term Gh exp(−H/T1/n) in order to explain the obvious deviation from the linear dependence of G(T) on T−1/n at low temperatures at which VRH is almost frozen out.16,17 As shown in Figure 3a, the conductance variation of C-RGO film is well fitted to eq 1 at high temperature. As the temperature decreases, however, deviations of the fitting results from the experimental data start to develop at approximately 100 K, and the disagreements increase further at low temperatures below 30 K. Our results suggest that the “quantum tunneling added VRH mechanism” of eq 1 is not sufficient to fully explain the charge transport behavior in RGO film. In the above model described by eq 1, two types of disordered regions are taken into account to explain the charge transport behavior through the parallel percolating conducting pathways. One is the large disordered regions where conducting electrons transport via hopping, and the other is small (thin) disordered regions, such as point and line defects, through which charges can tunnel quantum mechanically. The conducting pathways containing large disordered regions as a constituent give the VRH conductance in eq 1, whereas others giving the quantum tunneling conductance are assumed to be composed of only crystalline sp2 and small disordered regions. Baek et al. hypothesized potential barriers between crystalline sp2 and disordered regions whose height depends on the local density of oxygen functional groups on the disordered regions in order to explain the larger resistance than the VRH model predicts at low temperature.18 A recent electrostatic force microscopy study on charge propagation through a GO/RGO/ GO ribbon shows that the average potential energy is higher inside GO regions than RGO regions.19 It is easy to imagine that there are numerous disordered regions with diverse parameters, varying for example in size and local density of oxygen functional groups. Thus, we introduce another type of disordered region to the previously mentioned large and small disordered regions: medium disordered regions, where potential barriers are a bit thicker for the quantum tunneling but not large enough for conducting electrons to go across them exclusively by a hopping mechanism. A schematic diagram of the various types of disordered regions that compose percolating conducting pathways is shown in Figure 4. We modified eq 1 by adding the contribution of thermal activation over the potential barriers. As shown with dotted lines in Figure 3, our data are well fitted to eq 2 over the measured temperature range between 10 and 300 K. ⎛ H ⎞ ⎛ T⎞ G(T ) = G h exp⎜ − 1/ n ⎟ + Ga exp⎜ − a ⎟ + Gt , ⎝ T⎠ ⎝ T ⎠

Figure 4. Schematic diagram of potential barriers and disordered regions composing percolating conducting pathways. (a) A small (thin) disordered region such as a point or line defect, (b) a medium disordered region, and (c) a large disordered region. The background is a crystalline sp2 region. In the case of a medium disordered region, the potential barriers at both boundaries are partially overlapped to make the width of the potential barrier wide, through which tunneling events become suppressed. Both tunneling and thermal activation conduction are possible for the disordered regions of (a) and (b). However, the dominant conduction mechanisms across them are shown.

where the thermal activation conductance Ga exp(−Ta/T) corresponds to the contribution from the conducting electrons through the medium disordered regions. kBTa (kB is the Boltzmann constant) is the average height of potential barriers (activation energy) which is considered to depend on the local density of oxygen functional groups. Our results indicate strongly that the percolating conducting pathways in RGO film contain all three types of disordered regions shown in Figure 4. The fitting parameters extracted by fitting our data to eq 2 are summarized in Table 1. Table 1. Fitting Parameters of Eq 2 with n = 4 Gh (μS)

H (K1/4)

Ga (μS)

Ta (K)

Gt (μS)

15 s

1351

13.44

−24.92

203.4

1.98

3 min 15 s

1438 1585

11.53 12.52

−41.39 −4.50

198.5 6.0

6.61 100.98

3 min

1510

11.38

−9.65

5.9

107.18

reduction timea C-RGO film T-RGO film a

The reduction time denotes the chemical reduction time.

Among several interesting points to note, we highlight that the parameter Ga has negative values. Ravi et al. followed the same analysis to explain the temperature-dependent conductance variation for carbon-nanotube networks by including a thermal activation term.37,38 Although the fitting parameters were not available, they assumed three types of parallel conducting pathways along which the dominant conducting mechanisms are VRH, quantum tunneling, and thermal activation. However, we argue that the negative values of Ga imply that the medium disordered regions giving the thermal activation term in eq 2 do not form their own separate conducting pathways but are embedded as a constituent in the VRH and/or quantum tunneling pathways. We suggest a modified model of two parallel conducting pathways for the charge transport in RGO. One conducting pathway contains large disordered regions (“VRH pathway”), while the other is composed of small and medium disordered regions and

n=4 (2) 28688


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parallel percolating conducting pathways in RGO film. One is a “VRH-pathway” containing large disordered regions as one of the constituents, so its conductance is determined dominantly by VRH. The other is a “tunneling and activation pathway”, where small and medium disordered regions are connected in series with crystalline sp2 regions, which are responsible for the conductance, especially at low temperatures. The height of potential barriers gets lower upon the removal of oxygen functional groups from RGO film. Thus, the contribution of thermal activation to total conductance does not appear evidently for highly reduced GO film having low potential barriers, whereas it causes the conductance of moderately reduced film to change continuously even at low temperatures where VRH is almost frozen out.

crystalline sp2 regions (“tunneling and activation pathway”) whose contribution is given by the sum of the second and third terms in eq 2. The thermal reduction process decreases the hopping parameter H, and the average activation energy decreases from approximately 17.3 meV (Ta ∼ 200 K) to approximately 0.52 meV (Ta ∼ 6 K). The activation energy of C-RGO film is approximately equal to that (Ta = 217 K) of a single multilayer RGO flake annealed at 215 °C for 2 h in an Ar atmosphere.18 The decrease of activation energy explains why the conductance of the T-RGO film is well fitted to eq 1 even at low temperatures below 30 K where considerable deviations between experimental and fitting results are observed for CRGO film. At temperatures between 10 and 30 K, which is higher than Ta but low enough to suppress VRH events, the conductance of T-RGO film having Ta ∼ 6 K is approximately flattened as shown in Figure 3b. In this temperature range, conducting charges easily pass over both the small and medium disordered regions (it is no longer important to separate the contributions of tunneling and thermal activation), so the temperature dependence of the conductance of the “tunneling and activation pathway” becomes very weak. Thus, the parameter Gt of the T-RGO film is greatly enhanced, while the parameter Ga becomes so small that the contribution of the second term in eq 2 is almost negligible. We have observed approximately the same results from other thermally reduced RGO films with similar film thickness (Figure S5). The conductance at room temperature is in the same order of magnitude as those shown in Figure 3, and Ta values are below 10 K. It is worth pointing out that our model also successfully explained the charge transport in other types of RGO-coated materials: chemically reduced GO-coated polyester (PE) fiber bundles (Figure S6).7 Higher Ta values from chemically reduced GO-coated PE fiber bundles compared with those (Ta = 203.4 and 198.5 K) of chemically reduced GO films on SiO2/Si substrates suggest that the chemical reduction of GO flakes on fiber bundles is not as effective as the case of GO films on SiO2/Si substrates. Additional thermal reduction procedure for RGO-coated PE fiber bundles, however, was not carried out since the PE fiber bundles are vulnerable to excessive heat treatment. We compared our data with numerical fittings on the basis of 2D transport, which resulted in results outwardly similar to those in Figure 3. However, we found that some of the fitting parameters are not physically meaningful (Figure S7 and Table S1). In addition, the numerical fittings of our data to eqs 1 and 2 with n = 2 was not satisfactory, and physically meaningless fitting parameters were obtained (Figure S8 and Table S2). Therefore, we consider that VRH conduction in thick RGO film follows the Mott-VRH mechanism based on the constant DOS near Fermi energy (EF), but it does not follow the Efros and Shklovskii-VRH mechanism based on vanishing DOS with energy.39,40

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10734. Images of GO flakes used for the preparation of RGO films on Si substrate; XRD patterns and FT-IR spectra of RGO films; numerical fitting of the experimental conductance to equations describing charge transport in RGO; (1) n = 3 and (2) n = 2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.J.). *E-mail: [email protected] (D.H.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Y. I. Kim for his help with XRD measurement. This research was supported by the Fusion Research Program for Green Technologies through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2012M3C1A1048861).

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CONCLUSIONS We have prepared relatively thick RGO film by attaching small RGO flakes onto SiO2/Si substrate in aqueous solution using BSA as an adhesive layer. Disordered regions composing of the percolating conducting pathways were categorized as small, medium, and large. With temperature-dependent conductance measurements and numerical analyses, we showed that charge transport occurs in three dimensions, and there are two types of 28689


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