Letter pubs.acs.org/Langmuir
Thin-Film Transistors with a Graphene Oxide Nanocomposite Channel S. Mahaboob Jilani, Tanesh D. Gamot, and P. Banerji* Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India ABSTRACT: Graphene oxide (GO) and graphene oxide−zinc oxide nanocomposites (GO−ZnO) were used as channel materials on SiO2/Si to fabricate thin-film transistors (TFT) with an aluminum source and drain. Pure GO-based TFT showed poor field-effect characteristics. However, GO−ZnO-nanocompositebased TFT showed better field-effect performance because of the anchoring of ZnO nanostructures in the GO matrix, which causes a partial reduction in GO as is found from X-ray photoelectron spectroscopic data. The field-effect mobility of charge carriers at a drain voltage of 1 V was found to be 1.94 cm2/(V s). The transport of charge carriers in GO−ZnO was explained by a fluctuationinduced tunneling mechanism.
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However, it is found that the development of flexible electronics is moving very fast irrespective of inorganic, organic, and hybrid materials though low-cost organic electronic appliances on plastic are ahead of the others. An outstanding review by Forrest10 on low-cost flexible electronics has been found in the literature. A significant development in the performance of organic semiconductors for TFT applications has been explained in that work. The mobility of these materials is found to be comparable to that of many conventional semiconductors for TFT applications, making it suitable for roll-to-roll processing in large-area flexible electronics with low cost.10 In the area of inorganic materials, Kim et al.11 have shown that even silicon can be made to use in stretchable and foldable electronics. The authors have shown that nanoribbons of single-crystalline silicon can be integrated with plastic and elastomeric materials as substrates. The authors have developed integrated circuits with this stretchable format and have shown the operations of the devices. Thus, the concept of using organic or polymeric materials, as the only materials for flexible electronics will soon face challenges though cost optimization need to be done for widespread applications. Another trend is found in using carbon-based materials. Sun et al.12 have reported carbon-nanotube-based TFTs and their integrated circuits on flexible and transparent substrates with high performance. Apart from its flexibility, these TFTs showed a large on/off ratio, on the order of 106, together with high mobility compared to that of many inorganic materials. Nowadays, graphene oxide (GO), a carbon-based material with a tunable band gap ranging from semimetal to semiconductor, is emerging for large-area flexible and transparent electronics.13
INTRODUCTION Graphene, a single layer of carbon atoms in a honeycomb structure, has fascinated the research community because of its unique electronic, mechanical, and thermal properties since its discovery by Novoselov et al. in 2004.1 Because of its flexibility, transparency, thickness, and high mobility of charge carriers,2 graphene was widely reported as an active material for flexible electronics including thin-film transistors (TFT). The first fieldeffect characteristics of graphene were reported by Novoselov et al.1 on a 300 nm thin back-gate SiO2/Si surface. However, because of its large parasitic capacitance the performance was not satisfactory and additional top-gate geometry was conceived. The first top-gate graphene field-effect transistor was reported by Lemme et al.,3 and subsequently many reports were available on top-gate graphene field-effect transistors fabricated with mechanically exfoliated,4 chemical vapor deposition (CVD)-grown,5 and epitaxially grown graphene.6 For large-area, low-cost, and solution-processed applications, chemically synthesized graphene was used as a channel layer for transistor fabrication.7 Owing to the zero band gap of graphene, the above-reported transistors showed a very low (∼10) on/off ratio and linear output current−voltage characteristics without any saturation region, resulting in poor switching and weak field-effect carrier transport through the graphene channel layer. To incorporate a band gap and current saturation in graphene TFTs, recently AB-stacked bilayer graphene with a tunable band gap has been found to enhance the performance of graphene TFTs. In that work, Wu et al.8 showed that these bilayer graphene layers could be deposited onto foils of Cu or Cu−Ni alloy, using a CVD process, and could easily be transferred onto large-area flexible substrates. Similarly, Liu et al.9 have shown that the mobility of CVD-grown bilayer graphene TFTs along with their high on/off ratio are comparable to that of mechanically exfoliated graphene TFTs. © 2012 American Chemical Society
Received: September 3, 2012 Revised: November 8, 2012 Published: November 15, 2012 16485
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mercury lamp (λ = 350 nm) for 5 min. The dispersion was slowly stirred in the presence of the uniform exposure of UV radiation. Before irradiation, the dispersion was dark brown in color; however, it turned slightly black after reduction. Separately, graphite oxide was dispersed in DI water at a concentration of 1.0 mg/mL to make the GO channel. Two SiO2/Si substrates were degreased by heating in trichloroethylene for 3 min, followed by 3 min of heating in acetone. The acetone residue was removed by boiling the substrate in methanol for 3 min, and the methanol residue was removed by thoroughly cleaning in DI water. The dispersed GO−ZnO composite and GO were spin-coated separately onto cleaned SiO2 surface at 1000 rpm for 30 s and then annealed at 120 °C for 40 min. The surface morphology of GO−ZnO was observed via a field-effect scanning electron microscope image (FESEM, Zeiss). The X-ray photoelectron spectroscopy (XPS) data of GO and GO−ZnO were recorded with a PHI Versa probe II scanning XPS microprobe (VLVA-PHI Inc.). Two Al electrodes of channel length (L) 300 μm and width (W) 600 μm were deposited on GO and GO−ZnO surfaces as source and drain contacts. An Al gate electrode was deposited by thermal evaporation through a shadow mask at the bottom of a SiO2/Si substrate. The output and transfer characteristics of GO−ZnO−TFT were obtained with a Keithley 2400 in voltage sweeping mode through the LabView (NI) protocol.
Graphene oxide, an intermediate material for obtaining chemically reduced graphene, has attracted researchers because of its band gap and conductivity, which can be engineered by varying the ratio of carbon to oxygen functional groups (the C/ O ratio). It may be mentioned that GO is a monolayer of graphene attached by oxygen functional groups such as carbonyl, hydroxyl, and epoxides on both edges and surfaces of the basal plane through sp3-hybridized electrons. Because of sp3-hybridized electrons, the electronic conduction in GO layers deteriorates, and hence completely oxidized GO is an insulator.14 The oxygen functional groups impart a hydrophilic nature to GO, and as a result, it can be dispersed in a wide range of solvents including deionized (DI) water. On application of a reducing agent such as hydrazine in a dispersed mode, its electrical conductivity could be regained.15 Recently, Jin et al.16 have reported pristine GO (without reduction)based thin-film field-effect transistors on SiO2/Si substrates. The authors reported that the partially oxidized (oxidation time 5 min) GO with a band gap of 1.7 eV showed p-type semiconducting behavior at room temperature under ambient argon whereas the more highly oxidized (oxidation time 60 min) GO showed insulating characteristics with a band gap of 2.1 eV. Thus, completely oxidized GO is not suitable as a channel material for transistor applications because of its extremely poor electrical conductivity. To change the optical, dielectric, mechanical, and electrical properties of GO, different nanomaterials have been reported as intercalated and filler materials in GO-based nanocomposites.17 Graphene oxide-based nanocomposites with ZnO and TiO2 are widely reported for photocatalytic applications;18,19 however, because of its high electron mobility, ZnO is the most suitable compared to TiO2 as far as the enhanced electrical conductivity of the composite is concerned. Kamat et al.20 reported that large-band-gap semiconductors such as ZnO could be embedded onto GO surfaces through carboxylic functional groups to enhance the optical and electrical properties of GO nanocomposites. However, the graphene oxide−zinc oxide (GO−ZnO) nanocomposite is not investigated as a channel material for thin-film transistor (TFT) applications. In our present investigation, we have embedded ZnO nanostructures in the GO matrix to enhance the conductivity and to enable better field-effect transport of charge carriers and have compared its performance with that of pristine GO as a channel material for TFT applications. An attempt has also been made to understand the mechanism of charge-carrier transport in the GO−ZnO-nanocomposite-based channel region. For proof-of-concept purposes, the GO−ZnO composite and GO were deposited on a back-gate SiO2/Si substrate. We have used the notation (i) GO−ZnO−TFT, and (ii) GO− TFT to represent, respectively, the TFTs with (i) the GO− ZnO composite and (ii) pristine (completely oxidized) GO as channel materials.
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RESULTS AND DISCUSSION Figure 1a shows a schematic representation of GO−ZnO− TFT. The morphology of ZnO nanostructures on a GO surface
Figure 1. (a) Schematic representation of GO−ZnO−TFT on SiO2/ Si in back-gate configuration with an aluminum source and drain. (b) FESEM image of a GO−ZnO nanocomposite with 140 nm × 50 nm ZnO nanostructures embedded in a GO matrix.
can be observed via an FESEM image of GO−ZnO nanocomposites as shown in Figure 1b. Here it is observed that ZnO nanostructures dispersed in a GO matrix are very close to each other compared to their dimensions. Figure 2a shows the deconvoluted C 1s XPS spectra of pristine GO. The spectra were fitted with different peaks representing various bondings such as sp2 carbon (C−C at 284.6 eV), hydroxyl (C−
EXPERIMENTAL DETAILS
Graphene oxide was synthesized by the Hummers method.21 ZnO was synthesized by a sol−gel technique22 except that instead of triethanolamine and NaOH, respectively, ethanolamine and KOH were used. For solution processing, 7.5 mg of GO and 2.5 mg of ZnO was dispersed in 10 mL of DI water (Millipore, 18.2 MΩ·cm) and then it was stirred for an hour at 80 °C, making 10 mL of GO−ZnO composites. After the GO−ZnO composite was dispersed in DI water, 3 mL of ethanol was added and stirred for 15 min. The dispersed composite (in water and ethanol) was irradiated with a 110 mW/cm2
Figure 2. Deconvoluted C 1s XPS spectra of (a) pristine GO with a high degree of oxidation and (b) the GO−ZnO nanocomposite, showing a partial reduction in GO due to ZnO nanostrucures. 16486
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OH at 286.4 eV), and carbonyl (CO at 287.8 eV) functional groups. These values are found to be consistent with the literature.23 The C 1s XPS spectra of GO−ZnO are shown in Figure 2b. The positions of the peaks correspond to carbon, and different functional groups are observed to be nearly the same as that of pristine GO. However, an additional peak at 1022 eV (shown in the inset of Figure 2b) has been found in the spectra assigned to Zn present in GO−ZnO composites. A decrease in the intensities of the functional groups, particularly the hydroxyl group (peak at 286.4), is observed in the GO− ZnO composite compared to that of pristine GO. This reduction in the intensities of the functional groups is attributed to the partial reduction of GO due to ZnO nanostructures attached to GO.19 Consequently, because of the availability of very few free electrons, the conductivity of the GO−ZnO composite is enhanced. The XPS spectra of ZnO (inset in Figure 2b) in the GO−ZnO composite is similar to that of pristine ZnO, showing no traces of OH or C on the ZnO surface. The photocatalytic reduction of GO with different metal and metal oxide nanoparticles is well known. In our present investigation, the reason behind the OH loss is the photocatalytic reduction of GO by ZnO nanostructures in ethanol. Photocatalytic reduction is one of the many methods employed for the reduction of GO. The significance of photocatalytic reduction is that the degree of oxidation of GO can be controlled by varying the irradiation time to UV radiation, and thus the fine tuning of the conductivity in GO is achieved.20 In the present case of the GO−ZnO composite, the possible reduction mechanism is as follows:24
Figure 3. Output and transfer characteristics of thin-film transistors with (a) pristine GO as a channel showing poor electrical performance. The inset shows output characteristics of GO−TFT in which the current remained the same with increased gate bias and (b) the GO−ZnO nanocomposite as a channel material for TFT showing ambipolar transfer characteristics with dominant n-type conduction. The output characteristics shown in the inset represent an enhancement in the drain current with gate bias. The linear increase in the current indicates the semimetallic characteristics of the channel material after photocatalytic reduction with ZnO.
Completely oxidized pristine GO acts as an insulator, showing no field-effect charge-transport characteristics. After ZnO nanostructures are anchored onto the GO surface, the improved output and transfer characteristics of GO−ZnO− TFT are shown in Figure 3b. The output characteristics (inset of Figure 3b) were obtained by sweeping the drain-source voltage (Vds) from 0 to 5 V in steps of 0.05 V for each gate voltage (Vgs) of 2, 4, and 6 V. The output characteristics showed a linear variation in the drain current with Vds; however, a significant enhancement in the magnitude of the drain current with the applied gate voltage was observed. The transfer characteristics were studied by sweeping Vgs from −5 to 5 V at Vds = 1 and 6 V. The on/off ratio and threshold voltage are found to be 136 and 1.37 V, respectively. The output and transfer characteristics show that GO−ZnO is an n-type channel material. The transfer characteristics of GO−ZnO TFT showed partial ambipolar characteristics; however, it is predominantly n-type. Jin et al.16 reported that pristine partially oxidized graphene oxide showed ambipolar conductivity in vacuum and p-type characteristics under normal atmospheric conditions. Because we have carried out I−V measurements in a normal atmosphere and because GO is reduced by ZnO, we believe that feeble ptype conduction observed from 0 to −5 V is due to partially reduced GO in GO−ZnO composite. However, the n-type conduction observed from 0 to 5 V is due to ZnO nanostructures because as-grown ZnO is n-type. This situation is analogous to boron-doped graphene TFT reported by Tang et al.25 where in the case of pristine graphene TFT the conduction is ambipolar. As the doping concentration of boron increases, the conduction is strongly driven by p-type carriers. The field-effect mobility of charge carriers (μFE) was extracted from the transfer characteristics using the following relation26
ZnO + hν → ZnO(e− + h+) C2H5OH
ZnO(e− + h+) ⎯⎯⎯⎯⎯⎯⎯⎯→ ZnO(e−) + •C2H4OH ZnO(e−) + GO → ZnO + RGO
Because the band gap of ZnO is 3.37 eV, when irradiated with UV radiation, the electron in the valancy band will make a transition to the conduction band, resulting in the generation of electron−hole pairs. The holes will create ethoxy radicals in the presence of ethanol, and the photogenerated electrons on the surface of ZnO will be trapped by oxygen functional groups in the GO matrix, leading to the partial reduction of GO-restoring sp2 electrons. Therefore, the conductivity of GO−ZnO increases. Prior to the fabrication of TFT, the GO−ZnO composite was characterized by resistivity and Hall measurements under the van der Pauw configuration. The GO−ZnO composite of ∼10 μm thickness was drop cast onto a 5 mm × 5 mm quartz substrate and then annealed at 120 °C. Four aluminum contacts, each with a diameter of ∼700 μm, were thermally deposited onto the edges of GO−ZnO/quartz substrates. From these measurements, the mobility, conductivity, sheet resistance, and carrier concentrations of the GO−ZnO composite were found to be respectively 2.12 cm2/V s, 4.87 × 10−8 S, 443 ± 27 kΩ/square, and 1.563 × 1011 cm−3. The transfer and output (inset) characteristics of GO−TFT are shown in Figure 3a. From the output and transfer characteristics, no significant enhancement in the drain current with gate voltage was observed. The lack of saturation (and linearity) in output characteristics indicates that GO−ZnO behaves like a semimetal because of the reduction of GO.
gm =
∂Id ∂VG
= VD
W CGμFE Vds L
(1) −8
where gm is the transconductance and CG (= 1.15 × 10 F/ cm2; for a 300 nm thin SiO2 gate dielectric) is the gate capacitance. Above the threshold voltage, the value of gm is found to be 4.46 × 10−8 S and the field-effect mobility μFE of 16487
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CONCLUSIONS A photocatalytic partially reduced GO−ZnO nanocomposite has been used as a channel material for TFT applications, and its output and transfer characteristics were compared to those of pristine GO-based TFT. From these characteristics, it is observed that GO−ZnO−TFT gives a better field-effect performance than simply GO−TFT because of the anchoring of ZnO nanostructures in the GO matrix. From the XPS spectra, a partial reduction in oxygen functional groups was observed, resulting in an enhancement of the conductivity of the GO−ZnO composite. The GO−ZnO TFT showed slight ambipolar transfer characteristics with dominating n-type conduction. An enhancement in the drain current with applied gate voltage was also observed from the output characteristics. The linear variation of drain current with the source-drain voltage represents the semimetallic characteristics of the GO− ZnO composite after partial photocatalytic reduction. The transport of charge carriers in the GO−ZnO composite is explained by the FIT mechanism.
charge carriers in GO−ZnO TFT has been determined to be 1.94 cm2/V s at Vds = 1 V. The enhancement in the electrical conductivity in the GO− ZnO channel material compared to that of pristine GO is due to two possible factors. The first one is the availability of few free conducting electrons as a result of the partial reduction of GO by ZnO nanostructures (as observed in XPS spectra, Figure 2b). The second one is the transport of charge carriers between ZnO nanostructures that are separated by a very small barrier compared to their dimensions. The mechanism of charge transport in the GO−ZnO nanocomposite has been explained by the fluctuation-induced tunneling conduction mechanism (FIT).27,28 According to FIT theory in disordered materials such as insulator−conductor composites where the large conducting islands (filler) are separated by thin insulating regions (matrix), the conduction mechanism is dominated by thermal activation at high temperatures and simple elastic tunneling at low temperatures. Let us consider two successive conducting islands separated by a width w and area A. The potential barrier between these two is expressed by the equation27 V (x) = Vo −
⎛ 4Vo ⎞ 2 ⎜ ⎟x ⎝ w2 ⎠
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. (2)
Notes
The authors declare no competing financial interest.
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where Vo is the potential at the center of the junction. When an external electrical field is applied, the width and height will be, respectively, narrowed and lowered. GO−ZnO is an insulator−semiconductor composite in which semiconducting ZnO nanostructures are anchored on the surface of an insulating GO matrix. To understand the FIT mechanism in the GO−ZnO nanocomposite, let us consider that ZnO acts like conducting regions (as-grown ZnO is an ntype semiconductor21) separated by a narrow barrier in an insulating GO matrix. When the concentration of ZnO is very high (in the absence of GO), the electron-conducting network is continuous and the charge transport is dominated by a hopping conduction mechanism. When the concentration decreases, an insulating gap (a small potential barrier) will develop between semiconducting ZnO nanostructures on the GO surface and conduction is dominated by the electrons tunneling across these small barriers separating large conducting regions of ZnO. As a result, the conductivity of GO−ZnO− TFT will increase compared to that of pristine GO−TFT. We have observed better field-effect transport properties in GO− ZnO nanocomposites compared to that of TFTs with pristine graphene oxide as the channel. However, relatively cheap, easyto-obtain GO, which forms a composite with ZnO, encourages electrical properties that may be improved further with proper optimization. However, ZnO is a direct band gap semiconductor having a band gap of 3.37 eV at room temperature and a large exciton binding energy22 of 60 meV. Thus, its composite with GO has the potential to use transparent electronics over a large area even above room temperature (thermal energy at 300 K = 25.6 meV). Moreover, its fabrication/synthesis and processing are very simple and costeffective. Also, the exciton binding energy and oscillator strength of this composite system could be substantially increased (in fact could be varied) by differences in the permittivities of individual components (such as Coulomb interaction engineering), and thus with the proper engineering it could be useful in optical displays.
ACKNOWLEDGMENTS We are grateful to Prof. S. Banerjee of the Materials Science Centre, IIT, Kharagpur, for his help with the synthesis of graphite oxide and the analysis of the XPS data. We also acknowledge the help from the Department of Physics and Meteorology, IIT, Kharagpur, for providing the XPS facility.
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