TB, TM, US, JMM/457951, 1/05/2013

IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 23 (2013) 000000 (7pp) UNCORRECTED PROOF

All-carbon-based field effect transistorsfabricated by aerosol jet printing onflexible substratesRui Liu1, Fangping Shen1, Haiyan Ding1, Jian Lin2, Wen Gu1,Zheng Cui2 and Ting Zhang1,3

1 iLab, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123,People’s Republic of China2 Printable Electronics Research Center, Suzhou Institute of Nano-tech and Nano-bionics,Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China

E-mail: rliu2010@sinano.ac.cn and tzhang2009@sinano.ac.cn

Received 22 December 2012, in final form 17 April 2013Published DD MM 2013Online at stacks.iop.org/JMM/23/000000

AbstractAn all-carbon-based field effect transistor (FET) was fabricated on flexible polyethyleneQ1terephthalate substrates by aerosol jet printing method in this paper. Three different types ofhomogeneous conductive inks were made and then printed layer-by-layer to form the FETchips. The conducting-reduced graphene oxide was used as electrodes (source and drain) andchannel, respectively. Graphene oxide was used as dielectrics while multi-walled carbonnanotubes acted as the gate electrode. The all-carbon-based FET shows a good mobility of350 cm2 (V s)–1 at a drain bias of −1 V. This simple and novel method explores a promisingway to fabricate all-carbon-based, flexible and low-cost electronic devices.

Q2 (Some figures may appear in colour only in the online journal)

1. Introduction

Graphene has attracted tremendous attention due toits outstanding electronic, optoelectronic and mechanicalproperties in recent years [1–8]. Considerable efforts havebeen made to fabricate electronic devices, such as field effecttransistors (FETs), radio frequency transistors, etc, basedon graphene materials [9–14]. However, how to prepare acost-effective large-scale graphene film is still a difficulttask. Chemical vapor deposition (CVD) is the most popularmethod for making a graphene film, but the process requireshigh temperature and during synthesizing produces toxicgases [7]. Mechanically the exfoliated method is one of thecommon methods for preparing high-quality graphene film,however this method cannot be used in large-scale production[8]. Therefore, the methods of preparing graphene limitthe development of graphene-based devices to some extent,especially for flexible electronic devices. Fortunately, thereduced graphene oxide (rGO) has been demonstrated to be

3 Author to whom any correspondence should be addressed.

used in many electronic devices. rGO is a 2D carbon materialwhich is suitable for applications in electronics because itpreserves the unique properties of graphene and has versatilefunctionalities, such as high solubility in various solvents, easeof processing and rich chemical moieties [15]. Moreover, rGO-based electronic devices can be fabricated conveniently byprinting or simply by drop-deposition method. Therefore, low-cost large-scale production of rGOs can be easily obtained bychemical exfoliation from grapheme oxide (GO) [16–19]. Soit is a great advantage to make use of rGO to fabricate flexibleelectronic devices for large-scale and low-cost applications.

The development of printing technique has provided inrecent years a potential opportunity to fabricate devices basedon soluble nanomaterials for high-performance, low-cost andversatile applications. Printed electronics has been a rapidlyexpanding research area for the last couple of years, especiallyin flexible devices. This technique makes use of printingmethods to deposit a wide variety of materials onto differentsubstrates without conventional masks or thin-film depositionequipment. Various electronic devices have been fabricated

0960-1317/13/000000+07$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA

J. Micromech. Microeng. 23 (2013) 000000 R Liu et al

by this promising method including FET, RFID and sensors[20–23].

Some recent studies reported the fabrication of graphene-based transistors with flexible or rigid substrate includingink-jet printing and roller-style electrostatic printing; therewere a small number of effective methods to fabricate good-performance rGO-based transistors with lost-cost, convenientand environment-friendly process [24–26]. Aerosol jetprinting is a direct write technology to be used in a numberof electronic manufacturing applications, and can deposit awide variety of materials onto different substrates withoutconventional masks or thin-film equipment. Moreover, theaerosol jet printing technology can form a very small droplet(about 0.0001–0.0005 pL) which only contains finite nano-particles compared with common printing technology, andthe droplets evaporate rather quickly. During the aerosoljet-printing process, the small droplets are formed by anormal ultrasonic transducer or an air-operated one. The linewidth of electronic devices fabricated by aerosol jet-printingprocess can be controlled in the range 5–30 μm. Therefore,micro/nano devices can be fabricated with prepared nano-material ink by aerosol printing conveniently.

In this paper, an all-carbon-based top-gated FET isfabricated by aerosol jet printing on flexible polyethyleneterephthalate (PET) substrate. The source and drain electrodesare formed by printing aqueous GO solution which then isreduced by hydriodic acid. The channel of all-carbon-basedFET is rGO with very low concentration which has excellentsemiconductor characteristics. In addition, the GO is printedas the gate dielectrics with the dielectric constant in the range3–3.5 [27]. It is reported that GO materials have excellentproperties, mechanical and optical, suitable for preparingflexible and transparent electronic devices since these canbe formed on a grapheme channel by a solution-based ordirect oxidation process at room temperatures. Moreover,the electronic device with a GO insulator can be operatedstably [27]. The multi-walled carbon nanotubes (MWNTs) areprinted onto the GO dielectric films to form the gate electrode.MWNT is comparatively suitable for gate electrode becauseof its unique properties including high surface area, corrosionresistance, electrochemical stability, high conductivity andrelatively low manufacturing costs [28].

This flexible all-carbon-based FET is fabricated entirelywith environment-friendly printing process, and is mobile andflexible.

2. Experimental details

Figure 1 schematically shows the fabrication process of theall-carbon-based FET by aerosol jet printing on the flexiblesubstrate. Firstly, the GO flakes are dispersed in the aqueoussolution and then sonicated for a few minutes by an ultrasonicequip at 150 W (BILON92-�, Shanghai, China). The preparedGO ink with high concentration (10 mg mL−1) is printedon the flexible PET substrate by aerosol jet printing to formsource and drain electrodes after water evaporation. Secondly,the low concentration (0.5 mg mL−1) of GO ink is printedbetween source and drain electrodes as the semiconducting

channel. In the third step, the printed GO electrodes and thechannel are reduced for 10 min by hydriodic acid at 100 ◦C. Inorder to control the electrical characteristics of source, drainand channel, resistances of rGO with different concentrationand two printing passes are measured as shown in figure 2.The forth step is to print GO to form dielectric film on therGO channel and dried at room temperature. Finally, solubleMWNTs are printed onto the GO dielectric films to form aconductive thin film to be used as the gate electrode.

All components of the flexible all-carbon-based FET arefabricated by aerosol jet printing including source electrode,drain electrode, dielectric layer and gate electrode. Therefore,different inks are prepared for fabricating all-carbon FETon flexible substrate. Figure 3(a) presents the three inksprepared by GO and MWNTs in aqueous solution. Thelow GO is the GO aqueous solution with low concentration(0.5 mg mL−1) which forms rGO after reduction to act as thesemiconducting channel. The thickness of the rGO channel(with the concentration of 0.5 mg mL−1) is about 400 nm withone printing pass (by Veeco Dektak). The high GO is the GOaqueous solution with high concentration (10 mg mL–1) whichis printed as a thin film to be the dielectrics. Moreover, it canbe used as highly conductive source and drain electrodes afterbeing fully reduced (rGO). In addition, MWNTs’ aqueoussolution is printed on the GO dielectric film acting as the gateelectrode. Figure 3(b) shows the GO thin films printed on PETand the resulting reduced rGO films by reducing with hydriodicacid for 10 min at 100 ◦C. In figure 3(b) 1, 2, 3 and 4 are thenumbers of printing passes with GO aqueous solution on PETsubstrate. It can be seen that the rGO becomes light gray. Thephenomenon indicates that the C/O atomic ratio is increasingwith proper reduction time [29]. The surface morphology andcross-section image of rGO on PET substrate are shown infigures 3(c) and (d), respectively. The surface morphology ofrGO reduced by hydriodic acid is similar to that of graphenefabricated by hydrazine. Figure 3(d) shows that the thicknessof the printed rGO is in the range 3–5 μm which can becontrolled by the number of printing passes.

Figure 4 shows the surface roughness of rGO films byAFM measurement. It can be seen that the rGO films have agood surface flatness whose height difference is in the range0–35 nm.

3. Results and discussion

Figure 5(a) shows the Raman spectra of the printed GO filmsbefore and after the 10 min hydriodic acid reduction. It canbe seen that the intensity ratio ID/IG becomes higher afterreduction, which means more functional groups are removed.The increase in ID/IG ratio after reduction process on theGO is different from some of the existing results becauseof the different reduction method. It is reported that rGOobtained by the chemical reduction method of GO exhibitstwo characteristic main peaks (the D band at ∼1350-1, arisingfrom a breathing mode of k-point photons of A1g symmetry,and the G band at 1575 cm−1, corresponding to the first-order scattering of the E2g phonon of sp2 C atoms) [30]. Theincrease in ID/IG ratio indicated that the reduction process

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(a)

(b)

(c)

(d)

(e)

Figure 1. (a) Schematic of all-carbon-based FET fabricated by aerosol jet printing method. (b) Fabrication process of all-carbon-based FETon the flexible PET substrate.

(a) (b)

Figure 2. (a) The resistances of rGO with different concentration. (b) Electrical characteristics of GOs.

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J. Micromech. Microeng. 23 (2013) 000000 R Liu et al

(a)

(b)

(c)

(d)

Figure 3. (a) Different conductive inks for fabrication of all-carbon-based FET by printing process. The low GO is the GO aqueous solution(0.5 mg mL−1). The high GO is the GO aqueous solution (10 mg mL−1). (b) Printed GO (10 mg mL−1) before and after reduction byhydriodic acid for 10 min at 100 ◦C. (c) Top view and (d) cross-section SEM images of printed rGO (10 mg mL−1) thin film with twoprinting passes.

Figure 4. AFM image of the rGO surface morphology.

altered the structure of G–O with high quantity of structuraldefects [31]. In addition, GO was reduced using thermalreduction at 1000 ◦C in the recent study [26]; it is differentreduction methods that lead to different results in Ramancharacterization.

The infrared absorption spectrum of GO and rGO infigure 5(b) indicates that the oxygen-contained groups (suchas phenolic hydroxyl) are reduced by hydriodic acid becausethe absorption peak induced by oxygen-contained groupsdisappear in the rGO absorption spectrum. The resistanceof printed GO films with different reduction time is alsoinvestigated in this experiment. The sheet resistance of rGOwith different reduction time and two printing passes is shownin figure 5(c). When the reduction time increases from 5to 10 min, the sheet resistances change significantly, i.e.decreases from 1.4 to 1 K�. When the reduction time increasesto 60 min, the sheet resistance of rGO is reduced about 0.2 K�

compared to a 10 min increase in reduction time. However,long reduction times have an adverse effect on the adhesionstrength between the rGO film and the flexible substrate.Therefore, the optimized reduction time of GO is about 10 minin this experiment. The sheet resistance of rGO with differentnumber of printing passes is shown in figure 5(d). It can beseen that the sheet resistance of rGO decreases with increasein the number of printing passes gradually. This result may berelated to densification of printed rGO films which has goodconductivity with dense structure. Therefore, the resistance ofprinted rGO can be controlled to some extent by number ofprinting passes. In this paper, the resistance of rGO electrodeis in the range of 600–800 �/� in this all-carbon-based FETdevice.

The confocal microscopy image of the printed GO filmswith two printing passes is shown in figure 6. It can be seenthat the oxide layer which is printed by aerosol jet method has

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(a) (b)

(c) (d)

Figure 5. (a) Raman spectra of GO and rGO. (b) Infrared absorption spectrum of GO and rGO: 3394 is H2O (adsorbed water); 1727 is C=O(carbonyl/carboxy); 1619 is C=C (aromatics); 1388 is C–O (carboxy); 1249 is C–O (epoxy); 1064 is C–O (alkoxy). (c) Resistance of rGOfilm with different reduction time. (d) Resistance of rGO film with different number of printing passes.

Figure 6. Confocal microscopy image of printed GO films.

good condensation and flatness, and the electronic propertiesof the GO films have been studied in the above-mentionedparagraph. Hence, the printed GO film can spring up the goodfunction in dielectrics in all-carbon-based FET devices.

Source-to-gate I–V curve of all-carbon-based FETsQ3Figures 7(a) and (b) show optical images of the arrays

of all-carbon-based FET on the PET substrate fabricated byaerosol jet printing technique. The source and drain of thisFET are printed with two printing passes, and the channel isprinted with one printing pass. The effective channel lengthand width of the all-carbon-based FET is about 80 and 40 μm,respectively. Electrical characterization is performed with aKeithley 4200 semiconductor analyzer at room temperature.Figure 7(c) shows the source-to-gate I–V curve of rGO-basedtransistors. It can be seen that the GO has good insulationproperty which is suitable for the dielectric layer.

Figures 7(d) and (e) show the I–V and I–Vg curves ofthe all-carbon-based FET on the PET substrate. The on–off ratio of these devices is low (1.2–1.8) because of theintrinsic property of zero-band gap of graphene and rGO. Theeffective mobility (μ) of FET is estimated by the followingequation [32]:

μ = Lt

VDSCt• dID

dVG, (1)

where Lt is the length of the rGO in the channel, Ct is thegate capacitance per unit length for the rGO, dID/dVG isthe transconductance obtained from the linear regime. Thefield effect mobility of this all-carbon-based FET is about350 cm2 (V · s)–1 based on this equation at a drain bias of −1 V.This result indicates that the printed all-carbon-based FET hasa good mobility. In addition, the performance of the printedall-carbon-based FET is better than the device prepared by

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(a) (b)

(c) (d)

(e)

Figure 7. (a, b) Optical images of all-carbon-based FET array. (c–e) IVgs, IVds and IVg curves of an all-carbon-based FET on PET substrate.

Langmuir–Blodgett’s method whose field effect mobility isabout 250–300 cm2 (V · s)–1 [27], but the AJP printing methodis more convenient and environment friendly. The electronictransport properties of this printed all-carbon-based FET donot exhibit typical Dirac point at room temperature. In thisexperiment, the oxygen-containing groups (such as phenolichydroxyl, epoxy) in the GO film are almost removed by HIacid reduction, and more defects will be induced during thereduction process. It is reported that rGO-based FET devicesshows p-type behavior due to the polarization of trapped waterand oxygen molecules between the layers, and transforms toambipolar after annealing [33]. This characteristic is relatedto trapped water and oxygen molecules for these are removed

after annealing, and defects are repaired partly. In this paper,rGO-based FET devices show p-type behavior mostly likelybecause of room temperature environment test. So the transportcharacteristics of the device are affected by trapped water,oxygen molecules and defects.

4. Conclusions

In summary, a top-gated FET with all-carbon structure isfabricated by all-printing technique. This flexible all-carbon-based FET uses rGO with high concentration as source anddrain electrodes while low concentration of rGO acts as asemiconducting channel. GO and MWNT are used as the

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dielectric layer and gate electrode, respectively. This all-printing FET device promises high throughput fabricationof all-carbon, flexible, environment-friendly and low-costelectronic devices.

Acknowledgments

This work was supported by the National NaturalScience Foundation of China (91123034, 21107132,51205390), the Science and Technology Program of Foshan(2011BY100255), and National Ministry of Science andTechnology Support Programs (2012BAF13B05-402).

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