1224 | Chem. Commun., 2014, 50, 1224--1226 This journal is©The Royal Society of Chemistry 2014

Cite this:Chem. Commun., 2014,

50, 1224

Facile preparation of an n-type reduced grapheneoxide field effect transistor at room temperature†

Luyang Wang, Younghun Park, Peng Cui, Sora Bak, Hanleem Lee, Sae Mi Lee andHyoyoung Lee*

We introduce a facile method to prepare an n-type reduced gra-

phene oxide field effect transistor at room temperature via a typical

Benkeser reduction using lithium and ethylenediamine.

Graphene is a popular material which has attracted considerableattention for its unique properties1 and applications in trans-parent electronics,2 drug delivery,3 and in the catalysis of organicchiral reactions.4 In modern electronics, the doping of graphenetransistors is a promising process for realistic applications.5 Bothn-type and p-type doped graphene field effect transistors (FETs)are necessary for a complete electronic circuit. It is easy to preparep-type graphene FETs under ambient conditions. Oxygen isspontaneously adsorbed on the graphene FET surface, leadingto typical p-type doping via both chemisorption and physisorp-tion.6 In the fabrication of n-type doped graphene FETs, bothsubstitutional doping7 and surface doping8 methods are effective.When considering industrial production, nucleophilic substitu-tion doping is preferable to volatile surface doping, the latter canbe rapidly eliminated by changing the environment and removingthe dopants.9 Substitutional doping can be maintained for a longtime because the doped heteroatoms are connected to carbonatoms by covalent bonds.10 In addition, the doped heteroatomscan affect the physical properties of graphene materials.

A combination of chemical oxidation,11 exfoliation, andreduction12 is ideal for the mass production of graphene. W. Gaoet al. tried various methods to convert graphene oxide (GO) into theoriginal graphene honeycomb structure by chemical reduction.13

The conventional method for producing rGO is to treat GO withhydrazine in aqueous solution.14 However, the hydrazine reductantrequires specific heating conditions to reduce most of the epoxyand hydroxyl groups of GO and intensively promote the mobility of

chemically converted graphene (CCG).12,13 Recently, I. K. Moonet al. developed an HI/weak acid method that provides high mobilityand low sheet resistance CCG at room temperature and even lowertemperatures.12,15,16 Although this was an excellent achievement,researchers continue to develop new GO reduction methods at lowtemperature with low boiling point solvents, such as an alkalimetal and liquid ammonia (Birch reduction).17 S. Some et al. havedeveloped in situ nitrogen doped graphene during GO reductionwith various chemicals and reducing methods. The doped nitrogenatoms are added to the graphene backbone with chemical bondingduring GO reduction to rGO. In addition, this concept was used tofabricate an n-type reduced graphene oxide field effect transistor(rGOFET),18 however heating was required with hydrazine. Thus,the development of a low temperature process for the fabricationof n-type rGOFETs is necessary. A novel and facile method forproducing an n-type rGOFET at room temperature has not yet beenreported.

Herein, we demonstrate a facile method to produce highlyN-doped rGO and in situ n-type doped graphene FETs. Ethylene-diamine (EDA) was meticulously chosen as the solvent for thereduction system because it is in the liquid phase at roomtemperature, is a strong base with di-amine terminal groups toaccept the solvated electrons, and acts as a nucleophile in thesubstitution reaction. With Li and EDA, Li–rGO and n-typedoped rGOFETs were easily prepared at room temperature bydipping the pre-made GO channel device into the reductantsolution for a short amount of time. After the dipping process,both Li and EDA were easily removed from the surface byquenching and rinsing with de-ionized (DI) water and ethanol.

Li pieces were dissolved in EDA at room temperature, formingLi+ cations and free solvated electrons. This blue solution is a verystrong reducing agent and commonly used in the Benkeserreduction to open aromatic rings and hydrogenate carbon atomsin organic reactions. Benkeser reduction using the Li–EDA systemwas much more powerful than Birch reduction using the Na–NH3

system because the final product (rGO) obtained via the Benkeserreduction was reduced further than that of obtained by the Birchreduction. The work up process after the Li–EDA reaction is more

National Creative Research Initiative, Center for Smart Molecular Memory,

Department of Chemistry and Energy Science and SKKU Advanced Institute Nano

Technology, Sungkyunkwan University, 2066 Seoburo, Jangan-Gu, Suwon,

Gyeonggi-Do 440-746, South Korea. E-mail: hyoyoung@skku.edu;

Fax: +82-31-299-5934; Tel: +82-31-299-4566

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc47224h

Received 21st September 2013,Accepted 30th October 2013

DOI: 10.1039/c3cc47224h

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convenient for purification at room temperature than that of theBirch reduction.

X-ray diffraction (XRD) is an efficient and convenient method toquantify the reducing degree of rGO. The Li–rGO preparation wascharacterized by XRD, as shown in Fig. 1a. A typical GO peak near10.891 (d-spacing B8.11 Å) and different from the graphite peak(26.341, d-spacing B3.38 Å) was observed, indicating that theinterlayer spacing increased during oxidation. The Li–rGO peakshowed an obvious shift to higher 2y angles (23.391; d-spacingB3.80 Å) compared to GO, suggesting that the Li–rGO was wellordered in a two-dimensional plane by the removal of surfacefunctional oxygen groups during reduction. The quality of theLi–rGO was directly related to the reaction time (ESI,† Fig. S1). TheGO peak in XRD was completely eliminated when the reactiontime was more than 60 min.

Raman spectroscopy is a universal method used to for the non-destructively characterization of graphene. The main Raman fea-ture bands of graphene are the G-band, D-band, and 2D-band, eachof which has individual physical origins. Raman spectra of GO andLi–rGO powder are shown in Fig. 1b. The ID/IG ratio of Li–rGOnotably increased from 0.86 (in GO) to 1.06, indicating that thereduction process changed and repaired the structure of GO.The 2D band of Li–rGO was stronger than that of GO, supportingthe idea of better graphitization in the reduced product.

X-ray photoemission spectroscopy (XPS) was used to investi-gate the elemental composition and amount of samples. Fig. 1cshows the C1s spectra of GO and Li–rGO samples. GO generallyshowed bimodal XPS peaks since the oxidation process generatedmany carbon–oxygen functional groups. In contrast, the XPSof Li–rGO exhibited a spectrum similar to that of natural gra-phite (ESI,† Fig. S2a). The CQC bonding domain showed a singlepeak around 284.5 eV, confirming suitable reconstruction ofpi-conjugation in Li–rGO during the reduction process. Interest-ingly, the amount of N could not be detected when the amount ofLi was less than 1.67 mg ml�1 (Fig. S2b, ESI†). After doubling the

concentration of Li in EDA (from 3.33 mg ml�1 to 6.67 mg ml�1),the amount of N increased linearly as the Li concentrationincreased from 1.1% to 2.1% (Table S1, ESI†).

Thermogravimetric analysis (TGA) was used to characterize thelevel of reduction of GO powder by measuring the weight loss. TheTGA thermograms in Fig. 1d shows the weight loss at differenttemperatures for GO and Li–rGO (nitrogen atmosphere, heating rateof 10 1C min�1). The elimination of interlayer water in GO wasobserved in the curve at around 100 1C. Further intensive weightloss of the GO sample occurred around 150 1C to 200 1C. This wasattributed to the loss of oxygen groups. The same elimination ofinterlayer water did not occur in the Li–rGO curve since commonrGO is hydrophobic. Li–rGO demonstrated thermal stability becausethe Li–EDA reduction removed most of the oxygen functionalgroups and resulted in suitable graphitization of the rGO layer.

Atomic force microscopy (AFM) was used to characterizeand investigate the topographic surface of GO (Fig. S7, ESI†)and Li–rGO (Fig. 2a). A graph of the height profile shows therestacking and aggregation of Li–rGO (Fig. 2a). Scanning elec-tron microscopy (SEM) also revealed the surface morphology ofLi–rGO in which the restacking could be easily observed after thereduction (Fig. 2b). To demonstrate the distribution of N atomson the Li–rGO surface, the element mapping in the selected areais shown in Fig. S6 (ESI†). From the element mapping, we couldclearly observe the distribution of C, O and N atoms. Transmis-sion electron microscopy (TEM) also confirmed the morphologyof Li–rGO. Fig. 2c clearly shows the mono-sheet-like structureof the material. During reduction, a portion of the Li–rGOrestacked, which contributed to the well-reduced pi-conjugation(Fig. 2d). To characterize the electronic properties of Li–rGO,Li–rGO FETs were fabricated from GO FETs. GO was spin-coated on the device to fabricate top channels using the samemethod as reported in our previous paper.19

Fig. 1 Characterization of Li–rGO powder. (a) X-ray diffraction (XRD) patternsof GO, graphite, and Li–rGO. (b) Raman spectra of GO and Li–rGO. (c) X-rayphotoemission spectroscopy (XPS) C 1s spectra of GO and Li–rGO. (d) TGAthermograms for Li–rGO and GO.

Fig. 2 Surface characterization: (a) Atomic force microscopy (AFM) image ofLi–rGO powder. (b) Scanning electron microscopy (SEM) image of the Li–rGOsheets. Scale bar: 30 mm (c) Transmission electron microscopy (TEM) image ofwrinkled Li–rGO. (d) High-resolution (HR) TEM image of Li–rGO layers.

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1226 | Chem. Commun., 2014, 50, 1224--1226 This journal is©The Royal Society of Chemistry 2014

After reducing the device in a solution of Li–EDA, a constantsource-drain bias at 0.5 V was applied while the gate voltage wasswept from �40 V to 40 V. Under ambient conditions, all channelsexhibited p-type doping behaviour because oxygen could sponta-neously adsorb onto the graphene surface. After removing oxygenand moisture at 10�4 Torr for 2 h, the Li–rGO changed to n-typedoping behaviour with a typical Dirac point shift to�23.8 V (Fig. 3).This proved that Li–EDA effectively reduced GO to rGO andsupplied suitable electronic properties to induce n-type dopingduring the reduction. Most gaps in the Li–rGO FET showed typicaln-type doping behaviour, which confirmed the reproducibility ofthis method.

To compare Li–rGO FET to other FETs reduced by the hydra-zine and HI methods, rGO FET devices were prepared underdifferent reducing conditions. As shown in Fig. S5a and b (ESI†),rGO FETs reduced from HI and hydrazine showed ambipolarstates with a threshold voltage at around 0 V in the vacuum. Thiscommon phenomenon indicates that graphene is a zero bandgapmaterial in which the conduction and valence bands meet at theFermi level. Another control experiment was performed by dip-ping GO FETs into an EDA solution to test the solvent reducingeffect. As expected, there was no significant surface doping and noreduction with only EDA as solvent (ESI,† Fig. S3).

The Li+[EDA]n cation and free solvated electrons, e�[EDA]n, wereformed after dissolving Li in EDA. EDA demonstrates strongernucleophility and basicity than ammonia. A possible mechanism

of in situ n-type doping by Li–EDA (Scheme 1) is quite different fromthe previously reported one for doping by Na–NH3. It is proposed thattwo kinds of competitive reduction reactions occurred concurrentlyin the Li–EDA system. Radical reduction would affect the epoxy groupwhen electrons were added to the oxygen and carbon atoms. Withhelp from the solvent, protons would be transferred to newly formednegatively charged oxygen and carbon atoms to give a hydroxy groupand C–H bond. Finally, the strong base solution would eliminate thehydroxyl group to produce a new double bond. The radicals couldexist until the reaction mixture was quenched with water and alcohol.The other possible route is that the strong nucleophile e�[EDA]triggers a SN2 reaction to the epoxy groups under basic conditions.Nucleophilic SN2 substitution of e�[EDA] to an epoxide ring wouldgenerate a C–N bond, and the negatively charged oxygen groupwould transform into a hydroxy group with proton transfer in thesolution phase. Finally, the hydroxy group would be removed togenerate a new CQC double bond next to the C–N bond. Thus,restoration of the pi-conjugated CQC double bonds and new C–Nbonds results in a high-quality n-type rGOFET at room temperature.

In summary, we demonstrate a facile and novel method forproducing in situ nitrogen-doped rGO during reduction of GOwith Li and EDA at room temperature. A high quality n-typeLi–rGOFET was produced by dipping a GO FET into Li–EDAsolution at room temperature. This is a suitable method for themass production of n-type graphene electronics at room tem-perature in the near future. The National Research Foundationof Korea (NRF) grant funded by the Korean government (MSIP)(Grant No. 2006-0050684) supported this work.

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Fig. 3 I–V characterization of a Li–rGO FET under vacuum conditions.

Scheme 1 Possible mechanism of the reduction of GO and nitrogennucleophilic substitution-reduction on GO.

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