Anisotropic Electrical Transport Properties of Graphene

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Anisotropic Electrical Transport Properties of Graphene Nanoplatelets/Pyrene Composites by Electric-Field-Assisted Thermal Annealing Marta Cardinali,† Luca Valentini,*,† and Jose M. Kenny†,‡ † ‡

Dipartimento di Ingegneria Civile e Ambientale, Universita di Perugia, UdR INSTM, Pentima 4, 05100 Terni, Italy Institute of Polymer Science and Technology, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain

bS Supporting Information ABSTRACT: Dispersion of graphene nanoplatelets (GNPs) with a pyrene derivative compound (1,3,6,8-tetrakis(4-t-butylphenyl)pyrene (TBPP)) in tetrahydrofuran as solvent medium is investigated. Field-emission electron microscopy and atomic force microscopy demonstrated the π π stacking interaction between the TBPP compound and the GNPs resulting in the deposition of the TBPP compound onto the exfoliated graphene sheets. We found that the electrical conductivity of the films obtained by the prepared dispersions can be activated in temperature and oriented by the application of an external electric field. We found an increase of the composite conductivity measured along the substrate with respect to the composite conductivity measured perpendicular to the substrate. This result provides an initial understanding of how electric fields can be used to control the bulk physical properties of such composites.

’ INTRODUCTION Due to its mechanical, electrical and thermal properties such as Young’s modulus (∼1100 GPa), carrier mobility (200000 cm2/V 3 s), and thermal conductivity (∼5000 W/mK), graphene is undoubtedly the material that has recently received greater attention among researchers.1 4 A single, isolated graphene sheet having macroscopic dimensions is difficult to realize; several processes to produce graphene have been described in the literature such as micromechanical cleavage of graphene,5 exfoliation and chemical reduction of graphene oxide,6 and chemical vapor deposition.7 Systems consisting of multiple graphene sheets prepared starting from graphite and converted into few-layer graphene nanoplatelets (GNPs) by chemical and physical methods have been reported.8 These GNPs can be deposited to form macroscopically thick layers on common substrate materials such as silicon. However, achieving macroscopically anisotropic properties due to self-organization of GNPs in a fluid phase is still a big challenge due to the lack of a scalable assembly method. Moreover, discotic liquid crystals have been reported with a large number of discoid cores including pyrene.9 Liquid crystals are anisotropic fluids with the isotropic properties of a liquid and the ordering of a solid phase. Self-assembled liquid crystals exhibit orientational order with anisotropic physical properties, while maintaining the flow properties of their liquid component.10 This self-organizing property may be used to impart orientation on dispersed aromatic systems such as graphene. The ease of film formation could be a key feature of stimuli-responsive materials for organic-based electronic and optoelectronic devices. r 2011 American Chemical Society

Figure 1. Vials containing dispersions of GNPs in THF made without (left) and with (right) tip sonication treatment.

The use of π π stacking interactions usually occurring between pyrene units and aromatic macromolecules such as carbon nanotubes11 15 and graphene oxide16 has been reported in literature. The strategy of the functionalization of GNPs using pyrene via π π stacking has the advantage to retain the conjugated structure of graphene and therefore the electrical conductivity.17 20 Received: June 20, 2011 Revised: July 14, 2011 Published: July 18, 2011 16652

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Figure 2. (a) FESEM and (b) AFM images (30 μm  30 μm) with AFM height profile of GNPs drop cast onto Si substrate from a THF dispersion obtained without tip sonication treatment. (c) AFM image (30 μm  30 μm) and AFM height profile of GNPs drop cast onto Si substrate from a THF dispersion processed with tip sonication treatment.

Moreover, the possibility to dynamically and collectively change the orientation direction of ensembles of GNPs/pyrene composites under the action of an external electric field could open the direction toward optical and especially also electrical anisotropies. In this paper we will demonstrate the realization of electrically controlled GNP switches by use of pyrene GNP dispersions.

’ EXPERIMENTAL METHODS The general synthesis of 1,3,6,8-tetrakis(4-t-butylphenyl)pyrene (TBPP) was done as reported elsewhere.21,22

Figure 3. FESEM images of (a) TBPP and (b) GNPs/TBPP samples. The inset of Figure 3a shows the TBPP structure. (c) AFM image (30 μm  30 μm) and AFM height profile of the GNsP/TBPP sample. (d) TEM image of the GNPs/TBPP sample. 16653

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Figure 4. (a) Current versus time of a TBPP sample under the application of an external bias of 5 V from room temperature to 130 °C perpendicular to the sample surface. (b) Current versus time of a GNPs/TBPP sample under the application of an external bias of 5 V from room temperature to 130 °C perpendicular to the sample surface. (c) Current versus time of a TBPP sample under the application of an external bias of 5 V from room temperature to 130 °C parallel to the sample surface. (d) Current versus time of a GNPs/TBPP sample under the application of an external bias of 5 V from room temperature to 130 °C parallel to the sample surface.

Figure 5. Optical photographs (400 μm  200 μm) showing the morphology of a GNPs/TBPP composite under electric-field-assisted thermal annealing (a) onto the Al electrode, (b) near the same electrode, (c) between the electrodes, and (d) near the opposite electrode. The dashed line delimitates the Al electrode area.

GNPs were purchased from Grafen (GNP-HZN-BS grade). Tetrahydrofuran (THF, purchased from Aldrich, 1 mg/1 mL) dispersion (1 mg/1 mL) of GNPs was prepared by tip sonication (750 W, 60% amplitude) for 1 h to yield a stable dispersion. After sonication, the solution was transferred to a vial, and it was centrifuged for 30 min at 600 rpm; this procedure was repeated twice. After that, the top of the 12 mL solution (≈ 10 mL) was carefully extracted to remove residual aliquot of native dispersion in order to use a pristine one for the experimental process.

Solutions consisting of TBPP dispersed in THF and a combination of the GNPs and TBPP moiety (1(GNPs):1(TBPP)) in THF were drop casted onto silicon wafers previously cleaned in an ultrasonic bath of acetone and ethanol and dried under nitrogen. The morphology of the prepared samples was observed by fieldemission scanning electron microscopy (FESEM), transmission electron microscopy (TEM; Philips EM 400), and atomic force microscopy (AFM) operated in tapping mode. The thickness of the GNPs was estimated by AFM analysis by measuring the height profile between covered and uncovered areas of the Si substrate. The infrared (IR) spectrum of the TBPP deposit was recorded in transmission mode between 250 cm 1 and 3500 cm 1. For the electrical characterization performed using a computercontrolled Keithley 4200 source measure unit, Al electrodes (1 mm  5 mm) were deposited on a glass substrate by vacuum evaporation (≈ 10 6 Torr) with an optimized thickness of 60 nm and spaced of 1 mm. TBPP and GNPs/TBPP solutions were drop-cast on the Al electrodes. For the electric field thermal-assisted measurements, the samples were contemporarily heated up to 130 °C (heating rate 10 °C/min) and exposed to an electric voltage of 5 V applied between two neighboring aluminum electrodes. The sample was left at 130 °C for 90s and then cooled to room temperature leaving the electric field applied. An identical procedure was then repeated by applying the electric field between two electrodes consisting of a silver paste dot deposited on the top of the sample surface and the bottom Al electrode (sample thickness 500 μm).

’ RESULTS AND DISCUSSION Regarding the composite preparation process, it was found that the TBPP is soluble in THF. Thus the first investigation was 16654

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The Journal of Physical Chemistry C devoted to understand whether THF can be used for the preparation of GNP dispersions (Figure 1). The vial with GNPs dispersed into THF without tip sonication treatment contains visible precipitates (left panel), while that with GNPs dispersed through tip sonication does not contain visible precipitates (right panel). This finding indicated that the THF acts as good dispersant of GNPs. Figure 2a shows a typical FESEM image of GNPs obtained from THF dispersion without the tip sonication treatment. It is interesting to note the stacking of nanoplatelets, consistent with a dense distribution over large areas of the substrate. AFM is one of the most direct methods of quantifying the degree of exfoliation of graphene sheets. Tapping-mode AFM images of the GNPs dispersed in THF with or without tip sonication treatment are reported in Figure 2. The dispersions containing the GNPs were cast and left to dry on a silicon support before their observation by AFM. The AFM image shows thick and densely packed GNPs (Figure 2b). On the contrary, isolated and exfoliated graphene sheets have been obtained by tip sonication as reported in Figure 2c. GNPs with an average thickness of about 10 nm were observed, which is characteristic of an exfoliated graphene sheet.23,24 FESEM was also utilized to analyze the surface morphology of the TBPP drop-cast from THF solution. The FESEM analysis of the TBPP drop-cast from THF (Figure 3a) shows that the morphology consists of discoid crystals. The morphology of the GNPs/TBPP sample shows that the TBPP crystalline structure was transferred onto the GNP sheet (Figure 3b) as recorded by AFM and TEM analyses (Figure 3c,d). Under the electric field application perpendicular to the sample surface at room temperature, the electrical current values of TBPP and GNPs/TBPP samples remain constant (Figure 4a,b) and exhibit an insulating behavior. Moreover, it is reported how the current of both samples increases when the temperature increases, this effect being slightly enhanced for the TBPP containing GNPs. Cooling the samples to room temperature leaving the electric field applied, the samples were found to again be electrically insulating. This finding probably implies that the temperature induces an orientational distribution that alters the electrical conductivity. To support this theory, we should observe an anisotropic electrical conductivity depending on the orientation of the electric field, i.e., parallel or perpendicular to the sample surface. For the TBPP and GNPs/TBPP samples, the current (Figure 4a,b) measured when the electric field is perpendicular to the sample surface is lower than that (Figure 4c,d) measured when the applied electric field is parallel to the sample surface. A possible explanation may be the electric-field-enhanced ordering toward the electrode areas. The effect of the electricfield -ssisted thermal annealing on the composite morphology is reported on optical photographs reported in Figure 5 at the same magnification for ease of comparison. The GNPs/TBPP sample onto the Al electrode (Figure 5a) consists of large grains; between the electrodes these grains start to coalesce into fewer and smaller grains protruding from the electrode (Figure 5b,c). Figure 5d shows how the employment of an external electric field resulted in the formation of the chain-like structure that is mainly evident near the opposite electrode. The higher conductivity along the composite surface is due to this interconnected morphology, probably due to molecular orientation directed parallel to the substrate.

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’ CONCLUSIONS We have successfully modified GNPs with a pyrene-based compound via π π stacking interactions. It was found that the electrical properties of this GNPs/pyrene composite are sensitive to the temperature and to electric field direction. In particular, the composite shows anisotropic electrical transport properties. These results demonstrate the possibility to obtain pyrene/GNP switches activated by electric field assisted thermal annealing. ’ ASSOCIATED CONTENT

bS

Supporting Information. FTIR and AFM characterization of the TBPP compound. UV vis spectra of TBPP and GNPs/TBPP samples. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +39 0744 492924. Fax: +39 0744 492950. E-mail address: [email protected].

’ ACKNOWLEDGMENT The synthesis of the organic compound was performed by Prof. Bilal R. Kaafarani at the Department of Chemistry, American University of Beirut, Lebanon. The authors are grateful for this support. The authors also acknowledge Dr. Ibrahim Mutlay of Grafen Chemical Industries Co. and Hayzen Engineering Co. Ankara, Turkey for the supply of graphene nanoplatelets. ’ REFERENCES (1) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385–388. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V. Science 2004, 306, 666–669. (3) Balandin, A. A.; Ghosh, S.; Bao, W. Z.; Calizo, I.; Teweldebrhan, D.; Miao, F.; lau, C. N. Nano Lett. 2008, 8, 902–907. (4) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Solid State Commun. 2008, 146, 351–355. (5) Schniepp, H. C.; Li, J.; McAllister, M.; Sai, H.; Herrera-Alonso, M.; Adamson, D.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 110, 8535–8539. (6) Zhu, Y.; Stoller, M. D.; Cai, W.; Velamakanni, A.; Piner, R. D.; Chen, D.; Chen, D.; Ruoff, R. S. ACS Nano 2010, 4, 1227–1233. (7) Shang, N. G.; Papakonstantinou, P.; McMullan, M.; Chu, M.; Stamboulis, A.; Potenza, A.; Dhesi, S. S.; Marchetto, H. Adv. Funct. Mater. 2008, 18, 3506–3514. (8) Yu, A.; Ramesh, P.; Itkis, M. E.; Bekarova, E.; Haddon, R. C. J. Phys. Chem. C 2007, 111, 7565–7569. (9) Hayer, A.; De Halleux, V.; Koehler, A.; El-Garoughy, A.; Meijer, E. W.; Barbera, J.; Tant, J.; Levin, J.; Lehmann, M.; Gierschner, J.; Cornil, J.; Geerts, Y. H. J. Phys. Chem. B 2006, 110, 7653–7659. (10) Collings, P. J.; Hird, M. Introduction to Liquid Crystals; Taylor & Francis: London, 1997. (11) Liu, Z.; Sun, X. M.; Nakayama-Ratchford, N.; Dai, H. J. ACS Nano 2007, 1, 50–56. (12) Marquis, R.; Greco, C.; Sadokierska, I.; Lebedkin, S.; Kappes, M. M.; Michel, T.; Alvarez, L.; Sauvajol, J. L.; Meunier, S.; Mioskowski, C. Nano Lett. 2008, 8, 1830–1835. (13) Zhao, H.; Yuan, W. Z.; Mei, J.; Tang, L.; Liu, X. Q.; Yan, M.; Shen, X. Y.; Sun, J. Z.; Qin, A. J.; Tang, Z. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4995–5005. 16655

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