Formation of Thin, Flexible, Conducting Films Composed of Multilayer Graphene

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1 ISSN , Bulletin of the Russian Academy of Sciences. Physics, 04, Vol. 78, No., pp Allerton Press, Inc., 04. Formation of Thin, Flexible, Conducting Films Composed of Multilayer Graphene A. V. Alaferdov a, b, S. M. Balashov c, M. A. Canesqui a, S. Parada a, Yu. A. Danilov b, and S. A. Moshkalev a a Center for Semiconductor Components, State University of Campinas, Campinas, São Paulo , Brazil b Lobachevsky State University, Nizhny Novgorod, Russia c Renato Archer Center for Information and Technology, Campinas, São Paulo , Brazil andrei.alaferdov@ccs.unicamp.br Abstract The possibility of fabricating high-quality thin (0 00 nm), flexible, semitransparent films of multilayer graphene and graphite nanoplates (ultrathin graphite) using a modified Langmuir Blodgett method is demonstrated. The high quality of the resulting samples is confirmed via Raman spectroscopy and high-resolution scanning electron microscopy. Measurements of the films surface resistance show values of ~00 Ω/sq or less. DOI: 0.303/S INTRODUCTION Over the last 0 years, carbon nanostructures have drawn increasing attention and are considered one of the basic elements of future electronics. In addition to one- or two-layer graphene, multilayer graphene (MLG) or ultrathin graphite with low defect content is of considerable practical interest as well, due to its exceptional electrical, thermal, and optical properties [ 3]. Current key problems are manipulating the individual graphene plates and creating discrete elements of electronic schemes based on them, along with using them to create macrostructures (thin films). These problems can be solved by such methods as the chemical deposition of gas phase on Ni and Cu substrates [4, 5], restoring prepared films of graphene oxide [6, 7], and the electrophoresis of restored graphene oxide from aqueous solution [8]. Major disadvantages of the above methods are, in the first case, the island-like growth of the films, the high cost, and the great amounts of energy consumed in production; in the second and third case, they include the complexity and unreliability of the process, the incomplete restoration of the graphene oxide, and the resulting defects in the final product. Nowadays, thin films of graphene oxide are often created using a modified Langmuir Blodgett method [9 ]. This method is attractive due to its simplicity and the low cost of creating D structures. When the graphene oxide films are restored to graphene, however, the problem of defects arises once again. In forming graphene films from suspensions of intercalated graphene via the modified Langmuir Blodgett method [3], the technical complexity of the process is a great disadvantage. This work demonstrates the possibility of creating thin films via the Langmuir Blodgett method directly from MLG (nanoplates 3 5 nm thick) and ultrathin graphite (plate thickness, 5 50 nm) on various kinds of substrates (SiO, glass, or polymer) with low electrical and thermal resistance [, 4]. Note that in the traditional approach to the deposition of graphene films via the modified Langmuir Blodgett method, the problem arises of monitoring the surface pressure in the graphene layer, because (strictly speaking) there is no monolayer of amphiphilic molecules on the surface of water with well-controlled surface tension. To solve this problem, we propose ensuring the constancy of the surface pressure during film deposition by using a special inclined Langmuir cell wall. MATERIALS AND METHODS Thin films were created using an MLG suspension (~3 mg ml ) prepared by peeling graphite in the liquid phase [5], or commercial suspensions of ultrathin graphite (UTG) (~3 mg ml ) (Nacional de Grafite Ltda, Brazil) in N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF). These solvents belong to the class of popular (hydrophilic) aprotic solvents suitable for preparing MLG suspensions followed by use of the latter as the phase for creating films by the Langmuir Blodgett method. Silicon dioxide, glass, and polymer polydimethylsiloxane (PDMS) were used as substrates for the films. Substrates of silicon dioxide were processed in H SO 4 and H O solutions (4 : ) to remove organic compounds and improve their hydrophilic properties. Good hydrophilic properties of the substrate are necessary for the best wettability of the latter in the process of MLG film 357

2 358 ALAFERDOV et al. 5 tion scanning electron microscopy (SEM) on an FEI Nova 00 Nanolab apparatus, and confocal Raman spectroscopy (NT MDT) on an INTEGRA Spektra device. Surface resistance was measured via the fourprobe method using a Jundle RM3 setup. 3 RESULTS AND DISCUSSION 4 Fig.. Cuvette with MLG suspension distributed over a surface of water; deposition of the MLG film onto a Si/SiO substrate: () body of the cuvette, () water, (3) water drainage, (4) inclined wall of the cuvette, (5) substrate, (6) Langmuir Blodgett film. deposition. Each of the glass substrates was subsequently processed in an ultrasonic bath for 5 minutes in a molar solution of HCl, isopropanol, and deionized water. PDMS substrates were prepared by the standard method used in [6]. A trapezoidal Teflon cuvette was made in order to deposit thin MLG (UTG) films on the substrate (Fig. ). The cuvette was filled with deionized water (the subphase). Using a microsyringe, MLG (5 μl) or UTG (50 μl) suspension was slowly and carefully distributed in a thin layer (phase) on the surface of water filling the cuvette. Ten to fifteen minutes after depositing the suspension, a film of uniformly distributed graphene sheets floating freely on the water s surface formed (Fig. a). Reducing the subphase surface by slowly draining the water through a hole in the cuvette s bottom led to the convergence of the MLG plates with the formation of an incraesingly compact film (Fig. b). The compaction coefficient (the ratio of the initial area to the area of the formed film) was 3. As the water level fell at a rate of ~ mm min, the film was deposited onto a vertically oriented substrate (Fig. b) placed at the bottom of the cuvette. The cuvette wall opposite the sample was at an angle 45 (Fig. a). Due to this geometry, the reduced area of the water mirror covered with the film exactly matched the area of the film deposited onto the sample as the water drained, keeping the film density constant during deposition. The thicknesses of the films were measured by means of atom power microscopy, APM (NT MDT) in the semi-contact operating mode. The quality of the resulting structures were studied via optical microscopy on an Olympus MX5 unit, high-resolu- 6 Using the modified Langmuir Blodgett method, semi-transparent one- and two-layer MLG (UTG) films were deposited on three kinds of substrates: silicon dioxide, glass, and PDMS. As can be seen from the images in Fig., the coverage area for all three kinds of substrates was no less than 70 80% after a single deposition of the film. Together with the films stability, this testifies to the equally good adhesion of the films on different substrates. Note too that upon the deposition of MLG (UTG) films from different solvents (NMP and DMF), the coverage area remained the same, demonstrating the virtually full evaporation (and dissolution in water) of the solvents that we used. Three lines typical of carbon structures were observed in the films Raman spectra: a low-intensity D-line (~345 cm ) corresponding to the breathing mode of hexahydric carbon ring oscillations (TO-mode) and requiring structural defects for its activation; a high-intensity G-line (~575 cm ) corresponding to the LO- and transverse TO-modes of the phonon oscillations; and a medium-intensity D-line (70 cm ) that was an overtone of the D-line corresponding to oscillations of the phonons with oppositely oriented wave vectors and not requiring defects for its activation. The ratio of D- and G-line intensities in the Raman spectra (Fig. 3) of the samples did not exceed 0., indicating a low content of defects [7] and the high quality of the structures (both for the commercial suspension of Nacional de Grafite Ltda, and for the MLG suspension prepared by peeling in the liquid phase). From the heat conductivities measured in [, 8], the high level of percolation between the MLG (UTG) plates, and the uniform distribution of the plates in the films (the main heat losses in which occur due to heat dissipation at points where the MLG (UTG) plates are in contact) we can see their thermal properties were good. The films were 0 30 nm thick in single MLG deposition (Fig. 4) and up to ~00 nm in UTG deposition. The surface resistance of single-layer films was ~00 Ω/sq or less, due in our opinion to the high level of percolation between the plates in the films. Compared to the data presented in other works [5,, 3, 9], this value was one of the best that we obtained. It is worth mentioning that the surface resistance of flexible semitransparent MLG (UTG) films obtained on PDMS (Fig. 5) did not change after reversible deformation of the substrate (straining and bending). BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES. PHYSICS Vol. 78 No. 04

3 FORMATION OF THIN, FLEXIBLE, CONDUCTING FILMS (c) (d) 359 Fig.. SEM image of the MLG film on SiO; optical image of the MLG film on glass; (c) SEM image of the MLG film on PDMS; (d) optical image after single and double deposition of the UTG film (scale, 00 µm). D D G Raman shift, cm 800 Fig. 3. Raman spectrum of films obtained from a commercial suspension of Nacional de Grafite Ltda (lower spectrum, line ) and from a suspension prepared by peeling in the liquid phase (upper spectrum, line ). BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES. PHYSICS Vol. 78 No. 04

4 360 ALAFERDOV et al. 3 nm Fig. 4. APM image of part of the UTG film; profile of nanoplate thickness. Fig. 5. Picture of the semi-transparent MLG film on PDMS. The insert shows a SEM image of the film; scale, 50 µm. CONCLUSIONS The possibility of creating high-quality thin (0 00 nm), flexible, conducting (~00 Ω/sq), semitransparent films of multilayer graphene (ultrathin graphite) using a modified Langmuir Blodgett method was demonstrated. The film samples obtained by this method can be used to create flexible (on polymer substrate) thermofilters and electrodes in supercapacitors. ACKNOWLEDGMENTS This work was performed as part of the INCT NAMITEC project, National Institute of Science and Technology for Nano- and Microelectronics, National Research Board, Brazil. REFERENCES. Kopelevich, Y. and Esquinazi, P., Adv. Mater., 007, vol. 9, no. 4, pp Ermakov, V.A., et al., Nanotecnology, 03, vol. 4, no. 5, p Orlita, M. and Potemski, M., Semicond. Sci. Technol., 00, vol. 5, no. 6, p Li, X., et al., Science, 009, col. 34, no. 593, pp BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES. PHYSICS Vol. 78 No. 04

5 FORMATION OF THIN, FLEXIBLE, CONDUCTING FILMS Kim, K.S., et al., Nature, 009, vol. 457, no. 730, pp He, Q., et al., ACS Nano, 0, vol. 5, no. 6, pp He, Q., et al., ACS Nano, 00, vol. 4, no. 6, pp Guo, C.X., et al., Angew. Chem., 00, vol. 49, no. 7, pp Cote, L.J., Kim, F., and Huang, J., J. Am. Chem. Soc., 009, vol. 3, no. 3, pp Liu, H., et al., Proc. th IEEE Int. Conf. on Nanotechnology (IEEE-NANO), Birmingham, 0, pp. 5.. Zheng, Q., et al., ACS Nano, 0, vol. 5, no. 7, pp Aleksenskii, A.E., et al., Zh. Tekh. Fiz., 03, vol. 83, no., pp Park, K.H., et al., Nano Lett., 0, vol., no. 6, pp Rouxinol, F.P., et al., Appl. Phys. Lett., 00, vol. 97, no. 5, p Alaferdov, A.V., et al., Carbon, 04, vol. 69, pp Mata, A., Fleischman, A.J., and Roy, S., Biomed. Microdev., 005, vol. 7, no. 4, pp Pimenta, M.A., et al., Phys. Chem. Chem. Phys., 007, vol. 9, no., pp Alaferdov, A.V., et al., J. Surf. Investigation. X-Ray, Synchrotron Neutron Techn., 03, vol. 7, no. 4, pp Park, J.S., et al., ACS Appl. Mater. Interfaces, 0, vol. 3, no., pp Translated by O. Shmagunov SPELL:. ok BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES. PHYSICS Vol. 78 No. 04