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22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium
Chemical functionalization of graphene by plasma processes G.V. Bianco, M. Losurdo, M. M. Giangregorio, P. Capezzuto, and G. Bruno Institute of Inorganic Methodologies and of Plasmas, CNR-IMIP, Chemistry Department of University of Bari “Aldo Moro”, via Orabona 4, 70126 Bari, Italy Abstract: H 2 , O 2 , N 2 and SF 6 modulated plasma processes were applied for the covalent binding of functional groups on graphene without introducing structural defects related to ion radiative damaging. Real time monitoring of graphene optical properties by spectroscopic ellipsometry was used for allowing an unprecedented control over the degree of functionalization proven by structural, chemical and electrical characterizations. Keywords: graphene, functionalization, plasma treatment
The peculiar properties of graphene such as high carrier mobility, optical transparency, fleixibility and high chemical resistance have stimulated a vast amount of research in several technological fields. However, the diffusion of graphene technologies is still limited by several challenges. The opening of a gap in the graphene band structure is fundamental for its exploitation in electronic applications (especially for the development of graphene-based transistor devices). Similarly, the control of the doping state of graphene is needed for applications as low resistance transparent conductive layer substituting conductive transparent oxides. Moreover, since graphene is a relatively inert material, it provides weak interactions with adsorbates and, hence, this limits its potential for sensing applications. The chemical functionalization of graphene has been reported to be effective in addressing these issues by tailoring both the chemical and electrical properties. Functional groups covalently linked to graphene are effective for doping via charge-transfer processes [1] and for gap opening via the C-sp2 to C-sp3 conversion [2]. Moreover, the graphene functionalization locally changes the surface reactivity with chemical species also providing selective binding with specific target molecules [3]. Several experimental routes have been explored for decorating graphene with functional groups. In particular, the covalent attachment of hydrogen [3] and halogen [2] atoms has received great attention due to their twofold potential for (i) tailoring the graphene electrical properties by doping and band-gap opening, and (ii) activating the graphene basal plane toward a controlled functionalization with more complex organic groups. Depending on the degree of functionalization, the opening of an energy gap up to 3.0 eV has been estimated for fluorinated graphene [4]. Moreover, the fluorine atoms can be easily replaced with various types of functional groups through nucleophilic substitution reactions by using alcohols, amines, Grignard reagents, or alkyl lithium compounds. In the same way, a band-gap up to 3.5 eV can be opened in graphene by hydrogenation [5],
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which also locally activates the material towards diazonium functionalization.
Fig. 1. Chemical functionalization of graphene by plasma processes. Typically, the covalent modification of graphene is carried out by photochemical [6], thermal [4] or plasma generation [2] of atomic or molecular species which combine with graphene by free radicals addiction reaction. In particular, plasma-functionalization of graphene is attractive since it involves fast, dry and scalable processes. However, the control of the functionalization degree by plasma treatment is still challenging. In particular, the minimization of structural damaging by ion bombardment or etching processes (especially for oxygen plasma) is difficult to achieve. In this contribution, we present plasma-chemical routes for tailoring electrical properties in large area chemical vapor deposition (CVD) graphene by functionalization with several chemical groups including oxygen and nitrogen groups as well as hydrogen and fluorine atoms. Our functionalization processes have been developed and optimized with the twofold aim: (i) the fine tuning of graphene electrical properties, and (ii) the strong minimization of induced structural damage. To this purpose, we have explored modulated plasmas of H 2 , O 2 , N 2 , and SF 6 for the controlled modification of graphene. We demonstrate that the use of a pulsed plasma source allows a good control of the functionalization kinetics without introducing structural defects. Specifically, graphene functionalization by a modulated plasma process can take advantage from the different life-times of plasma generated species for minimizing the
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concentration of ions impacting the graphene surface. In the same way, the interaction between graphene and vibrationally excited species can be strongly reduced, and this limits the graphene chemical heating which favours carbon etching processes. Our plasma treatments were performed on monolayer CVD graphene transferred on corning glass. The reactor is equipped with an in situ spectroscopic ellipsometer to monitor in real time the graphene optical properties during functionalization (see Fig. 2) and to provide direct measurements of the functionalization degree. The latter was estimated by the analysis of Raman spectra of functionalized graphene (see Fig. 3). Specifically, the relationship proposed by Cancado et al. [7] between the intensity of D peak and the average distance between sp3 carbon atoms was exploited for estimating the surface density of functional groups covalently attached to graphene. This analysis was also supported by XPS studies to attest the chemical nature of functional groups attached to graphene and to provide further insights on the interaction between plasma generated species and graphene. Finally, electrical characterizations of functionalized graphene, including sheet resistance, carrier mobility, and Hall resistance were performed across a wide temperature range to investigate doping effects. In particular the potential of plasma functionalization for the work function engineering and band-gap opening of graphene was demonstrated.
Fig. 3. Evolution of the Raman spectra of monolayer graphene at different hydrogenation times. The pristine graphene spectrum shows the G band around 1580 cm-1 and the 2D band around 2700 cm-1. After hydrogen plasma exposure, the D peak (corresponding to defect activated bands) appears around 1350cm-1. Acknowledgement The authors acknowledge funding from the National Laboratory Sens&Micro LAB Project (POFESR 2007– 2013, code number 15) funded by Apulia Region, and from the European Community's 7th Framework Programme under grant agreement no. 314578 MEM4WIN (www.mem4win.org).
Fig. 2. Evolution of the spectra of the pseudoextinction coefficient of the pristine graphene on corning at different plasma-hydrogenation times. At higher energy (> 4.6 eV), the spectra are dominated by a broad band arising from interband transition in graphene. For increasing plasma-treatment times, these band decreases in intensity and shifts to higher energy, thus providing a measure of the hydrogenation level.
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References [1] G.V. Bianco, et al. Phys. Chem. Chem. Phys., 16, 3632 (2014) [2] G. Bruno, G.V. Bianco, et al. Phys. Chem. Chem. Phys., 16, 13948 (2014) [3] Z. Sun, et al. Nature Comm., 2, 559, (2011) [4] J.T. Robinson, et al. Nano Lett., 10, 3001 (2010) [5] R. Balog, et al. Nature Mat., 9,315 (2010) [6] B. Li, et al. ACS Nano, 5, 5957 (2011) [7] L.G. Cancado, et al. Nano Lett., 11, 3190 (2011)
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