Thinning and functionalization of few-layer graphene sheets by CF4 plasma treatment
© Shen et al.; licensee Springer. 2012
Received: 27 February 2012
Accepted: 25 April 2012
Published: 24 May 2012
Structural changes of few-layer graphene sheets induced by CF4 plasma treatment are studied by optical microscopy and Raman spectroscopy, together with theoretical simulation. Experimental results suggest a thickness reduction of few-layer graphene sheets subjected to prolonged CF4 plasma treatment while plasma treatment with short time only leads to fluorine functionalization on the surface layer by formation of covalent bonds. Raman spectra reveal an increase in disorder by physical disruption of the graphene lattice as well as functionalization during the plasma treatment. The F/CF3 adsorption and the lattice distortion produced are proved by theoretical simulation using density functional theory, which also predicts p-type doping and Dirac cone splitting in CF4 plasma-treated graphene sheets that may have potential in future graphene-based micro/nanodevices.
81.05.ue; 73.22.Pr; 52.40.Hf.
KeywordsGraphene sheets Plasma Thinning Functionalization Raman spectroscopy Density functional theory Dirac cone splitting
Graphene is one layer of C atoms, arranged in a hexagonal lattice [1–3]. Since it was first produced by mechanical exfoliation in 2004 , graphene has been studied both theoretically and experimentally, and demonstrates highly attractive properties [5–10]. Especially, the Dirac equation predicts that unique electronic properties should arise from the hexagonal honeycomb lattice structure, making the electrons behave as massless relativistic fermions [1, 5, 9]. The corresponding high mobility and velocity have great potential in future electronics [11, 12]. However, several problems need to be solved before it can be ultimately employed in practical applications. For instance, the zero bandgap as well as bad wettability of graphene might cause problems in device fabrication [13–15]. Under such circumstances, plasma treatment is considered to be one of the tricks to overcome the difficulties. The corresponding surface functionalization changes not only the surface status, but also the structure of graphene sheets [16, 17]. Previous researches have already demonstrated that oxygen plasma has the ability to tune the properties of graphene sheets [14, 17]. On the other hand, fluorine plasma, which may provide additional advantages [16, 18], is seldom experimentally investigated in detail, although a strong p-doping behavior was predicted [19, 20]. In this work, structural changes of few-layer graphene sheets induced by CF4 plasma treatment are studied by optical microscopy and Raman spectroscopy. Our results suggest an obvious thickness reduction effect in few-layer graphene sheets treated with CF4 plasma as well as surface fluorine functionalization by formation of covalent bonds between the top graphene layer and the ions. The produced disorder in graphene lattice is well reflected in the Raman spectra, and the corresponding mechanism is studied theoretically. The results presented in this work provide a possible direction to obtain giant single-layer graphene sheets with necessary surface functionalization to realize the p-type doping level and to open the Dirac cone for future graphene-based micro/nanodevices.
The few-layer graphene sheets used in the current study were produced by mechanical exfoliation from highly ordered pyrolytic graphite and transferred on a silicon substrate covered with 300-nm SiO2. The samples were then cleaned by ultrasonication in acetone for 30 s to remove the residual of the scotch tape and unattached graphite pieces. The CF4 plasma treatment was carried out in the reaction chamber of a Jupiter III reactive-ion etching setup. The samples were exposed to 0.8-Torr CF4 under radio-frequency plasma (20 W) with different times ranging from 1 to 5 s with a step of 1 s. In our experiment, the number of layers in the few-layer graphene sheets can be estimated by a combination of optical microscopy and Raman spectroscopy. Raman spectra were taken on a Renishaw inVia micro-Raman spectrometer with the 514-nm line of an Ar+ laser as light source (Shanghai, China). All the measurements were carried out at room temperature.
Results and discussion
The evolution of the D band is the most important concern of many researches. The increase of its intensity is generally considered to be evidence that lattice disorder exists in the graphene sheet . An interesting phenomenon arises when we look close to the intensity of the D band in a few-layer graphene sheet subjected to CF4 plasma for different times. The relative intensity of the D band (ID/IG) in the sample treated for 5 s is smaller than that for 2 s, implying that the disorder is even remarkable for shorter treatment. To understand this peculiar behavior, we must first make clear the mechanism of disorder production during plasma treatment. The disorder and the corresponding emergence of D and D' bands in our samples may arise from two processes: (1) physically, the order of pristine C atoms in graphene is disrupted by the CF4 plasma and some of the C atoms may be sputtered out, which is named as ion bombardment effect [29, 30]; and (2) chemically, covalent bonds of fluorine-related species to the graphene lattice form during plasma treatment, leading to corresponding surface modification and functionalization. Both processes contribute to the increase of the disorder. As for the chemical process, the bond energies need to be considered. It was disclosed that the C-C bond in graphene owns higher bond energy (607 kJ/mol) than the C-F bond (485 kJ/mol). Thus, the C-C bond can be hardly broken, and hence CFn (n = 1 to 3) or F species were only adsorbed on the top graphene layer by formation of covalent bonds, which will become saturated because the number of available active C atoms decreases with time. It is worth noting that the formation of covalent bonds can be evidenced by the upshift of 2D peak from 2,685 to 2,691 cm−1. One may infer that the few-layer graphene sheet subjected to CF4 plasma treatment therefore possesses disorder features from both the physical and chemical interactions. The chemical interaction takes place only on the topmost layer, while the physical interaction accumulated with time and a long treatment can remove the top layer (i.e., thinning effect, as is reflected in the optical microscopy and Raman spectroscopy), exposing the beneath layer. The emergence and intensification of the D and D' bands in the sample subjected to 5 s of treatment thus originates from the disorder created in this new top layer and should increase gradually. Consequently, the disorder probed by Raman spectroscopy is even smaller in the sample treated by CF4 plasma for 5 s than in the sample treated for 2 s (see Figure 2a,b).
In conclusion, few-layer graphene sheets were prepared by mechanical exfoliation and the structural evolution during CF4 plasma treatment was studied in detail by optical microscopy and Raman spectroscopy. The experimental results indicate a thickness reduction under prolonged plasma treatment while short treatment leads only to fluorine functionalization on the surface layer. The combination of both physical and chemical reactions in the plasma treatment leads to structural modifications which can be well probed in Raman spectra. Theoretical simulation suggests a F/CF3 functionalization by formation of covalent bonds and also predicts a corresponding p-type doping and Dirac cone opening after the F/CF3 adsorption. Although further characterizations are needed to evaluate the electronic properties of treated samples, the current work of thinning and functionalizing few-layer graphene sheets by CF4 plasma under control represents an integrative pathway to industrial fabrication of graphene-based micro/nanodevices.
This work is supported by the Natural Science Foundation of China (Nos. 61008029 and 51102049), Program for New Century Excellent Talents in University (No. NCET-10-0345), and Shanghai Pujiang Program (No. 11PJ1400900).
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