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ARTICLE Scanning Tunneling Microscopy and X-Ray Photoelectron Spectroscopy Studies of Graphene Films Prepared by Sonication-Assisted Dispersion ^ ) Elena Y

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ARTICLE Scanning Tunneling Microscopy and X-Ray Photoelectron Spectroscopy Studies of Graphene Films Prepared by Sonication-Assisted Dispersion ^ ) Elena Y ARTICLE Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy Studies of Graphene Films Prepared by Sonication-Assisted Dispersion ^ ) Elena Y. Polyakova (Stolyarova),†,* Kwang Taeg Rim,‡ Daejin Eom,§ Keith Douglass, Robert L. Opila, Tony F. Heinz,§ Andrew V. Teplyakov,^ and George W. Flynn‡ †Graphene Laboratories Inc, Reading, Massachusetts 01867, United States, ‡Department of Chemistry and Nanoscale Science and Engineering Center, Columbia University, 3000 Broadway, MC 3109 New York, New York 10027, United States, §Department of Physics and Nanoscale Science and Engineering Center, Columbia University, 3000 Broadway, MC 3109 New York, New York 10027, United States, ^Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States, and Department) of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States raphene, a two-dimensional carbon crystal, is a new addition to the family ABSTRACT We describe scanning tunneling microscopy and X-ray photoelectron spectroscopy G 1,2 fi of nanoscale carbon materials. It studies of graphene lms produced by sonication-assisted dispersion. Defects in these samples are À has a unique set of physical properties3 5 not randomly distributed, and the graphene films exhibit a “patchwork” structure where and is being considered for potential use in unperturbed graphene areas are adjacent to heavily functionalized ones. Adjacent graphene layers many practical applications such as gra- are likely in poor mechanical contact due to adventitious species trapped between the carbon sheets phene-based electronics,1 optical transis- of the sample. tors,6 liquid crystal7 and electromechanical 8 9 10 devices, chemical and biological sensors, KEYWORDS: . 11,12 graphene sonication-assisted dispersion scanning tunneling solar batteries, micro-electro-mechanical microscopy . X-ray photoelectron spectroscopy systems (MEMS) and nano-electro-mechan- ical systems (NEMS),13 and energy storage,14 to name a few. on transition metal surfaces20,22,28 at elevated Repeatable and reliable production of temperatures. Second, graphene can be pro- nanomaterials is a well-recognized techno- duced by chemical splitting of graphite, an logical challenge. For this reason, large- abundantly available material.15,29À31 Initi- scale production of high-quality graphene ally, a graphite crystal is oxidized and split represents a critical step for commercializa- into soluble graphene oxide (GO).32 GO can 15 tion of this novel material. In most experi- be either restacked as a durable, flexible, ments reported so far, graphene has been nonconducting transparent GO paper11,33 16À18 produced by mechanical exfoliation. or reduced back to conductive graphene.31,34 fi This method produces graphene lms of Such chemically converted graphene sheets 19 excellent crystalline quality, but the yield can be deposited on virtually any substrate of thin graphene sheets is extremely low, and processed by standard nanofabrication and the technique cannot be adapted for techniques. Several variations of this tech- industrial use. nique have been reported recently by multi- An active search for alternative methods ple research groups.35À37 Unfortunately, the of graphene production is underway. Very observed resistivity of reduced RO filmsistoo recently, macroscopic scale, high-quality gra- high for many practical applications. fi phene lms have been made through sev- Alternatively, graphite can be split into eral alternative, scalable, and cost-effective thin graphene pallets in the liquid phase by * Address correspondence to [email protected]. methods. First, graphene layers can be sonication of graphite crystals in organic 7 grown on top of a metal and later transferred solvent. Such sonication-assisted disper- Received for review April 29, 2010 20À22 to the desired substrate. This method sion methods are inexpensive, straightfor- and accepted July 1, 2011. relies on thermally induced epitaxial growth ward, and, as a result, extremely attractive Published online July 01, 2011 23À27 of graphene on a SiC surface or chemi- for industrial production of graphene coat- 10.1021/nn1009352 cal vapor deposition (CVD) growth of gra- ings. However, sonication is a relatively phene by decomposition of hydrocarbons harsh process that might cause high local C 2011 American Chemical Society POLYAKOVA ET AL.VOL.5’ NO. 8 ’ 6102–6108 ’ 2011 6102 www.acsnano.org ARTICLE Figure 1. Typical AFM image of a continuous graphitic conductive film prepared by sonication-assisted dispersion. fi Individual graphene flakes overlap. The image size is Figure 2. Raman spectrum of a graphene lm prepared by 1.8 μm  1.8 μm. sonication-assisted dispersion. The inset shows second- order Raman peaks. The doublet structure of the 2D peak is missing, indicating turbostratisity of the film. temperatures and pressures with resulting dissociation of molecules in solution.38 The conductivity of gra- in various carbon materials. The Raman spectrum of phene has been found to decrease upon sonication the sample studied here is shown in Figure 2 and treatment, presumably due to the introduction of provides a signal averaged over an area of ∼1 μm2. defects in the sample, even though no direct chemical A 20% variation in the relative intensities of spectral lines treatment is involved in this preparative method.7 has been observed for Raman spectra collected over Moreover, in-plane functionalization is critical for ef- randomly selected areas of our sample. Raman spectra fective suspension of graphene films in solution, as routinely collected on a single layer graphene flake unfunctionalized graphene platelets tend to agglom- have only two prominent features: a first-order G band erate, forming graphitic slurries.39 To date, there is only near 1581 cmÀ1 and a 2D line near 2680 cmÀ1. The limited understanding of the nature and origin of the spectrum shown in Figure 2, on the other hand, has a disorder introduced in graphene films by sonication. rich structure typical of disordered carbon material in The present work provides visualization of the atomic which the sp2 character of local carbon bonding is structure as well as a quantitative XPS analysis of the partially lost.45,46 chemical composition of ultrathin graphene films pre- In particular, the D0 band appears on the blue side of pared by sonication-assisted dispersion. the G band, and several second-order bands, such as The samples for this study were supplied by the 2D, DþD0, and 2D0, are observed. (See inset, Figure 2.) Manchester group, and the procedure used for pre- The intensity of these second-order bands is substan- paration of ultrathin graphene films by sonication- tially lower than that of the first-order ones, as would assisted dispersion has been described in a previous be expected for disordered sp2 carbon films.45,47 All publication by this group.7 A brief summary of sample spectral lines have contributions from several over- preparation is given in the Materials and Experimental lapping flakes and are significantly broadened when Methods section. compared to their counterparts recorded on mechani- A typical atomic force microscopy (AFM) image of cally exfoliated graphene and graphite.48 However, the such a film is shown in Figure 1. Individual flakes with observed Raman lines are sharper than the identical surface areas in the range 0.001À0.1 μm2 can be ones in samples of heavily functionalized films such as observed. AFM investigation shows that during spray graphene oxide and reduced graphene oxide.34,44,47 deposition graphene sheets self-assemble in a contin- Raman spectra similar to those reported here have uous thin film with root mean square (rms) size Sq (nm) been observed for hydrogenated graphene49,50 and = 6.74 ( 0.98 . The vast majority of the graphene flakes graphite intercalation compounds in which guest spe- comprising the film have their basal planes aligned cies are found between most of the graphene planes.51 with the substrate surface. AFM images show no All these observations suggest that local disorder has wrinkles or out-of-plane edges. A similar process has been introduced into the graphene planes as a result of been previously utilized to prepare graphene oxide sonication. membranes40 and paper.41 Clustering of this kind is The shape of the 2D peak is also known to be a also exhibited by carbon nanotubes that tend to form sensitive probe of the stacking disorder in graphitic bundles due to strong van der Waals attraction be- materials.52 This peak forms as a result of a two-photon tween the walls of neighboring tubes.42 resonant process and has a sharp single peak for Raman microscopy provides a fast, nondestructive single-layer graphene. In multilayer graphene, due to way to measure graphene thickness,43 as well as to splitting of the electronic band structure, the 2D lines monitor chemical changes44 and structural damage45 take on a complex shape that evolves as a function of POLYAKOVA ET AL.VOL.5’ NO. 8 ’ 6102–6108 ’ 2011 6103 www.acsnano.org ARTICLE Figure 3. (a) 3D STM topographic image of a 20 nm  20 nm area for the graphene film. Note the strong local bending of graphene. Arrows point at the center of the enlarged image shown in parts b and c. (b) 3D STM image of 10 nm  10 nm area for the film from the center of part a. A symmetrical hexagonal pattern is resolved in the lower left corner, while other areas do not show any resolved atomic scale structure. (c) STM image of superstructure near an isolated defect in the lower left corner of part a, for a sample prepared by sonication-assisted dispersion. (d) High-resolution STM image of a border between “perfect” and “functionalized” regions of a graphene film prepared by sonication-assisted dispersion. the number of layers.45 In a graphite crystal with an found between STM images recorded in these widely unperturbed ABAB stacking sequence along the separated areas. A typical topographic STM image on c-direction of the bulk material, the 2D line has a the 20 nm  20 nm scale (Figure 3a) reveals strong characteristic doublet shape.43 The doublet structure local buckling.
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