Layer-dependent morphologies of silver on n-layer graphene
© Huang et al.; licensee Springer. 2012
Received: 11 September 2012
Accepted: 27 October 2012
Published: 9 November 2012
The distributions of sizes of silver nanoparticles that were deposited on monolayer, bilayer, and trilayer graphene films were observed. Deposition was carried out by thermal evaporation and the graphene films, placed on SiO2/Si substrates, were obtained by the mechanical splitting of graphite. Before the deposition, optical microscopy and Raman spectroscopy were utilized to identify the number of the graphene layers. After the deposition, scanning electron microscopy was used to observe the morphologies of the particles. Systematic analysis revealed that the average sizes of the nanoparticles increased with the number of graphene layers. The density of nanoparticles decreased as the number of graphene layers increased, revealing a large variation in the surface diffusion strength of nanoparticles on the different substrates. The mechanisms of formation of these layer-dependent morphologies of silver on n-layer graphene are related to the surface free energy and surface diffusion of the n-layer graphene. The effect of the substrate such as SiO2/Si was investigated by fabricating suspended graphene, and the size and density were similar to those of supported graphene. Based on a comparison of the results, the different morphologies of the silver nanoparticles on different graphene layers were theorized to be caused only by the variation of the diffusion barriers with the number of layers of graphene.
KeywordsGraphene Nanoparticle growth mechanisms Diffusion difference barriers 68.65.Pq (graphene films) 68.70.+w (whiskers and dendrites) 78.67.Wj (optical properties of graphene)
A single atomic layer of graphene is the thinnest sp2 allotrope of carbon. It, therefore, has various unique electrical and optical properties of interest to scientists and technologists[1–3]. Graphene samples are widely fabricated by the micromechanical cleavage of highly oriented pyrolytic graphite (HOPG) with scotch tape. Layers of oxides such as SiO2 and Al2O3 with special thickness between graphene and the substrate are typically used to make graphene optically visible[4–7]. The effect of the substrate on Raman measurements has been widely investigated. Raman and surface-enhanced Raman spectroscopy have been widely utilized to elucidate the vibration properties of materials[9–14]. Recently, they have been used as powerful techniques for characterizing the phonons of graphene[15–20]. The profile and peak position of the Raman second-order (2D) band can be used to determine the number of graphene layers[21, 22].
In this work, layers of graphene were fabricated in a sample by micromechanical cleavage. The number of layers of graphene was determined by micro-Raman spectroscopy and optical microscopy. After silver nanoparticles were deposited on the sample using a thermal deposition system, the distribution and sizes of the particles on flakes with different numbers of layers were systematically analyzed. To analyze the effect of the substrate, suspended graphene was fabricated, and the size and density thereof were found to be similar to those of supported graphene. The different results for the mono-, bi-, and tri-layer graphene are theorized to be caused only by the variation among the diffusion barriers of the various graphene layers, which provides a method of determining the number of graphene layers and provides information that can be utilized to elucidate the interaction between a graphene flake and its substrate.
Results and discussion
To investigate the density and size of the nanoparticles, average size and density were determined by histogram analysis. The histograms, on the left-hand sides of Figure2a,b,c,d, demonstrate the distributions of nanoparticles on the SiO2/Si substrate, monolayer, bilayer, and trilayer graphene. The sizes of the nanoparticles on the monolayer graphene flake are distributed in the range of 0 to 50 nm, whereas those of the nanoparticles on the trilayer graphene flake were distributed in the range of 10 to 70 nm. Whereas the sizes of the nanoparticles on the SiO2/Si substrate were distributed in the range of 0 to 50 nm, the majority of them were in the range of 10 to 30 nm.
In this work, the distribution of sizes of silver nanoparticles that were deposited on graphene films with different numbers of atomic layers of graphene was investigated. A systematic analysis revealed that the average size of the nanoparticles increased, and the area density and the difference between diffusion barriers of the nanoparticles decreased as the number of graphene layers increased. To analyze the effect of a substrate such as SiO2, suspended graphene was also fabricated. The size and density of suspended graphene were found to be similar to those of the supported graphene. According to these results, only variations in the interactions between n-layer graphene and the silver nanoparticles were responsible for the variation in their distribution.
C-WH received his BS degree in Electrical Engineering from the National University of Kaohsiung, Kaohsiung, Taiwan, in 2008. He studied his MS degree in 2008 and Ph.D. degree directly in 2009. Currently, he is a Ph.D. candidate in the Department of Photonics, National Cheng Kung University, Tainan, Taiwan. He focuses on the property of graphene and surface plasmon resonance of nanoparticles. H-YL received her doctoral degree from the Institute of Biomedical Engineering, National Cheng Kung University in 2010. Her current research interest focuses on the SERS properties of graphene and possible sensing application. C-HH received his doctoral degree from the Institute of Electro-Optical Science and Engineering, National Cheng Kung University in 2009. His current research interest focuses on the Raman and SERS properties of graphene incorporating with metal nanoparticles.
We wish to acknowledge the support of this work from the National Science Council, Taiwan under contact numbers NSC 98-2112-M-006-004-MY3 and NSC100-2622-E-006-039-CC3. Ted Knoy is appreciated for his editorial assistance.
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