Raman Spectrum of Graphene Coated Nano-Holes
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Raman Spectrum of Graphene Coated Nano-Holes Amrita Banerjee, Ruiqiong Li, and Haim Grebel Electronic Imaging Center, New Jersey Institute of Technology, 161 Warren Street, Newark, NJ, 07102 ABSTRACT It was assumed in the past that aluminum is not a platform of choice for surface enhanced Raman scattering (SERS) experiments: this was in spite of its large negative permittivity value, larger than gold or silver at optical wavelength. It was also assumed that any oxide on top of metallic platform significantly hinders SERS signals. In addition, graphene has not been studied on perforated substrates. Here we use periodically perforated and oxidized aluminum surfaces to examine electrical and optical properties of multi layered graphene. Linear and nonlinear optical methods were used to characterize single and multi layered structures.
INTRODUCTION Graphene is a monolayer of carbon atoms with a two-dimensional honeycomb structure. It is thermodynamically unstable in a free-standing form and therefore, it was assumed in the past that graphene cannot exist [1]. Recently though, single and few layer graphene were successfully made into films on solid substrates [2, 3, 4]. At the same time, anodized aluminum oxide (AAO) gained interest for its organized nano hole structure [5]. One may attempt to lay graphene on such unique and perforated substrates and study them. Mono-layered and multilayered graphenes were assessed by its distinct Raman signature [8]. Raman spectroscopy is a widely used tool to study the vibration states of molecules. Surface enhanced Raman scattering (SERS) is a modified version of Raman spectroscopy. Periodic structures are sometime used to couple the pumping laser light to surface charge waves (surface plasmons) [6] at resonance condition. Such resonance conditions were utilized in the past to analyze Raman spectra of carbon nanotubes and bio-species [7]. We employ these resonance conditions to analyze Raman spectra of graphene as well.
EXPERIMENT The substrate was prepared according to a well-established process. High purity aluminum (99.999 + %) from Sigma-Aldrich was ultrasonically cleaned in de-ionized (DI) water for 30 min, then it was degreased in acetone for approximately 24 h. After that the sample was rinsed in ethanol followed by an electro-polishing in a mixture of ethanol, perchloric acid (HClO4) and DI water at 4 oC and 50 V to decrease surface roughness. Then, the anodic oxide layer was remove by using a mixture of phosphoric acid (6 wt %) and chromic acid (1.8 wt %) at 65 oC for 3 h. The aluminum sample was again anodized for 5 min under the same condition as before. The highly ordered pyrolitic graphite material, deposited on AAO samples, was purchased from SPI Supplies. The bulk graphite was deposited on AAO by exfoliation. Several
layers of graphene were deposited by use of this process. High temperature annealing at 850 oC was used to controllably thin and smooth the graphene layers. Argon (Ar) ion laser at 514.5 nm was used to pump the sample for the Raman spectro
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