Spectroscopic Raman Nanometrology of Graphene and Graphene Multilayers on Arbitrary Substrates

Raman spectroscopy is known to be an effective tool for characterization of graphene and graphene multilayers on the standard Si/SiO2 (300 nm) substrates, which allows one to determine non-destructively the number of the graphene layers and assess their quality. The Raman phonon peaks undergo modification when graphene is placed on other substrates due to changes in the nature and density of the defects, surface charges and different strength of the graphene – substrate bonding. We show that despite the spectrum variations the deconvolution of the double-resonant 2D band allows one to identify the number of graphene layers even on amorphous glass substrates. The results extend the application of Raman spectroscopy as nanometrology tool for graphene and graphene-based devices. 1. Raman spectroscopy as nanometrology tool for graphene layers The unique properties of graphene have recently attracted major attention of the physics and device research communities [1-2]. Raman spectroscopy has been successfully utilized as a convenient technique for identifying and counting graphene layers on the Si/SiO2 substrates [3-7]. It was shown that the evolution of the 2D-band Raman signatures with the addition of each extra layer of graphene can be used to accurately count the number of layers [3] together with the position of G peak, which up-shifts with the increasing 1/n, where n is the number of graphene layers [4]. Most of the Raman spectroscopy studies of graphene were limited to graphene layers on Si/SiO2 substrates with a carefully selected thickness W of the SiO2 layer (W•300 nm). The latter is explained by the fact that graphene becomes visible in an optical microscope when placed on top of Si wafer with the 300-nm thick oxide layer due to the light interference. At the same time, it is not obvious that the features of the 2D band and G-peak position will be preserved when graphene is transferred to another substrate due to possible changes in the nature and density of the defects, surface charges and different strength of graphene – substrate bonding. Here we demonstrate that the sensitivity of the 2D band to the number of graphene layers allows one to count the atomic layers when graphene is transferred to different substrates. As an example, we selected an “inconvenient” substrate of a very different type such as glass. The single-layer graphene 4 To whom any correspondence should be addressed. International Symposium on Advanced Nanodevices and Nanotechnology IOP Publishing Journal of Physics: Conference Series 109 (2008) 012008 doi:10.1088/1742-6596/109/1/012008 c © 2008 IOP Publishing Ltd 1 (SLG) and few-layer graphene (FLG) were obtained by micro-mechanical cleavage of bulk graphite. The same procedure was used to place graphene on reference Si/SiO2 (300 nm) and glass substrates. The glass substrates were selected as representatives of the unconventional amorphous substrates characterized by many defects. The number of layers was determined from the visual inspection of graphene on the standard Si/SiO2 substrate (see Figure 1 (a)) and atomic force microscopy (AFM) of the flakes placed on glass. The optical imaging and AFM were followed by Raman spectroscopy. The procedure of separating SLG and FLG on Si/SiO2 substrates from graphitic flakes is well established [3-6] and relies on differences in the G-peak and 2D-band features (see Figure 1 (b-c)). The AFM inspection of graphene on glass revealed flakes with the thickness below 2 nm, which indicates n<4. This information was used in the analyses of 2D band of FLG on glass substrates (see Figure 1 (d)). Figure 1: (a) Optical microscopy image of graphene flake on Si/SiO2 substrate; (b) Raman spectrum of the bulk graphite used to produce graphene and graphene multilayers; (c) Raman spectrum of the few-layer graphene on Si/SiO2 substrate; (d) atomic force microscopy image of the few-layer graphene (number of layers n<4) on the glass amorphous substrate. 120