A Confocal Raman-AFM Study of Graphene
U. Schmidt,* T. Dieing, W. Ibach, and O. Hollricher WITec GmbH, Lise-Meitner Str. 6, 89081 Ulm, Germany *
ute.schmidt@
witec.de
Introduction Te discovery by Novoselov and Geim [1] of a simple
method to transfer a single atomic layer of carbon from the c-face of graphite to a substrate suitable for measurements of its electrical and optical properties has led to an increased interest in studying and employing two-dimensional model systems. An overview of electron and phonon properties of graphene and their relationship to the one-dimensional form of carbon known as nanotubes can be found in [2]. Te unique chemical, mechanical, electrical, and optical properties of graphene lead to its many application possibilities such as: single molecule detectors, high-strength low-weight new materials, design of new semiconductor devices, etc. An important goal however is the detection of such
angstrom-thick two-dimensional sheets and precisely determining the number of layers forming the graphene flake. Te aim of this contribution is to show how a confocal Raman AFM can contribute to the characterization of such small materials and devices. In the past two decades, AFM (atomic force microscopy) was one of the main techniques used to characterize the morphology of nano-materials spread on nanometer-flat substrates. From such images it is possible to gain information about the physical dimensions of the material on the nanometer scale, without additional information about their chemical composition, crystallinity, or stress state. On the other hand, Raman spectroscopy is known to be used for unequivocally determining the
chemical composition of a
material. By combining chemically sensitive Raman spectroscopy with high-resolution confocal optical microscopy, the analyzed material volume can be reduced below 0.02 µm3, leading to the ability to acquire Raman images with diffraction-limited resolution from very flat surfaces [3, 4]. Using the combination of confocal Raman microscopy with AFM, the high spatial and topographical resolution obtained with an AFM can be directly linked to the chemical information provided by confocal Raman spectroscopy [5].
Experimental Te confocal Raman AFM alpha300 RA (
www.witec.de)
was used for AFM imaging in ambient conditions (24 ± 2°C). For high-resolution topographic imaging, the AFM was operated in AC-Mode, also known as Tapping Mode. In this imaging mode the cantilever is oscillated at its resonance frequency with a free amplitude A0. While the cantilever is approaching the surface, the oscillating amplitude is reduced to a value A, which depends on the distance to the surface. Te ratio r = A/A0 defines the damping of the amplitude while the tip is in contact with the surface and is proportional to the applied force. By keeping the damping of the amplitude constant, the surface topography can be mapped. Te AFM images were acquired with Arrow Force Modulation cantilevers from NanoWorld (
www.nanoworld. com). Te nominal spring constant of these cantilevers was 2.8 N/m, and the resonance frequency was 70–80 kHz. For confocal Raman measurements, the alpha300 RA was equipped with a 100× (NA = 0.90) air objective and a
30
frequency-doubled Nd:YAG laser (excitation at 532 nm). Te same graphene flake previously imaged with AFM was analyzed in Raman imaging mode. In this mode, Raman images were obtained by collecting a complete Raman spectrum at every image pixel with typical integration times below 50 ms/pixel. Spectral features (sum, peak position, peak width, etc.) were used to generate the Raman images.
Results and Discussion An exfoliated graphene sheet deposited on the SiO2 top layer
of a Si wafer by a method described in reference 1 was examined using confocal Raman AFM. Figure 1a shows the topography of the graphene flake recorded in AFM-AC Mode. Te cross section 1 highlighted in Figure 1a shows the topography varia- tion over a bi-, mono-, and no-graphene layer. Tis cross section is presented in Figure 2 (top). Te height difference between the SiO2/Si wafer and the first graphene layer is 0.8 ± 0.2 nm, whereas the double-layer of graphene is 1.2 ± 0.2 nm high. Tese
Figure 1: Atomic force microscope (AFM) topography image of a graphene flake (a) and Raman image of the same graphene flake showing the integrated intensity of the G-band (b).
doi:10.1017/S1551929511001192
www.microscopy-today.com • 2011 November
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