RAMAN SPECTROSCOPY continued
Figure 3 – a) White light image of graphene flakes dispersed on a 1 cm × 1 cm wafer. b) Corresponding composite Streamline Raman image high- lighting single-layer, bi-layer, and multilayer graphene flakes.
Figure 2 – Raman spectra illustrating the 2-D band of graphene. These spectra were collected from mechanically exfoliated graphene flakes con- sisting of: a) a single layer of graphene, b) a bi-layer of graphene, and c) multiple layers of graphene. With increasing layer number, the 2-D band broadens and changes in shape.
power to be used without causing any laser-induced modification of the sample. Because the Raman signal is proportional to the laser power used, measurement times are decreased by a factor of 20.
Conventional Raman mapping techniques superimpose an array of points on the sample area and take measurements at each point. This means that if the points do not overlap, areas of the sample are not measured. StreamLine can be operated in a mode called Slalom. In this mode, the laser line is zigzagged over the sample, ensuring that every part of the sample is measured and even small features contribute to a Raman spectrum. This makes StreamLine well-suited for conducting low spatial resolution survey work on samples of this type.
Raman spectra were collected from mechanically exfoliated graphene flakes dispersed on a ~1 cm × 1 cm SiO2
/Si substrate. Because this area
is much larger than the field of view of a 5× objective lens, a composite image or montage was created by moving the sample underneath the microscope and stitching the images together. WiRE spectrometer soft- ware (Renishaw) can be used to generate these images and conduct the stitching. The image, shown in Figure 3a, was used to define the Raman collection area. A StreamLine image was collected over a 10,490 µm × 10,724 µm area using a 20× objective lens and a 532-nm laser excitation source. In less than 90 min, 74,520 spectra were collected.
The data set was analyzed using a statistical technique—direct classical least squares (DCLS). The spectra of single-layer, bi-layer, and multilayer graphene shown in Figure 2 were used as references. This enabled each spectrum in the Raman data set to be identified as single-layer, bi-layer, or multilayer graphene. Figure 3b is a Raman image illustrating the dis- tribution of graphene flakes of different thickness on the silicon wafer. Here it can be seen that the majority of the graphene flakes consist of
multilayer graphene. Using particle statistics, it was possible to calcu- late the relative percentage of flakes that are single-layer, bi-layer, and multilayer as 2.2%, 5.7%, and 92.1%, respectively. It is also possible to calculate the average area of the single-layer and bi-layer flakes as 3944 µm2
and 4036 µm2 .
Mechanical exfoliation is a common technique for making graphene flakes because of its low cost and ease of use. This technique may pro- duce only a small percentage of graphene suitable for materials science research or for making test devices (single-layer and bi-layer graphene flakes). StreamLine Raman imaging has been demonstrated to be a valu- able technique for surveying large areas of graphene flakes and locating these single-layer and bi-layer flakes. Since StreamLine can be applied to graphene made using any technique, it is an effective tool for graphene research. The technique has been also used to investigate the growth
Figure 4 – a) White light image of graphene flake; b) Raman image illus- trating areas of single-layer and bi-layer graphene in green and white, respectively; c) Raman image illustrating change in graphene G-band position, which is an indicator of stress and doping.
AMERICAN LABORATORY • 16 • SEPTEMBER 2013
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