Graphene comprises a monolayer of carbon atoms packed into a two-dimensional (2D) honeycomb lattice. It possesses many unique properties that make it an ideal material for fundamental studies as well as potential applications. Today, graphene serves as a testbed for examining core quantum mechanics principles and forms the basis for the development of fundamentally new functional devices that are structurally smaller than those relying on conventional metals and semiconductors.
What kind of uses does graphene have?
- Graphene-based transistors can operate at higher frequencies and more efficiently than silicon transistors.
- Gas sensors that are sensitive to a single atom or molecule.
- Support membranes for transmission electron microscopy.
- Inert coatings providing objects with an atomically thin protective layer that offers protection against many powerful acids and alkalis.
Several approaches have been successfully developed to fabricate graphene. The most common method involves forming graphene on a silicon substrate randomly. When the substrate has a certain amount of SiO₂ film on it, graphene flakes can be detected using optical microscopy. However, it is challenging to determine exactly how many layers each graphene flake consists of.
A quick and precise method for determining the number of layers of graphene flakes is essential for accelerating research and exploration in the field of graphene. Although atomic force microscopy (AFM) is the most direct way to identify the number of graphene layers, this method has a very low throughput. In contrast, Raman spectroscopy has proven to be the most efficient technique for identifying different layers of graphene without damaging the crystal lattice.
Raman Mapping of Graphene
With Raman spectroscopy, the number of graphene layers can be distinguished by examining the intensity ratio of the G-band and the 2D-band — the two well-characterized and understood peaks in the Raman spectra of graphene.
Mapping a substrate with formed graphene allows for the visualization of the substrate’s surface based on the intensities of the graphene Raman lines.
In every part of the world, scientists are experimenting with new ways of fabricating graphene. In many cases, Raman spectroscopy proves to be the only reliable tool to validate the efficiency of a new method. The spectrum of graphene obtained through natural graphite exfoliation is shown in Figure 1. The spectrum has distinct peaks (G and 2D bands) at 1580 cm−1^{-1}−1 and 2680 cm−1^{-1}−1. The spectrum obtained from a film of nanocrystalline graphite deposited by the plasma-enhanced chemical vapor deposition (PECVD) method is shown in Figure 2.
Figure 3 illustrates the spectra of diamond-like films deposited with ions of different energies, which affect the surface. Another series of experiments where detecting graphene is crucial is the research of graphene’s electrical conductivity. As shown in Picture 2, contacts to graphene are formed in a Hall bar geometry.
Picture 2
Raman Microscope RamMics M532® by Enhanced Spectrometry is a truly indispensable tool for various applications and researches. The spatial resolution of 1 µm, spectral resolution 4-6 cm-1, and high efficiency provide precise quality of measurements at shorter acquisition time. Optionally equipped with a motorized sample stage with adjustable step (from 0.36 µm) RamMics M532® allows mapping of large surfaces at the instant quality. RamMics M532® efficiently works on low laser power (tunable) which secures avoiding damaging samples.
Benefits of RamMics M532:
- Focusing: sample positioning and focusing can be done with a digital camera
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- Safety: no damage to the sample due to low laser power