Let’s Look at Graphene

Leaders in industry and academia, including Samsung, Ford, IBM, Harvard, and MIT consider graphene as a choice material for the future of electronics, energy storage, drug delivery, and composite materials. Can graphene live up to this promise? How can you measure for quality as a raw material, and in finished goods? One analytical technique has been proven to be useful in characterizing graphene.

Graphene

Properties of graphene

Graphene is:

Electrically conductive
Thermally conductive
Mechanically strong
Contains no metals

This material is a single-atomic-layer of carbon atoms arranged in a hexagonal lattice. However, quality varies considerably based on how the material is produced. Manufacturers looking to scale-up and functionalize graphene production need rapid, reliable feedback on structure and quality to assure product quality. And those companies using graphene as a source material also need tools to assess the quality built into their products.

Products currently under development

Products currently under development and marketing that incorporate graphene include wearable electronics, RFID tags, conductive inks, antennas, composites and energy storage. Many more applications are currently undergoing research and development by public and private institutions.

Graphene field effect transistors

Once considered an immediate replacement for silicon-based digital electronics, graphene has so far been disappointing in this field. To be useful here, graphene would need a bandgap, that is the energy required to excite an electron stuck in the valence band, where it cannot conduct electricity, to the conduction band, where it can. In other words, graphene would need to behave not just as a conductor but to also have an insulator mode. One field in electronics where graphene does show promise is field-effect transistors (GFETs), which can be switched very fast, operating at the gigahertz speeds demanded in radiofrequency applications. While GFETs are generally unusable in digital electronics because they can’t be fully switched off, this is not a problem in radio receivers and amplifiers where complete switch-off is not essential for analog technologies.

Essential Tools for Graphene Characterization

Three analytical tools that have been shown to be instrumental in characterizing raw graphene and devices fabricated from it are transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy.

Transmission Electron Microscopy (TEM) measures amounts of graphene, graphite/soot and nanotubes.

X-ray Photoelectron Spectroscopy (XPS) reveals sp²/sp³ carbon ratios, quantification of surface chemical modifications, and measures the reduction of graphene oxide.

Raman Spectroscopy quantifies defect levels and graphene sheet quality (G/D and G/G’ ratios), sheet thickness (number of layers).

Raman spectroscopy

Raman spectroscopy is used in the study of allotropes of carbon as they differ only in the nature of bonding and position of the carbon atoms—one element, many geometric structures. It is also capable of differentiating multilayers and thicknesses up to four layers. Raman spectra exhibit simple structures characterized by two principle bands G and 2D bands, while a third D band determines if there are defects in the carbon lattice.

Looking at graphene with Raman, the Stokes phonon energy shift caused by laser excitation creates two main peaks in the Raman spectrum: G (1580 cm^-1), a primary in-plane vibrational mode, and 2D (2690 cm^-1), a second-order overtone of a different in-plane vibration, D (1350 cm^-1) [8]. D and 2D peak positions are dispersive (dependent on the laser excitation energy). The positions cited are from a 532 nm excitation laser.

The D-mode is caused by disordered structure of graphene. The presence of disorder in sp²-hybridized carbon systems results in resonance Raman spectra, and thus makes Raman spectroscopy one of the most sensitive techniques to characterize disorder in sp² carbon materials.

The G-mode is at about 1583 cm^-1 and is due to E2g mode at the Γ-point. G-band arises from the stretching of the C-C bond in graphitic materials and is common to all sp² carbon systems.

If there are some randomly distributed impurities or surface charges in the graphene, the G-peak can split into two peaks, G-peak (1583 cm^-1) and D’-peak (1620 cm^-1). The main reason is that the localized vibrational modes of the impurities can interact with the extended phonon modes of graphene resulting in the observed splitting.

All kinds of sp² carbon materials exhibit a strong peak in the range 2500 – 2800 cm^-1 in the Raman spectra. Combined with the G-band, this spectrum is a Raman signature of graphitic sp² materials and is called 2D(G*)-band. 2D-band is a second-order two-phonon process and exhibits a strong frequency dependence on the excitation laser energy.

The Thermo Scientific DXR 2 Raman microscope is an ideal instrument for graphene characterization. This tool provides high level of stability, control, and sensitivity needed to produce confident results. No other technique provides as much information about the structure of graphene samples as Raman spectroscopy. Raman spectroscopy is a great tool for the characterization of graphene, especially in layer thickness.

To learn more, read “Characterization of Graphene Using Raman Spectroscopy” on AZOM.com.