Working with 2D materials is difficult as in their purest form they consist of only a sheet of atoms, one layer thick. This makes analysis a delicate matter, with the use of low energy techniques necessary to prevent degradation and damage. When analyzing 2D materials, the most important considerations are the chemistry, assessment of surface contamination, and the confirmation of the structure of the material.
Ever since the rediscovery and isolation of graphene in 2004, there has been explosive growth in the research of 2D materials. Although graphene is still the material that springs to mind when 2D materials are mentioned, there are several others that are attracting interest due to their own unique properties and behavior.
These also consist of sheets of atoms, with other elements or compounds such as boron nitride and molybdenum disulfide being the motif instead of carbon. While graphene excels in many parameters, it is notoriously difficult to work with, and current research into scaling up production has identified boron nitride as a potential substrate for the deposition of graphene1. However, this must also be single layer and of high purity, demonstrating again the need for accurate chemical analysis.
The simple composition of 2D materials belies the complexity of working with them. Although there are only two significant parameters, chemical composition and number of layers, any change in either of these will have a drastic effect on the viability of the material. Typically, the properties of these materials approach those of the bulk material after only a few layers, so it is critical to be able to determine the precise number present in these advanced materials2.
This makes it crucial to be able to accurately measure the material. The standout techniques for this are XPS along with Raman spectroscopy. XPS can be used to identify any specific compounds or elements present at the surface and to determine their location. It allows understanding of both the elemental and chemical composition of the surface of the material.
Raman allows the layer number to be determined through the peak splitting at characteristic peaks. In AB stacked graphene, as the layer number increases, the forces from the interactions between them increases, and the 2D Raman peak splits into several modes giving a wider, shorter, higher frequency peak. Thus, for AB-stacked graphene, the number of layers can be derived from the ratio of peak intensities, I2D/IG, as well as the position and shape of these peaks3.
The Thermo Scientific™ Nexsa™ Surface Analysis System is the newest surface analysis device and is an excellent tool for the analysis of 2D materials. Although primarily an XPS device, it has the novel ability to be able to analyze a material using Raman spectroscopy, as well as two more modes in UV photoelectron spectroscopy (UPS), and ion scattering spectroscopy.
UPS is similar to XPS but uses UV photons instead of X-rays. It is typically used to analyze the valence bonding in material band gaps and can also be used to detect hydrogen, a feature that isn’t possible using XPS alone. Ion scattering is also similar to XPS but only looks at the first layer. This makes it useful for looking at ultra-thin film composition.
The Nexsa has three laser wavelengths available for use, and the versatile laser grating and blocking filter system makes it easy to swap to another laser wavelength. By having both techniques on the same platform, and the correct wavelengths of laser available almost instantly, it is possible to analyze the same positions simultaneously and gain a complete understanding of the material.
Not only is Nexsa the ideal machine for analysis of 2D materials, it is also applicable to a large variety of other types of samples. For a more complete description of the working principles of XPS and its use in accurate measurement of 2D materials, along with how Nexsa can assist in this regard, please watch this webinar here.
References and Further Reading
- Wang, H., Zhang, W. & Wang, C. Hexagonal boron nitride : a promising substrate for graphene with high heat dissipation.
- Shahil, K. M. F. & Balandin, A. A. Thermal properties of graphene and multilayer graphene : Applications in thermal interface materials. Solid State Commun. 152, 1331–1340 (2012).
- Wall, M. The Raman spectroscopy of graphene and the determination of layer thickness. Thermo Sci. Appl. note 52252 (2011).
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).
For more information on this source, please visit Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).