Studying 2D Materials Using Thermo Scientific Nexsa X-Ray Photoelectron Spectrometer (XPS) System

In this interview, Tim Nunney, Marketing Manager at Thermo Fisher, speaks about the capabilities of the Thermo Scientific ™ Nexsa™ Surface Analysis System, and how it can be used to gain insight into 2D materials.

Please can you introduce yourself and the Nexsa?

My name is Tim Nunney and I am the marketing manager for the surface analysis products from Thermo Fisher Scientific. I am going to be discussing how the use of X-ray photoelectron spectroscopy (XPS) with Raman Spectroscopy on our new Thermo Scientific Nexsa Surface Analysis System which can give new insight into the analysis of 2D materials.

Nexsa is our newest surface analysis instrument. It is primarily an XPS system which uses X-ray photoelectron spectroscopy to analyze the surface chemistry of materials. It also has the capability to use other complementary techniques such as UV photoelectron spectroscopy, ion scattering spectroscopy, or reflected electron energy loss spectroscopy.

A novel feature of the instrument is the ability to carry out Raman spectroscopy.

What is XPS?

XPS allows you to understand both the elemental and chemical composition of the surface of the material.

It works by shining a beam of X-rays, soft X-rays, and aluminum K-alpha click, which can be easily monochromated to give good energy resolution. When this surface is irradiated with this X-ray beam, photoelectrons are emitted from the sample.

The kinetic energy that these photoelectrons have is dependent on the element and its electronic configuration - the orbital from which the photoelectron has come. It is also dependent on the chemical state that the atom the electron came from was in. The influence of any other elements bonded to the atom also changes the kinetic energy that is measured by a small amount.

By using a very simple equation, we can calculate the binding energy that the photoelectron had within the atom. From that, we can determine the elements and the chemical state of the material.

The signal we detect is directly related to the amount of material that is present in the sample, and so by measuring the spectrum and the area of the peaks we detect, we can understand the composition of the surface of the material.

XPS allows us to understand the surface chemistry and quantify it. Techniques such as imaging XPS could also be used by moving the sample under the X-ray beam so that images could be generated by collecting spectra each pixel.

For more information about the basics of XPS, you can watch on-demand Thermo Scientific webinars, on the xpssimplified.com website.

Depth profiling can also be carried out, whereby we remove material from the sample using an ion beam and go deeper into the surface in order to understand how the chemistry changes at interfaces and when going into the bulk of the material.

Nexsa X-Ray Photoelectron Spectrometer (XPS) System

What other analytical techniques does Nexsa offer?

In addition to XPS, Nexsa offers four other analytical techniques. These are Raman spectroscopy, UV photoelectron spectroscopy, reflected electron energy loss spectroscopy, and ion scattering spectroscopy.

UV photoelectron spectroscopy (UPS) is very similar to XPS, but it uses UV photons rather than X-ray photons to excite the photoelectrons. UPS is typically used to analyze the valence bonding in a material.

Reflected electron energy loss spectroscopy or REELS is used to look at band structures and band gaps and can also be used to detect hydrogen which can't be done with XPS.

Ion scattering spectroscopy (ISS), sometimes known as low energy ion scattering (LEIS), is extremely surface sensitive, whereas the information in XPS comes from the outer 10 nanometers of the material.

ISS looks at the first atomic layer and so it can be very useful for looking at ultra-thin film composition. For example, for looking at how layers are building up through an ALD process, or for looking at how segregation has occurred from materials moving from the bulk to the surface.

All of these techniques can be added as options onto the Nexsa instrument.

How can Raman spectroscopy be used with XPS to analyze 2D materials?

In Raman spectroscopy, a laser is used to irradiate the sample. The light that scattered back from that material is then looked at in order to analyze the energy losses. In some ways, it is similar to what we do with REELS and ISS.

The majority of the light that is scattered back loses no energy and is what is known as Rayleigh scatter. This is filtered out. However, one in 10,000 or so photons that do lose energy through the Raman scattering process are collected and produce a spectrum.

Experimentally, the small form factor iXR spectrometer is integrated onto the system. The beam spot that is obtained on the sample is at around about 10 microns, which is very similar to the smallest X-ray spot size that can be obtained on the Nexsa, which is again 10 microns.

Nexsa X-Ray Photoelectron Spectrometer (XPS) System

The X-ray beam and the Raman laser can be colored differently so that we can see how well the areas match. This gives us good coincidence of the XY position for the analysis and allows us to analyze the same position using the two techniques.

The laser grating and blocking filter is also easy to swap to another laser wavelength. There are three laser wavelengths available for this system. The system enables you to swap quickly between the wavelengths and use whichever radiation is most useful for your analysis.

How do you use Raman spectroscopy with XPS to analyze boron nitride?

Take the example of the 2D material boron nitride analysis where the aim is to look at the chemistry, any surface contamination that may be present, and to confirm the structure of the material.

The XPS Raman analysis of boron nitride shows that hexagonal boron nitride is composed of nanosheets of sp2-bonded boron and nitrogen. It is analogous to graphene and a similar hexagonal structure is expected.

A layer that has been deposited onto a copper substrate is examined to understand whether boron nitride has been made and whether it has been made in the correct form - hexagonal as opposed to cubic.

To start with, XPS can be used to identify whether there is any nitride presence at the surface and to determine its location. This is called the SnapMap, a rapid XPS map of the surface. It allows us to look at whether there is a precursor to a nitride present or actual nitride formation.

A spectra can be extracted from the different regions, and different colored areas allow identification of organic nitrogen residue not changed into the nitride, and of nitride formed. The image obtained is used as a guide to further analysis.

The next step is to see which elements are present in the two areas that were identified. Areas where nitride has been found and areas where it has not are compared in terms of the elements that are detected. In this example, boron can be detected in the area where we detected the nitride. However, in the area where no nitride can be seen, boron is not detected.

By looking at high-resolution scans in the boron and nitrogen regions with XPS, the chemistry can be analyzed and the amount quantified can be seen. A strong boron peak and a strong peak for the nitride can be seen. However, there can sometimes be oxygenation of the film leading to some surface boron oxynitride present in addition to the boron nitride.

Raman spectroscopy can then be used to collect data. In this case the peak from the area where nitride has been identified as present corresponds to the hexagonal form of boron nitride – there is no peak in the position which would be expected if there was any cubic form of boron nitride present.

This will allow the identification of the structure of the material as well as the surface chemical information obtained from XPS. By using the two techniques in concert, this can be done without having to hunt around for the same positions and move from one instrument to another.

By having both techniques on the same platform, the same positions can be analyzed at the same time in order to get a full understanding of the material.

Nexsa X-Ray Photoelectron Spectrometer (XPS) System

How do you use Raman spectroscopy with XPS to analyze molybdenum disulfide?

Another useful example is that of a molybdenum disulfide sample deposited onto a silicon wafer, where there are flakes of molybdenum disulfide distributed across the surface of the wafer. Using the SnapMap capability of Nexsa, areas where the flakes are present and where there are no flakes can be seen and identified, before being used to collect XPS spectra from points that have been identified.

Using Raman, the peaks corresponding to molybdenum disulfide can be looked at. Here, there is a splitting of two peaks detected, which is indicative of the number of layers that are present.

On a bulk sample, the splitting between the two peaks would be seen at around 25 wavenumbers, whereas in the position in this example, there is a splitting at around 18 wavenumbers which corresponds to just one or two layers of Molybdenum disulfide.

Another area on the sample can be looked at in order to see the variation. In this example, there appears to be much more in the upper SnapMap. The image obtained in this example seems to correspond to an increase in the layer thickness of the molybdenum disulfide, which is seen as splitting between the two peaks in the Raman spectrum.

There is also a lot more oxidation in the upper layer. One of the peaks indicates detection of oxidized molybdenum, and that corresponds to some additional surface oxidation of the film that is present in that area compared with the initial area we looked at.

About Tim Nunney

Dr Tim Nunney is the Marketing Manager for the Thermo Scientific Surface Analysis (X-ray photoelectron spectroscopy) & Microanalysis (EDS, WDS & EBSD) product lines. His role involves all aspects of product marketing, including collateral development, customer evaluations, product development and commercial support. He has been with Thermo Fisher Scientific since 2004, previously holding positions as an applications scientist, and in the operations group. Prior to joining Thermo Fisher Scientific, Tim worked as a post-doctoral research fellow at the University of Southampton, investigating the dynamics of molecular dissociation on metal surfaces. He completed a PhD in Surface Science at the University of Liverpool, studying the surface catalysed decomposition of methylamines on transition metal surfaces using reflection absorption infra-red spectroscopy (RAIRS) and XPS.