Secondary ion mass spectrometry, otherwise known as SIMS, is a well-established analytical technique for determining the elemental composition of a sample through ion bombardment and sputtering approaches.
A team of Researchers from Poland have now invented a new SIMS technique- Graphene Enhanced Secondary Ion Mass Spectrometry (GESIMS), currently patent pending (European patent application no. EP 16461554.4), is an adapted version of traditional SIMS methods which utilizes a graphene layer above the substrate’s surface and analyzes the ejected secondary anions through mass spectrometry.
Secondary ion mass spectrometry is a widely used and precise analytical technique used in the determination of a sample by bombarding its surface with a primary ion beam, which leads to sputtering of the surface.
Small particles of the sputtered surface become ionized, leading to the formation of secondary ions, upon which they are determined by mass spectrometry (MS) analysis.
SIMS is a particularly useful tool for determining the depth profile of a material as the sputtering removes layers from the sample and monitors the change in its composition as a function of its depth.
Current SIMS methods possess an excellent sensitivity and detection limits for trace elements. However, to achieve the optimum detection possible and to analyze 2D materials, modified SIMS versions are required- dynamic SIMS (dSIMS) and static SIMS (sSIMS), respectively.
However, in the case of dSIMS, the ion beam is very dense which leads to a poor resolution, whereas sSIMS has a poor detection limit due to a lack of extracted secondary ions.
The new proposed technique was seen as an attempt to combine the high detection limits of dSIMS with the ability that sSIMS has to analyze 2D materials. The new invention, termed Graphene Enhanced Secondary Ion Mass Spectrometry (GESIMS), has been developed by the Researchers to enhance the detection limits of thin materials by analyzing the ejected secondary anions using mass spectrometry.
The use of GESIMS on a sample follows a logical progressive order, with the first step being the application of a layer of graphene over the sample surface. The graphene coating is then annealed under high temperatures and low pressures before sputtering occurs in dSIMS mode, leading to a partial destruction of the graphene layer. The secondary ions are then ejected and detected by mass spectrometry in sSIMS mode.
In this new method, the graphene coating blocks the emission of the substrate but significantly increases the ionization probability. The creation of defects within the graphene layer causes the emissions to increase whilst preserving the ionization ability so more ions can reach the detector, enhancing the SIMS signal.
Because the graphene layer acts as a kind of filament and emits a large number of secondary ions, the Researchers found the technique produces a SIMS signal greater than two orders of magnitude than a sample which was not coated with graphene.
In addition, it was found that by using this method, the dopant and contamination signals can be enhanced by up to 35 times that of other methods, with a detection limit of 1015 atoms/cm3 – similar to that of the high detection limit dSIMS methods.
The method is ideal for the analysis of surfaces, 2D materials and ultra-thin films, but not for depth profiling of a thick material– as the graphene layer would be destroyed under such methods and the associated enhancements would be nulled. It can also not be used for high resolution imaging, as the partial destruction to the graphene layer would compromise the spatial resolution of the image.
In short, the new patent pending techniques is going to be an ideal choice for Researchers when they want to perform SIMS experiments on atomically thin and 2D materials.
The technique brings together the two positive approaches from both dSIMS and sSIMS and negates the associated drawbacks of each. As such, it will have great potential for many SIMS applications in the future, and will be seen as a commercial endeavour once the patent has been cleared.
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“Graphene Enhanced Secondary Ion Mass Spectrometry (GESIMS)”- Michalowski P. P. et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-07984-1
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