I am excited to announce and launch a new blog mini-series where we will be talking about a variety of analytical instruments, which we use on a daily basis to answer materials questions and in turn, offer better products.
The mini-series will run bi-weekly for the next few months. I hope you enjoy the images. I also hope that you will be able to utilize the information provided to advance your research programs.
I will kick-off the mini-series with a technique, Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) that I use to analyze the surface chemistry of materials at very high (ppm to ppb) chemical sensitivity. In my post below and with the posts that follow, we will dissect these instruments in three ways:
1. What the instrument does
2. How the instrument works
3. Why we use the instrument
We’ll also share with you some of the unique samples we have analyzed with these instruments along with references to published manuscripts for readers wishing to learn more about them. In two weeks, check back for the next part of the mini-series that will explain another technique. I hope you’ll be able to learn more about these very powerful analytical instruments from our characterization team and the below technique, which is one I use to analyze materials, day in and day out!
Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
1. What does it do? ToF-SIMS analyzes the top surface of materials.
2. How does it work? A pulsed ion beam strikes sample surface under high vacuum conditions and surface species are ejected and detected. See the animated schematic below provided by ION-TOF, which describes the operation of the instrument.
3. Why would I use it?
ToF-SIMS is frequently used to detect and/or image species present at very low concentrations (often times, below the detection limit of other analytical techniques). In the left panel of the figure below, you will find a visual photo of a device (taken inside the ToF-SIMS instrument). My colleague needed to know if Cu was present on the gold pad (the center square). Within minutes, we could clearly see that Cu is indeed present on the gold pad (see the spectral peak colored in green at a nominal mass of 63 amu). The high mass resolution of ToF-SIMS allows one to separate Cu from C5H3 which are both at a nominal mass of 63 amu. In addition to determining that Cu is present at a concentration of less than 900 ppb, we were also able to show that Cu is quite uniformly distributed across the gold (Au) pad. The lower right panel shows both the spectral and image representation of Au.
ToF-SIMS is also often used to obtain molecular information about materials.
The left panel of the figure below shows a mass spectrum of a polymer which included an additive BnPOSS. BnPOSS has a number of unique high mass peaks (shown by the blue box) as well as the molecular peak at 1144.9amu (identified by the blue arrow). The observation of the molecular peak and its high mass fragments allows for a positive identification of the additive. The right panel of the figure shows a two color overlay where red represents the base polymer and blue represents the region(s) of the sample that are enriched in BnPOSS. For additional details, see: V.S. Smentkowski, H. M. Duong, R. Tamaki, M.R. Keenan, J.A. (Tony) Ohlhausen, and P.G. Kotula, “Using Time-of-Flight Secondary Ion Mass Spectrometry and Multivariate Statistical Analysis to Detect and Image Octabenzyl-Polyhedral Oligomeric Silsesquioxane in Polycarbonate”, Appl. Surf. Sci. 253 (2006) 1015
ToF-SIMS is one of the few characterization techniques that are able to detect and image hydrogen.
The figure below shows two ToF-SIMS images of hydrogen. The image on the left is of hydrogen in a metal alloy sample while the image on the right represents hydrogen in a solid oxide fuel cell assembly.References: V.S. Smentkowski, S.G. Ostrowski, “Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) – a Powerful High Throughput Screening Tool”, Review of Scientific Instruments, 78(1) (2007) 072215 (left panel). V.S. Smentkowski “Surface Analysis of Ceramics by Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS)” American Ceramics Society Bulletin 91 (1) (2012) 30 (right panel).
ToF-SIMS is able to detect and image unexpected species that are present in trace amounts.
The left panel of the figure below shows a light microscope image of a thin film sample which delaminated after being weathered. A ToF-SIMS 3 color overlay is provided on the right hand side (Red is the Si substrate, Green is Ti, and blue is an unexpected contaminant). The 3 color overlay clearly identified where and how the delamination occurred.
ToF-SIMS is also able to provide three dimensional analysis of materials, when the top surface layer is eroded (removed) between the collection of successive images. 3D analysis often show that certain regions of samples have contaminants or a non-uniform composition. A 3D ToF-SIMS movie clip is provided below (blue is the SiO2 film, green is the GaAs substrate, yellow is the SiO2/GaAs interface, red is surface contaminants, and light blue shows deposits).
Reference: V.S. Smentkowski, S.G. Ostrowski, M.R. Keenan, J.A. (Tony) Ohlhausen, and P.G. Kotula, and E. Braunstein “Multivariate Statistical Analysis of Three Spatial Dimension ToF-SIMS Raw Data Sets”, Analytical Chemistry. 79(20) (2007) 7719
We have analyzed a number of unique samples by ToF-SIMS at GE Global Research including human hair, butterfly wings and tissue slices, among others. Any thoughts or questions about ToF-SIMS? Feel free to share them below.