Vacuum benefits for high-end research using FT-IR techniques

Atmospheric disturbance is a major problem in Fourier-transform infrared (FT-IR) spectroscopy. The different vibrational modes of water vapor and carbon dioxide in a laboratory atmosphere have their absorption bands in the whole mid-infrared and far-infrared (FIR) spectral range. This reduces instrument sensitivity and stability and can even mask relevant spectral features of the sample.

In this SelectScience webinar, now available on demand, Dr. Xia Stammer, FT-IR application scientist at Bruker Optics, and Dr. Dan Wu, senior application scientist at Bruker Optics, describe how a vacuum not only terminates the water lines, but essentially increases stability, sensitivity, reproducibility, and thus the overall performance of an FT-IR spectrometer.

Read on for the live Q&A session, or register to watch the webinar at a time that suits you.

In which situations would a reference measurement of atmospheric air not be possible?

XS: This is for example the case for emission experiments where there is no reference measurement possible. What you measure is just a single channel spectrum of the emitting sample. Or in FT-Raman experiment, where you are detecting the Raman scattering in light using an FT-IR spectrometer, this is also a kind of emission detection and so no references are possible. In some situations, a reference sample preparation is just too difficult. Just like in thin-film or ultrathin film characterization, where you have a little amount of a very thin monomolecular layer and in best case you want to use the bare substrate as a reference. Anyhow, it is not easy to get a clean substrate with no physisorbed layers due to their high adsorption affinity.

What is the difference between attenuated total reflection (ATR) and FT-IR, and do you have any recommendations for the best specifications for an ATR-FTIR spectrometer?

XS: ATR is one of the measurement modes for FT-IR. In the modern spectrometer, you can insert the ATR measurement accessory into the sample compartment to carry out an ATR measurement. There are different kinds of ATR accessories, ATR with big crystals to achieve higher sensitivity, micro-ATR units with almost no sample preparation and high throughput, ATR unit with adaptive reaction cells to monitor a chemical reaction, or heated ATR crystals to measure at different temperatures. And there are different ATR crystal materials, they are each optimized for different spectral ranges, and they have different chemical resistance. The ATR you require depends on your project and what you want to know from your sample.

Would you recommend using ATR or the transmission mode for thin-film studies?

XS: It depends on what you need. I think ATR for thin-film studies, if we are talking about the thin film on a flat substrate, then it is quite difficult and limited only to certain kinds of ordered thin films. Transmission will give you much better sensitivity. Of course, it's not available for all kinds of substrates. I would rather recommend reflection or IRRAS for thin film measurement.

Can you use this technique to determine the bandgap for doped materials?

XS: It depends on the material itself. For the photoluminescence (PL) materials, as long as the bandgap is in the range of the IR energy range, we can see it. And for some other semiconductors or superconductor materials, we can also see the interband gap values from the IR measurement, but that's something different from the PL measurement. We can also use transmission and reflection techniques to see the interband transitions.

There is one purged spectrometer, the VERTEX 80 in the VERTEX series, can the V80 also achieve the same temporal resolution as the vacuum version?

XS: The specified temporal resolution for the VERTEX 80v Vacuum FT-IR spectrometer and for the VERTEX 80 purge spectrometer is the same, since they have the same interferometer and linear scanning. If we look at the important criteria for an excellent step-scan stability and performance, then a vacuum spectrometer is better. Reasons have been explained in the presentation.

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