The IR source is tunable from 900 to 2000 cm-1 and from 2234 to 3600 cm-1. The peak pulse energies are under 7 mJ to be under Class 1 laser safety limits. (The instrument is rated as a Class 2 laser product due to other visible laser sources.)
Q: Please comment on nanoIR vs FTIR (even ATR) vs Raman spatial resolution.
The spatial resolution of nanoIR is sample dependent. The best resolution we have seen is of order 50 nm, and we usually see resolution in the 100 to 200 nm range. FTIR has a fundamental limit of twice the wavelength i.e. 2λ, with a practical rule of thumb resolution limit around 3λ. For the wavelength range of the nanoIR instrument, 3-10 μm, the FTIR would then have a practical resolution limit around 10-30 μm. For ATR, the fundamental resolution limit is around λ/2, with a practical limit ranging from 3-10 μm depending on wavelength. Confocal Raman microscopy can be applied to some samples on the submicron scale, although fluorescence and photon migration can limit some applications and practical spatial resolution. See for example Neil Everall, Pavel Matousek, Neil MacLeod, Kate L. Ronayne, and Ian P. Clark, Applied Spectroscopy, Vol. 64, Issue 1, pp. 52-60.
Q: How does this technique compare to optical methods of breaking the diffraction limit such as SNOM (scanning near-field optical microscopy)?
SNOM has shown excellent ability to perform optical measurements below the diffraction limit. To date, SNOM has not been used for spectroscopic measurements over a wide spectral range. Another limitation of the technique is that the light collected in the far field depends on both the real and imaginary index of refraction of the material and sophisticated modeling may be required to extract a spectrum that is analogous to a conventional IR absorption spectrum obtained for example, by FTIR.
Q: What is the spatial resolution of the AFM and the thermal analysis measurements on the nanoIR platform?
The spatial resolution of the AFM is limited by the tip radius of the AFM probe as well as limitations due to the contact imaging mode. With a sharp probe, this resolution can range from a few nanometers up to larger length scales depending on how soft the sample is. Our thermal analysis probes have a sub-30 nm end radius and we routinely make measurements on the sub-100 nm scale, although resolution depends on the thermal and mechanical properties of the sample.
Q: What are the sample size constraints?
Samples may be placed anywhere in a 6 x 12 mm rectangular area on the surface of the ZnSe prism. We recommend sample thicknesses in the range of 100 nm – 1 um. We prepare most samples by either microtome or drop casting films from solution.
Q: How smooth does the surface have to be?
For best measurements, the sample should have a surface roughness less than 1 micron. Our AFM measurement module has a vertical range of 10 um, but better results can be obtained on smoother samples. Samples prepared by microtome or drop casting generally are sufficiently smooth. Care should be taken when transferring microtomed samples that they lay generally flat on the ZnSe prism or the user should select flat areas to measure.
Q: What microtome do you use?
Internally we use a microtome manufactured by Microstar Technologies which works well for preparing samples for the nanoIR. Other microtomes that are used for preparing samples for AFM and/or TEM measurement should be suitable assuming they generate sections within the required thickness.
Q. Does the material of interest have to be in direct contact with the ZnSe crystal?
It is important that the sample is well attached to the prism to be stable for AFM measurements. We generally select areas for measurement that are in direct contact with the ZnSe prism, but we have also made successful measurements in cases where the material is rippled away from the prism’s surface.
Q: Can the prism be removed for sample application?
Yes, the prism can be removed and exchanged in a few seconds with no tools. The prism is held in a self-leveling mount, secured with a simple thumbscrew. The prism mount is held on the AFM scanner with a kinematically aligned magnetic mount.
Q: Is IR sensitivity related to the sample thermal characteristics?
Yes, the cantilever amplitude is proportional to the thermal and physical properties of the sample. The sensitivity increases directly with the thermal expansion coefficient and inversely proportional to the heat capacity and density.
Q: How do you normalize the IR signal from the AFM to the IR incident laser power which must vary as you scan the laser?
We have integrated the ability to perform a laser power calibration at any time. The nanoIR system can direct the IR laser beam either to the AFM or to an integrated IR sensitive photodetector. When the user selects a power calibration, the laser is swept through the wavenumber range of interest and the power level at each wavenumber is stored in a calibration file. IR spectra can then be normalized to a constant power level.
Q: How much heat do you apply in a localized region due to nanoIR measurements?
The heat we produce is a tradeoff between signal to noise and eliminating sample damage. The IR source itself is powerful enough such that it can melt polymer materials at full power. So we turn down the power to reduce the chance of any sample damage. The simultaneous mechanical stiffness measurement lets us ensure the temperature doesn’t rise high enough that mechanical changes occur.
Q: Can the nanoIR spectroscopy capability be added as an accessory to an existing AFM?
The nanoIR system is a standalone platform that integrates a tunable IR source, beam delivery and control optics and an AFM measurement module. It is not compatible with other existing AFM systems
Q: Can the nano-TA cantilever be used for nano-IR measurements?
Yes. Other cantilevers can have a higher sensitivity, but we have made spectroscopic measurements with ThermaLever cantilevers designed for nanoTA.
Q: How does the stiffness of the cantilever play into the measurement?
In general, softer cantilevers give greater measurement sensitivity. The optimal cantilever stiffness can also depend on the sample stiffness, especially in the case where you would like to use the contact resonance method to map the relative sample elasticity.