Q: What is the spectral resolution?
The spectral resolution depends on the source selected for the nanoIR or nanoIR2. We can combine a Quantum Cascade Laser (QCL) or Optical Parametric Oscillator (OPO) source with either system. The QCL source has a line width which is <1 cm-1 over the full range. The OPO covers a broader range of the mid-IR and does this using two stages. The spectral resolution is <8 cm-1 over the full range and ~4 cm-1 over most of the range.
Q: What is the maximum wavenumber range of the OPO IR source, peak power and laser class?
The OPO 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 AFM-IR vs FTIR (even ATR) vs Raman spatial resolution.
The spatial resolution of AFM-IR 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 nanoIR2 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 nanoIR2 platform?
The spatial resolution of the AFM is limited by the tip radius of the AFM probe which typically provides resolution of a few nm. 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?
The nanoIR2 and nanoIR each have their own optimum sample size:
nanoIR2 – Samples as large as 25 mm diameter and 10 mm thick are easily accommodated.
nano-IR - 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. For the nanoIR, 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: Can AFM-IR be done in tapping mode?
The AFM-IR measurements are performed in contact mode. Other imaging modes that can be performed simultaneously include contact resonant frequency imaging and fixed wavelength absorption imaging. With appropriate cantilevers, nanoIR and nanoIR2 can operate in tapping mode for topographic imaging.
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 and nanoIR2 systems 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 AFM-IR 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 sample damage. The simultaneous mechanical stiffness measurement lets us ensure the temperature doesn’t rise high enough that mechanical changes occur.
Q: Can the AFM-IR spectroscopy capability be added as an accessory to an existing AFM?
The nanoIR and nanoIR2 systems are standalone platforms that integrate 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 nanoTA cantilever be used for AFM-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.