Adding chemical information through IR to AFM
Hybrid or correlative microscopy methods are in vogue. My last blog was about joining the complementary worlds of AFM and SEM, and I mentioned some other hybrid AFM methods including AFM-Raman and AFM-TOF SIMS. But today I want to focus on AFM-IR (AFM-infrared spectroscopy) technology, one of the most powerful combinations that provides chemical information with AFM. Infrared or IR spectra are the cornerstone of analytical identification of chemical functionality. The mid-IR is the fingerprint region for organic chemistry where important functional groups such as carbonyl (C=O), carboxyls (COOH), alkanes (CH), hydroxyls (OH), and amines (N-H), nitro (N-O) compounds and others have their own identifiable IR spectra. The mid-infrared portion of the electromagnetic spectrum is from 400-4000cm-1 (2.5-25um). [Note that chemists, who are the primary users of infrared spectroscopy, like to speak in wavenumbers or cm-1].
While AFM provides very useful information on the topography and mechanical properties of the sample, any sort of chemical identification is lacking from the technique. Without this, unambiguous identification of the material is not possible. For organic components, the ability to provide infrared spectra simultaneously with the other AFM data provides a powerful and more complete characterization tool.
There are essentially two main commercial technologies for conducting AFM-IR. The first technique is based on a photo-thermal method and was commercialized by Anasys Instruments with its nanoIR2. This method takes advantage of a laser at a fixed frequency that heats the sample, which then absorbs the energy at that wavelength. The locally heated sample knocks against the cantilever, and its ring down spectrum is recorded and converted to an absorption spectrum. There are also new resonance enhanced modes that avoid the ring-down process and enable faster spectral acquisition. Neaspec now offers a photothermal measurement but at locations only outside the US.
The other approach to AFM-IR is based on near field scattering technique, and its commercialization was pioneered by Neaspec, but is now also offered by the Bruker Inspire and by Anasys Instruments’ nanoIR2-s. With this method, the tip acts as a nano-antenna that converts the incident laser radiation into a very intense near-field light source. See schematic below where the tip (T) is illuminated by the laser light (L), generating a very focused light at the apex (N) locally exciting the vibrational resonances in the sample. These resonances modify the tip-scattered field, which is recorded with an interferometer and results in IR amplitude and phase images (these are the real and imaginary part, respectively, of the scattered field). The IR amplitude image contains the spectroscopic information most useful for analytical measurements.
NeaSNOM – how it works [courtesy of Neaspec]
From an AFM perspective, it can be important what imaging mode is used to image the sample as especially for soft samples (polymeric/bio), you do not want to induce damage on the sample. The scattering methods are all tapping mode based. The photothermal was originally contact mode based, compromising some of its capabilities to image soft matter, but recently a tapping mode version has been introduced. There is also a third method termed photo-induced force microscopy that also provides chemical specificity from Molecular Vista.
In this AFM image below you see bands of one material within a matrix of another. This was a blend of rubber and nylon –which component is which? Let’s ask the AFM-IR (run in the photo-thermal method)! Spectra were collected on the ribbon-like features and displayed a characteristic fingerprint, revealing that they are in fact nylon while the background is rubber.
Rubber-nylon figure [courtesy of Anasys Instruments]
Another example - in this image of pentacene, an organic semiconducting molecule, the topography is on the left and the infrared image on the right (collected in the near-field method). The infrared image exhibits different crystal structure and orientations that were only visible in the IR image, revealing important effects of granularity and crystallinity on this organic semiconducting thin film.
[C. Westermeier, Nature Comm, 5, 4101, 2014)]
So what is the resolution of these methods if you are trying to match AFM, which is generally at 10nm, with the IR? The scattering IR method gives 10nm resolution, while the photothermal method is slightly worse. But IR resolution on a scale of ten or tens of nanometers is truly impressive. Consider that before these technologies came along, the resolution of IR based methods was limited by the classic diffraction limit of l/2, which in the mid infrared spectrum would be on the order of 1 micron. So these AFM-based techniques provide a resolution improvement in infrared spectroscopy of over 100x!
Dalia Yablon, Ph.D.