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An Introduction to AFM-IR

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What is in the name

AFM-IR is a term used to categorize a wide range of nanoscale chemical characterization techniques. The name is a combination of Atomic Force Microscopy—AFM—and infrared spectroscopy—IR. The goal is to achieve high-resolution IR spectra and IR absorption maps with spatial resolutions smaller than the diffraction limit of infrared light. Therefore, the term nano-IR is often used synonymously with AFM-IR. Data such as these aid analysis of a wide range of samples such as complex nano-composite materials or nanoparticles. An AFM is chosen as the platform for such experiments because there is no specialized sample preparation required, and the physical tip of the AFM is used in clever ways to beat the diffraction limit of light

AFM background

Atomic force microscopy is a scanning probe technique that can produce incredibly high-resolution images of a sample surface. The best AFMs approach atomic resolution, but that usually takes a great deal of care and special environments such as ultra-high vacuum. In ambient conditions, typical resolutions are less than 10 nm, which is well below the diffraction limit of visible or infrared light.

As a scanning probe technique, one can imagine an AFM working like a record player. Where a record player has a sharp needle that is deflected to produce sound, an AFM has a sharp needle whose deflection is observed to record a topography. By raster scanning the AFM tip over a sample surface, the topographical image is easily recorded.

Atomic force microscope diagram
Figure 1 The tip of the AFM “feels” the surface of the sample. The topography is recorded by observing the deflection of the cantilever using a laser feedback system.

Because an AFM image is constructed by “feeling” the sample, the raw data is simply a height map with black representing low areas and white representing high areas. However, the data can be used to generate a 3D surface which can be rotated to highlight distinct features.

AFM topography heightmap data
Figure 2 AFM images are essentially height maps that can be used to render the 3D topography.

AFMs have several practical advantages: just about any reasonably flat sample can be imaged almost immediately. No special preparations are required. Lastly, AFM imaging is non-destructive when used in non-contact mode as the sample is not altered during the scanning process. These qualities make AFM an extremely attractive imaging technique, made even better with the addition of infrared capabilities.

Spectroscopy background

There are several ways to perform spectroscopy, and there are many reasons one might want to—everything from looking at color shifts from distant stars to understanding atomic energy transitions. The most relevant spectroscopy technique to AFM-IR is Fourier transform infrared spectroscopy, often called FTIR. FTIR is used to probe molecular vibrational states that provide information about what types of bonds are present in a compound. This helps study changes in bond structure, or identify unknown materials based on their unique spectra. This versatility means FTIR has become nearly synonymous with chemical characterization. Therefore, one might get a taste of the incredible capabilities of AFM-IR by thinking what is possible when FTIR capabilities are combined with the imaging prowess and spatial resolution of AFM.

AFM-IR uses

AFM-IR techniques occupy a specialized middle-ground between spectroscopy and microscopy. Therefore, this field represents a new and growing sector of expertise. As such, most researchers or technicians who are already well versed in either microscopy or spectroscopy techniques may need to adjust their approach when using AFM-IR techniques.

For example, a chemist well versed in IR spectroscopy will be aware of the power that techniques like FTIR offer for complex chemical analysis. For example, IR spectra can be used to investigate bond or material changes in a sample by looking at characteristic peaks for specific bonding combinations. For these reasons, FTIR has a place in forensic, failure, or investigative type analysis as a way to identify materials based on their unique IR spectra. However, for FTIR samples must be relatively large and thick, and the spectra produced represent an average of all the nanomaterials present in the sample.

This is where a change in thinking is required for a FTIR-literate chemist exploring AFM-IR techniques. Nano-IR imaging and spectroscopy allow one to spatially isolate chemical changes on the surface of a sample with incredible resolution. The utility and power may be immediately obvious to some, but to drive home the differences take this: as an analogy, consider an RGB LCD display showing a yellow object. RGB devices physically cannot produce yellow wavelengths of light, so they trick the brain by displaying a combination of green and red light with pixels smaller than the human eye can resolve. However, with proper magnification, it becomes possible to see each individual pixel. The transition from using FTIR to AFM-IR is similar. Nano-IR data can see the individual pixels so to speak that FTIR cannot. Where an FTIR spectrometer might record the aggregate spectrum from a complex sample, AFM-IR can isolate individual spectra and map out their locations.

AFM-IR vs ATR-FTIR resolution
Figure 3 The lateral resolution of AFM-IR techniques is so much higher than FTIR, that it can isolate and probe individual chemical components at the nanoscale.

This means that when investigating materials that are truly homogeneous at the nanoscale, spectra from nano-IR techniques tend to look almost identical to FTIR data of the same material. However, many materials show some inhomogeneity at such small scales, and nano-IR has the resolution to see the difference. Therefore, for investigating complex and varied surface chemistry there is no better option than nano-IR imaging. The results will not always match what is seen at larger scales, but that is because factors like molecule orientation or phase make a difference. When the probe is only measuring the response from a handful of molecules, the results offer an incredible glimpse into chemistry that is not available any other way.

AFM IR chemical analysis example
Figure 4 AFM-IR techniques open new possibilities for chemical analysis by providing the resolution necessary to explore single layer chemical films, complex nano-composite materials, nanoparticles, and more.

For a microscopist, the transition to AFM-IR is different than for a spectroscopist. Microscopists are adept at investigating samples at the smallest scales possible. People well-versed in AFM will already be used to interpreting the images that come from a scanning probe and recognizing any scanning artifacts that may crop up. However, microscopists are perpetually plagued by having to guess or deduce what features in an image are made of. AFM-IR now allows microscopists to not only colorize data with chemical absorption maps but take point-specific spectra that can be used to identify unknown features. Therefore, defect analysis on semiconductor masks, bio-mineral inclusions, or contaminant identification are possible when using AFM-IR techniques.

AFM-IR vs AFM topography images
Figure 5 AFM-IR allows microscopists to colorize images with detailed chemical information. Here is a sample with two materials—one of which is a crystal with two different phases.

The cross-disciplinary nature of AFM-IR makes it a bit of an adjustment for some people well-versed in other disciplines. However, the incredible power of this hybrid nature makes it capable of analyses that are not possible any other way. The application notes below provide some scientific examples of what is possible.

Specific techniques

There are a handful of techniques that fall under the category of AFM-IR. The three most popular are nano-FTIR, tapping mode photo-thermal IR (TM PTIR), and photo-induced force microscopy (PiFM). More details about how each of these techniques work and what they are capable of can be found in our comparison articles. However, as the company who developed PiFM, the first and only non-contact (and therefore non-destructive) AFM-IR technique, we think that this technology is the most capable AFM-IR technique on the market today. PiFM relies on detecting attractive force interactions between the sample and the AFM tip. PiFM not only consistently produces some of the best nanoscale chemical characterization data possible, but it does so easily on a wider variety of materials than any other nano-IR system. For more depth on this technique, read our scientific principles to learn how PiFM leverages mechanical force detection to achieve incredible signal to noise ratios.

If you are curious why we think PiFM produces some of the best data available, read our application notes to see real-world examples and data. Alternatively, consider sending us a sample for evaluation. There really is no better way to see what AFM-IR is capable of!

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