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Careful preparations are crucial for getting high-quality data from any sensitive measurement. PiF-IR nanoscale spectroscopy and PiFM nanoscale chemical mapping provide lots of important data. Maximize your results by using PiFM and PiF-IR after you have identified a region of interest, minimized contaminants, and properly fixtured the sample. This guide will explain and address the most common pitfalls that Molecular Vista sees in customer’s samples.
Because PiFM and PiF-IR are based off an AFM, the physical sample preparation requirements are the same as for any standard AFM. Therefore, the best way to ensure that your sample can be measured is to take some AFM images on the region of interest.
If taking your own AFM measurements is not possible, then there are some guidelines to follow.
Because AFMs use mechanical detection, there is a limit to the surface roughness that is possible to image.
Molecular Vista has a Z-scanner range of 12 µm. However, some of that range is needed to account for Z-drift and the angle of the sample surface relative to the scanning axes. Therefore, in general samples with 4-6 µm of height variation should be possible to image depending on the specific topography. Samples with 7-9 µm of height variation may be possible, but very challenging.
If the sample is too rough, or large scans are desired, then you will have to make the sample flatter. For example, malleable materials can be pressed to make them denser and flatter. Additionally, some materials can be microtomed (or cryo-microtomed!) without smearing to smooth the surface.
Most samples are relatively easy to fixture. If the sample is not taller than it is wide, simply use an adhesive that does not outgas to attach it to a metal AFM disc. Five-minute epoxy is preferred, but double-stick adhesive tabs can work as well. Avoid double-stick tape from a roll since that can cause the sample to drift.
For cross sections, tall samples, flakes/powders, and other challenging surfaces, there are some special techniques discussed below.
Samples are most stable when they are not taller than they are wide. However, sometimes this is unavoidable. If the height is less than 5 times the width, it may still be possible to image. However, supporting a sample in a small vice, or cutting it down to improve the aspect ratio is preferred.
Casting resin around a sample can be an effective way to fixture difficult materials or cross sections of thin films. However, the resin itself can cause contamination issues. Even a small layer of resin will make AFM-IR measurements impossible. Therefore, follow these steps for best results.
Powders, flakes, and other small amounts of material can be difficult to prepare. Often the best method is to deposit the flakes on top of some resin.
For imaging and spectroscopy, the AFM probe will have to land on the surface of the flakes or particles. Depending on the size of the individual pieces, these surfaces may be unstable and therefore require more rigid fixturing such as being cast in resin. Also, porous samples may wick some epoxy, and contaminate the surface if the sample is very thin. That is why it is important to smear the thinnest possible layer that will hold the material securely.
It is often helpful to analyze some pure reference materials for comparison to the primary sample. For example, comparing individual constituents to a manufactured composite material. Or, comparing some suspect materials to a defect to aid in identifying the process error.
When supplying Molecular Vista with reference materials, please provide enough for ATR-FTIR spectroscopy to complement the nano-IR measurements. For solids or powders make sure there are a few thousand cubic millimeters of material. For thin films, make sure the film is at least 500 nm thick for ATR-FTIR. Glass, silicon, mica, and sapphire are all good substrates that do not conduct too much.
Contamination is a common issue many customers are unaware of. PiF-IR is sensitive enough to measure contaminants at the single-molecule-level. Therefore, samples that are not packed carefully may produce spurious results. Here are some tips to avoid contamination.
PiFM and PiF-IR chemical analysis can provide a wealth of information. To make sure you get the most value, it is important to minimize sample preparation issues. Most samples are easy to mount in a matter of minutes. However, paying attention to more difficult cases and taking a little extra time to prepare a sample for both analysis and shipping will go a long way to getting the best results in the first attempt.