When using a scanning electron microscope (SEM) for the first time, you might have doubts about what can be imaged. You might also struggle to get the image quality you were expecting. Luckily, you can easily improve your results by following the simple yet powerful sample preparation techniques for SEM in this blog. Read on!
SEM: samples & vacuum
First things first: SEM’s operate in a vacuum. Samples can be seriously affected by a vacuum. For example, loose particles can detach from any surface, all liquids will evaporate immediately, and delicate materials will outgas. This is the reason why a fly can be easily imaged.
But do the same with a fly larva, and you will most likely get a splatter scenario. Larvae can be understood as small liquid pockets: if the water inside evaporates, the skin can collapse due to the internal pressure and, in the worst cases, it might ‘explode’. Gasses and particles can then access the electron column and compromise image quality for good.
How to image high moisture content samples with SEM
- Operate at a lower vacuum level;
- Freeze your sample;
- Dry your sample.
In addition, you should consider the effect of electrons accumulating on your sample. This translates into an image with bright areas where all the details can no longer be observed. This is typical when imaging polymers and other non-conductive materials.
Once again, sample preparation is the key to a good image.
How to image non-conductive samples with SEM
- Create a connection to the ground using metallic tape and image your sample in the nearby area;
- Sputter coat your sample with gold or other conductive coating materials;
- Operate at a lower vacuum level.
How to image powder samples with SEM
To ensure that downstream analysis of particle properties is accurate, it is not only necessary to separate individual particles, but also to break apart agglomerates. Figure 1 shows the difference between adding a powder sample directly onto the carbon adhesive, and using the Nebula™ vacuum dispersion system.
Once the particles in your sample have been evenly dispersed, you can start analyzing your sample using a SEM. The ParticleMetric software was developed in house and was specially designed with particle analysis in mind. This software package enables users to analyze particles in terms of parameters such as diameter, circularity or grayscale level. By revisiting outliers in the dataset, the user can perform elemental analysis via EDX for a powerful solution.
Recently, a group of researchers (Kotronia et al., available here) studied the encapsulation of oregano essential oils in β-Cyclodextrin. Using ParticleMetric, the average size of particles was determined, validating their experimental procedure.
In another use case, the Nebula™ was used for dispersing printer toner particles and particle size was determined using ParticleMetric, as part of a US patent application.
As can be seen from these examples, there are numerous solutions to problems you might encounter in your laboratory or industrial processes. If you would like to download the ParticleMetric & Nebula™ brochure, please click here.
How to image fragile or coated samples with SEM
The purpose of embedding is to protect fragile or coated materials during preparation, and to obtain good edge retention. Embedding is also used to produce specimens of a uniform size, such as minerals, clay or other particles and can also be used to section a material and investigate its interior.
Preparation of materialographic samples
Mechanical preparation is the most common method for preparing these so-called materialographic or metallographic samples for microscopic examination. After embedding, the samples are cut, grinded and polished. Abrasive particles are used in successively finer steps to remove material from the surface, until the required result is reached.
The mechanical preparation of these samples is a specialist task. Increasingly, however, more fully automated systems are available to make things easier. It can take a considerable amount of time to section, grind, mount and polish a sample.
Grinding and polishing
Grinding removes saw marks and levels and cleans the surface of the specimen. Polishing eliminates the artifacts of grinding but removes very little material. Grinding uses fixed abrasives – the abrasive particles are bonded to the paper or plates – for fast material removal. Polishing uses abrasive particles in a liquid, which are suspended on a cloth.
In summary, cutting the sample will take up to 1 hour, depending on the hardness. The grinding and polishing step may take approximately 2 – 2 ½ hours. Embedding, cutting and polishing are common techniques used to create flat samples for microscopic investigation.
Typically, these resin samples have a few standard size diameters. 32mm or 1 ¼ inch is the diameter most commonly used for optical, as well as electron optical, (SEM) investigation.
Resin mounted samples have a perfectly prepared flat top surface, which is a requirement for quantitative EDS results. However, the bottom surface can be very rough or even skewed. The Phenom desktop SEM sample holders and inserts have been designed in such a way that the top surface of these samples is always nicely leveled.
The Phenom desktop SEM can accommodate standard resin samples in diameter sizes between 25mm up to 40mm.
Sample preparation is key for obtaining flawless images
Sample preparation is a crucial step for obtaining flawless images. The best practices and more sample preparation tips and tricks are described in more detail in our sample preparation e-Guide.
Find out more about the optimal way to prepare your samples for electron microscopy imaging by downloading this sample preparation e-guide. The extended guide helps you obtain good results from the most common samples through tried-and-tested tips and tricks.
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