Monday, July 16, 2018

20 nm Nanoparticles

In this image gold nano-particles were imaged at 2MX using the through lens detector (TLD) in field immersion using secondary electrons. The sample was drop-cast directly onto an aluminum stub and then subsequently heated at over 100 C to drive off any physisorbed water and organic material. Sub 20 nm particles are clearly imaged and differentiated from each other. This is remarkable as TEM or AFM is generally required to image particles of this size.

There are several concerns in taking images at this high magnification. One is that there needs to be sufficient secondary electron contrast to image the objects. This is Au on Al so there is sufficient secondary electron contrast to differentiate the nanoparticles from the stub. Note that the stub looks fairly featureless while an AFM image of the same stub would show structure. There is just no secondary electron contrast from Al on Al at these feature scales. Thus Al particles drop cast on the aluminum SEM stub would impossible to image.

Another concern is sample preparation and chamber vacuum. At very high magnifications the current density is exceedingly large. Organic material on the sample surface and in the SEM chamber will darken the image where the SEM beam interacts with the sample. At low magnifications this effect can be minimal. At the very worst leaving behind dark rectangles of reduced secondary electron contrast when the magnification is subsequently reduced. However at 2MX the image will just turn dark and featureless in seconds.

Another concern is stability. This includes acoustic noise in the SEM lab, mechanical stage drift, thermal drift of the sample as well as of the SEM column, and electromagnetic interference in the SEM lab. Mitigating the effects of electromagnetic interference may require active field compensation or shielding in the SEM lab. The effects from mechanical vibration and stage drift may require changing scan speeds and applying the stage clamp which helps lock the stage down. The objective lens is not a constant-current lens, and there is always some image drift until it comes to thermal equilibrium. Another significant instability is magnetic hysteresis in the objective lens. Remnant magnetic field in the lens can lead to drift. This is mitigated using the lens reset function.

The point of this application note is that 2MX magnification may seem very exciting-- but results are entirely a matter of sample preparation and operator skill!

Sunday, July 15, 2018

Sample Preparation: Dilution, Dispersion and Surface Charge

Successful SEM imaging requires not only competent microscope operation but also expert sample preparation. In the case of imaging nanostructures, sample preparation is generally the most difficult task. In this example cellulose nanofibers were imaged at low energy due to their low density. In the first image we see an undifferentiated mass as the undiluted but dried fibers were imaged after being dried on a stub and coated with 4 nm of iridium to suppress charging.

The individual fibers are largely unobserved. This is due to the surface charge of the fibers causing them to aggregate to not only each other but also the stub itself. At this point successful imaging comes down to successful sample preparation.

The first plan of attack in any dispersion problem is dilution. In the second image the fibers were imaged after significant dilution. Dilution helps disperse nano-structures by simply keeping them from each other so that they can not aggregate. Dilutions of 1:100 to 1:1000 or more can be necessary.

It was found that dilution alone did not solve the aggregation problem. Dilution provided small patches of agglomerated fibers that resembled the first image. This was because cellulose fibers are generally tangled together and can not be separated upon dilution-- only individual clumps can be separated.

To mitigate surface charge the nanocellulose fibers were subject to washing in DI water and drying with a polar solvent to screen surface charge. Then the material was placed in a non-polar solvent to maintain space between the fibers. This mixture was then quickly dried so that free space between fibers was preserved.

In practice the surface potential of nanoparticles may need to be measured using an instrument that measures zeta-potential, and the surface potential neutralized using a solvent of an appropriate pH. this is often the case with such nanoparticle systems as aluminum oxide where surface potential is highly dependent upon synthesis and post synthesis handling.

The moral of the application note is to not give up when faced with useless images-- success is often a matter of a little experimentation with sample preparation.

Natural Nanopillars: Cicada Wing

Sometimes it is useful to have a source of regular nano-structures for a variety of imaging and metrology tests. Dispersing structures can be difficult due to aggregation due to surface change or substrate charge. Luckily nature provides arrays of these on cicada wings. The natural nanopillars on cicada wings have a natural anti-microbial affect as micro-organisms are torn apart by these pillars.

In this image taken at lower beam energy (3 kV) using the through lens detector in field immersion a large field of regular nanopillars is seen at 30 kX magnification. The pillars have local order, falling into regular domains without long term order.

The second image taken at 300 kX shows the faceted structure of the nanopillars and confirms their diameter as less than 100 nm on average.

The cicada wing was pulled off a dead insect and coated with 4 nm of iridium to suppress charging without any additional sample preparation.


Wednesday, July 11, 2018

Subsurface Imaging with SE2 Secondary Electrons

The range of electrons in materials is dependent upon their energy. This is a physical property that can be exploited to image sub-surface structures. This is a thermoplastic coated with 4 nm of Ir to suppress charging. In the first image the sample is imaged at 50kX at 15 kV using the through-lens (TLD) detector and field-immersion using secondary electrons.

Brighter regions are visible, indicating regions of higher secondary electron emission. A close examination seems to suggest that these particles are not resting on the top of the surface. In the second image the beam energy is dropped down to 2 kV and these bright regions disappear showing a fairly smooth surface. What is going on?

These images show the difference between SE1 and SE2 secondary electrons. In the first image taken at 15 kV there are secondary electrons coming from the sample surface, but also back-scattered electrons scattering from denser objects just under the sample surface. These elastically scattered electrons have considerable energy as they leave the surface, nearly 15 kV, and can generate secondary electrons themselves. These secondaries are called SE2 and appear in the vicinity of the dense sub-surface particles.

By dropping down to 2 kV these SE2 are no longer generated because the cross section for elastic scattering is greatly reduced, as is the cross section for generating SE2's from lower energy electrons. Thus the lower energy image fails to show these subsurface structures.

The point of this application note is to show that SE2's can reveal subsurface structure. This can be exploited to see objects under the surface of a sample. It can also be a source of confusion-- and imaging at several beam energies may be required to resolve this confusion.

The final images shows a back-scattered electron image (BSED) confirming the presence of higher-density inclusions in this material.



Thursday, July 5, 2018

Stigmators: Adjusting and Optimizing Astigmatism

The correction of astigmatism is essential to high resolution imaging, particularly at higher magnifications. Astigmatism is an aberration where the electron column has different focal lengths in two orthogonal directions. This means the image will be focused in one direction at one focal length, and focused in an orthogonal direction at a different focal length. In pragmatic terms this can also be understood as the objective lens (OL) being able to focus to a "blob" but not a point at the optimal focus.

Stigmators are octopole lenses (sometimes 12-pole lenses) that compensate for astigmatism at the level of the objective lens.  This series of images shows the effect of stigmator adjustment. The top image shows the best possible focus of a small particle at 40kX. The working distance, reflecting the OL focal length, is 19.4 mm.

The next image the OL focal length is reduced to 19.2 mm and the image shows streaks going from bottom right towards top left. Another way of looking at that is the image is focused along a roughly 45 degree diagonal from bottom left to top right.

The third image shows the OL focal length increased to 19.5 mm and the image shows streaks going from the bottom left to the top right. Another way of looking at that is the image is focused sharply along a roughly 135 degree diagonal from bottom right to top left.

This streaking in one direction followed by streaking in an orthogonal direction at a different focus is a classic sign of a significant amount of astigmatism.

The final image shows the same particle with the stigmators properly adjusted. Without astigmatism correction this would have looked like a particle on a smooth background, not a background of nanoparticles.

The best approach in astigmatism correction is to find the OL focus settings that provide the second and third images, and then focus in between. This should provide the first image. Then adjust the X- and Y-stigmators, followed by fine focus, iteratively, until one arrives at the last image.