Utilizing Microline Indentation for Controlled Cleaving of Crystalline Materials

This article explains how the LatticeAx cleaving machine was used by scientists at the University of Cambridge to support their research into a three-dimensional material with extremely unique properties.

It was an unexpected discovery made by Dr. Suchitra Sebastian of the University’s Cavendish Laboratory and her team of PhD students. Depending on the physical properties measured, both conducting and insulating behaviors in the same material were revealed by experiments which were focused on measuring the electronic properties of samarium hexaboride (SmB₆). This innovative research, which was featured in the July, 2015, issue of Science Magazine, is currently challenging the accepted principles of material behavior, and poses some extremely interesting questions on how this material could be used.

To perform this research, tiny samples of SmB₆ measuring 1 mm² x 300 µm, and oriented along specific crystallographic directions, had to be prepared from a bigger ingot measuring 4 mm long and 2.5 mm in diameter, without causing any damage to the material. Alternatives to techniques utilized earlier, which included electro-polishing and manual cleaving, were sought to accomplish the required orientation and size. The irregular shape and hardness of the three-dimensional material highlighted the fact that the sectioning could not be conducted solely using hand tools, while electro-polishing held a risk of modifying material properties, or missing the target facet in the process of over-polishing. These techniques failed to offer a guarantee that they could repeatedly produce the damage-free samples of the required orientation and dimensions.

Figure 1. Crystal shown after cleaving with the LatticeAx. The SmB₆ cleaved crystal is now ready for experiments to measure electronic properties.

LatticeAx® Cleaving Machine

Yu-Te Hsu, a PhD student in the Quantum Matter Group, was searching for a means to increase the success rate to at least 50% yield for the desired sample parameters. This was particularly significant because the SmB₆ crystals were being grown at the University of Warwick (Sci. Rep. 3:3071) specifically for the study. Hsu heard about the LatticeAx® from Dr. Suchitra Sebastian, his PhD supervisor, and together they hoped that it could offer the solution to the challenge of achieving precise orientation and damage-free samples of the required dimensions more consistently.

Figure 2. The SmB₆ sample is approximately 4 mm in width and 1 mm in height, with both cleaved [001] planes. It is sliced from the ingot; the curved side is the perimeter of the ingot. Using the optical microscope and computer display, it is possible to confirm the shape and dimension for the slice of the material.

The LatticeAx uses a technique called microline indentation to deliver a method for controlled cleaving of crystalline materials. Both the location and depth for the indent can be controlled by users with the help of a fine positioning mechanism. Operation of the LatticeAx; offering a high degree of flexibility for sample dimension and size, can be learned in a matter of minutes and mastered in just a few hours. In order to further enable assured placement accuracy for the initial indent (Figure 4), Cavendish Laboratory connected its own high magnification optical microscope to the LatticeAx.

To begin with Hsu used a very fine wire-saw to cut a disk of ~1 mm thickness in order to initiate the downsizing process for the ingot. He followed this by cleaving the disk using the LatticeAx; making parallel cleaves perpendicular to the disk plane to arrive at the desired dimension of 1 mm x 1 mm x 300 µm. The sample was then ready for surface, magnetic and electrical characterization. The crystals with superior quality were selected for measurements at the National High Magnetic Field Laboratory in the United States, under incredibly intense magnetic fields (up to 100 T) and very low temperatures (down to 0.03 K).

The LatticeAx was capable of repeatedly and reliably producing the required precisely-oriented and superior quality small samples, even on this three-dimensional material. Hsu attributes this success to two factors. To begin with, the SmB₆ material has extremely high crystallinity, which facilitates well-defined cleaved surfaces. However, without the fine placement control delivered by the LatticeAx, it would not have been possible to indent and cleave at the preferred site.

"The LatticeAx allowed us to prepare samples meeting our experiment’s specifications, with repeatable accuracy that simply isn’t possible with hand-held manual cleaving tools." – Yu-Te Hsu, PhD Student.

Different crystallographic planes may exhibit varied behaviors for this unusual material, so it was vital for Hsu to resolve the [001] direction on the sample. It was essential for the researchers to be confident of the sample’s crystallographic direction with respect to the magnetic field they were applying. For this reason, LatticeAx’s ability to deliver the precisely-oriented crystal facets reproducibly made it a vital component in this research.

Hsu discovered that the LatticeAx can be used very easily. He had experience of using both electro-polishing and manual cleaving, and therefore had a good idea of the steps needed to get the samples to the correct size and cleaved along the correct crystal facet. The most appropriate way to use the LatticeAx for this three-dimensional material was ascertained in just a single afternoon.

The portability of the LatticeAx itself is considered to be one interesting aspect of this story. Hsu was able to carry the LatticeAx in his backpack when he occasionally travelled to visit a collaborator’s laboratory. This was possible because of the compact size of the LatticeAx. With collaborators located in both the United States and England, this portability allowed the research to continue when new material samples were available.

Figure 3. The SmB₆ crystal before the final cleave (top), and after (bottom), with damage-free surface at the precise orientation required for the experiments

Research Results

Ultimately, Hsu succeeded in achieving his goal of 50% yield for samples, with the help of the LatticeAx. He was able to rapidly complete the sample preparation required for the research, with an increased degree of accuracy and repeatability. This accomplishment would not have been possible with hand tool-based manual cleaving tools. Hsu avoided the introduction of artifacts in addition to the chance of overshooting the target facet which could have occurred with electro-polishing. Significantly, it was possible for Hsu to shape the crystal as he downsized, discover the orientation and develop a damage-free crystal facet at the same time - using the LatticeAx; tasks which would have taken almost a week using conventional methods.

Figure 4. Cavendish Laboratory pairs the LatticeAx base platform with a Cavendish-supplied optical microscope and computer to further enable assured placement accuracy for the initial indent. This cleaving workstation ocupies only a small area on a work surface and is highly portable.

Conclusion

Currently, Hsu and his colleagues are working on further studies on SmB₆ and other closely related materials in the hope of seeing further exotic behaviors, and LatticeAx will continue to work as their “Excalibur”.

This information has been sourced, reviewed and adapted from materials provided by LatticeGear

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