Microline Indentation for Cleaving of Crystalline Materials

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

It was a surprising discovery made by Dr. Suchitra Sebastian and her team of PhD students at the University’s Cavendish Laboratory. Based on the physical properties measured, conducting as well as insulating behaviors in the same material were revealed by experiments which were focused on determining the electronic properties of samarium hexaboride (SmB₆). Published in the July 2015 issue of Science Magazine, this ground-breaking study is currently challenging the accepted principles of material behavior and posing some fascinating questions on how this material could be used.

In order to conduct this research, minute 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 and 2.5 mm in length and in diameter, respectively, without causing any material damage. Alternatives to previously used techniques, such as manual cleaving and electro-polishing, were sought to achieve the required size and orientation. The irregular hardness and shape of the three-dimensional material emphasized the fact that the sectioning could not be performed with hand tools alone, while electro-polishing held a risk of altering material properties, or missing the target facet in the process of over-polishing. These methods do not ensure repeated production of damage-free samples of the required dimensions and orientation.

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 looking for a method to improve the success rate to at least 50% yield for the preferred sample parameters. This was especially important since the SmB₆ crystals were being grown at the University of Warwick (Sci. Rep. 3:3071) particularly for the research. Dr. Suchitra Sebastian, who was Hsu’s PhD supervisor, introduced to him the LatticeAx^® cleaving machine and jointly they believed that it could provide the answer to the challenge of accomplishing precise orientation and damage-free samples of the required dimensions more reliably.

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.

Using a technique known as microline indentation, the LatticeAx delivers a method for controlled cleaving of crystalline. Users can control both the location and depth for the indent by means of a fine positioning mechanism. Operation of the LatticeAx, which provides a high degree of flexibility for sample dimension and size, can be learned within a few minutes and mastered in just a few hours. In order to further allow guaranteed placement accuracy for the initial indent (Figure 4), Cavendish Laboratory coupled its own high magnification optical microscope to the LatticeAx.

In order to initiate the downsizing process for the ingot, Hsu initially used a very fine wire-saw to cut a disk of approximately 1 mm thickness. He then cleaved the disk using the LatticeAx, making parallel cleaves at right angles to the disk plane to arrive at the preferred dimension of 1 mm x 1 mm x 300 µm. The sample was then ready for electrical, magnetic, and surface characterization. Superior quality crystals were chosen for measurements at the National High Magnetic Field Laboratory in the United States, under very low temperatures (down to 0.03 K) and extremely intense magnetic fields (up to 100 T).

Even on this three-dimensional material, the LatticeAx could reliably and consistently generate the required precisely-oriented and better quality small samples. According to Hsu, this success is attributed to two factors. Firstly, the extremely high crystallinity of the SmB₆ material, which enables well-defined, cleaved surfaces. However, indention and cleaving at the desired site, was achieved only because of the fine placement control delivered by the LatticeAx.

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

Since different crystallographic planes may display different behaviors for this unusual material, it was essential for Hsu to resolve the [001] direction on the sample. The researchers had to be certain of the sample’s crystallographic direction with regards to the applied magnetic field they were applying. Therefore, LatticeAx’s ability to deliver the precisely-oriented crystal facets reproducibly made it a crucial component in this study.

Hsu found that it was easy to use the LatticeAx. Since he had experience of using both manual cleaving and electro-polishing, he was familiar with the steps required to get the samples to the right size and cleaved along the correct crystal facet. The most suitable method to use the LatticeAx for this three-dimensional material was determined in just a single afternoon.

The portability of the LatticeAx itself is regarded as a fascinating aspect of this story. Hsu could carry the LatticeAx in his backpack when he sometimes traveled to visit a collaborator’s laboratory. This was possible due to the compact size of the LatticeAx. With collaborators based in both England and the United States, this portability enabled the study to continue when there was availability of new material samples.

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 was able to achieve the target 50% yield for samples by using the LatticeAx. He could quickly complete the sample preparation needed for the research, with an improved degree of repeatability and accuracy. This accomplishment could not have been realized with hand tool-based manual cleaving tools. Hsu avoided introducing artifacts in addition to the possibility of overshooting the target facet which could have taken place with electro-polishing. Most importantly, Hsu was able to shape the crystal as he downsized, discover the orientation, and develop a damage-free crystal facet simultaneously — all with the help of the LatticeAx; tasks which would have taken nearly a week using traditional 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 occupies only a small area on a work surface and is highly portable.

Conclusion

Presently, Hsu and his team are working to further examine SmB₆ and other closely related materials, in the anticipation of seeing further exotic behaviors, and LatticeAx will continue to serve as their “Excalibur”.

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

For more information on this source, please visit LatticeGear.