Observing the Kirkendall Effect with TEM and SEM

Electron microscopes use a beam of electrons to examine objects on a very small scale. These scientific instruments help to examine chemical components in materials and to image nanoscale features.

Properties of materials such as crystalline phase can be examined through electron diffraction, whilst x-ray analysis (EDS) can detect constituent elements.

TEM and SEM

The scanning electron microscope (SEM) has two types of imaging techniques that can be used to resolve nanoscale features. Backscatter electron imaging technique can be used to achieve element specific contrast and secondary electron imaging technique can be used to study surface topology.

EDS can also be utilized in the SEM to detect a variety of elements. Combining SEM and transmission electron microscopy (TEM) provides a different way of studying samples and correlates information. This provides an overall picture of the material properties.

The Aduro heating and electrical biasing platform utilizes E-chipTM semiconductor devices. These serve as the sample support and active area. This platform can be used with TEM and SEM instruments, converting both new and existing microscopes into in situ nanoscale laboratories.

Based on the type of experiment used, users can select between Electrical E-chips for electrical biasing experiments or Thermal E-chips for heating experiments. Protochips has developed software which can be used to control the electrical or thermal stimulus. The software communicates with an electronics control unit (ECU). This then transfers the stimulus to the E-chip via the SEM or TEM holder. Using this configuration, it is possible to use the E-chips, software and ECU with the SEM or TEM. Different holders are used for each instrument.

In this analysis, the Kirkendall effect was seen in zinc oxide and alumina concentric layers. The effect generally takes place when metal atoms spread at high temperatures and displace an interface. In this example, Zn diffuses into alumina and creates a spinal structure at greater than 700 oC. In addition, this process generates Kirkendall voids. These are a well-known phenomenon used to make hollow spherical shells, nanotubes, dendrites, and other nanomaterials. It was possible to capture the formation of the spinal structures and the resultant voids in situ in real time using the Aduro heating and electrical biasing platform in the TEM and SEM instruments.

Experimental Procedure

A hollow tube containing three coaxial layers of 30 nm ZnO, 35 nm Al2O3 and 38 nm Al2O3 was formed using the atomic layer deposition (ALD) technique. During the SEM imaging experiment, a polymer fiber was electrospun onto a Thermal E-chip membrane. Then, using the ALD technique, the fibers were coated with the three layers. Next, the polymer fibers were dissolved to leave Al2O3/ZnO/Al2O3 microtubes.

The tubes were then imaged in secondary electron mode in a Hitachi SU-6600 SEM. For the TEM imaging experiment, microtubes were prepared and their thin sections were produced using the focused ion beam (FIB) method. Next, these sections were mounted on the Thermal E-chip membrane. Images were obtained in bright field mode with the help of a JEOL 2010F operating at 200 kV.

Discussion

An SEM image of a tube prior to heating is shown in Figure 1. The arrows show the coaxial layers of ZnO and Al2O3. After being heated, the ZnO diffuses into the Al2O3 layers and produces the ZnAl2O3 spinel.

Void formation was viewed in real time over a span of 10 minutes in the middle layer where the ZnO layer formed initially. Void formation was also imaged and viewed in real time.

SEM image of a tube before heating.

Figure 1. SEM image of a tube before heating.

A TEM image of the thin section produced using the FIB method is shown in Figure 2, along with the three layers. The Kirkendall effect was observed in high resolution after heating to 750 oC for 10 minutes.

The formation of an image in the TEM is completely different from that of the SEM. Therefore, different features can be seen in this image. The Zn diffuses into the Al2O3 and this leads to larger crystal gains. Variations in the contrast were also observed. Grain growth in the microtubes could be seen, although voids were not seen in this case.

TEM image of the thin section created with FIB.

Figure 2. TEM image of the thin section created with FIB.

These correlative imaging experiments show the Kirkendall effect in real time. Therefore, they can be applied for research and development in batteries, semiconductors and drug delivery.

Conclusion

As well as the processes discussed, correlative imaging experiments can be applied to different types of material processes. The Aduro platform offers excellent flexibility and this makes it possible to carry out fast and easy correlative experiments, without compromising on data quality.

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

For more information on this source, please visit Protochips.

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