Using TEM and SEM to Observe the Kirkendall Effect in Zinc Oxide/Alumina Spinels

Electron microscopes are scientific instruments that use a beam of electrons to examine objects on a very fine scale. They help in imaging nanoscale features and also aid in examining chemical components in materials.

In fact, properties of materials like crystalline phase can be examined through electron diffraction and constituent elements can be detected through x-ray analysis (EDS).

TEM and SEM

In the scanning electron microscope, also known as SEM, two types of imaging techniques 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.

In addition, EDS can be utilized in the SEM to detect various elements. The combination of transmission electron microscopy (TEM) and SEM provides different means to study samples and correlate information to achieve an overall picture of the material properties.

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

Users can now select between Thermal E-chips for heating experiments or Electrical E-chips for electrical biasing experiments, based on the type of experiment used. Protochips has developed a software, which can be used to control the thermal or electrical stimulus. The software communicates to an electronics control unit (ECU), which in turn transfers the stimulus to the E-chip via the SEM or TEM holder. With the help of this configuration, it is possible to use the E-chips, ECU, and software with the TEM or SEM, using different holders for each instrument.

In this analysis, the Kirkendall effect was seen in alumina and zinc oxide concentric layers. Generally, this effect takes place when metal atoms spread at high temperatures and displace an interface. In this analysis, Zn diffuses into alumina and forms a spinel structure at greater than 700°C. This process also generates Kirkendall voids, a well-known phenomenon used to make dendrites, nanotubes, hollow spherical shells, and other nanomaterials. By using the Aduro heating and electrical biasing platform in the SEM and TEM instruments, it was possible to capture the formation of the spinel structures and the resultant voids in situ in real time.

Experimental Procedure

The atomic layer deposition (ALD) technique was used to form a hollow tube containing three coaxial layers of 30nm ZnO, 35nm Al2O3, and 38nm Al2O3. For the SEM imaging experiment, a polymer fiber was electrospun onto a Thermal E-chip membrane and the fibers were subsequently coated with the three layers using the ALD technique. Next, the polymer fibers were dissolved leaving Al2O3/ZnO/ Al2O3microtubes.

These 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. These sections were then mounted on the Thermal E-chip membrane. With the help of a JEOL 2010F operating at 200kV, images were obtained in bright field mode.

Discussion

Figure 1 shows an SEM image of a tube prior to heating. The arrows denote the coaxial layers of Al2O3 and ZnO. Subsequent to heating to 750°C, the ZnO diffuses into the Al2O3 layers and produces the ZnAl2O3 spinel.

The formation of void was viewed in real time over the span of 10 minutes in the middle layer where the ZnO layer initially formed. The formation of voids were also imaged and viewed in real time.

Figure 1. SEM image of a tube before heating.

Figure 2 displays a TEM image of the thin section produced using the FIB method and also shows the three layers. The Kirkendall effect was observed in high resolution following heating to 750°C for a period of 10 minutes.

The formation of image in the TEM is entirely different from that of the SEM, and therefore different features can be viewed in this image. Since the Zn diffuses into the Al2O3, this leads to larger crystal grains. Variations in the contrast were also observed. Though voids were not seen in this case, grain growth in the microtubes could be seen.

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

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

Conclusion

Besides the processes described above, correlative imaging experiments can be applied to different types of material processes. The Aduro platform provides excellent flexibility, which makes it possible to carry out fast and easy correlative experiments without affecting the data quality.

About Protochips

Protochips, Inc. is a rapidly growing early-stage company focused on providing the world's leading materials and life sciences research breakthrough analytical tools for targeted research and development of nano-scale materials.

Using its proprietary technology, Protochips is addressing the market need by transforming the most widely used tools in nanotechnology – electron and optical microscopes - from cameras into complete nano-scale laboratories.

Protochips' core competency lies in the application of semiconductor techniques to development of MEMS devices capable of providing heat, electrical, liquid and gas environments to samples in situ.

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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