Low Voltage-STEM vs TEM

Introduction

LV-STEM imaging and TEM imaging have been introduced in their technique pages. In this page, example images of both techniques from a thin section lamella prepared by FIB-SEM are shown for comparison to showcase their respective advantages. 

The sample is nerves fixed by osmium tetroxide and then embedded in resin, and its lift-out section has an estimated thickness of 100–110 nm. 

More examples will follow...

TEM imaging

The images are taken from JEOL 1400 TEM at 120 kV at different magnifications. The resolution permitted from this conventional TEM is sub-nanometer level. For biological specimen which consists of mainly light elements, staining is necessary to enhance the contrast of the target constituent. 

The bright parts in the images on the right are axons and the dark regions are myelin. As shown in the high-resolution image (right), the fibrous structure in the myelin can be clearly observed and measured if needed. 

Theoretically, the resolution of the imaging can be further improved if the sample thickness reduces. 

LV-STEM imaging

Thanks to the various imaging modes available from the STEM system in Zeiss Crossbeam 550, images presenting different contrast can be obtained. The availability of imaging with various contrast is often essential to gain a better understanding of the material. 

For instance in material science, HAADF (High Angle Annular Dark Field) imaging is of great importance since it's free of diffraction contrast which is typical in crystalline materials. The images shown are obtained from BF (Bright Field) and HAADF respectively. 

Moreover, the samples are better preserved under beam irradiation since the highest acceleration voltage in SEM is 30 kV. 

Comparison between TEM and Low Voltage-STEM

TEM imaging in conventional BF mode offers better resolution with much higher electron acceleration voltages.

STEM imaging in SEM provides more imaging modes (SE, BF, DF, HAADF) with various contrast, working distance adaption, elemental analysis capability, less damage to the sample, and an option to large area imaging

Comparison between Low Voltage-STEM systems

The resolutions can be achieved in both systems of JEOL 7001FE and Zeiss Crossbeam 550 are rather similar. 

STEM in Crossbeam 550 offers a full range of imaging modes: SE, BF, DF, ADF, HAADF, while 7001FE has SE, BF, DF. 

Both systems can work with elemental analysis. However, due to the different detectors equipped, the speed of such analysis for achieving the same signal counts is also different. 

References

[1] D. ACEVEDO‐REYES, M. PEREZ, C. VERDU, A. BOGNER, T. EPICIER (2008) Characterization of precipitates size distribution: validation of low‐voltage STEM. Journal of Microscopy, vol.232 (1) DOI

[2] Y. Beyer, R. Beanland, P. A. Midgley (2012) Low voltage STEM imaging of multi-walled carbon nanotubes. Micron, vol.43, pp 428-434 DOI

[3] Brodusch N., Demers H., Gauvin R. (2018) Low Voltage STEM in the SEM. In: Field Emission Scanning Electron Microscopy. SpringerBriefs in Applied Sciences and Technology. Springer, Singapore LINK.