Enhancement of low-spatial-frequency components by a new phase-contrast STEM using a probe formed with an amplitude Fresnel zone plate.

Electron microscopy is a powerful tool for visualizing the shapes of sub-nanometer objects. However, Contrast Transfer Function (CTF) principally restricts lower frequency components in the image. To overcome this problem, phase-plate techniques have been proposed and currently Hole Free Phase Plate (HFPP) and Volta Phase Plate (VPP) are widely used especially for biological specimens to retrieve low frequency information of the sample's potential distributions. In this report, we have developed a new phase-contrast scanning transmission electron microscope (STEM) in which a probe beam including side robes is formed with an amplitude Fresnel zone plate (FZP) and the interference patterns produced by the zero and first order diffracted waves generated by the FZP are detected. We name it FZP Phase Contrast STEM (FZP-PC-STEM) hereinafter. The amplitude FZP was manufactured by using focused ion beam (FIB) equipment, and the diffraction data were collected by using diffraction imaging technique. The validity of our proposed optical model was confirmed by comparing experimental and simulated images. Observations of carbon nanotube (CNT) bundles by using this method showed that the contrast of low-spatial-frequency components in the CNT image was significantly enhanced. This method does not, in principle, require the post-image processing used in the diffraction imaging method, and it can be easily introduced into pre-existing equipment without major modifications. The stability and robustness of the FZP inserted in condenser system were also confirmed during long-time operation. We expect that the FZP-PC-STEM will be widely applicable to high-contrast observations of low-Z samples with simple and easy operation.

Transmission Electron Microscope Using a Linear Accelerator.

High-voltage transmission electron microscopes (HVTEMs), which can visualize internal structures of micron thick samples, intrinsically have large instrument sizes because of the static voltage isolation. In this Letter, we develop a compact HVTEM, employing a linear accelerator, a subpicosecond beam chopper, and a linear decelerator. 100 kV electrons initially accelerated by a static field are accelerated at radio frequency (rf) up to 500 kV, transmitting through the sample and finally rf decelerated down to 200 kV to be imaged through a 200 kV energy filter. 500 kV imaging, as well as subnanometer resolution at 200 kV, have been demonstrated.