Contrast enhancement of nanomaterials using phase plate STEM

First step toward complex observations by 4D-STEM with phase plate

Quantitative measurements by electron microscopy are becoming increasingly important because we are often concerned with establishing quantitative relationships between the properties and structures of materials. This paper presents a method to derive the scattering and phase contrast components from scanning transmission electron microscope (STEM) images using a phase plate and two-dimensional electron detector and to quantitatively evaluate the amount of phase modulation. The phase-contrast transfer function (PCTF) modifies the phase contrast because it is not unity over all spatial frequency regions; therefore, the amount of phase modulation observed in the image becomes smaller than the actual value. We applied a filter function to the Fourier transform of image to perform PCTF correction and evaluated the phase modulation of the electron waves, which was quantitatively agreement with the values expected from the thickness estimated from the scattering contrast within 20% error. So far, few quantitative discussions on phase modulation have been conducted. Although the accuracy needs to be improved, this method is the first step toward quantitative complex observations.

Cathodoluminescence of green fluorescent protein exhibits the redshifted spectrum and the robustness

Green fluorescent protein (GFP) and its variants are an essential tool for visualizing functional units in biomaterials. This is achieved by the fascinating optical properties of them. Here, we report novel optical properties of enhanced GFP (EGFP), which is one of widely used engineered variants of the wild-type GFP. We study the electron-beam-induced luminescence, which is known as cathodoluminescence (CL), using the hybrid light and transmission electron microscope. Surprisingly, even from the same specimen, we observe a completely different dependences of the fluorescence and CL on the electron beam irradiation. Since light emission is normally independent of whether an electron is excited to the upper level by light or by electron beam, this difference is quite peculiar. We conclude that the electron beam irradiation causes the local generation of a new redshifted form of EGFP and CL is preferentially emitted from it. In addition, we also find that the redshifted form is rather robust to electron bombardment. These remarkable properties can be utilized for three-dimensional reconstruction without electron staining in focused ion beam/scanning electron microscopy technology and provide significant potential for simultaneously observing the functional information specified by super-resolution CL imaging and the structural information at the molecular level obtained by electron microscope.