Investigation of hole-free phase plate performance in transmission electron microscopy under different operation conditions by experiments and simulations

In an analysis of material structures, transmission electron microscopy (TEM) stands out as an essential tool. A wide variety of information can be obtained from a sample by appropriately using TEM. Recently, phase contrast transmission electron microscopy (PC‐TEM) with a hole‐free phase plate (HFPP) has been established [1‐4]. The HFPP is placed at the back focal plane of the objective lens. The incident beam passing through the thin film of HFPP generates secondary electrons, and they in turn lead to an electrostatic charging (potential). As the intensity distribution of electron beam on the HFPP has sharp peak at the center of optical axis, the charging occurs only at region of the beam crossover. This causes an additional phase shift for the direct beam, resulting in high contrast images of samples. The difference of phase shift between scattered electrons and direct electrons, made by the charging, enhance the image contrast as the common phase microscopy. Thus, the contrast of materials made of light elements, e.g., polymer samples and biological samples, can be significantly enhanced by HFPP. Furthermore, we expect that it may be possible to derive phase, i.e., the mean inner potential, from the images obtainable from the PC‐TEM with HFPP. In this study, we demonstrate a couple of experimental results of PC‐TEM with HFPP. First example is an observation of nano‐scale periodic structures of block copolymer (BCP). BCPs consist of light elements and thus often require “staining” of one of the phases for contrast enhancement under TEM observations. Because the staining might change nano‐structures, it would be better not to use this method for increasing contrast. We used a field emission TEM (JEM‐2200FS) operated at 200 kV with the PC‐TEM with HFPP in this experiments. Figure.1 shows TEM images of a unstained block copolymer (poly(styrene‐ b ‐isoprene)) obtained with and without HFPP. The lamellar structure was hardly seen in the TEM images without HFPP at in‐focus, i.e., Figures 1a. Under‐focusing slightly enhanced the contrast between the polystyrene (PS) and polyisoprene (PI) phases in Figure 1b. In contrast, it was obvious that the use of HFPP significantly increased contrast of the two phases in lamellar structure. Next, we explore the possibility of HFPP imaging for phase retrieval. As a first trial we selected the simple case of nanoparticles, for which we can assume a weak phase object approximation. As far as the phase values is small (typically <0.5 rad), we propose to retrieve the phase from two images with and without HFPP at the same defocus condition. The knowledge of the two images allows then for a simple inversion of the contrast transfer function that directly leads to a phase image of the area. Figure 2a and 2b show images of platinum nanoparticles with and without HFPP. Figure 2c is the phase image retrieved from the two images using the simple inversion. Phase images have been also retrieved for sets of images taken at different defocus (0, ‐40nm, ‐80 nm): the same phase images were obtained (though with a better signal/noise ratio at zero defocus). For comparison, an in line holography processing earlier validated on gold nanoparticles [5] has been applied: a good agreement on the phase value has been observed when comparing to the phase image of the same area obtained with and without HFPP. The measured phase values are close to 0.3 rd for 2‐nm particles as expected from simple calculation based on the mean inner potential (V Pt = 25 V). The advantage of the PC‐TEM with HFPP is easier usability than conventional phase retrieving methods, such as off axis electron holography. From these applications, we emphasize that PC‐TEM with HFPP would be one of prospective analysis tools for materials characterization.

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.

In this paper, we review the current state of phase plate imaging in a transmission electron microscope. We focus especially on the hole-free phase plate design, also referred to as the Volta phase plate. We discuss the implementation, operating principles and applications of phase plate imaging. We provide an imaging theory that accounts for inelastic scattering in both the sample and in the hole-free phase plate.

In Marangoni–Bénard convection with mass transfer from gas phase to liquid phase, it is found that solitary waves, excited and sustained by Marangoni stresses, are very stable during interactions with others, keeping their original shapes and celerities after interactions, thus experiencing at most a phase shift. Both positive and negative phase shifts have been observed for different types of oblique interaction. A ‘‘critical’’ angle, at which zero phase shift occurs as a result of the interaction, has been defined to delineate regions of negative and positive phase shifts.

Toward the quantitative the interpretation of hole-free phase plate images in a transmission electron microscope.

In this paper, unified modeling method for dual-active-bridge (DAB) bidirectional converters (BDCs) considering bidirectional phase shift is proposed. There is only one model, and it is shared by both positive and negative phase shift. DAB and its extended converters are widely used in energy storage systems. These converters are regulated by duty cycle plus phase shift. The existing modeling methods model these converters in positive and negative phase shift directions respectively. Though the two models are built in similar ways, they are not the same. Hence they are hard to be utilized in applications where phase shift direction changes. Therefore, a modeling method to build only one model for both phase shift directions is desired. In this paper, a conjugate shifted phase based superposition technique is proposed to facilitate the proposed bidirectional unified modeling method. By this technique, voltage on the shifted side of the transformer is expressed in a superposition form, which represents both situations of positive and negative phase shift simultaneously. This superposition form will collapse to actual form, once the shift direction is given. Hence situations in both directions are unified. Besides, a newly proposed current division technique is integrated into this method for high model accuracy. A buck-boost coupled dual-active-half-bridge (DAHB) converter is selected to describe the method. The only one large-signal model for both phase shift directions is established, so as a small-signal model. Simulations and experiments have validated the proposed modeling method and its high accuracy

Optimizing the phase shift and the cut-on periodicity of phase plates for TEM

With contrast enhancement technique, cryo-transmission electron microscopy (cryo-TEM) can be more advantageous. A Zernike phase plate (ZPP) and a hole-free phase plate (HFPP) are reported to have capability to enhance contrast [1, 2, 3, 4]. The ZPP is made of thin amorphous carbon film with a center hole, and the HFPP is made of thinner carbon film with no hole. In this study, we report the results with these phase plates on cryo-microscopic techniques such as cryo-TEM tomography and single particle analysis. With ZPP and HFPP, images in a close-to-focus condition exhibited higher contrast compared to defocus phase-contrast images (see Fig. A-C). The image with HFPP shows reduced fringe contrast (Fig. C), compared to the image with ZPP (Fig. B). In addition, the HFPP does not require any alignment procedure due to lack of a center hole. Thus, the data acquisition with HFPP could be easier than that with ZPP. [1] R Danev and K Nagayama, Ultramicroscopy 88 (2001) 243-252. [2] M Malac et al, Ultramicroscopy 118 (2012), 77-89. [3] M Malac et al, U.S. Patent US 8,785,850. [4] M Malac et al, JP. Patent 2011-151019. Figure: Cryo-TEM images of ice-embedded GroEL particles acquired with a few microns of defocus (A), ZPP (B) and HFPP (C). The images with ZPP and HFPP were taken with a close-to-focus condition. C2-P-07 doi:10.1093/jmicro/dfv303

Sub-3 Å apoferritin structure determined with full range of phase shifts using a single position of volta phase plate

The optimal carbon nanotube (CNT) bundles with a hexagonal arrangement were synthesized using thermal chemical vapor deposition (TCVD). To enhance the electron field emission characteristics of the pristine CNTs, the zinc oxide (ZnO) nanostructures coated on CNT bundles using another TCVD technique. Transmission electron microscopy (TEM) images showed that the ZnO nanostructures were grown onto the CNT surface uniformly, and the surface morphology of ZnO nanostructures varied with the distance between the CNT bundle and the zinc acetate. The results of field emissions showed that the ZnO nanostructures grown onto the CNTs could improve the electron field emission characteristics. The enhancement of field emission characteristics was attributed to the increase of emission sites formed by the nanostructures of ZnO grown onto the CNT surface, and each ZnO nanostructure could be regarded as an individual field emission site. In addition, ZnO-coated CNT bundles exhibited a good emission uniformity and stable current density. These results demonstrated that ZnO-coated CNTs is a promising field emitter material.

ABSTRACTCarbon nanotubes (CNTs) were synthesized by thermal chemical vapor deposition (thermal CVD) on n-type Si (100) at 700 ° under C2H2 gas flow ratio of 30 sccm. Fe catalysts were pre-deposited by RF sputtering system with RF power 150 W. Two kinds of as-grown CNTs were used to detect N2: the vertically oriented carbon nanotubes (CNTs) mat and horizontally oriented CNTs bundle. Two-terminal electrical measurements were performed at room temperature of 25 °. The electrical resistance of CNTs mat or bundle was found to increase when exposed to N2 environment, and to return back after the N2 pumping, respectively. However, the CNTs bundle had better sensitivity and possessed faster response and recovery time. This could be ascribed to that the CNTs bundle, with more effective grooves on the surface, provided more lower binding-force sites to absorb N2 molecules than the CNTs mat dose, which prominently had interstitial sites.

Investigation of the third-order nonlinear refraction using 4f coherent imaging system with positive–negative bar phase objects

We demonstrate the use of a hole-free phase plate (HFPP) for magnetic imaging in transmission electron microscopy by mapping the domain structure in PrDyFeB samples. The HFPP, a Zernike-like imaging method, allows for detecting magnetic signals in-focus to correlate the sample crystal structure and defects with the local magnetization topography, and to evidence stray fields protruding from the sample. Experimental and simulated results are shown and are compared with conventional Fresnel (out-of-focus) images without a phase plate. A key advantage of HFPP imaging is that the technique is free from the reference wave distortion from long-range fields affecting electron holography.

This paper proposes novel triangular cross-sectioned geometry of carbon nanotube (CNT) bundles for crosstalk and delay reduction in CNT bundle interconnects for VLSI circuits. First, we formulate the equivalent single conductor (ESC) transmission line models of the interconnects. Through SPICE analysis of the ESC circuits, we find the propagation delays of the proposed CNT bundles. Next, we model the capacitively coupled interconnects for crosstalk analysis. It is found that the coupling capacitance of triangular CNT bundle is 29% lesser than the traditionally used square CNT bundles. Further, the crosstalk-induced delay of triangular interconnects is found to be 30% lesser when compared to square bundle interconnects. The reduction in delay is found to increase as the number of CNTs in the bundle increases. So, we suggest that triangular CNT bundles are the most suitable candidates as global interconnects.

Phase plates (PPs) in transmission electron microscopy (TEM) improve the contrast of weakly scattering objects under in-focus imaging conditions. A well-established PP type is the Zernike (Z)PP, which consists of a thin amorphous carbon (aC) film with a microscaled hole in the centre. The mean inner potential of the aC film is exploited to shift the phase of the scattered electrons while the unscattered electrons in the zero-order beam propagate through the hole and remain unaffected. However, the abrupt thickness increase at the hole edge induces an abrupt change of the phase-shift distribution and leads to fringing, that is, intensity oscillations around imaged objects, in TEM images. In this work, we have used focused-ion-beam milling to fabricate ZPPs with abrupt and graded thickness profiles around the centre hole. Depending on the thickness gradient and inner hole radius, graded-ZPP-TEM images of an aC/vacuum interface and bundles of carbon nanotubes (CNTs) show strongly reduced fringing. Image simulations were performed with ZPP-phase-shift distributions derived from measured thickness profiles of graded ZPPs, which show good agreement with the experimental images. Fringing artefacts, that is, intensity oscillations around imaged objects, are strongly reduced for Zernike phase plates with a graded thickness profile around the centre hole. Focused-ion-beam milling is used to fabricate graded Zernike phase plates with specific inner hole radius and thickness gradients. The phase-shift distribution is obtained from measured thickness profiles around the centre hole. Image simulations based on experimentally measured thickness/phase-shift distributions show good agreement with experimental Zernike phase-plate TEM images.