196,156 research outputs found
Magnetic Imaging with a Novel Hole-Free Phase Plate
No full textOne of the main interests in phase plate imaging is motivated by a decrease in irradiation dose needed to obtain desired signal to noise ratio, a result of improved contrast transfer [1]. The decrease in irradiation improves the imaging of biological materials [2]. Here we demonstrate that phase plate imaging of magnetic samples (phase objects), using a hole-free phase plate (HFPP) [3], is superior to conventional Fresnel imaging with significantly improved signal to background ratio under in-focus or near-in-focus conditions.In principle, phase plate imaging should make it possible to image most phase objects, including magnetic and electrostatic fields in vacuum. The requirement for phase plate imaging, including that by HFPP, is that the object spectrum in the back focal plane of the objective lens must not be broadened via the effect of chromatic aberration. In other words, the imaged samples must be thin. Recently, the imaging of magnetic samples, including magnetic field in vacuum, proved possible using a HFPP [4]. The data shown here were obtained on a JEOL 2100 FM-LM microscope, equipped with low-field objective lens that is dedicated for magnetic imaging without affecting a sample's magnetic state. TheHFPP implementation of phase plate imaging was employed [3] due to its convenience, stability and possibility to achieve a semi-quantitative agreement between experiment and image simulations [4]. A 10 nm thick carbon film placed over one of the objective aperture opening was used as the HFPP. TheHFPP was maintained at room temperature.Figure 1 shows a PrFeB hard magnet imaged in focus (a), 120 μm under focus (b), and in-focus with the HFPP (c). The HFPP (Fig. 1c) allows for clear identification of domain walls and sample edge simultaneously, as well as imaging of the stray field in vacuum. Figure 2 shows a cobalt thin film, imaged close to in-focus with the HFPP (Fig. 2a) compared to an in-focus image without HFPP (Fig. 2b). Furthermore, the HFPP preserves structural information when compared to in-focus images (c,d), which is typically lost in the Fresnel imaging mode. Figure 3 further shows a patterned cobalt square, exhibiting a landau domain pattern. While the Fresnel mode can be used to accurately determine thisstructure (Fig. 3d,e), it lacks distinct contrast from nanoscale grains seen with the HFPP (Fig. 3f,g).The use of a HFPP allows us to image magnetic structure and field and their interactions with microstructure and defects under in-focus condition that is not possible with any other existing imaging methods. A semi-quantitative agreement between obtained images and simulations was achieved [4]. The use of phase plate imaging may open new opportunities in imaging of wide variety of samples far beyond the current applications in biology [5]
Imaging of radiation-sensitive samples in transmission electron microscopes equipped with Zernike phase plates
No full textWe have optimized a bright-field transmission electron microscope for imaging of high-resolution radiation-sensitive materials by calculating the imaging dose n(0) needed to obtain a signal-to-noise ratio (SNR)=5. Installing a Zernike phase plate (ZP) decreases the dose needed to detect single atoms by as much as a factor of two at 300 kV. For imaging larger objects, such as Gaussian objects with full-width at half-maximum larger than 0.15 nm, ZP appears more efficient in reducing the imaging dose than correcting for spherical aberration. The imaging dose n(0) does not decrease with extending of chromatic resolution limit by reducing chromatic aberration, using high accelerating potential (U(0)=300 kV), because the image contrast increases slower than the reciprocal of detection radius. However, reducing chromatic aberration would allow accelerating potential to be reduced leading to imaging doses below 10 e(-)/A(2) for a single iodine atom when a CS-corrector and a ZP are used together. Our simulations indicate that, in addition to microscope hardware, optimization is heavily dependent on the nature of the specimen under investigation.Peer reviewed: YesNRC publication: Ye
Low-dose performance of parallel-beam nanodiffraction
No full textWe evaluate the low-dose performance of parallel nano-beam diffraction (NBD) in the transmission electron microscope as a method for characterizing radiation sensitive materials at low electron irradiation dose. A criterion, analogous to Rose's, is established for detecting a diffraction spot with desired signal-to-noise ratio. Our experimental data show that a dose substantially lower than in high-resolution bright-field imaging is sufficient to determine structure and orientation of individual nanoscale objects embedded in amorphous matrix. In an instrument equipped with a cold field-emission gun it is possible to form a probe with sub-3 nm diameter and sub-0.3 mrad convergence angle with sufficient beam current to record a diffraction pattern with less than 0.2 s acquisition time. The interpretation of NBD patterns is identical to that of selected area diffraction patterns. We illustrate the physical principles underlying good low-dose performance of NBD by means of a phase grating. The electron irradiation dose needed to detect a diffraction peak in NBD is found proportional to 1/N(2), where N is the number of lattice planes contributing to the peak. (C) 2008 Elsevier B.V. All rights reserved
Bright-field TEM imaging of single molecules: Dream or near future?
No full textWe examine the suitability of spherical aberration (Cs)-corrected (CS) and uncorrected (UC) transmission electron microscopes (TEM) for conventional bright-field imaging of radiation-sensitive materials. We have chosen an individual molecule suspended in vacuum as a hypothetical example of a well-defined radiation-sensitive sample. We find that for this particular sample, CS instruments provide about 30% improvement over an UC instrument in terms of signal/noise ratio per unit electron dose at 300 kV. The lowest imaging doses can be achieved in CS instruments equipped with high-brightness electron source operated at low incident electron energies. Our calculations suggest that it may be possible to image individual, iodine- or bromine-substituted organic molecules in bright-field mode, at doses lower than the accepted values for radiation damage of aromatic molecules. Crown Copyright (c) 2006 Published by Elsevier B.V. All rights reserved
Convenient contrast enhancement by a hole-free phase plate
No full textDecrease of the irradiation dose needed to obtain a desired signal-to-noise ratio can be achieved by Zernike phase-plate imaging. Here we present results on a hole-free phase plate (HFPP) design that uses the incident electron beam to define the center of the plate, thereby eliminating the need for high precision alignment and with advantages in terms of ease of fabrication. The Zernike-like phase shift is provided by a charge distribution induced by the primary beam, rather than by a hole in the film. Compared to bright-field Fresnel-mode imaging, the hole-free phase plate (HFPP) results in two- to four-fold increase in contrast, leading to a corresponding decrease in the irradiation dose required to obtain a desired signal-to-noise ratio. A local potential distribution, developed due to electron beam-induced secondary-electron emission, is the most likely mechanism responsible for the contrast-transfer properties of the HFPP
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