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Project supported by the National Basic Research Program of China (Grant No. 2012CB825801) and the National Natural Science Foundation of China (Grant Nos. 11505188, and 11305173).
We develop an element-specific x-ray microscopy method by using Zernike phase contrast imaging near absorption edges, where a real part of refractive index changes abruptly. In this method two phase contrast images are subtracted to obtain the target element: one is at the absorption edge of the target element and the other is near the absorption edge. The x-ray exposure required by this method is expected to be significantly lower than that of conventional absorption-based x-ray elemental imaging methods. Numerical calculations confirm the advantages of this highly efficient imaging method.
X-rays have been employed as a quantitative mapping tool of elemental constituents since the last century.[1,2] At the absorption edge of an element, the complex x-ray atomic scattering factor experiences rapid and substantial change.[3,4] This elemental specific change has been exploited using x-ray absorption contrast imaging technique to obtain element specific image.[5–9] The technique typically requires two x-ray images: one is at x-ray energy slightly below the absorption edge energy and the other is at x-ray energy slightly above the absorption edge energy. After proper normalization of the two images in intensity, subtraction of the normalized images will yield an image of the target element. This elemental imaging technique is simple, straight forward, and fast, especially when a synchrotron x-ray source is used.[10–13]
For high Z elements, however, the change of the real part of their atomic scattering factors across an absorption edge between 0.3 keV–2 keV can be larger than that of the corresponding imaginary part,[3,4] and we refer to the change as resonance peak. Figure
Here, we propose a new elemental imaging method by using Zernike phase contrast (the changes of both the real and the imaginary parts of the atomic scattering factor) instead of absorption contrast (solely the change of the imaginary part). In our proposed method, two phase-contrast images are collected by using Zernike phase contrast setup: one is at energy corresponding to the resonance peak and the other is close to the resonance peak. The two images are normalized and subtracted to obtain an image of the target element. Our proposed method can be readily performed on existing Zernike phase contrast x-ray microscopes that are available by using synchrotron and laboratory x-ray sources. In Zernike phase contrast x-ray microscopes, the sample is placed on the focus of the condenser, a zone plate is used as an objective, and a phase ring is placed in the back focal plane of the zone plate to achieve π/2 or 3π/2 phase shift.[14–17] Only the working energy needs to be changed once in the system to perform the new elemental imaging experiment. Possible problems, such as different magnifications of the images at different energies, in the experiment could be solved just as in the absorption based method.[18]
To demonstrate the effect of the proposed method, we simulate the imaging procedure of different metal particles within a model cell as illustrated in Fig.
For in-depth analysis and quantifying the advantages of elemental imaging with using the phase contrast, we consider PND to achieve given SNR, as a criterion. The PND for conventional TXM images is calculated by[15]
In order to show the performance improvement in terms of the dose, we calculate the PND ratio: nr = PND(absorption)/PND(phase). The model consists of a small Zn particle in a 5-μm cell. Working energies for the calculation are 1022.3 eV and 973.7 eV, 1022.3 eV and 992.9 eV, 1022.3 eV, and 1012.4 eV for the phase contrast and 1022.3 eV and 1012.4 eV for the absorption. The variations of PND ratio with nanoparticle size are shown in Fig.
We also perform similar calculations for other metals such as Cu, Fe, and Ti, data based on Ref. [4]. Results indicate that for small size particles (<100 nm), elemental imaging based on the phase contrast is more efficient, i.e., at constant SNR the PND for phase contrast is halved or even quartered compared with that in the absorption method.
In addition, we evaluate the performance of the new method in terms of the size of the particles, and the results show that our phase contrast technique is always more effective than the absorption method, as long as the object particle size is smaller than 100 nm. Our new method is suitable for the soft x-ray range. Also, we should mention that the data from Ref. [4] are not exactly accurate near absorption edges, but for metals they are close to the realistic ones. Since no molecular orbitals are involved, we will consider these situations in our following work.
Our proposed elemental x-ray phase contrast imaging method exploits the intrinsic phase changes of resonance peaks, which enables us to achieve low dose element-specific x-ray imaging. Despite several limitations, this new imaging method can be used in a wide range of biological applications, such as three-dimensional (3D) imaging of metal protein clusters, and even in material sciences, such as transition process of metal clusters in battery materials. Finally, our proposed method can serve as a good complement to XANES imaging and XFM techniques, and easily implement with current TXM microscopes.
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