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Determining Crystal Orientation via Reflection High Energy Electron Diffraction
1 Purpose of the Research Nanocrystal thin films exhibit many useful properties, including electrochromicity and superconductivity. When synthesised via Molecular Beam Epitaxy (MBE), selection of substrate, specifically knowledge of crystal orientation, is critical. Reflection High Energy Electron Diffraction (RHEED) is an in situ crystal characterisation method highly compatible with MBE. This study explores a new method of RHEED analysis to determine crystal orientation. 2 Procedure/Theoretical Framework RHEED characterization is the incidence of a beam of high-energy electrons at a low angle with respect to the sample surface. Electrons diffract, and interfere to form patterns on the detector. Traditionally, studies of RHEED analyse one static image as a representation of the surface structure, or observations of RHEED patterns over time. The approach to RHEED analysis in this study exploits changes in RHEED patterns given a rotating substrate. Having specific rotational symmetries along different axes, crystal structures can be differentiated by determining rotational symmetry through RHEED. Electrons scatter upon incidence with crystal planes within the crystal to form Kikuchi lines on the RHEED detector (Fig. 2). The orientation of crystal with respect to incident electron beam affects the Kikuchi line patterns. If the crystal is rotated, crystal planes change orientation, and electrons would diffract from crystal planes in different directions. As such, as the crystal is rotated, the Kikuchi lines move. When the degree of rotation of the crystal corresponds to the rotational symmetry of the crystal (Fig. 1), the Kikuchi lines return to their original position. As crystals with different crystal plane orientations exhibit different orders of symmetries, analyzing the Kikuchi line patterns of the crystal at different degrees of rotation can reveal the rotational symmetry and consequently crystal plane orientation of a crystal. 3 Data/Experimental Testing In order to assess the practical viability of the proposed method, experiments were conducted on SrTiO3 (001), (110), and (111). SrTiO3 exists as a typical perovskite structure (Fig. 3), often used in the synthesis of superconductors via MBE. 3.1 Methodology RHEED images of each sample were taken at 0◦, 60◦, 90◦ and 180◦. Curves were fit to each Kikuchi line observed in the image (Fig. 4). These Kikuchi line approximations are compared by superimposing the curves traced and qualitatively assessing the degree of similarity between the Kikuchi lines of 2 images, to verify the order of symmetry and crystal orientation of the crystal. In the images of the superimposed Kikuchi lines illustrated in Fig. 5, there is similarity between the Kikuchi lines when only when the sample has been rotated by an angle corresponding its degree of symmetry. 4 Conclusions This study offers a method to determine the crystal orientation of thin film through determining the degree of rotational symmetry of the sample, by observation of Kikuchi lines in the RHEED pattern as the sample is rotated. Experimental data was analyzed qualitatively to verify the viability of this theoretical method in practice. This method could be extended to analyze the symmetry of other crystal structures. As it does not require information on the machine settings or usage of complex functions to produce a reliable output, this method is fast and straightforward, opening doors to more streamlined RHEED analysis.
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Microspheres (塑膠載體)對酸鹼值的敏感性
隨著製藥工業之發展及生活品質之提升,高療效及低副作用之藥物劑型已成為消費者主要的訴求,藥物控制釋放技術提供從事製藥技術開發研究者新的領域,包括在適當的時間、適當的藥量釋放,而不致在其輸送過程中產生不必要的支外反應;此外,一般藥劑在使用後常因快速的代謝而達到藥效極限,如欲維持較長時效必須多次使用,麻煩費時又不經濟。因此,發展出克服上述缺點的技術為制放技術,藉此而可達到延長藥程,降低副作用及增強藥效之目的,本實驗欲研發一高分子載體來應用於控制釋放過程。各式材料目前已逐漸受到重視,又以蛋白質及塑膠材料,由於其良好的生物相容性質及生物可分解性,更為受到重視。 近年來,利用手術疏通血管的病患人數節節升高,但是手術疏通後,血管再次堵塞的人也不在少數,如果能用控制溶解速度的載體承載藥物,將之存放在心臟週邊的靜脈壁中,適時釋放出適量藥物,延緩脂肪沉澱,則疏通血管手術的效果可大幅提高。
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