用於觀察流體內物質之電子顯微鏡石墨烯視窗的製作

液體槽(Liquid cell)的發展使臨場觀察流體或其內部物質的技術到達了一個新的里程碑，無論是在生物領域得以觀察細胞於水溶液中的真實樣貌，或是在材料科學領域觀察金屬奈米粒子於溶液中的成長與融合。而石墨烯液胞(Graphene liquid cell)利用石墨烯的高強度、高電子穿透率、高化學穩定性且為人類已知最薄的二維材料等特質，將其應用於密封液體供電子顯微鏡觀察之用途，此發明在以電子顯微鏡觀察液體內物質的解析度方面再度向前推進了一步。然而以上兩種觀察液體內物質的方法各有其優缺，如液體槽因為使用氮化矽薄膜為視窗，故無法達到同石墨烯液胞一般優良的解析度，而石墨烯液胞則是在樣品準備上較為繁複，並且因為製備方法上的限制，犧牲了許多液體槽可以整合的功能性包括液體流動、施加電壓或是加熱等等。本研究嘗試結合兩者的優點，製作石墨烯視窗液體槽微晶片(Graphene liquid cell microchip)，保持液體槽的晶片設計形式，並將電子可穿透視窗由氮化矽改為石墨烯薄膜，在改善解析度的同時，保留與各種功能整合的可能性。實驗的製程步驟依序為晶圓分割與清洗、視窗及支撐薄膜的尺寸定義、矽基板的化學蝕刻以及化學氣相沉積法成長石墨烯與其逐層堆疊之轉印。石墨烯視窗液體槽微晶片製備完成後，我們使用掃描式電子顯微鏡、穿透式電子顯微鏡及拉曼光譜儀檢查視窗的覆蓋率與石墨烯薄膜的品質，並設計微晶片適用之載台於真空腔體內測試其密封可靠性，最後於電子顯微鏡中觀察噴灑於表面後烤乾之金奈米粒子以及懸浮於溶液中之金奈米粒子，以測試其可行性，並與常見的氮化矽視窗薄膜加以比較。

關鍵字

石墨烯；液體槽；電子顯微鏡；液體觀察；製程設計

並列摘要

The development of liquid cells has marked a milestone of in-situ observation. It is a very useful tool in many research fields, such as observing cells in aqueous solution in the field of biology, and watching the growth and coalescence behavior of nanoparticles in materials science. Recently, the invention of graphene liquid cell provides a better resolution for observing liquid samples in an electron microscope. The outstanding physical properties of graphene, including its high strength, high electron transparency, high chemical stability, and the thickness of one atomic layer, are utilized as the membrane material in the liquid cells. On the other hand, the process to use silicon nitride as the window membrane on liquid cell microchips is already mature. Besides, other functions, e.g. liquid flow, electrical biasing, or heating, can be integrated. However, the thickness of the silicon nitride membrane lowers the image resolution. In this research, we aim to make a graphene liquid cell microchip, which exhibits the merits of liquid cell microchips and the graphene liquid cell. We replace the silicon nitride window membrane with a multilayer graphene film in order to obtain better image resolution, and, meanwhile, the possibility of integrating multiple functions on silicon-based microchip is preserved. The first step in the fabrication process of graphene liquid cell microchip is wafer dicing and cleaning. The second step is to produce the holey pattern of the silicon nitride supporting window membrane by means of photolithography and reactive ion etching of the standard semiconductor processes. Than silicon substrate underneath the silicon nitride membrane is removed by anisotropic etching using potassium hydroxide solution. Finally, CVD graphene sheets are transferred onto the microchip layer by layer to make a multilayer graphene film as the membrane on the microchip. Scanning electron microscope, transmission electron microscope and Raman spectrometer are used to examine the coverage and quality of graphene on the holey supporting window. A holder is designed for the graphene liquid cell microchip for testing the leakage in vacuum and the observations of dispersed gold nanoparticles in aqueous solution in a scanning electron microscope.

並列關鍵字

graphene ； liquid cell ； electron microscopy ； observation of liquid phase ； microfabrication process

參考文獻

1. S. Thiberge et al., Scanning electron microscopy of cells and tissues under fully hydrated conditions, P. Natl. Acad. Sci. USA. 101, 3346-3351 (2004).

2. H.G. Liao, and H.M. Zheng, Liquid cell transmission electron microscopy, Annu. Rev. Phys. Chem. 67, 719-747 (2016).

3. E.A. Ring, and N. de Jonge, Microfluidic system for transmission electron microscopy, Microsc. Microanal. 16, 622-629 (2010).

4. J.M. Yuk et al., High-resolution EM of colloidal nanocrystal growth using graphene liquid cells, Science 336, 61-64 (2012).

5. X.Y. Yu, B.W. Liu, and L. Yang, Imaging liquids using microfluidic cells, Microfluid. Nanofluid. 15, 725-744 (2013).

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用於觀察流體內物質之電子顯微鏡石墨烯視窗的製作

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