- 學位論文
低能量電子點投影顯微術:貴金屬覆蓋W(111)單原子場發射電子源之研究
Low-Energy Electron Point Projection Microscope: Noble-Metal Covered W(111) Single-atom Emitters
摘要
The coherence properties of electron beams field emitted from pyramidal single-atom tips (SATs) have been investigated with a custom-made low-energy electron point projection microscope (PPM). Individual single-walled carbon nanotubes (SWNTs) are also imaged with the coherent electron beam. In the first instance we have testing our PPM at room temperature and 90 K by using the normal tungsten tips and pyramidal single-atom tips. These results demonstrate a sufficient rigidity of the mechanical structure of the PPM and the experiments can be operated stably at room temperature and at 90 K. Furthermore, in the process of our experiments, we also find that the coherence of the single-atom emitter is higher than that of the normal tungsten field emitter. The characteristic of electron beams emitted from the pyramidal single-atom tips is investigated. These electron sources exhibit small opening angles, small energy dispersion, high brightness and full coherence. We have tried to quantify the properties of these single-atom emitters, such as the coherent length and the effective source size from the visibility and K factor. We then use PPM based on the single-atom emitters to image individual SWNT and SWNT bundles. We have seen the semitransparent fringes resulting from an individual SWNT and bright fringes resulting from a SWNT bundle at z1>10 μm. Through theoretical simulations, we find that at z1>10 μm, the phase contrast dominates the interference patterns and at z1<5 μm, the induced charge on the individual SWNT dominates the interference patterns. Therefore, theoretical simulation proves that it is necessary to take the phase contrast and the charge effect into consideration for clarifying the interference patterns.
並列摘要
本論文是利用自製低能量電子點投影顯微儀研究金字塔型單原子針的電子場發射同調特性，以及以同調電子束對單管奈米碳管進行成像。首先，利用傳統場發射鎢針以及單原子針，在298 K與90 K進行儀器的測試。研究結果顯示，儀器在不同的溫度下都可穩定操作。另外，在實驗過程中亦發現單原子針的同調性，在不同的溫度下都比傳統場發射鎢針的同調性來的高。由單原子針所發射的電子束具有張角小，能量分佈小，高亮度以及全同調性等性質。由可見度及K factor嘗試量化單原子針的同調長度，以及發射源的有效尺度。 在單管奈米碳管成像的研究中發現，單原子針與樣品在不同的距離下，干涉條紋會顯現出不同的明暗與寬度。經由理論計算結果得知，單原子針與樣品在10微米的距離外，干涉條紋會受到樣品本身的相位影響；反之在5微米的距離內，干涉條紋則會受到電場在樣品上產生的感應電荷影響。因此，在詮釋干涉條紋影像時必須要考慮到樣品本身的相位與電場在樣品上產生的感應電荷影響。