纳米粉体高分辨成像的荷电效应与应对策略

Charge effect of high resolution imaging of nano-powders and its countermeasures

为应对纳米粉体在扫描电镜成像中出现的荷电效应,分析荷电效应的形成机理;推导样品与入射电子束响应之间的关系式,揭示样品表面累积的有效电荷密度与电流、每帧扫描时间、放大倍率和电子总产额的关系;在荷电场达到近似于稳态后,建立表面累积电势与束流、放大倍数、电阻率以及电子总产额的关系表达式;分析缓解或消除荷电效应的各种措施的利弊,揭示当代场发射扫描电镜的低电压和高分辨的成像能力,探索场发射扫描电镜的较佳成像策略。结果表明:根据关系表达式可以指导电镜参数设置或者探测器的选择,以克服荷电现象;对纳米粉体表面镀导电膜虽然可以消除荷电效应,但会影响观察纳米粉体的真实形貌;现代场发射扫描电镜能够实现低电压、高分辨成像,可以克服镀膜对粉体形貌造成的影响。

In order to cope with the charge effect of nano-powders in scanning electron microscopy, the mechanism of charge effect was analyzed. The relationship between the sample and the incident electron beam response was derived to reveal the relationship between the accumulated effective charge density on the sample surface and the current, scanning time per frame, magnification and total electron production. After the charge field reached a steady state, the expression of the relationship between the accumulated surface potential and beam current, amplification, resistivity and total electron production was established. The advantages and disadvantages of various measures to alleviate or eliminate the charge effect were analyzed, the low voltage and high resolution imaging ability of modern field emission scanning electron microscopy was revealed and the optimal imaging strategy of field emission scanning electron microscopy was explored. The results show that the relationship expression can be used to guide the electron microscope parameter setting or detector selection to overcome the charge phenomenon. Plating conductive film on the surface of nano-powder can eliminate the charge effect, but it will affect the observation of the real morphology of nano-powder.Modern field emission scanning electron microscopy can realize low voltage and high resolution imaging, which can overcome the influence of coating on the morphology of powder.