小尺寸纳米颗粒的SEM高分辨成像模式探究——以Ni_(2)P为例

Exploration of SEM High-Resolution Imaging Modes for Small-Sized Nanoparticles——Taking Ni_(2)P as an Example

相比透射电子显微镜(TEM),扫描电子显微镜(SEM)具有批量进样、成本低、三维成像的优势,但小尺寸纳米颗粒在进行SEM成像时存在容易积碳、分辨率不足、形貌结构信息弱等技术难题。文章以超小空心(34.4 nm)、实心(13.3 nm)Ni_(2)P纳米颗粒为例,重点探究了如何通过调控工作模式、工作距离、加速电压、束流等参数获得分辨率高、形貌结构清晰的SEM图像。研究表明,高分辨模式下T2和超高分辨模式下T3探头所成二次电子(SE)像的分辨率明显优于标准模式下ETD探头的,且SE像表面形貌信息多、立体感较好。其中由于T3探头位置最高且仅收集高位二次电子信号,这部分二次电子信号进入透镜的角度准直且能量较低。因此超高分辨模式-T3探头所成SE像分辨率和信噪比最高、形貌衬度良好,但几乎观察不到颗粒内部结构。而高分辨和超高分辨模式下T1(背散射信号)探头所成BSE像成分衬度好,虽景深较小但最有利于观察空心结构。同时探究发现工作距离太小,所得SE像分辨率好但景深差,工作距离太大,SE像分辨率稍弱但景深较好。结合图像质量测量结果,对于文中的Ni_(2)P纳米颗粒,选择适中的工作距离(~8 mm)可获得质量较高的SE像。提升加速电压可有效提高图像分辨率(空心Ni_(2)P在30 kV下可达2.3 nm)。信噪比会随束流的增大而增强,其过大会导致颗粒边缘模糊,选择适中的束流(~0.2 nA)成像效果较好。以上研究结果对小尺寸及中空结构纳米颗粒的SEM成像选择具有一定的借鉴作用。

Compared to transmission electron microscopy(TEM),scanning electron microscopy(SEM)has the advantages of batch sample loading,lower cost,and three-dimensional imaging.However,during the SEM imaging of small-sized nanoparticles,several technical problems,including easy carbon deposition,insufficient resolution,and weak information on morphological structure,restrict the applications.Taking ultra-small sized hollow(34.4 nm)and solid(13.3 nm)Ni_(2)P nanoparticles as the example,this article focuses on exploring how to obtain SEM images with high resolution and clear morphology by adjusting parameters such as operating mode,working distance,acceleration voltage,and beam current.The results show that the resolution of secondary electron(SE)images obtained by Optiplan-T2 and Immersion-T3 modes are significantly better than that of the ETD probe in standard mode.Additionally,the SE images have abundant surface morphology information and good threedimensional sense.This is because the T3 probe is positioned the highest and only collects high-altitude secondary electron signals,which enter the lens at a more collimated angle and with lower energy.Therefore,the Immersion-T3 probe produces SE images with the highest resolution,signal-to-noise ratio,and good morphological contrast,but the internal structure of the particles is almost invisible.In the Optiplan and Immersion mode,the T1 probe produces BSE(backscattered electron)images with good composition contrast.Although the depth of field is smaller,it is most advantageous for observing hollow structures.The study also found that if the working distance is too small,the SE images have good resolution but poor depth of field.Conversely,if the working distance is too large,the SE image resolution is slightly weaker,but the depth of field is better.Based on the measurement results of image quality,for the Ni_(2)P nanoparticles,selecting an appropriate working distance(~8 mm)can obtain highquality SE images.Increasing the acceleration voltage can effectively improve the image resolution(for hollow Ni_(2)P,it can reach 2.3 nm at 30 kV).The signal-to-noise ratio will increase with the increase of beam current.But if it is too large,the particle edges will get blurring.Therefore,selecting the appropriate beam current(~0.2nA)can obtain a better imaging effect.The above research results provide a reference for selecting SEM imaging for smallsized and hollow structured nanoparticles.