Schamm S, Bonafos C, Coffin H, Cherkashin N, Carrada M, Ben Assayag G, Claverie A, Tencé M, Colliex C
nMat Group, CEMES-CNRS, 31055 Toulouse, France.
Ultramicroscopy. 2008 Mar;108(4):346-57. doi: 10.1016/j.ultramic.2007.05.008. Epub 2007 May 29.
Fabrication of systems in which Si nanoparticles are embedded in a thin silica layer is today mature for non-volatile memory and opto-electronics applications. The control of the different parameters (position, size and density) of the nanoparticles population is a key point to optimize the properties of such systems. A review of dedicated transmission electron microscopy (TEM) methods, which can be used to measure these parameters, is presented with an emphasis on those relying on electron energy-loss spectroscopy (EELS). Defocused bright-field imaging can be used in order to determine topographic information of a whole assembly of nanoparticles, but it is not efficient for looking at individual nanoparticles. High-resolution electron imaging or dark-field imaging can be of help in the case of crystalline particles but they always provide underestimated values of the nanocrystals population. EELS imaging in the low-energy-loss domain around the Si plasmon peak, which gives rise to strong signals, is the only way to visualize all Si nanoparticles within a silica film and to perform reliable size and density measurements. Two complementary types of experiments are investigated and discussed more extensively: direct imaging with a transmission electron microscope equipped with an imaging filter (EFTEM) and indirect imaging from spectrum-imaging data acquired with a scanning transmission electron microscope equipped with a spectrometer (STEM-PEELS). The direct image (EFTEM) and indirect set of spectra (STEM-PEELS) are processed in order to deliver images where the contribution of the silica matrix is minimized. The contrast of the resulting images can be enhanced with adapted numerical filters for further morphometric analysis. The two methods give equivalent results, with an easier access for EFTEM and the possibility of a more detailed study of the EELS signatures in the case of STEM-PEELS. Irradiation damage in such systems is also discussed.
如今，在非易失性存储器和光电子应用中，将硅纳米颗粒嵌入薄二氧化硅层的系统制造技术已经成熟。控制纳米颗粒群体的不同参数（位置、尺寸和密度）是优化此类系统性能的关键。本文综述了可用于测量这些参数的专用透射电子显微镜（TEM）方法，重点介绍了那些依赖电子能量损失谱（EELS）的方法。散焦明场成像可用于确定整个纳米颗粒组件的形貌信息，但对于观察单个纳米颗粒效率不高。对于晶体颗粒，高分辨率电子成像或暗场成像可能会有所帮助，但它们总是会低估纳米晶体群体的值。在硅等离子体激元峰周围的低能量损失区域进行EELS成像，会产生强烈信号，这是可视化二氧化硅薄膜内所有硅纳米颗粒并进行可靠的尺寸和密度测量的唯一方法。本文研究并更广泛地讨论了两种互补类型的实验：使用配备成像滤波器的透射电子显微镜进行直接成像（EFTEM），以及从配备光谱仪的扫描透射电子显微镜采集的光谱成像数据进行间接成像（STEM-PEELS）。对直接图像（EFTEM）和间接光谱集（STEM-PEELS）进行处理，以生成二氧化硅基质贡献最小化的图像。所得图像的对比度可以通过适用的数值滤波器增强，以便进行进一步的形态计量分析。这两种方法给出了等效的结果，EFTEM更容易使用，而在STEM-PEELS的情况下，可以更详细地研究EELS特征。本文还讨论了此类系统中的辐照损伤。
