Nanowires (NWs) are a typical example of nano-objects that can be functionalized in order to obtain multiple applications: transistors, solar cells, lasers, gas sensors … Moreover, Park et al. [1] showed that silicon NWs coated by gold nanocrystals (NCs) may be used in photothermal therapy in vivo where cancer cells are captured and destroyed. By controlling the size and the density of the superficial gold NCs population one can obtain a sufficient plasmon coupling and thus achieve an even unprecedented efficiency. The development, control and optimization of these functional nanomaterials therefore require advanced characterization tools able to quantify these nano-object populations. In this context, TEM has remained for decades the only imaging technique with sub-nanometer resolution. But the small fraction of the analyzed region with respect to the sample size makes it difficult to obtain representative measurements from a statistical point of view. On the other hand the high resolution achieved by modern SEMs allows them to compete clearly with TEM.

In a previous work [2], we showed the first results obtained with a Zeiss Gemini 500 ultra-high resolution FESEM just installed in CP2M at the Aix-Marseille University. By combining low kV In-lens SE and Energy Selective Backscattered (EsB) imaging to EBSD and Annular BF-DF-HAADF-STEM, we demonstrated the possibility of obtaining, in the same instrument, topographical, chemical and structural information from the same NW with a resolution as good as the STEM-HAADF one [3].

Figures 1 and 2 show examples of In-Lens SE images obtained at 1kV on a MBE Si NW grown on Si(111) with Au as catalyst. The saw-tooth faceting of one over two faces of the NW is visible with high resolution. Furthermore, one can see that it is also possible to resolve nanometric gold nanocrystals and their non-homogeneous repartition onto the different surfaces.

The outstanding quality of this SE image, with a high signal to noise ratio, demonstrates not only the possibility to image these nano-objects but also to quantify them. Indeed, we will show that coupling low kV mode to image processing and analysis allows studying the nanometric object population with a resolution better than one nanometer, i.e. even better than the nominal resolution of this microscope at this acceleration voltage.

This methodology can be applied to perform a systematic study of the distribution of gold NCs on the substrate and on different facets of the nanowires, and can be extended to any problem involving nano-objects quantification.

References:

[1] G-S. Park et al., Full Surface Embedding of Gold Clusters on Silicon Nanowires for Efficient Capture and Photothermal Therapy of Circulating Tumor Cells, Nano Lett. 12, (2012) 1638−1642. DOI: 10.1021/nl2045759

[2] C. Alfonso et al., Low kV high resolution Scanning Electron Microscopy study of silicon nanowires surfaces

Microscopy and Microanalysis, 21 (Suppl.3) (2015) 1261-1262. DOI: 10.1017/S1431927615007096

[3] T. David et al., Gold coverage and faceting of MBE grown silicon nanowires, J. Cryst. Growth 383 (2013) 151-157. DOI: 10.1016/j.jcrysgro.2013.08.023

Figures:

Figure 1: In-Lens Secondary Electron image of a MBE Si NW grown on Si(111) with Au as catalyst. Accelerating voltage = 1kV, work distance = 1mm.

Figure 2: Higher magnification In-Lens Secondary Electron image. The surface faceting is clearly resolved. Moreover, gold particles of a few nanometers appear bright and their repartition on different facets can be quantified. Accelerating voltage = 1kV, work distance = 2 mm.

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EMC Abstracts - https://emc-proceedings.com/abstract/high-resolution-scanning-electron-microscopy-study-of-au-nanocrystals-on-si-nanowire-surfaces/