To fully understand structure-property relationships in nanostructured materials, it is important to reveal the three dimensional (3D) structure at the nanometre scale.

Scanning electron confocal microscopy (SCEM) was introduced as an alternative approach to 3D imaging in 2003¹. The confocal method was originally developed in optical microscopy to image the 3D structure of biological samples². The incident beam is focused at a certain depth in a thick sample, and the excited fluorescence signal is imaged onto the detector plane through the imaging system. A small pin hole before the detector only allows the signal from the confocal plane to reach the detector and blocks the out-of-focus signal. Critically, by using a fluorescent signal, the incident and outgoing waves lose their phase relationship and an incoherent 3D point spread function can be achieved.

The optical setup in SCEM is analogous to fluorescence confocal microscopy and also requires an incoherent signal to achieve the incoherent 3D point spread function. Several groups have developed different approaches to using inelastically scattered electrons to achieve an incoherent confocal condition in the TEM^3,4,5, however, difficulties remain. Core-loss electrons have a suitably limited coherence length, however, the excitation probability is extremely low which leads to a poor signal to noise ratio (SNR). Low loss electrons give a much better SNR but still have a significant coherence length due to the collective nature of the  excitation.

In this work, we introduce a different approach to achieve 3D imaging in a confocal mode by using the elastically scattered, coherent electrons. This method exploits the depth sensitivity of electrons that have suffered a specific momentum change, rather than an intensity change.  According to Fourier optics, when a thin object is inserted at a distance z above the confocal plane, the new wave function at the confocal plane will be the original probe convoluted with the Fourier transform of the object function, together with a scale factor related to z (defined in fig.1). For crystalline specimens, the Fourier transform of the object function is a set of delta functions, so a diffraction-like pattern will be generated at the confocal plane. Importantly, the separation between the diffraction spots is proportional to the distance z (see fig.1), so that the resulting diffraction contrast is very sensitive to depth. This strong depth sensitivity is combined with the very strong SNR due to the use of the elastically scattered signal. Applications to the imaging of 3D engineered nanostructures are demonstrated (fig. 2 and 3).

1. S. P. Frigo, Z. H. Levine, and N. J. Zaluzec, Appl. Phys.Lett. 81, 2112 (2002 and N. J. Zaluzec, U.S. Patent No. 6,548,810 B2 (2003).

2. T. Wilson and C. Sheppard, Theory and practice of scanning optical microscopy (Academic Press, London ; Orlando, 1984)

3. Wang, P., Behan, G., Takeguchi, M., Hashimoto, A., Mitsuishi, K., Shimojo, M., & Nellist, P. D. (2010). Phys. Rev. Lett. 104(20), 200801.

4. Xin, H. L., Dwyer, C., Muller, D. A., Zheng, H., & Ercius, P. (2013). Microsc. Microanal., 19(04), 1036-1049.

5. C Zheng, Y Zhu, S Lazar, J Etheridge, Phys. Rev. Lett. (2014) 112 (16), 166101

Acknowledgement: The authors thanks staff at the Monash Centre for Electron Microscopy. The double-aberration Titan³ 80-300 FEGTEM was funded by ARC grant LE0454166.

Figures:

Fig. 1: Illustration of real space diffraction SCEM. The lateral distance between the diffraction spots in real space is proportional to the distance from the crystalline sample to the confocal plane.

Fig.2: Experimental real space diffraction SCEM imaging of SiO/SiO2/Ag Swiss roll structure. The left and right images are focused at different height separated by 570 nm.

Fig. 3: Reconstructed 3D side view of the SiO/SiO2/Ag nanostructure.

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