Electron holography (EH) is a TEM method which can quantitatively measure electromagnetic fields of various samples [1-3]. In this study, we tried observing a local two-dimensional electric distribution, formed in multilayer organic electroluminescence (OEL) quantitatively with phase-shifting EH [4-5] by HF-3300EH Cold-FE TEM operated at 300 kV equipped with multiple biprism system.
An OEL multilayer sample (CuPc/α-NPD/Rubrene/Alq3/LiF/Ag) was fabricated on a Si substrate using a vacuum evaporation method. Each layer’s thickness and the surface morphology of the OEL multilayer sample were evaluated by X-Ray Reflectometry (XRR) and Atomic Force Microscopy (AFM), respectively.
The sample for phase-shifting EH observation was fabricated by focused ion beam (FIB) technique. A part of the multilayer sample, which formed a multilayer structure on a Si substrate, was picked up by the microsampling technique and was fixed onto a W deposition on the mesh for TEM observation. Then, a thin film sample of thickness 450 nm was fabricated by FIB processing. Generally, the OEL sample is vulnerable to water, and the structure may change in quality in reaction to atmospheric water vapor, depending on the formed materials. Therefore, in this study, after thin film processing, contact with the atmosphere was prevented by using an air protection mesh holder.
Figure 1 shows a TEM image, and a hologram by double-biprism EH technique [6]. The model structure of an OEL multilayer is inserted in the TEM image. Sample thickness was about 450 nm. From the TEM image, some contrast is observed in the position of the CuPc layer, but we cannot confirm the image contrast corresponding to the other layers. 
Figure 2 shows the result of the visualization of the 2-dimensional potential map of an OEL multilayer. We can clearly observe the contrast of the OEL multilayer, as shown in the reconstruction image (Figure 2b). Because the inner potential of the materials in each layer is constant, we can consider phase shifts to be an electric potential change in the sample. In the future, we will investigate the accuracy of this experimental data by comparing our data with a simulation.

References

[1] Z. Wang et al., Appl. Phys. Lett., 80 (2002) 246.
[2] T. Hirayama et al., Appl. Phys. Lett., 63 (1993) 418.
[3] K. Yamamoto et al., Angewandte Chemie., 49 (2010) 4414.
[4] Q. Ru et al., Appl. Phys. Lett., 59 (1991) 2372.
[5] Q. Ru et al., Ultramicroscopy, 55 (1994) 209.
[6] K. Harada et al., Journal of Electron Microscopy, 54 (2005) 19.

Figures:

Figure 1. (a) TEM image and (b) hologram of an OEL sample fabricated by FIB technique.

Figure 2. (a) TEM image, (b) phase image (color) and (c) model structure of OEL sample. Reconstruction image used 32 holograms. The phase difference change characteristic of the inner Alq3 layer was detected. A drop in the local electric potential at the interface between the Alq3 and the Rubrene layer was observed on the 2-dimensional map.

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EMC Abstracts - https://emc-proceedings.com/abstract/visualization-of-2-dimensional-potential-map-in-multilayer-organic-electroluminescence-materials-by-phase-shifting-electron-holography/