Abstract text (incl. figure legends and references)

Aberration-corrected scanning transmission electron microscopy (STEM) allows the imaging of single atoms in bulk crystals, usually with high angle annular dark field (HAADF) images [1], but in special cases also with STEM spectroscopy methods [2]. However, the detection of single atoms in a bulk crystal is challenging and depends on various experimental parameters such as specimen thickness, crystal orientation, composition of the matrix and the type of dopants. For substitutional dopants, the channeling of the electron waves in the crystal must be considered, whereas for single atoms on interstitial sites such as in porous materials, other difficulties such as the electron probe confinement arise [3].

In this work, we show how single atoms in the channels of a beryl crystal can be detected and quantified. Beryl (Be₃Al₂Si₆O₁₈) has a hexagonal ring structure with crystal channels of about 0.5 nm diameter aligned parallel to the c-axis. Foreign atoms such as alkali ions or H₂O molecules can be embedded in these channels. Recently, atoms were detected in the channels using STEM-HAADF and it was assumed that these were Fe²⁺ ions [4].

Here we study a beryl that had Fe and Cs concentrations of 0.3% and 0.03%, respectively. The ion-milled crystal was investigated with a probe-corrected Titan³ (Thermo Fisher) operated with an X-FEG at a convergence angle of 15 mrad. The angular ranges of the detectors are from 62 to 214 mrad (FEI) and from 20 to 73 mrad (Gatan). The experimental results were validated by density functional theory (DFT) calculations and the images were simulated with the program QSTEM (C. Koch, Berlin).

The experimental HAADF images in Fig. 1 show the crystal structure parallel to the c-axis. The specimen thickness is 10 nm. The central channel is occupied by at least one atom, it shows the highest brightness of all occupied channels in this crystal, whereas the other channels in this image are unoccupied. The occupied channels are unevenly distributed over the crystal. Since several ions can be incorporated into these channels, we have simulated images of Na, K and Cs but also with Fe in the top crystal layer. The intensity ratio of the Si and Al columns with the central channel clearly shows that we have a single Cs ion present very close to the uppermost crystal plane. This is also in agreement with crystal chemical considerations. Thus, the qualitative assumption of Arivazhagan [4] that the contrast in the channels is due to Fe ions can be clearly refuted.

Fig. 2 shows the intensity distributions of the channels and the Al and Si columns. While the Si columns show a narrow distribution, the intensities of the Al columns vary strongly, which can be attributed to Fe atoms that preferentially sit on the octahedral Al sites. This is confirmed by DFT calculations and by STEM-EDX measurements.

References:

[1] J. Hwang et al., Phys. Rev. Lett. 111 (2013) 266101.

[2] M. Varela et al., Phys. Rev. Lett. 92 (2004) 095502.

[3] K. Kimoto et al., Appl. Phys. Lett. 94 (2009) 041908.

[4] V. Arivazhagan et al., J. Microsc. 265 (2017) 245.

Fig. 1 Experimental STEM images of the brightest channel occupant compared with multi slice simulations viewed along the [001] direction; with various dopants in the channel position Z=0 for a specimen thickness of 10 nm.

Fig. 2 Intensity distributions of the columns in the FEI HAADF image (left), intensity integration map (middle) and histogram showing the channels, Si-O and of Al-Fe columns (right).