Atomic resolution imaging

The Nion UltraSTEM at 100 kV is capable of sub-Å imaging by virtue of its 3rd-generation C₃/C₅ corrector. An experimental Ronchigram obtained in the UltraSTEM is shown below, demonstrating correction up to 40 mrad half-angle.  The region in the center is the aberration-free region (sweet spot): electrons passing through it do not undergo excessive phase shift that would cause the STEM probe to blur.

Additional circles indicate the best sweet spot theoretically obtainable with the UltraSTEM correcting to 5^th order (dashed); the sweet spot obtainable with Nion’s previous (C₃-only) corrector; and the sweet spot of the microscope running with the corrector turned off.

HAADF image of SrTiO₃ taken at 100 keV with 320 pA probe current with 5 ms dwell time per pixel.  The FFT (insert) shows that spacings of 1.23 Å (b) and 0.97 Å (a) were resolved. Courtesy Cornell University.

High-speed chemical mapping

The UltraSTEM's high brightness gun and efficient detector coupling optics enable rapid acquisition of electron energy loss spectra. This is demonstrated in the following background-subtracted electron energy loss spectrum of the Ti L_2,3 edge from a Ti column in SrTiO₃.

The coupling optics collected angles up to about 50 mr half-angle into the Gatan Enfina parallel-detection spectrometer. Dispersion is 0.2 eV/channel and acquisition time is 0.5 sec. The splitting into four individual peaks is clearly resolved, and S/N ratio is excellent for the short acquisition time.

The high efficiency at collecting spectra means that the UltraSTEM can obtain elemental maps more rapidly than ever before. The images below are atomic resolution chemical maps of a La_0.7Sr_0.3MnO₃/SrTiO₃ multilayer showing La, Ti, and Mn composition at the interface (figs a, b, and c, respectively). These maps, when combined into composite RGB image (fig. d), elucidate the chemistry at the interface.

The green and red dots show that La and Mn occupy different columns in the La_0.7Sr_0.3MnO₃layers.  The red dots near the left interface indicate that Ti in the SrTiO₃ layers occupies columns that are equivalent to those occupied by Mn in the La_0.7Sr_0.3MnO₃ layers.  Interestingly, in the composite image purple dots are visible at the interface between the two layers. This is evidence that, at this particular interface, there was a mixing of Ti and Mn within individual atomic columns.

The spectrum image was acquired using 40 mr illumination half-angle giving a 0.8 nA, 1.2 A probe.  The EELS coupling efficiency was such that 0.6 nA of current entered the electron spectrometer. The high current allowed the acquisition of the 64x64 pixel map in only 29 s (7 ms per pixel) with 10 s of additional processing time.

Nion UltraSTEM 100. Courtesy Prof. D.A. Muller, L. Fitting Koukoutis and M.F. Murfitt, multilayer structure courtesy Drs H.Y. Hwang and J.H. Song.

The UltraSTEM 100 offers atomic resolution imaging at reduced accelerating voltage. Examples of the resolving power at 60 kV are shown at left. The top image is a high-angle annular dark field (HAADF) image of a multiply-twinned gold nanoparticle showing atomic columns. Insets with modified contrast show that the UltraSTEM resolves individual atoms on the carbon substrate. Below, a filtered image of silicon <110> dumbbells demonstrates that the microscope is reaching 1.36 Å resolution at 60 kV.

Nanotube characterization is an area in which reduced primary voltage greatly reduces radiation damage. 60 kV accelerating voltage eliminates knock-on radiation damage in many materials containing low -Z elements such as B, C and N. The following image contains bright field (BF) and HAADF STEM images of a carbon nanotube acquired simultaneously at 60 kV. Sample courtesy of Mathieu Kociak.

Point diffraction patterns can be recorded on the UltraSTEM's 1k x 1k CCD camera when the convergence angle of the beam is reduced. Because the diffraction pattern does not shift during scanning, lower-resolution HAADF images can be observed at the same time as a diffraction pattern.

Nanoanalysis by dark-field STEM

The < 1 Å probe available with the Nion C_s corrector enables direct interpretation of structure in interfacial studies. In the HAADF image to the left, two Si₃N₄ grains of differing orientation meet at an intergranular oxide film rich in lutetium (the right Si₃N₄grain is not oriented to the zone axis). Lu atoms at the boundary show a strong preference for specific sites at the terminating Si₃N₄  plane, resulting in a periodic structure in the intergranular film.

ORNL C_s-corrected HB603 (300 kV), courtesy N. Shibata and S.J.Pennycook.

This image of a CoSi₂ epitaxial layer on a Si [110] substrate shows a number of classic defect structures between two different interface types. The jog in the center contains a (-111) facet that has only a few layers arranged in a microtwin boundary. The crystal accomodates this with a misfit dislocation (arrowed).

The interface structure Si and disilicide crystal changes from a (1x1) structure on the left to a (2x1) reconstruction on the right. The reconstruction model of alternating ")(" shapes was previously proposed by Yu et al.,¹ and was directly confirmed by this study. The inset shows the strain field for the interface structures, with dislocation indicated.

SuperSTEM 1 (100 kV), courtesy A. Bleloch.

1. B. D. Yu et al., J. Vac. Sci. Technol. B19, 1180 (2001).

Nanoanalysis by bright-field STEM

BF image of α-Al₂O₃ showing the characteristic zig-zags of the structure. A stacking fault of the (1120) type occurs in this crystal, where an extra zig-zag layer originating from the left terminates in a partial dislocation in the center of the image. The dislocation has a single O column at its core.

ORNL Cs-corrected HB603 STEM (300 kV), courtesy N. Shibata and S.J.Pennycook

Near atomic-resolution imaging by EDXS

The figure at left shows an EDXS concentration profile of Zr across a grain boundary in a Ni-Cr-Fe alloy. The peak concentration of Zr at the grain boundary was only  0.3%, which means that the X-ray signal originated from fewer than 10 atoms.

Lehigh University Cs-corrected HB603 (300 kV), courtesy Masashi Watanabe