Simultaneous atomic-resolution electron ptychography and Z-contrast imaging of light and heavy elements in complex nanostructures

Abstract

The aberration-corrected scanning transmission electron microscope (STEM) has emerged as a key tool for atomic resolution characterization of materials, allowing the use of imaging modes such as Z-contrast and spectroscopic mapping. The STEM has not been regarded as optimal for the phase-contrast imaging necessary for efficient imaging of light materials. Here, recent developments in fast electron detectors and data processing capability is shown to enable electron ptychography, to extend the capability of the STEM by allowing quantitative phase images to be formed simultaneously with incoherent signals. We demonstrate this capability as a practical tool for imaging complex structures containing light and heavy elements, and use it to solve the structure of a beam-sensitive carbon nanostructure. The contrast of the phase image contrast is maximized through the post-acquisition correction of lens aberrations. The compensation of defocus aberrations is also used for the measurement of three-dimensional sample information through post-acquisition optical sectioning.

Figure 1. Simultaneous atomic resolution incoherent and coherent imaging.

(a) An ADF detector collects the dark-field signal to form (b) an incoherent Z-contrast image. (c) The simultaneous phase image is reconstructed using ptychography. Simultaneously, a fast pixelated detector records the coherent BF diffraction pattern at every probe position forming a 4D data set. Taking the Fourier transform of the 4D data set with respect to probe position results in a complex 4D matrix G(K_f, Q_p), which carries the phase information of the interference between diffracted and undiffracted beams. (d,e) An example of the modulus and phase of the complex matrix G(K_f, Q_p) at a single spatial frequency Q_p where two diffracted beams +Q_p and −Q_p (indicated by dashed lines) overlap with the undiffracted direct beam. (d) The areas labelled as area I are double-overlap regions where one diffracted beam interferes with the direct beam and area II is the triple-overlap region where both diffracted beams and the direct beam interfere. Area III has no interfering beams and is only noise and is therefore not used in the ptychography reconstruction. By analysing the phase information for all spatial frequencies in the image, (c) the phase image can be reconstructed. Scale bar, 1 nm.

Figure 2. Scheme of the intended hybrid synthesis.

CNTs were oxidized to introduce carboxylic acid groups for functionalization. CNT-COOH 1 then underwent carboxyl preactivation and subsequent intended reaction with amine-bearing modified C₆₀ peptido-fullerene fC₆₀, to give the target tethered hybrids 2 and 3. Ahx denotes aminohexanoyl residue. Fo all other amino acid residues, standard 3-letter amino acid codes are used.

Figure 3. Simultaneous Z-contrast and phase images of a double-wall CNT peapod.

(a) Incoherent Z-contrast ADF image clearly shows the locations of the single iodine atoms indicated by the arrows. (b) The reconstructed phase image shows the presence of fullerenes inside the CNT. (c) Annotated phase image with the fullerenes labelled using dotted circles and iodine atoms labelled using cross marks based on their locations in the ADF image. It is clear that the iodine atoms are located close to but outside the fullerenes. For comparison, conventional phase-contrast images including BF, ABF, DPC and the DPC using the centre of mass (COM) approach were synthesized from the data and shown in d–h, respectively. The detector area of each imaging method is shown in white colour in d–g. The experiment was performed at an electron probe current of ∼2.8 pA, pixel dwell time of 0.25 ms and a dose of ∼1.3 × 10⁴ e⁻ Å⁻². Scale bar, 1 nm; the grey scale of the phase in b is in unit of radians.

Figure 4. Imaging a double-wall CNT at atomic resolution.

(a) The ADF image shows the presence of single iodine dopants, whereas (b) the simultaneous BF image is rather noisy and shows little contrast of the CNT. (c,d) The phase image before and after correcting residual aberrations, respectively. This comparison shows the improved visibility of the CNT atomic fringes and the damaged carbonaceous structure inside the CNT due to aberration correction. Scale bar, 2 nm.

Figure 5. WDD 3D optical sectioning of crossed CNTs of different heights.

(a) The simultaneous ADF image shows that none of the tubes is exactly in focus in the experimental data. By deconvolving a series of defocus aberrations (C₁) from the 4D data set, a set of reconstructed phase images are obtained. The best phase image contrast of the two crossed nanotubes (labelled as 1 and 2 in a) are found to be at defocus (b) +9 nm and (d) +25 nm, respectively, with ‘+' being over-focus relative to the experimental focal plane, and the mid-plane between the two optimal focus is shown in c as a comparison. (e) At defocus +39 nm, a small CNT filled with fullerene (indicated by the arrow) becomes visible. WDD optical sectioning offers a depth sensitivity that is not equivalent to the simple Fresnel wave propagation of the reconstructed ‘exit wave' and this is evidenced from f,g,h obtained by propagating the ptychographically reconstructed complex object function in b to the corresponding heights in c–e. The white arrow points to the small CNT, which becomes visible using WDD optical sectioning in e, but it remains invisible after applying Fresnel propagation in h, in comparison. Scale bar, 5 nm.