A three-dimensional (3D) chemical characterization of nanomaterials can be obtained using tomography based on high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) or energy dispersive X-ray spectroscopy (EDS) STEM. These two complementary techniques have both advantages and disadvantages. The Z-contrast images have good image quality but lack robustness in the compositional analysis, while the elemental maps give more element-specific information, but at a low signal-to-noise ratio and a longer exposure time. Our aim is to combine these two types of complementary information in one single tomographic reconstruction process. Therefore, an imaging model is proposed combining both HAADF-STEM and EDS-STEM. Based on this model, the elemental distributions can be reconstructed using both types of information simultaneously during the reconstruction process. The performance of the new technique is evaluated using simulated data and real experimental data. The results demonstrate that combining two imaging modalities leads to tomographic reconstructions with suppressed noise and enhanced contrast.

a b s t r a c tElectron tomography is a powerful technique for the 3D characterization of the morphology of nanostructures. Nevertheless, resolving the chemical composition of complex nanostructures in 3D remains challenging and the number of studies in which electron energy loss spectroscopy (EELS) is combined with tomography is limited. During the last decade, dedicated reconstruction algorithms have been developed for HAADF-STEM tomography using prior knowledge about the investigated sample. Here, we will use the prior knowledge that the experimental spectrum of each reconstructed voxel is a linear combination of a well-known set of references spectra in a so-called direct spectroscopic tomography technique. Based on a simulation experiment, it is shown that this technique provides superior results in comparison to conventional reconstruction methods for spectroscopic data, especially for spectrum images containing a relatively low signal to noise ratio. Next, this technique is used to investigate the spatial distribution of Fe dopants in Fe:Ceria nanoparticles in 3D. It is shown that the presence of the Fe 2 þ dopants is correlated with a reduction of the Ce atoms from Ce 4 þ towards Ce 3 þ . In addition, it is demonstrated that most of the Fe dopants are located near the voids inside the nanoparticle.

Energy-dispersive X-ray spectroscopy (EDS) tomography is an advanced technique to characterize compositional information for nanostructures in three dimensions (3D). However, the application is hindered by the poor image quality caused by the low signal-to-noise ratios and the limited number of tilts, which are fundamentally limited by the insufficient number of X-ray counts. In this paper, we explore how to make accurate EDS reconstructions from such data. We propose to augment EDS tomography by joining with it a more accurate high-angle annular dark-field STEM (HAADF-STEM) tomographic reconstruction, for which usually a larger number of tilt images are feasible. This augmentation is realized through total nuclear variation (TNV) regularization, which encourages the joint EDS and HAADF reconstructions to have not only sparse gradients but also common edges and parallel (or antiparallel) gradients. Our experiments show that reconstruction images are more accurate compared to the non-regularized and the total variation regularized reconstructions, even when the number of tilts is small or the X-ray counts are low.

The 3D spatial resolution, the material contrast and the evolution of the noise are analyzed in the reconstructed volume of a combined scanning transmission electron microscopy (HAADF-STEM) and energy dispersive x-ray spectroscopy (EDS) tomography experiment. Standard simultaneous iterative reconstruction technique and HAADF-EDS bimodal tomographic reconstruction are considered for the +/−90°tomography series of a pillar shaped sample embedding a full nanowire device. With a high number of iterations, a spatial resolution for both HAADF and EDS down to 5 nanometer can be reached for this volume. Best material's contrast and minimum noise are obtained for medium number of iterations. Improvement of the signal-tonoise and contrast can be obtained by filtering the EDS data while the spatial resolution is not impacted. A fast and reliable preparation methodology for rectangularly shaped pillar samples for tomography analysis is discussed.

Low-energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy measurements have been performed on GaAs samples cleaved in ultra-high vacuum and subject to in situ isochronous anneals up to 850 degrees C. The properties of the cleaved (110) surface remain unchanged up to about 350 degrees C. Around this temperature, a dissociation occurs inducing a local reconstruction of the surface; at the same time, new states appear which may be attributed to Ga vacancies. Then, around 580 degrees C, the well known dissociation of GaAs sets in with formation of As2, and the Ga-Ga bonds which then form, determine the electronic properties of the sample. Evaporation of arsenic would become dominant around 650 degrees C. Beyond about 780 degrees C, the excess Ga coalesces into Ga-metal droplets and the GaAs substrate undergoes faceting.