A soft touch with electron beams: Digging out structural information of nanomaterials with advanced scanning low energy electron microscopy coupled with deep learning.

Nanostructured materials continue to find applications in various electronic and sensing devices, chromatography, separations, drug delivery, renewable energy, and catalysis. While major advancements on the synthesis and characterization of these materials have already been made, getting information about their structures at sub-nanometer resolution remains challenging. It is also unfortunate to find that many emerging or already available powerful analytical methods take time to be fully adopted for characterization of various nanomaterials. The scanning low energy electron microscopy (SLEEM) is a good example to this. In this report, we show how clearer structural and surface information at nanoscale can be obtained by SLEEM, coupled with deep learning. The method is demonstrated using Au nanoparticles-loaded mesoporous silica as a model system. Moreover, unlike conventional scanning electron microscopy (SEM), SLEEM does not require the samples to be coated with conductive films for analysis; thus, not only it is convenient to use but it also does not give artifacts. The results further reveal that SLEEM and deep learning can serve as great tools to analyze materials at nanoscale well. The biggest advantage of the presented method is its availability, as most modern SEMs are able to operate at low energies and deep learning methods are already being widely used in many fields.

Contrast mechanism at landing energy near 0 eV in super low energy scanning electron microscopy.

In recent years, the technique of scanning electron microscopy (SEM) observation with low landing energy of a few keV or less has become common. We have especially focused on the drastic contrast change at near 0 eV. Using a patterned sample consisting of Si, Ni and Pt, threshold energies where total reflection of incident electrons occur was investigated by SEM at near 0 eV. In both the cases of in-situ and ex-situ sample cleaning, drastic changes in the brightness of each material were observed at near 0 eV, with threshold energies in the order Si < Ni < Pt. This order agreed with the order of the literature values of the work functions and the surface potentials measured by Kelvin force probe microscopy. This result suggests that the difference of the threshold energy is caused by the difference in surface potential due to the work function difference of each material. Although the order of the threshold energies also agreed with those of work functions reported in literatures, the work functions of air exposed surfaces should be rather considered as "modified work functions", since they could be significantly altered by the adsorbates etc. Nevertheless, the difference of threshold energy for each material was observed with commercial SEM at landing energy near 0 eV, which opens new possibility to distinguish materials, although the difference should be rather recognized as "fingerprints", since surface potentials are sensitive to conditions of surface treatments and atmospheric exposure. Mini-abstract In this study, we utilized a commercial SEM with near 0 eV landing energy to explore threshold energies where total reflection occurs for various materials in air-exposed model samples. Our results demonstrate the potential of threshold energy as a distinctive fingerprint for material differentiation.

Microstructural Characterization of Laser Weld of Hot-Stamped Al-Si Coated 22MnB5 and Modification of Weld Properties by Hybrid Welding.

The presence of Al-Si coating on 22MnB5 leads to the formation of large ferritic bands in the dominantly martensitic microstructure of butt laser welds. Rapid cooling of laser weld metal is responsible for insufficient diffusion of coating elements into the steel and incomplete homogenization of weld fusion zone. The Al-rich regions promote the formation of ferritic solid solution. Soft ferritic bands cause weld joint weakening. Laser welds reached only 64% of base metal's ultimate tensile strength, and they always fractured in the fusion zone during the tensile tests. We implemented hybrid laser-TIG welding technology to reduce weld cooling rate by the addition of heat of the arc. The effect of arc current on weld microstructure and mechanical properties was investigated. Thanks to the slower cooling, the large ferritic bands were eliminated. The hybrid welds reached greater ultimate tensile strength compared to laser welds. The location of the fracture moved from the fusion zone to a tempered heat-affected zone characterized by a drop in microhardness. The minimum of microhardness was independent of heat input in this region.

Effect of native oxide on the crystal orientation contrast in SEM micrographs obtained at hundreds, tens and units of eV.

This paper aims to elucidate the effect of native air-formed oxide on the crystallographic contrast between differently oriented copper grains in scanning electron microscope images obtained at energies from 0 eV up to 1 keV. The contrast between the Cu grains is strongly affected by the presence of native oxide. The crystallographic orientation contrast between the grains without covering the native oxide layer is relatively weak at hundreds of eV, negligible at tens of eV, and dramatically increases at energies below 10 eV. At extremely low landing energies, say below ~ 1 eV, the surface potential differences caused by work function variations between the differently oriented Cu grains affect the primary electrons, which enables us to obtain the micrographs with high crystallographic contrast. This contrast becomes surprisingly visible even if the grains are covered by a several nm thick native oxide layer. The presence of the native air-formed oxide layer on the Cu surface is inconsiderable for the contrast formation at energies close to the mirror conditions (< 1 eV). The surface potential differences originating in the substrate can affect the incident electrons through the native oxide film situated on the Cu surface. Scanning low-energy electron microscopy is a powerful tool for mapping local work function differences with a spatial resolution slightly better than 30 nm due to high sensitivity to local electrical potentials.

Visualization of three different phases in a multiphase steel by scanning electron microscopy at 1 eV landing energy.

In this study, we investigated an observation technique by super low energy scanning electron microscopy (SLESEM) at below 5 eV and its contrast mechanism for analyzing complex microstructures of a multiphase steel consisting of ferrite, martensite and austenite. With SLESEM at 1 eV, the three phases were observed as different brightness levels, ferrite as the darkest contrast, martensite as the second brightest and austenite as the brightest. These contrasts disappeared at 2 eV or higher. Similar contrasts and phenomena were also observed in the results of low energy electron microscopy (LEEM). According to the energy dependences of the LEEM intensities of the three phases, the threshold energies of the transition from electron reflection to surface impact were determined to be 0.00 eV, 0.15 eV and 0.39 eV for ferrite, martensite and austenite, respectively. These differences in thresholds indicate that the potentials on the surfaces of each phase are different, which is considered to result in the different brightness of each phase. This potential differences are probably due to the contact potentials generated when phases with different work functions contact each other. Although the sample is covered by a thin native oxide film (several nm thickness), the potentials can affect the incident electrons through the oxide film.

Contrast of positively charged oxide precipitate in out-lens, in-lens and in-column SE image.

Modern scanning electron microscopes are usually equipped with multiple detectors and enable simultaneous collection of two or even three secondary electron images. The secondary electrons become divided between the detectors in dependence on their initial kinetic energy and emission angle. In this study, sharing of the secondary electrons by out-lens, in-lens and in-column detectors has been systematically investigated. Energy filtering of the signal electrons is demonstrated by separation of the voltage and the topographical contrast in the micrographs obtained by out-lens and in-lens/in-column detectors. The presence of two detectors inside the electron column enables further filtering of the low kinetic energy secondary electrons, which results to unusual contrasts and phenomena. In this paper, inversion of the contrast sign between a positively charged oxide particle and conductive steel matrix (i.e. voltage contrast) in SE images collected under specific imaging conditions is demonstrated.

Dual-phase steel structure visualized by extremely slow electrons.

Mechanical properties of complex steels are affected by their multi-phase structure. Scanning electron microscopy (SEM) is routinely used for characterizing dual-phase (DP) steels, although the identification of steel constituents is not straightforward. In fact, there are several ways of enabling the ferrite-martensite segmentation by SEM, and a wide range of electron energies can be utilized. This study demonstrates the phase identification of DP steels at high, low and extremely low landing energies of the primary electrons from tens of keV to tens of eV. Visualization of the specimen surface at very low landing energies has been achieved by inserting an earthed detector between the pole piece and the negatively biased specimen. This 'cathode lens mode' enables the use of the full energy range up to the primary electron energies. It has been found that extremely slow electrons (<100 eV) are exceptionally suitable for separation of the martensite from the ferrite matrix due to high surface sensitivity, enabling visualization of very fine features. Moreover, the channelling contrast is significantly suppressed at the landing energy of tens of eV of the primary electrons, which enables separation of the phases clearly even in the images acquired at low magnification. The contrast between the phases at tens of eV can be explained by the different thickness of native oxide covering the martensite and the ferrite phase.

TRIP steel microstructure visualized by slow and very slow electrons.

The aim of the present paper is to demonstrate the ability of the scanning low-energy electron microscopy to visualize the transformed induced plasticity steel microstructure with extremely high sensitivity. Using the retarding mode in the scanning electron microscope, the high contrast between the individual phases has been obtained, which enables us to differentiate the retained austenite and the other phases. The sets of the micrographs have been collected from the sample at a wide range of landing energies of primary electrons from 50 eV to 10 keV and the dependence of the contrast between the phases on the landing energy has been calculated. Upon a comparison of these contrast curves, the optimal conditions for achieving of maximum contrast have been established.