Resolving hydrogen atoms at metal-metal hydride interfaces

Hydrogen as a fuel can be stored safely with high volumetric density in metals. It can, however, also be detrimental to metals causing embrittlement. Understanding fundamental behavior of hydrogen at atomic scale is key to improve the properties of metal-metal hydride systems. However, currently, there is no robust technique capable of visualizing hydrogen atoms. Here, we demonstrate that hydrogen atoms can be imaged unprecedentedly with integrated differential phase contrast, a recently developed technique performed in a scanning transmission electron microscope. Images of the titanium-titanium monohydride interface reveal remarkable stability of the hydride phase, originating from the interplay between compressive stress and interfacial coherence. We also uncovered, thirty years after three models were proposed, which one describes the position of the hydrogen atoms with respect to the interface. Our work enables novel research on hydrides and is extendable to all materials containing light and heavy elements, including oxides, nitrides, carbides and borides

High Resolution Imaging of Chalcogenide Superlattices for Data Storage Applications:Progress and Prospects

Phase-change materials (PCMs) based on Ge–Sb–Te alloys are a strong contender for next-generation memory technology. Recently, PCMs in the form of GeTe–Sb 2 Te 3 superlattices (CSLs) have shown superior performance compared to ordinary PCM memory, which relies on switching between amorphous and crystalline phases. Although detailed atomic structure switching models have been developed with the help of ab-initio simulations, there is still fierce scientific debate concerning the experimental verification of the actual crystal structures pertaining to the two CSL memory states. One of the strongest techniques to provide this information is (scanning) transmission electron microscopy ((S)TEM). The present article reviews the analyses of CSLs using TEM-based techniques published during the last seven years since the seminal 2011 Nature Nanotechnology paper of Simpson et al., showing the superior performance of the CSL memory. It is critically reviewed what relevant information can be extracted from the (S)TEM results, also showing the impressive progress that has been achieved in a relatively short time frame. Finally, an outlook is given including several open questions. Although debate on actual switching mechanism in CSL memory is clearly not settled, still there is consensus in this field that CSL research has a bright future

Clinicopathological and targeted exome gene features of a patient with metastatic acinic cell carcinoma of the parotid gland harboring an ARID2 nonsense mutation and CDKN2A/B deletion

We describe the presentation, treatment, clinical outcome, and targeted genome analysis of a metastatic salivary acinic cell carcinoma (AciCC). A 71-year-old male presented with a 3 cm right tail of a parotid lesion, first detected as a nodule by the patient seven months earlier. He had a right total parotidectomy with cranial nerve VII resection, right facial nerve resection and grafting, resection of the right conchal cartilage, and right modified radical neck dissection. The primary tumor revealed AciCC with two distinct areas: a well-differentiated component with glandular architecture and a dedifferentiated component with infiltrative growth pattern associated with prominent stromal response, necrosis, perineural invasion, and cellular pleomorphism. Tumor staging was pT4 N0 MX. Immunohistochemistry staining showed pankeratin (+), CD56 (−), and a Ki67 proliferation index of 15%. Upon microscopic inspection, 49 local lymph nodes resected during parotidectomy were negative for cancer cells. Targeted sequencing of the primary tumor revealed deletions of CDKN2A and CDKN2B, a nonsense mutation in ARID2, and single missense mutations of unknown significance in nine other genes. Despite postoperative localized radiation treatment, follow-up whole body PET/CT scan showed lung, soft tissue, bone, and liver metastases. The patient expired 9 months after resection of the primary tumor

Real space imaging of hydrogen at a metal - metal hydride interface

Hydrogen as a prospective fuel can be stored safely with high volumetric density in metals. It can, however, also be detrimental to metals causing embrittlement. For a better understanding of these metal-metal hydride systems, and in particular their interfaces, real-space imaging of hydrogen with atomic resolution is required. However, hydrogen has not been imaged before at an interface. Moreover, to date, a robust technique that is capable to do such light-element imaging has not been demonstrated. Here, we show that integrated Differential Phase Contrast (iDPC), a recently developed imaging technique performed in an aberration corrected scanning transmission electron microscope, has this capability. Atomically sharp interfaces between hexagonal close-packed titanium and face-centered tetragonal titanium monohydride have been imaged, unambiguously resolving the hydrogen columns. Exploiting the fact that this monohydride has two types of columns with identical surrounding of the host Ti atom we have, 30 years after they were first proposed, finally resolved which one of the proposed structural models holds for the interface. Using both experimental and simulated images, we compare the iDPC technique with the currently more common annular bright field (ABF) technique, showing that iDPC is superior regarding complicating wave interference effects that may lead to erroneous detection of light element columns

Real space imaging of hydrogen at a metal - metal hydride interface

Hydrogen as a prospective fuel can be stored safely with high volumetric density in metals. It can, however, also be detrimental to metals causing embrittlement. For a better understanding of these metal-metal hydride systems, and in particular their interfaces, real-space imaging of hydrogen with atomic resolution is required. However, hydrogen has not been imaged before at an interface. Moreover, to date, a robust technique that is capable to do such light-element imaging has not been demonstrated. Here, we show that integrated Differential Phase Contrast (iDPC), a recently developed imaging technique performed in an aberration corrected scanning transmission electron microscope, has this capability. Atomically sharp interfaces between hexagonal close-packed titanium and face-centered tetragonal titanium monohydride have been imaged, unambiguously resolving the hydrogen columns. Exploiting the fact that this monohydride has two types of columns with identical surrounding of the host Ti atom we have, 30 years after they were first proposed, finally resolved which one of the proposed structural models holds for the interface. Using both experimental and simulated images, we compare the iDPC technique with the currently more common annular bright field (ABF) technique, showing that iDPC is superior regarding complicating wave interference effects that may lead to erroneous detection of light element columns

Controlling phase separation in thermoelectric Pb1-xGexTe to minimize thermal conductivity

Intensive studies have been carried out over the past decade to identify nanostructured thermoelectric materials that allow the efficient conversion of waste heat to electrical power. However, less attention has been paid to the stability of such materials under operating temperatures, typically 400 degrees C or higher. Conventionally nanostructured ceramics tend to undergo grain growth at high temperature, lowering the density of interfaces and raising the thermal conductivity, which is detrimental to device performance. Therefore it is preferable to identify materials with stable nanostructures, for example systems that undergo spontaneous phase separation. Here we investigate PbTe-GeTe alloys, in which spinodal decomposition occurs on initial cooling from above 580 degrees C, forming complex nanostructures consisting of Ge-rich and Pb-rich domains on different size scales. The resulting dense arrangement of interfaces, combined with mass fluctuation associated with Pb-Ge mixing, enhances phonon scattering and strongly reduces the thermal conductivity. Here we focus on the nominal composition Pb0.49Ge0.51Te and show that by tuning the synthesis procedure, we are able to control the pattern of compositional domains and the density of interfaces between them. This allows low lattice thermal conductivities to be maintained even after thermal cycling over the operating temperature range

Interface formation of two- and three-dimensionally bonded materials in the case of GeTe-Sb2Te3 superlattices

GeTe–Sb2Te3 superlattices are nanostructured phase-change materials which are under intense investigation for non-volatile memory applications. They show superior properties compared to their bulk counterparts and significant efforts exist to explain the atomistic nature of their functionality. The present work sheds new light on the interface formation between GeTe and Sb2Te3, contradicting previously proposed models in the literature. For this purpose [GeTe(1 nm)–Sb2Te3(3 nm)]15 superlattices were grown on passivated Si(111) at 230 °C using molecular beam epitaxy and they have been characterized particularly with cross-sectional HAADF scanning transmission electron microscopy. Contrary to the previously proposed models, it is found that the ground state of the film actually consists of van der Waals bonded layers (i.e. a van der Waals heterostructure) of Sb2Te3 and rhombohedral GeSbTe. Moreover, it is shown by annealing the film at 400 °C, which reconfigures the superlattice into bulk rhombohedral GeSbTe, that this van der Waals layer is thermodynamically favored. These results are explained in terms of the bonding dimensionality of GeTe and Sb2Te3 and the strong tendency of these materials to intermix. The findings debate the previously proposed switching mechanisms of superlattice phase-change materials and give new insights in their possible memory application

Free-standing nanolayers based on Ru silicide formation on Si(100)

Free-standing layers of nanoscale thickness are essential in numerous applications but challenging to fabricate for all but a small selection of materials. We report a versatile, chemical-free pathway of exfoliating centimeter-sized free-standing nanolayers from Si(100) with native oxide based on the spontaneous delamination of thin Ru and Ru-based films upon annealing at temperatures as low as 400 °C. Combining results from X-ray photoelectron spectroscopy (XPS), and transmission and scanning electron microscopy (TEM, SEM), we identify that the element Ru, a thin SiO2 layer, and the Si(100) substrate are essential ingredients for the delamination and propose a stress-based mechanism to explain the effect. The diffusion of Si into the layer upon annealing leads to the formation of a Ru-Si compound at the thin-film side of the Ru/Si(100) interface and pyramidal cavities in the Si(100) substrate. Moreover, the uptake of Si results in an increase in layer thickness and the buildup of in-plane compressive stress, which is reduced by local buckling and finally by the separation of the full layer from the substrate at the SiO2-Si(100) interface. The use of a thin Ru-buffer layer allows us to apply this delamination process to produce free-standing nanolayers of Mo and HfMoNbTiZr in this simple, chemical-free, and vacuum-compatible manner. These results indicate the potential of the reported effect for the fabrication of free-standing layers using a wide range of compositions, deposition techniques, and growth conditions below the onset temperature of delamination