We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early ...Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka.
In GaN high-electron-mobility transistors, electrical degradation due to high-voltage stress is characterized by a critical voltage at which irreversible degradation starts to take place. Separately, ...cross-sectional TEM analysis has revealed significant crystallographic damage for severely degraded devices. Furthermore, a close correlation between the degree of drain current degradation and material degradation has been reported. However, the role of the critical voltage in physical degradation has not been explored. In this work, we investigate the connection between electrical degradation that occurs around and beyond the critical voltage and the formation of crystallographic defects through detailed electrical and TEM analysis, respectively. We find that a groove in the GaN cap starts to be generated around the critical voltage. At higher voltages, a pit develops that penetrates into the AlGaN barrier. The size of the pit increases with stress voltage. We also observe a good correlation between electrical and material degradation.
Electron tomography based on Z-contrast scanning transmission electron microscopy (STEM) can be applied to study 3D morphology of nanomaterials at high resolution, that is, 1 nm in all three spatial ...dimensions, to provide comprehensive insights into the structure of nanomaterials and their interfaces. Here, we report the 3D characterization of Au-catalyzed Ge and Si nanowires using a full-space tilting holder to address the “missing wedge” problem in STEM electron tomography. Electron tomography specimens were prepared by a novel two-step sample preparation process to minimize surface damage induced by focused ion beam (FIB) milling. The quality of specimen preparation protocol is demonstrated by the clear visibility of {112} facets in the reconstructed volume, and 3D morphology of Au nanoparticles on the nanowire surface. The 3D distribution of the Au nanoparticles on the coated Ge nanowires is also established. The integrated combination of innovative specimen preparation and full-tilt tomography represents a useful advance in the 3D analysis of nanostructures.
Reinforced self‐assembled nano‐dielectrics (R‐SANDs) are fabricated by depositing a thin protective layer on high‐capacitance SAND films, thus significantly improving SAND durability and expanding ...SAND compatibility with a broader range of semiconductor deposition techniques. Fully transparent TFTs with excellent field‐effect mobilities ∼140 cm2/V·s and low operating voltages ∼1.0 V are demonstrated by combining the R‐SAND and an amorphous Zn‐In‐Sn‐O transparent oxide semiconductor.
BaCuSeF/ZnTe is a model system to investigate physical and chemical properties of the interfaces of non-oxide wide-bandgap
p-type semiconductors with materials used in chalcogenide solar cells. The ...BaCuSeF/ZnTe interface was studied using electron microscopy and photoelectron spectroscopy. Both techniques indicate that Se and Cu from BaCuSeF diffuse into ZnTe creating an interdiffused layer between these two materials. The interdiffusion may be attributed to the differences in materials formation enthalpies and to Fermi level pinning in BaCuSeF.
The structure of amorphous In2–2xZnxSnxO3 (a‐ZITO, x = 0.2, 0.3, 0.4) was investigated with transmission electron microscopy (TEM), the total scattering pair‐distribution function (PDF) and X‐ray ...absorption spectroscopy (XAS), which revealed a well‐defined short‐range structure that differed from the crystalline bixbyite and corundum ZITO polymorphs. The X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) were used to detect InO6 and SnO6 octahedra and ZnO4 tetrahedra in a‐ZITO, which differ from the InO6, SnO6 and ZnO6 pseudo‐octahedra that are observed in the bixbyite and corundum ZITO polymorphs. The formation of the ZnO4 tetrahedron likely inhibits the crystallization of ZITO when synthesized under mild conditions. in situ XRD showed a gradual transition from a‐ZITO to a mixture of the bixbyite and corundum ZITO polymorphs as the temperature was raised from 300 °C to 568 °C.
By controlling the specimen aspect ratio and strain rate, compressive strains as high as 80% were obtained in an otherwise brittle metallic glass. Physical and mechanical properties were measured ...after deformation, and a systematic strain-induced softening was observed which contrasts sharply with the hardening typically observed in crystalline metals. If the deformed glass is treated as a composite of hard amorphous grains surrounded by soft shear-band boundaries, analogous to nanocrystalline materials that exhibit inverse Hall-Petch behavior, the correct functional form for the dependence of hardness on shear-band spacing is obtained. Deformation-induced softening leads naturally to shear localization and brittle fracture.
SignificanceThe exploration of gold-based colorants in glass and glazes led Nobel Laureate Richard Zsigmondy to the study of colloids, and to the development, with Henry Siedentopf, of the earliest ...microscopes capable of resolving such small length scales. Zsigmondy's studies were preceded by alchemical investigations starting in the 17th century that yielded the gold-based Purple of Cassius, and experiments in the early 18th century resulting in an unusual purple iridescent porcelain overglaze, called Böttger luster, at the Meissen Manufactory. We discuss the first nano-scale characterization of Böttger luster, its successful replication, and propose an explanation for its optical properties based on the physics of scattering and interference of nanoparticle arrays.
A specimen size effect related to elastic spring-back and strain softening is reported on the deformation and fracture behavior of bulk metallic glass (BMG). Unlike in large specimens where unstable ...shear band propagation usually leads to catastrophic fracture, in small specimens yielding is followed by stable shear band propagation and extensive plastic deformation. Additionally, the fracture surfaces of the small specimens are smooth without the characteristic vein patterns seen in large specimens. The present results demonstrate that it is important to take specimen size into account when interpreting plasticity and fracture of BMGs, especially when considering the effects of composition on ductility.
The mechanical properties of submicron scale columnar zinc structures, with average diameters between 130 and 1060 nm, were characterized by uniaxial microcompression tests. The zinc pillars were ...fabricated by electron beam lithography and electroplating and were found to be generally single crystalline, with a preferred out-of-plane orientation close to the 0001 directions. Post deformation microstructural analysis suggests that the zinc pillars maintain their single-crystalline structure, but without twin boundary formation. Interestingly, the engineering flow stress results indicate that small-scale zinc structures are insensitive to both strain rate and size.
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