RESUMEN
Elastic strain has the potential for a controlled manipulation of the band gap and spin-polarized Dirac states of topological materials, which can lead to pseudomagnetic field effects, helical flat bands, and topological phase transitions. However, practical realization of these exotic phenomena is challenging and yet to be achieved. Here we show that the Dirac surface states of the topological insulator Bi2Se3 can be reversibly tuned by an externally applied elastic strain. Performing in situ X-ray diffraction and in situ angle-resolved photoemission spectroscopy measurements during tensile testing of epitaxial Bi2Se3 films bonded onto a flexible substrate, we demonstrate elastic strains of up to 2.1% and quantify the resulting changes in the topological surface state. Our study establishes the functional relationship between the lattice and electronic structures of Bi2Se3 and, more generally, demonstrates a new route toward momentum-resolved mapping of strain-induced band structure changes.
RESUMEN
Extraordinarily high mobility of Si and Ge atoms at semiconductor (Si, Ge)-metal (Al) interfaces is observed at temperatures as low as 80 K during thin metal film deposition. In situ x-ray photoemission spectroscopic valence-band measurements reveal a changed chemical bonding nature of the semiconductor atoms, from localized covalentlike to delocalized metalliclike, at the interface with the Al metal. The resulting delocalized bonding nature of the interfacial semiconductor atoms brings about the observed extreme enhancement of their mobility. The finding opens avenues for tailoring reaction kinetics and phase transformations in nanostructured materials, as functional thin-film systems, at ultralow temperatures by dedicated interfacial design.
RESUMEN
Three-dimensional coherent diffraction patterns of an isolated, single-crystalline Ag/Au core-shell nanowire were recorded at different X-ray beam energies close to the Au LIII absorption edge. Two-dimensional slices of the three-dimensional diffraction pattern, with the diffraction vector oriented perpendicular to the wire axis, were investigated in detail. In reciprocal space, facet streaks with thickness fringes were clearly observed in the two-dimensional diffraction patterns, from which the shape and size of the corresponding cross sections of the nanowire could be revealed. Comparison with simulated diffraction patterns exhibited the coherency strain field in the nanowire. During in situ annealing at temperatures which would lead to significant intermixing by volume diffusion in bulk material, according to literature data, a core-shell morphology was preserved; that is, intermixing in the nanowire was pronouncedly decelerated compared to bulk diffusion.
RESUMEN
Convective transportation of materials in the solid state occurring in a prototype solid bilayer system of Al and Si with negligible mutual solubility has been directly imaged in real time at nanoscale using a valence energy-filtered transmission electron microscope. Such solid-state convection is driven by the stress gradient developing in the bilayer system due to the amorphous to crystalline phase transformation of the Si sublayer. The process is characterized by compression experienced in the Si phase crystallizing within the Al sublayer, as well as by the development of mushroom-shaped "plumes" of Al nanocrystals in the Si sublayer as a result of compressive stress relaxation and discrete, new nucleation of crystalline Al. The real-time, atomistic observation and the thus-obtained fundamental understanding of solid-state convection enable highly sophisticated applications of such a complex process in advanced fabrication and processing of nanomaterials and solid-state devices.
RESUMEN
Functionalization of thin-film heterostructures on the basis of their electrical, optical and magnetic properties, requires precise control of the film stresses that develop during the growth process. By using real-time in situ stress measurements, the present study reveals strikingly that the in-plane film stress oscillates with increasing film thickness at the initial stage of epitaxial Al(111) film growth on a Si(111)-â3×â3-Al surface, with a periodicity of 2 times the Fermi wavelength of bulk Al and a stress variation from maximum to minimum as large as 100 MPa. Such macroscopic stress oscillations are shown to be caused by quantum confinement of the free electrons in the ultrathin epitaxial metal film. The amplitude, period, and phase of the observed stress oscillations are consistent with predictions based on the free electron model and continuum elasticity.
RESUMEN
Needle-shaped atom probe specimens containing a single grain boundary were produced using the focused ion beam (FIB) of a two-beam FIB/SEM (scanning electron microscope) system. The presented specimen preparation approach allows the unprecedented study of a grain boundary which is well characterised in its crystallographic orientation by means of the field ion microscope (FIM) and the tomographic atom probe (TAP). The analysis of such specimens allows in particular the determination of solute excess atoms at this specific grain boundary and hence the investigation of the segregation behaviour. The crucial preparation steps are discussed in detail in the present study for the Sigma 19 a {331} 110 grain boundary of a 40 at.ppm-Bi doped Cu bi-crystal. Transmission electron microscope (TEM) images and TAP analyses of the atom probe tips demonstrate unambiguously the presence of the selectively prepared grain boundary in the apex region of some of the specimens.
RESUMEN
The microstructure of the compound ("white") layer formed on the surface of Fe-4wt.%V alloy, by nitriding in a gas mixture of ammonia and hydrogen at 580 °C, has been investigated by employing light and scanning electron microscopy, X-ray diffraction and electron probe microanalysis. The compound layer is dominantly composed of γ|-Fe4N nitride. Quantitative analysis of the composition data demonstrated that V is present in the compound layer as VN precipitates, i.e. V is not taken up significantly in (Fe, V) nitrides. A mechanism for compound-layer formation has been proposed.