Icosahedra clustering and short range order in Ni-Nb-Zr amorphous membranes.

Crystalline Pd/Pd-Ag membranes are widely used for hydrogen separation from CO2 and other gases in power generation applications. To substitute these high cost noble metal alloy membranes, the Ni-Nb-Zr amorphous alloys are being developed that exhibit relatively high permeability of hydrogen between 200-400 °C. Atom probe tomography (APT) experiments performed on these ribbons revealed nm-scale Nb-rich and Zr-rich regions (clusters) embedded in a ternary matrix, indicating phase separation within the Ni-Nb-Zr amorphous alloy. Density functional theory (DFT) simulations have predicted that these clusters are composed of icosahedral coordination polyhedra. The interatomic distances and correlation lengths of the short range order of these alloys were determined by neutron total scattering which match well with our DFT based molecular dynamics (DFT-MD) simulations.

Phonon Engineering in Isotopically Disordered Silicon Nanowires.

The introduction of stable isotopes in the fabrication of semiconductor nanowires provides an additional degree of freedom to manipulate their basic properties, design an entirely new class of devices, and highlight subtle but important nanoscale and quantum phenomena. With this perspective, we report on phonon engineering in metal-catalyzed silicon nanowires with tailor-made isotopic compositions grown using isotopically enriched silane precursors (28)SiH4, (29)SiH4, and (30)SiH4 with purity better than 99.9%. More specifically, isotopically mixed nanowires (28)Si(x)(30)Si(1-x) with a composition close to the highest mass disorder (x ∼ 0.5) were investigated. The effect of mass disorder on the phonon behavior was elucidated and compared to that in isotopically pure (29)Si nanowires having a similar reduced mass. We found that the disorder-induced enhancement in phonon scattering in isotopically mixed nanowires is unexpectedly much more significant than in bulk crystals of close isotopic compositions. This effect is explained by a nonuniform distribution of (28)Si and (30)Si isotopes in the grown isotopically mixed nanowires with local compositions ranging from x = ∼0.25 to 0.70. Moreover, we also observed that upon heating, phonons in (28)Si(x)(30)Si(1-x) nanowires behave remarkably differently from those in (29)Si nanowires suggesting a reduced thermal conductivity induced by mass disorder. Using Raman nanothermometry, we found that the thermal conductivity of isotopically mixed (28)Si(x)(30)Si(1-x) nanowires is ∼30% lower than that of isotopically pure (29)Si nanowires in agreement with theoretical predictions.

Preparation of nanowire specimens for laser-assisted atom probe tomography.

The availability of reliable and well-engineered commercial instruments and data analysis software has led to development in recent years of robust and ergonomic atom-probe tomographs. Indeed, atom-probe tomography (APT) is now being applied to a broader range of materials classes that involve highly important scientific and technological problems in materials science and engineering. Dual-beam focused-ion beam microscopy and its application to the fabrication of APT microtip specimens have dramatically improved the ability to probe a variety of systems. However, the sample preparation is still challenging especially for emerging nanomaterials such as epitaxial nanowires which typically grow vertically on a substrate through metal-catalyzed vapor phase epitaxy. The size, morphology, density, and sensitivity to radiation damage are the most influential parameters in the preparation of nanowire specimens for APT. In this paper, we describe a step-by-step process methodology to allow a precisely controlled, damage-free transfer of individual, short silicon nanowires onto atom probe microposts. Starting with a dense array of tiny nanowires and using focused ion beam, we employed a sequence of protective layers and markers to identify the nanowire to be transferred and probed while protecting it against Ga ions during lift-off processing and tip sharpening. Based on this approach, high-quality three-dimensional atom-by-atom maps of single aluminum-catalyzed silicon nanowires are obtained using a highly focused ultraviolet laser-assisted local electrode atom probe tomograph.

Morphology control of nanostructures: Na-doped PbTe-PbS system.

The morphology of crystalline precipitates in a solid-state matrix is governed by complex but tractable energetic considerations driven largely by volume strain energy minimization and anisotropy of interfacial energies. Spherical precipitate morphologies are favored by isotropic systems, while anisotropic interfacial energies give energetic preference to certain crystallographically oriented interfaces, resulting in a faceted precipitate morphology. In conventional solid-solution precipitation, a precipitate's morphological evolution is mediated by surface anchoring of capping molecules, which dramatically alter the surface energy in an anisotropic manner, thereby providing exquisite morphology control during crystal growth. Herein, we present experimental evidence and theoretical validation for the role of a ternary element (Na) in controlling the morphology of nanoscale PbS crystals nucleating in a PbTe matrix, an important bulk thermoelectric system. The PbS nanostructures formed by phase separation from a PbI(2)-doped or undoped PbTe matrix have irregular morphologies. However, replacing the iodine dopant with Na (1-2 mol %) alters dramatically the morphology of the PbS precipitates. Segregation of Na at PbTe/PbS interfaces result in cuboidal and truncated cuboidal morphologies for PbS. Using analytical scanning/transmission electron microscopy and atom-probe tomography, we demonstrate unambiguously that Na partitions to the precipitates and segregates at the matrix/precipitate interfaces, inducing morphological anisotropy of PbS precipitates. First-principles and semiclassical calculations reveal that Na as a solute in PbTe has a higher energy than in PbS and that Na segregation at a (100) PbTe/PbS interface decreases the total energy of matrix/precipitate system, resulting in faceting of PbS precipitates. These results provide an impetus for a new strategy for controlling morphological evolution in matrix/precipitate systems, mediated by solute partitioning of ternary additions.

Effects of 10 MeV electron irradiation at high temperature of a Ni-Mo-based Hastelloy.

A Hastelloy alloy was irradiated with 10 MeV electrons at 650 degrees C for 700 h to a total dose of 2 x 10(-3) displacements per atom (dpa). The microstructure of irradiated and non-irradiated specimens of this alloy were investigated by transmission electron microscopy (TEM). The non-irradiated specimens were analyzed by 3-D atom probe tomography (APT) in a local-electrode atom-probe (LEAP). TEM analysis before the irradiation detects small precipitates with a mean diameter of 22 nm, which are coherent with the FCC matrix. The number density of these precipitates is approximately 7 x 10(18) m(-3). Electron diffraction patterns from these precipitates exhibit superlattice reflections corresponding to the L1(2) ordered structure. The chemical composition of the precipitates, as measured by APT, is around 75 at% Ni with additions of Al, Ti and Mo. After electron irradiation, small precipitates with an irregular morphology are observed. The number density of these new precipitates about 10(20) m(-3) is greater than that of the L1(2) ordered precipitates before irradiation. The L1(2) superlattice reflections disappear completely, instead diffuse diffraction spots are observed at 1(1/2)0(FCC), which is attributed to compositional short-range order (SRO). The results are discussed with respect to the influence of the electron irradiation on the morphology and structure of the ordered precipitates.

Mg segregation at Al/Al3Sc heterophase interfaces on an atomic scale: experiments and computations.

Microscopic factors governing solute partitioning in ternary two-phase Al-Sc-Mg alloys are investigated combining three-dimensional-atom-probe (3DAP) microscopy measurements with first-principles computations. 3DAP is employed to measure composition profiles with subnanometer-scale resolution, leading to the identification of a large enhancement of Mg solute at the coherent alpha-Al/Al(3)Sc (fcc/L1(2)) heterophase interface. First-principles calculations establish an equilibrium driving force for this interfacial segregation reflecting the nature of the interatomic interactions.

Nanometer-scale solute segregation at heterophase interfaces and microstructural evolution of molybdenum nitride precipitates.

The interrelationship between coherency and solute segregation at metal/metal-nitride heterophase interfaces is studied on a subnanometer scale by both atom-probe field-ion and electron microscopies for molybdenum nitride precipitates in Fe-2 at% Mo-X, where X = 0.4 at% Sb or 0.5 at% Sn. Internal nitridation at 550 degrees C generates thin platelet-shaped molybdenum nitride precipitates, while nitridation at 600 degrees C produces, in addition to the small-scale structure with precipitates of the thin-platelet type, a much coarser structure of thick plates and spheroidal precipitates. The solute species Sn and Sb segregate at the heterophase interfaces of the coarse precipitates and Gibbsian interfacial excesses of up to 7 x 10(18) M(-2) are measured. The segregation is related to the presence of misfit dislocations at the interfaces of the coarse preciptitates, while the thin plates remain coherent with no detectable segregation.

Subnanometer three-dimensional atom-probe investigation of segregation at MgO/Cu ceramic/metal heterophase interfaces.

Three-dimensional atom-probe (3DAP) microscopy has been applied to the study of segregation at ceramic/metal (C/M) interfaces. In this article, results on the MgO/Cu(X) (where X = Ag or Sb) systems are summarized. Nanometer-size MgO precipitates with atomically clean and atomically sharp interfaces were prepared in these systems by internal oxidation. Segregation of the ternary component (Ag or Sb) at the MgO/Cu heterophase interface was enhanced by extended low-temperature anneals. Magnesia precipitates in the 3DAP reconstructions were delineated as isoconcentration surfaces, and segregation of each ternary component at the C/M interfaces was analyzed with the proximity histogram method developed at Northwestern University. This method allows the direct extraction of the Gibbsian interfacial excess of solute at the C/M interfaces from the experimental data. A value of (3.2+/-2.0) x 10(17)m(-2) at 500 degrees C is obtained for the segregation of Ag at a MgO/Cu(Ag) interface, while a value of (2.9+/-0.9) x 10(18) m(-2) at 500 degrees C is obtained for the segregation of Sb at a MgO/Cu(Sb) interface. The larger Gibbsian excess for Sb segregation at this ceramic/metal heterophase interface is most likely due to the so-called pdeltaV effect.

A differentially pumped low-energy ion beam system for an ultrahigh-vacuum atom-probe field-ion microscope.

An ultrahigh-vacuum (UHV) differentially pumped low-energy (50-3000 eV) ion beam system for the in situ irradiation of specimens in a UHV atom-probe field-ion microscope (FIM) was designed and constructed. The ion beam system consisted of a Finkelstein-type ion source, an Einzel lens, and a magnetic mass analyzer. The ion source was connected to the analyzer chamber by small apertures which resulted in differential pumping between the ion source and the analyzer chamber; during a typical in situ irradiation of a specimen in the atom-probe FIM the total pressure was maintained at approximately 10(-7) Torr. In the case of helium ion irradiation the optimum ion-current density was approximately 0.5 microA cm(-2) for 300-eV He+ ions at the atom-probe FIM specimen. After the completion of a helium ion irradiation the pumpdown time from 5 x 10(-7) to approximately 3 x 10(-10) Torr in the atom-probe FIM chamber was 0.5 h.