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Lupini, Andrew R

Publications and source records attributed to Lupini, Andrew R.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

A pathway for the growth of core-shell Pt-Pd nanoparticles

In this study, the aging of both Pt-Pd nanoparticles and core-shell Pt-Pd nanoparticles has been reported to result in alloying of Pt with Pd. In comparison to monometallic Pt catalysts, the growth of Pd-Pt bimetallics is slower; however, the mechanism of growth of particles and the mechanism by which Pd improves the hydrothermal durability of bimetallic Pd-Pt particles remains uncertain. In our work on hydrothermal aging of core-shell Pt-Pd nanoparticles, synthesized by solution methods, with varying Pd:Pt ratio of 1:4, 1:1, and 4:1, we compare the growth of core-shell Pt-Pd nanoparticles and find that particles grow by migrating and joining together. The unique feature of the observed growth is that Pd shells from both particles open up and join, allowing the cores to merge. At high temperatures, alloying occurs in good agreement with reports by other workers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Combined Scanning Transmission Electron Microscopy Tilt- and Focal Series

In this study, a combined tilt- and focal series is proposed as a new recording scheme for high-angle annular dark-field scanning transmission electron microscopy (STEM) tomography. Three-dimensional (3D) data were acquired by mechanically tilting the specimen, and recording a through-focal series at each tilt direction. The sample was a whole-mount macrophage cell with embedded gold nanoparticles. The tilt focal algebraic reconstruction technique (TF-ART) is introduced as a new algorithm to reconstruct tomograms from such combined tilt- and focal series. The feasibility of TF-ART was demonstrated by 3D reconstruction of the experimental 3D data. The results were compared with a conventional STEM tilt series of a similar sample. The combined tilt- and focal series led to smaller missing wedge artifacts, and a higher axial resolution than obtained for the STEM tilt series, thus improving on one of the main issues of tilt series-based electron tomography.

Dahmen, Tim↗
DOE OSTI2014

Band Gap Engineering and Layer-by-Layer Band Gap Mapping of Selenium-doped Molybdenum Disulfide

Ternary two-dimensional dichalcogenide alloys exhibit compositionally modulated electronic structure and hence, control of dopant concentration within each layer of these layered compounds provides a powerful way to modify their properties. The challenge then becomes quantifying and locating the dopant atoms within each layer in order to better understand and fine-tune the desired properties. Here we report the synthesis of selenium substitutionally doped molybdenum disulfide atomic layers, with a broad range of selenium concentrations, resulting in band gap modulations of over 0.2 eV. Atomic scale chemical analysis using Z-contrast imaging provides direct maps of the dopant atom distribution in individual MoS2 layers and hence a measure of the local band gaps. Furthermore, in a bilayer structure, the dopant distribution of each layer is imaged independently. We demonstrate that each layer in the bilayer contains similar doping levels, randomly distributed, providing new insights into the growth mechanism and alloying behavior in two-dimensional dichalcogenide atomic layers. The results show that growth of uniform, ternary, two-dimensional dichalcogenide alloy films with tunable electronic properties is feasible.

Gong, Yongji↗
DOE OSTI2013

Nanostructured columnar heterostructures of TiO{sub 2} and Cu{sub 2}O enabled by a thin-film self-assembly approach: Potential for photovoltaics

Graphical abstract: Display Omitted Highlights: ► Material self-assembly in phase-separated oxides is exploited. ► Three-dimensionally nanostructured epitaxial films are grown using sputtering. ► Films are composed of well-ordered oriented nanopillars of n-type TiO{sub 2} and p-type Cu{sub 2}O. ► Observed interfaces at adjacent TiO{sub 2}–Cu{sub 2}O columns are nearly atomically distinct. ► Absorption profile of the composite film captures a wide range of the solar spectrum. -- Abstract: Significant efforts are being devoted to the development of multifunctional thin-film heterostructures and nanostructured material architectures for components with novel applications of superconductivity, multiferroicity, solar photocatalysis and energy conversion. In particular, nanostructured assemblies with well-defined geometrical shapes have emerged as possible high efficiency and economically viable alternatives to planar photovoltaic thin-film architectures. By exploiting phase-separated self-assembly, here we present advances in a vertically oriented two-component system that offers potential for future development of nanostructured thin film solar cells. Through a single-step deposition by magnetron sputtering, we demonstrate growth of an epitaxial, composite film matrix formed as self-assembled, well ordered, phase segregated, and oriented nanopillars of n-type TiO{sub 2} and p-type Cu{sub 2}O. The composite films were structurally characterized to atomic resolution by a variety of analytical tools, and evaluated for preliminary optical properties using absorption measurements. We find nearly atomically distinct TiO{sub 2}–Cu{sub 2}O interfaces (i.e., needed for possible active p–n junctions), and an absorption profile that captures a wide range of the solar spectrum extending from ultraviolet to visible wavelengths. This high-quality materials system could lead to photovoltaic devices that can be optimized for both incident light absorption and carrier collection.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

AC/AB Stacking Boundaries in Bilayer Graphene

Boundaries, including phase boundaries, grain boundaries, and domain boundaries, are known to have an important influence on material properties. Here, dark-field (DF) transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) imaging are combined to provide a full view of boundaries between AB and AC stacking domains in bilayer graphene across length scales from discrete atoms to the macroscopic continuum. Combining the images with results obtained by density functional theory (DFT) and classical molecular dynamics calculations, we demonstrate that the AB/AC stacking boundaries in bilayer graphene are nanometer-wide strained channels, mostly in the form of ripples, producing smooth low-energy transitions between the two different stackings. Our results provide a new understanding of the novel stacking boundaries in bilayer graphene, which may be applied to other layered two-dimensional materials as well.

Lin, Junhao↗
DOE OSTI2012

Electron transfer and ionic displacements as the origin of the 2D electron gas at the LAO/STO interface: Direct measurements with atomic-column spatial resolution.

We present direct, atomic-column-resolved scanning transmission electron microscopy and electron energy loss measurements of atomic displacements and Ti valence in abrupt, conductive LAO/STO interfaces. We find that two distinct but interrelated mechanisms are responsible for screening the diverging electric potential in the LAO film: 1) charge injection in the interfacial Ti planes, and 2) dielectric relaxation in both LAO and STO through ionic displacements. The injected charge density decays over a length of nearly 3 unit cells within the STO substrate. The total injected charge is lower than predicted by pure electronic reconstruction. The origin of this discrepancy is attributed to cation and oxygen displacements, which we observe in both LAO and STO, and generate a polarization opposite to the intrinsic polarization of the LAO film. Our data attribute a minor role to oxygen vacancies and cation intermixing.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Tuning fifth-order aberrations in a Quadrupole-Octupole Corrector

The resolution of conventional electron microscopes is usually limited by spherical aberration. Microscopes equipped with aberration-correctors are then primarily limited by higher-order, chromatic, and misalignment aberrations. In particular the Nion third-order aberration correctors installed on machines with a low energy spread and possessing sophisticated alignment software were limited by the uncorrected fifth-order aberrations. Here we show how the Nion fifth-order aberration corrector can be used to adjust and reduce some of the fourth and fifth-order aberrations in a probe-corrected scanning transmission electron microscope.

Lupini, Andrew R↗