Scanning Transmission Electron Microscopy Energy Research Articles

Subsurface Enrichment of Highly Active Dealloyed Pt-Ni Catalyst Nanoparticles for Oxygen Reduction Reaction

We present the synthesis of homogenous PtNi and PtNi3 alloy nanoparticles by organic solution approach, which showed 4-5 fold increases in Pt-mass normalized activity of oxygen reduction reaction (ORR) compared to conventional Pt catalyst after electrochemical dealloying. Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, we found that there is a strong tendency of subsurface enrichment of Ni in the dealloyed Pt-Ni nanoparticles at a higher initial Ni composition (PtNi3), which therefore resulted in unusual core-shell fine structures. Based on this, a correlation of near-surface compositions to the ORR activities of the dealloyed Pt-Ni catalyst nanoparticles was established.

Nanoscale Probing of Voltage Activated Oxygen Reduction/Evolution Reactions in Nanopatterned (LaxSr1‐x)CoO3‐δ Cathodes

AbstractBias‐dependent mechanisms of reversible and irreversible electrochemical processes on a (La0.5Sr0.5)2CoO4±δ modified (LaxSr1‐x)CoO3‐ surface are studied using dynamic electrochemical strain microscopy (D‐ESM). The reversible oxygen reduction/evolution process is activated at voltages as low as 3–4 V and the degree of transformation increases linearly with applied bias. The irreversible processes associated with static surface deformation become apparent above 10–12 V. Post‐mortem focused‐ion milling combined with atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy is used to establish the mechanisms of irreversible transformations and attribute it to amorphization of the top layer of material. These studies both establish the framework for probing irreversible electrochemical processes in solids and illustrate rich spectrum of electrochemical transformations underpinning electrocatalytic activity in cobaltites.

Core–shell heterostructures of SnM (M = (Fe, Ni, and Cr) or Cu) alloy nanowires @ CNTs on metallic substrates

Sn alloy nanowires encapsulated in carbon nanotubes (SnM (M=(Fe, Ni, and Cr) or Cu) @ CNTs) were prepared in situ by a chemical vapor deposition (CVD) method, in which Sn came from a vaporized precursor while the alloy elements were supplied by the substrate. The heterostructures were grown on two types of substrates including stainless steel with high catalytic effectiveness and Cu substrates with low catalytic effectiveness for generating graphite layers, respectively. Pure Sn powder and C2H4 were employed to provide Sn and carbon precursors. The products were investigated by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM–EDS) mapping. The morphology, structure and composition of the nanomaterials depended significantly on the surface conditions of the substrates. While SnCu alloy nanowires encapsulated in carbon nanotubes were grown on the Cu substrate, carbon nanotubes filled with alloy nanowires and porous carbon fibers decorated internally with alloy particles were observed on the stainless steel substrate. The growth mechanisms of the heterostructures were proposed.

Microstructures of Long-Period Stacking Ordered Phase of Mg–Zn–Y Alloy

The microstructures of i-Mg, long-period stacking ordered (LPSO) phases, and kink bands in a Mg­Y­Zn alloy were observed by transmission electron microscopy (TEM). The results showed that extruded Mg97Zn1Y2 alloy included different kinds of phases: 2H-Mg, 2H-Mg with many stacking faults, 14H and 18R. Kink bands tended to occur in areas where there were many intermetallic compounds. The element distribution of Zn/Y/Mg in the LPSO phase including kink boundaries was also analyzed by scanning TEM energy dispersive X-ray spectroscopy. The results showed that the concentrations of atomic Zn/Y decreased, while the concentration of atomic Mg increased at kink boundaries. This can be understood in terms of the dislocation structure of kink boundaries where extended a-dislocations lying on sequential stacking faults in the LPSO phase cause the annihilation of Zn/Y-rich stacking faults between two partial dislocations. [doi:10.2320/matertrans.MI201219]

Quantum confinement of volume plasmons and interband transitions in germanium nanocrystals

The plasmonic properties of individual quantum-sized Ge nanocrystals (NCs) were observed and systematically analyzed by aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). For this purpose, Ge NCs embedded in an SiO${}_{2}$ matrix with controllable size, density, and structure were fabricated using magnetron sputtering. The size dependence of the Ge plasmon energies in the size range of 5--9 nm is shown to be well depicted by the so-called medium quantum confinement (QC) model, with an effective mass of 0.57${m}_{0}$ (contrary to expectations of a stronger quantum effect). In the very low-loss region of the EEL spectra, an apparent blue shift of the ${E}_{2}$ interband transition peak up to 2 eV and a strong reduction in the oscillator strength were measured for the NCs in the size range of 4--6 nm. It indicates for this smaller size range a transition to a QC regime where the band structure and the density of states are modified dramatically. These trends are explained by a combination of low-loss and core-loss EELS results, which show that the Ge NCs are surrounded uniformly by nearly stoichiometric SiO${}_{2}$. This local chemistry is shown to provide an infinite potential barrier and to confine electrons and holes in the spherically shaped Ge NCs. In addition to pure QC effects in the Ge NCs, the SiO${}_{2}$ matrix thus plays an important role in the strength of the observed QC and interband transitions.

Single Event Transients Induced by Picosecond Pulsed X-Ray Absorption in III–V<newline/> Heterojunction Transistors

We perform measurements which show that focused, picosecond pulses of x-rays can be used to generate single event transients (SET) in a GaAs heterostructure field effect transistor (HFET) and a GaN high electron mobility transistor. X-ray pulses with photon energies of 8, 10 and 12 keV from the Advanced Photon Source at Argonne National Laboratory were used to excite transients. SETs are observed when x-ray pulses are incident upon metal layers above sensitive areas on the transistors. We use focused ion beam (FIB) cross-sectioning and scanning transmission electron microscopy energy dispersive x-ray spectroscopy (STEM-EDXS) to determine the compositional structure of the devices. We present a first order analysis of energy deposition in the devices and correlate it to the transient response to make preliminary interpretations of the results. We compare the x-ray transients from the GaAs HFET with transients generated by 750 nm and 870 nm femtosecond laser pulses. We also present results on the total dose susceptibility of the GaN HEMTs.

Interfacial Ferromagnetism and Exchange Bias inCaRuO3/CaMnO3Superlattices

We have found ferromagnetism in epitaxially grown superlattices of CaRuO(3)/CaMnO(3) that arises in one unit cell at the interface. Scanning transmission electron microscopy and electron energy loss spectroscopy indicate that the difference in magnitude of the Mn valence states between the center of the CaMnO(3) layer and the interface region is consistent with double exchange interaction among the Mn ions at the interface. Polarized neutron reflectivity and the CaMnO(3) thickness dependence of the exchange bias field together indicate that the interfacial ferromagnetism is only limited to one unit cell of CaMnO(3) at each interface. The interfacial moment alternates between the 1 μ(B)/interface Mn ion for even CaMnO(3) layers and the 0.5 μ(B)/interface Mn ion for odd CaMnO(3) layers. This modulation, combined with the exchange bias, suggests the presence of a modulating interlayer coupling between neighboring ferromagnetic interfaces via the antiferromagnetic CaMnO(3) layers.

Synergistic Effect of a Carbon Black Supported PtAg Non‐Alloy Bimetal Nanocatalyst for CO Preferential Oxidation in Excess Hydrogen

AbstractCarbon black (CB) supported PtAg non‐alloy bimetal catalysts were prepared by incipient wetness impregnation and evaluated for CO preferential oxidation in excess H2 (PROX). PtAg/CB catalysts exhibited an evident synergistic effect. The catalyst support selection is crucial to the synergistic effect. X‐ray diffraction (XRD), transmission electron microscopy (TEM), and scanning TEM energy dispersive X‐ray analysis (STEM‐EDX) characterization indicates no alloy was formed over the PtAg/CB catalyst after activation in H2 at 500 °C for 2 h. However, temperature‐programmed reduction (TPR) and XRD data evidenced strong interactions between platinum and silver, as a result of their simultaneous reduction and high temperature activation. The PtAg interactions result in high catalytic performance and the synergistic effects. This is another example of model catalyst research translating to real world catalysis. PtAg non‐alloy catalysts may also be effective in eliminating or alleviating catalyst poisoning by CO‐like intermediates in direct liquid fuel cells.

Core–Shell Compositional Fine Structures of Dealloyed PtxNi1–x Nanoparticles and Their Impact on Oxygen Reduction Catalysis

Using aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy line profiles with Ångstrom resolution, we uncover novel core-shell fine structures in a series of catalytically active dealloyed Pt(x)Ni(1-x) core-shell nanoparticles, showing the formation of unusual near-surface Ni-enriched inner shells. The radial location and the composition of the Ni-enriched inner shells were sensitively dependent on the initial alloy compositions. We further discuss how these self-organized Ni-enriched inner shells play a key role in maintaining surface lattice strain and thus control the surface catalytic activity for oxygen reduction.

고온 플라즈마를 이용한 붕소 함유 나노입자 제조에 관한 연구

A new method for producing boron-containing nanoparticles is described. Boron trichloride () and methane () are dissociated through injection into a thermal plasma followed by a nucleation process producing boron or boron carbide nanoparticles. X-ray photoelectron spectroscopy was used to detect B-C bonds related to the carbide state and to probe the ratio of boron to carbon in the B-C bond structure. In addition, nanoparticles were characterized with scanning transmission electron microscopy and electron energy loss spectroscopy. It was found that nanoparticles were in the range 30-70 nm and a boron to carbon ratio in the B-C bond structure of up to 2 can be reached when of 20 sccm and of 25 sccm were used.

Electron Transfer and Ionic Displacements at the Origin of the 2D Electron Gas at the LAO/STO Interface: Direct Measurements with Atomic‐Column Spatial Resolution

Using state-of-the-art, aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy with atomic-scale spatial resolution, experimental evidence for an intrinsic electronic reconstruction at the LAO/STO interface is shown. Simultaneous measurements of interfacial electron density and system polarization are crucial for establishing the highly debated origin of the 2D electron gas.

Strain relaxation defects in perovskite oxide superlattices

Abstract

Electronic properties of W-encapsulated Si cluster film on Si (100) substrates

We have fabricated thin films composed of W-encapsulated Si clusters (WSi10) on Si substrates and investigated their electronic properties using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). An epitaxial layer was observed at the interface with the Si substrate, and an amorphous layer was on top. The bulk plasmon of the WSi10 cluster film was measured and compared with those of crystalline Si (c-Si) and WSi2 films. We found similar plasmon energies in the epitaxial and amorphous layers of the WSi10 cluster film. The plasmon peak of the WSi10 cluster film is shifted to higher energy compared with that of c-Si, which is related to the electron density increase in the valence band. The Si-L23 absorption edge spectra show that the conduction-band density of states in Si was modified by hybridization between Si and W atoms.

Ferromagnetic InMnSb multi-phase films study by aberration-corrected (scanning) transmission electron microscopy

In this work we report a structural and compositional study of ferromagnetic In0.78Mn0.22Sb films correlated to the magnetic properties as determined by superconducting quantum interference device magnetometer. The epilayers grown by metalorganic vapor phase epitaxy on GaAs(001) substrates showed two active magnetic components with Curie temperatures of approximately 300 K and in excess of 570 K. Secondary phases driven by the high manganese concentration (10 at. %) were identified by high-resolution (scanning) transmission electron microscopy imaging and energy dispersive X-ray spectroscopy. Most of the Mn was found to be incorporated in metallic manganese nanoprecipitates surrounded by an InMnSb matrix with Mn at 1 at. % concentration. The origin of the two Curie temperatures of the film is associated with the presence of three magnetic components: hexagonal MnSb nanoprecipitates, non-stoichiometric MnAsSb, and the InMnSb matrix.

Structural and Chemical Characterization of Al(Ga)N/GaN Quantum Well Structures Grown by Plasma Assisted Molecular Beam Epitaxy

Growth of high quality GaN/AlN heterostructures by plasma assisted molecular beam epitaxy (PAMBE) is possible with excess of Ga on the surface. During growth of AlN this additional Ga acts as surfactant and improves mobility of the Al adatoms on the growing surface, at the possible cost of Ga segregation and creation of mixed AlGaN interlayer. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were used to determine chemical composition of high crystallographic quality GaN-AlN multilayer structure. It was shown that segregation occurs at AlN-GaN heterointerfaces, while GaN-AlN interfaces have abrupt stepwise change of the chemical composition. HRTEM results show creation of trench defects at the periphery of growing AlN islands in the case of nonoptimized growth.

Chemical synthesis of oriented ferromagnetic LaSr-2 × 4 manganese oxide molecular sieve nanowires

We report a chemical solution based method using nanoporous track-etched polymer templates for producing long and oriented LaSr-2 × 4 manganese oxide molecular sieve nanowires. Scanning transmission electron microscopy and electron energy loss spectroscopy analyses show that the nanowires are ferromagnetic at room temperature, single crystalline, epitaxially grown and self-aligned.

Resonant x-ray reflectivity study of perovskite oxide superlattices

Resonant x-ray reflectivity (XRR) is utilized to characterize the structural properties of La0.7Sr0.3MnO3/La0.7Sr0.3FeO3 superlattices. XRR spectra at the Mn and Fe absorption edges provide additional structural information compared to spectra obtained with a conventional Cu x-ray source. We demonstrate that this technique provides individual layer thicknesses and intermixing behavior of perovskite superlattices with small density contrast in a non-destructive manner. These results are consistent with scanning transmission electron microscopy and electron energy loss spectroscopy.

Aberration-corrected STEM and EELS of semiconducting nanostructures

We review some applications of aberration–corrected electron microscopy for the detailed characterization of semiconducting nanostructures using a combination of high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy. The study of self-assembled quantum wires shows that it is possible to determine the composition of the nanostructures with better than 1 nm resolution down to the atomic level while the contrast in the high-angle annular dark-field images is used to determine the presence of wetting layers separating quantum wires and the strain field arising from the local compositional changes. The local measurements of energy loss spectra demonstrate the shift of plasmon peaks consistent with the changes in lattice parameters. High-angle annular dark-field images are also used to study the contrast in GaSb thin films deposited and study the presence of anti-phase domain boundaries. These examples show that aberration-corrected microscopy combined with electron energy loss spectroscopy provide not only enhanced resolution but also increased sensitivity to atomic site compositional changes.

Scanning Transmission Electron Microscopy Analysis of Grain Structure in Perpendicular Magnetic Recording Media

The key component of a hard disk medium is a Co-based magnetic layer (ML) grown on a Ru seed layer. The ML nanostructure, composed of less than 10 nm grains, is believed to be controlled by this seed layer. We successfully used scanning transmission electron microscopy energy dispersive spectrometry simultaneous composition-based imaging and Moiré pattern analysis for determining the mutual structural and orientation relationship between the two layers revealing a grain-to-grain agreement. The method presented here can be utilized for observing structural correlations between consecutive polycrystalline thin film layers in general.

Grain-Boundary Segregation and Phase-Separation Mechanism in Yttria-Stabilized Tetragonal Zirconia Polycrystal

Microstructure development during sintering in 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) was systematically investigated in two sintering conditions: (a) 1100-1650°C for 2 h and (b) 1300°C for 0-50 h. In the sintering condition (a), the density and grain size in Y-TZP increased with the increasing sintering temperature. Scanning transmission electron microscopy (STEM) and nanoprobe X-ray energy dispersive spectroscopy (EDS) measurements revealed that the Y3+ ion distribution was nearly homogeneous up to 1300°C, i.e., most of grains were the tetragonal phase, but cubic-phase regions with high Y3+ ion concentration were clearly formed in grain interiors adjacent to the grain boundaries at 1500°C. High-resolution transmission electron microscopy (HRTEM) and nanoprobe EDS measurements revealed that no amorphous or second phase is present along the grain-boundary faces, and Y3+ ions segregated not only along the tetragonal-tetragonal phase boundaries but also along tetragonal-cubic phase boundaries over a width below about 10 nm, respectively. These results indicate that the cubic-phase regions are formed from the grain boundaries and/or the multiple junctions in which Y3+ ions segregated. We termed this process a “grain boundary segregation-induced phase transformation (GBSIPT)” mechanism. In the sintering condition (b), the density was low and the grain-growth rate was much slow. In the specimen sintered at 1300°C for 50 h, the cubic-phase regions were clearly formed in the grain interiors adjacent to the grain boundaries. This behavior shows that the cubic-phase regions were formed without grain growth, which can be explained by the GBSIPT model.