Cecile S. Bonifacio

Field assisted sintering of nickel nanoparticles during in situ transmission electron microscopy

This study reports the in situ transmission electron microscopy (TEM) observation of pressure-less field-assisted sintering of agglomerated nanometric nickel particles. Scanning tunneling microscopy inside the TEM was used to apply an electrical current directly to the powder particles. Electrical testing during the experiment reveals that consolidation occurs in the absence of an external heat source. Neck formation between adjacent particles and attendant increase in local Joule heating causes rapid densification. The results represent a first stepping stone towards achieving a fundamental mechanistic understanding of the atomic-scale processes that enable field-enhanced sintering of conductive nanogranular materials.

In situ transmission electron microscopy study of dielectric breakdown of surface oxides during electric field-assisted sintering of nickel nanoparticles

The removal of ultra-thin oxide surface layers on nanometric nickel particles is investigated in the framework of electric field-induced dielectric breakdown. In situ transmission electron microscopy was used to directly apply electrical biasing to agglomerates of nanoparticles during simultaneous imaging of the contact area between two adjacent particles. The applied electrical field initiated dielectric breakdown of the surface layers through percolation of oxygen vacancies and the migration of oxygen away from the particle contact, which leads to the formation of metallic necks and their subsequent growth. The experimental results represent direct evidence for surface cleaning effects during electric field-assisted sintering.

Ultra-long Magnetic Nanochains for Highly Efficient Arsenic Removal from Water.

The contamination of drinking water with naturally occurring arsenic is a global health threat. Filters that are packed with adsorbent media with a high affinity for arsenic have been used to de-contaminate water - generally iron or aluminium oxides are favored materials. Recently, nanoparticles have been introduced as adsorbent media due to their superior efficiency compared to their bulk counter-parts. An efficient nanoadsorbent should ideally possess high surface area, be easy to synthesize, and most importantly offer a high arsenic removal capacity. Achieving all the key features in a single step synthesis is an engineering challenge. We have successfully engineered such a material in the form of nanochains synthesized via a one step flame synthesis. The ultra-long γ-Fe2O3 nanochains possess high surface area (151.12 m2 g-1), large saturation magnetization (77.1 emu g-1) that aids in their gas phase self-assembly into long chains in an external magnetic field, along with an extraordinary arsenic removal capacity (162 mg.g-1). A filter made with this material exhibited a relatively low-pressure drop and very little break-through of the iron oxide across the filter.

Characterization of defect evolution in ultrathin SiO2 layers under applied electrical stress

The structural evolution of ultrathin dielectric SiO2 layers within a Co-silicide/poly-Si/SiO2/Si multilayer system was studied by in situ transmission electron microscopy (TEM). The interface structure represents a model system for field effect transistors with a SiO2 dielectric layer. Electrical bias was applied across the interfaces of cross sectional TEM samples using a scanning tunneling microscopy (STM) tip. Atomic structure modifications of the dielectric layer due to the applied electrical field were observed by this in situ STM-TEM technique. Constant bias (+5.0 V) and ramped bias (+3.0 to +10.5 V) stresses applied to the CoSi2 gate electrode resulted in a loss in capacitance of the dielectric layer consistent with descriptions of soft dielectric breakdown (SBD) and hard dielectric breakdown (HBD). It was found that SBD events are characterized by fluctuations within uniform current step increase of 21 nA and increased roughness of the SiO2 film due to oxygen vacancy percolation. HBD, however, wa...

Targeted Ion Milling of Ex Situ Lift-Out FIB Specimens

The semiconductor industry is constantly investigating new methods that can improve both the quality of TEM lamella and the speed at which they can be created. To improve throughput, a combination of FIB-based preparation and ex situ lift-out (EXLO) techniques have been used. Unfortunately, the carbon support on the EXLO grid presents problems if the lamella needs to be thinned once it is on the grid. In this paper, we present low-energy (<1 keV), narrow-beam (<1 μm diameter), Ar ion milling as a method of preparing electrontransparent and gallium-free EXLO FIB specimens.

Computationally Assisted STEM and EXAFS Characterization of Tunable Rh/Au and Rh/Ag Bimetallic Nanoparticle Catalysts

The acceleration of rational catalyst design by computational simulations is only practical if the theoretical structures identified can be synthesized and experimentally verified. Of particular interest are bi-functional/bimetallic catalysts, which can have the potential to exceed the selectivity and efficiency of a single-component system [1]. However, adding a second metal greatly increases the complexity of the system; variation in the elements’ mixing patterns and reconfiguration can affect the reaction mechanisms and thus catalytic performance [2].

Corrigendum: Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene.

Nature Communications 7 Article number: 12123 (2016); Published 30 June 2016; Updated 12 September 2016 An incorrect version of the Supplementary Information was inadvertently published with this Article in which the image for Supplementary Fig. 7 was missing. The Article has now been updated to include the correct version of the Supplementary Information.

Structural Change of a Cu/ZnO Catalyst under Methanol Observed by ETEM

Cu/ZnO-based catalysts have been used in methanol synthesis and methanol oxidation for decades [1, 2]. In order to improve the stability of these catalysts and catalytic performance, much effort has been invested to understand the impact of copper loading and metal promoters [3]. However, the structural evolution of the Cu/ZnO catalyst under industrially relevant conditions and interplay of the morphological-reactivity are still under debate [4, 5]. More in situ studies at reaction conditions are necessary for the elucidation of reaction mechanism and eventual optimization of next-generation Cu/ZnO catalysts. We synthesized a 30% wt Cu/ZnO by co-precipitation methodology. The particle size change under heat and methanol exposure was investigated using in and ex situ transmission electron microscopy (TEM) techniques.

Sample preparation for aberration-corrected microscopy of high-quality TEM specimens of advanced integrated circuits

Advanced integrated circuits (ICs) are complex due to the fin field effect transistors (FinFETs), which comprise multigate transistors with the source/drain (S/D) channels (fins) surrounded by a threedimensional gate. State-of-the-art ICs are at the 10 nm and 7 nm nodes, with the later at the ramping stage of production[1]. At the 10 nm node, the S/D fins are 25% taller and 25% more closely spaced than 14 nm node technology [2]. Transmission electron microscopy (TEM) is a critical characterization tool for the semiconductor industry given the decreasing device size. Ga focused ion beam (FIB) is frequently used for advanced IC TEM specimen preparation due to its rapid, site-specific sample preparation capabilities. However, FIB milling typically results in specimen artifacts, such as surface amorphization and Ga implanted layers, both of which may limit analytical and high-resolution electron microscopy. Furthermore, 20 nm or less specimen thickness is required to characterize the 3D structures of the FinFET gate oxide in the TEM [3]. In this work, we present targeted, small spot (< 1μm ), low energy Ar milling for reproducible specimen preparation of advanced ICs with specimen thicknesses of less than 20 nm that removes FIB-induced artifacts.

Comparison of Spinel and Monoclinic Crystal Structures of γ-Al2O3 for Simulation of Electron Energy Loss Spectra

γ-Al2O3 is one of the metastable polymorphs of alumina that is used in many applications, such as in adsorbents and catalyst supports [1]. However, an accurate description of the crystal structure of γ-Al2O3 is still being disputed. Early studies of the structure of γ-Al2O3, including the seminal study by Zhou and Snyder [2], determined γ-Al2O3 has a spinel structure similar to the structure of MgAl2O4. As there are fewer cations in γ-Al2O3 per unit cell than MgAl2O4, some of the cation positions are vacant to satisfy stoichiometry. Recently, theoretical studies have suggested non-spinel structures as the more accurate structural description of γ-Al2O3, the most commonly cited of which was given by Digne et al. [3]. These theorists combined molecular dynamics (MD) and density functional theory (DFT) calculations to simulate the thermal dehydration of boehmite to γ-Al2O3, which is the most common synthesis method of γ-Al2O3. They predicted that the γ-Al2O3 structure was distorted into a monoclinic unit cell. Although subsequent experimental studies suggest that the spinel model is the more accurate structural description [4], it is also more computationally intensive to use in simulations because of the partial occupancy of the cation sites. As a result, the monoclinic model of γ-Al2O3 is commonly used in simulations, including of the electronic properties. Gauging the accuracy of this approach has been complicated by the nature of γAl2O3 produced by the dehydration of boehmite, which usually exhibits poor crystallinity and significant amounts of impurities. This has hindered the ability to directly compare experimental results with simulations, which uses simple, well-defined structures for input.