Aberration-corrected Scanning Transmission Electron Research Articles

Metals on BN Studied by High Resolution Transmission Electron Microscopy

Metal impurities, gold and nickel, have been deliberately introduced into boron-nitride (BN) sheets. The structural and topographic properties of doped BN have been studied by aberration corrected scanning transmission electron microscopy (STEM). Analysis revealed that metal atoms cluster preferentially in/on contaminated areas. The metal coverage on BN is almost the same for the same evaporated amount of 1 Å.

Scanning Transmission Electron Microscopy of Metal-Graphene Interaction

Metal impurities, gold, chromium and titanium, have been deliberately introduced into graphene. The structural and topographic properties of doped graphene have then been studied by using conventional transmission electron and aberration corrected scanning transmission electron microscopy. Analysis revealed that metal atoms cluster preferentially in/on contaminated areas. Contrarily to observations that gold atoms do not adhere to clean patches of monolayer graphene, chromium and titanium were found to be more reactive with clean monolayer graphene, and the coverage was higher than for gold for the same evaporated amount.

A Study of the relationship between brown colour and extended defects in diamond using core-loss electron energy loss spectroscopy

There is strong evidence to support the theory that the vacancy cluster, a by-product of dislocation interaction, is the extended defect responsible for brown colour in natural type IIa diamonds. A characteristic of these open volume structures is the hybridised sp2 bonding of the 3-fold coordinated carbon atoms that line their cage-like surfaces. When present in sufficiently large concentrations the sp2 bonds alter the electronic and optical properties of type IIa diamond dramatically. Similar sp2 bond concentrations are found in dark brown CVD diamond, although the defects in this material appear much larger in size than in the natural diamond. In this study we employ core-loss electron energy loss spectroscopy (EELS) and aberration corrected scanning transmission electron microscopy (AC-STEM) to compare the relative sp2 content of brown coloured natural type IIa diamonds and dark brown CVD diamonds, using 100% sp2 bonded graphite as a reference. The effect of high-pressure high-temperature (HPHT) treatment on sp2 content is also investigated.

Atomic resolution analysis of porous solids: A detailed study of silver ion-exchanged zeolite A

Currently, aberration corrected scanning transmission electron microscopy is a powerful technique to observe with the maximum resolution of all kinds of materials. Unfortunately, this advanced technique has failed in its application to porous solids such as zeolites, which are extremely beam sensitive inorganic solids. By controlling the beam current we have successfully managed to image the most beam sensitive zeolite, zeolite A (LTA type, Si/Al=1) and observe the distribution of silver within the cages, observing for the first time silver octahedra composed of six atoms. These results open up the possibility of applying this technique to any zeolitic framework with the potential capability of even observing the oxygen distribution. Some preliminary results using an annular bright field (ABF) in combination with the HAADF detectors on ZSM-5 (MFI type) zeolite are also presented in this manuscript demonstrating the possibility of obtaining real atomic resolution images of the zeolitic framework.

Direct Experimental Evidence of Metal-Mediated Etching of Suspended Graphene

Atomic resolution high angle annular dark field imaging of suspended, single-layer graphene, onto which the metals Cr, Ti, Pd, Ni, Al, and Au atoms had been deposited, was carried out in an aberration-corrected scanning transmission electron microscope. In combination with electron energy loss spectroscopy, employed to identify individual impurity atoms, it was shown that nanoscale holes were etched into graphene, initiated at sites where single atoms of all the metal species except for gold come into close contact with the graphene. The e-beam scanning process is instrumental in promoting metal atoms from clusters formed during the original metal deposition process onto the clean graphene surface, where they initiate the hole-forming process. Our observations are discussed in the light of calculations in the literature, predicting a much lowered vacancy formation in graphene when metal ad-atoms are present. The requirement and importance of oxygen atoms in this process, although not predicted by such previous calculations, is also discussed, following our observations of hole formation in pristine graphene in the presence of Si-impurity atoms, supported by new calculations which predict a dramatic decrease of the vacancy formation energy, when SiO(x) molecules are present.

Defect chemistry of Ti-doped antiferroelectric Bi0.85Nd0.15FeO3

Aberration corrected scanning transmission electron microscopy revealed that Bi0.85Nd0.15Fe0.9Ti0.1O3 ceramics contain coherent Nd-rich precipitates distributed throughout the perovskite lattice, implying charge compensation is obtained by the creation of VNd/// and not VBi///. At low concentrations, therefore, Ti4+ replace Fe2+ with the creation of 2/3VNd///, and at higher concentrations (when Fe2+ have been eliminated and the conductivity suppressed), Fe3+ with the creation of 1/3VNd///. The switch in ionic compensation mechanism from 2/3VNd/// at low Ti concentrations (∼1%) to 1/3VNd/// at higher concentrations (>1%) results in a decrease in the magnitude of ΔTC/Δx, as the disruption of long range anti-polar coupling declines.

Polarity Assignment in ZnTe, GaAs, ZnO, and GaN-AlN Nanowires from Direct Dumbbell Analysis

Aberration corrected scanning transmission electron microscopy (STEM) with high angle annular dark field (HAADF) imaging and the newly developed annular bright field (ABF) imaging are used to define a new guideline for the polarity determination of semiconductor nanowires (NWs) from binary compounds in two extreme cases: (i) when the dumbbell is formed with atoms of similar mass (GaAs) and (ii) in the case where one of the atoms is extremely light (N or O: ZnO and GaN/AlN). The theoretical fundaments of these procedures allow us to overcome the main challenge in the identification of dumbbell polarity. It resides in the separation and identification of the constituent atoms in the dumbbells. The proposed experimental via opens new routes for the fine characterization of nanostructures, e.g., in electronic and optoelectronic fields, where the polarity is crucial for the understanding of their physical properties (optical and electronic) as well as their growth mechanisms.

Direct observation of the passive layer on high nitrogen stainless steel used as bipolar plates for proton exchange membrane fuel cells

The purpose of this report is to directly observe the passive layer and concentration behavior of nitrogen interstitially incorporated in Ni-saving high nitrogen stainless steel (HNS) using an aberration corrected scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) or – electron energy loss spectroscopy (EELS). The thickness of the passive layer barely changed after 1000h single cell operation, compared with the as-polished state. The observed passive layer was thin (3nm) and mainly composed of chromium oxide, as confirmed by STEM-EDS. It was confirmed that nitrogen was not present in the passive layer, but was concentrated at the interface between the passive layer and the metal bulk. The concentrated area ranged approximately 2nm to steel bulk from the interface. With help of the STEM-EDS and EELS, we were able to understand the nature of the passive layer for Ni-saving HNS, which caused remarkable improvement of the cell performance due to superior corrosion resistance.

Synthesis and characterization of ultra-small magnetic FeNi/G and NiCo/G nanoparticles

Ultra-small magnetic nanoparticles consisting of NiCo and FeNi alloys enclosed within graphitic shells (NiCo/G and FeNi/G) have been synthesized. The particles, which retained the face centered cubic (fcc) symmetry of the original bulk metals, together with the graphitic coating were characterized by means of aberration corrected scanning transmission electron microscopy (STEM), obtaining mean particle sizes of 2.6 nm and 6.2 nm for NiCo/G and FeNi/G, respectively. Due to the enhancement of the thermal stability by the graphite shell, the graphite coated FeNi and NiCo were stable under oxygen atmosphere up to 170 °C. The effectiveness of the graphite shell was confirmed when unprotected bimetallic FeNi and NiCo were prepared and chemical characterization revealed that more than 60 at.% of the samples was oxygen due to the massive oxidation of the bimetallic nanoparticles.

Absence of Lateral Composition Fluctuations in Aberration-corrected STEM Images of an InGaN Quantum Well at Low Dose

ABSTRACTBy using aberration corrected scanning transmission electron microscopy we have found no small scale lateral In composition fluctuations exist in the In0.15Ga0.85N active region of a light emitting diode. Images were acquired at 2% of the electron dose known to create electron beam damage, so the acquired images reflect the intrinsic structure of the InGaN active region. Position averaged convergent beam electron diffraction reveals the local sample thickness where images were acquired is 4.8 nm, eliminating the possibility that the absence of composition variation was observed due to projection through a thick sample. In addition, 2-3 atomic layer steps were observed in the top surface of In0.08Ga0.92N layers and the In0.15Ga0.85N active layers, providing a possible mechanism for lateral carrier confinement.

Atomic resolution structural insights into PdPt nanoparticle–carbon interactions for the design of highly active and stable electrocatalysts

Interfacial interactions between sub-4nm metal alloy nanoparticles and carbon supports, although not well understood at the atomic level, may be expected to have a profound influence on catalytic properties. Pd3Pt2 alloy particles comprised of a disordered surface layer over a corrugated crystalline core are shown to exhibit strong interfacial interactions with a ∼20–50nm spherical carbon support, as characterized by probe aberration corrected scanning transmission electron microscopy (pcSTEM). The disordered shells were formed from defects introduced by Pd during arrested growth synthesis of the alloy nanoparticles. The chemical and morphological changes in the catalyst, before and after cyclic stability testing (1000 cycles, 0.5–1.2V), were probed with cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and pcSTEM. The strong metal–support interaction, along with the uniform alloy structure raised the mass activity by a factor of 1.8 versus pure Pt. The metal–support interactions also mitigated nanoparticle coalescence, dissolution, and ripening, resulting in only a 20% loss in mass activity (versus 60% for pure Pt on carbon) after the cyclic stability test. The design of alloy structure, guided by insight from atomic scale pcSTEM, for enhanced catalytic activity and stability, resulting from strong wetting with a deformable disordered shell, has the potential to be a general paradigm for improving catalytic performance.

On the structure of bimetallic noble metal nanoparticles as revealed by aberration corrected scanning transmission electron microscopy (STEM)

In the present manuscript we present the feasibility of two different routes to obtain metal nanoparticles: (1) inert gas deposition technique (IGC) and (2) chemical routes. These two routes can enable the production of metal nanoparticles with a well-controlled size and shape distribution. In addition, Co/Au and Pd/Au nanoparticles have been deposited on three different grids with the aim of studying the coalescence of the different metals. The as-synthesized materials, that initially were forming isolated, core–shell, were sintered by thermal treatment (Co/Au) or by strong electron beam irradiation and the subsequent characterization was carried out in situ in an aberration corrected STEM.

Analysis of partially oxidised epitaxial silicon mono-layers on germanium virtual substrates using aberration corrected scanning transmission electron microscopy

We report the application of aberration corrected STEM imaging to study the interface structure of a partially oxidised epitaxial-Si passivation layer, nominally 2 mono-layers thick, deposited on a Si(001)/Ge virtual substrate. HAADF imaging revealed evidence of 1-2 mono-layers of crystalline Si between the Ge virtual substrate and overlying SiO2. An interface roughness of 1-2 mono-layer steps was also apparent between the epitaxial-Si and Ge virtual substrate. The total thickness of the residual crystalline Si and overlying SiO2 suggests that the initial epitaxial-Si layer may have been 1-2 mono-layers thicker than the nominal value.

(V)EELS Characterization of InAlN/GaN Distributed Bragg Reflectors

Ten-period InAlN/GaN distributed Bragg reflectors are examined by aberration corrected scanning transmission electron microscopy and by valence electron energy-loss spectroscopy (VEELS) with sub-nanometric spatial resolution and sub-eV energy dispersion. Deconvolution and peak subtraction methods, implemented in Matlab routines, are applied to the low loss region of the obtained VEEL spectra to retrieve information about the band gap energy and chemical composition, whereas a Kramers-Kronig transformation is used to retrieve the complex dielectric function of the examined material. The VEEL measurements reveal significant compositional variations in InAlN layers and show a ∼2nm thick InAlN layer with high indium content at each GaN/InAlN interface.

C/CrC nanocomposite coating deposited by magnetron sputtering at high ion irradiation conditions

CrC with the fcc NaCl (B1) structure is a metastable phase that can be obtained under the non-equilibrium conditions of high ion irradiation. A nano-composite coating consisting of amorphous carbon embedded in a CrC matrix was prepared via the unbalanced magnetron sputtering of graphite and Cr metal targets in Ar gas with a high ionized flux (ion-to-neutral ratio Ji/Jn = 6). The nanoscale amorphous carbon clusters self-assembled into layers alternated by CrC, giving the composite a multilayer structure. The phase, microstructure, and composition of the coating were characterized using x-ray diffraction, transmission electron microscopy, and aberration corrected scanning transmission electron microscopy coupled with electron energy loss spectroscopy. The interpretation of the true coating structure, in particular the carbide type, is discussed.

The effect of growth parameters on the intrinsic properties of large-area single layer graphene grown by chemical vapor deposition on Cu

We present a comprehensive study of the parameter space for single layer graphene growth by chemical vapor deposition on Cu. The temperature is the most widely recognized control parameter in single layer graphene growth. We show that the methane-to-hydrogen ratio and the growth pressure also are critical parameters that affect the structural perfection and the cleanliness of graphene. The optimal conditions for suppressing double and multilayer graphene growth occur near 1000 °C, 1:20 methane-to-hydrogen ratio, and a total pressure in the range from 0.5 to 1 Torr. Raman mapping of a 40 × 30 μm 2 area shows single layer domains with 5–10 μm linear dimensions. Atomic resolution imaging of suspended graphene by aberration corrected scanning transmission electron microscopy shows that the single layer graphene consists of areas of 10–15 nm linear dimensions and smaller patches of residual contamination that was undetected by other characterization methods.

Atomic Resolved Secondary Electron Imaging with an Aberration Corrected Scanning Transmission Electron Microscope

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Observation of Defects of CuInSe2 by 300 kV Aberration Corrected Scanning Transmission Electron Microscope

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Preparation and electrocatalytic activity of palladium–platinum core–shell nanoalloys protected by a perfluorinated sulfonic acid ionomer

Palladium–platinum nanoalloys with a core–shell and nano-network structure were successfully synthesized by a hydrogen sacrificial protective method in an aqueous solution directly using a perfluorinated sulfonic acid (PFSA) ionomer as a protecting agent. The structure, local composition and electrocatalytic activity for the oxygen reduction reaction of the Pd/Pt/PFSA nanoalloys were investigated by transmission electron microscopy (TEM), aberration corrected scanning transmission electron microscopy (Cs-STEM), energy-dispersive X-ray spectrometry (EDS) and voltammetry. The core–shell structure was completed without contaminating reducing agents, organic solvents, useless protecting agents and a mediator. The Pd/Pt/PFSA core–shell nanoalloys realized a high electrochemical surface area and better electrocatalytic mass-activity for the oxygen reduction reaction than the Pt/PFSA nanoparticles.

Atomically Resolved Mapping of Polarization and Electric Fields Across Ferroelectric/Oxide Interfaces by Z‐contrast Imaging

Direct atomic displacement mapping at ferroelectric interfaces by aberration corrected scanning transmission electron microscopy(STEM) (a-STEM image, b-corresponding displacement profile) is combined with Landau-Ginsburg-Devonshire theory to obtain the complete interface electrostatics in real space, including separate estimates for the polarization and intrinsic interface charge contributions.