Umananda M. Bhatta

Cationic surface reconstructions on cerium oxide nanocrystals: an aberration-corrected HRTEM study.

Instabilities of nanoscale ceria surface facets are examined on the atomic level. The electron beam and its induced atom migration are proposed as a readily available probe to emulate and quantify functional surface activity, which is crucial for, for example, catalytic performance. In situ phase contrast high-resolution transmission electron microscopy with spherical aberration correction is shown to be the ideal tool to analyze cationic reconstruction. Hydrothermally prepared ceria nanoparticles with particularly enhanced {100} surface exposure are explored. Experimental analysis of cationic reconstruction is supported by molecular dynamics simulations where the Madelung energy is shown to be directly related to the binding energy, which enables one to generate a visual representation of the distribution of reactive surface oxygen.

Chlorine gas sensors using one-dimensional tellurium nanostructures

Tellurium nanotubes have been grown by physical vapor deposition under inert environment at atmospheric pressure as well as under vacuum conditions. Different techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical absorption have been utilized for characterization of grown structures. Films prepared using both types of tellurium nanotubes were characterized for sensitivity to oxidizing and reducing gases and it was found that the relative response to gases depends on the microstructure. Nanotubes prepared at atmospheric pressure (of argon) showed high sensitivity and better selectivity to chlorine gas. Impedance spectroscopy studies showed that the response to chlorine is mainly contributed by grain boundaries and is therefore enhanced for nanotubes prepared under argon atmosphere.

Oxide mediated liquid–solid growth of high aspect ratio aligned gold silicide nanowires on Si(110) substrates

Silicon nanowires grown using the vapor-liquid-solid method are promising candidates for nanoelectronics applications. The nanowires grow from an Au-Si catalyst during silicon chemical vapor deposition. In this paper, the effect of temperature, oxide at the interface and substrate orientation on the nucleation and growth kinetics during formation of nanogold silicide structures is explained using an oxide mediated liquid-solid growth mechanism. Using real time in situ high temperature transmission electron microscopy (with 40 ms time resolution), we show the formation of high aspect ratio ( approximately 15.0) aligned gold silicide nanorods in the presence of native oxide at the interface during in situ annealing of gold thin films on Si(110) substrates. Steps observed in the growth rate and real time electron diffraction show the existence of liquid Au-Si nano-alloy structures on the surface besides the un-reacted gold nanostructures. These results might enable us to engineer the growth of nanowires and similar structures with an Au-Si alloy as a catalyst.

Atomic motion on various surfaces of ceria nanoparticles in comparison

Aberration corrected HRTEM has been successfully applied to image the atomic motion at the edges and surfaces of nanoparticles of ceria of various types under electron irradiation. Here we identify movements not only on {100} facets, but also on {110} and even {111} facets, previously considered stable. However, the degree of movement varies strongly and HRTEM is evidently the preferred technique to measure relative stability at high spatial resolution as it does not require extended surfaces as in scanning probe microscopy (SPM) or chemical methods. The advantage of aberration correction shows in suppression of contrast from the carbon support films and the absence of delocalisation fringes at particle edges, apart from improving point resolution.

Formation of embedded indium nitride and indium oxide nanoclusters in silica samples sequentially implanted with indium and nitrogen ions

This article discusses the formation of embedded indium nitride (InN) nanoclusters (NCs) in silica matrix through sequential implantation of 890u2009keV In2+ and 140u2009keV N+ ions. The implanted samples were subjected to post-implantation annealing at 500°C in nitrogen atmosphere. Investigations carried out on the implanted samples using glancing incidence X-ray diffraction, high-resolution transmission electron microscopy and Raman spectroscopy gave clear evidence for the formation of InN nanoclusters. Alongside with InN NCs, we also notice the presence of indium oxide nanoclusters in the sample.

A route for periodic nanodot fabrication in substrates using nanochanel alumina membranes as masks for ion implantation

Progress in ion implantation of metal ions into substrates of amorphous silica or Si-nitride with respect to lateral periodic patterning is presented. We use a 2D-nanoporous membrane of anodic aluminium oxide (AAO) as mask to conduct the Co ion implantations. The criteria for successful masked implantation and main problems are presented, including testing of the masks in a focused ion beam (FIB) system. It is proposed that electron transparent thin windows are the most suitable substrate for methods development, as TEM observation can be followed without any further sample milling. Co clusters are found to exhibit the same lateral order as the pores, and first annealing tests to achieve Co nanoparticles are shown using an in-situ heating TEM holder.

Electron beam induced surface morphology changes of CeO 2 nanocrystals: An in-situ aberration corrected TEM study

Hydrothermally prepared ceria cuboid nanoparticles were imaged in a 300 keV aberration corrected TEM to study the instabilities of different facets of nanoscale ceria at the atomic level. Real-time video recording at a rate of 30fps enables to closely monitor and quantify individual atomic movements. It is proposed to use quantification of electron beam induced cation movements and reconstructions as a novel probe for measurements of oxygen surface activity with nm-scale resolution, with applications in e.g. catalysis.