David Hernandez-Maldonado

Compositional Analysis with Atomic Column Spatial Resolution by 5th-Order Aberration-Corrected Scanning Transmission Electron Microscopy

We show in this article that it is possible to obtain elemental compositional maps and profiles with atomic-column resolution across an InxGa1-xAs multilayer structure from 5th-order aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. The compositional profiles obtained from the analysis of HAADF-STEM images describe accurately the distribution of In in the studied multilayer in good agreement with Murakis segregation model [Muraki, K., Fukatsu, S., Shiraki, Y. & Ito, R. (1992). Surface segregation of In atoms during molecular beam epitaxy and its influence on the energy levels in InGaAs/GaAs quantums wells. Appl Phys Lett 61, 557-559].

Growth of Low-Density Vertical Quantum Dot Molecules with Control in Energy Emission

In this work, we present results on the formation of vertical molecule structures formed by two vertically aligned InAs quantum dots (QD) in which a deliberate control of energy emission is achieved. The emission energy of the first layer of QD forming the molecule can be tuned by the deposition of controlled amounts of InAs at a nanohole template formed by GaAs droplet epitaxy. The QD of the second layer are formed directly on top of the buried ones by a strain-driven process. In this way, either symmetric or asymmetric vertically coupled structures can be obtained. As a characteristic when using a droplet epitaxy patterning process, the density of quantum dot molecules finally obtained is low enough (2 × 108 cm−2) to permit their integration as active elements in advanced photonic devices where spectroscopic studies at the single nanostructure level are required.

STEM Optical Sectioning for Imaging Screw Dislocations Core Structures in BCC Metals

It is well known that the low-temperature plastic deformation of Body-Centred Cubic (BCC) metals is controlled by the glide of 1⁄2[111] screw dislocations. Their low mobility is caused by the delocalized nature of their cores [1] which are extended into several planes in the zone of the Burgers vector. This non-planar core spreading has been demonstrated by a number of atomistic studies made in the last 46 years [2]. However, attempts at experimental observation have been hindered by the Eshelby twist effect [3,4]. The aim of this work is to investigate whether the edge and screw displacements associated with 1⁄2[111] screw dislocations in BCC metals can be detected by optical sectioning in high-angle annular darkfield (HAADF) Scanning Transmission Electron Microscope (STEM) imaging conditions.

Evaluation of Aberration-corrected Optical Sectioning for Exploring the Core Structure of ½[111] Screw Dislocations in BCC Metals

The introduction of spherical-aberration correctors in the Scanning Transmission Electron Microscope (STEM) has allowed an improvement in spatial resolution to the sub-angströn scale accompanied by a reduction of the depth of focus (due to the increase in probe convergence angles),which in a modern instrument is just a few nanometers, thus often less than the sample thickness. This can be exploited to extract information along the beam direction by focusing the electron probe at specific depths within the sample. Optical sectioning has been proved to be a powerful tool for the study of the core structure of screw dislocations. It has been used to observe the depth-dependence of the strain field due to the Eshelby twist associated with dislocations containing a screw component in thin STEM samples. The measurement of the magnitude of the displacement confirmed the screw Burgers vector for dislocations in GaN [1] and allowed the identification of a new dissociation reaction of mixed [c+a] dislocations [2]. The optical sectioning approach has also been applied to the direct observation of the c-component of the dissociation reaction of mixed [c+a] dislocations in GaN by imaging a dislocation lying transverse to the electron beam [3].

Detection of oxygen sub-lattice ordering in A-site deficient perovskites through monochromated core-loss EELS mapping

Perovskite oxides are widely studied for a variety of applications, from theromoelectrics to fuel cells. Part of the attraction lies in the fact that perovskite ceramics are relatively easy to dope chemically over a wide range of compositions, resulting in various degrees of structural ordering [1]. As a consequence, the properties and functionalitiesof such materials can be readily tailored [2]. For instance in systems proposed for thermoelectric applications, the presence of superlattices, or domain boundaries vacancies can suppress the thermal conductivity due to increased phonon scattering [3,4]. Understanding therefore the mechanisms behind the formation of such types of ordering in ceramic systems is crucial for their implementation in engineering applications.

Determination of Local Chemistry Composition of Low-Dimensional Semiconductor Nanostructures Through the use of High-Resolution HAADF images

Departamento de Ciencia de los Materiales e I.M. y Q.I., Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro, s/n, 11510 Puerto Real, Cádiz, Spain SuperSTEM Laboratory, STFC Daresbury Campus, Daresbury WA4 4AD, UK Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Compositional mapping by Z-contrast imaging

This research was sponsored by the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (SJP, MV), by the Spanish MCI (projects CONSOLIDER INGENIO 2010 CSD2009-00013 andTEC2008-06756-C03-02/TEC,) and the Junta de Andalucia (PAI research’s groups TEP-120 and TIC-145; project P08-TEP-03516).