Joaquim Portillo

Orientation and phase mapping in the transmission electron microscope using precession‐assisted diffraction spot recognition: state‐of‐the‐art results

A recently developed technique based on the transmission electron microscope, which makes use of electron beam precession together with spot diffraction pattern recognition now offers the possibility to acquire reliable orientation/phase maps with a spatial resolution down to 2 nm on a field emission gun transmission electron microscope. The technique may be described as precession‐assisted crystal orientation mapping in the transmission electron microscope, precession‐assisted crystal orientation mapping technique–transmission electron microscope, also known by its product name, ASTAR, and consists in scanning the precessed electron beam in nanoprobe mode over the specimen area, thus producing a collection of precession electron diffraction spot patterns, to be thereafter indexed automatically through template matching. We present a review on several application examples relative to the characterization of microstructure/microtexture of nanocrystalline metals, ceramics, nanoparticles, minerals and organics. The strengths and limitations of the technique are also discussed using several application examples.

Precession Electron Diffraction Assisted Orientation Mapping in the Transmission Electron Microscope

Precession electron diffraction (PED) is a new promising technique for electron diffraction pattern collection under quasi-kinematical conditions (as in X-ray Diffraction), which enables “ab-initio” solving of crystalline structures of nanocrystals. The PED technique may be used in TEM instruments of voltages 100 to 400 kV and is an effective upgrade of the TEM instrument to a true electron diffractometer. The PED technique, when combined with fast electron diffraction acquisition and pattern matching software techniques, may also be used for the high magnification ultra-fast mapping of variable crystal orientations and phases, similarly to what is achieved with the Electron Backscattered Diffraction (EBSD) technique in Scanning Electron Microscopes (SEM) at lower magnifications and longer acquisition times.

A new approach for 3D reconstruction from bright field TEM imaging: Beam precession assisted electron tomography

The successful combination of electron beam precession and bright field electron tomography for 3D reconstruction is reported. Beam precession is demonstrated to be a powerful technique to reduce the contrast artifacts due to diffraction and curvature in thin foils. Taking advantage of these benefits, Precession assisted electron tomography has been applied to reconstruct the morphology of Sn precipitates embedded in an Al matrix, from a tilt series acquired in a range from +49° to -61° at intervals of 2° and with a precession angle of 0.6° in bright field mode. The combination of electron tomography and beam precession in conventional TEM mode is proposed as an alternative procedure to obtain 3D reconstructions of nano-objects without a scanning system or a high angle annular dark field detector.

Investigation of the hydroxyapatite obtained as hydrolysis product of α-tricalcium phosphate by transmission electron microscopy

Alpha tricalcium phosphate (α-TCP) is widely used as a reactant in many calcium phosphate cements, where the setting reaction occurs through its hydrolysis to a calcium deficient hydroxyapatite (CDHA), at room or body temperature. Transmission electron diffraction studies of cement single CDHA crystals produced at 37 °C have revealed the systematic presence of unexpected diffraction spots other than those assigned to the archetypal P63/m apatite. Three possible causes can explain the existence of these extra spots: electron beam irradiation damage (inducing phase transformation to monoclinic apatite), presence of octacalcium phosphate (OCP) and presence of monoclinic apatite in the hydrolyzed product. Determination of the occurrence of any of these phases would have great implications in the properties of the hydrolyzed product. However, the identification of their corresponding electron diffraction patterns is not straightforward due to coincidental spots under basal zone axis orientation. In the present work, off-zone axis electron diffraction in the transmission electron microscope, TEM, has been used for phase identification, clearly indicating the presence of the monoclinic apatite phase. These findings could be a strong asset reinforcing the possibility of the monoclinic phase as a constituent of hard bone tissue.

Assessment of misorientation in metallic and semiconducting nanowires using precession electron diffraction.

Precession electron diffraction (PED) allows for diffraction pattern collection under quasi-kinematical conditions. The combination of PED with fast electron diffraction acquisition and pattern matching software techniques is used for the high magnification ultra-fast mapping of variable crystal orientations and phases, similarly to what is achieved with the Electron Backscattered Diffraction technique in Scanning Electron Microscopes at lower magnifications and longer acquisition times. Here we report, for the first time, the application of this PED-based orientation mapping technique to both metallic and semiconducting nanowires.

Hafnium-silicon precipitate structure determination in a new heat-resistant ferritic alloy by precession electron diffraction techniques.

The structure determination of an HfSi4 precipitate has been carried out by a combination of two precession electron diffraction techniques: high precession angle, 2.2°, single pattern collection at eight different zone axes and low precession angle, 0.5°, serial collection of patterns obtained by increasing tilts of 1°. A three-dimensional reconstruction of the associated reciprocal space shows an orthorhombic unit cell with parameters a = 11.4 Å, b = 11.8 Å, c = 14.6 Å, and an extinction condition of (hkl) h + k odd. The merged intensities from the high angle precession patterns have been symmetry tested for possible space groups (SG) fulfilling this condition and a best symmetrization residual found at 18% for SG 65 Cmmm. Use of the SIR2011 direct methods program allowed solving the structure with a structure residual of 18%. The precipitate objects of this study were reproducibly found in a newly implemented alloy, designed according to molecular orbital theory.

Quasi-parallel precession diffraction: Alignment method for scanning transmission electron microscopes

A general method to set illuminating conditions for selectable beam convergence and probe size is presented in this work for Transmission Electron Microscopes (TEM) fitted with µs/pixel fast beam scanning control, (S)TEM, and an annular dark field detector. The case of interest of beam convergence and probe size, which enables diffraction pattern indexation, is then used as a starting point in this work to add 100 Hz precession to the beam while imaging the specimen at a fast rate and keeping the projector system in diffraction mode. The described systematic alignment method for the adjustment of beam precession on the specimen plane while scanning at fast rates is mainly based on the sharpness of the precessed STEM image. The complete alignment method for parallel condition and precession, Quasi-Parallel PED-STEM, is presented in block diagram scheme, as it has been tested on a variety of instruments. The immediate application of this methodology is that it renders the TEM column ready for the acquisition of Precessed Electron Diffraction Tomographies (EDT) as well as for the acquisition of slow Precessed Scanning Nanometer Electron Diffraction (SNED). Examples of the quality of the Precessed Electron Diffraction (PED) patterns and PED-STEM alignment images are presented with corresponding probe sizes and convergence angles.

Analysis of the HF-Etihanol Cleaning Process on Silicon Wafers with 100 Vicinal Surfaces

X-ray photoelectron spectroscopy (XPS) measurements on silicon surfaces cleaned by HF/Ethanol treatments are described using silicon wafers with (100) vicinal surfaces. The composition of the surfaces (Silicon, Oxygen, Fluorine and Carbon) has been measured leading to the conclusion of good passivated surfaces. Moreover, for each vicinal direction, the F 1s , C 1s ,O 1s and Si 2p lines have been decomposed into several components corresponding to different chemical bonds. The more significant results show that the percentages of the two components for Fluorine depend on the total amount of Oxygen and are related to the Si-F and Si-O-F bonds. On the other hand, the two complementary doublets necessary to fit the Si 2p band can be related to the Si-H and Si-F bonds and change with orientation and/or roughness of the considered surface. The native oxide formation also points out significant differences in the behaviour of each vicinal direction. The thickness and structure of the oxide and the remaining amount of Fluorine reflect the influence of the initial RMS-microroughness determined by Atomic Force Microscopy which changes from 0.05nm for (100) to 0. lnm for (911).