bluuebunny/crossref_metadata_2025_split huggingface dataset:
TEM
study of defect reduction in the growth of semipolar
GaN
grown on patterned substrates
Authors: Vennéguès Philippe , Tendille Florian , Khoury Michel , De Mierry Philippe , Mante Nicolas , Perez Jesus Zuniga , Feuillet Guy , Delaye Vincent , Martin Denis , Grandjean Nicolas 
Published: December 2016

III‐nitride heteroepitaxial thin films and heterostructures suffer from the presence of large densities of structural defects, which are detrimental for the development of efficient devices. These defects result from differences between III‐nitride films and foreign substrates (structure, chemistry, lattice parameters). Transmission electron microscopy (TEM) is the technique of choice for studying such crystalline defects. The understanding of the origin and behavior of structural defects may allow their tailoring and the development of low defect density materials.
III‐nitrides are classically grown along the polar c‐direction. In this case, internal electric fields play a major role in the properties of heterostructures. In order to eliminate or at least to reduce the influence of internal electric fields, growth along alternative directions with the c‐direction in the growth plane (nonpolar) or inclined to it (semipolar) have been developed. Heteroepitaxial nonpolar and semipolar films contain large densities of basal plane stacking faults (BSF) and related partial dislocations together with prismatic stacking faults.

In this presentation, results of TEM studies of semipolar GaN films deposited on patterned substrates will be presented. Different TEM techniques from diffraction contrast classical imaging, high resolution TEM and scanning TEM to analytical techniques as energy dispersive X‐ray spectroscopy (EDS) have been employed to obtain a complete view of semipolar GaN microstructures. The understanding of the nucleation and propagation of defects allowed us to develop several growth processes resulting in a drastic reduction of the defect density:

A 3‐step growth process for (11‐22) GaN deposited on patterned r‐sapphire leads to high quality GaN films with dislocation densities as low as 7x10
7
cm
‐2
and BSF densities below 10
2
cm
‐1
(figure 1)
1,2
.

A method based on the introduction of Si at an intermediate stage of the growth (before coalescence of nucleation islands) allows, in the case of (10‐11) GaN on patterned (001) 7° off‐axis Si, blocking the propagation of dislocations (figure 2). A 4nm thick Si‐rich (5% Si from EDS analysis) layer is revealed by HRSTEM (figure 3). This layer has the wurtzite structure of the surrounding GaN and does not introduce significant strain (as revealed by GPA analysis).

Selective growth on deeply grooved sapphire substrate results in GaN (11‐22) bands with a dislocation density in the mid 10
6
cm
‐2
on 100µm‐wide regions compatible with the fabrication of laser diodes.

Besides presenting results on the improvement of material quality through innovative growth processes, this presentation emphasizes the importance of TEM studies for the developments of heteroepitaxial semiconductors structures.
CelDi
: Development of an advanced solid / fluid reaction stage for
SEM
Authors: Salacroup Johan , Gonnet Gautier , Candeias Antoine , Brau Henri‐Pierre , Szenknect Stéphanie , Ivaldi Paul , Podor Renaud 
Published: December 2016

In numerous scientific fields such as life, materials and Earth sciences, or quality controls of industrial processes, there is a growing interest for the direct observation ‐ at the submicroscopic scale ‐ of processes occurring at solid / liquid and solid / gas interfaces. So far, only few experimental cells were designed to address this challenging issue. Most of them are devoted to a specific use in Transmission Electron Microscopy (TEM) and are not suitable for observation of large (or thick) samples and the other cells designed to be used in a Scanning Electron Microscope (SEM) chamber do not allow fluid flow.
To address this issue, a dedicated device was developed according to the following requirements: 1) The sample holder must be suitable for large samples. 2) The device must allow the renewal of the fluid through a continuous flow. 3) The device should be sufficiently efficient and secure to be used in any type of conventional SEM. 4) The device should be easy to implement and user friendly. A first prototype (Fig. 1) was recently tested and patented [1].

For the proof of concept, it was used to perform
in situ
experiments during which series of images was recorded with a SEM, using the back scattered electron detector, high vacuum in the SEM chamber and e‐beam acceleration voltage of 30kV. Fig. 2. presents several images of the growth of NaCl crystals obtained from a supersaturated solution. The image resolution is good enough to see details of a size of 50 nm. The liquid system that is inside the stage remained isolated from the SEM chamber during the complete experiment.

The CelDi project aims at the development of the second generation of this tool, integrating more safety protections, increasing the resolution of the images, finding solutions to achieve an easy and friendly use of the device. It will be possible to integrate a specific and fast BSE detector in combination with the stage, and work is under progress to develop automatized image processing through dedicated software.
In parallel, several tests will be carried out for different scientific applications in material science (corrosion) and life science (observation of live cells) to demonstrate the capabilities of the CelDi device.
Multi
ADF
detector tomography for
3D
characterization of heterostructures
Authors: Sentosun Kadir , Sanz Ortiz Marta N. , Batenburg K. Joost , Liz‐Marzán Luis M. , Bals Sara 
Published: December 2016

Characterization of core‐shell type nanoparticles in 3 dimensions (3D) by transmission electron microscopy (TEM) can be very challenging. Especially when both heavy and light elements co‐exist within the same nanostructure, artefacts in the 3D reconstruction are often present. A representative example would be a particle comprising an anisotropic metallic (Au) nanoparticle coated with a (mesoporous) silica shell. To obtain a reliable 3D characterization of such an object, we collected high angle annular dark field scanning TEM (HAADF‐STEM) and annular dark field tilt series (ADF‐STEM) for tomography (Figure 1A and 1B respectively). Although the series acquired by ADF‐STEM shows both the Au and the SiO
2
, artefacts are clearly present in the 3D reconstruction (Figure 2). Since the observed artefacts may cause loss of information or may lead to misinterpretation, it is extremely challenging to obtain reliable 3D results for core‐shell hybrid materials using conventional electron tomography. When selecting an optimal value for the collection angle, a compromise is needed between optimal contrast, produced by the atomic number of coexisting elements, and the minimization of diffraction contrast.

We here overcome this limitation by exploiting the flexibility of modern TEM instruments that enable one to collect multiple (HA)ADF‐STEM series simultaneously, by using different (HA)ADF detectors at the same time. This multi‐mode approach is very dose‐efficient, as one is able to collect 2 images while keeping the necessary electron dose the same. Tilt series were simultaneously acquired using an ADF detector with inner and outer collection angles of 35 and 125 mrad and a HAADF‐STEM detector using inner and outer collection angles of 150 and 220 mrad, respectively. To remove the artefacts that appear in the ADF‐STEM tilt series, we removed the complete Au nanoparticle from the ADF‐STEM projection images. Next, a technique known as inpainting was applied[1]. This approach replaces the absent information by a continuation of the texture of the surrounding area. This procedure was performed for each projection image of the tilt series separately (Figure 3A). The processed tilt series was then used as an input for 3D reconstruction using the SIRT algorithm implemented in the ASTRA toolbox[2]. Finally, the 3D HAADF‐STEM and ADF‐STEM reconstructions are combined into one single visualization using the AMIRA software as illustrated in Figure 3B [3]. In this manner, we were able to reliably characterize the structure of mesoporous SiO
2
Au nanoparticles. It must be noted that the methodology we propose here is generally applicable to a broad range of core shell hybrid nanostructures.
Laser triggered microfabricated ultrafast electron beam blanker
Authors: Weppelman Gerward , Moerland Robert , van Tol Ruud , Heerkens Carel , Hoogenboom Jacob , Kruit Pieter 
Published: December 2016

Femtosecond electron pulses are typically created by illuminating a flat photocathode with femtosecond laser pulses. [1] However, flat photocathodes have a low reduced brightness, 2 orders of magnitude lower than a Schottky electron source. A higher brightness can be achieved using a cold field emitter illuminated with femtosecond laser pulses. [2] Using a cold field emitter illuminated with UV pulses the group of Zewail has realized an ultrafast SEM. [3] However, such an USEM cannot easily be switched back to continuous beam operation. In addition, the pulse has to be accelerated from the tip onwards which leads to a broadened pulse at the sample.
Here, we propose a beam blanker for use in regular EMs that allows switching between continuous‐beam and ultrafast modes of operation. Previous approaches to ultrafast beam blanking were based on beam blankers using GHz magnetic or electric fields. [4,5] These GHz cavities are still relatively large and can't be inserted directly in a standard commercial SEM.
We use a miniaturized beam blanker controlled by a photoconductive switch, illuminated with femtosecond laser pulses, as schematically depicted in Figure 1. Hence, the blanker is locked jitter‐free to the laser. We show that such a beam blanker needs to have micrometer scale dimensions for ultrafast operation. COMSOL simulation results, including the full 3D blanker design, are used to evaluate the time response of the system.
We fabricated and integrated the deflector plates and the photoconductive switch in a one micrometer‐scale device, see Figure 2. We will show fabrication results of the ultrafast blanker and its incorporation on an insert for a FEI Quanta FEG 200 SEM. We will also show alignment of both laser and electron beam on the ultrafast beam blanker. Also results will be presented showing laser triggered deflection of the electron beam.
Element partitioning and atom location of alloying elements in
Co
‐base superalloys
Authors: Wang Li , Oehring Michael , Lorenz Uwe , Pyczak Florian 
Published: December 2016

The discovery of an ordered L1
2
precipitate (Co
3
(Al, W)) phase in the ternary Co‐Al‐W system in 2006 which is stable up to temperatures of 950 °C attracts significant research on microstructure design for a new class of load‐bearing Co‐base high‐temperature alloys. The low mismatch between the fcc γ‐Co solid‐solution phase and the L1
2
γ'‐Co
3
(Al, W) phase helps to establish a microstructure with coherent cuboidal γ' precipitates embedded in a continuous γ matrix, analogous to Ni‐base superalloys. These new Co‐base superalloys have the potential to exhibit excellent high‐temperature mechanical properties and are considered to be ideal turbine blade materials.

In the ternary Co‐Al‐W system, it is reported that the L1
2
phase is in equilibrium with the B
2
‐CoAl and D0
19
‐Co
3
W phases and transforms into those after extended annealing
[1]
. In order to increase the temperature capability and stabilize the γ'‐L1
2
phase, additional alloying elements are added to the Co‐Al‐W system. With additions of Cr, Mo, Ni, Re, Ta and V, the solidus and liquidus temperatures of Co‐Al‐W alloys are 100‐150 °C higher than those of advanced Ni‐base single‐crystal alloys strengthened by the L1
2
phase
[2]
. It is speculated in literature that Mo, V, Nb, Ta and Ti increase the γ' solvus temperature of Co‐Al‐W superalloys because they occupy B‐sites in the A
3
B ordered γ' phase and thereby increase its volume fraction, while Fe, Mn and Cr tend to distribute to the γ phase and decrease the amount of γ' phase
[3]
. Because the distribution of additional elements will influence the morphology and amount of the γ' phase, it is important to investigate their partitioning behavior and atomic site occupation in the Co‐Al‐W system. Therefore the high‐temperature strength and thermal stability of Co‐base superalloys are optimized.

In this study, alloy samples with nominal compositions Co‐9Al‐9W‐2X (X=Ti, Nb, V, Ta, Cr and Mo, at.%) were produced. The partitioning behavior of the alloying elements between γ and γ' phases were investigated by energy‐dispersive X‐ray spectroscopy (EDS) in the transmission electron microscope (TEM). In order to analyze the element occupation at atomic sites, atom location by channeling enhanced microanalysis (ALCHEMI) technique was applied.
The details about the partitioning behavior and the atomic site occupation of alloying elements will be discussed in the presentation.
Precise
STEM
measurement of defocus and aberration in monolayer graphene
Authors: Sawada Hidetaka , Kirkland Angus 
Published: December 2016

Improvements to the Z height resolution in a scanning transmission / transmission electron microscope (S/TEM) are important for 3D reconstruction of local specimen topography and for quantitative image analysis using comparisons of experimental images to simulations. The lateral position perpendicular to the direction of the electron trajectory in aberration corrected S/TEM instruments has a sensitivity comparable to most atomic spacing which can be directly determined in experimental images to an accuracy of a few pm. However, the resolution in the Z direction parallel to the electron trajectory is of the order of only a few nm as determined by direct experimental measurement using confocal electron microscopy of extended objects [1] or in HAADF STEM images of point like objects [2]. This resolution is an order of magnitude larger most atomic spacing and therefore to accurately determine Z height information at atomic resolution, a method for precise measurement of the specimen height is required.
Toward obtain better accuracy for Z direction measurements, we have used the lateral magnification change of an ideal specimen of monolayer graphene, where in reciprocal space with a probe forming mode, the magnification of the lattice depends on the value of the defocus. Ronchigrams have been recorded with a focused STEM probe of atomic dimensions at a large camera length of 80 cm at 80 kV [Fig. 1(a)]. The Ronchigram was divided into local angular areas to analyze the local magnification dependent on the probe‐forming aberrations. Importantly the lattice parameter of graphene is accurately known and changes in the lattice pattern with beam tilt or specimen tilt are smaller than for other, thicker crystalline specimens. The local magnification in each local area was analyzed using an auto‐correlation function [Fig.1(b)] [3]. Six peaks surrounding the center peak of the auto‐correlation function from a graphene image were identified and were fitted with an oval [Fig.1(c)]. Using the fitted parameters describing the oval shapes at local angular areas, the distance between the probe and the graphene monolayer (defocus) was measured precisely to be ‐ 58.52 nm. Since the lattice parameter of a specimen is known, only one Ronchigram pattern is required to derive the value of defocus. Other aberration coefficients for the probe forming lens also can be calculated using this method [Table. 1].
Structure of short period
In(Ga)N/GaN
superlattices comprising ultra‐thin quantum wells
Authors: Dimitrakopulos George , Bazioti Calliope , Karakostas Theodoros , Kioseoglou Joseph , Pavloudis Theodoros , Kret Slawomir , Koziorowska Julita , Suski Tadek , Dimakis Emmanouil , Moustakas Theodore , Komninou Philomela 
Published: December 2016

InGaN quantum wells (QWs) grown along the polar c‐axis are currently the principal structural elements of III‐nitride optoelectronic device active regions. Further advancement of their operational wavelength range with high internal quantum efficiency in the green part of the spectrum may be facilitated through the use of short period In(Ga)N/GaN superlattices comprising QWs with a minimal number of monolayers (MLs). At the same time, ultra‐thin InN/GaN QWs have recently generated considerable interest, due to the theoretical prediction that they may exhibit topological insulator properties [1], thus opening up new prospects for applications in quantum computing and spintronics. In recent work, two‐dimensional electron gas properties were demonstrated in ML‐thick, nominally InN QWs, as well as a temperature‐independent behavior in the diagonal resistance, indicating the topological nature of the 2DES [2].
We have studied a series of In(Ga)N/GaN short period superlattice (SPS) heterostructures deposited at low growth temperatures. The examined samples comprised SPS with nominally 1, 2, and 4 ML QW thicknesses grown by molecular beam epitaxy (MBE) on (0001) GaN/sapphire MOVPE templates. High resolution transmission electron microscopy (HRTEM), and high resolution high‐angle annular dark field imaging in scanning TEM mode (HRSTEM) have been employed in order to ascertain the compositional homogeneity due to issues such as the desorption, clustering, and diffusion of indium into the GaN barriers. Such issues may be aggravated by the large elastic strain associated with the pseudomorphic growth of the ultrathin QWs. On the other hand, it is also possible that the stress due to the misfit may in fact stabilize the growth of InN at unusually high temperatures. Fig. 1 illustrates HRTEM and HRSTEM images of a 1 ML InN / 10 ML GaN SPS and of a 4 ML‐thick QW. Strain and compositional mappings were implemented by using peak finding with the peak‐pairs method, and geometrical phase analysis. Z‐contrast image calibration was performed using reference samples and image simulations.
The results were compared with energetic calculations of pertinent supercells using both molecular dynamics with a modified Tersoff interatomic potential, and ab initio density functional theory. In both cases, deviation from the biaxial stress state of InN was identified for these QWs in agreement with the experimental observations.
Spin‐multislice simulation of an electron inside the objective lens of a TEM
Authors: Grillo Vincenzo , Schachinger Thomas , Karimi Ebrahim , Schattschneider Peter 
Published: December 2016

Spin filtering of an unpolarized beam in a TEM is a fascinating field of research. Bohr conjectured that it is impossible to spin filter an electron beam or, using Bohr words, “to observe the spin of the electron, separated fully from its orbital momentum, by means of experiments based on the concept of classical particle trajectories”[1].
However, the principle seems to be violated by theoretical calculations [2,3]. One of the most convincing proposals for free electron polarization is a multipolar Wien filter. But the fields involved are typically very large [2] while multipolar Wien filters in microscopy are still rare. The device, together with the diffractive elements, is called as “q‐filter” where q hints at the topologic charge of the field.
The objective lens of the microscope provides a very large field with the potentiality of introducing a spin‐orbit coupling, we performed spin‐multislice simulations [4], where a Bessel beam was propagated through the objective lens (modeled as a Glaser field) in order to quantify the degree of spin polarization.
We will discuss in particular that the spin‐orbit conversion in the pre‐ and post‐field can be understood in terms of the q‐filter.
Fig 1 a shows on the left a scheme of the objective lens and of the electron wavefunction (blue) passing through it. A schematic “Bohmian” trajectory is indicated by a curve. The image also features arrows indicating the classical spin orientation along the curve for an initial state with spin |↑> along the optic axis.

The fig 1b indicates the multislice calculated evolution of the wavefunction. While the expectation value of the spin operator S has components < S
x
>=< S
y
> =0 we can track the expectation of the x,y vector P=(S.r,S.t) (with r being the in plane position versor and t its orthogonal in plane versor). The result is shown in fig 2. P represents a sort of local in plane projection of the spin operator. To a good degree of approximation here |P| is equal to the rate of conversion from |↑> to |↓>.

The results indicates a net, typically weak , increase of P as an effect of the objective lens .
The overall final wavefunction is described in Fig 3 as a non‐separable spin‐orbital angular momentum state.
The multislice results are in quantitative agreement with ray tracing calculations, confirming the reliability of both methods in this case. However, the multislice approach enables us to use less classical states like Laguerre Gauss beams, to explore possible advantages and more quantum physical effects.
Multi‐modal in vivo visualization of single cell dynamics by
1P
,
2P
, light‐sheet, and on‐chip technologies
Authors: Nishimura Satoshi 
Published: December 2016

We made fluorescence multi‐scale imager including 1P, 2P, light‐sheet, microscope, and on‐chip sensors for visualization of XYZT single cell dynamics in vivo. We integrated five microscope for systematic evaluation of living animals.
First visualization system is super‐resolution imaging based on non‐linear optics, with X‐ (resonance), Y‐(galvano), and Z‐(piezo) axis scanning. Real‐time, multi‐color XYZT multi‐photon imaging enabled us to visualize single blood cell behavior in vessels and stroma. Spatial and time resolution was improved by pattern illumination. We analyzed thrombus formation, and inflammatory responses under microscope. We developed thrombus formation animal models, and elucidated cellular mechanisms of cardiovascular diseases. We directly manipulated cells by photo‐chemical reactions, and two photon lasers to induce and observe thrombotic reactions. Using this system, we elucidated contribution of endothelial injuries to thrombus formation.
Second, high‐resolutions and broad‐imaging field was simultaneously enabled using 8K CMOS technologies, and 1P spinning disk confocal. We visualized whole organs and single cell in one image, and revealed complicated cell‐cell interactions networks in single view. 8K, 60fps, and multi‐color imaging visualized single platelet dynamics and tissue structural changes in single image.
Third, we performed light‐sheet imaging for living mice, and enabled high‐speed observation of living animals.
Fourth, macro imaging system for awake mice, pigs and humans was also developed, and free behavior monitoring revealed the tight association between metabolism and vascular reactions during daily life. Bioluminescent and fluorescent imaging from body surface using CMOS camera, image intensifier, and macro‐lens enabled us to visualize cellular dynamics without anesthesia.
Last, wearable, implantable, and minimized devices for non‐invasive recording were also developed using lens‐less and on‐chip technologies. We utilized SCMOS, micro‐lens array, and LED illumination technologies.
In sum, we developed multi‐scale imaging system which can evaluate cellular networks dysregulations in diseased conditions.