bluuebunny/crossref_metadata_2025 huggingface dataset:
Dislocation mobility in
GaN
nanowire arrays by in‐situ heating in the
TEM
Authors: McLaren Mathew , Zubialevich Vitaly , Parbrook Peter , Shen John , Arredondo Miryam
Published: December 2016

Wide bandgap semiconductors are a current area of interest for a new generation of high‐temperature, high‐voltage and high‐power semiconductor‐based electronics. Gallium nitride (GaN) is a promising candidate and is already extensively used in blue and UV LEDs[1,2]. Thin films are grown on substrates such as silicon and sapphireas to produce transistors with high electron mobility. However, these films suffer from large dislocation densities (10
8
‐10
10
cm
‐2
)[3,4] due to the intrinsic strain of the substrate, resulting in devices with reduced reliability because of increased current leakage, disrupted electric field distribution and a premature breakdown of microplasma[5].

Therefore, novel methods of GaN device manufacturing using nanostructured systems such as nanowires and nanorods are a promising solution to minimise the presence of structural defects. Understanding how growth conditions affect the nanowire's structure is important for future device fabrication, in particular the effect of annealing conditions on the dislocation density.

Here, we investigate unannealed GaN nanowiresgrown by metalorganic vapour phase epitaxy on Si substrates[6]. The nanowires are doped with varying levels of Si, subjected either an N
2
or N
2
+NH
3
atmosphere upon growth.

The annealing process is seen to reduce dislocation density[7]. Ex‐situ annealing during growth shows a slight reduction in dislocation density. Figure 1 shows a comparison of dislocation density between an unannealed sample and a sample annealed at 900°C. Dislocation densities are measured to be 1.6x10
9
cm
‐2
and 1x10
9
cm
‐2
for unannealed and annealed nanowires respectively. The drop in dislocation density is, however, lower than expected for annealing at this temperature. In‐situ heating experiments allow the direct observation of dislocation mobility during annealing, giving a valuable insight into dislocation bending.
Ultracytochemical detection of iron in plant tissues by
TEM
Authors: Machado Silvia , Arrivabene Hiulana , Milanez Camilla
Published: December 2016

Iron (Fe) is an essential micronutrient operating in several important processes. It is found in high concentrations in mangrove sediment, and in contaminated areas, the level of this element may increase substantially. Mangrove vascular plants have strategies to deal with the high concentration of this metal, involving Fe‐plaque formation on the surface of the root, metal retention in the epidermis and endoderm cells, and compartmentalization in the vacuoles. Some methods have been used for the intracellular localization of iron, such as electron spectroscopic imaging and energy‐dispersive X‐ray analysis. Perl's reagent has been traditionally used to detect iron in tissue plants by light microscopy; however, its use in ultrastructural studies has been limited to animal cells. This study aimed to assess whether the Perl's reagent can be used successfully in plant material under TEM. Samples of
Rhizophora mangle
L. (Rhizophoraceae) mature leaves were incubated for 24 h at 4°C in a fixative solution containing 3% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.3). Then, materials were immersed for 30 min at 25°C in Perl's reagent (1% potassium ferrocyanide and 2% HCl, 1v/1v). Samples were rinsed using a sucrose‐cacodylate buffer, postfixed for 1 h at 4°C with 1% O
s
O
4
(0.1 M cacodylate buffer pH 7.3), dehydrated in alcohol series and propylene oxide, and embedded in Spurr's resin. Thin sections (80 nm) were cut with a diamond knife, and stained with uranyl acetate and lead citrate, and viewed with a Tecnai Spirit TEM. Sections without contrast were analyzed for enhanced identification of the reaction. Dense iron deposits, cuboid or irregular in shape, were detected in the peripheral cytoplasm and in cell walls (Fig. 1). The use of this method was able to show intracellular sites of iron compartmentalization in plant cells, and indicates the apoplastic and symplastic routes for the iron movement throughout the leaf tissues.
Simulation of
STEM
‐
HAADF
image contrast of Ruddlesden Popper faulted
LaNiO3
thin films
Authors: Coll Catalina , López‐Conesa Lluís , Magén Cesar , Sanchez Florencio , Fontcuberta Josep , Estradé Sònia , Peiró Francesca
Published: December 2016

Perovskite nickelates are interesting candidates to be used as electrode materials or in the research towards artificial superconductors. Among them, LaNiO
3
(LNO) is of high interest because of its exceptional transport properties, with a resistivity lower than 100 µΩ·cm, and the potential tuning of electrical and magnetic interactions through appropriated strain engineering. A precise control of the stoichiometry, structural phases, lattice distortions and the presence/absence of dislocations/defects is critical to make LNO electrodes and related heterostructures suitable for electronic applications
1–3
.

Frequent defects present in perovskite structures are Ruddlesden Popper (RP) faults which consist on the relative displacement of two perfect defect free perovskite ABO
3
blocks a distance of half‐ unit cell along the [111] direction. The earlier observations of RP faults were based on diffraction results, but they have also been imaged using advanced microscopy tools
4,5
. When observed along the [001] zone axis, this defect appears as a zig‐zag arrangement of the A cations with a BO
2
plane lost at the defect boundary.

In the present work we focus our attention on the characterization of RP faults observed in LNO thin films 14 nm and 35 nm thick, grown by pulsed laser deposition on (001) oriented LaAlO
3
(LAO) single crystal substrates at an oxygen pressure P = 0.15 mbar and at a temperature T = 700°C. The preliminary characterization of the layers by high resolution transmission electron microscopy (HREM) and electron diffraction, confirmed the good pseudo‐cube‐on‐cube epitaxial growth with atomic‐sharp interfaces and the expected [010]LNO(001)//[010]LAO(001) epitaxial relationship, despite the compressive strain driven by the 1% mismatch between LNO (3.838 Å) and LAO (3.795 Å). Nevertheless, defects identified as potential RP faults oriented in both [100] and [010] directions were also observed.

Detailed high angle annular dark field (HAADF) imaging of these defects enabled a better identification of the defects as Ruddlesden Popper type through the appropriated correlation of the contrast of the atomic columns with cationic La(A) and Ni(B) positions as shown in the figure 1.

We will systematically address the interpretation of the contrast of these HAADF images through STEM‐HAADF simulations calculated through the multislice procedure
6
from atomistic models based on different arrangements of defect free perovskite blocks with octaetrahedral Ni
3+
coordination (Fig. 2). Gradual variation of Z‐contrast is interpreted in terms of the average of the atomic numbers of the La and Ni in A and B sites in RP displaced overlapping perovskite crystals
7
. The good agreement between the experimental images and the simulated ones (Fig. 3) validates the proposed geometrical configurations of the RP faulted crystals.
In situ tracking of the heat‐induced replacement of
GaAs
by
Au
in nanowires
Authors: Fauske Vidar , Huh Junghwan , Divitini Giorgio , Munshi Mazid , Dheeraj Dasa Lakshmi , Ducati Caterina , Weman Helge , Fimland Bjørn‐Ove , van Helvoort Antonius
Published: December 2016

For devices, the junctions between the semiconductors and any metal contacts are crucial for the device performance. A heat treatment is commonly applied to improve the quality of the contacts. For GaAs nanowires with a Au‐based contact, annealing can lead to a well‐defined metal‐GaAs junction within the nanowire [1]. Here, we report an
in situ
heating
,
high‐angle annular dark‐field scanning transmission electron microscopy (HAADF STEM) study on the formation and structural characteristics of such junctions. The nanowires are dispersed on a high‐stability TEM heating chip, and local Au contacts are made by lithography before
in situ
heat treatment within the microscope (Figure 1) [2]. A replacement of GaAs by Au can take place and our study determined key aspects of the reaction mechanism and its kinetics such as the reaction rate and the activation energy. In general, the replacement proceeds one GaAs(111) bi‐layer at a time, as demonstrated by lattice resolved HAADF STEM (Figure 2). Ga dissolves in Au and As desorbs, as was previously reported for planar GaAs‐Au structures [3]. Using scanning precession electron diffraction (SPED) it was found that there is no fixed epitaxial relation between the newly formed 1D Au‐phase and the original GaAs nanowire. The reaction rate and the activation energy for the exchange are accurately determined by tracking the interface between the two phases over relatively long (~0.5 μm) distances.

The morphology of the solid 1D Au‐phase is the same as for the original GaAs nanowire. Within it, growth twins are observed. The morphology and growth twins do not alter upon cooling and reheating. For the case where the nanowire is attached to a relatively large Au contact (Au reservoir), the contact acts as a Ga diffusion sink, and only negligible amounts of Ga are found in the formed 1D Au‐phase. The
in situ
STEM and electron diffraction results prove that the replacement reaction takes place in the solid state. For the case where the nanowire is attached to a limited Au supply, as would be the case for a small volume Au deposition onto the nanowire or a Au catalyst droplet used for the nanowire growth, the growing metal segment gradually becomes richer in Ga as the exchange reaction proceeds. The reaction rate is slowed down over time due to the Ga enrichment. Eventually and at sufficiently high temperatures, the Au‐Ga segment becomes liquid. Upon cooling of such segments, different Au‐Ga intermetallic phases form and the main phases could be identified using a combination of SPED and machine learning (Figure 3).
Effects of dose and image registration on exit wave reconstruction of low‐dose focal series
Authors: Huang Chen , Sawada Hidetaka , Kirkland Angus
Published: December 2016

Dose effects are an important topic in electron microscopy (EM) due to the close connection to radiation damage and hence to quantitative image analysis [1]. Two main problems restricting the application of exit wave reconstruction to radiation‐sensitive materials are the high noise level in low‐dose images and the contrast reversals close to zero defocus. Both problems cause difficulty in correctly registering a focal series of images and a well‐aligned image series is a prerequisite for valid wave restoration [2].

In this work, a simulation‐assisted cross‐correlation function (SA‐XCF) registration scheme is proposed and tested with five focal series of the same cerium oxide (CeO
2
) nanoparticle taken under identical imaging conditions except for varying electron dose between different focal series. The registration results demonstrate the superiority of the new registration scheme over a simple neighboring‐reference cross‐correlation function (NR‐XCF) registration (Figure 1). The impact of registration quality on exit wave reconstruction is explored by the comparing the IQ factor, an image quality measurement calculated from the power spectrum of an image of the phase restored from the same focal series aligned by two different registration methods. Better phase restoration results are obtained with the focal series registered by SA‐XCF in comparison to NR‐XCF.

With improved image alignment exit waves reconstructed from focal series data at variable dose were compared. The comparison result confirms the natural hypothesis that higher electron dose series give improved restored exit waves than lower dose series. However, this improvement with increased dose appears to plateau beyond a certain dose threshold. Exit wave reconstruction from focal series at very low dose tends to preserve more noise from the images and leads to worse IQ factor values (Figure 2).

It has also been observed that the restored exit waves from focal series at very low dose can be unreliable for quantitative interpretation (Figure 3) where the phase shift is noticeably attenuated. This leads to the question of how a critical dose can be determined in order to obtain a quantitatively interpretable exit wave subject to the most efficient use of an allowed electron dose budget.
Atomic structure and electronic configurations of
2D
thermoelectric cobaltates
Authors: Bocher Laura , Gloter Alexandre , Stéphan Odile , Hébert Sylvie , Pelloquin Denis
Published: December 2016

Among the numerous different oxide systems, the cobaltites present fascinating and complex physical properties directly correlated with their electronic structure. A strong interplay exists in these Co‐based systems between (i) valence and spin states and (ii) low dimensionality and structural anisotropy, all of these yielding the emergence of unexpected magnetic and thermoelectric (TE) functionalities as illustrated in Na
x
CoO
2
[1]. The misfit‐layered cobalt oxides built from similar cobalt layers have triggered intensive works regarding their remarkable TE properties at high temperatures. A real driving force of this misfit family is the CoO
2
2D hexagonal layers turning out to be at the origin of its metallicity combined with a large thermopower, and more largely of interesting magnetic and electronic phenomena [2].

Another cobalt oxide, Bi
4
Sr
12
Co
8
O
28‐δ
, derived from the 2201‐type cuprates reveal an original tubular structure. These cobaltates present a peculiar 2D structure composed of two defined sub‐lattices interconnected through oxygen‐deficient CoO
x
pillars. This complex network has the ability to accommodate a certain structural and chemical flexibility by tuning locally its oxygen stoichiometry and therefore modulates its concentration of charge carriers. Large oxygen stoichiometry variations are achieved in these Co‐based systems after post‐annealed synthesis inducing dramatic changes in their physical properties, e.g. the emergence of two magnetic transitions at ≈ 50 K and 450 K. As in misfit cobaltites, the Co‐tubular family reveals peculiar transport properties and magnetic transitions while their local electronic configurations remain unresolved to date. In these bidimensional cobaltites, one of the pivotal questions is the key role played by each type of Co sites.

The recent advances in electron spectromicroscopy have considerably improved both spatial and spectral resolutions toward acquiring 2D elemental maps at sub‐Angström resolution over large unit cells and more recently mapping mixed‐valence state modulations down to the level of the atomic columns. Here we shed light on the [Bi
2
Sr
2
CoO
6
]
n
[Sr
8
Co
6
O
16‐δ
] n=2 member by resolving its atomic structure using a Cs‐corrected scanning transmission electron microscope (STEM), the NION UltraSTEM200, coupled with electron energy‐loss spectroscopy (EELS). Relying on high‐energy resolution, we map locally the Co mixed‐valence state modulations and further probe the effect of electronic charge transfer on individual Co atomic columns by tracking the subtle evolution of the Co‐L
2,3
and O‐K near‐edge fine‐structures which are highly sensitive to the mixed valence‐states and the hybridization geometries (see Fig.).
Induction of conoid extrusion is asociated to
MIC
secretion in Toxoplasma gondii tachyzoites
Authors: del Carmen Manuel Gonzalez , Calvo Lizbeth Cariño , Mondragón Ricardo
Published: December 2016

INTRODUCTION

Toxoplasma
gondii
is an intracellular protozoan parasite which affects animals and humans causing encephalitis, chorioretinitis and death. The tachyzoite has structures that allow it to perform cell invasion. During the invasion, the tachyzoite adheres to the target cell membrane through proteins MIC that come from micronemes and then projects a highly dynamic structure called conoid [1,2]. Subsequently the parasite enters the cell and is housed in a parasitophorous vacuole proliferating by endodyogeny. After several cycles of replication, the parasites leave the infected cell by mechanisms such as conoid extrusion and secretion, which enables the parasite to invade neighboring cells. This replication cycle results in cell destruction and is responsible for the major clinical manifestations of toxoplasmosis. Ethanol is a well‐characterized inductor calcium‐dependent events in different cell models. In this work we used ethanol and we analyze the asociation between conoid extrusion and MIC secretion.

METHODS

Cell culture
. The cell model used for our invasion, proliferation and egress assays was HEP‐2 cells (human epithelial laryngeal carcinoma cells) (Hep‐2,ATCCCCL‐23).

Parasites
and conoid extrusion
. The RH strain of
T. gondii
was maintained in BALB/c mice, purified and incubated with 0.5M ethanol, fixed, and processed for electron microscopy and immunofluorescence.

Eletronic
microscopy
. The samples were fixed, dehydrated and infiltrated in resin or processed for scanning electron microscopy. Thin sections obtained were contrasted with uranyl acetate and examined in a transmission electron microscope (Jeol 2000 EX).

Immunodetection
. MIC2 protein detection was performed without permeating in extracellular tachyzoites. The samples were fixed and incubated with specific primary antibodies and analyzed using confocal microscopy.

RESULTS

The exposure of tachyzoites to ethanol induces reversible conoid extrusion, clearly observed the projection of the apical part of tachyzoites (Fig. 1). We also observed the presence of vesicle components accumulating in the back of tachyzoites, associated with some changes in the plasma membrane. We could detected that these vesicular components presenting on its surface was always associated with the extruded conoid(Fig. 1). For immunofluorescence, we showed that the secreted products during extrusion conoid micronemes come from. These results suggest a similar mechanism of secretion and induction for extrusion.
Sn
catalysts and
Sn
dopants for
Ge
Nanowire Growth
Authors: Marshall Ann F. , Chan Gerentt , Meng Andrew C. , Braun Michael , McIntyre Paul C.
Published: December 2016

Vapor‐liquid‐solid (VLS) growth of semiconductor nanowires has been extensively studied as an avenue to control composition, crystallinity, strain and doping of nanowires for potential applications in Si device processing. As evidenced by studies of the Ge nanowire/Au catalyst system, an advantage of nanowire growth as compared with bulk synthesis is the generally large departure from equilibrium, which allows for supersaturation and undercooling during growth
1,2
and formation of metastable structures and compositions.
3,4,5
Non‐equilibrium growth also offers possiblities for metastable solute trapping of dopants. Here we show results using Sn both as the growth catalyst, and as a dopant in the nanowires using SnCl
4
as the dopant gas. Incorporating Sn into the nanowires offers the possibility of increasing the carrier mobility, and of achieving a direct band‐gap for efficient light absorption and emission by pushing the concentration of Sn in Ge beyond the equilibrium value.
6
Fig. 1 shows the morphology and single crystallinity of nanowires grown using Sn as the catalyst. The catalyst, formed by evaporation of thin Sn layers on Ge substrates, produces wires with typical diameters <10 nm and a <110> growth axis. The Ge‐Sn binary eutectic liquid occurs at a composition that is close to pure Sn so that the expected composition of the liquid droplet from which the Ge NW grows is very tin‐rich. However, the tip that remains after growth degrades rapidly in the electron beam, preventing reliable EDS analysis. Nanowires that are grown using Au catalysts, with Sn added via the introduction of SnCl
4
gas partway through the growth process, are shown in Fig. 2. Sn is incorporated into the liquid catalyst droplet, which enlarges the catalyst and the nanowire diameter. During end‐of‐growth cool‐down Sn and Ge are rejected from the catalyst resulting in tapered ends, with Au remaining at the tip. Fig. 3 and 4 show the effect of higher and lower concentrations of SnCl
4
gas flow respectively. Higher Sn concentration causes the Sn to precipitate out at the surface of the nanowires. Lower Sn concentration allows the wire to form a coreshell structure with stacking faults, presumably dislocation loops, forming in the outer region. Preliminary EDS results suggest a Sn concentration of 2‐5 at%, well above the <1 at% equilibrium value. We are currently investigating the nature of the stacking faults, and the growth parameters needed to achieve high Sn concentrations while minimizing defects.
In‐situ deformation of
Ti6Al4V
in electron microscopes
Authors: Lu Xinyu , Liu Zhaoran , Chiu Yu Lung , Jones Ian
Published: December 2016

Ti6Al4V is the most widely used titanium alloy [1] and comprises two phases, viz. a hexagonally closely packed α phase and a body centred cubic β phase. For the lifing prediction of the alloy components and also for the future alloy development, it is of interest to understand the role of various interfaces such as grain boundaries and inter‐phase boundaries in the mechanical response of the alloy. The last ten years have seen the development of nano‐mechanical testing within electron microscopes on samples prepared using focused ion beam, which has not only offered the quantitative mechanical properties but also the simultaneous imaging analysis of samples containing specific structural features [2, 3]. In this work, in‐situ electron microscopy studies on miniaturised Ti6Al4V samples were used to achieve a better understanding of the plastic deformation micro‐mechanisms involved. Micron‐sized pillar samples were compressed in an SEM and the mechanical properties of both alpha and beta phases were evaluated, while sub‐micron sized pillars were compressed in a TEM to observe the dislocation activities. The nucleation of dislocations, the yielding of the samples and the work hardening observed in the samples with different sizes will be discussed.
STEM‐EELS
valence mapping and charge relaxation in
LiFePO4
cathode
Authors: Schneider‐Haefner Joseph , Su Dong , Wang Yuxuan , Fang Jiye , Omenya Fredrick , Chernova Natasha , Cosandey Frederic
Published: December 2016

LiFePO
4
has emerged as an important cathode material for Li‐ion batteries because of it stability and high rate capabilities. It is now well established that lithiation‐delithiation occurs via a two‐phase reaction. At high charge/discharge rates, the process of nucleation and growth of a two phase reaction is too slow and a non‐equilibrium single phase reaction has been proposed followed by relaxation into LiFePO
4
and FePO
4
end product phases [1]. In this study, we studied relaxation mechanisms and determined the spatial distribution of lithiated/delithiated phases by STEM/EELS spectrum imaging.

LiFePO
4
particles from partially charged or discharge electrodes were observed with a cold cathode field emission Hitachi HD2700C STEM and Gatan Enfina EELS spectrometer. The energy resolution of the combined STEM/EELS system was 0.5eV. The energy was calibrated with respect to the main O‐K peak at 539 eV. Typical EELS spectrum for LiFePO
4
and FePO
4
are shown in Fig.1a and 1b respectively. A characteristic feature of delithiated FePO
4
phase is the presence of an oxygen pre‐peak marked by an arrow in Fig.1b. This pre‐peak in non existant in LiFePO4. The existence of this O prepeak has been attributed to a transition from O 1s to 2p hybridized state with Fe 3d [2]. In addition the change in Fe valence state from LiFe
2+
PO
4
to Fe
3+
PO
4
is accompanied with a shift to higher energy of Fe‐L
3
peak position of about 1.5 eV. In this study we have quantified the existence of these two lithiated and delithiated phases from the shift in Fe‐L
3
peak energy, Fe L
3
/L
2
peak intensity ratio and from quantification of normalized O pre‐peak intensities. Measurements made from about 50 particles reveal two clusters of data with average Fe‐L
3
peak energy of 708.2 eV and 709.8 eV with O pre‐peak intensity ratio of 0.037 and 0.16 respectively. These two data clusters correspond to fully lithiated LiFePO
4
and delithiated FePO
4
phases. The spectrum images of the lithiated LiFePO
4
and delithiated FePO
4
expressed as the normalized O pre‐peak intensity are shown in Fig.2a and 2b respectively revealing uniform lithiation throughout the particles, i.e. the particles are either fully lithiated or fully delithiated in accordance with the non‐equilibrium solid solution transformation path followed by relaxation. An ADF‐STEM image taken from an area with many particles and the corresponding phase distribution map are shown in Fig.3a and 3b respectively, revealing a non‐uniform distribution of phases with agglomeration of fully lithiated and delithiated regions that include many nanoparticles clusters.
Effect of grain size, precipitation and texture on mechanical and corrosion properties of
ECAPed AZ80
Authors: Tse Yau Yau , Zhang Kaiyuan , Du Yang , Maradze Diana , Liu Yang
Published: December 2016

Magnesium alloys have increasing engineering applications due to its high specific strength, for instance in automobile, aerospace and electronics industries [1]. Grain refinement of AZ80 magnesium alloy was achieved by Equal Channel Angular Pressing (ECAP) at 573K for 1, 2 and 4 passes using Bc route [2]. Backpressure of 100MPa was applied to increase the workability and uniformity of the samples [3]. Grain diameter was reduced from 18±0.5 µm to ~6±0.5 µm and further reduced when low processing temperature of 423K was used. SEM/EBSD showed B‐fibre texture was obtained after 4 passes of the ECAP process (Figure 1). Number of passes had no significant effect on grain refinement but improve the homogeneity of the samples. Hardness was increased approximately 15% after ECAP. Corrosion resistance of the ECAPed samples was decreased due to the increase in the volume fraction of the grain boundaries and the presence of the uneven distribution of Mg
17
Al
12
precipitates [4]. Grain orientation also affected corrosion rate to some extent. The morphology and composition of the corrosion products was studied using SEM and EDX (Figure 2). The main corrosion products could be a mixture of magnesium hydroxide (Mg(OH)
2
) and magnesium carbonate (MgCO
3
).
EMCD
investigation of the Verwey‐transition in magnetite
Authors: Hetaba Walid , Stöger‐Pollach Michael
Published: December 2016

Magnetite is of interest in physics as well as in chemistry, for example because of its surface chemistry [1]. Furthermore, the investigation of the magnetic properties is worthwhile as the material exhibits a magnetic phase transition (the so‐called Verwey‐transition) at 125 K [2]. Magnetite shows a drop in its magnetisation when cooled down below the transition temperature, together with a change in electronic and structural properties. This behaviour was subject to a number of investigations [2]. However, the details of the transformation mechanism are still under discussion.

EMCD (energy‐loss magnetic chiral dichroism) is a versatile technique to investigate magnetic properties in the TEM on a nanoscale [3]. When performing EMCD on magnetite, the Fe L
2,3
‐edge, acquired using two different scattering conditions, is compared. The difference signal corresponds to the magnetic properties of the investigated material. Using current TEMs, EMCD can be performed in‐situ with a high spatial resolution, thus giving information about local changes of the magnetic moments [4].

We used a FEI Tecnai TF20 TEM, operated at 200 kV, equipped with a Gatan GIF Tridiem, and a Gatan Cryo‐Transfer sample holder to investigate magnetite samples close to the transition temperature. Thus, not the whole specimen was transformed. Figure 1(a) shows a bright field image of the investigated sample area. Due to variations from the perfect stoichiometry, regions that already transformed are visible alongside sample regions still in the high‐temperature phase. The diffraction patterns shown in Fig. 1(b,c) were acquired at two different regions, marked in Fig. 1(a), to check whether the material has already transformed.

The EMCD spectra that were recorded in the two marked sample regions are depicted in Fig. 2. It can be seen that in the non‐transformed region a strong EMCD signal of 13.5% is visible, while in the transformed region no EMCD effect is observed. This indicates changes in the local magnetic moments due to the phase transition. The results are faced to Bloch‐wave simulations [5], carried out for the same settings used in the EMCD measurements. Thus, the influence of different channelling conditions due to structural changes induced by the Verwey‐transition is compared to the measured EMCD signal. It is shown that the difference in the EMCD effect is mainly caused by local changes of the magnetic moments and not by different channelling conditions due to the structural changes.

EMCD in combination with the versatility of a wide variety of analytical techniques in a TEM is an ideal tool to gain knowledge about the magnetic properties of a material on the nanoscale. The necessary combination of experiments with simulations (as elastic scattering strongly influences the measured signal) provides insight into the mechanisms of magnetic phase transitions.
Multiscale investigation of
USPIO
nanoparticles in atherosclerotic plaques and their catabolism and storage in vivo
Authors: Maraloiu Valentin‐Adrian , Appaix Florence , Broisat Alexis , Guellec Dominique Le , Teodorescu Valentin Serban , Ghezzi Catherine , van der Sanden Boudewijn , Blanchin Marie Genevieve
Published: December 2016

Since applications of nanotechnologies for life and health sciences are booming, magnetic nanoparticles (NP) have undergone considerable development. They combine physical, chemical and magnetic properties that make them appropriate as contrast agents for diagnosis using medical imaging techniques such as Magnetic Resonance Imaging (MRI) or ultrasonic imaging. Nanometric contrast agents are made of metallic rare earth elements (Gd) or transition metal (Mn, Fe) oxide cores.

The storage and catabolism of Ultrasmall Super Paramagnetic Iron Oxide (USPIO) nanoparticles was analyzed through a multiscale approach combining Two Photon Laser Scanning Microscopy (TPLSM) and High‐Resolution Transmission Electron Microscopy (HRTEM) at different times after intravenous injection (iv) in an atherosclerotic ApoE
‐/‐
mouse model. The atherotic plaque features and the USPIOs heterogeneous biodistribution were revealed from organ's scale down to subcellular level. The biotransformation of the nanoparticle iron oxide (maghemite) core into ferritin, the non‐toxic form of iron storage, was demonstrated for the first time
ex vivo
in atherosclerotic plaques as well as in spleen, the iron storage organ. These results rely on an innovative spatial and structural investigation of USPIO's catabolism in cellular phagolysosomes. This study showed that these nanoparticles were stored as non‐toxic iron compounds: maghemite oxide or ferritin, which is promising for MRI detection of atherosclerotic plaques in clinic using these USPIOs [1].
Retrieving atomic structure from dynamical rocking curve measurements in both real and reciprocal space
Authors: Koch Christoph , Van den Broek Wouter , Wang Feng , Pennington Robert
Published: December 2016

Transmission electron microscopy (TEM) data, in particular high‐resolution TEM and conventional electron diffraction has the reputation of being not easily interpretable in a quantitative manner in terms of the object being probed by the fast electrons. The reason for this lies in the fact that multiple scattering makes the detected signal a non‐linear function of the scattering potential. In cases where the structure is approximately known, refinement of structure factors from convergent beam electron diffraction (CBED) data [1] or atom positions from HRTEM images [2] is possible. But the ab‐initio inversion of multiple scattering to recover the structure of an unknown object has not yet been shown to work in a routine manner for experimental data. Structure determination approaches thus typically revert to techniques which collect tilt‐averaged data, such as precession electron diffraction (PED) and scanning transmission electron microscopy (STEM) with a convergent probe. Integrating the signal over a range of different relative orientations between object and electron beam wave vector averages over different multiple scattering conditions – however, a large amount of structural information encoded in the multiple scattering signal gets lost, including valuable information about the object's 3D structure [3].

Fig. 1 demonstrates that in the case of experimental large‐angle rocking‐beam electron diffraction (LARBED) data [4], i.e. diffraction data for a large range of beam tilts (about 20 times larger than possible in conventional CBED of silicon), the projected potential can be recovered by straight forward gradient optimization from a starting guess in which all structure factors were initialized to the same value, i.e. initially, all Ug = (0.01 + 0.01i) Å
−2
[5]. Although rocking curves of only 121 diffraction spots were measured, 456 structure factors could be determined from this data, since, all possible difference vectors between reflections extracted from the diffraction pattern were also included in the dynamical diffraction calculation.

Although CBED and HRTEM are very different modes of operation of the microscope, the multiple scattering contribution to the signal in tilt series of HRTEM images allows us to retrieve the 3D scattering potential [6], where the position and height of peaks allows direct interpretation as atom species and positions (see Fig. 2). This inversion of multiple scattering is based on an interpretation of the multislice algorithm as an artificial neural network that is taught by feeding it TEM data recorded under different experimental conditions. This can be HRTEM tilt series, ptychography data sets, or scanning confocal electron microscopy (SCEM) data [7].[8]
Electron microscopy investigations of cation exchange in colloidal
P
b
S
e/
C
d
S
e nanocrystals
Authors: Yalcin Anil , Goris Bart , Fan Zaochuan , Vlugt Thijs , van Blaaderen Alfons , Vanmaekelbergh Daniel , Bals Sara , van Huis Marijn
Published: December 2016

Cation exchange is a very powerful method for creating heterogeneous nanocrystals (NCs), such as core‐shell or rod‐tip nanostructures. Here we present an overview of the experimental and simulation efforts made to elucidate the atomic mechanism underlying cation exchange in PbSe/CdSe nanocrystals.

When pure PbSe nanorods are in a colloidal solution, adding Cd precursors molecules to the solution will lead to the exchange of Pb ions by Cd ions, resulting in PbSe/CdSe core‐shell nanorods. High‐resolution high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) analysis of partly exchanged PbSe/CdSe core‐shell nanorods showed clearly defined interfaces in the nanorods, dominated by PbSe‐rocksalt/CdSe‐zinc blende {111}/{111} crystal planes [1]. However, from an atomic‐resolution 3D reconstruction of the PbSe core within such a core‐shell nanorod [2], it became clear that the interfaces are not as straight and defect‐free as they appear to be when considering only 2D projections.

The opposite process of Cd‐for‐Pb cation exchange was recorded in real time when PbSe‐tipped CdSe nanorods were heated
in‐situ
in the transmission electron microscope [3]. Stills of a movie recorded in HAADF‐STEM mode at a temperature of 180 °C are shown in Figure 1. Here the PbSe domains at the tips grew at the expense of the CdSe domains, a process driven by the evaporation of Cd (or Cd molecular complexes), as confirmed by energy‐dispersive X‐ray spectroscopy (EDX) chemical mapping. It became clear that cation exchange is a very dynamic process whereby excess Pb cations were consumed from Pb‐oleate complexes that were covering the surface of the NCs.

Finally, very recently a force‐field molecular dynamics (FF‐MD) simulation study was conducted which elucidates the role of both cation and anions in the underlying physical mechanism [3,4]. Figure 2 shows the result of a simulation whereby two types of interfaces are present in the nanorods, exhibiting very different defectiveness of the atomic planes at both sides of the interfaces.

Future opportunities for studying cation exchange, including Liquid Cell investigations and performing cation exchange of 2D of 3D assemblies of nanocrystals, are discussed as well.
Three‐dimensional electron imaging of dislocations from a single sample tilt
Authors: Oveisi Emad , Antoine Letouzey , Pascal Fua , Cecile Hebert
Published: December 2016

Linear crystal defects called dislocations are one of the most fascinating concepts in materials science that govern mechanical and optoelectronic properties of many materials across a broad range of application [1–3]. Three‐dimensional (3‐D) study of dislocation network is in principle accessible by conventional tomographic and stereoscopic techniques in Transmission Electron Microscopy (TEM) [4,5]. In these techniques in general the need to tilt the specimen for acquiring image series over large tilt ranges remain however an intricate problem, in particular when diffraction contrast or sensitivity to electron beam are involved [6].

Here, a novel method in scanning TEM (STEM) is presented that provides a reliable and fast assessment of the 3‐D configuration of dislocations using data acquired from just one sample tilt. This technique acquires a stereoscopic pair of images by selecting different ray paths of a convergent illumination in STEM mode. The resulting images are then treated with a dedicated stereovision reconstruction algorithm, yielding a full 3‐D reconstruction of dislocations arrangement. The success of this method is demonstrated by measurement of dislocation arrangements in two experimental cases.
Atomically‐resolved insight of unusual Sr‐Mn(V) oxyhydroxide
Authors: Gómez‐Recio Isabel , Torres‐Pardo Almudena , Hernando María , Varela Aurea , Parras Marina , Gonzalez‐Calbet Jose Maria
Published: December 2016

Inorganic compounds containing Mn(V) in tetrahedral coordination are known to show strong optical absorption, producing turquoise‐ to green‐colored compounds [1]. The existence of hypermanganate anion MnO
4
3‐
has been found, for example, in the Ba
5
(PO
4
)
3‐x
(MnO
4
)
x
Cl [2]. However, Mn(V) is very rarely found in oxides. The ability of Mn to be stabilized in mixed oxidation states leads in turn to interplay among spin, charge, and orbital degrees of freedom. In this sense, it has been recently reported that the electrochemical activity of the Mn(IV)/Mn(V) couple, plays a very important role in the discharge capacity of the nanostructured Li
4
Mn
2
O
5
material [3].

In this work we show the characterization of a Sr‐Mn (V) oxide with blue‐greenish color and nominal composition Sr
2
(MnO
4
)(OH) [4]. Atomically‐resolved high angle annular dark field (HAADF) images acquired in a JEOL JEMARM200cF microscope shows that the Sr‐Mn oxide crystallizes according to the apatite‐type structure [5]. In this structure, each Mn ion is coordinated by distorted oxygen tetrahedral giving rise to isolated MnO
4
tetrahedra (Figure 1). Evidence of Mn(V) was shown by high energy resolution Electron Energy Loss Spectroscopy (EELS) (Figure 2a). However, contrast variations in the hexagonal tunnels of the apatite structure suggest either the direct imaging of light OH groups in the HAADF image or the presence of a heaviest element in these positions. Image simulation is used to elucidate the origin of such contrast because of the low stability of the sample under the electron beam prevents point‐by‐point spectroscopic analysis (Figures 2b and 2c).
Chemical reactivity between sol‐gel deposited Pb(Zr, Ti)O3 layers and their GaAs substrates
Authors: Meunier Benjamin , Largeau Ludovic , Regreny Philippe , Penueals José , Bachelet Romain , Vilquin Bertrand , Wague baba , Saint‐Girons Guillaume
Published: December 2016

The combination on the same wafer of materials having different physical properties is a key challenge. In particular, functional oxides of the perovskite family are very attractive for applications in the micro‐optoelectronic field since they present a variety of physical properties (ferroelectricity, ferromagnetism, superconductivity, high Pockels coefficients, …). They are classically grown on SrTiO
3
(STO) substrates, but molecular beam epitaxy (MBE) allows their epitaxy on Si and GaAs platforms, providing that specific interface engineering strategies are used. However, even in this case, chemical reactions between the growing oxide and the substrate strongly impact the epitaxy process and impose using specific growth procedures that are detrimental to the oxide crystal quality. Amongst the oxides that can be integrated on STO templates, Pb(Zr,Ti)O
3
(PZT) is of particular interest because of its high remanent polarization (P
r
), its low coercive field (E
c
) and its outstanding piezoelectric properties

In this contribution, we report the study of sol‐gel prepared PZT thin films on STO/GaAs templates grown by molecular beam epitaxy (MBE). The spin‐coated layers were calcined at 350°C under air for 5 minutes to dty the sol film. The crystallization of the PZT have been studied in function of post‐annealings at three different temperatures ranging (405°C, 420°C and 510 °C). High angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) and energy dispersive X‐ray spectroscopy (EDXS) have been used to study the structure and the chemical composition of the layers. High resolution HAADF images coupling to geometrical phase analysis (GPA) show that PZT is locally strained on the STO/GaAs during the first steps of crystallization. Complete crystallization of the PZT laeyrs cannot be achieved due to a significant oxidation of the GaAs substrate which enhances As diffusion through the structure and further formation of parasitic PbAs and SrAs compounds.
Addressing Pseudo‐Symmetric Misindexing in
EBSD
Analysis of gamma ‐
TiAl
with High Accuracy Band Detection
Authors: Schmidt Niels‐Henrik , Mansour Haithem , Goulden Jenny , Palomares‐García Alberto , Muñoz‐Moreno Rocio
Published: December 2016

Technological developments in EBSD has enabled great improvements in indexing reliability and accuracy [1]. However, some individual phases continue to pose challenges, especially those that present extremely similar Kikuchi patterns to the EBSD camera for different crystallographic orientations. This phenomenon is called “pseudosymmetry”, as it commonly involves relatively high intensity bands in certain patterns with an apparent higher symmetry than the crystal structure actually possesses. In many cases, only very slight differences in inter‐band angle separate candidate solutions, and only robust and accurate band detection may identify the correct one among them.

Conventional Hough‐based band detection methods are sufficiently accurate for most indexing requirements. High accuracy Hough transform settings improves band detection accuracy, and is useful in mitigating pseudosymmetric misindexing [2]. However, these settings result in reduced data acquisition speeds, and may not completely eliminate pseudosymmetry problems.

New band detection refinement methods improve EBSD indexing performance for some of the most chronic cases, at reasonable data acquisition speeds. Higher accuracy band detection is achieved by iteratively comparing the positions of simulated bands with bands in the actual Kikuchi pattern image, using expected versus actual band widths and, accounting for the hyperbolic shape of bands. In addition to delivering high precision crystallographic orientation data and helping discriminate different phases with similar Kikuchi patterns, this method is sufficiently sensitive to resolve fine differences in inter‐band angle to nearly eliminate many cases of pseudosymmetric misindexing.

An important application is γ‐TiAl alloys, which are promising jet engine turbine materials, combining low density with good oxidation and creep resistance. The high temperature deformation behaviour of these alloys must be better understood before they can widely replace the higher density Ni‐base superalloys; For example, an improved knowledge of the fundamentals of crystallographic slip and its interaction with the γ/γ lamellar variants could be critical. Pseudosymmetry, however, is a major issue here (Fig. 1a), arising from γ‐TiAl's close tetragonal c:a unit cell parameter ratio of 1.018, giving the Kikuchi patterns it generates a pseudo‐cubic configuration, resulting in indexing inaccuracies. These mistakes show the same misorientations as boundaries between real γ‐TiAl lamellae, causing problems in revealing the true microstructure. Phase discrimination between coexisting γ(TiAl) andα
2
(Ti
3
Al) phases is also difficult (Fig. 2a). Application of a new, automated band detection tool and system knowledge of the confronting pseudo‐symmetry almost completely eliminate these issues (Fig.'s 1b and 2b) and are used in real‐time during data collection, at normal acquisition speeds.
Mn 4+ holes localization around divalent cations in low‐hole doped manganites
Authors: Ruiz‐Gonzalez Luisa , Cortés‐Gil Raquel , Gonzalez‐Merchante Daniel , Alonso Jose Maria , Gonzalez‐Calbet José M.
Published: December 2016

Magnetotransport properties of low Ca doping La
1‐x
Ca
x
MnO
3
perovskite related manganites have been described on the basis of the existence of isolated metallic (M)‐double exchange (DE) ferromagnetic (FM) nanoclusters embedded in a long range insulating (I) matrix. Taking into account the DE mechanism, long range FM order and M conduction are expected across the La
1‐x
Ca
x
MnO
3
series. However, the real behaviour differs from this ideal description as an I state is observed for x£0.2. This can be understood by assuming that the electronic itinerancy is not spread to the whole sample but restricted to FM‐M clusters embedded in the matrix. In spite of much effort devoted to their study, the behavior at atomic level is far from being satisfactorily explained. Here we report direct experimental evidence in La
0.9
Ca
0.1
MnO
3
, by means of STEM‐EELS characterization in a JEOL ARM 200 cFEG aberration corrected microscope, that holes and divalent cations tend to be closely correlated in the sample. Figure 1 shows a characteristic HAADF image and corresponding EELS compositional maps showing a heterogeneous distribution of La and Ca. The study of the Mn oxidation state suggests a shift towards Mn
3+
or Mn
4+
when the environment is La and Ca richer, respectively (figure 2).
[1]
This Ca‐hole attraction results in the formation of small clusters confined to one or two unit cells and would provide an answer to the long‐standing problem of the origin of the electric and magnetic behavior in the low calcium region of the La
1‐x
Ca
x
MnO
3
system.