bluuebunny/crossref_metadata_2025 huggingface dataset:
Lanthanide distribution in
NaLuF4:Gd,Yb,Er
upconversion nanocrystals by
EFTEM
and
EELS
Authors: Lu Elsa , Pichaandi Jothirmayanantham , Tong Lemuel , Winnik M. A.
Published: December 2016

Lanthanide‐doped nanoparticles (NPs) have gained interest within the last decade due to their photon upconversion properties. Upconversion is a multi‐photon process in which two or more lower energy photons are converted to a higher energy photon by step‐wise energy transfer between an absorber ion and an emitter ion (1). These upconversion nanoparticles (UCNPs) are a promising alternative to traditional organic fluorphores and quantum dots in the area of bioimaging because their excitation wavelength (980 nm) lies within an optical window where there is the least absorption and scattering by biomolecules (lower background signals), and they do not exhibit photobleaching or photoblinking.

One of the main challenges limiting their application is a trade‐off between luminescence intensity and size of the NP. Smaller NPs exhibit less toxicity as they can be excreted through the urinary system, and they are better for intracellular imaging as the smaller size prevents interference with molecular trafficking within the cell, pharmacokinetics, and protein function; however, upconversion luminescence intensity decreases with size due to, in part, surface quenching from the presence of surface defects (2).

Few reports exist on the synthesis of sub‐10 nm UCNPs, and even fewer exist on the synthesis of sub‐5 nm UCNPs that show visible upconversion emission (3‐6). We have recently synthesized two sizes of NaLuF
4
:Gd, Yb, Er UCNPs (ca. 4 nm and ca. 12 nm), through a facile one‐pot method, that both show bright upconversion luminescence upon excitation by a 980 nm laser.

In order to better understand their luminescent properties, we have used energy filtered transmission electron microscopy (EFTEM) and electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) to investigate the distribution of the different lanthanides within the nanocrystal. Measurements were performed on an HF‐3300 instrument operating at 300 kV.

The results suggest that Lu is enriched in the outer (shell) region of the nanocrystal, while Gd is enriched in the inner (core) region of the nanocrystal (Figure 1). Based on these results, we propose that the Lu shell protects the upconverting core from surface quenching, thereby allowing even 4 nm NPs to show upconversion luminescence. We also propose that the formation of the core‐shell structure is mediated by the way the individual lanthanides nucleate in solution – Gd nucleates first to form the core, while Lu nucleates later to form the shell. This proposal is also in accordance with previous reports on the nucleation of lanthanide NPs (7).

EFTEM and EELS in STEM are shown to be valuable tools to characterize the structure of lanthanide‐doped UCNPs and to determine structure‐property relationships, and can aid in the further development of these materials for various applications.
Quantitative approach to twin boundaries in
Cu3Pt
nanoparticles
Authors: Drazic Goran , Zepeda Francisco Ruiz , Lautar Anja , Jovanovic Primoz , Bele Marjan , Gaberscek Miran
Published: December 2016

Imperfections of the crystal structure, such as partial ordering, formation of core‐shell and Pt rich skin improve the electro‐catalytic activity of Pt‐based intermetallic nanoparticles used as a catalyst material in low‐temperature fuel cells [1, 2]. Impact on the surface reactivity by the core–shell nanostructures could be explained with the induced lattice strain [3, 4]. In the case of twinned structure, the lattice strain significantly influences the interatomic distances and consequently the energy levels of bonding electrons, which determines the catalytic, electrical and optical properties [5].

In this work we studied the local structure and chemistry near the lamellar (111) twin boundaries in sol‐gel prepared intermetallic Cu
3
Pt nanoparticles, specially tailored [6] to exhibit high amount of polysynthetic twins. Using HAADF‐STEM (Jeol ARM 200 CF) imaging in conjunction with image simulations we extracted the information about chemical composition and strain on the level of individual atom columns. Experimental Cu‐Pt column intensities were measured using modified approach by LeBeau and Stemmer [7]. Experimental intensities were compared to the intensities from simulated images where chemical composition and thickness of individual Cu‐Pt columns were varied.

In Fig. 1a a LAADF micrograph (20 – 80 mrad) of (111) twin boundaries in 60 nm sized Cu
3
Pt nanoparticle is displayed. Enlarged image of lamellar twins is shown in Fig. 1b. The bright contrast around planar defect indicates strain. From HAADF (70 – 180 mrad) images (Fig. 2a) the intensity ratios were extracted and correlated to the average chemical composition of individual columns. Fig. 2b is the distribution of normalized intensities of atomic columns around the twin boundary where a significant decrease in the intensity near the boundary indicating depletion in Pt content. For conversion of intensities to Pt/Cu ratio HAADF images were simulated using a multislice method with frozen phonon approximation (QSTEM software). Using DFT calculations the energy and lattice relaxation were calculated as a function of local chemical composition near the twin boundaries. Taking into account the local strain and chemical composition variation the influence of twin boundaries on electrocatalytic properties will be discussed in details.
Nanoparticle Structure from Genetic Algorithm Refinement Against Quantitative
STEM
Data
Authors: Voyles Paul , Song Zhewen , Zhou Dan , Xu Zhongnan , Yankovich Andrew , Morgan Dane
Published: December 2016

We have developed a structure refinement method based on genetic algorithm optimization to create structural models of individual nanostructures based on quantitative scanning transmission electron microscopy (STEM) data [1]. We defined a cost function
C
for a structural model
s
, as
C
(
s
) =
E
(
s
) + αχ
2
[
I
(
s
),

I
exp

], where
E
is the simulated potential energy of
s
, χ
2
is goodness‐of‐fit between the experimental STEM data

I
exp

and the simulated STEM data
I
(
s
), and αis a weighting parameter. A genetic algorithm (GA) is used to minimize
C
over structures
s
, resulting in a structure that is both at a local minimum in the (simulated) energy and in good agreement with experimental data. The advantage of combining the energy and goodness‐of‐fit to experiments over optimization on just one or the other is the ability to refine structures that are not at a global energy minimum (like most nanoparticles) from experimental data that does not completely constrain the three‐dimensional structure (like a STEM image in one orientation).

We have validated the approach and implementation using simulated experimental data from a metastable, 309‐atom Au inodecahedron, as shown in Figure 1. Figure 1(a) is the test structure, and Figure 1(b) is the simulated STEM image from that structure. The energy is calculated using an embedded atom method empirical potential for Au. Figure 1(d) shows the evolution of the two terms in the cost function and the total cost function over the course of the optimization. Neither term decreases monotonically for the entire optimization, but the entire
C
(
s
) does. Figure 1(c) shows the STEM image of the refined structure after 2200 generations, which is an essentially perfect match for the input image in (b). Figure 1(e) shows that the 3D structures are also a perfect match, with a maximum difference in atomic positions of 0.02 Å.

As a first test, we have refined the structure of a ˜6000 atom colloidal Au nanoparticle, as shown in Figure 2. Figure 2(a) is the experimental STEM image of the particle [2]. Figure 2(c) shows the evolution of the cost function as it converges over 4000 generations to reach the final structure in Figure 2(b). In this case, the optimization was allowed to change the number of atoms in the structure as well as their position. The result faithful reproduces the image of the sample, including the outline and the twin boundary. Figure 2(d) shows the displacement of matching atomic columns in the two images. The large displacements near 0.3 Å arise from surface atoms which are not well‐imaged in the experiment due to surface atom mobility under the electron beam, but which are recovered in the refined model. Additional applications to Pt and Pt‐Mo catalysts will be discussed.
Analytical electron microscopy characterization of light‐emitting diodes based on ordered
InGaN
nanocolumns
Authors: Torres‐Pardo Almudena , Gačević Žarko , García‐Lepetit Noemi , Müller Marcus , Metzner Sebastian , Bengoechea‐Encabo Ana , Albert Steven , Bertram Frank , Veit Peter , Christen Juergen , Calleja Enrique , González‐Calbet Jose M.
Published: December 2016

Self‐assembled nanocolumns (NCs) with InGaN/GaN disks constitute an alternative to conventional light emitting diodes (LED) planar devices [1]. However, their efficiency and reliability are hindered by a strong dispersion of electrical characteristics among individual nanoLED. Polychromatic emission derives from an inhomogeneous distribution of indium concentration due to the inherent tendency of InGaN alloys to develop composition fluctuations as a function of the polarity of the growth crystallographic planes [2]. The recent development of selective area growth of NCs by molecular beam epitaxy has allowed the achieving of highly homogeneous and controllable GaN/InGaN NCs with improved crystalline quality and higher control over the indium distribution [3].

In this work, we present the characterization performed on LEDs based on ordered NCs with InGaN active disks (figure 1). The detailed structural characterization of the nanostructures has been performed by scanning transmission electron microscopy (STEM) carried out on an aberration‐corrected JEOL‐JEMARM200 microscope. High crystal quality of the NCs is set by the analysis of atomically‐resolved high angle annular dark field (HAADF) images. The indium distribution within the InGaN disks is studied by EDS elemental mapping while the polarity of the semiconductor NCs is followed by locating the nitrogen atomic columns in annular bright field (ABF) images while (figure2). Direct correlation of the optical and structural properties on a nanometer‐scale was achieved using low temperature cathodoluminescence (CL) spectroscopy in an FEI STEM Tecnai F20 [4].
Multi‐color correlative PALM/STORM and electron tomography reveals micro‐domain organization of endosomes
Authors: Brouilly Nicolas , Kalaidzidis Yannis , Verbavatz Jean‐Marc , Zerial Marino
Published: December 2016

Correlative Light and Electron Microscopy (CLEM) is a method of choice to demonstrate the localization of a protein in a given organelle while revealing the ultrastructure of the organelle. However, a long‐lasting problem in CLEM is the mismatch of resolution between modalities. As it becomes more and more clear that most organelles show a sub‐compartmentalization of their membranes into distinct micro‐domains, the visualization of such micro‐domains at the molecular level using CLEM is a potential strategy to elucidate the molecular basis underlying the ultrastructure of organelles.

We developed a workflow that combines multi‐color Single Molecule Localization Microscopy (SMLM) and electron tomography on the very same sample. We applied this approach to study the compartmentalization of early endosomes in micro‐domains. As proof of principle, we visualized Transferrin and EGF as models of cargo molecules that follow the recycling and degradative route, respectively. Using single molecule CLEM tomography, we could visualize Transferrin and EGF molecules in distinct morphological compartments within the same endosome (see figure). We could also demonstrate the localization of Rab5 in micro‐domains on the globular membrane of the endosome and its exclusion from the tubular parts. Beyond endocytosis, this method can be applied to any biological question requiring both multi‐molecule nanoscale localization and 3D ultra‐structural information of the very same sub‐cellular structure.
Revealing nanoscale morphology of organic solar cell blend by analytical electron microscopy
Authors: Sedighi Mona , Löffler Markus , Zschech Ehrenfried
Published: December 2016

Organic photovoltaic as future technology with cost‐efficient production and printability is a promising research field. New materials and studying improved synthesis techniques over the past years lead to efficiencies of more than 13 % by organic solar cell [1]. In order to improve device efficiencies, the conversion rate of photo‐generated excitons to electron‐hole pairs should be increased. This critical aspect, depends on the complex morphology of distributed donor and acceptor materials.

In this work we resolve the morphology of an organic solar cell, where the active layer is a blend of two small molecules; ZnPc (ZnC
32
H
18
N
8
) as donor and C
60
as acceptor
.
The investigation of nanoscale morphology and phase distribution is conducted using Energy Dispersive X‐ray spectroscopy (EDX) and Energy selective Backscattered (EsB) imaging in SEM [2]. The results are confirmed using analytical TEM.

The unique aspect of this work is substituting complicated TEM method by SEM with these advantages:

• Possibility of resolving chemical composition in a real solar cell in contrast to commonly deposited blend layer on TEM grids, that allows to study the complete stack of glass substrate, Indium tine oxide (ITO) electrode, electron and hole transport layers and aluminum top electrode.

• No influence of thick (90 nm) layer of heavy ITO, which is under the donor acceptor blend layer, on the TEM study.

• Gaining insight into the height distribution and roughness beside the lateral distribution (as in TEM)

To correlate morphology and material contrast we combined EDX data of the pure materials (composition of specific structure) with EsB detector mapping (strong contrast, unknown corresponding material). EDX spectrum of two material phase is shown in Figure 1, the same features where imaged by EsB detector, as it can be seen in Figure 2. Taking advantage of this combined data, we overcome poor EDX lateral and depth resolution. Since exciting Zn‐K using high energy electrons, deteriorates the spatial resolution and damage the material.

Furthermore, we manage to observe the phase morphology, despite the very close mass average and quite similar chemical composition of the two phases.

In principle the technique can be extended to 3D mapping by use of slice‐and‐view approaches. Finally, our analytical TEM (EELS and EDX) investigations proved the corresponding morphology of each phase.
Accommodation of oxygen deficiency in
L
a0.5
C
a2.5
M
n2O7‐d and
L
a
S
r2
M
n2O7‐d Ruddlesden‐Popper Manganites
Authors: Cortés‐Gil Raquel , Gonzalez‐Merchante Daniel , Alonso Jose M , Ruiz‐Gonzalez Luisa , Gonzalez‐Calbet Jose M
Published: December 2016

Ruddlesden‐Popper (RP) (AO)(ABO
3
)
n
mixed oxides have recently attracted considerable insight
1
as a consequence of their potential for low‐dimensional physic arising from their structural configuration which results from the ordered intergrowth between n ABO
3
perovskite (P) blocks and one AO rock‐salt (RS) layer. This ordered arrangement of different chemical and structural unities induces intriguing behaviours that can be modified by controlling the number of intergrowing unities. For instance, colossal magnetoresistance and ferromagnetic ordering have been reported by Moritomo
2
et al
in n=2 La
2‐2x
Sr
1+2x
Mn
2
O
7
system. However, it should be noticed the difficulty to stabilize pure high ordered terms, which are frequently obtained as disordered intergrowths between the basic unities. We have prepared and characterized ordered n=2 terms for the La
0.5
Ca
2.5
Mn
2
O
7
and LaSr
2
Mn
2
O
7
compositions. For instance, figure 1 shows characteristic HAADF and ABF images atomically resolved for the strontium sample. In addition, compositional variations at the anionic sublattice are scarce compared to other manganese related perovskite systems in which different superlattices have been described as a consequence of the ordering on non‐occupied oxygen positions. In this work, we have also focused on the stabilization and characterization of new oxygen deficient La
0.5
Ca
2.5
Mn
2
O
7‐
d
and LaSr
2
Mn
2
O
7‐
d
phases.
3
The ensemble of XRD, ND, SAED, HREM and aberration corrected HAADF, ABF images as well as EELS techniques allows proposing a topotactic reduction process in both systems through different oxygen sites, apical and equatorial, for the strontium and calcium samples, respectively.
Characterization of calcifications in human kidney by spectromicroscopy at the nanometer scale
Authors: de Frutos Marta , Gloter Alexandre , Bazin Dominique , Verpont Marie‐Christine , Daudon Michel , Letavernier Emmanuel , Stephan Odile
Published: December 2016

Randall's plaques are calcium phosphate deposits, at the origin of kidney stones. To date, little is known about the mechanisms involved in their formation. µFTIR (µFourier Transformed InfraRed spectroscopy) on samples from different kidneys indicate the presence of carbo‐apatite, whitlockite and their co‐existence with amorphous calcium phosphate phases. In the present study, our aim was to localize Randall's plaques at the early stages of their formation and to characterize their composition and crystallinity as a function of their localization, at the nanometer scale. Small pieces of papilla tip from healthy papillae of human kidneys were chemically fixed and embedded in epoxy resin. Ultrathin sections were analyzed by Transmission Electron Microscopy and Electron Energy‐Loss Spectroscopy (EELS). Nano‐calcifications were identified within vesicles, in many cases in close contact with collagen bundles (figure 1). These vesicles whose role and nature is to be determine could be the first step toward plaque formation. Selected Area Electron Diffraction (SAED) evidence that microvesicles contained a few nanocrystals whose diffraction pattern is compatible with the presence of crystalline apatite or whitlockite. Basic maps of light elements of biological interest (Ca, P, N, O) confirm that dense deposits are mainly composed of CaP (figure 1). To go further, data were decomposed using the Hyperspy open source software for principal components analysis (PCA). The fine structure of the different EELS characteristic signalsallows to investigate the composition of the nano‐calcifications in order to try to discriminate between the various phases identified by µFTIR (carbo‐apatite, whitlockite, amorphous calcium phosphate).
Ion implantation into two‐dimensional materials for electronic tailoring ‐ observing the behaviour of individual implanted atoms
Authors: O'Connell Eoghan , Kardynal Beata , Tu Jhih‐Sian , Winkler Florian , Hofsaess Hans , Kepaptsoglou Demie , Ramasse Quentin M. , Amani Julian Alexander , Zan Recep , Bangert Ursel
Published: December 2016

Controlled manipulation of materials on the atomic scale is a continuing challenge in physics and material science. Doping of two‐dimensional (2D) materials via low energy ion implantation could open possibilities for fabrication of nanometre‐scale patterned devices, as well as for functionalization compatible with large‐scale integrated semiconductor technology. High resolution imaging and spectroscopic analysis of these electronic dopants at the atomic scale is fundamental for understanding sites, retention, bonding and resulting effects on Fermi level shifts. This has been made achievable with the advent of aberration corrected transmission electron microscopy (TEM) and scanning TEM, as well as low‐loss electron energy loss spectroscopy (EELS).

We show for the first time directly that 2Ds can be doped via ion implantation. Retention is in good agreement with predictions from calculation‐based literature values [1], as are initial results of the sites of dopants and their influence on the band structure of surrounding atoms. We present results of N‐ and B‐implantation in graphene [2,3] as well as current progress with ion‐implantation in TMDCs.
Atomic relaxation in ultrathin fcc metal nanowires
Authors: Roy Ahin , Müller Knut , Kaneko Kenji , Rosenauer Andreas , Weismüller Jörg , Singh Abhishek Kumar , Ravishankar N
Published: December 2016

Capillary forces affect the atomic structure, and can lead to radically different atomic configuration in nanoscale compared to bulk counterpart. In this work, we demonstrate such a phenomenon in ultrathin Au nanowires. These Au nanowires can be fabricated by a simple wet‐chemical approach, using oleylamine as a capping agent facilitating the anisotropic crystal growth. Detailed investigation of the atomic structure shows that the close‐packed plane normal to the [111]‐wire axis undergoes wrinkling leading to the formation of a saddle surface
1
. This phenomenon was captured in
ab initio
simulations (Fig. 1) and was corroborated with aberration‐corrected transmission electron microscopic studies (Fig. 2). The reason of such relaxation can be attributed to the anisotropic surface stress of the bounding facets leading to out‐of‐plane displacement of the atoms. In terms of electronic structure, such atomic scale structural relaxation forces the
d
‐band of the Au nanowire towards higher energy, and opens up a tantalizing possibility of using them as nanoscale sensors
2
, as electronic states near Fermi energy become available for hybridization.

Furthermore, we have generalized this phenomenon for other FCC metal nanowires such as Cu, Ag and Pt. Our recent simulations show that the strain in the FCC nanowire exhibits a systematic variation with the resultant stress along the nanowire orientation (Fig. 3). These results shed light on the atomic structure of FCC nanowires and open up a possibility of experimental investigation of the atomic structure for other FCC nanowires in detail.
HR(S)TEM
,
EELS
and Raman spectroscopy analysis of a
LCMO
and
PCMO
films
Authors: Roddatis Vladimir , Fischgrabe Florian , Merten Sebastian , Ifland Benedikt , Moschnyaga Vasily , Jooss Christian
Published: December 2016

Interplay between charge, orbital and magnetic orderings in perovskitemanganites
R
1‐x
A
x
MnO
3
(
R
– rare earth,
A
– alkaline element) is of fundamental interest to describe electronic correlations in transition metal oxides. In particular, the overdoped systems with x > 0.5 are a part of the phase diagram that is less explored as compared to the underdoped manganites (x~0.3), where colossal magnetoresistance is observed. Until now, the overdoped manganites were studied manily as the polycrystalline pellets or powders. Here we report on the study of highly doped heteroepitaxial La
0.25
Ca
0.75
MnO
3
/MgO films grown by metalorganic aerosol deposition (MAD) technique on MgO(100) substrate. Cross‐sectional and plan‐view TEM specimens were prepared by mechanical polishing followed by Ar
+
ion milling.

TEM studies reveal that the La
0.25
Ca
0.75
MnO
3
film consisted of domains with three different orientations (Fig. 1) described by epitaxial relationships with respect to the MgO substrate as: 1) [001]
MgO
||[010]
LCMO
and [100]
MgO
||[10‐1]
LCMO
; 2) [001]
MgO
||[‐101]
LCMO
and [100]
MgO
||[010]
LCMO
; 3) [001]
MgO
||[‐101]
LCMO
and [100]
MgO
||[101]
LCMO
. The domain interfaces are CaO layers as revealed by atomic resolution STEM imaging and EELS measurements (Fig. 2). These defects cause the appearance of streaks visible in the ED patterns.

Upon the cooling down to the temperature below 140K the material demonstrates a structural transition manifested by the elongation of diffraction spots (marked in Fig. 3 with arrows) and appearance of crosses along [10‐1]* and [010]* directions in the selected area diffraction patterns taken from plan‐view specimens (see, for example, selected spot in Fig.3). These changes evidence the formation of charge‐ordered (CO) domains, however their size is rather small due to the presence of numerous defects and compositional variations. The CO transition is also visible in the Raman spectra, which were recorded in four different polarizationconfigurations as shown in Fig.3. Three additional lines pop up at 521cm
‐1
, 646 cm
‐1
and 707 cm
‐1
and the P2
1
/m structure can be identified as a new emerging structure when the sample undergoes CO‐transition.

As a reference system we have also studied heteroepitaxial thin films of Pr
0.65
Ca
0.35
MnO
3
deposited on single crystalline SrTiO
3
substrates by sputtering deposition technique. Upon cooling this model system undergoes a phase transition to the CO state, which has been extensively studied, e.g. [1,2]. This CO state is revealed by the appearance of 2a and 2c superstructure reflexes in the diffraction pattern, and two emerging peaks at 650 cm
‐1
and 709 cm
‐1
which can be seen (Fig. 4) when the temperature falls below CO‐transition temperature.

Thus, besides the electron diffraction experiments Raman spectroscopy can also be used as a powerful tool to visualize CO‐transitions that are often accommodated by structural transitions.
STEM
optical sectioning for imaging screw dislocation core structures
Authors: Hernandez‐Maldonado David , Yang Hao , Jones Lewys , Gröger Roman , Hirsch Peter B , Ramasse Quentin M , Nellist Peter D
Published: December 2016

The introduction of spherical‐aberration correctors in STEM has allowed an improvement in spatial resolution up to the sub‐angstrom scale also 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. This technique has already 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 associated with 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 a mixed [c+a] dislocation in GaN by imaging a dislocation lying transverse to the electron beam [3].

Here we show how optical sectioning in high‐angle annular dark‐field (HAADF) STEM imaging conditions can be used to image the core structure of screw dislocations at atomic resolution. In particular, we evaluate using simulations whether the edge and screw displacements associated with the delocalization of ½[111] screw dislocations in body‐centered cubic (BCC) metals [4] can be detected. In Figure 1 we show that the helicoidal displacements around a screw dislocation can be imaged with the dislocation lying transverse to the electron beam by optically sectioning the plane containing the dislocation.

In order to reveal how the edge components of the dislocation contribute to the observed contrast we have created two atomistic models, one using the anisotropic linear‐elastic displacements around the dislocation, which is not capable of modelling the core delocalisation, and the other using the core structure relaxed using the Bond Order Potential for W which does predict a delocalised core. Figures 2 a) and b) show the respective HAADF simulated images. Figure 2 c) is the RGB image made from the (101) component of the Fourier Transform (FT) (shown in Figure 2 d)) of both images. It is possible to observe that the shifts in this Fourier component occur along two distinct lines lying parallel to [111]. The superposition of both filtered images shows that there is a discrepancy on both sides of the core between both models. It is therefore apparent that the delocalisation of the core can in principle be detected using electron‐optical sectioning [5,6].
Time‐resolved cathodoluminescence in a scanning electron microscope
Authors: Meuret Sophie , Zeijlemaker Hans , Neu Elke , Appel Patrick , Maletinsky Patrick , Polman Albert
Published: December 2016

The past decade has witnessed enormous progress in the design and fabrication of nanostructured materials with unique optical properties. These tiny physical structures allow geometrical control over the electromagnetic shape of light well below its own wavelength. A large variety of applications for this field of subwavelength optics known as nanophotonics have already begun to emerge, including solar photovoltaics, chemical sensing, quantum cryptography, and LED lighting. Many fundamental optical interactions are hidden beneath the diffraction limit, however, which makes the retrieval of subwavelength optical behavior impossible with standard microscopy. An alternative approach, known as cathodoluminescence (CL) [1], is to probe nanostructures with an electron beam, which excites optical resonances and transitions and then collects the emitted light. It has recently been used to characterize plasmonic structure [2] or III‐N heterostructures [3]. Thanks to a very efficient CL system, in a scanning electron microscope (SEM), working in free space, we can directly probed many key dimensions of light – including intensity, angle, polarization, and frequency – with ~10 nm resolution [4].

In this presentation we will show how we can use a blanker to modulate a continuous electron beam to generate electron pulses with a resolution of ~10 ns. This approach has several advantages compared to alternatives such as a laser driven pulsed electron gun [5]. It can be easily implemented in any SEM, it requires few experimental modifications, it does not require a dedicated microscope and the pulsed electron beam can be controlled at will (pulse duration and repetition rate). We will show how this pulsed electron gun can be used to explore the dynamics of a single photon emitter, such as a rare earth ion or the NV0center of nano‐diamond. An HBT interferometer allows recording of the autocorrelation function of the CL signal [6][7]. Combining secondary electron images with measurements of the CL spectra, emission polarizations, lifetimes and second order correlation function, we are able to fully characterize optically active systems at the nanometer scale.
Snapshots of co‐translational protein targeting complexes
Authors: Jomaa Ahmad , Boehringer Daniel , Leibundgut Marc , Ban Nenad
Published: December 2016

The membrane targeting and insertion of proteins is facilitated by a conserved machinery that acts concomitantly with translation. Nascent proteins are recognized by the signal recognition particle (SRP) and transferred to the SRP receptor (SR) on the membrane. A series of conformational rearrangements of SRP and SR coordinates the handover of the nascent chain to the translocon, which inserts proteins into the membrane or facilitates their secretion. Here we used single particle cryo electron microscopy to obtain snapshots of co‐translational protein targeting complexes at near atomic resolution [1]. We observe conformational changes of the SRP M domain induced by binding of the nascent chain, which are coupled to rearrangements of the SRP and SR. In addition we reconstructed a near atomic resolution structure of the translocon in complex with a translating ribosome. A detailed analysis of the ribosome interaction sites of SRP and translocon gives novel insight into the handover of the nascent chain to the translocon.
Study of amorphous silica by Electron Energy Loss Spectroscopy and electron diffraction
PDF
Authors: khouchaf Lahcen , boulahya khalid , Partha Das , Labar Janos , vis Viktoria , nicolopoulos stavros
Published: December 2016

Amorphous silica (a‐SiO
2
) has many industrial (glass former) and scientific applications. Structurally silica which has SiO
4
tetrahedral units connected with bridging O at the corners. To understand and predict interesting properties of such materials depends on the knowledge of the detailed atomic structure. The loss of structural ordering in amorphous materials, carry sharp Bragg diffraction reflections to disappear and only diffuse diffraction pattern are observed. Study of amorphous materials cannot be performed by routine crystallography and techniques like Pair Distribution Function (PDF) can be used for structure analysis using X‐Ray, neutron or electron diffraction (ED) [1]. ED related PDF (e‐PDF) in TEM has the big advantage over X‐ Ray PDF technique that allows studying local structural ordering of amorphous materials in nm scale by collecting (ED) patterns in very short time (msec instead of 15‐24 hours in X‐Ray case). Here we present how e‐PDF analysis can be used to study in detail hydrothermal reactions flow process in amorphous silica.

Amorphous silica samples were heated with water in hydrothermal container under different time scales and were studied by X‐ray, Electron Energy Loss Spectroscopy and e‐PDF. Hydrothermal reaction on amorphous silica was performed with reaction times 6, 16, 312 hours [2]. Figure 1 show the Si

L
2,3

edges for studied compounds. Si peaks are always observed but its giving rise to a broadening of spectra as the intensity increases. Such broadening in the Si L peaks is probably due to Si tetrahedral distortion, since in SiO
2
amorphous phase, distances between silicon and the four surrounding oxygen are slightly different. From the collected ED data, pair distribution function (G(r)) was calculated using dedicated ePDF software [3], developed to analyse ED patterns from amorphous and nanomaterials (Figure 2). Our analysis with ePDF has revealed that no peak (corresponding to interatomic distances) was found beyond 5 Å, which confirms that only short range ordering present in the material, even after several hours of hydrothermal reaction. Besides, small changes were observed in the PDF peak positions (corresponding to interatomic Si‐Si, Si‐O and O‐O distances). All such peaks/interatomic distances match well with the distances existing within the SiO
2
crystalline structure. Slight peak width change has also been observed with reaction time, which can possibly arise from local strain. Modelling of the amorphous silica reactive product is under process.
Crystal structure of 1
DC
u
C
l@
SWCNT
s determined by low voltage high resolution transmission electron microscopy
Authors: Orekhov Andrey , Chuvilin Andrey , Tonkikh Alexander , Obraztsova Elena
Published: December 2016

A wide variety of metal oxides, metal halides and other materials may be introduced into opened multiwalled or single‐walled carbon nanotubes (SWCNT). These experiments have permitted the study of the crystal growth of low‐dimensional materials whereby the incorporated crystals are constrained to just a few atomic layers in cross‐section by the internal van der Waals surface of the carbon nanotubes. The type of encapsulated crystal and its interaction with the carbon nanotube determines the electronic properties of the entire system. Change in the local crystal chemistry of incorporated crystals has been observed directly by HRTEM and has also been predicted from ab initio calculations. Therefore, the precise the composition and structure data of the encapsulated crystal is important for a correct interpretation of the filled nanotubes electronic properties.

In this work, the structure and composition of SWCNTs filled by CuCl molecules from gas‐phase was characterized by High‐Resolution Transmission Electron Microscopy (HRTEM), Scanning Transmission Electron Microscopy (STEM) and X‐ray Energy Dispersion Spectroscopy (EDS). The measurements were carried out on a Titan 60‐300 TEM/STEM microscope (FEI, The Netherlands) at acceleration voltage of 80 kV equipped with monochromator and Cs spherical aberration corrector. Characterization and interpretation of the experimental images were performed by comparison with simulated HRTEM images. The CuCl@SWCNT films were characterized by Raman spectroscopy (Ar–Kr ion laser at the wavelengths of 488 nm (2.54 eV), 514.5 nm (2.41 eV), 568 nm (2.18 eV), and 647 nm (1.92 eV)). The spectral resolution was 0.5 cm
‐1
.

A series of HRTEM images with different orientations of encapsulated CuCl crystal inside the nanotubes was obtained and analyzed. The chemical composition of the filled nanotubes was evaluated by EDS. It is shown that the composition ratio of copper and chlorine is 1:1. A typical fragment of filled CuCl@SWCNT is shown in Fig. 1. Fast Fourier transform was applied for the interplanar distances and angles between the vectors of reciprocal lattice estimation. The good coincidence between the calculated and experimental values of interplanar distances of encapsulated CuCl was observed for the case of NaCl type crystal lattice deformed along the [110] direction. Simulation of CuCl@SWCNT HRTEM images revealed a good correspondence of calculated and experimental data. It was found the significant changes in the Raman spectra of the CuCl@SWCNT films relative to unfilled nanotubes. The G Raman mode had a large shift (up to 20 cm
‐1
) and a different shape. This could be explained by efficient charge transfer between the nanotube surface and the encapsulated CuCl crystal.

The work was supported by RSF‐15‐12‐30041.
A protected inert‐gas sample manipulation and transfer environment for cryo electron microscopy and analytics
Authors: Alexander Rosenthal Georg , Tacke Sebastian , Lucas Falk , Wepf Roger Albert
Published: December 2016

The preservation of the native sample state during manipulation and exchange between sample preparation and imaging devices is a prerequisite for artefact and contamination free structure investigation. In particular during cryo‐transfer and (re‐)mounting of frozen hydrated samples, which often in electron microscopy (EM), have a very large surface area compared to its volume, can be very challenging when samples have to be exchanged or remounted several times. Some of the main challenges are: 1. preventing the sample from warming up to the de‐vitrification temperature of water (approximately −137°C), 2. Preventing the sample from being covered with ice or other contaminants condensing from the surrounding atmosphere and 3. Handling these samples without chemical or mechanical alteration of their native state.

Currently the handling and transfer of samples under vacuum and cryo conditions is solved by systems provided from different suppliers. However the transfer of frozen (vitrified) samples under environmental pressure (e.g. from a high‐pressure‐machine to a low temperature coater or cryo‐EM) is still a challenge and a satisfactory practical solution is missing.

Here, we present an ambient pressure inert‐gas sample mounting and exchange chamber (Glove‐Box) (Fig. 1 &B) that addresses all the needs for structure and composition preservation during the manipulation, mounting and transfer of mainly frozen or hydration/oxygen sensitive samples. Once mounted these samples can be transferred and exchanged between different cryo‐preparation and imaging systems (Fig. 1 E) under controlled inert‐gas and temperature conditions. This controlled inert‐gas “Glove‐Box” includes a LN2 immersed sample mounting area with various interfaces to different inert‐gas/cryo transfer systems for RT or low temperature TEM or FIB/SEM applications, Furthermore, monitoring of the environmental parameters (Fig. 1 C) during transfer (humidity, oxygen, carbon dioxide), on‐screen imaging to support sample manipulation (Fig. 1 D), heating of tools and recording of all experimental parameters for documentation are included into this dedicated LN2 and inert‐gas “Glove‐Box”. The interface to other sample carrier devices is open and flexible and can be adapted upon individual laboratory needs. Finally such a “Glove‐Box” can play an essential role in a dedicated modern cryo‐EM & analytical environment as sketched in Fig. 1 E.
Chromatic corrected
EFTEM
investigation on spinodal decomposition of
TiAlN
at 80
kV
with
PICO
Authors: Chen Yen‐Ting , Chang Keke , Mayer Joachim , Schneider Jochen M.
Published: December 2016

Chromatic aberration (Cc) in the modern transmission electron microscopy (TEM) plays an important role with the advancement the resolution up to atomic level
1
. Correction of Cc is especially important for high resolution energy ‐filtered TEM (EFTEM) with the requirement of large slit width and signal intensity to obtain an atomic resolved image, formatted from the inelastic scattered electrons in a limited time and stability constraint. As an advantage, large area and high resolution elemental map can be obtained in just tens of seconds, much faster than the traditional STEM‐EELS counterpart. The short acquisition time is very crucial in the practical applications of both material science and biological science where the samples are very often sensitive to the electron beam illumination, the interesting compositions and structures are unknown, or they are the mixture of different elements and phases.

In this report, an annealed TiAlN sample is prepared for the EFTEM investigation with FEI‐Titan PICO, focusing on the spinodal decomposition phenomenon. As a hard‐coating material, TiAlN has exceptional mechanical and thermal properties attractive for tool‐coating industry
2
. However at a higher temperature around 900 °C, the hardness is found to drop drastically owing to the formation of hexagonal AlN, which typically occurs after the spinodal decomposition. The spinodal decomposition contains the segregation of the composition while the crystal structure remains
3
. It is therefore typically difficult to prove the existence of this phenomenon without an elemental map with an atomic resolution, which is exactly a Cc corrected EFTEM image can provide.

In addition, different effects to the image resolution induced from factors considered in a Cc corrected EFTEM experiment will be discussed, including sample thickness, acceleration voltage, sample drifting, and atomic numbers of the sample. Compositional variation near the dislocation will be discussed as well. Preservation of elastic contrast
4
has been observed and analyzed, which is crucial to map the inelastic image into the final elemental map.
New insights on the fertile megagametophyte and female sterility obtained from Maytenus obtusifolia Mart. (Celastraceae) plants under
TEM
Authors: Haddad Isabella , Santiago‐Fernandes Lygia , Machado Silvia , Machado Silvia
Published: December 2016

Maytenus obtusifolia
Mart. is an abundant species in restingas (sandy coastal plains), mainly in Rio de Janeiro state, Brazil. Populations in restinga of Maricá are gynodioecious formed by female individuals with pistillate flowers and hermaphrodites with perfect flowers. Pistillate flowers bear a developed gynoecium and staminodes. Perfect flowers have gynoecium with varied development and fertile stamens. Hermaphrodite plants have lower fruiting rate than female plants which was associated to the presence of sterile ovules with hypertrophied synergids and egg cell. The aim of this study was to compare fertile and sterile megagametophytes under TEM. Preanthesis buds were collected at restinga of Maricá/RJ, fixed in Karnovsky, post‐fixed in osmium tetroxide, embedded in Araldite, sectioned, contrasted and observed in Tecnai Spirit TEM. In fertile ovules of pistillate and perfect flowers, the egg cell presents cytoplasm dense and polarized, with vacuole facing the micropilar pole. Synergids have thin walls with evident fibrillar apparatus; polarized cytoplasm, vacuome facing the chalazal pole; mitochondria; plastids; nucleus; linear RER concentrically arranged; well developed dictyosomes with enlarged borders forming secretory vesicles. The central cell has thin walls with slightly disorganized cellulose fibrils; cytoplasm arranged in the cell periphery; developed vacuome occupying almost the entire cell; polar nucleus; ribosomes; vesicles with associated ribosomes, derived from RER; dictyosomes; oil bodies and mitochondria. In sterile ovules of perfect flowers, synergids undergo a vacuolization process, with subsequent fusion of small vacuoles; nucleus with fragmented or degenerated nuclear envelope; mitochondria and RER with signs of degeneration; organelles debris and plastids. In these hypertrophied synergids and the egg cell, disruption of tonoplast promote the cell lyses. Vacuole membranes and organelles debris are found inside synergids and central cell. Egg cell cytoplasm has fragments of nucleus. This study reveals that female sterility in perfect flowers seems to be caused by a vacuolization process followed by tonoplast disruption suggesting the occurrence of massive autophagy in megagametophyte cells.
Short‐range‐order (
SRO
) in quenched
Ni
‐rich
Ni‐Ti
alloys
Authors: Pourbabak Saeid , Wang Xiebing , Verlinden Bert , Humbeeck Jan Van , Schryvers Dominique
Published: December 2016

Binary Ni‐Ti alloys have a wide application in industry and medicine due to their shape memory effect and superelasticity properties. These mechanical properties are known to be caused by a martensitic transformation of which the characteristics are strongly dependent on Ni
4
Ti
3
precipitates formed during aging.

In this study a Ni‐Ti alloy which is quenched immediately after the production and aged at room temperature is investigated. No precipitation is expected to form in the sample, which is confirmed by conventional TEM images. However, the alloy still shows a changing shape memory effect with ageing time at room temperature, which indicates there must exist some small structural changes not visible by conventional TEM. These are also expected from observed diffuse intensities (Fig. 1) arranged around particular geometrical loci in reciprocal space. Different techniques are proposed and used to identify these microdomains. The Cluster Model [1] that assigns the shape of the diffuse reciprocal intensity to that of microdomains is applied to analyze the results. In the present case the diffuse intensity can to a first order be approximated by {111}* reciprocal planes, which can be translated into atomic rows along the [111] crystallographic directions in the cubic Ni‐Ti lattice. Such rows of pure Ni are also present in the crystal structure of Ni
4
Ti
3
precipitates, as seen in Fig. 2. In other words, the diffuse intensity can be correlated with contiguous strings of Ni atoms in the cubic directions of the B2 matrix, which normally reveals a …‐Ni‐Ti‐Ni‐Ti‐… sequence along these directions. In order to observe such atomic strings in real space, aberration corrected HAADF‐STEM has been performed along a cubic direction. Simulations indicate that the clustering of heavier Ni atoms can be seen as an increment of appr. 2% of intensity of a single atomic column due to the Z‐contrast nature of the HAADF‐STEM imaging concept. The experimental image shown in Fig. 3a indeed shows some random but coagulated fluctuations in intensity of the columns, as can be seen from the line trace in Fig. 3b (as well as by slightly defocusing your eyes when looking at the picture). However, to what extend these can be attributed to the atomic clustering is still not clear. In the near future, also other advanced TEM techniques will be applied in order to further identify the columns containing contiguous strings of Ni atoms.