The impact of doping rates on the morphologies of silver and gold nanowires grown in helium nanodroplets.

Silver and gold nanowires are grown within superfluid helium nanodroplets and investigated by high resolution electron microscopy after surface deposition. The wire morphologies depend on the rate of metal atom doping in the pickup sequence. While high doping rates result in a polycrystalline face-centered cubic nanowire structure, at lower doping rates the initial fivefold-symmetry seems to be preserved. An explanation for this observation is given by computer simulations, which allow the derivation of timescales for the nanowire growth process inside helium nanodroplets.

Correction: The impact of doping rates on the morphologies of silver and gold nanowires grown in helium nanodroplets.

Correction for 'The impact of doping rates on the morphologies of silver and gold nanowires grown in helium nanodroplets' by Alexander Volk et al., Phys. Chem. Chem. Phys., 2016, DOI: 10.1039/c5cp06248a.

Single CuO nanowires decorated with size-selected Pd nanoparticles for CO sensing in humid atmosphere.

We report on conductometric gas sensors based on single CuO nanowires and compare the carbon monoxide (CO) sensing properties of pristine as well as Pd nanoparticle decorated devices in humid atmosphere. Magnetron sputter inert gas aggregation combined with a quadrupole mass filter for cluster size selection was used for single-step Pd nanoparticle deposition in the soft landing regime. Uniformly dispersed, crystalline Pd nanoparticles with size-selected diameters around 5 nm were deposited on single CuO nanowire devices in a four point configuration. During gas sensing experiments in humid synthetic air, significantly enhanced CO response for CuO nanowires decorated with Pd nanoparticles was observed, which validates that magnetron sputter gas aggregation is very well suited for the realization of nanoparticle-functionalized sensors with improved performance.

Thermal instabilities and Rayleigh breakup of ultrathin silver nanowires grown in helium nanodroplets.

Ag nanowires with diameters below 6 nm are grown within vortex containing superfluid helium nanodroplets and deposited onto a heatable substrate at cryogenic temperatures. The experimental setup allows an unbiased investigation of the inherent stability of pristine silver nanowires, which is virtually impossible with other methods due to chemical processes or templates involved in standard production routes. We demonstrate by experiment and by adaption of a theoretical model that initially continuous wires disintegrate into chains of spheres. This phenomenon is well described by a Rayleigh-like breakup mechanism when the substrate is heated to room temperature. Our findings clarify the recent discussions on the cause of the observed segmented patterns, where a breakup during deposition [Gomez et al., Phys. Rev. Lett., 2012, 108, 155302] or mechanisms intrinsic to the helium droplet mediated growth process [Spence et al., Phys. Chem. Chem. Phys., 2014, 16, 6903] have been proposed. The experimental setup confirms the validity of previous suggestions derived from bulk superfluid helium experiments [Gordon et al., Phys. Chem. Chem. Phys., 2014, 16, 25229] for the helium droplet system, and further allows a much more accurate determination of the breakup temperature.

Linking TEM analytical spectroscopies for an assumptionless compositional analysis.

The classical implementation for putting quantitative figures on maps to reveal elemental compositions in transmission electron microscopy is by analytical methods like X-ray and energy-loss spectroscopy. Typically, the technique in use often depends on whether lighter or heavier elements are present and-more practically-which calibrations are available or sample-related properties are known. A framework linking electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) signals such that absolute volumetric concentrations can be derived without assumptions made a priori about the unknown sample, is largely missing. In order to combine both techniques and harness their respective potentials for a light and heavy element analysis, we have set up a powerful hardware configuration and implemented an experimental approach, which reduces the need for estimates on many parameters needed for quantitative work such as densities, absolute thicknesses, theoretical ionization cross-sections, etc. Calibrations on specimens with known geometry allow the measurement of inelastic mean free paths. As a consequence, mass-thicknesses obtained from the EDX ζ-factor approach can be broken up and quantities like concentrations and partial energy-differential ionization cross-sections become accessible. ζ-factors can then be used for conversion into EELS cross-sections that are hard to determine otherwise, or conversely, connecting EDXS and EELS in a quantitative manner quite effectively.

High resolution electron microscopy of Ag-clusters in crystalline and non-crystalline morphologies grown inside superfluid helium nanodroplets.

We present a first investigation of structural properties of Ag clusters with a diameter of up to 5.5 nm grown inside superfluid helium nanodroplets (He(N)) and deposited on an amorphous C surface. With high resolution transmission electron microscope images we are able to show that in addition to the crystalline face centered cubic (fcc) structure, noncrystalline icosahedral (Ih), and decahedral (Dh) morphologies are grown. Relative abundances (56% fcc, 31% Dh, and 13% Ih) as well as the size distribution of each morphology (mean diameters d(fcc)=2.62(5) nm, d(Dh)=3.34(7) nm, and d(Ih)=3.93(2) nm) do not reflect the situation expected from pure energetic considerations, where small Ihs should be followed by medium sized Dhs and large fccs. Instead, kinetic factors seem to play an important role in the formation of these structures, as it appears to be the case for clusters formed by inert gas aggregation. Considering the low temperatures (0.37 K) and extremely high cooling rates, we discuss basic ideas that might lead to a qualitative picture of the cluster formation process inside He(N).

The Performance of EDXS at Elevated Sample Temperatures Using a MEMS-Based In Situ TEM Heating System.

Since the development of MEMS heating holders, dynamic in-situ experiments at elevated temperatures may be complemented by X-ray spectrometry for chemical analysis. Although the amount of IR radiation is small when compared to furnace holders, the influence of IR radiation emitted from the heating device on the quality of the X-ray spectra is significant. In this work, we systematically examine the influence of infrared (IR) radiation generated by MEMS-based in situ heating systems (DENSsolutions single- and double-tilt holders) on the results and interpretation of energy-dispersive X-ray (EDX) spectra through simulation and measurement. Focal points of interest in this study are the influence of holder geometry, shadowing and orientation with respect to the different emission characteristics of IR and X-ray photons and their interaction with a side-entry and a multi-detector system. IR photons substantially contribute to count rates, dead time, electronic noise levels, energy resolution, and detection efficiency of semiconductor detectors. At higher sample temperatures, they ultimately limit the feasibility of EDXS for elemental characterization and especially the traceability of low-Z elements. This work provides a quantitative insight into the influence of all relevant parameters related to in situ heating experiments on the spectral quality. Bearing this in mind, we aim to provide a guide to optimizing in situ heating experiments with respect to chemical EDXS analysis.

In-situ observation of lateral AlAs oxidation and dislocation formation in VCSELs.

Understanding how defects are generated and propagate during operation in modern vertical cavity surface emitting lasers (VCSEL) is an important challenge in order to develop the next generation of highly reliable semiconductor lasers. Undesired oxidation processes or performance degrading dislocation networks are typically investigated by conventional failure analysis after damage formation. In this works new approach to VCSEL failure analysis, oxide confined high power VCSELs are investigated in-situ at elevated temperatures in a transmission electron microscope. At high temperatures, lateral oxidation of the current confinement layer as well as formation and propagation of dislocations are observed. The experimental results may deepen the understanding of defect generation in VCSELs during stress tests or standard operating conditions. On the other hand, in-situ TEM proofed to be a promising technique to be utilised in future VCSEL failure analysis, possibly leading to the development of improved defect models and increased VCSEL reliability.

A method for a column-by-column EELS quantification of barium lanthanum ferrate.

High-resolution STEM-EELS provides information about the composition of crystalline materials at the atomic scale, though a reliable quantitative chemical analysis is often hampered by zone axis conditions, where neighbouring atomic column intensities contribute to the signal at the probe position. In this work, we present a procedure to determine the concentration of two elements within equivalent atomic columns from EELS elemental maps - in our case barium and lanthanum within the A-sites of Ba1.1La1.9Fe2O7, a second order Ruddlesden-Popper phase. We took advantage of the large changes in the elemental distribution from column to column and introduced a technique, which substitutes inelastic scattering cross sections during the quantification step by using parameters obtained from the actual experiment. We considered channelling / de-channelling effects via inelastic multislice simulations and were thereby able to count occupancies in each atomic column. The EELS quantification results were then used as prior information during the Rietveld refinement in XRD measurements in order to differentiate between barium and lanthanum.

Silicon: The key element in early stages of biocalcification.

Biocalcification is a widespread process of forming hard tissues like bone and teeth in vertebrates. It is also a topic connecting life sciences and earth sciences: calcified skeletons and shells deposited as sediments represent the earth's fossil record and are of paramount interest for biogeochemists trying to get an insight into the past of our planet. This study reports on the role of silicon in the early biocalcification steps, where silicon and calcium were detected on the surface of cyanobacteria (initial stage of lacustrine calcite precipitation) and in crustacean cuticles. By using innovative methodological approaches of correlative microscopy (AFM in combination with analytical TEM: EFTEM, EELS) the chemical form of silicon in biocalcifying matrices and organic-inorganic particles is determined. Previously, silicon was reported to be localized in active growth areas in the young bone of vertebrates. We have found evidence that biocalcification in evolutionarily distant organisms involves very similar initial phases with silicon as a key element at the organic-inorganic interface.

An investigation on focused electron/ion beam induced degradation mechanisms of conjugated polymers.

Irradiation damage, caused by the use of beams in the electron microscopes, leads to undesired physical/chemical material property changes or uncontrollable modification of structures that are being processed. Particularly, soft matter such as polymers or biological materials is highly susceptible and very much prone to react on irradiation by electron and ion beams. The effect is even higher when materials are subjected to energetic species such as ions that possess high momentum and relatively low mean path due to their mass. Especially when Ga(+) ions (used as the ion source in Focused Ion Beam (FIB) instruments) are considered, the end-effect might even be the total loss of the material's properties. This paper will discuss the possible types of degradation mechanisms and defect formations that can take place during ion and electron beam irradiation of the conjugated polymers: e.g. polyfluorene (PF) and poly-3-hexylthiophene (P3HT) thin films. For the investigation of the irradiation induced degradation mechanisms in this study, complementary analytical techniques such as Raman Spectroscopy (RS), Infrared Spectroscopy (IR), Electron Energy Loss Spectroscopy (EELS), Atomic Force Microscopy (AFM), and Fluorescence Microscopy including Photoluminescence (PL) and Electroluminescence (EL) Microscopy were applied.

Application of high-resolution EFTEM SI in an AEM.

In this work we show how energy-filtered imaging can be used to obtain spectrum images of electron energy-loss spectrometric data. Focus is placed on improved energy resolution within these data sets. Using two multilayer samples (GaN/AlN and InP/InAs), we demonstrate the advantages of spectrum-imaging and its extended mapping capabilities. Plasmon-ratio maps are used to quickly create high-contrast material maps with high signal-to-noise ratio, ratio-contrast plots are used to gain optimum settings for the ratio maps, and plasmon-position maps are used to map small shifts of the energy position of bulk plasmon peaks.

Quantitative EDXS: Influence of geometry on a four detector system.

The influence of the geometry on quantitative energy dispersive X-ray spectrometry (EDXS) analysis is determined for a ChemiSTEM system (Super-X) in combination with a low-background double-tilt specimen holder. For the first time a combination of experimental measurements with simulations is used to determine the positions of the individual detectors of a Super-X system. These positions allow us to calculate the detector's solid angles and estimate the amount of detector shadowing and its influence on quantitative EDXS analysis, including absorption correction using the ζ-factor method. Both shadowing by the brass portions and the beryllium specimen carrier of the holder severely affect the quantification of low to medium atomic number elements. A multi-detector system is discussed in terms of practical consequences of the described effects, and a quantitative evaluation of a Fayalit sample is demonstrated. Corrections and suggestions for minimizing systematic errors are discussed to improve quantitative methods for a multi-detector system.

EFTEM spectrum imaging at high-energy resolution.

This paper deals with the application of high-energy resolution EFTEM image series and the corrections needed for reliable data interpretation. The detail of spectral information gained from an image series is largely determined by the intrinsic energy resolution. In this work we show that energy resolution values of as low as 0.8 eV in spectra extracted from EFTEM image series can be obtained with a small energy-selecting slit. At this resolution level aberrations of the energy filter, in particular the non-isochromaticity, can no longer be neglected. We show that the four most prominent factors for EFTEM image series data correction--spatial drift, non-isochromaticity, energy drift and image distortion--must not be treated independently but have to be corrected in unison. We present an efficient algorithm for this correction, and demonstrate the applied correction for the case of a GaN/AlN multilayer sample.

Electron-irradiation damage in chromium nitrides and chromium oxynitride thin films.

The aim of this work is to monitor changes of the N-K electron energy-loss near-edge structure (ELNES) of chromium nitride layers (CrN) introduced by electron irradiation in a transmission electron microscope (TEM). These changes are different for each sample material and seem to give an indication for a particular composition. The CrN samples (CrN and Cr(0.47)N(0.53)) were prepared on silicon wafers by reactive magnetron sputtering of a metallic chromium target in nitrogen plasma. In addition, a CrON sample (Cr(0.5)O(0.2)N(0.3)) was also investigated. This sample was prepared by the addition of oxygen to the plasma during film deposition. The ELNES of the N-K ionization edge of stoichiometric CrN shows a typical fine structure (peaks at 399.0 and 401.1 eV) and remains nearly unaffected even after high-current-density irradiation. On the other hand the N-K fine structures of Cr(0.47)N(0.53) and Cr(0.5)O(0.2)N(0.3) show a change of the ELNES with irradiation dose. This presumably arises from a 1s-pi*-transition of molecular nitrogen located at interstitial positions in these samples.

Switching from weakly to strongly limited injection in self-aligned, nano-patterned organic transistors.

Organic thin-film transistors for high frequency applications require large transconductances in combination with minimal parasitic capacitances. Techniques aiming at eliminating parasitic capacitances are prone to produce a mismatch between electrodes, in particular gaps between the gate and the interlayer electrodes. While such mismatches are typically undesirable, we demonstrate that, in fact, device structures with a small single-sided interlayer electrode gap directly probe the detrimental contact resistance arising from the presence of an injection barrier. By employing a self-alignment nanoimprint lithography technique, asymmetric coplanar organic transistors with an intentional gap of varying size (< 0.2 µm) between gate and one interlayer electrode are fabricated. An electrode overlap exceeding 1 µm with the other interlayer has been kept. Gaps, be them source or drain-sided, do not preclude transistor operation. The operation of the device with a source-gate gap reveals a current reduction up to two orders of magnitude compared to a source-sided overlap. Drift-diffusion based simulations reveal that this marked reduction is a consequence of a weakened gate-induced field at the contact which strongly inhibits injection.

Advantages of a monochromator for bandgap measurements using electron energy-loss spectroscopy.

The practical advantages of a monochromator for electron energy-loss spectroscopy (EELS) in transmission electron microscopy are reviewed. The zero-loss peaks (ZLPs) of a monochromator and a cold field emission gun are compared in terms of bandgap measurement performance. The intensity of the ZLP tails at the bandgap energy is more important than the full-width at half maximum of the ZLP, and a monochromator is preferable to conventional electron sources. The silicon bandgap of 1.1eV is evaluated from the onset in the EEL spectrum obtained using the monochromator without a numerical procedure. We also show a high-speed instability-correction technique to realize the inherent energy resolution of the monochromator, in which instabilities of less than 335Hz are corrected using 512 EEL spectra obtained with an exposure time of 1.4ms. It will be useful in bandgap measurements and advanced studies for elucidating sub-eV EEL spectra.

Tunable Semicrystalline Thin Film Cellulose Substrate for High-Resolution, In-Situ AFM Characterization of Enzymatic Cellulose Degradation.

In the field of enzymatic cellulose degradation, fundamental interactions between different enzymes and polymorphic cellulose materials are of essential importance but still not understood in full detail. One technology with the potential of direct visualization of such bioprocesses is atomic force microscopy (AFM) due to its capability of real-time in situ investigations with spatial resolutions down to the molecular scale. To exploit the full capabilities of this technology and unravel fundamental enzyme-cellulose bioprocesses, appropriate cellulose substrates are decisive. In this study, we introduce a semicrystalline-thin-film-cellulose (SCFTC) substrate which fulfills the strong demands on such ideal cellulose substrates by means of (1) tunable polymorphism via variable contents of homogeneously sized cellulose nanocrystals embedded in an amorphous cellulose matrix; (2) nanoflat surface topology for high-resolution and high-speed AFM; and (3) fast, simple, and reproducible fabrication. The study starts with a detailed description of SCTFC preparation protocols including an in-depth material characterization. In the second part, we demonstrate the suitability of SCTFC substrates for enzymatic degradation studies by combined, individual, and sequential exposure to TrCel6A/TrCel7A cellulases (Trichoderma reesei) to visualize synergistic effects down to the nanoscale.

Engineering high-performance Pd core-MgO porous shell nanocatalysts via heterogeneous gas-phase synthesis.

We report on the design and synthesis of high performance catalytic nanoparticles with a robust geometry via magnetron-sputter inert-gas condensation. Sputtering of Pd and Mg from two independent neighbouring targets enabled heterogeneous condensation and growth of nanoparticles with controlled Pd core-MgO porous shell structure. The thickness of the shell and the number of cores within each nanoparticle could be tailored by adjusting the respective sputtering powers. The nanoparticles were directly deposited on glassy carbon electrodes, and their catalytic activity towards methanol oxidation was examined by cyclic voltammetry. The measurements indicated that the catalytic activity was superior to conventional bare Pd nanoparticles. As confirmed by electron microscopy imaging and supported by density-functional theory (DFT) calculations, we attribute the improved catalytic performance primarily to inhibition of Pd core sintering during the catalytic process by the metal-oxide shell.

Automated spatial drift correction for EFTEM image series.

Energy filtering transmission electron microscopy (EFTEM) is a widely used technique in many areas of scientific research. Image contrast in energy-filtered images arises from specific scattering events such as the ionization of atoms. By combining a set of two or more images, relative sample thickness maps or elemental distribution maps can be easily created. It is also possible to acquire a whole series of energy-filtered images to do more complex data analysis. However, whenever several images are combined to extract certain information, problems are introduced due to sample drift between the exposures. In order to obtain artifact-free information, this spatial drift has to be taken care of. Manual alignment by overlaying and shifting the images to find the best overlap is usually very accurate but extremely time consuming for larger data sets. When large amounts of images are recorded in an EFTEM series, manual correction is no longer a reasonable option. Hence, automatic routines have been developed that are mostly based on the cross-correlation algorithm. Existing routines, however, sometimes fail and again make time consuming manual adjustments necessary. In this paper we describe a new approach to the drift correction problem by incorporating a statistical treatment of the data and we present our statistically determined spatial drift (SDSD) correction program. We show its improved performance by applying it to a typical EFTEM series data block.