Beam Effects on Atomic Dynamics in Metallic Glasses Studied With Electron Correlation Microscopy.

Electron correlation microscopy (ECM) is used to investigate atomic dynamics in metallic glasses (MG) close to metastable equilibrium. It temporally correlates diffracted intensities of a time series of dark-field images to deduce a metric for structural decays. The measurement parameters, such as time and temperature, must be chosen according to the material of interest. In this work, ECM was extended to measurements at room temperature. To ensure, or select, a time window with quasi-thermodynamic equilibrium/steady-state measurement conditions, two-time correlation functions of diffracted intensities were calculated. The dynamics at room temperature are partly driven by the electron beam, thus affecting the material and the results. A systematic analysis of the influence of the electron beam is presented, revealing an inverse relation between electron dose rate and intensity correlation decay times at 300 kV acceleration voltage. However, the underlying dynamical mechanisms, described by a stretching exponent, are found to be independent of the applied electron dose rate for a Pd40Ni40P20 MG. An extrapolation of the results to infinite long measurement times and zero dose rate agrees with X-ray photon correlation spectroscopy data and justifies the application of beam-driven ECM at room temperature to study the dynamics of disordered systems.

Fluctuation Electron Microscopy on Amorphous Silicon and Amorphous Germanium.

Variable resolution fluctuation electron microscopy experiments were performed on self-ion implanted amorphous silicon and amorphous germanium to analyze the medium-range order. The results highlight that the commonly used pair-persistence analysis is influenced by the experimental conditions. Precisely, the structural correlation length Λ, a metric for the medium-range order length scale in the material, obtained from this particular evaluation varies depending on whether energy filtering is used to acquire the data. In addition, Λ depends on the sample thickness. Both observations can be explained by the fact that the pair-persistence analysis utilizes the experimentally susceptible absolute value of the normalized variance obtained from fluctuation electron microscopy data. Instead, plotting the normalized variance peak magnitude over the electron beam size offers more robust results. This evaluation yields medium-range order with an extent of approximately (1.50 ± 0.50) nm for the analyzed amorphous germanium and around (1.10 ± 0.20) nm for amorphous silicon.

Challenges of Electron Correlation Microscopy on Amorphous Silicon and Amorphous Germanium.

Electron correlation microscopy experiments were conducted on amorphous germanium (a-Ge) and amorphous silicon (a-Si) with the goal to study self-diffusion. For this purpose, a series of tilted dark-field images were acquired during in situ heating of the samples in a transmission electron microscope. These experiments show that the measurements are greatly affected by artefacts. Contamination, crystallization, electron beam-induced sputtering, and macroscopic bending of the samples pose major obstacles to the measurements. Other, more subtle experimental artefacts could occur in addition to these which makes interpretations regarding the structural dynamics nearly impossible. The data were nonetheless evaluated to see if some useful information could be extracted. One such result is that the distribution of the characteristic times τKWW, which were obtained from stretched exponential fits to the intensity autocorrelation data, is spatially heterogeneous. This spatial heterogeneity is assumed to be caused by a potential nonergodicity of the materials, the artefacts or an inhomogeneous amorphous structure. Further data processing shows that the characteristic times τKWW are moreover temperature independent, especially for the a-Ge data. It is concluded that the structural rearrangements over time are primarily electron beam-driven and that diffusive dynamics are too slow to be measured at the chosen, experimentally accessible annealing temperatures.

Treating Knock-On Displacements in Fluctuation Electron Microscopy Experiments.

This work investigates how knock-on displacements influence fluctuation electron microscopy (FEM) experiments. FEM experiments were conducted on amorphous silicon, formed by self-ion implantation, in a transmission electron microscope at 300 kV and 60 kV at various electron doses, two different binnings and with two different cameras, a CCD and a CMOS one. Furthermore, energy filtering has been utilized in one case. Energy filtering greatly enhances the FEM data by removing the inelastic background intensity, leading to an improved speckle contrast. The CMOS camera yields a slightly larger normalized variance than the CCD at an identical electron dose and appears more prone to noise at low electron counts. Beam-induced atomic displacements affect the 300 kV FEM data, leading to a continuous suppression of the normalized variance with increasing electron dose. Such displacements are considerably reduced for 60 kV experiments since the primary electron's maximum energy transfer to an atom is less than the displacement threshold energy of amorphous silicon. The results show that the variance suppression due to knock-on displacements can be controlled in two ways: Either by minimizing the electron dose to the sample or by conducting the experiment at a lower acceleration voltage.

Focused Ion Beam Sample Preparation for In Situ Thermal and Electrical Transmission Electron Microscopy.

A focused ion beam (FIB) technique describing the preparation of specimens for in situ thermal and electrical transmission electron microscopy is presented in detail. The method can be applied to a wide range of materials and allows the sample to be thinned down to electron transparency while it is attached to the in situ chip. This offers the advantage that the specimen can have a quality in terms of contamination and damage due to the ion beam that is comparable to samples prepared by means of conventional FIB preparation. Additionally, our technique can be performed by most commercially available FIB devices and only requires three simple, custom stubs for the procedure. This should enable a large userbase for this type of sample fabrication. One further benefit of our technique is that the in situ chip can be reused to create another sample on the same chip. The quality of the samples is demonstrated by high-resolution transmission electron microscopy as well as electron energy loss spectroscopy.

Plasmon energy losses in shear bands of metallic glass.

Shear bands resulting from plastic deformation in cold-rolled Al88Y7Fe5 metallic glass were observed to display alternating density changes along their propagation direction. Electron-energy loss spectroscopy (EELS) was used to investigate the volume plasmon energy losses in and around shear bands. Energy shifts of the peak centre and changes in the peak width (FWHM) reflecting the damping were precisely determined within an accuracy of a few meV using an open source python module (Hyperspy) to fit the shapes of the plasmon and zero-loss peaks with Lorentzian functions. The maximum bulk plasmon energy shifts were calculated for the bright and dark shear band segments relative to the matrix to be about 38 and 14 meV, respectively. The damping was observed to be larger for the denser regions. The analysis presented here suggests that the changes in the plasmons are caused by two contributions: (i) Variable damping in the shear band segments due to changes in the medium-range order (MRO). This affects the static structure factor S(k), which, in turn, leads to either reduced or increased damping according to the Ziman-Baym formula. (ii) The ionic density and the effective electron mass appearing in the zero-momentum plasmon frequency formula Ep(q=0) are coupled and give rise to small variations in the plasmon energy. The model predicts plasmon energy shifts in the order of meV.

Exploring the Phase Space of Multi-Principal-Element Alloys and Predicting the Formation of Bulk Metallic Glasses.

Multi-principal-element alloys share a set of thermodynamic and structural parameters that, in their range of adopted values, correlate to the tendency of the alloys to assume a solid solution, whether as a crystalline or an amorphous phase. Based on empirical correlations, this work presents a computational method for the prediction of possible glass-forming compositions for a chosen alloys system as well as the calculation of their critical cooling rates. The obtained results compare well to experimental data for Pd-Ni-P, micro-alloyed Pd-Ni-P, Cu-Mg-Ca, and Cu-Zr-Ti. Furthermore, a random-number-generator-based algorithm is employed to explore glass-forming candidate alloys with a minimum critical cooling rate, reducing the number of datapoints necessary to find suitable glass-forming compositions. A comparison with experimental results for the quaternary Ti-Zr-Cu-Ni system shows a promising overlap of calculation and experiment, implying that it is a reasonable method to find candidates for glass-forming alloys with a sufficiently low critical cooling rate to allow the formation of bulk metallic glasses.

Comparison of Experimental STEM Conditions for Fluctuation Electron Microscopy.

Variable-resolution fluctuation electron microscopy (VR-FEM) data from measurements on amorphous silicon and PdNiP have been obtained at varying experimental conditions. Measurements have been conducted at identical total electron dose and with an identical electron dose normalized to the respective probe size. STEM probes of different sizes have been created by variation of the semi-convergence angle or by defocus. The results show that defocus yields a reduced normalized variance compared to data from probes created by convergence angle variation. Moreover, the trend of the normalized variance upon probe size variation differs between the two methods. Beam coherence, which affects FEM data, has been analyzed theoretically using geometrical optics on a multi-lens setup and linked to the illumination conditions. Fits to several experimental beam profiles support our geometrical optics theory regarding probe coherence. The normalized variance can be further optimized if one determines the optimal exposure time for the nanobeam diffraction patterns.

Phosphorescent Pt(ii) complexes spatially arrayed in micellar polymeric nanoparticles providing dual readout for multimodal imaging.

In this paper we report phosphorescent Pt(ii) complexes as monomers which can be directly incorporated into growing polymers. Due to the amphiphilic nature of the polymers they can self-assemble into micellar nanoparticles, where the phosphorescent Pt(ii) complexes can arrange selectively in the core or shell of the nanoparticles. The complexes enable dual orthogonal imaging, made possible by the heavy metal, which enhances the contrast for these micelles in electron microscopy and facilitates spin-orbit coupling that turns on microsecond lifetime luminescence.

Corrigendum to "Formation of Silver Nanoclusters from a DNA Template Containing Ag(I)-Mediated Base Pairs".

[This corrects the article DOI: 10.1155/2016/7485125.].

A Modular System for the Design of Stimuli-Responsive Multifunctional Nanoparticle Aggregates by Use of Host-Guest Chemistry.

A self-assembly approach for the design of multifunctional nanomaterials consisting of different nanoparticles (gold, iron oxide, and lanthanide-doped LiYF4 ) is developed. This modular system takes advantage of the light-responsive supramolecular host-guest chemistry of ß-cyclodextrin and arylazopyrazole, which enables the dynamic and reversible self-assembly of particles to spherical nanoparticle aggregates in aqueous solution. Due to the magnetic iron oxide nanoparticles, the aggregates can be manipulated by an external magnetic field leading to the formation of linear structures. As a result of the integration of upconversion nanoparticles, the aggregates are additionally responsive to near-infrared light and can be redispersed by use of the upconversion effect. By varying the nanoparticle and linker concentrations the composition, size, shape, and properties of the multifunctional nanoparticle aggregates can be fine-tuned.

Palladium Nanoparticle Loaded Bifunctional Silica Hybrid Material: Preparation and Applications as Catalyst in Hydrogenation Reactions.

Bifunctional mesoporous silica was prepared by co-condensation of tetraethyl orthosilicate (TEOS) with functionalized organosilanes containing azides or alkoxyamines. Orthogonal functional groups at the particles were selectively addressed in subsequent chemical modifications through "click"-chemistry ("click to ligand" strategy) and radical nitroxide exchange. Palladation with PdCl2 delivered Pd nanoparticle-loaded silica material bearing sulfoxides and additional aminoamides as stabilizing ligands by means of in situ reduction of the PdII -salt. These functional particles were successfully applied to the hydrogenation of alkynes and alkenes. Aldehyde hydrodeoxygenation and benzyl ether cleavage were achieved with these hybrid catalysts under mild conditions. Particles were analyzed by IR, TEM/STEM, EDX, and solid-state NMR spectroscopy.

Formation of Silver Nanoclusters from a DNA Template Containing Ag(I)-Mediated Base Pairs.

A series of DNA double helices containing different numbers of silver(I)-mediated base pairs involving the artificial nucleobases imidazole or 2-methylimidazole has been applied for the generation of DNA-templated silver nanoclusters. The original Ag(I)-containing nucleic acids as well as the resulting nanoclusters and nanoparticles have been characterized by means of UV/Vis spectroscopy, circular dichroism (CD) spectroscopy, fluorescence spectroscopy, and transmission electron microscopy (TEM). The results show for the first time that metal-mediated base pairs can be used for the templated growth of metal nanoclusters.

Arylazopyrazoles as Light-Responsive Molecular Switches in Cyclodextrin-Based Supramolecular Systems.

A simple and high yield synthesis of water-soluble arylazopyrazoles (AAPs) featuring superior photophysical properties is reported. The introduction of a carboxylic acid allows the diverse functionalization of AAPs. Based on structural modifications of the switching unit the photophysical properties of the AAPs could be tuned to obtain molecular switches with favorable photostationary states. Furthermore, AAPs form stable and light-responsive host-guest complexes with ß-cyclodextrin (ß-CD). Our most efficient AAP shows binding affinities comparable to azobenzenes, but more effective switching and higher thermal stability of the Z-isomer. As a proof-of-principle, we investigated two CD-based supramolecular systems, containing either cyclodextrin vesicles (CDVs) or cyclodextrin-functionalized gold nanoparticles (CDAuNPs), which revealed excellent reversible, light-responsive aggregation and dispersion behavior. To conclude, AAPs have great potential to be incorporated as molecular switches in highly demanding and multivalent photoresponsive systems.

Quantitative Measurement of Density in a Shear Band of Metallic Glass Monitored Along its Propagation Direction.

Quantitative density measurements from electron scattering show that shear bands in deformed Al88Y7Fe5 metallic glass exhibit alternating high and low density regions, ranging from -9% to +6% relative to the undeformed matrix. Small deflections of the shear band from the main propagation direction coincide with switches in density from higher to lower than the matrix and vice versa, indicating that faster and slower motion (stick slip) occurs during the propagation. Nanobeam diffraction analyses provide clear evidence that the density changes are accompanied by structural changes, suggesting that shear alters the packing of tightly bound short- or medium-range atomic clusters. This bears a striking resemblance to the packing behavior in granular shear bands formed upon deformation of granular media.

Sequential surface modification of au nanoparticles: from surface-bound Ag(I) complexes to Ag(0) doping.

Gold nanoparticles (Au NPs) with tailor-made structures and properties are highly desirable for applications in catalysis and sensing. In this context, surface modifications of Au NPs are of particular relevance. Herein, we present a sequential surface modification of Au NPs with Ag(I) coordination complexes, which can be converted into Ag(0)-doped Au NPs by simple ligand-exchange reaction. The key innovative element of this surface modification is a multifunctional bioxazoline-based ligand that brings coordinated Ag(I) into close proximity to the particle surface.

Density changes in shear bands of a metallic glass determined by correlative analytical transmission electron microscopy.

Density changes between sheared zones and their surrounding amorphous matrix as a result of plastic deformation in a cold-rolled metallic glass (melt-spun Al88Y7Fe5) were determined using high-angle annular dark-field (HAADF) detector intensities supplemented by electron-energy loss spectroscopy (EELS), energy-dispersive X-ray (EDX) and nano-beam diffraction analyses. Sheared zones or shear bands were observed as regions of bright or dark contrast arising from a higher or lower density relative to the matrix. Moreover, abrupt contrast changes from bright to dark and vice versa were found within individual shear bands. We associate the decrease in density mainly with an enhanced free volume in the shear bands and the increase in density with concomitant changes of the mass. This interpretation is further supported by changes in the zero loss and Plasmon signal originating from such sites. The limits of this new approach are discussed.

pH-Switchable ampholytic supramolecular copolymers.

ß-sheet-encoded anionic and cationic dendritic peptide amphiphiles form supramolecular copolymers when self-assembled in a 1:1 feed ratio of the monomers. These ampholytic materials have been designed for on-off polymerization in response to pH triggers. The cooperative supramolecular self-assembly process is switched on at a physiologically relevant pH value and can be switched off by increasing or decreasing the pH value.