Reaction-diffusion study of electron-beam-induced contamination growth.

A time-dependent reaction-diffusion model was elaborated to better understand the dynamical growth of contamination on surfaces illuminated by an electron beam. The goal of this work was to fully describe the flow of hydrocarbon molecules, denoted as contaminants, and their polymerization in the irradiated area with the number of parameters reduced to a minimum necessary. It was considered that the diffusion process of contaminants is driven by the gradient of their surface density generated by the impact of a circular homogeneous electron beam. The contribution of the residual gas atmosphere in the instrument was described by the tendency to reestablish the initial equilibrium surface density of contaminants before irradiation. The four unknown parameters of the model, the electron interaction cross-section, the diffusion coefficient, the initial surface density of contaminants, and the frequency of the supply of contaminants from the residual gas atmosphere were determined by comparing the modeled contamination growth with experimental results. The experiments were designed such that the influence of the single parameters could be unequivocally separated. To follow the dynamical evolution of the system and to generate time-resolved distinct experimental data, successive contamination measurements were performed at short time intervals up to 20 min. The local height and shape of the grown contamination were quantified by evaluating high-angle annular dark-field (HAADF) scanning-transmission- electron-microcopy (STEM) image intensities and corresponding Monte-Carlo simulations. Our model also applies to nonhomogeneous initial conditions like the reduced local surface density of contaminants after previous beam-showering. The dynamic analyses of this process might provide hints regarding the relative size of the contaminant molecules and also indicate some measures for the reduction of contamination growth.

Sc, Zr, Hf, and Mn Metal Nanoparticles: Reactive Starting Materials for Synthesis Near Room Temperature.

Zerovalent scandium, zirconium, hafnium, and manganese nanoparticles are prepared by reduction of ScCl3, ZrCl4, HfCl4, and MnCl2 with lithium or sodium naphthalenide in a one-pot, liquid-phase synthesis. Small-sized monocrystalline nanoparticles are obtained with diameters of 2.4 ± 0.2 nm (Sc), 4.0 ± 0.9 nm (Zr), 8.0 ± 3.9 nm (Hf) and 2.4 ± 0.3 nm (Mn). Thereof, Zr(0) and Hf(0) nanoparticles with such size are shown for the first time. To probe the reactivity and reactions of the as-prepared Sc(0), Zr(0), Hf(0), and Mn(0) nanoparticles, they are exemplarily reacted in the liquid phase (e.g., THF, toluene, ionic liquids) with different sterically demanding, monodentate to multidentate ligands, mainly comprising O-H and N-H acidic alcohols and amines. These include isopropanol (HOiPr), 1,1'-bi-2-naphthol (H2binol), N,N'-bis(salicylidene)ethylenediamine (H2salen), 2-mercaptopyridine (2-Hmpy), 2,6-diisopropylaniline (H2dipa), carbazole (Hcz), triphenylphosphane (PPh3), N,N,N',N'-tetramethylethylenediamine (tmeda), 2,2'-bipyridine (bipy), N,N'-diphenylformamidine (Hdpfa), N,N'-(2,6-diisopropylphenyl)-2,4-pentanediimine ((dipp)2nacnacH), 2,2'-dipydridylamine (Hdpa), and 2,6-bis(2-benzimidazolyl)pyridine (H2bbp). As a result, 22 new compounds are obtained, which frequently exhibit a metal center coordinated only by the sterically demanding ligand. Options and restrictions for the liquid-phase syntheses of novel coordination compounds using the oxidation of base-metal nanoparticles near room temperature are evaluated.

Preventing cation intermixing enables 50% quantum yield in sub-15 nm short-wave infrared-emitting rare-earth based core-shell nanocrystals.

Short-wave infrared (SWIR) fluorescence could become the new gold standard in optical imaging for biomedical applications due to important advantages such as lack of autofluorescence, weak photon absorption by blood and tissues, and reduced photon scattering coefficient. Therefore, contrary to the visible and NIR regions, tissues become translucent in the SWIR region. Nevertheless, the lack of bright and biocompatible probes is a key challenge that must be overcome to unlock the full potential of SWIR fluorescence. Although rare-earth-based core-shell nanocrystals appeared as promising SWIR probes, they suffer from limited photoluminescence quantum yield (PLQY). The lack of control over the atomic scale organization of such complex materials is one of the main barriers limiting their optical performance. Here, the growth of either homogeneous (α-NaYF4) or heterogeneous (CaF2) shell domains on optically-active α-NaYF4:Yb:Er (with and without Ce3+ co-doping) core nanocrystals is reported. The atomic scale organization can be controlled by preventing cation intermixing only in heterogeneous core-shell nanocrystals with a dramatic impact on the PLQY. The latter reached 50% at 60 mW/cm2; one of the highest reported PLQY values for sub-15 nm nanocrystals. The most efficient nanocrystals were utilized for in vivo imaging above 1450 nm.

SMART RHESINs-Superparamagnetic Magnetite Architecture Made of Phenolic Resin Hollow Spheres Coated with Eu(III) Containing Silica Nanoparticles for Future Quantitative Magnetic Particle Imaging Applications.

Magnetic particle imaging (MPI) is a powerful and rapidly growing tomographic imaging technique that allows for the non-invasive visualization of superparamagnetic nanoparticles (NPs) in living matter. Despite its potential for a wide range of applications, the intrinsic quantitative nature of MPI has not been fully exploited in biological environments. In this study, a novel NP architecture that overcomes this limitation by maintaining a virtually unchanged effective relaxation (Brownian plus Néel) even when immobilized is presented. This superparamagnetic magnetite architecture made of phenolic resin hollow spheres coated with Eu(III) containing silica nanoparticles (SMART RHESINs) was synthesized and studied. Magnetic particle spectroscopy (MPS) measurements confirm their suitability for potential MPI applications. Photobleaching studies show an unexpected photodynamic due to the fluorescence emission peak of the europium ion in combination with the phenol formaldehyde resin (PFR). Cell metabolic activity and proliferation behavior are not affected. Colocalization experiments reveal the distinct accumulation of SMART RHESINs near the Golgi apparatus. Overall, SMART RHESINs show superparamagnetic behavior and special luminescent properties without acute cytotoxicity, making them suitable for bimodal imaging probes for medical use like cancer diagnosis and treatment. SMART RHESINs have the potential to enable quantitative MPS and MPI measurements both in mobile and immobilized environments.

Quantitative analysis of backscattered-electron contrast in scanning electron microscopy.

Backscattered-electron scanning electron microscopy (BSE-SEM) imaging is a valuable technique for materials characterisation because it provides information about the homogeneity of the material in the analysed specimen and is therefore an important technique in modern electron microscopy. However, the information contained in BSE-SEM images is up to now rarely quantitatively evaluated. The main challenge of quantitative BSE-SEM imaging is to relate the measured BSE intensity to the backscattering coefficient η and the (average) atomic number Z to derive chemical information from the BSE-SEM image. We propose a quantitative BSE-SEM method, which is based on the comparison of Monte-Carlo (MC) simulated and measured BSE intensities acquired from wedge-shaped electron-transparent specimens with known thickness profile. The new method also includes measures to improve and validate the agreement of the MC simulations with experimental data. Two different challenging samples (ZnS/Zn(Ox S1- x )/ZnO/Si-multilayer and PTB7/PC71 BM-multilayer systems) are quantitatively analysed, which demonstrates the validity of the proposed method and emphasises the importance of realistic MC simulations for quantitative BSE-SEM analysis. Moreover, MC simulations can be used to optimise the imaging parameters (electron energy, detection-angle range) in advance to avoid tedious experimental trial and error optimisation. Under optimised imaging conditions pre-determined by MC simulations, the BSE-SEM technique is capable of distinguishing materials with small composition differences.

Critical current density improvement in CSD-grown high-entropy REBa2Cu3O7-δ films.

High-entropy oxide (HEO) superconductors have been developed since very recently. Different superconductors can be produced in the form of a high-entropy compound, including REBa2Cu3O7-δ (REBCO). However, until now, mainly bulk samples (mostly in polycrystalline form) have been reported. In this work, the first CSD-grown high-entropy (HE) REBCO nanocomposite films were successfully synthesized. In particular, high-quality Gd0.2Dy0.2Y0.2Ho0.2Er0.2Ba2Cu3O7-δ nanocomposite films with 12 mol% BaHfO3 nanoparticles were grown on SrTiO3 substrates. The X-ray diffraction patterns show a near-perfect c-axis oriented grain growth. Both T c and 77 K J sf c, 91.9 K and 3.5 MA cm-2, respectively, are comparable with the values of the single-RE REBCO films. Moreover, at low temperatures, specifically at 30 K, the J c values are larger than those of the single-RE samples. A transmission electron microscopy (TEM) study, including energy-dispersive X-ray spectroscopy (EDXS) measurements, reveals that the different RE3+ ions are distributed homogeneously in the matrix without forming clusters. This distribution causes point-like pinning centres that explain the superior performances of these samples at low temperatures. Although still seen as a proof-of-concept for the feasibility of preparing such films, these results demonstrate that the HE REBCO films are a promising option for the future fabrication of high-performance coated conductors. In the investigated B-T range, however, their J c values are still lower than those of other, medium-entropy REBCO films, which shows that an optimization of the composition of the HE REBCO films is needed to maximize their performance.

Room-temperature liquid-phase synthesis of aluminium nanoparticles.

Aluminium nanoparticles, Al(0), are obtained via liquid-phase synthesis at 25 °C. Accordingly, AlBr3 is reduced by lithium naphthalenide ([LiNaph]) in toluene in the presence of N,N,N',N'-tetramethylethylenediamine (TMEDA). The Al(0) nanoparticles are small (5.6 ± 1.5 nm) and highly crystalline. A light yellow colour and absorption at 250-350 nm are related to the plasmon-resonance absorption. Due to TMEDA functionalization, the Al(0) nanoparticles are colloidally and chemically stable, but show high reactivity after TMEDA removal.

Reducing Impedance at a Li-Metal Anode/Garnet-Type Electrolyte Interface Implementing Chemically Resolvable In Layers.

Garnet-type Li7La3Zr2O12 (LLZO) is a potential electrolyte material for all-solid-state Li-ion batteries mainly because of its reported excellent chemical stability in contact with Li metal. But good wettability of LLZO and 100% surface coverage of lithium are still a challenge. This study elucidated the suitability of magnetron-sputtered indium in Li(In)/LLZO/Li(In) symmetrical model cells as one of the promising interfacial modifications reported in the literature. Importance was given to the impact of preparation parameters on the surface coverage of Li(In)/LLZO interfaces and the consequences of impedance, cycling stability, and critical current density. SEM and EDXS analyses of In layers of thickness 100 nm to 1 µm revealed complete dissolution of indium in the lithium anode after annealing; 300 nm In layers annealed at 220 °C/10 h provided a surface coverage of >80%, best reproducibility, and a supreme interface resistance Rint of 12.4 Ω·cm2. Presuming a surface coverage of 100%, an ultimate interface resistance close to 1 Ω·cm2 can be expected. The critical current density was determined as 200-500 µA/cm2 at a charge of 100-250 µAh, whereas 500 µA/cm2 and above affected cell stability. The increasing voltage plateau was assigned to the increase of the interface resistance Rint and the electrolyte resistance RG+GB. SEM, EDXS, and X-ray microtomography analyses after voltage breakdown confirmed Li-dendrite growth along grain boundaries into LLZO, often curved parallel to the interface, indicating short-circuiting of the solid electrolyte. Grain boundary characteristics are supposed to be decisive for lithium deposition in and failure of garnet-type solid electrolytes after cycling.

[Sm6O4(cbz)10(thf)6]·2C7H8: A Polynuclear Samarium Oxo Cluster Obtained from Carbazole-Driven Oxidation of Samarium Nanoparticles.

Zerovalent samarium nanoparticles (1.7 ± 0.2 nm in size) are used as the starting material to prepare single crystals of the novel polynuclear samarium oxo cluster [Sm6O4(cbz)10(thf)6]·2C7H8. The reaction is performed by oxidation with carbazole (CbzH) in tetrahydrofuran (THF) at 50 °C with subsequent crystallization in toluene (C7H8). The oxo cluster contains noncharged molecular units with a central Sm6O4 core. Single-crystal structure analysis and infrared spectroscopy confirm the oxidation of CbzH with the formation of (cbz)-. Polynuclear carbazole complexes are generally rare and here prepared using metal nanoparticles as a reactive starting material for the first time. The reaction with CbzH as a sterically demanding ligand exemplarily shows the feasibility of rare-earth-metal nanoparticles for obtaining new compounds with complex composition and structure.

Highly Efficient Recovery of H2 from Industrial Waste by Sunlight-Driven Photoelectrocatalysis over a ZnS/Bi2S3/ZnO Photoelectrode.

Hydrogen (H2) fuel production from hazardous contaminants is not only of economic importance but also of significance for the environment and health. Hydrogen production is exemplified in this work by using bismuth sulfide (Bi2S3) sandwiched in between zinc sulfide (ZnS) and zinc oxide (ZnO) as dual-heterojunction photoelectrode to photoelectrochemically extract H2 from sulfide- and sulfite-containing wastewater, which is emitted in enormous quantities from the petrochemical industries. The H2 evolution rate over the ZnS/Bi2S3/ZnO photoelectrode under solar illumination amounts to 112.8 µmol cm-2 h-1, of which the photocurrent density in the meantime reaches 10.7 mA cm-2, by far exceeding those reported for additional Bi2S3-based counterparts in the literature. Such superior performance is ascribed on one hand to the broadband sunlight-harvesting ability of Bi2S3 that gives rise to respectable photoexcited electron-hole pairs. These photogenerated charge carriers are subsequently rectified by the built-in electric field at the ZnS/Bi2S3 and Bi2S3/ZnO heterojunctions to flow in the opposite directions to well circumvent the recombination losses and, most importantly, in turn contribute substantially to the H2 evolution reaction.

Decagram-Scale Synthesis of Multicolor Carbon Nanodots: Self-Tracking Nanoheaters with Inherent and Selective Anticancer Properties.

Carbon nanodots (CDs) are a new class of carbon-based nanoparticles endowed with photoluminescence, high specific surface area, and good photothermal conversion, which have spearheaded many breakthroughs in medicine, especially in drug delivery and cancer theranostics. However, the tight control of their structural, optical, and biological properties and the synthesis scale-up have been very difficult so far. Here, we report for the first time an efficient protocol for the one-step synthesis of decagram-scale quantities of N,S-doped CDs with a narrow size distribution, along with a single nanostructure multicolor emission, high near-infrared (NIR) photothermal conversion efficiency, and selective reactive oxygen species (ROS) production in cancer cells. This allows achieving targeted and multimodal cytotoxic effects (i.e., photothermal and oxidative stresses) in cancer cells by applying biocompatible NIR laser sources that can be remotely controlled under the guidance of fluorescence imaging. Hence, our findings open up a range of possibilities for real-world biomedical applications, among which is cancer theranostics. In this work, indocyanine green is used as a bidentate SOx donor which has the ability to tune surface groups and emission bands of CDs obtained by solvothermal decomposition of citric acid and urea in N,N-dimethylformamide. The co-doping implies various surface states providing transitions in the visible region, thus eliciting a tunable multicolor emission from blue to red and excellent photothermal efficiency in the NIR region useful in bioimaging applications and image-guided anticancer phototherapy. The fluorescence self-tracking capability of SOx-CDs reveals that they can enter cancer cells more quickly than healthy cell lines and undergo a different intracellular fate after cell internalization. This could explain why sulfur doping entails pro-oxidative activities by triggering more ROS generation in cancer cells when compared to healthy cell lines. We also find that oxidative stress can be locally enhanced under the effects of a NIR laser at moderate power density (2.5 W cm-2). Overall, these findings suggest that SOx-CDs are endowed with inherent drug-independent cytotoxic effects toward cancer cells, which would be selectively enhanced by external NIR light irradiation and helpful in precision anticancer approaches. Also, this work opens a debate on the role of CD surface engineering in determining nanotoxicity as a function of cell metabolism, thus allowing a rational design of next-generation nanomaterials with targeted anticancer properties.

Interface Pattern Engineering in Core-Shell Upconverting Nanocrystals: Shedding Light on Critical Parameters and Consequences for the Photoluminescence Properties.

Advances in controlling energy migration pathways in core-shell lanthanide (Ln)-based hetero-nanocrystals (HNCs) have relied heavily on assumptions about how optically active centers are distributed within individual HNCs. In this article, it is demonstrated that different types of interface patterns can be formed depending on shell growth conditions. Such interface patterns are not only identified but also characterized with spatial resolution ranging from the nanometer- to the atomic-scale. In the most favorable cases, atomic-scale resolved maps of individual particles are obtained. It is also demonstrated that, for the same type of core-shell architecture, the interface pattern can be engineered with thicknesses of just 1 nm up to several tens of nanometers. Total alloying between the core and shell domains is also possible when using ultra-small particles as seeds. Finally, with different types of interface patterns (same architecture and chemical composition of the core and shell domains) it is possible to modify the output color (yellow, red, and green-yellow) or change (improvement or degradation) the absolute upconversion quantum yield. The results presented in this article introduce an important paradigm shift and pave the way toward the emergence of a new generation of core-shell Ln-based HNCs with better control over their atomic-scale organization.

Ultrafast Interface Charge Separation in Carbon Nanodot-Nanotube Hybrids.

Carbon dots are an emerging family of zero-dimensional nanocarbons behaving as tunable light harvesters and photoactivated charge donors. Coupling them to carbon nanotubes, which are well-known electron acceptors with excellent charge transport capabilities, is very promising for several applications. Here, we first devised a route to achieve the stable electrostatic binding of carbon dots to multi- or single-walled carbon nanotubes, as confirmed by several experimental observations. The photoluminescence of carbon dots is strongly quenched when they contact either semiconductive or conductive nanotubes, indicating a strong electronic coupling to both. Theoretical simulations predict a favorable energy level alignment within these complexes, suggesting a photoinduced electron transfer from dots to nanotubes, which is a process of high functional interest. Femtosecond transient absorption confirms indeed an ultrafast (<100 fs) electron transfer independent of nanotubes being conductive or semiconductive in nature, followed by a much slower back electron transfer (≈60 ps) from the nanotube to the carbon dots. The high degree of charge separation and delocalization achieved in these nanohybrids entails significant photocatalytic properties, as we demonstrate by the reduction of silver ions in solution. The results are very promising in view of using these "all-carbon" nanohybrids as efficient light harvesters for applications in artificial photocatalysis and photosynthesis.

Highly Selective Cu Staining of Sulfur-Containing Polymers Facilitates 3D Nanomorphology Reconstruction of Polymer:Fullerene Blends in Organic Solar Cells by FIB-SEM Tomography.

The distinction of different organic materials in phase mixtures is hampered in electron microscopy because electron scattering does not strongly differ in carbon-based materials that mainly consist of light elements. A successful strategy for contrast enhancement is selective staining where one phase of a material mixture is labeled by heavier elements, but suitable staining agents are not available for all organic materials. This is also the case for bulk-heterojunction (BHJ) absorber layers of organic solar cells, which consist of interpenetrating networks of donor and acceptor domains. The domain structure strongly influences the power conversion efficiency, and nanomorphology optimization often requires real-space information on the sizes and interconnectivity of domains with nanometer resolution. In this work, we have developed an efficient approach to selectively stain sulfur-containing polymers by homogeneous Cu infiltration, which generates strong material contrast in scanning (transmission) electron microscopy (S(T)EM) images of polymer:fullerene BHJ layers. Cross-section lamellae of BHJ layers are prepared for STEM by focused-ion-beam milling and are attached to a Cu lift-out grid as a copper source. After thermal treatment at 200 °C for 3 h in air, sulfur-containing polymers are homogeneously infiltrated by Cu, while the fullerenes are not affected. Selective Cu staining is applied to map the phase distribution in PTB7:PC71BM BHJ layers fabricated with different processing additives to tailor the nanomorphology. The strong contrast between polymer and fullerene domains is the prerequisite for the three-dimensional reconstruction of the domain structure by focused-ion-beam/scanning-electron-microscopy tomography.

Liquid-Phase Synthesis of Highly Reactive Rare-Earth Metal Nanoparticles.

The first liquid-phase synthesis of high-quality, small-sized rare-earth metal nanoparticles (1-3 nm)-ranging from lanthanum as one of the largest (187 pm) to scandium as the smallest (161 pm) rare-earth metal-is shown. Size, oxidation state, and reactivity of the nanoparticles are examined (e.g., electron microscopy, electron spectroscopy, X-ray absorption spectroscopy, selected reactions). Whereas the nanoparticles are highly reactive (e.g. in contact to air and water), they are chemically stable as THF suspensions and powders under inert conditions. The reactivity can be controlled to obtain inorganic and metal-organic compounds at room temperature.

Liquid-phase synthesis of highly oxophilic zerovalent niobium and tantalum nanoparticles.

Zerovalent niobium, Nb(0), and tantalum, Ta(0), nanoparticles are prepared via a one-pot, liquid-phase synthesis. For this, NbCl5/TaCl5 are dissolved in pyridine and reduced by lithium pyridinyl. Deep black suspensions of very small, highly uniform nanoparticles are obtained with average diameters of 2.1 ± 0.4 nm (Nb(0)) and 1.9 ± 0.4 nm (Ta(0)). Whereas suspensions are chemically and colloidally stable, powder samples are very reactive. TEM/HRTEM, XRD, FT-IR, and XANES are used for characterization.

Impact of synthesis conditions on the morphology and crystal structure of tungsten nitride nanomaterials.

Nanocrystalline tungsten nitride (WN x ) aggregates and nanosheets are synthesized with a new alkylamine-based synthesis strategy for potential applications in nanoelectronics and catalysis. These applications preferentially require crystalline materials with controlled morphology, which has been rarely demonstrated for WN x nanomaterials in the past. In the synthesis approach presented in this work, the morphology of nanoscale WN x is controlled by long-chained amines that form lyotropic or lamellar phases depending on the surfactant concentration. The structural and chemical properties of the WN x nanomaterials are studied in detail using different electron microscopic techniques in combination with electron spectroscopic analyses. Material synthesis and sample preparation for transmission electron microscopy (TEM) were performed in an argon atmosphere (Schlenk line and glovebox). The samples were inserted into the electron microscope via an air-tight TEM transfer holder to protect the material from hydrolysis and oxidation. From the lyotropic phase nanocrystalline WN x aggregates were obtained, which consist of 2.4 ± 0.8 nm small crystallites of the cubic WN x phase with a composition of WN0.7. The lamellar phase with a higher surfactant concentration yields WN x nanosheets with lateral dimensions up to 500 nm and a mean thickness of 2.1 ± 1.1 nm. The nanosheets are N rich with a composition WN1.7-3.7 and occur in the hexagonal crystal structure. The nanosheets are often stacked on top of one another with frequent rotations of 4-6° around the hexagonal c axis, thereby forming commensurate interface structures between nanosheets. High stacking-fault densities and signs of nanotwins can be repeatedly observed in WN x nanosheets.

Pyridine-based Liquid-Phase Synthesis of Crystalline TiN and ZnSiN2 Nanoparticles.

TiN and ZnSiN2 nanoparticles are obtained via a novel pyridine-based synthesis route. This one-pot liquid-phase route strictly avoids all oxygen sources (including starting materials, surface functionalization, solvents), which is highly relevant in regard of the material purity and material properties. Colloidally stable suspensions of crystalline, small-sized TiN (5.4±0.4 nm) and ZnSiN2 (5.2±1.1 nm) are instantaneously available from the liquid phase. Elemental analysis and electron energy loss spectroscopy confirm the purity of the compounds and specifically the absence of oxygen. The as-prepared ZnSiN2 show yellowish emission (500-700 nm) already at room temperature with its maximum at 570 nm.

Analyzing contrast in cryo-transmission electron microscopy: Comparison of electrostatic Zach phase plates and hole-free phase plates.

Phase plates (PPs) are beneficial devices to improve the phase contrast of life-science objects in cryo-transmission electron microscopy (TEM). The development of the hole-free (HF) PP, which consists of a thin carbon film, has led to impressive results due to its ease in fabrication, implementation and application. However, the phase shift of the HFPP can be controlled only indirectly. The electrostatic Zach PP uses a strongly localized and adjustable electrostatic potential to generate well-defined and variable phase shifts between scattered and unscattered electrons. However, artifacts in phase-contrast TEM images are induced by the presence of the PP rod in the diffraction plane. We present a detailed analysis and comparison of the contrast-enhancing capabilities of both PP types and their emerging artifacts. For this purpose, cryo-TEM images of a standard T4-bacteriophage test sample were acquired with both PP types. Simulated images reproduce the experimental images well and substantially contribute to the understanding of contrast formation. An electrostatic Zach PP was used in this work to acquire cryo-electron tomograms with enhanced contrast, which are of similar quality as tomograms obtained by HFPP TEM.