Mapping Polar Distortions using Nanobeam Electron Diffraction and a Cepstral Approach.

Measuring local polar ordering is key to understanding ferroelectricity in thin films, especially for systems with small domains or significant disorder. Scanning nanobeam electron diffraction (NBED) provides an effective local probe of lattice parameters, local fields, polarization directions, and charge densities, which can be analyzed using a relatively low beam dose over large fields of view. However, quantitatively extracting the magnitudes and directions of polarization vectors from NBED remains challenging. Here, we use a cepstral approach, similar to a pair distribution function, to determine local polar displacements that drive ferroelectricity from NBED patterns. Because polar distortions generate asymmetry in the diffraction pattern intensity, we can efficiently recover the underlying displacements from the imaginary part of the cepstrum transform. We investigate the limits of this technique using analytical and simulated data and give experimental examples, achieving the order of 1.1 pm precision and mapping of polar displacements with nanometer resolution.

Steam-created grain boundaries for methane C-H activation in palladium catalysts.

Defects may display high reactivity because the specific arrangement of atoms differs from crystalline surfaces. We demonstrate that high-temperature steam pretreatment of palladium catalysts provides a 12-fold increase in the mass-specific reaction rate for carbon-hydrogen (C­H) activation in methane oxidation compared with conventional pretreatments. Through a combination of experimental and theoretical methods, we demonstrate that an increase in the grain boundary density through crystal twinning is achieved during the steam pretreatment and oxidation and is responsible for the increased reactivity. The grain boundaries are highly stable during reaction and show specific rates at least two orders of magnitude higher than other sites on the palladium on alumina (Pd/Al2O3) catalysts. Theoretical calculations show that strain introduced by the defective structure can enhance C­H bond activation. Introduction of grain boundaries through laser ablation led to further rate increases.

Influence of Polymer Aggregation and Liquid Immiscibility on Morphology Tuning by Varying Composition in PffBT4T-2DT/Non-Fullerene Organic Solar Cells.

The temperature dependent aggregation behavior of PffBT4T polymers used in organic solar cells plays a critical role in the formation of a favorable morphology in fullerene-based devices. However, there has been little investigation into the impact of donor/acceptor ratio on morphology tuning, especially for non-fullerene acceptors (NFAs). Herein, the influence of composition on morphology is reported for blends of PffBT4T-2DT with two NFAs, O-IDTBR and O-IDFBR. The monotectic phase behavior inferred from differential scanning calorimetry provides qualitative insight into the interplay between solid-liquid and liquid-liquid demixing. Transient absorption spectroscopy suggests that geminate recombination dominates charge decay and that the decay rate is insensitive to composition, corroborated by negligible changes in open-circuit voltage. Exciton lifetimes are also insensitive to composition, which is attributed to the signal being dominated by acceptor excitons which are formed and decay in domains of similar size and purity irrespective of composition. A hierarchical morphology is observed, where the composition dependence of size scales and scattering intensity from resonant soft X-ray scattering (R-SoXS) is dominated by variations in volume fractions of polymer/polymer rich domains. Results suggest an optimal morphology where polymer crystallite size and connectivity are balanced, ensuring a high probability of hole extraction via such domains.

Crystallography, Morphology, Electronic Structure, and Transport in Non-Fullerene/Non-Indacenodithienothiophene Polymer:Y6 Solar Cells.

Emerging nonfullerene acceptors (NFAs) with crystalline domains enable high-performance bulk heterojunction (BHJ) solar cells. Thermal annealing is known to enhance the BHJ photoactive layer morphology and performance. However, the microscopic mechanism of annealing-induced performance enhancement is poorly understood in emerging NFAs, especially regarding competing factors. Here, optimized thermal annealing of model system PBDB-TF:Y6 (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]-thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) decreases the open circuit voltage (VOC) but increases the short circuit current (JSC) and fill factor (FF) such that the resulting power conversion efficiency (PCE) increases from 14 to 15% in the ambient environment. Here we systematically investigate these thermal annealing effects through in-depth characterizations of carrier mobility, film morphology, charge photogeneration, and recombination using SCLC, GIXRD, AFM, XPS, NEXAFS, R-SoXS, TEM, STEM, fs/ns TA spectroscopy, 2DES, and impedance spectroscopy. Surprisingly, thermal annealing does not alter the film crystallinity, R-SoXS characteristic size scale, relative average phase purity, or TEM-imaged phase separation but rather facilitates Y6 migration to the BHJ film top surface, changes the PBDB-TF/Y6 vertical phase separation and intermixing, and reduces the bottom surface roughness. While these morphology changes increase bimolecular recombination (BR) and lower the free charge (FC) yield, they also increase the average electron and hole mobility by at least 2-fold. Importantly, the increased µh dominates and underlies the increased FF and PCE. Single-crystal X-ray diffraction reveals that Y6 molecules cofacially pack via their end groups/cores, with the shortest π-π distance as close as 3.34 Å, clarifying out-of-plane π-face-on molecular orientation in the nanocrystalline BHJ domains. DFT analysis of Y6 crystals reveals hole/electron reorganization energies of as low as 160/150 meV, large intermolecular electronic coupling integrals of 12.1-37.9 meV rationalizing the 3D electron transport, and relatively high µe of 10-4 cm2 V-1 s-1. Taken together, this work clarifies the richness of thermal annealing effects in high-efficiency NFA solar cells and tasks for future materials design.

Cosolvent Effects When Blade-Coating a Low-Solubility Conjugated Polymer for Bulk Heterojunction Organic Photovoltaics.

The adoption of solution-processed active layers in the production of thin-film photovoltaics is hampered by the transition from research fabrication techniques to scalable processing. We report a detailed study of the role of processing in determining the morphology and performance of organic photovoltaic devices using a commercially available, low-solubility, high-molar mass diketopyrrolopyrrole-based polymer donor. Ambient blade coating of thick layers in an inverted architecture was performed to best model scalable processing. Device performance was strongly dependent on the introduction of either o-dichlorobenzene (DCB), 1,8-diiodooctane, or diphenyl ether cosolvent into the chloroform (CHCl3) solution, which were all shown to drastically improve the morphology. To understand the origin of these morphological changes as a result of the addition of the cosolvent, in situ studies with grazing-incidence X-ray scattering and optical reflection interferometry were performed. Use of any of the cosolvents decreases the domain size relative to the single solvent system and moved the drying mechanism away from what is likely liquid-liquid phase separation to solid-liquid phase separation driven by polymer aggregation. Comparing the CHCl3 + DCB cast films to the CHCl3-only cast films, we observed both the formation of small domains and an increase in crystallinity during the evaporation of DCB due to a high nucleation rate from supersaturation. This resulted in percolated bulk heterojunction networks that performed similarly well with a wide range of film thicknesses from 180 to 440 nm, making this system amenable to continuous roll-to-roll processing methods.

In situ oxidation and reduction of cerium dioxide nanoparticles studied by scanning transmission electron microscopy.

Cerium dioxide nanocubes and truncated octahedra were reduced and oxidized in the scanning transmission electron microscope. The reduction process was stimulated by the electron beam and oxidation was supported by background gases in the microscope environment. High-angle annular dark field imaging is sensitive to local lattice distortions that arise as oxygen vacancies are created and cerium cations reduce enabling high spatial resolution characterization of this process with temporal resolution on the order of seconds. Such measurements enable us to differentiate and infer that the observed behavior between the nanocubes and truncated octahedra may be due to the difference in crystallographic termination of surfaces. In situ measurements taken with different partial pressures of oxygen reveal the cerium oxidation state and the dose rate threshold for the onset of beam reduction are influenced by the environment. Increasing oxygen partial pressure reduces the Ce3+ content and decreases susceptibility to electron beam driven reduction.

Formation of the Conducting Filament in TaO x-Resistive Switching Devices by Thermal-Gradient-Induced Cation Accumulation.

The distribution of tantalum and oxygen ions in electroformed and/or switched TaO x-based resistive switching devices has been assessed by high-angle annular dark-field microscopy, X-ray energy-dispersive spectroscopy, and electron energy-loss spectroscopy. The experiments have been performed in the plan-view geometry on the cross-bar devices producing elemental distribution maps in the direction perpendicular to the electric field. The maps revealed an accumulation of +20% Ta in the inner part of the filament with a 3.5% Ta-depleted ring around it. The diameter of the entire structure was approximately 100 nm. The distribution of oxygen was uniform with changes, if any, below the detection limit of 5%. We interpret the elemental segregation as due to diffusion driven by the temperature gradient, which in turn is induced by the spontaneous current constriction associated with the negative differential resistance-type I- V characteristics of the as-fabricated metal/oxide/metal structures. A finite-element model was used to evaluate the distribution of temperature in the devices and correlated with the elemental maps. In addition, a fine-scale (∼5 nm) intensity contrast was observed within the filament and interpreted as due phase separation of the functional oxide in the two-phase composition region. Understanding the temperature-gradient-induced phenomena is central to the engineering of oxide memory cells.

Local degradation pathways in lithium-rich manganese-nickel-cobalt-oxide epitaxial thin films.

The electrochemical performance and microstructure of positive electrodes are intimately linked. As such, developing batteries resistance to capacity and voltage fade requires understanding these underlying structure-properties relationships and their evolution with operation. Epitaxial films of a Li-rich manganese-nickel- cobalt oxide cathode material were deposited on (100) and (111) orientated SrRuO3/SrTiO3 substrates. Cyclic voltammetry and impedance spectroscopy tracked the response of these positive electrode materials, while the microstructure of the pristine and cycled films was characterized using transmission electron microscopy. Energy-dispersive X-ray spectroscopy identifies compositional fluctuations in as-deposited films. Phase transformations and dissolution were observed after electrochemical testing. There is a correlation between both local composition and substrate orientation (i.e., surface faceting) and what degradation pathways are active. Regions with comparatively higher concentrations of Ni and Co were more resistant to dissolution and unfavorable phase transformations than those with relatively more Mn. As such, a global composition metric may not be an accurate predictor of degradation and performance. Rather possessing the synthetic ability to engineer the chemical profile as well as characterizing it, pose a challenge and opportunity.

Domain Formation in Lithium-Rich Manganese-Nickel-Cobalt-Oxide Epitaxial Thin Films and Implications for Interpretation of Electrochemical Behavior.

Due to the directional dependence of physical properties, it is advantageous to grow and then study materials in specific orientations. Films of battery materials grown in epitaxy offers the possibility to gain new insight into the role of physical structure on electrochemical behaviors. Here we demonstrate the growth, testing, and characterization of monoclinic-phase (space group C2/m) Li-Mn-Ni-Co-O epitaxial films. The monoclinic phase is a layered structure and as such lithium diffusion is favored along specific crystallographic directions. Films were grown by pulsed laser deposition onto SrRuO3/SrTiO3 substrates with (001) and (111) orientations. Cyclic voltammetry measured the response of these positive electrode materials, while the film structure was characterized using scanning transmission electron microscopy. A combination of imaging and diffraction identifies the presence of orientational variants. Variants disrupt the orientation anisotropy expected of these layered materials when grown in epitaxy, thereby masking differences in electrochemical behavior as a function of substrate orientation. Learning to control the domain structure now presents itself as a challenge to realize the potential of low symmetry battery materials grown in epitaxy on high symmetry substrates.

Reduced Bimolecular Recombination in Blade-Coated, High-Efficiency, Small-Molecule Solar Cells.

To realize the full promise of solution deposited photovoltaic devices requires processes compatible with high-speed manufacturing. We report the performance and morphology of blade-coated bulk heterojunction devices based on the small molecule donor p-DTS(FBTTh2)2 when treated with a post-deposition solvent vapor annealing (SVA) process. SVA with tetrahydrofuran improves the device performance of blade-coated films more than solvent additive processing (SA) with 1,8-diiodooctane. In spin-coating, SA and SVA achieve similar device performance. Our optimized, blade coated, SVA devices achieve power conversion efficiencies over 8 % and maintain high efficiencies in films up to ≈ 250 nm thickness, providing valuable resilience to small process variations in high-speed manufacturing. Using impedance spectroscopy, we show that this advantageous behavior originates from highly suppressed bimolecular recombination in the SVA-treated films. Electron microscopy and grazing-incidence X-ray scattering experiments show that SA and SVA both produce highly crystalline donor domains, but SVA films have a radically smaller domain size compared to SA films. We attribute the different behavior to variations in initial nucleation density and relative ability of SVA and SA to control subsequent crystal growth.

Multimodal Characterization of the Morphology and Functional Interfaces in Composite Electrodes for Li-S Batteries by Li Ion and Electron Beams.

We report the characterization of multiscale 3D structural architectures of novel poly[sulfur-random-(1,3-diisopropenylbenzene)] copolymer-based cathodes for high-energy-density Li-S batteries capable of realizing discharge capacities >1000 mAh/g and long cycling lifetimes >500 cycles. Hierarchical morphologies and interfacial structures have been investigated by a combination of focused Li ion beam (LiFIB) and analytical electron microscopy in relation to the electrochemical performance and physicomechanical stability of the cathodes. Charge-free surface topography and composition-sensitive imaging of the electrodes was performed using recently introduced low-energy scanning LiFIB with Li+ probe sizes of a few tens of nanometers at 5 keV energy and 1 pA probe current. Furthermore, we demonstrate that LiFIB has the ability to inject a certain number of Li cations into the material with nanoscale precision, potentially enabling control of the state of discharge in the selected area. We show that chemical modification of the cathodes by replacing the elemental sulfur with organosulfur copolymers significantly improves its structural integrity and compositional homogeneity down to the sub-5-nm length scale, resulting in the creation of (a) robust functional interfaces and percolated conductive pathways involving graphitic-like outer shells of aggregated nanocarbons and (b) extended micro- and mesoscale porosities required for effective ion transport.

Optical Asymmetry and Nonlinear Light Scattering from Colloidal Gold Nanorods.

A systematic study is presented of the intensity-dependent nonlinear light scattering spectra of gold nanorods under resonant excitation of the longitudinal surface plasmon resonance (SPR). The spectra exhibit features due to coherent second and third harmonic generation as well as a broadband feature that has been previously attributed to multiphoton photoluminescence arising primarily from interband optical transitions in the gold. A detailed study of the spectral dependence of the scaling of the scattered light with excitation intensity shows unexpected scaling behavior of the coherent signals, which is quantitatively accounted for by optically induced damping of the SPR mode through a Fermi liquid model of the electronic scattering. The broadband feature is shown to arise not from luminescence, but from scattering of the second-order longitudinal SPR mode with the electron gas, where efficient excitation of the second order mode arises from an optical asymmetry of the nanorod. The electronic-temperature-dependent plasmon damping and the Fermi-Dirac distribution together determine the intensity dependence of the broadband emission, and the structure-dependent absorption spectrum determines the spectral shape through the fluctuation-dissipation theorem. Hence a complete self-consistent picture of both coherent and incoherent light scattering is obtained with a single set of physical parameters.

Origins and demonstrations of electrons with orbital angular momentum.

The surprising message of Allen et al. (Allen et al. 1992 Phys. Rev. A 45, 8185 (doi:10.1103/PhysRevA.45.8185)) was that photons could possess orbital angular momentum in free space, which subsequently launched advancements in optical manipulation, microscopy, quantum optics, communications, many more fields. It has recently been shown that this result also applies to quantum mechanical wave functions describing massive particles (matter waves). This article discusses how electron wave functions can be imprinted with quantized phase vortices in analogous ways to twisted light, demonstrating that charged particles with non-zero rest mass can possess orbital angular momentum in free space. With Allen et al. as a bridge, connections are made between this recent work in electron vortex wave functions and much earlier works, extending a 175 year old tradition in matter wave vortices.This article is part of the themed issue 'Optical orbital angular momentum'.

Analytical Multimode Scanning and Transmission Electron Imaging and Tomography of Multiscale Structural Architectures of Sulfur Copolymer-Based Composite Cathodes for Next-Generation High-Energy Density Li-S Batteries.

Poly[sulfur-random-(1,3-diisopropenylbenzene)] copolymers synthesized via inverse vulcanization represent an emerging class of electrochemically active polymers recently used in cathodes for Li-S batteries, capable of realizing enhanced capacity retention (1,005 mAh/g at 100 cycles) and lifetimes of over 500 cycles. The composite cathodes are organized in complex hierarchical three-dimensional (3D) architectures, which contain several components and are challenging to understand and characterize using any single technique. Here, multimode analytical scanning and transmission electron microscopies and energy-dispersive X-ray/electron energy-loss spectroscopies coupled with multivariate statistical analysis and tomography were applied to explore origins of the cathode-enhanced capacity retention. The surface topography, morphology, bonding, and compositions of the cathodes created by combining sulfur copolymers with varying 1,3-diisopropenylbenzene content and conductive carbons have been investigated at multiple scales in relation to the electrochemical performance and physico-mechanical stability. We demonstrate that replacing the elemental sulfur with organosulfur copolymers improves the compositional homogeneity and compatibility between carbons and sulfur-containing domains down to sub-5 nm length scales resulting in (a) intimate wetting of nanocarbons by the copolymers at interfaces; (b) the creation of 3D percolation networks of conductive pathways involving graphitic-like outer shells of aggregated carbons;

Structural studies of Li1.2Mn0.55Ni0.15Co0.1O2 electrode material.

A pristine Li-rich layered electrode material with composition Li1.2Mn0.55Ni0.15Co0.1O2 was characterized by X-ray diffraction, transmission electron microscopy, and scanning transmission electron microscopy to determine whether it is a coherent mixture of monoclinic C2/m Li2MO3 and trigonal [Formula: see text] LiMO2 phases or a solid solution of the monoclinic phase. Contradictory results have been previously reported which can be attributed to the complexity and structural similarity of the monoclinic and trigonal phases. We resolved this uncertainty by combining diffraction and imaging techniques that probe complimentary length scales. Our results demonstrate that the structure is primarily monoclinic, supporting the solid solution model, although near surface structural alterations were also observed.

Dose-rate-dependent damage of cerium dioxide in the scanning transmission electron microscope.

Beam damage caused by energetic electrons in the transmission electron microscope is a fundamental constraint limiting the collection of artifact-free information. Through understanding the influence of the electron beam, experimental routines may be adjusted to improve the data collection process. Investigations of CeO2 indicate that there is not a critical dose required for the accumulation of electron beam damage. Instead, measurements using annular dark field scanning transmission electron microscopy and electron energy loss spectroscopy demonstrate that the onset of measurable damage occurs when a critical dose rate is exceeded. The mechanism behind this phenomenon is that oxygen vacancies created by exposure to a 300keV electron beam are actively annihilated as the sample re-oxidizes in the microscope environment. As a result, only when the rate of vacancy creation exceeds the recovery rate will beam damage begin to accumulate. This observation suggests that dose-intensive experiments can be accomplished without disrupting the native structure of the sample when executed using dose rates below the appropriate threshold. Furthermore, the presence of an encapsulating carbonaceous layer inhibits processes that cause beam damage, markedly increasing the dose rate threshold for the accumulation of damage.

Transient Thermometry and High-Resolution Transmission Electron Microscopy Analysis of Filamentary Resistive Switches.

We present data on the filament size and temperature distribution in Hf0.82Al0.18Ox-based Resistive Random Access Memory (RRAM) devices obtained by transient thermometry and high-resolution transmission electron microscopy (HRTEM). The thermometry shows that the temperature of the nonvolatile conducting filament can reach temperatures as high as 1600 K at the onset of RESET at voltage of 0.8 V and power of 40 µW. The size of the filament was estimated at about 1 nm in diameter. Hot filament increases the temperature of the surrounding high resistivity oxide, causing it to conduct and carry a significant fraction of the total current. The current spreading results in slowing down the filament temperature increase at higher power. The results of thermometry have been corroborated by HRTEM analysis of the as-fabricated and switched RRAM devices. The functional HfAlOx layer in as-fabricated devices is amorphous. In devices that were switched, we detected a small crystalline region of 10-15 nm in size. The crystallization temperature of the HfAlOx was determined to be 850 K in an independent annealing experiment. The size of the crystalline region agrees with thermal modeling based on the thermometry data. Scanning transmission electron microscopy (TEM) coordinated with electron energy loss spectroscopy could not detect changes in the chemical makeup of the filament.

Oxidation-state sensitive imaging of cerium dioxide by atomic-resolution low-angle annular dark field scanning transmission electron microscopy.

Low-angle annular dark field (LAADF) scanning transmission electron microscopy (STEM) imaging is presented as a method that is sensitive to the oxidation state of cerium ions in CeO2 nanoparticles. This relationship was validated through electron energy loss spectroscopy (EELS), in situ measurements, as well as multislice image simulations. Static displacements caused by the increased ionic radius of Ce(3+) influence the electron channeling process and increase electron scattering to low angles while reducing scatter to high angles. This process manifests itself by reducing the high-angle annular dark field (HAADF) signal intensity while increasing the LAADF signal intensity in close proximity to Ce(3+) ions. This technique can supplement STEM-EELS and in so doing, relax the experimental challenges associated with acquiring oxidation state information at high spatial resolutions.