Interferometric 4D-STEM for Lattice Distortion and Interlayer Spacing Measurements of Bilayer and Trilayer 2D Materials

Van der Waals materials composed of stacks of individual atomic layers have attracted considerable attention due to their exotic electronic properties that can be altered by, e.g., manipulating the twist angle of bilayer materials or the stacking sequence of trilayer materials. To fully understand and control the unique properties of these few-layer materials, a technique that can provide information about their local in-plane structural deformations, twist direction, and out-of-plane structure is needed. In principle, interference in overlap regions of Bragg disks originating from separate layers of a material encodes 3D information about the relative positions of atoms in the corresponding layers. Here, an interferometric 4D scanning transmission electron microscopy technique is described that utilizes this phenomenon to extract precise structural information from few-layer materials with nm-scale resolution. It is demonstrated how this technique enables measurement of local pm-scale in-plane lattice distortions as well as twist direction and average interlayer spacings in bilayer and trilayer graphene, and therefore provides a means to better understand the interplay between electronic properties and precise structural arrangements of few-layer 2D materials.

Atomistic insights into the nucleation and growth of platinum on palladium nanocrystals

Despite the large number of reports on colloidal nanocrystals, very little is known about the mechanistic details in terms of nucleation and growth at the atomistic level. Taking bimetallic core-shell nanocrystals as an example, here we integrate in situ liquid-cell transmission electron microscopy with first-principles calculations to shed light on the atomistic details involved in the nucleation and growth of Pt on Pd cubic seeds. We elucidate the roles played by key synthesis parameters, including capping agent and precursor concentration, in controlling the nucleation site, diffusion path, and growth pattern of the Pt atoms. When the faces of a cubic seed are capped by Br-, Pt atoms preferentially nucleate from corners and then diffuse to edges and faces for the creation of a uniform shell. The diffusion does not occur until the Pt deposited at the corner has reached a threshold thickness. At a high concentration of the precursor, self-nucleation takes place and the Pt clusters then randomly attach to the surface of a seed for the formation of a non-uniform shell. These atomistic insights offer a general guideline for the rational synthesis of nanocrystals with diverse compositions, structures, shapes, and related properties.

Swelling-Induced Symmetry Breaking: A Versatile Approach to the Scalable Production of Colloidal Particles with a Janus Structure

Janus particles are widely sought for applications related to colloidal assembly, stabilization of emulsions, and development of active colloids, among others. Here we report a versatile route to the fabrication of well-controlled Janus particles by simply breaking the symmetry of spherical particles with swelling. When a polystyrene (PS) sphere covered by a rigid shell made of silica or polydopamine is exposed to a good solvent for PS, a gradually increased pressure will be created inside the shell. If the pressure becomes high enough to poke a hole in the shell, the spherical symmetry will break while pushing out the swollen PS through the opening to generate a Janus particle comprised of two distinct components. One of the components is made of PS and its size is controlled by the extent of swelling. The other component is comprised of the rigid shell and remaining PS, with its overall diameter determined by the original PS sphere and the rigid shell. This solution-based route holds promises for the scalable production of complex Janus particles with a variety of compositions and in large quantities.

Machine Learning Method Reveals Hidden Strong Metal-Support Interaction in Microscopy Datasets

Forming an ultra-thin, permeable encapsulation oxide-support layer on a metal catalyst surface is considered an effective strategy for achieving a balance between high stability and high activity in heterogenous catalysts. The success of such a design relies not only on the thickness, ideally one to two atomic layers thick, but also on the morphology and chemistry of the encapsulation layer. Reliably identifying the presence and chemical nature of such a trace layer has been challenging. Electron energy-loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM), the primary technique utilized for such studies, is limited by a weak signal on overlayers when using conventional analysis methods, often leading to misinterpreted or missed information. Here, a robust, unsupervised machine learning data analysis method is developed to reveal trace encapsulation layers that are otherwise overlooked in STEM-EELS datasets. This method provides a reliable tool for analyzing encapsulation of catalysts and is generally applicable to any spectroscopic analysis of materials and devices where revealing a trace signal and its spatial distribution is challenging.

Kinetically Controlled Synthesis of Rhodium Nanocrystals with Different Shapes and a Comparison Study of Their Thermal and Catalytic Properties

We report the synthesis of Rh nanocrystals with different shapes by controlling the kinetics involved in the growth of preformed Rh cubic seeds. Specifically, Rh nanocrystals with cubic, cuboctahedral, and octahedral shapes can all be obtained from the same cubic seeds under suitable reduction kinetics for the precursor. The success of such a synthesis also relies on the use of a halide-free precursor to avoid oxidative etching, as well as the involvement of a sufficiently high temperature to remove Br^- ions from the seeds while ensuring adequate surface diffusion. The availability of Rh nanocrystals with cubic and octahedral shapes allows for an evaluation of the facet dependences of their thermal and catalytic properties. The data from in situ electron microscopy studies indicate that the cubic and octahedral Rh nanocrystals can keep their original shapes up to 700 and 500 °C, respectively. When tested as catalysts for hydrazine decomposition, the octahedral nanocrystals exhibit almost 4-fold enhancement in terms of H₂ selectivity relative to the cubic counterpart. As for ethanol oxidation, the order is reversed, with the cubic nanocrystals being about three times more active than the octahedral sample.

Engineering Tunneling Selector to Achieve High Non-linearity for 1S1R Integration

Memristor devices have been extensively studied as one of the most promising technologies for next-generation non-volatile memory. However, for the memristor devices to have a real technological impact, they must be densely packed in a large crossbar array (CBA) exceeding Gigabytes in size. Devising a selector device that is CMOS compatible, 3D stackable, and has a high non-linearity (NL) and great endurance is a crucial enabling ingredient to reach this goal. Tunneling based selectors are very promising in these aspects, but the mediocre NL value limits their applications in large passive crossbar arrays. In this work, we demonstrated a trilayer tunneling selector based on the Ge/Pt/TaN1+x/Ta2O5/TaN1+x/Pd layers that could achieve a NL of 3 × 105, which is the highest NL achieved using a tunnel selector so far. The record-high tunneling NL is partially attributed to the bottom electrode's ultra-smoothness (BE) induced by a Ge/Pt layer. We further demonstrated the feasibility of 1S1R (1-selector 1-resistor) integration by vertically integrating a Pd/Ta2O5/Ru based memristor on top of the proposed selector.

Direct visualization of anionic electrons in an electride reveals inhomogeneities

Electrides are an unusual family of materials that feature loosely bonded electrons that occupy special interstitial sites and serve as anions. They are attracting increasing attention because of their wide range of exotic physical and chemical properties. Despite the critical role of the anionic electrons in inducing these properties, their presence has not been directly observed experimentally. Here, we visualize the columnar anionic electron density within the prototype electride Y₅Si₃ with sub-angstrom spatial resolution using differential phase-contrast imaging in a scanning transmission electron microscope. The data further reveal an unexpected charge variation at different anionic sites. Density functional theory simulations show that the presence of trace H impurities is the cause of this inhomogeneity. The visualization and quantification of charge inhomogeneities in crystals will serve as valuable input in future theoretical predictions and experimental analysis of exotic properties in electrides and materials beyond.

A Simple Route to the Synthesis of Pt Nanobars and the Mechanistic Understanding of Symmetry Reduction

Noble-metal nanocrystals with anisotropic shapes have received increasing interest owing to their unique properties. Here, a facile route to the preparation of Pt nanobars with aspect ratios tunable up to 2.1 was reported by simply reducing a Pt^IV precursor in N,N-dimethylformamide (DMF) at 160 °C in the presence of poly(vinyl pyrrolidone) (PVP). In addition to its commonly observed roles as a solvent and a reductant, DMF could also decompose to generate CO, a capping agent capable of selectively passivating Pt{100} facets to promote the formation of nanobars. The size and aspect ratio of the nanobars could be tuned by varying the amount of Pt^IV precursor involved in the synthesis, as well as the concentration of PVP because of its dual roles as a stabilizer and a co-reductant. Our mechanistic study indicated that the anisotropic growth resulted from both particle coalescence and localized oxidative etching followed by preferential growth.

Elucidating Interfacial Stability between Lithium Metal Anode and Li Phosphorus Oxynitride via In Situ Electron Microscopy

Li phosphorus oxynitride (LiPON) is one of a very few solid electrolytes that have demonstrated high stability against Li metal and extended cyclability with high Coulombic efficiency for all solid-state batteries (ASSBs). However, theoretical calculations show that LiPON reacts with Li metal. Here, we utilize in situ electron microscopy to observe the dynamic evolutions at the LiPON-Li interface upon contacting and under biasing. We reveal that a thin interface layer (∼60 nm) develops at the LiPON-Li interface upon contact. This layer is composed of conductive binary compounds that show a unique spatial distribution that warrants an electrochemical stability of the interface, serving as an effective passivation layer. Our results explicate the excellent cyclability of LiPON and reconcile the existing debates regarding the stability of the LiPON-Li interface, demonstrating that, though glassy solid electrolytes may not have a perfect initial electrochemical window with Li metal, they may excel in future applications for ASSBs.

Redox-couple investigations in Si-doped Li-rich cathode materials

In this investigation, the improved electrochemical behavior in Si-doped Li-rich cathodes is studied with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Z-contrast images show a layered structure that develops a thin, spinel-like surface layer after the first charge cycle. Si-doping increases discharge capacity by ∼25% and appears to retard the surface phase transformation. Based on electron energy loss spectra, the surface layer in the doped material has an altered oxygen electronic environment, which supports the STEM findings. Furthermore, Si-doping changes the redox behavior during the activation cycle. Density functional theory calculations indicate that Si-doping can increase oxygen vacancy formation, and change the sequence of the redox couples by introducing more oxygen vacancies before or during the typical high voltage activation process. The results of this work indicate that the type of doping employed here is an effective strategy for controlling the complex charge compensation mechanisms in lithium-rich cathodes.

Controlling the Surface Oxidation of Cu Nanowires Improves Their Catalytic Selectivity and Stability toward C2+ Products in CO2 Reduction

Copper nanostructures are promising catalysts for the electrochemical reduction of CO₂ because of their unique ability to produce a large proportion of multi-carbon products. Despite great progress, the selectivity and stability of such catalysts still need to be substantially improved. Here, we demonstrate that controlling the surface oxidation of Cu nanowires (CuNWs) can greatly improve their C₂₊ selectivity and stability. Specifically, we achieve a faradaic efficiency as high as 57.7 and 52.0 % for ethylene when the CuNWs are oxidized by the O₂ from air and aqueous H₂ O₂ , respectively, and both of them show hydrogen selectivity below 12 %. The high yields of C₂₊ products can be mainly attributed to the increase in surface roughness and the generation of defects and cavities during the electrochemical reduction of the oxide layer. Our results also indicate that the formation of a relatively thick, smooth oxide sheath can improve the catalytic stability by mitigating the fragmentation issue.

Pt-Co truncated octahedral nanocrystals: a class of highly active and durable catalysts toward oxygen reduction

We report a facile and scalable synthesis of Pt-Co truncated octahedral nanocrystals (TONs) by employing Pt(acac)2 and Co(acac)2 as precursors, together with CO molecules and Mn atoms derived from the decomposition of Mn2(CO)10 as a reductant and a {111} facet-directing agent, respectively. Both the composition and yield of the Pt-Co TONs could be varied through the introduction of CHCl3. When tested at 80 °C using membrane electrode assembly (MEA), the 4 nm Pt2.6Co TONs gave a mass activity of 294 A gPt-1 at beginning-of-life (BOL) and it increased to 384 A gPt-1 during recovery cycles. The mass activity at BOL only dropped by 24% after 30 000 voltage cycles at end-of-life (EOL) in a metal dissolution accelerated stress test. The Pt2.6Co/C catalyst outperformed the commercial TKK Pt3Co/C (230 A gPt-1 at BOL and 40% loss after 30 000 cycles at EOL) in terms of both activity and durability. Our systematic analysis suggested that the enhancement in activity can be attributed to the combination of small, uniform size and well-defined {111} facets. This new class of catalysts holds promise for applications in proton-exchange membrane fuel cells.

Efficient upgrading of CO to C3 fuel using asymmetric C-C coupling active sites

The electroreduction of C₁ feedgas to high-energy-density fuels provides an attractive avenue to the storage of renewable electricity. Much progress has been made to improve selectivity to C₁ and C₂ products, however, the selectivity to desirable high-energy-density C₃ products remains relatively low. We reason that C₃ electrosynthesis relies on a higher-order reaction pathway that requires the formation of multiple carbon-carbon (C-C) bonds, and thus pursue a strategy explicitly designed to couple C₂ with C₁ intermediates. We develop an approach wherein neighboring copper atoms having distinct electronic structures interact with two adsorbates to catalyze an asymmetric reaction. We achieve a record n-propanol Faradaic efficiency (FE) of (33 ± 1)% with a conversion rate of (4.5 ± 0.1) mA cm⁻², and a record n-propanol cathodic energy conversion efficiency (EE_{cathodic half-cell}) of 21%. The FE and EE_{cathodic half-cell} represent a 1.3× improvement relative to previously-published CO-to-n-propanol electroreduction reports.

Catalysts for CO electroreduction have focused on Cu, and their main products have been C₂ chemicals. Here authors use the concept of asymmetric active sites to develop a class of doped Cu catalysts for C-C coupling, delivering record selectivity to n-propanol.

Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials

Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State-of-the-art atomic resolution or in situ scanning transmission electron microscopy and electron energy-loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single-atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room-temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.

Catalytic System Based on Sub-2 nm Pt Particles and Its Extraordinary Activity and Durability for Oxygen Reduction

Carbon-supported Pt nanoparticles are used as catalysts for a variety of reactions including the oxygen reduction reaction (ORR) key to proton-exchange membrane fuel cells, but their catalytic performance has long been plagued by detachment and sintering. Here we report the in situ growth of sub-2 nm Pt particles on a commercial carbon support via the galvanic reaction between a Pt(II) precursor and a uniform film of amorphous Se predeposited on the support. The residual Se could serve as a linker to strongly anchor the Pt nanoparticles to the carbon surface, leading to a catalytic system with extraordinary activity and durability toward ORR. Even after 20 000 cycles of accelerated durability test, the sub-2 nm Pt particles were still dispersed well on the carbon support and maintained a mass activity more than three-times as high as the pristine value of a commercial Pt/C catalyst.

Ternary CoPtAu Nanoparticles as a General Catalyst for Highly Efficient Electro-oxidation of Liquid Fuels

Efficient electro-oxidation of formic acid, methanol, and ethanol is challenging owing to the multiple chemical reaction steps required to accomplish full oxidation to CO₂ . Herein, a ternary CoPtAu nanoparticle catalyst system is reported in which Co and Pt form an intermetallic L1₀ -structure and Au segregates on the surface to alloy with Pt. The L1₀ -structure stabilizes Co and significantly enhances the catalysis of the PtAu surface towards electro-oxidation of ethanol, methanol, and formic acid, with mass activities of 1.55 A/mg_Pt , 1.49 A/mg_Pt , and 11.97 A/mg_Pt , respectively in 0.1 m HClO₄ . The L1₀ -CoPtAu catalyst is also stable, with negligible degradation in mass activities and no obvious Co/Pt/Au composition changes after 10 000 potential cycles. The in situ surface-enhanced infrared absorption spectroscopy study indicates that the ternary catalyst activates the C-C bond more efficiently for ethanol oxidation.

Construction of a Nanoporous Highly Crystalline Hexagonal Boron Nitride from an Amorphous Precursor for Catalytic Dehydrogenation

Hexagonal boron nitride (h-BN) is regarded as a graphene analogue and exhibits important characteristics and vast application potentials. However, discovering a facile method for the preparation of nanoporous crystalline h-BN nanosheets (h-BNNS) is still a challenge. Herein, a novel and simple route for the conversion of amorphous h-BN precursors into highly crystalline h-BNNS was achieved through a successive dissolution-precipitation/crystallization process in the presence of magnesium. The h-BNNS has high crystallinity, high porosity with a surface area of 347 m²  g^-1 , high purity, and enhanced thermal stability. Improved catalytic performance of crystalline h-BNNS was evidenced by its much higher catalytic efficiency in the dehydrogenation of dodecahydro-N-ethylcarbazole, compared with its amorphous h-BN precursor, as well as other precious-metal-loaded heterogeneous catalysts.

Ruthenium Nanoframes in the Face-Centered Cubic Phase: Facile Synthesis and Their Enhanced Catalytic Performance

Owing to their highly open structure and a large number of low-coordination sites on the surface, noble-metal nanoframes are intriguing for catalytic applications. Here, we demonstrate the rational synthesis of Ru cuboctahedral nanoframes with enhanced catalytic performance toward hydrazine decomposition. The synthesis starts from Pd nanocubes, which quickly undergo truncation at the corners as a consequence of oxidative etching caused by Br^- ions. Afterward, the galvanic replacement reaction between Pd and Ru(III) ions dominates, leading to the selective deposition of Ru atoms on the corners and edges and thereby the fabrication of Pd@Ru core-frame cuboctahedra. Significantly, the deposited Ru atoms are crystallized in a face-centered cubic (fcc) phase instead of the hexagonal close-packed (hcp) structure typical of bulk Ru. Upon the removal of Pd remaining in the core via chemical etching, we obtain Ru cuboctahedral nanoframes. By varying the amount of the Ru(III) precursor, the ridge thickness of the nanoframes can be tuned from a few atomic layers up to 10. Both the frame structure and fcc crystal phase of the Ru cuboctahedral nanoframes can be well preserved up to 300 °C. When compared with hcp-Ru nanoparticles, the fcc-Ru nanoframes displayed substantial enhancement in terms of H₂ selectivity toward hydrazine decomposition. This work offers the opportunity to engineer both the morphology and crystal phase of Ru nanocrystals for catalytic applications.

Ru Octahedral Nanocrystals with a Face-Centered Cubic Structure, {111} Facets, Thermal Stability up to 400 °C, and Enhanced Catalytic Activity

Ruthenium nanocrystals with both a face-centered cubic ( fcc) structure and well-controlled facets are attractive catalytic materials for various reactions. Here we report a simple method for the synthesis of Ru octahedral nanocrystals with an fcc structure and an edge length of 9 nm. The success of this synthesis relies on the use of 4.5 nm Rh cubes as seeds to facilitate the heterogeneous nucleation and overgrowth of Ru atoms. We choose Rh because it can resist oxidative etching under the harsh conditions for Ru overgrowth, it can be readily prepared as nanocubes with edge lengths less than 5 nm, and its atoms have a size close to that of Ru atoms. During the seed-mediated growth, the atomic packing of Ru overlayers follows an fcc lattice, in contrast to the conventional hexagonal close-packed ( hcp) lattice associated with bulk Ru. The final product takes an octahedral shape, with the surface enclosed by {111} facets. Our in situ measurements suggest that both the octahedral shape and the fcc crystal structure can be well preserved up to 400 °C, which is more than 100 °C higher than what was reported for Ru octahedral nanocages. When utilized as catalysts, the Ru octahedral nanocrystals exhibited 4.4-fold enhancement in terms of specific activity toward oxygen evolution relative to hcp-Ru nanoparticles. We also demonstrate that Ru{111} facets are more active than Ru{100} facets in catalyzing the oxygen evolution reaction. Altogether, this work offers an effective method for the synthesis of Ru nanocrystals with an fcc structure and well-defined {111} facets, as well as enhanced thermal stability and catalytic activity. We believe these nanocrystals will find use in various catalytic applications.

Osteoprotegerin mediate RANK/RANKL signaling inhibition eases asthma inflammatory reaction by affecting the survival and function of dendritic cells

INTRODUCTION

Asthma is a chronic inflammatory, heterogeneous airway disease affecting millions of people around the world. Dendritic cells (DCs) are considered the most important antigen-presenting cell in asthma airway inflammatory reaction. But whether osteoprotegerin (OPG) mediate RANK/RANKL signaling inhibition influences asthma development by affecting the survival and function of DCs remains unclear. In this study, we assessed the effects of OPG on DCs and asthma.

MATERIAL AND METHODS

BALB/c mice immunized with ovalbumin (OVA) were challenged thrice with an aerosol of OVA every second day for eight days. Dexamethasone (1.0mg/kg) or OPG (50μg/kg) was administered intraperitoneally to OVA-immunized BALB/c mice on day 24 once a day for nine days. Mice were analyzed for effects of OPG on asthma, inflammatory cell infiltration and cytokine levels in lung tissue. The expression of RANK and β-actin was detected by Western Blot. DCs were isolated from mouse bone morrow. Cell survival was assessed by cell counting. The content of IL-12 was detected by ELISA.

RESULTS

Results showed that OVA increased the number of inflammatory factors in BALF, elevated lung inflammation scores in mice. OPG reversed the alterations induced by OVA in the asthmatic mice. OPG inhibited the survival and function of DC via inhibition of RANK/RANKL signaling.

CONCLUSIONS

This research proved inhibition of RANK/RANKL signaling by OPG could ease the inflammatory reaction in asthma, providing new evidence for the application of OPG on asthma.

Photothermal transformation of Au-Ag nanocages under pulsed laser irradiation

Pulsed laser irradiation has emerged as an effective means to photothermally transform plasmonic nanostructures after their use in different biomedical applications. However, the ability to predict the products after photothermal transformation requires extensive ex situ studies. Here, we report a systematic study of the photothermal transformation of Au-Ag nanocages with a localized surface plasmon resonance at ca. 750 nm under pulsed laser irradiation at different fluences and a pulse duration of 5 ns. At biologically relevant laser energies, the pulsed laser transforms Au-Ag nanocages into pseudo-spherical, solid nanoparticles. The solid nanoparticles contained similar numbers of Au and Ag atoms to the parent Au-Ag nanocages. At increased laser fluences (>16 mJ cm-2) and number of pulses (>150), the average diameter of the resulting pseudo-spherical particles increased due to the involvement of Ostwald ripening and/or attachment-based growth. The changes in optical properties as a result of the transformation were validated using simulations based on the discrete dipole approximation method, where the spectral profiles and peak positions of the initial and final states matched well with the experimentally derived data. The results may have implications for the future use of Au-Ag nanocages in biomedicine, catalysis, and sensing.

An all-in-one Sn–Co alloy as a binder-free anode for high-capacity batteries and its dynamic lithiation in situ

An all-in-one Sn–Co alloy anode is reported, which exhibits a robust electrode structure confirmed by in situ transmission electron microscopy.

Migration of Iron Oxide Nanoparticle through a Silica Shell by the Redox-Buffering Effect

This study demonstrates that mineral redox buffer, an important concept in geology, can be used to manipulate the migration of nanoparticles and produce nanostructures of unexpected morphologies. Using a silica shell as a redox buffer, we show that iron oxide nanoparticles can be relocated from inside to the outer surface of the silica shell. The migration of iron oxide through silica was initiated by manipulation of the oxygen fugacity conditions at an elevated temperature. During the treatment, iron oxide was absorbed and then separated from the silica shell by the formation and then decomposition of iron silicate (Fe₂SiO₄). Tuning the relative dimensions of the iron oxide core and silica shell allows control of the shape of the iron oxide-silica composite structures. It is believed that the discovery of the nanoscale redox buffering effect can be extended to control the morphological configuration of other multivalent metal oxide nanocomposite structures by this particular type of template synthesis through manipulation of the chemical-transport properties of nanoscale templates.

Direct in Situ Observation and Analysis of the Formation of Palladium Nanocrystals with High-Index Facets

Synthesizing concave-structured nanoparticles (NP) with high-index surfaces offers a viable method to significantly enhance the catalytic activity of NPs. Current approaches for fabricating concave NPs, however, are limited. Exploring novel synthesis methods requires a thorough understanding of the competing mechanisms that contribute to the evolution of surface structures during NP growth. Here, by tracking the evolution of Pd nanocubes into concave NPs at atomic scale using in situ liquid cell transmission electron microscopy, our study reveals that concave-structured Pd NPs can be formed by the cointroduction of surface capping agents and halogen ions. These two chemicals jointly create a new surface energy landscape of Pd NPs, leading to the morphological transformation. In particular, Pd atoms dissociate from the {100} surfaces with the aid of Cl^- ions and preferentially redeposit to the corners and edges of the nanocubes when the capping agent polyvinylpyrrolidone is introduced, resulting in the formation of concave Pd nanocubes with distinctive high-index facets. Our work not only demonstrates a potential route for synthesizing NPs with well-defined high-index facets but also reveals the detailed atomic-scale kinetics during their formation, providing insight for future predictive synthesis.

Confined Lithium-Sulfur Reactions in Narrow-Diameter Carbon Nanotubes Reveal Enhanced Electrochemical Reactivity

We demonstrate an unusual electrochemical reaction of sulfur with lithium upon encapsulation in narrow-diameter (subnanometer) single-walled carbon nanotubes (SWNTs). Our study provides mechanistic insight on the synergistic effects of sulfur confinement and Li⁺ ion solvation properties that culminate in a new mechanism of these sub-nanoscale-enabled reactions (which cannot be solely attributed to the lithiation-delithiation of conventional sulfur). Two types of SWNTs with distinct diameters, produced by electric arc (EA-SWNTs, average diameter 1.55 nm) or high-pressure carbon monoxide (HiPco-SWNTs, average diameter 1.0 nm), are investigated with two comparable electrolyte systems based on tetraethylene glycol dimethyl ether (TEGDME) and 1,4,7,10,13-pentaoxacyclopentadecane (15-crown-5). Electrochemical analyses indicate that a conventional solution-phase Li-S reaction occurs in EA-SWNTs, which can be attributed to the smaller solvated [Li(TEGDME)]⁺ and [Li(15-crown-5)]⁺ ions within the EA-SWNT diameter. In stark contrast, the Li-S confined in narrower diameter HiPco-SWNTs exhibits unusual electrochemical behavior that can be attributed to a solid-state reaction enabled by the smaller HiPco-SWNT diameter compared to the size of solvated Li⁺ ions. Our results of the electrochemical analyses are corroborated and supported with various spectroscopic analyses including operando Raman, X-ray photoelectron spectroscopy, and first-principles calculations from density functional theory. Taken together, our findings demonstrate that the controlled solid-state lithiation-delithiation of sulfur and an enhanced electrochemical reactivity can be achieved by sub-nanoscale encapsulation and one-dimensional confinement in narrow-diameter SWNTs.

Charge Transport Modulation in PbSe Nanocrystal Solids by Au xAg1- x Nanoparticle Doping

Nanocrystal (NC) solids are an exciting class of materials, whose physical properties are tunable by choice of the NCs as well as the strength of the interparticle coupling. One can consider these NCs as "artificial atoms" in analogy to the formation of condensed matter from atoms. Akin to atomic doping, the doping of a semiconducting NC solid with impurity NCs can drastically alter its electronic properties. A high degree of complexity is possible in these artificial structures by adjusting the size, shape, and composition of the building blocks, which enables "designer" materials with targeted properties. Here, we present the doping of the PbSe NC solids with a series of Au _xAg_{1- x} alloy nanoparticles (NPs). A combination of temperature-dependent electrical conductance and Seebeck coefficient measurements and room-temperature Hall effect measurements demonstrates that the incorporation of metal NPs both modifies the charge carrier density of the NC solids and introduces energy barriers for charge transport. These studies point to charge carrier injection from the metal NPs into the PbSe NC matrix. The charge carrier density and charge transport dynamics in the doped NC solids are adjustable in a wide range by employing the Au _xAg_{1- x} NP with different Au:Ag ratio as dopants. This doping strategy could be of great interest for thermoelectric applications taking advantage of the energy filtering effect introduced by the metal NPs.

Antisite Pairs Suppress the Thermal Conductivity of BAs

BAs was predicted to have an unusually high thermal conductivity with a room temperature value of 2000  W m^{-1} K^{-1}, comparable to that of diamond. However, the experimentally measured thermal conductivity of BAs single crystals is still lower than this value. To identify the origin of this large inconsistency, we investigate the lattice structure and potential defects in BAs single crystals at the atomic scale using aberration-corrected scanning transmission electron microscopy (STEM). Rather than finding a large concentration of As vacancies (V_{As}), as widely thought to dominate the thermal resistance in BAs, our STEM results show an enhanced intensity of some B columns and a reduced intensity of some As columns, suggesting the presence of antisite defects with As_{B} (As atom on a B site) and B_{As} (B atom on an As site). Additional calculations show that the antisite pair with As_{B} next to B_{As} is preferred energetically among the different types of point defects investigated and confirm that such defects lower the thermal conductivity for BAs. Using a concentration of 1.8(8)% (6.6±3.0×10^{20}  cm^{-3} in density) for the antisite pairs estimated from STEM images, the thermal conductivity is estimated to be 65-100  W m^{-1} K^{-1}, in reasonable agreement with our measured value. Our study suggests that As_{B}-B_{As} antisite pairs are the primary lattice defects suppressing thermal conductivity of BAs. Possible approaches are proposed for the growth of high-quality crystals or films with high thermal conductivity. Employing a combination of state-of-the-art synthesis, STEM characterization, theory, and physical insight, this work models a path toward identifying and understanding defect-limited material functionality.

Sub-Ångstrom electric field measurements on a universal detector in a scanning transmission electron microscope

Scanning transmission electron microscopy (STEM) excels in accessing atomic-scale structure and chemistry. Enhancing our ability to directly image the functionalities of local features in materials has become one of the most important topics in the future development of STEM. Recently, differential phase contrast (DPC) imaging has been utilized to map the internal electric and magnetic fields in materials from nanoscale features such as p-n junctions, skyrmions, and even from individual atoms. Here, we use an ultra-low noise SCMOS detector in as the diffraction plane camera to collect four-dimensional (4D) datasets. The high angular resolution, efficient high-SNR acquisition, and modifiability of the camera allow it to function as a universal detector, where STEM imaging configurations, such as DPC, bright field, annular bright field, and annular dark field can all be reconstructed from a single 4D dataset. By examining a distorted perovskite, DyScO3, which possesses projected lattice spacings as small as 0.83 Å, we demonstrate DPC spatial resolution almost reaching the information limit of a 100 keV electron beam. In addition, the perovskite has ordered O-coordinations with alternating octahedral tilts, which can be quantitatively measured with single degree accuracy by taking advantage of DPC's sensitivity to light atoms. The results, acquired on a standard Ronchigram camera as opposed to a specialized DPC detector, open up new opportunities to understand and design functional materials and devices that involve lattice and charge coupling at nano- and atomic-scales.