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Merging the properties of VO2 and van der Waals (vdW) materials has given rise to novel tunable photonic devices. Despite recent studies on the effect of the phase change of VO2 on tuning near-field optical response of phonon polaritons in the infrared range, active tuning of optical phonons (OPhs) using far-field techniques has been scarce. Here, we investigate the tunability of OPhs of α-MoO3 in a multilayer structure with VO2. Our experiments show the frequency and intensity tuning of 2 cm-1 and 11% for OPhs in the [100] direction and 2 cm-1 and 28% for OPhs in the [010] crystal direction of α-MoO3. Using the effective medium theory and dielectric models of each layer, we verify these findings with simulations. We then use loss tangent analysis and remove the effect of the substrate to understand the origin of these spectral characteristics. We expect that these findings will assist in intelligently designing tunable photonic devices for infrared applications, such as tunable camouflage and radiative cooling devices.

In situ electron microscopy is an effective tool for understanding the mechanisms driving novel phenomena in 2D structures. However, due to practical challenges, it is difficult to address these technologically relevant 2D heterostructures with electron microscopy. Here, we use the differential phase contrast (DPC) imaging technique to build a methodology for probing local electrostatic fields during electrical operation with nanoscale spatial resolution in such materials. We find that, by combining a traditional DPC setup with a high-pass filter, we can largely eliminate electric fluctuations emanating from short-range atomic potentials. Using a method based on this filtering algorithm, a priori electric field expectations can be directly compared with experimentally derived values to readily identify inhomogeneities and potentially problematic regions. We use this platform to analyze the electric field and charge density distribution across layers of hBN and MoS(2).

Molybdenum carbides have been expected to be one of the promising catalysts for the hydrogen evolution reaction (HER) due to their similar d-band electronic structures to the Pt-group metals. However, the weaker hydrogen-adsorption ability of MoC severely hinders its applications. Guided by density functional theory calculations, we put forward a strategy to design the novel MoC-based electrocatalyst with surface reconstruction through sulfur doping. The incorporation of minor sulfur not only greatly increases the number of active sites and intrinsic activity but also optimizes the electronic structure to improve the electron transfer efficiency. As a result, the as-prepared sulfur-substituted MoC tackles the limitation of the Volmer step and exhibits superior HER performance with a small Tafel slope of 48 mV dec-1. Theoretical investigations demonstrate that the terminal sulfur plays a critical role in facilitating a close to zero hydrogen adsorption energy (ΔGH*) and a lower hydrogen release barrier.

Copper (Cu) is a catalyst broadly used in industry for hydrogenation of carbon dioxide, which has broad implications for environmental sustainability. An accurate understanding of the degeneration behavior of Cu catalysts under operando conditions is critical for uncovering the failure mechanism of catalysts and designing novel ones with optimized performance. Despite the widespread use of these materials, their failure mechanisms are not well understood because conventional characterization techniques lack the necessary time and spatial resolution to capture these complex behaviors. In order to overcome these challenges, we carried out transmission electron microscopy (TEM) with a specialized in situ gas environmental holder, which allows us to unravel the dynamic behavior of the Cu nanowires (NWs) in operando. The failure process of these nanoscale Cu catalysts under CO2 atmosphere were tracked and further rationalized based on our numerical modeling using phase-field methods.

Nature materials (2021). doi:10.1038/s41563-021-01064-6, Published by Nature Publishing Group, Basingstoke

International audience; Two-dimensional (2D) hybrid lead iodide perovskites have gained prominence due to their remarkable structural tunability, optoelectronic features, and moisture stability, which have rendered them as attractive alternatives to 3D MAPbI3 for optoelectronic devices. 2D multilayer lead bromide perovskites remain an unfathomed phase space with the lack of systematic studies to establish the structure, photophysical properties and stability behavior of this family of 2D halide perovskites. Herein, we present new members of bilayer lead bromide perovskites (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 6–8) that belong to the Ruddlesden–Popper structure type, incorporating long chain alkyl-monoammonium cations (CmH2m+1NH3) of hexylammonium (m = 6), heptylammonium (m = 7), and octylammonium (m = 8). A universal solution synthetic methodology for bulk multilayer lead bromide perovskites is presented with all structures solved and refined using single crystal X-ray diffraction. The studied bilayer lead bromide perovskites demonstrate a decrease in the lattice rigidity and lattice match of the inorganic perovskite layer–organic layer, as the alkyl-monoammonium chain length increases. In comparison to their iodide analogues, the bilayer lead bromide compounds exhibit elongation of their stacking axis despite the smaller dimensions of the [PbBr6]4− lattice, while their internal lattice strain was calculated to be reduced, inferring a greater lattice match between the inorganic [PbBr6]4− perovskite layer and organic layer. The (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 4, 6–8) compounds exhibit narrow-band emission near 2.5 eV. Time-resolved photoluminescence (PL) displays longer carrier lifetimes on the nanosecond time scale comparing to their iodide analogues, where electronic structure calculations indicate that the increase of the alkyl chain length and, thus, lattice softness enhances nonradiative recombinations. A complete set of air, light, and heat stability tests on unencapsulated thin films of (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 4, 6–8) and MAPbBr3 show they are stable in ambient air for at least 5 months, exhibiting greater extrinsic stability than the 2D lead iodide congeners. Extraordinarily, 3D MAPbBr3 films prove to be more stable than films of 2D lead bromide perovskites, in contrast to MAPbI3 which is less stable than the 2D lead iodide perovskites.

Hard carbon is the most promising candidate material for lithium-ion batteries (LIBs) owing to its excellent cyclability and high stability. However, unlike graphite used in most of the commercial LIBs, most of the details of the electrochemical reaction mechanism in hard carbon remains unknown. Here, we report the dynamic structural evolutions of hard carbon in lithiation at the atomic scale by in-situ transmission electron microscopy. In the early stage of lithiation, Li-metal particles formed on the surface of hard carbon, while Li-ion intercalation was observed near the end of lithiation evidently by a slight lattice expansion of the graphitic layers in the hard carbon. These observations show that the Li-storage mechanism consist of a Li-metal surface absorption followed by the intercalation of Li-ions, namely a hybrid Li-metal and Li-ion storage mechanism. Furthermore, the optimized hard carbon (carbonized at 1000 °C for 2 h) delivers a high reversible capacity of 366.2 mAh g−1 at 50 mA g−1 for 100 cycles and 221.5 mAh g−1 at 1000 mAg−1 for 1000 cycles, respectively. Our investigation will provide insights in designing and fabricating more effective carbon-based nanostructured anode materials for the next generation LIBs with high capacity and cyclability.

Abstract In this work, we highlight the often-overlooked effects of doping on the microstructure and performance of bulk thermoelectric materials to offer a broader perspective on how dopants interact with their parent material. Using PbSe doped with Na, Ag, and K as a model material system, we combine original computational, experimental, and microscopy data with established trends in material behavior, to provide an in-depth discussion of the relationship between dopants, processing, and microstructure, and their effects on thermoelectric efficiency and thermal stability. Notable observations include differences in the microstructure and mass loss of thermally treated samples of Na- and Ag-doped PbSe, as well as findings that Na and K cations exist predominantly as substitutional point defects while Ag also occupies interstitial sites and exhibits lower solubility. We discuss how these differences in point defect populations are known to affect a dopants’ ability to alter carrier concentration and how they may affect the mechanical properties of PbSe during processing. Graphic Abstract

Many strategies have been employed to maintain an “optimal carrier concentration” to achieve a high thermoelectric power factor in n-type PbTe. Here, we show that a high power factor can also be ac...

Understanding the behavior of high-entropy alloy (HEA) materials under hydrogen (H2) environment is of utmost importance for their promising applications in structural materials, catalysis, and energy-related reactions. Herein, the reduction behavior of oxidized FeCoNiCuPt HEA nanoparticles (NPs) in atmospheric pressure H2 environment was investigated by in situ gas-cell transmission electron microscopy (TEM). The reduction reaction front was maintained at the external surface of the oxide. During reduction, the oxide layer expanded and transformed into porous structures where oxidized Cu was fully reduced to Cu NPs while Fe, Co, and Ni remained in the oxidized form. In situ chemical analysis showed that the expansion of the oxide layer resulted from the outward diffusion flux of all transition metals (Fe, Co, Ni, Cu). Revealing the H2 reduction behavior of HEA NPs facilitates the development of advanced multicomponent alloys for applications targeting H2 formation and storage, catalytic hydrogenation, and corrosion removal.

To improve their synthesis and ultimately realize the technical promise of two-dimensional covalent organic frameworks (2D COFs), it is imperative that a robust understanding of their structure be developed. However, high-resolution transmission electron microscopy (HR-TEM) imaging of such beam-sensitive materials is an outstanding characterization challenge. Here, we overcome this challenge by leveraging low electron flux imaging conditions and high-speed direct electron counting detectors to acquire high-resolution images of 2D COF films. We developed a Fourier mapping technique to rapidly extract nanoscale structural information from these TEM images. This postprocessing script analyzes the evolution of 2D Fourier transforms across a TEM image, which yields information about polycrystalline domain orientations and enables quantification of average domain sizes. Moreover, this approach provides information about several types of defects present in a film, such as overlapping grains and various types of grain boundaries. We also find that the pre-eminent origin of defects in COF-5 films, a prototypical boronate ester-linked COF, arises as a consequence of broken B-O bonds formed during polymerization. These results suggest that the nanoscale features observed are a direct consequence of chemical phenomena. Taken together, this mapping approach provides information about the fundamental microstructure and crystallographic underpinnings of 2D COF films, which will guide the development of future 2D polymerization strategies and help realize the goal of using 2D COFs in a host of thin-film device architectures.

The octahedral structure of 2D molybdenum disulfide (1T-MoS2 ) has attracted attention as a high-efficiency and low-cost electrocatalyst for hydrogen production. However, the large-scale synthesis of 1T-MoS2 films has not been realized because of higher formation energy compared to that of the trigonal prismatic phase (2H)-MoS2 . In this study, a uniform wafer-scale synthesis of the metastable 1T-MoS2 film is performed by sulfidation of the Mo metal layer using a plasma-enhanced chemical vapor deposition (PE-CVD) system. Thus, plasma-containing highly reactive ions and radicals of the sulfurization precursor enable the synthesis of 1T-MoS2 at 150 °C. Electrochemical analysis of 1T-MoS2 shows enhanced catalytic activity for the hydrogen evolution reaction (HER) compared to that of previously reported MoS2 electrocatalysts 1T-MoS2 does not transform into stable 2H-MoS2 even after 1000 cycles of HER. The proposed low-temperature synthesis approach may offer a promising solution for the facile production of various metastable-phase 2D materials.

The effective detection of formaldehyde is of great importance to people's daily life. Titanium oxide (TiO2) based nanomaterials have great promise for formaldehyde detection due to their wide availability, easy utilization and abundant surface reactions. However, currently their actual application is still limited by their relatively low sensitivity, high working temperature and poor selectivity. To address these issues, herein we develop a template-assisted self-assembly strategy to realize the simultaneous chemical doping and morphological control of TiO2 nanostructures. The resultant Co-doped TiO2 nanospheres show a cobalt content of up to 2.95 wt% and a unique raspberry-like mesoporous morphology composed of numerous superfine nanopores of 4.9–7.8 nm, which provides a large specific surface area (∼175 m2 g−1) and high porosity (0.438 cm3 g−1) to the product. The gas sensor device shows a sensitivity of ∼84.8 for 10 ppm formaldehyde, which exceeds that of most of the similar metal oxides reported recently and likely stands for the state-of-the-art merit of formaldehyde sensors. Meanwhile, the sensor also shows significantly decreased working temperature (∼86 °C) and unique selectivity. All this makes our chemically doped raspberry-like mesoporous TiO2 product a very promising candidate for future formaldehyde sensors.

Metal heteroanionic compounds such as oxynitrides, oxysulfides, and oxyhalides have emerged as promising photocatalysts for water splitting, owing to their reduced band gaps compared to conventiona...

Electrically responsive drug delivery is an attractive approach for the localised, tuneable release of drugs to meet therapeutic demands. Conducting polymers and hydrogels have been explored individually as materials for drug delivery applications with hybrids of these two materials offering potential to extend the range and amount of drug delivered, thereby creating new opportunities to achieve real-world benefit. Although accurate and long-term on-demand release of drugs through conducting polymer hydrogels still presents challenges, these are promising materials for the next generation of electrically responsive drug delivery devices. Here we review the fabrication methods and properties of conducting polymer hydrogels, relevant to drug delivery. In addition, the mechanisms behind drug loading and release are discussed, and applications for these systems presented. The current state of the field is discussed, alongside future steps required to achieve successful translation of these materials to the clinic.

Although high-entropy alloys (HEAs) have shown tremendous potential for elevated temperature, anticorrosion, and catalysis applications, little is known on how HEA materials behave under complex service environments. Herein, we studied the high-temperature oxidation behavior of Fe0.28Co0.21Ni0.20Cu0.08Pt0.23HEA nanoparticles (NPs) in an atmospheric pressure dry air environment by in situ gas-cell transmission electron microscopy. It is found that the oxidation of HEA NPs is governed by Kirkendall effects with logarithmic oxidation rates rather than parabolic as predicted by Wagner's theory. Further, the HEA NPs are found to oxidize at a significantly slower rate compared to monometallic NPs. The outward diffusion of transition metals and formation of disordered oxide layer are observed in real time and confirmed through analytical energy dispersive spectroscopy, and electron energy loss spectroscopy characterizations. Localized ordered lattices are identified in the oxide, suggesting the formation of Fe2O3, CoO, NiO, and CuO crystallites in an overall disordered matrix. Hybrid Monte Carlo and molecular dynamics simulations based on first-principles energies and forces support these findings and show that the oxidation drives surface segregation of Fe, Co, Ni, and Cu, while Pt stays in the core region. The present work offers key insights into how HEA NPs behave under high-temperature oxidizing environment and sheds light on future design of highly stable alloys under complex service conditions.

Natural biomolecules such as peptides and DNA can dynamically self-organize into diverse hierarchical structures. Mimicry of this homopolymer self-assembly using synthetic systems has remained limited but would be advantageous for the design of adaptive bio/nanomaterials. Here, we report both experiments and simulations on the dynamic network self-assembly and subsequent collapse of the synthetic homopolymer poly(propylene sulfone). The assembly is directed by dynamic noncovalent sulfone–sulfone bonds that are susceptible to solvent polarity. The hydration history, specified by the stepwise increase in water ratio within lower polarity water-miscible solvents like dimethylsulfoxide, controls the homopolymer assembly into crystalline frameworks or uniform nanostructured hydrogels of spherical, vesicular, or cylindrical morphologies. These electrostatic hydrogels have a high affinity for a wide range of organic solutes, achieving >95% encapsulation efficiency for hydrophilic small molecules and biologics. This system validates sulfone–sulfone bonding for dynamic self-assembly, presenting a robust platform for controllable gelation, nanofabrication, and molecular encapsulation., Natural biomolecules such as peptides and DNA can dynamically self-organize into diverse hierarchical structures. Here the authors report experiments and simulations on the dynamic network self-assembly and subsequent collapse of the synthetic homopolymer poly(propylene sulfone).

Summary The realization of lithium-ion battery (LIB) anodes with high volumetric energy densities and minimal Li plating at high rates remains a key challenge for emerging technologies, including electric vehicles and grid-level energy storage. Here, we present graphene-functionalized Li2TiSiO5 (G-LTSO) as a high volumetric energy and power density anode for LIBs. G-LTSO forms a dense electrode structure with electronically and ionically conductive networks that deliver superior electrochemical performance. Upon lithiation, in situ transmission electron microscopy reveals that graphene functionalization yields minimal structural changes compared with pristine LTSO, resulting in high cycling stability. Furthermore, G-LTSO exhibits not only high charge and discharge capacities but also low overpotentials at high rates with minimal voltage fading due to reduced formation of a solid-electrolyte interphase. The combination of highly compacted electrode morphology, stable high-rate electrochemistry, and low operating potential enables G-LTSO to achieve exceptional volumetric energy and power densities that overcome incumbent challenges for LIBs.

Engineering perovskites with anions The bandgap of the perovskite top layer in tandem silicon solar cells must be tuned to ∼1.7 electron volts. Usually, the cation composition is varied because the bromine-rich anion compositions with wide bandgaps are structurally unstable. Kim et al. show that by using phenethylammonium as a two-dimensional additive, along with iodine and thiocyanate, bromine-rich perovskite films can be stabilized. A tandem silicon cell delivered >26% certified power conversion efficiency, and a perovskite device maintained 80% of its initial power conversion efficiency of >20% after 1000 hours under illumination. Science , this issue p. 155

Complex bismuth chalcogenides are narrow band gap semiconductors with intrinsic low thermal conductivity, which exhibit high potential as highly efficient thermoelectric materials. Here, we assess ...

Solution preparation provides a versatile platform to extend the applications of transition-metal dichalcogenides (TMDs) beyond those achievable with traditional vapor-based preparation methods. Ho...

Many thermoelectric materials feature irregular electrical conductivity with thermally activated transport below ∼600 K and metallic behavior at high temperatures, despite possessing degenerate carrier concentrations. The suppression of the electrical conductivity ultimately degrades the thermoelectric performance on the cold side and limits the device energy conversion efficiency. As such, establishing the origin of the low temperature scattering and developing strategies to mitigate its effect are paramount issues. To date, the correct microscopic description of the low temperature carrier scattering remains an open issue, and there is little work addressing why some thermoelectric materials are more susceptible to the deleterious behavior. Here, we use the promising thermoelectric alloys of PbQ and NaSbQ2 (Q = S, Se, Te) as model systems to address these concerns. We directly show the thermally activated transport stems from the scattering of charge carriers by the grain boundaries (GBs), and that the expected metallic electrical conductivity is recovered by preparing large grained samples with reduced densities of GBs. We furthermore study the electrical properties NaPbmSbSem+2 as a function of NaSbSe2 fraction, as well as those of the chalcogenide analogues, PbTe–NaSbTe2 and PbS–NaSbS2, and demonstrate that the strength of GB scattering can be understood by utilizing simple chemical principles. By considering the polarizability of the host atoms, we directly relate the magnitude of GB scattering to the relative degree of charge carrier screening in each material, and demonstrate that GB scattering is strongest in the ionic NaSbQ2-rich compounds and weakest in more polarizable PbQ-rich phases. We finally show how these chemical arguments elegantly explain the strong GB scattering in numerous other thermoelectric materials. By uniting the deleterious charge transport properties exhibited by many different compounds into a common picture, we discuss how our work gives design principles for proper microstructure engineering in emerging thermoelectric materials.

Because of its unique transport properties, CuInTe2 has been considered as a promising p-type material for thermoelectric applications. However, its diamondoid structure gives it a high intrinsic lattice thermal conductivity that greatly limits its thermoelectric performance. In this study, we demonstrate that Ag alloying gives rise to an extremely low lattice thermal conductivity of 0.47 W m−1 K−1 for Cu0.8Ag0.2InTe2 at 850 K. Moreover, we found Cu doping significantly improves the carrier mobility while simultaneously increasing the carrier concentration. As a result, the power factor of Cu0.8Ag0.2InTe2 increases and a maximum ZT of ∼1.6 is achieved at 850 K. Both DFT calculations and low temperature heat capacity measurements suggest a strong interaction between low frequency optical phonons and heat carrying acoustic phonons, which is derived from the weak Ag–Te bonding. This strong phonon coupling decreases the Debye temperature and induces a low sound velocity.

CeO2–x quantum dots with massive oxygen vacancies are obtained by a one-step single molecular synthesis strategy. The yield of dimethyl carbonate from CO2 and methanol is more than 5 times that for commercial CeO2 nanoparticles.

We report a novel hierarchical microstructure in the PbSe–CdSe system, which collectively contributes to significant enhancement in thermoelectric performance, with ZTave ∼ 0.83 across the 400–923 K temperature range, the highest reported for p-type, Te-free PbSe systems. We have investigated the local atomic structure as well as the microstructure of a series of PbSe–xCdSe materials, up to x = 10%. We find that the behavior of the Cd atoms in the octahedral rock salt sites is discordant and results in off-center displacement and distortion. Such off-centered Cd in the PbSe matrix creates (1) L–Σ electronic energy band convergence, (2) a flattened L band, both contributing to higher Seebeck coefficients, and (3) enhanced phonon scattering, which leads to lower thermal conductivity. These conclusions are supported by photoemission yield spectroscopy in air (PYSA), solid state 111Cd, 77Se NMR spectroscopy and DFT calculations. Above the solubility limit (>6%CdSe), we also observe endotaxial CdSe nano-precipitates with core–shell architecture formed in PbSe, whose size, distribution and structure gradually change with the Cd content. The nano-precipitates exhibit a zinc blende crystal structure and a tetrahedral shape with significant local strain, but are covered with a thin wurtzite layer along the precipitate/matrix interface, creating a core–shell structure embedded in PbSe. This newly discovered architecture causes a further reduction in lattice thermal conductivity. Moreover, potassium is found to be an effective p-type dopant in the PbSe–CdSe system, leading to an enhanced power factor, a maximum ZT of ∼1.4 at 923 K for Pb0.98K0.02Se–6%CdSe.

A new bismuth metal–organic framework (MOF), bismuth-NU-901 (Bi-NU-901), featuring the scu topology and a pore with a diameter of ∼11 A, was solvothermally synthesized, and its use as an X-ray comp...

In the context of metal particle catalysts, composition, shape, exposed facets, crystal structure, and atom distribution dictate activity. While techniques have been developed to control each of these parameters, there is no general method that allows one to optimize all parameters in the context of polyelemental systems. Herein, by combining a solid-state, Bi-influenced, high-index facet shape regulation strategy with thermal annealing, we achieve control over crystal structure and atom distribution on the exposed high-index facets, resulting in an unprecedentedly diverse library of chemically disordered and ordered multimetallic (Pt, Co, Ni, Cu, Fe, and Mn) tetrahexahedral (THH) nanoparticles. Density functional theory calculations show that surface Bi modification stabilizes the {210} high-index facets of the nanoparticles, regardless of their internal atomic ordering. Moreover, we find that the ordering transition temperatures for the nanoparticles are dependent on their composition, and, in the case of Pt(3)Fe(1) THH nanoparticles, increasing Ni substitution leads to an order-to-disorder transition at 900 °C. Finally, we have discovered that ordered intermetallic THH Pt(1)Co(1) nanocatalysts exhibit a catalytic performance superior to disordered THH Pt(1)Co(1) nanoparticles and commercial Pt/C catalysts toward methanol electrooxidation, highlighting the importance of crystal structure and atom distribution control on high-index facets in nanoscale catalysts.