Corrected Transmission Electron Microscopy Research Articles

Photocatalytic Co-Reduction of N2 and CO2 with CeO2 Catalyst for Urea Synthesis.

The effective conversion of carbon dioxide (CO2 ) and nitrogen (N2 ) into urea by photocatalytic reaction under mild conditions is considered to be a more environmentally friendly and promising alternative strategies. However, the weak adsorption and activation ability of inert gas on photocatalysts has become the main challenge that hinder the advancement of this technique. Herein, we have successfully established mesoporous CeO2-x nanorods with adjustable oxygen vacancy concentration by heat treatment in Ar/H2 (90 % : 10 %) atmosphere, enhancing the targeted adsorption and activation of N2 and CO2 by introducing oxygen vacancies. Particularly, CeO2 -500 (CeO2 nanorods heated treatment at 500 °C) revealed high photocatalytic activity toward the C-N coupling reaction for urea synthesis with a remarkable urea yield rate of 15.5 μg/h. Besides, both aberration corrected transmission electron microscopy (AC-TEM) and Fourier transform infrared (FT-IR) spectroscopy were used to research the atomic surface structure of CeO2 -500 at high resolution and to monitor the key intermediate precursors generated. The reaction mechanism of photocatalytic C-N coupling was studied in detail by combining Density Functional Theory (DFT) with specific experiments. We hope this work provides important inspiration and guiding significance towards highly efficient photocatalytic synthesis of urea.

Interaction mechanism between immiscible Fe-Mg element diffusion and crystal defect annihilation in the steel/Mg composite interface region

Crystal defect-induced elemental diffusion between immiscible metals is the key to achieve metallurgical bonding. However, the interaction between immiscible element diffusion and crystal defect annihilation is unclear. In this paper, in-situ heat treatment experiments were conducted at the steel/Mg composite interface by ambient atmosphere spherical aberration corrected transmission electron microscope. The immiscible element diffusion phenomenon and crystal defect annihilation phenomenon in the composite interface region were studied, and the interaction mechanism was revealed. The results show that during the heat treatment, the diffusion of Mg element to the steel side is obvious, while the diffusion of Fe element to the Mg side is not obvious, which is called “one-way selective diffusion effect”. The annihilation rate of steel crystal defects is much smaller than that of Mg. The stable crystal defects of steel in the interface region can provide a channel for Mg elements diffusion, while the faster annihilation rate of Mg crystal defects causes Fe elements to hardly diffuse into Mg. The Mg elements that have diffused into the steel side have a pinning effect on dislocations and grain boundary movement, which in turn increases the thermal stability of the steel crystal defects. This study provides theoretical guidance for promoting the element diffusion at the immiscible metals composite interface.

Observation of in-plane oriented Guinier-Preston zones in Al-Cu

Known for a long time, the first metastable precipitate which forms in Al-Cu alloys during natural ageing are the so-called Guinier-Preston zones (GPZ), platelets of Cu on {100} planes of only one atomic layer thickness. Only with the development of aberration corrected transmission electron microscopes (TEM), direct observation and imaging of these platelets was possible, but with the restriction, that only edge view was possible. Here we show that under appropriate conditions an observation in a plan-view is possible and allows further insight into the shape and arrangement of the GP-zones. Furthermore, this demonstrates that single atom detection of one Cu atom in a column of Al atoms is possible.

Recognizing the formation of single-atom vanadium by enthalpy and its advantage in selective catalytic reduction of nitrogen oxide

Developing a more effective catalyst for the environmental pollutant is always the pursuit of worldwide researchers. In previous reports, most works were focused on prepared catalysts, and more efforts should explore process from a precursor to the formed catalyst. In this work, V supported TiO2 catalysts were synthesized with different V loadings, and compared in selective catalytic reduction of nitrogen oxide. For the first time, differential scanning calorimetry was used to analyze the forming enthalpy for these catalysts. As a result, turnover frequency (TOF) was decreased from 0.05 to 0.001 s−1 when the loading was increased from 0.05 to 15 mol%. At the same time, the forming enthalpy was decreased from 16.53 to 1.80 KJ·mol−1. Spherical Aberration Corrected Transmission Electron Microscope recognized both single-atom and cluster V even at the lowest loading. What’s more, density functional theory revealed that single-atom V had a lower dehydrogenation energy (1.088–1.152 eV) than cluster V during the catalysis (1.834–1.850 eV). According to these evidences, single-atom V had bigger formation enthalpy, lower dehydrogenation energy, and larger TOF than cluster V. These results help to optimize the preparation process for an effective catalyst, which is in favor of hazardous-pollutant treatment.

Origin of the bias instability in CsPbI3 light-emitting diodes

With the rapid development of all-inorganic CsPbX3 (X = I, Br, and Cl) perovskite light-emitting diodes (PeLEDs), their external quantum efficiency (EQE) has made great progress in the last few years. However, severe performance degradation at the operation condition impedes its large-scale industrial application. In this text, by using the advanced spherical aberration corrected transmission electron microscope (TEM), the degradation pathway of PeLEDs under bias loading was more systematically and comprehensively, especially from the viewpoint of its microstructural evolution pathway, which showed that iodine vacancy migration triggered the structural collapse was the primary inducement of performance decay. To solve this problem, inorganic termination ZnI2 was employed to passivate the surface halide vacancy defects and supplement the halide ion into the lattice, which significantly improves the photoluminescence (PL) lifetime and stability of CsPbI3 quantum dots (QDs). As expected, not only the EQE of PeLEDs has been greatly improved, but also the operating half-lifetime is enhanced by about 4 times as compared with the original one. Microstructural characterization and density functional (DFT) calculations confirm that the preferential segregation of Zn element on the surface of QDs was the principal cause to passivate the defect state of QDs, which exhibits lower formation energy and better operational stability.

Atomically dispersed bismuth on ZnIn2S4 Dual-Functional photocatalyst for photocatalytic hydrogen production coupled with oxidation of aromatic alcohols to aldehydes

It is of great significance to develop a simple method of non-noble metal atomical dispersion to establish a dual-function photocatalysis system for the oxidation of aromatic alcohols to aldehydes, and to couple it with the photocatalytic hydrogen production, so as to fully utilize the photogenerated carriers. Herein, we report a noble metal-free photocatalyst decorated with atomic dispersed Bi atoms in ZnIn2S4 hierarchical structures by a one-pot method, for selective oxidation of R-PhCH2OH (R = H, Me, OH, Cl) coupled with high H2 evolution. Aberration corrected transmission electron microscopy and theoretical calculations demonstrated that Bi atoms are atomically dispersed and bridged with two S atoms. Atomically dispersed Bi provides the maximum atom efficiency and promotes light absorption to the near-infrared region. By excluding all wasteful sacrificial agents, Bi/ZnIn2S4 exhibits a high hydrogen evolution rate of 3658.8 µmol g-1h−1, which is approximately 14 times of that of bare ZnIn2S4 (256 µmol g-1h−1). This work provides insights into the effective utilization of photogenerated carriers in one system and the designing of promising photocatalysts with atomic precision.

Effect of vacancy ordering on the grain growth of Ge2Sb2Te5 film

Ge2Sb2Te5 (GST) is the most widely used matrix material in phase change random access memory (PCRAM). In practical PCRAM device, the formed large hexagonal phase in GST material is not preferred, especially when the size of storage architecture is continually scaling down. In this report, with the aid of spherical-aberration corrected transmission electron microscopy (Cs-TEM), the grain growth behavior during the in situ heating process in GST alloy is investigated. Generally, the metastable face-centered-cubic (f-) grain tends to grow up with increasing temperature. However, a part of f-phase nanograins with {111} surface plane does not grow very obviously. Thus, the grain size distribution at high temperature shows a large average grain size as well as a large standard deviation. When the vacancy ordering layers forms at the grain boundary area in the nanograins, which is parallel to {111} surface plane, it could stabilize and refine these f-phase grains. By elaborating the relationship between the grain growth and the vacancy ordering process in GST, this work offers a new perspective for the grain refinement in GST-based PCRAM devices.

Highly efficient electrooxidation of ethanol on CuPtPd trimetallic catalyst

Pt-based ternary metal catalysts have excellent catalytic performance and high stability for ethanol oxidation (EOR). In this paper, Cu-Pt-Pd ternary alloy catalysts are prepared by a facile electrochemical deposition method. The morphology and elemental composition are probed by spherical aberration corrected transmission electron microscope (ACTEM) and X-ray photoelectron spectroscopy (XPS). Many hollow nanosphere are uniformly distributed on the surface of Cu-Pt-Pd catalysts. In various Cu-Pt-Pd catalysts, the peak current density and the mass activity of Cu-Pt1Pd3.2 are 166.7 mA⋅cm−2 and 2436 mA⋅mgPt+Pd-1 for EOR, which are 13.4 times and 8.1 times as those of commercial Pt/C catalysts. The Cu-Pt1Pd3.2 also exhibits excellent CO toxicity resistance. The strong interaction and synergistic effect among Cu, Pt and Pd would enhance its electro-catalytic activity. The above research results will be potential useful value in expanding application fields of Pt-based ternary metal catalysts for EOR.

Tremendous Acceleration of Plant Growth by Applying a New Sunlight Converter Sr4 Al14- x Gax O25 :Mn4+ Breaking Parity Forbidden Transition.

Majority of Mn4+ activated oxide phosphors have the wavelength of excitation and emission suitable for acceleration of plant growth as light converter from sunlight to deep red. Here, it is observed that 60% increase of red emission of Sr4 Al14 O25 :0.01Mn4+ is found by substituting 0.1Ga3+ . It is clarified that the increase is originated from a unique mechanism of breaking parity forbidden transition under the substitution of cation in d-d transition by using the tool of special aberration corrected transmission electron microscope(AC-STEM), pre-edge peak (1s→3d) Mn K-edge X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), Rietveld analysis of X-ray diffraction (XRD) patterns, and reflection spectra. Further, a combination of substituted Ga, Mg, and special double flux H3 BO3 /AlF3 is found to tremendously increase the emission intensity (355% up). Actual growth of chlorella and rose is examined by a combination of the cheap Sr4 Al14 O25 :0.01Mn4+ ,0.007Mg2+ ,0.1Ga3+ and a unique reflection typed phosphor-film system as sunlight converting system. Optical density of chlorella and height of rose grass is increased by 36±14% and 174±80% compared with nonphosphor-film, respectively.

Pt–Ru bimetallic nanoparticles anchored on carbon nanotubes/polyaniline composites with coral-like structure for enhanced methanol oxidation

In order to surmount the poor stability and low utilization efficiency of Pt catalysts, the Pt-Ru bimetallic nanoparticles/polyaniline/multi-walled carbon nanotube composites (PRPM) were fabricated. Structural characterization data by transmission electron microscope, scanning electron microscope, spherical aberration corrected transmission electron microscope, X-ray photoelectron spectroscopy analysis revealed PRPM with coral-like structure. The particle size of Pt-Ru bimetallic nanoparticles were about 3.76 nm. It exhibited higher mass activity of Pt 10 Ru 1 /PANI/CNTs catalyst (987.9 mA·mgPt −1 ) compared to Pt 10 Ru 1 /CNTs catalyst (619.2 mA·mgPt −1 ) for MOR. The Pt 10 Ru 1 /PANI/CNTs catalyst showed lower onset potentials, lower peak current potential compared to Pt-Ru/C in CO stripping experiment. The calculated results were confirmed by the above experimental data. This work provided scientific guidance for fabricating new functional materials in fuel cells field. • The Pt-Ru/polyaniline/carbon nanotubes composites have coral-like structure. • The Pt 10 Ru 1 /PANI/CNTs catalysts had the lower potential and higher current density for MOR. • The prepared composites showed higher performance in methanol and CO oxidation. • The prepared bimetallic catalysts are potential candidate for MOR.

Enhanced tensile properties and intersecting nanotwins formation mechanism of Ti44Al48Nb8 alloy via electromagnetic levitation processing

Combining the electromagnetic levitation (EML) method with materials processing techniques (pressure die-casting and copper mold solidification), bulk Ti44Al48Nb8 alloy samples with different shapes were prepared, and their microstructure evolution, tensile properties and deformation behavior were systematically investigated. Assisted with a double aberration corrected transmission electron microscope (TEM) and molecular dynamics (MD) simulation, three types of intersecting nanotwins were observed and the formation mechanism of step-like and intersecting nanotwins under both tensile loading and at highly undercooled state were discussed. For the 35 g cone-shaped Ti44Al48Nb8 sample by EML, the microstructure consists of lamellar α2-Ti3Al+γ-TiAl matrix, Nb-rich and Al-rich region, showing a typical peritectic morphology. The average grain size has been significantly reduced after levitation die-casting and copper mold casting, due to the relatively high cooling rate and external stress. The rapidly solidified microstructure of a 0.6 g sample undercooled at 61 K has the similar microstructure with large samples. With the increase of bulk undercooling at 300 K, elemental segregation has been effectively inhibited and the intersecting nanotwins were formed, which shows a unique hierarchical structure. The samples processed by copper-mold casting have good tensile properties, which can be attributed to the refined microstructure, the generation of dislocation and stacking faults, and the formation of intersecting nanotwins. Based on the results of MD simulation and atomic-scale quasi-in-situ TEM observation, the analogous dissociation of dislocation dominated by pole mechanism occurs, resulting in the formation of 1/6<112> Shockley partial dislocation, 1/6<110> stair-rod dislocation and 1/3<111> Frank partial dislocation. When samples were highly undercooled, the generated internal stress can facilitate twinning at a much lower stress level, which can be beneficial for the formation of nanotwins at undercooled state.

Stacking fault formation in perovskite Ti3AlC carbides in a TiAl based alloy during creep at 800°C

The interaction between dislocations and perovskite Ti3AlC precipitates in Ti-45Al-5Nb-0.75C after being crept at 800 °C was investigated by aberration corrected transmission electron microscopy. The results show that Ti3AlC carbides can be sheared by dislocations, but unlike dislocation activity reported in other perovskite structures, stacking faults and crossed stacking fault configurations form. The stacking fault formation is induced by the shear of 1/6<112> Shockley partial dislocations that resulted from the dissociation of 1/2<110> partial dislocations. Due to enhanced local atomic reordering processes at 800 °C, nearest neighbor atom violation of the complex stacking faults was relieved and thus stacking fault formation was promoted in Ti3AlC carbides. By glide of partial dislocations, stacking faults extended along {111} planes and encountered each other to form crossed stacking fault configurations. The resulting stair-rod arrangement might further strengthen the precipitates, and thus make them strong obstacles for dislocation gliding at 800 °C.

Investigation on influence of Ta on microstructure evolution of directionally solidified Ni-based superalloys

The microstructure evolution in directionally solidified Ni-based superalloys DZ411 with different Tantalum (Ta) compositions (2.72, 3.10, and 4.00 wt%) was investigated to develop Ta-containing hot corrosion superalloys. It is found that the addition of Ta promote the element segregation in the interdendritic region, thus, leading to the increase in volume fraction of MC carbides and eutectics, while the morphology of them are not obviously changed. In addition, both the volume fraction of regions increases with increasing Ta composition, which is attributed to the reason that Ta is the main forming element of γ' phase. Besides, the previous controversy on the dependence of both the primary and secondary dendrite arm spacing on Ta composition was solved. The increase of dendrite arm spacing with increasing Ta composition can be attributed to increasing solidification range of these superalloys through both JMatPro and Different Scanning Calorimetry (DSC) analysis. Furthermore, the atomic arrangement of γ and γ' phases in alloys was characterized by spherical aberration corrected transmission electron microscopy. Both the saturation of refractory elements in γ matrix and the lattice constant of γ matrix was found to increase with increasing Ta composition, leading to the increase of lattice mismatch of γ/γ' phase and more regular shape of γ' phase which has also been confirmed by JMatPro analysis.

Ce addition enabling superior strength and ductility combination of a low-carbon low-manganese transformation-induced plasticity steel

Transformation-induced plasticity (TRIP) steels doped with rare earth elements have attracted great attraction in the engineering field. Here we study microstructures and strengthening mechanisms of a low-carbon low-manganese TRIP steel containing Ce by applying electron back-scattered diffraction, energy dispersive spectroscopy and spherical aberration corrected transmission electron microscopy. The effect of Ce is three-fold: First, the Ce-rich inclusion forms as Ce is segregated at grain boundaries to absorb the impurities such as Ca and S, which reduces the interface energy for driving grain growth. Thus, the average grain size is refined from 2.1 μm to 1.4 μm, contributing to the elevated yield strength. Second, the addition of Ce leads to an increased density of geometrically necessary dislocations and strengthens the steel. Besides, Ce promotes the precipitation of cementite and refines the cementite, leading to the improved precipitation strengthening. As a result, the Ce-containing low-carbon low-manganese steel shows an increase of ∼100 MPa in yield strength compared with the Ce-free TRIP steel. In particular, the Ce addition improves the stability of the retained austenite, leading to the ductility increased from 38 ± 1% to 46 ± 1%. For the first time, the Ce inclusion was characterized at the atomic scale, revealing the multilayered structure composed of five atomic layers of Ca–S–Ce–Ce–S.

Fabricating ultralow-power-excitable lanthanide-doped inorganic nanoparticles with anomalous thermo-enhanced photoluminescence behavior

Trivalent lanthanide (Ln3+)-doped luminescent nanoparticles (NPs) have been extensively investigated as deep-tissue-penetration visual bioimaging agents owing to their exceptional upconversion and near-infrared (NIR) luminescence upon irradiation of NIR light. However, in most cases, the power density of irradiation used for in vivo biological imaging is much higher than that of the reported maximum permissible exposure (MPE) value of NIR light, which inevitably does great damage to the living organisms under study and thus impedes the plausible clinical applications. Herein, by using a facile syringe pump-aided shell epitaxial growth method, we construct for the first time a new class of Ln3+-doped KMgF3:Yb/Er@KMgF3 core-shell NPs that can be activated by utilizing a 980-nm xenon lamp or diode laser with an ultralow excitation power density down to 0.08 mW cm−2, a value that is approximately 4 orders of magnitude lower than the MPE value set by the American National Standards Institute (ANSI) for safe bioimaging in vivo. By combining the comparative spectroscopic investigations with atomic-resolved spherical aberration corrected transmission electron microscopy (AC-TEM) characterization, we find that the reduced crystallographic defects are the primary cause underlying such an ultralow-power-excitable feature of the KMgF3: Yb/Er@KMgF3 core-shell NPs. And, by the same token, the resultant KMgF3:Yb/Er@KMgF3 core-shell NPs also exhibit an anomalous thermo-enhanced photoluminescence (PL) behavior coupled with an excellent photothermal stability that cannot occur in other Ln3+-doped core-shell NPs. These findings described here unambiguously pave a new way to prepare high-quality Ln3+-doped luminescent NPs with desirable ultralow-power-excitable capability and photothermal stability for future biomedical applications.

Revealing Surface Restraint-Induced Hexagonal Pd Nanocrystals via In Situ Transmission Electron Microscopy.

Achieving metal nanocrystals with metastable phase draws much attention due to their anticipated fascinating properties, wheras it is still challenging because their polymorphism nature and phase transition mechanism remain elusive. Here, phase stability of face-centered cubic (fcc) Pd nanocrystals was studied via in situ spherical aberration (Cs)-corrected transmission electron microscopy (TEM). By constructing a well-defined Pd/C composite structure, Pd nanocrystals encapsulated by graphite, the dispersion process of fcc Pd was observed through a nucleation and growth process. Interestingly, Cs-corrected scanning TEM analysis demonstrated that the newly formed Pd nanocrystals could adopt a metastable hexagonal phase, which was considered challenging to obtain. Accordingly, formation mechanism of the hexagonal Pd nanocrystals was proposed, which involved the combined effect of two factors: (1) templating of graphite and (2) size effect. This work is expected to offer new insight into the polymorphism of Pd nanocrystals and pave the way for the future design of metastable metal nanomaterials.

Atomic layer deposition enabled PtNi alloy catalysts for accelerated fuel-cell oxygen reduction activity and stability

Structuring Pt-based alloy catalysts with uniform, dense dispersion, and low loading is a challenging work for efficient and relatively low-cost catalysts to promote oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). Herein, we present a strategy of active PtNi alloy structure formation with uniform metal alloy nanoparticles (NPs) using fluidized bed reactor atomic layer deposition (FBR-ALD). The compositions of the PtNi alloys were controlled by the ALD super-cycle method, where Pt75+XNi25-X alloy catalysts were successfully synthesized by tuning the super-cycle ratio. The as-deposited alloy catalysts consisted of well-dispersed and dense metal NPs, whose characteristics were maintained even after heat treatment at 700 °C with H2 to produce a well-mixed PtNi alloy. During heat treatment, the Pt-rich Pt3Ni alloy converted into the Pt skin-Pt3Ni alloy structure from the ALD growth mechanism, which is evidenced by spherical aberration (Cs)-corrected transmission electron microscopy (TEM), line profiling, X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS). The ALD synthesized Pt skin-Pt3Ni alloy structure featured a significantly higher electrochemical surface area (ECSA) as well as ORR activity, durability, and PEMFC performance compared to the commercial Pt and Pt3Ni catalysts due to their excellent uniformity, density, and well dispersity as well as the Pt skin-Pt3Ni structure. This study focuses on an atomic scale strategy for a new alloy architecture and marks a step toward a modern high-performing alloy catalyst for future PEMFC technology.

The origin of hexagonal phase and its evolution process in Ge2Sb2Te5 alloy

Ge2Sb2Te5 (GST) is the most important material for phase change random access memory (PCRAM) applications, while the formation of hexagonal (h-) phase results in low switching speed, large energy consumption, and worse endurance performance. Uncovering the formation mechanism of h-phase is beneficial for the further improvement of GST-based PCRAM devices. In this work, through advanced spherical aberration corrected transmission electron microscopy and transmission electron back-scattered diffraction technique, the mechanism of h-phase microstructure evolution is clearly clarified. We find that the vacancy ordering is more likely to appear around the grain boundary in a face-centered-cubic (f-) phase grain, which is the starting point for the generation of h-phase. More specifically, all the atoms in f-phase undergo a gradual shift into h-lattice positions to complete the f-to-h structural transition. By introducing an elemental dopant, for instance, carbon (C), the aggregation of C clusters prefers to distribute in the grain boundary area, which is the essential reason for postponing the generation and expansion of h-phase and greatly improving the thermal stability of C-GST material. In short, clarification of the origin of h-structure incubated from f-phase guides the optimization strategy of GST-based PCRAM devices.

Atomic-Scale Observation of Unusual Dislocations in GaAs-GaAsSb Heterostructured Nanowires.

Cognizing the structural characteristics of a heterointerface is significant to understand the growth mechanism of heterostructured nanowires. Here, the structural characteristics of a heterointerface in GaAs-GaAsSb heterostructured nanowires were investigated by employing spherical aberration (CS)-corrected transmission electron microscopy (TEM). It is found that some unusual dislocations are formed at the heterointerface, leading to the bending of nanowires. Further, the atomically inhomogeneous distribution of Sb content near the heterointerface is revealed, which is responsible for the formation of dislocations. By applying a thermal electric system equipped in the Cs-corrected TEM, a direct observation of structural evolution at the heterointerface was enabled and the stability of GaAs-GaAsSb heterostructured nanowires was evaluated. In situ high-resolution TEM imaging indicates that the destabilization of the heterointerface occurs during nanowire annealing. This study builds a direct correlation between the nanowire heterointerfacial structure with nanowire growth behavior and its stability, which is of importance for heterostructured nanowire design for practical use.

Bimetal single atom on defect-tailoring carbon nitride that boosts photocatalytic hydrogen evolution and superfast contaminant degradation

Bimetal single atom functionalized porous carbon nitride (K1Na1/pCN) is designed by one-step method of forming defective carbon nitride and anchoring single atom, which constructs an efficient transfer channel for charge carriers via selective tailoring and linking heptazine rings in frameworks. The existence of single atom over pCN substrate is proved by special aberration corrected transmission electron microscope, X-ray absorption near-edge structures and X-ray photoelectron spectroscopy. The as-prepared K1Na1/pCN presents superior activity towards hydrogen evolution (4,561 μmol/h/gPt, AQY is 21.8% at 420 nm) and ciprofloxacin degradation (apparent kinetic constant of 0.74 min−1), which are almost one-order and two-order of magnitude larger than the traditional carbon nitride, respectively. Results of experimental data and density functional theory reveal that the K1Na1/pCN can narrow bandgap, form midgap state and provide more effective active sites, which have a synergistic effect on inhibiting charge carrier recombination and improving their directional transfer. This paper proposes an alternative strategy to synthesize carbon nitride-supported single atom by a one-step method and extends to support other metal single atoms via facile alternatives of molten salts.