Hydrothermal synthesis is challenging in metal oxide systems with diverse polymorphism, as reaction products are often sensitive to subtle variations in synthesis parameters. This sensitivity is ...rooted in the non-equilibrium nature of low-temperature crystallization, where competition between different metastable phases can lead to complex multistage crystallization pathways. Here, we propose an ab initio framework to predict how particle size and solution composition influence polymorph stability during nucleation and growth. We validate this framework using in situ X-ray scattering, by monitoring how the hydrothermal synthesis of MnO
proceeds through different crystallization pathways under varying solution potassium ion concentrations (K
= 0, 0.2, and 0.33 M). We find that our computed size-dependent phase diagrams qualitatively capture which metastable polymorphs appear, the order of their appearance, and their relative lifetimes. Our combined computational and experimental approach offers a rational and systematic paradigm for the aqueous synthesis of target metal oxides.
► Cerium oxide, a surrogate for nuclear fuels, was used for atom probe tomography. ► Stoichiometric field evaporation of CeO2 was produced using laser pulsed APT. ► The effect of laser energy and ...specimen base temperature was studied. ► Mass resolving power was optimized for the run conditions with high specimen yield. ► Spatial resolution in the nm-scale was maximized by reducing laser energy.
Cerium oxide (CeO2) is an ideal surrogate material for trans-uranic elements and fission products found in nuclear fuels due to similarities in their thermal properties; therefore, cerium oxide was used to determine the best run condition for atom probe tomography (APT) of nuclear fuels. Laser-assisted APT is a technique that allows for spatial resolution in the nm scale and isotopic/elemental chemical identification. A systematic study of the impact of laser pulse energy and specimen base temperature on the mass resolution, measurement of stoichiometry, multiple detector hits, and evaporation mechanisms are reported in this paper. It was demonstrated that using laser-assisted APT stoichiometric field evaporation of cerium oxide was achieved at 1pJ laser pulse energy and 20K specimen base temperature.
Electron-selective layers (ESLs) and hole-selective layers (HSLs) are critical in high-efficiency organic–inorganic lead halide perovskite (PS) solar cells for charge-carrier transport, separation, ...and collection. We developed a procedure to assess the quality of the ESL/PS junction by measuring potential distribution on the cross section of SnO2-based PS solar cells using Kelvin probe force microscopy. Using the potential profiling, we compared three types of cells made of different ESLs but otherwise having an identical device structure: (1) cells with PS deposited directly on bare fluorine-doped SnO2 (FTO)-coated glass; (2) cells with an intrinsic SnO2 thin layer on the top of FTO as an effective ESL; and (3) cells with the SnO2 ESL and adding a self-assembled monolayer (SAM) of fullerene. The results reveal two major potential drops or electric fields at the ESL/PS and PS/HSL interfaces. The electric-field ratio between the ESL/PS and PS/HSL interfaces increased in devices as follows: FTO < SnO2-ESL < SnO2 + SAM; this sequence explains the improvements of the fill factor (FF) and open-circuit voltage (V oc). The improvement of the FF from the FTO to SnO2-ESL cells may result from the reduction in voltage loss at the PS/HSL back interface and the improvement of V oc from the prevention of hole recombination at the ESL/PS front interface. The further improvements with adding an SAM is caused by the defect passivation at the ESL/PS interface, and hence, improvement of the junction quality. These nanoelectrical findings suggest possibilities for improving the device performance by further optimizing the SnO2-based ESL material quality and the ESL/PS interface.
Electron-beam-induced damages in methylammonium lead triiodide (MAPbI3) perovskite thin films were studied by cathodoluminescence (CL) spectroscopy. We find that high-energy electron beams can ...significantly alter perovskite properties through two distinct mechanisms: (1) defect formation caused by irradiation damage and (2) phase transformation induced by electron-beam heating. The former mechanism causes quenching and broadening of the excitonic peaks in CL spectra, whereas the latter results in new peaks with higher emission photon energy. The electron-beam damage strongly depends on the electron-beam irradiation conditions. Although CL is a powerful technique for investigating the electronic properties of perovskite materials, irradiation conditions should be carefully controlled to avoid any significant beam damage. In general, reducing acceleration voltage and probing current, coupled with low-temperature cooling, is more favorable for CL characterization and potentially for other scanning electron-beam-based techniques as well. We have also shown that the stability of perovskite materials under electron-beam irradiation can be improved by reducing defects in the original thin films. In addition, we investigated effects of electron-beam irradiation on formamidinium lead triiodide (FAPbI3) and CsPbI3 thin films. FAPbI3 shows similar behavior as MAPbI3, whereas CsPbI3 displays higher resistance to electron-beam damage than its organic–inorganic hybrid counterparts. Using CsPbI3 as a model material, we observed nonuniform luminescence in different grains of perovskite thin films. We also discovered that black-to-yellow phase transformation of CsPbI3 tends to start from the junctions at grain boundaries.
The measurement of laser-induced temperature change and its influence on the field evaporation behavior in nanoscale CdTe and GaN specimens was assessed through systematic studies using laser pulsed ...atom probe tomography. For CdTe, the laser was shown to induce a linear thermal response in the material. Using the determined relationships, a phase map of the field evaporation behavior was created. This shows that at high base temperatures, high laser energies, or low fields, significant Cd sublimation occurs, leading to apparently Te-rich measured compositions. In contrast, the highest fields result in simultaneous evaporation of multiple Te species, leading to apparently Cd-rich measured compositions. For GaN, increasing laser energy reduced the applied bias necessary for a given detection rate, whereas base temperature changes produced no significant effect on the evaporation behavior, indicative of a largely athermal evaporation mechanism. Similarly, the laser energy and bias affected the measured compositions, whereas the base temperature did not. Additionally, the field evaporation behavior in GaN appears unusual in that there is a strong crystallographic dependence resulting in a nonuniform field being maintained across the apex of the specimen. These methods are useful beyond atom probe analyses for elucidating information about bonding and optoelectronic properties.
Inorganic nitrides with wurtzite crystal structures are well-known semiconductors used in optical and electronic devices. In contrast, rocksalt-structured nitrides are known for their superconducting ...and refractory properties. Breaking this dichotomy, herewe report ternary nitride semiconductors with rocksalt crystal structures, remarkable electronic properties, and the general chemical formula MgₓTM
1−xN (TM = Ti, Zr, Hf, Nb). Our experiments show that these materials form over a broad metal composition range, and that Mg-rich compositions are nondegenerate semiconductors with visible-range optical absorption onsets (1.8 to 2.1 eV) and up to 100 cm² V−1·s−1 electron mobility for MgZrN₂ grown on MgO substrates. Complementary ab initio calculations reveal that these materials have disorder-tunable optical absorption, large dielectric constants, and electronic bandgaps that are relatively insensitive to disorder. These ternary MgₓTM
1−xN semiconductors are also structurally compatible both with binary TMN superconductors and main-group nitride semiconductors along certain crystallographic orientations. Overall, these results highlight MgₓTM
1−xN as a class of materials combining the semiconducting properties of main-group wurtzite nitrides and rocksalt structure of superconducting transition-metal nitrides.
•The relationships between atom probe tomography reconstruction parameters are presented.•Atom probe tomography reconstructions where the values known from electron microscopy and the user input ...values are completely consistent with the output reconstructions are demonstrated.•A consistent power law relationship between the image compression factor and detection efficiency is found for all of the analyzed specimens.•This technique may be used to find the specimen evaporation field at the end of the atom probe analysis and to possibly determine how many atoms were undetected/unranged.•Concepts in this methodology should allow for straightforward incorporation into atom probe tomography reconstruction software.
Atom probe tomography reconstructions provide valuable information on nanometer-scale compositional variations within materials. As such, the spatial accuracy of the reconstructions is of primary importance for the resulting conclusions to be valid. Here, the use of transmission electron microscopy images before and after atom probe analysis to provide additional information and constraints is examined for a number of different materials. In particular, the consistency between the input reconstruction parameters and the output reconstruction is explored. It is demonstrated that it is possible to generate reconstructions in which the input and known values are completely consistent with the output reconstructions. Yet, it is also found that for all of the datasets examined, a particular power law relationship exists such that, if the image compression factor or detection efficiency is not constrained, a series of similarly spatially accurate reconstructions results. However, if one of these values can be independently assessed, then the other is known as well. Means of incorporating these findings and this general methodology into reconstruction protocols are also discussed.
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