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•O2 treatment of Mo2C catalysts suppresses metallic/alkaline catalytic activity.•O2 co-feed reversibly alters Brønsted acid site density on Mo2C by a factor of ∼30.•O∗–Mo2C Brønsted ...acid sites dehydrate IPA with a rate-determining step of β-hydrogen scission.•Bulk 2–5nm orthorhombic β-Mo2C crystallites were not affected by O2 exposure.
Acid site densities could be reversibly tuned by a factor of ∼30 using an O2 co-feed, which reversibly creates Brønsted acid sites on the carbide surface without altering the bulk crystal structure of 2–5nm Mo2C crystallites. Unimolecular isopropanol (IPA) dehydration at 415K, a probe reaction, occurred on Brønsted acid sites of these oxygen-modified carbides with an intrinsic activation energy of 93±1.3kJmole−1 via an E2 elimination mechanism with a kinetically-relevant step of β-hydrogen scission. Site densities were estimated via in situ 2,6-di-tert-butylpyridine (DTBP) titration and used to calculate a turnover frequency (TOF) of 0.1s−1, which was independent of site density. Oxygen co-processing allows for facile in situ tunability of acidic and metallic sites on highly oxophilic metal carbides.
In this work, we present an all-gas-phase approach for the synthesis of quantum-confined core/shell nanocrystals (NCs) as a promising alternative to traditional solution-based methods. Spherical ...quantum dots (QDs) are grown using a single-stage flow-through nonthermal plasma, yielding monodisperse NCs, with a concentric core/shell structure confirmed by electron microscopy. The in-flight negative charging of the NCs by plasma electrons keeps the NC cores separated during shell growth. The success of this gas-phase approach is demonstrated here through the study of Ge/Si core/shell QDs. We find that the epitaxial growth of a Si shell on the Ge QD core compressively strains the Ge lattice and affords the ability to manipulate the Ge band structure by modulation of the core and shell dimensions. This all-gas-phase approach to core/shell QD synthesis offers an effective method to produce high-quality heterostructured NCs with control over the core and shell dimensions.
Magic-sized semiconductor nanocrystals (MSNCs) grow via discrete jumps between specific sizes. Despite their potential to offer atomically precise structures, their use has been limited by poor ...stability and trap-dominated photoluminescence. Recently, CdSe MSNCs have been grown to larger sizes. We exploit such particles and demonstrate a method to grow shells on CdSe MSNC cores via high-temperature synthesis. Thin CdS shells lead to dramatic improvements in the emissive properties of the MSNCs, narrowing their fluorescence line widths, enhancing photoluminescence quantum yields, and eliminating trap emission. Although thicker CdS shells lead to decreased performance, Cd x Zn1–x S alloyed shells maintain efficient and narrow emission lines. These alloyed core/shell crystallites exhibit a tetrahedral shape, in agreement with a recent model for MSNC growth. Our results indicate that MSNCs can compete with other state-of-the-art semiconductor nanocrystals. Furthermore, these core/shell structures will allow further study of MSNCs and their potential for atomically precise growth.
“Magic-sized” nanocrystals (MSNCs) grow in discrete jumps between a series of specific sizes. Consequently, MSNCs have been explored as an alternative route to uniform semiconductor particles, ...potentially with atomic precision. However, because the growth mechanism has been poorly understood, the best strategies to control MSNC syntheses and obtain desired sizes are unknown. Experiments have found that common parameters, such as growth time and temperature, have limited utility. Here, we theoretically and experimentally investigate reactant supersaturation as a tool to control MSNC growth. We compare direct synthesis of CdSe MSNCs with ripening of isolated MSNCs or their mixtures. Surprisingly, we find that MSNCs readily synchronize to the same growth trajectory, even starting from distinct initial conditions, explaining the robustness of MSNC growth. Further, by understanding the synchronization mechanism, we demonstrate methods to control the final MSNC size. These results deepen our knowledge of MSNCs and indicate strategies to tailor their growth.
•By simultaneously fitting the pre- and post-edge regions of core-level EELS edges, reliable background fits are achieved.•This is beneficial for removing the background from noisy core-level EELS ...spectra.•This method can be extended to multiple EELS edges in a single spectrum.
We present a multi-region extension of power law background subtraction for core-level EEL spectra to improve the robustness of background removal. This method takes advantage of the post-edge shape of core-loss EEL edges to enable simultaneous fitting of pre- and post-edge background regions. This method also produces simultaneous and consistent background removal from multiple edges in a single EEL spectrum. The stability of this method with respect to the fitting energy window and the EELS signal to noise ratio is also discussed.
Metal oxide semiconductor nanocrystals (NCs) exhibit localized surface plasmon resonances (LSPRs) tunable within the infrared (IR) region of the electromagnetic spectrum by vacancy or impurity ...doping. Although a variety of these NCs have been produced using colloidal synthesis methods, incorporation and activation of dopants in the liquid phase has often been challenging. Herein, using Al-doped ZnO (AZO) NCs as an example, we demonstrate the potential of nonthermal plasma synthesis as an alternative strategy for the production of doped metal oxide NCs. Exploiting unique, thoroughly nonequilibrium synthesis conditions, we obtain NCs in which dopants are not segregated to the NC surfaces and local doping levels are high near the NC centers. Thus, we achieve overall doping levels as high as 2 × 1020 cm–3 in NCs with diameters ranging from 12.6 to 3.6 nm, and for the first time experimentally demonstrate a clear quantum confinement blue shift of the LSPR energy in vacancy- and impurity-doped semiconductor NCs. We propose that doping of central cores and heavy doping of small NCs are achievable via nonthermal plasma synthesis, because chemical potential differences between dopant and host atomswhich hinder dopant incorporation in colloidal synthesisare irrelevant when NC nucleation and growth proceed via irreversible interactions among highly reactive gas-phase ions and radicals and ligand-free NC surfaces. We explore how the distinctive nucleation and growth kinetics occurring in the plasma influences dopant distribution and activation, defect structure, and impurity phase formation.
Inspired by recent experimental subatomic measurements using analytical aberration-corrected scanning transmission electron microscopes, we study electron probe propagation in crystalline SrTiO3 at ...the subatomic length scale. Here, we report the existence of subatomic channeling and the formation of a helicon-type beam at this scale. The results of beam propagation simulations, which are performed at various crystal temperatures, STEM probe convergence angles (10–50 mrad), and beam energies (80–300 keV), showed that reducing the ambient temperature can enhance the subatomic channeling and STEM probe parameters can be used to control the features of helicon-type beams.
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•Intermetallic Ni–Zn catalysts were employed for acetylene semi-hydrogenation.•Isotopic labeling was used to evaluate reaction pathways.•Ethylene selectivity increases with increasing ...zinc content.•Zinc addition to nickel reduces the propensity for carbon–carbon bond formation.
Isotopic labeling and density functional theory (DFT) were used to determine the mechanism for acetylene hydrogenation and oligomerization on well-defined intermetallic nickel–zinc catalysts. The primary benefit of adding zinc to nickel is a reduction in oligomeric species formation which leads to higher ethylene selectivity. The production of ethane is not highly dependent on zinc content; therefore, ethane production is not a good descriptor of ethylene selectivity since acetylene may also be converted to higher molecular weight products. Analysis using DFT and Langmuir–Hinshelwood kinetics shows that the large decrease in the adsorption energy of acetylene on intermetallic NiZn compared to pure Ni is responsible for the observed increase in ethylene selectivity. The adsorption energy of acetylene appears to be a descriptor for carbon–carbon bond formation since a high adsorption energy leads to an increased coverage of C2 species and an increased rate of carbon–carbon bond formation.
•ADF-STEM is powerful technique for 3D location of substitutionally doped atoms.•The effects of specimen mistilt on ADF-STEM imaging of doped atoms are evaluated.•Visibility changes over 0–30 mrad ...mistilts are large enough to preclude 3D dopant location.•Dopant visibility is a strong function of specimen mistilt and cannot be ignored.
Quantitative ADF-STEM imaging paired with image simulations has proven to be a powerful technique for determining the three dimensional location of substitutionally doped atoms in thin films. Expansion of this technique to lightly-doped nanocrystals requires an understanding of the influence of specimen mistilt on dopant visibility due to the difficulty of accurate orientation determination in such systems as well as crystal movement under the beam. In this study, the effects of specimen mistilt on ADF-STEM imaging are evaluated using germanium-doped silicon nanocrystals as model systems. It is shown that dopant visibility is a strong function of specimen mistilt, and the accuracy of specimen orientation is an important factor in the analysis of three-dimensional dopant location, but the sensitivity to mistilt can be weakened by increasing the STEM probe convergence angle and optimizing ADF detector inner angle.
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