Colloidal Second Near-infrared-emitting Mn-doped Ag2S Quantum Dots

Incorporation of transition metal dopants within a semiconductor nanocrystal has a tremendous effect on the optical and magnetic properties of the semiconductor nanocrystals. Herein, we report on a novel synthesis of second near-infrared-emitting photoluminescent Mn2+-doped Ag2S quantum dots via co-pyrolysis of silver and manganese single-source precursors. The Mn2+ doping level was flexibly tuned in Ag2S quantum dots, which was confirmed by elemental analysis and electron paramagnetic resonance spectroscopy. The Mn2+ doping induced negligible change in the pristine monoclinic acanthite Ag2S crystal structure but significantly decreased the photoluminescent intensity. Mn2+-doped Ag2S QDs exhibit second near-infrared emission and ferromagnetic ordering, which show the potential applicability for multimodal fluorescence/MRI probes.11Nsciescopu

Ag2S Quantum Dots with Reduced Ag+ ion Vacancies

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Ultrafast Cation Exchange in Supra-Quantum Dots through Nanoporous Internal Structure

Cation exchange (CE) can convert the chemical composition of nanocrystals while it preserves their size, shape, and crystal phase. Here, we report a CE reaction in a porous nanostructure of supra-quantum dots (SQDs), which are threedimensional stepwise self-assembly of quantum dots (QDs) with several tens of nm size. It shows ultrafast and complete CE reactions in the SQDs from CdSe to Cu2-xSe or Ag2Se, conserving their size and shape. The complete suppression of the 1S excitonic peak of CdSe SQD and the complete conversion of their crystal structure and chemical composition dictate the complete CE reaction in SQDs even if it has near 100 nm diameter size. The conservation of size and shape after CE reactions reveals the existence of the internal void in porous SQDs which could compensate for the expected shrinkage or expansion due to the lattice constant change before and after CE reaction. The CE reaction rate of SQDs is estimated using temporal absorbance spectra in the course of CE reaction. The CE reaction rate of SQDs showed size-independent dynamics among the SQDs with various sizes from 63 to 83 nm. Their CE reaction rate was around 0.057 s-1 which is comparable to that of 4.8 nm sized QDs. For QDs, the reaction rate was critically size-dependent, showing slower CE reaction rate as their size increases. On the other hand, SQDs showed the CE reaction rate similar to those observed by QDs of 4-5 nm in size, which is phenomenal considering the size of SQDs at least 13 times larger than the QDs. Fully accessible external cations into the porous internal structure of SQDs and their direct interaction with the internal surface of SQDs can accelerate the CE reaction. The comparable primary unit size of SQDs, similar to 4 nm, to the size of QDs explains the ultrafast and size-independent CE reaction rate of SQDs.11Nsciescopu

Correction to “Ultrafast Cation Exchange in Supra-Quantum Dots through Nanoporous Internal Structure”

The author list should be “Hyunmi Doh, Juwon Park, Junhwa Lee, Jiwon Bang, Sanghwa Jeong, Wonseok Lee, Ho Jin, and Sungjee Kim”, with an addition of a co-author, Junhwa Lee. Hyunmi Doh, Juwon Park, and Junhwa Lee contributed equally to this work. This change was agreed to by all authors and is reflected in the authorship of this correction.11Nsciescopu

Formation and Stepwise Self-Assembly of Cadmium Chalcogenide Nanocrystals to Colloidal Supra-Quantum Dots and the Superlattices

Nearly monodisperse colloidal superstructures of cadmium chalcogenide quantum dots (QDs) are reported. The superstructures, which we named as supra quantum dot (SQD), are typically composed of hundreds of a-few-nanometer-sized QDs three-dimensionally (3D) assembled by oriented attachment. The synthesis route for SQD is quite universal and can be extended to CdS, CdSe, CdTe, and CdSeTe alloy. The size of SQD can be tuned from tens of nanometers to over a hundred nanometers. In the case of CdSe SQD, zinc-blende seeds (primary QDs) act as the building block for the formation of the 3D assembled structures, SQDs, with discrete intermediates nanostructures. Primary seeds, 4 nm tetrahedral shaped QDs, assembled into a large tetrahedron of 20 nm. The 20 nm tetrahedrons, in turn, self-assembled into a larger tetrahedron of 40 nm. The discrete-in-size and sequential assemblies were followed by conventional growth from the remaining precursors and ripening within the particles to result in spheroidal SQDs. SQDs allow surface ligand exchange without losing the structural integrity. Size selective precipitation of SQDs can provide monodisperse SQDs that can assemble into ordered superlattices. The size and composition tunability of SQDs and their capability to form superlattices can provide a new solution-processable building block for superstructure with programmable physical and chemical properties

Size-Dependent Photovoltaic Performance of CdSe Supraquantum Dot/Polymer Hybrid Solar Cells: ???Goldilocks Problem??? Resolved by Tuning the Band Alignment Using Surface Ligands

This study explored the size dependence of colloidal CdSe nanocrystals (NCs) on the photovoltaic properties of CdSe NC/poly(3-hexylthiophene) (P3HT) hybrid bulk-heterojunction (BHJ) solar cell devices. The size-dependent photovoltaic performance was achieved by utilizing CdSe supraquantum dots (SQDs), which are three-dimensionally interconnected colloidal superstructures composed of hundreds of CdSe quantum dots (QDs). The average size of the SQDs can span tens of nanometers, which allow the formation of percolation networks in BHJ films. The open-circuit voltage of the devices was observed to be proportional to the size of the SQDs because of their ideal percolation networks. The photocurrents were determined by the competition between the charge separation and charge transport abilities controlled by the SQD sizes. Overall, the 46 nm-sized CdSe SQD-device demonstrated the highest power conversion efficiency (PCE) of 0.95%, which was 3.2 times higher than that of the control 4.3 nm-sized CdSe QD device. However, further increasing the SQD size resulted in a decrease in the PCE because of the inherent carrier recombination loss within the SQDs. To overcome this ???Goldilocks problem,??? we tuned the energy level at the surface region of the CdSe SQDs via electron-donating 4-methylthiophenol (MTP) ligand exchange. The MTP-treated CdSe SQDs further improved the device performance by enhancing the charge separation and increasing the energy level offset at the CdSe SQD/P3HT interface

Influence of Cation Substitutions Based on ABO₃ Perovskite Materials, Sr_1–<i>x</i>Y_<i>x</i>Ti_1–<i>y</i>Ru_<i>y</i>O_3−δ, on Ammonia Dehydrogenation

In order to screen potential catalytic materials for synthesis and decomposition of ammonia, a series of ABO₃ perovskite materials, Sr_1–<i>x</i>Y_<i>x</i>Ti_1–<i>y</i>Ru_<i>y</i>O_3−δ (<i>x</i> = 0, 0.08, and 0.16; <i>y</i> = 0, 0.04, 0.07, 0.12, 0.17, and 0.26) were synthesized and tested for ammonia dehydrogenation. The influence of A or B site substitution on the catalytic ammonia dehydrogenation activity was determined by varying the quantity of either A or B site cation, producing <b>Sr</b>_{<b>1</b>–<b><i>x</i></b>}<b>Y</b>_{<b><i>x</i></b>}Ti_0.92Ru_0.08O_3−δ and Sr_0.92Y_0.08<b>Ti</b>_{<b>1</b>–<i><b>y</b></i>}<b>Ru</b>_{<b><i>y</i></b>}O_3−δ, respectively. Characterizations of the as-synthesized materials using different analytical techniques indicated that a new perovskite phase of SrRuO₃ was produced upon addition of large amounts of Ru (≥12 mol %), and the surface Ru⁰ species were formed simultaneously to ultimately yield <b>Ru</b>_{<b><i>z</i></b>}(surface)/Sr_0.92Y_0.08<b>Ti</b>_{<b>1</b>–<b><i>y</i></b>}<b>Ru</b>_{<i><b>y</b></i>–<b><i>z</i></b>}O_3−δ and/or <b>Ru</b>_{<b><i>z</i></b>–<b><i>w</i></b>}(surface)/Sr_<i>w</i>Ru_<i>w</i>O₃/Sr_{0.92–<i>w</i>}Y_0.08<b>Ti</b>_{<b>1</b>–<b><i>y</i></b>}<b>Ru</b>_{<b><i>y</i></b>–<b><i>z</i></b>}O_3−δ. The newly generated surface Ru⁰ species at the perovskite surfaces accelerated ammonia dehydrogenation under different conditions, and Sr_0.84Y_0.16Ti_0.92Ru_0.08O_3−δ exhibited a NH₃ conversion of ca. 96% at 500 °C with a gas hourly space velocity (GHSV) of 10 000 mL g_cat^–1 h^–1. In addition, Sr_0.84Y_0.16Ti_0.92Ru_0.08O_3−δ further proved to be highly active and stable toward ammonia decomposition at different reaction temperatures and GHSVs for >275 h