Impact of Bis(imino)pyridine Ligands on Mesoscale Properties of CdSe/ZnS Quantum Dots
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We investigate the effect of surface modification of CdSe/ZnS quantum dots (QDs) with bis(imino)pyridine (BIP) ligands. BIPs are a class of redox noninnocent ligands known to facilitate charge transfer in base metals on the molecular scale, but their behavior in nano- to mesoscale systems has been largely unexplored. Using electron microscopy, crystallography, and ultrafast spectroscopy, we reveal that structure-specific π–π stacking of the BIP molecules alters interdot separation in QD films, thereby leading to changes in optical and electronic properties. The three variations used are unsubstituted (BIP-H), dimethyl (BIP-Me), and diisopropyl (BIP-Ipr) BIP, and when compared with the native octadecylamine ligand, we find that both energy and charge transfer efficiencies between QDs are increased postligand exchange, the highest achieved through BIP-Ipr despite its larger unit cell volume. We further investigate charge transfer from QD films to conducting (indium tin oxide, ITO) and semiconducting (zinc oxide, ZnO) substrates using time-resolved spectroscopy and determine that the influence of the ligands is QD band gap-dependent. In QDs with a large band gap (2.3 eV), the BIP ligands facilitate charge transfer to both ITO and ZnO substrates, but in dots with a small band gap (1.9 eV), they pose a hindrance when ZnO is used, resulting in reduced recombination rates. These results highlight the importance of investigating multiple avenues in order to optimize surface modification of QDs based on the end goal. Finally, we verify that BIP ligands hasten the rate of QD photobrightening under continuous illumination, allowing the ensemble to achieve stable emission faster than in their native configuration. Our study sets the stage for novel charge transfer systems in the meso- and nanoscale, yielding a diverse selection of new surface ligands for applications such as conductive materials and energy production/storage devices employing QDs.Keywords:
Indium tin oxide
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Characterization
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Porous SiC Ceramics with Multiple Pore Structure Fabricated via Gelcasting and Solid State Sintering
Porous SiC ceramics with multiple pore structures were fabricated via gelcasting and solid state sintering.A novel gelling agent of Isobam was applied and PMMA was used as both foam stabilizer and pore forming agent.The mechanical properties of porous SiC ceramics were investigated as functions of PMMA content, rotating speed of ball mill, and sintering temperature.With PMMA content increasing from 5wt% to 20wt%, the foaming effect was inhibited while the stability of bubbles increased.When the rotating speed was 220 r/min, the open porosities of the as-prepared SiC ceramics sintered at 2100 varied ℃ from 51.5% to 72.8%, and compressive strength varied from 7.9 to 48.2 MPa.With the rotating speed increasing from 220 to 280 r/min, the foaming effect was aggravated and the porosities of SiC ceramics sintered at 2100 increased.℃ While the sintering temperature increasing from 2050 to 2150 , ℃ the SiC ceramics prepared with PMMA content of 20wt% at rotating speed of 220 r/min decreased in the open porosities while increased in compressive strength.
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ZrB2 based composites containing 10 vol.-% carbon nanotubes (CNTs) are synthesised by spark plasma sintering at temperatures ranging from 1600 to 18008C and at an applied pressure of 25 MPa. The effects of sintering temperature on densification behaviour, microstructural evolutions and mechanical properties are presented. Results indicate that ZrB2-CNT composites fabricated at 16508C have the optimal combination of dense microstructure and properties. The fracture toughness is sensitive to the temperature change and reaches 7.2 MPa m1/2 for the CNT toughened ZrB2 ceramics, which is higher than the measured result for monolithic ZrB2 (3.3 MPa m1/2). The crack deflection and CNT pullout are the dominant toughening mechanisms.
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Indium tin oxide (ITO) thin films deposited using the oblique angle deposition (OAD) technique exhibit a strong correlation between structural and optical properties, especially the optical bandgap energy. The microstructural properties of ITO thin films are strongly influenced by the tilt angle used during the OAD process. When changing the tilt angle, the refractive index, porosity, and optical bandgap energy of ITO films also change due to the existence of a preferential growth direction at the interface between ITO and the substrate. Experiments reveal that the ITO film's optical bandgap varies from 3.98 eV (at normal incident deposition) to 3.87 eV (at a 60° tilt angle).
Indium tin oxide
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Nanocrystalline material
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Many processes of interest in quantum dots involve charge or energy transfer from one dot to another. Energy transfer in films of quantum dots as well as between linked quantum dots has been demonstrated by luminescence shift, and the ultrafast time-dependence of energy transfer processes has been resolved. Bandgap variation among dots (energy disorder) and dot separation are known to play an important role in how energy diffuses. Thus, it would be very useful if energy transfer could be visualized directly on a dot-by-dot basis among small clusters or within films of quantum dots. To that effect, we report single molecule optical absorption detected by scanning tunneling microscopy (SMA-STM) to image energy pooling from donor into acceptor dots on a dot-by-dot basis. We show that we can manipulate groups of quantum dots by pruning away the dominant acceptor dot, and switching the energy transfer path to a different acceptor dot. Our experimental data agrees well with a simple Monte Carlo lattice model of energy transfer, similar to models in the literature, in which excitation energy is transferred preferentially from dots with a larger bandgap to dots with a smaller bandgap.
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