Topological Susceptibility under Gradient Flow

We study the impact of the Gradient Flow on the topology in various models of lattice field theory. The topological susceptibility $\chi_{\rm t}$ is measured directly, and by the slab method, which is based on the topological content of sub-volumes ("slabs") and estimates $\chi_{\rm t}$ even when the system remains trapped in a fixed topological sector. The results obtained by both methods are essentially consistent, but the impact of the Gradient Flow on the characteristic quantity of the slab method seems to be different in 2-flavour QCD and in the 2d O(3) model. In the latter model, we further address the question whether or not the Gradient Flow leads to a finite continuum limit of the topological susceptibility (rescaled by the correlation length squared, $\xi^{2}$). This ongoing study is based on direct measurements of $\chi_{\rm t}$ in $L \times L$ lattices, at $L/\xi \simeq 6$.Comment: 8 pages, LaTex, 5 figures, talk presented at the 35th International Symposium on Lattice Field Theory, June 18-24, 2017, Granada, Spai

Self-Precipitation of Highly Purified Red Emitting Carbon Dots as Red Phosphors

Colloidal carbon dots (C-dots) have attracted a great deal of attention for their unique optical properties. However, it is still a challenge to obtain highly purified C-dots without using multiple-step purification or postsize selection. In this work, a self-precipitation hydrothermal reaction was used to synthesize red-emitting C-dots (R-C-dots) using o-phenylenediamine (o-PDA) as a precursor without using any catalyst. The R-C-dots are able to precipitate on the wall of the reactor, which enables us to obtain solid-state C-dots with high purity. The R-C-dots have a photoluminescence quantum yield (PLQY) of as high as 36.75%, which is among the highest PLQY values reported previously for R-C-dots without using catalysts. The transient PL and transient absorption spectra revealed that 5,14-dihydroquinoxalino[2,3-b]phenazine linked on the surface of the C-dots determined the red luminescence behavior. This work provides a new path for the controllable synthesis of high-purity R-C-dots, showing potential applications in optoelectronic devices

Self-Precipitation of Highly Purified Red Emitting Carbon Dots as Red Phosphors

Colloidal carbon dots (C-dots) have attracted a great deal of attention for their unique optical properties. However, it is still a challenge to obtain highly purified C-dots without using multiple-step purification or postsize selection. In this work, a self-precipitation hydrothermal reaction was used to synthesize red-emitting C-dots (R-C-dots) using o-phenylenediamine (o-PDA) as a precursor without using any catalyst. The R-C-dots are able to precipitate on the wall of the reactor, which enables us to obtain solid-state C-dots with high purity. The R-C-dots have a photoluminescence quantum yield (PLQY) of as high as 36.75%, which is among the highest PLQY values reported previously for R-C-dots without using catalysts. The transient PL and transient absorption spectra revealed that 5,14-dihydroquinoxalino[2,3-b]phenazine linked on the surface of the C-dots determined the red luminescence behavior. This work provides a new path for the controllable synthesis of high-purity R-C-dots, showing potential applications in optoelectronic devices

Double Emissions in Boron-Doped Carbon Dots for Anticounterfeiting Applications

Carbon dots (C-dots) have excellent optical properties and serve as optical building blocks for various potential applications. However, the origin of the double emission in C-dots is still unclear. It is highly desirable to understand deeply the exciton dynamics in double-emission C-dots on the picosecond time scale. Herein, for the first time, double-emission boron-doped carbon dots (B–C-dots) are synthesized with a quantum yield as high as 41.1%. The B–C-dots have an excitation-dependent quantum yield and photoluminescent spectrum. Compared to one dominant energy state in undoped C-dots, B–C-dots exhibited multiple energy states, as proved by femtosecond transient absorption spectroscopy. The controllable double emissions advance the design of a high-level anticounterfeiting code. As a proof-of-concept, by the combination of single-emission and double-emission C-dots, we produced a highly bright excitation-dependent photoluminescence code, showing the great potential for anticounterfeiting systems

Exciton Dynamic in Pyramidal InP/ZnSe Quantum Dots for Luminescent Solar Concentrators

Colloidal indium phosphide (InP) quantum dots (QDs) have attracted a great attention for their excellent optical properties, and they are a promising candidate for potential applications in optoelectronic devices, such as photocatalysis, light emitting diodes, and bioimaging. However, their use for luminescent solar concentrators (LSCs) has been limited owing to their low optical efficiency, which is because of the energy loss due to the strong overlap between their emission and absorption spectra. Here, we synthesized pyramidal InP/ZnSe QDs based on aminophosphine as the phosphorus precursor by adding hydrofluoric acid to remove the oxides on the InP core surface before the growth of a ZnSe shell. Through researching the exciton dynamic of InP/ZnSe QDs with different shell thicknesses, we found that a thick shell can stabilize the 1Pe energy state of the InP core, thus decreasing the spectral overlap in the overall QDs. As a proof of concept, we demonstrated a highly efficient LSC based on “giant” InP/ZnSe QDs, which exhibited an external optical efficiency of 2.5% and a power conversion efficiency of 2% (5 × 5 × 0.5 cm3) upon simulated sunlight illumination (100 mW/cm2), which is among the best reported LSCs based on InP QDs. Pyramidal “giant” InP/ZnSe QDs hold great potential for the breakthrough development in the field of eco-friendly QD-based LSCs

Room-Temperature Synthesis of Carbon Dot/TiO₂ Composites with High Photocatalytic Activity

Benefiting from the wide-range absorption and adjustable energy gap, carbon dots (C-dots) have attracted a great deal of attention and they have been used to sensitize semiconductor nanocomposites to boost the efficiency of energy conversion devices, while there is still a lack of fundamental understanding of the interaction between such materials and their influence on the catalytic activity on the reaction process. In this study, C-dots were used to modify TiO2 to form a direct Z-scheme (DZS) junction for enhancement of the photocatalytic activity. The C-dot/TiO2 composite was prepared by ultrasonication at room temperature through coupling between the Ti–O–C bond and electrostatic interaction. The C-dots can dramatically enhance the absorption of the composite by forming the DZS, and the composite is enabled to generate more free radicals, which facilitate ∼10 times higher photocatalytic activity compared to that of TiO2. As a proof of concept, the as-prepared C-dot/TiO2 was used for textile wastewater dye degradation. This study provides an efficient approach for room-temperature preparation of C-dot/TiO2 composites with high photocatalytic activity

Tuning the Charge-Transfer Property of PbS-Quantum Dot/TiO₂-Nanobelt Nanohybrids via Quantum Confinement

A newly designed photoactive nanohybrid structure based on the combination of near-infrared PbS quantum dots (QDs) as light harvester and one-dimensional TiO₂ nanobelts (NBs) to guide the flow of photogenerated charge carriers is reported. Efficient electron transfer from photoexcited PbS QDs to TiO₂ NBs has been demonstrated to occur in the developed PbS-QD/TiO₂-NB nanohybrids, and the charge-transfer property can be tuned through the size quantization effect of PbS QDs. Moreover, the use of TiO₂ NBs instead of TiO₂ NPs permits a larger critical size of PbS QDs capable of injecting electrons into TiO₂ NBs, which, in turn, markedly extends the “effective” absorption of the PbS-QD/TiO₂-NB nanohybrids to a longer wavelength region up to 1400 nm. Such an extension of the “effective” absorption is a major asset for improving the overall photoconversion efficiency of PbS-QD/TiO₂-NB nanohybrids-based photovoltaic devices

Size Dependence of Temperature-Related Optical Properties of PbS and PbS/CdS Core/Shell Quantum Dots

The effect of PbS core size on the temperature-dependent photoluminescence (PL) of PbS/CdS quantum dots (QDs) in the temperature range of 100–300 K was thoroughly investigated and compared with shell-free PbS QDs. The core/shell QDs show significantly smaller PL intensity variation with temperature at a smaller PbS size, while a larger activation energy when the PbS domain size is relatively large, suggesting both different density and different distribution of defects/traps in the PbS and PbS/CdS QDs. The most remarkable difference consists in the PbS size dependence of the energy gap temperature coefficient (d<i>E</i>/d<i>T</i>). The PbS/CdS QDs show unusual non-monotonic d<i>E</i>/d<i>T</i> variation, resulting in the reversal of the d<i>E</i>/d<i>T</i> difference between the PbS and PbS/CdS QDs at a larger PbS size. In combination with theoretical calculations, we find that, although lattice dilation and carrier-phonon coupling are generally considered as dominant terms, the unique negative contribution to d<i>E</i>/d<i>T</i> from the core/shell interfacial strain becomes most important in the relatively larger-core PbS@CdS QDs

Role of Carbon Nanotubes to Enhance the Long-Term Stability of Dye-Sensitized Solar Cells

Improving the long-term stability of dye-sensitized solar cells (DSSCs) is a critical challenge which affects both their technical viability and future large-scale commercialization. Here, we investigate the role of multiwall carbon nanotubes (MWCNTs) in improving the long-term stability of DSSCs by comparing the performance of two series of devices made of (i) bare nanocrystalline TiO2 and (ii) MWCNTs-TiO2 composite anode, which are exposed to continuous simulated sunlight, indoor and ultraviolet (UV) light irradiation. The DSSCs based on the composite anode showed approximately 3 times longer stability compared with the standard device. To understand the degradation mechanisms that underpin these changes in device performance, both devices were characterized using various techniques. The results indicate that the MWCNTs can act as a conductive support, reinforcing the TiO2 nanoparticles’ matrix and offering a directional path to the photoinjected electrons, which enhances electron lifetime and reduces the carrier recombination rate. UV stability measurements demonstrated that MWCNTs can partially absorb and act as a blocking agent for UV light, thereby preventing degradation. The Raman spectra showed that dye desorption was decreased by the addition of MWCNTs. Our results provide a fundamental understanding of photoanode degradation mechanisms under illumination and offer a simple, low-cost, and large-area scalable approach to fabricate solar-energy-conversion devices with long-term stability

Silver Nanorice Structures: Oriented Attachment-Dominated Growth, High Environmental Sensitivity, and Real-Space Visualization of Multipolar Resonances

We have synthesized and investigated the anisotropic growth of interesting silver nanorice. Its growth is kinetically controlled at 100 °C, and both oriented attachment and Ostwald ripening are involved, with the former growth mode dominating the anisotropic growth of the nanorice along the ⟨111⟩ direction. This one-directional growth is initiated by an indispensable seed-selection process, in which oxygen plays a critical role in oxidatively etching twinned silver crystals. The inhibition of this process by removing oxygen essentially blocks the nanorice growth. Although increasing reaction temperature to 120 °C accelerates the one-dimensional growth along the ⟨111⟩ direction, further temperature increase to 160 °C makes the oriented attachment dominated one-directional growth disappear; instead, the diffusion-controlled two-dimensional growth leads to the emergence of highly faceted truncated triangular and hexagonal plates mainly bound by low energy faces of {111}. Interestingly, we also found that the longitudinal surface plasmon resonance of the nanorice structures is highly sensitive to the refractive index of surrounding dielectric media, which predicts their promising applications as chemical or biological sensors. Moreover, the multipolar plasmonic resonances in these individual nanorice structures are visualized in real space, using high-resolution electron energy-loss spectroscopy