Quantum dots (QDs) show great promise for use in nanotechnology, owing to their high quantum efficiency, color tenability, narrow emission, and high luminescence efficiency. As a new generation of light-emitting devices (LEDs), QD-LEDs have attracted a great deal of attention in displays and lighting. To meet the commercial requirements, the brightness of QD-LEDs needs to be further improved. In this work, we propose multilayer heterostructures on a SiO2 substrate, which provides multiple reflective bands with the very high reflective efficiency of nearly up to 100%. Electric field distributes mostly in the superficial layer. The proposed structure provides highly multiband reflection covering the emission peaks of QDs in LEDs; hence, it can eventually enhance QDs' fluorescence and enhance the brightness of QD-LEDs. We investigate four typical emission wavelengths, mainly aiming for red QD-LEDs and infrared QD-LEDs, which correspond to the applications of displays, infrared illumination, optical communication, and so on. The total reflection bands can be adjusted according to practical requirements by tuning the thickness of every layer. One fabrication procedure can be used for different kinds of QDs or the same kind of QD with different sizes without changing their processing properties. The proposed structure has fewer flat layers compared with 1-D photonic crystals, which leads to lower cost and easier fabrications.

Enhancing the Brightness of Quantum Dot Light-Emitting Diodes by Multilayer Heterostructures

A one-dimensional photonic crystal (1DPC), which is a periodic nanostructure with a refractive index distribution along one direction, has been widely studied by scientists. In this review, materials and methods for 1DPC fabrication are summarized. Applications are listed, with a special emphasis on sensing platforms and photovoltaic devices together with full color display. After that, some typical 3D ordered structures with stacked layers are highlighted, fabrication methods are also described, and remaining problems are pointed out. Lastly, the possibility of building 3D stacked structures based on 1D layers through chemical routes is discussed; a relatively convenient and flexible method. We believe such a method is a promising way to conduct 3D fabrication.

One-dimensional photonic crystals: fabrication, responsiveness and emerging applications in 3D construction

We have developed a simple assay for the sensing of cobalt ions (Co2+), based on the analyte induced photoluminescence (PL) quenching of carbon dots (C-dots). The C-dots (mean diameter 3.6 ± 0.3 nm) prepared from L-cysteine through a simple hydrothermal process at 300 °C for 2 h have a quantum yield of 13.2%. The C-dots have strong blue PL with a maximum PL intensity at 395 nm under an excitation wavelength of 325 nm. Through the reactions of Co2+ ions with cysteine molecules/residues on the surfaces of the C-dots, non-photoluminescent CoxSy nanoparticles are formed. As-formed CoxSy nanoparticles and the C-dots further form aggregates in the solution, leading to PL quenching. The C-dot probe allows detection of Co2+ ions over a concentration range from 10 nM to 100 μM (R2 = 0.992). This reliable, rapid, sensitive, and selective C-dot probe has been utilized for the determination of the concentrations of Co2+ ions in vitamin B12 and natural water samples.

Synthesis of photoluminescent carbon dots for the detection of cobalt ions

Photoluminescent carbon dots (CDs) as a novel carbon-based material have attracted much more attention. Here, we reported the synthesis of cellulose-based carbon dots by a facile and highly reproducible new rote from cellulose and urea. The as-obtained CDs were characterized via FTIR, XPS, TEM, UV-vis and PL spectra. The as-obtained CDs showed high nitrogen content, stable and excitation-independent blue-green photoluminesce. The quantum yield (QY) of the synthesized CDs is up to 21.7 %. Furthermore, the low cytotoxicity and great biocompatibility of CDs were proved via CCK-8 assay and fluorescence bioimaging. The results indicate that the cellulose-based CDs show promising applications for bioimaging in living cell systems.

Synthesis of Cellulose‐Based Carbon Dots for Bioimaging

In the present work, the upconversion luminescence properties of oleic acid capped NaYF4:Gd3+:Yb3+:Er3+ upconversion nanoparticles (UCNP) with pure β crystal phase and Nd3+ ions as an additional sensitizer were studied in the temperature range of 288 K < T < 328 K. The results of this study showed that the complex interplay of different mechanisms and effects, causing the special temperature behavior of the UCNP can be developed into thermometry on the nanoscale, e.g. to be applied in biological systems on a cellular level. The performance was improved by the use of Nd3+ as an additional dopant utilizing the cascade sensitization mechanism in tri-doped UCNP.

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Upconversion NaYF4:Yb:Er nanoparticles co-doped with Gd3+ and Nd3+ for thermometry on the nanoscale

Large normal-direction excitation and emission of dual-emitting quantum dots (QDs) are essential for practical application of QD sensors based on the ratiometric fluorescence response. We have numerically demonstrated an all-dielectric four-layer cascaded photonic crystal (CPC) structure (alternating TiO2 and SiO2/SU8 layers with two dimensional nanoscale patterns in each layer) which is capable of providing normal-direction high Q-factor leaky modes at excitation wavelengths of QDs and two low Q-factor leaky modes coinciding with the two emission peaks of a dual-emitting QD. Normal-direction excitation and far-field emission of the dual-emitting QDs are enhanced significantly when QDs are distributed on/in the top TiO2 layer of the CPC structure, especially in the spatial distribution areas of the resonant leaky modes. QDs can be positioned differently depending on the applications. Positioning QDs on the top TiO2 layer will improve the signal-to-noise ratios of QD biomedical/chemical/temperature sensors, while embedding QDs in the top TiO2 layer will increase the light extraction from the QD light emitting device, making our CPC a versatile optical coupling structure. Our CPC-QD structure is experimentally feasible and robust against the parameter perturbation in real fabrication.

Zhongyuan Yu

Papers

Enhanced normal-direction excitation and emission of dual-emitting quantum dots on a cascaded photonic crystal surface