Wenjin Zhang

Facile synthesis of ZnS-CuInS2-alloyed nanocrystals for a color-tunable fluorchrome and photocatalyst.

High-quality ZnS-CuInS(2) (ZCIS) alloy nanocrystals have been synthesized via reaction between the acetate salts of the corresponding metals and elemental sulfur in the presence of dodecanethiol in octadecene media at 230 °C. The PL emission wavelength can be tuned conveniently via variation of the stoichiometric ratio of their components. The influence of various experimental variables, including Zn/CuIn ratio, amount of sulfur and dodecanethiol, and reaction temperature, on the optical properties and composition of the obtained ZCIS NCs have been systematically investigated. The plain ZCIS NCs did show PL emission but with quite low PL quantum yield (typically below 3%). In order to improve the PL emission efficiency, the ZnS shell was subsequently overcoated around the ZCIS core NCs. With ZnS shell growth, the PL emission wavelength of the resulting ZCIS/ZnS NCs can cover from 518 to 810 nm with the maximum PL quantum efficiency up to 56%. Furthermore, the obtained ZCIS/ZnS NCs show promising photocatalytic activity in the degradation of rhodamine B.

Design and Synthesis of Highly Luminescent Near-Infrared-Emitting Water-Soluble CdTe/CdSe/ZnS Core/Shell/Shell Quantum Dots

Applications of water-dispersible near-infrared (NIR)-emitting quantum dots (QDs) have been hampered by their instability and low photoluminescence (PL) efficiencies. In this paper, water-soluble highly luminescent NIR-emitting QDs were developed through constructing CdTe/CdSe/ZnS core/shell/shell nanostructure. The CdTe/CdSe type-II structure yields the QDs with NIR emission. By varying the size of CdTe cores and the thickness of the CdSe shell, the emission wavelength of the obtained nanostructure can span from 540 to 825 nm. In addition, the passivation of the ZnS shell with a substantially wide bandgap confines the excitons within the CdTe/CdSe interface and isolates them from the solution environment and consequently improves the stability of the nanostructure, especially in aqueous media. An effective shell-coating route was developed for the preparation of CdTe/CdSe core/shell nanostructures by selecting capping reagents with a strong coordinating capacity and adopting a low temperature for shell deposition. An additional ZnS shell was deposited around the outer layer of CdTe/CdSe QDs to form the core/shell/shell nanostructure through the decomposition of single molecular precursor zinc diethyldithiocarbamate in the crude CdTe/CdSe reaction solution. The water solubilization of the initially oil-soluble CdTe/CdSe/ZnS QDs was achieved through ligand replacement by 3-mercaptopropionic acid. The as-prepared water-soluble CdTe/CdSe/ZnS QDs possess PL quantum yields as high as 84% in aqueous media, which is one of the best results for the luminescent semiconductor nanocrystals.

One-Pot Noninjection Synthesis of Cu-Doped ZnxCd1-xS Nanocrystals with Emission Color Tunable over Entire Visible Spectrum

Unlike Mn doped quantum dots (d-dots), the emission color of Cu dopant in Cu d-dots is dependent on the nature, size, and composition of host nanocrystals (NCs). The tunable Cu dopant emission has been achieved via tuning the particle size of host NCs in previous reports. In this paper, for the first time we doped Cu impurity in Zn(x)Cd(1-x)S alloyed NCs and tuned the dopant emission in the whole visible spectrum via variation of the stoichiometric ratio of Zn/Cd precursors in the host Zn(x)Cd(1-x)S alloyed NCs. A facile noninjection and low cost approach for the synthesis of Cu:Zn(x)Cd(1-x)S d-dots was reported. The optical properties and structure of the obtained Cu:Zn(x)Cd(1-x)S d-dots have been characterized by UV-vis spectroscopy, photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The influences of various experimental variables, including Zn/Cd ratio, reaction temperature, and Cu dopant concentration, on the optical properties of Cu dopant emission have been systematically investigated. The as-prepared Cu:Zn(x)Cd(1-x)S d-dots did show PL emission but with quite low quantum yield (QY) (typically below 6%). With the deposition of ZnS shell around the Cu:Zn(x)Cd(1-x)S core NCs, the PL QY increased substantially with a maximum value of 65%. More importantly, the high PL QY can be preserved when the initial oil-soluble d-dots were transferred into aqueous media via ligand replacement by mercaptoundeconic acid. In addition, these d-dots have thermal stability up to 250 °C.

Dual emissive manganese and copper Co-doped Zn-In-S quantum dots as a single color-converter for high color rendering white-light-emitting diodes.

Novel white light emitting diodes (LEDs) with environmentally friendly dual emissive quantum dots (QDs) as single color-converters are one of the most promising high-quality solid-state lighting sources for meeting the growing global demand for resource sustainability. A facile method was developed for the synthesis of the bright green-red-emitting Mn and Cu codoped Zn-In-S QDs with an absorption bangdgap of 2.56 eV (485 nm), a large Stokes shift of 150 nm, and high emission quantum yield up to 75%, which were suitable for warm white LEDs based on blue GaN chips. The wide photoluminescence (PL) spectra composed of Cu-related green and Mn-related red emissions in the codoped QDs could be controlled by varying the doping concentrations of Mn and Cu ions. The energy transfer processes in Mn and Cu codoped QDs were proposed on the basis of the changes in PL intensity and lifetime measured by means of steady-state and time-resolved PL spectra. By integrating these bicolor QDs with commercial GaN-based blue LEDs, the as-fabricated tricolor white LEDs showed bright natural white light with a color rendering index of 95, luminous efficacy of 73.2 lm/W, and color temperature of 5092 K. These results indicated that (Mn,Cu):Zn-In-S/ZnS QDs could be used as a single color-converting material for the next generation of solid-state lighting.

Quantum Dots Acting as Energy Acceptors with Organic Dyes as Donors in Solution

Quantum dots (QDs) usually act as energy donors in Förster resonant energy transfer (FRET) in various application fields. We report, for the first time, a FRET process from a conventional naphthalimide chromophore 1 to CdSe/ZnS core/shell QDs (acting as energy acceptors) in (1)-QDs hybrid system in solution. This FRET process is supported by various spectroscopies, such as steady-state and time-resolved photoluminescence (PL) as well as PL excitation spectra. The highest energy transfer efficiency is estimated to be about 0.37. Interestingly, the role of QDs in FRET can be specifically reversed (from energy acceptors to energy donors) with the use of QDs with larger band-gaps.

Noninjection facile synthesis of Gram-scale highly luminescent CdSe multipod nanocrystals.

Nearly all reported approaches for synthesis of high quality CdSe nanocrystals (NCs) involved two steps of preparation of Cd or Se stock solution in advance and then mixing the two reactants via hot-injection in high temperature. In this manuscript, Gram-scale CdSe multipod NCs were facilely synthesized in a noninjection route with the use of CdO and Se powder directly as reactants in paraffin reaction medium containing small amount of oleic acid and trioctylphosphine. The influence of various experimental variables, including reaction temperature, nature and amount of surfactants, Cd-to-Se ratio, and the nature of reactants, on the morphology of the obtained CdSe NCs have been systematically investigated. After deposition of ZnS shell around the CdSe multipod NCs, the PL QY of the obtained CdSe/ZnS can be up to 85%. The reported noninjection preparation approach can satisfy the requirement of industrial production bearing the advantage of low-cost, reproducible, and scalable. Furthermore, this facile noninjection strategy provides a versatile route to large-scale preparation of other semiconductor NCs with multipod or other morphologies.

Noninjection ultralarge-scaled synthesis of shape-tunable CdS nanocrystals as photocatalysts

CdS nanocrystals with a wide variety of shapes including spheres, tetrahedra, and branched and flower-like structures have been conveniently tuned by only changing the amount of trioctylphosphine (TOP) used in a noninjection synthetic approach, in which CdO and S powder were directly used as reactants in paraffin media. The noninjection approach could be facilely scaled up to tens of grams scale in one batch reaction. The shape-controlled growth mechanism could be explained by the TOP-concentration-dependent nuclei structure and monomer concentration. This is further confirmed by the effects of the amount of oleic acid and the Cd/S ratio on the morphology of the obtained CdS nanocrystals. All the CdS nanocrystal samples with different morphologies exhibit good photocatalytic activity for degradation of dyes. The observed lower photocatalytic activity of the sphere-shaped CdS nanocrystals could be ascribed to the higher PL emission efficiency relative to those with other morphologies, which results in low electron–hole separation efficiency. Our reported preparation approach can satisfy the requirements of industrial production bearing the advantage of low-cost, reproducibility and scalability.

Control of piezoelectric inertia-friction actuator with consideration of thermal effects

In this paper, two control strategies are developed to control the piezoelectric inertia-friction actuator (PIFA) systems with a great attention to counter for the thermal effect. The first strategy is to use two controllers: one is the main controller (used to control the actuation) and the other is the auxiliary controller to adjust the contact pressure (used to compensate the thermal effect). The second strategy is to use one controller, which serves for two purposes: to control the actuation and to compensate and the thermal effect. The two strategies are tested in a validated simulator. The result shows that the second strategy is better in terms of control accuracy and hardware cost. This paper may appear to be the first study on PIFA with consideration of the thermal effect.

On membership of black-box or white-box of artificial neural network models

Artificial Neural network (ANN) has been widely used in solving complex real world problems. It is commonly known as a black-box approach in the sense that examples (input and output pairs) are learned in the process of establishing an ANN model. However, there is a long debate in literature on whether ANN is a white-box or a black-box approach. In this article, we take a more conceptual view towards the nature of modelling with ANN, leading to a conclusion that ANN can be a black-box or grey-box approach, depending on the way to create an ANN model, in particular whether the knowledge of the problem and/or domain of the problem to be modeled by ANN is used in the creation of the ANN model. Another contribution of this paper is the outline of a methodology for designing ANN models, which is based on our proposed view in this paper that designing an ANN model is like designing a product and then the design theory and methodology for general products is learned to designing an ANN model.