Annealing structured Au nanoparticles enhanced light emission from CdSe quantum dots

Enhancement of the light emission of CdSe quantum dots was observed by coupling through localized surface plasmons from Au nanoparticles. The enhancement was found to be relative to the shapes and sizes of Au nanoparticles. Au nanoparticles of different sizes were synthesized by a citrate-seeded method. By varying the annealing temperature, worm-like Au nanoparticles of different aspect ratios from 1 to 5 were obtained. Samples of the CdSe coupled with Au with an aspect ratio of 2 and annealed at 500 °C exhibited the best photoluminescence emission efficiency. Furthermore, a stronger photoluminescence enhancement was observed with increasing the size of Au nanoparticles. It was also found that when the localized surface plasmons resonance absorption wavelength of Au nanopartiles was just a little smaller than the emission peak of CdSe, the CdSe quantum dots exhibited the strongest photoluminescence intensity, with an enhancement of 6 times.     

Temporal evolution of white light emission from CdSe quantum dots

A study of white light emission from magic sized CdSe quantum dots is reported. Quantum dots were synthesized using a hot solution method and exhibited strong white light with a quantum yield in the range of 10-30%. The emission of white light was attributed to photoluminescence (PL) from both excitons and surface states. The wavelength and intensity of the PL from both excitons and surface states were found to depend upon the particle size, reaction time, reaction temperature and precursor ratio.     

Green synthesis of nitrogen-doped fluorescent carbon quantum dots for selective detection of iron

Carbon quantum dots (CQDs) were synthesized by a simple and economic hydrothermal method using Lycium barbarum (LB-CQDs) as precursor. Ammonia was used as a dopant to prepare nitrogen modified LB-CQDs (N-LB-CQDs). The characterizations of atomic force microscope, transmission electron microscopy, X-ray photoelectron spectroscopy showed that N-LB-CQDs were spherical in shape with an average diameter of 2～5 nm. Its surface was rich in nitrogen-containing groups. The fluorescence spectrophotometer showed that N-LB-CQDs exhibited double peak emission. The two peak positions in photoluminescence were about 462 nm and 512 nm. The as-prepared N-LB-CQDs are high-fluorescent quantum yield and water soluble. In addition, iron selectively results in a strong fluorescence quenching of N-LB-CQDs. Theoretical research results verified that iron reduced energy gap between HOMO and LUMO of N-LB-CQDs greatly, which leads to its emission wavelength shift out of the visible range. Results from the study may shed light on the production of fluorescent and biocompatible CQDs with simple, economic and environmental benign strategy in which Lycium barbarum was used as a carbon source.     

InGaAs quantum dots grown by molecular beam epitaxy for light emission on Si substrates

The aim of this study is to achieve homogeneous, high density and dislocation free InGaAs quantum dots grown by molecular beam epitaxy for light emission on silicon substrates. This work is part of a project which aims at overcoming the severe limitation suffered by silicon regarding its optoelectronic applications, especially efficient light emission device. For this study, one of the key points is to overcome the expected type II InGaAs/Si interface by inserting the InGaAs quantum dots inside a thin silicon quantum well in SiO2 fabricated on a SOI substrate. Confinement effects of the Si/SiO2 quantum well are expected to heighten the indirect silicon bandgap and then give rise to a type I interface with the InGaAs quantum dots. Band structure and optical properties are modeled within the tight binding approximation: direct energy bandgap is demonstrated in SiO2/Si/InAs/Si/SiO2 heterostructures for very thin Si layers and absorption coefficient is calculated. Thinned SOI substrates are successfully prepared using successive etching process resulting in a 2 nm-thick Si layer on top of silica. Another key point to get light emission from InGaAs quantum dots is to avoid any dislocations or defects in the quantum dots. We investigate the quantum dot size distribution, density and structural quality at different V/III beam equivalent pressure ratios, different growth temperatures and as a function of the amount of deposited material. This study was performed for InGaAs quantum dots grown on Si(001) substrates. The capping of InGaAs quantum dots by a silicon epilayer is performed in order to get efficient photoluminescence emission from quantum dots. Scanning transmission electronic microscopy images are used to study the structural quality of the quantum dots. Dislocation free In50Ga50As QDs are successfully obtained on a (001) silicon substrate. The analysis of QDs capped with silicon by Rutherford Backscattering Spectrometry in a channeling geometry is also presented.     

Detection of bioconjugated quantum dots passivated with different ligands for bio-applications

Bioconjugation of quantum dots has resulted in a significant increase in resolution of biological fluorescent labeling. This intrinsic property of quantum dots can be utilized for sensitive detection of target analytes with high sensitivity; including pathogenic bacteria and cancer monitoring. The quantum dots and quantum dot doped silica nanoparticles exhibit prominent emission peaks when excited at 400 nm but on conjugation to model rabbit antigoat antibodies exhibit diminished intensity of emission peak at 600 nm. It shows that photoluminescence intensity of conjugated quantum dots and quantum dot doped silica nanoparticles could permit the detection of bioconjugation. Samples of conjugated and unconjugated quantum dots and quantum dot doped silica nanoparticles were subjected to enzyme linked immunosorbent assay for further confirmation of bioconjugation. In the present study ligand exchange, bioconjugation, fluorescence detection of bioconjugated quantum dots and quantum dot doped silica nanoparticles and further confirmation of bioconjugation by enzyme linked immunosorbent assay has been described.     

Experiments and modeling of alloying in self-assembled quantum dots

We review the recent advances in the experimental and theoretical investigation of alloy distribution in semiconductor quantum dots (QDs). X-ray diffraction analysis, as well as wet chemical etching, represent two powerful techniques that are able to measure the alloy distribution inside the dots. From a theoretical point of view, determination of the alloy distribution follows from consideration of the thermodynamic quantities involved in the formation and stability of the QD: strain energy, surface energy, internal energy and entropy. Starting from the alloy distribution, the investigation of its role in influencing the electronic and optical properties of QDs is possible. Tight binding and ab initio calculation show the band structure of non-uniform alloyed Ge/Si and InAs/GaAs quantum dots. While for Ge/Si the indirect bandgap does not offer a strong photoluminescence spectra, direct-bandgap materials offer intense light emission, including the range for telecom applications (1.77–1.37 μm). Control of alloying inside the QDs allows for the tailoring of their band structure and photoluminescence spectra, where high alloy gradients induce a blue-shift of the spectra, compared to a more uniform composition.     

Synthesis and photoluminescence of ZnS quantum dots

Single-phase zinc sulphide (ZnS) quantum dots were synthesized by a chemical method. The influence of the pH value of the Zn(CH3COO)2 solution on the size and photoluminescence properties of the ZnS quantum dots was evaluated. X-ray power diffraction, transmission electron microscopy, and ultraviolet-visible spectroscopy were used to characterize the structure, size, surface states, and photoluminescence properties of ZnS quantum dots. The results showed that the crystal structure of ZnS quantum dots was a cubic zinc blende structure, and their average diameter was about 3.0 nm. ZnS quantum dots with good distribution and high purity were obtained. A strong broad band centered at about 320 nm was observed in the excitation spectrum of ZnS quantum dots. Their emission spectrum peaking at about 408 nm, was due mostly to the trap-state emission. The relative integrated emission intensity of ZnS quantum dots decreased as the pH value of the Zn(CH3COO)2 solution increased, which could be ascribed to the increase in average diameter of the ZnS quantum dots as the pH value of Zn(CH3COO)2 solution increased.     

Indium Phosphide‐Based Quantum Dots with Shell‐Enhanced Absorption for Luminescent Down‐Conversion

It is shown that admixing small amounts of cadmium into the shell of InP/ZnSe core/shell quantum dots results in an increased absorption of blue light and a limited redshift of the band‐edge emission. These effects reflect the reduced bandgap of (Zn,Cd)Se alloys and their smaller conduction‐band offset with InP. Nevertheless, adjusting the InP core size enables InP/ZnSe and InP/(Zn,Cd)Se quantum dots with identical emission characteristics to be made. Processing both materials into remote phosphor disks, it is demonstrated that the shell‐enhanced absorbance of InP/(Zn,Cd)Se has the double benefit of suppressing self‐absorption and reducing the amount of quantum dots by weight needed to attain a given blue‐to‐red color conversion.     

Preparation of lanthanide doped CdS, ZnS quantum dots in natural polysaccharide template and their optical properties

Employing a biomimic method using polysaccharide as template, luminescent lanthanide ions doped CdS and ZnS quantum dots (QDs) were prepared. According to the results of TEM and absorbance, nanocrystals with an average size of 6 nm were formed under mild condition without any toxic and expensive agent applied. Differentiating from the un-doped CdS and ZnS QDs prepared in polysaccharide template, the lanthanide doped QDs exhibited obvious dopant emission in their photoluminescence spectrum. It was also found that the dopant PL became more prominent with increasing lanthanide doping concentration, while the highest PL intensity was obtained at a doping level of 1% for both of CdS and ZnS QDs. When different lanthanide ions were introduced into the CdS QDs in polysaccharide template, varied emission wavelength were able to be obtained. This study provides an easy, mild and environmental friendly alternative method to prepare doped quantum dots. In addition, the bioactivity and processabilities endowed by the polysaccharide template may expand the applications potential of this type of optical materials.     

Photoluminescence of carbon dots and their applications in Hela cell imaging and Fe<Superscript>3+</Superscript> ion detection

Carbon dots are a new and important form of zero-dimensional carbonaceous nanomaterials. In our present work, disperse water-soluble fluorescent carbon dots were prepared using coffee grounds as raw material by hydrothermal treatment. The as-prepared carbon dots exhibit an excitation-dependent and high photostable photoluminescence behavior which can be applied to Hela cell imaging. Notably, the fluorescence of the carbon dots can be quenched by Fe³⁺ ions, which can serve as a useful fluorescent probe for detecting Fe³⁺ ions.     

Effect of biocompatible glutathione capping on core–shell ZnS quantum dots

Glutathione capped quantum dots are a potential candidate for different applications like ligand exchange in living cells, cell imaging and detection of glucose levels. Keeping these in mind, glutathione capped ZnS quantum dots were synthesized by using the thiol group of the capping agent by chemical precipitation method. Morphological characterizations were done by XRD and TEM. X-ray diffraction (XRD) measurements showed that the nanocrystals have Zinc Blende structure. Grain size and particle size shows a little variation with glutathione capping. Optical characterizations were done by UV–visible absorption, FTIR and energy resolved photoluminescence. UV–visible studies shows that the band gap also shows a small variation with glutathione capping. FTIR studies confirm glutathione capping on the surface of ZnS quantum dots. Room temperature energy resolved photoluminescence spectrum of samples exhibited a defect-related blue emission band. However, the PL properties seem to start tunability at higher concentration of glutathione which is a very good sign for extending this research further.     

Deep Red Emissive Carbonized Polymer Dots with Unprecedented Narrow Full Width at Half Maximum

Development of high-performance carbon dots (CDs) with emission wavelength longer than 660 nm (deep red emission) is critical in deep-tissue bioimaging, yet it is still a major challenge to obtain CDs with both narrow full width at half maximum (FWHM) and high deep red/near-infrared emission yield. Here, deep red emissive carbonized polymer dots (CPDs) with unprecedented FWHM of 20 nm are synthesized. The purified CPDs in dimethyl sulfoxide (DMSO) solution possess quantum yield (QY) as high as 59% under 413 nm excitation, as well as recorded QY of 31% under 660 nm excitation in the deep red fluorescent window. Detailed characterizations identify that CPDs have unique polymer characteristics, consisting of carbon cores and the shells of polymer chains, and π conjugated system formed with N heterocycles and aromatic rings governs the single photoluminescence (PL) center, which is responsible for high QY in deep red emissive CPDs with narrow FWHM. The CPDs exhibit strong absorption and emission in the deep red light region, low toxicity, and good biocompatibility, making them an efficient probe for both one-photon and two-photon bioimaging. CPDs are rapidly excreted via the kidney system and hepatobiliary system.     

One‐Pot Exfoliation of Graphitic C3N4 Quantum Dots for Blue QLEDs by Methylamine Intercalation

Here, a simplified synthesis of graphitic carbon nitride quantum dots (g‐C₃N₄‐QDs) with improved solution and electroluminescent properties using a one‐pot methylamine intercalation–stripping method (OMIM) to hydrothermally exfoliate QDs from bulk graphitic carbon nitride (g‐C₃N₄) is presented. The quantum dots synthesized by this method retain the blue photoluminescence with extremely high fluorescent quantum yield (47.0%). As compared to previously reported quantum dots, the g‐C₃N₄‐QDs synthesized herein have lower polydispersity and improved solution stability due to high absolute zeta‐potential (?41.23 mV), which combine to create a much more tractable material for solution processed thin film fabrication. Spin coating of these QDs yields uniform films with full coverage and low surface roughness ideal for quantum dot light‐emitting diode (QLED) fabrication. When incorporated into a functional QLED with OMIM g‐C₃N₄‐QDs as the emitting layer, the LED demonstrates ≈60× higher luminance (605 vs 11 Cd m^?2) at lower operating voltage (9 vs 21 V), as compared to the previously reported first generation g‐C₃N₄ QLEDs, though further work is needed to improve device stability.     

Excitation‐Dependent Photoluminescence from Single‐Carbon Dots

Carbon dots (CDs) are carbon‐based fluorescent nanoparticles that can exhibit excitation‐dependent photoluminescence (PL) “tunable” throughout the entire visible range, interesting for optoelectronic and imaging applications. The mechanism underlying this tunable emission remains largely debated, most prominently being ascribed to dot‐to‐dot variations that ultimately lead to excitation‐dependent ensemble properties. Here, single‐dot spectroscopy is used to elucidate the origin of the excitation‐dependent PL of CDs. It is demonstrated that already single CDs exhibit excitation‐dependent PL spectra, similar to those of the CD ensemble. The single dots, produced by a facile one‐step synthesis from chloroform and diethylamine, exhibit emission spectra with several characteristic peaks differing in emission peak position and spectral width and shape, indicating the presence of distinct emission sites on the CDs. Based on previous work, these emission sites are related to the sp² subregions in the carbon core, as well as the functional groups on the surface. These results confirm that it is possible to integrate and engineer different types of electronic transitions at the nanoscale on a single CD, making these CDs even more versatile than organic dyes or inorganic quantum dots and opening up new routes toward light‐emission engineering.     

Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots

The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4 nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326 nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed.     

Photocatalytic degradation of methylene blue using novel flower-like zirconium bismuth molybdate thin film,grown by temperature-assisted CBD method

Thin films of a novel metal molybdate are successfully coated on soda lime glass by one step chemical bath deposition method. The deposition is carried out under different bath temperature ranging from 30 to 70 °C. In order to analyze the crystalline structure, X-ray diffraction study is carried out. Film thickness and morphology are examined through atomic force microscope and scanning electron microscope. Optical studies are done by UV–Vis–NIR absorbance spectra and photoluminescence spectra. All the samples have absorbance in the UV–Vis region and possess direct allowed transition with bandgap values ranging from 2.17 to 2.83 eV. Direct current show good ohmic response and possess resistivity in the range characteristic of wide bandgap materials. The material possesses three major photoluminescence emission peaks in the green and blue wavelength regions. Thin film grown at 50 °C showed increased absorbance width, minimum bandgap energy, larger electron–hole separation efficiency and more porous flower-like morphology which indicates its higher photocatalytic ability. The photocatalytic degradation process for the removal of methylene blue from waste water, under sunlight irradiation using thin film grown at 50 °C as photocatalyst has been discussed in detail.

     

三色碳点荧光防伪油墨的制备与性能研究

目的 通过优化碳点合成方法和油墨配方,制备一种具有优良防伪效果和印刷适性的环保丝网印刷油墨。方法 以邻苯二胺为碳源,水或乙醇为溶剂,采用溶剂(水)热法制备红色和黄色碳点。以柠檬酸钠和碳酸氢铵为碳源和氮源制备蓝色碳点,并对三色碳点的结构组成和光学性质进行表征和分析。以三色碳点作为荧光颜料,选择乙醇或水作为溶剂,水性环氧树脂或聚丙烯酸树脂作为连接料,通过实验获得最佳配比,制备三色荧光防伪油墨。结果 三色碳点均具有较为均匀的尺寸,在365 nm紫外光激发下分别发射725 nm的红色荧光、450 nm的蓝色荧光和570 nm的黄色荧光,且rCDs、bCDs和yCDs的荧光量子产率分别为56.63%、64.37%和78.26%。通过对pH、细度、黏度等性能测试,该荧光防伪油墨各项印刷适性指标良好。结论 通过优化碳点合成方法可控调节荧光发射光谱,制备出具有较宽的紫外吸收带、较窄的发射光谱带、荧光量子产率高的三色碳点。以此碳点作为荧光颜料可以制备出印刷适性良好的水性油墨,满足荧光防伪印刷的要求。     

CdS／PAMAM纳米复合材料的制备及光学性能

以聚酰胺－胺（PAMAM）树形分子为模板，原位合成CdS，AMAM树形分子纳米复合材料，探索不同负载比对其性能的影响及量子点表面的功能性。通过荧光光谱技术，分析Cu^2＋和Mn^2＋重金属离子对其光致发光性能的影响，利用光学性质变化、材料分析稳定度考察其反应机制。     

Assignment of Core and Surface States in Multicolor-Emissive Carbon Dots

It is important to reveal the luminescence mechanisms of carbon dots (CDs). Herein, CDs with two types of optical centers are synthesized from citric acid in formamide by a solvothermal method, and show high photoluminescence quantum yield reaching 42%. Their green/yellow emission exhibits pronounced vibrational structure and high resistance toward photobleaching, while broad red photoluminescence is sensitive to solvents, temperature, and UV–IR. Under UV–IR, the red emission is gradually bleached due to the photoinduced dehydration of the deprotonated surface of CDs in dimethyl sulfoxide, while this process is hindered in water. From the analysis of steady-state and time-resolved photoluminescence and transient absorption data together with density functional theory calculations, the green/ yellow emission is assigned to conjugated sp²-domains (core state) similar to organic dye derivatives stacked within disk-shaped CDs; and the broad red emission—to oxygen-containing groups bound to sp²-domains (surface state), whereas energy transfer from the core to the surface state can happen.     

Using layer-by-layer assembly to fabricate NaLa(MoO4)2@CdTe core–shell microspheres with high fluorescence

The development of fluorescent materials with low energy consumption, low cost and desirable optical properties is needed for the perspective of practical application. Here, functional NaLa(MoO₄)₂@CdTe core–shell microspheres with high fluorescence were prepared by layer-by-layer self-assembly technique. Through the consecutive electrostatic adsorption of charged cetyltrimethyl ammonium bromide and CdTe quantum dots (QDs), the uniform and regular multilayer shell of CdTe QDs was synthesized. The NaLa(MoO₄)₂@CdTe microspheres exhibited improved photoluminescence intensity and stability of red emission, compared with that of the CdTe QDs powder, and the fluorescence enhancement mechanism were investigated. The CdTe QDs multilayer shell is expected to supersede the Eu³⁺ ion for producing a novel red phosphor.