Water‐Dispersible Silicon‐Quantum‐Dot‐Containing Micelles Self‐Assembled from an Amphiphilic Polymer

The dispersion of silicon quantum dots (Si QDs) in water has not been established as well as that in organic solvents. It is now demonstrated that the excellent dispersion of Si QDs in water with photoluminescence (PL) quantum yields (QYs) comparable to those for hydrophobic Si QDs can be realized by combining the processes of hydrosilylation and self‐assembly. Hydrogen‐passivated Si QDs are initially hydrosilylated with 1‐dodecence. The toluene solution of the resulting dodecyl‐passivated Si QDs is mixed with the water solution of the amphiphilic polymer of Pluronic F127 to form an emulsion. Dodecyl‐passivated Si QDs are encapsulated in the micelles self‐assembled from F127 in the emulsion. The size of the Si‐QD‐containing micelles may be tuned in the range from 10 to 100 nm. Although self‐assembly in the emulsion causes the PL QY of Si QDs to decrease, after a few days of storage in ambient conditions, Si QDs encapsulated in the water‐dispersible micelles exhibit recovered PL QYs of ≈24% at the PL wavelength of ≈680 nm. The intensity of the PL from Si QDs encapsulated in the water‐dispersible micelles is >90% of the original value after 60 min ultraviolet illumination, indicating excellent photostability.     

Hydrothermal Preparation of Photoluminescent Graphene Quantum Dots Characterized Excitation‐Independent Emission and its Application as a Bioimaging Reagent

Zero‐dimensional photoluminescent (PL) graphene quantum dots (GQDs) that can be used as the cell‐imaging reagent are prepared by a hydrothermal route using the graphene oxide (GO) as the carbon source. Under the optimized hydrothermal conditions, an initial hydrogen peroxide concentration of 0.5 mg mL^?1 at 180 °C for 120 min, the GO sheets can be cut into nanocrystals with lateral dimensions in the range of 1.5–5.5 nm and an average thickness of around 1.1 nm. The as‐prepared GQDs exhibit an abundance of hydrophilic hydroxy and carboxyl groups and emit bright blue luminescence with up‐conversion properties in a water solution at neutral pH. Most interestingly, they indicate excitation‐independent emission characteristics, and the surface state is demonstrated to have a key role in the PL properties. The fluorescence quantum yield of the GQDs is tested to be around 6.99% using quinine sulfate as a standard. In addition, the as‐prepared GQDs can enter into HeLa cells easily as a fluorescent imaging reagent without any further functionalization, indicating they are aqueous stability, biocompatibility, and promising for potential applications in biolabeling and solution state optoelectronics.     

Optimum Quantum Yield of the Light Emission from 2 to 10 nm Hydrosilylated Silicon Quantum Dots

Optimizing the light‐emitting efficiency of silicon quantum dots (Si QDs) has been recently intensified by the demand of the practical use of Si QDs in a variety of fields such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light‐emitting efficiency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈2–10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum‐confinement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specific QD size for Si QDs. The optimum PL QY results from the interplay between quantum‐confinement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light‐emission efficiency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out.     

Synthesis of Blue‐, Green‐, Yellow‐, and Red‐Emitting Graphene‐Quantum‐Dot‐Based Nanomaterials with Excitation‐Independent Emission

A one‐pot method is described for the preparation of graphene quantum dots/graphene oxide (GQDs/GO) hybrid composites with emission in the visible region, through heteroatom doping and hydroxyl‐radical‐induced decomposition of GO. The NH₄OH‐ and thiourea‐mediated dissociation of H₂O₂ produces hydroxyl radicals. Treatment of GO with hydroxyl radicals results in the production of small‐sized GO sheets and GQDs, which self‐assemble to form GQDs/GO through strong π–π interactions. For example, the reaction of GO with a mixture of NH₄OH and H₂O₂ for 40, 120, and 270 min generates yellow‐emitting GQDs/GO (Y‐GQDs/GO), green‐emitting GQDs/GO, and blue‐emitting GQDs, while red‐emitting GQDs/GO (R‐GQDs/GO) are prepared by incubating GO with a mixture of thiourea and H₂O₂. From the analysis of these four GQD‐based nanomaterials by transmission electron microscopy, atomic force microscopy, and fluorescence lifetime spectroscopy, it is found that this tunable fluorescence wavelength results from the differences in particle size. All four GQD‐based nanomaterials exhibit moderate quantum yields (1–10%), nanosecond fluorescence lifetimes, and excitation‐independent emissions. Except for R‐GQDs/GO, the other three GQD‐based nanomaterials are stable in a high‐concentration salt solution (e.g., 1.6 m NaCl) and under high‐power irradiation, enabling the sensitive (high‐temperature resolution and large activation energy) and reversible detection of temperature change. It is further demonstrated that Y‐GQD/GO can be used to image HeLa cells.     

Highly Luminescent Carbon‐Nanoparticle‐Based Materials: Factors Influencing Photoluminescence Quantum Yield

Unravelling the factors influencing photoluminescence (PL) quantum yield of the carbon nanoparticles (CNPs) is the prerequisite for preparing highly luminescent CNP‐based materials. In this work, an easy and effective method is reported for preparing highly luminescent CNP‐based materials. Water‐soluble luminescent CNPs (CNP‐Cs) with large size distribution (1–60 nm) with PL quantum yields of 22% are synthesized through a microwave pyrolysis approach. Energy transfer (ET) is confirmed to occur from small size CNPs (CNP‐Ss:1‐7 nm, blue emitters) to large size CNPs (CNP‐Ls:10–60 nm, green emitters). Further centrifugally separating CNP‐Cs resulted in an enhancement of the PL quantum yield up to 39% of CNP‐Ss aqueous solution. The PL quantum yield of CNP‐Ss could even be further improved in high‐viscosity solvents. PL quantum yield higher than 90% is achieved in films of commercial glue water embedded with the CNP‐Ss at embedding ratio lower than 3 wt%. By contrast, the yield is greatly decreased in the CNP‐C‐embedding films with embedding ratio higher than 1 wt%, which is due to self‐absorption, as well as enhanced ET between CNP‐Ss and CNP‐Ls. High‐viscosity solvents and polymer matrix are proposed to act as surface passivation reagents to enhance PL quantum yield of CNPs.     

Effect of Spectral Overlap and Separation Distance on Exciton and Biexciton Quantum Yields and Radiative and Nonradiative Recombination Rates in Quantum Dots Near Plasmon Nanoparticles

Efficient biexciton (BX) photoluminescence (PL) from quantum dots (QDs) paves the way to the generation of entangled photons and related applications. However, the quantum yield (QY) of BX PL is much lower than that for single excitons (EX) due to efficient Auger-like recombination. In the vicinity of plasmon nanoparticles, the recombination rates of EX and BX may be affected by the Purcell effect, fluorescence quenching, and the excitation rate enhancement. Here, the effect of the plasmon resonance spectral position on the EX and BX PL is experimentally studied in two cases: when the plasmon band overlaps with the excitation wavelength and when it coincides with the QDs PL band. In the first case, the EX and BX excitation efficiencies are significantly increased but the EX QY reduced. As a result, the BX-to-EX QY ratio is higher than 1 at plasmon–exciton systems separations shorter than 40 nm. In the second case, the radiative recombination rates are enhanced by several orders of magnitude, which led to an increase in BX QY over distances of up to 90 nm. Finally, these two effects are obtained in the same hybrid structure, with the resultant increase in both excitation efficiency and QY of BX PL.     

Graphene Quantum Dots from Polycyclic Aromatic Hydrocarbon for Bioimaging and Sensing of Fe3+ and Hydrogen Peroxide

An easy approach for large‐scale and low‐cost synthesis of photoluminescent (PL) graphene quantum dots (GQDs) based on the carbonization of commercially available polycyclic aromatic hydrocarbon (PAH) precursors with strong acid and followed by hydrothermal reduction with hydrazine hydrate is reported. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) characterizations indicate that the size and height of GQDs are in the range of 5–10 nm and 0.5–2.5 nm, respectively. PAH, which has more benzene rings, generally forms GQDs with relatively larger size. The GQDs show high water solubility, tunable photoluminescence, low cytotoxicity, and good optical stability, which makes them promising fluorescent probes for cellular imaging. In addition, the fluorescence of GQDs shows a sensitive and selective quenching effect to Fe³⁺ with a detection limit of 5 × 10^?9m . By combination with the Fe²⁺/Fe³⁺ redox couple, the PL GQDs are able to detect oxidant, using H₂O₂ as an example. This study opens up new opportunities to make full use of GQDs because of their facile availability, cost‐effective productivity, and robust functionality.     

Decay processes of photoluminescence in a nanocrystalline SiC thin film

Nanocrystalline (nc)-SiC film has been deposited by helicon wave plasma enhanced chemical vapor deposition technique and intense blue-white light emission is obtained. Microstructure analyses show that the 3C–SiC particles are embed in amorphous SiC matrix, and the average size of the nc-SiC is 3.96 nm. The photon energy of the main photoluminescence (PL) band is higher than the band gap of bulk SiC, which indicates that the optical emission mainly occurs in quantum states of 3C–SiC nanocrystals. In addition, the band tail states of amorphous SiC also contribute to the optical emission. Three decay processes are obtained from time-resolved PL spectra by deconvolution treatment, and the decay components correspond to the quantum confinement effect (QCE), surface states of nc-SiC particles, and band tail of amorphous SiC, respectively. The fractional integrated PL intensity of QCE related decay process decreases dramatically in the lower PL photon energy, indicating that the QCE mainly contributes to the short wavelength optical emission.     

PbSe量子点荧光匹配气体吸收光谱方法研究

PbSe量子点(PbSe-QDs)是红外波段的典型纳米材料,其具有大的玻尔半径、小的体材料禁带宽度(玻尔半径是46 nm,体材料禁带宽度是0.28 eV),因此,在近红外区域,PbSe-QDs具有强大的尺寸受限效应和较高的量子产出率。本文对不同尺寸的PbSe量子点的荧光光谱特性进行了研究,提出了一种通过调节PbSe量子点的量子尺寸匹配气体吸收光谱的方法。采用配位溶剂的方法制备了尺寸为4.6和6.1 nm的PbSe量子点,将该PbSe量子点沉积到GaN发光芯片上并经过紫外光照处理和固化后制成了光致发光的近红外光源,其中4.6 nm的PbSe-QDs的沉积厚度为671.5 μm,而6.1 nm的PbSe-QDs的沉积厚度为48 μm。将制成的近红外光源应用到C₂H₂气体和NH₃气体的检测实验中,实验结果表明,通过改变PbSe量子点的尺寸可以调节光源光致发光峰的位置,从而覆盖目标气体在近红外波段的吸收谱线。4.6 nm的光源发射光谱包含了1 500～1 550 nm之间的C₂H₂气体的全部的吸收谱;6.1 nm的光源发射光谱包含了1 900～2 060 nm之间的NH₃气体的全部的吸收谱。这种利用PbSe量子点尺寸的可调性匹配对应气体吸收谱的方法是可行和有效的,具有广阔的应用前景。     

Effect of size variation on the cathodoluminescence characteristics of graphene quantum dots

Cathodoluminescence (CL) has been studied in graphene quantum dots (GQDs) by varying their average size (d) from 5 to 35 nm. The size dependence of CL peak wavelength is very analogous to that of photoluminescence (PL) peak wavelength unusually showing non-monotonic behaviors having a maximum at d = ∼17 nm. The CL behaviors can therefore be attributed to the novel feature of GQDs, i.e., the circular-to-polygonal-shape and corresponding edge-state variations of GQDs at d = ∼17 nm as d increases. However, the peak wavelengths of CL are especially much smaller than those of PL at both ends in the size range of GQDs, possibly resulting from the recombination of the electron-beam-excited e-h pairs at higher energy states before thermalization due to fast carrier-carrier scattering dominating over electron-phonon scattering in graphene.     

Tunable and High Photoluminescence Quantum Yield from Self‐Decorated TiO2 Quantum Dots on Fluorine Doped Mesoporous TiO2 Flowers by Rapid Thermal Annealing

Herein a novel approach is reported to achieve tunable and high photoluminescence (PL) quantum yield (QY) from the self‐grown spherical TiO₂ quantum dots (QDs) on fluorine doped TiO₂ (F‐TiO₂) flowers, mesoporous in nature, synthesized by a simple solvothermal process. The strong PL emission from F‐TiO₂ QDs centered at ≈485 nm is associated with shallow and deep traps, and a record high PL QY of ≈5.76% is measured at room temperature. Size distribution and doping of F‐TiO₂ nanocrystals (NCs) are successfully tuned by simply varying the HF concentration during synthesis. During the post‐growth rapid thermal annealing (RTA) under vacuum, the arbitrary shaped F‐TiO₂ NCs transform into spherical QDs with smaller sizes and it shows dramatic enhancement (≈163 times) in the PL intensity. Electron spin resonance (ESR) and X‐ray photoelectron spectroscopy (XPS) confirm the high density of oxygen vacancy defects on the surface of TiO₂ NCs. Confocal fluorescence microscopy imaging shows bright whitish emission from the F‐TiO₂ QDs. Low temperature and time resolved PL studies reveal that the ultrafast radiative recombination in the TiO₂ QDs results in highly efficient PL emission. A highly stable, biologically inert, and highly fluorescent TiO₂ QDs/flowers without any capping agent demonstrated here is significant for emerging applications in bioimaging, energy, and environmental cleaning.     

Multicolor Light‐Emitting Diodes with MoS2 Quantum Dots

Molybdenum disulfide (MoS₂) quantum dots (QDs) are known for their excitation‐wavelength‐dependent photoluminescent (PL) properties. However, the mechanism of this phenomenon is still unclear. Here, small size MoS₂ QDs with a narrow size distribution are synthesized. Based on the decay study and PL dynamics, a reasonable radiation model is presented to understand the special PL properties, i.e., the carrier recombination in the localized surface defect states generated the PL. Accordingly, this optical property is used to fabricate multicolor light‐emitting devices with the same MoS₂ QDs. The emission color covers the full visible spectrum from blue to red, only by adjusting the thickness of the down‐conversion QD layers.     

ZnO@GQDs核壳结构量子点的制备及性能研究

采用原位聚合法制备了以ZnO量子点为核、石墨烯量子点（GQDs）为壳的ZnO@ GQDs核壳结构量子点。通过TEM和HR-TEM对量子点进行形貌和结构的分析表征。结果表明,合成的ZnO@ GQDs核壳结构量子点为球形,粒径为～7 nm,且尺寸均匀。PL光谱研究表明,新型量子点的发射峰位于369 nm,发光峰窄、强度高;相对于ZnO的本征发射峰,GQDs的引入使得ZnO@GQDs核壳量子点的荧光发射峰出现蓝移、强度变高,从而使复合量子点的荧光具有较纯的色度和较高的强度,说明GQDs的引入具有协同优化效应。该量子点有望应用于LED显示器件。     

Versatile Graphene Quantum Dots with Tunable Nitrogen Doping

This paper reports a facile fabrication of N‐doped graphene quantum dots (N‐GQDs) showing controllable chemical properties through a hydrothermal treatment. The N‐GQDs have a uniform size of 3.06 ± 0.78 nm and prefer the equilibrium shapes of circle and ellipse due to the minimization of edge free energy. The N/C atomic ratio in N‐GQDs can be precisely tailored in a range from 8.3 at% to 15.8 at% by simply controlling the concentration of N source (ammonium hydroxide). One order of magnitude quantum yield of 34.5% is achieved by N‐GQDs, compared with the N‐free GQDs, as the substitutional N has an essential role in more effective radiative emission. Excessive N dopants in N‐GQDs can lead to photoluminescence quenching, through nonradiative transition back to the ground state. The N‐GQDs are further found to be suitable as photocurrent conversion materials due to benign energy matching with anatase nanofibers, the ultrafast electron injection at their interface, and efficient electron transfer. This work provides an efficient and inspiring approach to engineering both chemical components and physical properties of N‐GQDs, and will therefore promote their basic research and applications in energy conversion.     

Assessing Carrier Recombination Processes in Type‐II SiGe/Si(001) Quantum Dots

In this work, it is shown how different carrier recombination paths significantly broaden the photoluminescence (PL) emission bandwidth observed in type‐II self‐assembled SiGe/Si(001) quantum dots (QDs). QDs grown by molecular beam epitaxy with very homogeneous size distribution, onion‐shaped composition profile, and Si capping layer thicknesses varying from 0 to 1100 nm are utilized to assess the optical carrier‐recombination paths. By using high‐energy photons for PL excitation, electron‐hole pairs can be selectively generated either above or below the QD layer and, thus, clearly access two radiative carrier recombination channels. Fitting the charge carrier capture‐, loss‐ and recombination‐dynamics to PL time‐decay curves measured for different experimental configurations allows to obtain quantitative information of carrier capture‐, excitonic‐emission‐, and Auger‐recombination rates in this type‐II nano‐system.     

Photoluminescence dynamics of organic molecule-passivated Si nanoclusters

The size-selected phenyl-passivated Si nanoclusters with the mean diameters of 1.5 and 1.3 nm have been prepared in the solution route. The intense PL observed in Si nanoclusters, in which the band gap energies increase up to approximately 4 eV, originates from electron-hole pair recombination. From time-resolved PL, two kinds of exponential PL decay components with fast and slow lifetime were observed. The identical decays with slow lifetime will be derived from the surface states of Si nanoclusters passivated by phenyl molecules. The size-dependent zero-phonon assisted optical transitions by quantum size effects occur at a high rate of sub-nanosecond timescale.     

Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.     

Carrier dynamics in type-II GaAsSb/GaAs quantum wells

Time-resolved photoluminescence (PL) characteristics of type-II GaAsSb/GaAs quantum wells are presented. The PL kinetics are determined by the dynamic band bending effect and the distribution of localized centers below the quantum well band gap. The dynamic band bending results from the spatially separated electron and hole distribution functions evolving in time. It strongly depends on the optical pump power density and causes temporal renormalization of the quantum well ground-state energy occurring a few nanoseconds after the optical pulse excitation. Moreover, it alters the optical transition oscillator strength. The measured PL lifetime is 4.5 ns. We point out the critical role of the charge transfer processes between the quantum well and localized centers, which accelerate the quantum well photoluminescence decay at low temperature. However, at elevated temperatures the thermally activated back transfer process slows down the quantum well photoluminescence kinetics. A three-level rate equation model is proposed to explain these observations.     

Size-dependent radiative emission of PbS quantum dots embedded in Nafion membrane

PbS quantum dots (QDs) have been incorporated in a Nafion membrane, where the QD sizes were adjusted by changing the reaction time due to the steady growing process. The radiative emissions of the samples were investigated by optical absorption, photoluminescence (PL), and time-resolved PL spectroscopy. Size-tunable emissions are shown by the PL spectrum in a range of 1.84–1.65 eV, and the emission mechanism was investigated based on a four-band envelope-function model. Possible energy transitions for the radiative emission are listed. The PL lifetime depending on the particle size is about one microsecond, and PL decay curves exhibit a trend of decreasing decay time with an increase of the PbS QD size.     

Luminescence decay of broadband emission from CdS quantum dots

The stretched exponential photoluminescence decay of the energy-resolved broadband emission of purified and unpurified CdS quantum dots (QDs) made in reverse micelles is characterized as a function of photolysis time and thiol addition. Photolysis is found to proportionately increase both the lifetime and quantum yield of these QDs. This proportionality is consistent with a simple parallel channel model of the decay of the excited states. The ultimate QY of the purified sample is found to be as high as 24%, which is twice that previously reported for this preparation. At −70 °C both the QY and the lifetime increase by more than a factor of two, indicating that thermal quenching limits the QY at room temperature. Finally, the addition of alkanethiols is shown to red-shift and quench the emission while only modestly altering the lifetime. These thiolated QDs show an extremely large temperature dependence of QY, demonstrating stronger thermal quenching than the unfunctionalized QDs.